Direct carbon-13 nuclear magnetic resonance study of boron trifluoride complexes with alkyl and cyclic ketones

Abstract

A study of the born trifluoride complexes of the C₄–C₁₀ cycloalkanones and nine alkyl ketones has been completed using a direct, low-temperature carbon-13 n.m.r. technique. At temperatures in the rang –85 to –125 °C, exchange is slow enough on the n.m.r. time scale to permit the observation of separate ¹³C resonance signals for bond and free ligand. The chemical shift displacements induced by complex formation were ca. 24 p.p.m. to lower field for the carbonyl group signal, and about an order of magnitude less, to higher and lower field, for the other carbon signals. Doublets were observed for the signals of bond ligand with several members of both series, a feature which is best interpreted in terms of a slow exchange of the BF₃ between the two sp² orbitals of the carbonyl oxygen atom. Some evidence is presented for the much greater stability of only one of these cis-trans-isomers in four of the alkyl ketone cases.