Pyranoside Phosphite-Oxazoline Ligand Library: Highly Efficient Modular P,N Ligands for Palladium-Catalyzed Allylic Substitution Reactions. A Study of the Key Palladium Allyl Intermediates

摘要

We have screened a library of modular phosphite-oxazoline ligands for asymmetric allylic substitution reactions. The library is efficiently prepared from the commercially available and cheap d-glucosamine. The introduction of a phosphite moiety into the ligand design is highly advantageous for the product outcome. Therefore, this ligand library affords good-to-excellent reaction rates [TOFs up to 600 mol substrate x (mol Pd x h)~(-1)] and enantioselec-tivities (ees up to 99%) and, at the same time, shows a broad scope for mono-, di- and trisubstituted linear hindered and unhindered substrates and cyclic substrates. The NMR studies on the palladium allyl intermediates provide a deeper understanding about the effect of the ligand parameters on the origin of enantioselectivity.