Transition metal complexes epoxidation catalysis

The properties of siloxide as ancillary ligand in the system TM-O-SiRs can be effectively utilized in molecular catalysis, but predominantly by early transition metal complexes. Mono- and di-substituted branched siloxy ligands (e.g., incompletely condensed silsesquioxanes) have been employed as more advanced models of the silanol sites on silica surface for catalytically active centers of early TM (Ti, W, V) that could be effectively used in polymerization [5], metathesis [6] and epoxidation [7] of alkenes as well as dehydrogenative coupling of silanes [8]. 

Coordination catalysis via alkyl hydroperoxides is well documented (4, 31). Selective oxidations of olefins to epoxides (Reaction 16), using especially Group IV, V, and VI transition-metal complexes, can occur possibly via oxygen-transfer processes of the type... 

EPOXIDATIONS VIA CATALYSIS BY FIRST-ROW TRANSITION METAL COMPLEXES... 

The study of mixed-ligand 0x0 derivatives is closely related to the use of these species in catalytic oxidation systems (including the epoxidation of aUcenes and the oxidation of alkanes. See Oxidation Catalysis by Transition Metal Complexes). In such complexes, the 0x0 group can be terminal, doubly bridging, or triply bridging. 

One of the most exciting developments in asymmetric catalysis over the past 25 years has been the discovery of transition metal complexes that catalyze the oxidation of alkenes to chiral epoxides and 1,2-diols. Equations 12.16, 12.17, and 12.18 show examples of epoxidation and 1,2-dihydroxylation. 

A. Brunetta, G. Strukul, Epoxidation versus Baeyer-Villiger oxidation The possible role of Lewis acidity in tire conrol of selectivity in catalysis by transition metal complexes, Eur. ]. Inorg. Chem. 5 (2004) 1030. 

Especially noteworthy is the field of asymmetric catalysis. Asymmetric catalytic reactions with transition metal complexes are used advantageously for hydrogenation, cyclization, codimerization, alkylation, epoxidation, hydroformylation, hydroesterification, hydrosilylation, hydrocyanation, and isomerization. In many cases, even higher regio- and stereoselectivities are required. Fundamental investigations of the mechanism of chirality transfer are also of interest. New chiral ligands that are suitable for catalytic processes are needed. 

This year has again emphasized the growing importance of organo-transition metal complexes in organic synthesis. In catalysed reactions the major advances have been in asymmetric catalysis with the first reports of chiral induction in catalytic epoxidation and further reports on improved catalysts for asymmetric hydrogenation and allylic alkylation. The formation of carbon-carbon bonds continues to attract attention, and several novel and potentially useful synthetic applications of organometallic complexes have been reported. 

The types of reactions that can be catalyzed by transition metal complexes are now very numerous and are very widely used in synthesis. We have already met a number of them—osmium in catalysis of dihydroxylation reactions, titanium in Sharpless epoxidation, various metals in hydrogenation reactions of alkenes, and the Ziegler-Natta process for polymerization. In this section, we will just highlight a few types that have been popular—an oxidation, some hydrogenations, and some coupling reactions. Although outline reaction mechanisms will be given, this is for interest only—they are beyond the scope of this text, and many are more complicated than is shown here. 

The coordination properties of phosphine oxides has been explored with late transition-metal (Ru, Co, Rh, Ir, Pd, Pt, Cu, and Au),301 303 305 306 310 316 early transition-metal,317 lanthanide,304,318,319 and actinide307,320 ions. One interesting complex is the palladium(II) complex (148) (Scheme 10) which is an extremely rare example of a ds metal center with a tetrahedral geometry.313 Phosphine oxides have found uses in the extraction of alkali, alkaline earth, and actinide metals in catalysis (hydroformylation of alkenes and epoxides, carbonylation of methanol324) and as a useful crystallization aid (Ph3PO).325... 

Taken together, these results for epoxide opening promoted by chiral metal complexes [21] illustrate how much there is find out about asymmetric catalysis by transition metals. These valuable new processes are further proof, if any is required, of the rewards of searching for new reactivity amongst the transition metals. 

The epoxidation of olefins was discovered by a Russian chemist, N. Prileschajew, in 1909 using peracids RCO3H, and this reaction has been used for a long time. Per-acids often are dangerous (explosive), however, and must therefore imperatively be avoided. Transition-metal catalysis is now used together with hydroperoxides ROOH (most often R = t-Bu, in short TBHP) whose first example was discovered by Hawkins in 1950 with [V2OS] as catalyst. The catalysts most often are d° complexes with an oxophilic Lewis acid able to bind an oxygen atom of hydroperoxide. 

Abstract This chapter highlights the most recent and representative results on the use of organoaluminum compounds in polymerization catalysis with a special emphasis on discrete Al-incorporating catalysts. The first part of this contribution summarizes recent and noteworthy developments on well-defined Al-based initiators for the controlled (and stereocontrolled) polymerization of various monomers including isobutene, styrene, epoxides, methyl methacrylate, cyclic esters, and cyclic carbonates. The second part discusses the latest significant advances on the synthesis and structural characterization of polynuclear organoaluminum/transition (and f-block) metal complexes relevant to Ziegler-Natta-type catalysis. 

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