A common conformation of stabilized triphenyl phosphonium ylidic diesters with bulky alkoxy groups
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The phosphonium ylidic diesters, methyl and ethyl isopropyl and, methyl and ethyl t-butyl triphenylphosphoranylidene malonates, 1a,b and 2a,b, respectively, have the syn-anti conformation in solution, as in the crystal, and the bulkier alkoxy group is oriented towards phosphorus. The 1H NMR spectra show that in 1a,b, the isopropyl group is oriented towards the face of a phenyl group, consistent with shielding in the 1H signals, and examination of the 1H coupled 13C NMR spectra allows assignment of the acyl carbon signals. Computed bond lengths and angles for isolated molecules are similar to those in the crystal, and the geometry and the NMR spectra indicate extensive ylidic resonance. Estimated partial atomic charges on the ester oxygens are more negative when they are oriented towards, rather than away from, phosphorus.
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