Tin

Tin TADASH I SATO Waseda University, Tokyo,Japan 8.1 INTRODUCTION 8.1.1 Literature 8.1.2 Overview 8.1.2.1 General scope of organotin chemistry 8.1.2.2 Classification of tin reagents 8.1.2.3 Reaction types of the carbon-tin bond 8.1.2.4 Handling

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8.2 ORGANOTIN HYDRIDES AND ORGANODITINS 8.2.1 Reduction of Carbon-Heteroatom Bonds 8.2.1.1 Halogen compounds 8.2.1.2 Alcohols 8.2.1.3 Divalent heteroatom compounds 8.2.1.4 Nitrogen compounds 8.2.2 Reduction of Carbonyl and Carboxyl Groups 8.2.3 Addition to Multiple Bonds 8.2.4 Carbon-Carbon Bond Formation 8.2.5 Other Reactions

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8.3 ANIONIC TIN REAGENTS 8.3.1 Stannyl Anions (Sn~) 8.3.1.1 Preparation 8.3.1.2 Reactions 8.3.2 a-Stannyl Carbanions (Sn-C~) 8.3.2.1 Preparation 8.3.2.2 Reactions

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8.4 CATIONIC TIN REAGENTS 8.4.1 Reagents with Tin-Heteroatom Bonds (Sn-X) 8.4.1.1 Tin-oxygen bonds 8.4.1.2 Tin-halogen bonds 8.4.1.3 Tin-nitrogen bonds 8.4.1.4 Other heteroatoms 8.4.2 Carbocationic Tin Reagents (Sn-(C)n-X)

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8.5 ALKENYL- AND ARYLSTANNANES (Sn-C=C) 8.5.1 Preparation 8.5.2 Reactions 8.5.2.1 Transmetallation 8.5.2.2 Palladium-catalyzed cross-coupling reactions 8.5.2.3 Miscellaneous reactions

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8.6 ALLYLSTANNANES (Sn-C-C=C) 8.6.1 Preparation 8.6.2 Reactions 8.6.2.1 Transmetallation 8.6.2.2 Coupling with halides 8.6.2.3 Reactions with electrophiles 8.6.2.4 Miscellaneous reactions

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Tin

8.7 MISCELLANEOUS UNSATURATED STANNANES 8.7.1 Allenylstannanes 8.7.2 Alkynylstannanes

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8.8 FUNCTIONALIZED STANNANES (Sn-(C)n-F) 8.8.1 a-Stannyl Alcohols (n = 1, F = OH) 8.8.1.1 Preparation 8.8.1.2 Reactions 8.8.2 Ji-Stannyl Alcohols (n = 2, F = OH) 8.8.3 y-Stannyl Alcohols (n = 3, F - OH) 8.8.3.1 Preparation 8.8.3.2 Reactions 8.8.4 Acylstannanes (n = 0, F - CO) 8.8.5 a-Stannyl Ketones (n = /, F = CO) 8.8.6 P-Stannyl Ketones (n = 2, F= CO) 8.8.6.1 Preparation 8.8.6.2 Reactions 8.8.7 y-Stannyl Ketones (n = 3, F = CO) 8.8.8 Other Alkylstannanes Functionalized at the a-Position 8.8.9 Alkylstannanes Functionalized at Remote Positions

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8.9 PHOTOREACTIONS OF ORGANOTIN COMPOUNDS

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8.10 SUMMARY

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8.11 REFERENCES

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8.1 INTRODUCTION Since the introduction of boron and phosphorus to organic synthesis, as hydroboration and the Wittig reaction, the chemistry of many other elements has been pursued to develop novel selective reactions and reagents. Organotin chemistry has been playing an increasingly important role for this purpose. Since the last publication on organotin compounds in COMC-l} applications to organic synthesis have expanded enormously. Among many types of organotin compounds, tin hydride, a-heteroatomsubstituted tin compounds, and vinyl- and allylstannanes are the most widely known and, consequently, most extensively documented. In 1992 alone, approximately 230 papers appeared on "organotin in organic synthesis," excluding those dealing with tin reagents only in a particular step in multistep synthetic studies. More than 85% of these papers deal with one of the above four types of stannyl compounds. In this chapter, emphasis is on the less popular fields, as well as on the well-documented areas, to reveal new aspects of organotin chemistry. In the earlier sections (Sections 8.2-8.4), the chemistry of smaller molecules which are used as "tin reagents" is discussed and in later sections (Sections 8.5-8.9) the behavior of many types of organotin compounds is discussed.

8.1.1 Literature Numerous specialized books and reviews are available; those published prior to 1984 have been listed2 and are not duplicated here. A wide survey of organotin compounds from basic concepts to industrial applications, including synthetic applications, has been made.3 Original papers in many fields in organotin chemistry have been collected in Tetrahedron Symposia-in-Print, No. 36.4 In Comprehensive Organic Synthesis, the chemistry of allylstannanes has been compared with that of allyl compounds of related elements.5 In this series, various types of other tin chemistries have been discussed in many of the related sections. Review articles dealing with special fields will be referred to in the appropriate sections. Due to the limited space, only the most recent references are cited here and the earlier papers cited in these references, particularly those from the same laboratories, are omitted.

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8.1.2 Overview 8.1.2.1 General scope of organotin chemistry The most common form of tin in organotin compounds is tetravalent with sp3 hybridization. Divalent tin compounds, known as stannylenes, have been studied in only a few cases. When the tin atom bears electronegative substituents, its Lewis acidity increases and coordination with electron-rich sites leads to the hypervalent form of sp3d (trigonal bipyramid) or sp3d2 (octahedral). The covalent radius of tin is large (0.214 nm) and bonds to the tin atom are long (Sn-C, 0.22 nm; Sn-H, 0.17 nm) and mostly covalent with facile polarizability. In view of the relatively low carbon-tin bond dissociation energy (-210 kJ mol"1), it is no surprise that organotin compounds exhibit reactivities in both homolytic and heterolytic senses. Stabilization of an electron-deficient center at the (3-position, known as the "|3-effect" in silicon chemistry,6 is also observed in tin chemistry7 and defines many of the reaction patterns of organotin compounds. While being considered hard, tin is softer than silicon. Hence, like silicon, it has a tendency to associate with hard bases, such as fluoride, but it is also apt to associate with much softer bases such as sulfides. Actually, tin is known as a thiophile.

8.1.2.2 Classification of tin reagents There are several types of organotin compounds used as synthetic reagents. They can be classified into two categories, homolytic and heterolytic, depending on their reaction modes. Typical reagents of homolytic nature are trialkyltin hydrides, which are now the most frequently utilized tin reagents and they have been used as radical initiators, reducing reagents, or, to a lesser extent, as delivery reagents of tin atoms to organic substrates. The heterolytic reagents can be either cationic or anionic. Anionic tin compounds are widely used to stannylate electrophilic centers or multiple bonds. The counter-cation most commonly employed is lithium, but sodium, potassium, and magnesium are also widely used. Copper(II) complexes also behave anionically. The most popular cationic reagents are tin halides, tin alkoxides, and tin triflate, which are used as Lewis acids or counter-species of enolates. Cationic tin reagents can also be used to introduce the stannyl group to electron-rich substrates. However, this type of stannylation has limited use only, because the electron-rich substrates are usually none other than the species which it is intended to prepare from the tin compounds.

8.1.2.3 Reaction types of the carbon-tin bond Due to the electropositive character of tin compared with that of carbon and also because of the weak tin-carbon bond, the tin-bearing carbon reacts as a carbanion or radical. However, except in special cases,8 the normal tin-carbon bond is so stable that it is necessary to activate the bond in some way to initiate reaction. One of the activation methods is to introduce an "activating group," such as a vinyl or allyl group or a heteroatom-bearing carbon, directly to the tin atom. In this way, the tin atom can be either transmetallated with more reactive metals or brought into reaction directly. Usually, the tin is replaced with lithium to enhance nucleophilicity or activated by palladium catalysis to facilitate crosscoupling with halides or triflates. Such cross-coupling reactions are among the best methods available for allyl or vinyl group transfer to organic substrates. Another common activation method is to use other "activating groups," such as a carbonyl, hydroxy group, or some cationic center, located at various positions from the tin atom in the same molecule. Due to the very low reactivity of the carbon-tin bond, a wide variety of these functionalities can be introduced into organotin compounds, which are in many cases isolable as stable intermediates. The organotin compounds thus prepared in the first stage can be reacted with electrophiles in various ways by manipulating the second stage of the reaction through control of functionalities and reaction conditions. In contrast to reactions proceeding via tin-metal exchange, which are inherently those of the corresponding organometals, many of the reactions induced by the latter activation method have characteristics typical of organotin compounds.9

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Tin

8.1,2.4 Handling Most organotin compounds are stable in air, insensitive to moisture, and can be stored for long periods; ordinary distillation, chromatographic separation, and recrystallization procedures are tolerated. They are soluble in ordinary organic solvents, insoluble in water and can be treated without any special techniques other than those required for ordinary organic reactions. Generally, organotin compounds which are formed as by-products after a given reaction are tetraalkylstannanes or trialkyltin halides. These by-products can be separated from the reaction products by ordinary techniques, in most cases by distillation or column chromatography. If problems are encountered in separating tin by-products from desired products due to similar polarity, the separation can be done simply by shaking the reaction mixture with an aqueous solution of potassium fluoride and filtering off the insoluble tin fluoride thus formed10 or by treating the reaction mixture with I2/dbu (l,5-diazabicyclo[5.4.0]undec-5-ene) and removing the solid material by filtration.11 Most organotin compounds are toxic and the toxicity varies over a wide range depending on the types and nature of the substituents. Maximum toxicity occurs with methyl and ethyl derivatives, but decreases as the alkyl groups on tin become larger, such as butyl or phenyl. In the series RnSnY4_n, the toxicity decreases as the number of alkyl groups decreases and inorganic tin compounds are mostly nontoxic. Despite their higher toxicity, trimethylstannyl compounds are sometimes preferred as reagents to the tributyl compounds, because of the simplicity of their NMR spectra, in which the methyl signal appears as a singlet at S = 0, with small satellite signals due to 117Sn (7.54% abundance, 7 = 51 Hz) and Sn (8.62% abundance, J = 53 Hz). In some cases the reaction pattern differs between trimethyl- and tributylstannyl compounds.12'13 Due to their toxicity, organotin compounds should be handled with adequate safety measures: use of protective gloves, good hoods, and caution in waste disposal.

