Nucleophilicity and Electrophilicity Parameters for Predicting Absolute Rate Constants of Highly Asynchronous 1,3-Dipolar Cycloadditions of Aryldiazomethanes.

Kinetics of the reactions of aryldiazomethanes (ArCHN2) with benzhydrylium ions (Ar2CH+) have been measured photometrically in dichloromethane. The resulting second-order rate constants correlate linearly with the electrophilicities E of the benzhydrylium ions which allowed us to use the correlation lg k = sN( N + E) (eq 1) for determining the nucleophile-specific parameters N and sN of the diazo compounds. UV-vis spectroscopy was analogously employed to measure the rates of the 1,3-dipolar cycloadditions of these aryldiazomethanes with acceptor-substituted ethylenes of known electrophilicities E. The measured rate constants for the reactions of the diazoalkanes with highly electrophilic Michael acceptors ( E > -11, for example 2-benzylidene Meldrum's acid or 1,1-bis(phenylsulfonyl)ethylene) agreed with those calculated by eq 1 from the one-bond nucleophilicities N and sN of the diazo compounds and the one-bond electrophilicities of the dipolarophiles, indicating that the incremental approach of eq 1 may also be applied to predict the rates of highly asynchronous cycloadditions. Weaker electrophiles, e.g., methyl acrylate, react faster than calculated from E, N, and sN, and the ratio of experimental to calculated rate constants was suggested to be a measure for the energy of concert Δ G‡concert = RT ln( k2exptl/ k2calcd). Quantum chemical calculations indicated that all products isolated from the reactions of the aryldiazomethanes with acceptor substituted ethylenes (Δ2-pyrazolines, cyclopropanes, and substituted ethylenes) arise from intermediate Δ1-pyrazolines, which are formed through concerted 1,3-dipolar cycloadditions with transition states, in which the C-N bond formation lags behind the C-C bond formation. The Gibbs activation energies for these cycloadditions calculated at the PCM(UA0,CH2Cl2)/(U)B3LYP-D3/6-31+G(d,p) level of theory agree within 5 kJ mol-1 with the experimental numbers showing the suitability of the applied polarizable continuum model (PCM) for considering solvation.