Reactions of alkenes reactions of alkenes electrophilic catalytic carbene oxidative addition halohydrogenation acid catalyzed hydration oxymercurationdemercuration hydroborationoxidation hydrogenation cyclopropanation halogenation halohydrin formation epoxidation antihydroxylation synhydroxylation oxidative cleavage via. The sharpless epoxidation is an organic reaction used to steroselectively convert an allylic alcohol to an epoxy alcohol using a titanium isopropoxide catalyst, tbutyl hydroperoxide tbhp, and a chiral diethyl tartrate det. Feb 04, 2011 the vanadiumcatalyzed epoxidation reaction voacac 2 tbutyl hydroperoxide has become a very popular procedure for the stereoselective epoxidation of acyclic homoallylic alcohols. An example is the reaction of ethylene oxide with a grignard reagent discussed in section 15.
Stereoselective voacac2 catalyzed epoxidation of acyclic. Oxidation reactions in synthetic organic chemistry constitute one of the more important. These compounds contain an electrophilic oxygen in the oh group and react with alkenes by adding this oxigen to the double bond to form oxacycloprpanes. Vanadium catalyst gives preferential epoxidation of the less substituted alkene. The student should perform the extraction again, and run a tlc of the organic phase afterwards to. Epoxidation reaction with mchloroperoxybenzoic acid in water.
The regio and stereoselectivity of the mcpba epoxidation of methyl dimorphecolate and. In addition to the epoxide product, a carboxylic acid byproduct is formed. Enantioselective epoxidation of unfunctionalized olefins. By changing the catalyst it is now possible to convert simple alkenes to the corresponding epoxide with highasymmetric induction. The vanadiumcatalyzed epoxidation reaction voacac 2 tbutyl hydroperoxide has become a very popular procedure for the stereoselective epoxidation of acyclic homoallylic alcohols. However it is synthetically easier just to add x 2 to an alkene. If one face is hindered, the major product comes from attack on the unhindered face. In the laboratory, the epoxidation agent of choice is mchloroperbenzoic acid mcpba. Illustrated glossary of organic chemistry mcpba meta.
Other types of reaction have been substitution and elimination. Sharpless asymmetric epoxidation sae converts primary and secondary allylic alcohols into 2,3 epoxyalcoholsthe reaction is enantioselective only one enantiomer producedenantiomer formed depends on stereochemistry of catalyst. This video shows you how to synthesize an epoxide from an alkene using a peroxyacid such as mcpba or by means of the halohydrin formation reaction. The stereochemistry of the product is also important. The regio and stereoselectivity of the mcpbaepoxidation of. After completing this section, you should be able to. Alkene to epoxide reaction synthesis mechanism mcpba. A protonationpromoted pathway and its quantum chemical elucidation. Well, we do it by using a type of molecule called a peroxy acid. The sulfate as a good leaving group facilitates the ring closure to the dioxiranes. Epoxidation reaction abstract epoxide chemistry is used extensively in organic synthesis via reaction of an alkene with a peroxyacid, mchloroperoxybenzoic acid and oxone. Peroxycarboxylic acids are sometimes replaced in industrial applications by monoperphthalic acid, or the monoperoxyphthalate ion bound to magnesium, which gives magnesium monoperoxyphthalate mmpp. Takushi nagata, kiyomi imagawa, tohru yamada, teruaki mukaiyama.
Addition to the same side of a double bond is called syn addition. A good way to think of this is that the alkene is rotated so that some constituents are coming forward and some are behind. Stereospeciflcity in formation of epoxides from cyclic allylic alcohols. The syn addition is why the mechanism is the same for both symmetric and asymmetric alkenes. Oxidation with p methoxycarbonyl perbenzqic acid 1. Stereoselective reactions of alkenes massey university. The regio and stereoselectivity of the mcpbaepoxidation of methyl. The regio and stereoselectivity of the mcpbaepoxidation. The sharpless asymmetric epoxidation only works with allylic alcohols, as illustrated by the conversion of 156 to 157 or 158 to 159.
The migration does not change the stereochemistry of the group that transfers, i. Epoxidation, needless to say, is going to add that one oxygen to the double bond in order to make it into that threemembered ring. The stereochemistry of ethylene1,2d2 epoxidation over. The high yield would suggest a greener reaction, however that is not the case, the use of oxone is a greener reaction because mcpba is more expensive and time. The shi epoxidation allows the synthesis of epoxides from various alkenes using a fructosederived organocatalyst with oxone as the primary oxidant. Pdf oxidation with pmethoxycarbonyl perbenzqic acid 1. Apr 8, 2015 ericminikel cambridge, ma chem20 these are my notes from lecture 25 of harvards chemistry 20.
