Cyclizations in the Addition of Alkylmercury Halides to Dienes and Enynes

Photostimulated additions of tert -butylmercury halides to 1,6-dienes and enynes, including (CH 2 CHCH 2 ) 2 X with X = CH 2, C(COEt) 2, O, or NCH 2 CH CH 2, form the primary alkylmercury halides resulting from 5-exo-cyclizations of the adduct radicals, t -BuCH 2 CH • CH 2 XCH 2 CH CH 2 . The kinetic chain lengths of these free-radical processes are low and in the range of 3−7 at 35 °C in Me 2 SO. 5-Exo-cyclizations with kinetic chain lengths of ∼100 are observed in the photostimulated addition of t -BuHgI to CH 2 CHP(O)(OR)OCH 2 CH CH 2 (R = H or allyl), CH 2 CHC(O)N(R)CH 2 CH CH 2 (R = Me, Ph, or CH 2 CH CH 2 ), or CH 2 CHC(O)OC(Me) 2 CH CH 2 . The radicals t -BuCH 2 CH • C(O)C(Me) 2 (CH 2 ) n CH CH 2 cyclize mainly in the 6-endo mode for n = 1 and more rapidly in the 6-exo mode for n = 2, while the radicals t -BuCH 2 CH • C(O)OC(Me) 2 (CH 2 ) n CH CH 2 cyclize only in the exo mode for n = 0 or 1 but with a faster rate for 5-exo-cyclization. Gem -dimethyl substitution adjacent to the alkyl oxygen accelerates the 5-exo-cyclization of the ester radicals because of rotamer populations but retards their 6-exo-cyclization because of the boatlike transition state required to maintain planarity of the ester function in forming a δ-lactone.