Publication
Chiral cyclic alkyl amino carbenes (CAACs), an intriguing subclass of N-heterocyclic carbenes, hold promising potential in asymmetric catalysis. Yet, synthetic hurdles and a lack of understanding of dominant selectivity-inducing design elements prevent the development of widespread applications. We report the systematic synthesis of a chiral CAAC ligand family featuring a chiral backbone and a total of three independently controllable stereogenic centers. An enantioselective DAP-catalyzed Eschenmoser–Claisen rearrangement selectively delivered both diastereomers of the pivotal methallyl aldehydes with two vicinal stereogenic centers. Condensation with chiral primary amines enabled the selective synthesis of all possible α,β,β-chiral CAAC ligand diastereomers. The corresponding copper complexes provided structural insights and were assessed for their catalytic performance in benchmarking enantioselective 1,4-borylations of α,β-unsaturated esters. One ligand diastereomer displayed synergism of the three stereogenic centers, showing superior enantioselectivities and reactivity at a low catalyst loading. Highlighting the interplay of the stereocenters, the other three diastereomers suffered from various degrees of mismatched effects with poor catalytic outcomes. Crystallographic data analysis provided insights into the role of the ligand's chiral backbone in enhancing selectivity and reactivity. The best performing catalyst was exploited with an array of β,β-disubstituted Michael acceptors including unsaturated esters, thioesters, amides, and ketones, providing enantioselectivities of up to 98:2 er for the addition products. This work highlights the potential of introducing a stereogenic center in the backbone of the CAACs as a strategy to optimize both reactivity and selectivity in enantioselective CAAC-transition-metal catalysis.