Structural and conformational analysis of 2-triphenylphosphoranylidene succinic acid derivatives by 1H, 13C and 31P one and two dimensional NMR spectroscopy and molecular modelling

Abstract

The structure and conformation of 2-triphenylphosphoranylidenesuccinic acid derivatives have been studied by ¹H, ¹³C and ³¹P NMR spectroscopy, in various solvents and at variable temperatures. In 2-triphenylphosphoranylidenesuccinic anhydride (1) the conformation is constrained but in the corresponding monoethyl (2) and the diethyl (3) esters there is an anticlinal relationship between the phosphorus and hydrogens on C-3 and signals of Z and E isomers were observed with the diethyl ester 3 in solvent-dependent ratios based on ¹H, ¹³C and ³¹P resonances. Similar results were obtained with the diethyl ester of the 3-methyl derivative 4. Protonation (deuteronation) of 2 and 3 changes their conformation and P–H couplings. The NMR evidence on conformations of the phosphoranylidenesuccinic acid esters is consistent with T₁ relaxation times and results of molecular modelling.