Hydrogen abstractions, CIDNP

Maeda K, Terazima M, Azumi T and Tanimoto Y 1991 CIDNP and CIDEP studies on intramolecular hydrogen abstraction reaction of polymethylene-linked xanthone and xanthene. Determination of the... 

Both CIDNP and ESR techniques were used to study the mechanism for the photoreduction of 4-cyano-l-nitrobenzene in 2-propanol5. Evidence was obtained for hydrogen abstractions by triplet excited nitrobenzene moieties and for the existence of ArNHO, Ai N( )211 and hydroxyl amines. Time-resolved ESR experiments have also been carried out to elucidate the initial process in the photochemical reduction of aromatic nitro compounds6. CIDEP (chemically induced dynamic electron polarization) effects were observed for nitrobenzene anion radicals in the presence of triethylamine and the triplet mechanism was confirmed. 

The CIDNP experiments have been carried out to demonstrate an intervention of triplet radical pairs in many hydrogen abstraction reactions involving triplet car-benes. The following examples in reactions with halo compounds and ethers are of particular interest. 

Unlike acetone, diethyl ketone cleaves well even in cumene 22>. 2-Pentanone triplet undergoes considerable a-cleavage in competition with rapid intramolecular hydrogen abstraction, as judged by the buildup of an efficient triplet quencher 23>. Biacetyl is the only likely candidate for that quencher and is the major product of a-cleavage of methyl ketones at temperatures low enough that decarbonylation of the acetyl radical is slow. Isopropyl, tert-butyl, and benzyl ketones all cleave quite efficiently and various reports have appeared on the CIDNP spectra of products derived from the radicals 24>. 

FIGURE 9.6 Photosensitized hydrogen abstraction from triethylamine DH by 9,10-anthra-quinone A (for the formulas, see Chart 9.3). The rate constant of in-cage deprotonation as obtained from the polarity pattern, is shown as a function of the relativity permittivity e of the reaction medium (mixtures of acetonitrile and chloroform). The timescale of the CIDNP effect provides a kinetic window, within which such a quantitative treatment is apphcable. Further explanation, see text. 

Isotope effects in hydrogen abstractions from cyclohexane were studied by the concentration dependence of CIDNP for mixtures of protiated and perdeuterated... 

The diamagnetic products are observed and can be characterized very well because the detection method is high-resolution NMR spectroscopy. On the other hand, the signal enhancement by the CIDNP effect mitigates the inherent low sensitivity of NMR. In consequence, even diamagnetic species that are unstable and thus present only in low concentration can be captured. An early example is the enol of acetophenone formed in the photoreaction of acetophenone with phenol [45], others are vinylamines in photoinduced hydrogen abstractions from aliphatic amines [46] (see Section V.B) and in the sensitized photoreactions of amino acids (Section V.G.l). 

Azumi et al. carried out variable-field CIDNP experiments on the biradicals resulting from intramolecular hydrogen abstractions in polymethylene-linked xanthone and xanthene moieties [61a, 61b]. They also studied the temperature dependence of the mean exchange interaction for... 

Am = 2 is responsible for this polarization. The same authors studied the photolysis of benzoquinones in CDC13 [69] by CIDNP and DNP. They interpreted their results by a hydrogen abstraction from CHC13 present as an impurity in the solvent. On the basis of this chemistry, the polarization phase at low quinone concentrations agrees with Kaptein s rules, whereas the phase is reversed at high concentrations, which was again attributed to Am = 2 cross-relaxation. 

Hydrogen abstractions in ketone/amine and quinone/amine systems continue to attract the attention of CIDNP spectroscopists [94] despite the facts that the application of CIDNP to these reactions dates back to 1974 [95] and that the basic mechanism — electron transfer from the amine DH followed by deprotonation of the resulting aminium cation DH + to give an a-aminoalkyl radical D" has already been cleared up in those early investigations [46]. CIDNP spectroscopy is very well suited to probe the microscopic details of such reactions that involve more than one radical intermediate. Polarizations can arise in both DH + and D, but the spin density distributions of these two radicals differ strongly. Hence, the polar-... 

Hydrogen abstractions by deoxybenzoin from the solvent, undeuterated and perdeuterated cyclohexane, were studied by using stationary l3C CIDNP [96]. As was inferred from the polarizations, the products are formed to a large degree by secondary encounters of escaped radicals. These reactions are accompanied by a-cleavage of the ketone, which is of comparable rate as deuterium abstraction but significantly slower than hydrogen abstraction. 

