Reaction of a terminal phosphinidene complex with azulenes: η¹-Complexes C-H bond insertions, and 1,4-adducts

Reaction of an in situ generated phosphinidene complex [PhPW(CO) ₅] with the aromatic azulene and guaiazulene leads to unexpected 1,4-adducts of the seven-membered ring and to C-H bond insertion of the five-membered ring. A DFT analysis suggests that the reaction is initiated by formation of a η¹-complex between the phosphinidene and the five-membered ring of the aromatic substrate. Four conformations of this complex were identified. Two convert without barrier to the slightly more stable syn- and anti-1,2-adducts. These undergo pericyclic 1,7-sigmatropic rearrangements with remarkably low barriers to give 1,4-adducts, with an inverted configuration at the phosphorus center. An X-ray crystal structure is presented for one of the 1,4-adducts of guaiazulene. The other two η¹-complexes insert with modest barriers into a C-H bond of the five-membered ring.