Hydrogen naphthalene, abstraction

When 160 was heated at 150 °C in the presence of 1,4-CHD, the naphthalene derivative 163 was obtained in 45% yield (Scheme 20.32) [56, 67]. An initial [3,3]-sigmatro-pic rearrangement to form 161 followed by a Myers-Saito cyclization and hydrogen-atom abstractions from 1,4-CHD could account for the formation of 163. 

An explanation for the non-formation of aromatic products under entry 4 in Table 1 is that hydrogen atom abstraction from the corresponding dihydro derivative would lead to a /3-thio substituted radical, which would rapidly eliminate a thiyl radical, which in turn would readily polymerize. The last entry in the table is also surprising in that the cyclization takes place on the naphthalene /3-position when an a-position is free. The use of this photocyclization-oxidation sequence for the construction of heterohelicenes has been reviewed (71ACR65). 

The differences in rate for the two positions of naphthalene show clearly that an additional-elimination mechanism may be ruled out. On the other hand, the magnitude of the above isotope effect is smaller than would be expected for a reaction involving rate-determining abstraction of hydrogen, so a mechanism involving significant internal return had been proposed, equilibria (239) and (240), p. 266. In this base-catalysed (B-SE2) reaction both k and k 2 must be fast in view of the reaction path symmetry. If diffusion away of the labelled solvent molecule BH is not rapid compared with the return reaction kLt a considerable fraction of ArLi reacts with BH rather than BH, the former possibility leading to no nett isotope effect. Since the diffusion process is unlikely to have an isotope effect then the overall observed effect will be less than that for the step k. ... 

The reaction in Scheme 5.2 proceeds through the formation of the 1-bromonaphthalene anion-radical, which rapidly converts into the naphthyl radical. Thiophenolate intercepts the naphthyl radical and forms the anion-radical of l-(phenylthio)naphthalene. The reaction takes place in the preelectrode space. It competes with the formation of the unsubstituted naphthalene. The debromi-nation is a result of hydrogen abstraction from the solvent SolH by the naphthyl radical. The unreacted 1-bromonapthalene oxidizes the l-(phenylthio)naphthalene anion-radical formed. This leads to the neutral l-(phenylthio)naphthalene and the anion-radical of 1-bromonaphthalene. The reaction takes place in the bulk solution and is the key-point for the chain propagation. 

There is still considerable interest in the products of irradiation of hydrogen abstracting ketones for example, Plank96 has reported that benzophenone in ether gives benzophenone pinacol in higher yield when naphthalene is added. It was suggested that the pinacol is formed from some intermediate which is photochemically destroyed in the absence of a quencher. Challand97 has isolated products from addition of ether to acetophenone. 

This process competes favorably with benzylic hydrogen abstraction in toluene, less in ethylbenzene, and least in cumene (31). Such reactions do not seem significant in the oxidation of benzene derivatives. However, naphthalene reacts about 20 times as rapidly with phenyl radical as does benzene (16), and radical addition to the naphthalene nucleus may at least partly account for the slow oxidation rate in the methylnapthalenes. Among the minor products from both methylnaphthalene oxidations were compounds of molecular weight 296 ... 

The well-known photorearrangement of urtAo-nitrobenzaldehyde to orfAo-nitrosobenzoic acid458 may well involve hydrogen abstraction as the initial photoprocess. Nitrobenzene is photoreduced in reactive solvents to iV-phenylhydroxylamine,459 and the intermolecular hydrogen abstraction apparently proceeds from the triplet state, since, when perfluoronaphthalene is present in the sample, the ESR spectrum of the PhN02H radical is replaced by the typical triplet ESR spectrum of the naphthalene.460... 

Note. Both the rearrangement In t-Butanol) and the double bond isomerization of (114) In Benzene) are quenched in a diffusion-controlled process by suitable triplet acceptors (e.g., naphthalene or 2,5-dimethylhexa-2,4-diene). The rearrangement (114) - (118) + (120) is also observed on irradiation in pyridine, the double bond isomerization (114) -> (122) in trifluorotoluene. In benzonitrile both types of processes occur in parallel with similar efficiencies. In hydrogen-donating solvents (toluene, ether, dioxane, ethanol), hydrogen abstraction processes prevail [toluene (114) - (123)]. 

Abstract The problem of the low-barrier hydrogen bond in protonated naphthalene proton sponges is reviewed. Experimental data related to the infra-red and NMR spectra are presented, and the isotope effects are discussed. An unusual potential for the proton motion that leads to a reverse anharmonicity was shown The potential energy curve becomes much steeper than in the case of the harmonic potential. The isotopic ratio, i.e., vH/VD (v-stretching vibration frequency), reaches values above 2. The MP2 calculations reproduce the potential energy curve and the vibrational H/D levels quite well. A critical review of contemporary theoretical approaches to the barrier height for the proton transfer in the simplest homoconjugated ions is also presented. 

Perinaphthalyne (16) evidently prefers to react by hydrogen abstraction and insertion, giving naphthalene, biphenyl and phenylnaphthalene but relatively little anthracene (Reaction scheme 13) ... 

The a-hydrogen exchange, Reaction 1, and the e-hydrogen exchange, Reaction 2, are interrelated by a 1,2-hydrogen exchange as shown in Reaction 3. Each of these reactions may be more complicated than that shown above, since they probably involve solvent or coal radical intermediates. In reactions 1 and 2, for example, the coal probably abstracts 2H from the naphthalene-ds to form a naphthyl radical, and subsequently the naphthyl radical abstracts lH from a different part of the coal as shown in Reactions 4 and 5 ... 

All of these reactions presumably arise through free radical mechanisms. The abstraction of hydrogen from tetralin by coal to produce naphthalene is of course expected. What sets this reaction apart from the rest is the linear dependence of naphthalene yield on coal concentration. From the slope of the yield curve, we calculate that hydrogen was removed from tetralin in the amount of 2.5 wt. percent of the added coal. This is a reasonable amount of hydrogen to be transferred to coal under liquefaction conditions. However, we note a recent report on the decomposition of 1,2-diphenylethane in tetralin in which Benjamin states that over twice the amount of naphthalene required in the formation of toluene was produced (11). Presumably, the excess appears as molecular hydrogen. In the present case, we cannot rule out the possibility that some fraction of the naphthalene was produced by a similiar mechanism. 

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