The regio and stereoselectivity of the mcpba epoxidation of methyl dimorphecolate and derivatives. Henbest epoxidation epoxidation directed by a polar group. As expected, epoxidation by a peracid such as mcpba normally chooses the side. Since the transfer of oxygen is to the same side of the double bond, the resulting oxacyclopropane ring will have the same stereochemistry as the starting alkene. In this post, we will focus on the antidihydroxylation which is achieved by converting the alkene into an epoxide first. Advanced organic stereoselective reactions of alkenes earlier, we saw that stereospeci. Stereochemistry h 3 c ch 3 h 3 c ch 3 o h h h h oh h 2 n inversion of configuration at carbon being attacked by nucleophile. Somewhere in one of your exams, you will see at least one question on epoxidation of alkenes. There are a lot of aromatic peaks showing up in this nmr between 7. Epoxidation of alkenes with free study guide organic chemistry. Stereochemistry of epoxide preparation and how it relates to mechanism.
This is considered a syn addition since the oxygen is. The stereochemistry of the catalytic epoxidation of cisethylene1,2d2 to ethylene oxide over four unsupported and two supported silver catalysts as well as over ag2o and ago was studied. Alkene epoxidation is an exciting reaction in which a double bond reacts with a peroxy acid to form an epoxide. Influence of solvent on the stereo chemistry of epoxidation of cholesterol acetate. During the epoxidation of alkenes, an oxygen atom is transferred from the peracid to the cc double bond thus forming an oxirane ring. We also acknowledge previous national science foundation support under grant numbers 1246120. Iodolactonizations and stereoselectivity baldwins rules. The oxidant for the epoxidation is tertbutyl hydroperoxide. The reaction mixture is heated with stirring under reflux for 24 hours.
An epoxide is a 3membered ring containing two carbon atoms and one oxygen atom. The catalyst is easily prepared from fructose and displays broad generality. Stereoselective reactions of alkenes earlier, we saw that stereospeci. Pdf on jun 11, 2014, sergiy i okovytyy and others published stereochemistry of the epoxidation of bicyclo2. The epoxidation reaction is believed to occur in a single step with a transition state incorporating all of the bonding events shown in the equation. Sharpless asymmetric epoxidation mcmaster university. Epoxidation via vicinal halohydrins br 2 h 2 o oh naohnaoh.
Stereochemistry epoxidation is stereospecific the stereochemistry of the products depends on the stereochemistry of the reactants. Often abbreviated mcpba, it is a stable crystalline solid. Oligopeptides as catalysts for asymmetric epoxidation. Relative reactivity of different alkenes o stability of cation formed is key. This epoxidation adds in a syn addition, and the enantiomers of this, where the epoxide oxygen is either in the back or in the front of the plane of the molecule, are both formed. Epoxidation of trans2butene gives only trans2,3dimethyloxirane.
Epoxidation of alkenes with peracids mcpba meta chloroperbenzoic acid and the appropriate alkene in a ratio of 1. These are not indefinitely stable, but can be generated in situ by oxidation of a ketone with potassium peroxymonosulfate oxone. The stereochemistry of ethylenel,2dz epoxidation over. In this epoxidation reaction, mcpba oxidizes cyclohexene to the corresponding epoxide. Variations of the stereochemistry with reaction composition including addition of c,h,ci, to. Mar 25, 2015 ericminikel cambridge, ma chem20 these are my notes from lecture 19 of harvards chemistry 20. In independent work, katsuki and jacobsen showed that asymmetric epoxidation occurs using manganese. Reactions of alkenes ii reading study problems key concepts. This is usually done with peroxyacids such as mcpba meta chloroperoxy benzoic acid or peroxyacetic acid. However, mcpba, like most other peroxides, will detonate if it is not handled carefully. Hydrohalic acids epoxides react with hx to produce halohydrins, which react further with hx to generate 1,2dihalides. Since the abc ring system of 2b contains three of the five chiral centres of the alkaloid, its construction with the correct absolute stereochemistry should potentially set the stage. Reactions of alkenes ii reading study problems key.
