Pyrolytic Elimination of

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 ... 

Physical and spectral properties of batrachotoxins are presented in Table I. Mass spectra have been presented and interpreted (3,13,14). The parent ion of batrachotoxin is virtually nondetectable by direct probe methods, and instead an apparent molecular ion of miz 399 is seen, probably because of pyrolytic elimination of the pyrrole carboxylate moiety. Batrachotoxin alkaloids do not chromatograph on capillary gas chromatographic columns, but a pyrolysis product has been detected at 280°C on the temperature-programmed, packed OV-1 columns used for analysis of other dendrobatid alkaloids (see Appendix). The pyrrole carboxylate moiety is responsible for major ions of C7H9N02 (m/z 139), C6H9N ... 

The other type of pyrolytic elimination reaction involves free radicals. The steps are similar to those that we studied in the free radical substitution reactions, i.e. there is an initiation step, followed by several propagation steps, and then there are some termination steps. Free radical elimination is found in polyhalides and primary monohalides. For the general primary monohalide, R2CHCH2X,... 

The other type of pyrolytic elimination reaction involves free radicals, for example in polyhalides and primary monohalides. These reactions involve the normal sequence for radical reactions, namely initiation, propagation and termination steps. 

RELATIVE RATES OF PYROLYTIC ELIMINATION OF SOME ALKYL SUBSTRATES. 

Entry 5 is an example of a stereospecific anti elimination reaction of an alkyl halide in which the transition state requires the proton being removed and the bromide lost to be in an anti orientation with respect to each other. The dias-tereomeric threo- and eryfhro-l,2-diphenyl-l-bromopropanes undergo base-catalyzed jS-elimination to produce stereoisomeric products. Entry 6 is an example of pyrolytic elimination requiring a syn orientation of the proton being removed and the amine oxide nitrogen atom. This elimination reaction is a concerted one in which the proton is abstracted by the oxygen of the amine oxide group. 

Sol 3. The syn character of pyrolytic eliminations has been demonstrated in many ways. For example, pyrolysis of erythro and threo isomers of 1-acetoxy-2-deutero-l,2-diphenylethane gave in each case frans-stilbene as a major product, but the stilbene from the erythro compound retained nearly all its deuterium, whereas the stilbene from the threo compound had lost most of its deuterium. Either the hydrogen or deuterium could be syn to the acetoxy group, but the preferred conformations are those in which the phenyl groups are as far removed from each other as possible. 

Reactions of Sulphones.—The first report of pyrolytic elimination of aryl-sulphinic acid from an aryl alkyl sulphone has appeared, for 2-pentyl compounds these give pent-l-ene in constant proportion with cis- and t/ /i5-pent-2-ene, with exceptions for mesityl and p-nitrophenyl 2-pentyl sulphones. Base-catalysed elimination reactions leading to vinyl sulphones are well known, amine-promoted elimination from PhSOzCHa CHF-SPh following a jy/i-stereospecific course through an Elcb mechanism involving ion-pairs. A mild decarbomethoxylative elimination is the last step in a synthesis of ( )-versimide (131). ... 

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). 

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... 

A third category of syn eliminations involves pyrolytic decomposition of esters with elimination of a carboxylic acid. The pyrolysis of acetate esters normally requires temperatures above 400° C and is usually a vapor phase reaction. In the laboratory this is done by using a glass tube in the heating zone of a small furnace. The vapors of the reactant are swept through the hot chamber by an inert gas and into a cold trap. Similar reactions occur with esters derived from long-chain acids. If the boiling point of the ester is above the decomposition temperature, the reaction can be carried out in the liquid phase, with distillation of the pyrolysis product. 

Furthermore, the authors showed that compounds of type 1-78, easily accessible from 1-77, can be used as starting materials for the production of a new cyclo-propa[c]chromane framework 1-80 (Scheme 1.21). Oxidation of 1-77 to the corresponding sulfoxide and subsequent pyrolytic elimination generated the labile cyclobutene 1-78, which was directly epoxidized leading to the desired tricyclic compound 1-80 in 46% yield, probably via the epoxycyclobutane 1-79, again in a domino fashion. 

The pores of the silica template can be filled by carbon from a gas or a liquid phase. One may consider an insertion of pyrolytic carbon from the thermal decomposition of propylene or by an aqueous solution of sucrose, which after elimination of water requires a carbonization step at 900°C. The carbon infiltration is followed by the dissolution of silica by HF. The main attribute of template carbons is their well sized pores defined by the wall thickness of the silica matrix. Application of such highly ordered materials allows an exact screening of pores adapted for efficient charging of the electrical double layer. The electrochemical performance of capacitor electrodes prepared from the various template carbons have been determined and are tentatively correlated with their structural and microtextural characteristics. 

A third category of syn eliminations involves pyrolytic decomposition of esters with elimination of a carboxylic acid. The pyrolysis of acetate esters normally requires temperatures above 400° C. The pyrolysis is usually a vapor-phase reaction. In the... 

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