Identification by means of heavy-atom isotope effects of a photoactivated reaction as a ground-state process

Kwart, Harold; Benko, D. A.; Streith, Jacques; Schuppiser, J. L.

doi:10.1021/ja00488a041

Cited by 10 publications

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“…To determine reversibility, we irradiated azepines 2 and 31 with blue LEDs and monitored them by 1 H NMR and observed no decomposition within 24 h. Based on these observations, the electrocyclic opening of the postulated aza -NCD is irreversible under the reaction conditions. We propose that analogously to pyridine N -oxides and N -iminopyridinium ylides, pyridinium methylides undergo radical recombination from the singlet excited state giving aza -NCD intermediates that rapidly undergo 6π-electrocyclic ring opening, affording azepine products (Scheme ). Further rearrangement of polycyclic products 31 – 40 gives α-imino ester/nitrile isomers, which are useful functional groups themselves .…”

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Unified Synthesis of Azepines by Visible-Light-Mediated Dearomative Ring Expansion of Aromatic N-Ylides

et al. 2021

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Herein, we report a unified approach to azepines by dearomative photochemical rearrangement of aromatic N-ylides. Deprotonation of quaternary aromatic salts with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) or N,N,N′,N’-tetramethylquanidine (TMG) under visible light irradiation provides mono- and polycyclic azepines in yields up to 98%. This ring-expansion presents a new mode of access to functionalized azepines from N-heteroarenes using two straightforward steps and simple starting materials.

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Unified Synthesis of Azepines by Visible-Light-Mediated Dearomative Ring Expansion of Aromatic N-Ylides

et al. 2021

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“…(2). The photo-Wallach rearrangement then, appears to involve the formation, after excitation, of a ground-state intermediate in a step having an activation barrier.…”

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The photo-Wallach rearrangement. Heavy-atom kinetic isotope effects and mechanism

Shine¹,

Subotkowski²,

Gruszecka³

1986

Can. J. Chem.

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The photo-rearrangement of mixtures of azoxybenzene 4 and, successively, [15~,15N']4, [180]4, and [2-14C]4 were carried out. Kinetic isotope effects (KIE) were calculated from measurements of isotopic ratios in both recovered 4 and the product, 2-hydroxyazobenzene (6). Analogous rearrangement of mixtures of 2,2'-azoxynaphthalene (8) Introduction In recent years sporadic attempts have been made to detect intermediates in photochemical reactions in solution by measuring heavy-atom (other than hydrogen isotopes) kinetic isotope effects (KIE). The basis for this approach was expressed by Schutte and Havinga in 1967, who used the photo-Fries rearrangement to try to answer "the question that has been raised whether photochemical reactions after electron excitation pass through a transition state of appreciable energy of activation" (1). Schutte and Havinga used 4-metho~~phenyl-['~~]acetate for measuring the carbon KIE in the photorearrangement of the ester into 2-acetyl-4-methoxyphenol, and found that within experimental error the rearrangement did not exhibit an isotope effect. They concluded, therefore, that rearrangement began in the electronically excited state (singlet) and proceeded very rapidly through vibrationally excited states in a process that required no thermal energy of activation.The quest was later pursued by Kwart and co-workers (2), who sought evidence for a vibrationally excited ground state intermediate in the photochemical rearrangement of the ylide 1 into the diazepine 3. In this case an inverse carbon KIE was found with the use of specifically labeled [13c]1, and the result was attributed to the formation of the intermediate 2 in the pathway from excited 1 to product (3) (reaction [I]). To our knowledge this is the first time in which a heavy-atom KIE has been found in a photochemical rearrangement.

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N-(Benzoylimino)pyridinium Betaine (NAPBz)

Legault

2011

Encyclopedia of Reagents for Organic Synthesis

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Identification by means of heavy-atom isotope effects of a photoactivated reaction as a ground-state process

Cited by 10 publications

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Unified Synthesis of Azepines by Visible-Light-Mediated Dearomative Ring Expansion of Aromatic N-Ylides

Unified Synthesis of Azepines by Visible-Light-Mediated Dearomative Ring Expansion of Aromatic N-Ylides

The photo-Wallach rearrangement. Heavy-atom kinetic isotope effects and mechanism

N-(Benzoylimino)pyridinium Betaine (NAPBz)

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