Rings containing Silicon to Lead

14.19 Rings containing Silicon to Lead F. Sa˛czewski and A. Kornicka ´ ´ Medical University of Gdansk, Gdansk, Poland ª 2008 Elsevier Ltd. All rights reserved. 14.19.1

Introduction

979

14.19.2

Parent Rings with One Group 14 Heteroatom

979

14.19.2.1

Seven-Membered Rings

14.19.2.1.1 14.19.2.1.2

14.19.2.2 14.19.3

979

Monocyclic derivatives Fused benzo derivatives

979 983

Larger Rings

986

Rings with Two or More Heteroatoms

987

14.19.3.1

Rings Containing E–(CH2)n–E and Related Units

987

14.19.3.2

Rings Containing E–CUC–E and Related Units

990

14.19.3.3

Insertion into Si–Si and Ge–Ge Bonds

995

14.19.3.4

Rings Containing C–E–O and Related Units

999

14.19.3.5

Rings Containing O–E–O and Related Units

1005

14.19.3.6

Silacrown Ethers, Calixarenes, Cyclophanes, and Metallacenes

1011

14.19.3.7

Atranes and Related Compounds

1019

14.19.4

Reactivity and Transformations of Heterocyclic Rings

1021

14.19.5

Applications of Computational Methods

1027

14.19.6

Further Developments

1028

References

1033

14.19.1 Introduction This subject was covered previously in pages 993–1022 in CHEC-II(1996) (volume 9, chapter 36). This chapter is intended to update the previous work on major preparative and structural aspects of various types of rings containing silicon to lead that have been reported since 1995. As compared to previous work, two novel topics are covered: reactivity and transformations of heterocyclic rings in Section 14.19.4 and application of computational methods in Section 14.19.5. Moreover, silacrown ethers and related compounds such as calixarenes, cyclophanes, and metallacenes are covered in Section 14.19.3.6. During the past decade, the chemistry of heterocycles containing group 14 heteroatoms has been studied widely and many books and review articles have been published on this subject. Readers are advised to consult the monograph, The Chemistry of Silicon Compounds and following reviews: <2004CR5847, 2003COR691, 1999EJI373, 1995CCR157, 1999CRV3463, 2001T7237, 2004AGE4704, 1997AGE2426, 1996POL4311, 2002CCR47, 1995CRV813, 1998T2289, 2005AOM440, 2002AOM481, 2006T7951, 1997CSR453>.

14.19.2 Parent Rings with One Group 14 Heteroatom 14.19.2.1 Seven-Membered Rings 14.19.2.1.1

Monocyclic derivatives

Over the last decade, the synthesis and characterization of trivalent silyl cations (silylium ions), including silatropylium ion 1 (Figure 1), have remained as a major challenge in organosilicon chemistry, and, therefore, was the subject of intensive studies. The earlier suggestion that silatropylium ion 1 is more stable than silabenzyl cation 2 and may

979

980

Rings containing Silicon to Lead

Figure 1

exist in the gas phase <1993JA10805> was disapproved by both the ab initio molecular orbital calculations <1994JA9769> and experimental study showing that actually it was a rearranged adduct C6H?5SiHþ of type 3 <1997JA6376>. However, the first silatropylium ion 4 stabilized by rigid -frameworks has recently been prepared <2000JA9312, 2001T3645> having in mind the previously established guideline for the possible generation of silylium ions in condensed phase <1992JA7737, 1999JA5001>, which include (1) the silylium ion center should be surrounded by bulky substituents, (2) the cation should be prepared in the presence of a low-coordinating counteranion, and (3) a low-coordinating solvent should be used for the synthesis. The cyclic p-conjugated tropylium ion 4 synthesized from dibromo derivative 5 according to the procedure described in Scheme 1 was found to be stable at temperatures below 50  C and was characterized by 1H, 13C, and 29Si nuclear magnetic resonance (NMR) spectral data. As a clear evidence for the silylium ion, character of 4 was assumed by the presence of a signal at 142.9 ppm in 29Si NMR spectrum run in CH2Cl2 solution. For comparison, the corresponding signal in the spectrum of the precursor silepin 7, structure of which was determined by X-ray crystallography, was found at 49.3 ppm.

Scheme 1

The fully unsaturated seven-membered rings containing group 14 elements (1,1-dimethylmetallepins) 8–10 were also obtained (Equation 1) and characterized by ultraviolet (UV) and NMR spectroscopic data as well as X-ray crystallographic analysis. All of these compounds were found to have the central seven-membered ring in a boat form. Metallepins 8–10 are stable in the solid state but readily decompose under acidic conditions even during elution from a silica-gel column <1995JOC1309>.

ð1Þ

Rings containing Silicon to Lead

The transition metal-catalyzed hydrosilylation of olefins, which is one of the most versatile methods for the synthesis of alkylsilanes, has been known for many years . The reaction of 1,5-hexadiene with monosilane (SiH4) was carried out in an autoclave in the presence of a catalytic amount of Pt(PPh3)4 forming a 1:1 mixture of 5-hexenylsilane and silacycloheptane 11 in 31% yield (Equation 2). Product 11 was separated by distillation under reduced pressure and its structure was confirmed by 1H NMR, infrared (IR) and mass spectrometry (MS) spectral data <1999JOM241>. Hydrosilylation of 1,5-hexadiene with phenylsilane (PhSiH3) catalyzed by commercially available lanthanum tris[bis(trimethylsilyl)amide] led to the formation of silacycloheptane 12 and silamethylcyclopentane 13 as a 1:4 mixture in 95% yield (Equation 3) <2004OM12>.

Regioselective synthesis of silacycloalkanes can also be achieved under radical conditions. Thus, upon treatment of 2-(bromomethyl)-2-methyl-silahept-6-enyl-isobutyrate 14 with tributylstannane (HSnBu3) in the presence of azobisisobutyronitrile (AIBN) in boiling benzene for 48 h, a 56/44 mixture of 2,2-dimethyl-2-silahept-6-enyl isobutyrate 15 and (1-methyl-1-silacycloheptyl)methyl isobutyrate 16 was formed in 73% yield as shown in Equation (4) <2001MGM363>. In similar reactions of triallyl(3-bromopropyl)silane and (3-bromopropyl)tripropargylsilane, corresponding silacycloheptane 17 and silacycloheptene 18 were obtained in 48% and 28% yields, respectively <2001JOM160>.

ð4Þ

Intramolecular hydrosilylation of alkynes is a widely used method for the synthesis of various silacycles with a vinylsilane framework. As shown in Scheme 2, the reaction provides three different types of silacycles, depending on the mode of the cyclization and addition of SiH bond. The transition metal-catalyzed reaction proceeds in a cis-manner leading to exo-silacycles of general formula A <1999S921>. On the other hand, the Lewis acid-catalyzed hydrosilylation could produce the endo- (B) or exo- (C) silacycles depending on the substrates, to give five-, six-, seven-, or eight-membered rings (Scheme 2) <2000JOC8919>. Thus, the AlCl3-catalyzed reaction of the phenylsubstituted alkyne 19 having a tether of five methylene groups gave the seven-membered silacycle 20 as a result of the exo-mode intramolecular hydrosilylation (Equation 5) <2000JOC8919>.

981

982

Rings containing Silicon to Lead

Scheme 2

ð5Þ

Another method for the synthesis of silacycloalkenes involves carbon–carbon bond formation by a Ru(II)-catalyzed ring-closing metathesis (RCM). As shown in Equation (6), cyclization of dienyl silanes 21 conducted in benzene afforded the corresponding 1-silacyclohept-4-enes 22a–c and silaspirene 23 in 35–60% yield <2001JOM160>. The above procedure has also been applied to the preparation of cyclic siloxanes from appropriately functionalized alkoxysilanes <1997TL4757, 1997TL7861, 1998JOC6768>.

ð6Þ

Enantiomerically pure (E)-1,1,3,3,6,6-hexamethyl-1-sila-4-cycloheptene 25, the smallest nonbridged (E)-cycloalkene which can be isolated in a pure form at room temperature, was synthesized by the Corey–Winter elimination of hexamethyl-1-sila-trans-4,5-cycloheptanethiocarbonate 24 with 1,3-dimethyl-2-phenyl-1,3,2-diazaphospholidine (Equation 7) <1997AGE159, 1999TA3483>.

ð7Þ

Rings containing Silicon to Lead

A facile synthesis of cyclic alkylsilanes consisting in the electrochemical reduction of aliphatic dibromides in the presence of polychlorosilanes of the formula RnSiCl4 (n ¼ 0, 2) affords heterocyclic compounds in good yields <1995JOM213>. According to the procedure described in Equation (8), the compound 26 was obtained in 57% yield. In contrast to nonelectrochemical methods, which are based on the ring closure of terminal unsaturated compounds, the electrochemical route is claimed to be more efficient and selective.

ð8Þ

Reaction of vinyl(3-phenylaminopropyl)dimethylsilane 27 with Hg(OAc)2 followed by the treatment with NaBH4 afforded 4-silaazepane 28 and 3-silapiperidine 29 as a 2.7:1 mixture in 49% yield (Equation 9). Structures of these products were confirmed by 1H, 13C, and 29Si NMR spectroscopic data <2001ZOB1979>.

ð9Þ

14.19.2.1.2

Fused benzo derivatives

As noted in Section 14.19.2.1.1, the Lewis acid-catalyzed hydrosilylation of unactivated acetylenes provided a general method for the synthesis of alkenylsilanes. Hence, intramolecular trans-hydrosilylation of TMS-substituted alkyne 30 bearing a benzene ring spacer led to the formation of endo-cyclization product 31 (Equation 10) <2000JOC8919>.

ð10Þ

It should be pointed out that intermolecular hydrostannation of acetylene compounds which can be induced by radical initiators, transition metal catalysts, base catalysts, or Lewis acids to form vinylstannanes has also been known for many years. However, the first synthesis of 1-benzostannepines 34 was described in 1998 from (Z)-1-(o-bromophenyl)but-1-en-3-ynes 32 via the tin hydride intermediate 33 by the intramolecular 7-endo-dig-ring closure at the sp carbon atom of ethynyl moiety (Scheme 3) <1998CC767, 2000J(PI)1965>. 2-Alkyl-1-benzostannepins thus obtained are stable (not sensitive to air, light and moisture, and colorless oils).

Scheme 3

983

984

Rings containing Silicon to Lead

Fully unsaturated group 14 2-trimethylsilyl-1-heteropines 36 and C-unsubstituted parent rings 37 were obtained from the common starting compound (Z)-1-(bromophenyl)-4-trimethylsilyl-1-butyn-3-ene 35, as shown in Scheme 4 <1999CPB1108>. Structures of the newly prepared compounds 36 and 37 were confirmed by 1H NMR and MS spectral data.

Scheme 4

An alternative route for the preparation of 1-benzosilepines and 1-benzostannepines of type 39 utilized the conversion of 1-benzotellurepins 38 via the Te–Li exchange and successive coupling of dilithium intermediate with silicon or tin reagent <2000H(53)49>. Thus, tellurepines 38 were lithiated with 2.2 equiv of ButLi in the presence of tetramethylethylenediamine (TMEDA) followed by addition of dibutyltin or dichlorodimethylsilane to give corresponding products 39 in 15–35% yield (Scheme 5).

Scheme 5

Synthesis of 1,9-difluoro-5-methyl-5-phenyl-10,11-dihydro-5H-dibenzo[b,f ]silepin 40, the first silepin isolated with substituents adjacent to the ethano bridge, has been achieved starting from 2-chloro-6-fluorotoluene, as depicted in Scheme 6. The compound 40 was characterized in the solid state by X-ray crystallography and in solution by 1H, 13 C, 29Si, and 19F NMR spectroscopy. The effect of substituents adjacent to the ethano bridge was found to produce a heterocycle with a smaller butterfly angle (bend angle) than any other tricyclic silepin characterized crystallographically thus far <1995JOM113>.

