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Chemoselective Alkylating Esterification of Thiophosphinic Acids: An Experimental and Theoretical Study

Year: 2026

Type: article

Abstract: Preparative experiments including the alkylation of diphenylthiophosphinic acid with a series of C 1 ‐C 4 haloalkenes at 75/115°C in the presence of triethylamine in toluene suggested chemoselective formation of the corresponding S‐ (thiolic) ester. No O‐(thionic) ester was formed. For this, we wished to clarify the confusion regarding the outcome of such alkylations described in the literature. A few analogous derivatives were also synthesized and characterized for comparison. First in the literature, crystal structures of two thiolic esters were analyzed in detail. The alkylation led to non‐centrosymmetric crystal structures. Moreover, the two isostructural thiolic compounds formed chiral crystals, a rather unusual feature of achiral molecules. Weak interactions such as C–H … O bridges and dispersion dominate and tailor the structure of the crystals. As regards the mechanism, three possible pathways were investigated at the B3LYP/6‐31G(d,p)/PCM[toluene] level of theory. The activation enthalpy for the rate‐determining step was significantly lower for the triethylamine‐catalyzed route than for the base‐free variation. Moreover, in the base‐catalyzed protocol, the difference of 30.8 kJ mol –1 between the enthalpy of activation for the S and O‐alkylations justified the S‐selectivity. The pathway involving the anion from the thiophosphinic acid was found unfavorable and hence was excluded.

Source: ChemPlusChem