RING-OPENING OF VINYLCYCLOPROPANE RADICAL CATIONS VIA STEREOSELECTIVESIGMATROPIC SHIFTS

The electron transfer photoreaction of (1R,5R)-(+)-sabinene 4-methylen e-1-(1-methylethyl)bicyclo[3.1.0]hexane; 1) with 1,4-dicyanobenzene an d phenanthrene gives rise to (S)-(+)-beta-phellandrene (3-methylene-6- (1-methylethyl)cyclohexene; 3), whereas (1S,5S)-(+)-alpha-thujene 2-me thyl-5-(1-methylethyl)bicyclo[3.1.0]hex-2-ene; 2) is converted to (S)- (+)-alpha-phellandrene (2-methyl-5-(1-methylethyl)-1,3-cyclohexadiene; 7). Both products can be rationalized via stereoselective sigmatropic shifts in the corresponding radical cations. For 1(.+), a [1,3] sigma tropic shift transfers a hydrogen from C-6 to C-1 with inversion of st ereochemistry at C-1. For 2(.+), experiments with 2-3-d(1) indicate th at a [1,3] sigmatropic shift (from C-6 to C-1) competes with a homo-[1 ,5] sigmatropic shift (from C-2 to C-1); the latter migration is sligh tly favored. The stereochemical relationship between the chiral educts , 1 and 2, and the ring-opened products, 3 and 7, requires that the ra dical cations, 1(.+) and 2(.+), retain the chirality of their precurso rs; they cannot be ring-opened. The rearrangement of 2(.+) takes a sig nificantly different course from the thermal isomerization of its pare nt, 2, which proceeds via the vinylcyclopropane rearrangement. The ste reochemical course established for the rearrangement of 2(.+) is the f irst case of competing sigmatropic shifts in radical cations.