MECHANISM OF REDUCTIVE ELIMINATION - REACTION OF ALKYLPALLADIUM(II) COMPLEXES WITH TETRAORGANOTIN, ORGANOLITHIUM, AND GRIGNARD-REAGENTS - EVIDENCE FOR PALLADIUM(IV) INTERMEDIACY

Coupling products are obtained in good yields from the reaction of tetraorganotin compounds or Grignard reagents and organohalogenopalladium(II) complexes provided that a benzyl bromide is present. Low yields are obtained in the absence of the benzyl bromides, in which case other decomposition pathways (e.g., α elimination) take place, even in the presence of electron acceptors (e.g., oxygen, m-dinitrobenzene). The first step in the reaction of benzylhalogenobis(triphenylphosphine)- palladium(II) complexes with MeM (M = SnMe3, MgBr) is metathesis of the benzyl ligand rather than the halogen. This unique carbon-for-carbon transmetalation takes place at 25 °C and is facilitated by electron-donating substituents on the benzyl ligand. The products of this reaction subsequently react at higher temperature in the presence of a benzyl bromide to afford ethylbenzene. Optically active chloro-(α-deuteriobenzyl)bis(triphenylphosphine)palladium yields, upon reaction with tetramethyltin in the presence of p-nitrobenzyl bromide, optically activc α-deuterioethylbenzene in which overall retention of configuration at carbon has resulted. m-Dimethylbis(triphenylphosphine)palladiurn(II) reacts with benzyl bromide at 25 °C to afford ethylbenzene and bromomethy!bis(triphenylphosphine)palladium(ll) rather than ethane. When optically active α-deuteriobenzyl bromide is used in this reaction, optically active α-deuterioethylbenzene is formed, and inversion of configuration at carbon takes place. The reductive elimination process is proposed to take place preferentially from a palladium(IV) intermediate with retention of configuration at carbon. © 1979, American Chemical Society. All rights reserved.