FORMATION OF CYCLOHEXENONES BY OXIDATIVE CYCLIZATION OF ALKENE-FUNCTIONALIZED CARBYNE COMPLEXES

Deprotonation of the complexes (eta5-C5H5)(CO){P(OMe)3}Mo=CR (R = Me o r C-C3H5) with n-BuLi followed by reaction with allyl bromide generate s the carbynes (eta5-C5H5)(CO){P(OMe)3}Mo=CR (1a, R = CH2CH2CH=CH2; 1b , R = C(CH2CH=CH2)CH26H2). Photooxidation of 1a in CDCl3 yields 2-cycl ohexenone (2a). Protonation of 1a with HCI also affords 2a in addition to the eta2-acyl complex (eta5-C5H5){P(OMe)3}Cl2Mo[CO(CH2)3CH=CH2] (3 ), while thermal decomposition in CDCl3 yields 2a and 1,4-pentadiene ( 4a). Deuterium-labeling studies suggest that 2a forms by a mechanism i nvolving initial formation of a cationic carbene complex followed by c yclization via an intramolecular ene reaction. Carbyne 1b forms 2-cycl ohexenone-5-spirocyclopropane (2b) upon decomposition in chloroform, b ut the principal product is the metallacycle (eta5-C5H5)Cl{P(OMe)3}Mo{ CH2CH=C(C3H5)6H(CHO)} (5). Derivatives of la with substituents on the tethered olefin do not form cyclohexenones upon thermal decomposition, yielding instead substituted pentadienes.