Envelope conformation, 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. 

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. 

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. 

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... 

Envelope, twist, and (hypothetical) planar conformations of cyclopentane. 

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. 

Cyclopentanes, cyclohexanes, and all larger cycloalkanes exist in dynamic equilibrium between a set of puckered conformations.The lowest energy conformation of cyclopentane is an envelope conformation.The lowest energy conformations of cyclohexane are two interconvertible chair conformations. In a chair conformation, six bonds are axial and six are equatorial. Bonds axial in one chair are equatorial in the alternative chair, and vice versa. A boat conformation is higher in energy than chair conformations. The more stable conformation of a substituted cyclohexane is the one that minimizes axial-axial interactions. 

Cyclic alkanes undergo pseudorotation because rotation by 360° is not possible. Pseudorotation in cyclic alkanes leads to many conformations. Cyclopropane is planar, with relatively weak banana bonds. The lowest energy conformation of cyclobutane is a puckered conformation. The lowest energy conformation of cyclopentane is an envelope conformation. The lowest energy conformation of cyclohexane is an equilibrating mixture of two chair conformations. 

The lowest-energy conformation of cyclopentane is an envelope conformation. 

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