Cyclopropane ring, geometry

This increased stability has been explained between the bent orbitals of cyclopropyl rings and the vacant p orbital of the cation carbon. The vacant p orbital lies parallel to C2-C3 bond of the cyclopropane ring and not perpendicular to it. Thus the geometry becomes similar to that of a cyclopropane ring conjugated with an oblefinic bond. 

The geometry of the molecule favours the process. This reaction is also called homo-Diels-Alder reaction because a cyclopropane ring is formed rather than a n bond. 

Alkyl and cycloalkyl substituents seem to have little effects on the geometry of the cyclopropane ring. There are hardly any reliable data on ring distortion in simple molecules. Recent studies have concentrated on highly strained structures (Table 2). 

Park et al. [10] have recently reported an efficient photocyclization of 1-benzoyl-l-(o-ethylphenyl) cyclopropane (29) to the corresponding indanol 31. In this ketone, the cyclopropane ring bulk appears to be equivalent to that of two methyl groups, and forces the geometry of the 1,5-biradical intermediate suitable for cydization (Scheme 8.8). 

The quantities S and R are defined so as to be positive in sign. We shall be concerned here with the strain energy of the cyclopropane ring, in which the C—C—C bond angles are constrained by geometry to be 60° rather than their normal values, which are in the neighborhood of the tetrahedral angle 109.5°. 

The interaction with a cationic center, as in the cyclopropylcarbinyl cation, has a similar character. The main difference is that whereas a proton has no stereochemical requirements with respect to its bond to carbon, the cationic center may take different orientations with respect to the cyclopropane ring. Experiment and theory both agree that the preferred geometry is that known as bisected and that rotation of the cationic center by 90° will raise the energy by about 14 kcal mol The preferred conformation is that which allows the p orbital at the cationic center to interact with the in-plane carbon orbitals in the highest occupied MO. This type of interaction also may be seen in the energies of radical cation states of cyclopropane derivatives as determined by photoelectron spectroscopy . 

In cyclopropane rings three types of coupling constant are possible (1) vicinal (cis), (2) vicinal (trans) and (3) geminal. The geometry of the cyclopropane ring indicates that the dihedral angle between cis-vicinal protons is 0° whereas that between trans-vicinal... 

Cyclopropylidenemethyl systems, in which one carbon of the double bond is already part of the cyclopropane ring, give especially stable vinyl cations owing to favorable geometry for overlap with participating cyclopropyl orbitals. This subject has also been reviewed recently in detaiP " and thus will only be summarized here briefly. 

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