Metathesis interchange reactions

That was in fact for zirconium alkoxides that Bradley developed all the major synthetic approaches to the alkoxides ofhigh-valent metals—metathesis of ZrCl4or (more preferably [PyH]2ZrCl6)with ammonia [201] (method 5), alcoholysis of Zr(NR2)4 [1579] (method 4), alcohol interchange reactions... 

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... 

Scheme 5.17 Acetal interchange reaction a) and the same principle applied to polymerisation by the acetal metathesis polymerisation mechanism b)...

Interchange reactions are common in polycondensation chemistry, and occur when the functional groups in the polymer backbone continue to be active toward the condensation reaction. These reactions do not affect the average molecular weight of the polymer and, in that sense, are degenerate. In the case of ADMET, the internal carbon-carbon double bonds of the polymer are still metathesis-active, and can undergo reaction with either the metal methylidene or a polymeric metal alkylidene. 

The term olefin metathesis refers to an interchange reaction of alkylidene groups between alkenes. The total number of double bonds remains unchanged [1]. 

Olefin metathesis, an expression coined by Calderon in 1967,1 has been accurately described in Ivin and Mol s seminal text Olefin Metathesis and Metathesis Polymerization as the (apparent) interchange of carbon atoms between a pair of double bonds (ref. 2, p. 1). This remarkable conversion can be divided into three types of reactions, as illustrated in Fig. 8.1. These reactions have been used extensively in the synthesis of a broad range of both macromolecules and small molecules3 this chapter focuses on acyclic diene metathesis (ADMET) polymerization as a versatile route for the production of a wide range of functionalized polymers. 

Several transition metal complexes can catalyze the exchange of partners of two double bonds. Known as the olefin metathesis reaction, this process can be used to close or open rings, as well to interchange double-bond components. 

When alkenes are allowed to react with certain catalysts (mostly tungsten and molybdenum complexes), they are converted to other alkenes in a reaction in which the substituents on the alkenes formally interchange. This interconversion is called metathesis 126>. For some time its mechanism was believed to involve a cyclobutane intermediate (Eq. (16)). Although this has since been proven wrong and found that the catalytic metathesis rather proceeds via metal carbene complexes and metallo-cyclobutanes as discrete intermediates, reactions of olefins forming cyclobutanes,... 

In the metathesis reaction, alkenes are converted to mixtures of lower and higher alkenes through the interchange of groups on the double bonds in an equilibrium process. The number of moles of product lighter than the feed is roughly the same as the number of moles of heavier alkenes. Olefin metathesis is a thermoneutral reaction and produces an equilibrium mixture of products. In simple cases this mixture is close to the statistical distribution of substituent groups. 

Vlefin disproportionation or metathesis is a transition-metal catalyzed reaction involving the interchange of alkylidene units between olefins (Reaction 1) (1, 2, 3, 4). 

The alkoxides of both oxidation states were obtained by metathesis of germanium halides (chlorides and iodides) with alkali alkoxides [1488, 1142, 857, 1535]. The yields can be increased by application of GeCl4 solvates with Py or NH3 or amines [3, 222] (method 5) and also by alcohol interchange of ethoxides (method 6) or alcoholysis of Ge[Si(NR3)2]2 [568, 1543] (method 4). The application of alkali alkoxides in the preparation of Ge(OR)4is possible in contrast to that of analogous derivatives of Sn(IV) and Zr due presumably to the much lower stability of corresponding alkoxogermanates the intermediate products of the corresponding reactions because of stability — of the tetrahedral coordination for Ge. The direct electrochemical preparation of Ge(OEt)4... 

Metathesis, or double decomposition, is one of the main types of chemical reaction, and it takes place between two compounds, consisting merely in an interchange of radicals, the positive radical of the first compound pairing off with the negative radical of the second, and the negative radical of the first pairing off with the positive of the second. Such a reaction involves no change in the valence of any radical concerned. 

Analyses of the products obtained from the metathesis of 2-butene and 2-butene-dg (2, 5) and also from the metathesis of 3-hexene and 2-butene-d8 (5), proved that the interchange process in Reaction 1 definitely does not proceed via a simple transalkylation process—i.e.y a cleavage and reformation of the carbon-to-carbon single bond situated a to the double bond. The data are, however, consistent with a trans-alkylidenation process—i.e., a scheme which contemplates an interchange... 

These features are all well understood when it is realized that the above reactions of monomer and polymers are simply examples of the general olefin metathesis reaction which operates via a random interchange of alkylidene groups as described in our previous work (1, 2, 5, 6). 

Double displacement reactions are also called metathesis reactions. The word "metathesis" (pronounced with the stress on the second syllable meh-TATH-e-sis) means "interchange." In chemistry, a metathesis reaction occurs when ions or atoms are exchanged between different compounds. Chemists are not the only people who use this word. Use a dictionary or encyclopedia to find nonchemistry examples of metathesis. 

Metathesis of alkenes is essentially a class of reactions where an interchange of C atoms between pairs of double bonds takes place. A few representative examples are shown by the reactions listed in Fig. 7.10. The industrial use of metathesis reactions so far has been limited mainly to exchange metathesis (Fig. 7.10, top, backward reaction) as in the SHOP process, and ring-open metathesis polymerization (ROMP). As already mentioned (Section 7.5), Vas-tenamer is a polymer made by Hulls by ROMP from cyclooctene. Similarly, the polymer from norborene by ROMP is manufactured by CdF Chemie and is sold by the trade name of Norsorex . 

A metathesis reaction is defined as a chemical transformation in which atoms from different functional groups interchange with one another, resulting in the redistribution of functionality yielding similar bonding patterns for both molecules [6]. 

The metathesis reaction is still occasionally referred to as disproportionation although it bears no resemblance to disproportionation as a chain-growth termination step. Rather, it is analogous to interchange in step-growth polymerization (reaction 10.7 in Section 10.2.2) except that the reactants are broken apart at their double bonds and the fragments reconnected by double bonds. 

Alkene metathesis, more commonly called olefin metathesis, is a reaction between two alkene molecules that results in the interchange of the carbons of their double bonds. Two o and two n bonds are broken and two new o and two new n bonds are formed. 

Metathesis (Section 26.6) A reaction between two alkene molecules that results in the interchange of the carbons of their double bonds. 

The term olefin metathesis originally implied the interchange of sp2 carbon atoms between two olefins, as shown in Scheme 14.1. Since the elucidation of its mechanism, the reaction of a metal carbene with an olefin, in which the double bonds of the metal carbene and olefin are cleaved and a new metal carbene and a new olefin are constructed, the process illustrated in Scheme 14.2 has been referred to as olefin metathesis [Id]. 

With the preparation of lithium aluminum hydride, Schlesinger and coworkers not only opened wide fields of work in organic chemistry, but provided improved methods for the preparation of many hydrides (11). In general, the reaction of lithium hydride or lithium aluminum hydride to form other hydrides is a metathesis of the following form, where a metal hydride reacts with the salt of another metal to give a metal-metal interchange ... 

---