Disilane-Catalyzed and Thermally Induced Oligomerizations of Alkynes:  A Comparison

The alkynes RC⋮CR (R = H, Et, Ph), RC⋮CH (R = Me(CH2)5, Me(CH2)7, Ph, Me3Si, EtO2C), and RC⋮CR‘ (R = Ph, R‘ = C6F5; R = Me, R‘ = Ph) trimerize to corresponding benzene derivatives in 30−100% yields in the presence of Si2Cl6 as a procatalyst at 170−200 °C over 20−48 h. These reactions represent only the second example of nonmetal-catalyzed alkyne trimerizations. The unsymmetrical alkynes Me3SiC⋮CH, EtO2CC⋮CH, and PhC⋮CC6F5 gave symmetrical 1,3,5-substituted benzenes, while the others led to isomeric mixtures. A 1:2 molar mixture of PhC⋮CH and PhC⋮CPh provided an isomeric mixture (45% yield) consisting mainly of 1,2,4,5-tetraphenylbenzene. While Si2(OMe)6 also catalyzed alkyne trimerizations (though not as efficiently as Si2Cl6), Si2Me6 did not, suggesting an electronegativity influence in the formation of the Cl3Si• radicals shown to be involved in these reactions. Somewhat unexpectedly, however, neither Si2F6 nor sym-Si2Me2Cl4 catalyzed alkyne trimerizations. Experimental support for the radical pathway proposed for the alkyne trimerization observed herein is presented. In the absence of disilane procatalyst, PhC⋮CH gave an isomeric mixture of dimers, p-MeC6H4C⋮CH afforded predominantly a single dimer, and 1-ethynyl-1-cyclohexene provided exclusively a single dimer, whereas RC⋮CH (R = alkyl) and PhC⋮CMe did not react upon heating under the same conditions.