Addition of acetylenes to olefins. Oxidative coupling versus [2+2] cycloaddition to a vinylidene intermediate

The reaction of [RuCp (kappa(1)(P),eta(2)-PPh2CH2CH2CH=CH2)( CH3CN)] PF6 (1) with HC equivalent to CPh in the absence of base results in the formation of the eta(4)-butadiene complexes [RUCp(kappa(1)(P),eta(4)-(3Z,5E)-PPh2CH2CH2CH=CH-CH=CHPh)]PF6 (2a) and [RuCp(kappa(1)(P),eta(4)-(3Z)-PPh2CH2CH2CH=CH-CPh=CH2)]PF6 (2b). When the reaction is carried out in the presence of base (NaOEt), in addition to 2a and 2b, the eta(3)-butadienyl complex RuCp(kappa(1)(P),(3,4,5-eta)-PPh2CH2CH2CHCHCCHPh) (2c) is obtained. The C-C coupling reactions take place also with internal alkynes (RC)-C-1 equivalent to CR2 (R-1= R-2 = Ph, Et; R-1=Ph, R-2 = Et) to give the eta 4-butadiene complexes [RuCp(kappa(1)(P),eta(4)-(3Z,5Z)-PPh2CH2CH2CH=CH-CR1 equivalent to CHR2)]PF6 (3-5). In the case of terminal acetylenes two distinct reaction modes are observed proceeding via either metallacyclopentene complexes or the successive intermediacy of vinylidene and metallacyclobutane complexes. With internal alkynes, the metallacyclopentene route is followed. X-ray structures of representative complexes are reported.