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.
Yang et al. (Thu,) studied this question.