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Ch. 10 - Alkynes: Electrophilic Addition and Redox Reactions
Mullins - Organic Chemistry: A Learner Centered Approach 1st Edition
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Tutti i libri di testoMullins 1st EditionCh. 10 - Alkynes: Electrophilic Addition and Redox ReactionsProblema 55
Capitolo 9, Problema 55

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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Step 1: Analyze the starting material and identify the functional groups. The molecule contains an alkyne (≡C), a hydroxyl group (-OH), a ketone (C=O), and a trimethylsilyl group (SiMe₃). The reaction involves electrophilic addition of I-Cl to the alkyne.
Step 2: Recognize the role of I-Cl as an electrophile. The iodine atom (I) is less electronegative and will act as the electrophilic center, while chlorine (Cl) is more electronegative and will act as the nucleophilic center.
Step 3: Propose the first step of the mechanism. The alkyne undergoes electrophilic attack by the iodine atom, forming a cyclic iodonium ion intermediate. This step is facilitated by the electron-rich π-bond of the alkyne.
Step 4: Suggest the nucleophilic attack by the hydroxyl group (-OH). The hydroxyl group attacks the iodonium ion intermediate, leading to the formation of a five-membered lactone ring. This step involves intramolecular cyclization.
Step 5: Finalize the mechanism by considering the addition of chlorine (Cl) to the remaining carbon of the alkyne. The chlorine atom adds to the carbon adjacent to the iodine, completing the electrophilic addition and forming the final product with the lactone ring and halogen substituents.

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Electrophilic Addition

Electrophilic addition is a fundamental reaction mechanism in organic chemistry where an electrophile reacts with a nucleophile, resulting in the formation of a new bond. In this context, iodine (I) acts as the electrophile, adding to the alkyne, which is a nucleophilic site due to its electron-rich triple bond. This step is crucial for initiating the cyclization process leading to the formation of the five-membered lactone.
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1,2 vs 1,4 Addition

Cyclization Reactions

Cyclization reactions involve the formation of a cyclic compound from a linear precursor. In this case, the electrophilic addition of I-Cl to the alkyne facilitates the formation of a cyclic ester (lactone) by creating a new bond that closes the ring. Understanding the conditions and mechanisms that favor cyclization is essential for predicting the outcome of such reactions in organic synthesis.
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Cyclization Reaction

Lactone Formation

Lactones are cyclic esters formed from the reaction of a hydroxy acid or a carboxylic acid with an alcohol, resulting in the loss of water. The synthesis of five-membered lactones, as described in the question, typically involves intramolecular reactions where the hydroxyl group attacks the carbonyl carbon, leading to ring closure. Recognizing the structural features that promote lactone formation is vital for understanding the overall reaction mechanism.
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Lactones and Lactams
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