Computational Study Illustrating NCN-Palladium(IV) Involvement in Generating Pd⁰ Species to Facilitate Pd⁰/Pd^II Heck-Type Catalysis with Diphenyliodine(III) Species

Abstract

Density functional theory has been applied in a mechanistic study of the role of pincer complex PdII(NCN-N,C,N)(O2CPh-O) ([NCN]- = [2,6-(Me2NCH2)2C6H3]−) (3) in Heck-type catalysis in the presence of diphenyliodine(III) triflate as the oxidative arylating agent for CH2═CHAr and bicarbonate as the base to afford PhCH═CHAr (Ar = p-BrC6H4). The initially formed palladium(IV) complex PhPd(NCN-N,C,N)(OBz···HOCO2-O,O) (9) (ΔG‡ 31.6 kcal/mol) undergoes Ph···Cipso reductive elimination to form PdII{NC(Ph)N-N,C,N}(OBz···HOCO2-O,O) (11) (ΔG‡ 25.6 kcal/mol), which is reduced by bicarbonate to form palladium(0) species. Reduction to Pd0 occurs via deprotonation of one NMe2 group by bicarbonate to provide a “-CH2-N(Me)-CH2-PdII″ moiety (ΔG‡ 23.6 kcal/mol) followed by nucleophilic attack on this moiety by bicarbonate to give a Pd0 product with a “-CH2-NMe(CH2OCO2H)″ group (ΔG‡ 14.5 kcal/mol). The Pd0 complex undergoes exceptionally facile oxidative addition by Ph2I(HCO3) (ΔG‡ = 5.1 kcal/mol). Modeling the Pd0 complex as [Pd(benzene)(O2CPh)]− provides a similar result (ΔG‡ = 5.6 kcal/mol), allowing entry to PhPdII species to be able to undergo migratory insertion for CH2═CAr (ΔG‡ = 14.4 kcal/mol) and β-hydride elimination (ΔG‡ = 16.2 kcal/mol) processes of Pd0/PdII Heck-type catalysis. Activation barriers for reduction of PdIV to Pd0, and in the Heck-type process, are lower than the initial oxidation to form PdIV species, ensuring that only a small quantity of PdII(NCN)(OBz) (3) is consumed, in accord with its presence on completion of catalysis. Computational studies of PdIV-mediated Heck-type catalysis revealed energetically unfavorable processes and a preference for the formation of CH2═C(Ar)Ph rather than the experimentally reported PhCH═CHAr. This study reveals the role of a pincer complex as a precatalyst, the oxidation of PdII to PdIV followed by reductive elimination, the role of bicarbonate in reducing PdII to Pd0, the extremely facile oxidative addition of a diaryliodine(III) reagent to Pd0, and the selectivity differences in migratory insertion for PdII and PdIV centers.