0022-328X(91)86189-W
Copyright © 1991 Published by Elsevier Science B.V. All rights reserved.

Reversilbility of titanocene-catalyzed hydromagnesation of silylacetylenes

Marina A. Djadchenko and Kasimir K. Pivnitsky^*

Jorg Spanig and Hans Schick

Institute of Experimental Endocrinology of The All-Union Endocrinology Scientific Centre, USSSR Academy of Medical Sciences, ul Moskvorechje 1, Moscow 115522 USSR
Central Institute of Organic Chemistry, Academy of Sciences of the G.D.R. Rudower Chaussee 5, Berlin-O-1199 Germany

Received 14 May 1990.  Available online 21 June 2001.

Abstract

The regioisomeric silylvinylmagnesium compounds A and B, products of titanocene-catalyzed hydromagnesation of the 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 conditions of hydromagnesation, of 'HMgBr' from the vinylmagnesium compounds A/B to other acetylenes with regeneration of the starting acetylenes. Me₃Si(BrMg)CC(H)R (A)Me₃SiCCRMe₃Si(H)CC(MgBr) (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 equilibriation.
 

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