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Ring Strain and Other Factors Governing the Basicity of Nitrogen Heterocycles - An Interpretation by Triadic Analysis

The chemistry of strained organic molecules has been investigated extensively ever since Baeyer first introduced the concept of ring strain in the 1880's1 because such structures present challenges for the existing synthetic methodologies and may lead to new compounds with unusual and even unprecedented chemical behaviours. Features like bonding, property, reactivity, and synthetic accessibility have been the constant subject of research activities in this area. In the present work we report the results of the investigation of the intrinsic gas phase basicity of strained nitrogen heterocycles involving aziridine, azetidine, pyrrolidine and piperidine, together with their N-methyl and N-phenyl derivatives, NR(CH2)n (n = 2-5 ; R = H, Me and Ph) obtained at the M06-2X/6-311++G(2df, 2pd)//M06-2X/6-31+G(d) level of theory.2 Basicity constants were compared with the corresponding acyclic counterparts, NR(CH3)2 (R = H, Me and Ph), and were, based on triadic analysis, resolved into contributions mirroring features of both initial base and the final protonated form as well as their interplay, thus offering quantitative insight into the obtained results. In general, the N-methyl derivatives provided strongest bases investigated here, and, within each group of molecules, the basicity increased on going from three- to six-membered rings, consistent with a decrease in ring strain, with four-membered systems already surpassing or coming close to the basicity of the acyclic gauge molecule. Calculated basicities were found in a very good agreement with available experimental data, except for N-methylazetidine, where a remarkable discrepancy was revealed, suggesting that this system should be experimentally reassessed and its gas-phase basicity parameters revised. Triadic analysis showed different behaviour of individual contributions governing basicities, both among and within distinct families of molecules. It also convincingly demonstrated that, if a proper and a quantitatively accurate interpretation of observed basicity trends is desired, one should not rely only on the on concepts such as localized reactive hybrid orbitals (RHO) or thus derived nitrogen electron-donating ability, 3 which take into account only properties of the initial base in question, but rather consider protonation reaction in its entirety. REFERENCES: 1. A. Baeyer, Ber. Dtsch. Chem. Ges. 1885, 18, 2269. 2. N. Radić, I. Despotović, R. Vianello, Croat. Chem. Acta. 2012, 85, 495. 3. T. Ohwada, H. Hiaro, A. Ogawa, J. Org. Chem. 2004, 69, 7486.

Basicity; Nitrogen heterocycles; Triadic Analysis

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