Write the mechanism for the reaction of a 1.7 -diester with...

Key Concepts

Dieckmann Condensation

This is an intramolecular version of the Claisen condensation where a diester is treated with a strong base, such as an alkoxide ion, to form a cyclic beta-keto ester. It involves deprotonation at an alpha carbon to form an enolate, which subsequently attacks the carbonyl carbon of the other ester group within the same molecule.

Intramolecular Claisen Reaction

A Claisen condensation that occurs within a single molecule (intramolecularly) rather than between two different molecules. The mechanism includes enolate formation, nucleophilic attack on an electrophilic carbonyl carbon, and the collapse of a tetrahedral intermediate, resulting in ring closure and formation of a beta-keto ester.

Enolate Formation

This key step involves the removal of an acidic alpha-hydrogen from an ester by a strong base, such as an alkoxide ion, generating a resonance-stabilized enolate ion. This anionic species is crucial because it serves as the nucleophile that attacks an electrophilic carbonyl carbon to initiate the condensation.

Tetrahedral Intermediate and Its Collapse

Following enolate attack on the carbonyl group, a tetrahedral intermediate is formed. The instability of this intermediate causes it to collapse, expelling an alkoxide ion as a leaving group and forming a carbon–carbon bond. This step is essential in the formation of the cyclic beta-keto ester framework.

Cyclic ?-Keto Ester Formation

The final product of the reaction is a cyclic beta-keto ester, which contains both a ketone and an ester group separated by one carbon atom (beta-position relative to the ketone). Its formation is favored by the formation of a stable ring, which often minimizes strain and can be driven by thermodynamic factors.

Transcript

00:01 Okay, this problem is asking us to propose a mechanism for the reaction between a 1 -7 diester and an alk -oxide ion in order to form a cyclic beta -keto ester.

00:09 Okay, so let's go ahead and propose this mechanism.

00:11 So first off, we have the 1 -7 diester.

00:14 That means we have a 7 -carbon compound.

00:17 So 1 -2, 3, 4, 5, 6, 7, and then on the 7th carbon, we have the presence of my ester, as we do on the 1st carbon.

00:24 So first and 7th carbon, we have my esters.

00:28 Okay, so that is my 1 -7 diester, and we're going to run.

00:30 React out with an alcoxide ion.

00:32 So i'm just going to make my alcoxide the same thing that i have right here.

00:36 So that's going to be och3.

00:38 That's simply to avoid the transesterification.

00:42 So if i would have foxide, for example, that could potentially react with my carbonyl and a nucleophilic asyl substitution reaction.

00:50 So this is just to avoid that.

00:51 Because even if i did have a nucleophilic acal substitution, it would just replace this och3 with this och3.

00:57 So this is just to avoid that possibility.

01:00 Okay, but my och3, because it's a base, is going to deprotonate my most acidic hydrogen.

01:04 Okay, and because this is a symmetrical molecule, it doesn't matter as long as i deprotonate the alpha carbon.

01:09 Okay, so i'm going to use the alpha hydrogen over here.

01:13 I'm going to deprotonate that, moving the electrons onto this carbon.

01:16 Okay, and when i do that, i will have formed this product in which i have the same exact molecule over here.

01:23 I'm going to duplicate this.

01:27 Okay, and then on that alpha carbon, i'm going to have my set of lone pairs.

01:32 Okay, so the thing about a 1 -7 diester is that it can undergo an intramolecular reincyclization.

01:40 So, for example, if i see that i have lone pairs there and i see that i have a carbonyl slash ester over here, again, with kleisin connoissell, we're going to usually have a reaction between an alpha carbon that is deprotonated with another ester.

01:54 But if we have the ester on the same molecule as we do of my anion, my carbonyl, we're going to perform that intramolecular, clysen concession.

02:03 Okay, so i'm going to have my first carbon right here and that's going to attack this carbonyl right there, that ester.

02:09 Okay, and that is going to form a one, two, three, four, five, oops, five, six member drain...

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