O-Quinonoid intermediates

Shimizu et al report that while [2.2] paracyclophane (55) undergoes two-photon dissociation in low temperature matrices by way of the triplet state, in the gas phase, the efficient two-photon process proceeds via a hot molecule formed by internal conversion from the initially formed singlet excited state. The photocleavage of 2-nitrobenzyl ethers and ester has been widely reported and has now been evaluated as a deprotection methodology for indoles, benzimidazole, and 6-chlorouracil (Voelker et al). The mechanism of the cleavage of such compounds is considered to involve the o-quinonoid intermediate, but previously these had only been deduced from transient electronic spectra produced in flash photolysis experiments. Infrared spectral data from photochemical studies of 2-nitrobenzyl methyl ether in argon and nitrogen matrices have now been published which confirm that the intermediate does indeed have the o-quinonoid structure... 

Direct aromatization of the quinonoid intermediates is a photochemically allowed but thermally forbidden rearrangement (Scheme 5.6). When phenylethyl radicals are generated photochemically at 20 °C there is evidence95 of a-o coupling by way of the aromatized product 7. The products derived from these pathways can be trapped in thermal reactions by radical98 or acid1 catalyzed... 

The reductive cleavage of the C—O bond in 2- and 4-substituted hydroxy-methylpyridines was mentioned in Part I. The optically active l-(4-pyridyl)-alkanols are reduced in good yield to the optically inactive alkylpyridine, whereas the 2-substituted derivatives were reduced in lower yield to the optically active alkylpyridine (and some tetrahydropyridine derivatives).384,385 The difference in behavior was explained by orientation of the adsorbed pro-tonated pyridine, allowing contact between the electrode and the 2-substituent, but not the 4-substituent. This model was used in later work however, not excluded was the possibility that the higher stability of the p-quinonoid dihydropyridine intermediate, compared to the o-quinonoid dihydropyridine, played a role in the relative ease of reduction of the 4- and 2-substi-tuted pyridines. 

Cystathionine y-synthase (CGS) is a rather unique PLP-enzyme that catalyzes a transsulfuration reaction important in microbial methionine biosynthesis. It is the only known enzyme whose function is the catalysis of a PLP-dependent replacement reaction at the y-carbon of the amino acid substrate the succinyl moiety of O-succinyl-L-homoserine is replaced by i-Cys to give the thioether linkage of L,/.-cystathionine (scheme II). In the absence of L-Cys, the enzyme catalyzes a net y-elimination reaction from OSHS (scheme II). Because both reactions require the elimination of succinate, the catalytic pathways must diverge from a common reaction intermediate. It was originally hypothesized that a vinylglycine quinonoidal intermediate (structure 11)... 

NMR studies in a CDCl3 acetic acid solution clearly demonstrated an equilibrium between the dimers and the corresponding N-imines. Based on this observation, a mechanism for the formation of the 1,2-benzodiazepines via the diaziridines and o-quinonoid 2H-1,2-benzodiazepine intermediates is proposed. Considering their aromatic stability, the latter would be expected to undergo a photochemically allowed [l,7]-sigmatropic hydrogen shift to give the observed product (Eq. 42). 

Irradiation of quinoline JV-acetyl-239 and N-ethoxycarbonylimines215,240 in alcoholic solvents gives 2,3-dihydro-lH-l,2-benzodiazepines 114 and 115, respectively, as the major products. In these cases o-quinonoid 2H-l,2-benzodiazepine intermediates are stablized by incorporation of the solvent. Quinoline N-benzoylimine on irradiation in methanol yields exclusively 2-benzoylaminoquinoline (116).215,240... 

DTA showed that o-nitrophenol and sodium hydroxide gave a sharp exotherm at 46°, and p-nitrophenol at 95°, while m-nitrophenol gave no significant evidence of reaction below 200°C. o-Nitroaniline gave a sharp exotherm at 217°, and p-nitroaniline at 246°C [5]. Electronic excitation of TNT leads to formation of <7-quinonoid transient intermediates, and action of various bases leads to deflagration of molten TNT [6]. 

Based on the evidence obtained from the amount and nature of transformation products formed, a mechanism of melt stabilising action of tocopherol in PP and PE has been proposed, see Scheme 6 [34]. It is well known that, like other hindered phenols, a-tocopherol is rapidly oxidised by alkylperoxyl radicals to the corresponding tocopheroxyl radical (a-Toe, Scheme 6a). Further oxidation of the tocopheroxyl radical in the polymers leads to the formation of coupled and quinonoid-type products, e.g. SPD, TRI, DHD (see Figs. 8 and 9). Dimerisation of the intermediate o-quinone methide (QM) leads to the formation of the quinonoid-type dimeric coupled product, SPD (Scheme 6 reaction d). 

Further formal hydration at C-7 of the quinonoid forms of Scheme 3 leads analogously via 3,7-dihydroxy heterocycles and corresponding ionizable radical intermediates to structures of the form 245 which are dyes [e.g., 245 (X = O), resorufin (X = S), thionol]. 

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