Catalytic Asymmetric Reactions of Ketimines and Alkenes via [2 + 2] Cycloaddition: Chemical Reactivity Controlled by Switching a Heteroatom

Azetidine units are commonly found in natural products and biologically active drugs. The [2 + 2] cycloaddition of imines and alkenes has been extensively used in the synthesis of such structures, while enantioselective approaches remain elusive. Herein, an efficient B(C6F5)3/chiral phosphoric acid-catalyzed asymmetric [2 + 2] cycloaddition of ketimines and aryl vinyl selenides was presented, delivering valuable chiral azetidines with excellent stereoselectivities (>20:1 dr and up to 96:4 er). What's even more interesting was that when a "Se" atom was switched to an "S" atom, the reaction proceeded through a [2 + 2] cycloaddition/ring-opening cascade process, affording a range of chiral thioacetals with high enantioselectivities (up to 98:2 er), which were also important organic sulfur compounds. Mechanistic experiments, coupled with density functional theory (DFT) calculations, shed light on a mechanism involving stepwise [2 + 2] cycloaddition and ring-opening processes, with the initial alkenylation step identified as crucial for achieving stereoselective control.

Cooperative Hydrogen-Bond-Donor Catalysis with Hydrogen Chloride Enables Highly Enantioselective Prins Cyclization Reactions

Cooperative asymmetric catalysis with hydrogen chloride (HCl) and chiral dual-hydrogen-bond donors (HBDs) is applied successfully to highly enantioselective Prins cyclization reactions of a wide variety of simple alkenyl aldehydes. The optimal chiral catalysts were designed to withstand the strongly acidic reaction conditions and were found to induce rate accelerations of 2 orders of magnitude over reactions catalyzed by HCl alone. We propose that the combination of strong mineral acids and chiral hydrogen-bond-donor catalysts may represent a general strategy for inducing enantioselectivity in reactions that require highly acidic conditions.

N-Triflylphosphoramides: highly acidic catalysts for asymmetric transformations

N-Triflylphosphoramides (NTPA), have become increasingly popular catalysts in the development of enantioselective transformations as they are stronger Brønsted acids than the corresponding phosphoric acids (PA). Their highly acidic, asymmetric active site can activate difficult, unreactive substrates. In this review, we present an account of asymmetric transformations using this type of catalyst that have been reported in the past ten years and we classify these reactions using the enantio-determining step as the key criterion. This compendium of NTPA-catalysed reactions is organised into the following categories: (1) cycloadditions, (2) electrocyclisations, polyene and related cyclisations, (3) addition reactions to imines, (4) electrophilic aromatic substitutions, (5) addition reactions to carbocations, (6) aldol and related reactions, (7) addition reactions to double bonds, and (8) rearrangements and desymmetrisations. We highlight the use of NTPA in total synthesis and suggest mnemonics which account for their enantioselectivity.