Epoxidation of alkenes with mcpba

Lab Report

EPOXIDATION

OBJECTIVES: To perform an epoxidation reaction and synthesize a strawberry-flavored compound. To purify the compound using chromatography and analyze it with NMR

TIME ALLOCATED: TWO LAB PERIODS

Background

An epoxide is a cyclic ether with a three-member ring structure. Due to the strain in the ring, epoxides are quite reactive. A common procedure for the synthesis of epoxides is a reaction involving an alkene as a starting material and a peroxiacid. The reaction shown below is an example in which a disubstituted alkene reacts with a common peroxyacid, m-chloroperoxybenzoic acid (mCPBA), to form an epoxide and 3-chlorobenzoic acid.

Because substituents on a three-membered ring can be either pseudo-axial or pseudo-equatorial, 1,2- disubstituted oxacyclopropanes can have cis or trans stereochemistry. With more substitution, as in compound 1, E and Z isomers can potentially result. However, in the reaction shown, the geometry of the starting material determines the product stereochemistry because the mechanism is stereoselective. The oxygen atom is delivered to one side of the alkene at the same time as the double bond is being broken.

Experimental

Week 1. Set up of the epoxidation reaction

1. Into a clean, dry vial, weigh 176 mg (1.0 mmol) of ethyl trans cinnamate.

2. Add 4 mL of dichloromethane. On weighing paper, weigh out 670 mg mCPBA. Note: the mCPBA is not pure, the commercial material is 77% by weight and you will need to take that into account in your yield calculations.

3. Add the mCPBA to the vial, cap the vial and shake until the mCPBA completely dissolves.

4. Vent the vial by briefly removing the cap. Label the vial with the reagent structures, date, and

your initials and then seal it with parafilm. Give it to your instructor to place in the refrigerator.

Week 2. TLC, Column Chromatography, and NMR spectroscopy

1. Gravity-filter the solution into your separatory funnel and rinse the solid with 3 mL of CH2Cl2.

2. Add 8-10 mL of aqueous 10% aqueous sodium sulfite (Na2SO3) and shake the funnel.

3. Drain out the bottom organic layer into a small beaker, then drain off the aqueous layer. Add the organic layer back to the separatory funnel, and then wash with 8-10 mL of 5% aqueous sodium bicarbonate, NaHCO3, to remove any remaining acid.

4. Collect the bottom layer and dry it over CaCl2. Transfer the dry organic solution to a 50 mL round bottom flask (the one in your drawer, not the yellow kit) and remove most of the CH2Cl2 using a rotary evaporator or alternative method.

Column chromatography

1. Set up a 6-inch alumina column using 25% ethyl acetate in hexanes as eluting solvent

2. Load the concentrated sample using a pipet and rinse the flask with 0.5 mL of eluting solvent to rinse the round-bottomed flask, then add it to the column.

3. Collect five fractions (~5 mL each fraction) from the column, and analyze them using TLC.

TLC analysis

• Your product will be fairly dilute, so spot each fraction about 4x (put spots on top of each other)

• Spot the starting ester on the same plate using 25% ethyl acetate in hexanes as eluent.

• Use anisaldehyde stain to visualize the spots (in the hood, immerse the dry TLC plate in the anisaldehyde solution, wipe off excess stain, and heat on a hotplate until pink –

• Tare a round-bottom flask, and then combine the fractions containing only the green spot. Remove ALL of the solvent by rotovap. Weigh your flask and calculate the % yield.

NMR analysis

• Prepare an NMR sample of the epoxide product 3 in CDCl3 using one drop of the dry product in a clean NMR tube and adding CDCl3 as described in the NMR instructions. The spectrum should be analyzed 13C NMR

Real life example

C O 2 E t

H

H

P h +

C O 3 H

C l

C H 2 C l 2

C O 2 H

C l

O

H P h

H

C O 2 E t

+

1 2 3 4

2

Researchers at Glaxo-SmithKline recently reported an innovative synthesis of 1,2 amino alcohols (important intermediates in the synthesis of oncological drugs). This approach is particularly suitable for industrial applications that use large scale synthesis. The reported approach utilizes an epoxide.

Post lab assignment

1) Complete the worksheet

2) Complete the writing assignment “Full paper”

References

1) Lim, J.; Leitch, D. C.; Org. Process Res. Dev. 2018, 22, 641−649

2) Pageau, G. J.; Mabaera, R.; Kosuda, K. M.; Sebelius, T. A.; Ghaffari, A. H.; Kearns, K. A.; McIntyre, J. P.; Beachy, T. M.; Thamattoor, D. M. J. Chem. Educ., 2002, 79, 96-98.

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