Nonplanar interconversion

Cyclopentane is nonplanar, and the two minimum-energy geometries are the envelope and half-chair. In the envelope conformation, one carbon atom is displaced from the plane of the other four. In the half-chair conformation, three carbons are coplanar, vdth one of the remaining two being above the plane and the other below. The energy differences between the conformers are very small, and interconversion is rapid. All of the carbon atoms r idly move through planar and nonplanar positions. The process is called pseudorotation. 

Compound 156 adopted the 3H tautomer exclusively, even in hydrogen bond acceptor solvents, such as acetone-46. It was proposed by analogy with the 1,5-benzodiazepine system which adopts an analogous tautomer, the seven-membered ring of 156 is likely to be nonplanar and, regarding NMR spectra, interconversion between two equivalent forms may be present. 

More recently, the radical cation of the parent system has been characterized by ESR/ENDOR spectroscopy [340], These experiments revealed medium-sized hfcs for both the bridgehead (—11.4 G) and the equatorial protons (+11.4G) and large positive hfcs (+ 77 G) for the axial protons (which in 75 are replaced by the trimethylene bridge). The large difference between the couplings of the axial and equatorial protons indicates a nonplanar, puckered geometry of the radical cation. No evidence for interconversion is apparent up to 160 K this finding requires an inversion barrier of at least 12 kJ mol-1. 

Cyclobutane derivatives are usually, but not always, nonplanar. Cy-clobutane itself exists as two butterflylike conformers that are easily interconvertible by inversion. Cyclopentane is a cyclic five-membered ring structure that can exist as a set of half-chair (twist-boat) forms (C2) and a set of envelope (C5) conformations. These are low-energy conformations and are readily interconverted by twists about bonds without any bond angle changes, only changes in torsion angles. These interconversions are called pseudorotations. 

There are many other nonplanar conformations of cyclohexane, one of which is the boat conformation. You can visualize the interconversion of a chair conformation to a boat conformation by twisting the ring as illustrated in Figure 3.9. A boat conformation is considerably less stable than a chair conformation. In a boat conformation, torsional strain is created by four sets of eclipsed hydrogen interactions, and steric strain is created by the one set of flagpole interactions. Steric strain (also called nonbonded interaction strain) results when nonbonded atoms separated by four or more bonds are forced abnormally close to each other—that is, when they are forced closer than their atomic (contact) radii allow. The difference in potential energy between chair and boat conformations is approximately 27 kj/mol (6.5 kcal/mol), which means that, at room temperature, approximately 99.99% of all cyclohexane molecules are in the chair conformation. 

Neither rw-l,2-dimethylcyclopropane nor c -l,2-dimethylcyclohexane can be resolved, because the cyclopropane is a meso compound (Chapter 4, p. 168), and the cyclohexane flips into its mirror image. However, both 1,2-dimethyl-cyclopropane and /rawi-l,2-dimethylcyclohexane can be resolved they are not superimposable on their mirror images. The point is that in a practical sense the planar cyclopropanes and nonplanar cyclohexanes behave in the same way. This finding has important consequences. In deciding questions of stereochemistry, we can treat the decidedly nonplanar 1,2-dimethylcyclohexanes as if they were planar. Indeed, all cyclohexanes can be treated as planar for the purposes of stereochemical analysis, because the planar forms represent the average positions of ring atoms in the rapid chair-chair interconversions. 

PROBLEM 5.41 In Section 5.6b (p. 205), you saw that for purposes of stereochemical analysis you could treat the decidedly nonplanar cis- and /rarer-l,2-dimethylcyclohexanes as if they were planar. The planar forms represent the average positions of ring atoms in the rapid chair-chair interconversions. Use planar representations of the following cyclohexanes to determine which molecules are chiral. 

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