Chemistry of Ylides Answer the following questions to the best of your ability. Once you have completed the assignment, be sure to check your answers against the answer key.
1. Limonene is a terpene found in citrus foods such as lemons and oranges. It can be easily prepared by the Wittig reaction of 1-(4-methylcyclohex-3-enyl)ethanone with triphenylphosphine ylide. The reaction is shown below:
Provide the mechanism for the reaction.
Answer: The mechanism starts with the nucleophilic attack of the phosphorus ylide on the ketone. The result is this phosphorus zwitterion. The attraction of the positice charge to the negative charge drives the formation of the very important betaine intermediate. The last step is the formation of the desired alkene and the side product triphenylphosphine oxide.
2. How can you make the following compound using cyclohexene as a starting material while using a Witting reaction in one of the steps? CH2=CH(CH2)4CH=CH2.
Answer: The best reaction to open the cyclohexene ring is to oxidize it via ozonolysis. Since the desired product is an aldehyde, reductive work-up using Me2S is necessary. A Wittig reaction is followed to obtain the desired product.
3. What ylide can be used for the following reaction?
The Saylor Foundation 1
Answer: Sulfur ylides are the best reagent for formation of epoxides from aldehydes and ketones. The sulfur ylide that can be used for this reaction is:
Where R can be any carbon containing moiety such as CH3.
4. What is a ylide? What are the different types of ylides? What reactions can be achieved by using them?
Answer: a ylide is a neutral organic compound containing a positive and negative charge on adjacent atoms. The negatively charge atom is a carbanion and the positively charged atom can be phosphorus, sulfur or nitrogen. They are used as reagents in various organic reactions. Phosphorus ylides can be synthesized from the reaction of a phosphine with an alkyl halide. They are typically used in Wittig reactions. Sulfur ylides can be used in the synthesis of epoxides via the the Stevens rearrangement. Nitrogen ylides are a versatile reagent for various organic reactions.
5. Compare the Wittig reaction with the Horner-Wadsworth-Emmons reaction.
Answer: The Wittig reaction is an effective way to prepare Z-alkenes, whereas the Horner-Wadsworth-Emmons reaction provides the E-alkene. While both reactions require an aldehyde or ketone as a starting material, the Wittig reaction uses a phosphoru ylide while the Horner-Wadsworth-Emmons uses a phosphonate carbanion. Both reactions occur via similar mechanisms with a betaine intermediate.
6. What are the best reagents to prepare 3-ethyl-2-pentene via Wittig reaction?
Answer: The Wittig reaction is a very versatile method to synthesize alkenes. The starting materials are an aldehyde or ketone and a phosphorus ylide.
The structure of 3-ethyl-2-pentene is ; the best aldehyde is ; the best
phosphorus ylide is: , where R is most commonly a phenyl group.
The Saylor Foundation 2