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Syntheses and properties of cycloborazines and cyclocarborazines
Polyhedron 48 (2012) 9–20
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Syntheses and properties of cycloborazines and cyclocarborazines Miroslav Kavala, Peter Zálupsky´, Peter Szolcsányi ⇑ Dept. of Organic Chemistry, Slovak University of Technology, Radlinského 9, SK-812 37 Bratislava, Slovakia
a r t i c l e
i n f o
Article history: Received 28 May 2012 Accepted 4 September 2012 Available online 15 September 2012 Keywords: Borazines Synthesis Properties Hydrolysis
a b s t r a c t We present a review of preparative methods as well as physico-chemical properties of selected types of cyclic boron–(carbon)–nitrogen containing compounds. The review is compiled as a synthetic manual for preparation of compounds with the desired structure, which are to be further modified to novel, as yet unknown derivatives. The compilation is subdivided according to various types of ring sizes. Each section presents known methods of synthesis of respective cyclo(car)borazines highlighting their possible functionalisation. The important data on hydrolytic stability of compounds are also included. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Cycloborazines are cyclic compounds formed of alternating nitrogen and boron atoms [1–9]. Such systems are isoelectronic with their all-carbon analogues, however, due to the dipole moments of adjacent heteroatoms, the bonding contains a significant ionic component [10–13]. In contrast to cyclosilazanes, a family encompassing numerous derivatives with various ring size, 4- and 8-membered cycloborazines substituted at boron are as yet unknown. Therefore, we shall concentrate on properties and preparation of 6-membered cyclotriborazines. 2. Cyclotriborazines (borazines) Cyclotriborazines form 6-membered rings, the basic structure of which is shown on Fig. 1. Positions 1, 3 and 5 are occupied by nitrogen atoms, while positions 2, 4 and 6 are taken up by threevalent boron atoms. Although boron is normally three-valent, with its empty p-orbital it can form a four-valent anion. The name cyclotriborazines has not become a common name, they are routinely known under the name B-trisubstituted borazine. In this section we shall deal with cyclic borazines with unsubstituted nitrogen atoms, carrying at boron either bulky substituents, or such that allow further functionalisation. 2.1. Preparation of borazines Presently several methods for preparation of borazine derivatives are known. They encompass aminolysis of trichloroborane, ⇑ Corresponding author. Tel.: +421 2 59325162; fax: +421 2 524 953 81. E-mail address: [email protected] (P. Szolcsányi). 0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2012.09.003
functionalisation of hexahydroborazine or derivatisation of boric acid esters. 2.1.1. Cyclocondensation of iminoboranes Conceptually this is the most effective method of borazine preparation, because it is experimentally rather simple. In its first step, it involves reduction of alkylboroxines (anhydrides of boric acid) with LAH in the presence of trialkylamine, to give trialkylaminoalkylboranes 1–4 (Scheme 1) [14]. Next, the latter are treated with an excess of ammonia in diglyme, in the presence of ammonium chloride as catalyst. The reaction is accompanied by vigorous release of hydrogen and dimethylamine, forming the corresponding cyclic borazines. Although the reaction has not been described with alkylamines, authors claimed reactions proceeded also with various alkylsubstituted tertiary amines. Cyclic borazine is considered to be an isoelectronic analogue of benzene, however, its thus supposed aromaticity is still a matter of debate [15–21]. According to the postulated mechanism (Scheme 2), the first step involves transamination of trimethylamine by ammonia. The subsequent action of ammonia and catalytic amount of ammonium chloride elicits a sequential elimination of hydride anion from boron and proton from nitrogen, the evolving gaseous hydrogen shifting the reaction equilibrium towards products 5–13. The key step of the synthesis is the cyclocondensation reaction of three molecules of iminoborane. It turned out that although the above mentioned borazines did not succumb to air moisture, nevertheless after stirring with water for several hours they completely hydrolysed to boric acid with concomitant release of ammonia. In addition, storing samples in dry conditions under nitrogen atmosphere caused yellowing of the originally colourless compound and its transformation to a further unspecified solid.
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M. Kavala et al. / Polyhedron 48 (2012) 9–20
2.1.3. Ammonolysis of diethylaminodichloroborane The methodology has been demonstrated on a single substrate with free amino group at the borazine ring [23]. First, the diethylaminodichloroborane 15 was prepared by treating trichloroborane with diethylamine (Scheme 5). Compound 15 was then subjected to ammonolysis to give the cyclic product 16 in only average yield. This section also presents a method based on ammonolysis of aryldichloroboranes 17–19, accessible by reaction of aromatic chloromercury derivative with trichloroborane (Scheme 6) [24]. Although the ammonolysis itself furnishes very good yields of arylborazines 20–22 (80–97%), its major drawback is the use of toxic mercury(II) salts.
R2 HN B
R1
3
B4 5
2
6 1
NH B
R3
N H
R 1 - R 3 = chlorine, alkyl, aryl, N-alkyl, N-silyl, O(S)-alkyl Fig. 1. General formula of cyclotriborazines.
2.1.2. Ammonolysis of alkylthioboronic acid This sequential method was described for the synthesis of alkylborazine derivatives 5–7. Authors claim it to be a facile method of preparation with easy isolation of products and good yields [22]. In the first step, anhydride of alkylboric acid reacts with tribromoborane, giving rise to alkyldibromoboranes 8–10 (Scheme 3). It may be assumed that low yields can be accounted for by polysubstitution of the created monosubstituted borane. Next, dibromoboranes are transformed to n-butylesters of alkylthioboric acid 11–13; the experiment has however been described in detail in case of isopropyl derivative. These esters underwent ammonolysis already at laboratory temperature, giving the required borazines 5–7 in good yields (80%). The tentative mechanism of ammonolysis assumes in its first step a substitution of thiolate group with ammonia to give A and self-condensation in the next step to give B (Scheme 4). The thus formed intermediate next condenses to triborazane structure 14, which in the last releases a molecule of thiol to give the cyclic borazine.
2.1.4. Reaction of HMDS with dichloroborane derivatives The method was used to prepare an ethyl and phenyl derivative of borazine [25,26]. It is a ‘‘one-pot’’ reaction of ethyl- or phenyldichloroborane (23 and 24) with HMDS, releasing TMSCl and forming cyclic borazine derivatives 25 and 26 (Scheme 7). B-triphenyl borazine could be isolated in almost quantitative yield which makes this preparation synthetically very attractive. 2.1.5. Reaction of bis(diisopropylamino)ethynylborane with ammonium chloride Further functionalisation of borazine derivatives has been the topic of very few reports. This method was used to prepare triethynylborazine (30 or 31) [27], the triple bonds of which lend itself to further functionalisation. In its first step, the synthesis uses the reaction of BCl3 with diisopropylamine (DIPA) in toluene, leading to diaminochloroborane 27 (Scheme 8). Chloroborane 27 then reacts with
R O B
R
R
B
a
O B
O
H 3 R
B
b
N
H
R 1: 2: 3: 4:
HN B
R
R = n Pr (65%) R = i Pr (65%) R = n Bu (64%) R = s Bu (66%)
5: 6: 7: 8: 9:
B
N H
NH B
R 10 : R = 11 : R = 12: R = 13: R =
R = n Pr (88%) R = i Pr (79%) R = n Bu (91%) R = sBu (85%) R = i Bu (70%)
t
Bu (65%) Pen (88%) n Hex (86%) Bn (70%) n
a) Me3 N (3 equiv), LAH, Et 2 O, reflux, 3 h; b) NH 3 (g), NH 4 Cl, diglym, 100-150 °C, 2 h. Scheme 1.
H R
B
N
H
H R
B
N
+NH 3 - Me 3N
H
H B
N
H
+ NH 4
B
N H
+NH3 - NH 4
+ NH 4
H
-H2 , -NH 3
H
R
H B
H
R
H
+NH3
N H H
H
-H 2, -NH3 H
R
H R
R
H B
- NH 4
N
H
H
R
N B
B N
N B
R HN H
R Scheme 2.
R
B
B
N H
R NH B
HN R
R
B
B
N H
NH B
R
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M. Kavala et al. / Polyhedron 48 (2012) 9–20
R O R
B
B
O
R
B
Br
a
O
R
b
B
R
Br
R
8: R = n Pr 9: R = i Pr (59%) 10 : R = n Bu
S
n
S
n Bu
Bu
B
11: R = n Pr 12: R = i Pr (78%) 13: R = n Bu
c
HN R
B
B
N H
NH B
R
5: R = n Pr (86%) 6: R = i Pr (82%) 7: R = n Bu (80%)
a) BBr 3, r.t.; b) n BuSH, reflux, 12 h; c) NH3 (g), r.t., 1 h. Scheme 3.
R S R
n Bu
B S
n
Bu
+NH3 - nBuSH
n Bu
S R
HN
+A
B
-n BuSH
NH2
R
B R
R
- n BuSH
R
HN B
B HN
NH2
n Bu
S NH2
A
+A
B
B
R B S
HN - n BuSH R
n Bu
B
14
B
N H
NH B
R
5- 7 Scheme 4.
lithium acetylide (or TMS/lithium acetylide) to furnish the borane derivatives 28 and 29, which, in the final step afford the target borazine 30 and 31. It is evidently one very effective method for preparation of borazines with the propensity for further functionalisation. 2.1.6. Substitution reactions of B-trichloroborazine B-Trichloroborazine 32 is a labile compound, completely hydrolysed by water to boric acid and ammonium chloride [28]. In spite of the wealth of data in the literature, it has become commercially available, we shall concentrate here on derivatives 32. Derivatives 32 can be approached by four methods we shall describe in chronological order. The first described method involves treatment of trichloroborazine 32 with Grignard reagents MeMgI, EtMgI and PhMgBr (Scheme 9) [29]. Triethylborazine 25 and triphenylborazine 26 are isolated from the reaction mixture after the solvent has been distilled off and the crude reaction mixture pyrolysed at 150 °C. Compounds 25 and 26 are better prepared by the method given in Section 2.1.4.