8.2 ORGANOTIN HYDRIDES AND ORGANODITINS Organotin hydrides, particularly tri-n-butyltin hydride, are the most frequently used organotin compounds in the laboratory. Some of the tin hydrides can be prepared in situ by the reaction of tin chloride and borohydride.14 The chemistry of tri-n-butyltin hydride has been summarized in a review.15 Reactions involving radical species are the most common pattern, since tin hydrides generate tin radicals easily by means of 2,2'-azobisisobutyronitrile (AIBN) or other radical sources, by UV, or simply by heat. Ultrasonic16 and electrooxidative17 initiation are also successful. Triethylborane, which is unlikely to behave as a radical source, greatly enhances the reactivity of triphenyltin hydride.18 An involvement of molecular oxygen has been suggested for this initiation. Tin radicals either abstract heteroatoms from the C-X bond or add to carbon-carbon or carbon-heteroatom multiple bonds to generate carbon radicals. In some cases, the intermediate radicals undergo secondary reactions involving nearby functional groups which induces multistep C-C skeleton rearrangements.20 In view of the recent advances in free-radical chemistry in organic synthesis,21*22 the use of tin hydride in radical initiation is very important, while reaction pathways involving transition metal catalysis are also widely employed.23 Polymer-supported tin hydrides and ditins have been developed which control the succeeding reaction types and facilitate the separation of the reaction products. Hexaalkylditins are cleaved to tin radicals under thermal or photochemical conditions25 or in palladium-catalyzed reactions.26*27

8.2.1 Reduction of Carbon-Heteroatom Bonds The most common reaction of carbon radicals generated from C-X bonds in the presence of tin hydrides is hydrogen abstraction from the tin hydride, which results in net reduction of the C-X bond to a C-H bond. This process can be carried out with several types of C-X compounds.

8.2.1.1 Halogen compounds Free-radical halogen abstraction from a C-X bond presents a broad variety of synthetic possibilities. Almost all chlorine, bromine, or iodine (but not fluorine) atoms at saturated or alkenic carbons and bromine or iodine (but not chlorine) at aromatic carbons can be replaced by hydrogen by means of tin hydride. Reactivity is greatest with iodides and decreases in the order of bromides and then chlorides.

Tin

359

The initially formed radicals may undergo secondary reactions with neighboring groups prior to hydrogen abstraction. When acetoxyl or phosphoryl groups are present on the vicinal carbon, the initially formed radicals induce a 1,2-radical migration as shown in Scheme 1. This is an effective method for the synthesis of 2-deoxy sugars.28 With a-chloro epoxides, triphenyltin hydride affords a-chloro carbonyl compounds, while the reagent gives exclusively allyl alcohols from p-chloro epoxides.29 A ring expansion has been observed with iodo epoxides. o-Bromobenzyl bromide derivatives can serve as "self-oxidizing protecting groups" since alcohols protected with this reagent are oxidized to carbonyl compounds under reductive conditions when treated with tributyltin hydride, as outlined in Scheme 2.31 OAc

OAc Bu3SnH

o

OAc

O

OAc

O

O

AIBN or h\

AcO

OAc

OAc

Scheme 1 H R-)—O H

Bu3SnH

H R^—O H

R

>=o

AIBN

H

R

R

R Scheme 2

The stereochemistry of halogen-hydrogen exchange was examined with diastereomeric 2-halo sugars in connection with the development of a synthetic method for 2-deoxy sugars.32 The reaction proceeds via a common radical intermediate and the final stereochemistry is controlled by neighboring steric factors. Reaction with a-halo esters proceeds by inversion and this has been interpreted in terms of conformational analysis of the intermediate radical.33 The reduction of alkenyl iodides, which proceeds with a high degree of retention of configuration, can be achieved with tributyltin hydride under palladium catalysis. However, the triethylborane-induced reduction is not stereoselective.34

8,2.1.2 Alcohols The selective and mild replacement of a hydroxy group by hydrogen is an important process, particularly in carbohydrate chemistry. Although a direct removal of the hydroxy group is not possible, alcohol reduction can be achieved via derivatization and conversion into halides is a convenient choice. An alternative is conversion of the alcohol into esters or thioesters, particularly to xanthates, which is known as the Barton-McCombie reaction.35 Although the mechanism of R - 0 cleavage from xanthates is controversial, it is believed that the tin radical adds to the sulfur atom of the C=S bond to afford a thioketyl radical, which undergoes the desired R-O bond cleavage (Scheme 3).19 Primary alcohols can be easily deoxygenated by derivatization as polyhalophenyl thiocarbonate esters.36 In contrast to the general reaction pattern of hydrogen abstraction, xanthates derived from allyl alcohols undergo stannyl group abstraction to afford allylstannanes via [3,3]-sigmatropic rearrangement.37 Thermal reaction of trimethyltin hydride with allyl alcohols, on the other hand, results in hydride addition to afford 3-(trimethy lstanny l)propanol. Bu3SnH

O

SMe

AIBN

SSnBu3 R

O * SMe

SSnBu3 R

RH Scheme 3

O

SMe

Bu3SnSMe

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Tin

In contrast to ketyl radical formation in the case of thioesters, the reaction of selenocarbonates proceeds by Se-C bond cleavage. The resulting alkoxycarbonyl radicals can give either formates, alcohols, or alkanes (Scheme 4). o II

A

O

Bu3SnH

O

II

II

J

% A%HA H °r R°H

% %J -

SPh SePh ~ ^T ^T

Scheme 4

8.2.1.3 Divalent heteroatom compounds The bonds R'-X-R 2 , where X = S, Se, Te, or other heteroatom, are readily split by stannyl radicals. The reactivity increases in the order phenyl (not split)«methyl < primary < secondary < tertiary < allyl = benzyl,15 and therefore phenyl is generally used as the R2 group so that bond fission occurs only at the R'-X bond.40 The C-Hg bond in P-mercuriocycloalkanones is homolytically cleaved by tin hydride to afford either direct reduction products or one-carbon-expanded cycloalkanones, depending on the radical stability of the intermediate.41

8.2.1.4 Nitrogen compounds The nitro group can be removed homolytically by tin hydride. Since the nitro group can activate a-carbon anionically to effect C-C bond formation with carbon electrophiles, the tandem reactions of C-C bond formation followed by removal of the nitro group can serve as a versatile synthetic method.42 The amino group can be reductively eliminated if first converted to an isonitrile. Aliphatic, alicyclic, and steroidal isonitriles give the corresponding hydrocarbons in 80-90% yields.15

8.2.2 Reduction of Carbonyl and Carboxyl Groups Reduction of carbonyl groups to alcohols can be accomplished either through ionic or radical pathways. Ionic reactions can be induced by activating the carbonyl group with Lewis acids or by activating the tin hydride by coordination of Lewis bases to the tin atom to facilitate the liberation of hydride. A remarkable contrast in stereoselectivity has been observed in the reduction of a-alkoxy ketones (Equations (1) and (2)).43 The use of a Lewis base induced a syn reduction (Equation (1)), while the use of a Lewis acidic tin hydride resulted in anti reduction (Equation (2)). These results have been rationalized by assuming that the Lewis base effectively traps the stannyl cation, thus inducing Cramtype reduction, while the chlorostannyl group coordinates with two oxygen atoms to induce the antiCram reduction by chelation. OMe

OMe Bu3SnH/Bu4NF

R2

R

i

(1)

""

Bu2SnCl-H

(2) OH

The radical reactions proceed by addition of the tin radical to the carbonyl oxygen, as shown in Scheme 5/14 The stannyloxycarbon radicals thus formed abstract hydrogen directly to give alcohols or they may interact with a neighboring group prior to hydrogen abstraction. As an example of the latter case, an epoxide can participate in the reaction to give P-hydroxy ketones.45 a,p-Unsaturated aldehydes react with tin hydride in various ways: the conjugate reduction to saturated aldehydes occurs with palladium catalysis, one-carbon loss to give saturated ketones occurs with palladium/oxygen, while an aldehyde function is reduced to an allyl alcohol with ZnCl2 catalysis.46 Smooth conjugate reduction of a,P-enones is achieved by Et3B-induced reaction of triphenyltin hydride18 or by CuI/LiCl-assisted reaction of tributyltin hydride.12 A number of other functional groups

Tin R3SnO

361 R3SnO

O

OH

Scheme 5

including sulfoxide, ester, and nitrile are unaffected. The AIBN-induced reduction of 3-oxo-l,4-diene steroid derivatives results in B-ring fission, aromatization of the A-ring, and formation of 9,10secosteroids.47 The reduction of carboxylic acid derivatives to aldehydes by tin hydride is carried out using acid chlorides48 or thio- or selenoesters49 under palladium catalysis or by AIBN initiation.

8.2.3 Addition to Multiple Bonds Tin hydride adds easily to various types of multiple bonds, but the addition to alkynes or allenes is the most important, because it is a convenient method of vinylstannane preparation. The AIBN-induced radical method is frequently employed, while palladium catalysis is also used.8'50 The regio- and stereochemistry are greatly influenced by the nature of the substrates and the reaction conditions.8'51

8.2.4 Carbon-Carbon Bond Formation Hydrogen abstraction by primarily formed radicals can be competitive with other reaction types such as addition to an unsaturated group to form a C-C bond, which is known as the Giese reaction.52553 Reaction with allyl sulfides or allylstannanes results in a radical allylation.54 Although intermolecular C-C couplings are certainly useful,55 the intramolecular reactions have been used more frequently as powerful and versatile tools for ring construction. Much success has been achieved in stereochemical control in acyclic carbon-carbon bond formation56 and in cyclization.57 The unsaturated moiety is usually C=C,58 but C=C,59 C=O,60 imine,61 or enamine62 double bonds and aromatic rings63 or acylsilanes64 can also serve as radical acceptors. The majority of cyclizations involve five-membered ring formation from 8,£-unsaturated radicals via a 5-exo-trig cyclization,65 although the exo.endo ratio is sometimes controlled by the steric66 or electronic environment.11 The 5-exo-trig cyclization seems to be an extremely facile reaction, since the initially formed thiocarbonyl radical from xanthates of homoallylic alcohols cyclizes to thiolactones prior to the otherwise ensuing C-O bond cleavage.67 In cases where the intermediate radical is stabilized, endo cyclization is exclusive.68'69 The cyclized radicals can abstract hydrogen or a halogen atom to afford the final products.70'71 The halogen abstraction reaction is often referred to as "atom-transfer cyclization." u Under aerobic conditions, the cyclized radicals are oxygenated to give alcohols.72 Since halogen atoms are abstracted most readily compared with other heteroatoms, y-lactams having a sulfur substituent are obtained by the cyclization of the alkenic a-chloro-a-phenylthiocarbamates. The reaction of selenocarbonates which have a triple bond to give the corresponding a-alkylidene-ybutyrolactones is characterized by high exo cyclization and a high ratio of cyclization to reduction.39 When there is more than one double bond in a molecule, a variety of products, such as spiro and fused ring systems, can be prepared through tandem radical cyclizations. An effective cyclization is possible by the Et3B-induced process, since the reaction can be carried out at lower concentrations compared with other conventional radical-initiated reactions. This procedure has been applied to the stereoselective synthesis of a-methylene-y-butyrolactones (1) (Scheme 6).78