Scheme 6

It has been known that enantiometrically pure complexes of biphenolate 43 (H2[Biphen]) serve as catalysts for efficient asymmetric metathesis reactions. A method of derivatizing the biphenolate ligand found in these catalysts

Rings containing Silicon to Lead

has been developed <2001OM4705>. First, the readily available enantiopure ligand 41 was transformed using a three-step procedure into the ligand H2[Me2SiBiphen] 42. Then, optically pure molybdenium complex 43 was prepared by a standard procedure involving deprotonation of the ligand with benzyl potassium followed by addition of Mo(NAr)(CHCMe2Ph)(OTf)2(DME) (Scheme 7).

Scheme 7

The reaction of dilithium dialkyl bipyridines 44 derived from dilithiation of 3,39-dimethyl-2,29-bipyridine with (R1)2ECl2 (R1 ¼ Me or Ph; E ¼ Si, Ge, or Sn) afforded the axially symmetric seven-membered metallacycles 45, which were purified by column chromatography and characterized by their MS and NMR spectra and for 45c and 45e the X-ray structure has been determined. Subsequent reaction of 45d–f with W(CO)6 in toluene gave the corresponding bimetallic complexes 46a–c (Scheme 8) <1997OM4839>.

Scheme 8

Reactivity of functionalized arylcarbenes with the CH2–X–Ph (X ¼ CH2, O, SiMe2) group in the ortho-position was studied in terms of both the rate and regioselectivity of intramolecular carbene addition to the terminal phenyl group <1997T9935>. Phenylcarbene 48 with the Si(Me)2CH2Ph group in the ortho-position, generated thermally from tosylhydrazone precursor 47, underwent an insertion reaction into the 29,39-bond of the terminal phenyl group to give a norcardiene derivative 49 in 56% yield as a result of the unique donating effect of R3SiCH2 substituent (Scheme 9).

985

986

Rings containing Silicon to Lead

Scheme 9

Tin hydrides (R)-50 and (S)-50 were prepared in three steps starting from (R)- and (S)-2,29-bis(chloromethyl)-1,19binaphthyl, respectively, in an overall yield of 46% (Scheme 10). This sequence can be used for a wide variety of different alkyl substituents at the tin atom <2003TA3069>.

Scheme 10

When (2-bromophenyl)lithium 51, generated by the reaction of 1,2-dibromobenzene with n-butyllithium at 110  C, was treated with di-t-butyldichlorostannane and then the reaction was warmed to room temperature, an unexpected product tribenzostannepin 52 was obtained in 16% yield together with 9-stannafluorene (Equation 11). Although the mechanism of this reaction was not investigated in detail, the tribenzo skeleton of 52 was reasonably assumed to be formed by successive couplings of benzyne generated from 2-bromophenyllithium with 1,2-dibromobenzene.The structure of 52, which is the first example of a tribenzo-fused stannepin, was established by X-ray crystallographic analysis. The central seven-membered ring has a boat conformation <2001OM749>.

ð11Þ

14.19.2.2 Larger Rings 5,5-Dimethyl-6-(trimethylsilyl)-5,8,9,10-tetrahydro-5-silabenzocyclooctane 53 (Figure 2) was obtained in 48% yield via the Lewis acid-catalyzed intramolecular trans-hydrosilylation of 1-(dimethylsilanyl)-2-[5-(trimethylsilyl)-pent-4-enyl]benzene according to the procedure discussed above for its seven-membered ring analogue 31 <2000JOC8919>. Metallaacetylenes of general structure 54 represent an interesting class of derivatives because they are expected to show carbene-like character perturbed electronically by a neighboring divalent heavy group 14 element through the resonance forms shown in Scheme 11. The first successful generation of stannaacetylene 54 (E ¼ Sn) consisted in the reaction of arylchlorostannylene 55 with silyldiazomethyllithium (formation of aryldiazostannylene 56) followed

Rings containing Silicon to Lead

Figure 2

R1

R1

• •

E C R Metallaacetylene

E

R

54

C •

•

Carbene Pri

Li C N2 • •

Ar

Sn Cl

R13Si

• •

Ar

55

Pri

N2 Sn

SiR13

hν Ar

Sn C

56

SiR13

57

Me Pri

Pri

Sn • •

Pri

Pri

Si(Pri)3

Pri Pri

Ar =

58

Pri Pri Pri Scheme 11

by the photolysis of a benzene solution of 56 using a 500 W high-pressure mercury arc lamp at room temperature. Diazomethylstannylene 56 was obtained in 18% yield as thermally stable but air- and moisture-sensitive red crystals and its structure was confirmed by X-ray crystallography. The photolysis of 56 led to the formation of stannaacetylene 57, which showed singlet carbene character and underwent the intramolecular insertion of the carbene moiety to a proximate methyl C–H bond in an isopropyl group with the formation of stannacyclooctane 58 in 70% yield. Interestingly, the photocyclization proceeded stereoselectively and gave 58 with a cis-arrangement between methyl and triisopropylsilyl substituents on the heterocyclic ring <2004JA2696>. The sila-anti-Bredt olefins (olefins with bridgehead double bond) were obtained starting from 59, as a key intermediate. Thus, addition of dibromo- or chlorofluorocarbene to 59 gave the crude 60, which was converted in ethanol to 4-silabicyclo[5.3.1]undec-1(11)-ene 61 (Scheme 12). Molecular structure of 61 was confirmed by X-ray crystallographic studies <2001JOC1216>.

14.19.3 Rings with Two or More Heteroatoms 14.19.3.1 Rings Containing E–(CH2)n–E and Related Units Upon treatment of bis(dimethylsilyl)ethyne 62 with trialkylborane, 1,1-organoboration took place with formation of 63, which by hydrosilylation of the CTC bond of the alkyl group resulted in the formation of the new sevenmembered heterocycle 64 (Scheme 13). The presence of the SiH- - -B bridge in 63 was confirmed unequivocally by IR, 1H, 13C, and 29Si NMR spectroscopic evidence <1999AGE124>. Compound 64 was isolated by distillation as a colorless, extremely air-sensitive liquid.

987

988

Rings containing Silicon to Lead

Scheme 12

Scheme 13

Chelated diorganolithiate ion 65 was prepared and isolated as its [Li(TMEDA)]þ salt (Scheme 14). It was then transformed into the highly crowded neutral metallacycles 66: organomercury compound <1996OM1651>, plumbacycloalkane <1997OM5621>, and the chelated compounds of Zn and Yb <1999OM2342> (Scheme 14). On the other hand, the reaction of 65 with MnCl2 in tetrahydrofuran (THF) afforded complex 67 (Equation 12) <2000OM1190>. Structures of these compounds were confirmed by X-ray crystallographic studies.

Scheme 14

65

MnCl2 THF

(SiMe3)2 Me2 Si C Mn Cl Li(THF)3 C Si Me (SiMe3)2 2

67

ð12Þ

Rings containing Silicon to Lead

A series of seven- and eight-membered disilacycloalkenes 69 was prepared in 49–87% yield under mild reaction conditions using ruthenium-catalyzed ring-closing metathesis (RCM) of various ,!-bis(allyldimethylsilyl)-substituted compounds 68 (Equation 13). Interestingly, 5-oxa-4,6-disilacycloheptane and 5-aza-4,6-disilacycloheptane also could be obtained in this maner. The method failed, however, when the ring size was increased to a nine-membered disilaalkene. In order to deepen the understanding of the reaction mechanism and of the accessible ring size, the semiempirical PM3 conformational analyses were carried out <1999BCJ821>.

ð13Þ

Reactivity of the highly crowded silicon-substituted cyclic stannylene 70 was investigated in detail <2002OM2430>. Upon treatment with iodoalkanes, enones, and diones, the derivatives 71, 72, and 73 were obtained (Figure 3) and their structure was confirmed by 1H, 13C, 29Si, and 119Sn NMR spectroscopy as well as X-ray crystallographic studies.

Figure 3

The anti-pentasilane 78 was prepared starting from 1,3-dichlorosilane according to Scheme 15. First, the monocyclic structure 75 was constructed by the reaction of 74 with a di-Grignard reagent and after successive introduction of allyl and dimethylphenylsilyl groups at positions 1 and 3 via exhaustive dephenylchlorination and partial amination, RCM of 76 catalyzed by benzylidene ruthenium complex afforded the bicyclic 77. Hydrogenation at the unsaturated bond in the ‘tether’, followed by replacement of the phenyl groups in the termini by methyl groups, afforded pentasilane 78. The structure of 78 with its conformation rigidly constrained to all-anti on the basis of the bis(tetramethylene)-tethered bicyclic trisilane unit was confirmed by X-ray crystallographic studies <2004OM3375>. A series of cyclic diynes of type 79–81 with common dimethylsilyl or dimethylgermanyl units in the propargylic position of the ring were synthesized (Figure 4). Based on their photoelectron (PE) spectra, a strong interaction between the plane p-linear combinations of the triple bonds and the bridges has been postulated <1997CB1807, 1997TL8679, 1999J(P2)2093, 1999OM3615>. Reductive lithiation of trisilapentane 82 with lithium 4,49-di-tert-butylbiphenylide (LDBB) and silylation with 1,2dichloro-1,1,2,2-tetramethyldisilane at 78  C gave pentasilacycloheptane 83 in 69% yield (Scheme 16). A similar cyclization proceeded with dichlorodimethylsilane, while no cyclization occurred with 1,3-dichloro-1,1,2,2,3,3-hexamethyltrisilane due to steric hindrance <2000JOM12>.

989

990

Rings containing Silicon to Lead

Scheme 15

Figure 4

Scheme 16

14.19.3.2 Rings Containing E–CUC–E and Related Units One-sided sterically congested cyclic siladiynes were prepared according to two different protocols. Terminal diynes 84 were transformed into the corresponding Grignard derivatives with MeMgBr, which upon treatment with 1,2-dichloro1,1,2,2-tetramethyldisilane, 1,4-dichloro-1,1,4,4-tetramethyl-1,4-disilane, or 1,3-dichloro-1,1,2,2,3,3-hexamethyl-1,4-disilane at 25  C in THF were converted into cyclic siladiynes 85, 86, and 87, respectively (Scheme 17) <1995TL4603, 1997OM646>. Structure of 85 (n ¼ 1) was confirmed by X-ray crystallographic studies. Compounds 87 1,2,3-trisilacyclodeca-4,9-diyne (n ¼ 1), 1,2,3-trisilacycloundeca-4,10-diyne (n ¼ 2), and 1,2,3-trisilacyclododeca-4,11-diyne (n ¼ 3) were further treated with trimethylamine oxide in refluxing benzene to afford 2,4-dioxa-1,3,5-trisilacyclodeca-6,11-diyne, 2,4-dioxa-1,3,5-trisilacyclotrideca-6,12-diyne, and 2,4-dioxa-1,3,5-trisilatetradeca-6,13-diyne 88, respectively. Synthesis of 90 and 93 was realized by reaction of the dilithium salt of 3,3,4,4-tetramethyl-3,4-disilahexa-1,5-diyne 89 with corresponding diiodides at 40  C in THF. The yields of isolated products 90 and 93 were low (1% and 4%, respectively; Scheme 18) <1995TL4603>. Formation of side products 91, 92, and 94 was explained by the previously described <1988CC1079> fragmentation of 89 into (CH3)2SiLi(CUC)Li as an intermediate.

Rings containing Silicon to Lead

Scheme 17

Scheme 18

The dibenzo derivative of a cyclic acetylenic silane 96 was synthesized by mono-deprotonation of 1,2-diethynylbenzene with 1 equiv of lithium hexamethyldisilazide (LiHMDS) followed by treatment with 0.5 equiv of dichlorodiphenylsilane (formation of 95). A repeat of the deprotonation step, followed by silylation cycle, gave 96 in 77% yield after column chromatography (Scheme 19). One-step preparation of 96 using 2 equiv of base and 2 equiv of the silylation agent also yielded the desired product, however, the yield was 50% of those of the previous method

991

992

Rings containing Silicon to Lead

<2000TL2079>. Structure of 96 was fully characterized by MS, IR, and 1H and 13C NMR spectroscopic methods as well as X-ray crystallography. It should be pointed out that this macrocyclic compound was also mentioned in the patent literature <1995JPP07126394>.