NEt 2 Cl Cl
B
a
Cl Et 2 N B
Cl
Cl 15
b
HN Et2 N
B
B
N H 16
a) Et2 NH, benzene, -78 °C - r.t., 3 h, 81%; b) NH3(g), benzene, -78 °C - r.t., 20 h, 52%. Scheme 5.
NH B
NEt 2
In 1961 a synthesis was published [30] the first step of which involved substitution of chlorine atoms in 32 for butanethiolate coming from n-butyl lead mercaptide (Scheme 10). The thus obtained B-tri-n-butylmercaptoborazine 34, when treated with ammonia, dimethylamine, aniline and methanol respectively, gives the corresponding derivatives 35–38. However, certain limitations of this methodology lies in the use of toxic lead compounds and foul smelling sulfides and/or thiols. Gerrard and coworkers carried out reactions of trichloroborazine 32 with a series of secondary amines [31], leading in turn to the corresponding amino borazines (16, 36 and 39–44) in low yields (Scheme 11). Only substitution with diphenylamine gave product 44 in good yield (76%). Tris(diethylamino)borazine 16 arises from the reaction of 40 with trimethylsilyldiethylamine (Scheme 12) [32]. As far as the yields are concerned, there was hardly any improvement (54%).
2.1.7. Rhodium-catalysed hydroboration of alkenes All methods described so far produced derivatives with symmetrical substitution pattern at boron atoms. Sneddon carried out a series of experiments [33], in which he succeeded in preparing (apart from symmetrically trisubstituted borazines) also monoand disubstituted borazines by Rh-catalysed reaction of olefins with the borazine 45 (Scheme 13 and Table 1). The Sneddon methodology not only allowed preparation of monosubstituted borazines, it also allowed a step-by-step procedure introducing three different substituents into the molecule of borazine. Authors have also postulated a mechanism of such Rh-catalysed hydroboration of olefin (Scheme 14). The first step involves dissociation of fosfine ligand from the central rhodium metal (A), followed by coordination of olefin (B), addition of hydride to olefin (C), oxidative addition of the B–H group of olefin (D) and finally by reductive elimination of alkylborazine (E). This method has a considerable synthetic potential, since the metal catalysts is used only in 0.015–4.4 mol%, in addition to the possibility to introduce different substituents – a feature unique to this methodology but one. It gives fairly good yields of desired products, thus making it unequivocally the method for preparation of borazines. Table 2 compiles data on melting and/or boiling points of 39 borazine derivatives. Rhodium(I) complexes were also used in the catalytic dehydrocoupling methodology [34] that transforms ammonia–borane adducts to cyclic borazines under mild conditions (max. 45 °C, glymes). However, isolation of products from the reaction mixtures (by vacuum fractionation) proved difficult; pure borazines were isolated low yields (10–30%) with the major products being nonvolatile, oligomeric species.
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M. Kavala et al. / Polyhedron 48 (2012) 9–20
Ar a
Hg
Ar
Cl Ar
Cl
b
HN
B Cl
Ar
17: Ar = 2-Me-C 6H 4 (67%) 18 : Ar = 3-Me-C 6H 4 (57%) 19: Ar = 4-Me-C 6H 4 (74%)
Ar = 2-Me-C6 H4 , 3-Me-C 6H 4 4-Me-C 6H 4
B
B
N H
NH B
Ar
20: Ar = 2-Me-C6 H4 (85%) 21: Ar = 3-Me-C6 H4 (97%) 22: Ar = 4-Me-C6 H4 (80%)
a) BCl3, benzene, reflux, 4 h; b) NH3 (g), benzene, -78 °C - r.t., 20 h. Scheme 6.
R Cl R
B
a
HN
Cl
B
R
B
N H
Cl
B
HN
a
NH
B
R
HN
B N H
Cl
25: R = Et 26: R = Ph
23: R = Et 24: R = Ph
R
B
NH
Cl
R
B
B
N H
NH B
R
33: R = Me 25: R = Et (70%) 26: R = Ph (60%)
32
a) HMDS 25 : benzene, -78 °C - r.t., 20 h, 61%; 26: CH 2Cl2, -78 °C - 55 °C, 10 d, 98%.
a) RMgX (X=I, Br), Et2 O, r.t. - 150 °C.
Scheme 7.
Scheme 9.
Borazine derivatives being relatively unstable compounds can also be expected to be fairly reactive. This has been amply demonstrated in their preparation starting from trichloroborazine. All reactions are carried out under the blanket of nitrogen or argon atmosphere, without access of oxygen and air moisture. We shall nevertheless try to map their hydrolytic stability.
lein-mannitol). The highest hydrolytic stability demonstrated the electron-rich tris(diphenylamino)borazine 52, which, after 30 minute reflux hydrolysed merely to 7% – a fact testifying to its relatively high stability towards hydrolysis. It also means that in manipulating other derivatives contact with water should be avoided which is a fairly serious practical limitation. In spite of this, various derivatives can be prepared and used in further functionalisation.
2.2. Reactions of borazines
3. Cyclocarborazines
2.2.1. Hydrolysis of aminoborazine compounds All borazines undergo hydrolysis, the outcome of which depends a great deal on volume of substituent and electronic effects. Table 3 summarises the results of the study by Gerrard, who studied the hydrolytic stability of aminoborazines [31] in refluxing water, or aqueous NaOH. The reaction products were naturally amines (neutralised by 0.1 N HCl), and boric acid, determined by titration (phenolphtha-
Cyclocarborazines are cyclic compounds, analogues of saturated carbon azaheterocycles – azetidine, pyrrolidine and piperidine. Their parent structure contains in positions 2 and (3 + n) boron atoms instead of carbon atoms (Fig. 2). The stability and reactivity is what makes these compounds truly interesting. The reports on cyclocarborazines in the literature are scarce, derivatives with free
R
R NH
a
N
B N
Cl
b
N
B
B HN
c
28: R = H (98%); 29: R = TMS (89%)
30: R = H (92%); 31: R = TMS (83%)
a) BCl3, toluene, 0 °C - r.t., 14 h, 90%; b) Li-C C-R, 12-crown-4 (cat.), THF or Et 2 O, 0 °C - 80 °C, 4 h; c) NH 4 Cl, toluene, reflux, 12-24 h. Scheme 8.
B N H
N R
27
NH
B
R
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M. Kavala et al. / Polyhedron 48 (2012) 9–20 n
Cl B
HN B
Cl
N H
a
NH B
Bu
B
HN n
Cl
Bu
S
R
S
B
N H
b
NH B
n
S
HN
Bu
35 : 36: 37: 38 :
34
32
B
R
B
N H
NH B
R
R = OMe (50%) R = NMe2 (97%) R = NHPh (70%) R = NH 2 (80%)
a) (n BuS)2 Pb, benzene, reflux, 2 h, 91% b) RH, benzene, r.t., 1 h. Scheme 10.
R2
Cl HN Cl
B
B
N H 32
R1 NH B
+ Cl
a
NH
HN
R2
R1
B
N R2
36 : 16 : 39: 40:
R1 R1 R1 R1
= R 2 = Me (51%) = R 2 = Et (58%) = R 2 = n Pr (47%) = R 2 = i Pr (52%)
R1
N B
NH B
N H
N
R2
R1
41: 42: 43: 44:
R1 R1 R1 R1
= R 2 = n Bu (42%) = R 2 = i Bu (28%) = Me, R2 = Ph (64%) = R 2 = Ph (76%)
a) benzene, r.t. Scheme 11.
amino group being practically unknown. We shall therefore concentrate here solely on preparation and properties of N-substituted cyclic derivatives. 3.1. Cyclomonocarbodiborazines Cyclomonocarbodiborazines form 4-membered rings with the basic structure shown in Fig. 3. The name of this group of compounds has been derived from the saturated azaheterocycle – azetidine. Instead of carbons atoms in positions 2 and 4 it contains boron atoms. The valence of boron allows it carry a substituent, thus a typical representative of this group of compound is a 2,4disubstituted 2,4-diboraazetidine, also called 2,4-disubstituted 2,4-dibora-1-azacyclobutane. All so far reported compounds of this type carry a substituent at nitrogen. 3.1.1. Preparations of 2,4-diboraazetidines Cyclomonocarbodiborazines are structurally and chemically intriguing compounds, so far accessible by only a handful of methods. There is no procedure so far capable of producing such heterocycle without a substituent at nitrogen. 3.1.1.1. Insertion of isonitriles and carbenes. The key substrate, on which this method of preparation of 4-membered rings is based [35], is the 1,2,3-tri-tert-butylazadiboridine 65. It is a highly reactive compound (owing to the extremely weak B–B bond), prepared in two steps (Scheme 15) [36,37]. Authors reported an aromatic character for this azadiboracyclopropane, the free electron pair at nitrogen being delocalised over the ring. The first of its synthetic transformations is the reaction of 65 with aromatic isonitriles, of which only one has been described in detail (Scheme 16). According to the postulated mechanism (Scheme 17) it involves an insertion of isonitrile 66 into the B–B bond, bringing about an expansion to a 4-membered ring of
N
Cl HN Cl
B
B
N H
a
NH B
N
+
HN
TMS
N
Cl
B
40
B
N H
NH B
N
16 a) toluene, r.t., 54%. Scheme 12.
HN HB
H B
N H
R2 NH
+
alkene
a
BH
HN R1
45
B
B
N H
NH B
R3
Products in Table 1
a) RhH(CO)(PPh 3 )3 (0.015-4.4% mol), neat / CH2Cl2, -196 °C - r.t. Scheme 13.
