This experiment performs a modified Wittig reaction using a phosphorus-containing Hornes-Emmons-Wittig reagent to generate an enolate anion of trimethyl phosphonoacetate instead of a phosphorus ylide. The methyl trans-4-methoxy cinnamate produced is then analyzed using melting point and 1H NMR spectroscopy. Theory
The Wittig reaction prepares alkenes from carbonyl compounds by attacking a phosphorus ylide with a nucleophilic carbon atom stabilized by a triphenylphosphonium group. An ylide is a compound that contains two oppositely charged atoms bonded together with complete octets and is generated through phosphonium salts. This experiment is a modified Wittig reaction that uses a Horner-Emmons-Wittig reagent that is more acidic than the Wittig salts. This allows a weaker base to be used and gives an advantage when isolating and purifying the product because the final phosphorus byproduct is water soluble. The overall reaction can be best explained through the mechanism below:
The stability of the Wittig reagent dictates the stereochemistry of the reaction. Wittig reagents stabilized by a carbonyl group that share the carbanion’s negative form the E isomer as the major product and unstabilized Wittig reagentsform the Z isomer as a product. The E isomer has a melting point of 88°C while the Z isomer exists as an oil at room temperature. Results
The experiment produced 0.033 g of white crystal product, an 11.7 % percent yield. The melting point range of the product was 80.0°C-82.3°C. Small amounts of yellow oil was also produced before recrystallization. Attached are the 1H NMR spectroscopy of both the product and the oil side product from the reaction. Discussion
This reaction proved to be inefficient, yielding only 11.7% of product. Perhaps letting the reaction run longer than an hour would have helped increased the percent yield. The reagents used in this experiment were also exposed to the air for a long period of time as my peers have left the stock uncapped and this could have deactivated the reagents and made them unreactive.
The melting point range of the product was 80.0°C-82.3°C, whereas the desired E isomer of methyl-4-methoxy cinnamate has a literature melting point of 88°C. This indicates impurity in the product, as the melting point range is fairly below the literature value. This, again, could be due to unreactive reagents or the oil produced in the reaction could have mixed in with the recrystallization. The Z isomer of the product is oil in room temperature. We observed some oil product after the reaction was complete and before recrystallizing. The 1H NMR spectroscopy of the E isomer shows hydrogens more upfield as opposed to the Z isomer, indicating that the E isomer is the more favored product.
This experiment primarily yields the E isomer because the steric bulk of the phosphonate and the electron-withdrawing group favor the E isomer. The reaction is slow enough to produce the thermodynamically favored product, which is the E isomer. Conclusion
This experiment was inefficient in producing a respectable amount of E isomers, the desired product. Our product contained some impurities, with its melting point range significantly below the literature value of the E isomer melting point. The E isomers are favored over the Z isomer for the Horner-Emmons-Wittig reaction because of steric hinderance with the Z isomer.