Scheme 19

Dissociative addition of a silicon–silicon linkage to unsaturated carbon units is well known as the bis-silylation reaction. Silole-containing cyclic disilanes, such as 4,5,10-trisilabicyclo[6.3.0]undeca-1(11),8-diene-2,6-diynes 97, underwent the Pd-catalyzed bis-silylation reactions with acetylenes and 1,3-diynes producing cyclic adducts <1995OM1089>. Addition of butadiyne units occurred regiospecifically, depending on the nature of substituents on the triple bonds. As shown in Scheme 20, in this manner 4,7,12-trisilabicyclo[8.3.0]trideca-1(13),5,10-triene-2,8diynes 98 and 4,9,14-trisilabicyclo[10.3.0]pentadeca-1(15),5,6,7,12-pentaene-2,10-diyne 99 were prepared. Structure of the latter was confirmed by X-ray crystallographic analysis <1999OM3792>.

Scheme 20

Rings containing Silicon to Lead

1,10-Distanna-cyclo-octadeca-2,8,11,17-tetrayne 100 was obtained in 84% yield from the reaction of 1,7-octadiyne with bis(diethylamino)dimethyltin or by reacting 1,8-bis(trimethylstannyl)-1,7-octadiyne 101 with dimethyltin dichloride or bromide in 75% and 80%, respectively (Scheme 21). Structure of 100 was confirmed by MS, IR, 1H and 119Sn NMR spectroscopic data <1997MGM573>.

Scheme 21

The first silyl-substituted radialene 103 was obtained in 11% yield from the macrocyclic tetrayne 102 by irradiation with a 500 W high-pressure mercury lamp under the reflux temperature in THF in the presence of 3 molar excess of [Mn(CO)3(Me–Cp)] (Equation 14) <1998BCJ1705>. From the reaction mixture, the trimethylenecyclopentene derivative 104 was also isolated in 17% yield. The octasilyl[4]radialene 103 was then reduced to the lithium salt 105 upon treatment with excess lithium in THF at room temperature (Equation 15). The octasilyl[4]radialene anion has an eight-centered, 10-electron p-system, which was characterized by means of X-ray crystallography, 1H, 13C, 29Si, and 6Li NMR spectroscopic data <1998AGE1662>. Variable-temperature 13C NMR experiment revealed that in the temperature range between 173 and 298 K Liþ ions underwent a walk on the [4]radialene framework, as shown in Equation (15). At 173 K, however, the Liþ ion walk is suppressed so that Liþ is fixed at one site of the framework. Me2 Si

Me2 Si

Me2 Si SiMe2

Me2Si

i

Me2Si

SiMe2 Si Me2

Si Me2

102

Me2 Si

Me2 Me2 Si Si Me2Si

SiMe2

Me2Si

+ Me2Si

SiMe2

Me2Si

SiMe2

SiMe2 Si Me2

Si Me2

ð14Þ Si Me2

103

Si Me2

104

i, 3 equiv [Mn(CO)3(Me–Cp)], hν (λ > 300 nM), THF, reflux, 2.5 h

ð15Þ

Persilylated [5]radialene 106 was obtained analogously by the intramolecular reaction of hexadecamethyl3,6,8,11,14,16,19,21-octasilacycloicosa-1,4,9,12,17-pentayne with an excess of [Mn(CO)3(C5H4Me)] by irradiation in

993

994

Rings containing Silicon to Lead

refluxing THF <1998CL1101>. Subsequent reaction of 106 with lithium metal in THF gave air- and moisturesensitive dark red crystals of the tetralithium salt 107 with 10-centered, 14-p-electron system stabilized by silyl groups (Equation 16) <1999CC1981, 2000BCJ2129>.

ð16Þ

Synthesis of 9,10-disilaanthracene 109 was performed by the treatment of a cis/trans-mixture of 108 with 2 equiv of Li in THF containing TMEDA at room temperature for 48 h. The structure of dimer 109 was confirmed by MS, and 1 H, 13C, and 29Si NMR spectroscopy as well as X-ray crystallographic analysis. Upon reduction of 109 with an excess of lithium or potassium in THF solution, the dianions 110 were formed (Scheme 22). Subsequent reaction of 110 with ,!-dichloropolysilane or dichlorodimethylsilane led to the formation of corresponding cyclic 111–113 <1995OM3625>.

Me Si

Me

H Si

Li/TMEDA THF, rt, 48 h

Si H

Me

Me

109

2–

Me –

Si

+ 2M

SiMe2

Me2Si

Cl(SiMe2)3Cl

SiMe2

Me Si

Si

Si

– Me

Me

110

111

M = Li, K

50%

Me2SiCl2

Me Si SiMe2 Me2Si

Si

Me Si

SiMe2

Me

Me +

Si Si Me

Me

Scheme 22

Me

Si

108

Si

M, THF, 24 h

Si

Me Si

112

113

18%

12%

Rings containing Silicon to Lead

The first example of stable plumbylplumbylene 115 was obtained and its structure was confirmed by X-ray crystallographic analysis <2005OM5484>. Compound 115, which can be regarded as an isomeric form of a ‘diplumbene’, was synthesized by reacting terphenyl ‘diplumbylene’ [PbArTrip2]2 114, where ArTrip2 ¼ -C6H3(C6H2-2,4,6Pri)2 with N3SiMe3, as shown in Equation (17). The activation of the Pri–CH3 groups resulted in the incorporation of the Pb(1)–Pb(2) unit into a Pb2C6 eight-membered ring, which along with steric factors is responsible for the stability of 115. The most prominent structural feature is the Pb(1)–Pb(2) bond, which links two-coordinate Pb(1) and fourcoordinate Pb(2) lead centers with the formal oxidation states of þ1 and þ3, respectively. The 1H NMR spectrum of 115 was consistent with the determined structure. However, 207Pb NMR signal was not observed, apparently due to the large anisotropy associated with the lead environment and poor solubility of 115 in hydrocarbons.

ð17Þ

14.19.3.3 Insertion into Si–Si and Ge–Ge Bonds Palladium-catalyzed reactions of cis- and trans-1,2-diphenyl-1,2-disilacyclopentanes 116 with phenylacetylene and diphenylacetylene were carried out in a sealed glass tube at 200  C resulted in the formation of corresponding 1,4disilacyclohept-2-enes 120 (Scheme 23). The stereospecific formation of 120 was explained in terms of the insertion of a palladium species into a silicon–silicon bond in disilacyclopentanes to give a 2-pallada-1,3-disilacyclohexane 117 with retention of the configuration, followed by coordination of an alkyne to the palladium atom in the intermediate 118. Then, insertion of the alkyne on the palladium afforded stereospecifically the 2-pallada-1,5-disilacyclooct-3-ene 119. This process was completed by reductive elimination of the palladium species to give 1,4-disilacyclohept-2-ene 120 with retention of the configuration <2001OM1204>.

Scheme 23

995

996

Rings containing Silicon to Lead

It is well known that bis(tert-butyl isocyanide)–palladium(0) catalyzes a selective intramolecular Si–Si -bond metathesis of some bis(silanyl)methanes <1994OM4148>. The same catalyst induced an oligomerization of 1,1,2,2tetramethyl-1,2-disilacyclopentane 121 via a formal insertion of an –Me2Si(CH2)3SiMe2– unit of 121 into the Si–Si bonds of oligomers produced. Cyclic oligomers up to the 40-membered octamer could be obtained according to this method. Structure of 20-membered tetramer 122 (Scheme 24) was confirmed by single crystal X-ray diffraction. Compound 122 was further elaborated by transition metal complex-catalyzed isocyanide insertion into all Si–Si bonds to give 123 <1995JA1665>.

Scheme 24

Palladium-catalyzed double-silylation reactions of 3,4-carboranylene-1,1,2,2-tetraethyl-1,2-disilacyclobut-3-ene 124, obtained from the reaction of 1,2-dilithiated o-carborane with 1,2-dichlorotetraethyldisilane, have been described <2001OM5537>. Thus, the reaction of 124 with trans-cinnamaldehyde in the presence of Pd(PPh3)4 yielded the insertion compound 125, whose structure was determined by single crystal X-ray crystallography (Equation 18).

ð18Þ

In a similar way, 126 was converted into the corresponding disilane metathesis product 127 (Equation 19) <1995OM2556>. The reaction of equimolar amounts of bis(isopropylidene)disilacyclobutane 128 and 3,4-benzo1,2-disilacyclobutane 126 in the presence of a catalytic amount of Pd(PPh3)4 gave the cross-metathesis product 129 accompanied by a minor amount of homo-metathesis product 127. The structure of 129 was assigned by X-ray structure analysis (Equation 20) <1996JOM335>.

Rings containing Silicon to Lead

ð19Þ

ð20Þ

Addition reactions of the Si–Si -bonds of disilanes 121, 131, and 133 to the CUC bonds of various arynes were found to be promoted by a palladium-1,1,3,3-tetramethylbutyl isocyanide complex. Diverse 1,2-disilylated arenes 130, 132, and 134 were obtained from five-membered and benzo-condensed six-membered cyclic disilanes (Equations 21–23). The 1H, 13C, and 29Si NMR spectroscopic data as well as X-ray crystallographic analysis were used to confirm the above structures <2005OM156>.

ð21Þ

ð22Þ

ð23Þ

The reactions of dithienodisilacyclohexadiene derivative 135 with acetylenes, such as diphenylacetylene, dimethyl acetylenedicarboxylate, and phenylacetylene, were carried out at 150  C for 24 h in the presence of a palladium

997

998

Rings containing Silicon to Lead

catalyst to afford respective adducts 136, resulting from the insertion of a triple bond of the alkynes into the Si–Si bond of 135. The main product was accompanied by a small amount of the oxygen-insertion product 137, derived from oxidation of the Si–Si bond of 135 by molecular oxygen (Scheme 25). Compound 137 was obtained in 91% yield by the oxidation of 135 with trimethylamine oxide. The crystal structure as well as optical and electrochemical properties of these products were determined <2006OM48>.

Scheme 25

In contradistinction to the chemical and physical properties of the Si–Si bond, which have been studied extensively, the chemistry of its higher analogue Ge–Ge -bond has attracted much less attention. It is well recognized that the group 14 dimetalloid -bond is reactive as the corresponding CTC p-bond. However, to study reactivity of E–E bond (E ¼ Si, Ge), it should be activated by, for example, ring strain. Thus, a strained germacycle, 3,4-benzo-1,1,2,2tetramethyl-1,2-germacyclobut-3-ene 138, was obtained by the treatment of 1,2-bis(chlorodiethylgermanyl)benzene with sodium in toluene <1996OM2014>. Compound 138 proved to be thermally labile and, upon thermolysis, conducted in an evacuated sealed tube at 160  C for 20 h, gave 3,4:6,7-dibenzo-1,1,2,2,5,5-hexaethyl-1,2,5-trigermacyclohepta-3,6-diene 142 in 53% yield together with 1,2,3-trigermacyclopent-4-ene 141 (Scheme 26). The reaction presumably proceeded via germyl diradical 139 resulting from homolytic scission of the activated Ge–Ge bond and the intermolecular attack on the Ge–Ge bond of a second molecule 138 (formation of 140) <1996OM2014>.

Scheme 26

Compound 138 underwent palladium-catalyzed Ge–Ge -metathesis at room temperature to give the dimeric germacycloocta-1,5-diene 143 (Equation 24). Moreover, at 160  C in a sealed tube containing a catalytic amount of

Rings containing Silicon to Lead

Pd(PPh3)4, 138 gave the unsymmetrical dimer, dibenzo-3,6,7,8-tetragermacycloocta-1,4-diene 144, in 24% yield (Equation 25) <2000JOM420>.

14.19.3.4 Rings Containing C–E–O and Related Units Intramolecular iodosilyletherization of alkenylsilanols 145 with bis(2,4,6-trimethylpyridine) iodine(I) hexafluorophosphate afforded a mixture of exo-ring mode cyclization 146 and endo-mode cyclization products 147 in 35–74% combined yields (Equation 26) <1996TL6781>.

