Why is copper(I) complex more competent than dirhodium(II) complex in catalytic asymmetric O-H insertion reactions? A computational study of the metal carbenoid O-H insertion into water.J Am Chem Soc. 2009 Dec 16; 131(49):17783-5.JA
The asymmetric O-H insertion reaction is an ideal synthetic strategy for preparing optically pure alpha-alkoxy, alpha-aryloxy, and alpha-hydroxy carboxylic acid derivatives, which are valuable building blocks for the construction of natural products and other biologically active molecules. Surprisingly, to date there have been no reports of significant levels of enantiocontrol in the O-H insertions using chiral dirhodium(II) catalysts, which are powerful for asymmetric C-H insertions. Only recently, through the use of chiral copper catalysts, have highly enantioselective insertions of alpha-diazocarbonyl compounds into O-H bonds been achieved. To explain these interesting phenomena, density functional theory calculations have been conducted. The results show that in the Cu(I)-catalyzed system, the [1,2]-H shift process (the stereocenter formation step) favors the copper-associated ylide pathway. This ensures that when a chiral copper complex is used as the catalyst, the stereocenter forms in a chiral environment, which is the prerequisite for achieving enantioselectivity. In contrast, the free-ylide pathway is favored in the Rh(II)-catalyzed system. This significant difference renders the copper(I) complexes more competent than the dirhodium(II) complexes in catalytic asymmetric O-H insertions. In addition, it has been found for the first time that in transition-metal-catalyzed X-H insertions, water acts as an efficient proton-transport catalyst for the [1,2]-H shift.