Bimolecular Hydrogen Abstraction from Phenols by Aromatic Ketone Triplets†

Abstract

Absolute rate constants for hydrogen abstraction from 4-methylphenol (para-cresol) by the lowest triplet states of 24 aromatic ketones have been determined in acetonitrile solution at 23 degrees C, and the results combined with previously reported data for roughly a dozen other compounds under identical conditions. The ketones studied include various ring-substituted benzophenones and acetophenones, alpha,alpha,alpha-trifluoroacetophenone and its 4-methoxy analog, 2-benzoylthiophene, 2-acetonaphthone, and various other polycyclic aromatic ketones such as fluorenone, xanthone and thioxanthone, and encompass n,pi*, pi,pi*(CT) and arenoid pi,pi* lowest triplets with (triplet) reduction potentials (E(red)*) varying from about -10 to -38 kcal mol(-1). The 4-methylphenoxyl radical is observed as the product of triplet quenching in almost every case, along with the corresponding hemipinacol radical in most instances. Hammett plots for the acetophenones and benzophenones are quite different, but plots of log k(Q) vs E(red)* reveal a common behavior for most of the compounds studied. The results are consistent with reaction via two mechanisms: a simple electron-transfer mechanism, which applies to the n,pi* triplet ketones and those pi,pi* triplets that possess particularly low reduction potentials, and a coupled electron-/proton-transfer mechanism involving the intermediacy of a hydrogen-bonded exciplex, which applies to the pi,pi* ketone triplets. Ketones with lowest charge-transfer pi,pi* states exhibit rate constants that vary only slightly with triplet reduction potential over the full range investigated; this is due to the compensating effect of substituents on triplet state basicity and reduction potential, which both play a role in quenching by the hydrogen-bonded exciplex mechanism. Ketones with arenoid pi,pi* states exhibit the fall-off in rate constant that is typical of photoinduced electron transfer reactions, but it occurs at a much higher potential than would be normally expected due to the effects of hydrogen-bonding on the rate of electron-transfer within the exciplex.

A reatividade do estado excitado triplete de 1,3-indanodiona em benzeno

“…A abstração de hidrogênio fenólico por estados excitados triplete de carbonilas normalmente é mais rápida que aquela observada para toluenos, uma vez que fenóis apresentam tanto uma menor energia de dissociação da ligação O-H, comparada à C-H, quanto um menor potencial de oxidação que toluenos. Assim, o mecanismo da reação de abstração de hidrogênio de fenóis deve ocorrer através da formação inicial de um exciplexo triplete, estabilizado por ligação de hidrogênio, seguido por uma transferência seqüencial de elétron e de próton [17][18][19][20][21][22][23][24][25] . 1,4-diazabiciclo[2.2.2]octano (DABCO) suprime de maneira eficiente (k q =5,5x10 9 L mol -1 s -1 , Tabela 1, Figura 4) o estado excitado triplete de 1,3-indanodiona (1) através um processo de transferência de elétron, levando à formação do ânion radical da dicetona, isto é 4, e do cátion radical derivado da amina (Esquema 2).…”

Phenolic hydrogen abstraction by the triplet excited state of thiochromanone: a laser flash photolysis study

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] The mechanism for this process is now well established and involves the initial formation of a hydrogen-bonded exciplex in the triplet excited state, followed by electron transfer and, subsequently, an ultrafast proton transfer. 4,9-16 DFT calculations clearly showed that for the triplet complex (exciplex) ketone-phenol the hydrogen transfer is predominantly occurring.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] The mechanism for this process is now well established and involves the initial formation of a hydrogen-bonded exciplex in the triplet excited state, followed by electron transfer and, subsequently, an ultrafast proton transfer. 4,[9][10][11][12][13][14][15][16] DFT calculations clearly showed that for the triplet complex (exciplex) ketone-phenol the hydrogen transfer is predominantly occurring. 8 The very fast quenching rate constants of triplet carbonyls towards phenols is consequence of both the low reduction potential of the former as well as the low oxidation potential of the later.…”

“…8 The very fast quenching rate constants of triplet carbonyls towards phenols is consequence of both the low reduction potential of the former as well as the low oxidation potential of the later. 4,16 Experimentally, this coupled electron/proton transfer mechanism ultimately results in the formation of the ketyl-aryloxyl radical pair.…”

“…8 The very fast quenching rate constants of triplet carbonyls towards phenols is consequence of both the low reduction potential of the former as well as the low oxidation potential of the later. 4,16 Experimentally, this coupled electron/proton transfer mechanism ultimately results in the formation of the ketyl-aryloxyl radical pair.Triplet ketones are known to oxidize biological substrates such as amino acids, nucleosides and DNA. Due to this, topical or systemic administration of drugs containing the carbonyl chromophore can lead to sever sideeffects upon exposure to sunlight.…”