Isolation of Cationic η 3 -Allenylnickel(II) Key Intermediate Complexes: Origins of Enantioselectivity and Regioselectivity in Nickel(0)-Catalyzed Asymmetric Propargylic Substitutions.

Herein, we report the synthesis and isolation of cationic η 3 -allenylnickel(II) complexes that bear rac -BINAP as a bidentate ligand for the first time via Me 3 SiOTf-promoted C-O bond cleavage of propargylic tert -butyl carbonate. In contrast, in the presence of the monodentate phosphine ligand PEt 3 , treatment of propargylic tert -butyl carbonate with Ni(cod) 2 resulted in a gradual C-O bond cleavage leading to η 1 -allenylnickel(II) complexes, i.e., trans -(PEt 3 ) 2 Ni(η 1 -CPh═C═CHR)(OBoc). X-ray diffraction and NMR spectroscopy studies of [(η 3 -RCH-CCPh)Ni( rac -BINAP)](OTf) revealed that the complex adopts an η 3 -allenyl coordination mode both in the crystal lattice and in solution. A thorough structural comparison between [(η 3 -RCH-CCPh)Ni( rac -BINAP)](OTf) and palladium and platinum analogues revealed that the η 3 -allenyl moiety in the nickel complex is similar to that observed in palladium and platinum complexes, albeit that each Ni-C bond is shorter than the corresponding Pd-C and Pt-C bonds due to the smaller ionic radius of nickel to that of Pd or Pt. The reactions of either N -methylaniline or sodium N -methylanilide with [(η 3 -RCH-CCPh)Ni(( R )-BINAP)](OTf) furnished ( R )-PhC≡CCH(NMePh)Me as an asymmetric propargylic substitution (APS) product with excellent enantioselectivity. Furthermore, when the nickel-catalyzed APS reaction of propargylic tert -butyl carbonate with N -methylaniline was conducted in DMSO at 60 °C in the presence of 5 mol % of [(η 3 -RCH-CCPh)Ni(( R )-BINAP)](OTf) and 7.5 mol % of sodium N -methylanilide as a catalytic precursor and an additive, respectively, ( R )-PhC≡CCH(NMePh)Me was obtained in 79% yield with 90% ee. The experimental results and computational calculations strongly suggest that the nickel-catalyzed APS reaction might proceed via a cationic η 3 -allenylnickel(II) species as the key reaction intermediate.