Precursors dehydrohalogenation

The first /3 -lactam was produced by addition of a ketene to an imine and there are now many examples of this type of approach. The ketenes are most frequently generated in situ from acid chlorides by dehydrohalogenation, but have also been produced from diazo ketones, by heating of alkoxyacetylenes and in the case of certain cyanoketenes by thermolysis of the cyclic precursors (162) and (163). 

The sulfonium precursor route may also be applied to alkoxy-substituted PPVs, but a dehydrohalogenation-condensation polymerization route, pioneered by Gilch, is favored 37]. The polymerization again proceeds via a quinomethide intermediate, but die syndicsis of the conjugated polymer requires only two steps and proceeds often in improved yields. The synthesis of the much-studied poly 2-methoxy-5-(2-ethylhexyloxy)-l,4-phenylene vinylene], MEH-PPV 15 is outlined in Scheme 1-5 33, 35]. The solubility of MEH-PPV is believed to be enhanced by the branched nature of its side-chain. 

There have been a number of different synthetic approaches to substituted PTV derivatives proposed in the last decade. Almost all focus on the aromatic ring as the site for substitution. Some effort has been made to apply the traditional base-catalyzed dehydrohalogenation route to PTV and its substituted analogs. The methodology, however, is not as successful for PTV as it is for PPV and its derivatives because of the great tendency for the poly(u-chloro thiophene) precursor spontaneously to eliminate at room temperature. Swager and co-workers attempted this route to synthesize a PTV derivative substituted with a crown ether with potential applications as a sensory material (Scheme 1-26) [123]. The synthesis employs a Fager condensation [124] in its initial step to yield diol 78. Treatment with a ditosylate yields a crown ether-functionalized thiophene diester 79. This may be elaborated to dichloride 81, but pure material could not be isolated and the dichloride monomer had to be polymerized in situ. The polymer isolated... 

Potential Non-Cvcllc Precursors of Preceramic Polymers. Boranes such as bis(trimethylsilyl(aminotrimethylsilylaminochloroboranes can be viewed as monomers for preceramic polymer and, ultimately, boron nitride production. Intermolecular dehydrohalogenation of this borane would be thus expected to yield either the dimer or the polymeric system. 

Co(CO)4] ) dehydrohalogenation [Eq. (15)] followed by addition of HCo(CO)4, or whether splitting out of HCo(CO)4 occurs from the alkyl-cobalt [Eq. (14)], which is the malonate precursor, followed by HCo(CO)4 addition in the opposite direction. In one case [Eq. (15)], olefin formation proceeds directly from the bromide and no reversibility of any steps is required, while according to Eq. (14) olefin formation proceeds from elimination of HCo(CO)4. 

Dehydrochlorination of poly vinylidene chloride and chlorinated polyvinyl chloride was carried out. High chlorine content in the polymers (more than 60%) provides the formation of chlorinated conjugated polymers, polychlorovinylenes. The reactivity of chlorinated polyvinylenes contributes to the sp carbon material formation during heat treatment. Synthesis of porous carbon has been carried out in three stages low-temperature dehydrohalogenation of the polymer precursor by strong bases, carbonization in the inert atmosphere at 400-600°C and activation up to 950°C. 

O ) By formation of seven-from three-membered rings This is the most widely used synthetic route to monocyclic oxepins. The key step in the synthesis of oxepin-benzene oxide (7) is the dehydrohalogenation of a dibromoepoxide precursor (64AG(E)S10). Since the benzene oxide valence tautomer is formed initially the valence tautomerization of the latter to oxepin (equation 51) may be considered as a ring expansion reaction. 

Instead of 13C labeling, 12C-enriched precursors with a 13C abundance of less than 1.1 % can be used for mechanistic studies. In this case, the spectra are more easily analyzed as no homonuclear carbon-carbon coupling occurs. An illustrative example is the base-catalyzed dehydrohalogenation of 7,7-dichloronorcarene to benzocyclopropene [407]. For this reaction, two pathways A and B have been proposed, mechanism B involving a skeletal rearrangement ... 

Another common method for the preparation of benzene and naphthalene epoxides is by introduction of double bonds into the dihydro precursors. This is generally achieved by bromination-dehydrobromination. Thus unsubstituted benzene oxide-oxepin (86 96) can be obtained by dehydrohalogenation of the dibromocyclohexane epoxide 99, using a basic amine or sodium methoxide in ether.53... 

However, the ready availability of halocyclopropanes has led to extensive studies of their 1,2-dehydrochlorination, and amines are now rarely used as cyclopropene precursors. Although the reaction of 1,1-dichlorocyclopropanes with strong base does in certain situations lead to cyclopropenes, it is frequently the case that the initially formed 1-halocyclopropene does not survive under the reaction conditions, undergoing either addition of a nucleophile to the alkene bond or prototropic shifts followed by further dehydrohalogenation. Two main variations on this method are available which proceed under conditions where further reaction does not, in general, occur, that is 1,2-dehalogenation and 1,2-dehalosilylation. Each of these three alternatives will be considered in turn. 

Chlorocyanoketene (1), 8, 88 9, 103. The pseudoisopropyl ester of the azidofuranone (2) is now preferred over the pseudomethyl ester used originally as the precursor because it is less prone to detonation. The ketene can be generated by dehydrohalogenation of the... 

There are four main general methods [40 2] for the preparation of perfluorinated alkenes, namely dehydrohalogenation, dehalogenation, pyrolysis and halogen exchange reactions of appropriate fluorinated precursors. The overall features of the mechanisms of each of these processes have already been discussed (Chapters 6 and 7, Sections 1 and 11). Representative examples of each of these types of synthesis are collated in Table 7.5 clearly the method of choice for the synthesis of a particular fluoroalkene will depend... 

Conjugated polymer preparation by CVP is a two step process. The first step involves chemical vapor depostion of an unconjugated precursor polymer. The second involves the conversion of the unconjugated polymer into a conjugated polymer. This is done by dehydrohalogenation, or dehydrogenation, of the unconjugated polymer to induce the formation of a carbon-carbon double bond. 

One of the most innovative and useful techniques for the preparation of conducting polymers has been the synthesis of highly soluble precursor polymers that can be easily handled in solution, purified, and then later converted to the less tractable conducting polymer. The first example of such an approach was the dehydrohalogenation of poly(vinyl chloride) (102). This reaction, like most elimination reactions on polymers, rarely goes to completion and is not well suited for the synthesis of useful conducting... 

Carbodiphosphoranes (bistriorgano)phosphoranylideneniethanes, hexaorganocarbodiphosphoranes) may be generated from suitable phosphonium salt precursors by dehydrohalogenation or dehalogena-tion. Since the first synthesis of hexaphenylcarbodiphosphorane, a series of symmetrical, unsymmetri-cal, mixed alkyl/phenyl, cyclic and difunctional carbodiphosphoranes have been prepared. Compounds synthesized until 1984 have been listed. 

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