Catalytic asymmetric deprotonation of a phosphine borane: comparison of two-ligand and one-ligand catalysis

The catalytic asymmetric deprotonation of tert-butyldimethylphosphine borane using s-BuLi or n-BuLi and sub-stoichiometric amounts of (−)-sparteine under one-ligand and two-ligand manifolds has been investigated. Using s-BuLi, slightly higher enantioselectivity was obtained using two-ligand catalysis (use of sub-stoichiometric (−)-sparteine in the presence of a stoichiometric amount of a second achiral ligand) compared to one-ligand catalysis (use of sub-stoichiometric (−)-sparteine only). With n-BuLi, two-ligand catalysis using LiDMAE (DMAE = dimethylaminoethanol) as the stoichiometric ligand was the only method for obtaining good yield and enantioselectivity. In this case, one-ligand catalysis failed as the (−)-sparteine was not turned over.

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(R)-tert-Butylmethylphosphineborane(methyl)-dimethylphenylsilaneC14H28BPSiEr 79:21[α]D = +6.3 (c 1.1, CHCl3)Source of chirality: asymmetric deprotonationAbsolute configuration: (R)

(R)-P-(2,2-Diphenyl-2-hydroxyethyl)-P-methyl-tert-butylphosphineC19H28BPOEr 87:13[α]D = +18.7 (c 1.0, CHCl3)Source of chirality: asymmetric deprotonationAbsolute configuration: (R)