A Proton-Responsive Pyridyl(benzamide)-Functionalized NHC Ligand on Ir Complex for Alkylation of Ketones and Secondary Alcohols

Mandeep Kaur, Noor U Din Reshi, Kamaless Patra, Arindom Bhattacherya, Sooraj Kunnikuruvan, Jitendra K. Bera

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27 Scopus citations

Abstract

A Cp*Ir(III) complex (1) of a newly designed ligand L1 featuring a proton-responsive pyridyl(benzamide) appended on N-heterocyclic carbene (NHC) has been synthesized. The molecular structure of 1 reveals a dearomatized form of the ligand. The protonation of 1 with HBF4 in tetrahydrofuran gives the corresponding aromatized complex [Cp*Ir(L1H)Cl]BF4 (2). Both compounds are characterized spectroscopically and by X-ray crystallography. The protonation of 1 with acid is examined by 1H NMR and UV-vis spectra. The proton-responsive character of 1 is exploited for catalyzing α-alkylation of ketones and β-alkylation of secondary alcohols using primary alcohols as alkylating agents through hydrogen-borrowing methodology. Compound 1 is an effective catalyst for these reactions and exhibits a superior activity in comparison to a structurally similar iridium complex [Cp*Ir(L2)Cl]PF6 (3) lacking a proton-responsive pendant amide moiety. The catalytic alkylation is characterized by a wide substrate scope, low catalyst and base loadings, and a short reaction time. The catalytic efficacy of 1 is also demonstrated for the syntheses of quinoline and lactone derivatives via acceptorless dehydrogenation, and selective alkylation of two steroids, pregnenolone and testosterone. Detailed mechanistic investigations and DFT calculations substantiate the role of the proton-responsive ligand in the hydrogen-borrowing process.

Original languageEnglish
Pages (from-to)10737-10748
Number of pages12
JournalChemistry - A European Journal
Volume27
Issue number41
DOIs
StatePublished - 21 Jul 2021
Externally publishedYes

Bibliographical note

Publisher Copyright:
© 2021 Wiley-VCH GmbH

Funding

We thank the anonymous reviewers for useful suggestions. Financial support from the Science and Engineering Research Board (SERB) is gratefully acknowledged. J.K.B. thanks SERB, India, for a J. C. Bose fellowship. M.K., K.P and A.B thank the CSIR (India), UGC (India), and IIT Kanpur for fellowships, respectively. N.U.D.R. thanks IIT Kanpur for a post-doctoral fellowship. SK thanks the department of science and technology (DST), India for an INSPIRE faculty fellowship and IISER Thiruvananthapuram for providing the HPC facilities. We thank the anonymous reviewers for useful suggestions. Financial support from the Science and Engineering Research Board (SERB) is gratefully acknowledged. J.K.B. thanks SERB, India, for a J. C. Bose fellowship. M.K., K.P and A.B thank the CSIR (India), UGC (India), and IIT Kanpur for fellowships, respectively. N.U.D.R. thanks IIT Kanpur for a post‐doctoral fellowship. SK thanks the department of science and technology (DST), India for an INSPIRE faculty fellowship and IISER Thiruvananthapuram for providing the HPC facilities.

FundersFunder number
Indian Institute of Technology Kanpur
Department of Science and Technology, Ministry of Science and Technology, India
Council of Scientific and Industrial Research, India
University Grants Committee
Science and Engineering Research Board
Indian Institute of Science Education and Research Pune

    Keywords

    • Borrowing-Hydrogen
    • Homogeneous Catalysis
    • Hydrogen transfer
    • Metal-Ligand Cooperation
    • Proton-Responsive Ligand
    • Reaction Mechanism

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