Highly regio- and chemoselective [2+2+2] cycloaddition of electron-deficient diynes with allenes catalyzed by nickel complexes: A novel entry to polysubstituted benzene derivatives

Diynes 1a-c [X(CH(2)Cequivalent toCCO(2)Me)(2): X = (CH2)(2), 1a, X = CH2, 1b and X = O, 1c] undergo [2 + 2 + 2] ene-diyne cycloaddition reactions with a variety of allenes (n-butylallene 2a, phenylallene 2b, (4-chlorophenyl)allene 2c, (4-bromophenyl)allene 2d, (3-methoxyphenyl)allene 2e, 1-naphthylallene 2f, cyclohexylallene 2g and cyclopentylallene 2h) in the presence of Ni(dppe)Br-2 and Zn powder in CH3CN at 80 degreesC for 8 h to give the corresponding polysubstituted benzene derivatives 4a-l in good to excellent yields. Under similar reaction conditions, unsymmetrical diynes 5a-c (HCequivalent toCCH(2)-XCH(2)Cequivalent toCCO(2)Me) react with allenes 2 to afford exclusively the corresponding meta-isomers 6a-g in 73-86% yields. The catalytic reaction is highly regioselective and completely chemoselective. This synthetic method is compatible with many functional groups such as Cl, Br, and OMe on the phenyl group of the allene moiety and an ether linkage in a diyne moiety. In this catalytic reaction, allenes are synthetically equivalent to terminal alkynes. Interestingly, unsymmetrical diyne 7 (MeCequivalent toC(CH2)(4)Cequivalent toCCO(2)Me) undergoes 2:1 cocyclotrimerization with allenes 2a and 2g to afford the corresponding polysubstituted benzene derivatives 9a,b in 87% and 82% yields, respectively. A plausible mechanism involving a nickelacycloheptadiene intermediate is proposed to account for this nickel-catalyzed reaction.