Elimination and Isomerization Reactions

KliminaKon and Isomerization Reactions.— Tables 7 and 8 summarize some recent investigations on elimination and isomerization reactions. The pre-exponential 

C2H5CI C2H4 + HCl i-C3H,Cl CjHs -f- HCl t-C HsCl C4H8 + HCl CH2FCHF2 - CH2CF2 -t- HF /ra M-CHFCHF -t- HF - cir-CHFCHF -I- HF CH3CFj - CH2CF2 + HF cto-2-Butene - butadiene -I- H2 

3- Diethyloxetan - 2-ethylbut-l-ene + CH2O (CH2)3S02 - (CH2 3 + SOa 3-Methylsulpholane - C3H6 + CjH4 -h SO2 

Tschuikow-Roux and K. H. Jung, Internal. J. Chem. Kinetics, 1975,7,649 B. T buikow-Roux and K. R. Mailman, ibid., p. 363. 

---

Szmant, H. H. Mechanism of the Wolff-Kishner reduction, elimination, and isomerization reactions. Angew. Chem., Int. Ed. Engl. 1968, 7, 120-128. 

Figure 6.7 Elimination and isomerization reactions of ODC6 DO and resulting intermediates.

Table 4.1. Arrhenius parameters for decomposition, elimination, and isomerization reactions...

The Wolff-Kishner reduction is an important alternative method to the Clem-mensen reduction, and is especially useful for the reduction of acid-labile or high-molecular substrates. Yields are often below 70%, due to various side-reactions such as elimination or isomerization reactions. ... 

The HDO and isomerization reactions were previously described as bimolecular nucleophilic substitutions with allylic migrations-the so-called SN2 mechanism (7). The first common step is the fixation of the hydride on the carbon sp of the substrate. The loss of the hydroxyl group of the alcohols could not be a simple dehydration -a preliminar elimination reaction- as the 3-butene-l-ol leads to neither isomerization nor hydrodehydroxyl at ion (6). The results observed with vinylic ethers confirm that only allylic oxygenated compounds are able to undergo easily isomerization and HDO reactions. Moreover, we can note that furan tetrahydro and furan do not react at all even at high temperature (200 C). 

The aromatic residue may be any of a large number of such units but the favourite for academic study has been the perfluoromethylxylene derivative shown, which smoothly eliminates at around room temperature to give a polyacetylene containing 25 % of trans- and 75 % of m-units. After transformation and isomerization at 80 °C, the polyacetylene produced is a continuous dense film. The physical chemistry of the transformation and isomerization reactions has been studied in detail229,230) and the properties of the polyacetylene are reviewed 231). The great advantage of this route is that the precursor is a soluble polymer so that it can be characterized and the physical form of the polyacetylene can be controlled. 

Palladium-catalyzed oxidative cyclization of aryl homoallyl ethers affords 4-methyl-2//-chromenes in moderate yield. The reaction is proposed to proceed via activation of the alkene by coordination to Pd(ll) followed by intramolecular nucleophilic attack by the arene. Subsequent [1-hydride elimination and isomerization then affords 4-methyl-27/-chromenes (Scheme 13). Electron-rich aryl homoallyl ethers give the best yield and good regio-selectivity is observed for the reaction of unsymmetrical arenes <2005OL3355>. 

All these ligands have extensive chemistry here we note only a few points that are of interest from the point of view of catalysis. The relatively easy formation of metal alkyls by two reactions—insertion of an alkene into a metal-hydrogen or an existing metal-carbon bond, and by addition of alkyl halides to unsaturated metal centers—are of special importance. The reactivity of metal alkyls, especially their kinetic instability towards conversion to metal hydrides and alkenes by the so-called /3-hydride elimination, plays a crucial role in catalytic alkene polymerization and isomerization reactions. These reactions are schematically shown in Fig. 2.5 and are discussed in greater detail in the next section. 

The catalytic activity of Hg in elimination and substitution reactions of transition-metal complexes continues to be the subject of study. Kinetic studies on the Hg" catalysed aquation of both cis-[Co(en)2(CN)Cl], cis-[(chloro)(aniline)CO(en)2], and [PdLX] (L = tetramethyldiethylenetriamine X = Cl, Br, or I) are consistent with intermediate adduct formation at the halide substituent. Mercury(ii) reacts reversibly with the [Co(NH3)5SCN] ion to yield [Co(NH3)5(SCN)Hg] which irreversibly undergoes either aquation to yield [Co(NH3)sH20] or isomerization to yield [Co(NH3)5(NCS)Hg] at approximately equal rates. The kinetics of the reaction... 

Elimination, reductive elimination, reductive alkylation, and isomerization reactions have been employed in order to create the fluoroolefin moiety in molecules already containing one or more fluorine atoms. In general, these reactions were developed for a specific purpose and are often not generally applicable to other systems. For example, base catalyzed HF elimination converted 1,2,2-trifluoroethylcyclohexane (107) to the difluoroolefin (108)(50) (Scheme 32). Reductive elimination reactions were employed to prepare the interesting fluoroallene (110) (51)... 

It has further been demonstrated that by appropriate presentation of point charges on haptens, general acid/ base catalysis could be achieved in a variety of catalytic antibody reactions, including condensation, isomerization, elimination, and hydrolytic reactions. 

