Is the Mechanistic Scheme of the Heck‐Type Coupling with Unsaturated Substrates needed to be revised?

Abstract

Patterns of differential selectivity of two competing unsaturated substrates along with differential regioselectivity of α‐ and β‐regioisomers of arylated products formed in Heck reactions with aryl halides or with aromatic carboxylic anhydrides as well as differential regioselectivity of C2‐ and C3‐arylated indoles in direct C−H arylation with aryl halides unambiguously evidence that the commonly accepted mechanistic scheme of unsaturated substrate activation the Heck‐type coupling reactions needs to be revised. Comprehensive kinetic study points to the activation of aryl halide and unsaturated substrate by two distinct Pd species, as well as existence of an additional intermediate of their catalytic cycles being not taken into consideration earlier in the mechanistic schemes of the reactions.

Unusual Regioselectivity in Palladium‐Catalyzed Tandem C−H Arylation and C−H Amidation of cis‐Cinnamyl Hydroxamates: Facile Synthesis of 3‐Aryl‐2‐quinolones

“…[20] We observed that when the reaction was conducted in oxygen atmosphere the course of the reaction was not hampered although there was little rate-acceleration. However, in view of the known [21] uncertainties in the mechanism of Heck-type coupling of unsaturated esters, other possibilities may exist. We became interested to see whether the developed onepot single metal catalyzed arylation-cyclization sequence will be applicable to trans-cinnamyl hydroxamates.…”

“…We observed that when the reaction was conducted in oxygen atmosphere the course of the reaction was not hampered although there was little rate‐acceleration. However, in view of the known [21] uncertainties in the mechanism of Heck‐type coupling of unsaturated esters, other possibilities may exist.…”

Latest Developments on Palladium- and Nickel-Catalyzed Cross-Couplings Using Aryl Chlorides: Suzuki–Miyaura and Buchwald–Hartwig Reactions

“…Schmidt et al reported that the oxidative addition of aryl chlorides to ligand-free Pd species was a reversible process. 19 Thus, under ligand-free conditions, oxidative addition was not the rate-determining step. In 2020, Zeng and Liu reported the NHC-Pd(II)-azole complex C8 as a pre-catalyst for the SM reaction.…”