Pyrolytic eliminations,

Double bonds can be created by a number of available stereospecific reactions (E2 elimination, pyrolytic elimination, e.g. Cope, Chugaev reactions), stereoselective Wittig and related reactions, reduction of triple bonds, by cisitrans isomerisation of existing double bonds either photochemically or by wet chemistry, e.g. the Corey-Winter procedure. [17]... 

Enby 6 is an example of a stereospecific elimination reaction of an alkyl halide in which the transition state requires die proton and bromide ion that are lost to be in an anti orientation with respect to each odier. The diastereomeric threo- and e/ytAra-l-bromo-1,2-diphenyl-propanes undergo )3-elimination to produce stereoisomeric products. Enby 7 is an example of a pyrolytic elimination requiring a syn orientation of die proton that is removed and the nitrogen atom of the amine oxide group. The elimination proceeds through a cyclic transition state in which the proton is transferred to die oxygen of die amine oxide group. 

Occasionally it is difficult to find dehydrating conditions which do not give rise, at least in part, to rearrangement e.g. ref. 199, 200). In such cases, recourse can be made to pyrolytic elimination of an ester (see section VII-6). 

Ester eliminations are normally one of two types, base catalyzed or pyrolytic. The usual choice for base catalyzed j5-elimination is a sulfonate ester, generally the tosylate or mesylate. The traditional conditions for elimination are treatment with refluxing collidine or other pyridine base, and rearrangement may occur. Alternative conditions include treatment with variously prepared aluminas, amide-metal halide-carbonate combinations, and recently, the use of DMSO either alone or in the presence of potassium -butoxide. 

A rule for the correlation of ease of pyrolytic elimination with structure has been suggested by DePuy. The order of ease of decomposition among the commonly used esters is ... 

The enol ethers (56) and (57) are prepared from the corresponding ketals (54) and (55) in both 5a- and 5j5-series by pyrolytic or catalytic elimination of one molecule of alcohol. 

Elimination reactions of fluorine compounds are not limited to the removal of simple molecules Frequently, large molecules or combination of smaller ones are formed as by-products, especially in pyrolytic reactions For example perhalo genated acid chlorides lose not only carbon monoxide but also chlorine fluoride [106, 107] (equations 74 and 75)... 

Enamines derived from aldehydes can usually be obtained by the reaction of 2 equivalents of a secondary amine with the carbonyl compound, in the presence of anhydrous potassium carbonate, followed by pyrolytic distillation of the aminal with elimination of one of the amine groups (10,15, 30-36). Ketones are directly converted to enamines under the conditions of aminal formation. The azeotropic removal of water with excess aldehyde has also been described (32,37). 

Pyrolytic elimination from isoindoline A -oxides also affords iso-indoles, but yields were found to be generally lower than those obtained by Kreher and Seubert s procedure. The considerable amount of polymeric material formed in the pyrolytic reaction makes isolation of the isoindole difficult, but a convenient method for separation of the product was found utilizing complex formation with 1,3,5 -trin i tro benzene. 

These topics are discussed in more detail in other chapters of this text. Formally, the pyrolytic elimination of sulphur dioxide from a sulphone, with the concomitant formation of a new carbon-carbon bond, constitutes a reduction at sulphur. These reductions have been valuable in the formation of new molecules, especially macrocycles and cyclophanes, and have been reviewed by Vogtle and Rossa205. Pyrolytic elimination of sulphur dioxide has been used by Julia and co workers in the formation of mixtures of isoprenoids206, and by Takayama and collaborators in the stereoselective synthesis of vitamin D, 19-alkanoic acids207. 

Several types of compound undergo elimination on heating, with no other reagent present. Reactions of this type are often run in the gas phase. The mechanisms are obviously different from those already discussed, since all those require a base (which may be the solvent) in one of the steps, and there is no base or solvent present in pyrolytic elimination. Two mechanisms have been found to operate. One involves a cyclic transition state, which may be four, five, or six membered. Examples of each size are... 

The layer of titanium and ruthenium oxides usually is applied to a titanium substrate pyrolytically, by thermal decomposition (at a temperature of about 450°C) of an aqueous or alcoholic solution of the chlorides or of complex compounds of titanium and rathenium. The optimum layer composition corresponds to 25 to 30 atom % of ruthenium. The layer contains some quantity of chlorine its composition can be written as Ruq 2sTio 750(2- c)Cl r At this deposition temperature and Ru-Ti ratio, the layer is a poorly ordered solid solution of the dioxides of ruthenium and titanium. Chlorine is completely eliminated from the layer when this is formed at higher temperatures (up to 800°C), and the solid solution decomposes into two independent phases of titanium dioxide and ruthenium dioxide no longer exhibiting the unique catalytic properties. 

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