The dye-sensitized photo-oxidation of phenols has been examined using CIDNP techniques.148 It is concluded from the observed polarizations of nuclear spin that sensitization by xanthene dyes (such as Rose Bengal) is the result of reversible hydrogen abstraction by the triplet dye molecule from the phenolic hydroxyl group. Any resulting photochemical reactions (e.g. with oxygen) arise from irreversible reactions of the phenoxy-radicals produced, rather than from direct reactions of the phenols with 102. Dye-sensitized photo-oxidation of p-hydroxyphenylpyruvic acid (94) in solution at pH 7 (conditions under which... 

More informative is a process described by Roth (1972). The direct and the sensitized (benzophenone) photolysis of diazomethane in toluene yields ethylbenzene, but a CIDNP effect can only be observed from the sensitized reaction. This result leads to the conclusion that the direct photolysis is a concerted CH insertion of a highly reactive species, whereas the sensitized reaction takes place by a hydrogen abstraction, followed by a radical addition (8-24). It is, therefore, likely that it is the singlet that reacts in the first reaction, but the triplet in the second process. 

A CIDNP study of the irradiation of (7) has shown that a-fission occurs on irradiation. The resultant radical (8) eliminates diethyl phosphoric acid and yields the radical cation (9). When irradiation is carried out in methanol this species is trapped as the ether (10). Other products are methyl benzoate and the acetal (II). A laser irradiation study of the reaction of acetophenone with amines constrained in NaY zeolites has been carried out and evidence has been collected that shows that ketyl and amino radicals are formed by hydrogen abstraction pathways. ... 

The exclusive addition of the xanthene moiety to the sterically less hindered carbonyl group was observed in 15b, in accordance with the lower reactivity of dimethyl-substituted 15c. The CIDNP experiment revealed that 16 and 17 are recombination products via an initially formed triplet radical pair. The proposed mechanism is shown in Scheme 11. The hydrogen abstraction of the excited triplet state of 15 from xanthene generates ketyl and xanthenyl radicals. A possible product derived from outer cyclopropane bond cleavage was not obtained because of the lower stability of a primary radical compared with the conjugated radical formed by inner cyclopropane bond cleavage. 

The excited triplet states of quinones can be fairly readily populated by irradiation and nuclear polarization observed (Cocivera, 1968). Hydrogen atom abstraction leads to the relatively stable semiquinone radicals and, in alkaline media, radical anions. Recombination of radical pairs formed in this way can give rise to CIDNP signals, as found on irradiation of phenanthraquinone (20) in the presence of donors such as fluorene, xanthene and diphenylmethane (Maruyama et al., 1971a, c Shindo et al., 1971 see also Maruyama et al., 1972). The adducts are believed to have the 1,2-structure (21) with the methine proton appearing in absorption in the polarized spectrum, as expected for a triplet precursor. Consistently, thermal decomposition of 21 as shown in equation (61) leads to polarization of the reactant but now in emission (Maruyama... 

Chemically induced dynamic nuclear polarization (CIDNP) is a very powerful tool for establishing the existence of radical pair intermediates and their spin. CIDNP has reinforced the view that singlet carbene undergoes direct insertion into C—H bonds and that the triplet abstracts hydrogen. 

Observation of emission and absorption in the NMR benzylic proton signal from an irradiated solution of diphenyldiazomethane in toluene was the first example showing the importance of the CIDNP technique. Triplet diphenylcarbene generated by photolysis of diphenyldiazomethane in toluene abstracts a hydrogen atom to generate the triplet radical pair, which either recombines to give 31, or diffuses apart, ultimately to produce dimers of each fragment (32, 33, Scheme 9.7,... 

The photochemistry of benzaldehyde (90% 13C=0), 519, deoxybenzoin (99%) 13C=0), 521, and / -chloro benzoin (99% 13C=0), 522, in cyclohexane-Dn solution has been studied633 by spectroscopic techniques, such as XH chemically induced dynamic nuclear634 or electron polarization635 (CIDNP/CIDEP) or dynamic nuclear polarization636 (DNP). In all these cases the formation of benzaldehyde-D with emissive 13C=0 polarization has been observed and the results rationalized by intermolecular hydrogen (deuterium) abstraction by the photoexcited ketones from the solvent molecules and by reactions of cage-escaped radicals (equations 303-308), Benzoin, 520, is formed also. 

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