Photoaddition of water and methanol to 2,2,4,6-tetramethyl-1,2-dihydroquinoline

Abstract

Products of the steady-state photolysis of 2,2,4,6-tetramethyl-1,2-dihydroquinoline (1) in water, methanol, and water--ethanol were isolated for the first time and identified by IH and J3C NMR spectroscopy, IR spectroscopy, and mass spectrometry. As a result of the photolysis, the molecule of the solvent is added to the double bond of the heterocycle with formation of 4-hydroxy-(2) or 4-methoxy-2,2,4,6-tetramethyl-1,2,3,4-tetrahydroquinoline (3) in water and methanol, respectively. Compounds 2 and 3 are converted gradually into 1 in the dark. The rate of the back reaction depends on the solvent and the concentration of the product.Comparison of the products of the photolysis in methanol and hexane at 45 ~ and of the azoisobutyronitrile-initiated oxidation of 1 at the same temperature has shown that unlike the photolysis in hexane, aminyl radicals are not precursors of the product of the photolysis in methanol. The reaction proceeds via an excited singlet state.Key words: t,2-dihydroquinolines, photoaddition to the double bond, steady-state photolysis, effect of a solvent, lH and 13C NMR.Effect of the medium on the mechanism and kinetics of chemical transformations is one of the key problems of modern physical chemistry. The polarity of medium and hydrogen bonding between a reagent and a solvent play the most important role in this phenomenon. We have found earlier that the nature of the solvent affects the direction and the kinetics of photolysis of 2,2,4,6-tetramethyl-1,2-dihydroquinoline (1). t 6-Substituted and partly hydrogenated quinolines of general formula A, especially ethoxyquin (R I = EtO and R 2 = H), are widely used as antioxidants and antiozonants for rubbers, 2 as stabilizers for vitamin A and 13-carotene in feed, and as a post-harvest dip for apples. 3,4 Upon photolysis of these compounds in organic solvents (alkanes, benzene, toluene, and propan-2-ol), the hydrogen atom is split out from the molecule of dihydroquinoline and an aminyl radical is formed, which decaysat the ambient temperature predominantly in recombination reaction s giving a l-8"-dimer B. 6,7 The rate constants of the recombination of the transient radicals measured by flash-photolysis s and by ESR 7 are close and vary from 106 to 107 L tool -1 s -l depending on the substituent in position 6 and the solvent character. If the parent 1,2-dihydroquinoline has a substituent R 2 ~ H in position 8, the rate constant for radical decay dramatically decreases and becomes equal to 102 L tool -I s-I. 7The photolysis of aqueous solutions of compound I (A with R 1 = Me and R 2 = H) gives a product the absorption spectrum of which significantly differs from that of the dimer. 1 The kinetics of the decay of the transient species changes crucially: it follows the firstorder law with the rate constant depending on pH and varying from 25 s -t in acidic and neutral solutions to 3.9 s -~ in alkaline solutions i at the pH range corresponding to the value of pK a 10.4 measured for the transient species. ~ These experiments pose a number of questions con...

10.1007/s10975-008-2008-z

“…The steady-state photolysis of 1 in the SDS micellar solutions is similar to photolysis in aqueous solutions: the quantitative formation of product characterized by a UV spectrum similar to the spectrum of the hydroxy adduct formed in water ( λ max = 305, 245, and 214 nm) [6] is observed. Qualitatively, the kinetic regularities of the reaction and the intermediate species generated in the flash photolysis are similar to those observed in water: intermediate species decay is a pseudofirst-order reaction, and the intermediate species structure depends drastically on pH.…”

“…Detailed studies of the photolysis mechanism for 2,2,4-trimethyl-1,2-dihydroquinolines (DHQ) with alkyl and alkoxy substituents in the C(6) and C (8) positions of the aromatic ring showed that the reaction mechanism in water and methanol is radically different [6][7][8]. In these solvents, no aminyl radicals are generated under irradiation and the primary photochemical reaction is double proton transfer from the water or methanol molecule to the excited singlet DHQ molecule ( S 1 ) in the C(3) position of the heterocycle and from the N-H bond to the solvent (Scheme 1).…”

Laser flash photolysis study of the decay kinetics of carbocations of 1,2,2,4-tetramethyl-1,2-dihydroquinoline in the porous glass in the presence of methanol

“…1), which is characteristic of the CC generated from 1,2 dihydro quinolines. [15][16][17][18][19] When comparing the absorption intensi ties of the CC at 480 nm in methanol and the triplet state of benzophenone at 525 nm in benzene (quantum yield 100%, molar absorption coefficient 7200 L mol -1 cm -1 ) 27 detected upon the laser photolysis of the corresponding solutions with the same absorbance at the excitation wave length, we obtain the molar absorption coefficient of the CC equal to ∼4000 L mol -1 cm -1 if the quantum yield of the CC is 40%. 18, 19 The kinetics of CC decay is described by a law corre sponding to parallel reactions of the first and second or ders.…”

Phototransformations of non‐toxic antioxidants, the derivatives of 1,2‐dihydroquinolines, in homogeneous and micellar solutions

“…The preparative-scale steady-state photolysis was carried out, products 2 and 3 were isolated and analyzed by 1 H and 13 C NMR, IR, and mass-spectroscopy [8]. The analysis of the structure of compounds 2 and 3 has shown that corresponding tetrahydroquinoline is formed as a result of the addition of solvent to the double bond of heterocycle.…”

“…The rate of the conversion depends on the solvent, concentration, and weakly on the temperature [8].…”