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 HC⋮CH and RC⋮CH (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, R2-OCT). Two molecules of HC⋮CH 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 RC⋮CD to
produce both R-HEX-d1 (RCDC(CHCH2)2) and R2-OCT-d2 (H2CCH−CRCD−C(−CHCH2)CDR). R-HEX are exclusively obtained from the reactions of RC⋮CH 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 η4-R-HEX-d1
complexes [Ir(η4-RCDC(CHCH2)2)(CO)(PPh3)2]+ (5-d1) from which R-HEX-d1 are obtained
in the presence of a base. Di- and trinuclear alkynyl−bis(alkenyl) complexes [L5Ir−C⋮C-p-C6H4-C⋮C−IrL5] (7, L5 = (−CHCH2)2(CO)(PPh3)2) and [L5Ir−C⋮C-m,m-C6H3-(C⋮C−IrL5)2] (8, L5 = (−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.