Alkenyl-tin Compounds

The most common routes to alkenyltin compounds involve reaction between an alkenyl-metallic compound and a tin halide, or the hydrostannation of an alkyne (see Section 4.4). In recent years, the stannylmetallation of alkynes has become increasingly important. Direct hydrogenation of an alkynylstannane to a vinylstannane does not appear to be practicable, presumably because the catalyst is poisoned by the tin compound, but cis addition of dihydrogen can be achieved by hydrozirconation followed by hydrolysis,1 or in some cases, by cyclotitanation followed by hydrolysis2 (Section 8.2.2 below). 

Some examples of the first route are shown in equations 8-1,3 8-2,4 8-3,5 and 8-4.6 The formation of vinylstannanes, RCH=CHSnR 3, from the reaction between RCH=0 and R 3SnCHBr2 in the presence of CrCI2 and Lil (equation 6-6) is thought to involve the initial formation of the R 3SnCH(Crm)2 derivative which then adds to the carbonyl group, followed by elimination of an 0(Crm)2 species.7 

Organotin Chemistry, Second Edition. Alwyn G. Davies Copyright 2004 Wiley-VCH Verlag GmbH Co. KGaA. ISBN 3-527-31023-1 

The general principles of hydrostannation are described in Section 4.4. The most common tin hydride to be used is tributyltin hydride, which is commercially available. Al-kynes are more reactive than alkenes. With non-polar alkynes, the reaction follows a radical chain mechanism, and is typically carried out at 60-80 °C, often with AIBN as a radical initiator, but reactions can be carried out at room temperature or below with initiation with UV light or sonication,8 or with a trialkylborane in the presence of a trace of air.9 

The Z-product is formed initially, probably because the tin hydride donates its hydrogen at the less hindered face of the vinyl radical, but as the reaction progresses, this Z-adduct is partially rearranged to the -adducl by reversible addition/elimination of a tin radical, particularly at elevated temperatures or prolonged reaction times (equation 8-5). The variation of product composition with reaction conditions is illustrated in equation 8-6. 

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