Hydrogenations Mediated by Phase-transfer Catalysts

Extensive work showed that this methodology can be applied to many hydrogenations alkenes or alkynes [33], substituted benzene — substituted cyclohexane [33, 34], deuterium exchange [34], selective reduction of polycyclic aromatics [35, 36], selective double-bond reduction in the presence of nitro groups (e.g., PhCH=CHN02 — PHCH2CH2N02) [37], and reduction of double bonds in a,[i-unsaturated esters and carbonyls [38, 39]. 

Rhodium(III) complexes L2Rh(H)Cl2 (L = tricydohexylphosphine or triisopro-pylphosphine) were also shown to be excellently suited to a PTC-mediated hydro-genolysis of chloroarenes. The solvent mixture was 40% aqueous NaOH/toluene, and TEBA was the PT catalyst. Functional groups such as OR, CF3, COR, COOH, and NH2 were compatible with the C—Cl — C- H process [40]. 

Quite a different catalyst and technique were used by Wilkinson and co-workers for the hydrogenation of benzylideneaniline in water/diethyl ether. The water-soluble complex salt [ Rh(PPh3)2(cod)]PF6 was extracted by Triton X-100, a non-ionic detergent [41]. 

A unique application comprises a facile hydrodehalogenation (hydrogenolysis) of aromatic compounds in 15% KOH with a heterogeneous Pd/C catalyst and Ali-quat 336 as PT catalyst [42], The latter results in a marked rate enhancement in many cases. The ease of halide removal falls from iodide through chloride to fluoride. It is believed that the lipophilic Q+ promotes the transport of the halide ion produced, from the Pd site to the alkaline aqueous phase. 

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