Donohoe, oxidation and reduction in organic chemistry. The stereochemistry of basecatalyzed epoxidation journal. Ready enantioselective epoxidation with dioxiranes several groups have developed chiral ketones as catalysts for asymmetric epoxidation. The sharpless epoxidation reaction is an enantioselective chemical reaction to prepare 2,3epoxyalcohols from primary and secondary allylic alcohols. The reaction is catalyzed by tioipr 4, which binds the hydroperoxide, the allylic alcohol group, and the asymmetric tartrate ligand via oxygen atoms putative transition state depicted below. Stereochemistry of alkene starting material is retained in epoxide product. Stereochemistry diastereoselective and enantioselective reactions. Since the transferred oxygen atom carries a positive charge, peracids must be considered electrophilic oxidizing agents. Illustrated glossary of organic chemistry mcpba meta chloro peroxy benzoic acid. Aerobic enantioselective epoxidation of unfunctionalized olefins catalyzed by optically active salenmanganeseiii complexes bulletin of the chemical society of japan 1994, 67 8, 22482256. These are not indefinitely stable, but can be generated in situ by. Epoxidation of cis2butene gives only cis2,3dimethyloxirane.
Ch08 reacns of alkenes landscape page 1 reactions of alkenes since bonds are stronger than bonds, double bonds tend to react to convert the double bond into bonds this is an addition reaction. Epoxidation epoxide oxirane formation takes place when an alkene is treated with an organic peracid. Sep 22, 2012 stereochemistry of epoxide preparation and how it relates to mechanism. It is unlikely that a dipolar intermediate, as shown above, is actually formed. Mcpba gives preferential epoxidation of the more substituted alkene. A popular peroxycarboxylic acid is metachloroperoxybenzoic acid mcpba.
The stereochemistry of the resulting epoxide is determined by the enantiomer of the chiral tartrate diester usually diethyl tartrate or diisopropyl tartrate employed in the reaction. All reactions involve nucleophilic attack at carbon and lead to opening of the ring. Groups that are cis on the alkene will end up cis on the epoxide product. These indicate that not all of the mcpba byproduct was removed during the extraction. Anti dihydroxylation of alkenes with mcpba and other. The epoxidizing species is believed to be a dioxirane, which is a powerful epoxidation reagent. Epoxides using mcpba, voacac 2tbuooh, or twostep methods that rely on bromoalcohol or iodoalcohol intermediates to pick the right epoxidation method, pay attention to starting material and product stereochemistry. An exception is metachloroperoxybenzoic acid, shown in the mechanism above. Iodolactonizations and stereoselectivity baldwins rules for ring closure epoxidations and stereoselectivity. Oct 22, 2015 this video shows you how to synthesize an epoxide from an alkene using a peroxyacid such as mcpba or by means of the halohydrin formation reaction. Nov 26, 2014 alkene epoxidation is an exciting reaction in which a double bond reacts with a peroxy acid to form an epoxide.
General procedure mcpba meta chloroperbenzoic acid and the appropriate alkene in a ratio of 1. This is considered a syn addition since the oxygen is bound to 2 carbon atoms. This oxidation reduces mcpba to metachlorobenzoic acid. Aim to develop an asymmetric epoxidation catalyst which would operate on substrates with no functionality for precoordination a number of reasonably efficient porphyrin based oxotransfer reagents were developed but the real success story has been the us of salenbased reagents. Consequently, epoxidations by peracids always have synstereoselectivity, and seldom give structural rearrangement. Alkenes epoxidation, dihydroxylation, cyclopropanation. Peroxy acids are the molecules that are used to make them. Many students like to remember it as a cyclic ether. The epoxidation reactions of cyclohexene with mcpba in homogeneous solutions of nhexane and dichloromethane give quantitatively the epoxycyclohexane after 8 hrs and 0. However, mcpba can be explosive under some conditions. Alkene epoxidation tutorial video reaction mechanism in. The geometry of the alkene is preserved in the product recall the cyclopropanation of alkenes, ch.
Epoxidation is a onestep process concerted bond formation and bond breaking, and the. Variations of the stereochemistry with reaction composition including addition of c4h2cl2 to moderate the catalyst and the use of n2o as the oxidant were also explored. The mechanism of epoxidation is similar to the formation of halonium ion and oxymercuration that we discussed in. Relative stereochemistry o matters only if both the first and the second alkene carbons are transformed into chiral centers 3.
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