ð26Þ

Free radical conjugate additions of carbon radicals onto alkenes which results in formation of carbon–carbon bonds have been important reactions in organic chemistry. This type of reaction was first explored in an intramolecular fashion, but quickly gained attention of heterocyclic chemists and a plethora of novel ring systems have been synthesized by this methodology <2001T7237>. Radical cyclization of allylsiloxy derivatives 148 to 1-oxa-2-silacycloheptanes 149 was achieved by treatment of 2-(allyldimethylsiloxy)-1,1-dibromoalkanes with Bun3SnH in the presence of a catalytic amount of triethylborane in benzene (Equation 27). An interesting stereochemical outcome was observed in the cyclization of 1-allyldimethylsiloxy-2,2-dibromo-1-phenylpropane (R1 ¼ H, R2 ¼ Ph) which gave a stereoisomeric mixture of 2,2,6-trimethyl-7phenyl-1-oxa-2-silacycloheptane (cis/trans = 87/13). These seven-membered cyclic silyl ethers and acetals were stable and could be isolated by silica-gel column chromatography <1997BCJ2255>.

ð27Þ

999

1000 Rings containing Silicon to Lead Highly diastereoselective 7-endo-radical cyclization of (bromomethyl)dimethylsilyl ethers 150, derived from ethyl -hydroxy--methylenecarboxylates, bearing a bulky -substituent such as isopropyl, cyclohexyl, and tert-butyl in THF gave cyclic silyl ethers 151 bearing preferentially the ethoxycarbonyl group anti to the -substituent (Equation 28) <2004TL4329>.

ð28Þ

Acylsilanes of type 152 with radicalphiles attached to the silicon atom underwent tandem radical cyclization to give 7,7,10-trimethyl-6-oxa-7-silaspiro[4.6]undecane (153: n ¼ 1) and 8,8,11-trimethyl-7-oxo-8-sila-spiro[5.6]dodecane (153: n ¼ 2) (Equation 29) <2005T2037>.

ð29Þ

1-Oxa-2-silacyclopentane derivative 154 subjected to the oxidation reaction with Oxone at room temperature followed by treatment with silica gel gave the eight-membered product 155 (Equation 30). On the other hand, sevenmembered silyl ether 156 was formed in diastereometrically pure form by the reaction of 154 with N-bromosuccinimide (NBS) in acetone–water solution (Equation 31) <1997JOC4206>.

RCM of acyclic silicon connected dienes 157 using molybdenum and ruthenium alkylidene catalyst afforded the corresponding cyclic silyloxy olefins 158 in good to excellent yields (Scheme 27). Interestingly, the formation of the cyclized eight-membered products (158f and 158g) did not require high dilution conditions commonly used for the formation of eight-membered rings <1997TL4757>. Molybdenum-catalyzed asymmetric ring-closure metathesis (ARCM) has been successfully applied for the synthesis of seven-membered siloxanes 160 bearing tertiary ether centers from trienes 159 (Equation 32). In effecting the ARCM, it performed especially well with siloxanes bearing bulky substituents at -carbon atom (R ¼ Ph, Cy, Pri) <2002JA2868>.

Rings containing Silicon to Lead

Scheme 27

ð32Þ

Cyclic seven-membered vinyl silanes 161 were obtained by regio- and stereoselective hydrosilylation of internal alkynes catalyzed by the ruthenium complex [Cp* Ru(MeCN)3]PF6, as shown in Equation (33) <2005JA10028>. Hydrosilylation of 2,2-divinyladamantane with bis(hydrosilane) species 162 in the presence of Zeise’s dimer [Pt2Cl4(CH2CH2)2] gave the disilacyclic 163 in high yields (Equation 34) <1998OM4267>.

ð33Þ

1001

1002 Rings containing Silicon to Lead

ð34Þ

Although many syntheses and reactions of group 14 doubly bonded compounds are well recognized, little is known about the reaction mechanisms. However, elegant mechanistic studies of the addition of carbonyl compounds to disilenes and germasilenes 164 have recently been described <2003OM1603>. Thus, when tetramesitilyldisilene and tetramesitilylgermasilene were subjected to the reaction with trans-2-phenylcyclopropane carbaldehydes, a 1:1 mixture of oxasilametallacycloheptanes 165 (E ¼ Si, Ge) and oxapentadienylsilylmetallanes 166 was obtained (Scheme 28). Unequivocal evidence was obtained for the presence of diradical intermediates in this addition reaction. The structure of 5-methoxyoxa-2,3-disilacyclohept-6-ene 165 (R ¼ OMe) was confirmed by X-ray crystallography.

Scheme 28

Functionalized oxosilacycloheptane 168 was obtained by silylformylation of alkynol 167, in which the catalytic intermediate of an intramolecular hydrosilylation is intercepted by carbon monoxide (Equation 35) <1995JA6797>.

ð35Þ

The first example of the sila-Pummerer rearrangement, which consists in the thermal conversion of sulfoxide 169 into O-silylated cyclic O,S-acetal 170, has been described <1999TL185>. As shown in Scheme 29, a 1,3-migration of silicon atom to a sulfoxide oxygen resulted in ring expansion.

Me

THF Si

S

Me

O

169 Scheme 29

reflux, 2 h

Me Me

Si

– H2C O

S

+

Me Si H2C S Me O–

+

Me Si Me O

170

S

Rings containing Silicon to Lead

During the last 15 years, the silylative coupling reactions of vinylsilane derivatives in the presence of ruthenium, rhodium, cobalt, and iridium complexes have been developed <2003COR691>. The mechanism of this reaction involving -silyl elimination and insertion of a CTC bond into the resulting M–Si bond has been proven by insertion of ethylene and vinylsilane into M–Si bonds (where M ¼ Ru, Rh, Co). Intramolecular disproportionation of ,!bis(vinylsilyl) compounds 171 gave disilacycles of various ring sizes 172 and 173 (Scheme 30) <1998CC699>.

Scheme 30

A facile and efficient method for the synthesis of cyclic silyl ethers 175 consisted of the ruthenium-catalyzed silylative coupling cyclization of 1,2-bis(dimethylvinylsilyloxy)ethane 174 <2005JOC370>. Upon treatment with Grignard reagents, these compounds were converted into alkyl-, aryl-, or alkenyl-substituted 1,1-bis(silyl)ethenes 176 (Scheme 31). Similarly, silylative coupling cyclization of N,N9-dimethyl-N,N9(dimethylvinylsilyl)ethane-1,2-diamine 177 catalyzed by [Ru-HCl(CO)(PCy3)2] in toluene under argon atmosphere gave 1,2,2,4,4,5-hexamethyl-3-methylene-1,5-diaza-2.4-disilacycloheptane 178, which upon subsequent treatment with alcohols afforded 1,1-bis(alkoxydimethylsilyl)ethanes 179 (Scheme 32) <2005SL1105>.

Scheme 31

Scheme 32

1003

1004 Rings containing Silicon to Lead 1,19-Binaphthyl derivatives bearing chlorosilyl or chlorogermanyl substituents at the 2,29 positions have been prepared and converted into oxadisilepin (E ¼ Si) and digermepin (E ¼ Ge) 180 upon treatment with NaOH or Et2O/H2O (Figure 5) <1996JOM15, 1997CB923>. 2,2-Dimethyl-4-methyl-1-oxa-4-aza-silabenzocycloheptan-5-one 181 was obtained (Figure 5) by treatment of salicylic acid N-ethylamide with hexamethyldisilazane ((Me3Si)2NH) and dimethylchloromethylchlorosilane (ClCH2SiMe2Cl) in boiling o-xylene <2002KGS127>.

Figure 5

Hexakis(2,4,6-triisopropylphenyl)tetragermabuta-1,3-diene 182 was obtained and converted upon treatment with sulfur into thiatetragermacyclopentene 183 with an endocyclic GeTGe bond. The reaction of 183 with dry air furnished a 2,4,7,8-tetraoxa-1,3,5,6-tetragermabicyclo[4.1.1]octane derivative 184 (Scheme 33). Structures of 183 and 184 were confirmed by X-ray crystallographic analysis <2003OM1302>.

Scheme 33

1,2,3,4,5,6-Pentathiagermepane 186 was obtained by reacting germabenzene 185 with elemental sulfur in benzene at room temperature, together with 1,2,3,4-tetrathiagermolane 187 (Equation 36). Structure of 186, which was separated by gel permeation liquid chromatography and subsequent preparative thin-layer chromatography, was confirmed by X-ray crystallography <2003JOM66>.

ð36Þ

Rings containing Silicon to Lead

An eight-membered cyclic C,N-bis(germadiyl)bis(ketenimine) 190 was prepared by the reaction of tert-butyllithium with (fluorodimesitylgermyl)phenylacetonitrile 188 leading to the lithium salt 189, which then underwent an elimination of lithium halide. Compound 190, the first ring containing two ketenimine moieties, was characterized by IR and 13C NMR spectroscopy as well as X-ray structure determination (Scheme 34) <1998OM1517>.

Scheme 34

14.19.3.5 Rings Containing O–E–O and Related Units 2-Functional 1,3-dioxa-2,4,7-trisilacycloheptanes 192 were prepared by the reaction of corresponding silane diols 191 with methyldichlorosilane in the presence of pyridine, as an HCl acceptor, and diethyl ether, as solvent (Scheme 35). The subsequent reaction of 192 (R1 ¼ Me, R2 ¼ OSi(Me)3) with chlorine in the presence of pyridine afforded chlorosilane 193, which upon treatment with an alcohol gave alkoxide 194. Siloxanes 192–194 were characterized by 1H, 13C, and 29Si NMR spectroscopic data <1995JOM29, 1996KGS1590>.

Scheme 35

Treatment of the allylic alcohols with diphenyldichlorosilane and 2,6-lutidine afforded the bis-alkoxysilanes 195 in excellent yield. These silicon-tethered compounds were treated with Grubbs’ catalyst to induce RCM reaction, furnishing the seven-membered silacycles 196 in 87–95% yield (Equation 37) <1998JOC6768>.

ð37Þ

1005

1006 Rings containing Silicon to Lead The wide applicability of the RCM and its remarkable tolerance to functional groups allowed the preparation of unsymmetrical eight-membered silaketals in the form of a 1:1 mixture of diastereoisomers 197 (S) and 197 (R) (Figure 6). Compound 197 was further used for the synthesis of spiroketals which constitute the C28–C38 portion of okadaic acid <2001TL239>. A new approach to long-range asymmetric induction using the diastereoselective temporary silicon-tethered (TST) RCM reaction of mixed bisalkoxy silanes derived from an allylic prochiral alcohol was applied to the construction of cis-1,4-silaketals 198 <2003AGE1734>. Analogous cyclic silaketals 199 and 200 with ring sizes from 9 to 11 members were also obtained by RCM cyclization of symmetrical silaketals (Figure 6) <1999TL1429>.

Figure 6

Unsymmetrical silaketals 201 having a 1,2-disubstituted double bond and a nitro group were prepared and transformed upon treatment with phenyl isocyanate in the presence of Et3N under mild reaction conditions into 2-oxazoline derivatives 202 by a regiospecific intramolecular 1,3-dipolar cycloaddition, as shown in Scheme 36 <1997SL1208>. The trans-stereochemistry of the alkene 201 was transported to the 2-isoxazoline product 202, which was confirmed by the 1H NMR spectroscopic data.

Scheme 36

Diazoacetic acid silyl esters can be prepared by trans-esterification of tert-butyl diazoacetate with trialkylsilyl triflate <1985JOM33>. Analogously prepared (alkenyloxy)silyl 203 and (alkynyloxy)silyl diazoacetates 206 underwent silicon-tethered 1,3-dipolar cycloaddition reactions as shown in Scheme 37 and Equation (38). Compound 205 resulted from a lateral criss-cross cycloaddition of the intermediate azine 204, which was formed from two molecules of 203 by diazo þ diazo or diazo þ carbene reaction <2000T4139>. On the other hand, when silyl diazoacetates 206 were kept in xylene at 142  C for 1 h, bicyclic pyrazoles 207 were obtained (Equation 38).

