Spectroscopic studies of the structure of carbanionic species in nonaqueous solutions: Identification of reactive intermediates
The comparison of the structure of the chelated carbanions of acetylacetone, acetylacetate keto and esterphosphonate throuQh X—ray, IR and NtIR data sho that the oxygen atom cx to the carbonyl ester group plays an important role in the stability of the chelated ion pair. The charge delocalization beti.€en the carbonyl oxygen atoms and the central carbon atom is more sensitive to the cation field effect in esters as the extra cycle oxygen atom may participate in that delocalization. The phosphorus atom reinforces this effect as it already drains the electrons from this oxygen in the parent phosphonate ester. In the corresponding free anions like in the neutral phosphorus ylids, the activation barrier relating the Z and the E conformer is higher in the ketonic compounds than in esters i.èiere the C±—C bond has a lowar double bond character due to the larger delocalization possibilities and also to the lowar oxygen charge repulsion in phosphonates compounds. The analysis through IR and NMR of the formation mechanism of the ester phosphonate from an organic base or from an amine (DBU) actiuated" lithium salt showa the formation of different intermediate species I. The underprotonated phosphonate is strongly bonded as a bidentate ligand to the lithium cation of the activated lithium salt in these species. Although the protons of the neutral phosphonate part of I becomes more acidic than those of the free ligand, this intermediate species does not participate to the fast exchange observed betwaen the free ligand and the anionic species in the presence of DBU. Such intermediates seem not to be formed with ketophosphonates (study underway) probably due to the lowar basicity of its carbonyl group or to the larger stability of the derived carbanionic chelated ion—pair.