Predicting Absolute Rate Constants for Huisgen Reactions of Unsaturated Iminium Ions with Diazoalkanes.

The kinetics and stereochemistry of the reactions of iminium ions derived from cinnamaldehydes and MacMillan's imidazolidinones with diphenyldiazomethane and aryldiazo-methanes were investigated experimentally and with DFT calculations at the (SMD=DCM)//B3LYP-D3BJ/def2svp level of theory. The reactions of diphenyldiazomethane with iminium ions derived from MacMillan 2nd generation catalysts gave 3-aryl-2,2-diphenylcyclopropanecarbaldehydes with yields >90 % and enantiomeric ratios of >/=90:10. Predominantly 2:1 products were obtained from the corresponding reactions with monoaryldiazo-methanes. The measured rate constants are in good agreement with the rate constants derived from the one-bond nucleophilicity parameters N and sN of diazomethanes and the one-bond electrophilicity parameters E of iminium ions as well as with quantum chemically calculated activation energies.