Phenol 4-hydroxy-2,5-cyclohexadienone

As can be seen, the pathway from the cyclohexadienone to the ortho-rearranged phenol does not imply a jump over an energy barrier (34 kcal/mol), but a tunneling from the vibrational energy levels located at E(vo) = 0 kcal/mol and E(vi) = 3.9 kcal/mol (4.4 kcal/mol for the deuterated compound). Analogous results have been reported for the photo-Fries rearrangement of 2,4-dimethoxy-6-( ara-tolyloxy)-v-triazine (28) to give 2,4-dimethoxy-6-(2-hydroxy-5-meth-ylphenyl)-v-triazine (29) (Scheme 11) [43]. 

DFT and ab initio calculations have been used to study the mechanism of the gas-phase oxidation of phenol by HO. Addition of HO to the ort/io-position forms P2, which subsequently combines with O2 at the ip o-position to form adduct P2-1-00. A concerted HO2 elimination from P2-1-00 forms 2-hydroxy-3,5-cyclohexadienone (HCH) as the main product and is responsible for the rate constants for the reaction between P2 and O2 to be about two orders of magnitude higher than those between other aromatic-OH adducts and O2. The HCH subsequently isomerizes to catechol, which is thermodynamically more stable than HCH, possibly through a heterogeneous process. Reaction of P2 with NO2 proceeds by addition to form P2-n-N02 ( = 1, 3, 5) followed by HONO elimination from P2-1/3-N02 to form catechol. The barriers for HONO elimination and catechol formation are below the separate reactants P2 and NO2, being consistent with the experimental observation of catechol in the absence of O2, while H2O elimination from P2-I/3-NO2 forms 2-nitrophenol (2NP). The most likely pathway for 2NP is the reaction between phenoxy radical and N02." ... 

Isonitrile 19 was detected in ultraviolet (UV) and infrared (IR) studies at low temperatures. They speculated that azirine intermediate 21 may precede the formation of 19 and, indeed, LFP studies on 18 performed by Richard and co-workers allowed observation of an intermediate which was assigned to 21 on the basis of the similarity of its UV absorption spectra to that of the cyclohexadienone chro-mophore. The reaction was not quenchable by piperylene and is presumed to occur via the singlet manifold. The authors did not put forward a mechanism for the formation of 21, although it may be formed by initial ESIPT from the phenol to the cyano group to give tautomer 22, which can then rearrange to give 21. Similar products (substituted benzoxazoles) were observed via presumed initial ESIPT in structurally related o-hydroxy substituted aromatic oximes and oxime ethers. ... 

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