Trapping and Analysing Wheland–Meisenheimer σ Complexes, Usually Labile and Escaping Intermediates

Abstract

The reactions between 2,4-dipyrrolidin-1-yl-1,3-thiazole, a supernucleophilic reagent, and 4,6-dinitrobenzofuroxan (DNBF) or 4,6-dinitrotetrazolopyridine (DNTP), two superelectrophilic reagents, afforded new covalent complexes that are contemporaneously intermediates of an S_NAr reaction (a Meisenheimer complex) and of an S_EAr reaction (a Wheland complex). These compounds belong to a new class of covalent complexes, which we have named Wheland–Meisenheimer complexes (WM). The high stability of the complexes reported herein allowed the first X-ray diffraction analyses of WM complexes. In addition, the reactions are diastereoselective, probably because of the specific approach of the two starting partners. The WM complex obtained with DNBF unexpectedly evolved to a neutral substitution product, a furazan derivative. Probably, the protons bonded to two sp³ carbon atoms are lost together with the oxygen atom of the furoxan moiety to form water. This represents a unique example of the formation of a neutral substitution compound from a C–C WM complex. Finally, exchange of the DNBF moiety in WM8 with DNTP in a solution of CD₃CN was observed; the formation of the WM9 complex provided further evidence for the reversibility of the formation of the WM complexes.