Asymmetric Catalyzed Allylic Substitution Using a Pd/P–S Catalyst Library with Exceptional High Substrate and Nucleophile Versatility: DFT and Pd-π-allyl Key Intermediates Studies

A large library of furanoside phosphite/phosphinite/phosphine-thioether ligands L1–L17a–l has been applied in the Pd-catalyzed allylic substitution reactions of several substrate types using a wide range of nucleophiles. These ligands, which are prepared from inexpensive d-xylose, also incorporate the advantages of the heterodonor, the robustness of the thioether moiety, and the extra control provided by the flexibility of the chiral pocket through the presence of a biaryl phosphite group and a modular sugar backbone. By selecting the ligand components, we have been able to identify catalytic systems that can create new C–C, C–N, and C–O bonds in several substrate types (hindered and unhindered) using a wide range of nucleophiles in high yields and enantioselectivities (ee’s up to >99%). Of particular note are the excellent enantioselectivities obtained in the etherification of linear and cyclic substrates, which represent the first example of successful etherification of both substrate types. The DFT computational study is in agreement with an early transition state. Further studies on the Pd-π-allyl intermediates provided a deep understanding of the effect of ligand structure in the origin of enantioselectivity.