REVERSIBILITY OF TITANOCENE-CATALYZED HYDROMAGNESATION OF SILYLACETYLENES

The regioisomeric silylvinylmagnesium compounds A and B, products of titanocene-catalyzed hydromagnesation of trimethylsilylacetylenes, are in a rapid dynamic equilibrium with starting acetylene and, therefore, with each other due to the reversibility of the reaction. The existence of this equilibrium is demonstrated by transfer, under the condition of hydromagnesation 'HMgBr' from the vinylmagnesium compounds A/B to other acetylenes with regeneration of the starting acetylenes. Me3Si(BrMg)C = C(H)R (A) reversible Me3SiC = CR reversible Me3Si(H)C = C(MgBr)R (B). The position of the equilibrium between A and B was determined by deuterolysis and found to depend on the substituent, being 77:23 for R = n-C5H11 and 95:5 for R = cyclo-CH2C6H11 in diethyl ether. Reactions of the magnesium reagents A/B with electrophiles produce mixtures of linear (type A) and branched (type B) products. The ratios of regioisomeric products differ depending on the electrophile and the conditions (from 98:2 to 20:80). This is due to the preferential reaction of one of the regioisomers and to rapid equilibration.