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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 π 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. [ABSTRACT FROM AUTHOR]

The diacyl ylid, 3-triphenylphosphoranylidene-2,4-pentanedione 2 can adopt conformations with one oxygen syn- and the other anti- to phosphorus 2a as in the crystal, or with both oxygens syn- to phosphorus 2b. In solution, 1H and 13C NMR signals are sharp, but 13C signals in the solid confirm that the two acyl groups are in different positions, and a comparison of the 13C signals in solution and a solid shows that conformations differ. Ab initio geometrical optimizations give, for 2a, a geometry similar to that in a crystal, and structures of both conformers permit extended ylidic resonance through near planar ylidic and acyl moieties. In crystalline diethyl 2-triphenyl phosphoranylidene malonate 3, the torsional angles of the ester moieties to the plane of the near trigonal ylidic moiety are -149.2° and 170.4°, due to packing forces and interactions with adjacent molecules, which limit ylidic resonance. The 13C NMR spectrum in the solid differs from that in solution, with the two ester groups in different positions, but in solution, 13C and 1H NMR signals are sharp. Ab initio geometrical optimizations indicate that for an isolated molecule, the conformer with torsional angles as in the crystal is energetically unfavored but energetic barriers to interconversion are small, and rapidly equilibrating conformers probably exist in solution. [ABSTRACT FROM AUTHOR]