NMR of terminal oxygen. Part 10. 17O NMR spectra of carbonyl phosphonium ylides and diazocarbonyl compounds: resonance stabilization, bond order and excitation energy

Abstract

On the scale of carbonyl electrophilicities, measured by ¹⁷O NMR spectroscopy, carbonyl groups in aroyl (1), acetyl (2) and ethoxycarbonyl (3) phosphonium ylides are similar to those of amide groups, showing a considerable effect of resonance donation. The relative weight of the limiting formulae in the resonance in 1 and 3 is found to be close to earlier estimations which were based upon ¹H and ¹³C NMR shift values. For comparison, a series of phenacyl compounds PhCOCH₂X have been investigated; they show a very small effect of X on δ_o The δ_o values of aroyl (4) and ethoxycarbonyl (5) diazoalkanes are found to be less shielded than those of the corresponding ylides, demonstrating the importance of long-wave electronic transitions observable in the UV–VIS spectrum. However, the substituent sensitivity coefficient ρ⁺ of aroyldiazoalkanes 4 is similar to those of esters and amides.