Nitro-nitrite rearrangement

Derivatives of 9-nitroanthracens 115 undergo the nitro-nitrite rearrangement from their triplet n, r state to 9-anthrol derivatives 118 as shown66 in equation 57 nitrite photolysis is well known and ESR spectra for the anthryloxy radical 117 can be recorded at room temperature. 

A number of reactions of compounds containing the nitro group are postulated to occur by an isomerization to give the nitrite form —NO2 - —ONO, which is often followed by O—N homolytic bond fission releasing nitric oxide. The reaction has been extensively examined theoretically, for example in nitromethane. Calculations have led to values for the barrier height and characteristics of the transition state have been established (see, for example, Reference 87). The question of the nitro-nitrite rearrangement within the... 

Non-coplanarity of the aromatic (or olefinic) system and the nitro group attached to it has been thought to be a prerequisite to an efficient nitro-nitrite rearrangement si) (see ref. for a review of this well-known reaction). Exceptions to this rule have been found ... 

Van den Braken-van Leersum and co-workers (1987) reported that on irradiation in methanol solutions (A > 300 nm), 1-nitropyrene undergoes a rapid conversion via the nitro-nitrite rearrangement, forming 1-hydroxy-pyrene (88%) and l-hydroxy-2-nitropyrene (7%). Under the same conditions, the 4-nitro isomer is more stable than the 1-nitro isomer and 2-nitropyrene is very stable it does not react either with or without oxygen present. 

They determined the branching ratio of reaction I to reaction II to be 0.6 0.2. By modeling the reactions with RRKM theory they estimated the barrier to the nitro-nitrite rearrangement to be 55.5 1.5 kcal/mol, which is about 4 kcal/mol lower than that for C-N bond rupture. Previously, there had been ab initio predictions of the energy barrier to nitro-nitrite isomerization [89-91], but the values were significantly higher (by 20-30 kcal/mol) than that estimated by Wodtke et al. based on their data, as were the values published subsequently [92, 93]. 

AE for the dissociation represented by eq. (1) is very similar to the activation barrier for the nitro-nitrite rearrangement, eq. (2). Accordingly these processes should be competitive. 

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