Angle strain, conformational isomers

In Table 7 the six-membered monocyclic dienes are represented by the conjugated 1,3-cyclohexadiene and its isomer 1,4-cyclohexadiene. 1,3-Cyclohexadiene has a nonplanar equilibrium conformation that is primarily influenced by three factors -electron interaction (optimal for a planar conformation) angle strain and torsion strain (both optimal for a planar conformation). The reduced overlap between the two --orbital systems is, for the observed C=C—C=C angle of 18°, estimated at ca 10% and should therefore not influence the conjugation stabilization drastically, compared to a conformation with coplanar C=C bonds. 

Aromaticity in the larger (4A+2) annulenes depends on whether the molecule can adopt the necessary planar conformation. In the all-cis [10]annulene, the planar conformation requires an excessive amount of angle strain. The [10]annulene isomer with two trans double bonds cannot adopt a planar conformation either, because two hydrogen atoms interfere with each other. Neither of these [10]annulene isomers is aromatic, even though each has (4/V+2) pi electrons, with N = 2. If the interfering hydrogen atoms in the partially trans isomer are removed, the molecule can be planar. When these hydrogen atoms are replaced with a bond, the aromatic compound naphthalene results. 

The conformation is thus not only free of angle strain but free of torsional strain as well. It lies at an energy minimum, and is therefore a conformational isomer. The chair form is the most stable conformation of cyclohexane, and, indeed, of nearly every derivative of cyclohexane. 

The cyclohexane ring, either alone or as part of a more complex structural unit, occurs in certain natural products and, accordingly, cyclohexane is the most important saturated cyclic hydrocarbon. Two principal conformational isomers exist. The more stable is called the chair conformation 14, and the less stable the boat conformation (see Section 1.9). In both these conformations the C-C-C bond angles are close to the tetrahedral value of 109°28 consequently, cyclohexane has little angle strain Angle strain becomes significant in saturated hydrocarbons if there are meaningful departures from the above value. 

It is well known that iraras-bicyclo[3,3,0]octane is a rigid and strained system, whereas the cis-isomer is almost strain-free. Pyrrolizidine differs in that one of the carbon atoms is substituted by a trivalent nitrogen atom which does not rigidly fix the bicyclic system. For this reason, pyrrolizidine, although it probably occurs in the preferred cia-conformation,74 has no stereoisomers. The two rings of the pyrrolizidine system form a dihedral angle with the axis along the C(8)—N bond. 

Later, Dale used the extensive triple-bond migration, which takes place easily in macrocyclic dialkynes (C12-C20) when they are treated with KO-t-Bu in DMSO, to confirm his predictions for the relative stability of cyclic diyne isomers [4 c, 62]. These predictions were based on conformational considerations. Thus, 1,7-cyclotetradecadiyne (119) is almost completely rearranged to the 1,8-isomer 120 because in the latter compound both angle and terminal strain are absent, i.e., an ideal conformation is possible (Scheme 8-11). Dale s predictions, which were made long before the routine use of X-ray crystallography, were summarized in a seminal paper [63] and later confirmed by various X-ray investigations. The triple bond migration in cyclic diynes proceeds via allenic intermediates, which, however, are found only in small quantities in the reaction mixture. 

Introducing one or more sp carbon atoms into the six-membered ring introduces more strain, since these atoms require 120 ° angles. Any addition reaction that converts the system to a saturated cyclohexane will tend to be more favorable than for a comparable acyclic system. Thus, cyclohexanone is a little more susceptible to addition reactions than acetone. However, in cyclohexanone, two 1,3-diaxial interactions are removed (7.31, 7.32), This means that for substituted cyclohexanones, the axial conformation is less unfavorable than for a related cyclohexane. As noted earlier, the difference in energy between axial and equatorial conformations for methyl cyclohexane is 7.5 kj mol", and there is only about 5 % of the axial conformer at equilibrium. For 3-methylcyclohexanone, the energy difference is only 2.9 kJ mol", and at equilibrium, there is 25 % of the axial isomer (7.33). 

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