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

Abstract

α-Diazo ketones RC(O)CHN₂ react with tributyltin hydride at 60 °C in benzene to give the corresponding α-stannyl ketones RC(O)CH₂SnBu₃, which exist in equilibrium with the stannyl enol ether tautomers R(Bu₃SnO)CCH₂. The reactions are initiated by di-tert-butyl hyponitrite and follow a freeradical chain mechanism. Triphenyltin hydride and tris(trimethylsilyl)silane [(TMS)₃SiH] react similarly, the latter to yield the α-silyl ketone RC(O)CH₂Si(TMS)₃ which does not isomerise to the more stable silyl enol ether R[(TMS)₃SiO]CCH₂ under the reaction conditions. This result indicates that TMS₃Si˙ reacts at the α-carbon atom of the α-diazo ketone to give R(CO)ĊHSiTMS₃, probably via an initial diazenyl radical adduct; triorganotin radicals are assumed to react in the same way. When the group R in the α-diazo ketone is but-3-enyl, the intermediate α-metalloalkyl radical undergoes 5-exo-cyclisation. Allyltributylstannane reacts with α-diazo ketones and with ethyl α-diazoacetate in refluxing benzene, in the presence of 2,2′-azo(2-methylpropionitrile) as initiator, to give butenyl ketones RC(O)CH₂CH₂CHCH₂ and ethyl pent-4-enoate, respectively, after a hydrolytic work-up.