Treatment of the l-isopropenyl-2-(3-butenyl)cyclobutanol 508 with Pd(II) afforded the bicyclo[4.3.0]nonane 512. The reaction can be understood by domino -carbon elimination as shown by 509, followed by 5-exo and 6-exo cyclizations of 510 to give 511, and f-H elimination and isomerization afforded 512 [208], A different mechanism based on ring expansion was given before. 

In studying the linking of (401) with (402), it was recognized that when tetrahydrofuran was used as a solvent, the coupling reaction was suppressed. It is probably for this reason that the reaction of (401) with (407) in tetrahydrofuran gives (408) instead of the desired allene. However, the cuprate (407) cannot be prepared in other solvents. Allenic retinoids have also been postulated as reaction products in the elimination and isomerization of acetylenic retinoids (Sueiras and Okamura, 1980). 

Abstract The application of modem density functional theory techniques to the computational study of paUadium-catalyzed C-C formation reactions has led to a better mechanistic understanding of these processes of fundamental interest in organic chemistry. This chapter reviews the main contributions to the topic, analyzing the current knowledge on the different reaction steps oxidative addition, transmetalation, metalation, reductive elimination and isomerization. A special emphasis is placed on the metalation step, which is specific of C-C bond formation processes. 

The following types of unimolecular reactions can be distinguished cleavage of the ordinary bond and formation of two radicals elimination to form stable molecules isomerization reactions. Table 4.1 contains the examples and Arrhenius parameters of the rate constant for these types of reactions. The experimental studies presented in Table 4.1 were carried out in shock tubes except for the decomposition of CCI2HCH2CI when laser heating of the gas mixture was used. It is seen from these data that the highest pre-exponential factors belong to the rate constants of decomposition at the ordinary bond. Recombination reactions, which, as a rule, occur without a barrier, are inverse for these reactions. Unlike recombination reactions, inverse reactions of elimination and isomerization have substantial potential barriers. 

ViUano SM, Huynh LK, Carstensen HH, Dean AM. High-pressure rate rules for alkyl + O2 reactions. 2. The isomerization, cychc ether formation, and beta-scission reactions of hydroperoxy alkyl radicals./Phys Chem A. 2012 116 5068-5089. ViUano SM, Huynh LK, Carstensen HH, Dean AM. High-pressure rate rules for alkyl + 02 reactions. 1. the dissociation, concerted elimination, and isomerization channels of the alkylperoxy radical. / Phys Chem A. 2012 115 13425-13442. 

Ailyl enol carbonates derived from ketones and aldehydes undergo Pd-cat-alyzed decarboxylation-elimination, and are used for the preparation of a, /3-unsaturated ketones and aldehydes. The reaction is regiospecific. The regio-isomenc enol carbonates 724 and 726, prepared from 723, are converted into two isomeric enones, 725 and 727. selectively. The saturated aldehyde 728 can be converted into the a,/3-unsaturated aldehyde 730 via the enol carbonate 729[459]. 

Similar to IFP s Dimersol process, the Alphabutol process uses a Ziegler-Natta type soluble catalyst based on a titanium complex, with triethyl aluminum as a co-catalyst. This soluble catalyst system avoids the isomerization of 1-butene to 2-butene and thus eliminates the need for removing the isomers from the 1-butene. The process is composed of four sections reaction, co-catalyst injection, catalyst removal, and distillation. Reaction takes place at 50—55°C and 2.4—2.8 MPa (350—400 psig) for 5—6 h. The catalyst is continuously fed to the reactor ethylene conversion is about 80—85% per pass with a selectivity to 1-butene of 93%. The catalyst is removed by vaporizing Hquid withdrawn from the reactor in two steps classical exchanger and thin-film evaporator. The purity of the butene produced with this technology is 99.90%. IFP has Hcensed this technology in areas where there is no local supply of 1-butene from other sources, such as Saudi Arabia and the Far East. 

Potassium Amides. The strong, extremely soluble, stable, and nonnucleophilic potassium amide base (42), potassium hexamethyldisilazane [40949-94-8] (KHMDS), KN [Si(CH2]2, pX = 28, has been developed and commercialized. KHMDS, ideal for regio/stereospecific deprotonation and enolization reactions for less acidic compounds, is available in both THF and toluene solutions. It has demonstrated benefits for reactions involving kinetic enolates (43), alkylation and acylation (44), Wittig reaction (45), epoxidation (46), Ireland-Claison rearrangement (47,48), isomerization (49,50), Darzen reaction (51), Dieckmann condensation (52), cyclization (53), chain and ring expansion (54,55), and elimination (56). 

Sulfites. The Hterature concerning dialkyl sulfites is extensive, although less than for sulfates. Reactions involving alkylation are similar to those of sulfates. Sulfites also undergo elimination, transesterification, and isomerization. The last two parallel reactions of phosphites. 

The elimination from sulfonates of secondary alcohols is frequently easier than more direct methods applied to the free alcohols. As with the latter, there are the possibilities of isomeric olefin formation and rearrangement reactions. In addition, displacement and hydrolysis may occur, but these side reactions can usually be suppressed. 

The reverse reaction (formation of metal alkyls by addition of alkenes to M-H) is the basis of several important catalytic reactions such as alkene hydrogenation, hydroformylation, hydroboration, and isomerization. A good example of decomposition by y3-elimination is the first-order intramolecular reaction ... 

---