Homogeneous synthetic pathways

The international symposium on Recent Developments in Olefin Polymerization Catalysts was held in Tokyo in October 1989. This volume includes 38 i>apers fi"om the 31 lectures and 18 posters presented at the symposium, which covered the following topics Overview of super-active homogeneous and heterogeneous catalysts, kinetic profile of olefin polymerization including copolymerization, characterization of catalysts and polymers, methods for the determination of active center concentration, role of Lewis bases on the catalyst isospecificity, polymerization mechanisms, and synthetic pathways for functionalized polyolefins. We believe the contents are well balanced between fundamental research and application as well as between homogeneous and heterogeneous catalyst systems. 

Mechanistic studies of homogenous chemical reactions involving formation of (P)Rh(R) from (P)Rh and RX demonstrate a radical pathway(9). These studies were carried out under different experimental conditions from those in the electrosynthesis. Thus, the difference between the proposed mechanism using chemical and electrochemical synthetic methods may be due to differences related to the particular investigated alkyl halides in the two different studies or alternatively to the different reaction conditions between the two sets of experiments. However, it should be noted that the electrochemical method for generating the reactive species is under conditions which allow for a greater selectivity and control of the reaction products. 

Butadiene Hydrogenation. Rhodium complexes of the type Rh(diene)(dppe)+, where dppe = 1,2-bis(diphenylphosphino)ethane, are catalyst precursors for overall 1,2 and 1,4 addition of hydrogen to 1,3-butadienes. In these reactions the distribution of terminal and internal olefin products is kinetically regulated by the reaction pathways of a common RhH(R)(dppe)+ intermediate (13). Under homogeneous reaction conditions, the thermodynamically more stable internal olefin products (1,4-addition) are favored over the synthetically more useful terminal olefin products (1,2 addition). However, significant increases in the yield of 1,2 addition products can be achieved by intercalation of the catalyst precursor in hectorite. (14)... 

Allylic substitution is well known and other metal-based complexes have been reported to catalyze reactions with hydroxyl leaving groups/ Indeed, with homogeneous Au-catalysis, a variety of reactions that proceed via cationic pathways have been reported. " While discussion is beyond the scope of this chapter, it is worth noting that the mechanism for this reaction has been extensively studied and does not appear to proceed via an allylic cation/ Further developments in this area have been reported by a variety of groups and now involve the use of nitrogen nucleophiles, carbon nucleophiles, and inter-molecular reactions as well as enantioselective variants/ While many of those reports came from our laboratory, our interests took us in a different direction and we became keen on using modified substrates to develop spiro-ketalization methods for applications in synthetic schemes. 

Though this multistage modification on polymer is performed with improved homogeneity compared to the first one, whose functional heterogeneity had been only very roughly estimated, both of the examples demonstrate that the finally modified support will under no circumstances exhibit functional uniformity. The possibilities of handling a cross-linked polymer like a conventional chemical in a projected pathway of synthetic operations are limited, since each deviation like side reactions and incomplete conversions remains fixed to the insoluble support. In addition, small quantities of functional sites (< 10%) are scarcely detectable by IR spectroscopy and are hard to analyze by conventional methods of organic chemistry. 

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