Double bond shift reactions

The hydrogen atoms are as a whole antibonding with the remaining part in LU MO, so that the charge-transfer to LU from the base easily comes to release these hydrogens. Similar double-bond shift reactions have also been treated 133>. 

Traditionally, the same overall mechanisms of acid catalysis invoking carben-ium ions have been assumed to prevail both in heterogeneous (2) and in liquid homogeneous (3) systems. But these mechanisms do not adequately take into account the fact that adsorbed, rather than free, carbenium ions are formed in the pores of solid catalysts. Consequently, a quantum-chemical model that demonstrates how the interaction of carbenium ions with the sites of their adsorption can influence the reaction mechanism has been formulated by Kazansky (4), taking double-bond-shift reactions in olefins as a particular example. According to this view, adsorbed carbenium ions are best regarded as transition states rather than reaction intermediates, a notion that had also been proposed earlier by Zhidomirov and one of us (5). 

The formation of polymers containing [=CH(CH2)4CH=], units is possible through the ROMP of an appropriate cyclic diene, such as cycloocta-1,3-diene, or by a double-bond shift reaction of a polymer such as poly(l-pentenylene). Such units can be eliminated as cyclohexene so long as metathesis activity is present in the system360. The ROMP of 2,3-dihydropyran, initiated by Mo(CO)6/CBr4// v, has been reported361. 

On the other hand, H2S did not prevent cis-trans isomerization and double bond shift reactions from going to completion at 300° or 350°, and the olefin mixtures analyzed were always found to be close to thermodynamic equilibrium, whether they had been formed from thiophene, butadiene, or one of the olefins themselves. Nor did H2S prevent the total conversion of butadiene to butene, even when 10- to 20-cc. samples were used at reaction temperatures down to 200° C. and flow rates up to 10 liters per hour. This may be the explanation of the absence of butadiene from the thiophene reaction products over cobalt molybdate—that if it had been formed as it was over chromia, it would have reacted further too rapidly to survive. 

For a series of methylhex-l-enes, the reactivity decreases as the substituent(s) are placed closer to the double bond, no matter whether the catalyst is Re207/CsN03/ AI2O3 (Kawai 1989), WCl6/Me4Sn (Kawai 1994), or W(sCCMe3)(Cl)3(dme) (Weiss 1988c). 4,4-Dimethylhex-l-ene has a reactivity intermediate between that of 4-methyl- and 3-methylhex-l-ene (Kawai 1989). Double-bond shift reactions and dimerization tend to predominate when the substituent is close to the double bond (Kawai 1992, 1994). 

Such olefins exist in cis and trans forms, and can undergo cis/trans isomerization by the metathesis reaction. Indeed when the olefin is symmetrical this is the only observable change, provided that there is no concomitant double-bond shift reaction. When the olefin is unsymmetrical there is an added point of interest, namely the extent to which a cis reactant gives rise to cis products, and a trans reactant to trans products. We have already touched on this important question of stereoselectivity in Section 3.3, and here we shall elaborate further in Section 6.7, after summarizing the cis/trans ratio observed for the products of metathesis of both alk-l-enes and alk-2-enes. The selectivities in metathesis of internal olefins are usually very high. 

The simplest example of a productive cross-metathesis reaction between acyclic olefins is that between ethene and but-2-ene reaction (1). In this case only one product is possible, apart from cis/trans isomerization of the but-2-ene the equilibrium mixture thus consists of four compounds. At the other extreme, the reaction of two unsymmetrical olefins, R CH=CHR and R CH=CHR , with R, R, R, R all different, can produce cis/trans isomers of four different unsymmetrical olefins by cross-metathesis as well as four symmetrical olefins by self-metathesis. Counting the cis/trans isomers of the reactants as well, this means that the equilibrium mixture will contain 20 different compounds. Side reactions, such as double-bond shift reactions, will complicate the situation still further. The main value of cross-metathesis reactions, apart from their use in the proof of mechanism, lies in their application to the synthesis of olefins that are otherwise expensive or difficult to prepare. A number of higher olefins, useful as insect sex attractants, have been made in this way. 

E = ethene. Double-bond shift reactions are sometimes a complication with M0O3- and WOa-based catalysts. Many Mo and W compounds can be activated by exposure to NO for 30 min (suspended in chlorobenzene) followed by removal of excess NO by evacuation and then addition of dry pyridine. See Ch. 17. 

Cyclooctadienyl substituents at the methylene groups in c/s-1,4-polybutadiene can be introduced by the radical-induced reaction with cycloocta-1,5-diene. Metathesis degradation of the product with tra 5-oct-4-ene shows that the substitution reaction is accompanied by double-bond shift reactions (Hummel 1990a). 

Theoretical calculations indicate that a Dgj, (planar, antiaromatic) structure is a transition state in the double bond shifting reaction of cyclooctatetraene Hrovat, D. A. Borden, W. T. /. Am. Chem. Soc. 1992, 114, 5879. 

It was developed in order to valorize the C3 fraction of a local naphtha cracker. Shawinigan Chemicals Inc. Used this process in thewir plant near Montreal (at Varennes) in Canada from 1966 to 1972. The plant had a capacity of 50000 tons of feedstock per year. The catalyst used was tungsten oxide supported on silica and doped with sodium to prevent the double bond shift reaction of but-2-ene (which should decrease the selectivity). The operating temperature was between 370 and 450°C and the. conversion (40 - 43%) corresponded to near equilibrium conditions. The selectivity towards ethylene and but-2-ene was very high (> 95%). In addition, instead of pure propene, it was possible to use directly the C3 fraction (propane + propene) of the naphtha cracker. 

Stepanov AG, Arzumanov SS, Luzgin MV, Ernst H, Ereude D. In situ monitoring of n-butene conversion on H-ferrierite by H, H, and C MAS NMR kinetics of a double-bond-shift reaction, hydrogen exchange, and the C-label scrambling. J Catal 2005 229 243-51. 

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