Steric Demand and Rate‐determining Step for Photoenolization of Di‐<i>ortho</i>‐substituted Acetophenone Derivatives

Das, Anushree; Thomas, Suma S.; Garofoli, August A.; Chavez, Kevin A.; Krause, Jeanette A.; Bohne, Cornelia; Guđmundsdóttir, Anna D.

doi:10.1111/php.12996

Cited by 5 publications

(6 citation statements)

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“…In contrast, the decay was significantly slower in acetonitrile (5.1 × 10 3 s –1 , τ ∼190 μs; Figure B). H-bonding solvents such as methanol and viscous solvents such as HMPA generally impede 1,5-H atom shifts; − therefore, the observation that the lifetime does not increase in methanol or HMPA supports the assignment of the transient absorption to the T 1 of cis - 1K rather than cis - 1K . In addition, as trans - 1K in methanol is stable at ambient temperature and has an absorption band with a λ max at 405 nm, we assigned the residual absorption to trans - 1K .…”

Section: Resultsmentioning

confidence: 72%

“…Laser flash photolysis of ketones 5 and 6 allows for the direct detection of trans-5E and trans-6E but not cis-5E and cis-6E in solvents such as methanol and acetonitrile. 47 However, in more viscous HMPA, the 1,5-H atom shift in cis-5E and cis-6E becomes slower than the intersystem crossing of the T 1 of 5E and 6E. For the T 1 of 5E, the activation barrier for intersystem crossing was measured to be 4.7 kcal/mol, which is on the same order as that for the T 1 of cis-1K.…”

Section: Discussionmentioning

confidence: 88%

“…In detail, the triplet excited states of cis - and trans -photoenols (T 1 of 5E and 6E ), formed by intramolecular H-atom abstraction in ketones 5 and 6 , are longer-lived than the corresponding cis -photoenols ( cis - 5E and cis - 6E ), which decay through an efficient 1,5-H atom shift (Scheme ). Laser flash photolysis of ketones 5 and 6 allows for the direct detection of trans - 5E and trans - 6E but not cis - 5E and cis - 6E in solvents such as methanol and acetonitrile . However, in more viscous HMPA, the 1,5-H atom shift in cis - 5E and cis - 6E becomes slower than the intersystem crossing of the T 1 of 5E and 6E .…”

Section: Discussionmentioning

confidence: 99%

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Excited-State Intramolecular Proton Transfer in Salicylidene-α-Hydroxy Carboxylate Derivatives: Direct Detection of the Triplet Excited State of the cis-Keto Tautomer

et al. 2023

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Excited-state intramolecular hydrogen transfer on the triplet surface of salicylideneaniline derivatives has received much less attention than the corresponding ultrafast process on the singlet surface. To enhance the understanding of this triplet reactivity, the photochemical properties of a series of salicylidene-α-hydroxy acid salts with different substituents on the phenol moiety (1–3) were characterized. UV/vis absorption and phosphorescence measurements in ethanol revealed that 1–3 exist as both enol and keto tautomers, with the enol form being predominant. Irradiation of 1 at 310 nm in ethanol glass (77 K) yielded an absorption band with a λmax at ∼405 nm, which was assigned to the trans-keto tautomer (trans- 1K). In contrast, laser flash photolysis of 1–3 in methanol or acetonitrile resulted in a transient absorption with λmax at 440–460 nm. This transient, which decayed on the microsecond timescale and was significantly shorter lived in methanol than in acetonitrile, was assigned to the triplet excited state (T1) of the cis-keto tautomer (cis-1K–3K) and residual absorption of trans-1K–3K by comparison with TD-DFT calculations. The assignment of the T1 of cis-1K was further supported by quenching studies with anthracene and 2,5-dimethyl-2,4-hexadiene. Laser flash photolysis of 1 in the temperature range of 173–293 K gave an activation barrier of 6.7 kcal/mol for the decay of the T1 of cis-1K. In contrast, the calculated activation barrier for cis-1K to undergo a 1,5-H atom shift to reform 1 was smaller, indicating that intersystem crossing of the T1 of cis-1K is the rate-determining step in the regeneration of 1.

