Chain hydrostannation

Allylstannanes carrying functional substituents (e.g. CN, CChMe, or SC Ar) can be obtained by the radical-chain hydrostannation of the corresponding allyl sulfones in refluxing benzene,22 and l,3-bis(trimethylstannyl)-2-methylenepropane is formed in a similar reaction with the corresponding allylic sulfide.23... 

Results of a chemical activation induced by ultrasound have been reported by Nakamura et al. in the initiation of radical chain reactions with tin radicals [59]. When an aerated solution of R3SnH and an olefin is sonicated at low temperatures (0 to 10 °C), hydroxystannation of the double bond occurs and not the conventional hydrostannation achieved under silent conditions (Scheme 3.10). This point evidences the differences between radical sonochemistry and the classical free radical chemistry. The result was interpreted on the basis of the generation of tin and peroxy radicals in the region of hot cavities, which then undergo synthetic reactions in the bulk liquid phase. These findings also enable the sonochemical synthesis of alkyl hydroperoxides by aerobic reductive oxygenation of alkyl halides [60], and the aerobic catalytic conversion of alkyl halides into alcohols by trialkyltin halides [61]. 

Unlike hydrosilation (Section 8-5), hydrostannation proceeds via radical chains propagated by R3Sn radicals intermediate radicals R3SnCR2CR2 have been characterized. The rates for hydrostannation reactions are greatly increased (100-600 times) under sonication conditions.84... 

In 1962, Kuivila showed that the reaction of trialkyltin hydrides with alkyl halides (hydrostannolysis) (equation 1-6) was a radical chain reaction involving short-lived trialkyltin radicals, R3Sn, 12 and in 1964, Neumann showed that the reaction with non-polar alkenes and alkynes (hydrostannation) (equation 1-7) followed a similar mechanism,13, 14 and these reactions now provide the basis of a number of important organic synthetic methods. 

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). 

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... 

Scheme 15-2 Homolytic chain mechanisms of hydrostannolysis and hydrostannation.

Reduction of the halides with a metal hydride such as lithium aluminium hydride, sodium borohydride, or poly(methylhydrosiloxane) gives the corresponding organotin hydrides These have an important place in organic synthesis for the reduction of halides to hydrides (hydrostannolysis) and the addition to alkenes and alkynes (hydrostannation), by radical chain reactions. Further reactions may intervene between the pairs of reactions shown in Equations (1.1.3) and (1.1.4), and (1.1.4) and (1.1.5), and these reactions are particularly useful for inducing ring-closure reactions. 

Entirely different in concept are several useful methods based on alkynes. Hydrosilylation of alkynes, generally catalyzed by chloroplatinic acid and hydrostannation, generally in a radical chain process, give vinylsilanes and vinylstannanes (Scheme 88). Catalyzed hydrosilylation is stereoselectively syn, but radical chain hydrostannylation is stereoselectively anti only when under kinetic control. Under thermodynamic control, as when there is a small excess of tin hydride present, the reaction is often stereoselectively syn. Alternatively, the silyl or stannyl groups can be introduced using the corresponding cuprate... 

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... 

The increase in activity in the field of organotin hydrides is mostly due to the facility of addition of the tin-hydrogen bond to unsaturated systems and to the multiple mechanistic investigations of the hydrostannation reaction. Hev is a number of organometallic reactions, i.e., addition to transition metal complexes7 condensation with metal alkyls yielding tin-metal derivatives and with metal amides for synthesis of compounds with longer tin chains. 

---