Ethylene cyclobutane decomposition

Experiments on the decomposition of the ketone in the presence of oxygen (Table I) strongly indicate that the formation of ethylene, cyclobutane (and, by inference, carbon monoxide), and pentenal is not affected by even 35.5 mm. of oxygen. This may be compared with the... 

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. 

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). 

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... 

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... 

Although the picture of the photochemical primary processes in cyclopentanone which has been presented seems self-consistent, a number of minor points still have to be explained. These are (a) the dependence of the ratio of ethylene to cyclobutane on the geometry of the system (6) the puzzling fact that a constant fraction, between 2/10 and 3/10, of the initially excited molecules seem to return to the ground state without decomposition, by a route that is virtually unaffected by pressure. Before this can be explained it is necessary to confirm the value for the quantum yield for decomposition and (c) the fact that 2.5 kcal./mole of energy affects the reaction path profoundly. In the ground state the enthalpies of 2 and 3 differ by 19 kcal./mole at 25° while 3 and 4 may be estimated to differ by 15 to 20 kcal./mole. This phenomenon may be explained when a clear understanding of the excited state of the molecule is obtained. 

One reaction studied by Zewail is the decomposition of cyclobutane to two ethylene molecules ... 

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. ... 

Here I r resents a reactively inert diluent, R is reactant cyclobutane, ki and kr are bimolecular rate constants for energy transfer, and k is the rate constant for unimolecular decomposition. This particular system exhibits a relatively straightforward chemistry giving ethylene as the only decomposition product of excited cyclobutane. 

The situation changed in the 1980s when chemists at the California Institute of Technology began to use very short laser pulses to probe chemical reactions. Because transition states last only 10 to 1000 femtoseconds, the laser pulses needed to probe them must be extraordinarily short. (1 femtosecond, or 1 fs, is 1 X 10 s. To appreciate how short this time duration is, note that there are as many femtoseconds in one second as there are seconds in about 32 million years ) One of the reactions studied was the decomposition of cyclobutane (C4Hg) to ethylene (C2H4). There are two possible mechanisms. The first is a single step process in which two carbon-carbon bonds break simultaneously to form the product... 

Decomposition of metallacyclopentane compounds may occur either via reductive cycloelimination (reductive decoupling) (Table 1.4) with ethylene evolution or because of reductive elimination leading to the formation of cyclobutane. 

The decomposition of cyclobutane to form two ethylene molecules can take place in one of two ways, (a) The reaction proceeds via a single step, which involves the breaking of two C—C bonds simultaneously, (b) The reaction proceeds in two steps, with the formation of a shortlived intermediate in which just one bond is broken. There is only a small energy barrier for the intermediate to proceed to the final products. The correct mechanism is (b). 

That one of the two carbonyl groups, either that of the decarboxyketoluciferin or that of the carbon dioxide, should be formed in an excited state can be readily predicted from the orbital symmetry conservation principles of Woodward and Hoffmann. This reaction is formally like the decomposition of a cyclobutane ring and yields two ethylene molecules. In analyzing the forward course of that reaction. 

Two possible potential-energy curves for the decomposition of cyclobutane to ethylene... 

Nickel.—Metallocyclopentane complexes [Ni(CH2)4(PRs)2] undergo decomposition to give but-l-ene, cyclobutane, and ethylene. The complexes exist in solution as equilibria (22) and mechanistic studies suggest that but-l-ene is produced from the... 

Many researchers put forward proposals to explain how metathesis could take place, but it was Herisson and Chauvin who proposed the intervention of a metal bound to the carbon through a double bond (metal alkylidene). In the catalytic cycle of cross-metathesis, this active species first reacts with the olefin to form a four-membered ring (Scheme 7.5). This metalla-cyclobutane intermediate then cleaves, yielding ethylene and a new metal alkylidene, which reacts with a new alkene substrate to yield another metal-lacyclobutane. On decomposition in the forward direction, this second intermediate yields the internal alkene product and regenerates the initial metal alkylidene. ... 

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