Survey of Catalysts

With the exceptions of a few rhodium systems (see following), the catalytic pyridine-synthesis relies exclusively on cobalt as the active metal. The reaction can be carried out advantageously in a one-pot reaction by generating the cobalt catalysts in situ [Eq.(2)] (74GEP2416295, 74S575 75USP4006149). 

We (79TH1 81GEP3117363 84USP4588815) and others (87MI1) have studied acetylacetonato and rj -cp-rhodium complexes as catalysts in the pyridine formation [Eq.(l)]. Resin-attached cp-rhodium complexes are also active in the cocyclization of alkynes and nitriles, and the activity is 

Comparison of the different types of cobalt catalysts shows that the in situ system [Eq.(2)] is most accessible while the Rep-, R(ind)-, and bori-ninato ligands having electron-withdrawing substitutents are the most active. The difference between the 14e and the 12e core complexes makes itself apparent in the chemoselectivity of the reaction. Catalysts containing a 14-electron core favor pyridine formation, whereas those containing a 12-electron core (i.e., the rj -allylcobalt systems) favor the formation of benzene derivatives by cyclotrimerization of the alkynes. For example, in the reaction of propyne and propionitrile at 140°C in the presence of a 12-electron system (5), a 2 1 ratio of benzene to pyridine product is formed, whereas a catalyst containing the cpCo moiety (a 14-electron system) leads (under identical conditions) to the predominant formation of pyridine derivatives (84HCA1616). 

The cobalt-catalyzed pyridine synthesis is the only known one-step process for the selective preparation of the industrially significant 2-substituted-pyridine derivatives. Moreover, the method is applicable to a broad variety of substituted alkynes and nitriles, thereby giving access to a whole range of pyridine derivatives having 1,2,3 or 5 substituents in the ring. Selected examples follow and are compared to the prior state of the art. 

Nippon Steel has developed an interesting liquid-phase process for producing 2-methylpyridine from ethylene and ammonia (74MI1 81MI2, 81MI3). The catalyst is reminiscent of the well-known Wacker process, viz. Pd /Cu redox system [Eq.(5)]. 

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In 1942 the resolution of the microscope in the hands of James Hillier of the RCA Laboratories was 20 A. Now in the hands of Joseph H. Wall of the Brookhaven National Laboratory it is 2.5 A permitting visualization of the individual platinum atoms. A survey of catalysts made with the electron microscope in 1942(95) showed a diversity of size, shape and texture of catalytic substances. Many of the precious metals were large and consequently not very efficient—only a very small fraction of the atoms were available for surface reactions. However many of them were of colloidal size,(96) i.e. of one dimension at most of 2000A. The usual method of making the catalyst was to soak the support with a solution of the salt of the precious metal and then subject it to thermal treatment. The complex topoche-mical reactions that take place are difficult to control to obtain monodisperse particles of optimum size. Two questions arose in the 40 s and 50 s. What is the dependence of catalytic activity on particle size Is there a particle size below which there is no catalytic activity It was proposed to synthesize the metal particles in solution in colloidal form check their properties, both physical and chemical in solution then mount them on a suitable support to study their activity in heterogeneous catalytic reactions. However, the colloidal chemistry of platinum and palladium was complex, poorly understood and difficult to reproduce. 

From this survey of catalyst development for micro fuel processors, it can be seen that achieving targeted performances of MSRs requires investigating relationships... 

A survey of catalysts reveals the Ugandless palladium(O) source, Pd(dba)2 or Pd2(dba)3, to be superior to other palladium sources. The reactions are remarkable for the extremely mild conditions employed (about 10 min at ambient temperature)... 

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