Conformational differences in stabilized diketo and diester phosphonium ylides in a solid and in solution
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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, H-1 and C-13 NMR signals are sharp, but C-13 signals in the solid confirm that the two acyl groups are in different positions, and a comparison of the C-13 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 degrees and 170.4 degrees, due to packing forces and interactions with adjacent molecules, which limit ylidic resonance. The C-13 NMR spectrum in the solid differs from that in solution, with the two ester groups in different positions, but in solution, C-13 and H-1 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.