Epoxides transition metal catalysts

Physical and Chemical Properties. The (F)- and (Z)-isomers of cinnamaldehyde are both known. (F)-Cinnamaldehyde [14371-10-9] is generally produced commercially and its properties are given in Table 2. Cinnamaldehyde undergoes reactions that are typical of an a,P-unsaturated aromatic aldehyde. Slow oxidation to cinnamic acid is observed upon exposure to air. This process can be accelerated in the presence of transition-metal catalysts such as cobalt acetate (28). Under more vigorous conditions with either nitric or chromic acid, cleavage at the double bond occurs to afford benzoic acid. Epoxidation of cinnamaldehyde via a conjugate addition mechanism is observed upon treatment with a salt of /-butyl hydroperoxide (29). 

Chiral epoxides and their corresponding vicinal diols are very important intermediates in asymmetric synthesis [163]. Chiral nonracemic epoxides can be obtained through asymmetric epoxidation using either chemical catalysts [164] or enzymes [165-167]. Biocatalytic epoxidations require sophisticated techniques and have thus far found limited application. An alternative approach is the asymmetric hydrolysis of racemic or meso-epoxides using transition-metal catalysts [168] or biocatalysts [169-174]. Epoxide hydrolases (EHs) (EC 3.3.2.3) catalyze the conversion of epoxides to their corresponding vicinal diols. EHs are cofactor-independent enzymes that are almost ubiquitous in nature. They are usually employed as whole cells or crude... 

R)-epoxide. It is noteworthy that this stereoselective reaction cannot be accomplished with transition-metal catalysts. 

A variety of reactions have been conducted. Catalysts based on noble metals on Deloxan amino poly siloxane supports have been used. Hitzler et al. (1998) have reported alkylation of mesitylene with propylene or wopropanol in SC propylene or CO2 using a solid acid Deloxan catalyst. Pesiri et al. (1998) have carried out selective epoxidation in SC CO2 with transition metal catalysts (V, Ti, Mo) and tert-BHPO high conversion and selectivity have been reported. 

Silylformylation, defined as the addition of RsSi- and -CHO across various types of bonds using a silane R3SiH, CO, and a transition metal catalyst, was discovered by Murai and co-workers, who developed the Co2(CO)8-catalyzed silylformylation of aldehydes, epoxides, and cyclic ethers [26]. More recently, as described in detail in Section 5.3.1, below, alkynes and alkenes have been successfully developed as silylformylation substrates. These reactions represent a powerful variation on hydroformylation, in that a C-Si bond is produced instead of a C-H bond. Given that C-Si groups are subject to, among other reactions, oxidation to C-OH groups, silylformylation could represent an oxidative carbonylation of the type described in Scheme 5.1. 

Aiming at easier workup conditions, immobilization of several transition metal catalysts, which show activity for the epoxidation of allylic alcohols, on polymer support has been investigated. For example, Suzuki and coworkers incorporated an oxo-vanadium ion into cross-linked polystyrene resins functionalized with iminodiacetic acid or diethylenetri-amine derivatives (Scheme 57), which afforded a heterogeneous catalyst that can promote... 

L Ti, Zr, Hf. Characteristic for group IV transition metal catalysts for epoxidation reactions is the intermediate formation of a mono- or bidentate coordinated alkyl hydroperoxide, hydrogen peroxide or a bidentate coordinated peroxo group in the catalytically active species. 

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)... 

The epoxidation of enones using chiral phase transfer catalysis (PTC) is an emerging technology that does not use transition metal catalysts. Lygo and To described the use of anthracenylmethyl derivatives of a cinchona alkaloid that are capable of catalyzing the epoxidation of enones with remarkable levels of asymmetric control and a one pot method for oxidation of the aUyl alcohol directly into... 

About 50% of the current worldwide propylene oxide capacity is based on a newer process called hydroperoxide (coproduct or oxirene) route (Halcon-Arco technology919,920). According to this process, hydroperoxides synthesized by the oxidation of certain hydrocarbons, such as ethylbenzene or isobutane, are used for epoxidation of propylene in the presence of a transition-metal catalyst ... 

Oxygen may also be used as the oxidant for epoxidation. For example, a selective epoxidation, in the absence of metal ions, has been performed in excellent yield by treatment of alkenes with oxygen in the presence of a large excess of benzaldehyde36. Photooxygenation of alkenes, in the presence of transition metal catalysts, is a very general and synthetically useful reaction which leads to hydroxy-epoxidation (equation 4)37-39. 

The heading substitution reactions has been used to describe the conversion of a stereogenic center to another. Of course, this means that the substrate stereogenic center has had to be obtained by one of the reaction types outlined earlier, from the chiral pool, or by resolution. Reactions that fall into this category include epoxide and cyclic sulfate openings and iodolactonizations (Chapter 22). Perhaps the most important reaction of this type for asymmetric synthesis is allylic substitution in the presence of a transition metal catalyst. 

Cyclic sulfates provide a useful alternative to epoxides now that it is viable to produce a chiral diol from an alkene. These cyclic compounds are prepared by reaction of the diol with thionyl chloride, followed by ruthenium-catalyzed oxidation of the sulfur (Scheme 9.26).166 This oxidation has the advantage over previous procedures because it only uses a small amount of the transition metal catalyst.167168... 

