Coupled heterogeneously catalyzed hydrogenation

Hydride complexes of platinum have received considerable study since the preparation of PtHCl(PEt3)2- Spectroscopic studies by NMR techniques have been widely used because of the structural information which can be obtained from coupling constant data to Pt and other nuclei. Platinum is widely used as a heterogeneous catalyst, and vibrational studies on platinum hydride complexes have been useful for comparison of a hydrogen atom bonded to a single platinum with that bonded to a surface. Complexes of platinum have been used to catalyze hydrogenation, hydrosilylation and isomerization reactions with alkenes and alkynes, as well as H/D exchange reactions on alkanes. Hydride complexes are frequently proposed as intermediates in these reactions, and the pathways related to the known chemistry of hydride complexes. 

Liquid phase hydrogenation catalyzed by Pd/C is a heterogeneous reaction occurring at the interface between the solid catalyst and the liquid. In our one-pot process, the hydrogenation was initiated after aldehyde A and the Schiff s base reached equilibrium conditions (A B). There are three catalytic reactions A => D, B => C, and C => E, that occur simultaneously on the catalyst surface. Selectivity and catalytic activity are influenced by the ability to transfer reactants to the active sites and the optimum hydrogen-to-reactant surface coverage. The Langmuir-Hinshelwood kinetic approach is coupled with the quasi-equilibrium and the two-step cycle concepts to model the reaction scheme (1,2,3). Both A and B are adsorbed initially on the surface of the catalyst. Expressions for the elementary surface reactions may be written as follows ... 

The Pd )-catalyzed reaction of aryl diazonium salts with mono-substituted alkenes [1] was found to be an interesting alternative to the well-known Pd - catalyzed arylhalide alkene coupling (Heck type reaction) or the copper mediated reaction of aryl diazonium salts with alkenes (Meerwein arylation) [2], The reaction can be run without isolation of the diazonium salt in presence of only 0.5 to 1 mol% of the Palladium catalyst in a one pot procedure, in high yield and under nuld conditions. The resulting styrene is reduced in a subsequent hydrogenation step with an in situ generated heterogeneous Pd-catalyst. The combination of three reaction steps without isolation of intermediates and the virtually complete recovery of the Pd-metal at the end of the reaction sequence makes this process [4] extremely efficient. 

Irrespective of the phase system (homogeneous, aqueous biphasic, heterogeneous single-site), the hydrogenation mechanisms of thiophene (T) or BT catalyzed by metal complexes comprise the usual steps of H2 oxidative addition, f-C,C coordination of the substrate, hydride transfer to form dihydrobenzothienyl, and elimination of DHBT by hydride/dihydrobenzothienyl reductive coupling (Scheme 2) [8],... 

To present, silica-supported rhodium catalysts have been successfully used for hydrogenation, hydroformylation, and hydrosilylation reactions. Zhang and coworkers" developed a heterogeneous rhodium complexes 23 catalyzed carbon-heteroatom bond formation. The reaction couples disulfides 21 or diselenides with an alkyl or acyl halide to generate unsymmetrical sulfides (24) and selenides in good yields. The catalyst could be easily recovered and recycled by filtration of the reaction solution and re-used for five cycles without significant loss of activity (maintains over 90% yield)." ... 

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