Photochemistry of the o-nitrobenzyl system in solution: evidence for singlet-state intramolecular hydrogen abstraction

Yip, R. W.; Sharma, Dinkar; Giasson, R.; Gravel, Denis

doi:10.1021/j100271a002

Cited by 73 publications

(64 citation statements)

References 3 publications

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“…Consequently, the energy barrier separating the singlet biradical and the o-quinonoid species 2 is likely to be small and may even be insignificant, so that it may be more appropriate to consider the reaction from the singlet excited state as a concerted [1,5]sigmatropic shift (6). Our previous demonstration that the singlet state pathway is indeed involved in some of these systems, and that it proceeds extremely rapidly (7), is in accord with these views. Following the hydrogen transfer step, the primary intermediate(~): o-quinonoid 2 and (or) triplet biradical 3, proceed(s) respectively to the cyclized N-hydroxybenzisoxazolidine 4, the former by a polar cyclization mechanism, and the latter by intersystem crossing to the o-quinonoid 2 or, alternatively, by radical cyclization to 4 via the mesomeric form 3', following intersystem crossing.…”

Section: Introductionmentioning

confidence: 54%

Photochemistry of the o-nitrobenzyl system in solution: effects of distance and geometrical constraint on the hydrogen transfer mechanism in the excited state

Gravel¹,

Giasson²,

Blanchet³

et al. 1991

Can. J. Chem.

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. Can. J. Chem. 69, 1193Chem. 69, (1991. Two rigid nitrobenzene derivatives, 5-nitro-1,2,3,4-tetrahydro-1,4-methanonaphthalene (6), and 5-nitro-] ,2,3,4-tetrahydro-1,4-ethanonaphthalene (7), have been synthesized and studied by picosecond flash absorption spectroscopy, steady-state irradiation, and MMX molecular mechanics calculations. Transients with lifetimes of 770 and 410 ps have been detected and assigned to the excited triplet states of 6 and 7, respectively. Consistent with prediction, the o-quinonoid intermediate is not detected in the time domain between the laser excitation and the end of decay of the triplet state. Intramolecular abstraction of the bridgehead benzylic hydrogen by the cited triplet state proceeds with relative rates of 1:125 for 6 and 7, respectively. Since structural constraints prohibit the formation of an orthoquinonoid intermediate in these systems, formation of the usual nitrosoalcohol 13 represents the first substantiation of the biradical route to product (3' + 4, Scheme 1). Absorption of the intermediate triplet biradical resulting from hydrogen abstraction by the triplet excited state was not observed, and is therefore believed to be either very weak in the 400-700 nm region, or located outside the detection region. The kinetic results together with structural parameters determined by molecular mechanics show good correlation between structure and reactivity. Hydrogen atom transfer can proceed at up to 53" deviation from linearity of the C-H-0 nuclei, with transfer rates decreasing sharply as the 0. -.H distance increases from 1.3 to 1.6 A. It is suggested that the conditions of the reaction from the excited triplet state meet those required for a hydrogen tunneling mechanism.Key words: o-nitrotoluene photorearrangement, mechanism of; o-nitrobenzyl systems, mechanism of photorearrangement; 0. . .H distance, influence of, in hydrogen abstraction mechanism; picosecond flash absorption spectroscopy study of o-nitrobenzyl photorearrangement; radical pathway to product in o-nitrotoluene photorearrangement.

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Section: Introductionmentioning

confidence: 54%

Photochemistry of the o-nitrobenzyl system in solution: effects of distance and geometrical constraint on the hydrogen transfer mechanism in the excited state

Gravel¹,

Giasson²,

Blanchet³

et al. 1991

Can. J. Chem.

