Cyclobutane, a bonds

Photolysis of the substituted norbomen-7-one (84) and its exo-isomer gave the cyclohexa-1,3-diene (85). This isomerized to a cyclo-octatriene only on warming so showing that the cyclobutane a-bond was not involved in the loss of carbon mon-oxide. Benzocyclo-octatrienes were amongst the products obtained by irradiation of a mixture of 2-naphthonitrile and alkyl vinyl ethers. Photolysis of 1-phenylindan-2-one gave the dibenzocyclo-octadiene (86) amongst other products. 

The overall reaction includes allylic transposition of a double bond, migration of the allylic hydrogen and formation of a bond between ene and enophile. Experimental findings suggest a concerted mechanism. Alternatively a diradical species 4 might be formed as intermediate however such a species should also give rise to formation of a cyclobutane derivative 5 as a side-product. If such a by-product is not observed, one might exclude the diradical pathway ... 

With two y,8 double bonds, two a,/3 double bonds, and the possibilities of cis and trans ring fusions with syn and anti configurations, 20 isomeric dimers are possible. Surprisingly, only one product is formed in a head-to-tail fashion. The sole product of the irradiation of the 3,5-diene-7-ketosteroid (76), however, is the head-to-head dimer. The specificity and mode of addition arise presumably through the effect of the specific environment of the chromaphore. The dimerization of (75) is believed to involve the addition of the a,fi double bond of a photoexcited molecule to the less hindered y,8 double bond of a ground state molecule. The photocondensation of (76) with cyclopentene, in which steric hindrance should not be a controlling factor, was found to yield a cyclobutane product involving the a,/ bond of the steroid in contrast to dimerization across the y,8 bond. 

If one connects the two double bonded end groups X in form RED of general structure /I (n = 1) with two methylene bridges instead of the usual a-bond, then 4(5red should result. With well adapted X-groups this cyclobutane derivative 6red may... 

In contrast to cyclopropane, the cyclobutane C C bonds are only slightly bent, so that in order for the proton to form a bond, it must come close to the positively charged carbon nuclei, leading to increased Coulombic repulsion. Similarly, an attempt to bond one of the carbons does not lead to an ion with any apparent stabilization. This attempt to bond to one of the carbons leads to a relatively unstable... 

Structures of protonated cyclobutanes have been studied in the same fashion (see Figure 9 B). In the corner-protonated cyclobutane, the structure corresponds essentially to a methyl cation interacting with a trimethylene diyl, and is much less favorable than that for cyclopropane. Similarly, for the edge-protonated ion, the proton must come much closer to the carbons to form a bond than for cyclopropane, and as a result, cyclobutane is much less basic. 

A special case of the preparation of cyclobutanes from 1,5-dienes via valence isomerization is the use of acyclic or cyclic 1,5,7-trienes which give four-membered rings via an intramolecular [7t + 7ts2] cycloaddition (Diels-Alder reaction). This variant is illustrated for monocyclic tricnes 18 and 20 where two 71-bonds are transformed into a-bonds, resulting in tricyclic compounds 1968 and 21.09... 

Among the catalysts used are Lewis acids991 and phosphine-nickel complexes.992 Certain of the reverse cyclobutane ring openings can also be catalytically induced (8-40). The role of the catalyst is not certain and may be different in each case. One possibility is that the presence of the catalyst causes a forbidden reaction to become allowed, through coordination of the catalyst to the -it or a bonds of the substrate.993 In such a case the reaction would of course be a concerted 2S + 2S process. However, the available evidence is more consistent with nonconcerted mechanisms involving metal-carbon a-bonded intermediates, at least in most cases.994 For example, such an intermediate was isolated in the dimerization of nor-bornadiene, catalyzed by iridium complexes.995... 

To achieve a bond angle of 90°, cyclobutane must be planar. This would force the adjacent C-H bonds to be eclipsed and would also raise the energy of the system. As a compromise, cyclobutane folds diagonally by 35°. While this raises the angle strain somewhat, it decreases the eclipsing interactions so that the lowest possible energy is attained. While each methylene group is less strained than one in cyclopropane, the total molecular strain is similar to that of cyclopropane. 

Considering the actual cyclobutane ring opening step Pyr +OPyr Pyr+-Pyr (Scheme 4.5.4), however, where the C(6)-C(6 ) a bond is broken and a N(1)=C(6) n bond is simultaneously formed, this cleavage should be slower the more the involved orbitals, e.g. the SOMO and the c orbitals of the N(l)-C(6) and C(6)-C(6 ) bonds, deviate from coplanarity (Scheme 4.5.8). 

Paquette, L.A. and Leichter, L.M. (1971) Pyrolysis of anti-tricyclo[3.2.0.02,4] heptanes. The role of 2,4 substitution in the operation of a bond assisted cyclobutane fragmentations. Journal of the American Chemical Society, 93, 4922 -924. 

From organic chemistry it is known that cycloaddition reactions leading to cyclobutanes are required to be stepwise reactions, according to the Woodward-Hoffmann rules [131]. A bond is formed between the two olefins, leading to a tetramethylene intermediate (T). In a subsequent step, the second bond is formed, yielding the cycloadduct. Depending on the reactants, either zwitterionic or diradical tetramethylenes can be proposed as intermediates [132, 133]. 

The all cis. cyclobutane will yield only a cis alkene. Configuration is retained (or inverted) at both termini of the two a bonds involved, the reaction is a supra-supra process as in A. In B a cis alkene and a trans alkene are formed. Thus at both the termini of one c bond the configuration is retained only at one terminus and not at the other. The process will be supra-antara. 

Let us examine the correlation of the orbitals of reactant with those of the product (Fig. 8.22). There is a correlation of a bonding reactant level with an antibonding product level, and vice versa. But if orbital symmetry is to be conserved, two ground state ethene molecules cannot combine in a concerted reaction to give ground state cyclobutane nor can cyclobutane be decomposed in a concerted fashion to two ethene molecules. 

The four T-electrons of the two ethene double bonds must redistribute themselves to the a-oibitals of the newly formed cyclobutane single bonds. As it turns out in this case, the requirement of symmetry conservation does not allow two ground-state ethene molecules to combine in a concerted reaction to form a ground-state cyclobutane molecule The reaction is "forbidden." The only allowed interactions are destabilizing. 

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