The Hydrostannation of Alkenes and Alkynes

This section deals mainly with the hydrostannation of alkenes, and further details can be found in the appropriate subsequent sections on, for example, stannyl radicals (Section 20.1.3), and stannacycloalkanes (Chapter 10). The hydrostannation of alkynes, allyl compounds, and allenes, is dealt with in Sections 8.1.1 and 10.1. which cover the vinyl-and allyl-tin compounds which are produced. 

A clean separation between the homolytic and heterolytic processes can be achieved with trimethyltin hydride and methyl propiolate.87 In the dark at 50 °C, with acetonitrile as solvent, and galvinoxyl present as a radical inhibitor, only methyl 2-trimethyl-stannylacrylate is formed, but at 60 °C in cyclohexane, with AIBN as a radical initiator, a 4 1 mixture of methyl Z- and E-2-trimethylstannylacrylate is obtained (equation 4-49). 

The reaction of terminal olefins can be catalysed by a small amount of the corresponding trialkylaluminium compound (or of BuSAlII which is alkylated by the olefin).88 The mechanism of these reactions has not been studied in detail, but there appears to be no evidence for the involvement of free radicals, and the steps which are believed to be involved are shown in equations 4-50-4-52 (sn = l/4Sn, al = 1/3Al). 

In the presence of ZrCLt or HfCLt, alkynes react with Bu3SnH or Bu2SnH2 at 0 °C by anti addition.81 Plausible mechanisms involve direct electrophilic hydrostannation via a tin hydride-Lewis acid complex, or initial electrophilic hydrozirconation followed by transmetallation. 

The tin always is introduced at the P-position, and thus is playing the part of a nucleophile rather than an electrophile. No reaction occurs with isophorone, which is a cyclohexenone, and hence the reaction is suggested to involve a cyclic mechanism (equation 4-55). 

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The hydrostannation of alkenes and alkynes can be catalyzed by a number of transition metals (Ni, Pd, Pt, Mo, etc.), but most studies have involved palladium.106 The mechanism that is most commonly accepted is shown for an alkyne in Scheme 1 the model for an alkene is similar. This mechanism accounts for the observations that the reactions normally involve cir-addition, that the R3Sn group enters so as to avoid steric hindrance by the largest substituent group on the alkene or alkyne, and that the regioselectivity depends on the polarization of the palladium-hydrogen bond in the sense Pd -H15-. 

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