SnBu3

o

O

Bu3SnH EhB

R2

R3 (1) Scheme 6

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Tin

Although the tin radical initiated cyclization proceeds satisfactorily with enynes, that of dienes or trienes has been far less successful, mainly because the reaction results in bistannylation without cyclization. However, a successful cyclization from various activated or unactivated dienes and trienes (2) has been reported.79 A development of oxidative cleavage of the unactivated C-Sn bond in (3) to give an acetal definitely enhances the synthetic applicability of the method (Scheme 7; can, cerium(IV) ammonium nitrate). Me3Sn

MeCL

Me3SnCl NaBH3CN/AIBN

.OMe

can

O

O

H (2)

(3)

Scheme 7

co-Iodoalkyl propiolate esters (5) of n - 10-12 cyclized to 14-16-membered frans-c^p-unsaturated macrocyclic lactones (6), while the reaction afforded only reduction products (4) when n = 6-9 (Scheme 8).80 Due to slow hydrogen abstraction from the tin hydride, the primarily formed c/s-radical intermediate inverts to give the thermodynamically more favored trans geometry. This macrocyclization technique has been extended to even a 22-membered ring.81

n = 6-9

(4)

n= 10-12

O

(5)

(6) Scheme 8

Some radical intermediates undergo a couple of radical transfers by consecutive intramolecular hydrogen abstractions before affording final products. This has been developed into a useful way of constructing novel cyclic systems.14'25' The reaction of unsaturated acid chlorides with tin hydride is reagent dependent. Radical initiation by AIBN results in cyclization, while smooth reduction to aldehydes is realized by the palladium-catalyzed reaction.48 When the AIBN/Bu3SnH-induced reaction was applied to P-allenic oxime ethers, intramolecular radical attack occurred to produce stannylated aminocyclopentenes. Acid treatment easily eliminated the stannyl group to produce aminocyclopentenes.85

8.2.5 Other Reactions Heteroaromatic compounds having a suitable leaving group undergo ipso-substitution by the stannyl group in the AIBN-induced reaction with tributyltin hydride.86 The ipso-substitution also proceeds cleanly with phenylthio-substituted furanones. Surprisingly, neither the conventional reduction nor addition-elimination products were identified.87 Carbene species undergo insertion into the Sn-H a-bond. Aliphatic Fischer carbene complexes (7) having an a-stereogenic center afford the insertion products with considerable diastereoselectivity (Equation (3)). Notably, the stereochemistry was reversed compared to that obtained by Cram-type addition of Bu3SnLi to the structurally comparable aldehydes.88 OMe

OMe Bu3SnH

Bu3Sn (7)

(3)

Tin

363

Aminyl radicals can be produced by abstraction of a thio group from suIf enamides. Although the aminyl radical thus produced does not cyclize with ordinary alkenes, it cyclizes with aryl-activated alkenes to afford pyrrolidines.89 A stannyl radical initiated decomposition of azides results in cyclization with ensuing ring expansion.90 Aryl radicals generated from aryl iodides by iodine atom abstraction can react with an azo group intramolecularly to afford Af-aminocarbazoles.9i The allyloxycarbonyl group, a useful amine-protecting group, can be cleanly eliminated by palladium-catalyzed reaction with tributyltin hydride. Other conventional deprotections sometimes afford allylamines as by-products.92

8.3 ANIONIC TIN REAGENTS 8.3.1 Stannyl Anions (Sn ) 8.3.1.1 Preparation The most typical anionic stannyl reagents are tributylstannyllithium and trimethylstannyllithium. In a few cases, triphenylstannyl derivatives are used. Tetrahydrofuran solutions of the stannyl anions can be prepared from the corresponding hexaalkylditins in yields over 95% by treatment with butyllithium or methyllithium.93 Tributylstannyllithium can be prepared more conveniently by deprotonation of the tin hydride with lithium diisopropylamide.94 Chlorine-lithium exchange of trialkyltin chlorides with lithium metal is also a convenient method.95 Magnesium derivatives are best accessible from Bu3SnH by treatment with sterically hindered secondary alkylmagnesium halides.96 Stannylcuprates having several "dummy" ligands, such as CN and ROC, 9 7 CN, and Me,98'99 have been prepared. A convenient method for the preparation of higher order stannylcuprates from trimethyltin hydride or hexamethylditin has been described.12 The naked tributylstannyl anion can be prepared from (trimethylsilyl)tributylstannane by treatment with naked cyanide or with fluoride anion.101 These species are particularly interesting in two respects: first, the preparative method is very typical of tin chemistry and is not realizable with carbanions; second, the species generated have an entirely different chemistry to that of stannyl anions having metal counter-ions.

8.3.1.2 Reactions (i) Substitution Stannyl anions undergo nucleophilic substitution reactions with compounds containing leaving groups at the sp3 carbon. The reaction mechanism has been extensively studied102'103 and it is generally accepted that the reaction with acyclic104 and cyclic95 halides proceeds by an initial electron transfer, while the reaction with tosylates proceeds via an SN2 mechanism with complete inversion of configuration. Reactions with oxirane or tetrahydrofuran rings proceed with ring opening to produce P- or 5-stannyl alcohols.105 Reaction between a stannyl anion and haloarenes in liquid ammonia can lead either to substitution or halogen-metal exchange, depending on the ligands on the tin atom and the nature of the halogen(s) of the haloarenes.106

(ii) Addition to multiple bonds Since Piers found that trimethylstannylcuprates added to alkynic bonds, this method has been used as a useful preparative method for alkenylstannanes. In contrast to tin hydride, in which the partner of the tin atom is always hydrogen, reagents having tin-heteroatom bonds are potentially double functional, if both the heteroatom and tin atom are manipulated individually in succeeding steps. However, this has proved to be a rather difficult task with electrophiles other than the proton, because the stannylcupration step is reversible and, although the equilibrium is well to the product side, the original stannylcuprate reagents are more reactive towards most electrophiles than the vinylcuprate intermediates. The problem has been overcome by using ZnCl2, which promotes the formation of

364

Tin

vinylcopper-zinc intermediates,107 or by using higher order (cyano)(methyl)(tributylstannyl)cuprates under specific conditions.98'108 The regio-, stereochemistry and characterization of the intermediates have been discussed.109 The reaction is effectively controlled by neighboring polar substituents: terminal vinylstannanes are obtained from propargyl alcohols110 or propargyl acetals,111 while inner vinylstannanes are obtained from 1-alkynes.98 The regioselectivity of the stannylcupration of propargylic sulfides112 and amines113 has also been discussed. The geometry of the vinylstannanes from 2-alkynoates is dependent upon the reaction conditions: (Z)-isomers were obtained at 0 °C under aprotic conditions, while (£')-isomers were obtained at -78 °C in the presence of alcohols.97 Various types of stannylcuprates add to allenes114 or alkynes.115 Further in situ manipulation of the cuprates offers an easy entry to polyfunctionalized compounds such as exomethylenic 1,3-diketones.115 Stannylmetals other than stannylcuprates also undergo addition to triple bonds. The regio- and stereochemistry are controlled by the nature of the counter-metals.116

(Hi) Reactions with carbonyl compounds Trialkylstannyllithium adds to aldehydes to produce 1-hydroxyalkylstannanes.94 Although the free hydroxy compounds are unstable, the alkoxy derivatives are isolable by distillation or chromatographic separation and can be stored for several months. The stannyl anion reagent undergoes 1,2-addition to a,(3-unsaturated aldehydes to give a-alkoxyallylstannanes.117 On the other hand, the addition proceeds in a 1,4-fashion with a,|3-enones in THF, which is a simple and efficient entry to functionalized tetraorganostannanes. The apparent 1,4addition is believed to proceed through a primary 1,2-addition, followed by 1,3-migration of the stannyl group.118 Many types of stannylcuprates undergo 1,4-addition to a,|3-enones. The delivery of the stannyl group proceeds effectively even with cuprates which contain an alkyl group because the Sn-Cu bond is weaker than the C-Cu bond.119 The nature of the counter-cation controls the regio- and stereoselectivity. In contrast to the exclusive 1,4-addition by stannyllithium, both 1,2- and 1,4-adducts have been observed with trialkylstannylmagnesium chloride and the reaction of bicyclic enones (9) affords cis adduct (8) with stannylcuprate, while it affords the trans adduct (10) with stannyllithium (Scheme 9).120 Some (E)and (Z)-y-silyloxyallylic stannanes can be prepared separately from enals by using stannylcuprates and stannyllithium, respectively.121

Me3Sn[Cu] 69%

C T ^ ^ ^ ^

SnMe3 (8)

87%

SnMe3 (9)

(10)

Scheme 9

The stereochemistry of stannyl addition to 2-cyclohexenone derivatives is characterized by the rigid axial approach of the reagent.122 Subsequent treatment of the enolates with electrophiles results in high anti selectivity in the acyclic as well as in cyclic systems.123 The electrophiles can be a proton, alkyl halides, or aldehydes.124 Conjugate addition to vinyl sulfones, followed by trapping with aldehydes, gives y-hydroxyvinylstannanes. The Michael addition of R!3SnLi to 2-cyclohexenone SAMP ((S)-l-amino-2-methoxymethyl-lpyrrolidine) and RAMP ((/?)- l-amino-2-methoxymethyl-l-pyrrolidine) hydrazones (11), followed by trapping of the resulting azaenolates with alkyl halides and oxidative removal of the chiral auxiliary, affords frans-tandem adducts (12) with high diastereomeric excess (Scheme 10). Remarkably, the quenching of the azaenolate with protons (R = H) affords the product with only modest diastereomeric excess (de = 42^4%). 126 The stannyl anion prepared from fluoride-induced desilylation of a silylstannane abstracts halogens from aryl or vinyl halides much faster than it adds to the carbonyl group of (13), and thus an interesting annulation has been accomplished as shown in Equation (4).101

