Abstract
Enantioselective transition metal catalysis is an area very much at the forefront of contemporary synthetic research. The development of processes that enable the efficient synthesis of enantiopure compounds is of unquestionable importance to chemists working within the many diverse fields of the central science. Traditional approaches to solving this challenge have typically relied on leveraging repulsive steric interactions between chiral ligands and substrates in order to raise the energy of one of the diastereomeric transition states over the other. By contrast, this Review examines an alternative tactic in which a set of attractive noncovalent interactions operating between transition metal ligands and substrates are used to control enantioselectivity. Examples where this creative approach has been successfully applied to render fundamental synthetic processes enantioselective are presented and discussed. In many of the cases examined, the ligand scaffold has been carefully designed to accommodate these attractive interactions, while in others, the importance of the critical interactions was only elucidated in subsequent computational and mechanistic studies. Through an exploration and discussion of recent reports encompassing a wide range of reaction classes, we hope to inspire synthetic chemists to continue to develop asymmetric transformations based on this powerful concept.
Highlights
The necessity for synthetic chemists to be able to access enantiomerically pure material remains as crucial as it has ever been
While the initial challenges faced by the pioneers of this general strategy were significant, over the years, transition metal complexes, typically bearing chiral ligands, have become the central workhorses of asymmetric synthesis.[2−4] Since the early 2000s, organocatalysis has risen quickly as an alternative branch of asymmetric synthesis, which can offer complementary activation strategies and new opportunities for asymmetric induction
The remarkable and diverse reactivity, coupled with the redox capability enjoyed by many transition metals makes them still as valuable as ever for those engaged in the design of new catalytic asymmetric chemical methods
Summary
The necessity for synthetic chemists to be able to access enantiomerically pure material remains as crucial as it has ever been. Reversible nucleophilic displacement by the palladium(0) species could interconvert the mismatched diastereoisomer in a proposed epimerization process to the favored isomer, prior to trapping with the indole.[102] Another noteworthy example of hydrogen-bond-directed enantioselective allylic alkylation was reported by Sawamura, who developed a protocol for the regio- and enantioselective addition of terminal alkynes to allylic phosphates using copper catalysis (Figure 23a).[103] Saturated N-heterocylic carbene (NHC) ligands with a chiral backbone were able to direct the incoming alkyne to give the branched product, forming a stereogenic center. DFT calculations, IR studies, and control experiments suggested that a single hydrogen bond between the urea unit of L17 and the N−H bond in L19
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