Jacobsen’s Method of Epoxidation of an Alkene
Various types of reactions were completed to first create and then use Jacobsen’s catalyst in the asymmetric epoxidation of an unknown alkene with bleach in the laboratory. The chiral epoxide synthesized was then characterized with GC/MS and NMR. With this information the unknown alkene was able to be identified as 4-chlorostyrene. Introduction
Organisms have evolved with mechanisms that use specific enantiomers of molecules. If the chirality of the molecules is incorrect, they may not be utilized or may even hurt the organism. For this reason a method to create chiral molecules is very important and for this reason we study asymmetric synthesis. One method in which a chiral epoxide can be synthesized is through the use of a Jacobsen-type catalyst. In order to synthesize Jacobsen’s catalyst, Jacobsen’s ligand must be created first which requires the use of 3,5-di-tert-butyl-salicylaldehyde . There are many methods by which this salicylaldehyde can be synthesized but one method with a relatively high yield starts with 2,4-di-tert-butylphenol. The reaction scheme is shown below in Figure 1.
Figure 1: (1) 2,4-di-tert-butylphenol ,(2) 2,4-di-tert-butyl-6-hydroxymethylphenol, (3) 3,5-di-tert-butyl-salicylaldehyde shown above. In this reaction (1) was reacted with formaldehyde utilizing a Lederer-Mannase reaction giving (2) with a good yield. This compound was then oxidized in order to form (3). One oxidizing agent which could be used is sodium hypochlorite (bleach) with phase transfer catalyst. This method of synthesizing the salicylaldehyde is advantageous because it has a very high yield of 88%.1 It also uses many of the techniques that undergraduate students have already learned such as vacuum filtration, drying over anhydrous sodium sulfate, and recrystallization. An alternative method in which one can synthesize chiral epoxides is through the use of a fructose-derived catalyst instead of the Jacobsen’s catalyst. The dioxirane that is formed in the derivation performs the oxidation. The reaction scheme for creating the fructose derived catalyst is shown below in Figure 2.
Figure 2: (4) D-fructose , (5) bis-ketal alcohol, (6) fructose-derived catalyst
The first step in synthesizing the catalyst is ketalization, which protects four of the five hydroxyl groups on the fructose. This ketalization allows for good stereoselectivity having an enantiomeric excess value ranging from 85-97%.2 The second and final step is the oxidation of the unprotected hydroxyl group using PCC, an oxidizing agent. The newly created catalyst can then be converted into a dioxirane, as shown in Figure 3, in the presence of potassium peroxomonosulfate. The dioxirane formed performs the epoxidation of the alkene.
Figure 3: Catalyst - top middle, dioxirane - bottom middle, alkene and epoxide - right side.
Though the yield of this reaction is high it utilizes percholoric acid and PCC which are potentially hazardous reagents. Other than that, the reaction allows the student to use such techniques as recrystallization to purify products, column chromatography to separate products from reactants, and allows students to propose designs of other catalysts that could be used employing different sugars and expected yields and enantiomeric excesses. Results/Discussion
In order to undergo asymmetric synthesis, the reactants must be as optically pure as possible to produce a product with high optical purity, but because a single enantiomer of 1,2-diaminocyclohexane is much more expensive and impractical for undergraduate laboratories, a mixture of trans-cis isomers is used. Therefore a resolution step is required to produce higher optical purity. The problem with using simple steps such as recrystallization is that enantiomers have the same physical properties. To circumvent this...
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