Tin

365

OMe O i, R^SnLi

O3

R2

ii, R2X 43-95%

64-97%

SnR1 de = 87- >96%

(11)

(12) de > 98% ee = 85- >96%

Scheme 10

Bu3SnTMS/F

(4) 86%

(13)

8.3.2 a-Stannyl Carbanions (Sn-C ) 8.3.2.1 Preparation Trialkylstannylmethyllithiums are another kind of typical anionic reagent and a review article including other organoelement groups is available.127 The reagents can be prepared by iodine-lithium exchange of trialkylstannylmethyl iodides, obtainable by reaction of trialkylstannyl chloride with the Simmons-Smith reagent. Trimethylstannylmethyllithium (15) can be prepared more conveniently and in larger scale by tin-lithium exchange of (14), which can be obtained from diiodomethane, tin(II) bromide, and methylmagnesium iodide, as shown in Scheme 11. The corresponding alkyl derivatives (17) can be obtained in the same way, but their application for synthetic purposes is not satisfactory because of the low yields obtained of the bis(stannyl) compounds (16). The corresponding zinc and copper reagents can be prepared from (18) through iodine-metal exchange, as shown in Scheme 12.128 The cuprate (19) undergoes a conjugate addition to a,P-enones to afford y-stannyl ketones.129 2SnBr 2 + R-CHI 2

R-CH(SnBr2I)2

MeMgl

" R-CH(SnMe3)2 R =H R = alkyl

BuLi

Me3SnCH(R)Li

(14) (16)

(15) (17)

Scheme 11 R3Sn-CH(R)I

Zn/Cu

R3Sn-CH(R)[Zn]

CuCN

R3Sn-CH(R)[Cu] (19)

(18) Scheme 12

8.3.2.2 Reactions (i) Reactions with carbonyl compounds Kauffmann et al.13° observed that trialkyl- or triarylstannylmethyllithiums or their a-thio-substituted derivatives reacted with carbonyl compounds to afford p-stannyl alcohols, which gave alkenes upon acid treatment or heating. Further studies revealed that trie reaction is sometimes superior to the Wittig reaction, particularly with enolizable ketones.131 Unlike the Wittig or Peterson reaction, the tin-based reaction is subject to the effects of neighboring leaving groups. The p-stannyl alcohols (21) produced from epoxide (20) react with a second molecule of the reagent at the tin center to afford the diol (22) (Scheme 13). A comparison of the reactivity patterns between the stannyllithium and silyllithium reagents is noteworthy. Although the first-stage reaction proceeds similarly, the second molecule of the silyl reagent attacks the oxirane ring of (23) to produce the ally lie alcohol (24).

Tin

366

CH2SnMe: HO

CH2SnMe3

HO

McSnCH

VOH o (22)

(21)

o CH2TMS

(20)

OH

TMS-CH2

CH2TMS CH2TMS (24)

(23) Scheme 13

(ii) Reactions with esters The reaction of conventional alkyllithiums with esters of simple alcohols usually produces tertiary alcohols because the initially formed ketones are more reactive toward the reagents than the starting esters. However, the reaction of the anion (25) leads to a special situation, since the presence of the tin group in the intermediate product (26) induces nucleophilic attack by a second molecule of the reagent at the tin atom, rather than at the carbonyl carbon, thus effecting heterolytic bond cleavage of the tin-carbon bond. The net result is the formation of enolates from esters with two equivalents of the 132 reagent. Quenching of the enolates with electrophiles affords the ketone (27) (Scheme 14). The electrophile can be a proton, an aldehyde, carbon dioxide, or methyl formate. No racemization was observed with suitably protected chiral a-hydroxy and a-amino esters. Obviously, the anion (25) serves as a conjunctive reagent, connecting an acyl group and the E moiety by a methylene group. This reaction contrasts with that of an a-silyl carbanion, which either terminates at the stage of a-silyl ketone formation or proceeds further to produce allylsilanes through reagent attack at the carbonyl carbon. The reaction with methyl formate as electrophile affords a P-keto aldehyde and this process has been successfully applied to D-mycaroside synthesis.

O

A OR R

CH2SnMe: SnMe3 R (26)

O

o-

-CH2SnMe3 (25)

R (27)

Scheme 14

8.4 CATIONIC TIN REAGENTS 8.4.1 Reagents with Tin-Heteroatom Bonds (Sn-X) One of the characteristic features of organotin compounds is that tin atoms attached to heteroatoms easily coordinate Lewis bases to give hypervalent tin compounds in which the nucleophilicity of the heteroatoms bound to tin is greatly enhanced.

367

Tin

8.4.1.1 Tin-oxygen bonds (i) Organotin alkoxides and oxides Enhancement of the nucleophilicity of the alkoxide group by coordination of the tin atom with Lewis bases leads to a facile exchange of the alkoxy groups in esters and acetals.133 The concept has also been successfully applied to glycosylation of carbohydrates.134-6 Trifluoroethyl esters are particularly suitable for transesterification, which can be used for the conversion of co-hydroxy esters to macrolactones.137 1,3-Disubstituted tetrabutyldistannoxanes have been developed as effective catalysts for esterification, acetalization, and deprotection of silyl ethers under mild conditions.138 Tin alkoxide induced reactions often display high chemoselectivity when applied to polyfunctional derivatives, due to the ability of the tin atom to coordinate with neighboring groups and thus to direct the reaction intramolecularly.139 The coordination is subject to a very significant solvent effect. When an O-stannylated glycal (28) was treated with NIS (AModosuccinimide) in benzene, allylic oxidation was the major pathway (Equation (5)), while use of the same reagent in acetonitrile resulted in iodocyclization (Equation (6)).140 The halostannyl group in y-hydroxystannanes can be replaced by a hydroxy group with retention of configuration by use of hydrogen peroxide. The chelating effect of the halogen-containing tin atom with the internal hydroxy group controls the stereochemistry.141'142

RO Bu 3 Sn /

NIS

o-

O CH2OSnBu3 benzene

^

(5)

(28) Bu3Sn

O NIS

Bu3SnO

(6) acetonitrile

RO

I OR

(28)

A remarkable "tin effect" has been observed, in which trimethylstannyl acetate greatly enhances the reactivity of palladium-catalyzed trimethylenemethane cycloaddition to carbonyl groups (Scheme 15). The effect has been rationalized by assumption of an enhancement of the nucleophilicity of the alcohol group for cyclization by formation of a tin alkoxide.13

TMS

OAc

PdL2+

RCHO

Me 3 Sn0

Me3SnOAc

Scheme 15

The enhanced nucleophilicity of tin alkoxides is also shown in the nucleophilic substitution of a sulfoxide group by tin benzyloxide143 and in tributyltin alkoxide addition to ketene144 or isocyanates145 to afford a-stannyl acetates or Af-stannyl carbamates. Tin oxides, particularly dibutyltin oxide, have evoked much interest due to their remarkable ability to protect specific hydroxy groups of polyhydroxy compounds.146 The most important application is in the regioselective protection of saccharides. The position of protection differs among saccharides and protecting groups and the results have been discussed based on an equilibrium of the tin intermediate and the kinetic behavior of the protecting reagent.147 This concept has been used for the regio- and stereospecific ring opening of carbohydrates to afford polyhydroxy-a,P-enals.148 Dibutyltin oxide has also been used for the preparation of macrocyclic oligolactones from glycosides and dicarboxylic acid dichlorides.149 Dibutyltin oxide undergoes sulfur-oxygen exchange with thiolactones. Dibutyltin bis(triflate) catalyzes the Michael addition of silyl enol ethers to a,|3-enones under mild conditions.151

Tin

368 (ii) Enolates

Since the first introduction of tin(II) enolates for the 5j«-selective aldol reaction by Mukaiyama in 1982, many applications have been reported. A series of tin(II) enolate reactions which led to the catalytic asymmetric aldol reactions has been overviewed.152 One of the recent and successful applications has been reported as a substrate-controlled aldol reaction, as shown in Scheme 16. The high selectivity is rationalized by selective formation of the (Z)-enolate and the rigidity of the tin(II) chelate (29).153 Such a high selectivity cannot be achieved with the corresponding boron and titanium enolates.

Sr 0

Sn(OTf)2

RCHO

F

OBz

R

OBz

1

Et3N

OBz

V

SrT

o

O

OTf (29) Scheme 16

Organotin(IV) enolates are generally prepared by transmetallation of the lithium enolate, by hydrostannation of a,P-enones, or by transesterification of enol acetates. Convenient methods for the preparation of organotin(IV) enolates have been developed using stannylcarbamates 145 or Bu3SnSnBu3/Bu2Snl2/HMPA as stannylating reagents. The reactions of a-stannyl ketones with a-bromo ketones have been extensively studied and the results show that the addition of hexamethylphosphoramide (HMPA) favors direct coupling to 1,4-diketones. In contrast, P-keto oxirane formation occurs without HMPA, but under otherwise similar conditions.155

8.4.1.2 Tin-halogen bonds Organotin halides of the type R4_nSnXn have been shown to be useful catalysts for dehydration processes, as well as being tin delivery reagents for anionic species. Particularly useful is BuSnCl3, which promotes bimolecular dehydration of allylie alcohols, cyclization of l,n-diols to cyclic ethers, cyclization of 1,4-diketones to furans, dehydration of cyclic diols, and acetalization of aldehydes and diols.156 As with the tin alkoxides, the nucleophilicity of the chloride is enhanced by use of coordinating Lewis bases. The complexes which result can be used as catalysts for regioselective ring opening of
8.4.1.3 Tin-nitrogen bonds Af-Stannylcarbamates are prepared by the reaction of tributyltin methoxide with ethyl isocyanate and have proved to be effective reagents for the preparation of organotin(IV) enolates.145 Although the chemistry of divalent tin compounds, known as stannylenes, has not been investigated as thoroughly as carbenes, a nitrogen compound was first synthesized in 1974 as a stable orange-yellow solid. Recently, ligand transfer was reported by the reaction of the tin(II) amide (30) with primary aldehydes to afford frans-enamines (31) (Scheme 17).163 The reaction is believed to proceed through carbonyl insertion into the Sn-N bond, followed by J3-elimination. Tin(II) amides have been used as convenient reagents for the conversion of esters into amides.164 Organotin azides are well known as 1,3-dipolar reagents that undergo [3 + 2]-cycloaddition with alkynes to give triazoles. Recently, reaction with isothiocyanates was found to proceed at the C=N functionality, in contrast to the reaction at the C=S moiety as observed with hydrogen azide.165

Tin

369

(TMS)2N — Sn Sn[N(TMS)2]2 (30)

)

^

H-i

/\

.

