Hydrogen bromide, carbon-catalyzed synthesis

The synthesis of hydrogen bromide from the elements is also catalyzed by carbons [147]. A few preliminary experiments were performed with carbon black Corax 3. A hydrogen stream satnrated with Br2 at 273 K was passed over the samples at 423 K. At this temperatnre, thermal dissociation of HBr is negligible, and the conversion is a measnre of catalytic activity. The HBr formed was absorbed in water and titrated with standard alkali [96]. The catalytic activity of the carbon black tripled after treatment of the oxidized black with NH3 at 873 K, and doubled when heat-treated at 1273 K. This behavior agrees very well with that observed in the oxidation reactions described in this section. 

The synthesis of gm-diallyl derivatives can be achieved by double alkylation of active methylene groups. We realized that installation of gem-diallyl functionality on a carbon atom, not activated by any electron-withdrawing group, is a difficult proposition. The problem becomes insurmountable on carbohydrate precursors because base-catalyzed reactions lead to tandem elimination of water molecules, resulting in the formation of complex mixtures, We observed interesting reactions with carbohydrate cyclopropyl precursors. For example, the radical-mediated cyclopropyl scission of the spirocyclopropyl bromide (86) with n-BusSnH gave the C-aUyl derivative (87) in a stereo-controlled fashion. On the other hand, hydrogenation of the cyclopropylaldehyde derivative (88) over Pd/C provided 89 with a quaternary chiral center (Scheme 30.14). 

Examples of the formylation of aryl halides with synthesis gas catalyzed by palladium complexes are summarized in Equation 19.90. These reactions relied upon the development of ligands with particular steric and electronic properties. The dia-damantyl-n-butyl phosphine shown in the equation, in combination with palladium acetate, leads to the formation of aromatic aldehydes in high yields from electron-rich and electron-poor aryl bromides. Reactions of nitroarenes and 2-bromopyridine provided the aldehydes in low yield, but other examples occurred in satisfactor) yield with only 0.1-0.75 mol % catalyst. The identity of the base is important in this process, and TMEDA was the most effective base. The mechanism of this process was not proposed in the initial work, but is likely to occur by oxidative addition of the aryl halide, insertion of the carbon monoxide into the palladium-aryl bond, and a combination of hydrogenolysis of the acyl intermediate and elimination of hydrogen halide to regenerate palladium(O). The base would then be involved in the hydrogenol5 sis and consumption of hydrogen halide. 

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