Synthesis of cross-conjugated olefins from alkynes: Regioselective C-C bond formation between alkynes

Reactions of cis-dihydrido complex [Ir(H)(2)(NCCH3)(2)(PPh3)(2)](+) (1) with HCdropCH and RCdropCH (R = C6H5, p-C6H4CH3, cyclohex-1-enyl, C(CH3)=CH2, C(CH3)(3)) produce cross-conjugated hexatrienes (RCH=C(CH=CH2)(2), R-HEX) and octatetraenes (H2C=CH-CR=CH-C(-CH=CH2)=CHR, R-2-OCT). Two molecules of HCdropCH are inserted into Ir-H bonds of 1 to give cis-bis(ethenyl) complex [Ir(CH=CH2)(2)(NCCH3)(2)(PPh3)(2)](+) (2), which reacts with RCdropCD to produce both R-HEX-d(1) (RCD=C(CH=CH2)(2)) and R-2-OCT-d(2) (H2C=CH-CR=CD-C(-CH=CH2)=CDR). R-HEX are exclusively obtained from the reactions of RCdropCH with [Ir(CH=CH2)(2)(CO)(2)(PPh3)(2)](+) (3). Alkynyl cis-bis(ethenyl) complexes Ir(CH=CH2)(2)(C=CR)(CO)(PPh3)(2) (4, R = C6H5 (a), p-C6H4CH3 (b), cyclohex-1-enyl (c)) react with D+ to give eta(4)-R-HEX-d(1) complexes [Ir(eta(4)-RCD=C(CH=CH2)(2))(CO)(PPh3)(2)](+) (5-d(1)) from which R-HEX-d(1) are obtained in the presence of a base. Di- and trinuclear alkynyl-bis(alkenyl) complexes [L5Ir-CdropC-p-C6H4-CdropC-IrL5] (7, L-5 = (-CH=CH2)(2)(CO)(PPh3)(2)) and [L5Ir-CdropC-m,m-C6H3-(CdropC-IrL5)(2)] (8, L-5 = (-CH=CH2)(2)(CO)(PPh3)(2)) react with H+ to produce extended cross-conjugated olefins, p-C6H4-(HEX)(2) and m,m-C6H3-(HEX)(3), respectively, in high yields. Plausible reaction pathways involve alkenyl-vinylidene complexes that undergo the C-C bond formation reaction between the two hydrocarbyl ligands to produce the cross-conjugated olefins.