The S,Ar substitutions of a number of 5-X-substituted indoles (1 a-f ), 5-X-substituted 2methylindoles (1 g-j) and N-methylindole (1 k) by 4,6-dinitrobenzofuroxan (DNBF) have been kinetically studied in 70-30 (v/v) H,O-Me,SO, 50-50 (v/v) H,O-Me2S0, methanol and acetonitrile. The absence of a significant dependence of the rates of reactions on the hydrogen or deuterium labelling at C-3 of the indole ring indicates that electrophilic attack (kyNBF) by DNBF at this position is the rate-limiting step of the substitutions. However, the kYNBf rate constants are strongly sensitive to the solvent polarity, the observed reactivity sequence being 70-30 H,O-Me,SO > 50-50 H,O-Me,SO > methanol > acetonitrile. This trend is consistent with a highly 10 7

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Rate constants have been determined in aqueous Me 2 SO mixtures for the reaction of super-electrophilic 4,6-dinitrobenzofuroxan (DNBF) with a series of hydroxy-and methoxy-substituted benzenes whose pK a values range between -3 and -9. The study extends the reactivity range of weakly basic aromatics with DNBF, from the family of indoles previously studied with pK a values ranging from -1 to -6. The overall rate constants for the reactions of DNBF as the electrophile are at least one order of magnitude greater than for the reactions of H 3 O with the same series of aromatics. This lends further credence to the notion that DNBF possesses super-electrophilic properties. An LFER is observed between logk DNBF 50%Me 2 SO and pK H 2 O a with slope 0.54. In the case of 1,3,5-trimethoxybenzene a significant kinetic isotope effect (KIE) is observed (k H /k D = 3.71 in 50% Me 2 SO). This system hence affords one of the few instances in which a KIE has been observed in S E Ar reactions. It follows from the observed KIE that the addition of DNBF to the aromatic is not rate-limiting and that reversion to reactants and proton loss from the arenonium intermediate occur at comparable rates. Structures of the products of electrophilic substitution have been confirmed by 1 H NMR. In all cases the regiochemistry of the reactions was identical to that observed in protonation studies of the starting aromatics.

The reaction of ethyl vinyl ether (2 equiv) with 4,6-dinitrobenzofuroxan (DNBF, 1 equiv) in dichloromethane affords a mixture of two diastereomeric dihydrooxazine N-oxide adducts, 5a and 5b, in a 4:1 ratio. Further addition of the enol reagent to this mixture results in a second cycloaddition process with formation of a bis(dihydrooxazine N-oxide) product 6, which can also be obtained directly upon treatment of DNBF with excess ethyl vinyl ether. The observed condensations are accounted for in terms of inverse electron demand Diels-Alder cycloaddition processes between the enol ether dienophile and the heterodienyl moieties of DNBF, constituted first, by the 6-NO(2) group conjugated to the 6,7-double bond and then by the 4-NO(2) group and the 4,5-double bond of the carbocyclic ring. The configurations of the cycloadducts 5a and 5b have been determined on the basis of collected (1)H NMR parameters, as well as NOE experiments. It thus appears that the configuration of the major diastereomer corresponds to the endo product while that of the minor one corresponds to the exo product of the first cycloaddition process. The results obtained emphasize a low aromatic character for the DNBF molecule.