Fragmentation dehydrochlorinative

The base-catalyzed rearrangement of the cyclopentadiene-dichloroketene adduct gives tropolone. The reaction course passes through an SN2 displacement of the enol, fragmentative dehydrochlorination, and enolization [85],... 

Chloroadamantan-1 -ol undergoes fragmentative dehydrochlorination more than 15 times faster than the corresponding thiol (81). The larger frangomeric effect of the former is due to the greater tendency to form a C=0 bond rather than the C=S bond. 

In earlier investigations chain ends were suggested to be important initiation sites for dehydrochlorination. Provided there are no transfer reactions during polymerization, at least half the polymer chain ends will carry initiator fragments. In practice, transfer reactions swamp the normal termination processes and <30% of the chain ends carry initiator residues [59]. 

The second method for the chain branching is based on the preparation of the alternative triangulane derivative 54, which also possesses a methylenecyclopropyl fragment and a tetrahydropyranyloxymethyl substituent73. This kind of precursor was prepared by cycloaddition of chloro(tetrahydropyranyloxyethyl)carbene to a methylenetriangulane followed by dehydrochlorination (equation 40). 

Instability of the polymer is responsible for the primary step in decomposition and is attributed either to fragments of initiator or to branched chains or to terminal double bonds. The appearance of branching is the result of reactions of chain transfer through the polymer, while that of unsaturated terminal groups results from reaction of disproportionation and chain transfer through the monomer. During thermal and thermo-oxidative dehydrochlorination of PVC, allyl activation of the chlorine atoms next to the double bonds occurs. In this volume, Klemchuk describes the kinetics of PVC degradation based on experiments with allylic chloride as a model substance. He observed that thermal stabilizers replace the allylic chlorine at a faster ratio than the decomposition rate of the allylic chloride. 

Guyot and Benevise also recorded the chlorine content of the dissolved and reprecipitated polymer obtained from degraded resin samples. Although the degraded sample, as a whole, showed appreciable loss of chlorine, the reprecipitated samples had contents close to that of un-dehydrochlorinated polymer. According to the authors, this precipitation procedure left in solution small amounts of low molecular weight fragments which did not contain chlorine. 

Heat stabilisers retard dehydrochlorination and autoxidation and reduce fragmentation. In addition, they also cure existing damage. 

Strong bases (e.g., alkoxide) can dehydrochlorinate mustard and its sulfoxide and sulfone, converting one or both of their 2-chloroethyl fragments to vinyl groups. 

The surface area of cement catalysts, which carries aluminum- and calcium-containing oxide fragments, exhibits pronounced acid—base properties. These properties can manifest itself as a catalytic activity to the reactions of dehydrochlorination, which proceed via the formation of donor—acceptor complexes between the substrate and acid or base sites at the catalyst surface. 

Delay in dehydrochlorination is also observed with graft copolymers, but some chain fragments are produced at the same temperature as is HC1. 

The course of the condensation between an acylphosphonate and a 1-nitroalkane car-banion depends on the nature of the acyl group. Base catalysts are commonly employed, but an acylphosphonate derived from an aromatic acid requires an acidic catalyst otherwise the intermediate anion fragments to hydrogenphosphonate and nitroketone (Scheme 20)207 jYiQ reaction between a nitroalkane carbanion and a dialkyl (trichloroacetyl)phos-phonate results in a rearrangement to phosphate with additional dehydrochlorination (Scheme 21) . ... 

The specific ion current decreased with increasing molecular weight as would be expected. There was some evidence of the evolution of aliphatic hydrocarbons, but they were not characterised. The evolution profiles for some of the aromatic hydrocarbon fragments were bimodal and the first evolution maximum occurred concomitantly with the dehydrochlorination step. These results confirm that the ion (PhCHCl) was formed in secondary gas-phase reactions since it was not observed in the positive ion spectra. It is interesting to note that not all the ions have this bimodal evolution profile. 

Not only a planar x-conjugated molecule but also nonplanar x-conjugated molecules, highly curved buckybowls containing a corannulene fragment, were also synthesized via the rhodium-catalyzed [2+2+2] cycloaddition followed by dehydrochlorination (Scheme 21.15) [18]. 

The chlorine atom produced can abstract hydrogen from the allylic methylene group to begin dehydrochlorination by the free radical chain reaction described previously. Alternatively, the initiator fragment may abstract a methylenic hydrogen atom from some point along a polymer chain subsequent elimination of a chlorine atom then leads to a carbon-carbon double bond ... 

Unzipping may then proceed from the new allyl group by either a radical process or unimolecular elimination. In addition, of course, the radicals produced by thermal decomposition of the hydroperoxide may directly initiate the radical chain dehydrochlorination process in a manner similar to that described previously for initiator fragments. 

Under thermal load, PVC sustains damage caused by hydrogen chloride elimination (dehydrochlorination), autoxidation, and mechano-chemical fragmentation. Specifically effective stabilizers and costabilizers and/or mixtures of stabilizers delay dehydrochlorination and autoxidation. They neutralize the eliminated hydrogen chloride during thermoplastic processing and help maintain service properties [86]. The technically most important thermostabilizers and costabilizers are described in Section 3.7.5. 

Therefore, stabilizers always have to be added when processing PVC. They delay dehydrochlorination by binding HCI and eliminating initiation points, suppress autoxidation, and minimize mechano-chemical fragmentation by an internal lubricating action [20],... 

The formation of a rearrangement product can be represented by Scheme 6.51. According to this scheme, the initial step involves the nucleophilic displacement of the Cl atom by the pyridine N atom, which is followed by a cascade transformation. The latter involves (a) dehydrochlorination with the abstraction of a methyl H atom of a-picohne, (b) intramolecular nucleophilic addition of the methyUdene C atom of the a-picoline fragment to the azomethine C atom of the quinoxaUne system... 

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