R

L°

~

^^N(TMS)2

^ \

(31)

R

N(TMS)2

Scheme 17

8.4.1.4 Other heteroatoms Allylthiostannanes undergo nucleophilic substitution of aryl iodides under palladium catalysis to afford aryl allyl sulfides.166 Fluoride ions smoothly destannylate organotin chalcogenides to liberate highly nucleophilic chalcogenide ions. Thus, the tin compounds are equivalents to oxide-transfer (O2~) or selenide-transfer (Se2~) reagents.167 Fluoride activation has been applied to a selenoaldehyde synthesis. Although silylstannanes are barely regarded as anionic tin reagents, they behave as stannyl anions when activated by naked cyanide catalysis100 or by (Et2N)3S+-SiMe3F2~ (tasf) or CsF in DMF.101 Silylstannanes add to alkynes regio- and stereoselectively under palladium catalysis, while they undergo 1,4-addition to dienes with platinum catalysis.168 Selective destannylation of the adducts by HI or desilylation by fluoride affords (£)-vinylsilanes or vinylstannanes, respectively.169

8.4.2 Carbocationic Tin Reagents (Sn-(C)n-X) The most popular carbocationic tin reagent in this category is Me3SnCH2I. It is used to prepare a-alkoxyalkylstannanes from alcohols as a synthon of a-alkoxy carbanions.170 3-Stannylpropanal dimethylacetal, another reagent in this category, undergoes the Mukaiyama aldol reaction with silyl enol ethers to afford e-stannyl ketones, which cyclize in situ to cyclopentanol derivatives. The reaction is a formal, one-pot [3 + 2]-annulation.171 y-Chloroallylstannanes (32) react with aldehydes in the presence of Lewis acids to afford aldehydes (33) and 1,2-migration of the R group has been postulated (Scheme 18). The reaction is a formal insertion of allylic carbon into the C-C a-bond of the aldehyde.172

CHO S11R3

Lewis acid (LA)

Cl (32)

Cl '

(33)

Scheme 18

8.5 ALKENYL- AND ARYLSTANNANES (Sn-C=C) 8.5.1 Preparation Alkenylstannanes are usually prepared by the hydrostannation or stannyInstallation (R3SnMXn: M = Al, B, Cu, Mg, Si, and Zn) of alkynes173 or by treatment of alkenylorganometallics with Bu3SnOTf or R3SnCl.174'175 (Z)-Alkenylstannanes are'prepared by the selective ds-addition of Cp2Zr(H)Cl to alkynylstannanes, followed by proton quenching.176 The addition is regioselective to afford 1,1-dimetallo compounds. They are viewed as stereodefined 1,1-vinyl dianions, into which two distinct electrophiles can be introduced sequentially by using the difference in reactivities between vinylstannanes and vinylzirconates. Enol triflates or aryl/alkenyl halides couple with hexamethylditin under palladium catalysis to afford alkenylstannanes.177 The reaction of vinyl triflates with high-order stannylcuprates offers a convenient preparative method for vinylstannanes.178 (E)-Alkenylstannanes can be prepared from aldehydes by one-carbon homologation, using Bu3SnCHBr2 through CrCl2-mediated alkenation179 or by dehydroiodination of iodoalkylstannanes.173 The radical-initiated hydrostannylation of propargylic alcohols (34) affords (Z)-p-stannylated allyl alcohols (35), while titanium-catalyzed hydromagnesiation followed by treatment with tributyltin

370

Tin

chloride affords (Z)-y-stannylated allyl alcohols (36) (Scheme 19).51 The radical-initiated addition of tin hydride to propargylamine affords (£)-y-(trialkylstannyl)allylamine exclusively.180 The preparation of the corresponding (Z)-isomer has been attained from the corresponding (Z)-vinyllithium with the cationic tin reagent, Bu3SnCl. Bu3SnH AIBN

SnBu 3 (35) R (34)

2

Bu'MgCl " TiCp2Cl2

OH MgCl I I R* ^^r R2 I H

OH

SnBu 3

Bu3SnCl *" R' H (36)

Scheme 19

The reaction of a-stannyl-a-silyl acetates is characteristic. Deprotonation by lithium diisopropylamide (LDA) and subsequent reaction with aldehydes results in deoxysilylation to afford a-alkoxycarbonyl-substituted vinylstannanes as (E)/(Z) mixtures. Remarkably, under these conditions desilylation precedes destannylation.181 The nucleophilic nature of the tributyltin radical induces the displacement of sulfonyl groups from electron-deficient heteroaryl tosylates86 or vinyl sulfones to produce aryl- or vinylstannanes.182 Nucleophilic attack of Bu3SnLi on chiral epoxysilanes affords enantiomerically pure alkenylstannanes.183

8.5.2 Reactions 8.5.2.1 Transmetallation (i) Alkali metals The tin atom of alkenylstannanes184 or arylstannanes185 can be replaced easily by lithium by treatment with alkyllithiums. The overall process is an equilibrium in which the driving force is the relative difference in base strengths of the organolithium species (Equation (7)). With the correct choice of solvents, tin compounds of allyl, benzyl, vinyl, a-heteroatom-substituted alkyl, or even cyclopropyl undergo complete tin-lithium exchange by alkyl- or aryllithiums. This method is far superior to other conventional methods for the preparation of organolithium compounds, such as lithium-halogen exchange, in avoiding contamination by lithium halides. The formation of by-product tetraalkylstannanes is not a problem, since these hydrocarbon-like species are virtually unreactive and in general are easily separated at the end of the process. Since the tin-lithium exchange in vinyl- or allylstannanes proceeds so fast, it can be accomplished successfully even in the presence of electrophilic centers within the molecule, such as a carbonyl group186 or chlorine.184 When a silyloxy187 or silylamino group180 is present in the molecule, silyl migration from these heteroatoms to carbon proceeds upon transmetallation of tin to lithium in vinylstannanes. The latter reaction has been used as a synthetic method for (Z)-allylamines and nitrogen heterocycles. R ! 3 SnR 2 + R3Li

•

R2Li + R»3SnR3

(7)

Tin-lithium exchange of o-stannylbenzyl alcohol derivatives can be applied to the preparation of highly strained benzocyclopropanes.188 (ii) Other metals Tin-boron exchange proceeds cleanly on treatment of alkenylstannanes with 9-BBN bromide. This reaction constitutes a simple method for the synthesis and manipulation of vinylboranes that avoids the problems otherwise encountered frequently.1

Tin

371

Alkenyl- or allylstannanes can be transmetallated to cuprates by treatment with higher order alkylcuprates. The resulting cuprates undergo smooth conjugate addition to a,P-enones12 or nucleophilic substitution.190 Tin-lead exchange of vinylstannanes proceeds with lead tetraacetate to afford vinyllead derivatives, which react with carbon nucleophiles, such as P-dicarbonyl compounds, to give C-vinylated products.191

8.5.2.2 Palladium-catalyzed cross-coupling reactions Stille192 developed the palladium-catalyzed cross-coupling of organotin compounds (Equation (8)) and published a detailed review on this subject. Results obtained since then in this rapidly developing field have been summarized.193 Alkenyl-, alkynyl-, and arylstannanes are by far the most important tin compounds, with decreasing reactivity in this order. Heteroaromatic stannanes have also been utilized.194 Allylstannanes and alkylstannanes8'195 also undergo the cross-coupling, albeit less frequently. The presence of oxygen196"8 or nitrogen atoms199 in the a-position does not interfere with the cross-coupling. The substrates R3 are typically aryl, heteroaryl,200 alkenyl, or alkynyl,201 but ring-strained allyl202 can also be used. Alkyl groups other than methyl cannot be used, because P-elimination occurs rapidly. Acyl chlorides,203 a-halo ethers or sulfides,204 and chloroformates or carbamates205 can be used as the R3X moiety. -R 2 + R 3 -X

[Pd]

•

R 2 -R 3 + R^Sn-X

(8)

The X group is usually a halide or triflate,206'207 but hypervalent iodine,208 arenesulfonate,209 and epoxide210 are also effective. The reaction of aryl triflates proceeds quite readily even with highly hindered o,o'-disubstituted aryl triflates.211 Enol triflates of P-keto esters also react smoothly.212 When the reactions of enol triflates with vinylic or alkynic stannanes are allowed to proceed in the presence of CO, high yields of divinyl ketones are obtained by carbonylative coupling, although the reaction conditions must be carefully controlled.206 The carbonylation of vinylstannanes has also been realized with aryl iodides.213 The reaction is generally stereospecific and the geometry of the vinyl halides is retained.214 The palladium complexes which result from the intramolecular Heck reaction of alkynic aryl triflates or iodides couple in situ with vinylstannanes to afford (Z)-indanylidenes with high stereoselectivity.215 The cross-coupling of aryl- or allylstannanes with the palladium intermediates generated from a radical cyclization has been realized.204 The coupling reaction can be accomplished by employing various combinations of Pd/solvent/ additive. Both Pd° and Pd2+ are effective as catalysts, but use of copper(I) iodide and triphenylarsine or silver(I) oxide216 as cocatalyst and use of highly polar solvents and soft ligands are effective.217'218 The presence of a tertiary amino group in the vicinity accelerates the reaction 100-fold through coordination to the tin atom.219 Cyclization can be accomplished by carrying out the cross-coupling intramolecularly220 and can be applied to macrolide synthesis.221 Tandem palladium-catalyzed addition of tin hydride to alkynes and cyclization of the resulting vinylstannanes has been used for the preparation of benzocyclobutanone derivatives (Scheme 20).222

Scheme 20

The reaction of the bifunctional organometallic (37) with acid chlorides is noteworthy (Scheme 21).223 When the reaction was induced by palladium catalysis, (37) reacted as a vinylstannane to afford the enone (38). However, when the reaction was induced by Lewis acid catalysis, (37) reacted as an allylsilane to produce initially the cation (39). This intermediate carbocation underwent a 1,2-silyl migration spontaneously, probably due to the double stabilization of the cationic center in (40) by the silyl and tin p-effect, and gave vinylsilane (41) as the final product. The preferential elimination of the stannyl group is a reflection of the more labile nature of the C-Sn bond as compared with the C-Si bond. The reaction of (£)-l,2-bis(tri-n-butylstannyl)ethene with acyl chlorides yields 1,2-diacylethenes initially, but upon prolonged reaction the double bond is reduced to produce 1,4-diketones.203 The

