Radical addition to carbenoids. Chain reactions of alpha-diazo carbonyl compounds with triorganotin hydrides, tris(trimethylsilyl)silane and allyltributylstannane

alpha-Diazo ketones RC(O)CH=N-2 react with tributyltin hydride at 60 degrees C in benzene to give the corresponding alpha-stannyl ketones RC(O)CH(2)SnBu(3), which exist in equilibrium with the stannyl enol ether tautomers R(Bu(3)SnO)C=CH2. The reactions are initiated by di-tert-butyl hyponitrite and follow a free-radical chain mechanism, Triphenyltin hydride and tris(trimethylsilyl)silane [(TMS)(3)SiH] react similarly, the latter to yield the alpha-silyl ketone RC(O)CH2Si(TMS)(3) which does not isomerise to the more stable silyl enol ether R[(TMS)(3)SiO]C=CH2 under the reaction conditions. This result indicates that TMS(3)Si reacts at the alpha-carbon atom of the alpha-diazo ketone to give R(CO)CHSiTMS(3), probably via an initial diazenyl radical adduct; triorganotin radicals are assumed to react in the same way, When the group R in the alpha-diazo ketone is but-3-enyl, the intermediate alpha-metalloalkyl radical undergoes 5-exo-cyclisation. Aliyltributylstannane reacts with a-diazo ketones and with ethyl alpha-diazoacetate in refluxing benzene, in the presence of 2,2'-azo(2-methylpropionitrile) as initiator, to give butenyl ketones RC(O)CH2CH2CH=CH2 and ethyl pent-4-enoate, respectively, after a hydrolytic work-up.