Four-center elimination reactions

Tertiary alcohols such as r-butyl alcohol ° and t-amyl alcohol , decompose at fairly high temperatures [T 800 °K) to water and the corresponding olefins. These reactions are believed to be concerted and to proceed via four-center, polar, transition states. Thus for t-butyl alcohol 

This conclusion is further supported by calculations of the reaction rate of the free radical induced decomposition of /-butyl alcohol. The most reasonable mechanism is the self inhibiting chain process 

The radical abstraction activation energy (Ei) was estimated from the thermochemistry (Ai/° i = 12.6 kcal.mole ) and a back abstraction activation energy of, = 9 kcal.mole Other estimates follow from the initiation reaction thermodynamics and by analogies with similar reactions . The chain carrier is the resonance stabilized, 2-methyl ally radical, and this should give the slowest (maximally inhibited) chain. From the steady state, one obtains 

In contrast to the tertiary alcohols, primary (e.g., ethyl alcohol) and secondary alcohols e.g., isopropyl alcohol) decompose to products at temperatures above 800 °K via complex free radical chain processes . This mechanistic inversion is not surprising. Based on the magnitude of substituent effects in four-center elimination reactions, particularly the variations found in the series r-BuCI, i-PrCl, EtCl - , one would estimate that the isopropyl alcohol unimolecular elimination of water should have an activation energy about 6 kcal.mole higher than that for r-butyl alcohol. The. 4-factor can be estimated by transition state methods, and one obtains for the unimolecular decomposition 

This should be compared to the slowest possible chain path for the isopropyl alcohol decomposition, which (like /-butyl alcohol) is one propagated by allyl type radicals. A calculation, similar to that for /-butyl alcohol (shown above), where the major chain process is 

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Figure 21 Surprisal plot for the vibrationally hot HF product from the four-center elimination reaction CH,CF, - CH2 = CF + HF. The energy rich, long living, CH,CF, is produced via two routes as shown. The HF vibrational distribution is rather nonstatistical, but is almost the same for both routes. (Adapted from E. Zamir and R. D. Levine, Chem. Phys. 52 253 (1980).) For recent experimental studies of elimination reactions see E. Arunan, S. J. Wategaonker, and D. W. Setser, J. Phys. Chem. 95 1539 (1991) T. R. Fletcher and R. Leone, J. Chem. Phys. 88 4720 (1988).)...

Infrared chemiluminescence from the HX product has been used to infer the dynamics of three- and four-centered elimination reactions (94,95). Three-centered elimination has been observed from chemically activated halomethanes (94) which were produced by recombination of an atom and a radical. The radical was the product of an abstraction process. Thus in the F + CH2CI2 system, the following reactions leading to three-centered elimination of vlbrationally excited HCl were observed ... 

S. Kato and K. Morokuma, Potential energy characteristics and energy partitioning in chemical reactions Ah initio MO study of four-centered elimination reaction CH3CH2F -> CH2=CH2 + HF,... 

The degree of vibrational excitation in a newly formed bond (or vibrational mode) of the products may also increase with increasing difference in bond length (or normal coordinate displacement) between the transition state and the separated products. For example, in the photodissociation of vinyl chloride [9] (reaction 7), the H—Cl bond length at the transition state for four-center elimination is 1.80 A, whereas in the three-center elimination, it is 1.40 A. A Franck-Condon projection of these bond lengths onto that of an HCl molecule at equilibrium (1.275 A) will result in greater product vibrational excitation from the four-center transition state pathway, and provides a metric to distinguish between the two pathways. 

Whereas phosphonium ylides normally react with carbonyl compounds to give alkenes, dimethylsulfonium methylide and dimethylsulfoxonium methylide yield epoxides. Instead of a four-center elimination, the adducts from the sulfur ylides undergo intramolecular displacement of the sulfur substituent by oxygen. In this reaction, the sulfur substituent serves both to promote anion formation and as the leaving group. 

