Conformation of cyclopentane

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

Envelope (Section 3.6) One of the two most stable conformations of cyclopentane. Four of the carbons in the envelope conformation are coplanar the fifth carbon lies above or below this plane. 

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.

Most stable puckered conformation of cyclopentane Most H atoms are nearly staggered... 

In the envelope conformation four of the carbon atoms are coplanar. The fifth carbon is out of the plane of the other four. There are three coplanar carbons in the half-chair conformation, with one carbon atom displaced above that plane and another below it. In both the envelope and the half-chair conformations, in-plane and out-of-plane carbons exchange positions rapidly. Equilibration between conformations of cyclopentane is very fast and occurs at rates similar to that of rotation about the carbon-carbon bond of ethane. 

Figure 12-14 Nonplanar conformation of cyclopentane. Notice that the forward carbon is out of the plane of the other four.

The conformation of cyclopentane is important because ribose and deoxyri-bose, the sugar components of RNA and DNA, respectively, assume cyclopentanelike ring conformations. These conformations are crucial to the properties and reactions of RNA and DNA. 

The conformation of cyclopentane is slightly folded, like the shape of an envelope. This puckered conformation reduces the eclipsing of adjacent CH2 groups. 

The usually high stereoselectivity on formation of a new double bond or a stereogenic center is due to the stereodifferentiating ability of envelope-like transition states of this concerted suprafacial process. The envelope conformation of cyclopentane serves as a model for the transition state of the [2,3] sigmatropic rearrangement6,7. 

Figure 14. Conformers of cyclopentane and schematic representation of possible singly occupied molecular orbitals (SOMOs) of cyclopentane radical cation.

The symmetric puckered conformations of cyclopentane are the Cs symmetric envelope (E) (10) with four carbon atoms in a plane and the C2 s)mrmetric twist (T) (11) with three carbon atoms in a plane [88]. Unlike cyclohexane, these conformations are of almost equal energy and are separated by barriers of about RT or less [89]. There are ten envelope conformations, each with one of the five carbon atoms out of the plane in one of the two directions, and ten corresponding twist conformations. The individual conformations freely exchange which atom or atoms are out of the plane, a process termed pseudorotation, and the whole sequence of conformations is called the pseudorotational itinerary (O Fig. 9). 

Envelope conformer One of the puckered conformations of cyclopentane, the other being the half... 

In a frozen conformation of cyclopentane, there are five torsion angles, Oq. .. 04 which are fixed (Fig. 2.10a). During pseudo-rotation, one of them, say Oq, varies between two extreme values, c/n) and -o for example, going through 0o, O, O, 00- From this came the idea of evaluating this periodical function... 

---