Ab initio molecular orbital study of conformational properties of cyclohexyne, cycloheptyne, and cyclooctyne

Abstract

ABSTRACT:The structures and relative energies for the basic conformations of cyclohexyne (1), cycloheptyne (2), and cyclooctyne (3) have been calculated by the HF/ 6-31G*, MP2/6-31G*, and B3LYP/6-31G* methods. The C 2 symmetric twist conformer of 1 is calculated to be more stable than the planar C 2v geometry. Ring inversion of the envelope conformation of 2 takes place via C 2 symmetric twist transition state, which is 34.0 kJ mol Ϫ1 higher than the envelope form. The C 2 symmetric chair conformation of 3 is calculated to be 11.7 kJ mol Ϫ1 more stable than the unsymmetrical twist-boat (3-TB) geometry. Interconversion of the chair conformation and 3-TB takes place via an unsymmetrical transition state, which is 37.6 kJ mol Ϫ1 less stable than the chair form. Conformational racemization of the chiral 3-TB takes place via the boat transition state.

Chemisorption of a Strained but Flexible Molecule: Cyclooctyne on Si(001)

“…The conversion energy barrier from twist-boat to chair was calculated to be 21.2 (PBE-D3) and 23.0 kJ mol À1 (CCSD(T)) in electronic energies and 20.2 kJ mol À1 (PBE-D3) in Gibbs energies. These values are in good agreement with a previous MP2/6-31G * study [27] that found 11.7 kJ mol À1 for the energy difference and 25.9 kJ mol À1 for the energy barrier. Furthermore, electron diffraction experiments, [28] force field calculations [29] and a 13 C NMR study showing only four signals [30] also suggest that the twist-boat confor-mation can be neglected over a wide temperature range including room temperature.…”

“…Structural parameters of the optimized molecule in chair conformation are found in Table 2. The CÀC bond lengths and C-C-C angles are in good agreement with previous calculations at the Hartree-Fock/6-31G * level [27] and experimental results. [28] Differences, the largest ones to experiment being 0.062 and 8.18, can be attributed to missing temperature and anharmonicity effects that have not been considered in our calculations as well as the simplified structural model assumed in the data refinement of the experimental study.…”

A Theoretical Study of Cyclohexyne Addition to Carbonyl–Cα Bonds: Allowed and Forbidden Electrocyclic and Nonpericyclic Ring-Openings of Strained Cyclobutenes

“…Our calculations show that the alkyne ( 1 ) is distorted from linearity by 48°, which is in agreement with past theoretical studies of cyclohexyne using Hartree-Fock, DFT, or Møller-Plesset perturbation theory methods. 14–16 The C-C and C-O bond lengths in cyclopentanone enolate ( 2a ) are representative of enolate character. Figure 2 shows the geometries of cyclopentanone enolate nucleophilic addition to cyclohexyne optimized in the gas phase with B3LYP/6-31G(d) and B3LYP/6-311++G(d,p).…”

Highly Strained Unsaturated Carbocycles

“…Yavari et al investi- gated the possible conformations of cycloheptyne 45, using ab initio calculations. [43] Cycloheptyne has two main geometries: the envelope (C s ) conformation -corresponding to an energy minimum -and the twist-transition state (C 2 ), which corresponds to an energy maximum of 34.0 kJ mol -1 (Figure 4). Krebs and co-workers synthesized compound 45 and other more stable analogues 46 and 47 by irradiation of the corresponding cyclopropenones (Scheme 12).…”

“…[7,48] It was found that the C-C C-C unit in cyclooctyne is still deformed, with a C C-C angle of 154.5°, with different accessible conformations. Work by Yavari and co-authors [43] showed that the chair conformation with C 2 symmetry is more stable than the twisted conformation. The authors used DFT calculations to investigate the interconversion energy between the two forms.…”