Dehydrohalogenation partial

Isoxazoles and their partially or fully saturated analogs have mainly been prepared, both in solution and on insoluble supports, by 1,3-dipolar cycloadditions of nitrile oxides or nitrones to alkenes or alkynes (Figure 15.10). Nitrile oxides can be generated in situ on insoluble supports by dehydration of nitroalkanes with isocyanates, or by conversion of aldehyde-derived oximes into a-chlorooximes and dehydrohalogenation of the latter. Nitrile oxides react smoothly with a wide variety of alkenes and alkynes to yield the corresponding isoxazoles. A less convergent approach to isoxazoles is the cyclocondensation of hydroxylamine with 1,3-dicarbonyl compounds or a,[3-unsatu-rated ketones. 

Tables IV through IX summarize the data that are currently available on the rates of bimolecular substitution and dehydrohalogenation reactions between sulfur nucleophiles and halogenated aliphatic substrates in aqueous solution (i.e., either measured in water or extrapolated to water from a non-aqueous or partially aqueous solvent). The sulfrir nucleophiles considered in these tables are HS-, S2-, S42-, S52- (Table IV), S2032 (Tables V and VIII), SO32-, HSO3 (Table VI), thiolate anions (Tables VII, VIII, and IX), thiols, thioethers, and thioadds (Table VII).

Why is a stronger base needed in this dehydrohalogenation The transition state for the second elimination reaction includes partial cleavage of a C - H bond. In this case, however, the carbon atom is sp hybridized, and sp hybridized C-H bonds are stronger than sp hybridized C-H bonds. As a result, a stronger base is needed to cleave this bond. 

A drawback of these bases is that in the preparation of terminal aliphatic acetylenes they may cause, at the high temperatures used (100-200 °C), partial prototropic rearrangement to the 2-alkynes via the intermediate allenes, as illustrated in equation (13) . Hence, these dehydrohalogenation reagents are preferably used to... 

Incorporation of a benzylic halide into the structure of the alternate-substrate lactone (12-4) led to the bifunctional lactones (13-1, Table 2.13), and (13-2), which showed rapid and irreversible inactivation of a-chymotrypsin and PPE [178]. It was postulated that the intermediate acyl-enzyme formed from attack of Ser-195 on the lactone carbonyl dehydrohalogenated to form a reactive quinone methide that coupled with His-57. If this mechanism were followed, then lactone (13-2) would be an example of a mechanism-activated inhibitor. However, lactone (13-2) is sufficiently reactive as an alkylating agent to directly couple with imidazole while the lactone ring is intact. Because of this, it is not clear, from the published data, whether acylation of Ser-195 precedes alkylation, a prerequisite for this compound to be confirmed as a mechanism-activated inhibitor. Interestingly, the corresponding coumarin (13-3) was both less potent and only provided partial inactivation of a-chymotrypsin [179, 180]. It was shown that the lactone linkage in this coumarin was stable in the presence of a-chymotrypsin and that the modified enzyme retained its intact active-site. These facts led to the postulate that, like the action of phenacyl bromides or benzyl bromides on a-chymotrypsin, the partial inactivation by (13-3) involves alkylation of Met-192 [179]. 

Cyclohexene can be synthesized by partial hydrogenation of benzene, by partial dehydrogenation of cyclohexane, or by dehydrohalogenation of cyclohexyl halides. The hydrogenation of benzene is the most viable route in the Asahi process, cyclohexene is obtained with a 60% yield and 80% selectivity, with the remainder being converted into cyclohexane. Asahi has developed a process for the addition of water to cyclohexene to produce cyclohexanol that can then be oxidized to AA using the conventional nitric acid oxidation. 

It appears that dehydrohalogenation of XVII in the presence of lithium chloride is accompanied by partial epimerization of the methyl group at C-16, for reduction of the product XVIII over palladium gave a... 

The majority of grafting from syntheses have been accomplished by converting polymeric halides into macro-initiators. Allyl halides provide suitably stable carbocation derivatives and consequently the partial dehydrohalogenation of poly(vinyl chloride) has been found to enhance the rate of styrene grafting in the presence of aluminium chloride. Similar sites available in chloroprene were activated by silver hexafluorophosphate to induce grafting of IBVE. Nitrosyl and... 

The occurrence of such side reactions can partially account for the fact that the tuffy resin has a lower epoxide value than is implied by the measured molecular weight distribution [11]. The fact that the stoichiometric balance cannot be maintained due to incomplete dehydrohalogenation and the presence of monofunctional epoxides in the reacting mixture can appreciably limit the molecular weight attainable. 

Partial dehydrohalogenation. A mixture of 1,1,2,2-tetrachlorethane, ethylene oxide, and tetra-n-butylammonium chloride heated 5 hrs. at 100° in a sealed tube or autoclave -> trichlorethylene. Y 92% (purity 95%). - Oxido compds. act as bases in the presence of halide ions. F. e. with lower yields and limitations s. J. Buddrus and W. Kimpenhaus, B. 106, 1648 (1973). 

In certain cases the fully conjugated polymer may not be soluble. It is then possible to use a precursor variant of the Gilch synthesis, where only one equivalent of base is used. This produces an intermediate soluble halide-substituted polymer that can be dehydrohalogenated thermally (Scheme 13) [104-107] or partially eliminated in an alcoholic solvent [108]. However, the intermediate halide-containing polymers may not be very stable, though this can depend on storage conditions [104]. 

Bromoform heated lOhrs. at 80° with 0.5 equivalent triethylsilane and a catalytic amount of benzoyl peroxide in a sealed tube methylene dibromide. Y ca. 100% based on startg. m. consumed. F. e., also with phenyldimethyl-silane, s. Y. Nagai, K. Yamazaki, and I. Shiojima, Bull. Ghem. Soc. Japan 2210 (1967) also partial dehydrohalogenation by addition of FeGlg s. E. T. Ghukovskaya, N. A. Kuz mina, and R. K. Freidlina, Dokl. Akad. Nauk SSSR 175, 1301 (1967) G. A. 68, 59024 with trichlorosilane/tri-n-butylamine s. R. A. Benkeser and W. E. Smith, Am. Soc. 90, 5307 (1968). 

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