Cycloreversion, of cyclobutanes

ET-induced cycloadditions of polycyclic olefins and cycloreversions of cyclobutane species have been studied by ESR spectroscopy [266]. Upon chemical and electrochemical reduction, 2,2 -distyrylbiphenyl rearranges by intramolecular coupling into a bis-benzylic dihydrophenanthrene dianion (Scheme 1), which can be either protonated to a 9,10 -dibenzyl-9,10-dihydrophenanthrene or oxidatively coupled to a cyclobutane species. It is interesting to note that the intramolecular bond... 

Intramolecular bond formations include (net) [2 + 2] cycloadditions for example, diolefin 52, containing two double bonds in close proximity, forms the cage structure 53. This intramolecular bond formation is a notable reversal of the more general cycloreversion of cyclobutane type olefin dimers (e.g., 15 + to 16 +). The cycloaddition occurs only in polar solvents and has a quantum yield greater than unity. In analogy to several cycloreversions these results were interpreted in terms of a free radical cation chain mechanism. 

The cycloaddition of two ethylenes or the cycloreversion of cyclobutane is one of the textbook examples used in the illustration of the Woodward-Hoffman rules [20] of orbital symmetry. Studies on the cyclobutane radical cation [21,22] showed a low activation energy for the cycloaddition of an ethylene radical cation to ethylene, in remarkable contrast with the high activation energy for the corresponding neutral reaction [23]. The dissociation reaction of cyclobutane radical cation is endothermic. Although there is a cyclobutane ring in the pyrimidine dimer, its electronic structure is likely to be different from cyclobutane itself, because of the presence of the two pyrimidine rings. 

Aloisi, G.G., Mazzucato, U., Bartocci, G., Cavicchio, G., Maravigna, R, and Montaudo, G., Luminescence and photolytic cycloreversion of cyclobutane derivatives cinnamic acid dimers and their diamides, Z. Phys. Chem., 138, 207, 1983. 

Cleavage of a C—C bond gives a distonic radical cation as an intermediate, while concerted cleavage of two C—C bonds yields the corresponding ArO and ArO in cycloreversion of aryl-substituted cyclobutane Therefore, the cycloreversion mechanism is related to dimerization of ArO where tt- and a-dimers are detected during PR of ArO such as... 

In marked contrast to that of cyclobutanes, the cycloreversion of cyclobutanones to ethene and ketene88 is most probably a concerted process.89-94 For example, the fact that the pyrolysis of 2-propylcyclobutanone (13) at 350 °C gives ethene and pent-l-ene in the ratio of 3.8 1 is not easily explained by a diradical mechanism.90 This transformation presumably involves two competing cycloreversion reactions. As expected for a concerted process, the conversion through the less sterically congested transition structure is favored. For this reason, ethene is generated as the major alkene.90... 

A great deal is already known about the pyrolysis of pinenes," which constitutes a perfect case for the study of cyclobutane cycloreversion reactions. In practice, this avenue was first explored with the hope of obtaining products with commercial value.99 Unfortunately, the application of these reactions to organic synthesis is somewhat restricted, because complex product mixtures cause complications. For the sake of clarity Table 6100 110 outlines only the cycloreversion products and their straightforward secondary derivatives nevertheless, it demonstrates some of the synthetic uses of these thermal cleavage reactions. 

The pyrolysis of pinenes is mechanistically similar to that of cyclobutane, giving initially a 1,4-diradical. Subsequent C-C bond fission of this 1,4-diradical thus generates a diene. The stepwise cycloreversion of 7,7-dimethylbicyclo[3.1.1]heptan-2-one (20) at 600 °C is a good example.100 106... 

Despite the fact that the outcome of cycloreversion reactions of cyclobutane derivatives is usually unpredictable, there have been ample examples that demonstrate the usefulness of these reactions synthetically. Some of these reactions are summarized in Table 7.111-158 Indeed, a practical synthesis of methyl buta-2,3-dienoatc by this cycloreversion strategy has been recorded in a detailed format.111... 

The synthetic potential of cyclobutane cycloreversions can be demonstrated by the reactions shown below. For example, the strain inherent in hexacyclo[5.4.1.02 6.03,1°.05,9.08,11]dodecane-... 

The mechanistic and synthetic groundwork has been unequivocally established for the consecutive cycloreversion transannular ene reaction sequence, from which /ra/w-decalin derivatives with a hydroxyl group at the ring junction are produced.146,147 As an example, the thermally induced cycloreversion of the ester 43 at 200°C affords 45 in an astonishing 96% yield.146 Presumably the initial cycloreversion product 44 is converted by a transannular ene reaction to generate the decalin 45.146 However, not all the cycloreversion reactions proceed to give a single product as predicted, as can be shown by the examples collected in Table 7. In fact, closer inspection of work already reported has shown that complex product mixtures are usually obtained from cyclobutane cycloreversion reactions.143,148 152... 

