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 = C6H5, p-C6H4CH3) give linear conjugated dienynes (RC⋮CCHCHCHCH2 (3) and (5)) and benzene derivatives (RC6H5 (4) and RC6H3C6H8 (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 = C6H5 (a), p-C6H4CH3 (b), cyclohex-1-enyl (c)) with HOTf, respectively. The di- and trinuclear alkynyl iridacyclopentadienes (9a), (9b), and (10) (L3 = (CO)(PPh3)2) react with HOTf to produce the extended and conjugated dienynes p-C6H4(C⋮CCHCHCHCH2)2 (11), (12), and m,m-C6H3(C⋮CCHCHCHCH2)3 (13), respectively, in high yields. Iridacyclohexadienes (17, R‘ = H (a), CH3 (b)) are obtained from the reactions of 7 with HBF4 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.