Debromination

X0 group present in hecogenin (79) is transferred to C-11 (84). Bromination of hecogenin in ben2ene yields the lla,23a-dibromo derivative (80). The latter is treated with sodium hydroxide in aqueous /-butanol to yield the crystalline 23-bromo-ketal (81) which is acetylated and then debrominated with... 

Urea and uracil herbicides tend to be persistent in soils and may carry over from one season to the next (299). However, there is significant variation between compounds. Bromacil is debrominated under anaerobic conditions but does not undergo further transformation (423), linuron is degraded in a field soil and does not accumulate or cause carryover problems (424), and terbacd [5902-51-2] is slowly degraded in a Russian soil by microbial means (425). The half-hves for this breakdown range from 76 to 2,475 days and are affected by several factors including moisture and temperature. Finally, tebuthiuron apphed to rangeland has been shown to be phytotoxic after 615 days, and the estimated time for total dissipation of the herbicide is from 2.9 to 7.2 years (426). 

Pigment Red 177 [4051-63-2] 65300 anthraquinone bimolecular debromination of l-amino-4-bromoanthraquiQone-2-sulfonic acid, foUowed by desulfonation... 

Radiolabeled folate provides a powerful tool for folate bioavaHabiUty studies in animals and for diagnostic procedures in humans. Deuteration at the 3- and 5-positions of the central benzene ring of foHc acid (31) was accompHshed by catalytic debromination (47,48) or acid-cataly2ed exchange reaction (49). Alternatively, deuterium-labeled fohc acid (32) was prepared by condensing pteroic acid with commercially available labeled glutamic acid (50). 

Tetracyanoethylene is colorless but forms intensely colored complexes with olefins or aromatic hydrocarbons, eg, benzene solutions are yellow, xylene solutions are orange, and mesitylene solutions are red. The colors arise from complexes of a Lewis acid—base type, with partial transfer of a TT-electron from the aromatic hydrocarbon to TCNE (8). TCNE is conveniendy prepared in the laboratory from malononitrile [109-77-3] (1) by debromination of dibromoma1 ononitrile [1855-23-0] (2) with copper powder (9). The debromination can also be done by pyrolysis at ca 500°C (10). 

Rates of debromination of bromonitro-thiophenes and -selenophenes with sodium thio-phenoxide and sodium selenophenoxide have been studied. Selenophene compounds were about four times more reactive than the corresponding thiophene derivatives. The rate ratio was not significantly different whether attack was occurring at the a- or /3-position. As in benzenoid chemistry, numerous nucleophilic displacement reactions are found to be copper catalyzed. Illustrative of these reactions is the displacement of bromide from 3-bromothiophene-2-carboxylic acid and 3-bromothiophene-4-carboxylic acid by active methylene compounds (e.g. AcCH2C02Et) in the presence of copper and sodium ethoxide (Scheme 77) (75JCS(P1)1390). 

Nitration in 80% sulfuric acid of 4-bromopyrazoles gives rise to considerable nitro-debromination (formation of 4-nitropyrazoles) (79AJC1727). The reaction takes place on the protonated pyrazolium ion (Section 4.04.2.1.4(ii)). 

Selenophene, bis(TV-chlorothioimino)-molecular structure, 4, 939 Selenophene, 2-bromo-mercuration, 4, 946 Selenophene, 3-bromo-lithiation, 4, 949 synthesis, 4, 955 Selenophene, 3-bromo-2-lithio-synthesis, 4, 955 Selenophene, bromonitro-debromination, 4, 78 synthesis, 4, 955 Selenophene, 2-chloromethyl-solvolysis, 4, 952 Selenophene, 3-cyano-synthesis, 4, 955 Selenophene, deutero-deuterium exchange, 4, 949 Selenophene, 2,5-diacyl-3,4-dihydroxy-synthesis, 4, 964... 

Thiophene, 2-(bromomethyl)-5-methyl-reactions, 4, 70 Thiophene, bromonitro-debromination, 4, 78 reactions, 4, 78 reactivity, 4, 826 3-substituted reactivity, 4, 827 5-substituted reactivity, 4, 826-827 Thiophene, 2-bromotetrahydro-synthesis, 4, 857... 

Thiophene-4-carboxylic acid, 3-bromo-debromination, 4, 78 Thiophenecarboxylic acids acidity, 4, 71... 

Triazine-3,5-dione, 6-bromo-debromination, 3, 416 nucleophilic reactions, 3, 417... 

Butyl acrylate has been prepared by direet esterifieaLion/ by debromination of -butyl ,/3-dibromopropionate with zinc, by treatment of either butyl /3-chloropropionate or butyl /3-bromopropionate with diethylaniline, and by the pyrolysis of butyl (3-acetoxypropionated Direct esterification and alcoholysis of methyl or ethyl acrylate have been recommended for the preparation of the higher alkyl acrylates. ... 

