Olefin dismutation

Aluminium and other metal alkyls also activate tungsten and molybdenum compounds (particularly oxychlorides), to generate homogeneous or supported olefin dismutation catalysts. It is now believed that an initial M-alkyl (M = W, Mo) group is converted to a metal alkylidene group by a-hydrogen abstraction. Coordinated olefin now gives a metallocyclobutane (isolable in some cases) ... 

Unlike all previous mechanisms for olefin metathesis, the metallocyclo-butane mechanism does not involve the pairwise exchange of alkylidene units between two alkenes undergoing dismutation. The nonpairwise exchange of alkenes in the olefin dismutation reaction has now been established (Grubbs et al., 1975 Katz and McGinnis, 1975 Katz and Rothchild, 1976). 

Olefin Metathesis. The olefin metathesis (dismutation) reaction (30), discovered by Eleuterio (31), converts olefins to lower and higher molecular weight olefins. For example, propylene is converted into ethylene and butene... 

Historically, NHC complexes were investigated for the first time as catalysts and discussed as catalytic intermediates in the dismutation of electron rich tetraamino-ethylenes [Eq, (49)] Mixtures of two differently substituted olefins were reacted in the presence of rhodium(I) complexes and the products obtained showed mixed substitution patterns. Starting from Wilkinson s catalyst [(Ph3P)3RhCl], NHC complexes are formed as intermediates which could be isolated and used as even more active catalysts. In this first example, however, the NHC actively participates in... 

At temperatures above 250°C, the olefin-forming reactions are irreversible but the transformations in the first line of Scheme 3 are reversible. Thus, starting with an arbitrary amine, all other derivatives are obtained by these reactions, called disproportionations, transalkylations or dismutations (the nomenclature is also inconsistent in that the analoguous formation of ethers from alcohols is named dehydration). 

When alkaline hydrogen peroxide is added to organonitriles, the hydrogen peroxide decomposes this dismutation is known as the Radzizsewski reaction.136 The initial product formed is a percarboximidic acid [RC(NH)OOH], which can react either with more hydrogen peroxide to liberate oxygen and the amide, or oxidize a substrate such as an olefin (Figure 2.43) in a similar way to a percarboxylic acid. 

Olefin isomerization and dismutation reactions 47. Hydroformylation and related reactions... 

Olefin metathesis offers a means of shifting olefins to olefins with a different number of carbon atoms. Olefin metathesis is the disproportionation or dismutation of olefins over a catalyst, usually based on molybdenum or tungsten. For example, propylene gives ethylene and... 

Arene complexes have been found to be catalysts for the polymerization, hydrogenation, dismutation, and oxidative dimerization of olefins... 

Metathesis The reaction when two alkenes react in the presence of a catalyst to interchange their alkylidiene, R2C=, groups. Also called dismutation and disproportionation of olefins. The... 

A last question that arises now is the mode of generation of the metallocarbenes. Besides the indirect formation by C-C bond rupture, in metallocyclobutane dismutation, for instance (Scheme 59a), two other possible ways are a-hydrogen elimination (Scheme 59b) and hydrogen shift in ii-adsorbed olefin (Scheme 59c) (see also Scheme 48). 

These deuterium distributions were interpreted by assuming that in 11,12, and 13, rapid interconversions between a,y-diadsorbed and a,a,y-triadsorbed species, attached to the metal at the two gemdimethyl carbon atoms, were followed by metallocyclobutane dismutation. The resulting adsorbed methylene-cycloalkane may be deuterated via either n-alkyl or tertiary alkyl species. In the former case, rapid desorption would occur, with the formation of a d4 molecule, while in the latter case, a number of interconversions between cr-alkyl and 7r-olefinic cyclic adsorbed species would yield the perdeutero compound. In the case of demethylation at a tertiary center. 

Alkene Metathesis.77 Although these reactions are not polymerizations, it is convenient to deal with them here. The olefin metathesis or dismutation is of the following type ... 

The only necessary information is that both filled (bonding) and empty (anti-bonding) MO s exist of i, h, 2, and 62 symmetry. We will assume, with previous workers, that a key step is the formation of a complexed cyclobutane-like molecule, which can then decompose to the original complex or a new complex with the dismuted olefin molecules as ligands. These steps are all allowed by either orbital correlation, or perturbation theory. 

However, some oxides, such as supported Re207 (at 50-120°) or molybdenum oxide (at higher temperatures), show dismutation activity without alkyl treatment. Presumably, partial reduction of the surface by olefin exposes suitably liganded (M—O—M=CR2) centres. Further reduction (slow anion migration to the surface) leads to deactivation, but activity is restored by reoxidation. 

Olefin metathesis or dismutation was discovered in the laboratories of Phillips Petroleum in the late 1950s. In one of the early experiments propene was passed over an alumina catalyst which had been pretreated with Mo(CO). Ethene and 2-butene (cis and trans isomers) were obtained with negligible by-product formation. Without a catalyst, such reactions require temperatures above 700°C and do not occur cleanly. 

The catalytic dismutation of olefins is thermoneutral and results from the rupture and reformation of carbon—carbon double bonds with a statistical redistribution of the alkylidene moities. It has also been extended to substituted (functionnalized) alkenes, diolefins, polyenes and alkynes. The reaction with alkenes, to produce macrocyclic polyenes and polymers, is recognized as a special case of disproportionation. 

The binuclear complexes of chromium and vanadium, (21), exhibit restricted rotation of the cyclooctatetraene (COT) ring below 2(X)K. In related cyclooctatriene ligand complexes a diamagnetic-paramagnetic equilibrium exists suggesting Cr—Cr multiple bonding. Olefin rotation, in addition to other dynamic processes, has been reported in the (p-benzo-quinone)copper(I) complexes (22). The complex dismutates in solution according to the equilibrium 2[Cu(bq)L] [LCu(p-bq)CuL]+ bq. Intra-... 

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