Electrophilic aromatic substitutions: reactions of hydroxy- and methoxy-substituted benzenes with 4,6-dinitrobenzofuroxan: kinetics and mechanism

Abstract

Rate constants have been determined in aqueous Me2SO mixtures for the reaction of super‐electrophilic 4,6‐dinitrobenzofuroxan (DNBF) with a series of hydroxy‐ and methoxy‐substituted benzenes whose pKa 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 pKa 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 H3O+ 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 50%Me 2SODNBF and pK aH 2O with slope 0.54. In the case of 1,3,5‐trimethoxybenzene a significant kinetic isotope effect (KIE) is observed (kH/kD = 3.71 in 50% Me2SO). This system hence affords one of the few instances in which a KIE has been observed in SEAr 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 1H NMR. In all cases the regiochemistry of the reactions was identical to that observed in protonation studies of the starting aromatics. © 1998 John Wiley & Sons, Ltd.

A new feature in the chemistry of nitrobenzofuroxans: ambident reactivity in Diels–Alder condensations

“…1. 6 The same principle applies in all reported interactions of DNBF with oxygen, nitrogen or sulfur nucleophiles. 1,3,4,[8][9][10] Interestingly it has been argued that the low aromatic character of the benzofuroxan system should be one of the major factors responsible for the exceptional electrophilic reactivity of DNBF and nitrobenzofuroxans in general.…”

“…1,3-dimethoxybenzene (pK a CH = 29) or 3-methoxythiophene (pK a CH = 26.5). 6,7 In all of the above processes, covalent addition of the carbon nucleophile takes place at C(7) of the carbocyclic ring of DNBF to give stable anionic s-complexes, e.g. 1.…”

Synthesis and Facile Dearomatization of Highly Electrophilic Nitroisoxazolo[4,3-b]pyridines

“…Such compounds have been extensively studied in recent decades due to their interesting, sometimes exceptional, properties. Their high susceptibility to undergoing nucleophilic addition or substitution processes with very weak nucleophiles has raised considerable interest, leading to numerous synthetic, biological, and analytical applications [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31].…”

Covalent hydration of nitrobenzofuroxans compounds: Kinetic and theoretical study using reactivity–selectivity descriptor

“…A new development in the area of electron-deficient aromatics in the last two decades is the discovery of very powerful electrophilic heteroaromatic structures, such as 4,6-dinitrobenzofuroxan (DNBF) [1][2][3][4][5][6][7][8].…”