Comparing Methods of Grignard-based Ring Opening of an Epoxide

Comparing Methods of Grignard-based Ring Opening of an Epoxide

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Lab 4: Comparing Methods of Grignard-based Ring Opening of an Epoxide Manuel Vazquez 4/1/12
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INTRODUCTION: The field of Organometallics in Chemistry is a very fascinating area of ongoing research which has provided a formidable task to those working to fully understand it. Although it is usually taught in basic organic chemistry courses that a Grignard solution simply consists of the R-MgX compound alone, in practice one often observes that the Grignard undergoes secondary changes, such as forming R 2 -Mg, where the halogen is replaced by another R group, or performs other chemistries with the compound to be reacted with the Grignard. In the case of this experiment, styrene oxide, a phenyl with a benzylic epoxide, is reacted with phenyl magnesium bromide, PhMgBr (Figure 1); the issue at hand is that the MgR 2 in solution (where R here either means a phenyl group or a bromide) can act as catalyst for the isomerization of styrene oxide to an aldehyde form (the mechanism by which the isomerization occurs is detailed in Figure 2, and consists of the MgR 2 acting as a Lewis Acid). In the epoxide reaction, the Grignard can open the ring from either the more or less substituted carbon to form either the primary or secondary alcohol as the product; in addition, the reaction of the Grignard with the aldehyde proceeds to form solely the secondary alcohol. In this experiment, the reaction was to be carried out in two distinct styles: ILUVW± LQ WKH ³QRUPDO´ addition, the Grignard reagent was to be prepared from phenyl bromide in solution in diethyl ether (Et 2 O) reacting with magnesium turnings inside the reaction flask, followed by the drop wise addition of the styrene oxide. The other VW\OH± ³LQYHUVH±´ FRQVLVWHG RI JHQHUDWLQJ WKH *ULJQDUG DV GHVFULEHG± EXW WKHQ removing it from the reaction flask, adding to the flsk styrene oxide, and then finally adding back in the Grignard drop-wise into the flask. The crude products of the reaction mixture were then to be analyzed via NMR to determine product ratios. To assist this end, the predicted primary and secondary alcohols to be formed were to be synthesized via the NaBH 4 reduction of diphenylacetaldehyde and of deoxybenzoin, Figure 1: The Three Possible Reactions Figure 2: Catalytic Rearrangement of Styrene Oxide by MgBr 2
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respectively; the products would then be purified and then have an NMR taken of each for comparisons. Because Grignards tend to open up epoxides by attacking the least substituted carbon, the expected major product of reaction would be the secondary alcohol, which in this case is also the major product expected if there is rearrangement due to the magnesium compounds in solution. The largest concern for determining the reaction set-up was the presence of water and oxygen; since Grignards are capable of reacting with both, the reaction environment must be as void of them as possible. To achieve this end, a closed system with a nitrogen atmosphere was established as shown in Figure 3. In this set-up, the main reaction occurs in a three-neck,
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  • Spring '12
  • ProfessorHyatt
  • Biology, Grignard, Magnesium, styrene oxide, OH peak

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