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“…1 B, species in brackets) (Yip et al, 1985;Schupp et al, 1987;Zhu et al, 1987;Yip et al, 1991), which has a decay that is commonly taken to be concomitant with cleavage of the caging group and release of product (Walker et al, 1988;McCray and Trentham, 1989). The kinetic spectroscopy trace for laser flash photolysis of compound 6 in 0.1 M sodium phosphate buffer, pH 7.4, is shown in Figure 2 B. Photolysis by a single 8.6 ns pulse of 355 nm light gave rise to a transient absorption that decayed with single-exponential time course, showing exponential time constant ϭ 27.2 Ϯ 0.7 s (equivalent to a half-life of t 1/2 ϭ 18.9 Ϯ 0.5 s).…”

Section: Resultsmentioning

confidence: 99%

Endocannabinoid Signaling Dynamics Probed with Optical Tools

Heinbockel¹,

Brager²,

Reich³

et al. 2005

J. Neurosci.

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Intercellular signaling dynamics critically influence the functional roles that the signals can play. Small lipids are synthesized and released from neurons, acting as intercellular signals in regulating neurotransmitter release, modulating ion channels on target cells, and modifying synaptic plasticity. The repertoire of biological effects of lipids such as endocannabinoids (eCBs) is rapidly expanding, yet lipid signaling dynamics have not been studied. The eCB system constitutes a powerful tool for bioassaying the dynamics of lipid signaling. The eCBs are synthesized in, and released from, postsynaptic somatodendritic domains that are readily accessible to whole-cell patch electrodes. The dramatic effects of these lipid signals are detected electrophysiologically as CB1-dependent alterations in conventional synaptic transmission, which therefore serve as a sensitive reporter of eCB actions. We used electrophysiological recording, photolytic release of caged glutamate and a newly developed caged AEA (anandamide), together with rapid [Ca 2ϩ ] i measurements, to investigate the dynamics of retrograde eCB signaling between CA1 pyramidal cells and GABAergic synapses in rat hippocampus in vitro. We show that, at 22°C, eCB synthesis and release must occur within 75-190 ms after the initiating stimulus, almost an order of magnitude faster than previously thought. At 37°C, the time could be Ͻ50 ms. Activation of CB1 and downstream processes constitute a significant fraction of the total delay and are identified as major rate-limiting steps in retrograde signaling. Our findings imply that lipid messenger dynamics are comparable with those of metabotropic neurotransmitters and can modulate neuronal interactions on a similarly fast time scale.

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“…Another possibility is a biradical, as suggested from picosecond transient absorption spectroscopic studies of o-nitrobenzyl compounds. 22 In contrast to the normally accepted proton-transfer mechanism for R-D-NBP photochromism, 2 with the o-nitrobenzyl systems studied, it was suggested that hydrogen abstraction may be involved and that this may proceed through a biradical intermediate. Could P be such a biradical?…”

Section: Discussionmentioning

confidence: 81%

New Insights on the Photochromism of 2-(2‘,4‘-Dinitrobenzyl)pyridine

2002

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The photochromic behavior of 2-(2′,4′-dinitrobenzyl)pyridine (R-DNBP) has been followed in poly(methyl methacrylate) (PMMA) films and benzene solutions to clarify the behavior of a precursor state, previously identified in studies on crystalline R-DNBP at low temperatures. In PMMA films, photolysis at temperatures e50 K led to the concurrent formation of a NH tautomer and a colorless intermediate, which was stable for several hours. On irradiation at low temperatures and warming the sample, the colorless intermediate was seen to react to produce the NH tautomer in a higher yield than that found in the direct photolysis. Further information on this intermediate has come from flash photolysis studies in benzene solution, in which a new transient absorption has been observed at 335 nm and assigned to this species. This decays within a few microseconds at room temperature to form an OH tautomer, which then interconverts to the NH tautomer. The precursor state is not quenched by oxygen or naphthalene. From consideration of the kinetic and spectral data, it is suggested that this new species corresponds to a nonrelaxed tautomeric form of the OH state of R-DNBP.

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Photochemistry of the o-nitrobenzyl system in solution: evidence for singlet-state intramolecular hydrogen abstraction

Cited by 73 publications

References 3 publications

Photochemistry of the o-nitrobenzyl system in solution: effects of distance and geometrical constraint on the hydrogen transfer mechanism in the excited state

Photochemistry of the o-nitrobenzyl system in solution: effects of distance and geometrical constraint on the hydrogen transfer mechanism in the excited state

Endocannabinoid Signaling Dynamics Probed with Optical Tools

New Insights on the Photochromism of 2-(2‘,4‘-Dinitrobenzyl)pyridine

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