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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*
Weitere Informationen

Publikationsverlauf

Received: 09. März 2015

Accepted after revision: 07. Mai 2015

Publikationsdatum:
09. Juni 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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  • 11 Preparation of pyro-Borate 8 (Procedure B) To a flame-dried 25 mL Schlenk flask equipped with a stir bar and flushed with nitrogen was added phenol (113 mg, 1.20 mmol, 12.00 equiv), dry toluene (2 mL), and BH3·SMe2 (2 M in toluene, 150 μL, 0.300 mmol, 3.00 equiv). The flask was sealed, and the resulting mixture was stirred at 80 °C for 1 h. The volatiles were then carefully removed via high vacuum and left under high vacuum at r.t. for 0.5 h yielding a white solid. To this solid was added VAPOL (54 mg, 0.10 mmol, 1.0 equiv), dry toluene (2 mL), and H2O (1.8 μL, 0.10 mmol, 1.0 equiv). The mixture was stirred at 80 °C for 1 h. The volatiles were carefully removed via high vacuum and left under high vacuum at 80 °C for 0.5 h. The residue was then dissolved in ca. 0.7 mL CDCl3 and transferred to a quartz NMR tube, and the pyro-borate 8 was subjected to 1H NMR and 11B NMR analysis. To locate the five aromatic protons in the phenol moiety in 8, the procedure B described above was repeated with phenol-2,3,4,5,6-d 5 (120 mg, 1.20 mmol, 12.00 equiv). The ratio of pyro-/meso-borates with phenol was 8:1 and with the pentadeuterophenol was 8:1. Spectral Data for 8 (from Phenol, Figure 3, a) 1H NMR (500 MHz, CDCl3): δ = 6.49 (d, 4 H, J = 7.8 Hz), 6.99 (tt, 2 H, J = 7.2, 1.3 Hz), 7.05 (t, 4 H, J = 7.8 Hz), 7.09–7.18 (m, 10 H), 7.68–7.74 (m, 4 H), 7.80 (s, 2 H), 7.82 (d, 2 H, J = 9.0 Hz), 7.88 (d, 2 H, J = 9.0 Hz), 8.01 (dd, 2 H, J = 7.3, 2.0 Hz), 9.29–9.34 (m, 2 H). Spectral Data for 8 (from Pentadeuterophenol, Figure 3, b) 1H NMR (500 MHz, CDCl3): δ = 7.09–7.18 (m, 10 H), 7.68–7.74 (m, 4 H), 7.80 (s, 2 H), 7.81 (d, 2 H, J = 9.0 Hz), 7.88 (d, 2 H, J = 9.0 Hz), 8.01 (dd, 2 H, J = 7.3, 2.0 Hz), 9.29–9.34 (m, 2 H).
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