Question

Show the product expected when the following unsaturated \(\delta\)-ketoester is treated with each reagent.

Short answer

Question: Predict the products when an unsaturated \(\delta\)-ketoester reacts with a Grignard reagent and sodium borohydride. Answer: When an unsaturated \(\delta\)-ketoester reacts with a Grignard reagent, a tertiary alcohol is formed with a new carbon-carbon bond at the ketone carbonyl group. When treated with sodium borohydride, the ketone carbonyl group is selectively reduced to form an alcohol without reducing the ester functional group.

Step-by-step solution

Identify the Unsaturated \(\delta\)-ketoester Structure

An unsaturated \(\delta\)-ketoester is an organic molecule containing a conjugated system formed by an ester functional group with a ketone group \(\delta\) (4) carbons away and a carbon-carbon double bond between them. The general structure of such a molecule is as follows: $$\ce{-COOR-} \ce{C=C} \ce{R1} \ce{C=O} \ce{-R2-}$$ To determine the product expect when treated with certain reagents, we will consider this unsaturated \(\delta\)-ketoester as a generic structure.

Determine the Reagents

The given reagents are not specified, so to continue with the exercise, let us consider two common reagents: 1. Grignard reagent: \(\ce{R-MgX}\) 2. Sodium borohydride: \(\ce{NaBH4}\)

Reaction with Grignard Reagent (\(\ce{R-MgX}\))

The Grignard reagent is a strong nucleophile and a very strong base. The reaction between the Unsaturated \(\delta\)-ketoester and the Grignard reagent mainly occurs at the ketone carbonyl group (\(\ce{C=O}\)) due to its high reactivity toward nucleophiles. The mechanism involves the Grignard reagent attacking the carbonyl carbon, thus breaking the \(\ce{C=O}\) bond and forming a new carbon-carbon bond. The product is an alkoxide intermediate, which is then protonated with water or an acidic workup to form a tertiary alcohol. Considering the generic structure for the unsaturated \(\delta\)-ketoester and the Grignard reagent, the reaction proceeds as follows: $$\ce{-COOR-} \ce{C=C} \ce{R1} \ce{C=O} \ce{-R2-} + \ce{R-MgX} \rightarrow \ce{-COOR-} \ce{C=C} \ce{R1} \ce{C(O^-)(R)(R2)} \rightarrow \ce{-COOR-} \ce{C=C} \ce{R1} \ce{C(OH)(R)(R2)}$$

Reaction with Sodium Borohydride (\(\ce{NaBH4}\))

Sodium borohydride (\(\ce{NaBH4}\)) is a mild reducing agent that selectively reduces aldehydes and ketones to their corresponding alcohols. In this case, the reaction would occur at the ketone carbonyl group (\(\ce{C=O}\)). However, it does not reduce esters, amides, or nitriles. The reaction mechanism involves the hydride ion from sodium borohydride attacking the carbonyl carbon to form a new \(\ce{C-H}\) bond, consequently forming an alcohol. For the generic unsaturated \(\delta\)-ketoester treated with sodium borohydride, the reaction proceeds as follows: $$\ce{-COOR-} \ce{C=C} \ce{R1} \ce{C=O} \ce{-R2-} + \ce{NaBH4} \rightarrow \ce{-COOR-} \ce{C=C} \ce{R1} \ce{C(OH)(H)(R2)}$$ Now, based on the chosen reagents, we have determined the expected product for each reaction. For any other specific reagent, a similar approach can be applied after understanding their reactivity and the possible reaction mechanism.