Dehalogenation Subject

Halogen dealkylation mimics O-dealkylation both in terms of mechanism and the commonality of the process. Virtually any drug that contains a carbon-hydrogen bond adjacent to a halogen atom will be subject to P450-catalyzed oxidative dehalogenation (Fig. 4.61). 

A project at the University of Arizona (FEDRIP 1996) will study microbial dehalogenation of several compounds, including chloroform. A major part of the study will focus on the facultative anaerobic bacteria Shewanella putrefaciens sp., which is known to catalyze the transformation of carbon tetrachloride to chloroform and other as yet unidentified products. The organic substrates will also contain metals. It is hoped that the end-products from the biochemical treatment can be subjected to a photolytic finishing process that will completely mineralize any remaining halogenated compounds. 

Dwell time, or the time the molecule was actually in the lamp unit, and concentration were two parameters that affected the rate of degradation. Mass spectra of the trimethylsilyl (TMS) derivatives of atrazine subjected to UV-ozonation revealed a number of dehalogenated, dealkylated s -triazines, paraquat yielded the 4-picolinic acid, and 2,4-D gave oxalic acid, glycolic acid and several four-carbon oxidation products. The economics of UV-ozonation as a pretreatment for land disposal compares favorably with incineration and other options open to the small pesticide user. 

A mild method for generating reactive ketenes has been developed59 which involves the dehalogenation of cc-bromo carboxylic acid chlorides at low temperatures (— 80 C) with pentacarbonylmanganese(IV) anion [Mn(CO),]. For example, subjecting 16 to these conditions provides a practical synthesis of the corresponding eyclobutanediones 18. 

Perhalogenated aliphatic compounds such as hexachloroethane and perchloro-ethene (PCE) are highly oxidized compounds that are subject to reductive dehalogenation... 

One of the earliest reported preparations of the requisite glycosidation precursor 5-deoxy-l,2,3-tri-0-acetyl-p-D-ribofuranoside (17) was published by Kissman and Baker in 1957.23 D-Ribose was heated at reflux in a methanol/acetone mixture in the presence of concentrated HCI to provide methyl 2,3-O-isopropylidene-D-ribofuranosidc (21), which was in turn converted to the corresponding 5-O-mesyl ribofuranoside 22 with methanesulfonyl chloride in pyridine in 63% yield. The sulfonate moiety of 22 was then displaced with sodium iodide in refluxing DMF to provide 5-deoxy-5-iodo derivative 23 in 76% yield on a multigram scale. Reductive dehalogenation of 23 was accomplished under heterogeneous catalytic hydrogenation conditions to provide the reduced 2,3-0-protected intermediate 24 in 56% yield, which was subjected to hydrolysis conditions in... 

The photochemistry of aqueous 2-iodophenol was the subject of a subsequent study [19]. Although ring contraction, carbene formation and photohydrolysis took place in this case too, they all were minor pathways (

comparison with homolytic C -1 cleavage forming phenyl radicals and I" atoms (

radical anions. It was concluded that the heterolytic dehalogenation mechanism observed for aqueous 2-chloro- and 2-bromophenol is overruled in the case of 2-iodophenol by a homolytic one promoted by the comparatively weaker carbon-halogen bond [19]. 

Since eliminations cover a very wide spectrum of chemical reactions, this chapter is a selective discussion of the subject. The mechanistic basis of (3-eliminations is discussed, largely with reference to dehydrohalogenation, and a variety of a-eliminations are included here. Other eliminations, such as dehalogenation, are described throughout later chapters with reference to specific syntheses. 

The free-radical chemistry of this class of compound has been the subject of study by radiation chemists (c/. [11, 27-29]) ever since it was observed that they are reductively dehalogenated by electron attachment. 

Having developed an efficient route to the common intermediate 91, we next turned our attention to the further transformation of 91 into quinolizidine-type alkaloids (Scheme 18). The amido group in 91 was reduced with BH3 THF in THF under reflux to give amine 100. Without purification, 100 was treated with SOCI2 to yield chloride 101, which was subjected to the next reaction without purification because of instability on SiOa gel. Reductive dehalogenation of chloride 101 with LiAlH4 afforded cermizine C (87) [51]. 

Unlike the well researched subject of the stabilisation of PVC, only a few researchers have investigated the dehalogenation of plastic waste. The studies which have been performed [2] have come up with the following results ... 

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