Redistribution reactions compounds

Typical initiators for living anionic polymerization of siloxanes include conventional organoalkali compounds and lithium siloxanolates22). Initiators containing lithium counterions are preferable to sodium or potassium counterions due to the lower catalytic activity of lithium in siloxane redistribution reactions. Living anionic polymeriza-... 

Redistribution reactions in chemistry, Ann. N.Y. Acad. Sci. 159,1-334(1969). Transition metal compounds feature in one paper only. 

The combination of equimolar amounts of tris(trimethylsilyl)methyllithium and zinc bromide in a THF/diethyl ether mixture, Scheme 27, furnished tris(trimethylsilyl)methylzinc bromide, as a lithium bromide/ether adduct.43 The compound, which may also be formulated as a lithium alkyldibromozincate, showed no ligand redistribution reactions. It is monomeric in solution and can be treated with 1 equiv. of an organolithium reagent to afford heteroleptic diorganozinc compounds. 

These redistribution reactions are possible at atmospheric pressure under the action of MW irradiation is performed for a few minutes in the presence of the same catalysts [57]. These reactions with the less volatile germanium tetrabromide (44b) (b.p. 184 °C) have also been performed by use of the GS/MW process, without added catalyst (Tab. 7.4, entries 1 and 3) [15, 16]. In this instance, despite the use of weaker incident power, the temperature reached 420 °C, very much higher than that obtained under the action of MW irradiation of a reaction mixture containing AlBr3 (200 °C to 250 °C) (Tab. 7.4, entries 2 and 4). The presence of this catalyst considerably favors redistribution towards the dibrominated products (46b) (84% for R = nBu, 85% for R = Ph) relative to the monobrominated compounds (46a), which are the major products of the GS/MW process (78% and 43% respectively). The tri-brominated products (46c), the most difficult to prepare, have been obtained with a rather high selectivity (73 to 80%) by use of the catalytic process under the action of MW [57]. In this reaction, therefore, the GS/MW process seems less effective than the MW-assisted and AlX3-catalyzed process. 

The first redistribution reactions were introduced early by Kocheshkov334. They remain still the subject of extended interest in the field of organometallic syntheses of Ge(IV), Sn(IV) and Pb(IV) compounds291,335,336. For example, the redistribution reactions occurring between allyltrialkyltin or crotyltributyltin compounds and SnCU at — 50 °C were studied by NMR335 ... 

Redistribution reactions between 28 and 29 to form mixed-imido compounds were not detected. On the other hand, oxo-imido interchange reactions readily take place between MeRe03 and 29. As a consequence, new compounds are formed, MeReO(NR)2, 30, and MeRe02NR, 31. The product obtained is largely under stoichiometric control, as represented by these equations (62) ... 

Allyltin chlorides, allylSnIL/T-, are more reactive in carbonyl addition than are the allyltrialkylstannanes, allylSnRj, and for this purpose, the latter can be converted into the former by the Kocheshkov redistribution reaction with BuSnCl3 or S11CI4 the /ra r-stannylation can be carried out with the carbonyl compound in situ in a one-pot process (Equation (97)).270... 

The second route (Scheme 1) is a redistribution reaction, in fact the Schlenk equilibrium. This route may be used in the reverse direction for the preparation of pure diorganomagnesium compounds from organomagnesium halides. Addition of a ligand, usually dioxane, that forms an insoluble complex with magnesium dihalide, shifts the Schlenk equilibrium completely to the left side and allows isolation of pure diorganomagnesium compounds from the remaining solution. ... 

This review summarizes some earlier qualitative work as well as recent quantitative studies of redistribution equilibria and describes the principles underlying the mathematical treatment of such equilibria as well as the general implications of these equilibria with respect to general chemistry. In line with the general objective of the Advances in Organometallic Chemistry series, this article is limited to carbon-metal-bonded systems, metal hydrides, metal carbonyl compounds, metallocenes, and similar complexes. Excluded therefore are halogen-, sulfur-, nitrogen, and phosphorus-based systems.Various aspects of redistribution reactions were reviewed previously (42, 74, 87, 88,150,186, 285, 286, 288). 

In addition, this procedure was quite tedious and time consuming. Therefore, in recent years when physical methods for assaying molecules in mixtures—methods such as nuclear magnetic resonance (NMR), gas chromatography, and others—have become available, a renaissance in the study of redistribution reactions has taken place. These methods allowed a rapid, quantitative, and precise determination of all of the reaction products present in a mixture. Also, equilibrium reactions could be carried out in much smaller sample sizes, thus permitting the study of exotic, hard-to-obtain compounds. Redistribution reactions—the kinetics as well as the equilibria—can now be measured directly in sealed NMR tubes. Furthermore, the relatively recent widespread availability to chemists of high-speed computers, in addition to these modern analytical tools, has facilitated the use of the appropriate mathematics even when highly complicated. 

In the last decade, an immense amount of experimental material has been generated describing the preparation and the chemical and physical properties of transition metal n complexes and coordination compounds. Recently great emphasis has been placed on the study of the kinetics and the reaction mechanisms involving such compounds. Although redistribution reactions as defined earlier in this review and as exemplified specifically by the reaction of Eq. (168) (M = transition metal, L=coordinated ligand)... 

These, to some extent, are related to redistribution reactions but, of course, do not lie within the scope of this review. Probably one of the first redistribution reactions of ir-bonded compounds is the exchange of carbonyl groups with 7r-bonded benzene (85) on chromium leading to the mixed compound, benzenechromium tricarbonyl. 

Langer, H. G., Redistribution reactions with Group IV metal compounds in dimethyl-sulfoxide (DMSO), Tetrahedron Letters, 43 (1967). 

The mixed methyl-ethyl tetraalkyllead compounds have become quite important commercially in the past several years because of their excellent antiknock effects and volatilities (see Extent of Antiknock Effect). Sometimes tetramethyllead and tetraethyllead are simply mixed together in motor fuel, but usually the compounds are subjected to "redistribution of the methyl and ethyl groups. The redistribution reaction involves the exchange of the organic groups between the compounds, yielding practically a random equilibrium production of all the possible products in a statistical distribution, as shown in the equation ... 

Many such redistribution reactions are known between organome-tallic compounds the discovery of this type of reaction is due to Calin-gaert et al 70 71>. Redistribution reactions involving lead and other Group IVb metals have been reviewed by Moedritzer224>, who calculated equilibrium constants for several of the reactions. The enthalpy change for reactions on the same metal is nearly zero, and the entropy of the system is then the principal driving force. 

The redistribution reaction in lead compounds is straightforward and there are no appreciable side reactions. It is normally carried out commercially in the liquid phase at substantially room temperature. However, a catalyst is required to effect the reaction with lead compounds. A number of catalysts have been patented, but the exact procedure as practiced commercially has never been revealed. Among the effective catalysts are activated alumina and other activated metal oxides, triethyllead chloride, triethyllead iodide, phosphorus trichloride, arsenic trichloride, bismuth trichloride, iron(III)chloride, zirconium(IV)-chloride, tin(IV)chloride, zinc chloride, zinc fluoride, mercury(II)chloride, boron trifluoride, aluminum chloride, aluminum bromide, dimethyl-aluminum chloride, and platinum(IV)chloride 43,70-72,79,80,97,117, 131,31s) A separate catalyst compound is not required for the exchange between R.jPb and R3PbX compounds however, this type of uncatalyzed exchange is rather slow. Again, the products are practically a random mixture. 

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