Alkylations of Heterocycles by Alkylmercurials

Abstract

Alkylmetcurials can participate in free radical chain reactions in which the alkyl group substitutes for hydrogen or halogen at a heterocyclic vinyl or aromatic carbon atom, adds in a conjugate manner to o,B-unsaturated lactones or lactams, or undergoes addition-elimination ( ~~~l i z a t i m ) reactions leading to a new heterocyclic ring.Alkylmercurials (RHgX, R2Hg) are an excellent source of alkyl radicals upon photolyeis or thernolysib.Electron transfer to an alkylmercury h a ~i d e , ~. ~ or attack of an electrophilic radical upon the mercury also generates the alkyl radical (Reactions 1 and 2).These reactions appear to be concerted processes whose rates increase with the stability of the incipient R. (I-Bu > 2 P r > n-~u).576Chain reactions can be achieved if a donor (D.) or acceptor (A') radical can be formed by further reactions of an alkyl radical which is itself not a strong donor or acceptor species.Some examples of D' end A' radicals which have been observed to participate in Reactions 1 or Z are:'-' mnor Species: RNO2'-; R C M ~~~( O -) P ~; BP(Ph)2.-;R C H ~~H ( C ~H ~O M ~-~) ~; pyridinyl' Acceptor Species: PhS02'; RS' , PhSe.; PhTe' ; halogen atoms; I-BuO'; SO4'-; RCH~~H(EWG), EWG -(Et0)2P0, PhS02, L -O ~N C ~H ~, Ph3Si, CH20P(0)(OEt)2.CH2OZCR, CH20SiMe3, CH20~~eZf; RCH-t~, Y -~h , C O ~E ~, COP^, CH~OTS, CH~OP(O)(OE~)~ Evidence for Reaction 2 is the observation of strong CIDNP signals in the A/E mltiplet mode for RH, R(-H) and R-R when a ~olution of RHgCl (R = I * , Z ' , 3'-alkyl) and (NH4)2S208 is heated in d6-Me2S0 at 80'C (Scheme I).Reaction of Me3CC02H with ~~0 ~-~ gives a CIDNP signal for Me2C-CH2, but completely in the emission mode from the diffusive encounter of I-Bu' and SO4'-.t Dedicated to Sir Oerek Barton on the occasion of his 70th birthday.