372

Tin SnR3

[Pd]

+ R-C0C1

(37)

•

R-^V^TMS

(38)

\

SnR3

O "

TMS

R

v

(39)

SnR3 I—TMS

^ (40)

+

O

TMS

R (41)

Scheme 21

reduction has been assumed to be induced by a palladium hydride formed from a butylpalladium intermediate through P-elimination. A formal Michael addition of a vinyl anion has been accomplished by reaction of an enal and tributylvinylstannane under catalysis by Ni(cod)2 and trialkylsilyl chloride (Scheme 22). A mechanism involving vinyl insertion into the 7i-allylnickel intermediate (42) hasbeen proposed.224 OSiR3 0SiR

Ni(cod)2 R3SiCl T

'

'xn

SnBu3 (42) Scheme 22

8.5.2.3 Miscellaneous reactions The stannyl group in vinylstannanes or arylstannanes can be replaced by fluorine,177'182'225'226 chlorine, bromine, or iodine. The trialkylstannyl group in arylstannanes is a good leaving group in electrophilic substitution by isocyanates, diazonium salts, or sulfonyl chlorides. Normally, ipso substitution takes place and the reaction has been applied to the preparation of regiochemically defined diaryl sulfones, arenesulfonamides, andarenesulfonic acids. Alkenylstannanes undergo a Simmons-Smith-type reaction to afford cyclopropylstannanes. The reaction is stereocontrolled by the presence of hydroxy groups.230 Alkenylstannanes substituted by electron-withdrawing groups act as Michael acceptors1 or as dienophiles.231 Hydrogenation of alkenylstannanes can be carried out with cationic rhodium complex catalysis. The accompanying reductive C-Sn bond cleavage can be minimized by correct choice of conditions. The stereochemistry of the hydrogenation can be controlled by a neighboring hydroxy group to afford y-stannyl alcohols with high diastereoselectivity.232

8.6 ALLYLSTANNANES (Sn-C-C=C) 8.6.1 Preparation Simple allylstannanes have been prepared by: (i) the reactions of conventional allylorganometallic compounds with organotin halides or oxides; (ii) reactions of stannyl anions with allylic halides or their equivalents; (iii) 1,4-hydrostannation of 1,3-dienes; and (iv) Wurtz-type cross-coupling between organotin halides and allylic halides with zinc.233 Palladium/samarium-induced cross-coupling of tributyltin chloride with allyl acetate belongs to this category.234 As with alkenylstannane preparation, nucleophilic radical attack by tin hydride on allyl sulfones affords allylstannanes.235 The direct coupling of allyl groups and aldehydes to give homoallylic alcohols via allylstannanes has been carried out with allyl alcohol >237 or with allyl chloride by treatment with SnCl2 under catalysis by palladium or iodide, respectively. Metallic tin can be used for the stannylation of allyl alcohols239 or allyl halides240 in aqueous solution and the reagents thus generated react in situ with carbonyl compounds.

Tin

373

a-Alkoxyallylstannanes can be prepared by 1,2-addition of a stannyllithium to a,p-enals; the addition products undergo a 1,3-stannyl rearrangement to afford y-alkoxyallylstannanes upon treatment with BF3 etherate.241 Chiral products are available by BINAL-H (lithium(l,r-binaphthalene-2,2'diolato)(ethanolato)hydridoaluminate) reduction of the acylstannanes obtained by in situ oxidation of racemic a-hydroxyallylstannanes.117

8.6.2 Reactions 8.6.2.1 Transmetallation Similarly to vinylstannanes, allylstannanes can be transmetallated with lithium even in the presence of a carbonyl group.186 A smooth transmetallation of tin to copper was realized using higher order alkylcuprates. Although the chemistry of allylie cuprates has not been documented in as much detail as alkyl- or vinylcuprates, they do undergo typical coupling reactions with several electrophiles.12

8.6.2.2 Coupling with halides Carbon-carbon coupling between organic halides and allylstannanes can be effected either by radical235'242 or palladium-catalyzed pathways.243 A comparison between these two activation methods has been made with a-chloro ketones.244 An important feature is the compatibility of this allylation method with the presence of other functionalities such as alcohol, acetal, ether, ketone, and some others. A free-radical carbonylation of alkyl halides was extended to three- or four-component coupling involving allylstannanes and electron-deficient alkenes.245 Acyl chlorides also couple with allylstannanes under dibutyltin dichloride catalysis.246

8.6.2.3 Reactions with electrophiles Allylstannanes have been extensively used as allyl anion equivalents and a number of review articles are available.5'247'248 The 7C-nucleophilicity of the alkenic bonds in the allylic groups towards cationic centers has been investigated and the relative activating effect of trialkylstannyl groups against hydrogen is 3 x 109, which is much larger than that of trialkylsilyl groups (5 x 10 ).249 The most widely documented reactions of allylstannanes are SE2' reactions with aldehydes. The reaction is induced by heat,250 high pressure,251 Lewis acids,252 or cationic transition metal complexes.253 The last method could open a way to an enantioselective preparation of homoallylic alcohols, albeit that asymmetric induction is still low (17% ee) at the moment. Spectroscopic investigation of the reaction intermediates has been reported.254 The BF3-induced reaction generally favors syn adducts regardless of the double-bond geometry of the starting allylstannanes and an open-chain transition state has been suggested.255 On the other hand, the pressure-induced reaction proceeds through a cyclic six-membered transition state, affording and products from the (E)-allylstannanes and syn products from the (Z)-isomers.256 When SnCl4, BuSnCl3, and Bu2SnCl2 were used as Lewis acids, an initial SE2'-type transmetallation proceeded to yield a reactive allylmetal halide (43), which then reacted with aldehydes, again in an SE2' manner (Scheme 23). The regio- and stereoselectivity of this apparent a-substitution are, however, subject to the experimental conditions.257"9 No such transmetallation was observed with silicon Lewis acids.252

BU3S11

^ s .

Sncis

S11CI4

RCHO

(43) Scheme 23

Nucleophilic allylation by allylstannanes proceeds stereospecifically with cyclic acetals,260 a-alkoxyaldehydes, or 2-methoxyoxazolidines containing stereogenic centers. Using the enhanced ability of a halogen-bearing tin atom to coordinate with Lewis bases, as discussed in Section 8.4.1.2, 1,5-remote asymmetric induction has been attained by neighboring ether substituents.263 The reactions of a-264 and y-alkoxy-substituted117 allylstannanes with aldehydes proceeds similarly through SE2'

Tin

374

reaction to afford stereochemically defined vinyl ethers or 1,2-diol derivatives, respectively, under thermal or Lewis acid conditions. The stereochemistry has been investigated using enantiomerically enriched substrates.265 In contrast to the Lewis acid induced activation of C=O groups, activation of the C=N group can be effected by acid chlorides and this method has been elegantly applied to a one-pot, three-component bicycloannulation to construct a yohimban nucleus (Scheme 24).

SnBu3

H SnBu3 Scheme 24

The Lewis acid induced reaction of 2,4-pentadienyltrimethylstannane with aldehydes is characteristic in that the regiochemistry is dependent upon the (E)/(Z) geometry of the diene moiety.267 Two-stage reaction of a double allylstannane is possible with (44), which can be used as a conjunctive reagent for imines and aldehydes, using the difference in reactivities of the two-stage reaction.268 Allylstannanes undergo conjugate addition to a,p-enones under TMS-OTf (trimethylsilyltriflate) treatment.269

Bu3Sn

SnBu3 (44)

8,6.2.4 Miscellaneous reactions Allylation of the imidazole ring with allylstannanes has been carried out by enhancing the electrophilicity of the ring by quaternization of the nitrogen atom.270 Treatment of allylstannanes with mcpba affords allyl alcohols with 1,3-migration.37 This is in sharp contrast to the mcpba oxidation of allylsilanes, which produces epoxysilanes. The reaction, when combined with SH2-type stannylation of allyl alcohols, constitutes a 1,3-transposition of allyl alcohols. Allylstannanes, which are considered as nucleophiles, react with other nucleophiles in the presence of tin(IV) chloride, which probably functions as an oxidant.271

8.7 MISCELLANEOUS UNSATURATED STANNANES 8.7.1 Allenylstannanes The synthesis of enantio-enriched allenylstannanes can be effected from the (/?)- or (S)-propargylic alcohol (45) by SN2' reaction (Scheme 25). The allenylstannane (46) reacts readily with aldehydes under Lewis acid catalysis to afford optically active alcohol (47) with good syn selectivity.272 Hydrozirconation of allene (48) generates the vinylstannane (49), which reacts with aldehydes and ketones in a highly regio- and stereoselective manner, and subsequent treatment with BF3-OEt2 effects (3-elimination to give terminal 1,3-dienes with high (Zs)-selectivity (Scheme 26).273

Tin OTs

R

Bu3SnLi»CuBfMe2S

R

375 RCHO

>= Bu3Sn

Lewis acid

(47)

(46)

(45)

Scheme 25

OZrClCp2 Cp2Zr(H)Cl

SnBu3

_ _ SnBu3

RCHO

-

BF 3

j^

* R SnBu3

(49)

(48)

Scheme 26

8.7.2 Alkynylstannanes Alkynylstannanes undergo palladium-catalyzed cross-coupling with many substrates in the same way as alkenylstannanes.274 The cross-coupling of alkynylstannanes with the vinylpalladium intermediate (50), formed in situ from alkynes and trimethylsilyl iodide in the presence of a palladium catalyst, gives stereochemically defined enynes (51) (Scheme 27).275

R1 1

R

H

TMS-I

H [Pd]

L2Pd

>=<

R2

SnBu3

TMS

I

R2

(50)

(51)

Scheme 27

Zirconocene hydride adds to alkynylstannanes and the products afford (Z)-alkenylstannanes upon proton quenching.276 When the adducts of dialkylboranes to alkynylstannanes were treated with aryllithiums, tin-lithium exchange proceeded to give boron-stabilized alkenyl carbanions, which underwent boron-Wittig reaction with aldehydes to give allenes.277 Alkynyltrichlorostannanes are prepared directly from a 1-alkyne, tin(IV) chloride, and a base and are efficient alkynylation reagents for aldehydes and a,P-enones. 1,2-Distannylacetylene undergoes Diels-Alder reaction with oxazoles to give bis(stannylfurans), which can be converted into disubstituted furans.279 Alkynylstannanes are subject to electrophilic substitution by highly electrophilic iodonium reagents.280

