Question

Wittig reactions with the following \(\alpha\)-chloroethers can be used for the synthesis of aldehydes and ketones.(a) Draw the structure of the triphenylphosphonium salt and Wittig reagent formed from each chloroether. (b) Draw the structural formula of the product formed by treating each Wittig reagent with cyclopentanone. Note that the functional group is an enol ether or, alternatively, a vinyl ether. (c) Draw the structural formula of the product formed on acid-catalyzed hydrolysis of each enol ether from part (b).

Short answer

Answer: The final products after acid-catalyzed hydrolysis of the enol ethers are substituted cyclopentanones. An example of the final product structure is O=C(CH2-CH2-Cl)(CCCC).

Step-by-step solution

Draw the structure of the triphenylphosphonium salt formed from each chloroether.

To do this, we first need to draw the structure of the given α-chloroether. Then, we will react it with triphenylphosphine (Ph3P) to generate the corresponding triphenylphosphonium salt. The structure of α-chloroether is: CH2Cl-CH2-O-R (where R is the alkyl or aryl group) For simplicity, let's assume R is an alkyl group (CH3, named as methyl). The reaction with triphenylphosphine is: CH2Cl-CH2-O-CH3 + Ph3P → Ph3PCH2-CH2-O-CH3 + Cl- And thus, the triphenylphosphonium salt formed from the α-chloroether is: Ph3PCH2-CH2-O-CH3

Draw the structure of the Wittig reagent formed from each triphenylphosphonium salt.

The next step is to generate the Wittig reagent, which is a ylide. To form the ylide, the triphenylphosphonium salt is treated with a strong base, for example, n-BuLi. Ph3PCH2-CH2-O-CH3 → Ph3P=CH-CH2-O-CH3 + -Li+ The resulting Wittig reagent is Ph3P=CH-CH2-O-CH3.

Draw the structural formula of the product formed by treating each Wittig reagent with cyclopentanone.

Now, we will react the Wittig reagent with cyclopentanone to form the product: Wittig reagent: Ph3P=CH-CH2-O-CH3 Cyclopentanone: O=C(CCCC) The reaction takes place between the ylide and the carbonyl group of cyclopentanone through a [2+2] cycloaddition followed by a retro-[2+2] cycloaddition: Ph3P=CH-CH2-O-CH3 + O=C(CCCC) → O=C(CH2-CH=CH2-O-CH3)(CCCC)

Draw the structural formula of the product formed on acid-catalyzed hydrolysis of each enol ether from part (b).

Finally, we will hydrolyze the enol ether under acidic conditions. The reaction between the enol ether and a strong acid, such as HCl, results in the formation of aldehydes or ketones. In this case, we will obtain a ketone: O=C(CH2-CH=CH2-O-CH3)(CCCC) + HCl → O=C(CH2-CH2-Cl)(CCCC) + CH3-OH The final product is a substituted cyclopentanone: O=C(CH2-CH2-Cl)(CCCC)

Key concepts

Synthesis of Aldehydes and Ketones via Wittig Reaction

The Wittig reaction is a robust method for forming carbon-carbon double bonds and is particularly useful in the synthesis of aldehydes and ketones. The process involves the reaction of phosphorus ylides, or Wittig reagents, with carbonyl compounds, typically aldehydes or ketones themselves. Utilizing the Wittig reaction, chemists can construct a wide array of aldehyde and ketone derivatives by varying the structure of the ylide and the carbonyl compound.

For example, when a Wittig reagent interacts with cyclopentanone, a common ketone, the result is the formation of a new carbon-carbon double bond. This bond is juxtaposed between the α-position of the cyclopentanone and the carbon atom of the ylide. Given the versatility of this method, it's a cornerstone in organic synthesis, allowing for the targeted production of complex molecules.

Triphenylphosphonium Salt: The Starting Point of Wittig Reaction

In the first step of a Wittig reaction, a triphenylphosphonium salt is formed. This important intermediate is usually derived from a halide precursor and triphenylphosphine. The salt, characterized by a phosphorus atom with a positive charge, is a key component in creating the reactive ylide.

The structure of the salt directly affects the outcome of the Wittig reaction. For instance, if we start with an \(\alpha\)-chloroether, reacting it with triphenylphosphine displaces the chloride ion, resulting in a triphenylphosphonium salt with an ether functional group. This group plays a crucial role in the reactivity and stabilization of the subsequent ylide, ultimately influencing the final product's structure.

Ylide Formation: The Reactive Intermediate in Wittig Reaction

Ylides are the reactive intermediates in the Wittig reaction and are formed when triphenylphosphonium salts react with a strong base. The resulting structures have both a positively charged phosphorus atom and a negatively charged carbon atom adjacent to it, which accounts for their unique reactivity.

The proper choice of base is critical for successful ylide generation. A common choice is n-butyl lithium (n-BuLi), which effectively abstracts a proton from the salt, resulting in the corresponding ylide. With the ylide ready, it can then undergo a [2+2] cycloaddition with a ketone or aldehyde, leading to the formation of a new carbon-carbon double bond.

Enol Ether Hydrolysis: Unlocking the Final Product in Wittig Reaction

The Wittig reaction can result in the formation of enol ethers when reacting with ketones such as cyclopentanone. However, to obtain the desired ketone or aldehyde, these enol ethers must undergo hydrolysis. Acid-catalyzed hydrolysis is a common method used to convert enol ethers into their corresponding carbonyl compounds.

This final step involves the reaction of the enol ether with a strong acid, typically HCl. It breaks the ether linkage and results in the reformation of the carbonyl group, thus yielding the stable aldehyde or ketone. The acid-catalyzed hydrolysis is an essential part of the process that ensures the Wittig reaction truly reflects its value as a synthetic tool for constructing complex carbonyl compounds.