1,2,4-azadiboretidine 67 as red oily liquid. The method has one substantial limitation though, being demonstrated on a single aromatic isonitrile. In case of compound 67 NMR spectra revealed an interesting feature, namely the non-equivalence of tert-butyl groups. This can be accounted for by assuming presence of a C@N bond without free rotation. Such structure would also be responsible for a single 1H NMR signal for ortho-methyl groups at the benzene ring, thus their magnetic equivalence.
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M. Kavala et al. / Polyhedron 48 (2012) 9–20
Table 1 Compounds prepared by Rh-catalysed reaction of olefins with borazine 45.
a b c
Compounds
Alkene
45/alkene
Time (h)
R1
R2
25 46 47 5 48 49 50 51 52 53 54 55 56
ethylene propene propenea
1/3 8/1 1/3.3
2 2 1
1-butene Z-2-butene
5.8/1 5/1
3 47
E-2-butene styrene 4-allylanisol ethyleneb ethylenec acetylenec
6/1 7/1 10.5/1 1/2.2 1/3.4 1/3.9
185 2.5 96.5 2 2 120
Et H H n Pr H H H H H H Et Et vinyl
Et H Pr n Pr H H n Bu H H H Et n Pr n Pr
R3
Yield (%)
Et Pr Pr n Pr n Bu n Bu n Bu s Bu (CH2)2C6H5 (CH2)3C6H4-4-OMe n Pr n Pr n Pr
92 98 43 55 91 90 3 72 84 57 97 97 54
n
n
n
Di- and trisubstituted products are formed in the ratio 44/56. Starting substrate is 2-propylborazine. Starting substrate is 2,4-dipropylborazine.
HB HN
H N
B
H
P = PPh3
P Rh P P CO
BH NH
(A) +P
R
-P
H
HB HN H R
P
H N
B
R
P H Rh OC P
(E)
(B) BH
R
NH
H Rh P CO B3N 3H 6
H
(D)
H Rh P P CO
(C) P
R
Rh
CO P
(A) release of phosphine ligand (B) olefin coordination (C) hydride addition to olefin (D) oxidative addition of borazine (E) reduction elimination of alkylborazine Scheme 14.
Compound 67 is rather unstable and exposed to air quickly decomposes. Authors do not report about its thermal stability, or moisture sensitivity. It can be assumed though, that the imino bond quickly hydrolyses, making the derivatives hydrolytically unstable. The second type of transformation of 67 to 4-membered ring is the carbene insertion of lithiated a-bromoalkanes. Using this method, authors prepared the 4-membered spiro-compound 69, starting from the cyclopropane derivative 68 (Scheme 18). This transformation starts with lithiation of the dibromocyclopropane 68, followed by insertion of organometallic cyclopropylidene compound 68A into the B–B bond of 65 to give the 1,2,4-azadiboretidine 69 (80%) and free LiBr (Scheme 19). 3.1.1.2. Cycloaddition reaction of iminoboranes. An interesting method of preparation of 1,2,4-azadiboracyclobutanes is the cycloaddition of iminoborane with unsaturated alkylidene tantalum complex [38]. Alkyl(tert-butylimino)boranes 70–72 add to the tantalum complex 73 (in a molar ratio 2:1) to give the intermediacy diazadiboratantalocyclohexanes 74–76 (Scheme 20) [39]. When heated, these 6-membered derivatives eliminate the tantalum
Table 2 Compiles data on melting and/or boiling points of 39 borazine derivatives. Compounds
mp (°C)
bp (°C)
5 6 7 8 9 10 11 12 13 16 20 21 22 25 26 30 33 35 36 37 38 39 40 41 42 43 44 47 52
– – – – – – – – –
70 (0.6 Torr) 70 (0.5 Torr) 102–105 (0.2 Torr) 94 (0.7 Torr) 72 (0.03 Torr) 60 (10 Torr) 125 (0.07 Torr) 140 (0.05 Torr) not given 120 (0.1 Torr) – – – 56–57 (2 Torr) – 90 (75 Torr) sublimation 35 (20 Torr) – – – – 170–172 (0.4 Torr) – 200 (0.05 Torr) 167 (0.3 Torr) – – 119–124 (760 Torr) –
89–92 140 189–190 46 180–185 132 36 114 112–115 204–206 >250 32–35 138–143 – 47–52 128–132 152–155 – 123–125
Table 3 Compiles date on the hydrolytic stability of aminoborazines. Compounds
R2 N
Reagent
Hydrolysis (%)
44 48 48 51 52 52
Me2N i Pr2N i Pr2N PhMeN Ph2N Ph2N
NaOH solution (reflux) water (reflux) water (cold) water (reflux) water (reflux) NaOH solution (reflux)
100 100 55 58 7 29
R2 N R1 B 2
1
4
3
B
R3
n
R1, R 2, R3 = alkyl, aryl Fig. 2. General formula of cyclocarborazines.
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M. Kavala et al. / Polyhedron 48 (2012) 9–20
R2 N R1 B 2
1 3
4
NC
R3
B
B
R 1, R2, R 3 = alkyl
N B
N
+
B
N 65
Fig. 3. General formula of cyclomonocarbodiborazines.
complex 77 causing a ring contraction to the target 1,2,4-azadiboracyclobutanes 78–80. Although they could not be analysed in pure form, their presence was deduced from NMR spectra of reaction mixture. The reaction of analogous tantalum complex 82 with iminoborane 81 (in 1:1 molar ratio) gives rise to the 1-aza-2,4-diboracyclobutane derivative 83 (Scheme 21), which could be isolated by multiple crystallisations in 26% yield. Authors give the tentative structure of leaving tantalum complex 84 without being able to confirm its structure. 1,2,4-Azadiboracyclobutanes have been fully characterised by NMR spectroscopy, but were not further chemically transformed due to their high instability (Table 4). 4. Cyclodicarbodiborazines Cyclodicarbodiborazines form 5-membered rings with the core structure shown in Fig. 4. The name of this group of compounds has been derived from the saturated azaheterocycle – pyrrolidine. Instead of carbons atoms in positions 2 and 5 it contains boron atoms. The valence of boron allows it carry a substituent, thus a typical representative of this group of compound is a 2,5-disubstituted-1,2,5-azadiborolane, also called 1,2,5-azadiborolidines or 1aza-2,5-diboracyclopentane. All so far reported compounds of this type carry a substituent at nitrogen. 4.1. Preparation of 2,5-diborapyrrolidines So far, only two methods of preparation of diborolanes are known. One of them involves cyclisation of dichlorodiboralkanes with a tertiary silazane, the other relies on thermal cyclisation of aminoboranes. Similarly as in the earlier section on 4-membered derivatives, these methods were so far able to furnish only Nsubstituted cyclic compounds. 4.1.1. Cyclisation of chloroboranes with silazanes Syntheses of 5-membered nitrogen heterocycles containing a B–N–B moiety described in the literature are few and far between. Only several derivatives have been known. A method reported for the preparation of such heterocycles starts from dichlo- or tetrachlorodiboralkane derivative [40]. The action of diborane on ethylene at 80 °C gives rise to 1,2-bis(dichloroboryl)ethane 85 [41],
66 67 a) CH 2Cl2, -78 °C, 2 h, 61%. Scheme 16.
which next undergoes a reaction with hexamethyldisilazane 86 to give cyclic 2,5-dichloro-1,2,5-azadiboracyclopentane 88 as colourless liquid in 95% yield (Scheme 22). However, the compound decomposes on standing at laboratory temperature. However, a reaction of 85 with hexamethylsiladimethylborazines 87 failed to produce isolable target compound 89. Its formation was only proved by experiment carried out in a NMR tube. Substrates 86 and 87 were prepared from LiHMDS and the corresponding electrophiles (methyl iodide, or dimethylchloroborane) [42]. Compound 88 decomposes on standing too, but boron atoms, being shielded by a nitrogen bridge, are no longer attacked by active reagents such as SbF3, or Me4Sn. Dimethyldiborazine 90 is obtained from tetrachloroalkyldiborane 85 by double methylation with tetramethyltin 91. Dimethyl derivative cyclodicarbodiborazines 92 arises from cyclisation reaction of 90 and 86, taking place without solvent, and in good yield (Scheme 23). 4.1.2. Reductive cyclisation Another known methodology leading to azadiboracyclopentane is based on a reaction of tetraalkyldiboranes with dialkylaminoboranes [43]. The diethyl-N-boroaniline 95 arises in the dehydrogenation reaction of aniline 93 in 88% yield (Scheme 24). After isolation and purification, the secondary amine 95 reacted with 1.5 equiv. of triethylborane 96 in the presence of 0.3 equiv. of tetraethyldiborane 94 in an autoclave for 16 h at 180 °C and 9 h at 200 °C. Subsequently, the reaction mixture was reheated to 225 °C for another 2 h. This procedure, followed by a fraction distillation furnished the desired cyclic product 97 in 8% yield. However, authors did not propose any plausible mechanistic course of reaction. Table 5 displays boiling points of isolated cyclic products. 5. Cyclotricarbodiborazines Cyclotricarbodiborazines are 6-membered rings with the parent structure shown in Fig. 5. Once again, positions next to nitrogen (2, 6) are occupied by boron atoms. The difference in valence be-
Cl Br
Sn N Sn
B
a
B
+
B
a
Cl
N
Br
B
Cl
b
N B
B
Cl 65 a) CH2Cl2, -30 °C, 30 min., 97%; b) Na-K, hexane, reflux, 4 h, 33%. Scheme 15.
16
M. Kavala et al. / Polyhedron 48 (2012) 9–20
N
R R B R
N
R'
N
+
B
B
N R
B
C
B
R
B N
C
R 65
N
66
R'
67
R = t Bu, R' = 2,6-(CH3) 2-C6 H3 Scheme 17.
