Electrochemically Generated Carbanions Enable Isomerizing Allylation and Allenylation of Aldehydes with Alkenes and Alkynes

The direct coupling of aldehydes with petrochemical feedstockalkenesand alkynes would represent a practical and streamlined approach forallylation and allenylation chemistry. However, conventional approachescommonly require preactivated substrates or strong bases to generateallylic or propargylic carbanions and only afford branched allylationor propargylation products. Developing a mild and selective approachto access synthetically useful linear allylation and allenylationproducts is highly desirable, albeit with formidable challenges. Wereport a strategy using hydrogen evolution reaction (HER) to generatea carbanion from weakly acidic sp(3) C-H bonds (pK (a) similar to 35-40) under mild reactionconditions, obviating the use of strong bases, Schlenk techniques,and multistep procedures. The cathodically generated carbanion reverses the typical reaction selectivity to afford unconventionalisomerizing allylation and allenylation products (125 examples). Thegeneration of carbanions was monitored and identified by insitu ultraviolet-visible (UV-vis) spectroelectrochemistry.Furthermore, we extended this protocol to the generation of othercarbanions and their application in coupling reactions between alcoholswith carbanions. The appealing features of this approach include mildreaction conditions, excellent functional group tolerance, unconventionalchemo- and regioselectivity, and the diverse utility of products,which includes offering direct access to diene luminophores and bioactivescaffolds. We also performed cyclic voltammetry, control experiments,and density functional theory (DFT) calculations to rationalize theobserved reaction selectivity and mechanism.

Automated summary

This study presents a strategy using hydrogen evolution reaction (HER) to generate carbanions from weakly acidic sp(3) C-H bonds. This approach allows for selective functionalization of C-H bonds without the use of strong bases or complicated procedures, enabling mild and efficient allylation and allenylation reactions.