Puckered, conformation of cyclobutane

FIGURE 3.11 Nonpl anar ("puckered") conformation of cyclobutane. The nonplanar conformation avoids the eclipsing of bonds on adjacent carbons that characterizes the planar conformation. 

Problem 9.10 Depict the flexible puckered conformation of cyclobutane in a Newman projection. 

The question of planarity of cyclobutane systems has been studied by Lambert and Roberts (1963). If two puckered conformations of cyclobutane exist then a substituent R has two possible environments, pseudo-axial and equatorial to the ring ... 

Aspects of cycloalkane conformational analysis. A. The two puckered conformations of cyclobutane. Note how the substituents (X and Y) exchange positions when puckered forms interconvert. B. Left The envelope form of cyclopentane—the conformer contains a mirror plane in the plane of the page. Right The half-chair form of cyclopentane, looking down the two-fold rotation axis. 

Cycloalkanes with more than three carbon atoms are nonplanar and have puckered conformations. In cyclobutane and cyclopentane, puckering allows the molecule to adopt the most stable conformation (with the least strain energy). Puckering introduces strain by making the C—C—C angles a little smaller than they would be if the molecules were planar however, less eclipsing of the adjacent hydrogens compensates for this. 

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. 

Therefore, there are two different values of the N-H bond distance, N-N-H bond angles, C-N-N-H dihedral angles, etc., for the air-isomer. For both the conformers, the four-membered rings are puckered, as in cyclobutane. However, the air-isomer has C symmetry, whereas the /ra .r-isorncr has C2 symmetry. 

In cyclopentane, as opposed to cyclobutane and cyclopropane, the bond angles have values close to the optimum. Therefore, the strain in the molecule arises essentially from bond opposition and is partly relieved by puckered conformations. Two flexible forms of cyclopentane exist, namely the so-called envelope (LXXVlIIa) and half-chair (LXXVIIIb) forms. The former has four carbons in the same plane, and... 

The cyclobutane ring exists in a non-planar, open-book conformation, which may flex further open or shut. The cyclopentane ring exists in two puckered conformations, the half chair and the envelope, both of which are non-planar. The position of the puckering may migrate around the ring, and this is called pseudorotation. 

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. 

The strain in cyclopropane and cyclobutane is largely a result of angular strain due to compression of the tetrahedral angle of 109 28 in the carbon-carbon bonding. In cyclopentane and cyclohexane this angular strain is reduced by the molecule adopting a puckered conformation. Thus, the conformation of the cyclopentane molecule is that of four carbon atoms in a plane, with one carbon atom some 0 5 A below this plane Fig. 1 (a). This is a mean value about which the puckering oscillates, and in addition there is a... 

The steric energy for cyclobutane is calculated to be 29.22 kcal/mol. Because the molecule is cyclic, there should not be free rotation about any bonds. Nevertheless, the conformational dynamics associated with inversion of puckered geometries (Figure 3.17) means that some internal energy must be included in the calculation of the heat of formation, and the total correction for POP and TOR amounts to 0.36kcal/mol. Thus, the strain energy of cyclobutane should be greater than the amoimt shown by 0.36 kcal/mol, or a total of 26.95 kcal/mol. 

Two conformations are possible for cyclobutanes, a flat planar structure or a puckered one. Cyclobutane itself has been determined to possess a puckered geometry in the gas phase by electron diffraction. A puckered conformation is also consistent with equilibration studies of 1,3-disubstituted cyclobutanes, which indicate that the ds-isomer is more stable than the trans. Such observations are consistent with the equatorial-like orientation of cis substituents in a nonplanar... 

The energy difference between puckered and planar conformations in cyclobutanes appears to be small. The geometries of many-substituted cyclobutanes in the crystal have been determined to be planar by X-ray diffraction, trans-1,3-Cyclobutanedicarboxylic acid, when crystallized by itself, has a planar four-membered ring, but, when cocrystallized with its disodium salt, has a puckered four-membered ring. ... 

The orbital correlation diagram for the formation of cyclobutane from two ethylene molecules, using two perpendicular mirror planes as diagnostic symmetry elements, [1, p. 19] is too familiar to have to be reproduced. We recall that when the reactant molecules are set up in the coplanar [ 2s + 7r2s] orientation (Fig. 1.1b), other symmetry elements exist as well, including two rotational axes which also bisect bonds made or broken in the process [1, p. 31]. The use of these symmetry elements instead of the mirror planes is also more realistic, because cyclobutane is most stable in a puckered conformation [53], in which the rotational axes are retained but the mirror planes are not. 

The principal component of the reaction coordinate is the approach of the two ethylene molecules towards one another with retention of the full symmetry assumed in the construction of the correspondence diagram as Fig. 6.2 Wcts set up in D2/1, this least motion approach has the irrep ag. The diagram then tells us that the reaction coordinate for concerted conversion of the two tt bonds into the two (T bonds of cyclobutane also has to include a 625 component. Several symmetry coordinates, and the subgroups of D2/1 to which they desymmetrize the reaction path, are shown in Fig. 6.3. If the correspondence diagram had called for an displacement, the relatively facile formation of cyclobutane in its stable puckered D2 conformation would have been expected. If a b u component were required to induce the neccesary correspondence, the favored pathway would generate a cisoid biradical, which would immediately collapse to cyclobutane. The nominally stepwise reaction would then be kinetically indistinguishable from one in which the formation of both bonds is synchronous. 

Cyclobutane has less angle strain than cyclopropane. However, it has more torsional strain, because there are four sets of eclipsing H s rather than just three. To alleviate some of this additional torsional strain, cyclobutane can adopt a sfightly puckered conformation without gaining too much angle strain ... 

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