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Borylated N‐heterocyclic carbenes: rearrangement and chemical trapping

DOI: 10.1002/chem.201804808 DOI Help

Authors: Simon Aldridge (University of Oxford) , Lilja Kristinsdottir (University of Oxford) , Petra Vasko (University of Oxford) , Haoyu Niu (University of Oxford) , Eugene Kolychev (University of Oxford) , Jesus Campos (University of Oxford) , Maria Angeles Fuentes (University of Oxford) , Jamie Hicks (University of Oxford) , Amber L. Thompson (University of Oxford)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Chemistry – A European Journal

State: Published (Approved)
Published: December 2018
Diamond Proposal Number(s): 13639

Abstract: This study details attempts to access N‐heterocyclic carbenes (NHCs) featuring the diazaborolyl group, {(HCNDipp)2B}, as one or both of the N‐bound substituents. Attempts to isolate N‐heterocyclic carbenes based around imidazolylidene or related heterocycles, are characterized by facile N‐to‐C migration of the boryl substituent. In the cases of imidazolium precursors bearing one N‐bound diazaboryl group and one methyl substituent, deprotonation leads to the generation of the target carbenes, which can be characterized in situ by NMR measurements, and trapped by reactions with metal fragments and elemental selenium. The half‐lives of the free carbenes at room temperature range from 4‐50 h (depending on the pattern of ancillary substituents) with N‐to‐C2 migration of the boryl function being shown to be the predominant rearrangement pathway. Kinetic studies show this to be a first order process that occurs with an entropy of activation close to zero. DFT calculations imply that an intramolecular 1,2‐shift is mechanistically feasible, with calculated activation energies of the order of 90‐100 kJ mol‐1 reflecting the retention of significant aromatic character in the imidazole ring in the transition state. Trapping of the carbene allows for evaluation of steric and electronic properties via systems of the type LAuCl, LRh(CO)2Cl and LSe. A highly unsymmetrical (but nonetheless bulky) steric profile and moderately enhanced σ‐donor capabilities (compared to IMes) are revealed.

Journal Keywords: N-heterocyclic carbene; boryl group; boron; rearrangement; metal complex

Subject Areas: Chemistry

Instruments: I19-Small Molecule Single Crystal Diffraction

Added On: 08/01/2019 08:41

Discipline Tags:

Molecular Complexes Organic Chemistry Chemistry

Technical Tags:

Diffraction Single Crystal X-ray Diffraction (SXRD)