Epoxidation late transition metal

Finally, the structural characteristics of the 1,8-naphthyridines favour their behavior as bidentate and dinucleating ligands, thus leading to short metal-metal separations in late transition-metal complexes,24"26 and to the stabilization of mixed-valence complexes.27,28 The dinuclear ruthenium complex [ Ru(napy)(H20)2 (/i-Cl)(/i-0H)](C104)2 is a stable and active catalyst for the oxidation of alcohols and the epoxidation of alkenes, while its mononuclear precursor is much less active.29... 

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... 

HOMOGENEOUS EPOXIDATION BY LATE TRANSITION METALS (REDOX MECHANISM)... 

The group of Busch also presented evidence that the Mn(n) complex of a cross-bridged cyclam ligand, 4,ll-dimethyl-l,4,8,ll-tetraazabicyclo[6.6.2]hexadecanc, denoted as Mn"(Me2EBC)CL, forms a Mn(IV) adduct with lodosylbenzene which is a new active intermediate in epoxidation reactions [600,609,610]. These examples with Mn, and in the previous section with Fe, are instances of late transition metals catalyzing epoxidation reactions by both the redox and the Lewis acid mechanisms (see Chapter 3). 

Late transition metal ions that can accommodate a two-electron rise in their oxidation state, like Cr(III), Mn(III), and Fe(III), and likely Ru(I), operate by a redox mechanism of epoxidation. They receive an oxygen atom from a TO to form an oxene species (MO) which then transfers the oxygen to an olefin by the intermediacy of a metallacycle, or a radical or cation species. Interestingly, these systems are not inhibited by water or alcohol as are the Lewis acid metals. 

Mid- to Late Transition Metal Catalysts also Perform Epoxidation Reactions by the Lewis Acid Mechanism... 

Key Words Lewis acid adducts, Radical oxidations, Epoxidation, Hydrogen peroxide, Bond dissociation energy, Catalyst durability, Methyltrioxorhenium, Cross-bridged cyclam, Mn(IV), Late transition metal. Propylene oxide. Titanium silicalite (TS-1) catalyst, Ethylanthrahydroquinone/H2 process, Polyoxometallates, Mn(IV) catalyst. Hydrogen abstraction. Rebound mechanism, Isotopic label, t-BuOOH, Peroxide adduct. 2008 Elsevier B.v. 

This listing illustrates the common perspective that "early transition metal ions" perform oxidations by the Lewis acid pathway while "late transition metal ions" produce epoxides by the rebound mechanism. That generality has... 

MID- TO LATE TRANSITION METAL CATALYSTS ALSO PERFORM EPOXIDATION REACTIONS BY THE LEWIS ACID MECHANISM... 

The formation of an intermediate organometaUacycle, a metalla-2,3-dioxo-lane, is a central part of the mechanism proposed in the pioneering work of Mimoun and co-workers (1970) for the olefin epoxidation with Mo diperoxo complexes (Fig. 4). The initial step of the suggested mechanism is the coordination of the olefin with the d° metal. The second step is the formation of the organometaUacycle upon cycloinsertion of the coordinated olefin into a metal-oxygen bond of the metal diperoxo complex. Note that metalla-2,3-dioxolanes of late transition metals are known [73,74]. The final step is the cycloreversion of the organometaUacycle, yielding the oxirane. 

Tosylhydrazone salts could also be deconposed into the corresponding diazo compounds in the presence of a phase transfer catalyst. The addition of late transition metal complexes leads to the formation of metal carbenoid species which undergo various reactions such as cyclopropanation, aziridination, epoxidation, and C-H insertion. While both cyclopropanation and aziridination work equally well in the presence of the sodium and lithium tosylhydrazone salts, it was established that the sodium salt provided higher yields and selectivities in the reaction with aldehydes which led to the formation of epoxides (eq 28). ... 

Peroxometal pathways are typically observed with early transition metal ions with a d configuration, for example, Mo(VI), W(VI), Ti(IV), and Re(VII), which are relatively weak oxidants. Oxometal pathways are characteristic of late transition elements and first row transition elements, for example, Cr(VI), Mn(V), Os(VIII), Ru(VI), and Ru(VIII), which are strong oxidants in high oxidation states. Some metals can operate via both pathways depending, among other things, on the substrate for example, vanadium(V) operates via a peroxometal pathway in alkene epoxidations, but an oxometal pathway is involved in alcohol oxidations [31]. 

The Sharpless and Jacobsen epoxidations both rely on toxic transition metals, which can present problems in pharmaceutical manufacturing. Final APIs are held to rigorous standards for trace metals content, so a late stage asymmetric epoxidation with either of these methods would require purification and analysis to ensure that acceptable standards were met. The... 

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