Lecture 4b - UCLA

Lecture 4b - UCLA

Lecture 4b Enantioselective Epoxidation II Theory of Epoxidation R1 R1 O 5-10 mol% JC NaOCl R2 R2 The epoxidation is carried out in a biphasic system Organic layer: ethyl acetate, Jacobsen catalyst, unknown alkene assigned by TA (list of possible unknowns can be found online, hints meeting 5), pyridine N-oxide (optional) The alkenes have to be handled under the hood because they are suspected carcinogens Aqueous layer: water, bleach, disodium hydrogen phosphate solution + 1 drop sodium hydroxide (pH= ~11.3) Experimental I Prepare a solution of 0.5 mL of an unknown alkene (measured with a syringe in the hood) and the catalyst in ethyl acetate

How much of the catalyst is used? 4-8 mol % (minimum: ~100 mg) When should pyridine N-oxide If the amount of catalyst is very low be added? (m<<100 mg) Prepare a mixture of bleach, Na2HPO4 and NaOH What is thea buffer function of Na2HPO They form to stabilize the 4 pH-value of the aqueous solution and NaOH? Combine both solutions in an Erlenmeyer

flask or a round bottomed flask (do not use a beaker here!), close the flask and stir vigorously This is a biphasic system that means that efficient stirring is necessary to Why this mass necessary? have aisgood transfer Experimental II Monitor reaction using TLC (make sure to optimize the solvent mixture) Why is this necessary? The reaction mixture has to be worked up as soon as the epoxidation is completed to minimize losses of product due to hydrolysis of the epoxide

Which solvent should be used here? EtOAc:hexane (1:4) as starting point How are the compounds visualized? 1. UV-light: Alkene 2. Ceric staining: Epoxide What does optimizing the solvent mixture imply? The desirable Rf-values are of Rf= ~0.3-0.4 for the epoxide and Rf= ~0.7-0.8 for the alkene Experimental III After the reaction is completed, hexane is added to the reaction mixture How long does the reaction usually take? 1-2 hours Why is hexane added? To increase amount of the organic

layer and precipitate the catalyst, which has to be removed by filtration After drying the organic layer, the solvents are removed using the rotary evaporator The crude product is transferred into a vial and stored over a small amount of anhydrous Na2SO4 What Anhydrous sodium is used for dryingsulfate here? Why is the rotary evaporator used here? What remains behind? A small amount of a beige or brown oil How is the product transferred? Why is the crude over Na To remove

the stored remaining water 2SO4? from the epoxide Experimental III Pack the 25 mL burette with a small piece of cotton, seasand (0.5-1 cm) on the bottom, dry silica and (up to the 7 mL mark) and more seasand (0.5-1 cm) on the top Use hexane to wet column (Hint: Use adapter and air pressure to Seasand push the solvent through the column. The glass tube might have to be wrapped with some parafilm again to get a better seal. The pressure has to be properly regulated using the regulator (setting 10 psi)). Silica Pretreat the column with 20 mL of a 1 % solution of NEt3 in hexane Rinse the column with 20 mL of hexane

Seasand Apply the entire crude epoxide dissolved in a small hexane to the column Cotton Use the solvent mixture as determined by TLC to elute fractions (~10*10 mL each) Close stopcock before filling the column Experimental IV Identify the fractions containing epoxide with TLC 1 Which fractions should be combined here? Combine the fractions and remove the solvent using the rotary evaporator 2 3 4 5

Fractions 5-7 6 7 8 9 10 Characterization I No infrared spectrum No UV-Vis spectrum Submit sample for NMR Analysis Use a clean and dry NMR tube with a minimum length of 16 cm The solution should be as concentrated as possible The solution level in the NMR tube has to be 5 cm exactly The compound is dissolved in CDCl3 and not in any other

solvent (handle only under the hood) The NMR tube has to be properly labeled on the upper third of the NMR tube using a Sharpie. The label has to contain your name, the unknown alkene, Chem 30CL, and the section. The sample has to be signed into the log sheet with the exact same name as written down on the NMR tube. Characterization II Submit a sample for GC Spectrum Prepare a solution of the epoxide in hexane (1-2 mg/mL) Fill the GC vial to the 1.5 mL mark The sample cannot contain any solids or water Close the vial with a cap and submit into tray Place the letter of the unknown on the label as well Sign the sample in on sign-in sheet (student name, code on the vial (make sure not to remove it) and unknown alkene) Record the code in your notebook as well All GC samples have to be submitted by January 22, 2016 at 5 pm

Recently Viewed Presentations

  • Billions and billions

    Billions and billions

    Bank is a trusted ALGORITHMIC BEING. Checks ids. Ensures no double dipping by Bob or Alice . Keeps a ginormous central ledger of the entire history of all customer transactions . This ginormous ledger helps the bank track all transaction...
  • Quarterly Results and FY 2006 Financial Outlook

    Quarterly Results and FY 2006 Financial Outlook

    Now to make matters worse, he was in such a rush to get into his bed, that he just didn't brush! So day after day, and night after night, his teeth just got yellowier instead of snow white. Then into...
  • EVM tools at CERN - Agenda (Indico)

    EVM tools at CERN - Agenda (Indico)

    The AIS group provides CERN with a set of integrated and reliable Corporate Information Systems. The group ensures coherent, reliable, comprehensive and accurate management information across CERN. The group provides optimal support for all business processes at CERN. ... EDH....
  • Hyperlinks, Images and Tables - De Montfort University

    Hyperlinks, Images and Tables - De Montfort University

    When clicking an email link, it will open the default mail client on the computer such as Outlook, Thunderbird etc. To create this kind of link we use the following text. What if we wanted to auto fill the subject?...
  • Introduction to Robotics - Penn State Engineering

    Introduction to Robotics - Penn State Engineering

    Introduction to Robotics A common view : Robots as Humanoids We will be studying Industrial manipulator type Robots. Agenda Introduction to Robotics Classification of Robots Robot accessories Robot coordinates Work volumes and Reference Frames Robot Programming Robot Applications in Lean...
  • PERSONIFICATION - North Thurston Public Schools

    PERSONIFICATION - North Thurston Public Schools

    PERSONIFICATION A figure of speech which endows animals, ideas, or inanimate objects with human traits or abilities. EXAMPLES The Train I like to see it lap the miles, And lick the valleys up, And stop to feed itself at tanks;...
  • MODULE 1 Why do We Need to Talk

    MODULE 1 Why do We Need to Talk

    A patriarchal social system is dominated largely by the voice and authority of men. A patriarchal social system is centered on males; the world is portrayed with men as the main actors in life and reflects their ideas and values....
  • Historical Context: A Victorian Christmas

    Historical Context: A Victorian Christmas

    the problems of the poor, create enormous social problems. They create children like this. Want: because the population has 'boomed' in Victorian times, many people ' want for ' (go without) the basic necessities such as food and shelter. Combined...