Assessing the superelectrophilic dimension through σ-complexation, SNAr and Diels–Alder reactivity

Abstract

In the domain of organic chemistry, S_NAr substitutions represent a class of reactions of overwhelming importance, both in synthesis and in the understanding of structure–reactivity relationships, especially the role of σ-complex intermediates. The primary factor necessary for achievement of S_NAr reactions is the presence of a good leaving group, which allows facile rearomatization of the ring undergoing nucleophilic attack. Consistent is the finding that the superelectrophilic chloronitrobenzofuroxans—or furazans—exhibit a very high S_NAr reactivity, allowing a number of C–C, C–N, C–O couplings to be achieved that are not accessible with the classical series of nitro-substituted aromatics. Of particular interest is the synthesis of a number of indoles, indolizines, pyrroles and extended π-excessive aromatic structures like azulene substituted by superelectrophilic moieties. The remarkable driving force for the facile completion of these reactions is the 10 orders of magnitude greater reactivity of 10π-electron-deficient heteroaromatics such as 4,6-dinitrobenzofuroxan (DNBF) than of the most reactive trinitrobenzene derivatives in σ-adduct complexation. Among the factors that have been recognized as governing superelectrophilicity, there is the poor aromaticity of 6-membered 10π-electron structures investigated, with a common origin for σ-complexation and pericyclic processes. A remarkable capacity of these structures is actually to contribute to a variety of Diels–Alder reactions. As an example, the DNBF molecule formally behaves as a nitroalkene, being susceptible to act as a dienophile as well as a heterodiene. Another remarkable Diels–Alder pathway is the capacity of the 6-membered carbocyclic ring of DNBF to act as a carbodiene. Also noteworthy is the successful Diels–Alder trapping of the dinitroso intermediate associated with 1-oxide/3-oxide tautomerism of the furoxan moiety of 4-aza-6-nitrobenzofuroxan. A point of fundamental importance in taking advantage of the reactivity of superelectrophilic structures at hand has been a successful calibration of their reactivity within the electrophilicity E scale developed by Mayr to describe nucleophile–electrophile combinations in general. It has thus been established that the E parameters measuring the electrophilicity of neutral heteroaromatics lie in the same region of the E scale as a number of highly reactive cationic reagents. Besides a reactivity rather similar to that of the 4-nitrobenzenediazonium cation (vide supra), the most electrophilic neutral molecules (DNBF, DNTP, DNBZ) are as electrophilic as tropylium cations or a number of metal-coordinated carbenium ions. Furthermore, there is a remarkable link between the pK_a^H₂O and E scales, as evidenced by the existence of a unique linear relationship spanning more than 20 orders of reactivity. This relationship appears as being a nice probe to predict the feasibility of S_NAr substitutions and related σ-complexation processes. Also revealing in terms of feasibility of the reactions is the existence of a close correlation between the electrochemical oxidation potential E° of σ-adducts and their positioning on the pK_a^H₂O scale. Our data can also be used to evaluate the potential of a theoretical model recently derived from DFT calculations, namely the global electrophilicity index ω, for the description of nucleophile–electrophile combinations. While showing several significant deviations, a reasonably linear ωvs. pK_a^H₂O relationship is obtained when restricting the correlation to structurally similar electrophilic moieties. On this basis, valuable information could be derived regarding the polar character of some DA reactions. Overall, the global electrophilicity (ω) approach may be a promising avenue in future work of electrophile–nucleophile combinations.