Dehalogenation, pyridine

However, treatment of cortisone 3,20-bissemicarbazone with acetic anhydride and pyridine removes the 20-semicarbazone group preferentially. Selective removal of a protecting group can be also achieved by a selective reaction to give a new intermediate which can be converted into the desired product ketone. Thus progesterone 20-monoenol acetate (42) is prepared from the 3,20-bisenol acetate (40) via selective electrophilic attack of iodine at C-6 followed by reductive dehalogenation of (41). ... 

Finally, these reactions do not seem to be appreciably dependent on the structure of the substrate since methoxy- and ethoxy-dehalogen-ation in their respective alcohols proceed at nearly the same rate with each of the substrates, 2- and 4-chloroquinoline and 2-bromo-pyridine. ... 

Lee CH, Lewis TA, Paszczynski A, Crawford RL (1999) Identification of an Extracellular Catalyst of Carhon Tetrachloride Dehalogenation from Pseudomonas stutzeri Strain KC as Pyridine-2,6-bis(thiocarboxylate). Biochem Biophys Res Commun 261 562... 

Several of the methods of synthesis of 2,2 -bipyridines have their counterpart in the preparation of 4,4 -bipyridine. The Ullmann reaction has been used to prepare 4,4 -bipyridine. Thus 4-halogenated pyridines afford 4,4 -bipyridine. Dehalogenation and dimerization of 4-bromopyridine may be accomplished too with hydrazine and alkali at 65°C in the presence of a palladium catalyst, whereas 4-chloropyridine is converted to 4,4 -bipyridine in 46% yield by reaction with alkaline sodium formate in the presence of palladium on charcoal and a surfactant. Several extensions of the Ullmann reaction have recently been reported, especially for the synthesis of substituted 4,4 -bipyridines. Thus 4-iodo-2-methylpyridine gives 2,2 -dimethyl-4,4 -bipyridine, 3-nitro-4-chloropyridine affords 3,3 -dinitro-4,4 -bipyridine, 4-bromo- or 4-iodotetrafluoropyridine gives octafluoro-4,4 -bipyridine, and 4-iodo- or 4-bromotetrachloropyridine gives octachloro-4,4 -bipyridine. Related syntheses have been de-... 

In several examples the reductive halide-hydrogen exchange has been studied on a preparative scale. For example, the indirect electroreduction of 2-chloropyridine in DMF using anthracene as mediator gives pyridine in 83-86 % yield 2 . For the dehalogenation of 1-chlorohexane (80% yield), naphthalene is applied as redox catalyst. Similarly, 6-chloro-hexene yields 1-hexene (60%) and methylcyclopentane (25%), which is the product of the radical cyclization . The indirect electrochemical reduction of p- and y-bromocarboxylic esters forms coupling and elimination products besides the dehalogenated products... 

Catalytic reduction of the derivatives of triazolopyridines invariably attacks the pyridine ring, leaving the triazole. There is no report of reduction of compound 1, but 3-nitrotriazolopyridine (154) gives as major product the tetrahydrotriazolopyridinamine 155.229 Reduction with palladium-charcoal takes a different course (see Section IV,F). The mesoionic derivatives of [l,2,4]triazolo[l,5-a]pyridine 156-15854 and 159230 are reduced to py-tetrahydro derivatives, as is compound 3,231 although it had previously been reported that 3-substituted derivatives of compound 3 were not reduced.65 Chloro[l, 2,3]triazolo[4,5-6]pyridine (160) is reductively dehalogenated with reduction of the pyridine ring by platinum or palladium catalysts.146 By addition of base the reduction can be stopped at compound 4 (see Section... 

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... 

Direct photolysis of 2-halopyridines in methanol, ethanol or acetonitrile (with 1% water) affords pyridine, together with alkoxy-, or hydroxypyridines376. The formation of the substitution products will be discussed in Section IV.C. The reductive dehalogenation is considered to proceed via homolysis of the carbon-halogen bond. The efficiency of pyridine formation decreases in the order Br > Cl > I. The 3- and 4-halopyridines produce pyridine exclusively. 

Oxidative addition of C2 - H bonds of imidazolium salts to low valent metals was first observed by Nolan and coworkers in 2001, who proposed a NHC - Pd - H intermediate in the catalytic cycle of the dehalogenation of aryl halides with Pd(dba)2 in the presence of imidazolium salts [154]. More direct evidence of this process was described by Crabtree and coworkers two years later [155]. The reaction between a pyridine-imidazolium salt and Pd2(dba)3 afforded the preparation of bis-NHC - Pd(II) complexes by C2 - H oxidative addition (Scheme 40). The presumed Pd - H intermediates were not detected. The authors proposed a mechanism via two successive C - H oxidative additions followed by reductive elimination of H2 [ 155]. 

The chromatographic mobility of a series of chlorinated isothiazolo[5,4-/ ]pyridines compared to the fully dehalogenated system using the same chromatographic conditions is in the order 4,6-dichloro-3-methylisothiazolo[5,4-6]pyridine > 4-chloro-3-methylisothiazolo[5,4-h]pyridine > 6-chloro-3-methylisothiazolo[5,4-6]pyridine> 3-methylisothiazolo[5,4-6]pyridine <90JCS(P1)1477>. 

The mixture of Cp2TiCl2 and a hydride reagent catalyzes the dehalogenation of monohalopyridines at room temperature to give pyridine.1259... 

---