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(C(3)H(5))(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-CH(2)-C(5)H(4)N)(2)(THF)] (2b⋅(THF)), [Ca(4-CH(2)-C(5)H(4)N)(2)(THF)(2)] (3b⋅(THF)(2)), [Ca(2-CH(2)-C(5)H(3)N-6-CH(3))(2)(THF)(n)] (4b⋅(THF)(n); n=0, 0.75), [Ca{2-C(5)H(3)N-4-C(CH(3))(3)}(2)(THF)(2)] (5c⋅(THF)(2)), [Ca{4,4'-(C(3)H(5))(2)-(C(10)H(8)N(2))}(THF)] (6a⋅(THF)), [Ca(NC(13) H(9)-9-C(3)H(5))(2)(THF)] (7a⋅(THF)), [Ca(4-C(3) H(5)-C(9) H(7)N)(2)(THF)] (8b⋅(THF)), and [Ca(1-C(3)H(5)-C(9)H(7) N)(2)(THF)(3)] (9a⋅(THF)(3)) have been characterized by NMR spectroscopy and metal analysis. 9a⋅(THF)(4) and 4b⋅(THF)(3) were additionally characterized in the solid state by X-ray diffraction experiments. 4b⋅(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.