Br
Br
Br
Li
a
B 68
N
b
N
+
B
B
B
68A 65
69
a) LiBr, BuLi, THF/Et 2O/pentane (3:1:1), -105 °C, 30 min.; b) -110 °C to 20 °C, 8 h, 80%. Scheme 18.
R R N
N
Br
B R
R B Li
+ Li
B
N B
B
R
B
Br
R 68A
65
69
R = t Bu Scheme 19.
H R
B
N
+
R
THF Cl C Ta Cl Cl
H -78 °C
B
THF
N Cl Ta Cl
N 60 °C
N Cl
R
B
N B
R
+ Cl
78 : R = Et 79: R = Pr 80 : R = Bu
74: R = Et 75: R = Pr 76: R = Bu
Ta Cl
H
R
73
70: R = Et 71: R = Pr 72: R = Bu
B C
77
Scheme 20.
H N
B
TMS
N
+
PMe3 Cl C Ta Cl Cl
N
a
N B TMS
81
83
82
a) toluene,-78 °C - r.t., 16 h, 26%. Scheme 21.
N
+ TMS
H
PMe3
B
N Me 3P Ta PMe3 Cl Cl Cl
84
Cl
17
M. Kavala et al. / Polyhedron 48 (2012) 9–20 Table 4 Boiling points.
Table 5 Boiling points.
Compounds
bp (°C)
Compounds
bp (°C)
67 69 83
– 60 (0.002 Torr) –
88 92 97
20 (0.5 Torr) 20 (22 Torr) 65–73 (0.2 Torr)
R2
R2
N
R1
1
B2
5 4
3
R3
B
R1
N
B2
1 6
3
4
R 1, R2, R 3 = alkyl
N R
+
TMS
Cl
Fig. 5. General formula of cyclotricarbodiborazines.
R
Cl B
a
Cl
B
Cl
N B
5
R 2 = H, alkyl R1 , R 3 = alkyl, N, S
Fig. 4. General formula of cyclodicarbodiborazines.
TMS
R3
B
B
Cl
5.1. Preparation of 1-aza-2,6-diboracyclohexanes
Cl 86: R = Me 87: R = B(Me) 2
85
Preparations of cyclotricarbodiborazines parallel those described in Sections 3.1.1 and 4.1.1.
88 : R = Me (95%) 89: R = B(Me)2 (0%)
a) isopentane, -78 °C - r.t. Scheme 22.
tween carbon and boron predetermines the substitution pattern. Substituents are carried in positions 2 and 6, thus giving rise to 2,6-disubstituted 1-aza-2,6-diboracyclohexanes, or else 2-aza1,3-diborinanes. In this section, we shall deal with compounds having (un)substituted nitrogen.
5.1.1. Cyclisation of chloroboranes with silylamines Syntheses of 6-membered nitrogen heterocycles containing a B–N–B moiety described in the literature are scarce. The reported synthesis of trialylborane 99 starts with the reaction of alylmagnesium bromide 98 with BF3.OEt2 (Scheme 25) [44]. Trialylborane was next subjected to a transformation with diborane to a polymer of unknown composition, which was nevertheless subjected to reaction at high temperature with trichloroborane, to give the 1,3-bis(dichloroboryl)propane 101 [45]. It was expected that the action of hexamethyldisilazane 86 on organoborane 100 in pentane would furnish the cyclic 2,6-dichloro-1-aza-2,6-diboracyclo-
Cl Cl
B
Cl +
B
Sn
TMS
a Cl
B
+
B
Cl 91
N B
B
TMS
Cl
85
b
N
90
86
92
a) pentane, -78 °C - r.t. - -78 °C, fraction distillation, 82% b) -78 °C - r.t., low-temp distillation, 70%. Scheme 23.
H +
B
B H
a
H + HN
B
+
B
B
b
H
B
N B
B
NH 2 93
94
95
96
94
a) -78 °C to 80 °C, 90 min., fraction distillation, 88% b) 200 °C - r.t. - 220 °C, 25 + 2 h., fraction distillation, 98%. Scheme 24.
97
18
M. Kavala et al. / Polyhedron 48 (2012) 9–20
Br
a
b, c
Cl
yield (Scheme 27). Action of tert-butylamine on 104 elicits cyclisation to cyclotricarbodiborazane derivative 105 (73%).
Cl
B
B
B
Cl
Cl
98
5.1.2. Cyclisation of thioboranes with amines 1,1,5,5-Tetramethoxy-1,5-diboropentane 106 serves as the starting compound in the methodology. It is prepared by the reaction of 100 with methanol (Scheme 28) [45]. The subsequent reaction with tris(ethylsulfanyl)boranes 106 and 107 affords the borasulfane 108. 1,1,5,5-Tetraethylmercapto-1,5-diborapentane 108 represents a sulfur analogue of boric acid esters and allows the exchange of mercaptoethyl groups by substitution reaction with ammonia, or amines [48]. The action of gaseous ammonia on 108 affords the cyclic product – 2,6-diamino-1-aza-2,6-diboracyclohexane 109 in 44% yield, with concomitant loss of ethanethiol. If methylamine or ethylamine are used, analogous Nsubstituted cyclic compounds 110 and 111 are obtained in 70% and 93% yield, respectively. The cyclic product 112 could be obtained in 38% yield (Scheme 29) by treating the tetraethyl ester of the propane-1,3dithioboronic acid 108 with 1 equiv. of methylamine. Table 6 displays an overview of physical properties of isolated cyclic products. As in the case of earlier mentioned 4- and 5-membered cyclocarborazines, the products given here were not further transformed and their preparation only served to demonstrate the synthetic viability of the suggested methodology.
100 99
Cl
TMS
Cl B
B
Cl
Cl
+
Cl
d
N
B
N
Cl
B
TMS
100
86
101
a) BF 3 .OEt 2 , I 2, Mg, Et 2 O, reflux, 3 h, Kugel-Rohr, 84%; b) B2H6 , THF, 1 h, concentrated 3 h, 100 °C; c) BCl3 , 200 °C, 20 h, dist., 92%; d) pentane, r.t., 11B-NMR analysis if reaction mixture. Scheme 25.
hexane 101 [46]. Nevertheless, authors failed to isolate the target compounds and thus tried to prove the presence of compound 101 in reaction mixture by 11B NMR monitoring. The reported signals that may well have been assigned to the derivative 101, but it is still an assumption. A similar attempt was undertaken with the 1,3-bsi(chloromethylboryl)propane 102, obtained by treating 100 with tetramethyltin (Scheme 26). The situation repeated itself in that no desired cyclic product 103 could be isolated and its presence was assumed from the 11B NMR analysis of the reaction mixture. It appears that the described methodology cannot be used for preparation of 1-aza-2,6-diboracyclohexanes after all. The only success of this method was the preparation of 1-aza2,6-diboracyclohexane derivative 105 [47]. In its first step 101 was functionalised by diisopropylamine to compound 104 in 98%
Cl
B
B
Cl
Cl
Cl
6. Summary As can be seen from facts given so far, cyclomono- and cyclodicarbodiborazines are hydrolytically unstable (with the exception of 69, 88, 92 and 97) and thus, none of the compounds could be isolated in pure state. On the other hand, cyclotricarbodiborazines are the most stable in this group, their synthesis by cyclisation of chloroboranes or thioalkylboranes is straightforward and effective. These compounds can be isolated from reaction mixtures by stan-
TMS
a
B
B
Cl
Cl
100
+
102
N
b
N
B
B
TMS 103
86
a) Me4 Sn, pentane, -78 °C - r.t. - -78 °C, fraction dist., 70%; d) pentane, r.t., 11 B-NMR analysis of reaction mixture. Scheme 26.
Cl
B
B
Cl
Cl
Cl
Cl
a Cl
B
N
b N
B
B
N
N 100
105 104 a) (i Pr) 2NH, pentane, reflux, 2 h, 98%; b) tBuNH2 , pentane, 0 °C - reflux, 5 h, 73%. Scheme 27.
B
N
19
M. Kavala et al. / Polyhedron 48 (2012) 9–20
Cl Cl
O
B
B
a
Cl
S
B
O
B
Cl
O
B
+
S
S
b
S
B
S
B
O
100
S
106
107
108 R
S S
S
B
B
S
+
R
NH2
c
R
B
N
B
R
S 109 : R = H (44%) 110 : R = Me (70%) 111 : R = Et (93%)
108
a) MeOH, hexane, 0 °C, distillation, 95%; b) reduced pressure, 120-140 °C, distillation, 88%; c) 109 and 111 : CHCl 3; 110: benzene, 60-70 °C. Scheme 28.
S
S
+ Me
B
B
S
S
a
NH2
N
S B
S B
(1 equiv) 112
108
a) Et2 O, 0 °C, 70 min., fraction distillation, 38%. Scheme 29.
Table 6 Displays an overview of physical properties of isolated cyclic products. Compounds
mp (°C)
bp (°C)
105 109 110 111 112
– 63–66 – – –
115 (0.01 Torr) – 90–91 (7 Torr) 120–121 (15 Torr) 124–126 (3 Torr)
dard procedures, they even withstand higher temperatures (e.g. during distillation), but similarly as 4- and 5-membered cycles remain sensitive to water.
Acknowledgments We thank Prof. Roman Bocˇa for helpful discussions. This work was supported by the Science and Technology Assistance Agency under contract No. APVV-0014-11.
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[35] S. Luckert, E. Eversheim, M. Müller, B. Redenz-Stormanns, U. Englert, P. Paetzold, Chem. Ber. 128 (1995) 1029. [36] S. Diemer, H. Noeth, W. Storch, Eur. J. Inorg. Chem. 9 (1999) 1765. [37] R. Boese, B. Kroeckert, P. Paetzold, Chem. Ber. 120 (1987) 1913. [38] G.A. Rupprecht, L.W. Messerle, J.D. Fellmann, R.R. Schrock, J. Am. Chem. Soc. 102 (1980) 6236. [39] H. Braunschweig, P. Paetzold, T.P. Spaniol, Chem. Ber. 126 (1993) 1565. [40] W. Haubold, U. Kraatz, Chem. Ber. (1979) 1083.
