Expt #2. Epoxidation of (E)-ethyl 3-phenylbut-2-enoate using column chromatography for product purification.

edited by TN on 2/6/2014

Relevant textbook reading - Klein, Chapter 14.7-14.10, Mohrig, Chapter 17, 18.

Literature References - (a) Domagala, J. M.; Bach, R.D. J. Org. Chem. 1979, 44, 3168. (b) Valente, V. R.; Wolfhagen, J. L. J. Org. Chem. 1966, 31, 2509. (c) Pageau, G. J.; Mabaera, R.; Kosuda, K. M.; Sebelius, T. A.; Ali H. Ghaffari , Kearns, K. A.; McIntyre, J. P.; Beachy, T. M.; Thamattoor, D. M. J. Chem. Educ. 2002, 79, 96

Overview – The title compound (also known as ethyl trans-β-methylcinnamate) will be epoxidized using meta-chloroperbenzoic acid (MCPBA) to form a constituent of a popular food flavoring agent known as "strawberry aldehyde" (eq 1). The product will be purified by column chromatography and analyzed by NMR and GC-MS. From the results, we will be able to determine whether the reaction is diastereoselective (as shown in eq 1). You will also analyze a commercial sample of strawberry aldehyde and use the NMR spectrum of the product to identify resonances in the strawberry aldehyde spectra.

Background

Column chromatography, unlike gas chromatography, is a useful technique for purifying compounds on a preparative scale. In column chromatography, the stationary phase (silica gel or alumina) is packed into a straight glass column and the sample is applied at the top of that column. A mobile phase consisting of an appropriate solvent is then introduced to the top of the column to "elute" the sample out. As you might guess, the different compounds in the sample travel down the column at rates that are dictated by their relative polarities. Fractions of the "eluent" are simply collected and analyzed for the presence of the desired compound. Fractions that contain the compound are combined and the solvent is evaporated off to yield the purified product.

In order to analyze the fractions for the presence of product we will use Thin Layer Chromatography (TLC). TLC is very similar to column chromatography. The main differences are:

• The stationary phase is coated onto a plastic or glass plate rather than packed into a column.
• The solvent enters at the bottom of the plate and rises up the plate due to capillary action.
• The compounds are not eluted completely out of the stationary phase. Instead they are left on the plate where they can be detected as spots.
• TLC is usually done on a very microscale and not generally useful on a preparative scale.

Make sure to read Chapters 17 and 18 in Mohrig carefully in preparation for this experiment. Also see http://orgchem.colorado.edu/Technique/Procedures/TLC/TLC.html for a nice step-by-step description of how to do TLC.

Procedures

Running the reaction. (Week 1)

• Add 0.50 g of 85% m-chloroperbenzoic acid to a clean, dry 20 mL scintillation vial

• Add 2.5 mL of dichloromethane (DCM) to the vial and gently stir the mixture.

• Prepare a solution of 0.25 mL of ethyl trans-β-methylcinnamate (EMC) in 2.5 mL DCM.

• Add this solution by pipet to the stirring MCPBA solution.

• Continue stirring throughout the remainder of the lab period. (The MCPBA should eventually completely dissolve and you should note when this occurs.)

• Cap the vial and store in a refrigerator until next week.

Analysis. (Week 1)

• Analyze a commercial E/Z mixture of ethyl 3-methyl-3-phenylglycidate (aka "strawberry aldehyde" by GC-MS and NMR. Also analyze a sample of the starting material, EMC. We will delegate the above tasks to individual teams so as to make the lab move more quickly.

  • NMR. Use CDCl₃ as the NMR solvent. If you are assigned the C-13 spectrum then make your NMR sample fairly concentrated by using ca 0.5 cm height of solute in the tube. For the proton NMR use a much smaller volume (e.g. one drop) of compound.
    
