Cyclopentane, planar

The model has been extended to cyclobutane hydrogenolysis and to cycloheptane reactions. Cyclobutane is assumed to undergo, like cyclopentane, planar adsorption on the surface (114). The stretching, however, is more pronounced than in the case of the C5 rings and propagates to all four C-C bonds of the molecule. These can then be broken easily, and that accounts for the absence of selectivity in the hydrogenolysis of alkylcy-clobutanes (114,115). 

Structural Considerations and Spectroscopic Properties.—The geometry of three conformations of cyclopentane, planar half chair (C2), and envelope (CJ, has been investigated by means of a floating spherical Gaussian orbital model which allows for extensive optimization of geometry. Relative to the half-chair the and planar forms were calculated to be 2.6 and 9.8 kcal mol less stable, values which are larger than found previously. 

The most common stereoselective syntheses involve the formation and cleavage of cyclopentane and cyclohexane derivatives or their unsaturated analogues. The target molecule (aff-cts)-2-methyl-l,4-cyclohexanediol has all of its substituents on the same side of the ring. Such a compound can be obtained by catalytic hydrogenation of a planar cyclic precursor. Methyl-l,4-benzoquinone is an ideal choice (p-toluquinone M. Nakazaki, 1966). 

FIGURE 3 12 The (a) planar (b) envelope and (c) half chair conformations of cyclopentane... 

At one time all cycloalkanes were believed to be planar It was expected that cyclopentane would be the least strained cycloalkane because the angles of a regular pentagon (108°) are closest to the tetrahedral angle of 109 5° Heats of combustion established that this is not so With the exception of cyclopropane the rings of all cycloalkanes are nonplanar... 

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. 

Identify the lowest-energy conformer from among those provided cyclopropane, planar and puckered cyclobutane, planar and puckered cyclopentane and chair, half-chair, boat and twist-boat cyclohexane. (If... 

Figure 4.6 The conformation of cyclopentane. Carbons 1, 2, 3, and 4 are nearly planar, but carbon 5 is out of the plane. Part (c) is a Newman projection along the C1-C2 bond, showing that neighboring C-H bonds are nearly staggered.

Problem 4,10 How many H eclipsing interactions would be present if cyclopentane were planar Assuming an energy cost of 4.0 kj/mol for each eclipsing interaction, how much torsional strain would planar cyclopentane have Since the measured total Strain of cyclopentane is 26 kj/mol, how much of the torsional strain is relieved by puckering ... 

Cyclopropane (115 kj/mol strain) and cyclobutane (110.4 kj/mol strain) have both angle strain and torsional strain. Cyclopentane is free of angle strain but has a substantial torsional strain due to its large number of eclipsing interactions. Both cyclobutane and cyclopentane pucker slightly away from planarity to relieve torsional strain. 

The six-membered rings in these T.S.s are more flexible than the five-membered T.S.—(81) above—and need not be planar (cf cyclohexanes v. cyclopentanes). Elimination may thus proceed, in part at least, from conformations other than the syn-periplanar, with the result that the degree of SYN stereoselectivity in these eliminations may sometimes be lower than that observed in the Cope reaction. Both reactions require higher temperatures than for the Cope reaction, carboxylic esters particularly so. 

Ashton, J.G., Fink, H.L., Schumann, S.C. (1943a) The heat capacity, heats of transition, fusion and vaporization and the vapor pressures of cyclopentane. Evidence for a non-planar structure. J. Am. Chem. Soc. 65, 341-346. 

Two non-planar conformations are believed to exist for cyclopentane which is the envelope form 10.9(Z>), and half chair form 10.9(c). 

Five- and six-membered rings formed by coordination of diamines with a metal ion have the stereochemical characteristics of cyclopentane and cyclohexane. The ethylenediamine complexes have puckered rings and the trimethylenediamine complexes have chair conformations. The methylene protons are nonequivalent in these nonplanar conformations, taking on the character of equatorial and axial substituents. They are made equivalent as the result of rapid conformational inversion at room temperature, just as in the alicyclic compounds (Fig. 7.1). This has been observed in nmr studies of planar and octahedral complexes of ethylenediamine-type ligands with a number of metals. 

Cyclopentane with a planar ring would have five pairs of eclipsed H s producing considerable eclipsing strain. This strain is reduced, at the expense of some increase in angle strain, when one CHj pushes out of the plane of the ring (Fig. 9-4). The puckering is not fixed but alternates around the ring. 

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