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Contents lists available at SciVerse ScienceDirect
Polyhedron journal homepage: www.elsevier.com/locate/poly
Syntheses and properties of cycloborazines and cyclocarborazines Miroslav Kavala, Peter Zálupsky´, Peter Szolcsányi ⇑ Dept. of Organic Chemistry, Slovak University of Technology, Radlinského 9, SK-812 37 Bratislava, Slovakia
a r t i c l e
i n f o
Article history: Received 28 May 2012 Accepted 4 September 2012 Available online 15 September 2012 Keywords: Borazines Synthesis Properties Hydrolysis
a b s t r a c t We present a review of preparative methods as well as physico-chemical properties of selected types of cyclic boron–(carbon)–nitrogen containing compounds. The review is compiled as a synthetic manual for preparation of compounds with the desired structure, which are to be further modified to novel, as yet unknown derivatives. The compilation is subdivided according to various types of ring sizes. Each section presents known methods of synthesis of respective cyclo(car)borazines highlighting their possible functionalisation. The important data on hydrolytic stability of compounds are also included. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Cycloborazines are cyclic compounds formed of alternating nitrogen and boron atoms [1–9]. Such systems are isoelectronic with their all-carbon analogues, however, due to the dipole moments of adjacent heteroatoms, the bonding contains a significant ionic component [10–13]. In contrast to cyclosilazanes, a family encompassing numerous derivatives with various ring size, 4- and 8-membered cycloborazines substituted at boron are as yet unknown. Therefore, we shall concentrate on properties and preparation of 6-membered cyclotriborazines. 2. Cyclotriborazines (borazines) Cyclotriborazines form 6-membered rings, the basic structure of which is shown on Fig. 1. Positions 1, 3 and 5 are occupied by nitrogen atoms, while positions 2, 4 and 6 are taken up by threevalent boron atoms. Although boron is normally three-valent, with its empty p-orbital it can form a four-valent anion. The name cyclotriborazines has not become a common name, they are routinely known under the name B-trisubstituted borazine. In this section we shall deal with cyclic borazines with unsubstituted nitrogen atoms, carrying at boron either bulky substituents, or such that allow further functionalisation. 2.1. Preparation of borazines Presently several methods for preparation of borazine derivatives are known. They encompass aminolysis of trichloroborane, ⇑ Corresponding author. Tel.: +421 2 59325162; fax: +421 2 524 953 81. E-mail address: [email protected] (P. Szolcsányi). 0277-5387/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.poly.2012.09.003
functionalisation of hexahydroborazine or derivatisation of boric acid esters. 2.1.1. Cyclocondensation of iminoboranes Conceptually this is the most effective method of borazine preparation, because it is experimentally rather simple. In its first step, it involves reduction of alkylboroxines (anhydrides of boric acid) with LAH in the presence of trialkylamine, to give trialkylaminoalkylboranes 1–4 (Scheme 1) [14]. Next, the latter are treated with an excess of ammonia in diglyme, in the presence of ammonium chloride as catalyst. The reaction is accompanied by vigorous release of hydrogen and dimethylamine, forming the corresponding cyclic borazines. Although the reaction has not been described with alkylamines, authors claimed reactions proceeded also with various alkylsubstituted tertiary amines. Cyclic borazine is considered to be an isoelectronic analogue of benzene, however, its thus supposed aromaticity is still a matter of debate [15–21]. According to the postulated mechanism (Scheme 2), the first step involves transamination of trimethylamine by ammonia. The subsequent action of ammonia and catalytic amount of ammonium chloride elicits a sequential elimination of hydride anion from boron and proton from nitrogen, the evolving gaseous hydrogen shifting the reaction equilibrium towards products 5–13. The key step of the synthesis is the cyclocondensation reaction of three molecules of iminoborane. It turned out that although the above mentioned borazines did not succumb to air moisture, nevertheless after stirring with water for several hours they completely hydrolysed to boric acid with concomitant release of ammonia. In addition, storing samples in dry conditions under nitrogen atmosphere caused yellowing of the originally colourless compound and its transformation to a further unspecified solid.
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M. Kavala et al. / Polyhedron 48 (2012) 9–20
2.1.3. Ammonolysis of diethylaminodichloroborane The methodology has been demonstrated on a single substrate with free amino group at the borazine ring [23]. First, the diethylaminodichloroborane 15 was prepared by treating trichloroborane with diethylamine (Scheme 5). Compound 15 was then subjected to ammonolysis to give the cyclic product 16 in only average yield. This section also presents a method based on ammonolysis of aryldichloroboranes 17–19, accessible by reaction of aromatic chloromercury derivative with trichloroborane (Scheme 6) [24]. Although the ammonolysis itself furnishes very good yields of arylborazines 20–22 (80–97%), its major drawback is the use of toxic mercury(II) salts.
R2 HN B
R1
3
B4 5
2
6 1
NH B
R3
N H
R 1 - R 3 = chlorine, alkyl, aryl, N-alkyl, N-silyl, O(S)-alkyl Fig. 1. General formula of cyclotriborazines.
2.1.2. Ammonolysis of alkylthioboronic acid This sequential method was described for the synthesis of alkylborazine derivatives 5–7. Authors claim it to be a facile method of preparation with easy isolation of products and good yields [22]. In the first step, anhydride of alkylboric acid reacts with tribromoborane, giving rise to alkyldibromoboranes 8–10 (Scheme 3). It may be assumed that low yields can be accounted for by polysubstitution of the created monosubstituted borane. Next, dibromoboranes are transformed to n-butylesters of alkylthioboric acid 11–13; the experiment has however been described in detail in case of isopropyl derivative. These esters underwent ammonolysis already at laboratory temperature, giving the required borazines 5–7 in good yields (80%). The tentative mechanism of ammonolysis assumes in its first step a substitution of thiolate group with ammonia to give A and self-condensation in the next step to give B (Scheme 4). The thus formed intermediate next condenses to triborazane structure 14, which in the last releases a molecule of thiol to give the cyclic borazine.
2.1.4. Reaction of HMDS with dichloroborane derivatives The method was used to prepare an ethyl and phenyl derivative of borazine [25,26]. It is a ‘‘one-pot’’ reaction of ethyl- or phenyldichloroborane (23 and 24) with HMDS, releasing TMSCl and forming cyclic borazine derivatives 25 and 26 (Scheme 7). B-triphenyl borazine could be isolated in almost quantitative yield which makes this preparation synthetically very attractive. 2.1.5. Reaction of bis(diisopropylamino)ethynylborane with ammonium chloride Further functionalisation of borazine derivatives has been the topic of very few reports. This method was used to prepare triethynylborazine (30 or 31) [27], the triple bonds of which lend itself to further functionalisation. In its first step, the synthesis uses the reaction of BCl3 with diisopropylamine (DIPA) in toluene, leading to diaminochloroborane 27 (Scheme 8). Chloroborane 27 then reacts with
R O B
R
R
B
a
O B
O
H 3 R
B
b
N
H
R 1: 2: 3: 4:
HN B
R
R = n Pr (65%) R = i Pr (65%) R = n Bu (64%) R = s Bu (66%)
5: 6: 7: 8: 9:
B
N H
NH B
R 10 : R = 11 : R = 12: R = 13: R =
R = n Pr (88%) R = i Pr (79%) R = n Bu (91%) R = sBu (85%) R = i Bu (70%)
t
Bu (65%) Pen (88%) n Hex (86%) Bn (70%) n
a) Me3 N (3 equiv), LAH, Et 2 O, reflux, 3 h; b) NH 3 (g), NH 4 Cl, diglym, 100-150 °C, 2 h. Scheme 1.
H R
B
N
H
H R
B
N
+NH 3 - Me 3N
H
H B
N
H
+ NH 4
B
N H
+NH3 - NH 4
+ NH 4
H
-H2 , -NH 3
H
R
H B
H
R
H
+NH3
N H H
H
-H 2, -NH3 H
R
H R
R
H B
- NH 4
N
H
H
R
N B
B N
N B
R HN H
R Scheme 2.
R
B
B
N H
R NH B
HN R
R
B
B
N H
NH B
R
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M. Kavala et al. / Polyhedron 48 (2012) 9–20
R O R
B
B
O
R
B
Br
a
O
R
b
B
R
Br
R
8: R = n Pr 9: R = i Pr (59%) 10 : R = n Bu
S
n
S
n Bu
Bu
B
11: R = n Pr 12: R = i Pr (78%) 13: R = n Bu
c
HN R
B
B
N H
NH B
R
5: R = n Pr (86%) 6: R = i Pr (82%) 7: R = n Bu (80%)
a) BBr 3, r.t.; b) n BuSH, reflux, 12 h; c) NH3 (g), r.t., 1 h. Scheme 3.
R S R
n Bu
B S
n
Bu
+NH3 - nBuSH
n Bu
S R
HN
+A
B
-n BuSH
NH2
R
B R
R
- n BuSH
R
HN B
B HN
NH2
n Bu
S NH2
A
+A
B
B
R B S
HN - n BuSH R
n Bu
B
14
B
N H
NH B
R
5- 7 Scheme 4.
lithium acetylide (or TMS/lithium acetylide) to furnish the borane derivatives 28 and 29, which, in the final step afford the target borazine 30 and 31. It is evidently one very effective method for preparation of borazines with the propensity for further functionalisation. 2.1.6. Substitution reactions of B-trichloroborazine B-Trichloroborazine 32 is a labile compound, completely hydrolysed by water to boric acid and ammonium chloride [28]. In spite of the wealth of data in the literature, it has become commercially available, we shall concentrate here on derivatives 32. Derivatives 32 can be approached by four methods we shall describe in chronological order. The first described method involves treatment of trichloroborazine 32 with Grignard reagents MeMgI, EtMgI and PhMgBr (Scheme 9) [29]. Triethylborazine 25 and triphenylborazine 26 are isolated from the reaction mixture after the solvent has been distilled off and the crude reaction mixture pyrolysed at 150 °C. Compounds 25 and 26 are better prepared by the method given in Section 2.1.4.