The first catalytic asymmetric cyclopropanation using an ylide as catalyst was reported by Aggarwal et al. in 1997 [95, 96]. Phenyl diazomethane was added slowly to a mixture containing sulfide 12, an enone and Rh2(OAc)4 (1 mol%). A sulfur ylide was generated in situ from the sulfide and phenyl diazomethane in the presence of the transition-metal catalyst (see Scheme 10.20), as in the epoxidations discussed earlier (see Section 10.2.1.3). 

Hie first of Sharpless s reactions is an oxidation of alkenes by asymmetric epoxidation. You met vanadium as a transition-metal catalyst for epoxidation with r-butyl hydroperoxide in Chapter 33, and this new reaction makes use of titanium, as titanium tetraisopropoxide, Ti(OiPr)4, to do the same thing. Sharpless surmised that, by adding a chiral ligand to the titanium catalyst, he might be able to make the reaction asymmetric. The ligand that works best is diethyl tartrate, and the reaction shown below is just one of many that demonstrate that this is a remarkably good reaction. 

The activation of oxygen in oxygen transfer reactions is usually mediated by a suitable transition metal catalyst which has to be sufficiently stable under the reaction conditions needed. But also non-metal catalysts for homogeneous oxidations have recently been of broad interest and several of them have been compiled in a recent review.2 Other examples for well known alkene oxidation reactions are the ozonolysis, hydroboration reactions or all biological processes, where oxygen is activated and transferred to the substrate. Examples for these reactions might be cytochrome P450 or other oxotransferases. Of these reactions, this contribution will focus on transition-metal mediated epoxidation and dihydroxylation. 

Various strategies have been pursued in order to immobilise chiral epoxidation catalysts and these encompass covalent attachment to solid supports,[41] steric occlusion in nanosized cages of zeolites,[42 44] entrapment in a polydimethylsiloxane membrane145,461 and fluorous biphasic systems.1471 However, these approaches frequently require tedious ligand modifications and often lead to a marked decrease in both selectivity and activity of the transition metal catalyst. 

The exceptions are epoxidations using less common transition metal catalysts which involve free radicals1 -14. epoxidations by Michael addition forming free carbanions (Section 4.5.1.3.), or, within the halohydrin route (Section 4.5. (.1.1.), in processes which involve free carbenium ions. 

The compehtion of one-electron pathways is sometimes detectable in the epoxidations catalyzed by transition metal catalysts [67]. However, in the epoxidahon of unhindered olefins on TS-1, the typical radical products are below the detection limits. Their presence could no longer be neglected when the rate of epoxidation is so slow as to become comparable to that of homolytic side reactions, for example with bulky olefins (see also Section 18.11). It is possible that, within these limits only, the epoxide is produced in part through the addition of a radical peroxy intermediate to the double bond [68, 69]. Even so, a homolytic pathway has again been proposed as a generally vahd epoxidation mechanism [7]. 

Epoxides can isomerize under the influence of transition metal catalysts. This formal 1,2-hydride shift is a method to prepare unsaturated carbonyl compounds from epoxides (Equation 54) <1998T1361>. This method has been extended as a double epoxide isomerization-intramolecular aldol condensation (Equation 55) <1996JOC7656, 1998TL3107>. m-Epoxides are isomerized to /ra r-epoxides under ruthenium catalysis <2003TL3143>. 

The alkynyl epoxide 52 cycloisomerized to a furan in the presence of a group VI transition metal catalyst <1994JA9363>. The analogous reaction with alkynyl thiirane 53 was also observed although thermal extrusion of sulfur was the major competing side reaction (Scheme 19) <2000S970>. 

Epoxidation of allylic alcohols with peracids or hydroperoxide such as f-BuOaH in the presence of a transition metal catalyst is a useful procedure for the synthesis of epoxides, particularly stereoselective synthesis [587-590]. As the transition metal catalyst, molybdenum and vanadium complexes are well studied and, accordingly, are the most popular [587-590], (Achiral) titanium compounds are also known to effect this transformation, and result in stereoselectivity different from that of the aforementioned Mo- and V-derived catalysts. The stereochemistry of epoxidation by these methods has been compared for representative examples, including simple [591] and more complex trcMs-disubstituted, rrans-trisubstituted, and cis-trisubstituted allyl alcohols (Eqs (253) [592], (254) [592-594], and (255) [593]). In particular the epoxidation of trisubstituted allyl alcohols shown in Eqs (254) and (255) highlights the complementary use of the titanium-based method and other methods. More results from titanium-catalyzed diastereoselective epoxidation are summarized in Table 25. 

With chiral ligands, the transition-metal catalyst-hydroperoxide complex yields optically active oxiranes. " One of the most significant advances in the formation of chiral epoxides from allyl alcohols has recently been reported by the Sharpless group. Using (-l-)-tartaric acid, ferf-butylhydroperoxide, and titanium isopropoxide, they were able to obtain chiral epoxides in very high enantiomeric excess. The enantiomeric epoxide can be obtained by employing (—)-tartaric acid (Eq. 33a). 

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