Synthesis and Characterization of Carbon-Bridged (1(n))(2,6)pyridinophanes.

Pyridine-containing heterophanes, which possess a rigid non-flexible framework, similar to that of porphyrin backbones, are ideal structures to probe the electronic and/or steric effects within a highly electron-rich cavity. The syntheses of heteromacrocycles incorporating the 2,6-pyridino moiety are described. 2-Bromo-6-lithiopyridine, generated from 2,6-dibromopyridine and n-butyllithium, was used to synthesize 2,6-bis (2$\sp\prime$-(6$\sp\prime$-bromopicolinoyl)) pyridine and bis-2-(6-bromopyridyl)ketone in 36% and 63% yield, respectively. Ketalization of these ketones was accomplished by either standard acidic or basic conditions. To model a nucleophilic substitution route for cyclization of the resulting ketals, lithioacetonitriles were allowed to react with bromopyridines to produce symmetrical and unsymmetrical cyanomethine adducts in more than 47% yield. Reaction of 2,6-bis- (2$\sp\prime$-(6$\sp\prime$-bromopyridyl)-1,3-dioxolan-2-yl) pyridine or 2,2-bis-2$\sp\prime$-(6$\sp\prime$-bromopyridyl)-1,3-dioxolane with lithioacetonitrile afforded (1$\sb{\rm n}$) (2,6)pyridinophanes (n = 3,4), in which the pyridine rings were coupled with ketal and cyanomethine functionalities. At 80$\sp\circ$C, cyclocondensation via nucleophilic substitution favors macrocycle formation because the intermediates are held in the desired syn-conformation by a metal ion template effect. The ketal and nitrile groups of the initially generated macrocycles were hydrolyzed under acidic conditions. Hydrolysis of the nitrile was accompanied by decarboxylation to produce methylenic intermediates (143 and 152), which were oxidized with SeO$\sb2$ to afford the desired triketone 115 and tetraketone 125, respectively. Alternatively, oxidation of the $\alpha$,$\beta$-unsaturated nitrile tautomers with m-chloroperbenzoic acid to a keto group; followed by deketalization under acidic conditions afforded the same ketones. Triketone 115 and tetraketone 125 contain only sp$\sp2$ carbon atoms and should be essentially planar; however, due predominantly to N,N-electron repulsions within the confines of the cavity, deformations from planarity were observed. The dihedral angles of pyridines in triketone 115 are 35.4, 41.4, and 46.5$\sp\circ$, respectively. Wittig reactions and the Knoevenagel condensations on the bridging carbonyl groups in triketone 115 were unsuccessful, but facile monohemiketalization of 115 was observed. X-ray analysis of a Cu(II) complex isolated from ethanol confirmed the presence of a hemiethyl ketal (178). Upon exposure to air, precursor 143 of triketone 115 underwent oxidization to afford dimeric (1$\sb3$) (2,6)-pyridinophane, which was subsequently dehydrogenated with either DDQ or air. X-ray data of these dimers confirm the juxtaposition of the two electron-rich cores