Mechanistic Insights into the ReIO2(PPh3)2‑Promoted Reductive Coupling of Alcohols

A mechanistic investigation of the reductive coupling of benzylic and allylic alcohols by triphenylphosphine catalyzed by ReIO2(PPh3)2 (1) is disclosed utilizing (1) stoichiometric reaction studies of 1 with alcohols, with PPh3 and with OPPh3; (2) rate law determination of the reaction of benzhydrol with PPh3 catalyzed by 1; (3) substrate structure-dependent reactivity/selectivity studies; and (4) DFT computational analysis of various potential reaction pathways in the benzyl alcohol/PPh3 reaction. In situ NMR monitoring of reactions of 1 with PPh3 and various alcohols demonstrate (a) facile, reversible PPh3 dissociation from 1; (b) association of various alcohols to form Re-alcohol/alkoxide complexes, (Ph3P)­IReO2(ROH) and (Ph3P)­IReO­(OH)­(OR); and (c) thermal conversion of these alcohol­(ate)-rhenium complexes to Ph2CH–CHPh2 and OPPh3 at >50 °C. Under pseudo-first-order conditions, the initial rate kinetics of reductive coupling of Ph2CHOH/PPh3 catalyzed by 1 shows (a) a reaction rate that is first-order each in ROH, catalyst and first-order (or higher) in PPh3 and (b) the reaction is inhibited by OPPh3. Alcohol structure effects show (a) relative reactivity of sec-, tert-benzylic = allylic > prim-benzylic/allylic ≫ sec-, prim-alkyl and (b) low regioselectivity of the dimers from unsymmetrical allylic alcohols. A DFT computational study of the reaction of benzyl alcohol/PPh3 with 1 reveals a preferred pathway involving: (a) formation of rhenium–alcohol and −alkoxide intermediates, (Ph3P)­IReO2(ROH) and (Ph3P)­IReO­(OH)­(OR); (b) reduction of the latter by PPh3 to form (OPPh3)­(Ph3P)­IRe­(OH)­(OBn) (E); (c) association of a second BnOH with E to give (Ph3P)­IRe­(OBn)2 (K); (d) facile dissociation of a benzyl radical from K by C–O homolysis; and (e) a second rhenium–O–Bn homolysis from (PPh3)­IRe­(H2O)­(OBn) (O), giving bibenzyl via benzyl radical recombination and regenerating (PPh3)­ReIO2.