Solvent effects on the sensitized photoxygenation of lidocaine

The Singlet Oxygen Oxidation of Chlorpromazine and Some Phenothiazine Derivatives. Products and Reaction Mechanisms

A kinetic and product study of the reactions of chlorpromazine 1, N-methylphenothiazine 2, and N-ethylphenothiazine 3 with singlet oxygen was carried out in MeOH and MeCN. 1 undergoes exclusive side-chain cleavage, whereas the reactions of 2 and 3, in MeOH, afforded only the corresponding sulfoxides. A mechanism for the reaction of 1 is proposed where the first step involves an interaction between singlet oxygen and the side-chain dimethylamino nitrogen. This explains why no side-chain cleavage is observed for 2 and 3.

Solvent effect on the sensitized photooxygenation of 2,3-dihydropyrazine derivatives

Detection of O(2)((1)Delta(g)) phosphorescence emission, lambda(max) = 1270 nm, following laser excitation and steady-state methods was employed to determine the total rate constant, k(T), and the chemical reaction rate constant, k(R), for reaction between 5,6-disubstituted-2,3-dihydropyrazines and singlet oxygen in several solvents. Values of k(T) ranged from 0.26 x 10(5) M(-1) s(-1) in hexafluoro-2-propanol to 58.9 x 10(5) M(-1) s(-1) in N,N-dimethylacetamide for 5,6-dimethyl-2,3-dihydropyrazine (DMD) and from 5.74 x 10(5) M(-1) s(-1) in trifluoroethanol to 159.0 x 10(5) M(-1) s(-1) in tributyl phosphate for 5-methyl-6-phenyl-2,3-dihydropyrazine (MPD). Chemical reaction rate constants, k(R), for DMD are similar to k(T) in polar solvents such as propylencarbonate, whereas for MPD in this solvent, the contribution of the chemical channel to the total reaction is about of 4%. Dependence of the total rate constant on solvent microscopic parameters, alpha and pi, for DMD can be explained in terms of a reaction mechanism that involves formation of a perepoxide exciplex. Replacement of the methyl by a phenyl substituent enhances dihydropyrazine ring reactivity toward singlet oxygen and modifies the dependence of k(T) on solvent parameters, specially on the Hildebrand parameter. These results are explained in terms of an additional reaction path, involving a perepoxide-like exciplex stabilized by the interaction of the negative charge on the terminal oxygen of the perepoxide with the aromatic pi system.

Sensitized photoxygenation of piroxicam in neat solvents and solvent mixtures

Detection of O(2)(1Delta(g)) phosphorescence emission, lambda(max)=1270 nm, following laser excitation and steady state methods were employed to determine the total rate constant, k(T), for the reaction between the non-steroidal anti-inflammatory drug piroxicam (PRX) and singlet oxygen in several solvents. Values of k(T) ranged from 0.048+/-0.003 x 10(6) M(-1) s(-1) in chloroform to 71.2+/-2.2 x 10(6) M(-1) s(-1) in N,N-dimethylformamide. The chemical reaction rate constant, k(R), was determined by using thermal decomposition of 1,4-dimethylnaphthalene endoperoxide as the singlet oxygen source. In acetonitrile, the k(R) value is equal to 5.0+/-0.4 x 10(6) M(-1) s(-1), very close to the k(T) value. This result indicates that, in this solvent, the chemical reaction corresponds to the main reaction path. Dependence of total rate constant on the solvent parameters pi* and beta can be explained in terms of a reaction mechanism that involves the formation of a perepoxide intermediate. Rearrangement of the perepoxide to dioxetane followed by ring cleavage and transacylation accounts for the formation of N-methylsaccharine and N-(2-pyridyl)oxamic acid, the main reaction products. Data obtained in dioxane-water (pH 4) mixtures with neutral enolic and zwitterionic tautomers of piroxicam in equilibrium show that the zwitterionic tautomer reacts with singlet oxygen faster than the enolic tautomer.