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.
Benzotrifuranone (BTF), bearing three symmetry-equivalent lactone rings, is unique in its ability to undergo highly selective and sequential aminolysis reactions in one-pot to afford multifunctionalized molecules (>80% overall yield). New insight into this behavior is presented through kinetics measurements (by stopped-flow IR spectroscopy), X-ray crystal structure analysis, quantum chemical calculations, and comparison of BTF to other benzoate esters, including its ring expanded congener benzotripyranone (BTP). While the structure-property investigation confirms stepwise electronic/inductive lactone deactivation for both BTF and BTP, the unusually fast and selective aminolysis of BTF is only fully explained through synergistic ring strain effects. Experimental signatures of the significant ring strain of BTF (∼28 kcal mol based on DFT calculations vs 17 kcal mol for BTP) include its high lactone carbonyl stretching energy (1821 cm in acetonitrile vs 1777 cm for BTP) and bond length alternation within its benzenoid ring. While ring strain is relieved upon the sequential aminolysis of both BTF and BTP, it is only for the former that a ring strain gradient is established that contributes to the stepwise aminolysis rate differences and enhanced selectivity. The work shows how a combination of electronic effects and ring strain can underpin the design of small molecules capable of stepwise functionalization, of which there are notably few examples.
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