Path b, a four-center elimination process, reported in the diethyl ether decomposition , is a very dubious reaction, viz. 

A useful variant of the reflected shock technique has been to add a chemical substance in trace amounts whose unimolecular rate constant is well known. The products of this trace reaction are then used as an internal standard for measuring the mean temperature in the system (66, 67, 68, 69). A number of rate constants for four-center and six-center elimination reactions measured in this way have been found to be in excellent agreement with rate constants obtained more conventionally at lower temperatures. However, almost all of the rate constants for simple fission of branched hydrocarbons obtained in this way have yielded anomalies (48) which have not yet been resolved. 

The examples covered in this chapter demonstrate that multistep catalytic cycles are simply composed of the following mixture of elementary organometallic reaction steps (i) ligand addition/dissociation, (ii) oxidative addi-tion/reductive elimination, (iii) hydrogenolysis (four-center concerted reactions, heterolytic H2 activation), and (iv) migratory insertion/elimination. The vast majority of catalytic reactions can be described using these basic reactions. 

Is an example of a four-centered elimination from a chemically activated molecule. In this case there is a significant potential energy release since the activation energy of the reverse reaction (HF + CH2 = CF - CH3CF3) is v42 kcal/mole endoergic. 

There is an important difference between the reactions of these sulfur ylides and the analogous phosphorus ylides with carbonyl compounds. Whereas phosphorus ylides react with aldehydes and ketones to yield alkenes, dimethylsulfonium methylide and dimethyloxosulfonium methylide yield epoxides. Instead of a four-center elimination in the zwitterionic intermediate, intramolecular nucleophilic displacement by oxygen occurs. 

In Europe, interest has centered particularly on polyhydroxybutyrate, which can be made into films for packaging as well as into molded items. The polymer degrades within 4 weeks in landfills, both by ester hydrolysis and by an ElcB elimination reaction of the oxygen atom p to the carbonyl group. The use of polyhydroxybutyrate is limited at present by its cost—about four times that of polypropylene. 

These results are in accordance with the findings of Boothe and coworkers26, who found that the reactions of four diastereomeric 2-bromo-3-phenylsulfinylbutanes with tributyl-tin radicals generate /3-phenylsulfinyl sec-butyl radicals (8) which eliminate PhSO radicals to form the 2-butenes in a stereoselective manner. The stereoselectivities observed in this free radical elimination must result from the fact that the rate constant for elimination is greater than that for rotation about the C—C bond. Furthermore, a neighboring phenyl group on the radical center seems to stabilize the radical enough so that the internal rotation can compete with the -elimination reaction. It is also noteworthy that the small... 

Molecular reactions are generally more difficult to treat because of the complexity of the possible transition states. The most widely studied complex molecular reaction class is HX elimination from halogenated hydrocarbons. These reactions proceed primarily via the formation of polar, four-centered tight transition states, and examples include... 

Intramolecular alkylation of a lithiated species has been exploited in the synthesis of [1.1.0]- and [1. l.l]-propellanes (equation 45).143-145 In what appears to be a related reaction, the bis(dibromocarbene) adduct (11) on treatment with methyllithium undergoes a ring closure by 1,6-elimination, presumably via a four-center transition state, to give (12 equation 46), which was subsequently converted to a bis(ho-mobenzene).146... 

As with C-alkylation, the mechanism of amino group replacement can follow the elimination/addition or the nucleophilic replacement path. Both mechanisms are indeed mentioned in the literature and are cxrcasionally claimed to occur concurrently. The elimination/addition path is suggested in the reaction of p-aminoketones with uracils, and a four-centered transition state is proposed for the. same reaction with indole Mannich bases. ... 

The reaction might be concerted or could initially proceed by one four-center step in which one H2O molecule was eliminated, followed by a second four-center step in which N2 and the other H2O molecule were produced. 

The bimolecular elimination reaction (E2) also requires a specific arrangement (B) of four atomic centers. In B, the groups X and Y are coplanar with the two carbon atoms and antiparallel (anti-trans, or true... 

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