LITERÁK, Jaromír, Anna DOSTÁLOVÁ and Petr KLÁN. Chain Mechanism in the Photocleavage of Phenacyl and Pyridacyl Esters in the Presence of Hydrogen Donors. Journal of Organic Chemistry. Columbus, OH, USA: the American Chemical Society, 2006, vol. 71, No 2, p. 713-723. ISSN 0022-3263.
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Basic information
Original name Chain Mechanism in the Photocleavage of Phenacyl and Pyridacyl Esters in the Presence of Hydrogen Donors
Name in Czech Řetězový mechanismus fotochemického štěpení fenacyl a pyridacyl esterů
Authors LITERÁK, Jaromír (203 Czech Republic), Anna DOSTÁLOVÁ (203 Czech Republic) and Petr KLÁN (203 Czech Republic, guarantor).
Edition Journal of Organic Chemistry, Columbus, OH, USA, the American Chemical Society, 2006, 0022-3263.
Other information
Original language English
Type of outcome Article in a journal
Field of Study 10401 Organic chemistry
Country of publisher United States of America
Confidentiality degree is not subject to a state or trade secret
Impact factor Impact factor: 3.790
RIV identification code RIV/00216224:14310/06:00015580
Organization unit Faculty of Science
UT WoS 000234837500034
Keywords in English Photochemistry; Photoremovable protecting groups
Tags Photochemistry, photoremovable protecting groups
Tags International impact
Changed by Changed by: prof. RNDr. Petr Klán, Ph.D., učo 32829. Changed: 23/6/2009 15:18.
Abstract
Excited phenacyl and 3-pyridacyl esters of benzoic acid react with an excess of aliphatic alcohols in a chain reaction process, involving hydrogen transfer from the ketyl radical intermediates, leading to benzoic acid in addition to acetophenone and 3-acetylpyridine, respectively, as the by-products. While the maximum quantum yields reached 4 in both cases, the 2- or 4-pyridacyl ester photoreduction proceeded with the efficiency below 100% under the same conditions. The investigation indicates that a radical coupling between ketyl radicals, both formed from the excited ester by hydrogen abstraction from an alcohol, is accompanied by the elimination of benzoic acid from the ester ketyl radical itself. A partitioning between two reactions was found to be remarkably sensitive to the chromophore nature, such as a position of the nitrogen atom in the pyridacyl moiety. The magnitude of a radical chain process is dependent on the efficiency of consecutive steps that produce free radicals capable of a subsequent ester reduction. The driving force of a possible electron transfer from the ketyl radicals to the ester has been excluded on the basis of cyclic voltametry measurements. The observed quantum yields of photoreduction were found to be diminished by formation of relatively long-lived light absorbing transients, co-products obtained apparently by secondary photochemical reactions. Besides, it is shown that basic additives such as pyridine can further increase the efficiency of the photoreduction by a factor of 4. A radical nature of the reduction mechanism was supported by finding a large kinetic chain length of an analogous reaction initiated by free radicals generated thermally, yet again when phenacyl or 3-pyridacyl benzoate was used. Both phenacyl and pyridacyl chromophores are pronounced to be valuable as the photoremovable protecting groups, when high quantum and chemical yields of carboxylic acid elimination are important but higher concentrations of the hydrogen atom donors are not destructive for a reaction system or experimentally impractical.
Abstract (in Czech)
Excited phenacyl and 3-pyridacyl esters of benzoic acid react with an excess of aliphatic alcohols in a chain reaction process, involving hydrogen transfer from the ketyl radical intermediates, leading to benzoic acid in addition to acetophenone and 3-acetylpyridine, respectively, as the by-products. While the maximum quantum yields reached 4 in both cases, the 2- or 4-pyridacyl ester photoreduction proceeded with the efficiency below 100% under the same conditions. The investigation indicates that a radical coupling between ketyl radicals, both formed from the excited ester by hydrogen abstraction from an alcohol, is accompanied by the elimination of benzoic acid from the ester ketyl radical itself. A partitioning between two reactions was found to be remarkably sensitive to the chromophore nature, such as a position of the nitrogen atom in the pyridacyl moiety. The magnitude of a radical chain process is dependent on the efficiency of consecutive steps that produce free radicals capable of a subsequent ester reduction. The driving force of a possible electron transfer from the ketyl radicals to the ester has been excluded on the basis of cyclic voltametry measurements. The observed quantum yields of photoreduction were found to be diminished by formation of relatively long-lived light absorbing transients, co-products obtained apparently by secondary photochemical reactions. Besides, it is shown that basic additives such as pyridine can further increase the efficiency of the photoreduction by a factor of 4. A radical nature of the reduction mechanism was supported by finding a large kinetic chain length of an analogous reaction initiated by free radicals generated thermally, yet again when phenacyl or 3-pyridacyl benzoate was used. Both phenacyl and pyridacyl chromophores are pronounced to be valuable as the photoremovable protecting groups, when high quantum and chemical yields of carboxylic acid elimination are important but higher concentrations of the hydrogen atom donors are not destructive for a reaction system or experimentally impractical.
Links
GA203/05/0641, research and development projectName: Vývoj a aplikace fotolabilních chránících skupin
Investor: Czech Science Foundation, Design and Applications of Photoremovable Protecting Groups
MSM0021622413, plan (intention)Name: Proteiny v metabolismu a při interakci organismů s prostředím
Investor: Ministry of Education, Youth and Sports of the CR, Proteins in metabolism and interaction of organisms with the environment
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