Trimethylstannyllithium

Me3SnLi

[17946-71-3]  · C3H9LiSn  · Trimethylstannyllithium  · (MW 170.77)

(synthesis of allylstannanes,1 vinylstannanes,2 acylstannanes,3 a-hydroxyalkylstannanes,4 and b-stannyl ketones5)

Alternate Name: trimethyltinlithium.

Preparative Methods: prepared by the reaction of Hexamethyldistannane and Methyllithium, Chlorotrimethylstannane and Lithium metal, or by deprotonation of Trimethylstannane.

Handling, Storage, and Precautions: should be used immediately after preparation. For some applications the purity of the starting material is critical. Trimethylstannane should be distilled immediately prior to deprotonation. Sensitive to air and moisture. Volatile organostannanes are toxic and should be used always in a well ventilated fume hood. Contact with eyes and skin should be avoided.

General Discussion.

Trimethyltinlithium is prepared by the reaction of trimethyltin chloride and lithium6 or by the deprotonation of trimethylstannane using Lithium Diisopropylamide.5,7 In each case the generation of the reagent is a complicated reaction process, leading to several other reactive species in addition to trimethyltinlithium itself (eq 1).8

The reaction of trimethyltinlithium with alkyl halides has been the subject of intensive mechanistic study. There are three potential reaction mechanisms for the displacement of an alkyl halide with trimethyltinlithium: direct SN2 displacement, initial electron transfer leading to the formation of radical intermediates, or initial metal-halogen exchange leading to a carbanion intermediate.9 Several additives have been explored as a means to delineate radical versus carbanionic pathways: however, the influence of additives such as dicyclohexylphosphine or t-butylamine on the mechanism of the reaction has been called into question.9 Evidence for radical pathways has been obtained by several studies using internal radical traps such as 6-bromohexene,10 6-bromoheptene (eq 2),11 and cyclopropylmethyl bromide.12 Several studies have used stereochemical probes to determine the possible mechanisms for trimethyltinlithium displacement of alkyl halides.13 Displacement of a nonracemic secondary alkyl bromide or tosylate also results in a nonstereoselective reaction for the alkyl bromide (~40% inversion) and a stereoselective (100% inversion) process for the tosylate.14 4-Methyl- or 4-t-butylcyclohexyl bromide and tosylate have been examined as substrates for trimethyltinlithium reactions.15 In each of these studies the cyclohexyl bromides undergo substitution with loss of stereochemistry, implying a radical process (eq 3). The corresponding tosylates are displaced with complete inversion of stereochemistry by an SN2 process (eq 4). Radical chain processes are also implicated in the reaction of trimethyltinlithium and 1-bromo-4-iodobicyclo[2.2.2]octane.16 In a related study using 1,4-dihalobicyclo[2.2.1]heptanes, trimethyltinlithium substitution by an SRN1-type mechanism has been postulated; however, a polar mechanism involving formation of a carbanion effectively competes with the chain process.17

Trimethyltinlithium can be used for the synthesis of allylstannanes by the displacement of allyl halides,1 and in the synthesis of 1-trimethylstannyl-2,4-dienes by reaction with 5-tetrahydropyranoxy-1,3-dienes (eq 5).18 Direct displacement of vinyl halides can also be realized, providing access to 1,1-distannyl-1-alkenes.2 Acylstannanes are obtained by the reaction of trimethyltinlithium and ethyl esters or thiophenyl esters in the presence of Boron Trifluoride Etherate.3 Direct displacement of a-chloro boronates can be accomplished leading to a-trimethylstannyl boronate esters with inversion of configuration.19 The trimethylstannyl-substituted boronate thus obtained can undergo transmetalation with an additional equivalent of trimethyltinlithium to provide the a-lithio boronate ester. Silylation of the alkoxide intermediate obtained by addition of trimethyltinlithium to carbonyl compounds provides a-silyloxystannanes which are useful precursors for the synthesis of (a-alkoxyalkyl)trialkylsilanes.4 (a-Alkoxyalkyl)stannanes and (a-alkoxy)trialkylsilanes can be employed in the stereoselective synthesis of acyclic ethers by means of a directed Mukaiyama aldol reaction.20

Trimethyltinlithium undergoes conjugate addition reactions with enones in THF or THF/NH3.5,21 This approach has been used in the synthesis of several potential chiral Lewis acids derived from conjugate addition of trimethyltinlithium to verbenone, carvone, and pinene. Subsequent regioselective cleavage of one of the methyl groups from the trimethyltin moiety is possible using Tin(IV) Chloride or Mercury(II) Chloride (eq 6).21

Oxidative cleavage of the trimethyltin moiety to give an alcohol has been accomplished with retention of configuration.22 Initial 1,4-addition of trimethyltinlithium to cyclohexenone, followed by alkylation of the enolate intermediate and reduction of the ketone, provides a b-hydroxy trimethylstannyl-substituted cyclohexane ring. Subsequent reaction with Bromine and m-Chloroperbenzoic Acid results in cleavage of the trimethyltin moiety to give the cyclohexane diol (eq 7).

The 1,4-addition product obtained by the reaction of trimethyltinlithium and cyclohexenone has also provided a substrate for the study of the g-effect of tin in solvolysis reactions.23 Providing that the Sn-C-C-C-X bonds adopt a planar W orientation, the tin substituent serves to enhance solvolysis through an extended hyperconjugation effect. Oxidative fragmentation of b-hydroxy trimethylstannyl-substituted cyclohexanes by Lead(II) Acetate has also been employed in the stereospecific synthesis of (E)- and (Z)-keto alkenes (eq 8).24 1,4-Addition of trimethyltinlithium to cyclohexenone, followed by reduction of the ketone and oxidative fragmentation, has resulted in the synthesis of exo- and endo-brevicomin. Treatment of b-trimethylstannylcyclohexanones with Lewis acids results in the destannylated cyclohexanone or in a ring-contracted cyclopentanone through the intermediacy of a cyclopropanol (eq 9).25 If the ketone is first transformed into an epoxide, Lewis acid treatment results in the synthesis of cyclopropane products (eq 10).26,27 The Lewis acid-catalyzed reaction proceeds through an ionic intermediate since there is no stereochemical restriction on the reaction. Cyclopropane formation can also be induced by protic acid treatment; however, a concerted 1,3-elimination reaction occurs which places a stereoelectronic restriction on the reaction. Only stannyl epoxides which can adopt a planar W configuration of the Sn-C-C-C-OH bonds can undergo 1,3-elimination. Other conformationally restricted isomers undergo a 1,2-shift.27 Trimethyltinlithium can undergo transmetalation to a variety of copper species which have been extensively studied by NMR.28

Related Reagents.

Tri-n-butylstannyllithium.


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20. Linderman, R. J.; Anklekar, T. V. JOC 1992, 57, 5078.
21. Krishnamurti, R.; Kuivila, H. G. JOC 1986, 51, 4947.
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24. O'Shea, M. G.; Kitching, W. T 1989, 45, 1177.
25. Sato, T.; Watanabe, T.; Hayata, T.; Tsukui, T. T 1989, 45, 6401.
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27. Plamondon, L.; Wuest, J. D. JOC 1991, 56, 2076.
28. (a) Sharma, S.; Oehlschlager, A. C. JOC 1991, 56, 770. (b) Sharma, S.; Oehlschlager, A. C. T 1991, 47, 1177.

Russell J. Linderman

North Carolina State University, Raleigh, NC, USA



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