ELIMINATION-REACTIONS OF HYDRAZONIUM SALTS - EXPERIMENTAL AND THEORETICAL EVIDENCE FOR A LARGE STEREOELECTRONIC EFFECT OF NITROGEN

Elimination of trimethylamine from hydrazonium salts (8, R = Me) is promoted by base (methoxide ion in methanol). The mechanism is characterized as E2 as shown by substituent effects (p-+2.57), Bronsted coefficients, a primary kinetic isotope effect (kH/kD= 2.10), and solvent effects. Variation in the leaving group (8, R = arylmethyl) shows that N-N bond cleavage is less well advanced in the transition state than C-H bond breaking. The elimination to give the nitrile is syn-periplanar and, using the phthalazinium salt 4 as a model for the anti elimination, an anti/syn ratio of ca. 102 is found. The reactions have been modelled using ab initio methods with the transition structures located at the HF/3-21G level and relative energies computed using the MP2/6-31G* method. Using both H20 and NH3as model bases to induce elimination on [H2C=NNH3]+, the calculated energy for anti elimination is lower (in the range 11–13 kcal mol-1) than that for syn elimination. The implications of the results for the ease of formation of nitriles from aldehydes via the hydrazonium salt route are discussed. © 1990, American Chemical Society. All rights reserved.