Addition mechanism, organotin hydrides

Detailed studies on the mechanisms of addition of organotin hydrides to oleflns and acetylenes remain to be made. Certain inferences can be drawn, however, on the basis of information now available. The catalysis by free radical sources demonstrates that the reaction can proceed by a free radical chain mechanism, since only a few mole per cent of initiator is needed in many cases for complete reaction. An attractive reaction sequence is shown in Eqs. (25-28), in which R is a carbon free radical and Sn is a trisubsti-tuted tin radical. Reaction (26) is the chain initiation step and reactions (27) and (28) comprise the propagation steps. Most of the characteristics of the hydrostannation reaction can be accommodated in such a scheme. For example, free radicals attack terminal oleflns at the terminal carbon atom because the resulting secondary or tertiary radical is more stable than the... 

Organotin hydrides add to alkenes and alkynes (equations 22 and 23). Unless strongly electron-withdrawing groups are present in the unsaturated species, the additions have radical-chain mechanisms (Scheme 2). For l-aUcenes, tin usually adds predominantly to the terminal carbon (equations 24 and 25). 

Other Nitrile Complexes. While similar additions of iron (i) and rhodium (11) hydrides to acrylonitrile to form 1-cyanoethylmetal complexes have been reported, 2-cyanoethylmetal complexes also form in certain cases (43, 51). Organotin hydrides may add to acrylonitrile in either direction, depending on the conditions of the reaction (25). Formation of the 2-cyanoalkyltin adduct apparently involves a radical mechanism, whereas a polar mechanism is operative in forming the 1-cyano-alkyl adduct. A four-center transition state was not considered probable in the latter case. 

With a metal hydride as the nucleophile, the organotin hydrides, R SnH4. are formed, which, by addition to an alkene or alkyne (hydrostannation), usually by a radical chain mechanism involving stannyl radicals, RaSn", provide the second way of generating the tin-carbon bond (Scheme 1-2). 

Since triphenyltin hydride tends to interact with benzoyl peroxide to yield triphenyltin benzoate and other products (44) > a peroxide-catalyzed addition of this hydride to olefins is overshadowed by decomposition products. However, polar unsaturated compounds such as acrylonitrile add triphenyltin hydride readily via a supposed ionic mechanism (122,123). The reaction offers a new pathway to functional organotin derivatives of high biocidal activity ... 

Organotin hydrides add to alkenes and alkynes by both polar and radical mechanisms, frequently concurrently, according to the conditions. Such additions when carried out between dienes and dihydrides R2SnH2 lead to polymers incorporating tin in the polymer backbone... 

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