8.8 FUNCTIONALIZED STANNANES (Sn-(C)B-F) 8.8.1 a-Stannyl Alcohols (n = 1, F = OH) 8.8.1.1 Preparation Still94 found that trialkylstannyllithiums add to aldehydes to produce 1-hydroxyalkylstannanes. Although the free hydroxy compounds are unstable, the alkoxy derivatives are isolable by distillation or chromatographic separation and can be stored for several months. The a-alkoxyalkylstannanes can be prepared from acetals by acetal cleavage with acetyl chloride to a-chloro ethers, followed by displacement of the chlorides by the stannyl group with Bu3SnLi.281

376

Tin

8.8.1.2 Reactions An alkoxy group at the a-position enhances the reactivity of the neighboring C-Sn bond and rapid tin-lithium exchange can be effected upon addition of butyllithium to yield the corresponding a-alkoxyorganolithium compounds. When reacted with an electrophile, such as cyclohexanone, the lithium compounds give the expected 1,2-diols. Significantly, no 1-butylcyclohexanol was detected in the crude product, indicating that the equilibrium in the tin-lithium exchange is far on the side of the 1-alkoxyalkyllithium.94 It was found that both the tin-lithium exchange and trapping of the anion with electrophiles proceed with retention of configuration of the tin-bearing carbon. Addition of the stannyl anion to 1-alkoxyaldehydes proceeds under chelation control to produce syn adducts as major products, particularly in the presence of zinc or copper salts.283 Treatment of the resulting 1-stannyl-1,2-diol derivatives with butyllithium induced selective 1,2-ant/-elimination of stannyl and alkoxy groups to produce vinyl ethers with (E)- stereochemistry. 1-Stannylalkyl allyl ethers (52) undergo stereocontrolled 2,3-Wittig rearrangement to give homoallylic alcohols (53) upon transmetallation (Scheme 28). It was shown that the tin-lithium exchange proceeded with retention of configuration, but the Wittig rearrangement proceeded with inversion of the carbanion carbon.284'285 The reaction has been used for the synthesis of (S)-adamascone.286

BuLi

R (52)

(53) Scheme 28

a-Silyloxystannanes (54), upon transmetallation with butyllithium, undergo silyl migration from oxygen to carbon (reverse Brooke rearrangement) (Scheme 29).287 The reaction is tolerant to several functional groups. . RCHO

.

O-TMS

i, Bu3SnLi

ii,TMS-CN

|

R

OH BuLi

SnBu3

R

TMS

(54) Scheme 29

a-Alkoxyorganocopper(I) compounds are prepared from the corresponding stannanes by successive transmetallations via lithium compounds.288 The cuprates can transfer chiral ligands to triple bonds with retention of configuration to give products that are otherwise accessible only by multistep sequences. MOM-protected a-hydroxyorganocuprates behave as carbonyl ylide equivalents and undergo annulation with cyclic enones (MOM = methoxymethyl). When the hydroxy group is converted into a better leaving group by tosylation, nucleophilic substitution by alkylcopper occurs with the carbon-tin bond left intact.289

8.8.2 P-Stannyl Alcohols (n = 2, F = OH) (3-Stannyl alcohols can be prepared by the reaction of stannylmethyllithiums with carbonyl compounds171 or of stannyllithiums with oxiranes,105 and give alkenes upon acid treatment or by heating.290 The deoxystannylation of alcohol (55) affords allenes.291 Chiral allenes (56) have been prepared by this methodology (Scheme 30). HO

—=^-R

Bu3SnH AIBN

,

(55) Scheme 30

M

R

(56)

377

Tin 8.8.3 y-Stannyl Alcohols (n = 3, F = OH) 8.8.3.1 Preparation

Although y-stannyl alcohols can be prepared in several ways, such as tin hydride addition to allyl alcohols,38 the preparation from (3-stannyl ketones by addition of nucleophiles R3 (including hydride) to the carbonyl group, as shown in Scheme 31, represents a wide range of application292 because antiregulated introduction of various groups R1 is possible with a large choice for R1 and R3. Enantioselective catalytic addition of functionalized dialkylzincs to p-stannylaldehydes affords chiral y-stannyl alcohols.293

o

i, R23SnLi

x>*

R 3 OH R1

R3-Li

I1" or Pb lv

O

ii, R'-X

O

Bu3SnH

R1

Scheme 31

8.8.3.2 Reactions In 1970, Davis reported that y-stannyl tertiary alcohols produced cyclopropanes upon treatment with thionyl chloride or phosphorus trichloride. In subsequent studies it was established that the reaction proceeded with inversion of configuration at both carbon centers. The cyclopropanation is also observed by Lewis acid treatment of y,5-epoxystannanes.120 In 1984, Ochiai292 and Isoe295 independently found that y-stannyl alcohols undergo C-C bond cleavage upon oxidation with iodonium(III) or with lead tetraacetate to produce alkenes (Scheme 31). Notably, the geometry of the double bond is unambiguously determined by the relative configuration of the stannyl group and R1. Evidently the reaction proceeds under rigid stereoelectronic control of the antiperiplanar relationship with respect to the Sn-C bond and the C-C bond cleaved. Although the exact mechanism of the oxidative bond cleavage is unknown, a radical pathway is feasible because the process is induced by radical initiation, as shown in Scheme 31. 2% The reaction has a variety of applications for i • 297 298 synthetic purposes. ' Treatment of 3-hydroxyalkylstannanes with BuLi provides y-lithiated lithium alkoxides. Although the reactivity of the y-position is low because of chelation, it can be enhanced by derivatization of the alcohols as methoxyethoxymethyl (MEM) ethers. The enantiomeric MEM-protected y-lithio alcohols react with various electrophiles to afford polyfunctionalized alcohols in optically active form.293

8.8.4 Acylstannanes (n = 0, F = CO) In contrast to the intensive studies reported on acylsilanes, only little is known about acylstannanes. Although there are several preparative methods, each has its own limitations. Palladium-catalyzed coupling between acyl chlorides and hexaalkylditins has been reported recently.26 Chiral cyclic a-stannylacetals undergo stereoselective ring cleavage upon treatment with 299 organocopper reagents. a-Tributylstannylthioacetals, prepared from the lithium salts of thioacetals with tributyltin chloride, react with silyl enol ethers to afford thioaldols, which are in turn converted stereoselectively into (Z)-P-tributylstannyl-a,P-enones upon thiol elimination by KH treatment.300 The carbon-tin bond in a-tributylstannylthioacetals is oxidatively cleaved by cerium(IV) ammonium nitrate (can) to afford thionium cations, which react with silyl enol ethers to give thioacetals of P-ketoaldehydes.301

Tin

378

As acylstannane derivatives, a-silyloxyvinylstannanes302 and C-alkyl-C-stannylimines303 have been prepared and transmetallated with lithium and used as acyl anion precursors.

8.8.5 a-Stannyl Ketones (n = 1, F = CO) a-Stannyl ketones are in equilibrium with tin enolates (Section 8.4.l.l(ii)). Although highly stable, ethyl tributylstannylacetate (57) reacts as a carbon nucleophile to cleave chiral oxazolidines under Lewis acid catalysis to afford P-amino esters with remarkable stereocontrol (Equation (9)).304 H N RlH

Ph

EtO2C Bu3SnCH2CO2Et (57)

R

Lewis acid

O

Ph ^

I •

(9)

N i

H

OH

8.8.6 p-Stannyl Ketones (n = 2, F = CO) 8,8.6.1 Preparation P-Stannyl ketones are prepared most conveniently by conjugate addition of stannyl anions to ot,penones as discussed in Section 8.3.1.2(iii) The TMS-Cl-induced addition of SnCl2 to a,p-enones affords P-trichlorostannyl ketones, which can be chemoselectively alkylated with Grignard reagents at the tin atom.305

8.8.6.2 Reactions The Lewis acid induced reaction of P-stannyl ketones (59) proceeds via cyclopropanol intermediates (60) (Scheme 32, path I) and affords saturated ketones of type A (61) or type B (62), according to the position of the bond cleavage of the cyclopropanol ring of intermediate (60).124 Generally, the type B reaction becomes predominant when the p-carbon is quatemized and TMS-OTf is used as the Lewis acid. Only when an alkyl group of sufficient migratory aptitude occupies a position antiperiplanar to the stannyl group does 1,2-alkyl migration compete with cyclopropanation to afford an allylic alcohol (58) (path II).306 Since the type B reaction (Scheme 32) involves a carbon skeleton rearrangement, usually with high stereoselectivity and yields, it can be an effective synthetic reaction124 and has been applied to the synthesis of (+)-P~cuparenone.307 O

•,Az R

type A

H (61) ii, RX

R OH

OLA path II

path I

typeB

SnMe3 (58)

(59)

(60)

(62)

Scheme 32

In contrast to acidic activation of the carbonyl group, direct anionic activation of the tin-carbon bond can be realized only when the carbonyl group is suitably deactivated to nucleophilic attack. Deactivation of the carbonyl group is possible by conversion to an amide,308 to a lithium or silyl310 enolate, or to an enamine.

Tin

379

Conjugate addition of tributylstannyllithium to 2-phenylseleno-2-cycloalkenones, followed by trapping of the resulting enolate with allylic halides and subsequent destannylselenylation, gives 2-substituted 2-cycloalkenones in high yields in a one-pot procedure. The destannylselenylation can be performed by F~, Lewis acids, or silica gel as well as by thermal or photochemical treatment.312 When silyl enol ethers of P-stannyl ketones are treated with mild oxidants, such as mcpba, periodic acid, anhydrous iron(III) chloride, or manganese dioxide, the parent a,P~enones are obtained. Clearly, this process represents a method for protection of the enone function and has been applied in a synthesis of (±)-periplanone-B.313 In cases where the bulky trialkylstannyl group exerts an influence on the equilibrium among conformers of the silyl enol ether in a macrocyclic system, the net result of the addition-elimination reaction is cis-trans isomerization of the double bond in the a,p-enone.314 With a stronger oxidant such as CrO3/pyridine, the carbon-tin bond is oxidatively cleaved to produce a ketone. This process constitutes a dialkylative enone transposition, as shown in Scheme 33. R1 i, Me3SnLi

Me

3Sn

R1 0

R^MgX

Me

3Sn

CrCtypy

ii.R'X

Scheme 33

8.8.7 y-Stannyl Ketones (n = 3, F = CO) y-Stannyl ketones are prepared by conjugate addition of a-stannylalkylcuprates to a,p-enones, as discussed in Section 8.3.2.1. TMS-SPh/TiCl4 treatment of y-stannyl ketones proceeds via a possible cyclobutanation, while EtAlCl2 treatment induces a C-C bond cleavage, as shown in Equation (10).315 Notably, such a captodative-type bond cleavage occurs only with assistance of opening of the strained ring (three- or four-membered ring) in silyl compounds,316 while the reaction with stannyl compounds proceeds in the six-membered system.

