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, Zhixiang Wang

Research output: Contribution to journalArticlepeer-review

154 Scopus citations

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 (<32.0 kcal/mol) in PNN-Ru system are not unreachable. Another significant difference between the two systems is that the addition of alcohol to 1A giving an alkoxo complex is endergonic by 8.1 kcal/mol, whereas the addition to 2A is exergonic by 8.9 kcal/mol. The thermodynamic difference could be the main reason for PNP-Ir system requiring lower catalyst loading than the PNN-Ru system. We discuss how the differences are resulted in terms of electronic and geometric structures of the catalysts and how to use the features in catalyst development. © 2014 American Chemical Society.
Original languageEnglish (US)
Pages (from-to)4974-4991
Number of pages18
JournalJournal of the American Chemical Society
Volume136
Issue number13
DOIs
StatePublished - Mar 24 2014

Bibliographical note

KAUST Repository Item: Exported on 2020-10-01
Acknowledgements: This work is financially supported by National Natural Science Foundation of China (nos. 21173263 and 21373216). We greatly appreciate the insightful suggestions of the anonymous reviewers which helped us to improve the study.

ASJC Scopus subject areas

  • Biochemistry
  • Colloid and Surface Chemistry
  • Chemistry(all)
  • Catalysis

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