Cyclobutane decomposition

In the Lindemann-Hinshelwood theory the Lindemann expression for the uni-molecular rate constant, Eq. (9), is still assumed to be correct, but an improved activation rate coefficient is obtained from the Hinshelwood formulation. The shape of the fall-off curve should therefore still be the simple form predicted by Lindemann. Reference to Fig. 2 shows that, for the cyclobutane decomposition reaction, the change in the activation rate coefficient brings the theory much closer to the experimental results, particularly at low pressure. However, the shape of the fall-off curve is still not correct the Lindemann-Hinshelwood model predicts a faU-off region that is too narrow, the true fall-off is broader. 

The hydrogen-deuterium kinetic isotope effects in the cyclobutane decomposition were first investigated by Langrish and Pritchard ", who co-pyrolyzed QHg... 

Thermal decomposition of unsubstituted 3,4,5,6-tetrahydropyridazine at 439 °C in the gas phase proceeds 55% via tetramethylene and 45% via a stereospecific alkene forming pathway. The thermal decomposition of labelled c/s-3,4,5,6-tetrahydropyridazine-3,4- f2 affords cfs-ethylene-l,2- f2, trans-ethylene-l,2-if2, c/s-cyclobutane-l,2- f2 and trans-cyclo-butane-1,2- /2 (Scheme 57) (79JA3663, 80JA3863). 

Thermal decomposition of cis- and frans-3,6-dimethyl-3,4,5,6-tetrahydropyridazines affords propene, cis- and frans-l,2-dimethylcyclobutanes and 1-hexene. The stereochemistry of the products is consistent with the intermediacy of the 1,4-biradical 2,5-hexadienyl. The results indicate that thermal reactions of cyclic azo compounds and cyclobutanes of similar substitution proceed with similar stereospecificity when compared at similar temperatures 79JA2069). 

At high temperatures, the decomposition of cyclobutane is a first-order reaction. Its activation energy is 262kJ/mol. At 477°C, its half-life is 5.00 min. What is its half-life (in seconds) at 527°C ... 

It is clear that a detailed mechanism for the metathesis reaction of alkenes cannot yet be given with certainty. In view of the fact that, for similar reactions which are formally cyclobutane-dialkene transformations, a nonconcerted reaction pathway has been demonstrated, a concerted fusion of two alkenes to form a cyclobutane complex and its decomposition in the same way with a change in the symmetry plane is less probable. On the basis of the information on the two other mechanisms to date, the mechanism involving a metallocyclic intermediate is more plausible than a mechanism involving carbene complexes. 

C15-0028. Estimate the rate constant for the decomposition of cyclobutane at 25 °C (see Extra Practice Exercise). 

This work has been compared with analogous cyclobutane thermolytic decompositions. The siletanes were found to fragment more readily than the cyclobutanes. Although fragmentation via propene formation (most substituted C—C bond) was favored in both classes of compounds, it was more dominant with the siletanes. These effects are apparent from the kinetic data in Table IV.144-147... 

The extreme hazards involved in handling this highly reactive material are stressed. Freshly distilled material rapidly polymerises at ambient temperature to produce a gel and then a hard resin. These products can neither be distilled nor manipulated without explosions ranging from rapid decomposition to violent detonation. The hydrocarbon should be stored in the mixture with catalyst used to prepare it, and distilled out as required [1], The dangerously explosive gel is a peroxidic species not formed in absence of air, when some l,2-di(3-buten-l-ynyl)cyclobutane is produced by polymerisation [2], The dienyne reacts readily with atmospheric oxygen, forming an explosively unstable polymeric peroxide. Equipment used with it should be rinsed with a dilute solution of a polymerisation inhibitor to prevent formation of unstable residual films. Adequate shielding of operations is essential [3],... 

As depicted in Scheme 11, ylides 39 derived from 4-methyl-[l,2,3]triazolo[l,5- ]pyridine react with Michael acceptors, which, upon nucleophilic attack at C3 and ring opening, lead to nucleophilic displacement of nitrogen. The intermediate diradical led to a mixture of compounds, including alkenes and a cyclobutane derivative when methyl acrylate was used, and the indolizine 40 with methyl propiolate as the electrophile <1998T9785>. Heating 4-methyl triazolopyridine with benzenesulfonyl chloride in acetone also confirmed decomposition via a radical pathway. 

Several other types of photochemical reactions involving unsaturated carbohydrates have been reported. One of these is38 photochemical, E -Z isomerization of the groups attached to a double bond (see Scheme 5). A second is the internal cycloaddition between two double bonds connected by a carbohydrate chain.39-41 Although the carbohydrate portion of the molecule is not directly involved in this cycloaddition, its presence induces optical activity in the cyclobutane derivatives produced photochemically. Finally, a group of acid-catalyzed addition-reactions has been observed for which the catalyst appears to arise from photochemical decomposition of a noncarbohydrate reactant.42-44... 

That the situation is different for photochemical reactions is indicated by a particularly interesting recent study of some dialkylketones (239). In solution, 5-nonanone, 152, reacts photochemically to yield the cyclobutanol 153 and its isomer 154 in comparable amounts. Within the urea clathrate, however, 153 is the dominant product, with only traces of 154 being formed. The cyclobutanols analogous to 153, that is, having methyl and hydroxyl cis, also predominate in the urea-clathrate-mediated photocyclization of 2-hexanone and 2-undecanone. It might be expected that the bulky cyclobutane derivatives, which almost certainly cannot be crystallized in a urea clathrate, would also not be formed in such a clathrate. There are decomposition pathways (cleavage reaction 0 of the diradical intermediate that occur both in the clathrate and in solution. Nevertheless, the ring closure is a major pathway of reaction even in the clathrate. 

