Ring Opening of Vinylcyclopropane Radical Cations via Stereoselective Sigmatropic Shifts

The electron transfer photoreaction of (1R,5R)-(+)-sabinene (4-methylene-1-(1-methylethyl)bicyclo[3.1.0]-hexane; 1) with 1,4-dicyanobenzene and phenanthrene gives rise to (S)-(+)-β-phellandrene (3-methylene-6-(1-methylethyl)cyclohexene; 3), whereas (1S,5S)-(+)-α-thujene (2-methyl-5-(1-methylethyl)bicyclo[3.1.0]hex-2-ene; 2) is converted to (S)-(+)-α-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] sigmatropic shift transfers a hydrogen from C₆ to C₁with inversion of stereochemistry at Ci. For 2^•+, experiments with 2-3-d₁ indicate that a [1,3] sigmatropic shift (from C₆ to C₁) competes with a homo-[1,5] sigmatropic shift (from C2 to C₁); the latter migration is slightly favored. The stereochemical relationship between the chiral educts, 1 and 2, and the ring-opened products, 3 and 7, requires that the radical cations, 1^•+ and 2^•+, retain the chirality of their precursors; they cannot be ring-opened. The rearrangement of 2^•+ takes a significantly different course from the thermal isomerization of its parent, 2, which proceeds via the vinylcyclopropane rearrangement. The stereochemical course established for the rearrangement of 2^•+ is the first case of competing sigmatropic shifts in radical cations.