Photoenolization of ortho-alkyl-substituted acetophenones: evidence for the enol triplet state

Findlay, David M.; Tchir, Morris F.

doi:10.1039/f19767201096

Cited by 34 publications

(18 citation statements)

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“…-The results of this investigation provide a fairly complete and consistent picture for the evolution of enolization after excitation of the orthomethylphenyl ketones 1 to 3. Both, 2-methylacetophenone (1) and 2,4-dimethylbenzophenone (3), have already been studied by the technique of flash photolysis [3] [4] [6] and, apart from a single observation stated below (see p. 2603 and Footnote 3 therein), our experimental results are in agreement with those of previous workers. However, we are convinced that our additional findings, as well as the comparison with the conformationally locked tetralone 2, necessitate a reassignment of most transients observed previously.…”

supporting

confidence: 94%

“…However, we are convinced that our additional findings, as well as the comparison with the conformationally locked tetralone 2, necessitate a reassignment of most transients observed previously. Some partly conflicting suggestions for such reassignments have been made in the recent literature: Porter's attribution of transients C and D [3] to E-E (3) and Z-E (3), respectively, has been questioned by Sammes [2] on the basis of chemical trapping experiments, and two contradictory identifications of the transient B [3] were proposed [6] [17] of which we support the latter. Further, a redefinition of Lindqvist's 320 nm transient arising from flash photolysis of 1 [4] was suggested by Tchir [6].…”

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confidence: 65%

“…Tchir [6] has noted that the yield of E-E (l), formed by flash photolysis of 1 in cyclohexane, is increased in the presence of oxygen. This unusual effect was interpreted in terms of the formation of an oxygen quenchable enol triplet which can decay either to enol or ketone ground state.…”

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confidence: 98%

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The Photoenolization of 2‐Methylacetophenone and Related Compounds

Haag¹,

Wirz²,

Wagner³

1977

Helvetica Chimica Acta

195

141

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SummaryA reinvestigation of 2-methylacetophenone (1) by ns flash photolysis has provided detailed evidence for the reaction sequence of photoenolization. The triplet reaction proceeds adiabatically from the lowest excited triplet state of the ketone, 3K (l), to the enol excited triplet state, 3E (l), which decays both to enol and ketone ground state. The Z-and E-isomers of the photoenol, Z-E (1) and E-E (1) are formed in about equal yield by the triplet pathway, while direct enolization from the lowest excited singlet state of 1 yields (predominantly) the 2-isomer. Intramolecular reketonization from 2-E (1) to 1 proceeds at a rate of ca. loss-' in cyclohexane, but can be retarded to ca. 104s-' in hydrogen-bond-acceptor solvents. The proposed mechanism is summarized in Scheme 1 and rationalized on the basis of a state correlation diagram, Scheme 2. 3,3,6,8-Tetramethyl-l-tetralone (2) was used as a reference compound with fixed conformational position of the carbonyl group, and some results from a brief investigation of 2,4-dimethylbenzophenone (3) are also reported.We have recently found [ 11 that the photoenolization of 5-methyl-l,6naphtho-quinone takes place with high efficiency and that the thermal reketonization to starting material is very fast at room temperature. The great retardation of the back reaction from a rate of ca. 105s-' in cyclohexane to ca. lo's-' in hexamethyl phosphoric acid triamide (HMPA) was striking but not surprising: The solvation of the enolic proton in hydrogen-bond-acceptor (HBA) solvents obviously provides an I)

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confidence: 94%

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confidence: 65%

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The Photoenolization of 2‐Methylacetophenone and Related Compounds

Haag¹,

Wirz²,

Wagner³

1977

Helvetica Chimica Acta

195

141

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“…Previously, o -methylarylketones were shown to be activated by UV light and to form a reactive o -quinodimethane species, which can subsequently undergo a Diels–Alder reaction with a dienophile. − The reaction has been widely adopted as an efficient photoligation. ,,− Herein, we initially probe the conversion of the photoinduced reaction of the o -methylbenzaldehyde-containing model compound A1 with N -ethylmaleimide (refer to Figure a) in its photoactive range of 285–390 nm. A 1.1-fold excess of the dienophile was used to ensure that the transiently generated photoenols are quantitatively trapped.…”

Section: Resultsmentioning

confidence: 99%

Wavelength Dependence of Light-Induced Cycloadditions

et al. 2017

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The wavelength-dependent conversion of two rapid photoinduced ligation reactions, i.e., the light activation of o-methylbenzaldehydes, leading to the formation of reactive o-quinodimethanes (photoenols), and the photolysis of 2,5-diphenyltetrazoles, affording highly reactive nitrile imines, is probed via a monochromatic wavelength scan at constant photon count. The transient species are trapped by cycloaddition with N-ethylmaleimide, and the reactions are traced by high resolution mass spectrometry and nuclear magnetic resonance spectroscopy. The resulting action plots are assessed in the context of Beer-Lambert's law and provide combined with time-dependent density functional theory and multireference calculations an in-depth understanding of the underpinning mechanistic processes, including conical intersections. The π → π* transition of the carbonyl group of the o-methylbenzaldehyde correlates with a highly efficient conversion to the cycloadduct, showing no significant wavelength dependence, while conversion following the n → π* transition proceeds markedly less efficient at longer wavelengths. The influence of absorbance and reactivity has critical consequences for an effective reaction design: At high concentrations of o-methylbenzaldehydes (c = 8 mmol L), photoligations with N-ethylmaleimide (possible for λ ≤ 390 nm) are ideally performed at 330 nm, whereas at high light penetration regimes at lower concentrations (c = 0.3 mmol L), 315 nm irradiation leads to the highest conversion. Activation and trapping of 2,5-diphenyltetrazoles (possible for λ ≤ 322 nm) proceeds best at a wavelength shorter than 295 nm, irrespective of concentration.

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“…Discussion. -The photoenolization of aromatic ketones such as ortho-methyl aceto-or benzophenones has been thoroughly studied [4] but the results obtained by different workers are partly conflicting and details of the reaction mechanism are still a matter of debate [5] [6]. The rigid molecular framework of 1 eliminates one complicating feature, namely the conformational degree of freedom of the carbonyl group.…”

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confidence: 99%

The photoenol tautomer of 5‐methyl‐1, 4‐naphthoquinone

Rommel

Wirz

1977

Helvetica Chimica Acta

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SummaryIrradiation of pale yellow 5-methyl-l,4-naphthoquinone (1, Scheme I ) yields the blue photoenol 4-hydroxy-5-methylidene-1(5 H)-naphthalenone (2) which is stable at 77 K, At room temperature the enol retautomerizes to starting material, the reaction rate being strongly dependent on the hydrogen-bond-acceptor basicity of the solvent. The enol is trapped in the presence of acid by protonation at the remaining carbonyl oxygen atom and subsequent electrophilic reaction of the exocyclic methylene group.

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Photoenolization of ortho-alkyl-substituted acetophenones: evidence for the enol triplet state

Cited by 34 publications

References 0 publications

The Photoenolization of 2‐Methylacetophenone and Related Compounds

The Photoenolization of 2‐Methylacetophenone and Related Compounds

Wavelength Dependence of Light-Induced Cycloadditions

The photoenol tautomer of 5‐methyl‐1, 4‐naphthoquinone

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