Conformational analysis of 3-triphenylphosphoranylidene-2,4-pentanedione by NMR spectroscopy and geometrical optimization
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The diacyl ylide, 3-triphenylphosphoranylidene-2,4-pentanedione (2) can adopt conformations stabilized by ylidic resonance with both acyl oxygens syn- or with one oxygen syn- and the other anti- to cationoid phosphorus, 2a, Z, Z and 2b, Z, E respectively. In solution the CH3 groups give single H-1 and C-13 NMR signals indicating either that 2a is dominant, or that conformational equilibration is fast on the NMR time scale. However, two sets of C-13 signals of CH3 and CO are observed in the solid and comparison of the chemical shifts shows that 2a is the dominant conformer in solution and 2b in the solid. Calculations with A initio HF basis sets indicate that the heat of formation of 2a is slightly more favorable than of 2b, and that the rotational barrier for interconversion is low. These energetic differences are much smaller than for the otherwise similar monoacyl ylides and are related to differences in ylidic resonance in conformational equilibration in mono-and diacyl ylides.