Rings containing Silicon to Lead

Scheme 37

ð38Þ

The first synthesis of 3-sila-1,2,4-trioxepane 208 was achieved by a two-step procedure involving initial ozonolysis of undecen-4-ol followed by bissilylation with But2(SiOTf)2 in the presence of imidazole (Equation 39). Compound 208 was separated by flash chromatography in 45% yield. Alkylation of 208 with alkyltrimethylsilane in the presence of SnCl4 gave alkylated silatrioxepane 209 in 5% yield <2005T4657>.

ð39Þ

Chiral silyl ether 210 was prepared and subjected to azidonation with trimethylsilyl azide (TMSN3) and iodobenzene diacetate [PhI(OAc)2], which served as a substitute of IN3, to give -azido-dioxasilepine 211 as a colorless oil, in diastereoselective ratio 1:3 (Equation 40) <2005OBC816>.

ð40Þ

1007

1008 Rings containing Silicon to Lead A new class of hydride organic/silica compounds such as organically bridged polysilsesquioxanes 213 were prepared from the hydrolysis and condensation of triethoxysilanes 212 linked by a hydrocarbon spacer, as shown in Equation (41).

ð41Þ

Previously, it has been shown that acid- or base-catalyzed polymerization of ,!-bis(triethoxysilyl)alkanes proceeds to highly condensed rigid gels within a few hours <1993CM943>. However, when the alkyne bridge is ethylene, propylene, or butylene, as in 212, gelation under acidic conditions requires 2–6 months to form seven-membered rings 213, that are designated as ‘cyclic disilsesquioxanes’ <1999JA5413, 1996JA8501>. Dichlorodimethylsilane was used for the synthesis of amides from unprotected amino acids by a simultaneous protection–activation strategy <2002TL9203>. However, sterically crowded di-tert-butyldichlorosilane reacted with phenylalanine in the presence of Et3N to give 5-aza-1,3-dioxa-2,4-disilacycloheptan-7-one 214 in 40% yield (Equation 42), whose structure was confirmed by MS, 1H, and 13C NMR spectral data.

ð42Þ

Stable dioxadiazasilepines and dioxadiazastannapines 216 were prepared in 60–77% yields from vicinal oximes 215 via dianion intermediates, which are intramolecularly trapped with dielectrophilic diorganodichlorosilanes and diorganodichlorostannanes, respectively (Equation 43) <2005TL315>.

ð43Þ

The reaction of germylenes GeR2 bearing the bulky amide group (R ¼ N(SiMe3)2, N(SiMe3)But) with p-benzoquinone in n-hexane at 0  C followed by exposing to air for 12 h afforded cyclic peroxides 218 in 69% and 50%, respectively.The reaction proceeded via the semiquinone radicals 217 which are trapped by an oxygen molecule. The molecular structure of 218 was definitively determined by X-ray crystallographic analysis (Scheme 38) <1995JA2187>.

Scheme 38

Rings containing Silicon to Lead

When the disodium salt of benzilmonoxime thiosemicarbazone was treated with R2SnCl2 in a 1:1 molar ratio, the nine-membered cyclic compounds 219 were obtained (Equation 44); their 119Sn NMR spectra exhibited a sharp 119 Sn resonances at 136.72 (R ¼ Me) and 134.86 ppm (R ¼ Bun) confirming the presence of tetracoordinated tin <2002IJB419>.

ð44Þ

The addition of 1 equiv of the dilithio salt of rac-1,19-bi-2-naphthol to an equivalent amount of 1,1-dichloro-1silacyclobutane in ethyl ether at 78  C led to the formation of 1,1-(rac-1,19-bi-2-naphthoxy)-1-silacyclobutane 220 as a white solid in 71% yield (Figure 7). The structure of 220 was confirmed by 1H, 13C, and 29Si NMR spectroscopy and X-ray crystallographic studies <2005JOM2272>.

Figure 7

Chiral metal alkoxides M(OR)4 have been developed as asymmetric variants of ordinary Lewis acids, such as AlCl3 and ZrCl4, and are used as catalysts for selective carbon–carbon bond formation. Thus, starting from bidentate 1,19-bi2-naphthol derivatives (BINOL) and SnCl4, a series of chiral tin(IV) aryloxides 221 (Figure 7) was prepared and successfully applied to the enantioselective Diels–Alder reaction <2006TL873>. Similar silocanes obtained from menthone- or camphor-derived 2,29-biphenols have been obtained and their configuration was analyzed by NOE differential spectroscopy (NOEDS) <1997JOC7156>. Novel helicene-like quinones 222 were prepared from silylene-tethered binaphthols based on the newly developed chromium-templated [3þ2þ1] benzannulation reaction (Figure 7) <2005EJO1541>. The tin(II) and germanium(II) amides 223 were obtained from corresponding dilithio diamide and SnCl2 or GeCl2–diox (diox ¼ dioxane). Analogous reaction with SiCl4 afforded the appropriate cyclic diaminodichlorosilane 224 (Figure 7) <1996JCD3595>.

1009

1010 Rings containing Silicon to Lead The racemic dithiobinap was reacted with either BuSnCl3 or BuSn(OPri)2Cl to produce dithiotin chloride (rac)-225 (E ¼ Sn) (Equation 45). Similarly, upon treatment of (rac)-dithiobinap and (R)-dithiobinap with ButGeCl3 in the presence of Et3N in THF solution, (rac)-225 and (R)-225 (E ¼ Ge) were obtained <2001SL1038>. Dithiogerman chlorides (E ¼ Ge), but not dithiotin chloride (E ¼ Sn), could be reduced to the hydrides 226 with sodium or lithium borohydride under carefully controlled conditions (Equation 45). Structures of the germanium chloride 225 and hydride 226 were confirmed by X-ray crystallographic analysis <2003JOC5013>.

ð45Þ

Photolysis of phenanthraquinone (PQ) in the presence of disilane precursors such as 7,8-disilabicyclo[2.2.2]octa2,5-diene using two 500 W tungsten–halogen lamps led to the formation of 227 and 228 as silylene-transfer products (Equation 46) <2001JOM63>.

ð46Þ

The synthesis and conformation of the sterically congested seven-membered ring containing tetracoordinate germanium(IV) have been described <2001IC3830>. Thus, new dibenzo[d,f][1,3,2]dioxa-germapin 229 and bis(1,19biphenylen-2,29-dioxy)germanium 230 were obtained from the reaction of tetra-tert-butyl-substituted biphenyldiol with dimethyl(diphenyl)germanium dichloride and germanium tetrachloride, respectively (Figure 8). Using variabletemperature 1H NMR spectroscopy, the free energy of activation (
G* 288) for ring inversion was determined.

Figure 8

Rings containing Silicon to Lead

The spirotetraaza silane 231 was prepared by treatment of the corresponding o-phenylenediamine derivative with SiCl4. When 231 (X ¼ H) was treated with bromotrichloromethane and AIBN, the monocyclization product 232 was isolated in low yield as a mixture of stereoisomers (Scheme 39). This radical-induced cyclization process was explored with respect to the impact of Si on the chemo- and regioselectivity <2002JOC8906>.

Scheme 39

Diorganotin(IV) derivatives of diphenic acid 233 were prepared by reaction of dialkyltin(IV) oxide with diphenic acid and its sodium salt in 1:1 molar ratio (Figure 9). Structure of these compounds was confirmed by IR, 1H, and 13C NMR spectroscopic data <1995AOM121>.

Figure 9

14.19.3.6 Silacrown Ethers, Calixarenes, Cyclophanes, and Metallacenes [5.5][2.6]Pyridinophanes and cyclophanes 234a–d containing silylene units (Figure 10) were prepared by sonication of pyridine- or benzenemethanol with dichlorosilanes R2SiCl2 (R ¼ Me, Ph) in benzene at room temperature for 3 h. As evidenced from X-ray crystallographic studies, in the solid state diphenylsilylene pyridinophane 234b (X ¼ N, R ¼ Ph) and dimethylsilane cyclophane 234c (X ¼ CH, R ¼ Me) were found to adopt an anti-conformation, while their congener 234a (X ¼ N, R ¼ Me) exhibited a syn-arrangement <1998OM2656>. A polymorphic form of 234a was claimed to be obtained by other authors <1996IC4342>.

Figure 10

1011

1012 Rings containing Silicon to Lead The transition metal-catalyzed 1,4-double silylation of ,-unsaturated ketones and aldehydes with 1,2-bis(dimethylsilyl)carborane 235 gave the di-insertion product 236 of a carbonyl group into each of the C–Si bonds <1988JA5579, 1991OM3173>. A similar reaction of 235 with Pt(CH2TCH2)(PPh3)2 gave the cyclic bis(silyl)platinium complex 237, which reacted with a variety of substrates such as alkyne, dione, and nitrile to give heterocyclic compounds incorporating an alkene, ketonate, imine, or amine moiety (Scheme 40). Thus, trans-cinnamaldehyde reacted with 237 to give the disilylation product 238 in 56% yield and insertion of phenanthrenequinone into 237 produced the eight-membered ring 239 <1999OM1818>.

Scheme 40

Sterically hindered cyclotetradeca-1,8-diynes 242 were prepared in a straightforward manner as shown in Scheme 41. First, the reaction of 2-methyl-3-butyn-2-ol with either dimethyldichlorosilane or diphenyldichlorosilane yielded the dialkynes 240. Then, the bis(lithium) salt of 240 was converted into the bis(alcohols) 241 by reaction with acetone or benzophenone. Finally, condensation of 241 with R32SiCl2 afforded the products 242 in 4-25% yield <2003EJOC3051>. Ring conformations of 242 in the solid state were determined by the rigid and linear 1,1,4,4tetrasubstituted 2-butyne units. The geometry of 14-membered ring was determined by X-ray crystallographic studies. Cyclic diynes 242 were converted into the hexacarbonyldicobalt complexes 243 by refluxing with CpCo(CO)2 in decalin <2004OM2225>. Reaction of 243 with ceric ammonium nitrate (CAN) gave the dinitrato cobalt-stabilized cyclobutadienes 244 <2004JCD4146>. Similarly, the reaction of 88 with Fe2(CO)9 gave the tricyclic complex 245 in low yield (Equation 47). Structures of cyclobutadienes 244 and 245 were established by X-ray crystallographic studies <1997OM646>.

Rings containing Silicon to Lead

Scheme 41

ð47Þ

Chemistry of compounds in which metal group 14 atoms are joined by organic groups with delocalized p-systems represent an area of considerable interest as a result of their unusual physical and chemical properties. The trisilacalix[3]arenes 246 and tetrasilacalix[4]arenes 247 (Figure 11) were prepared using a one-pot procedure consisting in the condensation of 1,3-dibromobenzene and dimethyldichlorosilane with magnesium in refluxing THF. Due to the formation of linear oligomers, yields of the above products were low. Their structures were confirmed by MS, 1H, 13C, and 29Si NMR spectra as well as by X-ray crystallography. Compounds 246 and 247a exhibited p-cryptand character since the cation–p-interactions between the silacalixarenes and a silver cation were observed by FAB mass spectrometry <1999OM1465>. Macrocyclic tetramers of type 247 and hexamers 248 were obtained by deprotonation of furan, thiophene, and N-methylpyrrole in the 2- and 5-position with 2 equiv of BunLi/ TMEDA/KOBut (1:1:1) in hexane, followed by addition of a solution of R2ECl2 in hexane (R ¼ Me, Ph, E ¼ Si, Ge, Sn) <1995JOC7406, 1997ICA11, 1997MGM775>. Moreover, methoxy-directed ortho-lithiation of 1,2-dimethoxybenzene followed by addition of Me2GeCl2 afforded GeMe2-bridged arene 249 in good yield. Subsequent reaction of lithiated 249 with an equimolar amount of Me2GeCl2 furnished octamethoxy[14]dimethylgerma-1,4-calixarene 247g, as a white solid (Scheme 42) <1997CB421>.

