How Strained are Carbomeric-Cycloalkanes?

Abstract

The ring strain energies of carbomeric-cycloalkanes (molecules with one or more acetylene spacer units placed into carbon single bonds) are assessed using a series of isodesmic, homodesmotic, and hyperhomodesmotic chemical equations. Isodesmic bond separation reactions and other equations derived from the explicitly defined hierarchy of homodesmotic equations are insufficient for accurately determining these values, since not all perturbing effects (i.e., conjugation and hyperconjugation) are fully balanced. A set of homodesmotic reactions is proposed, which succeeds in balancing all stereoelectronic effects present within the carbomeric rings, allowing for a direct assessment of the strain energies. Values calculated from chemical equations are validated using an increment/additivity approach. The ring strain energy decreases as acetylene units are added, manifesting from the net stabilization gained by opening the C-CH(2)-C angle around the methylene groups and the destabilization arising from bending the C-C identical withC angles of the spacer groups. This destabilization vanishes with increasing parent ring size (i.e., the angle distortion is less in the carbomeric-cyclobutanes than in the carbomeric-cyclopropanes), leading to strain energies near zero for carbo(n)-cyclopentanes and carbo(n)-cyclohexanes.

A Photo-Favorskii Ring Contraction Reaction: The Effect of Ring Size

“…The same approach connected with the levels of theory used (vide infra) has proven to give satisfactory results. 52,53 The enthalpy change calculated according to Scheme 6 is summarized in Table 4. In the case of 20a , optimizations at the RB3LYP or RMP2 levels of theory with the 6-31G+(d) or 6-311+G(d,p) basis sets resulted, however, in a spontaneous cleavage of the spiro bond (highlighted in red, Scheme 6) and the formation of an open planar structure resembling that of the corresponding triplet biradical.…”

Synthesis and structural study of organic two-photon-absorbing cycloalkanone chromophores

“…Thus, according to the X-ray diffraction data (Table 1 and Scheme 3), upon increasing the size of the central ring, some structural trends can be revealed: (i) the increase of the angles at the -bridges, (ii) the decrease of the exocyclic (O )C-C C angles and (iii) the increase of the deviation of atoms from the mean plane of the acceptor fragment. It should be mentioned also that a conventional estimate for the ring strain of cycloalkanes according to experimental thermochemical data (Pedley et al, 1986) and quantum chemical ab initio calculations (Wodrich et al, 2010) indicates significantly higher ring strain in the four-membered ring than in the larger rings (cyclopropane À26.70 > cyclopentane À6.64 > cyclo-hexane À0.10 kcal mol À1 ). These data reflect structural features (geometric and electronic) of the cyclic groups, such as the deviation of the bond angles from tetrahedral values, a 'banana' electronic bond structure etc.…”

Strained Molecules: Insights from Force Distribution Analysis

“…Recent examples range from the cleavage of carbon-carbon bonds by strain in the ring molecule [4], cycloaddition reactions enhanced by ring strain [5], or highly strained carbomeric cycloalkanes [6]. High strain on the one hand challenges its chemical synthesis, but on the other hand provides an exciting route for coupling chemistry and mechanics.…”