Isomer elimination reaction

The reaction of isoprene with MeOH catalyzed by Pd(acac)2 and Ph3P is not regioselective, giving a mixture of isomers[37]. However, l-methoxy-2,6-dimethyl-2,7-octadiene (35), the head-to-tail dimer, was obtained in 80% yield, accompanied by the tail-to-tail dimer (15%) using 7r-allylpalladium chloride and BU3P. On heating, 35 was converted into 2.6-dimethyl-1,3,7-octatriene (36) by an elimination reaction[38]. 

We have previously seen (Scheme 2.9, enby 6), that the dehydrohalogenation of alkyl halides is a stereospecific reaction involving an anti orientation of the proton and the halide leaving group in the transition state. The elimination reaction is also moderately stereoselective (Scheme 2.10, enby 1) in the sense that the more stable of the two alkene isomers is formed preferentially. Both isomers are formed by anti elimination processes, but these processes involve stereochemically distinct hydrogens. Base-catalyzed elimination of 2-iodobutane affords three times as much -2-butene as Z-2-butene. 

The presence of an additional -substituent engenders the possibility of formation of the ( > and (Z)-isomers. The (F)-isomers, ( >3, are formed preferentially with E/Z ratios often exceeding 90 10. Both methoxy and acetoxy derivatives may be isolated and chromatograph-ically purified prior to the elimination reaction. 

Isomerization and elimination reactions of alkyls and aryls Isomerizations of mono-alkyls and aryls have been widely studied [107] many ds-Pt(PR3)2ArCl undergo rapid isomerization in the presence of free phosphine, a reaction inhibited by Cl- with a mechanism believed to involve a 3-coordinate Pt(PR3)2Ar+ intermediate that is then attacked by Cl-. The cis- and trans-isomers of Pt(PEt3)2(Ph)Cl undergo reversible isomerization when irradiated at the wavelength of charge-transfer transitions (254 and 280 nm). 

The pentacoordinate molecules of trigonal bipyramidal form, like PF5, are a very nice example for the study of the formal properties of stereoisomerizations. They are characterized by an appreciable nonrigidity and they permit the description of kinetics among a reasonable number of isomers, at least in particular cases (see below). Therefore the physical and chemical properties of these molecules have been thoroughly investigated in relation to stereoisomerization. Recent reviews may be found in the literature on some aspects of this problem. Mislow has described the role of Berry pseudorotation on nucleophilic addition-elimination reactions and Muetterties has reviewed the stereochemical consequences of non-rigidity, especially for five- and six-atom families as far as their nmr spectra are concerned. 

Then again, many elimination reactions are found to occur much, more readily in that member of a pair of geometrical isomerides in which the atoms or groups to be eliminated are trans to each other than in the isomer in which they are cis (p. 255). As is seen in the relative ease of elimination from and and syn aldoxime acetates to yield the same cyanide ... 

A detection of p-isomer HCH in water in relatively larger quantities in comparison with other isomers shows a high degree of insecticide transformation in the soil (mainly by microorganisms), and hence loss or leaching of insecticide residues deposited some decades ago. It is known that p-isomer HCH is the most stable compound among others of HCH isomers, i.e., it is not or very weakly exposed to elimination reaction—dehydrochlorination (Cristol, 1947). High persistence of HCH p-isomer is... 

Cristol, S. J. (1947) The kinetics of the alkaline dehydrochlorination of the benzene hexachloride isomers. The mechanisms of second-order elimination reactions. Journal of the American Chemical Society, 69, 338-342. 

Compounds with axial and equatorial orientation of substitutents not only differ in physical properties but in reactivity as well, because the rates of formation of equatorial and axial isomers and rates of substituion and elimination reactions are different. 

The geometry of the cis-alkylcyclohexanol is favorable for trans elimination since the hydroxyl and the neighboring trans hydrogen are coplanar, but this is not true for the l,i-trans isomer hence the molecular conformation has to flip over, to set the hydroxyl group in the axial position for the trans elimination to occur. This would require a few kilocalories of energy and for frans-lert-butylcyclohexanol it would be more difficult to achieve than for IroMs-methylcyclohexanol. It is, therefore, possible that the trans elimination from a boat conformation, or possibly even an epimerization from the trans to the cis isomer which then undergoes a trans elimination reaction. Such an epimerization was found to occur under conditions of dehydration of certain alcohols over alumina, as will be seen under 1,4-cyclohexanediol. The more facile elimination of the cis-i-tert-butylcyclohexanol system as compared with the trans system in solution was also reported in the literature 63). 

The difficulty of distinguishing mechanisms at the ElcB-E2 borderline has also been discussed for reactions of secondary halides (1-X and 2-X) which feature a f-hydrogen made acidic by incorporation of an a-indenyl substituent (Scheme 1). 1,2-Elimination reactions of (/f,5 )-l-(l-X-ethyl)indene (1-X, X = Cl, Br, I, OBs) and the corresponding R,R isomers (2-X) promoted by water containing 25 vol.% acetonitrile... 

In order to strengthen evidence in favour of the proposition that concerted inplane 5n2 displacement reactions can occur at vinylic carbon the kinetics of reactions of some /3-alkyl-substituted vinyliodonium salts (17) with chloride ion have been studied. Substitution and elimination reactions with formation of (21) and (22), respectively, compete following initial formation of a chloro-A, -iodane reaction intermediate (18). Both (17) and (18) undergo bimolecular substitution by chloride ion while (18) also undergoes a unimolecular (intramolecular) jS-elimination of iodoben-zene and HCl. The [21]/[22] ratios for reactions of (18a-b) increase with halide ion concentration, and there is no evidence for formation of the -isomer of (Z)-alkene (21) iodonium ion (17d) forms only the products of elimination, (22d) and (23). 

The reductive elimination step has undergone much less examination, with the majority of authors considering that the acyl species produces CH3COI and regenerates the active anionic [Irl2(CO)2] species. When a DFT study was carried out by Kinnunen and Laasonen [39], the Jac,cis-[Ir(COCH3)l3(CO)2] isomer was seen to be the dominant intermediate for the anionic route, whereas for the neutral pathway the mer,ds-[lr(COCHi)l2(CO)i] isomer allowed a faster reductive elimination reaction. 

R ) (85d). The isomers (86) and (87) were obtained in 47% and 32% yields respectively, establishing the consecutive cycloaddition elimination reactions shown in path (b). 

A practical synthesis of bicyclo[1.1.0]butane-l-carbonitrile (67) from 3-chlorocyclobutane-l-carbonitrile by treatment with potassium Ze/7-butoxide in ttvh-butyl alcohol has been reported.25 Detailed mechanistic studies have also been carried out on the elimination of hydrogen chloride from bww-3-chlorocyclobutane-l-carbonitrile (66) and cis-3-chlorocyclobutane-l-carbonitrile (68), yielding 67.26 28 It has been reported that the elimination reactions of the two isomers are likely to proceed via a common carbanion intermediate. However, in the presence of a crown ether, the carbanion intermediates derived from the trans-isomer 66 and the ds-isomer 68 are found experimentally not to be identical. These intriguing results are attributed to the presence or absence of the potassium cation in assisting the expulsion of the leaving chloride ion.26... 

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