Meisenheimer-Wheland complexes between 1,3,5-tris(N,N-dialkylamino)benzenes and 4,6-dinitrotetrazolo[1,5-a]pyridine. Evidence of reversible C-C coupling in the S(E)Ar/S(N)Ar reaction.

Reactions between a superelectrophilic carbon reagent, 4,6-dinitrotetrazolopyridine, and 1,3,5-tris(N,N-dialkylamino)benzenes, a supernucleophilic carbon reagent series, afford C-C coupling products which are "double sigma-complexes" (W-M), Wheland-like on the 1,3,5-tris(N,N-dialkylamino)benzene moiety and Meisenheimer-like on the 4,6-dinitrotetrazolopyridine moiety. These complexes were moderately stable at low temperature, and they were characterized by NMR spectroscopy methods. (1)H NMR experiments at variable temperature strongly indicate that the formation of these complexes by a nucleophile/electrophile attack is a reversible process.

Regioselectivity in the addition of vinylmagnesium bromide to heteroarylic ketones: C- versus O-alkylation.

The reactivity of heteroarylic ketones toward vinylmagnesium bromide (2) and the regiochemistry of the addition were investigated. The reactivity drastically increases when the carbonyl is conjugated with at least one aza group and the regiochemistry of the addition of the vinyl Grignard reagent depends on the carbonyl compound: in the series of di(heteroazolyl) ketones the O-alkylation product was observed as unique with di(1,3-benzothiazol-2-yl) ketone, and in different relative ratios with respect to the classic C-alkylation product with di(1,3-thiazol-2-yl) ketone, (1,3-benzothiazol-2-yl) (1,3-thiazol-2-yl) ketone, and di(1,3-benzoxazol-2-yl) ketone, whereas di(N-methylbenzimidazol-2-yl) ketone gave the exclusive formation of the carbinol. This behavior can be explained by the intervention of the delocalization power of the heterocyclic ring and this was confirmed by the results obtained from the reaction between vinylmagnesium bromide and a series of mixed (1,3-benzothiazol-2-yl) (para-substituted phenyl) ketones, that showed a relative O-/C-alkylation ratio dependent on the nature and on the electronic effect of the substituent on the phenyl ring. The results are in agreement with the existence of intermediate species bearing a negative charge on the benzylic carbonyl carbon atom, and make the O-alkylation reaction between vinyl Grignard reagents and carbonyl compounds no longer a rare case, since it was observed with a number of heterocyclic carbonyl compounds, such as (1,3-benzothiazol-2-yl) aryl ketones and di(heteroaryl) ketones of the pentatomic 1,3-heteroazolic series.