Palladium(0) mediated coupling of bromophosphaalkenes with Grignard reagents

Abstract

Attempts to subject the phosphaalkene Mes * P CBr 2 ( 1) (Mes * = supermesityl = 2,4,6-tri- tert-butylphenyl) to a palladium(0) catalyzed Stille-type coupling reaction with phenylmagnesium chloride failed due to elimination of palladium bromide and rearrangement to the phosphaacetylene Mes * C P ( 2). To prevent this undesired reaction, the monobromophosphaalkene Mes * P C(H)Br ( 6) was used. Although both isomers are known, a new method for the synthesis of ( E)-Mes * P C(H)Br (( E)- 6) has been developed and the compound tested in Stille-type coupling reactions with organometallic reagents. Best results were obtained in combination with Grignard reagents; aromatic, olefinic and alkynyl groups could be introduced. Most unexpected was the result when ( Z)-Mes * P C(Br)H (( Z)- 6) was subjected to this coupling reaction: in all cases, isomerization occurred to give ( E)-Mes * P C(H)R ( 7–17) in high yield and purity. This method offers a convenient access to a variety of new functionalized phosphaalkenes with potentially interesting coordinating properties. The mechanism of the coupling reaction appears not to involve the usual oxidative addition step, assumed to occur in the normal Stille coupling. Attempts to elucidate the mechanism are reported and the η 2-palladium complex 19 has been tentatively identified as an intermediate.