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

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

• References and Notes

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• 10 Preparation of VAPOL-meso-borate 2 (Procedure A) To a flame-dried 25 mL Schlenk flask equipped with a stir bar and flushed with nitrogen was added (R)-VAPOL (1, 54 mg, 0.10 mmol, 1.0 equiv), phenol (9.4 mg, 0.10 mmol, 1.0 equiv), dry toluene (2 mL), and BH₃·SMe₂ (2 M in toluene, 0.10 mmol, 1.0 equiv). The flask was sealed, and the resulting mixture was stirred at 100 °C for 1 h. The volatiles were then carefully removed via high vacuum and left under high vacuum at 100 °C for 0.5 h. The residue was dissolved in ca. 0.7 mL CDCl₃ and transferred to a quartz NMR tube and was subjected to ¹H NMR and ¹¹B NMR analysis. To locate the five aromatic protons in the phenol moiety in the meso-borate 2, procedure A described above was repeated with phenol-2,3,4,5,6-d ₅ (10 mg, 0.10 mmol, 1.0 equiv), and the resultant meso-borate 2 was subjected to ¹H NMR and ¹¹B NMR analysis. The ratio of meso-/pyro-borates with phenol was 17:1 and with the pentadeuterophenol was 13:1. Spectral Data for 2 (from Phenol, Figure 2, a) ¹H NMR (500 MHz, CDCl₃): δ = 6.64 (dd, 4 H, J = 8.3, 1.0 Hz), 7.00 (t, 4 H, J = 7.8 Hz), 7.13–7.18 (m, 4 H), 7.30 (tt, 1 H, J = 1.1, 7.5 Hz), 7.35–7.40 (m, 4 H), 7.55 (s, 2 H), 7.61 (ddd, 2 H, J = 7.8, 7.0, 1.0 Hz), 7.72 (d, 2 H, J = 9.0 Hz), 7.83 (d, 2 H, J = 8.5 Hz), 7.95 (dd, 2 H, J = 7.8, 1.2 Hz), 9.60 (d, 2 H, J = 8.5 Hz). Spectral Data for 2 (from Pentadeuterophenol, Figure 2, b) ¹H NMR (500 MHz, CDCl₃): δ = 6.64 (dd, 4 H, J = 8.3, 1.0 Hz), 7.00 (t, 4 H, J = 7.8 Hz), 7.15 (t, 2 H, J = 7.3 Hz), 7.37 (ddd, 2 H, J = 8.7, 7.0, 1.4 Hz), 7.55 (s, 2 H), 7.61 (ddd, 2 H, J = 7.8, 7.0, 1.0 Hz), 7.72 (d, 2 H, J = 9.0 Hz), 7.83 (d, 2 H, J = 8.5 Hz), 7.95 (dd, 2 H, J = 7.8, 1.2 Hz), 9.59 (d, 2 H, J = 8.5 Hz).
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