Cyclobutane, conformational isomers

Cyclopropane (Fig.J) is a flat molecule in respect of C-atoms, with the hydrogen atoms situated above and below the plane of the ring, so it has no conformational isomers. Cyclobutane can form three distinct shapes-a planar shape and two butterfly shapes (fig.K). Cyclopentane can also form a number of shapes or conformations. The planar structures for cyclobutane and cyclopentane are too strained to exist in practice because of eclipsed C-H bonds. 

Exercise 12-14 Given the favored nonplanar conformation of cyclobutane (Figure 12-15), predict whether c/ s-1,2-dibromocyclobutane will be more, or less, stable than the corresponding trans isomer. Do the same for the c/s- and trans-1,3-dibromocyclo-butanes. Give your reasoning. 

The molecular structures of cyclobutane and several of its simply substituted derivatives have been investigated by i.r. and Raman spectroscopy. The energy barriers between the two puckered minima for cyclobutane and [ Hg]-cyclobutane have been determined (518 + 5cm" and 508 8cm , respectively). With chloro-, bromo-, and cyano-substituents, asymmetry is introduced into the potential function for ring inversion, and conformational isomers become possible. However, the best fit of the far-i.r. data was for asymmetric potentials with single minima. It was presumed that the minima correspond to puckered conformations with equatorial substituents. The Raman spectra of chloro-, fluoro-, and methyl-cyclobutane lead to similar conclusions. The vibrational spectrum of crystalline cyclobutanecarboxylic acid has been reported and is best interpreted in terms of a hydrogen-bonded dimeric structure with a centre of symmetry. 

A single substituent on the cyclobutane ring leads to conformational isomers and introduces asymmetry into the potential function for ring inversion. As the ring inverts, the substituent goes from the axial conformation to the equatorial, or vice versa. These two conformations will of course have different energies and the spectra can be interpreted using a potential function of the form... 

Solution 11 All the hydrogen atoms are eclipsed in the planar conformation of cyclobutane. The folded ring system has skewed hydrogen interactions. There are six possible isomers of dimethyl-cyclobutane. Since the ring is not held in one particular folded conformation, deviations of the ring planarity need not be considered in determining the number of possible dimethyl structures. 

Cyclobutane adopts a puckered conformation in which substituents then occupy axial-like or equatorial-like positions. 1,3-Disubstituted cyclobutanes show small energy preferences for the cis isomer since this places both substituents in equatorial-like positions. The energy differences and the barrier to inversion are both smaller than in cyclohexane. 

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. 

The similarity in the product ratios from the two 3.2.0 dideuterio isomers would appear to minimize the role of isotope effects here. It is possible that the 1,3- and 3,3-shift pathways are separate, but ring opening of cyclobutanes by the coupled motions depicted in Scheme 9.59 would give conformationally distinct, incompletely equilibrated, bisallyl biradicals which could rationalize the preferences. 

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 conformations and relative stabilities of the cis- and trans-isomers of 1,3-difluorocyclobutane and 1,3-dimethylcyclobutane have been investigated by ab initio methods using the STO-3G basis set. The cis-isomers are found to be the more stable, by 1.39 kcalmolfor the fluorinated cyclobutanes and by 1.48 kcalmol" for the methylated cyclobutanes. These values are within the range of known experimental values. The ring puckering angles in the cis-isomers were calculated to be 19.9° and 23.7° respectively. The trans-isomers were foimd to be less puckered and more flexible than the cis-isomers. 

Molecules of cyclobutane, cyclopentane, and cyclohexane are nonplanar. Besides determining configuration at each carbon atom in these compounds, conformational aspects of the whole molecule should also be considered. Figure 2.15 shows the most stable conformations of rings in these molecules. Of course, such molecules can also exist in any intermediary conformations between those shown. In monosubstituted rings of cyclobutane, cyclopentane, and cyclohexane there is no asymmetric carbon atom. In disubstituted isomers of these cyclic compounds, optical activity can appear even if the substituents are identical. The cis forms will not be optically active in any case since they exist in meso form. On the other hand, the trans forms will be optically active (they occur as two... 

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