  • GC-MS. Add 2 uL of compound by syringe to 10 mL of clean DCM in a clean vial. The instructor will assist you in the operation of the GC-MS instrument.
• Carry out TLC analysis of both the E/Z strawberry aldehyde mixture and the starting material EMC:
  • Prepare dilute solutions (1 mg per mL) of each substance in DCM. (We have recently found that 10 mg/mL works better for the SA solution.)
    
  • Make side-by-side spots of each solution on the provided TLC plate.
  • Prepare a developing chamber with toluene solvent and a piece of filter paper as discussed in Mohrig and at the CU Boulder link.
  • Place the plate in the developing chamber and allow the solvent to rise while not disturbing the chamber. Remove the plate when the solvent reaches about 0.5 cm from the top.
  • Allow the toluene solvent to evaporate off the plate in the hood. Warming gently on a thermowell can hasten this process. 
  • Use the UV lamp to visualize the plate. Lightly outline all spots visible under UV with a pencil. The pencil marks form a permanent visible record of where the spots are.
  • Measure the Rf values of all spots and record the data in your lab notebook.
    

Work-up procedures. (Week 2)

• Carry out a gravity filtration to remove precipitate solids from the reaction mixture. Wash the filter paper through with an additional 5 mL DCM.

• Transfer the filtrate to a separatory funnel, and wash with 10 mL portions of 10% sodium sulfite until the washings give a negative test with starch-iodide paper (stays white). This ensures that all excess peracid has been destroyed.
• Wash the organic layer with two 10 mL portions of 5% NaHCO₃(aq). This step removes the benzoic acid byproduct of the reaction.
• Dry the organic layer over sodium sulfate.
• Make a spot of the above solution on a TLC plate along with a spot of the EMC and SA solutions prepared last week. Develop the plate while carrying out the following procedures.
  
• Transfer the solution to a preweighed round bottom flask and remove the solvent using the miniscale rotary evaporator.
• Determine the crude yield.

Product Purification. (Week 2)

• Pack the glass column that is provided to you with a slurry of neutral alumina in DCM, 10 g of alumina in 30 mL of DCM should be sufficient. The packing should be approximately 13 cm long. Add a thin layer (about 3 mm) of sand at the top. Make sure that the solvent level in the column does not drop below the level of the sand.

• Drain the solvent from the column until the liquid is just level with the top of the sand layer. Introduce your crude sample, with a pipet, at the top of the column.

• Add DCM to the column and collect ten 5-mLfractions using small labeled test tubes. Be careful to never let any part of the column packing to go dry.

• Analyze the fractions by TLC for the presence of the epoxide product.
  

• If the TLC results show it to be necessary, continue to run the column and collect more fractions.

• Combine the fractions containing the epoxide in a pre-weighed round bottom flask and evaporate the solvent using the rotary evaporator.

• Determine the final yield of the purified epoxide

Characterization of Final Product (Week 2)

• Obtain the ¹H and ¹³C NMR spectra using CDCl₃ as the solvent.
• Prepare a sample for GC-MS analysis using the same method as in week 1.

Report

Proton and C-13 NMRs were obtained by various groups for EMC and the strawberry aldehyde mixture (SA). Make sure to include all of these in your report. GC-MS data for EMC and SA were also obtained and need to be included. Use all of the above as standards for comparison when looking at your product's proton NMR, C-13 NMR (use another groups if necessary), and GC-MS (printouts will be provided in lecture).

After assigning peaks in your product NMRs you should be able to go back to the SA NMRs and label every peak as to which H (and which C on the C13) on which diasteromer (E or Z) causes it.

Assigned Questions

• Discuss the stereoselectivity of the reaction carried out in context with the accepted mechanism of alkene epoxidation by  MCPBA. Although we did not obtain evidence for this, MCPBA epoxidations are always diastereospecific. Why? As part of your explanation, make sure to be clear about the contrasting definitions of stereoselective and stereospecific.
• Is the product epoxide the major or minor constituent of the commercial strawberry aldehyde mixture?
• How does washing the crude product solution with NaHCO₃(aq) remove the benzoic acid byproduct?