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Stereoinduction in complex organic reactions often involves the influence of multiple stereocontrol elements. The interaction among these can often result in the observation of significant cooperative effects that afford different rates and selectivities between the matched and mismatched sets of stereodifferentiating chiral elements. The elucidation of matched/mismatched effects in ground-state chemical reactions was a critically important theme in the maturation of modern stereocontrolled synthesis. The development of robust methods for the control of photochemical reactions, however, is a relatively recent development, and similar cooperative stereocontrolling effects in excited-state enantioselective photoreactions have not previously been documented. Herein, we describe a tandem chiral photocatalyst/Brønsted acid strategy for highly enantioselective [2 + 2] photocycloadditions of vinylpyridines. Importantly, the matched and mismatched chiral catalyst pairs exhibit different reaction rates and enantioselectivities across a range of coupling partners. We observe no evidence of ground-state interactions between the catalysts and conclude that these effects arise from their cooperative behavior in a transient excited-state assembly. These results suggest that similar matched/mismatched effects might be important in other classes of enantioselective dual-catalytic photochemical reactions.

One of the most active current areas of chemical research is centered on how to synthesize handed (chiral) compounds in a selective manner, rather than as mixtures of mirror-image forms (enantiomers) with different three-dimensional structures (stereochemistries). Nature points the way in this endeavor: different enantiomers of a given biomolecule can exhibit dramatically different biological activities, and enzymes have therefore evolved to catalyze reactions with exquisite selectivity for the formation of one enantiomeric form over the other. Drawing inspiration from these natural catalysts, chemists have developed a variety of synthetic small-molecule catalysts that can achieve levels of selectivity approaching, and in some cases matching, those observed in enzymatic reactions., Although the principles underlying asymmetric catalysis with enzymes and small molecules are fundamentally the same (1), some striking and rather surprising differences have been noted. William S. Knowles, a pioneer [...]

The use of the anionic halocuprate(II) complexes as catalysts has enabled the efficient aminohydroxylation of electronically and sterically deactivated styrenes and has also enabled the use of nonstereogenic 3,3-dialkyl oxaziridines as terminal oxidants in the aminohydroxylation reaction. A modified mechanism is proposed that has involved a hemolytic reaction of the olefin with a copper(II)-activated oxaziridine.

The report of a light-activated catalyst that dictates the three-dimensional shape --the stereochemistry--of molecules formed in an organic reaction suggests a new strategy for controlling such reactions using visible light [...]

The di-π-methane (DPM) rearrangement is an important organic photorearrangement that converts 1,4-diene-containing compounds to vinyl cyclopropanes, often resulting in extensive, synthetically valuable restructuring of the substrate's carbon framework. We investigated the influence of Lewis and Brønsted acids on the DPM rearrangement of dibenzobarrelenes. These studies have culminated in the identification of a dual chiral Brønsted acid-iridium photosensitizer system that enables the first highly enantioselective catalytic all-carbon DPM rearrangement.

Catalysis is central to contemporary synthetic chemistry. There has been a recent recognition that the rates of photochemical reactions can be profoundly impacted by the use of Lewis acid catalysts and co-catalysts. Herein, we show that Brønsted acids can also modulate the reactivity of excited-state organic reactions. Brønsted acids dramatically increase the rate of Ru(bpy) 3 2+ -sensitized [2 + 2] photocycloadditions between C -cinnamoyl imidazoles and a range of electron-rich alkene reaction partners. A combination of experimental and computational studies supports a mechanism in which the Brønsted acid co-catalyst accelerates triplet energy transfer from the excited-state [Ru*(bpy) 3 ] 2+ chromophore to the Brønsted acid activated C -cinnamoyl imidazole. Computational evidence further suggests the importance of driving force as well as geometrical reorganization, in which the protonation of the imidazole decreases the reorganization penalty during the energy transfer event., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)