Kinetic studies on the thermal cis-to-trans isomerization of 1-phenyltriazenes derived from cyclic amines.

Rate constants for thermal cis-to-trans isomerization of N-(phenylazo)-substituted nitrogen ring heterocyles were determined as a function of phenyl ring substitution, cyclic amine ring size, and organic solvents. Observed first-order rate constants are found to increase with increasing electron-withdrawing character of the para substituent, with larger amine rings, and with increasing solvent polarity. Overall, trends observed are consistent with geometrical isomerization taking place through rotation around the nitrogen-nitrogen double bond via a polarized transition state.

On the alcohol- and water-catalyzed tautomerization of vitamins K1- and K2-derived quinone methide intermediates.

Rates of conversion of 1,3-quinone methides into the corresponding 1,2-quinone methide tautomers, formed upon laser-flash excitation of vitamins K(1) and K(2) in CH(3)CN solutions, were determined in the presence of hydroxylic solvents (ROH; R = H, alkyl). In all cases, the tautomerization process is accelerated in the presence of ROH, and the corresponding observed rate constants show a cubed dependence on ROH concentration. This high-order dependency is attributed to a proton-relay transfer involving 3 equiv of ROH in each case.

Substituent effects on the thermal cis-to-trans isomerization of 1,3-diphenyltriazenes in aqueous solution.

The thermal cis-to-trans isomerization of some symmetrically p,p'-disubstituted 1,3-diphenyltriazenes has been studied by means of laser-flash photolysis techniques. The geometric isomerization is catalyzed by general acids and general bases as a result of acid/base-promoted 1,3-prototropic rearrangements. Acid catalysis becomes more prominent as the electron-donating character of the para substituent increases, while base catalysis becomes more important as the electron-withdrawing character of the para substituent increases. In addition, the rate ascribed to the interconversion of neutral cis rotamers through hindered rotation around the nitrogen-nitrogen single bond is found to decrease as the electron-withdrawing character of the para substituent increases. Rates of interconversion of neutral cis rotamers are also found to decrease with decreasing solvent polarity, which is indicative of the involvement of a polar transition state. On the other hand, kinetic investigations of the acid-catalyzed decomposition of target triazenes are consistent with an A1 mechanism.

On the Mechanism of the Acid/Base-Catalyzed Thermal Cis-Trans Isomerization of Methyl Orange.

The thermal cis-trans isomerization of methyl orange (i.e., sodium 4-[4'-(dimethylamino)phenylazo]benzenesulfonate) in acidic aqueous solutions has been investigated by means of laser-flash photolysis techniques. The thermal cis-trans isomerization is found to be catalyzed by general acids and general bases. Catalysis is attributed to acid/base-assisted tautomerization of cis-ammonium ions (formed by rapid protonation of cis-methyl orange generated upon photochemically induced trans-cis isomerization) into cis-azonium ions. The latter can easily isomerize via rotation around the -N=N- bond as a result of the concomitant decrease in the double bond character. Furthermore, the acidity of cis-ammonium ions is estimated to be significantly lower than that reported for the trans isomers (pK(a) values are 5.0 and 2.70-2.87, respectively). This result is attributed to a decrease in resonance interactions of the two aryl rings in the cis isomer compared with the trans form due to the nonplanar conformation of the former.