Hydrodehalogenation of aryl halides

It has not been shown unambiguously, however, whether the rhodium remains coordinated to the ligands 2 and 3 during the catalysis. Kantam and associates [23] have used a Pd(II) catalyst anchored onto MCM-41/silylamine (MCM-41 a molecular-sieve) for hydrodehalogenation of aryl halides by atmospheric hydrogen. The catalyst was reused for several cycles with constant activity. 

Scheme 7. Factors controlling selectivity for amination vs. hydrodehalogenation of aryl halides.

It has also been proposed that reaction CD is the rate limiting step in hydrodehalogenation of aryl halides in the presence of homogeneous and metallocomplex catalysts [7,8]. Based on this knowledge it has been suggested that the introduction of palladium into the support not in metallic but in ionic form should increase the rate of hydrodehalogenation provided the mode of stabilization of the ionic form of palladium can be found. 

The scope of the multiphasic system was extended to coupling reactions—like the Heck reaction—using a heterogeneous supported catalyst, such as Pd/C. The rationale here lay in the observation that aryl halides were activated in the multiphasic system (as seen for hydrodehalogenation), and that therefore they should also be activated toward C-C coupling reactions. 

The reaction of the carbanion derived from diethyl methylphosphonate with perhalogenated aromatics may result in substitution of halide to yield perhaloaryl(hetaryl)methylphosphonates, which can be converted into tris- or bis-(perhaloaryl)methanes. Displacement of fluoride ion has been reported in the reaction of dimethoxycarbene with l-fluoro-2,4-dinitrobenzene and with hexafluorobenzene. The hydrodehalogenation of halogenated aryl ketones may be facilitated using hydrogen over a Pt/C catalyst. " ... 

Hydrodehalogenation. Aryl halides are reduced when heated in formic acid with catalytic amounts of Pd/C. Nitro group are also reduced, but carbonyl groups are not. Yields are 80-95%.4... 

Hydrodehalogenation. Triethylborane (10 mole %) can initiate reduction of organic halides, particularly iodides or bromides, by Bu3SnH even at —78°. Alkenyl iodides are also reduced under these conditions. Aryl halides require room temperature or higher. Yields are comparable to those obtained by radical initiators. 

Aryl chlorides are widely used for C-C bond creation under Pd(0)-Pd(II) catalytic cycle conditions [62], As a consequence, use of cheaper ArCl in benzoic acid arylation should also be possible if appropriate ligands are used and catalytic cycle for the arylation is changed from Pd(II)-Pd(IV) to a Pd(0)-Pd(II) couple. The method involves use of catalytic palladium acetate in combination with butyl-di-1-adamantylphosphine ligand, cesium carbonate base, and an aryl chloride coupling partner [61]. Benzoic acids of any electronic properties are reactive (Scheme 15). Electron-poor benzoic acids react well, and it is possible to couple them with both electron-poor and electron-rich aryl chlorides. Both electron-poor and electron-rich aryl chlorides can be coupled with electron-rich benzoic acids. However, the combination of an electron-rich aryl chloride with an electron-rich benzoic acid is occasionally problematic due to product decarboxylation or aryl halide hydrodehalogenation. The nitro group is compatible with the reaction conditions, as is the ester group. 

Mechanistically related to iron-catalyzed cross-coupling is a hydrodehalogenation reaction via P-hydride elimination. Both reactions may be competitive. However, the reaction conditions can be adjusted to allow for selective hydrodehalogenation. Aryl halides are dehalogenated by teft-butylmagnesium chloride or -butylmagnesium chloride in the presence of a catalytic amount of tris(acetylacetonato)iron or iron(III) chloride under mild conditions (Scheme 4-225). Selective transformation... 

Marques, C. A., Selva, M. Tundo, P. (1994). Facile hydrodehalogenation with H2 and Pd/C catalyst under multiphase conditions 2. Selectivity and kinetics. Journal of Organic Chemistry, 59,14, 3830-3837, ISSN 0022-3263 Marques, C. A., Selva, M. Tundo, P. (1995). Facile hydrohalogenation with H2 and Pd/C catalyst under multiphase conditions 3. Selective removal of halogen from functionalized aryl ketones. 4. Aryl halide-promoted reduction of benzyl alcohols to alkanes. Journal of Organic Chemistry, 60, 8, 2430-2435, ISSN 0022-3263... 

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