Catalysis hydrostannation

Geminal bis(hydrostannation) is attributed to quenching of the bis-2-tributylstannylethyl radical by the thiol, with polarity reversal catalysis (Equation (16)). 

Hydrostannation of carbonyl compounds with tributyltin hydride is promoted by radical initiation and Lewis or protic acid catalysis.The activation of the carbonyl group by the acidic species allows the weakly nucleophilic tin hydride to react via a polar mechanism. Silica gel was a suitable catalyst allowing chemoselective reduction of carbonyl groups under conditions that left many functional groups unchanged. Tributyltin triflate generated in situ from the tin hydride and triflic acid was a particularly efficient catalyst for the reduction of aldehydes and ketones with tributyltin hydride in benzene or 1,2-di-chloromethane at room temperature. Esters and ketals were not affected under these conditions and certain aldehydes were reduced selectively in preference to ketones. 

Tin hydrides are used extensively for organic functionalization of tin via hydrostan-nation ° . These reactions resemble the corresponding hydrogermylation reactions rather than hydrosilation. Although many hydrostannation reactions occur in the absence of catalysts, reflecting the lower Sn-H bond energy, they are most commonly carried out under free radical catalysis. Some typical hydrogermylation and hydrostannation reactions are illustrated in Table 2 . 

There are a few reports of the catalysis of the hydrostannation of active alkenes by soluble palladium catalysts,105- 106 but, with less active alkenes, the major product is the hexaalkyldistannane. Good yields of hydrostannation products, however, can be obtained with heterogeneous catalysts, and Table 4-2 shows examples of the hydrostannation of alkenes in the presence of a PdjOHVC catalyst.107 A recent, thorough, survey in Chemical Reviews is available.108 The mechanism of the palladium-catalysed hydrostannation is not known in detail, but presumably it involves oxidative addition and insertion of the alkene by stannylmetallation (or hydrometallation) as shown, followed by reductive elimination (Scheme 4-4). 

The tetrakis(triphenylphosphine)palladium - catalysed hydrostannation of hex-3-yn-2-one (298) with trimethylstannane proceeds with high regio- and stereoselectivity to afford 299218 stannylalkenes 301 (R = Bu, Ph, HOCH2CH2 or Me2NCH2) are produced regioselectively from the silicon-tin compound 300 and terminal acetylenes under catalysis by palladiumtetrakis(triphenylphosphine), followed by desilylation with tetrabutylammonium fluoride ... 

Unlike hydrosilation discussed in the preceding subsection, which requires catalysis in essentially all cases, hydrostannation can be observed typically at or above 50 °C under thermal conditions. Under such conditions, however, the reaction tends to be capricious and unpredictable. Thus, for example, the reaction of terminal alkynes with HSnMes, HSnBus, and other triorganylstannanes tends to produce mixtures of a-, cis-f -, and trans-j8-stannyl-substimted alkenes except for some special cases, such as those shown in Scheme 19.W7H49]... 

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... 

---