Epoxidation early transition metals

This chemical bond between the metal and the hydroxyl group of ahyl alcohol has an important effect on stereoselectivity. Asymmetric epoxidation is weU-known. The most stereoselective catalyst is Ti(OR) which is one of the early transition metal compounds and has no 0x0 group (28). Epoxidation of isopropylvinylcarbinol [4798-45-2] (1-isopropylaHyl alcohol) using a combined chiral catalyst of Ti(OR)4 and L-(+)-diethyl tartrate and (CH2)3COOH as the oxidant, stops at 50% conversion, and the erythro threo ratio of the product is 97 3. The reason for the reaction stopping at 50% conversion is that only one enantiomer can react and the unreacted enantiomer is recovered in optically pure form (28). 

Epoxidations of Olefins Catalyzed by Early Transition Metals... 

The phenomenon that early transition metals in combination with alkyl hydroperoxides could participate in olefin epoxidation was discovered in the early 1970s [30, 31]. While m-CPBA was known to oxidize more reactive isolated olefins, it was discovered that allylic alcohols were oxidized to the corresponding epoxides at the same rate or even faster than a simple double bond when Vv or MoVI catalysts were employed in the reaction [Eq. (2)] [30]. 

During the last three decades, peroxo compounds of early transition metals (TMs) in their highest oxidation state, like TiIV, Vv, MoVI, WV1, and Revn, attracted much interest due to their activity in oxygen transfer processes which are important for many chemical and biological applications. Olefin epoxidation is of particular significance since epoxides are key starting compounds for a large variety of chemicals and polymers [1]. Yet, details of the mechanism of olefin epoxidation by TM peroxides are still under discussion. 

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]. 

Allylic alcohols can also be epoxidized with methyltrioxorhenium (MTO). However, in contrast to the early transition metal catalysts, metal-alcoholate binding does not appear to be operative, but rather straightforward hydrogen bonding, as demonstrated by the epoxidation of geraniol (20)... 

Codeposition experiments have shown that vapors the early transition metals Ti, V, and Cr deoxygenate desulfurate organic compounds. Epoxides yield alkenes with these metal atoms. The early transition metals are generally more efficient for these types of reactions. Aminoxides and DMSO undergo deoxygenation, while nitro compounds and nitroarenes yield coupled azo and azoxy products ... 

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... 

High valent early transition metal derivatives are known to catalyze the epoxidation of alkenes by peroxides (e.g., t-butylhydroperoxide) through the formation of active metal-peroxy species, which deliver the metal linked oxygen, acting as an electrophile, to the alkene. More substituted alkenes are then more reactive tlian monosubstituted ones. Hydrogen peroxide may also give the epoxidation by this same mechanism. The formed epoxides are... 

Ti carries out epoxidation with hydrogen peroxide transferring the proximal oxygen atom, like the early transition metals. However, in the case of Ti, the transfer is promoted by protic substances such as alcohols or water, making the use of dilute hydrogen peroxide possible. 

HOMOGENEOUS EPOXIDATION BY EARLY TRANSITION METALS (LEWIS ACID MECHANISM)... 

Early transition metal ions in their highest oxidation states, such as Ti(IV), V(V), W (VI), and Mo(VI), tend to be stable toward changes in their oxidation states. Consequently, in epoxidation reactions with hydrogen peroxide or alkyl hydroperoxides they form adducts (M-OOH and M-OOR) that are the key intermediates in the... 

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... 

The most well-known example is the catalytic epoxidation of olefins with alkyl hydroperoxides that is used for the commercial production of propylene oxide (see earlier). The reaction is catalyzed by high-valent compounds of early transition metals, e.g. Movl, WVI, Vv and TiIV, and involves a peroxometal type mechanism [6,7] as shown (reaction 12). Mo compounds are particularly effective homo-... 

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