C/s-Elimination

The use of mesyl chloride for the dehydration of C-ll alcohols has already been mentioned, and mesylates can certainly be intermediates at least in the a-series. The preference for a coplanar trans arrangement is demonstrated by the well-known elimination reactions of tosylates of epimeric 20-alcohols (ref. 185, p. 616), although this does not restrict the usefulness of the reaction, and in some cases (sulfonates of 1 la-alcohols, for example) c/s-elimination occurs (ref. 216, p. 293 ref. 224, 225, 226). 

Dehydrohalogenation by oxygen bases from vinyl halides proceeds readily by trans elimination via an 2-type of mechanism. c/s-Elimination is sluggish or does not occur at all. Thus trans elimination from bromovinyl ethers 2 (R = alkyl) furnishes the acetylenes 3 in a fast reaction in about 90% yield, whereas elimination from 4 (R = alkyl) is very sluggish and yields a mixture of the acetylene 3 and allene 5 (equations 17 and 18). Similar observations were shown in Schemes 1 and... 

A number of studies were done to assess thermal stability of aromatic polyesters. Some of these studies describe flash pyrolysis [27-32]. Some studies are dedicated to slow thermal degradation in an inert atmosphere, and others describe the decomposition in specific conditions such as in the presence of humidity or in the presence of catalysts [33]. For example, thermal decomposition of poly(butylene terephthalate) was significantly influenced by the presence of water vapor, and the amount of the residues decrease with increasing the partial pressure of water in the atmosphere [34]. In another study, thermal stability of some small molecule phthalate esters was studied [35]. The results can be used for inferring information on the thermal stability of related polymers. The influence of substitution on the p-carbon atom was evaluated on compounds such as bis(2-aminobutyl) phthalate, bis(2-nitrobutyl) phthalate, bis(2,4-diphenylbutyl) phthalate, and dineopentyl phthalate. Only the phenyl groups were found to improve the heat resistance by the obstruction of the planar configuration necessary for the c/s-elimination and the hindrance of the formation of a six-membered cyclic transition state. 

The formation of some p-terphenyl (13) besides the expected 3-benzoyl-p-terphenyl (12) in No. 9 from the highly overcrowed intermediate (11) may find an explanation in the c/s-elimination of dimethylbenzamide. 

Earlier it was observed that the treatment of pyrylium salts (14) with enamines of cycloalkanones (15) in boiling acetonitrile results in the formation of substituted cycloalkenobenzophenones (17) if the pyrrolidino residue in (15) is used33. However, with enamines of morpholine preferentially the cycloalkenobenzenes (18) could be isolated. The heptatrienones (16), although not isolated, were supposed to be intermediates consistently with our experience, and a disrotatory ring-closure with c/s-elimination of the corresponding benzamide was proposed for the formation of (iS)33 (Table 4). 

What remains after all is the indication that this reaction also follows an electro-cyclic mode, passing through the energy-rich intermediate of formula (363") with subsequent c/s-elimination of hydrogen halide. 

It has been reported that the thermal degradation of PLA predominantly consists of random main-chain scission and unzipping depolymerization reactions. The random degradation reaction involves hydrolysis, oxidative degradation, c/s-elimination, and intramolecular and intermolecular transesterification. Almost all the active chain-end groups, residual catalysts, residual monomers, and other impurities enhance the thermal degradation of PLA. As a consequence... 

Scheme 12. Degradation of PA-6 via c/s-elimination Reprinted from [a.114] with permission from Elsevier...

Poly((] )-3-hydroxybutyrate) (PHB), poly(E-lactide) (PLEA) and poly(e-caprolactone) (PCL) are biodegradable polyesters used in practical applications mainly because they combine remarkable physico-mechanical performances with biodegradability, compostability and compatibility with different forms of waste disposal a.439. As shown in Scheme 17(a), thermal degradation of PHB has been suggested to occur almost exclusively by a nonradical random chain scission reaction (c/s-elimination) involving a six-membered ring transition state a.l50] 852663 832485. ... 

A highly diastereoselective alkcnylation of c/s-4-cyclopentene-l,3>diols has been achieved with 0-protected (Z)-l-iodo-l-octen-3-ols and palladium catalyst (S. Torii, 1989). The ( )-isomers yielded 1 1 mixtures of diastcrcomcric products. The (Z)-alkenylpalladium intermediate is thought to undergo sy/i-addition to the less crowded face of the prochiral cyclopentene followed by syn-elimination of a hydropalladium intermediate. 

This mechanism not only accounts for the substitution of the more labile chlorine atom on the polymer chain, it also results in the elimination of a new potential initiation site by moving the double bond out of conjugation with any adjacent chlorine atoms. The newly formed C—O or C—S bonds, with AH > 484 kJ/mol (100 kcal/mol), are significantly more thermally stable than even the normal C—Cl bonds in PVC at about 411 kj/mol (85 kcal/mol) (11). 

D. Sharp, and S. Diamond, SHPP-C-343 Eliminating or Minimising Alkali-Silica Reactivity, National Research Council, Washiagton, D.C., 1993. 

Diaziridinones (167) are obtained from di-t-alkylureas by Af-chlorination and base-promoted elimination of hydrogen chloride from (279) (69JOC2254). The very unstable c/s-fused diaziridinone (280) could be prepared by a similar but milder procedure (76JOC2813). 

Two disadvantages are associated with the use of S-acetyl or 5-benzoyl derivatives in peptide syntheses (a) base-catalyzed hydrolysis of 5-acetyl- and 5-benzoylcys-teine occurs with /S-elimination to give olefinic side products, CH2=C-(NHPG)CO—(b) the yields of peptides formed by coupling an unprotected amino group in an 5-acylcysteine are low because of prior S-N acyl migration. ... 

For example, c/s-4-t-butyleyelohexyl bromide undergoes E2 elimination at a rate about 500 times greater than the tram isomer because only the cis isomer permits anti elimination from the favored chair conformation. ... 

Ring opening of the heteroring (C—S bond cleavage), 300, or even ring opening with elimination of the heteroatom, 301 and 302, happens in the reaction of benzothiophene with Ru3(CO)l2 (94AGE1381). 

Two-component methods represent the most widely applied principles in sulfone syntheses, including C—S bond formation between carbon and RSOz species of nucleophilic, radical or electrophilic character as well as oxidations of thioethers or sulfoxides, and cheletropic reactions of sulfur dioxide. Three-component methods use sulfur dioxide as a binding link in order to connect two carbons by a radical or polar route, or use sulfur trioxide as an electrophilic condensation agent to combine two hydrocarbon moieties by a sulfonyl bridge with elimination of water. 

Rossol I, A Ptihler (1992) The Corynebacterium glutamicum aecD gene encodes a C-S lyase with alpha-beta-elimination activity that degrades aminoethylcysteine. J Bacteriol 174 2968-2977. 

Yoshida Y, Y Nakano, A Amano, M Yoshimura, H Fukamachi, T Oho, Y Koga (2002) led from Streptoccus anginosus encodes a C-S lyase with a,P-elimination activity that degrades L-cysteine. Microbiology (UK) 148 3961-3970. 

A possible reaction mechanism shown in Scheme 7-10 includes (a) oxidative addition of the S-H bond to Pd(0), (b) insertion of the allene into the Pd-H bond to form the tt-allyl palladium 38, (c) reductive elimination of allyl sulfide, (d) oxidative addition of the I-aryl bond into the Pd(0), (e) insertion of CO into the Pd-C bond, (f) insertion of the tethered C=C into the Pd-C(O) bond, and (g) P-elimination to form 37 followed by the formation of [baseHjI and Pd(0). 

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