Rate and Mechanism of the Heck Reactions of Arylpalladium Complexes Ligated by a Bidentate P,P Ligand with an Electron-Rich Alkene (Isobutyl Vinyl Ether)

Abstract

Two main intermediate complexes, PhPd(OAc)(dppp) and PhPdI(dppp), are generated in palladiumcatalyzed Heck reactions performed from PhI and an acetate salt used as a base, in DMF. Both complexes react with the electron-rich alkene isobutyl vinyl ether (CH 2 dCHO i Bu) exclusively by an ionic mechanism, i.e., via the cationic complex [PhPd(dppp)(DMF)] + formed by dissociation of AcOor Ifrom the parent complexes, as already established for the reaction of PhPd(OAc)(dppp) with styrene and methyl acrylate and the reaction of PhPdI(dppp) with styrene. A mechanism is proposed which rationalizes the regioselectivity observed when reacting the electron-rich isobutylvinyl ether in palladium-catalyzed Heck reactions.

Mechanisms of the Mizoroki–Heck Reaction

“…PhPdS(dppp) is, via an ionic mechanism (Scheme 1.32) [43], as with isobutylvinyl ether (Scheme 1.33) [53]. The mechanism of the reaction of PhPdI(dppp) with alkenes is substrate dependent.…”

“…The mechanism of the reaction of PhPdI(dppp) with alkenes is substrate dependent. PhPdI(dppp) reacts with styrene via PhPd(DMF)(dppp) + in the ionic mechanism (Scheme 1.32) [43], as with isobutylvinyl ether (Scheme 1.33) [53], whereas PhPdI(dppp) and PhPd(DMF)(dppp)…”

“…In all cases, the linear product (E)-PhCH CHR is formed as: (i) the unique product for R = CO 2 Me from PhPdX(dppp) (X = I, OAc) [43]; (ii) the major product for R = Ph from PhPdI(dppp) (80% selectivity) and PhPd(OAc)(dppp) (82% selectivity) [47]. The branched product CH CH(Ph)R is formed as the major product for R = OiBu from PhPdI(dppp) (90% selectivity) [53]. In a first approach, the slower formation of the minor product has not been considered and the alkene insertion step has been regarded as irreversible, as postulated in textbook mechanisms (Schemes 1.27 and 1.29).…”

“…In a first approach, the slower formation of the minor product has not been considered and the alkene insertion step has been regarded as irreversible, as postulated in textbook mechanisms (Schemes 1.27 and 1.29). The equilibrium and rate constants for the formation of the major product have been determined in DMF (Table 1.2) with the following reactivity orders [43,53]: r whatever the complex PhPdX(dppp) (X = I, OAc) CH 2 CH CO 2 Me > CH 2 CH O-i-Bu > CH 2 CH Ph r whatever the alkene PhPd(DMF)(dppp) + PhPdX(dppp) (X = I, OAc) …”

“…under the conditions of the textbook neutral mechanism of Scheme 1.27 proposed by Cabri and Candiani [1g]) give a mixture of branched and linear products in DMF, whereas the branched product is exclusively produced in ionic liquids (in the absence of halide scavengers) in a faster reaction. Whatever the medium, the cationic complex ArPdS(dppp) + is always the sole reactive complex with electron-rich alkene (Scheme 1.33) [53]. Consequently, the regioselectivity should not vary with the experimental conditions.…”