The thermal decomposition of cyclobutane to yield ethylene has been very extensively investigated (Genaux and Walters, 1951 Kem and Walters, 1952, 1953). The reaction is homogeneous and kinetically first order. Addition of inhibitors to the reactant does not affect the rate, and... 

The decomposition of cyclobutane can be discussed in terms of two quite distinct types of transition complex. In the first we imagine the simultaneous lengthening of two of the carbon-carbon bonds and the contraction of the other two to yield a complex very like the product molecules, viz. ... 

We first consider the decomposition reactions leading to olefins in terms of an intermediate biradical. The cleavage of the cyclobutane can take place in two ways, to give two distinet biradicals, viz. ... 

Both cis- and frana-butene-2 are formed from each of the dimethyl-cyclobutanes. They are not however formed in equilibrium amounts. Further, more a -butene-2 than the equilibrium amoimt is formed in the decomposition of cis-l,2-dimethylcyclobutane. The fact that the cis- and cyclo-butanes, this does imply that either the lifetime of the free biradical is of the same magnitude as the time for one rotation of the groups in the biradical, or that the biradical is never strictly a free biradical . In either case the configmation of the reactant will, to some extent, determine the stereochemistry of the products. 

The relative percentage yields of products at 135-7°C are (A) ll-l, (B) 1-3, (C) 45-7, (D) 32-1, (E) 9-8. The yield of isoprene has been scaled down by a factor of 2 since the decomposition of one molecule of the cyclobutane yields two molecules of isoprene. The products and rates of reaction of this cyclobutane and the previous one discussed are both consistent with the reaction paths already mentioned. 

The very low value of the energy of activation for this isomerization is of considerable interest. Comparison with the decomposition of cyclobutane shows a reduction of 30 kcal mole caused by the presence of the double bond. If a similar transition state were involved in both reactions, then this difference would be a measure of the extra strain energy of the cyclobutene. This is quite unrealistically high. Thus we eliminate the possibility that the reaction path is as shown below ... 

Apparently the fusion of a benzocyclopropene to two cyclobutanes represents the upper limit for isolable benzocyclopropenes. Several attempts to synthesize dicyclopropabenzenes failed. Thus, attempted aromatization of the carbene adducts of the isomeric cyclohexadienes, 99 and 101 afforded none of the expected 100 and 102, respectively, and attempted aromatization of 103 resulted only in uncharac-terizable decomposition products without any evidence for the intermediacy of 104. ... 

Comparable results are known for the decomposition of nickela-cyclopentane complexes wMch lead, depending on the ligand-to-metal ratio to cyclobutane, 1-butene and ethylene... 

This is so, as shown by Grubbs reports of the reactions of metallacycles of type L Ni(CH2)4 It is reported that the preferred decomposition pathway depends on the degree of coordination (Scheme 2). Here the formation of cyclobutane from... 

Bartlett and Porter197 studied the stereospecificity of the photochemical decomposition of 42. Direct irradiation led to nearly complete retention of stereochemistry in the cyclobutane products but thioxanthone, xanthone, or... 

The thermal decomposition of 8 in tetrachloroethene at 134 C gave a chromatographically separable mixture of cyclobutane 9 and 1,8-divinylnaphthalene 10 in 7 1 ratio. Although several experiments have been carried out to identify the spin multiplicity of the intermediate diradical, the results were inconclusive.17 A recent report stated that while triplet-sensitized photolysis resulted in predominant denitrogenation, laser/liquid jet photochemical reaction also gave cyclopentenes by 1,2-hydrogen shift.18 Indications are that the amounts of cyclopentenes increase with increasing lifetime of the intermediary 1,3-cyclopentadiyl triplet diradical.18... 

A solution of tris(triphenylphosphanc)tetramethylenenickel(II) in toluene was treated with ethene (3.83 kPa), and cyclobutane and butenes were formed, The highest selectivity in favor of cyclobutane (72% in addition to 28% of butenes) was obtained with a 0.0023 M concentration of the catalyst in toluene at 0 A in 72 hours (turnover C4/Ni = 13). Substituted alkenes gave stable metallacycles. which could be converted to cyclobutane derivatives by oxidative decomposition.6... 

One of the problems associated with thermal cyclodimerization of alkenes is the elevated temperatures required which often cause the strained cyclobutane derivatives formed to undergo ring opening, resulting in the formation of secondary thermolysis products. This deficiency can be overcome by the use of catalysts (metals Lewis or Bronsted acids) which convert less reactive alkenes to reactive intermediates (metalated alkenes, cations, radical cations) which undergo cycloaddilion more efficiently. Nevertheless, a number of these catalysts can also cause the decomposition of the cyclobutanes formed in the initial reaction. Such catalyzed alkene cycloadditions are limited specifically to allyl cations, strained alkenes such as methylenccyclo-propane and donor-acceptor-substituted alkenes. The milder reaction conditions of the catalyzed process permit the extension of the scope of [2 + 2] cycloadditions to include alkene combinations which would not otherwise react. 

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