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Epoxidation to 1,2-diols

Epoxides are cleaved by treatment with acid just as other ethers are, but under much milder conditions because of ring strain. As we saw in Section 7.8, dilute aqueous acid at room temperature is sufficient to cause the hydrolysis of epoxides to 1,2-diols, also called vicinal glycols. (The word vicinal means "adjacent/ and a glycol is a diol.) The epoxide cleavage takes place by SK2-like backside attack of a nucleophile on the protonated epoxide, giving a trans- 1,2-dio) as product. [Pg.662]

Some such pathway is necessary to account for the migration of oxygen that is found. It may involve a protonated epoxide, a 1,2-diol, or simply a 1,2 shift of an OH group. [Pg.1474]

Diols are applied on a multimilhon ton scale as antifreezing agents and polyester monomers (ethylene and propylene glycol) [58]. In addition, they are starting materials for various fine chemicals. Intimately coimected with the epoxidation-hydrolysis process, dihydroxylation of C=C double bonds constitutes a shorter and more atom-efficient route to 1,2-diols. Although considerable advancements in the field of biomimetic nonheme complexes have been achieved in recent years, still osmium complexes remain the most efficient and reliable catalysts for dihydroxylation of olefins (reviews [59]). [Pg.90]

In contrast to Red-Al reductions, DIBAL-H or LiBH4/Ti(OPr1)4 reduction of epoxides yields 1,2-diols as the major products.32 When treated with DI-BAL-H, ratios of 1,3- to 1,2-diol ranging from 1 6 to 1 13 have been observed. [Pg.209]

Cleavage of Si—C bonds (12,243-245). This oxidation can be used to convert vinylsilanes in three steps to syn- or anti-1,2-diols. Thus Grignard reagents cleave epoxides of vinylsilanes selectively to (3-hydroxy silanes, which can be oxidized with retention of configuration to 1,2-diols. When applied to an (E)-vinylsilane, the sequence results in the syn-l,2-diol the an -l,2-diol is obtained from a (Z)-vinylsilane by the same reactions. [Pg.176]

To overcome this issue Kureshy et al. [55, 56] reported dimeric form of Jacobsen s catalysts 3, 4. They used the concept of solubility modification by altering the molecular weight of the catalyst so that in a post catalytic work-up procedure the catalyst is precipitated, filtered and used for subsequent catalytic runs. The complexes 3, 4 (0.2 mol % of Co(lll)-salen unit) (Figure 2) were effectively used for HKR of racemic epoxides, e.g., styrene oxide, epichlorohydrin, 1,2-epoxypropane, 1,2-epoxyhexane, 1,2-epoxyoctane, and 1,2-epoxydodecane to achieve corresponding epoxides and 1,2-diols in high optical purity and isolated yields. In this process, once the catalytic reaction is complete the product epoxides were collected by reduced pressure distillation. Addition of diethylether to the residue precipitated the catalyst which was removed by filtration. However, the recovered catalyst was required to be reactivated by its treatment with acetic acid in air. The catalysts were reused 4 times with complete retention of its performance. [Pg.303]

It ib often customary to hydrolyze 1,2-diol monoesters t tinted from olefins directly to 1,2-diols, without determining the site of attack in the presumed epoxide intermediate. This iB regrettable in those instances where information on the direction of ring lniAMj. might be useful. Where the structure of the 1,2-diol monoesters cannot be specified, only the formula of the 1,2-diol ultimately isolated is shown here (ejg. Eq. 747). [Pg.189]

The development of simple systems that allow for the asymmetric oxidation of allyl alcohols and simple alkenes to epoxides or 1,2-diols has had a great impact on synthetic methodology because it allows for the introduction of functionality with concurrent formation of one or two stereogenic centers. This functionality can then be used for subsequent reactions that usually fall into the... [Pg.8]

The reaction of an epoxide with hydroxide ion leads to the same product as the acid-catalyzed opening of the epoxide a 1,2-diol (glycol), with anti stereochemistry. In fact, either the acid-catalyzed or base-catalyzed reaction may be used to open an epoxide, but the acid-catalyzed reaction takes place under milder conditions. Unless there is an acid-sensitive functional group present, the acid-catalyzed hydrolysis is preferred. [Pg.653]

The introduction of oxygen atoms into unsaturated organic molecules via dihydroxylation reactions leads to 1,2-diols. 1,2-Diols can be synthesized by the reaction of alkenes either with peracids via corresponding epoxides and subsequent hydrolysis or with OSO4, KMn04, RUO4 and Cr(VI) compounds. [Pg.297]

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. [Pg.545]

The hydrolytic kinetic resolution (HKR) of racemic terminal epoxides catalyzed by chiral (salen)-Co(III) complexes provides efficient access to epoxides and 1,2-diols, valuable chiral building blocks, in highly enantioenriched forms. While the original procedure has proved scalable for many substrates, several issues needed to be overcome for the process to be industrially practical for one of the most useful epoxides, epichlorohydrin. Combined with kinetic modelling of the HKR of epichlorohydrin, novel solutions were developed which resulted in linearly scalable processes that successfully addressed issues of catalyst activation, analysis and reactivity, control of exothermicity, product isolation, racemization, and side-product formation. [Pg.165]

The hydrolytic kinetic resolution addressed a long-standing problem in enan-tioselective epoxide synthesis. The ability to access almost any terminal epoxide or 1,2-diol in high enantiopurity greatly expanded the chiral pool of compounds available for asymmetric synthesis. Equally important was the demonstration of practicality and efficiency that renders the ARO of a racemic mixture a synthetically viable approach. [Pg.1250]


See other pages where Epoxidation to 1,2-diols is mentioned: [Pg.720]    [Pg.117]    [Pg.720]    [Pg.117]    [Pg.489]    [Pg.150]    [Pg.257]    [Pg.254]    [Pg.253]    [Pg.315]    [Pg.165]    [Pg.302]    [Pg.1022]    [Pg.1083]    [Pg.1022]    [Pg.1083]    [Pg.150]    [Pg.786]    [Pg.189]    [Pg.240]    [Pg.254]    [Pg.144]    [Pg.135]    [Pg.189]    [Pg.189]    [Pg.194]    [Pg.186]    [Pg.165]    [Pg.579]    [Pg.404]    [Pg.341]   
See also in sourсe #XX -- [ Pg.236 , Pg.259 ]




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Diol epoxide

Diol epoxides

Epoxide To diol

Epoxide To diol

To epoxide

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