EtAlCl2

(10)

SnBu3

8.8.8 Other Alkylstannanes Functionalized at the a-Position Chiral a-aminoalkylstannanes can be prepared by (S)-BINAL-H reduction of acylstannanes, followed by Mitsunobu reaction with phthalide. The a-aminoalkylstannanes undergo facile transmetallation with lithium and the enantiomerically enriched a-aminoorganolithiums are configurationally stable at -95 °C and can be trapped with various electrophiles.318 When the lithio compounds are treated with one or one half equivalent of CuCN, they afford lower or higher order cyanocuprates, respectively, which undergo effective conjugate addition to enones.319 2-Azaallylstannanes (64), easily prepared from azides (63) and aldehydes320 or from a-azidostannanes by aza-Wittig reaction,321 can be transmetallated to 2-azaallyl anions, which can serve as 4TC systems to afford pyrrolidine derivatives through [4 + 2]-cycloaddition with alkenes (Scheme 34). a-Diazostannanes react smoothly with triphenylmethyl chloride to afford a nitrilimine as air-stable yellow crystals, which undergoes 1,3-dipolar addition with alkenes to give pyrazoline derivatives.322

SnR3

N-

Ph3P

BuLi

N

SnR3

O

(63)

(64) Scheme 34

N

Tin

380

Electron transfer from a-heteroatom-substituted (O, N, and S) stannanes is facile and chemical323 and electrochemical324'325 oxidation produces a cationic intermediate, which couples easily with heteroatom or carbon nucleophiles. Treatment of diethyl alkylphosphonates (65) with two equivalents of lithium diisopropylamide (LDA) and Bu3SnCl affords the a-lithiostannane (66), which reacts in situ with aldehydes to produce (E)- or (Z)-alkenylphosphonates (67) (Scheme 35).326 The geometry of the product is dependent upon the nature of R1, R2, and R3.

EtO

EtO

OEt

\

i,LDA

R1

EtO

OEt SnR2

o

2

ii, R 3SnCl

R CHO

R1

Li 2

(65)

OEt

3

(67)

(66) R = Bu Scheme 35

8.8.9 Alkylstannanes Functionalized at Remote Positions Macdonald et al.327 observed that the Lewis acid induced reactions of stannyl ketones having the carbonyl group and tin atom separated by more than three carbons proceed by either cyclization or P-hydride shift, depending upon the types of the substrates and the reaction conditions. Thionium cation containing stannanes also undergo hydride shift or cyclization and the balance between the two is sensitive to the class of the tin-bearing carbon and the number of carbons between the tin atom and the cationic center.328 With primary alkylstannanes (68), cyclization predominates when n = 0 or 2, while hydride shift predominates when n = 3 or 4 (Scheme 36). With secondary alkylstannanes (69), however, only hydride shift occurs and there exists a strict selectivity between Ha- and Hb-shifts, depending on the alkyl chain length: when n = 1 or 2, only Ha-shift proceeds, while only Hb-shift proceeds when n = 3 or 4. The high selectivity was explained by assuming a cyclic transition state with substantial rigidity, in which the trimethylstannyl group and the migrating hydrogen atom are antiperiplanar. This has been verified by a 1,4-chirality transfer in the 1,5-hydride shift of a chiral stannane. The Cul-catalyzed 1,4addition of the Grignard (70) to a,(3-enones affords ketone (71), which undergoes 1,5-hydride shift stereospecifically to produce the alcohol (72) on treatment with TiCl4 (Scheme 37). Remarkably, the introduction of two stereogenic centers at the 1- and 3-positions is controlled by the stereochemistry of the 5-position.307 H

H

n = 0, 2 cyclization

n = 3,4

PhS

n

PhS

H-shift

SnR3 (68)

n = 1, 2

PhS

R ~

Ha-shift

« = 3,4

PhS

Hb-shift

PhS

SnMe3 (69) Scheme 36 OH Me3Sn

MgBr (70)

TiCL

Cul

SnMe3 (71)

(72)

Scheme 37

Oximes of |3-stannyl ketones undergo two types of carbon-carbon bond cleavage. Treatment with pyridine/SOCl2 induces a Beckmann fragmentation to give unsaturated nitriles,330 while treatment with

Tin

381

lead tetraacetate affords nitrile oxides through the same type of bond cleavage as that of y-hydroxystannanes.331 Tin-directed regioselective bond cleavage is also observed in the Baeyer-Villiger fragmentation of p-stannyl ketones.330 p-Sulfonylstannanes can be prepared by Michael addition of Bu3SnLi to vinyl sulfones332 or by the reaction of R3SnCH2I with carbanions stabilized by a sulfone or nitrile group.333 Stereospecific desulfonylstannylation with silica gel or fluoride ion affords alkenes. The stereochemistry in the cyclization and hydride shift of several types of stannanes having electrophilic centers has been investigated and it was found that the selectivity was dependent upon the ring size.334

8.9 PHOTOREACTIONS OF ORGANOTIN COMPOUNDS Although the most convenient method for tin radical formation is to use tin hydride as mentioned in Section 8.2, a problem is sometimes encountered in that the intermediate radicals may be quenched by hydrogen abstraction prior to undergoing the desired slow processes such as cyclization or intermolecular additions, due to the high radical-scavenging power of the tin hydride. This drawback can be avoided by using a small amount of hexaalkylditins or polymer-supported ditin336 under UV irradiation. The tin radical thus generated serves as an initiator for the radical chain reaction. Even though light induced, the reactions cannot be strictly classified as real photochemical reactions of organotin compounds, since the light energy is used only to create a tin radical. A genuine type of photoreaction has been observed in the reactions of allylstannanes with dicyanobenzenes, electrondeficient alkenes, and several other electron-deficient substrates.337 An extensive literature survey is given by Mizuno et a/.,337 and the reaction is believed to proceed through a single-electron-transfer mechanism. As discussed in Section 8.6.2.3, the Lewis acid induced thermal reactions of allylstannanes with aldehydes proceed by an SE2'-type mechanism and involve y-attack. On the other hand, the reaction of allylstannanes with aldehydes proceeds by an SE2-type mechanism under UV irradiation to afford products of a-attack.338 The reaction has also been postulated to involve a single-electron transfer from allylstannanes to the triplet state of aldehydes. Photochemical activation of P-stannyl ketones has been realized by carbonyl excitation. The reactivity pattern has been compared with those of the Lewis acid induced reactions.339

8.10 SUMMARY The reaction patterns of organotin compounds can be classified as summarized in Table 1. The most typical reaction is transmetallation with lithium or palladium. Transmetallations with other metals have also been used frequently, but they are usually carried out secondarily via the organolithium compounds. Transmetallation with lithium is used primarily to enhance nucleophilicity, while that with palladium is used to induce reductive elimination to give cross-coupling products. Thus, reactions involving transmetallation are actually those of the corresponding organometals, rather than those of the organotin compounds themselves. The direct reaction of carbon-tin bonds is another important reaction type of organotin compounds. Although intermolecular reactions are mostly confined to those of allylstannanes, intramolecular reactions can be realized with various types of compounds by activation of respective cationic functional groups present in the same molecule in various ways. The characteristic feature of this reaction is that the reaction types can be varied by changing the nature of the cationic centers, the reaction conditions, and the activation methods. Among the many tin reagents described in this chapter, the tin hydrides are the most widely used. Although still less popular, anionic and cationic reagents are also promising as synthetic tools, because various types of organostannanes can be prepared by using these reagents. The organostannanes thus prepared are ideal substrates for performing the intramolecular activation described in the bottom row of Table 1. In view of the anionic or radical nature of the tin-bearing carbon, the cationic reagent (73) can be viewed as an anion cation (74) or a corresponding radical cation equivalent, while the anionic reagent (75) can be viewed as a double anion (76) or a radical anion equivalent (77). The characteristic features of these reagents can be represented more clearly by depicting some of the reactions described in the main text with these synthons as shown in Table 2. These types of reactions are typical of organotin compounds and their chemistry is a developing field embracing great potential for novel synthetic methodology.

Tin

382

Table 1 Reaction patterns of organotin compounds. Reaction type

Reaction scheme

Typical substrate

I Transmetallations Lithium

R-Sn

R-Li

Palladium

R*-Sn

-

R 2 -X

R = allyl, vinyl, a-heteroatom; E = carbonyl, halide

R-E R'-R 2

R*-PdLn

* R2-PdL,,

R2

R = vinyl; R = vinyl, aryl; X = halides, triflate

// Direct reactions Homolytic C-C coupling R2-X

Ri_Sn

R1 = allyl; R 2 = alkyl; X = halide

R»-R2

*

C-C fission SnC.

OH Ox (rS

SnC. •*

O (c\

y-Stannyl alcohol

"* Bond fission

Anionic intermolecular R-Sn

R = allyl; E = carbonyl

R-E

intramolecular (C)n

Various types

E = various electrophiles

Table 2 Reaction patterns of heterolytic organotin reagents. Reagent

Synthon

Reaction in text

2(76) (n = 0)

Scheme 32 type B

Synthon representation of reaction O

R3SnLi (75) (n = 0)

O 2-

CH22~

R3SnCH2Li

Scheme 14

R1COCH2E" + R2O~

(75) (/i = 1)

R R3SnLi + Ox (75) (n = 0)

O-X

R

O-X

(77) {n = 0)

+ X«

O H

H R3Sn

MgBr

(75) (n = 3)

H

Scheme 37 (76) in = 3)

Cl

R-CHO Scheme 18

SnR3 (73) in = 3)

R

Scheme 31

(74) (n = 3)

XHO

Tin

383

8.11 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. 67. 68. 69. 70. 71.

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Comprehensive Organometallic Chemistry II