PALLADIUM(0) MEDIATED COUPLING OF BROMOPHOSPHAALKENES WITH GRIGNARD-REAGENTS

Attempts to subject the phosphaalkene MesP = CBr2 (1) (Mes* = superme sityl = 2,4,6-tri-tert-butylphenyl) to a palladium(0) catalyzed Stille -type coupling reaction with phenylmagnesium chloride failed due to el imination of palladium bromide and rearrangement to the phosphaacetyle ne MesC = P (2). To prevent this undesired reaction, the monobromopho sphaalkene MesP = C(H)Br (6) was used. Although both isomers are know n, a new method for the synthesis of (E)-MesP = C(H)Br ((E)-6) has be en developed and the compound tested in Stille-type coupling reactions with organometallic reagents. Best results were obtained in combinati on with Grignard reagents; aromatic, olefinic and alkynyl groups could be introduced. Most unexpected was the result when (Z)-MesP = C(Br)H ((Z)-6) was subjected to this coupling reaction: in all cases, isomer ization occurred to give (E)-MesP = C(H)R (7-17) in high yield and pu rity. This method offers a convenient access to a variety of new funct ionalized phosphaalkenes with potentially interesting coordinating pro perties. The mechanism of the coupling reaction appears not to involve the usual oxidative addition step, assumed to occur in the normal Sti lle coupling. Attempts to elucidate the mechanism are reported and the eta(12)-palladium complex 19 has been tentatively identified as an in termediate.