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 CH 2 CHCH 2 YCH 2 CN [Y = CH 2 , O, CMe 2 , C(CO 2 Et) 2 , NCH 2 CN] and CH 2 CHCH 2 CH 2 YCH 2 CN [Y = CH 2 , O, C(CO 2 Et) 2 ] in Me 2 SO in the presence of hydriodic acid. Only with Y = C(CO 2 Et) 2 does the adduct radical, t ‐BuCH 2 ĊHCH 2 YCH 2 CN, undergo facile 5‐ exo cyclization in the absence of a proton donor. The other 5‐ exo and all 6‐ exo cyclizations require substrate protonation to yield t ‐BuCH 2 ĊH(CH 2 ) n YCH 2 CNH + ( n  = 1, 2), which cyclizes readily to the iminium radical cation followed by electron transfer with I − or t ‐BuHgI 2 − to form the imine as a precursor to the cyclopentanone or cyclohexanone upon hydrolysis. For CH 2 CHCH 2 C(CO 2 Et) 2 CH 2 CN the formation of the cyclopentanone is dramatically promoted by NH 4 I in the dark in the absence of any other acid. In this case, where cyclization of the adduct radical occurs readily without substrate activation, protonation of the cyclized iminyl radical allows the electron transfer with I − or t ‐BuHgI 2 − to occur with regeneration of t ‐Bu ċ . A similar effect is observed with CH 2 CHCH 2 C(CO 2 Et) 2 CH 2 N 3 where only a slow reaction is observed upon photolysis with t ‐BuHgI in the absence of NH 4 I, although apparently cyclization of t ‐BuCH 2 ĊHCH 2 C(CO 2 Et) 2 CH 2 N 3 (with loss of N 2 ) occurs readily. In the presence of NH 4 I the cyclized aminyl radical can be protonated and the resulting amine radical cation readily reduced by I − or t ‐BuHgI 2 − to continue a chain process. With the thioesters CH 2 CHCH 2 YCH 2 C(O)SPh [Y = O, CH 2 , CMe 2 , C(CO 2 Et) 2 ], significant cyclization upon photolysis with t ‐BuHgX occurred only for Y = C(CO 2 Et) 2 . © 1998 John Wiley & Sons, Ltd.