Recent Studies using Transition Metal Catalysts

Recent Studies using Transition Metal Catalysts 

The advantage of proceeding exclusively via pathway (2) is the absence of ether linkages in the resultant copolymer, leading to improvements in its physical properties. 

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Recent Studies using Transition Metal Catalysts... 

The oxidation of alcohols is an important reaction in organic chemistry. While this transformation is traditionally performed in organic solvents, the use of aqueous orgarric solutions has just recently become a field of intense study (1-6). The effect of water on transition metal-catalyzed reactions, however, remains widely unexplored as most of these reactions require dry organic solvents to avoid decomposition of the transition metal catalyst, of water sensitive reagents, and/or intermediates by a nucleophilic attack of water (1). Comparative studies focusing on the effect of water as a co-solvent on the catalyst and the proceedings of a reaction are therefore rare (7). 

As discussed in Section 6.9 1, 3-dienes and dienophiles in which multiple bonds are not activated by electron-withdrawing or electron-releasing substituents fail to undergo cycloaddition except under the most severe conditions. Particular difficulty is encountered in the cycloaddition of two unactivated species since homodimerization can be a competitive and dominant reaction pathway. The use of transition-metal catalysts, however, has proved to be a valuable solution. Complexation of unactivated substrates to such catalysts promotes both inter- and intramolecular cycloadditions. Consequently, the cycloaddition of such unactivated compounds, that is, simple unsubstituted dienes and alkenes, catalyzed by transition metals is a major, important area of study.655 In addition, theoretical problems of the transformation have frequently been addressed in the more recent literature. 

Whereas most carbonylation studies have focused on the use of transition metal catalysts, one recent report describes the facile synthesis of 4-hydroxycoumarins using sulfur-assisted carbonylation (equation 50). Yields are good for a variety of substrates while reaction conditions are relatively mild. 

Another important issue in olefin polymerization is copolymerization of different types of monomers. If one can freely produce copolymers of non-polar and polar monomers, which are difficult to copolymerize with conventional initiators, it would provide useful polymer materials. The Ziegler type catalysts using trialkylaluminum is not suitable for polymerizing polar monomers, whereas late transition metal catalysts are more tolerant of polar monomers. Recently catalysts using late transition metal catalysts have been intensively studied [89]. Because of the obvious importance of these polymeric materials in industrial use, further studies are expected on the applicability of late transition metal complexes for polymerization. 

In 2010, the groups of Charette and Lei independently reported an iron/diamine-catalyzed reaction of aryl iodides or bromides with a solvent amount of arene at 80-90°C (Eqs. 24 and 25) [60, 61]. A mixture of ortho-, meta-, and para-isomers was obtained when substituted arenes were used as the substrate, the ort/jo-isomer being the major product. The Charette group reported a KIE value of 1.04, while Lei group measured a KIE of 1.7. Based also on reaction inhibition by a radical scavenger, Charette suggested that radical processes are involved. Recent studies have revealed that cross-coupling of an aryl halide with an arene can proceed in the absence of a transition metal catalyst ([62] and references therein). 

More recent studies included the low-temperature phenylytterbium iodide-induced cross-coupling with organic halides in the presence of transition metal catalysts (Yokoo et al., 1984a) and the use of methylytterbium iodide to convert active hydrogen compounds such as phenylacetylene or fluorene to corresponding acids or alcohols (Yokoo et al., 1984b). 

This chapter describes recent advances in transition metal-catalyzed crosscouplings involving the cleavage of C(sp )-0 bonds in non-sulfonylated phenol and enol derivatives. The chapter is divided into three main sections The first details the most intensively studied nickel-based catalyst system. The second deals with C-O bond transformations using metals other than nickel. Finally, the last section describes several examples that exemplify the potential utility of these classes of cross-coupling reactions. 

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