Light in Aminocatalysis: The Asymmetric Intermolecular α‐Alkylation of Aldehydes

Dual catalysis: combining photoredox and Lewis base catalysis for direct Mannich reactions

A dual catalytic system combining photoredox and Lewis base catalysis has been developed. By the appropriate choice of light source and catalyst, the photoredox cycle can be optimally modulated to match the base catalyzed reaction cycle to provide the corresponding products under mild reaction conditions.

Enantioselective catalytic alpha-alkylation of aldehydes via an SN1 pathway

Primary aminothiourea derivatives are shown to catalyze enantioselective alkylation of alpha-arylpriopionaldehdyes with diarylbromomethane. Evidence for a stepwise, S(N)1 mechanism in the substitution reaction induced by anion binding to the catalyst is provided by catalyst structure-activity studies, kinetic isotope effects, linear free-energy relationship studies, and competition experiments.

Transition states and origins of 1,4-asymmetric induction in alkylations of 2,2,6-trialkylpiperidine enamines

The asymmetric C-alkylation of chiral enamines derived from terminal epoxides and lithium 2,2,6-trialkylpiperidides has previously been shown to provide alpha-alkylated aldehydes by intermolecular nucleophilic substitution in good levels of asymmetric induction. We now report a computational study of the origins of asymmetric induction in these reactions. Computational modeling with density functional theory (B3LYP/6-31G(d)) agrees closely with the experimental observations. This stereoselectivity is attributed to a preferential conformation of the enamine and the piperidine ring that places the C-6 alkyl substituent in an axial position due to A(1, 3) strain. Preferential attack occurs away from the axial group, for steric reasons. The effects of changing the C-6 substituent from methyl to isopropyl were studied, and twist transition states were found to contribute significantly in the latter alkylations.

Controlling stereoselectivity in the aminocatalytic enantioselective Mannich reaction of aldehydes with in situ generated N-carbamoyl imines

A simple and convenient method for the direct, aminocatalytic, and highly enantioselective Mannich reactions of aldehydes with in situ generated N-carbamoyl imines has been developed. Both alpha-imino esters and aromatic imines serve as suitable electrophilic components. Moreover, the judicious selection of commercially available secondary amine catalysts allows selective access to the desired stereoisomer of the N-tert-butoxycarbonyl (Boc) or N-carbobenzyloxy (Cbz) Mannich adducts, with high control over the syn or anti relative configuration and almost perfect enantioselectivity. Besides the possibility to fully control the stereochemistry of the Mannich reaction, the main advantage of this method lies in the operational simplicity; the highly reactive N-carbamate-protected imines are generated in situ from stable and easily handled alpha-amido sulfones.