1013

1014 Rings containing Silicon to Lead

R2 E Ar

Ar R2E

ER2

ER2

Ar

Ar

Ar

R2E

ER2

Ar R2E Ar E R2

R2 E

Ar

Ar ER2

R2E Ar

Ar

Ar E Ar R2

E R2

246

247

248

Calix[3]arene

Calix[4]arene

Calix[6]arene

Compound E

Ar R

246a

Si A

247a 247b 247c 247d 247e 247f 247g 247h

Si Si Si Si Si Si Ge Ge

A B C D E F B D

Yield (%) Reference

Me 12 Me Me Me Me Me Me Me Me

1999OM1465

2.3 12 16 16 18 12 3 48

1999OM1465 1995JOC7406 1995JOC7406 1995JOC7406 1995JOC7406 1997CB421 1997CB421 1997ICA11

MeO

Ar R

Yield (%) Reference

247i 247j 247k 247l 247m 247n

Ge Ge Ge Ge Ge Sn

E E F F G G

Me 2-Thiophenyl Me Ph Ph Bun

53 51 4 9 47 31

1997ICA11 1997ICA11 1997MGM775 1997MGM775 1997MGM775 1997MGM775

248a 248b

Si Si

D E

Me Me

10 17

1995JOC7406 1995JOC7406

But

OMe

, B=

A=

Compound E

, C=

, D=

, O

OMe Ph

Ph E=

, F= S

, G=

N

N

N Me S

Figure 11

Scheme 42

Rings containing Silicon to Lead

In search for an electronic equivalent of the CO ligand, which could be incorporated into sophisticated structures such as polydentate or macrocyclic ligands, it was found that dicoordinate phosphinine would be a highly advantageous alternative <1998SCI1587>. Thus, silacalix[3]phosphinine 250 and silacalix[4]phosphinine 251 shown in Figure 12 were prepared starting from 1,3,2-diazaphosphinines, and their structures were confirmed by X-ray crystallography <1999CEJ2109>. In the solid state, 250 adopted a partial cone-type structure. Two phosphorus atom lone pairs point toward the top of the cavity and are located above the plane defined by the three silicon atoms, and the third one points below this plane. Macrocycle 251 adopted a opened-out partial cone conformation. Two opposing phosphinine subunits lie almost in the plane defined by the four silicon atoms, whereas the other two subunits are located in two roughly parallel planes that are perpendicular to the first, and their phosphorus atoms point in opposite directions. The strategy devised for the synthesis of 251 was then extended to derivatives containing furan and thiophene (Figure 12, Ar ¼ 2,5-furyl, 2,5-thienyl).

Figure 12

As shown in Equation (48), mixed phosphinine–ether macrocycles 252 were also prepared in 50% (X ¼ O) and 25% (X ¼ OCH2CH2O, OCH2C(Me2)CH2O) by reacting diynes (PhCTCSiO)2X with 4,6-bis(tert-butyl)-1,3,2-diazaphosphinine <2001JOC1054>. Reduction of 252 (X ¼ O) with sodium naphthalenide afforded diamagnetic dianion 253 with a new type of phosphorus–phosphorus bond <2001JA6654>.

ð48Þ

Silabridged cyclobutadiene superphanes 254 and 255 were obtained in good yields and high stereoselectivity by reacting unsymmetrically bridged disiladiynes 85 and 91 <1995TL4603> with (5-cyclopentadienyl)cobalt complexes (Equations 49 and 50) <1995TL4607>. The molecular structure of 254 (n ¼ 1) was confirmed by X-ray crystallography.

1015

1016 Rings containing Silicon to Lead

ð49Þ

ð50Þ

The synthesis of small strained cyclophanes attracted the attention of chemists for many years because the forced proximity of atoms leads to unusual chemical and physical properties. Using the Suzuki reaction to couple a three-legged borane intermediate derived from a triallylsilane 256 to 1,3,5-tribromobenzene, the analogue of Pascal’s hydrocarbon was obtained in a single step <1994OM3728>. This strategy has been applied to the synthesis of 257 (4% yield) from trisborabicyclo[3.3.1]nonane (9-BBN) adduct of methyltriallylsilane 256 and 1,3,5-tribromobenzene (Scheme 43).

Scheme 43

An in stereoisomer of fluorosilaphane 260 was obtained as depicted in Scheme 44 in overall 0.4% yield. First, tri(o-tolyl)fluorosilane 258 was fluorinated at silicon with AgF and then brominated with NBS to give tris[2(bromomethyl)phenyl]fluorosilane 259. Condensation of 259 with 1,3,5-tris(mercaptomethyl)benzene yielded the final product 260 <1998JA6421, 1999JOC5626>. The inside location of the fluorine atom was established by X-ray ˚ compared to the mean Si–F distance for all crystallographic analysis. The Si–F bond distance is very short (1.591 A) tetracoordinate tris[(alkyl(aryl)]fluorosilanes found in the Cambridge Structural Database. The 19F NMR resonance for 260 appears at  ¼ 5.3 ppm, that is, 155 ppm upfield from that of tri(o-tolyl)fluorosilane 258. 5,5,6,6,21,21,22,22-Octamethyl-5,6,21,22-tetragerma[10.10]paracyclophane 261 (R1 ¼ R2 ¼ Me) was obtained in 0.8% yield by a double Wurtz coupling of 1,4-bis[4(bromodimethylgermyl)butyl]benzene using Na in toluene at reflux (Figure 13). The structure of 261 (R1 ¼ R2 ¼ Me) was confirmed by EIMS spectrum, which contained molecular ion peak at m/z ¼ 786 accompained by peaks that form a characteristic pattern that is associated with species containing four germanium atoms. Attempts made to prepare trace amounts of congeners with R1 ¼ R2 ¼ Ph and R1 ¼ Ph, R2 ¼ Me were also successful; however, this method was impractical since analytically pure samples were not obtained <2003JOM61>.

Rings containing Silicon to Lead

Scheme 44

Figure 13

The metallamacrocycles have attracted attention due to their potential applications such as catalysis, sensing, molecular electronics, and host–guest chemistry. The first silicon-bridged [1.1]ferrocenophane [{Fe(-C5H4)2SiMe2}2] 262 has been obtained from disilylated ferrocene [Fe{-C5H4SiMe2(C5H5)}2] upon treatment with 2 equiv of BunLi in THF at 5  C followed by the reaction of the resulting Li2[Fe{-C5H4SiMe2Ph)}2] with FeCl2 in THF (Figure 14) <1995JCD1893>.

Figure 14

Tetrasilaferracyclohexane 263 (Figure 14) was prepared in 24% yield by treatment of tetrasilaferracycle Fp(SiMe2)4Cl (Fp ¼ [(5-C5H5)Fe(CO)2]2) with lithium diisopropylamide (LDA) in THF at 0  C. Structure of 263 was confirmed by IR and 1H, 13C, and 29Si NMR spectral data <2006OM3969>. A novel pentanuclear heterodimetallic metallamacrocycle 266 was prepared in three steps starting from Na2[pC5H4C(O)]2-C6H4 264, as shown in Scheme 45. First, 264 reacted with Mo(CO)6 followed by addition of PhSnCl3 to yield the tetranuclear heterodimetallic complex 265. Reaction of the latter with Na2S9H2O in boiling ethanol resulted in the formation of 266, which was characterized by IR and NMR spectra as well as X-ray structural analysis

1017

1018 Rings containing Silicon to Lead <2002OM3675>. The formation of the SnPh2 unit was attributed to the redistribution reaction of the organotin moiety from the partial decomposition or reductive elimination of complex 265 under reductive conditions. The ˚ and average Sn–S distance (2.417 A) ˚ are within normal range. The structure of 266 is average bond distances (2.799 A) a rare example of an organotin metallamacrocycle involving the molecular architecture and simultaneously fusing a small heterocycle. A similar procedure was applied for preparation of metallamacrocycles 268 via 267 (Scheme 45) <2003JOM57>.

Scheme 45

When the dianion [(CH2(5-C5H4)Fe(CO)2}2]2, obtained by the reductive cleavage of Fe–Fe bond in the dimer CH2{(5-C5H4)Fe(CO)2}2 with 1% Na/Hg amalgam in THF, was subjected to the reaction with Ph2SnCl2 at 78  C for 2 h, the complex 269, which is soluble in hexane and diethyl ether, was obtained in 80% yield. Structure of 269 was confirmed by 1H NMR spectrum and X-ray crystallographic studies (Figure 15) <1999ICA252>.

Figure 15

Rings containing Silicon to Lead

A series of silicon- and tin-containing ferrocenophanes 270 such as 1,1,3,3,14,14,16,16,18,18,29,29-dodecamethyl3,14,18,29-tetrasila-1,16-distanna[5.5]ferrocenophane were obtained as novel Lewis acids (Figure 15) <2000OM430, 1998AXC1425>. The molecular structure of these compounds was confirmed by 1H, 13C, 29Si, and 119Sn NMR spectroscopy as well as X-ray crystallographic studies of congeners with R1 ¼ R2 ¼ Me, R1 ¼ Ph, R2 ¼ Cl, and R1 ¼ Me, R2 ¼ Cl. In solution the halogen-substituted ferrocenophanes undergo cis–trans-isomerism, the rate of which was enhanced by addition of halide ions. Compounds 271a and 271b (Figure 16), containing 10-membered rings, were obtained by reacting Li(THF){C(SiMe3)2SiMe2C5HN}2 with CuI and AuClSiMe2, respectively. Structures of these compounds were confirmed by X-ray crystallographic studies <2002JCD2467>.

Figure 16

Heteronuclear tin(IV)–silver(I) complexes 272 with phosphinothiolate ligands (Figure 16) were obtained in good yields by the reaction of [SnR2(SC6H4PPh2)2] (R ¼ Me, Ph) with Ag(CF3SO3)(PR3) (PR3 ¼ PPh3, PPh2Me) as a result of the coordination of [AgPR3]þ units to the starting material. The molecular structure of 272 (R ¼ PPh2Me) has been established by X-ray diffraction of its dichloromethane solvate <2001JOM274>.

14.19.3.7 Atranes and Related Compounds Atranes as a special class of compounds with N ! E transannular bond have attracted a considerable amount of attention due to their intriguing molecular structure, biological activities, and patterns of chemical reactivity. In contrast, germocanes, closely related analogues of germatranes, have been little studied. Therefore, 1,6-diaza-2,2dimethoxy-2-silacyclooctane 273 was prepared in 90% yield by heating of 1,4-diaza-8,8,8-trimethoxy-8-silaoctane at reflux, and its structure was confirmed by X-ray crystallographic studies (Figure 17) <1996JOM29>. Dichalogermocanes MeN(CH2CH2O)GeX2 (X ¼ Cl, Br) were obtained by the reaction of GeX4 with N(CH2CH2OSiMe3)2, while dimethylgermocane MeN(CH2CH2O)GeMe2 was formed upon treatment of Me2Ge(NMe2)2 with MeN(CH2CH2OH)2 <2003ZNB1165>.

Figure 17

Synthesis of allylic germatranes 274 (Figure 17) has been achieved by two complementary routes. The first of these consists in the preparation of the precursor germanium trichlorides by a transmetallation reaction between germanium(IV) chloride and the corresponding tributylstannanes, followed by alcoholysis and reaction with triethanolamine. The second route is through the palladium-catalyzed hydrogermylation of a suitable diene using germatrane

1019

1020 Rings containing Silicon to Lead <2004JOM2565>. Germatranes 275 with germanium–carbon bonds containing functional groups neighboring to the germanium atom – allyl <1997ZNB30>, phenylacetylenyl <1997ZFA1144, 2001ZFA1>, fluorenyl <1997ZFA1144, 1998ZNB1247, 1999ZFA655>, benzyl <1999ZOB518>, alkoxycarbonylmethyl <1997CB739>, and 1-trimethylsilyloxy <2000JOM387> – were also prepared. The reactions of germatranyltriflates and 1-trimethylsilyloxygermatranes, which are even more reactive than 1-bromogermatrane, with lithium reagents such as (Me3)2NLi, cyclopentadienyllithium, indenyllithium, and fluorenyllithium were studied in detail <2000JOM387>. Synthesis and characterization of 3- and 4-phenylgermatranes by X-ray crystallography were also described <2003JOM8>. A new route for the synthesis of 5-coordinated germanium compounds such as (1,3-dioxa-6-aza)-2-germoctanes 278 consisted of the reaction of diethanolamine with di(2-thienyl)germane 277 prepared by the reduction of di(2-thienyl)diethoxygermane 276 with lithium chloride in pentane under phase-transfer conditions (Scheme 46). Germocane 278 (R ¼ Me) was also obtained by transesterification of 276 with N-methyldiethanolamine. Molecular structure of 278 was confirmed by X-ray crystallography. In the solid state, the eight-membered ring has a crown conformation. In contrast to germatranes, germocane 278 is characterized by a considerable lengthening of the N ! Ge transannular ˚ The coordination polyhedron of the germanium atom is a strongly distorted trigonal bipyramid bond (2.446 A). <1996JOM41>.