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Section: Resultsmentioning

confidence: 72%

Section: Discussionmentioning

confidence: 88%

Section: Discussionmentioning

confidence: 99%

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Excited-State Intramolecular Proton Transfer in Salicylidene-α-Hydroxy Carboxylate Derivatives: Direct Detection of the Triplet Excited State of the cis-Keto Tautomer

et al. 2023

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“…Furthermore, previous computational investigations into this process are limited in scope. 4 In recent findings, steric effects on the (E)/(Z) isomerization (Scheme 1, B/C) were identified by comparison of the photoenolization of 4′-tert-butyl-2′,6′dimethylacetophenone with 2′ ,4′,6′-triisopropylacetophenone, 44 while charged substituents on the aromatic ring were shown to alter PEDA reactivity of benzophenone derivatives with CO 2 . 12 These studies highlight the potential for manipulating the wavelength of activation and promoting or inhibiting the subsequent reactivity.…”

Section: ■ Introductionmentioning

confidence: 99%

Tuning Photoenolization-Driven Cycloadditions Using Theory and Spectroscopy

et al. 2022

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The first broad spectrum investigation into the photoenolization/Diels-Alder (PEDA) sequence was carried out using M06-2X/6-31+G(d,p) in conjunction with SMD solvation and supported by experimental UV−vis spectroscopy. A test set of 20 prodienes was chosen to examine the role of the H atom acceptor group (substituted and unsubstituted carbonyl, thiocarbonyl, and imine), the H atom donor group, and bystander ring substituents. As reaction partners for the photogenerated dienes, a diverse test set of 20 dienophiles was examined, comprising electron rich, electron poor, neutral, strain activated, hydrocarbon, and heteroatom-containing molecules including CO 2 and CO. A key finding of this work is the demonstration that the PEDA sequence of carbonyl based prodienes is tolerant of most substitution patterns. Another is that thiocarbonyl derivatives should behave analogously to the carbonyls but are likely to do so much more slowly, due to an inefficient intersystem crossing, an endothermic 1,5-hydrogen atom transfer (HAT) step, and a [1,5] sigmatropic H shift to regenerate the starting material that outcompetes the [4 + 2]cycloaddition. In contrast, the T 1 state of the ortho-alkyl imines displays the incorrect orbital symmetry for 1,5-HAT and is correspondingly accompanied by higher barriers, even in the excited state. However, provided these barriers can be overcome, the remaining steps in the PEDA sequence are predicted to be facile. The Diels−Alder reaction is predicted to be of much broader scope than reported synthetic literature: while electron poor dienophiles are expected to be the most reactive partners, ethylene and electron rich alkenes should react at a synthetically useful rate. CO is predicted to undergo a facile (4 + 1)cheletropic addition instead of the normal [4 + 2]cycloaddition pathway. This unique photoenolization/cheletropic addition (PECA) sequence could provide metal-free access to benzannelated cyclopentanones.

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“…Intermediate 4 is a disfavored dienyne-type intermediate for 6π-photocyclization. The lifetime of 4 is around 50 μs, which means it has little chance to coordinate with 1 mol % of Cu(II) to form a favored triene-type intermediate. Thus, the majority of 4 may de-excite and return to 1 .…”

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confidence: 99%

Cu(II)-Catalyzed 6π-Photocyclization of Non-6π Substrates

et al. 2020

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This research successfully achieved a Cu(II)-catalyzed 6π-photocyclization of non-6π substrates. The photoenolization converts ortho-alkylphenyl alkynl ketones into a triene-type intermediate which undergoes the subsequent 6π-photocyclization to give naphthol as the final product. Cu(II) catalyst facilitates both photoenolization and 6π-photocyclization. This research highlighted the tandem reaction strategy and the importance of metal catalysis in photochemistry.

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Steric Demand and Rate‐determining Step for Photoenolization of Di‐ortho‐substituted Acetophenone Derivatives

Cited by 5 publications

References 50 publications

Excited-State Intramolecular Proton Transfer in Salicylidene-α-Hydroxy Carboxylate Derivatives: Direct Detection of the Triplet Excited State of the cis-Keto Tautomer

Excited-State Intramolecular Proton Transfer in Salicylidene-α-Hydroxy Carboxylate Derivatives: Direct Detection of the Triplet Excited State of the cis-Keto Tautomer

Tuning Photoenolization-Driven Cycloadditions Using Theory and Spectroscopy

Cu(II)-Catalyzed 6π-Photocyclization of Non-6π Substrates

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