Isolation of Cationic η³-Allenylnickel(II) Key Intermediate Complexes: Origins of Enantioselectivity and Regioselectivity in Nickel(0)-Catalyzed Asymmetric Propargylic Substitutions

Abstract

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.

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