title: Regioselectivity in the addition of vinylmagnesium bromide to heteroarylic ketones: C- versus O-alkylation.
abstract: 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.
authors.affiliation: Dipartimento di Chimica Organica A. Mangini, Viale Risorgimento 4, 40136-Bologna, Italy. boga@ms.fci.unibo.it
authors.forename: Carla
authors.initials: C
authors.lastname: Boga
authors.forename: Rayk
authors.initials: R
authors.lastname: Stengel
authors.forename: Rodolphe
authors.initials: R
authors.lastname: Abdayem
authors.forename: Erminia
authors.initials: E
authors.lastname: Del Vecchio
authors.forename: Luciano
authors.initials: L
authors.lastname: Forlani
authors.forename: Paolo E
authors.initials: PE
authors.lastname: Todesco
mesh_terms: D000478:Alkylation; D006571:Heterocyclic Compounds / Q000737:chemistry; D007659:Ketones / Q000138:chemical synthesis / Q000737:chemistry*; D008274:Magnesium / Q000737:chemistry*; D008968:Molecular Conformation; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D013237:Stereoisomerism; D014753:Vinyl Compounds / Q000138:chemical synthesis / Q000737:chemistry*
chemical_list: D006571:Heterocyclic Compounds; D007659:Ketones; D009942:Organometallic Compounds; D014753:Vinyl Compounds; D008274:Magnesium