Debromination reactions

The presence of metal oxides have a different influence on PBDD/F concentration and pattern (ref. 11), see Figure 8. Oxides of zinc and copper reduce the PBDD/F yields due to debromination reactions. The two oxides of copper show... 

Scheme 13.16. Protection-deprotection strategy in microwave-enhanced fluoro-debromination reaction.

All of the initial CGC organobromine degradation products assignments were based on the use of relative retention time standards. The relative retention times for the partially brominated degradation products were established either by the synthesis of the expected degradation products or by the use of selective debromination reactions. 

In extension of this work, the amino-debromination reaction of 4-methoxy-, 4-phenyl-, 4-t-butyl-, and 4-(A-methylanilino)-5-bromopyrimi-dine was investigated. It was found that in all these reactions, good yields of the corresponding 6-aminopyrimidines are obtained no indication for the formation of a 4-substituted 5-aminopyrimidine was observed (Scheme II.3) (64TL2093 65TL555 68TL9). 

Note that log (k,, /k I20) versus X.o, produces a better fit to the experimental data for both the dechlorination and debromination reactions than does log kOH or log kH20 versus X.o,. This is probably because the former approach eliminates steric effects. 

Treating 113 (easily available from 45 by reaction of its dilithio derivative with formaldehyde and treatment of the resulting diol with phosphorus dibromide) with the so-called Mori reagent [72], a trimethylsilyl(tri-n-butyl)stannane/cesium fluoride combination that has been used very successfully in many debromination reactions [73], results in the production of the [6.6]paracyclophane 115 in acceptable yield. Since p-xylylene (p-quinodimethane) is known to dimerize to [2.2]paracyclophane, it may well be that the bis(cumulene) 114 is formed as an intermediate in this reaction. Since a stepwise process is also conceivable, further clarification of the mechanism is obviously desirable (Scheme 25). 

The special effects of a mesoionic system as a substituent have been noted in the reactions with nucleophiles of 3-7V (4-chloro-3-nitrophenyl)sydnone. A synthesis, using two amino-debromination reactions, has been used to prepare phenothiazines analogous to methylene blue (23). An imusual susceptibihty to the nature of the counteranion has been observed in the kinetics of the reaction of iV-(2,4-dinitrophenyl)-4-dimethylaminopyridinium salts (24) with piperidine in acetonitrile, and may indicate participation of the anion in stabilising the intermediate. An ANRORC mechanism is imphcated in the reaction of A-(2,4-dinitrophenyl)-4-(4-pyridyl)pyridinium cations with arylamines. Ring-opening and ring-closure reactions... 

Why might old zinc dust not react in the debromination reaction ... 

Debromination. Reaction of 1,2-dibromoethylene (cis/trans = 64 36) in refluxing THF with 2 moles of lithium diphenylphosphide gives acetylene in 80% yield. This debromination also occurs with lithium diphenylarsenide. ... 

The debromination reaction was carried out with a series of cyclic and acyclic vic-dibromides (general formula RCHBr-CHBrR) in CDCI3 or CD3CN at 90-100°C (sealed tube) and the kinetics were followed by H-NMR spectroscopy. 

The kinetic study suggests a first-order dependence in both telluride and vic-dibromide for the debromination reaction. The rate law can be expressed as ... 

As we have seen, the study of the stereochemistry of the debromination reaction is the key choosing between the two mechanistic pathways. Both proposals could justify the kinetic data (rate law, nucleophilicity of the telluride, effects of solvent polarity) however, only Mechanism 2 could satisfactorily explain the stereoselectivity in all cases. The intermediacy of a bromonium ion and the role of the telluride as a scavenger of the Br seem to be the best option with all the data in hand. 

Another application of [pmim][BFJ for the debromination reaction has been reported by Ranu et al. (2007). a-Bromoketones were selectively debrominated to either monobromo or debromoketones under microwave irradiation. The utilization of [pmim][BF J was also found appropriate for the dehalogenation of a-haloketones esters and v/c-bromoacetals. 

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