Reductive Dimerization of Alkenes and Allenes Enabled by Photochemically Activated Zinc-Zinc Bonded Compounds

Metal radicals have shown versatile reactivity in modern synthetic chemistry. However, the use of zinc radicals for molecular synthesis has been barely explored. Here, we show that a transient zinc radical can be formed through photoactivation of a zinc-zinc bonded compound, which is able to mediate the selective dimerization of alkenes and allenes. Treatment of dizinc compounds [L₂Zn₂] [L = CH₃C(2,6-Pr₂C₆H₃N)CHC(CH₃)(NCH₂CH₂PR₂); R = Ph (L^Ph) or Pr (L)] with a diverse array of aromatic alkenes under UV irradiation (365 nm) facilely afforded the head-to-head coupling products, i.e., 1,4-dizinciobutanes in high yields. In addition, arylallenes could also be selectively dimerized by the dizinc compound to give 2,5-dizincyl-functionalized 1,5-hexadienes under the same conditions. Control reactions of [L^Ph₂Zn₂] in the presence of UV irradiation isolated a zinc phenyl complex and a trimeric zinc phosphide complex resulting from C-P bond cleavage at the tridentate ligand. Reactions of photoactivated dizinc compounds with organic spin traps, i.e., 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) and 2,2'-bipyridine (2,2'-bpy), successfully isolated zinc radical trapping products [LZnOTEMP] and [L^PhZn(2,2'-bpy)^·], respectively. The profile of alkene dimerization was elucidated by density functional theory calculations, which confirmed that a transient zinc radical [LZn·] was initially generated through homolytic Zn-Zn bond cleavage via photoactivation followed by single-electron transfer and radical dimerization. The unique selectivity of the current reaction was also studied computationally.