Synthesis of Cross-Conjugated Olefins from Alkynes:  Regioselective C−C Bond Formation between Alkynes

Reactions of cis -dihydrido complex [Ir(H) 2 (NCCH 3 ) 2 (PPh 3 ) 2 ] + ( 1 ) with HC⋮CH and RC⋮CH (R = C 6 H 5, p -C 6 H 4 CH 3, cyclohex-1-enyl, C(CH 3 ) CH 2, C(CH 3 ) 3 ) produce cross-conjugated hexatrienes (RCH C(CH CH 2 ) 2, R- HEX ) and octatetraenes (H 2 C CH−CR CH−C(−CH CH 2 ) CHR, R 2 - OCT ). Two molecules of HC⋮CH are inserted into Ir−H bonds of 1 to give cis -bis(ethenyl) complex [Ir(CH CH 2 ) 2 (NCCH 3 ) 2 (PPh 3 ) 2 ] + ( 2 ), which reacts with RC⋮CD to produce both R- HEX - d 1 (RCD C(CH CH 2 ) 2 ) and R 2 - OCT - d 2 (H 2 C CH−CR CD−C(−CH CH 2 ) CDR). R- HEX are exclusively obtained from the reactions of RC⋮CH with [Ir(CH CH 2 ) 2 (CO) 2 (PPh 3 ) 2 ] + ( 3 ). Alkynyl cis -bis(ethenyl) complexes Ir(CH CH 2 ) 2 (C⋮CR)(CO)(PPh 3 ) 2 ( 4, R = C 6 H 5 ( a ), p -C 6 H 4 CH 3 ( b ), cyclohex-1-enyl ( c )) react with D + to give η 4 -R- HEX - d 1 complexes [Ir(η 4 -RC D C(CH CH 2 ) 2 )(CO)(PPh 3 ) 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 [L 5 Ir−C⋮C- p -C 6 H 4 -C⋮C−IrL 5 ] ( 7, L 5 = (−CH CH 2 ) 2 (CO)(PPh 3 ) 2 ) and [L 5 Ir−C⋮C- m,m -C 6 H 3 -(C⋮C−IrL 5 ) 2 ] ( 8, L 5 = (−CH CH 2 ) 2 (CO)(PPh 3 ) 2 ) react with H + to produce extended cross-conjugated olefins, p -C 6 H 4 -( HEX ) 2 and m,m -C 6 H 3 -( 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.