Puckered envelope conformation

Cyclopentane, (a) Structural formula, (b) In the planar conformation, there are 10 pairs of eclipsed C—H interactions. (c)The most stable conformation is a puckered "envelope" conformation. 

Hypothetical planar conformation Puckered envelope conformation... 

Five-membered rings with one double bond are normally found to be in an envelope conformation, or they are planar. For the non-planar molecules a puckering potential will exist. The barrier to puckering is generally quite low. For example, in cyclopentene a barrier of about 2.4 kJ/mol has been determined239. ... 

Generally, the 1,2,4-trioxolane ring can adopt either an envelope (A) or a puckered (B) conformation. 

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). 

Depending on steric factors and the coordination sphere of the metal, metallacyclopentanes occur in three structural isomers. They can have an open-envelope conformation (compound Ia ), in which the carbon frame C-1 —C-4 is close to being planar, or a twisted open-envelope structure (compound 19 ° ). A third possibility is a puckered conformation (com-... 

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

In accordance with the characteristics of conformer B, the thiazine ring in 92a assumes a skew-boat shape shifted somewhat toward the twist-boat form, whereas in 92b it has an almost perfect skew-boat conformation. The Cremer-Pople puckering parameters (< > = 89-%° and 0 = 69-77°) and the asymmetry factors for these conformations indicate a less (92a) and a more (92b) perfect symmetry axis bisecting the C-4—S-5 bond. Thus, the cw-fused triazoline ring with its N-2—C-3 double bond has an envelope conformation in 92a with the C-10 on the flap, but a halfchair conformation in 92b with a twofold axis bisecting C-3. Together,... 

When the 3-aryl substituent is replaced by a COOEt group as in 92c, the flexible thiazine ring moves via pseudorotation from the skew-boat/ twist-boat form to an intermediate shape between a half-chair and an envelope conformation (< > = 45.5° and 0 = 52.8°) with the lowest puckering... 

If cyclopentane were planar, as Baeyer had predicted, it would have essentially no angle strain, but its 10 pairs of eclipsed hydrogens would be subject to considerable torsional strain. So cyclopentane puckers, allowing the hydrogens to become nearly staggered. In the process, however, it acquires some angle strain. The puckered form of cyclopentane is called the envelope conformation because the shape resembles a squarish envelope with the flap up. 

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