THE PHOTOCHEMICAL NUCLEOPHILE-OLEFIN COMBINATION, AROMATIC-SUBSTITUTION (PHOTO-NOCAS) REACTION .8. METHANOL-2-CARENE, AND 1,4-DICYANOBENZENE

The structure and reactivity of the radical cation of(+)-2-carene ((1S,6R)-3,7,7-trimethyl-cis-bicyclo[4.1.0]hept-2-ene (3)) have been studied. The radical cation was generated by photoinduced single electron transfer to the first electronically excited singlet state of 1,4-dicyanobenzene in acetonitrile-methanol (3: 1). The 1:1:1 (methanol:2-carene: 1,4-dicyanobenzene) adducts were formed: trans-3-(4-cyanophenyl)-4-(1-methoxyl-methylethyl)-1-methylcyclohexene (14), and cis- (15) and trans-3-(4-cyanophenyl)-6-(1-methoxy-1-methylethyl)-3-methylcyclohexene (16) in a combined yield of 80%. The efficiency of the reaction and the yield of products were increased by the addition of biphenyl, serving as a codonor. These photo-NOCAS adducts formally result from cleavage of the three-membered ring of the 2-carene radical cation, at the C1-C7 bond, forming the tertiary carbocation and allylic radical. Reaction of the cation with methanol and coupling of the allylic radical with the 1,4-dicyanobenzene radical anion at the ipso position, followed by loss of cyanide ion, completes the sequence. There was no evidence for cleavage of the C1-C6 bond under these conditions; however, when the irradiation was carried out in acetonitrile (no methanol) the (+)-2-carene was partially racemized. Racemization is indicative of C1-C6 bond cleavage. The results of ab initio molecular orbital calculations (STO-3G) provide insight into the extent of C1-C7 bond cleavage in the radical cation. The calculated spin and charge distribution, on the 2-carene radical cation global minimum (3a(+.)), is consistent with the observed regiospecificity of adduct formation.