The chromium photocatalyzed cyclopropanation of diazo reagents with electron rich alkenes is described. The transformation occurs under mild conditions and features specific distinctions from traditional diazo-based cyclopropanations (e.g., avoiding β-hydride elimination, chemoselectivity considerations, etc.). The reaction appears to work most effectively using chromium catalysis, and a number of decorated cyclopropanes can be accessed in generally good yields.

A direct (3+2) cycloaddition between alkenes and vinyl diazo reagents using either Cr or Ru photocatalysis is described. The intermediacy of a radical cation species enables a nucleophilic interception by vinyl diazo compounds, a departure from their traditional electrophilic behavior. A variety of cyclopentenes are synthesized using this method, and experimental insights implicate a direct cycloaddition instead of a cyclopropanation/rearrangement process.

Catalytic asymmetric oxidation-chemistry involving heteroatom transfer from a reagent to a substrate is perhaps unparalleled in synthetic utility for the construction of enantioenriched materials.[1] Conversely, there is a significant deficiency of asymmetric two-electron oxidations that do not involve heteroatom transfer. Some potentially valuable reactions of this type include the oxidation of secondary alcohols and oxidative heterocyclizations (Scheme 1). The design of efficient processes of this nature requires an abundant, inexpensive, and effective stoichiometric oxidant, and a solvent that is amenable to asymmetric catalysis. To begin to address this general synthetic problem, we recently developed a Pd-catalyzed oxidative kinetic resolution of secondary alcohols in toluene that uses molecular oxygen as the terminal oxidant (Scheme 1). [2,3] Herein we demonstrate the utility of this simple system (Pd catalyst, ligand, PhCH 3 , O 2 ) for the construction of a range of heterocycles by catalytic oxidative cyclization. We also demonstrate for the first time that aerobic cyclizations of this type are amenable to asymmetric catalysis, and thereby establish a critical proof of concept for the further development of catalytic asymmetric oxidative cyclizations that use molecular oxygen as the sole stoichiometric oxidant.Palladium-catalyzed bond-forming constructions have become indispensable in organic chemistry.[4] A favorable property of palladium is that it can serve as both a nucleophile (i.e., Pd 0 ) and an electrophile (i.e., Pd II ), which produces many opportunities for catalysis. Although both modes are prevalent, electrophilic oxidative catalysis by Pd II has garnered less attention in the asymmetric arena. Adding to the disparity is the fact that until recently, cocatalysts (e.g., copper salts) or organic oxidants (e.g., benzoquinone) were necessary for the reoxidation of Pd 0 to Pd II , thus creating a nearly intractable situation for asymmetric catalysis. For example, the use of the traditional copper/O 2 reoxidation system introduces a secondary catalytic cycle, while the benzoquinone system requires the removal of stoichiometric quantities of organic compounds at the end of the reaction. In contrast, reactions that proceed under direct dioxygen coupled catalysis produce H 2 O as the sole byproduct. Despite the difficulties of the traditional systems, seminal works by Hosokawa and Murahashi, [5] Hayashi, [6] Sasai, [7] and Bäckvall [8] have established the potential for enantioselective Pd II -catalyzed oxidative cyclizations and dialkoxylations. [9] To the best of our knowledge, however, there were no examples of direct dioxygen-coupled enantioselective Pd II

We observe photocatalyst-based regioselectivity distinctions in Diels–Alder reactions using an electron-poor dienophile. We find that one complex, [Cr­(Ph2phen)3]­(BF4)3, offers significantly higher selectivity than other Cr- and Ru-containing photosensitizers. Computational results, supported by excited-state quenching and reaction quantum yield studies, indicate that similar electron- and energy-transfer pathways are available to all photosensitizers studied, but [Cr­(Ph2phen)3]3+ prefers an energy-transfer pathway that selectively orients the electron-poor alkene for cycloaddition, while an unselective outer-sphere radical-cation/electron-transfer pathway is favored for other photosensitizers.

The photooxidizing capabilities of selected Cr III complexes for promoting radical cation cycloadditions are described. These complexes have sufficiently long-lived excited states to oxidize electron-richalkenes,therebyinitiating [4+ +2] processes.T hese metal species augment the spectrum of catalysts explored in photoredox systems,a st hey feature unique properties that can result in differential reactivity from the more commonly employed ruthenium or iridium catalysts.