CYCLIZATIONS IN THE HOMOLYTIC REACTIONS OF UNSATURATED NITRILES WITH TERT-BUTYLMERCURY HALIDES IN THE PRESENCE OF PROTON DONORS

Cyclizations are observed in the homolytic reactions of t-BuHgI with C H2=CHCH2YCH2CN [Y = CH2, O, CMe2, C(CO2Et)(2), NCH2CN] and CH2=CHCH2CH 2YCH2CN [Y = CH2, O, C(CO2Et)(2)] in Me2SO in the presence of hydriodi c acid. Only with Y = C(CO2Et)(2) does the adduct radical, t-BuCH2(C) over dot HCH2YCH2CN, undergo facile 5-exo cyclization in the absence o f a proton donor. The other 5-exo and all 6-exo cyclizations require s ubstrate protonation to yield t-BuCH2CH(CH2)(n)YCH2C=NH+ (n = 1, 2), w hich cyclizes readily to the iminium radical cation followed by electr on transfer with I- or t-BuHgI2- to form the imine as a precursor to t he cyclopentanone or cyclohexanone upon hydrolysis. For CH2=CHCH2C(CO2 Et)(2)CH2CN the formation of the cyclopentanone is dramatically promot ed by NH4I in the dark in the absence of any other acid. In this case, where cyclization of the adduct radical occurs readily without substr ate activation, protonation of the cyclized iminyl radical allows the electron transfer with I- or t-BuHgI2- to occur with regeneration of t -Bu-.. A similar effect is observed with CH2=CHCH2C(CO2Et)(2)CH2N3 whe re only a slow reaction is observed upon photolysis with t-BuHgI in th e absence of NH4I, although apparently cyclization of t-BuCH2(C) over dot HCH2C(CO2Et)(2)CH2N3 (with loss of N-2) occurs readily. In the pre sence of NH4I the cyclized aminyl radical can be protonated and the re sulting amine radical cation readily reduced by I- or t-BuHgI2- to con tinue a chain process. With the thioesters CH2=CHCH2YCH2C(O)SPh [Y = O , CH2, CMe2, C(CO2Et)(2)] significant cyclization upon photolysis with t-BuHgX occurred only for Y = C(CO2Et)(2). (C) 1998 John Wiley & Sons , Ltd.