Chemistry
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Facile Preparation of (2Z,4E)-Dienamides by the Olefination of Electron-deficient Alkenes with Allyl Acetate
Chapters
Summary June 21st, 2017
The ruthenium-catalyzed olefination of electron-deficient alkenes with allyl acetate is described here. By using aminocarbonyl as a directing group, this external oxidant-free protocol has high efficiency and good stereo- and regioselectivity, opening a novel synthetic route to (Z,E)-butadiene skeletons.
Transcript
The overall goal of this ruthenium catalyzed and oxidant free protocol is to synthesize Z, E-butadienes by olefination of electron-deficient alkenes using aminocarbonyl as a directing group. This method that can help answer key questions in a C-H bound functionalization, such as the olifination of alkenes using allyl acetate. The main advantage of this technique is that Z, E-butadiene can be obtained directly from acrylic amide and allyl acetate result in antioxidant using inexpensive ruthenium in catalyst.
Demonstrating the procedure will be Chunbing Yu, a graduate student in our laboratory;Prior to starting this procedure, dry an eight milliliter screw cap reaction vial with a compatible magnetic stir bar in an oven at 120 degrees Celsius for over two hours. After removing the vial from the oven, cool it to room temperature with inert gas before use. Using an analytical balance, weigh 3.7 milligrams of ruthenium cyme chloride dimer and 13.7 milligrams of silver hexafluoroantimonate into the reaction vial.
Next, add one milliliter of dry 1, 2-dicholoroethane to the reaction viae. Using the analytical balance, weigh 0.2 millimoles of acrylamide into the reaction vial. Then, add 43 microliters of allyl acetate to the reaction vial with a micro syringe.
Following this, blanket the reaction vial with argon and immediately cover it with a compatible screw cap. Stir the reaction mixture at room temperature for five minutes. Then heat the reaction mixture to 110 degrees Celsius in an oil bath with stirring for 16 to 18 hours.
After cooling the vial down, use thin layer chromatography or TLC, to monitor the progress of the reaction by comparing the reaction mixture to an acrylamide standard. Use a 1:2 ratio of ethyl acetate and petroleum ether to develop the TLC plate. Now dissolve the crude product in a minimum volume of dichloromethane and load it onto a silica gel column wet with petroleum ether.
Separate the cross coupling products by column chromatography, using a mixture of ethyl acetate and petroleum ether as the eluent. After connecting the eluent containing product in a flask, evaporate the solvent on a rotary evaporator. Then place the flask under a high vacuum for a minimum of two hours to obtain 20 to 50 milligrams of product for characterization by NMR spectroscopy.
The optimization of conditions for the preparation of 1, 3-butadiene, including the screening of various additives and solvents, using your ruthenium cymene chloride dimer as the catalyst is shown here. The scope of the reaction was explored by submitting various acrylamides to the optimized conditions in the presence of allyl acetate 2a. Modest to excellent yields were obtained with good ratio selectivity and alkene selectivity.
The reactivity of different allyl derivatives and branched allyl acetates, was also examined. Alpha and beta substituted allyl acetates were completely inert for cross coupling. While gamma substituted allyl acetate afforded only trace product.
Other allyl carboxylic esters showed decreased reactivity compared to allyl acetate 2a. And allyl methyl carbonate 2f was more inactive, forming product in 24%yield. To investigate the reaction mechanism, two deuterium labeled experiments were conducted.
When acrylamide 1g was subjected to a standard catalytic system in the presence of acetic acid D4, the cationic ruthenium species led to a Z selective hydrogen deuterium exchange on a acrylamide, indicating a reversible cyclometalation event. Moreover, a kinetic isotope effect of 3.2 was observed in the intermolecular isotopic study, suggesting that the olefinic carbon-hydrogen bond metalation step, is probably involved in the rate determining step.
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