STUDY OF SUBSTITUENT EFFECTS ON NITROGEN ELIMINATION FROM AZO-COMPOUNDS VIA CATIONIC INTERMEDIATES - ACETOLYSIS OF (5-ARYL-3,5-DIMETHYL-1-PYRAZOLIN-3-YL)METHYL TRIFLUOROMETHANESULFONATES

Acetolyses of (3, 5, 5-trimethyl-1-pyrazolin-3-yl)methyl trifluoromethanesulfonate and a series of (5-aryl-3, 5-dimethyl-1-pyrazolin-3-yl)methyl trifluoromethanesulfonates containing P-CH3O, p-CH3, p-H, and p-Cl substituents on the aryl ring were compared to see if the charge-stabilizing 5-aryl groups could induce a concerted one-step ionization-nitrogen elimination mechanism with C5-N bond breaking in the rate-determining transition state. All acetolysis kinetics were strictly first order. The total reactivity spread for all of the pyrazoline trifluoromethanesulfonates was ca. 6. This compares to an expected total reactivity difference of 107 if appreciable C5-N bond breaking occurs in the rate-determining step. A study with (3, 5-dimethyl-5-phenyl-1-pyrazolin-3-yl)methyl and (3-methyl-5-methyI-d3-5-phenyl-1-pyrazolin-3-yl)methyl trifluoromethanesulfonates showed a kinetic kH/kD value of 0.98 ± 0.01 for acetolysis. The products from acetolysis of (3, 5-dimethyl-5-phenyl-1-pyrazolin-3-yl)methyl trifluoromethanesulfonate were three isomeric dienes of unrearranged carbon skeleton and a quantitative yield of nitrogen. For the 5-aryl-substituted pyrazoline trifluoromethanesulfonates, all of this is evidence that C5-N bond breaking is not of importance in the rate-determining step. The results show that a stepwise mechanism involving C3-N bond breaking in the rate-determining transition state to give a diazonium ion intermediate followed by formation of a 2-aryl-4-methyl-4-penten-2-yl cation is the strongly favored reaction route. © 1979, American Chemical Society. All rights reserved.