Cascade catalytic reactions ruthenium catalysts

Cascade Catalytic Reactions Involving Ruthenium Catalysts. 296... 

Recently, a catalytic system consisting of a second generation Grubbs catalyst or an in situ non-carbenic ruthenium complex have allowed a cascade catalytic reaction of cyclopropanation/ring closing metathesis of dienynes containing a malonate or bissulfone moiety. In this reaction, the interaction between the triple bond and one double bond gives a bicyclic product via cyclopropanation, and then the subsequent diene RCM produces the last cyclization step [16] (Scheme 6). 

Abstract Ruthenium holds a prominent position among the efficient transition metals involved in catalytic processes. Molecular ruthenium catalysts are able to perform unique transformations based on a variety of reaction mechanisms. They arise from easy to make complexes with versatile catalytic properties, and are ideal precursors for the performance of successive chemical transformations and catalytic reactions. This review provides examples of catalytic cascade reactions and sequential transformations initiated by ruthenium precursors present from the outset of the reaction and involving a common mechanism, such as in alkene metathesis, or in which the compound formed during the first step is used as a substrate for the second ruthenium-catalyzed reaction. Multimetallic sequential catalytic transformations promoted by ruthenium complexes first, and then by another metal precursor will also be illustrated. 

During the last decade, molecular ruthenium catalysts have attracted increasing interest for organic synthesis due to their ability to perform specific new reactions with a large panel of applications. Beside individual catalytic transformations, a variety of multi-step catalytic transformations in one pot have appeared. These transformations present practical and economic advantages as far as they are efficient, selective and proceed with atom economy. Ruthenium catalysis has entered this field with a variety of cascade and sequential catalytic transformations. 

In the past decade, chemists have become more and more interested in ruthenium catalysts for organic synthesis, since ruthenium catalysts are able to perform specific new reactions with a large number of applications [54]. A variety of cascade reactions and sequential catalytic transformations have been developed based on this powerful catalyst system, which can promote several different types of reactions. 

A very powerful cascade reaction had been developed by Cho and Lee in their approach to the total synthesis of (3I ,9i ,10/ )-Panaxytriol 179 (Scheme 7.37) [81], which was isolated from Panax ginseng in 1983 [82]. The cascade sequence was initiated by relay metathesis, which is then followed by metallotropic [l,3]-shift and cross-metathesis. This approach has become an efficient way for the synthesis of natural products with highly unsaturated carbon skeletons. Treatment of 174 with Grubbs second-generation catalyst in CH Clj at 40 °C in the presence of 2.0 equiv of alkene 175 generated the expected prodnet 178 in 61% yield as a mixture of Z E-isomers. Surprisingly, ruthenium alkylidene 176 was isolated in 10% yield and could be converted to 178 upon treatment with 175. This confirms that complex 176 is a catalytically viable intermediate in the catalytic cycle. 

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