ChemInform Abstract: Structure and Electronic Nature of the Benzaldehyde/Boron Trifluoride Adduct

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-OC-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/BF, 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 CH,CHO/BF,, 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. A wide variety of C-C bond-forming reactions of carbonyl compounds are mediated by Lewis acids such as BF3, AICI,, EtAlCI,, TiCI4, SnC1,. and ZnX,. They include carbonyl additions of allylsilanes, enosilanes, cyanotrimethylsilane, and other silyated' and stannylated* carbon nucleophiles, as well as ene reaction^,^ Diels-Alder addition^,^ and hetero-Diels-Alder cycloconden~at ions.~ It is generally accepted that the Lewis acid activates the carbonyl component by forming an adduct prior to C-C bond formation. Whereas X-ray crystallographic data of a TiCI4 adduct of a chiral acrylic acid ester has recently been reported and discussed with regard to stereoselective Diels-Alder reactions,6 precise structural information concerning complexes of aldehydes with the above-mentioned Lewis acid remains to be presented. N M R , UV, and I R data of common aldehyde/Lewis acid complexes are available, but they do not answer the question of anti vs. syn complexation (1 vs. 2; MX, = Lewis acid).' A great deal of experimental and theoretical work concerning the interaction of formaldehyde with Li+ and other Lewis acids has accumulated ' Philipps-Universitat. 'Universitat Essen. R H KMX