Merging HAT with Propadiene Telomerization: A Palladium Metallaphotoredox Dual Catalyzed Approach for the Radical Allylation and Dienylation of Styrenes

Propadiene, together with its isomer propyne, accounts
for up to
6 mol % of the crude C3 fraction obtained from steam cracking.
Despite this abundance, its current industrial use as a welding gas
is neither economically or environmentally optimal. Consequently,
propadiene deserves greater attention as a versatile building block
in chemical synthesis and catalysis. Herein, we report a multicomponent
allylation and dienylation of styrene derivatives in which propadiene
serves as either an allylic or dienylic precursor under palladium-catalyzed
metallaphotoredox conditions. A hydrogen atom transfer (HAT) strategy
was employed to generate the initial radical species, with the decatungstate
anion (DT) selected as a direct HAT catalyst (d-HAT).
Upon light irradiation, DT forms an electrophilic oxygen-centered
radical capable of performing regioselective HAT at the most hydridic
and sterically accessible C–H sites. Using this approach, 30
distinct C–H units, including α-heteroatom C(sp3)-H, unactivated C(sp3)-H, and formyl C(sp2)-H bonds, were selectively activated and functionalized. Moreover,
the self-assembling ligand 3-DPICon, developed by our
group two decades ago, proved to be the optimal choice for allylic
transformations. The practicality of these protocols was demonstrated
through diverse downstream derivatizations, and preliminary mechanistic
studies were conducted to elucidate the reaction pathway.