Izvorni znanstveni članak

https://doi.org/10.5562/cca2121

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

Nena Radić ; Quantum Organic Chemistry Group, Department of Organic Chemistry and Biochemistry, Ruđer Bošković Institute, Bijenička cesta 54, HR–10000 Zagreb, Croatia
Ines Despotović ; Quantum Organic Chemistry Group, Department of Organic Chemistry and Biochemistry, Ruđer Bošković Institute, Bijenička cesta 54, HR–10000 Zagreb, Croatia
Robert Vianello orcid.org/0000-0003-1779-4524 ; Quantum Organic Chemistry Group, Department of Organic Chemistry and Biochemistry, Ruđer Bošković Institute, Bijenička cesta 54, HR–10000 Zagreb, Croatia

Sažetak

M06–2X/6–311++G(2df,2pd)//M06–2X/6–31+G(d) computations were employed to investigate 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). 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, con-sistent 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 concepts such as localized reactive hybrid orbitals (RHO) or thus derived nitrogen electron-donating ability (T. Ohwada et al., J. Org. Chem. 69 (2004) 7486), which take into account only properties of the initial base in question, but rather consider protonation reaction in its entirety. (doi: 10.5562/cca2121)

Ključne riječi

Baeyer ring strain; protonation; substituent effect; superbases; triadic formula

Hrčak ID:

94447

URI

https://hrcak.srce.hr/94447

Datum izdavanja:

17.12.2012.

Posjeta: 8.219 *