Scheme 46

The chemistry of hypercoordinate silicon compounds has stimulated the synthesis of novel compounds for applications such as rodenticides and anticancer drugs. A novel approach to switch between penta- and hexacoordination of the silicon atom by adding Brønsted acid to Si-complexes with enamine-functionalized salen-type ligands has been extensively explored <2002AG1825, 2003AGE1732, B-2003MI317>. Silicon complexes 279 with enaminefunctionalized salen-type ligands reacted with Brønsted acid in 1,4-addition reaction to yield hexacoordinate silicon complexes 280 (Equation 51). The Si-compounds were characterized by multinuclear NMR spectroscopy and the structures of 280 (X ¼ benzoate, picrate, 8-oxyquinolinate) were confirmed by X-ray crystallographic studies <2005ICA4270>.

ð51Þ

The new stable formal metallanimines, matallanethiones, and -selones 281 were prepared by the reactions of the divalent species L2E (L2 ¼ tetradentate Schiff base; E ¼ Ge, Sn) with Me3SiN3, elemental S8, or Se (Figure 18) <1999MGM703>.

Rings containing Silicon to Lead

Figure 18

Starting from new bivalent germylenes and stannylenes 282, various stable cyclic organometallic compounds 283–285 were obtained as shown in Scheme 47 <1997MGM791>.

Scheme 47

14.19.4 Reactivity and Transformations of Heterocyclic Rings Organometallic compounds with group 14 heteroatoms enjoy a wide range of applications in the synthesis of diverse open-chain organic products. Treatment of 1,10-distannacyclodeca-2,8,11,17-tetrayne 100 with electrophiles leads to cleavage of the Sn–C bonds. The reaction of 100 with three different boron halides giving rise to the formation of acetylene derivatives 286 is summarized in Scheme 48 <1997MGM573>.

1021

1022 Rings containing Silicon to Lead

Scheme 48

As shown in Scheme 49, cyclic silyl ethers bearing the ethoxycarbonyl group anti to the -substituents 151 upon treatment with silica gel gave acyclic ethyl -hydroxy--[2-(hydroxydimethylsilyl)]carboxylates 287, and the subsequent reduction with diisobutylaluminium (DIBAL) followed by Tamao oxidation <1990OS96> gave the corresponding acyclic triols 288 <2004TL4329>.

Scheme 49

Ring cleavage of cyclic silyloxy olefins 158 using the oxidative conditions developed by Tamao efficiently afforded the corresponding cis-olefinic dihydroxy compounds 289 in good to excellent yield (Equation 52) <1997TL4757>.

ð52Þ

The nonracemic chiral seven-membered ring siloxanes 160 were employed to prepare numerous difficult-to-attain tertiary alcohols. Thus, on treatment with MeLi in THF at 22  C, tertiary alcohol 290 was obtained (93% ee). Subjection of the siloxane 160 to m-chloroperbenzoic acid (MCPBA) led to the diastereoselective formation of epoxide 291, which reacts further with tetrabutylammonium fluoride (Bun4NF ¼ TBAF) to give 1,3-tertiary diol 292 (Scheme 50) <2002JA2868>. Diphenyl silaketals 196 can be readily converted to the C2-symmetrical 1,4-diol 293 by reduction with Raney-Ni, followed by desilylation with TBAF in 65% yield. Another potentially powerful application of cyclic alkoxysilanes 196 consists in dihydroxylation with catalytic amount of osmium tetraoxide in the presence of N-methylmorpholine N-oxide (NMO), followed by treatment with TBAF which gives D-altriol 294 (Scheme 51) <1998JOC6768>. The parallel synthesis of an exhaustively stereodiversified library of cis-1,5-enediols by silyl-tethered RCM has also been described <2001OL2157>.

Rings containing Silicon to Lead

Scheme 50

Scheme 51

A general method for the synthesis of stereodefined amino polyols consists of highly regioselective and diastereoselective intramolecular chiral nitrone cycloaddition reactions with a vinyl group tethered by a silicon atom. The reaction sequence is shown in Scheme 52 <2000JA7633>. This strategy features a series of one-pot reactions involving (1) diisobutylaluminium hydride (DIBAH) reduction of the carbonyl group of chiral -hydroxy carbonyl compounds 295, in which the hydroxyl group is protected as diphenylvinyl silyl ethers, to give an aldehyde; (2) condensation of the aldehyde with N-benzylhydroxylamine to furnish nitrone 296; and (3) intramolecular [3þ2] dipolar cyclization reaction between the nitrone and the silicon-tethered vinyl group to give isoxazoline derivative 297 as direct precursor of amino polyols 298. Novel IP3 receptor ligands having an -C-glycosidic structure were synthesized via a radical cyclization reaction with a temporary connecting allylsilyl group as the key step (Scheme 53). Thus, phenyl 2-O-allyldimethylsilyl-3,4bis-O-TBS-1-seleno--D-glucopyranoside 299 was treated with Bu3SnH/AIBN to form the -cyclization product 300 in almost quantitative yield. The latter was converted into the corresponding penta-O-benzoate 301 by successive treatment under Tamao oxidation conditions, HCl/MeOH, and BzCl/pyridine <2005T3697>. An effective cross-coupling of alkynes using ‘silicon-tethered’ Fe(CO)5-promoted cyclocarbonylation was shown to provide a seven-membered ring dialkoxysilane 302, which subsequently upon treatment with Me3NO in acetone at 0  C was converted to cyclopentadienones 303 with variable substituents (Scheme 54) <2002OL2837>.

1023

1024 Rings containing Silicon to Lead

Scheme 52

Scheme 53

Scheme 54

Rings containing Silicon to Lead

Regioselective synthesis of unsymmetrical C-aryl glycosides using silicon tethers, as disposable linkers, has been developed (Scheme 55). Deprotonation of 304 with ButLi led to the formation of an intermediate benzyne 305 that underwent cycloaddition to deliver 306. When 306 was treated with TBAF in DMF at 70  C, the tether was cleaved and 307 was obtained in 80% yield. The acid-catalyzed opening of the oxabicycloheptadiene ring afforded the glycosyl naphthols 308 in quantitative yield <2003JA12994>.

Scheme 55

The Sonogashira reaction is a powerful tool in synthesis for the preparation of arylethynyl compounds. On the other hand, organosilicon and organotin compounds can be successfully applied in cross-coupling reactions. The reactions were extended on the utility of arylalkynyl germatranes 309 as reagents for the Sonogashira-type coupling reaction using aryl chlorides and triflates as substrates (Scheme 56) <2003TL451>. In comparison with other Sonogashiratype reactions, germatrane reagents enable the reaction to proceed at lower temperatures than are required for the related reactions using triorganosilicon compounds.

Scheme 56

The first example of the coupling of germatranes 310 with aryl bromide has been described (Equation 53) <1996JOM255>. The cross-coupling reaction is possible due to transannular coordination of nitrogen to germanium, which enhances the reactivity of the carbon–germanium bond <2005ICC131>.

1025

1026 Rings containing Silicon to Lead

ð53Þ

It is well known that stannanes are useful reagents in palladium-catalyzed cross-coupling reactions with aryl iodides. The potential of using organogermatranes in this analogue of Stille–Migita–Kasugi coupling <2002ACR835, 2002CPB1531, 2006CEJ4954> has been investigated <2002OM5911>. The hypervalent germanium species produced from the germatranes 311 was found to provide a more efficient and more easily handled reagents than trialkylgermanium analogues (Equation 54).

ð54Þ

The 1,1-binaphthyl ring system is a key component of a number of chiral ligands that have been used as catalysts for asymmetric synthesis <1992S503>. Chemo- and stereoselective (S)-stannepin-catalyzed monobenzoylation of terminal 1,2-diols 312 afforded (S)-enantiomer-enriched 2-benzoylated diols 313 in moderate selectivity. Only a trace of 1-benzoylated diols 314 was observed (Equation 55). Thus, the method was successfully applied to kinetic resolution of racemic 1-phenyl-1,2-ethanol using a chiral organotin catalyst <2000JOC996>.

ð55Þ

Both enantiomers of 4-tert-butyl-4,5-dihydro-3H-dinaphtho[2,1-c:19,29-e]stannepin 52 were synthesized and used for the enantioselective reduction of -bromo esters 315. By using the achiral hydride sodium cyanoborohydride (NaBH3CN), the radical reductions proceeded enantioselectively with only catalylic amount of chiral tin hydride (Equation 56) <2003TA3069>.

ð56Þ

The molybdenum complex Mo(NAr)(CHCMe2Ph)[(S)-Me2SiBiphen] 43 was used for catalytic asymmetric olefin metathesis reactions such as desymmetrization of trienes, kinetic resolution of allylic ethers, tandem catalytic asymmetric ring-opening metathesis/cross-metathesis. Interestingly, tandem catalytic asymmetric ring-opening

Rings containing Silicon to Lead

metathesis/ring-closing metathesis proceeded with an enantioselectivity comparable to that of catalyzed by Mo(NAr)(CHMe2Ph)[(S)-Biphen] <2001OM4705>. A simple and reliable method, based on derivatization and NMR, for stereochemical assignment of 1,4-diols having a cis-2,3-methano bridge 316 has been described (Equation 57). The two protons H1 and H19 always have a quasitrans-diaxial relationship in the trans- (anti-)isomers of silicon derivatives 317, and thus exhibit a larger coupling constant than the cis- (syn-)isomers <2001NJC676>.

ð57Þ

14.19.5 Applications of Computational Methods Computational methods have been widely used as valuable tools for structure elucidation, investigation of reaction mechanisms, and interpretation of spectroscopic data. The representative applications in the area of heterocyclic compounds described in this chapter with group 14 heteroatom are presented in Table 1.

