The addition of alkynes to a tetrasilyldisilene — Evidence for a biradical intermediate

The addition of two newly developed mechanistic probes, (trans,trans-2-methoxy-3-phenylcyclopropyl)ethyne ( 1 ) and (trans,trans-2-methoxy-1-methyl-3-phenylcyclopropyl)ethyne ( 2 ), to tetrakis(tert-butyldimethylsilyl)disilene ( 3 ) has been investigated. The addition of 1 to 3 gave 1-[2-(cis-2-methoxy-3-phenylcyclopropylidene)vinyl]-1,1,2,2-tetrakis(tert-butyldimethylsilyl)disilane ( 5 ) as the major product; whereas addition of alkyne 2 to the disilene gave three stereoisomers of 1,1,2,2-tetrakis(tert-butyldimethylsilyl)-6-methoxy-5-methyl-7-phenyl-1,2-disilacyclohepta-3,4-diene ( 7 – 9 ) and 1,1,2,2- tetrakis(tert-butyldimethylsilyl)-3-(trans,trans-2-methoxy-1-methyl-3-phenylcyclopropyl)-1,2-disilacy-clobut-3-ene ( 10 ) as the major products. The formation of cycloheptaallenes 7 – 9 provides convincing evidence that the addition of alkynes to tetrasilyldisilenes involves the formation of a biradical intermediate. Key words: disilene, alkyne, cycloaddition, reaction mechanism, mechanistic probe.