NEt 2 Cl Cl
B
a
Cl Et 2 N B
Cl
Cl 15
b
HN Et2 N
B
B
N H 16
a) Et2 NH, benzene, -78 °C - r.t., 3 h, 81%; b) NH3(g), benzene, -78 °C - r.t., 20 h, 52%. Scheme 5.
NH B
NEt 2
In 1961 a synthesis was published [30] the first step of which involved substitution of chlorine atoms in 32 for butanethiolate coming from n-butyl lead mercaptide (Scheme 10). The thus obtained B-tri-n-butylmercaptoborazine 34, when treated with ammonia, dimethylamine, aniline and methanol respectively, gives the corresponding derivatives 35–38. However, certain limitations of this methodology lies in the use of toxic lead compounds and foul smelling sulfides and/or thiols. Gerrard and coworkers carried out reactions of trichloroborazine 32 with a series of secondary amines [31], leading in turn to the corresponding amino borazines (16, 36 and 39–44) in low yields (Scheme 11). Only substitution with diphenylamine gave product 44 in good yield (76%). Tris(diethylamino)borazine 16 arises from the reaction of 40 with trimethylsilyldiethylamine (Scheme 12) [32]. As far as the yields are concerned, there was hardly any improvement (54%).
2.1.7. Rhodium-catalysed hydroboration of alkenes All methods described so far produced derivatives with symmetrical substitution pattern at boron atoms. Sneddon carried out a series of experiments [33], in which he succeeded in preparing (apart from symmetrically trisubstituted borazines) also monoand disubstituted borazines by Rh-catalysed reaction of olefins with the borazine 45 (Scheme 13 and Table 1). The Sneddon methodology not only allowed preparation of monosubstituted borazines, it also allowed a step-by-step procedure introducing three different substituents into the molecule of borazine. Authors have also postulated a mechanism of such Rh-catalysed hydroboration of olefin (Scheme 14). The first step involves dissociation of fosfine ligand from the central rhodium metal (A), followed by coordination of olefin (B), addition of hydride to olefin (C), oxidative addition of the B–H group of olefin (D) and finally by reductive elimination of alkylborazine (E). This method has a considerable synthetic potential, since the metal catalysts is used only in 0.015–4.4 mol%, in addition to the possibility to introduce different substituents – a feature unique to this methodology but one. It gives fairly good yields of desired products, thus making it unequivocally the method for preparation of borazines. Table 2 compiles data on melting and/or boiling points of 39 borazine derivatives. Rhodium(I) complexes were also used in the catalytic dehydrocoupling methodology [34] that transforms ammonia–borane adducts to cyclic borazines under mild conditions (max. 45 °C, glymes). However, isolation of products from the reaction mixtures (by vacuum fractionation) proved difficult; pure borazines were isolated low yields (10–30%) with the major products being nonvolatile, oligomeric species.
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M. Kavala et al. / Polyhedron 48 (2012) 9–20
Ar a
Hg
Ar
Cl Ar
Cl
b
HN
B Cl
Ar
17: Ar = 2-Me-C 6H 4 (67%) 18 : Ar = 3-Me-C 6H 4 (57%) 19: Ar = 4-Me-C 6H 4 (74%)
Ar = 2-Me-C6 H4 , 3-Me-C 6H 4 4-Me-C 6H 4
B
B
N H
NH B
Ar
20: Ar = 2-Me-C6 H4 (85%) 21: Ar = 3-Me-C6 H4 (97%) 22: Ar = 4-Me-C6 H4 (80%)
a) BCl3, benzene, reflux, 4 h; b) NH3 (g), benzene, -78 °C - r.t., 20 h. Scheme 6.
R Cl R
B
a
HN
Cl
B
R
B
N H
Cl
B
HN
a
NH
B
R
HN
B N H
Cl
25: R = Et 26: R = Ph
23: R = Et 24: R = Ph
R
B
NH
Cl
R
B
B
N H
NH B
R
33: R = Me 25: R = Et (70%) 26: R = Ph (60%)
32
a) HMDS 25 : benzene, -78 °C - r.t., 20 h, 61%; 26: CH 2Cl2, -78 °C - 55 °C, 10 d, 98%.
a) RMgX (X=I, Br), Et2 O, r.t. - 150 °C.
Scheme 7.
Scheme 9.
Borazine derivatives being relatively unstable compounds can also be expected to be fairly reactive. This has been amply demonstrated in their preparation starting from trichloroborazine. All reactions are carried out under the blanket of nitrogen or argon atmosphere, without access of oxygen and air moisture. We shall nevertheless try to map their hydrolytic stability.
lein-mannitol). The highest hydrolytic stability demonstrated the electron-rich tris(diphenylamino)borazine 52, which, after 30 minute reflux hydrolysed merely to 7% – a fact testifying to its relatively high stability towards hydrolysis. It also means that in manipulating other derivatives contact with water should be avoided which is a fairly serious practical limitation. In spite of this, various derivatives can be prepared and used in further functionalisation.
2.2. Reactions of borazines
3. Cyclocarborazines
2.2.1. Hydrolysis of aminoborazine compounds All borazines undergo hydrolysis, the outcome of which depends a great deal on volume of substituent and electronic effects. Table 3 summarises the results of the study by Gerrard, who studied the hydrolytic stability of aminoborazines [31] in refluxing water, or aqueous NaOH. The reaction products were naturally amines (neutralised by 0.1 N HCl), and boric acid, determined by titration (phenolphtha-
Cyclocarborazines are cyclic compounds, analogues of saturated carbon azaheterocycles – azetidine, pyrrolidine and piperidine. Their parent structure contains in positions 2 and (3 + n) boron atoms instead of carbon atoms (Fig. 2). The stability and reactivity is what makes these compounds truly interesting. The reports on cyclocarborazines in the literature are scarce, derivatives with free
R
R NH
a
N
B N
Cl
b
N
B
B HN
c
28: R = H (98%); 29: R = TMS (89%)
30: R = H (92%); 31: R = TMS (83%)
a) BCl3, toluene, 0 °C - r.t., 14 h, 90%; b) Li-C C-R, 12-crown-4 (cat.), THF or Et 2 O, 0 °C - 80 °C, 4 h; c) NH 4 Cl, toluene, reflux, 12-24 h. Scheme 8.
B N H
N R
27
NH
B
R
13
M. Kavala et al. / Polyhedron 48 (2012) 9–20 n
Cl B
HN B
Cl
N H
a
NH B
Bu
B
HN n
Cl
Bu
S
R
S
B
N H
b
NH B
n
S
HN
Bu
35 : 36: 37: 38 :
34
32
B
R
B
N H
NH B
R
R = OMe (50%) R = NMe2 (97%) R = NHPh (70%) R = NH 2 (80%)
a) (n BuS)2 Pb, benzene, reflux, 2 h, 91% b) RH, benzene, r.t., 1 h. Scheme 10.
R2
Cl HN Cl
B
B
N H 32
R1 NH B
+ Cl
a
NH
HN
R2
R1
B
N R2
36 : 16 : 39: 40:
R1 R1 R1 R1
= R 2 = Me (51%) = R 2 = Et (58%) = R 2 = n Pr (47%) = R 2 = i Pr (52%)
R1
N B
NH B
N H
N
R2
R1
41: 42: 43: 44:
R1 R1 R1 R1
= R 2 = n Bu (42%) = R 2 = i Bu (28%) = Me, R2 = Ph (64%) = R 2 = Ph (76%)
a) benzene, r.t. Scheme 11.
amino group being practically unknown. We shall therefore concentrate here solely on preparation and properties of N-substituted cyclic derivatives. 3.1. Cyclomonocarbodiborazines Cyclomonocarbodiborazines form 4-membered rings with the basic structure shown in Fig. 3. The name of this group of compounds has been derived from the saturated azaheterocycle – azetidine. Instead of carbons atoms in positions 2 and 4 it contains boron atoms. The valence of boron allows it carry a substituent, thus a typical representative of this group of compound is a 2,4disubstituted 2,4-diboraazetidine, also called 2,4-disubstituted 2,4-dibora-1-azacyclobutane. All so far reported compounds of this type carry a substituent at nitrogen. 3.1.1. Preparations of 2,4-diboraazetidines Cyclomonocarbodiborazines are structurally and chemically intriguing compounds, so far accessible by only a handful of methods. There is no procedure so far capable of producing such heterocycle without a substituent at nitrogen. 3.1.1.1. Insertion of isonitriles and carbenes. The key substrate, on which this method of preparation of 4-membered rings is based [35], is the 1,2,3-tri-tert-butylazadiboridine 65. It is a highly reactive compound (owing to the extremely weak B–B bond), prepared in two steps (Scheme 15) [36,37]. Authors reported an aromatic character for this azadiboracyclopropane, the free electron pair at nitrogen being delocalised over the ring. The first of its synthetic transformations is the reaction of 65 with aromatic isonitriles, of which only one has been described in detail (Scheme 16). According to the postulated mechanism (Scheme 17) it involves an insertion of isonitrile 66 into the B–B bond, bringing about an expansion to a 4-membered ring of
N
Cl HN Cl
B
B
N H
a
NH B
N
+
HN
TMS
N
Cl
B
40
B
N H
NH B
N
16 a) toluene, r.t., 54%. Scheme 12.
