Transition metal catalysts cascade reactions

The Borstar process involves the use of two cascaded reactors. In the first stage, ethylene is polymerized in supercritical propane by the addition of a transition metal catalyst in a loop reactor, which leads to low-molecular-weight polyethylene. The reaction mixture is then transferred into a gas-phase reactor in which high-molecular-weight polymers are formed. The direct result of this two-stage process is an intimate mixing of the two polymer fractions, which differ in their molar masses. 

A review of the reactions of propargylic esters and phosphates with transition metal catalysts indicates that r-acidic metals, mainly gold and platinum salts, promote 1,2- or 1,3-acyloxy and phosphatyloxy migration to give reactive intermediates which undergo cascade reactions to a wide range of functionalized products (Scheme 134). 

In this chapter we described [2 + 2 + 2] and related cycloaddition reactions using palladium, iron, manganese, rhenium, and other transition metals. Palladium complexes are able to catalyze [2 + 2 + 2] and related cycloaddition reactions, which proceed via cascade-type mechanism or metallacycle intermediates. It is worthy of note that arynes are suitable substrates for this palladium catalysis. Iron complexes are promising catalysts for practical [2 + 2 + 2] cycloaddition reactions, owing to their low cost and nontoxicity, although both catalytic activity and substrate scope are not satisfactory. Manganese and rhenium complexes allow the use of 3-keto esters as a cycloaddition partner. To realize the practical process and broaden the product scope, further development of new transition-metal catalysts is expected in this research field. 

Coumarin derivatives have also been synthesized by a cascade reaction of phenols and 1-alkynecarboxylic acid esters in the presence of transition-metal catalysts such as Pd [27], Pt [28], and Fe [29], as shown in Scheme 18.28. For example, Pd(OAc)2-catalyzed reaction of sesamol and ethyl phenylpropiolate in TFA gave a coumarin product in 93% yield. The reaction proceeds via intramolecular hydroarylation followed by intramolecular transesteriflcation. 

As described in this chapter, transition-metal catalysts promote various types of cyclization reactions between C=N, C=0, N—H, O—H, and S—H bonds and alkynes in 5/6-endo/exo-dig manners. These reaction modes provide facile and atom-economical pathways to aromatic compounds such as pyrroles, indoles, isoquinolines, quinolines, furans, thiophenes, oxazoles, pyrones, and isoquinolones and their aza analogs and fused-ring congeners. Particularly notable is their utility in cascade reactions, which are step-economical approaches to target molecules, which increase rapidly in structural complexity. Therefore, these reactions can help provide solutions to meet the increasing demands of environmentally benign synthesis in modern organic chemistry. 

Significant progress has been made in the fields of ruthenium-, iron-, iridium-, rhodium-, and copper-catalyzed cascade reactions. Noticeably, these transition metal catalysts are critically important in numerous commercial chemical processes. Discoveries of new cascade processes along with improvements in the activity, selectivity, and scope of these catalysts could drastically reduce the environmental impact and increase the sustainability of chemical reactions. From the viewpoint of practical applications, iron and copper are the most abundant metals on Earth and, consequently, inexpensive and environmentally friendly. Moreover, many iron and copper salts and complexes are commercially available or are described in the literature. Due to these advantages, the development and applications of iron- and copper-catalyzed cascade reactions are becoming a thriving area of organic synthesis chemistry. 

Carbonylation of unsaturated substrates has been known for decades but the reaction selectivity has been progressively improved by tuning the coordination sphere of late transition metal-based catalysts. Palladium assumes a privileged place in this chemistry and its versatility allows the use of mild conditions for the selective incorporation of CO into acyclic and cyclic compounds. Further improvements open a path to more sophisticated reactions, particularly cascade reactions. Similarly, asymmetric versions of most of these carbonylations can be envisioned. Atom economy and the green character of the process will probably be the key criteria for evaluating any new catalytic system. 

Industrial polymerisation processes with the use of titanium-, cobalt- and nickel-based aluminium alkyl-activated Ziegler-Natta catalysts, which are employed for the manufacture of cis- 1,4-poly butadiene, involve a solution polymerisation in low-boiling aromatic hydrocarbons such as toluene or in a mixture of aromatic and aliphatic hydrocarbons such as n-heptane or cyclohexane. The polymerisation is carried out in an anhydrous hydrocarbon solvent system. The proper ratio of butadiene monomer and solvent is blended and then completely dried in the tower, followed by molecular sieves. The alkyla-luminium activator is added, the mixture is agitated and then the transition metal precatalyst is introduced. This blend then passes through a series of reactors in a cascade system in which highly exothermic polymerisation occurs. Therefore, the reaction vessels are cooled to slightly below room temperature. 

Abstract This review gives an insight into the growing field of transition metal-catalyzed cascades. More particularly, we have focused on the construction of complex molecules from acyclic precursors. Several approaches have been devised. We have not covered palladium-mediated cyclizations, multiple Heck reactions, or ruthenium-catalyzed metathesis reactions because they are discussed in others chapters of this book. This manuscript is composed of two main parts. In the first part, we emphasize cascade sequences involving cycloaddition, cycloisomerization, or ene-type reactions. Most of these reaction sequences involve a transition metal-catalyzed step that is either followed by another reaction promoted by the same catalyst or by a purely thermal reaction. A simple change in the temperature of the reaction mixture is often the only technical requirement to go from one step to another. The second part covers the cascades relying on transition metalo carbenoid intermediates, which have recently undergone tremendous... 

In this review, we have shown the major advances in the growing field of cascade chemistry that have led to regio-, chemo-, and stereoselective formation of several new carbon-carbon and carbon-heteroatom bonds in a stepwise economical fashion by using transition metal-catalyzed reactions. These approaches have already allowed very impressive and rapid construction of unnatural and natural polycyclic compounds of very high molecular complexity. These initial achievements should stimulate the synthetic community to pursue further works notably to develop more efficient and selective strategies that involve new generations of versatile catalysts. Next endeavors will have to focus on green processes as well as asymmetric catalysis. 

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. 

Tandem carbonyl ylide generation from the reaction of metallo carbenoids with carbonyl continues to be of great interest both mechanistically and synthetically. Effective carbonyl ylide formation in transition metal catalyzed reactions of diazo compounds depends on the catalyst, the diazo species, the nature of the interacting carbonyl group and competition with other processes. The many structurally diverse and highly successful examples of tetrahydrofuran formation cited in this mini-review clearly indicate that the tandem cyclization/cycloaddition cascade of metallo carbenoids has evolved as an important strategy in both carbo- and heterocyclic synthesis. 

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