Cyclopentane Newman Projection

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.

The steric effects become clear on inspecting the Newman projections of the transition states. The conformation 43 a leads to cyclopentane formation, while the conformation 43 b would give / -H elimination. As the rhodium carbenoid becomes larger, conformation 43 b is increasingly favored. Thus, as the steric bulk of the ligands on the Rh carbenoid increases on going from acetate (entry 1) to the TPA catalyst (entry 4), there is a significant increase in the proportion of /9-hydride elimination. 

Cyclopentane is appreciably less strained than cyclobutane and cyclopropane, and the strain energy relative to cyclohexane is ca. 6.45 kJ mol-1 per CH2 group. In order to lessen the torsion strain that would occur in a planar conformation, in which every C-H bond is involved in two eclipsing interactions, cyclopentane adopts a puckered conformation (see Dunitz, Further Reading). This has four carbons approximately planar, with the fifth carbon bent out of this plane in such a way that the molecule resembles a small near-square envelope 9. A Newman projection of 9 is shown in 10. 

Figure 3.2.4 Two Newman projections of the major conformers of the C20-C17 bond in progesterone. The flat conformer with the carbonyl oxygen almost in the cyclopentane plane is slightly less energetic and is also the biologically active conformer.

PROBLEM 5.1 Draw a Newman projection looking down one of the carbon-carbon bonds in planar and envelope cyclopentane. Use models ... 

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