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Synlett 2015; 26(11): 1606-1614
DOI: 10.1055/s-0034-1378707
letter
© Georg Thieme Verlag Stuttgart · New York

The Nature of meso- and pyro-Borate Precatalysts to the VANOL and VAPOL BOROX Catalysts

Wenjun Zhao
,
Xiaopeng Yin
,
Anil K. Gupta
,
Xin Zhang
,
William D. Wulff*
Further Information

Publication History

Received: 09 March 2015

Accepted after revision: 07 May 2015

Publication Date:
09 June 2015 (online)

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Abstract

The structures of the meso- and pyro-borate esters generated by treatment of the VANOL and VAPOL ligands with triphenylborate have been revisited. These species were previously identified as precatalysts that could be in situ converted into VANOL and VAPOL BOROX catalysts by an imine substrate. The complete assignment of all protons for both the meso- and pyro-borate esters of both ligands was aided by the 1H NMR spectrum of each generated from pentadeuterophenol. There were significant differences between the chemical shifts for certain protons in the meso- and pyro-borate species in both the VANOL and VAPOL derivatives. Optimized structures for the meso-borates and two different isomers of the pyro-borates were determined by DFT calculations for each ligand. For each ligand the cyclic pyro-borate was found to be lower in energy than the corresponding linear pyro-borate at the B3LYP/6-311+G(d,p) level of theory. The structures of the cyclic pyro-borate esters were more consistent with the observed 1H NMR chemical shifts than the linear pyro-borates for each ligand and thus the structures of the pyro-borates esters of VANOL and VAPOL have been re-assigned as the cyclic isomers.

Key words

meso- and pyro-borate esters - VANOL and VAPOL ligands - VANOL and VAPOL BOROX Catalysts - imine substrate

Supporting Information

    Supporting information for this article is available online at http://dx.doi.org/10.1055/s-0034-1378707. Included are experimental procedures, 1H NMR and 11B NMR spectra of compounds 2, 8, 11, and 13 and pdb files for DFT energy-minimized structures for 1–3, 8, 10–13.
  • Supporting Information

Primary Data

    Primary data for this article are available online at https://zenodo.org/record/4610225 and can be cited using the following DOI: 10.5281/zenodo.4610163. Please note that the DOI for the Primary Data associated with this article was updated on April 21, 2021.

  • Primary Data
 

  • References and Notes

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