Cyclobutane geometry

Cyclobutane has less angle strain than cyclopropane and can reduce the torsional strain that goes with a planar geometry by adopting the nonplanar puckered confer matron shown m Figure 3 11... 

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. ... 

The NMR spectra of cis and trans-diphenylbenzo-cyclobutene are nearly the same except for the chemical shifts of the hydrogens on the four-membered ring. The hydrogens in one compound produce a singlet at 6 4.44, while those in the other compound produce a singlet at 6 5.2. Examine the geometries of cis and trans diphenylbenzo-cyclobutane. Which compound is responsible for which NMR spectrum Explain your reasoning. 

Fig. 1. Geometry of the formation of cyclobutane from two ethene molecules.

Crotonaldehyde, hydrogenation of, 43-48 Cubane, isomerization of, 148 Cyclic dienes, metathesis of, 135 Cyclic polyenes, metathesis of, 135 Cycloalkenes, metathesis of, 134-136 kinetic model, 164 ring-opening polymerization, 143 stereoselectivity, 158-160 transalkylation, 142-144 transalkylidenation, 142-144 Cyclobutane configuration, 147 geometry of, 145, 146 Cyclobutene, metathesis of, 135 1,5,9-Cyclododecatriene, metathesis of, 135... 

A cycloaddition reaction produces a ring of atoms by forming two new G-bonds, for example the formation of a cyclobutane dimer from two alkene molecules. The direct photoreaction involves the concerted reaction of the singlet Jtpt ) excited state of one alkene with the ground state of the other. Stereospecific reactions in which the dimers preserve the ground-state geometry occur when liquid cis- or trans-but-2-ene are irradiated at low temperature ... 

Photodimerization of cinnamic acids and its derivatives generally proceeds with high efficiency in the crystal (176), but very inefficiently in fluid phases (177). This low efficiency in the latter phases is apparently due to the rapid deactivation of excited monomers in such phases. However, in systems in which pairs of molecules are constrained so that potentially reactive double bonds are close to one another, the reaction may proceed in reasonable yield even in fluid and disordered states. The major practical application has been for production of photoresists, that is, insoluble photoformed polymers used for image-transfer systems (printed circuits, lithography, etc.) (178). Another application, of more interest here, is the use that has been made of mono- and dicinnamates for asymmetric synthesis (179), in studies of molecular association (180), and in the mapping of the geometry of complex molecules in fluid phases (181). In all of these it is tacitly assumed that there is quasi-topochemical control in other words, that the stereochemistry of the cyclobutane dimer is related to the prereaction geometry of the monomers in the same way as for the solid-state processes. 

The concept of the conservation of orbital symmetry can be extended to intermolecular cycloaddition reactions which occur in a concerted manner. The simplest case is the dimerization of ethylene molecules to give cyclobutane, the 2n + 2je cycloaddition. The proper geometry for the concerted action would be for the two ethylene molecules to orient one over the other. Two planes of symmetry are thereby set up -perpendicular to the molecular plane bisecting the bond axes oy-parallel to the molecular plane lying in between the two molecules (Figure 8.10). 

TABLE 12. Geometries of propane, ethene, cyclobutane and some fused cyclopropanes"... 

Although the picture of the photochemical primary processes in cyclopentanone which has been presented seems self-consistent, a number of minor points still have to be explained. These are (a) the dependence of the ratio of ethylene to cyclobutane on the geometry of the system (6) the puzzling fact that a constant fraction, between 2/10 and 3/10, of the initially excited molecules seem to return to the ground state without decomposition, by a route that is virtually unaffected by pressure. Before this can be explained it is necessary to confirm the value for the quantum yield for decomposition and (c) the fact that 2.5 kcal./mole of energy affects the reaction path profoundly. In the ground state the enthalpies of 2 and 3 differ by 19 kcal./mole at 25° while 3 and 4 may be estimated to differ by 15 to 20 kcal./mole. This phenomenon may be explained when a clear understanding of the excited state of the molecule is obtained. 

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