Among the electron transfer induced reactions of cyclobutane systems, cycloreversions are the most prominent. These reactions are the reverse of the cycloadditions discussed in Sect. 4.1. The reactivity of the corresponding radical cations depends on their substitution pattern. We have mentioned the fast two-bond cycloreversion of quadicyclane radical cation as well as the ready ring closure of a tetracyclic system (3, Sect. 4.1). A related fragmentation of cis-, trans-, cis-1,2,3,4-tetraphenylcyclobutane (84) can be induced by pulse radiolysis of 1,2-dichloro-ethane solutions. This reaction produces the known spectrum of trans-stilbene radical cation (85) without a detectable intermediate and with a high degree of... 

The cycloreversion of the cyclobutane radical cation Pyr +oPyr could proceed in either a concerted or stepwise manner, and many attempts were made to determine the mechanism of this cleavage step. Because the radical cation is delocalized, it is not unreasonable that both the C(5)-C(5 ) and the C(6)-C(6 ) bonds are weakened by oxidation of PyroPyr. The observation of a substantial secondary deuterium isotope effect for the cleavage of the first bond [C(6)-C(6 )] and a small isotope effect for the cleavage of the second bond [C(5)-C(5 )] in various deu-terated uracil-derived cyclobutane dimers was, however, taken as an indication of a stepwise splitting mechanism via the distonic radical cation Pyr+-Pyr [9]. Theoretical studies performed by Rosch, Michel-Beyerle et al. also strongly support the assumption of a successive cycloreversion [10]. 

Miranda and his co-workers have studied the cycloreversion of the cyclobutanes (127) and (128) and of the oxetane (129). This work made use of the pyrylium salt sensitizers (130). The reactions arise from the triplet state of the sensitizers since there is clear evidence that the reactions are quenched by molecular oxygen. The ring opening involves an electron transfer, and the best sensitizer is the thiapyrylium salt (130b). The quantum yields for the three products using the three sensitizers are shown in Table 2. ... 

The majority of the intramolecular (2 + 2)-cycloreversions of hetero-bicyclic compounds proceed via ring opening of an annulated cyclobutene or azetine ring. Only in one instance has the same reaction of a cyclobutane ring been proposed as an intermediate step, viz., in the reaction of a thiirene 1,1-dioxide with enamines (Section II,B).33... 

The cycloreversion of the cyclobutane (278) to the olefin occurs from the singlet state on irradiation at 265 nm. Triplet-state reactivity is reported for cycloreversion using A = 347 nm. The isomerization of 1,2-diphenylcyclobutanes has been used as a means of establishing the efficiency of electron-transfer processes in the phenanthrene-dicyanobenzene system. 

A mechanophore (blue in Fig. 2a) is a strategically designed chemical entity which responds to mechanical force in a predictable and useful manner (Fig. 2d-f). The polymer strand here acts as an actuator to transmit macroscopic force to the target. For a fully extended polymer chain, the maximum tension force is at the middle point of the chain contour. So the mechanophore should be incorporated into the middle of the chain with its active bond along the chain contotu (Fig. 2a) [15, 29, 32]. Examples of mechanochemical reactions include homolytic scission of weak bonds (diazo [33]), electrocyclic ring-opening (benzocyclobutenes [29], spiropyrans [32, 34 5], gem-dichlorocyclopropanes [46-49], ge/n-difluorocyclo-propanes [30, 50], and epoxide [51]), cycloreversion reactions (cyclobutane derivatives [52-56], Diels-Alder adducts [57, 58], 1,3-dipolar adducts [59, 60], and 1,2-dioxetanes [61]), dative bond scission [62-64], and flex-activated reactions [34, 65, 66], as recently reviewed by Bielawski [67]. 

FIGURE 4.5 Correlation diagram for cycloaddition and cycloreversion of ethene-cyclobutane system. 

Paquette and Leichter have reported the first case in which a highly stereoselective cycloreversion of a cyclobutane to two olefins becomes energetically accessible, as a result of suprafacial participation of a third proximate o-bond. Thermolysis of the ant/-tricyclo[3,2,0,0 Jheptane (641) gives the cyclopentadiene (642) and olefin (643). The latter had a cis.trans ratio of 90 10, reflecting kinetically controlled product distribution. Biradicals are ruled out by the observed stereoselectivity and the two cyclobutane bonds must be broken more or less simultaneously. Paquette and Leichter favour a + A) process. 

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