HN HB
H B
N H
R2 NH
+
alkene
a
BH
HN R1
45
B
B
N H
NH B
R3
Products in Table 1
a) RhH(CO)(PPh 3 )3 (0.015-4.4% mol), neat / CH2Cl2, -196 °C - r.t. Scheme 13.
1,2,4-azadiboretidine 67 as red oily liquid. The method has one substantial limitation though, being demonstrated on a single aromatic isonitrile. In case of compound 67 NMR spectra revealed an interesting feature, namely the non-equivalence of tert-butyl groups. This can be accounted for by assuming presence of a C@N bond without free rotation. Such structure would also be responsible for a single 1H NMR signal for ortho-methyl groups at the benzene ring, thus their magnetic equivalence.
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M. Kavala et al. / Polyhedron 48 (2012) 9–20
Table 1 Compounds prepared by Rh-catalysed reaction of olefins with borazine 45.
a b c
Compounds
Alkene
45/alkene
Time (h)
R1
R2
25 46 47 5 48 49 50 51 52 53 54 55 56
ethylene propene propenea
1/3 8/1 1/3.3
2 2 1
1-butene Z-2-butene
5.8/1 5/1
3 47
E-2-butene styrene 4-allylanisol ethyleneb ethylenec acetylenec
6/1 7/1 10.5/1 1/2.2 1/3.4 1/3.9
185 2.5 96.5 2 2 120
Et H H n Pr H H H H H H Et Et vinyl
Et H Pr n Pr H H n Bu H H H Et n Pr n Pr
R3
Yield (%)
Et Pr Pr n Pr n Bu n Bu n Bu s Bu (CH2)2C6H5 (CH2)3C6H4-4-OMe n Pr n Pr n Pr
92 98 43 55 91 90 3 72 84 57 97 97 54
n
n
n
Di- and trisubstituted products are formed in the ratio 44/56. Starting substrate is 2-propylborazine. Starting substrate is 2,4-dipropylborazine.
HB HN
H N
B
H
P = PPh3
P Rh P P CO
BH NH
(A) +P
R
-P
H
HB HN H R
P
H N
B
R
P H Rh OC P
(E)
(B) BH
R
NH
H Rh P CO B3N 3H 6
H
(D)
H Rh P P CO
(C) P
R
Rh
CO P
(A) release of phosphine ligand (B) olefin coordination (C) hydride addition to olefin (D) oxidative addition of borazine (E) reduction elimination of alkylborazine Scheme 14.
Compound 67 is rather unstable and exposed to air quickly decomposes. Authors do not report about its thermal stability, or moisture sensitivity. It can be assumed though, that the imino bond quickly hydrolyses, making the derivatives hydrolytically unstable. The second type of transformation of 67 to 4-membered ring is the carbene insertion of lithiated a-bromoalkanes. Using this method, authors prepared the 4-membered spiro-compound 69, starting from the cyclopropane derivative 68 (Scheme 18). This transformation starts with lithiation of the dibromocyclopropane 68, followed by insertion of organometallic cyclopropylidene compound 68A into the B–B bond of 65 to give the 1,2,4-azadiboretidine 69 (80%) and free LiBr (Scheme 19). 3.1.1.2. Cycloaddition reaction of iminoboranes. An interesting method of preparation of 1,2,4-azadiboracyclobutanes is the cycloaddition of iminoborane with unsaturated alkylidene tantalum complex [38]. Alkyl(tert-butylimino)boranes 70–72 add to the tantalum complex 73 (in a molar ratio 2:1) to give the intermediacy diazadiboratantalocyclohexanes 74–76 (Scheme 20) [39]. When heated, these 6-membered derivatives eliminate the tantalum
Table 2 Compiles data on melting and/or boiling points of 39 borazine derivatives. Compounds
mp (°C)
bp (°C)
5 6 7 8 9 10 11 12 13 16 20 21 22 25 26 30 33 35 36 37 38 39 40 41 42 43 44 47 52
– – – – – – – – –
70 (0.6 Torr) 70 (0.5 Torr) 102–105 (0.2 Torr) 94 (0.7 Torr) 72 (0.03 Torr) 60 (10 Torr) 125 (0.07 Torr) 140 (0.05 Torr) not given 120 (0.1 Torr) – – – 56–57 (2 Torr) – 90 (75 Torr) sublimation 35 (20 Torr) – – – – 170–172 (0.4 Torr) – 200 (0.05 Torr) 167 (0.3 Torr) – – 119–124 (760 Torr) –
89–92 140 189–190 46 180–185 132 36 114 112–115 204–206 >250 32–35 138–143 – 47–52 128–132 152–155 – 123–125
Table 3 Compiles date on the hydrolytic stability of aminoborazines. Compounds
R2 N
Reagent
Hydrolysis (%)
44 48 48 51 52 52
Me2N i Pr2N i Pr2N PhMeN Ph2N Ph2N
NaOH solution (reflux) water (reflux) water (cold) water (reflux) water (reflux) NaOH solution (reflux)
100 100 55 58 7 29
R2 N R1 B 2
1
4
3
B
R3
n
R1, R 2, R3 = alkyl, aryl Fig. 2. General formula of cyclocarborazines.
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M. Kavala et al. / Polyhedron 48 (2012) 9–20
R2 N R1 B 2
1 3
4
NC
R3
B
B
R 1, R2, R 3 = alkyl
N B
N
+
B
N 65
Fig. 3. General formula of cyclomonocarbodiborazines.
complex 77 causing a ring contraction to the target 1,2,4-azadiboracyclobutanes 78–80. Although they could not be analysed in pure form, their presence was deduced from NMR spectra of reaction mixture. The reaction of analogous tantalum complex 82 with iminoborane 81 (in 1:1 molar ratio) gives rise to the 1-aza-2,4-diboracyclobutane derivative 83 (Scheme 21), which could be isolated by multiple crystallisations in 26% yield. Authors give the tentative structure of leaving tantalum complex 84 without being able to confirm its structure. 1,2,4-Azadiboracyclobutanes have been fully characterised by NMR spectroscopy, but were not further chemically transformed due to their high instability (Table 4). 4. Cyclodicarbodiborazines Cyclodicarbodiborazines form 5-membered rings with the core structure shown in Fig. 4. The name of this group of compounds has been derived from the saturated azaheterocycle – pyrrolidine. Instead of carbons atoms in positions 2 and 5 it contains boron atoms. The valence of boron allows it carry a substituent, thus a typical representative of this group of compound is a 2,5-disubstituted-1,2,5-azadiborolane, also called 1,2,5-azadiborolidines or 1aza-2,5-diboracyclopentane. All so far reported compounds of this type carry a substituent at nitrogen. 4.1. Preparation of 2,5-diborapyrrolidines So far, only two methods of preparation of diborolanes are known. One of them involves cyclisation of dichlorodiboralkanes with a tertiary silazane, the other relies on thermal cyclisation of aminoboranes. Similarly as in the earlier section on 4-membered derivatives, these methods were so far able to furnish only Nsubstituted cyclic compounds. 4.1.1. Cyclisation of chloroboranes with silazanes Syntheses of 5-membered nitrogen heterocycles containing a B–N–B moiety described in the literature are few and far between. Only several derivatives have been known. A method reported for the preparation of such heterocycles starts from dichlo- or tetrachlorodiboralkane derivative [40]. The action of diborane on ethylene at 80 °C gives rise to 1,2-bis(dichloroboryl)ethane 85 [41],
66 67 a) CH 2Cl2, -78 °C, 2 h, 61%. Scheme 16.
which next undergoes a reaction with hexamethyldisilazane 86 to give cyclic 2,5-dichloro-1,2,5-azadiboracyclopentane 88 as colourless liquid in 95% yield (Scheme 22). However, the compound decomposes on standing at laboratory temperature. However, a reaction of 85 with hexamethylsiladimethylborazines 87 failed to produce isolable target compound 89. Its formation was only proved by experiment carried out in a NMR tube. Substrates 86 and 87 were prepared from LiHMDS and the corresponding electrophiles (methyl iodide, or dimethylchloroborane) [42]. Compound 88 decomposes on standing too, but boron atoms, being shielded by a nitrogen bridge, are no longer attacked by active reagents such as SbF3, or Me4Sn. Dimethyldiborazine 90 is obtained from tetrachloroalkyldiborane 85 by double methylation with tetramethyltin 91. Dimethyl derivative cyclodicarbodiborazines 92 arises from cyclisation reaction of 90 and 86, taking place without solvent, and in good yield (Scheme 23). 4.1.2. Reductive cyclisation Another known methodology leading to azadiboracyclopentane is based on a reaction of tetraalkyldiboranes with dialkylaminoboranes [43]. The diethyl-N-boroaniline 95 arises in the dehydrogenation reaction of aniline 93 in 88% yield (Scheme 24). After isolation and purification, the secondary amine 95 reacted with 1.5 equiv. of triethylborane 96 in the presence of 0.3 equiv. of tetraethyldiborane 94 in an autoclave for 16 h at 180 °C and 9 h at 200 °C. Subsequently, the reaction mixture was reheated to 225 °C for another 2 h. This procedure, followed by a fraction distillation furnished the desired cyclic product 97 in 8% yield. However, authors did not propose any plausible mechanistic course of reaction. Table 5 displays boiling points of isolated cyclic products. 5. Cyclotricarbodiborazines Cyclotricarbodiborazines are 6-membered rings with the parent structure shown in Fig. 5. Once again, positions next to nitrogen (2, 6) are occupied by boron atoms. The difference in valence be-
Cl Br
Sn N Sn
B
a
B
+
B
a
Cl
N
Br
B
Cl
b
N B
B
Cl 65 a) CH2Cl2, -30 °C, 30 min., 97%; b) Na-K, hexane, reflux, 4 h, 33%. Scheme 15.
