DFT characterisation of a PdII → IIII adduct, and a PdII complex formed after oxidative alkenylation of PdII by [Ph(alkenyl)IIII]+, in Pd-mediated synthesis of benzofurans involving PdIV, annulation and chain-walking

Abstract

The synthesis of benzofurans by the reaction of the palladium(II) complex Pd{1-C₆H₄-2-OCH(CO₂Et)-C,C}(bipy) (bipy = 2,2′-bipyridine) with hypervalent iodine(III) reagents [Ph(CHCHR)I]⁺ has been examined by Density Functional Theory. Results highlight the role of oxidative alkenylation to form Pd^IV intermediates and the role of initial adduct formation in this process, an annulation process facilitated by Pd^II, and the role of ‘chain-walking’ at Pd^II centres to allow formation of the lowest energy product. Computation (R = Me) allows assignment of an initially formed adduct with a ‘Pd^II → I^III’ interaction at −50 °C, and, after oxidative alkenylation of Pd^II and reductive elimination from a Pd^IV centre via Ar⋯Alkenyl coupling, formation of a second intermediate with a structure consistent with NMR detection (R = n-hexyl) at −30 °C is obtained. This Pd^II complex, containing a coordinated alkene group in Pd{1-(RHC^γC^β)C₆H₄-2-OC^αH(CO₂Et)-η²-C^αC^β,C}(bipy), undergoes a 5-exo-trig annulation by forming a C^α–C^β bond to give a complex with a bicyclic carbon skeleton suitable for subsequent formation of benzofurans. A series of facile rearrangements including chain-walking results in formation of a lowest energy complex of three feasible hydrido(alkene)palladium(II) species, leading to decomposition and release of the observed benzofuran isomer isolated under synthesis conditions. The computational study allows reinterpretation of the NMR data reported previously, in particular the determination of barriers in the reaction pathway allowing assignment of structure for key intermediates.