Homogeneous Transition-Metal Catalyzed Reactions Under Phase-Transfer Conditions

2 Homogeneous Transition-Metal Catalyzed Reactions Under Phase-Transfer Conditions 

Palladium(II) chloride in conjuction with PEG-400 smoothly catalyzes the biphasic reduction of arylalkynes with sodium borohydride [18]. The reaction does not occur in the absence of PdCl2. In contrast to LiAlH, (a well-known reagent for the 

The anionic, water-soluble cobalt complex K3[Co(CN)5]H, under PTC conditions, effectively catalyzes the hydrogenation of dienes to monoenes [26-28]. In most cases 1,4-addition prevails, producing tran -alkenes in high yield. The catalytic pair Co /Q X also accelerates the biphasic hydrogenation of styrene to ethylbenzene [29]. 

In a series of papers [30-35], Blum and co-workers reported the hydrogenation of alkenes, alkynes, and arenes in the presence of a hydrated ion-pair [0ct3NMe] [RhCl4(H20)n] . The quaternary ammonium tetrachlororhodate also catalyzes HTR of alkynes, alkenes and a,j6-unsaturated carbonyl compounds with polymethylhydrosiloxane [36, 37]. Recently, heterogenized and therefore recyclable quaternary ammonium halometallates, namely glass-encapsulated catalysts [38] and polymer-bound tetrachlororhodate [39], have been described. Both types of the insoluble catalysts effectively promote various processes including the hydrogenation of alkenes and the HTR of a,/9-unsaturated carbonyl compounds [40]. 

A remarkably mild biphasic hydrogenation of aromatics to cyclohexanes has been carried out using a Rh complex and a PT agent (eq. (1)), [41]. 2-Fthylfuran, 2-methylpyridine, and quinoline are also easily reduced, the latter only undergoing hydrogenation of the heterocyclic ring [42]. 

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