Article pubs.acs.org/JACS
Catalytic Mechanisms of Direct Pyrrole Synthesis via Dehydrogenative Coupling Mediated by PNP-Ir or PNN-Ru Pincer Complexes: Crucial Role of Proton-Transfer Shuttles in the PNP-Ir System Shuanglin Qu,‡ Yanfeng Dang,‡ Chunyu Song,‡ Mingwei Wen,‡ Kuo-Wei Huang,§ and Zhi-Xiang Wang*,‡,∥ ‡
School of Chemistry and Chemical Engineering, University of the Chinese Academy of Sciences, Beijing, 100049, China KAUST Catalysis Center and Division of Physical Science and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia ∥ Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, 300072, China §
S Supporting Information *
ABSTRACT: Kempe et al. and Milstein et al. have recently advanced the dehydrogenative coupling methodology to synthesize pyrroles from secondary alcohols (e.g., 3) and β-amino alcohols (e.g., 4), using PNP-Ir (1) and PNN-Ru (2) pincer complexes, respectively. We herein present a DFT study to characterize the catalytic mechanism of these reactions. After precatalyst activation to give active 1A/2A, the transformation proceeds via four stages: 1A/2A-catalyzed alcohol (3) dehydrogenation to give ketone (11), base-facilitated C−N coupling of 11 and 4 to form an imine-alcohol intermediate (18), base-promoted cyclization of 18, and catalyst regeneration via H2 release from 1R/2R. For alcohol dehydrogenations, the bifunctional double hydrogen-transfer pathway is more favorable than that via β-hydride elimination. Generally, proton-transfer (H-transfer) shuttles facilitate various H-transfer processes in both systems. Notwithstanding, H-transfer shuttles play a much more crucial role in the PNP-Ir system than in the PNN-Ru system. Without H-transfer shuttles, the key barriers up to 45.9 kcal/mol in PNP-Ir system are too high to be accessible, while the corresponding barriers (
Catalytic Mechanisms of Direct Pyrrole Synthesis via Dehydrogenative Coupling Mediated by PNP-Ir or PNN-Ru Pincer Complexes: Crucial Role of Proton-Transfer Shuttles in the PNP-Ir System Shuanglin Qu,‡ Yanfeng Dang,‡ Chunyu Song,‡ Mingwei Wen,‡ Kuo-Wei Huang,§ and Zhi-Xiang Wang*,‡,∥ ‡
School of Chemistry and Chemical Engineering, University of the Chinese Academy of Sciences, Beijing, 100049, China KAUST Catalysis Center and Division of Physical Science and Engineering, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia ∥ Collaborative Innovation Center of Chemical Science and Engineering, Tianjin, 300072, China §
S Supporting Information *
ABSTRACT: Kempe et al. and Milstein et al. have recently advanced the dehydrogenative coupling methodology to synthesize pyrroles from secondary alcohols (e.g., 3) and β-amino alcohols (e.g., 4), using PNP-Ir (1) and PNN-Ru (2) pincer complexes, respectively. We herein present a DFT study to characterize the catalytic mechanism of these reactions. After precatalyst activation to give active 1A/2A, the transformation proceeds via four stages: 1A/2A-catalyzed alcohol (3) dehydrogenation to give ketone (11), base-facilitated C−N coupling of 11 and 4 to form an imine-alcohol intermediate (18), base-promoted cyclization of 18, and catalyst regeneration via H2 release from 1R/2R. For alcohol dehydrogenations, the bifunctional double hydrogen-transfer pathway is more favorable than that via β-hydride elimination. Generally, proton-transfer (H-transfer) shuttles facilitate various H-transfer processes in both systems. Notwithstanding, H-transfer shuttles play a much more crucial role in the PNP-Ir system than in the PNN-Ru system. Without H-transfer shuttles, the key barriers up to 45.9 kcal/mol in PNP-Ir system are too high to be accessible, while the corresponding barriers (
