Elimination reactions continued

Olefin Syntheses by Dehydrogenation and Other Elimination Reactions 139 Table 3, (Continued)... 

In order to follow progress of elimination, reactions were also performed on thin films in a special sealed glass cell which permitted in situ monitoring of the electronic or infrared spectra at room temperature (23°C). Typically, the infrared or electronic spectrum of the pristine precursor polymer film was obtained and then bromide vapor was introduced into the reaction vessel. In situ FTIR spectra in the 250-4000 cm-- - region were recorded every 90 sec with a Digilab Model FTS-14 spectrometer and optical absorption spectra in the 185-3200 nm (0.39-6.70 eV) range were recorded every 15 min with a Perkin-Elmer Model Lambda 9 UV-vis-NIR spectrophotometer. The reactions were continued until no visible changes were detected in the spectra. 

As with the alkenes, most chemical reactions of alkynes involve the elimination of the triple bond in favor of a double and a single bond. Normally the reactions continue so that the double bond is eliminated in favor of two more single bonds. 

Multistep catalytic cycles (continued) reductive elimination reactions, 1, 121 Muon spin resonance, metallocene-based magnets,... 

The most important synthetic routes continue to be (1) the elimination of an alkali halide between the salt of a transition-metal anion and a silicon halide, and (2) oxidative addition and addition-elimination reactions. The present position regarding the scope and limitations of these and other routes is outlined in this section. 

If the reaction can be directed mostly to the monoxide, step (1) above, the heat liberated is only 28 percent of that given off when the reaction continues to the dioxide. Thus glowing may be eliminated by an insufficient amount of heat to continue combustion. 

The salt-elimination reactions are usually performed in THF or related ethereal solvents and the range of metallates is very large. The method continues to be useful as noted from the additional recent examples collected in Table 10. 

Organometallic compounds are used widely as homogeneous catalysts in the chemical industry. For example, if the alkene insertion reaction continues with further alkene inserting into the M C bond, it can form the basis for catalytic alkene polymerisation. Other catalytic cycles may include oxidative addition and reductive elimination steps. Figure above shows the steps involved in the Monsanto acetic acid process, which performs the conversion... 

In Chapter 6, elimination reactions were presented. In the context of elimination reactions, the formation of double bonds was noted regardless of the elimination mechanism discussed. Continuing from the concept of using elimination reactions to form sites of unsaturation, one may reason that addition reactions can be used to remove sites of unsaturation. Thus, elaborating upon addition reactions, this chapter provides an introduction to relevant mechanisms applied to both carbon-carbon double bonds (olefins) and carbon-oxygen double bonds (carbonyls). 

Theoretically, it is possible for the process of olefin coordination and insertion to continue as in Ziegler-Natta polymerization (Chapter 52) but with palladium the metal is expelled from the molecule by a p-hydride elimination reaction and the product is an alkene. For the whole process to be catalytic, a palladium(O) complex must be regenerated from the palladium(ll) product of P-hydride elimination. This occurs in the presence of base which removes HX from the palladium(II) species. 

First, following the results of the 1,6-dioxa-spiro[2.5]octane rearrangement (5,19), continuous gas phase conditions were applied in a fixed bed reactor and secondly under liquid phase conditions in a slurry reactor. The catalytic experiments carried out showed that two main reactions took place rearrangement of 18 to the aldehyde 19 and a oxidative decarbonylation reaction to the olefine 1,3,3,4-tetramethyl-cyclohex-l-ene 20, which is assumed to be caused by a formaldehyde elimination reaction. Also observed was a deoxygenation reaction to the alkane 1,1,2,5-tetra-methylcyclohexane 21 (Eq. 15.2.7), explained by elimination of CO. There are several other side-products such as 2,2,3,6-tetramethylcyclohex-l-enyl-methanol, ringcontracting compounds and double bond isomers of dimethyl-isopropylene-cyclopentene. 

Cyclic products, and hence cyclic carbenium ions, seem to form more rapidly at higher conversion This is probably due to product olefin addition (alkylation) to small surface-resident ions, followed by cyclization and elimination reactions. The process of addition, cyclization and elimination can continue if no desorption takes place, until a large and undesorbable aromatic is formed on the surface, resulting in the complete deactivation of a site by "aromatic coke". Each such addition of an olefin changes the character of the adsorbed species and, in particular, its reactivity. Notice that in the processes proposed so far, only one active site will be deactivated per deactivating event. 

Over the years, several techniques have been developed to elucidate the structure of a new lantibiotic. The extensive post-translational modifications limit Edman degradation to a stretch of amino acids from the N-terminus to the first modification. Various chemical derivatization techniques have been used to allow continuation of the sequence and to reveal the position of the posttranslationaUy modified residues. Originally, these techniques relied on treatment with ethanethiol under highly basic conditions that result in elimination reactions of the thioethers... 

The stilbene-dihydrophenanthrene photocycllsatlon reaction continues to find synthetic applications. The tetra-oxygenated methyl phenanthrene skeleton (161) has been prepared by photocyclisation of the stilbene (162) aromatisation of the intermediate dihydrophenanthrene occurs by elimination of methanol. 2 nq photocyclisation was observed In the absence of the cyano group in this compound 2 although the closely related structures (163) and (164) are said to cyclise to the phenanthrenes (165) and (166). Triarylethylenes are important... 

In this case the starting material is considerably more difficult to oxidize than the product under comparable conditions benzene is oxidized at a potential 1.3 V higher than phenol. Consequently, it is observed that the reaction usually cannot be stopped at the phenol stage, but proceeds to benzoquinone [14,15]. Similarly, the oxidation of 9,10-dihaloanthracenes leads to anthraquinone in an addition-elimination reaction [16]. However, the problem may to some extent be overcome if the degree of substrate conversion is kept low (e.g., the anodic oxidation of phenol to hydroquinone [17]) or if the oxidation product is continuously removed (e.g., the indirect oxidation of methylarenes... 

Sequential elimination reactions, most of them being dehydration, involving the reaction at the anomeric center often produce various aromatic compounds [235] especially furans which have diverse use [236,237]. Explorations have been continued to open a new route to aromatics based on renewable biomass in place of fossilized material. 

Among much less nucleophilic substrates, alcoholates are very interesting. They are often very inexpensive, they allow the formation of carbanions, they generally do not compete with carbanions in condensation reactions, they lead to complex bases able to give elimination reactions (vide infra) and finally they allow a large modulation of the basic properties. For these reasons we studied and we are continuing to study a lot of them. 

This chain reaction continues as long as there is a significant concentration of radicals. Termination steps eliminate two radicals and can eventually terminate the reaction. 

In this system, the elimination reaction PR5 —PR3 + R2 was studied on the reaction path from the more stable pentavalent Dsh bipyramid trigonal structure to the products PR3 with a C3V structure. The apical-apical fragmentation starts from the D3h structure and evolves directly into PR3 possessing a planar D3h structure and eventually into the more stable pyramidal C y structure. Although this process appears sterically unlikely, it can be seen as a continuation of the Berry pseudorotation, in which the two apical ligands move to the equatorial positions. In the case of the equatorial-equatorial fragmentation, the first formed intermediate PR3 presents a T-shaped C2v structure, which eventually evolves into the C3V structure. (Scheme 2.4)... 

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