Cyclobutanes conformation

Figure 12-15 Nonplanar cyclobutane conformation with a substituent R in the less hindered, quasi-equatorial position. The dihedral angle between the two halves of the bent ring usually is 25° to 30°, that is, a 25° to 30° deviation from planarity.

Cyclobutane has less angle strain than cyclopropane (only 19.5°). It is also believed to have some bent-bond character associated with the carbon-carbon bonds. The molecule exists in a nonplanar conformation in order to minimize hydrogen-hydrogen eclipsing strain. 

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. 

Such a structure implies that there would be a barrier to rotation about the C(2)—C(3) bond and would explain why the s-trans and s-cis conformers lead to different excited states. Another result that can be explained in terms of the two noninterconverting excited states is the dependence of the ratio of [2 + 2] and [2 + 4] addition products on sensitizer energy. The s-Z geometry is suitable for cyclohexene formation, but the s-E is not. The excitation energy for the s-Z state is slightly lower than that for the s-E. With low-energy sensitizers, therefore, the s-Z excited state is formed preferentially, and the ratio of cyclohexene to cyclobutane product increases. ... 

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. 

Cyclobutane has less angle strain than cyclopropane and can reduce the torsional strain that goes with a planai geometry by adopting the nonplanai puckered conformation shown in Figure 3.11. 

Identify the lowest-energy conformer from among those provided cyclopropane, planar and puckered cyclobutane, planar and puckered cyclopentane and chair, half-chair, boat and twist-boat cyclohexane. (If... 

Figure 4.5 The conformation of cyclobutane. Part (c) is a Newman projection along the C1-C2 bond, showing that neighboring C—H bonds are not quite eclipsed.

Wehle, D. Fitjer, L. Tetrahedron Lett., 1986, 27, 5843, have succeeded in producing two conformers that are indefinitely stable in solution at room temperature. However, the other five positions of the cyclohexane ring in this case are all spiro substituted with cyclobutane rings, greatly increasing the barrier to chair-chair interconversion. 

The modification of molecular conformation from the highly strained non-isolable dimer molecule to the V-shaped dimer molecule (6 OPr-dimer) is explained in terms of relaxation of the strain energy due to the bond angle in the non-isolable dimer, which accumulated during the cyclobutane formation. Therefore, strictly speaking, the process going from the non-isolable dimer into the V-shaped dimer (6 OPr-dimer) is not a... 

Figure 4.11 (a) The folded or bent conformation of cyclobutane, (b) The bent or envelop form of cyclopentane. In this structure the front carbon atom is bent upward. In actuality, the molecule is flexible and shifts conformations constantly... 

The observation that the overwhelming product from the cycloaddition of 3 to 1,3-butadiene (12) is a cyclobutane derivative 31 and the proportion of the [4 -I- 2] adduct increases in the order 12 < 26 6 is in accord with the increasing diene reactivity in this series. Whereas cyclopentadiene readily combines with most dienophiles at low temperatures, 1,3-butadiene, mainly owing to its predominant s-trans conformation, enters into [4 + 2] cycloadditions only at elevated temperatures. 

The planar form of cyclobutane will be the energy maximum in the interconversion of conformers. 

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