1,4-Addition of a phosphinidene complex to cisoid 1,3-dienes

The first genuine 1,4-additions of the terminal phosphinidene complex PhPW(CO)₅ (2) to 1,3-dienes 3 are reported. Two of the 1,4-adducts, 15 and 17, were characterised by X-ray crystal structure determinations. A cisoid conformation, but not coplanarity, of 3 appears to be a prerequisite for the occurrence of a 1,4-addition. The reaction of 2 with the sterically hindered 3B yields 66.5% of the 1,4-addition product 13, the highest percentage of 1,4-addition thus far observed for the reaction of an electrophilic species with a 1,3-diene. The product composition is almost independent of the temperature, implying that the usual preference for the 1,2-mode is mainly entropy controlled. Steric factors appear to be the main cause of the higher tendency towards 1,4-addition of phosphinidines as compared to carbenes.