Selective Reductive Coupling of Vinyl Azaarenes and Alkynes via Photoredox Cobalt Dual Catalysis.

A visible light-induced Co-catalyzed highly regio- and stereoselective reductive coupling of vinyl azaarenes and alkynes has been developed. Notably, Hünig's base together with simple ethanol has been successfully applied as the hydrogen sources instead of commonly used Hantzsch esters in this catalytic photoredox reaction. This approach has considerable advantages for the straightforward synthesis of stereodefined multiple substituted alkenes bearing an azaarene motif, such as excellent regioselectivity (>20 : 1 for >30 examples) and stereoselectivity (>20 : 1 E/Z), broad substrate scope and good functional group compatibility under mild reaction conditions, which has been utilized in the concise synthesis of natural product monomorine I. A reasonable catalytic reaction pathway involving protolysis of the cobaltacyclopentene intermediate has been proposed based on the mechanistic studies.

Boryl Radical Activation of Benzylic C-OH Bond: Cross-Electrophile Coupling of Free Alcohols and CO2 via Photoredox Catalysis.

A new strategy for the direct cleavage of the C(sp3)-OH bond has been developed via activation of free alcohols with neutral diphenyl boryl radical generated from sodium tetraphenylborate under mild visible light photoredox conditions. This strategy has been verified by cross-electrophile coupling of free alcohols and carbon dioxide for the synthesis of carboxylic acids. Direct transformation of a range of primary, secondary, and tertiary benzyl alcohols to acids has been achieved. Control experiments and computational studies indicate that activation of alcohols with neutral boryl radical undergoes homolysis of the C(sp3)-OH bond, generating alkyl radicals. After reducing the alkyl radical into carbon anion under photoredox conditions, the following carboxylation with CO2 affords the coupling product.

Transition-Metal-Free Reductive Deoxygenative Olefination with CO2.

A new transition-metal-free reductive deoxygenative olefination of phosphorus ylides with CO2, an abundant and sustainable C1 chemical feedstock, is described. This catalytic CO2 fixation afforded ß-unsubstituted acrylates and vinyl ketones in good yields with broad scope and good functional group tolerance under mild reaction conditions. Cost-effective and easily handled polymethylhydrosiloxane was used as a reductant. Bis(silyl)acetal was proved to be the key intermediate in this reductive functionalization of CO2.