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M. Kavala et al. / Polyhedron 48 (2012) 9–20
N
R R B R
N
R'
N
+
B
B
N R
B
C
B
R
B N
C
R 65
N
66
R'
67
R = t Bu, R' = 2,6-(CH3) 2-C6 H3 Scheme 17.
Br
Br
Br
Li
a
B 68
N
b
N
+
B
B
B
68A 65
69
a) LiBr, BuLi, THF/Et 2O/pentane (3:1:1), -105 °C, 30 min.; b) -110 °C to 20 °C, 8 h, 80%. Scheme 18.
R R N
N
Br
B R
R B Li
+ Li
B
N B
B
R
B
Br
R 68A
65
69
R = t Bu Scheme 19.
H R
B
N
+
R
THF Cl C Ta Cl Cl
H -78 °C
B
THF
N Cl Ta Cl
N 60 °C
N Cl
R
B
N B
R
+ Cl
78 : R = Et 79: R = Pr 80 : R = Bu
74: R = Et 75: R = Pr 76: R = Bu
Ta Cl
H
R
73
70: R = Et 71: R = Pr 72: R = Bu
B C
77
Scheme 20.
H N
B
TMS
N
+
PMe3 Cl C Ta Cl Cl
N
a
N B TMS
81
83
82
a) toluene,-78 °C - r.t., 16 h, 26%. Scheme 21.
N
+ TMS
H
PMe3
B
N Me 3P Ta PMe3 Cl Cl Cl
84
Cl
17
M. Kavala et al. / Polyhedron 48 (2012) 9–20 Table 4 Boiling points.
Table 5 Boiling points.
Compounds
bp (°C)
Compounds
bp (°C)
67 69 83
– 60 (0.002 Torr) –
88 92 97
20 (0.5 Torr) 20 (22 Torr) 65–73 (0.2 Torr)
R2
R2
N
R1
1
B2
5 4
3
R3
B
R1
N
B2
1 6
3
4
R 1, R2, R 3 = alkyl
N R
+
TMS
Cl
Fig. 5. General formula of cyclotricarbodiborazines.
R
Cl B
a
Cl
B
Cl
N B
5
R 2 = H, alkyl R1 , R 3 = alkyl, N, S
Fig. 4. General formula of cyclodicarbodiborazines.
TMS
R3
B
B
Cl
5.1. Preparation of 1-aza-2,6-diboracyclohexanes
Cl 86: R = Me 87: R = B(Me) 2
85
Preparations of cyclotricarbodiborazines parallel those described in Sections 3.1.1 and 4.1.1.
88 : R = Me (95%) 89: R = B(Me)2 (0%)
a) isopentane, -78 °C - r.t. Scheme 22.
tween carbon and boron predetermines the substitution pattern. Substituents are carried in positions 2 and 6, thus giving rise to 2,6-disubstituted 1-aza-2,6-diboracyclohexanes, or else 2-aza1,3-diborinanes. In this section, we shall deal with compounds having (un)substituted nitrogen.
5.1.1. Cyclisation of chloroboranes with silylamines Syntheses of 6-membered nitrogen heterocycles containing a B–N–B moiety described in the literature are scarce. The reported synthesis of trialylborane 99 starts with the reaction of alylmagnesium bromide 98 with BF3.OEt2 (Scheme 25) [44]. Trialylborane was next subjected to a transformation with diborane to a polymer of unknown composition, which was nevertheless subjected to reaction at high temperature with trichloroborane, to give the 1,3-bis(dichloroboryl)propane 101 [45]. It was expected that the action of hexamethyldisilazane 86 on organoborane 100 in pentane would furnish the cyclic 2,6-dichloro-1-aza-2,6-diboracyclo-
Cl Cl
B
Cl +
B
Sn
TMS
a Cl
B
+
B
Cl 91
N B
B
TMS
Cl
85
b
N
90
86
92
a) pentane, -78 °C - r.t. - -78 °C, fraction distillation, 82% b) -78 °C - r.t., low-temp distillation, 70%. Scheme 23.
H +
B
B H
a
H + HN
B
+
B
B
b
H
B
N B
B
NH 2 93
94
95
96
94
a) -78 °C to 80 °C, 90 min., fraction distillation, 88% b) 200 °C - r.t. - 220 °C, 25 + 2 h., fraction distillation, 98%. Scheme 24.
97
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M. Kavala et al. / Polyhedron 48 (2012) 9–20
Br
a
b, c
Cl
yield (Scheme 27). Action of tert-butylamine on 104 elicits cyclisation to cyclotricarbodiborazane derivative 105 (73%).
Cl
B
B
B
Cl
Cl
98
5.1.2. Cyclisation of thioboranes with amines 1,1,5,5-Tetramethoxy-1,5-diboropentane 106 serves as the starting compound in the methodology. It is prepared by the reaction of 100 with methanol (Scheme 28) [45]. The subsequent reaction with tris(ethylsulfanyl)boranes 106 and 107 affords the borasulfane 108. 1,1,5,5-Tetraethylmercapto-1,5-diborapentane 108 represents a sulfur analogue of boric acid esters and allows the exchange of mercaptoethyl groups by substitution reaction with ammonia, or amines [48]. The action of gaseous ammonia on 108 affords the cyclic product – 2,6-diamino-1-aza-2,6-diboracyclohexane 109 in 44% yield, with concomitant loss of ethanethiol. If methylamine or ethylamine are used, analogous Nsubstituted cyclic compounds 110 and 111 are obtained in 70% and 93% yield, respectively. The cyclic product 112 could be obtained in 38% yield (Scheme 29) by treating the tetraethyl ester of the propane-1,3dithioboronic acid 108 with 1 equiv. of methylamine. Table 6 displays an overview of physical properties of isolated cyclic products. As in the case of earlier mentioned 4- and 5-membered cyclocarborazines, the products given here were not further transformed and their preparation only served to demonstrate the synthetic viability of the suggested methodology.
100 99
Cl
TMS
Cl B
B
Cl
Cl
+
Cl
d
N
B
N
Cl
B
TMS
100
86
101
a) BF 3 .OEt 2 , I 2, Mg, Et 2 O, reflux, 3 h, Kugel-Rohr, 84%; b) B2H6 , THF, 1 h, concentrated 3 h, 100 °C; c) BCl3 , 200 °C, 20 h, dist., 92%; d) pentane, r.t., 11B-NMR analysis if reaction mixture. Scheme 25.
hexane 101 [46]. Nevertheless, authors failed to isolate the target compounds and thus tried to prove the presence of compound 101 in reaction mixture by 11B NMR monitoring. The reported signals that may well have been assigned to the derivative 101, but it is still an assumption. A similar attempt was undertaken with the 1,3-bsi(chloromethylboryl)propane 102, obtained by treating 100 with tetramethyltin (Scheme 26). The situation repeated itself in that no desired cyclic product 103 could be isolated and its presence was assumed from the 11B NMR analysis of the reaction mixture. It appears that the described methodology cannot be used for preparation of 1-aza-2,6-diboracyclohexanes after all. The only success of this method was the preparation of 1-aza2,6-diboracyclohexane derivative 105 [47]. In its first step 101 was functionalised by diisopropylamine to compound 104 in 98%
Cl
B
B
Cl
Cl
Cl
6. Summary As can be seen from facts given so far, cyclomono- and cyclodicarbodiborazines are hydrolytically unstable (with the exception of 69, 88, 92 and 97) and thus, none of the compounds could be isolated in pure state. On the other hand, cyclotricarbodiborazines are the most stable in this group, their synthesis by cyclisation of chloroboranes or thioalkylboranes is straightforward and effective. These compounds can be isolated from reaction mixtures by stan-
TMS
a
B
B
Cl
Cl
100
+
102
N
b
N
B
B
TMS 103
86
a) Me4 Sn, pentane, -78 °C - r.t. - -78 °C, fraction dist., 70%; d) pentane, r.t., 11 B-NMR analysis of reaction mixture. Scheme 26.
Cl
B
B
Cl
Cl
Cl
Cl
a Cl
B
N
b N
B
B
N
N 100
105 104 a) (i Pr) 2NH, pentane, reflux, 2 h, 98%; b) tBuNH2 , pentane, 0 °C - reflux, 5 h, 73%. Scheme 27.
B
N
19
M. Kavala et al. / Polyhedron 48 (2012) 9–20
Cl Cl
O
B
B
a
Cl
S
B
O
B
Cl
O
B
+
S
S
b
S
B
S
B
O
100
S
106
107
108 R
S S
S
B
B
S
+
R
NH2
c
R
B
N
B
R
S 109 : R = H (44%) 110 : R = Me (70%) 111 : R = Et (93%)
108
a) MeOH, hexane, 0 °C, distillation, 95%; b) reduced pressure, 120-140 °C, distillation, 88%; c) 109 and 111 : CHCl 3; 110: benzene, 60-70 °C. Scheme 28.
S
S
+ Me
B
B
S
S
a
NH2
N
S B
S B
(1 equiv) 112
108
a) Et2 O, 0 °C, 70 min., fraction distillation, 38%. Scheme 29.
Table 6 Displays an overview of physical properties of isolated cyclic products. Compounds
mp (°C)
bp (°C)
105 109 110 111 112
– 63–66 – – –
115 (0.01 Torr) – 90–91 (7 Torr) 120–121 (15 Torr) 124–126 (3 Torr)
dard procedures, they even withstand higher temperatures (e.g. during distillation), but similarly as 4- and 5-membered cycles remain sensitive to water.
Acknowledgments We thank Prof. Roman Bocˇa for helpful discussions. This work was supported by the Science and Technology Assistance Agency under contract No. APVV-0014-11.
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