Table 1 Applications of computational methods Compound

Method

Problem

Reference

3

DFT, HF, ROHF, MP2, ROMP2

1997JA6376

4 8, 9, 10

B3LYP/6-31G(d) PM3

24

56 69 81

Combination of DFT and single excitation configuration interaction approach (DFT/ SCI) MM2 (Macromodel v. 3.0) PM3, ab initio HF-default options available within the MOPAC 93 QCISD/3-21G* PM3 RHF/3-21G*

Total energy from the optimized geometry of C6SiH7þ and C6SiH8?þ radical cation, hydride affinity Structure optimization Activation parameters for ring inversion and calculation of

Hf Theoretical calculation of CD spectrum, i.e., excitation energies
E and rotatory strengths R

136, 137

B3LYP/6-31þG(dp)

151 213 234

CONFLECþPM3 MM2 Monte Carlo style conformational search (MC), MM3 MM3

40 50

247 (E ¼ Si) 253 257 (R ¼ H)

B3LYP/6-31G* B3LYP/6-31þG* AM1 RHF/6-31G(d)

260 317

AM1, HF/3-21G* AM1, PM3

Conformational analysis, steric strain energy Structure optimization of the chiral tin hydride Total energy of a singlet carbene-like structure Conformational analysis of the RCM reaction Calculation of orbital energies based on the optimized geometries Geometry optimization, electronic structure – HOMO, LUMO energy levels Conformational analysis Calculation of steric energies Conformational analysis

2001T3645 1995JOC1309 1999TA3483

1995JOM113 1997AGE235 2003TA3069 2004JA2696 1999BCJ821 1999OM3615 2006OM48 2004TL4329 1999JA5413 1998OM2656

Conformational analysis

1995JOC7406

Conformational analysis, electronic structure, calculation of EPR parameters Examination of the factors that control the preference of the hydrogen for ‘inside’ vs. ‘outside’ orientation Geometry optimization, conformational analysis Conformational analysis

2001JA6654 1994OM3728

1999JOC5626 2001NJC676

1027

1028 Rings containing Silicon to Lead Perhaps the most spectacular application of quantum-chemical calculations dealt with clarification of the nature of trialkylsilicenium ion, the first long-lived chiral and highly Lewis-acidic silyl cationic catalyst claimed to be prepared in the condensed state <1998JA7637>. Initially, the ion prepared in acetonitrile solution was characterized as a nitrile-coordinated trialkylsilicenium ion 318 with the reported 29Si NMR chemical shift 34.0 ppm. However, very soon this finding proved to be incorrect <1999JA9615>. Based on optimized geometries obtained at the B3LYP/631G* level, NMR calculations were performed using individual gauge for localized orbital (IGLO) method. The calculated 29Si NMR shifts were 334.4 and 44.2 ppm for silicenium ion 318 and silanitrilium ion 319, respectively (Figure 19). The almost 300 ppm difference between the calculated and the experimentally obtained values clearly exclude the observation of a free tertiary alkylsilicenium ion 318.

Figure 19

Computational studies have also been focused on the nature of the transannular N ! E bond observed in altranes. These studies include density functional calculations of geometrical parameters of metallatranes, where E ¼ Si, Ge, Sn, and Pb <2005JMT31>, ab initio and DFT studies of silatranes <1996JMT199>, quantum-chemical study of silatranes and germatranes using modified neglect of diatomic overlap (MNDO), AM1, and ab initio methods <1999JOM205>, DFT calculations of 1-phenylethynylgermatranes <2005JST1>, DFT calculations of azametallatranes <2004JST261>, conformational analysis of silatranyl carboxylic acids <1995JST249>, and DFT calculations of N-methylgermocanes <2003ZNB1165>.

14.19.6 Further Developments The first kinetics and mechanistic study of the thermolysis of 1,1-dimethylgermacycloheptatriene 320 (Scheme 57) was described and the relationship between the reactivity and the group 14 elements in this type of compounds was discussed <2006OM4231>. Compound 320 underwent thermolytic extrusion of divalent dimethylgermylene 321

Scheme 57

Rings containing Silicon to Lead

which could be trapped with 2,3-dimethylbuta-1,3-diene to give 1,1,3,4-tetramethyl-1-germacyclopent-3-ene 322. The progress of the thermolysis was monitored by 1H NMR spectral measurements. The pyrolysis was observed to follow first-order kinetics. From Arrhenius plots, the activation energy of the germylene extrusion was estimated to be 21.2  0.1 kcal/mol. The values of activation enthalpy (
H* ) and the activation entropy (
S* ) were determined to be 20.5  0.1 kcal/mol and 8.1  0.1 cal/(mol K), respectively. These data indicate that germacycloheptatriene 320 is the most thermally labile among metallepins containing group 14 elements. Cyclic trisilane 323 upon steady-state photolysis (245 nm) was used for preparation of diphenylsilylene 324, the silicon analogue of singlet diphenylcarbene <2006JA14442>. Diphenylsilylene 324 was trapped by MeOH or triethylsilane to give diphenylmethoxysilane 325 and 1,1,1-triethyl-2,2-diphenyldisilane 326 in 72 and 69% yield, respectively.

Scheme 58

All-anti-octasilane 327 (Figure 20) composed of two bicyclic trisilane units with trimethylsilyl groups at the termini was prepared and its structure was confirmed by X-ray crystal structure. The spectroscopic data demonstrated the effective -delocalization over the silicon framework <2006JA6800>. Bridgehead allylsilanes 328 (Figure 20) were prepared and used for the synthesis of a variety of polyhalogenated monoterpene natural products isolated from marine algae Plocaminium sp. <1997JOC8962>. Synthesis of 1-aza-5-silabicyclo[5.2.0]nonan-9-one, a silylated bicyclic lactam 329 (Figure 20) which shows antimicrobial activity against Gram positive bacteria was also achieved <1995JOC8403>.

Figure 20

The photolysis of cis-1,2-dimethyl-1,2-diphenyl-1,2-disilacyclohexane 330 (Equation 58) affords the rearranged silene which reacts stereospecifically with isobutylene to give the ene-type adduct, cis-2,3-benzo-1-isobutyl-1,4dimethyl-4-phenyl-1,4-disilaoct-2-ene 332, while trans-1,2-dimethyl-1,2-diphenyl-1,2-disilacyclohexane 331 (Equation 59) gives formal [2þ2] cycloadduct 333 with trans-configuration <2002OM4206>.

1029

1030 Rings containing Silicon to Lead

ð58Þ

ð59Þ

Irradiation of 330 in the presence of tert-butyl alcohol with low-pressure mercury lamp bearing a Vycor filter gave cis-2,3-benzo-1-tert-butoxy-1,4-dimethyl-4-phenyl-1,4-disilacyclooct-2-ene 334 in 33% yield (Equation 60), while a similar reaction of 331 led to the formation of trans-2,3-benzo-1-tert-butoxy-1,4-dimethyl-4-phenyl-1,4-disilacyclooct2-ene 335 in 41% yield (Equation 61) <2006JOM2440>.

ð60Þ

ð61Þ

A new synthetic approach to functional eight-membered silicon-containing heterocycles 336 (Figure 21) have been developed. It was found that intramolecular coordination of the aminoaryl ligand enhances the reactivity of at least one of the two functionalities at the silicon atom <1997OM3878>. Reactivity and the influence of substituents on transannular interaction germanium–nitrogen in germocanes 337 (Figure 21) have also been investigated in detail <2006JOM5710>. As shown in Equation (62), 1,2-bis(methylthio)benzene 338 was dimetallated using butyllithium or superbasic mixture of butyllithium/potassium tert-butoxide (LICKOR), and then quenched with dichlorodimethylsilane to obtain 1,5,3-benzodithiasilepin 339 <1999T14069>.

Rings containing Silicon to Lead

Figure 21

ð62Þ

Seven-membered silasultones 341 were synthesized in good-to-moderate yields by dehydrative cyclization of disulfonic acid siloxanes 340, carried out via vacuum sublimation (Equation 63). This method was found to be superior to attempted dehydrative cyclization of the disulfonic acids by azeotropic removal of water in refluxing toluene <2005T7233>.

ð63Þ

Siloxane-bridged 8-membered cyclic phosphaethane 343 was prepared by the reaction of bis(1-bromo-2-phosphaethenyl)disiloxane 342 with butyllithium (2 equiv) followed by treatment with 1,1,3,3-tetramethyl-1,3-dichlorosiloxane (Equation 64). Structure of 343 was confirmed by 1H, 13C(1H) and 31P(1H) NMR spectra as well as X-ray crystallographic studies. The two MesPTC groups take a trans configuration in the crystalline state, alleviating steric congestion <2007JOM243>.

ð64Þ

1,5-Dichlorohexamethyltrisiloxane 344 undergoes Mg-promoted reductive coupling with aromatic carbonyl compounds such as benzaldehyde, ketones and esters to give cyclic siloxanes 345 in moderate to good yields (Equation 65). The above reaction represents an interesting example of one-pot selective formation of carbon–silicon and oxygen–silicon bonds initiated by electron transfer from Mg metal <2006T3103>.

1031

1032 Rings containing Silicon to Lead

ð65Þ

Three series of new 8–12-membered heterocycles (Figure 22) such as 1,5-dichalcogena-3,7-siloxanes 346, 1,6dichalcogena-3,4,8,9-tetrasilocanes 347 and 1,5,9-triselena-3,7,11-trisilacyclodecanes 348 that include mixed Si–S, Sn–S, Si–Se, Sn–Se and Si–Te systems were synthesized and characterized by 1H, 13C, 77Se and 125Te NMR as well as X-ray crystallography. Oxidation of mixed S(Se, Te)/Si eight membered mesocycles 346 with one electron oxidant nitrosyl hexafluorophosphate (NOPF6) or Br2 gave dications and bicyclic dibromides, respectively, structure of which was confirmed by NMR methods <2006JA12685, 2006JA14949>.

Figure 22

Silyl-tethered stilbazole derivatives 349 were synthesized and subjected to intramolecular photocycloaddition in benzene at room temperature to give compounds of type 350 with stereochemistry cis–trans–cis and cis–trans–trans (Equation 66). It was shown that complexation of pyridine-containing stilbazoles 349 with dicarboxylic acid or catechol enhanced both the efficiency and stereoselectivity of the photocycloaddition <2006TL7865>.

ð66Þ

Acyclic disilane-containing oligoether terminated by two vinyl groups undergo ring-closure metathesis reaction (RCM) in the presence of catalytic amount of RuCl2(¼CHPh)(PCy3)2 at room temperature to give 32-membered macrocycle 351 (Figure 23) in the isolated yield of 31% (trans/cis ¼ 84:16) <2006JOM5260>. A double ring closure metathesis approach was also applied to the synthesis of macrocycles 352 <2006JOM5517>.

Rings containing Silicon to Lead

A series of axially-disubstituted silicon-phthalocyanines 353 (silicon-Pcs, Figure 23) was obtained by the nucleophilic displacement of one or two chlorine leaving groups from either PhSi(Pc)Cl or Si(Pc)Cl2, respectively, by reaction with the acid or alkoxide derivative of the ligand. Structure of these compounds was confirmed by 1H and 13 C NMR, UV-vis absorption and emission spectra, electrospray or MALDI-ToF mass spectrometry as well as X-ray crystallography <2006T9433>.

Figure 23

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1033

1034 Rings containing Silicon to Lead

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Rings containing Silicon to Lead

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Rings containing Silicon to Lead

2005SL1105 2005T2037 2005T3697 2005T4657 2005T7233 2005TL315 2006CEJ4954 2006JA6800 2006JA12685 2006JA14442 2006JA14949 2006JOM2440 2006JOM5260 2006JOM5517 2006JOM5710 2006OM48 2006OM3969 2006OM4231 2006T3103 2006T7951 2006T9433 2006TL7865 2006TL873 2007JOM243

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1037

1038 Rings containing Silicon to Lead Biographical Sketch

Franciszek Sa˛ czewski was born in Sopot, Poland, in 1951. He graduated from Medical University ´ in 1974 with M.S. degree in pharmacy and that same year began his career at the of Gdansk Department of Organic Chemistry. In 1981 he received his Ph.D. and in 1988 D.Sc. degree in pharmaceutical chemistry. In 1999 he was promoted to full professor. During 1983–1984 and 1988–1989 he was working with Prof. Alan Roy Katritzky at the Department of Chemistry, University of Florida, USA. He is a member of the Royal Society of Chemistry, International Society of Heterocyclic Chemistry and Polish Pharmaceutical Society. Prof. F. Sa˛ czewski is currently the head of the Department of Chemical Technology of Drugs, ´ Medical University of Gdansk, Poland. His research interests are focused on the design and synthesis of nitrogen-containing heterocyclic compounds with potential circulatory, anticancer and anti-HIV activities.

´ Anita Kornicka was born in Gdansk, Poland on February 15, 1966. She graduated from Medicinal ´ in 1990 with M.S. degree in pharmacy. Since 1990 she has been working at University of Gdansk ´ the Department of Chemical Technology of Drugs, Medical University of Gdansk. In 2000 she received her Ph.D. degree in pharmaceutical sciences. For many years she has been conducting studies on chemical and biological properties of 2-mercaptobenzenesulfonamide derivatives. Nowadays, her research interests include the design and synthesis of imidazoline-containing compounds with potential circulatory and anticancer activities. Anita Kornicka is a member of the Polish Pharmaceutical Society.