Asymmetric activation of chiral BINOL-zirconium catalysts: Effect of a product-like activator

Abstract

The asymmetric activation of binaphthol (BINOL)‐zirconium catalysts is described on the enantioselective allylation of benzaldehyde with allyltin reagents. This model reaction has been used with the intention of establishing a protocol for a zirconium‐based catalyst system, to be followed by structural studies. Initial results are reported that characterise a number of interesting features, namely the influence of an induction period and a high level of asymmetric activation obtained after addition of a product‐like activator. Chirality 10:717–721, 1998. © 1998 Wiley‐Liss, Inc.

BINOL: A Versatile Chiral Reagent

“…Finally, Mikami proposed to use a catalyst prepared from BINOL and Zr(O t -Bu) 4 associated with a product-like activator, ( R )-(+)-α-methyl-2-naphthalene-methanol. Although the ee remained moderate (40−57%), the influence of the activator is clear …”

Modified BINOL Ligands in Asymmetric Catalysis

“…Mikami and co-workers described an asymmetric allylation in which a product-like activator is used to activate chiral BINOL-zirconium catalysts in the reaction of benzaldehyde with allyltin reagents (Scheme 121). 172 Without any activator, in the presence of an extra 1 equiv of (R)-BINOL as an additive, the product is obtained in 44% yield and 27% ee. Addition of the chiral activator (MNM*, Scheme 121) enhances the enantioselectivity of the reaction to 53%, with 35% yield.…”

Symmetry Breaking in Asymmetric Catalysis: Racemic Catalysis to Autocatalysis

“…Asymmetric activation was also observed in the combination of ( R )-BINOL and Zr(O t Bu) 4 -promoted enantioselective synthesis of homoallylic alcohols (Scheme ) . The use of ( R )-BINOL and Zr(O t Bu) 4 in a 2:1 ratio without any other chiral source afforded the homoallylic alcohol product in 27% ee and 44% yield.…”

“…Asymmetric activation was also observed in the combination of (R)-BINOL and Zr(O t Bu) 4 -promoted enantioselective synthesis of homoallylic alcohols (Scheme 13). 36 The use of (R)-BINOL and Zr(O t Bu) 4 in a 2:1 ratio without any other chiral source afforded the homoallylic alcohol product in 27% ee and 44% yield. Since addition of (R)-(+)-R-methyl-2-naphthalenemethanol ((R)-MNM) led to higher ee (53% ee) than those obtained by using only (R)-BINOL (27% ee), (R)-MNM can act as a chiral activator for the reaction.…”