Calcium-Mediated Dearomatization, C-H Bond Activation, and Allylation of Alkylated and Benzannulated Pyridine Derivatives

A facile and general synthetic pathway for the production of dearomatized, allylated, and C-H bond activated pyridine derivatives is presented. Reaction of the corresponding derivative with the previously reported reagent bis(allyl)calcium, [Ca(C3H5)(2)] (1), cleanly affords the product in high yield. The range of N-heterocyclic compounds studied comprised 2-picoline (2), 4-picoline (3), 2,6-lutidine (4), 4-tert-butylpyridine (5), 2,2'-bipyridine (6), acridine (7), quinoline (8), and isoquinoline (9). Depending on the substitution pattern of the pyridine derivative, either carbometalation or C-H bond activation products are obtained. In the absence of methyl groups ortho or para to the nitrogen atom, carbometalation leads to dearomatized products. C(sp(3))-H bond activation occurs at ortho and para situated methyl groups. Steric shielding of the 4-position in pyridine yields the ring-metalated product through C(sp(2))-H bond activation instead. The isolated compounds [Ca(2-CH2-C5H4N)(2)(THF)] (2b center dot(THF)), [Ca(4-CH2-C5H4N)(2)(THF)(2)] (3b center dot(THF)(2)), [Ca(2-CH2-C5H3N-6-CH3)(2)(THF)(n)] (4b center dot(THF)(n); n=0, 0.75), [Ca{2-C5H3N-4-C(CH3)(3)}(2)(THF)(2)] (5c center dot(THF)(2)), [Ca{4,4'-(C3H5)(2)(C10H8N2)} (THF)] (6a center dot(THF)), [Ca(NC13H9-9-C3H5)(2)(THF)] (7a center dot(THF)), [Ca(4-C3H5-C9H7N)(2)(THF)] (8b center dot(THF)), and [Ca(1-C3H5-C9H7N)(2)(THF)(3)] (9a center dot(THF)(3)) have been characterized by NMR spectroscopy and metal analysis. 9a center dot(THF) 4 and 4b center dot(THF)(3) were additionally characterized in the solid state by X-ray diffraction experiments. 4b center dot(THF)(3) shows an aza-allyl coordination mode in the solid state. Based on the results, mechanistic aspects are discussed in the context of previous findings.