Kinetics of direct and water-mediated tautomerization reactions of four-membered cyclic lactims to amides or lactams

Abstract

As part of a series of studies of hydrogen-atom transfer or tautomerization reactions of imidic acid-amide species, $H$─$O$─$C$═$N$─ $\rightleftharpoons O$═$C$─$\text{NH}$─, we report the rate constants for a set of 16 four-membered cyclic compounds at low, 50–300 K, and high, 500–1500 K, temperatures. The compounds are labeled according to the two ring groups X and Y, which can be ${\text{CH}}_2$, NH, CH, N, O, or C(O) and which are at some remove from the reactive site. These rate constants are for both the direct reaction and for that mediated by an additional water molecule, which facilitates the hydrogen transfer reaction. In the latter case, we show that the rate of reaction from a pre-reaction complex is rapid at temperatures down to 50 K and dominated by quantum mechanical effects as evaluated by small-curvature and quantized-reaction-states tunneling. In addition, we present thermochemical data such as enthalpies of formation, entropies, isobaric heat capacities, and enthalpy functions for these largely unknown species, which span a range of compounds from $\beta$-propiolactone to 1,3-diazetidine-2,4-dione.