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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.
