Ethylene from cyclobutane

Figure 3.5. Potential energy paths for formation ethylene from cyclobutane...

Fig. 3.5 Potential energy paths for formation ethylene from cyclobutane. (From http //www.zewail. caltech.edu/graphics/position.jpg. 2016 Caiifornia institute of Technoiogy).

It is not possible to estimate the energy difference which separates 2 and 3 because the value, which can be predicted to be less than 2.5 kcal./mole has to be measured from the slight changes in the ratio of ethylene to cyclobutane with temperature. The temperature dependence would be a function of the pressure also. 

Liberation of ethylene from the titanium cyclobutane complex Cp 2Ti[C(=CH2)CH2CH2] generates the corresponding titana-allene species, Cp 2Ti=C=CH2 which, surprisingly, behaves differently in trapping reactions with... 

The metallaallene Cp 2Ti=C=CH2 generated by liberation of ethylene from the titanium cyclobutane Cp 2Ti[C(=CH2)CH2CH2] (see Section 4.05.4.2.4.(i)) reacts with an excess of nitrile at high temperatures under insertion of two molecules to give Cp 2Ti[(NCR)2C=CH2] (Scheme 575).1357... 

Ethylene deriv. molecules from cyclobutane ring... 

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. 

Fluorinaied dienophiles. Although ethylene reacts with butadiene to give a 99 98% yield of a Diels-Alder adduct [63], tetrattuoroethylene and 1,1-dichloro-2,2-difluoroethylene prefer to react with 1,3-butadiene via a [2+2] pathway to form almost exclusively cyclobutane adducts [61, 64] (equation 61). This obvious difference in the behavior of hydrocarbon ethylenes and fluorocarbon ethylenes is believed to result not from a lack of reactivity of the latter species toward [2+4] cycloadditions but rather from the fact that the rate of nonconcerted cyclobutane formation is greatly enhanced [65]... 

The synthesis of 2-chloro-2,3,3-trifluorocyclobutyl acetate illustrates a general method of preparing cyclobutanes by heating chlorotrifluoroethylene, tetrafluoroethylene, and other highly fluorinated ethylenes with alkenes. The reaction has recently been reviewed.11 Chlorotrifluoroethylene has been shown to form cyclobutanes in this way with acrylonitrile,6 vinylidene chloride,3 phenylacetylene,7 and methyl propiolate.3 A far greater number of cyclobutanes have been prepared from tetrafluoroethylene and alkenes 4,11 when tetrafluoroethylene is used, care must be exercised because of the danger of explosion. The fluorinated cyclobutanes can be converted to a variety of cyclobutanes, cyclobutenes, and butadienes. 

Enhancement of the total butene yield is observed when various additives whose ionization potential falls below about 9.4 e.v. are present during ethylene radiolysis (35). This is consistent with the above interpretation (Figure 2). In the vacuum ultraviolet photolysis of cyclobutane the yield of butenes varies with the ionization potential of the additives in the same way as observed here (12). The maximum enhancement corresponds closely to the yield of C4H8+, as expected from our mechanism. 

The real reactions that most resemble these are the production of cyclobutane from two ethylene molecules ((112)2) and the Diels-Alder reaction between butadiene and ethylene ((H2)3). Even these cannot be made to react in the bare forms, but fairly simple activation by substituents will allow the (H2)3 analog to proceed. Apparently, no form of the (1 2)2 analog has ever been observed. Our analysis suggests that there is a fundamental difference between the four-electron and six-electron systems that produces the effect. The book by Woodward and Hoffrnan[58] may be consulted for a rationalization of these results based upon MO theory. 

Reduced pyrimidines are much less stable toward hydrolysis than the fully conjugated analogs, and this is often used synthetically to produce amino acids and diamines. The BH3 reduction of cyclic amidines (1,4,5,6-tetrahydropyr-imidines) to hexahydropyrimidines, and their subsequent hydrolysis was mentioned above <1999JFIC105>, but there are many more examples. For instance, m-cyclobutane /5-amino acids 544 can be prepared from the cyclobutane derivatives 542 formed by the [2-F2] photocycloaddition reaction between uracil and ethylene <2002TL6177, 2004TL7095, 2006SL1394>. 

The decay of the tetramethylene diradical derived from 2,2,5,5-t/4-cyclopenta-none is much slower than seen for the C4Hg diradical. Both principal decay modes, fragmentation to two ethylenes and ring-closure to cyclobutane, may be dependent dynamically on torsional motions of the terminal methylene groups. 

There are a number of other mechanisms by which alkenes can undergo photochemical f2 + 2) cycloaddition, one of which works well for electron-rich alkenes and electron-acceptor sensitizers. The pathway is through the radical cation of the alkene, which attacks a second, ground-state alkene molecule and then cydizes and accepts an electron to give the product cyclobutane. Typical of this group of reactions is the formation of 1,2-dialkoxycydobutanes from alkoxy-ethylenes with drcyanonaphthalene as sensitizer 12.78). 

Arc-generated carbon atoms abstract sulfur from tetrahydrothiophene, forming ethylene (91%) and cyclobutane (9%) (71JA3807). The photolysis of tetrahydrothiophene vapour with 2139 A radiation gave ethylene as the major product (71CJC1316). 

Orbital Symmetry Conservation in Bimolecular Cycloadditions. The cycloaddition reactions of carbonyl compounds to form oxetanes with ethylenes, as well as those of enones and their derivatives to form cyclobutanes, are examples of reactions which originate from triplet excited states and lead in the first step to biradical intermediates. Such reactions are of course not concerted, and they show little or no stereo-specificity. 

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