Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions

Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition

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Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions

Problem 60

Chapter 9, Problem 60

In 1973, Caine and Tuller reported a synthesis of racemic oplapanone, a sesquiterpene isolated from Oplopanax japonicus (a deciduous shrub) involving a reaction we learned in this chapter. Predict the product of the reaction shown. (Caine, D.; Tuller, F. N. J. Org. Chem. 1973, 38, 3663.)

Verified step by step guidance

Step 1: Analyze the reaction conditions. The reagents Hg²⁺, HgSO₄, and H₂O indicate that this is a hydration reaction of an alkyne, specifically oxymercuration-demercuration. This reaction typically converts an alkyne into a ketone via Markovnikov addition of water.

Step 2: Identify the functional group undergoing the reaction. The molecule contains a terminal alkyne group (-C≡CH) attached to a bicyclic structure. This alkyne will react with the Hg²⁺ catalyst and water to form a ketone.

Step 3: Predict the intermediate. In the presence of Hg²⁺, the alkyne undergoes electrophilic addition to form a mercurinium ion intermediate. Water then attacks the more substituted carbon of the alkyne, leading to the formation of an enol intermediate.

Step 4: Tautomerization of the enol. The enol intermediate undergoes keto-enol tautomerization, which is acid-catalyzed, to yield the final ketone product. The ketone will form at the more substituted carbon due to Markovnikov addition.

Step 5: Consider stereochemistry and the bicyclic structure. The reaction does not affect the stereochemistry of the other groups in the molecule, such as the hydroxyl group and methyl groups. The ketone will be incorporated into the bicyclic structure at the position of the original alkyne.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Synthesis of Sesquiterpenes

Sesquiterpenes are a class of terpenes consisting of three isoprene units, resulting in a 15-carbon structure. Understanding their synthesis involves recognizing the specific reactions and mechanisms that can form these complex molecules. The synthesis often includes cyclization and rearrangement reactions, which are crucial for constructing the unique carbon skeletons characteristic of sesquiterpenes.

Racemic Mixtures

A racemic mixture contains equal amounts of two enantiomers, which are molecules that are mirror images of each other. In organic synthesis, producing a racemic mixture can occur when a reaction does not favor one enantiomer over the other, leading to a 50/50 distribution. Understanding the implications of racemic mixtures is essential for predicting the properties and reactivity of the synthesized compound.

Reaction Mechanisms

Reaction mechanisms describe the step-by-step process by which reactants transform into products, detailing the bond-breaking and bond-forming events. Familiarity with common mechanisms, such as nucleophilic substitutions or electrophilic additions, is vital for predicting the outcome of a given reaction. Analyzing the mechanism helps in understanding how specific conditions or reagents influence the final product.

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The synthesis of five-membered lactones (cyclic esters) has been accomplished using the electrophilic addition of I―Cl to an alkyne. Suggest a mechanism for this cyclization reaction. (Structure modification of Yao, T.; Larock, R.C. J. Org. Chem. 2005, 70, 1432–1437.)

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A chemist attempted to do the following acetylide alkylation reaction but was unsuccessful in several attempts, producing only the original starting materials in each case. Explain why the reaction didn't work.

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When alkynes are treated with water and bromine a bromoketone is produced. Provide a plausible arrow-pushing mechanism that accounts for the formation of this product.

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