Regio- and Stereoselective C−C Bond Formation between Alkynes:  Synthesis of Linear Dienynes from Alkynes

Reactions of the iridacyclopentadienes ( 2a ) and ( 2b ) with RC⋮CH (R = C 6 H 5, p -C 6 H 4 CH 3 ) give linear conjugated dienynes (RC⋮CCH CHCH CH 2 ( 3 ) and ( 5 )) and benzene derivatives (RC 6 H 5 ( 4 ) and RC 6 H 3 C 6 H 8 ( 6, 6-aryl-1,2,3,4-tetrahydronaphthalene)). Linear conjugated dienynes 3 and 5 are exclusively obtained from the reactions of alkynyl iridacyclopentadienes ( 7 ) and ( 8 ) (R = C 6 H 5 ( a ), p -C 6 H 4 CH 3 ( b ), cyclohex-1-enyl ( c )) with HOTf, respectively. The di- and trinuclear alkynyl iridacyclopentadienes ( 9a ), ( 9b ), and ( 10 ) (L 3 = (CO)(PPh 3 ) 2 ) react with HOTf to produce the extended and conjugated dienynes p -C 6 H 4 (C⋮CCH CHCH CH 2 ) 2 ( 11 ), ( 12 ), and m,m -C 6 H 3 (C⋮CCH CHCH CH 2 ) 3 ( 13 ), respectively, in high yields. Iridacyclohexadienes ( 17, R‘ = H ( a ), CH 3 ( b )) are obtained from the reactions of 7 with HBF 4 through a C−C bond-forming reaction between the α-carbons of alkynyl and 1,3-butadiene-1,4-diyl ligands. Plausible reaction pathways are suggested for the C−C bond-forming reaction between RC⋮CH and 1,3-butadiene-1,4-diyl ligands of iridacyclopentadienes to produce linear dienynes.