The direct coupling of aldehydes with petrochemical feedstock
alkenes
and alkynes would represent a practical and streamlined approach for
allylation and allenylation chemistry. However, conventional approaches
commonly require preactivated substrates or strong bases to generate
allylic or propargylic carbanions and only afford branched allylation
or propargylation products. Developing a mild and selective approach
to access synthetically useful linear allylation and allenylation
products is highly desirable, albeit with formidable challenges. We
report a strategy using hydrogen evolution reaction (HER) to generate
a carbanion from weakly acidic sp3C–H bonds (pKa∼ 35–40) under mild reaction
conditions, obviating the use of strong bases, Schlenk techniques,
and multistep procedures. The cathodically generated carbanionreversesthe typical reaction selectivity to afford unconventional
isomerizing allylation and allenylation products (125 examples). The
generation of carbanions was monitored and identified byin
situultraviolet–visible (UV–vis) spectroelectrochemistry.
Furthermore, we extended this protocol to the generation of other
carbanions and their application in coupling reactions between alcohols
with carbanions. The appealing features of this approach include mild
reaction conditions, excellent functional group tolerance, unconventional
chemo- and regioselectivity, and the diverse utility of products,
which includes offering direct access to diene luminophores and bioactive
scaffolds. We also performed cyclic voltammetry, control experiments,
and density functional theory (DFT) calculations to rationalize the
observed reaction selectivity and mechanism.