NMR OF TERMINAL OXYGEN .10. O-17 NMR-SPECTRA OF CARBONYL PHOSPHONIUM YLIDES AND DIAZOCARBONYL COMPOUNDS - RESONANCE STABILIZATION, BOND ORDER AND EXCITATION-ENERGY

On the scale of carbonyl electrophilicities, measured by O-17 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 H-1 and C-13 NMR shift values. For comparison, a series of phenacyl compounds PhCOCH2X have been investigated; they show a very small effect of delta(o) on delta(o). The delta(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 rho+ of aroyldiazoalkanes 4 is similar to those of esters and amides.