Meisenheimer complexes spiro

As depicted in Scheme 53 the photo-Smiles rearrangement involves radical ion pairs. Intermediately the spiro-type Meisenheimer complex 277 is formed... 

NMR evidence has been provided for the formation of Meisenheimer adducts in DMSO solution from several mononitrothiophenes (70JHC1441, 74JHC97, 75JHC327). Interesting examples of the formation of spiro adducts have also been recorded (Scheme 137) (75JHC1083) one of these appears to be the first Meisenheimer complex observed at position 3 of thiophene. 

The relevant kinetic studies of spiro Meisenheimer complex formation from 2,4,6-trinitrophenyl ether of 3,6-dimethylcatechol (157, R = H or Me) was reported [72] (Scheme 50). It was found that the presence of methyl groups, accelerates the rearrangement, although the increase in the rate was not so dramatic as in the case of suhbne 153 (Scheme 49). It was suggested that the steric effect of the methyl groups increases the population of the conformation leading to rearrangement. The authors discussed two mechanisms for Meisenheimer complex formation—diffusion and pre-association controlled. 

However, it was found subsequently20 that the 4-tolyl analog gives the rearranged 7-nitro-3-(4-tolyl)-4/7-l,2,4-benzoxadiazine (2, R = H Ar = 4-Tol) in this reaction via a proposed spiro-Meisenheimer complex (Smiles rearrangement cf. Vol. E9a, p 533ff). 

NMe2 (9) has been unsuccessful. In continuing our search for this kind of isomerism we decided to study the reaction of sulfite ion with the spiro Meisenheimer complex which is formed by base catalyzed cyclization of N-methyl-N-3-hydroxyethyl picramide (10). As indicated in Scheme II, the two complexes MS and MS" formed by sulfite ion attack on M are geometric isomers in a similar way as the cis and trans isomers in Scheme I. 

Independent studies have confirmed that the stabilities of spiro-Meisenheimer complexes decrease with increasing size of the spiro-ring. The rates of Smiles rearrangement of l-(2-N-acetylaminoethoxy)-2-X-4-nitrobenzene with base depend only on the decomposition process of the anionic spiro cr-complexes. ... 

The n.m.r. spectra of the Meisenheimer complex of 2-nitrothiophen and 2-nitrofuran with methoxide ion have been compared. Evidence has been obtained for the formation of spiro-Meisenheimer compounds (130) and (131)... 

Reversible double Smiles rearrangement through intermediate formation of tautomeric Meisenheimer spiro-complexes was observed in the system of l-methylamino-3-picryloxy-2-propanol [18] (Scheme 9). 

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