Ruthenium catalysis mechanistic studies

Besides copper, rhodium, and ruthenium were able to perform similar chemistry (119). However, silver is an improvement versus these metals due to its lower toxicity to humans and cheaper price. More work is needed in order to understand these intriguing silver catalysis systems, specifically mechanistic studies. 

For a discussion of mechanistic studies of Ru-alkylidene catalysis of metathesis, see M. S. Sanford and J. A. Love, Mechanism of Ruthenium-Catalyzed Olefin Metathesis Reactions, in Handbook of Metathesis, Vol. 1, R. H. Grubbs, Ed., Wiley-VCH Weinheim, Germany, 2003, pp. 112-131. 

Two mechanistic pathways, which differed in the way of ruthenium-mediated initial cleavage of formyl C-H or amido N-H bond, were proposed for the catalytic cycle. As shown in Scheme 7.3, an irreversibly cleavage of formyl C-H bond by the active ruthenium complex was followed by reversible insertion of the olefin into the Ru-H bond, which afforded either six-membered or seven-membered ruthenacycle. After reductive elimination, indolin-2-ones or 3,4-dihydroquinolin-2-one was formed. According to isotopic studies, pathway leading to six-membered lactams is postulated to be less favored. Another cyclization process initiated by Ru-catalyzed oxidative addition of formyl N-H bond (Scheme 7.4) was similar to Carreira s proposal for their hydrocarbamoyla-tion reaction of allylic formamides under similar ruthenium catalysis conditions [7]. The 6-endo cyclization process is proposed to be favored under the catalytic system B. 

During the past few decades, a wide variety of molecules with transition metal-carhon mulhple bonds have been studied. The chemistry of doubly bonded species - carbenes - is particularly interesting because it leads to several synthetically important transformations, and for this reason, metal carbenes are the main subject of this chapter. Our discussion begins with a classification of metal-carbene complexes based on electronic structure, which provides a way to understand their reactivity patterns. Next, we summarize the mechanistic highlights of three metal-carbene-mediated reactions carbonyl olefinafion, olefin cyclopropanafion, and olefin metathesis. Throughout the second half of the chapter, we focus mainly on ruthenium-carbene olefin metathesis catalysts, in part because of widespread interest in the applications of these catalysts, and in part because of our expertise in this area. We conclude with some perspectives on the chemistry of metal carbenes and on future developments in catalysis. 

In this chapter, we have highlighted the important new developments within computational chemistry over the last decade or so, which have enabled near-quantitative description of transition metal-catalyzed reactions. We have also introduced some of the most popular experimental mechanistic tools such as determination of KlEs and Hammett substituent effects. The level of accuracy that can be expected is highlighted through a series of case studies performed over the last 5 years and includes catalysis performed by rhodium, iridium, palladium, and ruthenium. The combined use of experimental and theoretical methods have in all cases led to additional insights that would not have been possible using either of the two separately. The inclusion of experimental results... 

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