It is not possible to use zinc for reductive debromination in the presence of (x-halo ketones and for transformations involving these intermediates, sodium iodide has been used. ° In some instances, e.g. 5,6-dihalo-3-ketones, iodide does not always give a completely halogen-free product, and zinc does not give clean debromination. The use of chromous chloride has proved advantageous in such cases and is the reagent of choice for vicinal dichlorides, which are inert to iodide ... 

Some instances of incomplete debromination of 5,6-dibromo compounds may be due to the presence of 5j5,6a-isomer of wrong stereochemistry for anti-coplanar elimination. The higher temperature afforded by replacing acetone with refluxing cyclohexanone has proved advantageous in some cases. There is evidence that both the zinc and lithium aluminum hydride reductions of vicinal dihalides also proceed faster with diaxial isomers (ref. 266, cf. ref. 215, p. 136, ref. 265). The chromous reduction of vicinal dihalides appears to involve free radical intermediates produced by one electron transfer, and is not stereospecific but favors tra 5-elimination in the case of vic-di-bromides. Chromous ion complexed with ethylene diamine is more reactive than the uncomplexed ion in reduction of -substituted halides and epoxides to olefins. ... 

Steroid, followed by debromination of the resulting dibromo ketone with... 

The effect of a carboxy group is illustrated by the reactivity of 2-bromopyridine-3- and 6-carboxylic acids (resonance and inductive activation, respectively) (cf. 166) to aqueous acid under conditions which do not give hydroxy-debromination of 2-bromopyridine and also by the hydroxy-dechlorination of 3-chloropyridine-4-car-boxylic acid. The intervention of intermolecular bifunctional autocatalysis by the carboxy group (cf. 237) is quite possible. In the amino-dechlorination (80°, 4 hr, petroleum ether) of 5-carbethoxy-4-chloropyrimidine there is opportunity for built-in solvation (167) in addition to electronic activation. This effect of the carboxylate ion, ester, and acid and its variation with charge on the nucleophile are discussed in Sections I,D,2,a, I,D,2,b, and II,B, 1. A 5-amidino group activates 2-methylsulfonylpyridine toward methanolic am-... 

A comparison of ortho vs. para direct deactivation by a methoxy group has been made by Karmas and Spoerri in 2,3-dibromo-5,6-dimethyl- and 2,5-dibromo-3,6-dimethyl-pyrazine. The former gives monomethoxy-debromination with one equivalent of methanolic methoxide (65°, 6 hr) and disubstitution via 198 with excess reagent for a longer time (10 hr). In contrast, the isomeric 2,5-dibromo compound gave only monosubstitution, forming 199, under the latter conditions. 

The question of the occurrence of cine or aryne substitution in some of these reactions has been raised but not answered adequately. The normal product, 2-methoxynaphthalene was shown to be formed from 2-chloronaphthalene and methoxide ion, and the normal 6- and 8-piperidinoquinolines were proved to be products of piperidino-debromination of 6- and 8-bromoquinolines, all in unspecified yield. More highly activated compounds were then assumed not to react via the aryne mechanism. Even if the major product had been characterized, the occurrence of a substantial or predominant amount of aryne reaction may escape notice when strong orientation or steric effects lead to formation of the normal displacement product from the aryne. A substantial amoimt of concurrent aryne reaction may also escape detection if it yields an amount of cine-substituted material easily removed in purification or if the entire reaction mixture is not chromatographed Kauffman and Boettcher have demonstrated that activated compounds such as 4-chloropyridine do indeed react partially via the aryne mechanism (Section I,C,1). 

An abnormal substitution, not detected with the other isomers, is the major reaction in attempted methoxy-debromination of 6- and 8-bromoquinolines. A 50-70% yield of unsubstituted quinoline was isolated. ... 

The activation of all the positions in quinoline in relation to each other and to those in naphthalene can be judged from the kinetic study of piperidino-debromination (Tables IX and X) summarized in the following tabulation (res. = resonance activation, ind. = inductive activation). 

As summarized in the following tabulation, the relative rates of piperidino-debromination of the halo-l-nitronaphtholenes (data from Table XII, numbered to show relation to quinolines) provide a good confirmation of the relation of induction (ind.) to resonance activation (res.) and of the extent of transmission of activation to an adjoining ring. Here again, as in the quinoline series, the 8-isomer (346) is more reactive than its resonance-activated 5-bromo isomer (345) and its inductively activated 3- and 6-bromo isomers (351 and... 

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