Development of a Highly Enantioselective Catalytic Di-π-methane Rearrangement.

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.

Tailoring Dynamic Hydrogels by Controlling Associative Exchange Rates.

Dithioalkylidenes are a newly-developed class of conjugate acceptors that undergo thiol exchange via an associative mechanism, enabling decoupling of key material properties for sustainability, biomedical, and sensing applications. Here, we show that the exchange rate is highly sensitive to the structure of the acceptor and tunable over four orders of magnitude in aqueous environments. Cyclic acceptors exchange rapidly, from 0.

Asymmetric Photochemical [2 + 2]-Cycloaddition of Acyclic Vinylpyridines through Ternary Complex Formation and an Uncontrolled Sensitization Mechanism.

Stereochemical control of photochemical reactions that occur via triplet energy transfer remains a challenge. Suppressing off-catalyst stereorandom reactivity is difficult for highly reactive open-shell intermediates. Strategies for suppressing racemate-producing, off-catalyst pathways have long focused on formation of ground state, substrate-catalyst chiral complexes that are primed for triplet energy transfer via a photocatalyst in contrast to their off-catalyst counterparts.

Variable Temperature LED-NMR: Rapid Insights into a Photocatalytic Mechanism from Reaction Progress Kinetic Analysis.

A multitude of techniques are available to obtain a useful understanding of photocatalytic mechanisms. The combination of LED illumination with nuclear magnetic resonance spectroscopy (LED-NMR) provides a rapid, convenient means to directly monitor a photocatalytic reaction in situ. Herein, we describe a study of the mechanism of an enantioselective intermolecular [2 + 2] photocycloaddition catalyzed by a chiral Ir photocatalyst using LED-NMR.

Cooperative Stereoinduction in Asymmetric Photocatalysis.

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.

Discovery and Elucidation of Counteranion Dependence in Photoredox Catalysis.

Over the past decade, there has been a renewed interest in the use of transition metal polypyridyl complexes as photoredox catalysts for a variety of innovative synthetic applications. Many derivatives of these complexes are known, and the effect of ligand modifications on their efficacy as photoredox catalysts has been the subject of extensive, systematic investigation. However, the influence of the photocatalyst counteranion has received little attention, despite the fact that these complexes are generally cationic in nature.