Cyclobutane thermal decomposition

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). 

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 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... 

Cyclobutanes were also obtained from metallacyclopentanes by simple thermal decomposition, by treatment with other nucleophiles (rather than alkenes) such as phosphanes, nucleophilic solvents or by reaction with oxygen. Byproducts of these reactions are the respective alkenes or linear dimers. The extent of the formation of byproducts depends on the temperature of the decomposition, on the solvent and the nucleophile and on the coordination number of the metal. 

Thermal decomposition of l,l,l-tris(triphenylphosphanyl)nickelacyclopentane (9a) gave ethene (90%) and cyclobutane (10%), while thermal decomposition of l,l-bis(triphenylphos-phanyl)nickelacyclopentane (8b), under the same conditions, gave cyclobutanc ( 70%) in addition to ethene (5%) and but-l-ene (25%). Under related conditions, l,l-bis(tricyclo-hexylphosphanyl)nickelacyclopentane (8c) gave exclusively but-l-ene. However, under optimized conditions (see table below), cyclobutane (10) becomes the main product.126-127... 

A fragmentation reaction which appears to proceed via the generation of 1,4-diradicals is the decomposition of 1,1-tetramethylenediazenes. Unlike the more stable 1,2-diazenes (tetrahy-dropyridazines, see Section 4.2.1.), the 1,1-isomers are not usually isolated or characterized by physical methods but are proposed as intermediates in the thermal decomposition of iV-phenyl-sulfonamidopyrrolidines 1, giving 1,4-diradicals which recombine to yield cyclobutanes 3 and 4. 39 These intermediates are also formed in the photochemical decomposition at low temperature of 1,1-tetramethylenediazenes, prepared in situ from 1-aminopyrrolidines and /er/-butyl hypochlorite.141... 

Table 5. Examples of Cyclobutanes from Photochemical and Thermal Decompositions of Tetrahydropy-ridazine Derivatives...

Finally, the relative rates of cleavage and rotation of 1,4-diradicals have been directly studied by thermal decomposition of the tetrahydropyridazines 9 and 10 (R = Me or D) at 415 "C.85-87 Judging from the rate constants and product distributions obtained for various processes, it is likely that the fate of the diradicals 11 and 12 is identical to those generated by thermolysis of cyclobutanes.85 - 87 Obviously, a choice in favor of the nonconcerted diradical pathway can, therefore, be made on the basis of the aforementioned theoretical as well as experimental endeavors. 

Benzothiophene-1,1-dioxide (VI) in benzene solution, is converted in sunlight into a stable photodimer (Vila or VHb), which is considered to contain a central cyclobutane structure.56 196 Even though the monomer cannot be isolated on thermal decomposition of the dimer, the latter in boiling ethyl phthalate decomposes, forming sulfur dioxide and... 

From the slopes of plots of pentenal/CO vs. pressure of added gas, the relative efficiencies of the inert gas molecules for transfer of vibrational energy are obtained. These are shown in Table III. These relative values seem to follow the same order as the efficiencies of the same molecules in the thermal decomposition of cyclopropane and cyclobutane. 

Gas Photolysis of cyclopentanone Thermal decomposition Cyclopropane13 Cyclobutane ... 

In the thermal decomposition of aliphatic aldehydes free radicals are formed, while cyclobutane and its alkyl derivatives decompose in a unimolecular process. It is thus of interest to know the mechanism of the decomposition of cyclobutane carboxaldehyde containing both cyclobutyl and CHO groups. 

Allene and its decomposition products increase the rate of ketene consumption and that of CO formation, while they hardly influence the rate of CO2 formation. A similar effect was caused by addition of methylene cyclobutane, which decomposes into allene and ethylene. The decomposition of ketene is not inhibited significantly by the usual chain inhibitors, nor is it initiated by biacetyl. Thus, chains, if any, are very short in the thermal decomposition. 

The Wolff rearrangement of six- and five-membered a-diazocycloalkanones has been extensively applied to the synthesis of highly strained frameworks. The rearrangement of an (x-diazo-cyclobutanone was reported from 2-diazo-3,4-bis(diphenylmethylene)cyclobutanone (1). The diazo ketone (1) was prepared by treatment of the 3,4-bis(diphenylmethyl-ene)cyclobutane-l,2-dione tosylhydrazone with alumina in 95% overall yield from the corresponding cyclobutanedione. Irradiation in the presence of water, alcohols and aniline afforded 1-carboxy-, 1-alkoxycarbonyl- and 1-phenylcarbamoyl-substituted 2,3-bis(diphenyl-methylene)cyclopropanes 2, respectively, in 13-87% yields. Thermal decomposition in aqueous dioxane afforded the cyclopropanecarboxylic acid 2 (X = OH) in 52% yield. ... 

As a representative example, treatment of bicyclo[1.1.0]butane (1) with Zeise s dimer in acetone at 23°C for 20 minutes gives butadiene in quantitative yield. At — 45°C (48 hours) a 1 1 complex is formed in 97% yield, which upon further treatment with pyridine gives 2-[bis(pyridine)dichloroplatina]bicyclo[l.l.l]pentane (2) in 94% yield. Thermal decomposition of this complex 2 unselectively leads to a mixture of the starting hydrocarbon 1, cyclobutene (3), butadiene (4), 2-methylcyclopropene (5) and methylene cyclopropane (6). Reduction of the metallacyclic intermediate 2 with lithium aluminum deuteride leads to a bisdeuterated methylcyclopropane 7 accompanied by a bisdeuterated cyclobutane 8 and the starting material 1. ... 

This last reaction necessarily proceedes through a pentacoordinated intermediate, XII The reaction sequence of Eq. (j), coupled with the thermal decomposition of X to cyclobutane, provides a path for the unique catalytic dimerization of ethylene to cyclobutene. ... 

The type of pictorial, back-of-the-envelope, application of theory exemplified by the 4n + 2 rule would soon be applied toward understanding a wide range of no-mechanism reactions. The Diels-Alder reaction (see chapter 3) is one example. The reverse reaction ( retro-Diels-Alder ) also occurs readily. In one step (one transition state), three n bonds are broken and two new o bonds and one n bond are created. The simultaneous cleavage and formation of all bonds in one step is referred to as concerted and such reactions often occur under mild conditions and form specific stereoisomers. Since the bonds made and broken form a continuous cycle in the transition state, the Diels-Alder is an example of an electrocyclic reaction. In contrast to the Diels-Alder reaction, dimerizations of two alkene molecules (R2C=CR2) to cyclobutanes typically fail (as do the reverse thermal decompositions of cyclobutanes). These reactions often succeed when ultraviolet light (photochemistry) is used in place of heat (thermal chemistry). 

Rate constants for the thermal isomerisation of cyclopropane and for the thermal decomposition of cyclobutane... 

Thermal decomposition of cyclobutane under 383°C is a parallel first-order... 

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. 

Swinehart and coworkers and Wilson and coworkers have investigated the pyrolysis of methyl and ethyl 46) cyclobutanes respectively. The primary path for thermal and ionic decomposition of these compounds are shown in Eq. (24) and (25) respectively. 

---