Four-center elimination

Epoxides can be converted to alkenes by treatment with triphenylphosphine or triethyl phosphite P(OEt)3. The first step of the mechanism is nucleophilic substitution (10-50), followed by a four-center elimination. Since inversion accompanies the substitution, the overall elimination is anti, that is, if two groups A and C are cis in the epoxide, they will be trans in the alkene ... 

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. 

Molecular Fissions Three- and four-center eliminations ... 

If mechanism I is operative, the four-center elimination must be non-rate determining. Gordon and Norris have noted that the normal CH3O—H (the... 

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

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... 

Major reference sources for the topics shown below are general thermochemical theory of transition states refs. 39, 54, 301 thermodynamics reference source refs. 44, 100 four-center eliminations ref. 1 six-center rearrangements and eliminations refs. 1, 45 small ring compound decompositions and isomerizations ref. 60 cis-trans isomerizations refs. 39, 54 bond fissions refs. 39, 503. 

In our own laboratories, a new technique called very low pressure pyrolysis (VLPP) (60) has permitted us to make direct studies on high temperature rate processes at sufficiently low pressures that secondary processes are negligible. Under such conditions (1200°K, 10 4 torr), we have found that the four-center elimination of HI from isopropyl iodide can be as much as 1/1000 as fast as one would estimate from extrapolating the Arrhenius rate constant from lower temperatures. 

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).)...

The vibrationally excited HF molecule is believed to be formed primarily by the following four-centered elimination processes involving chemically activated intermediates ... 

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 ... 

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. 

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,... 

---