Hypervalent Iodine-Mediated Styrene Hetero- and Homodimerizations Initiation Proceed with Two-Electron Reductive Cleavage
A mechanistic insight into the hetero- and homodimerizations (HETD and HOMD) of styrenes promoted by hypervalent iodine reagents (HVIRs; DMP and PIDA) and facilitated by HFIP to yield all trans cyclobutanes is reported using density functional theory (DFT) calculations. The reaction is initiated with two-electron reductive cleavage of two I─O bond cleavages, affording I(III) (iodinane) and I(I) (iodobenzene) product with DMP and PIDA as oxidant, respectively. The resulting acetate groups are stabilized by the solvent HFIP through strong hydrogen bonding interaction, which promotes the electron transfer process. The initialization involving one-electron transfer was found to be highly unfavored, especially for the PIDA system. At this point, we found that two-electron process is the key initialization process, which is in accordance with literature report on alcohol oxidation. The reaction rate is determined by the initialization step: For I(III), the initiation is thermodynamically endergonic, whereas the endergonicity for I(V) is modest. The difference in reactivity is explained by the difference LUMO energies. Upon initialization, the reaction proceeds through a stepwise [2+2] pathway, involving a radical-cationic π-π stacked intermediate, either hetero- or homodimerized. DFT results supported by quasiclassical molecular dynamics simulations show that HOMD is dynamically competing pathway to HETD although the latter is relatively faster, in accordance with experimental observations.