Structure and Electronic Nature of the Benzaldehyde/Boron Trifluoride Adduct

Reetz, M. T.; Hüllmann, M.; Massa, W.; Rademacher, P.; Heymanns, P.

doi:10.1021/ja00269a044

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Structure and Electronic Nature of the Benzaldehyde/Boron Trifluoride Adduct

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Reetz, M. T., Hüllmann, M., Massa, W., Rademacher, P., & Heymanns, P. (1986). Structure and Electronic Nature of the Benzaldehyde/Boron Trifluoride Adduct. Journal of the American Chemical Society, 108(9), 2405-2408. doi:10.1021/ja00269a044.

Cite as: https://hdl.handle.net/21.11116/0000-000E-7E53-8

Abstract

The structure of the benzaldehyde/boron trifluoride adduct (6) has been determined by X-ray crystallography. Accordingly, the Lewis acid BF₃ is complexed anti to the phenyl group in benzaldehyde. The B-O-C-C fragment lies essentially in a common plane. Anti complexation also pertains in solution, as shown by a heteronuclear Overhauser experiment. MNDO calculations of the acetaldehyde/BF₃ adduct show that anti complexation does indeed lead to the lowest energy species. However, the syn adduct lies only 1.8 kcal/mol higher in energy. The linear form does not represent a minimum on the energy surface but rather the lowest energy transition state for intramolecular anti <=> syn isomerization. The calculations of CH₃CHO/BF₃, of 6, and of the free aldehydes clearly point to LUMO lowering and to an increased positive charge at the carbonyl carbon atom upon complexation. The results are discussed in light of Lewis acid mediated aldehyde additions involving allyl and enolsilanes, stannanes, and cyanotrimethylsilane as well as such processes as Diels-Alder, ene, and Grignard reactions.