Cyclopropanes Newman projection

Figure 4.4 The structure of cyclopropane, showing the eclipsing of neighboring C-H bonds that gives rise to torsional strain. Part (b) is a Newman projection along a C-C bond.

Norin (71) and Dauben and collaborators (72-74) have shown that the bond of the cyclopropane ring which is reductively cleaved corresponds to the bond which better overlaps with the n system of the adjacent carbonyl group. Thus, when the cyclopropyl ketone exists preferentially in the cisofd and transold conformations described by the Newman projections 238 and 239, the Cq — Cg bond is always cleaved in preference to the Cq— C3 bond. 

In addition to angle strain, cyclopropane has a significant amount of torsional strain. This can best be seen by looking at a Newman projection down any of the C—C bonds. As can be seen in this diagram, each C—C bond is held in an eclipsed conformation by the rigidity of the molecule. 

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. 

Torsional strain is the second factor in cyclopropane s large ring strain. The three-membered ring is planar, and all the bonds are eclipsed. A Newman projection of one of the carbon-carbon bonds (Fig. 3-16) shows that the conformation resembles the totally eclipsed conformation of butane. The torsional strain in cyclopropane is not as great as its angle strain, but it helps to account for the large total ring strain. 

FIGURE 4.9 (a) Orbital overlap in the carbon-carbon bonds of cyclopropane cannot occur perfectly end-on. This leads to weaker bent bonds and to angle strain, p) Bond distances and angles in cyclopropane. (c) A Newman projection formula as viewed along one carbon-carbon bond shows the eclipsed hydrogens. (Viewing along either of the other two bonds would show the same picture.) (d) Ball-and-stick model of cyclopropane. 

In addition, cyclopropane also exhibits significant torsional strain, which can best be seen in a Newman projection ... 

Of course, the carbon skeleton of cyclopropane is flat— three points determine a plane—and planarity causes still other problems for cyclopropane. Figure 5.4 shows a Newman projection looking down one of the three equivalent carbon-carbon bonds of cyclopropane. In cyclopropane, all carbon-hydrogen bonds are eclipsed. Remember. The solid wedges are coming toward you, and the dashed wedges are retreating from you. 

This Newman projection attempts to show the eclipsed C—H bonds in cyclopropane all the C—H bonds are eclipsed... 

Figure 17 Mono (50) and regio-isomeric bis-adducts (51, ( )-52 and ( )-53) formed by Bingel cyclopropanation of [70]fullerene. Also shown are Newman-type projections looking down the C s-symmetry axis of the C70 core on to the two polar pentagons, which show the relative orientations of the addends.

---