Benzyl hydrodehalogenation

Platinum on charcoal provided a further means for directing the selectivity of the multiphasic hydrodehalogenation reactions. For example, the same reaction in Figure 6.20 conducted using Pt/C instead of Pd/C, yielded selectively the dehalo-genated benzylic alcohol (Figure 6.22)." The same reaction was conducted using... 

Kinetic studies performed on model compounds were aimed at understanding the effect of different parameters on the selectivity. They showed that selectivity was achieved only when A336 was present. In fact, in the absence of A336 and of the base the hydrodehalogenation of p-chloroacetophenone proceeded aU the way to ethylcyclohexane in the biphasic aqueous-organic system. When A336 was added, selectivity was reversed—chloride was removed first—and the selective dehalogenated benzyl alcohol was obtained. ... 

Samarium iodide catalyzes the reduction of halides in damp THF120-122 and yields may be significantly improved if HMPA is added to the reaction mixture121,123. a-Halocarbonyl compounds are readily hydrodehalogenated by the use of this and similar methods124-126. Bis-cyclopentadiene complexes of samarium catalyze the reduction of benzylic and ally-lie halides via an organosamarium complex intermediate127-129. This reaction may be controlled so that monodeuteration occurs if the reaction is carried out under dry aprotic conditions followed by addition of D20 (equation 12). 

Hydrodehalogenation of aliphatic or benzylic halides were catalyzed by water soluble ruthenium phosphine complexes in the presence of sodium formate as hydrogen donor [194], Hydroxycarbonylations could also be performed with high palladium catalyst activities in biphasic systems [195-197]. 

The latter exemple is a very selective hydrogenation without hydrodehalogenation or hydrogenolysis of the benzylic group. 

In addition to the reaction pathways of the previously discussed substrates, 4-chlorobenzaldehyde 1c can be hydrodehalogenated. Indeed, the main product obtained using the Pd catalysts on fumed titania was toluene 6c which is the hydrogenolysis product of 4-chlorotoluene 3c. No benzyl alcohol 2c was formed. Apart from about 50-60% of toluene, 4-chlorotoluene 3c, ether 4b and saturated aromatic ring product 5c are present in the range of between 5 and 30%. In this case, the different acidic properties of the Pd catalysts on fumed titania are of minor importance. The release of hydrogen chloride from the hydrodechlorination reaction compensates for the initial differences of the catalyst acidity. 

Patzold, E. Ohme, G. (1993) Water-soluble palladium(ll) phosphine complexes as catalysts in the hydrodehalogenation of allyl and benzyl halogenides under biphasic and phase-transfer conditions, J. Prakt. Chem. - Chem. Ztg., 335,181-4. 

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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