The photochemical nucleophile–olefin combination, aromatic substitution (photo-NOCAS) reaction. Part 8: methanol–2-carene, and 1,4-dicyanobenzene

Abstract

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-methoxy-1-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 abinitio 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. Keywords: photoinduced electron transfer, radical ions, molecular orbital calculations, bond cleavage, 2-carene.