Dichlorocarbene addition to [5]metacyclophane; Experimental and calculational evidence for a [1,5] sigmatropic chlorine shift in a bridged cycloheptatriene

In contrast to the terminal phosphinidene complex PhPW(CO)(5) (2), which ad ds to [5]metacyclophane (1) in a 1,4-fashion, dichlorocarbene preferentiall y adds in a 1,2-fashion to the formal "anti-Bredt" type double bond of the aromatic ring of 1 to afford the norcaradiene 11b, which immediately rearra nges to the bridged cycloheptatriene 12b and further by a [1,5] sigmatropic chlorine migration to the isomeric 13b as the first observable product. Mo re slowly, the latter isomerizes via a dissociative mechanism to give 15b. A computational study supports the notion that the [1,5] chlorine migration in the rearrangement 12b --> 13b, for which an activation barrier of 70.2 kJ mol-l was calculated, is essentially concerted with minor charge separat ion. In contrast, the analogous [1,5] chlorine migration in the flat model compound 7,7-dichlorocycloheptatriene (12a) displays features of a dissocia tive pathway.