Assessable consequences of through-bond donor-acceptor interactions in beta-aminoketones

Reported are the syntheses of ester-functionalized (6-8) and alkyl-substituted (9) 1-aza-adamantanones; the easy handling of the compounds provides an opportunity to comprehensively study the fundamental changes in structure and reactivity that can accompany the donor-acceptor arrangement in rigid beta-aminoketones. X-ray structural analysis of trione 6 and dione 7 reveals bond length and angle variations consistent with through-bond (hyperconjugative) donor-acceptor interactions. Observed is a shortening of the C-N bond, elongation of the central C-C bond (to approximately 1.6 A), and a significant pyramidalization of the carbonyl carbon within the donor-sigma-acceptor pathway. UV/Vis spectra of 6-9 show a new absorption maximum (lambda(max)=260-275 nm in three solvents), the so-called "sigma-coupled transition"; the molar absorptivity scales with the number of carbonyl groups (for trione 6, epsilon approximately 3000, for dione 7, epsilon approximately 2000) and the band reversibly disappears upon addition of acid. IR and (13)C NMR spectroscopic data show trends consistent with through-bond donation to the carbonyl acceptor groups and commensurate weakening of the carbonyl pi bond. High yielding acid-mediated fragmentations are used to illustrate the effects of the donor-acceptor arrangement on the reactivity of the molecules. Given that donor-sigma-acceptor molecules have recently been found to show self-assembly behavior and macromolecular properties linked to their unusual structure, the current analysis encourages further consideration of the systems in advanced materials applications.