Photochemistry of lignin model compounds on solid supports

Abstract

. Can. J. Chem. 71, 1340 (1993).The photochemistry of several substituted a-(aryloxy)acetoveratrones that are models for chromophores in lignin has been studied using a combination of laser flash photolysis and product studies in solution and on silica, Na-X zeolite, and cellulose. The lifetimes of the triplet ketones vary substantially with the electron-donating ability of the substituents in the a-aryloxy ring, with values ranging from 30 ns for the 4-methoxy derivative to 5.5 ps for the 4-cyano ketone in acetonitrile. The triplet ketones are considerably longer lived on a silica surface than in solution and do not decay with first-order kinetics; however, they are all shorter lived than triplet acetoveratrone under the same conditions, indicating that P-phenyl quenching and (or) P-cleavage dominate their decay. The same triplets show quite different kinetic behavior in Na-X or cellulose and there is no indication in these systems for the importance of either P-phenyl quenching or P-cleavage in determining the rates of triplet decay. Product studies for a-guaiacoxyacetoveratrone indicate that Pcleavage from the singlet excited state is responsible for product formation; in contrast to solution results, the phenacyl-phenoxyl radical pairs react exclusively by geminate recombination to give rearranged ketones resulting from ortho and para ring coupling of the two radicals. Product formation is very inefficient for this ketone in cellulose, indicating either a low singlet reactivity or efficient geminate recombination of the initial radical pair to regenerate starting material. Utilisant une combinaison de photolyse Cclair au laser et une Ctude des produits en solution, sur de la silice, sur une zColite Na-X et sur de la cellulose, on a CtudiC la photochimie de plusieurs a-(aryloxy)acCtovCratrones qui sont des modkles pour les chromophores de la lignine. Les temps de vie dcs cCtones triplet varient beaucoup avec les capacitts des substituants du cycle aryloxy ?i donner des Clectrons; les valeurs varient de 30 ns pour le dCrivC 4-mtthoxy 5 5,5 ps pour la 4-cyanocCtone dans I'acCtonitrile. Les cCtones triplet ont des temps de vie beaucoup plus ClevCs sur une surface de silice qu'en solution et leur dCgCnCrescence n'est pas du premier ordre; toutcfois, leurs temps de vie sont tous plus courts que celui du triplet de I'acCtovCratrone dans les mCmes conditions et ces rksultats indiquent que la relaxation P-phknyle et (ou) un clivage P dominent leur dCgCnCrescence. Les mCmes triplets prtsentent un comporternent cinCtique trks diffCrent avec Na-X ou la cellulose et il ne semble pas que, dans ces systi-mes, la relaxation P-phCnyle ou un clivage P joue un r6le important dans la determination de la vitesse de dCgCnCrescence de I'Ctat triplet. Des etudes de produit pour l'a-guai'acoxyacttovCratrone indiquent que le clivage P B partir de 1'Ctat singulet excitC est responsable pour la formation des produits; par opposition aux rksultats obtenus en solution, Ics paires de radicaux phenacyle-phCnoxyle rkagissent exclusivement p...

Radical-induced degradation of a lignin model compound. Decomposition of 1-phenyl-2-phenoxyethanol

“…The approximate twofold increase in the yield of the ketone 4 upon introduction of oxygen is consistent with a mechanism in which direct reaction with oxygen (reaction [8]) competes with the bimolecular disproportionation of 1' to give one equivalent of 1 and one of 4 (reaction [9]). In the presence of oxygen all of the ketyl radicals are trapped by reaction [8] and subsequently tranformed into 4.…”

“…They proposed that up to 70% of the photogenerated chromophores were generated via path C in which ketyl radicals, generated from P-guaiacoxybenzyl alcohols, underwent fragmentation to give the phenoxyl radical directly and a substituted acetophenone (via the initally formed enol). While the formation and reactivity of phenoxyl radicals in the lignin matrix is not well understood, a number of photochemical studies (5)(6)(7)(8)(9)(10)(11)(12) using model compounds in solution or adsorbed on solid supports have provided some insights into the factors influencing all of the proposed mechanisms. In one of these studies (7) it was suggested that the fragmentation of 1 -phenyl-2-phenoxyethanol-1 -yl radical (1') formed from the reaction of photogenerated tert-butoxyl radicals with 1-phenyl-2-phenoxyethanol (1, eqs. [I], [2]) undergoes rapid fragmentation to yield phenoxyl radical and acetophenone (2, eq.…”

Electrochemistry of Electron-Transfer Probes. The Role of the Leaving Group in the Cleavage of Radical Anions of α-Aryloxyacetophenones1

“…In this work the rate constant for the fragmentation was estimated from a product study in which the α-phenoxy fragmentation occurred in competition with the loss of halide ion from the acetophenone ring. Thermally and photochemically induced fragmentations of the C−OPh bond in other α-aryloxyacetophenones also have received considerable attention as it is believed that these are important reactions in the degradation of lignin, − leading to the photoyellowing of paper. While most of the color reversion is thought to occur via photogenerated aryloxyl radicals, lignin also contains alkoxystilbene substructures which can act as photoreductants. , It is likely that some degradation of the lignin matrix occurs via the α-aryloxyacetophenone radical anions which are formed from a photoinduced electron transfer route in which electron-rich alkoxyl stilbenes are electron donors.…”

Design, synthesis and antifungal activity of carboxylic acid amide fungicides: part 2: substituted 1-phenyl-2-phenoxyethylamino valinamide carbamates

“…Melting points were determined on an X-4 binocular microscope melting point apparatus (Beijing Tech Instruments, Beijing, China) and were uncorrected. Compounds 4a-h were synthesized by reaction of an α-bromoacetophenone with the corresponding phenol in dry acetone and anhydrous K 2 CO 3 according to the literature, 21 and were recrystallized from ethanol. All reagents are analytical grade.…”

“…General synthetic procedure for 1-(3,4-dimethoxyphenyl)-2-(substituted phenoxy)ethanone (4a-h). Compounds 4a-h were synthesized by reaction of an a-bromoacetophenone with the corresponding phenol in dry acetone and anhydrous K 2 CO 3 according to the literature, 21 and were recrystallized from ethanol. The data for compounds 4a-h can be found in the ESI.…”