Dechlorination, aryl chlorides

The complete elimination of functional groups is often an undesirable side reaction in organic synthesis, but on the other hand it is a possibility for the recycling of environmentally harmful compounds, for example phenols and haloarenes such as polychlorinated dibenzodioxins (PCDDs or dioxins ). For example, aryl chlorides can be effectively dechlorinated with Pd(0) NPs in tetra-butylammonium salts with almost quantitative conversions also after 19 runs (entry H, Table 1.4) [96]. On the other hand, a C-0 bond cleavage reaction also seems suitable for the fragmentation of sugar-based biomass such as cellulose or cello-biose in that way, sugar monomers and bioalcohol can be derived from renewable resources (entry F, Table 1.4) [164]. 

Milstein and colleagues [100] have developed very efficient methods using basic, chelating phosphine ligands. Even aryl chlorides underwent reductive dechlorination to the corresponding arenes with ](dippp)2Pd] as catalyst (dippp,... 

However, arylmagnesium organometaUics can only be used in a few cases " like an iron-catalyzed homocoupling (equation 92) . Iron catalysis is further used for the dechlorination of electron-rich aryl chlorides by Grignard reagents . ... 

Aryl halides are often susceptible to photochemical degradation. As described in Chapter 10 later in this book, cleavage of the C-X bond occurs with low quantum yield for aryl chlorides (132), higher quantum yields for aryl bromides and iodides (133), and high quantum yields for some aryl fluorides (e.g., fluoroquinolones) (134). Aryl chlorides are photolabile to homolytic and/or heterolytic dechlorination (43). For sertraline hydrochloride, decomposition of the aryl dichloride moiety occurs in solution when exposed to light (ultraviolet and fluorescent conditions). As shown in the following proposed mechanism, the major photochemical decomposition products include mono-chloro- and des-chloro-sertraline via homolytic cleavage (Fig. 91) (86). 

The photochemical dechlorination of 4-chlorobiphenyl in the presence of anthracene and triethylamine is supposed to proceed via two electron transfer steps, first from triethylamine to excited anthracene and then from the anthracene radical anion to the aryl halide410. This type of mechanism is also operative in the dechlorination of 4-chlorobiphenyl, 4,4 -dichlorobiphenyl, 1,3,5-trichlorobenzene, 1,2,3,5-tetrachloroben-zene and pentachlorobenzene photosensitized by visible dyes (protoporphyrin IX, acriflavin, rose bengal, zinc protoporphyrin, methylene green) in the presence of triethylamine411. The various steps of the mechanism are summarized in equations 112-115. A mechanistic alternative is that the excited photosensitizer transfers an electron directly to the aryl chloride and becomes regenerated by electron transfer from the amine. 

Primary, secondary, and tertiary alkyl chlorides as well as aryl chlorides undergo reductive dechlorination using Sml2, and in the presence of CO ketones are formed by photocarbonylation. ° ... 

Dechlorination of chloroaromatics is an important chemical transformation from the synthetic as well as environmental point of view. Facile dehydrochlorination of aryl chlorides can be affected by a catalytic amount of the rhodium complex 74 under basic conditions and 2-butanol as a hydrogen source (Scheme 104). The reaction tolerates both electron-donating and electron-accepting substituents and gives high yields of the products 228 [142]. 

The kinetics and mechanism of the dechlorination of /V-aryl 2-oxo-2-phenylamino-ethanehydrazonyl chlorides (89) in triethylamine in aqueous dioxane at 25 °C giving the oxanilic hydrazide (91) and 1,4-diaryl-1,2,4,5-tetrazine (92) have been examined.80 The slow step of this interesting reaction is considered to be the breakup of the nitrilium imide (90). 

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