A description of the effects of silyl vs. alkyl substituents in ketones in the light of oxygen-17 NMR data and with the help of qualitative MO theory

Abstract

17O NMR spectra (natural abundance) for selected acylsilanes show high-frequency chemical shifts with respect to the corresponding ketones. An MO qualitative analysis clarifies the origin of the above experimental finding as being due to other well-known chemical and physical differences between the two types of compound. The importance of a relatively small HOMO—LUMO gap in acylsilanes is stressed. The alkyl and silyl substituents promote different electron distributions over the CO σ and π fragment orbitals which can ultimately contribute to the shift of the NMR resonances. The MO analysis addresses new viewpoints on acylsilanes: (i) the silyl groups, rather than strong σ-donors should be better defined as weak σ-acceptors; (ii) the carbonyl carbon atom is negatively charged, but the electron density is accumulated in the σ orbitals and not in the p π one; (iii) the IR data are rationalized in terms of a weakened in plane component of the CO bond, while the π ⊥ bond is unaffected; (iv) the carbonyl carbon atom keeps its electrophilic character in acylsilanes and it can exert nucleophilicity only upon departure of SiR + 3, a good leaving group.