Dehalogenation product forming step

The product-forming steps of dehalogenations by free radical pathways were discussed earlier (see Section 18.3.1.1). In non-radical mechanisms, the dehalo-genated products (RH) will be formed mostly by reductive elimination [193, 194] however, concerted processes lead directly from RX to RH (see Sections 18.3.1.2 and 18.3.1.3). 

In the presence of electron donors, photodechlorination of chlorobenzene390, 4-chloroanisole378 and other aryl halides is enhanced. The key step in this process is electron transfer from the donor to the excited halide. A kinetic study, based on determination of quantum yields and fluorescence quenching efficiencies, has revealed that in the triethyl-amine-assisted dehalogenation of chlorobenzene the product-forming interaction occurs with the triplet excited state (equations 97-99)390. 

The arylamines are generally formed in good yields (Table 1). Dehalogenation products are the only by-products observed, which pro-baly arise from base-induced )9-hydride elimination of the amido arylpalladium complex and subsequent reduction. Interestingly, the base employed has a decisive influence on the course of the reaction. In the amination of l-bromo-4-n-butylbenzene with free amines in the presence of silyl amides as base -in contrast to the coupling with tin amides -the rate-determining step in the catalytic cycle is the oxidative addition of bis(tri-o-tolylphosphine)palladium(O) to the aryl halide. However, when LiOrBu is used as base, the formation and reductive elimination of the amido arylpalladium complex is decisive for the rate of the reaction. In the presence of NaOtBu both reaction steps seem to take place at similar rates. [9]... 

Despite this correlation, different product distributions were observed when the reduction is run with AcrH2 vs. Fe(MeCsH4)2 (Fig. 43). This observation can be rationalized by considering the subsequent steps of the reduction mechanism (Fig. 44). The 1,2 reduction product forms as a result of hydrogen-atom transfer from the AcrH2 radical cation to yield the a-halohydrin product. Since this pathway is unavailable for the outersphere one-electron reductants, only the dehalogenated acetophenones are observed. 

For the preparation of aryl or heteroaryl acetylenic compounds, the reactions proceed in good yield, with little isomerization of the products. When a two-step process is used, a highly stable vinyl halogenide forms in the first step under mild conditions. Dehalogenation with a strong base in the second step yields the corresponding alkynes [Eqs. (1 8)]. 

This reaction was first reported by Hantzsch and Weber in 1887. It is the formation of thiazole derivatives by means of condensation of a-haloketones (or aldehydes) and thioamides. Therefore, it is generally known as the Hantzsch thiazole synthesis. In addition, other names, including the Hantzsch synthesis, Hantzsch reaction, and Hantzsch thiazole reaction are also used from time to time. Besides thioamides, other thio-ketone derivatives such as thiourea, dithiocarbamates, and ketone thiosemicarbazone can also condense with a-halo ketones (or aldehydes) to form thiazoles. This reaction occurs because of the strong nucleophilicity of the sulfur atom in thioamides or thioureas, and normally gives excellent yields for simple thiazoles but low yields for some substituted thiazoles, as of dehalogenation. This reaction has been proven to be a multistep reaction, and the intermediates have been isolated at low temperatures, in which the dehydration of cyclic intermediates seems to be the slow step. It is found that a variety of reaction conditions might result in the racemized thiazoles that contain an enolizable proton at their chiral center, and it is the intermediate not the final product that is involved in the racemization. Therefore, some modifications have been made to reduce or even eliminate the epimeriza-tion upon thiazole formation. In addition, this reaction has been modified using a-tosyloxy ketones to replace a-haloketones. ... 

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