Mechanistic Investigation of the Pd-Catalyzed Intermolecular Carboetherification and Carboamination of 2,3-Dihydrofuran: Similarities, Differences, and Evidence for Unusual Reaction Intermediates

4 years ago

Mechanistic Investigation of the Pd-Catalyzed Intermolecular Carboetherification and Carboamination of 2,3-Dihydrofuran: Similarities, Differences, and Evidence for Unusual Reaction Intermediates

Gustavo M. Borrajo-Calleja, Vincent Bizet, Céline Besnard, Clément Mazet

The mechanism of the Pd-catalyzed intermolecular syn carboetherification and syn carboamination of 2,3-dihydrofuran was investigated experimentally. Crystallographic, spectroscopic, and spectrometric methods have shed light on the nature of a number of catalytically competent palladium complexes. Several oxidative addition complexes as well as their cationic derivatives have been characterized by X-ray diffraction analyses. In the latter, the complexes derived from 2-bromophenol displayed an unorthodox η⁶ binding mode of the privileged Buchwald-type dialkylbiarylphosphine ligands. The hemilabile character of this interaction was found to facilitate coordination of the polarized olefinic substrate, as evidenced by NMR spectroscopy. In contrast, coordination of the pendant sulfonyl group in the cationic complexes derived from 2-bromo-N-sulfonylated anilines prevented direct binding of 2,3-dihydrofuran. Deprotonation of these species induced aggregation of monomeric units through various weak noncovalent interactions to generate trinuclear palladium clusters. The reversibility of this process was probed by conducting crossover experiments. The nature of the alkali ion was found to strongly influence the selectivity of the assembly phenomenon. Examination of the importance of the nucleophilicity in these intermolecular reactions revealed that the switch between syn carbofunctionalization and Heck arylation of 2,3-dihydrofuran certainly arose from a zwitterionic intermediate common to both catalytic manifolds. The understanding of these reactions gained through this study should certainly favor the design of novel Pd-catalyzed transformations for related systems.

Publisher URL: http://dx.doi.org/10.1021/acs.organomet.7b00483

DOI: 10.1021/acs.organomet.7b00483