The metalation/transmetalation strategy using [Zr(NMe2)4] as initial metalating reagent offers an efficient approach to the synthesis of CCC–NHC pincer complexes. Many CCC–NHC pincer complexes have been prepared via this methodology. As efficient as this methodology is, many questions remained as to the mechanism for the process, particularly the requirement of two equivalents of Rh per proligand for good yields. Previously, no intermediates have been reported to shed light on the mechanism. In the process of investigating an intermediate and the mechanism of the metalation/transmetalation methodology, a new mixed valent bimetallic CCC–NHC pincer Rh complex with two chloro ligands bridged between a [(CCC–NHC)Rh(III)] and a [Rh(I)(COD)] fragment was isolated and fully characterized. The investigation of the Rh(III)/Rh(I) bimetallic intermediate in the CCC–NHC pincer metalation/transmetalation methodology led to an improved stoichiometric synthesis of CCC–NHC pincer Rh complexes. It was found that switching the proligand from iodide to chloride counterion obviated the need for an extra equivalent of Rh. The iodide bridged Rh(III)/Rh(I) intermediate was much more stable and prevented further reaction in comparison to the chloride congener. When it was switched to only chloride present the reaction quickly gave efficient, complete transmetalation with only a 1:1 ratio of proligand:Rh.
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Synthesis of a CCC‐NHC pincer Re complex: An air stable catalyst for coupling ketones with primary alcohols via borrowing hydrogen
To date, no CCC‐NHC pincer complexes of Re have been reported in the literature. The first CCC‐NHC pincer complex of Re is reported. It was fully characterized by1H and13C nuclear magnetic resonance (NMR) spectroscopy, mass spectroscopy, elemental analysis, and X‐ray crystallographic methods to determine the molecular structure. It was synthesized via transmetallation from an isolated Zr precursor and was found to be air stable. The catalytic activity of the CCC‐NHC Re(I) pincer complex was demonstrated for the borrowing hydrogen coupling reaction between benzylic ketones and primary alcohols to generate a new C–C bond in an environmentally friendly catalysis requiring no activating groups for the alcohol functionality. This borrowing hydrogen coupling reaction produced a stoichiometric amount of water as the only by‐product and did not require the conversion of the primary alcohol to a leaving group. A broad range of substrates was examined, and isolated yields from 53% to 92% were obtained. A catalytic cycle for the CCC‐NHC Re(I) pincer complex catalyzed borrowing hydrogen coupling reaction is proposed.
more » « less- NSF-PAR ID:
- 10368933
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Applied Organometallic Chemistry
- ISSN:
- 0268-2605
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The role of ligands in rhodium‐ and iridium‐catalyzed
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