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 δo The δo 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.