Conformational analysis of stabilized phosphonium ylides by ¹H nuclear magnetic resonance spectroscopy

Rotational barriers (ΔG‡) for methoxycarbonyl groups in methyl 4-hydroxy-1,1-diphenyl-3-[(E)-phenylimino]-3H-λ5-phosphole-2- carboxylate (8) (ΔG‡ = 62 ± 4 kJ mol-1, T c = 283 ± 5 K) and its synthetic precursor, dimethyl 2-(methyldiphenylphosphoranylidene)-3-phenyliminosuccinate (6) (ΔG‡ = 45 ± 2 kJ mol-1, Tc = 208 ± 5 K), as well as the related compounds, dimethyl 3-benzylidene-2- (methyldiphenylphosphoranylidene)succinate (7) (ΔG‡ = 62 ± 3 kJ mol-1, Tc = 278 ± 5 K) and methyl 2-(methyldiphenylphosphoranylidene)-3-phenyliminopropionate (5) (ΔG‡ = 52 ± 2 kJ mol-1, Tc = 243 ± 5 K), were determined by low-temperature 1H n.m.r. Individual rotamers were identified and their n.m.r. signals assigned by means of low-temperature nuclear Overhauser effect (n.O.e.) studies on the ester (5). Protonation of the phenylimino derivatives (5) and (6) was found to occur at the nitrogen atom rather than at the phosphorane α-carbon atom.