Cyclobutadiene, angle strain

DE, since the 7c-electron energy is 4a + 4/3, the same as that for two independent double bonds. Thus, at this level of approximation, HMO theory prediets no stabilization for eyelobutadiene from delocalization and furthermore predicts that the molecule will have unpaired electrons, which would lead to very high reactivity. In addition, cyclobutadiene would suffer angle strain, whieh is not present in benzene. The extreme instability of eyelobutadiene is then understandable. Higher-level MO ealculations modify this picture somewhat and predict that eyelobutadiene will be a rectangular molecule, as will be diseussed in Chapter 9. These ealculations, nevertheless, agree with simple HMO theory in... 

One possibility for the lack of stability4 of cyclobutadiene is that the angle strain associated with having four sp2 carbons in a four-membered ring is much greater than estimated. However, the stable existence of many compounds with four such sp2 carbons, for example 27 and 28, make this argument weak, if not invalid ... 

The first synthesis of an isolable cyclobutadiene (71) kinetically stabilized by bulky substituents was achieved starting from the angle strained cycloalkyne (31) via the PdCl2 complex (69)9 198,199). Here again the high reactivities of (10), (31) and (79)... 

Cyclobutadiene has two pairs of rr electrons, and cyclooctatetraene has four pairs of tt electrons. Unlike benzene, these compounds are not aromatic because they have an even number of pairs of rr electrons. There is an additional reason why cyclooctatetraene is not aromatic—it is not planar but, instead, tub-shaped. Earlier, we saw that, for an eight-membered ring to be planar, it must have bond angles of 135° (Chapter 2, Problem 28), and we know that sp carbons have 120° bond angles. Therefore, if cyclooctatetraene were planar, it would have considerable angle strain. Because cyclobutadiene and cyclooctatetraene are not aromatic, they do not have the unusual stability of aromatic compounds. 

Like cyclobutadiene, planar cyclooctatetraene must have one electron in each of the degenerate nonbonding molecular orbitals, making it a diradical. Unlike cydobu-tadiene, however, there is a way for cyclooctatetraene to get out of its energetic misery. Cyclobutadiene must be planar, or very nearly so. By contrast, cyclooctatetraene can distort easily into a tublike form in which most of the angle strain and the diradical nature of the planar form is avoided (Fig. 13.23). 

Even fusion with four-membered rings has been achieved, at least with the smaller of the two furan projection angles as in the cyclobutadiene equivalent (32) (72JA8950) the presumably more strained (and antiaromatic ) isomer (33) remains unknown. 

Valence bond theory, in the terms defined by Pauling, is not able to account for the 4n+2 rule, and the properties of cyclobutadiene and cyclooctatetraene. It has been suggested that the problem with these molecules is the strain associated with the bond angles in the planar structures.10 However, this was shown to be incorrect by the observation that the addition of two electrons to cyclooctatetraene leads to the planar dianion. It is only recently that it has been recognized that cyclic permutations must be included in order to properly treat cyclic systems via valence bond theory.11 One of Pauling s few failures in structural theory is his nonrecognition of the problems associated with the 4n molecules. 

If cyclobutadiene were square or rectangular, the C-C-C angles must be 90°. If the molecule is diamondshaped, two of the C-C-C angles must be less than 90°. Both of these situations result in a great deal of distortion and strain in the molecule. Cyclobutadiene is very unstable for these and other reasons. 

Since the preparation of [I8]annulene, many other annulenes have been made. As long as they are (nearly) planar and delocalized, those with (4n + 2) tt electrons, such as benzene and [18]annulene, are aromatic, whereas those with 4n tt electrons, such as cyclobutadiene and [16]annulene, are antiaromatic. When cyclic delocalization is prohibited by angle or steric strain, such as in cyclooctatetraene or [10]annulene (Exercise 15-15), the systems are nonaromatic. Of course, cyclic polyenes in which there is no contiguous array of p orbitals are not annulenes and therefore also nonaromatic. 

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