Structural studies of carbanionic species α to a phosphoryl group; anion from ketophosphonate comparison with related enoxysilanes and p-ylid

The structure of the carbanionic species 4 formed from diethyl(2-oxopropyl)phosphonate and t-BuOK has been studied by ¹H, ¹³C and ³¹P NMR. In an associating medium (pyridine), a single chelated (C) carbanionic species 4Z (C) is observed. Two slowly interconverting species 4Z and 4E co-exist when the anion-cation interaction is loose, i.e. in Me₂SO and when the gegenion K⁺ is complexed to (2,2,2)cryptand in Me₂SO. The anionic carbon of 4Z and 4E is planar, and they have a strong enolate character as shown by the barrier to rotation around the C-1? C-2 bond (ΔG_c?～92 KJ mol^?1). Their structures are compared to those of the related E and Z 1-diethylphosphonato-2-trimethylsilyloxypropenes and to (acetylmethylene)triphenylphosphorane.