Imine/Enamine formation

[663697]

This is Q41 from TPR test 3.

They say that p-TSA is an acid and don't mention the presence of a base. When I wrote out the mechanism I got an imine, so am I supposed to assume that water will act as a base to abstract a proton to produce the C=C bond?

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[77deuce]

This is Q41 from TPR test 3.

They say that p-TSA is an acid and don't mention the presence of a base. When I wrote out the mechanism I got an imine, so am I supposed to assume that water will act as a base to abstract a proton to produce the C=C bond?

Primary amines normally react in acidic conditions with a carbonyl to form imines, so I agree with you there.

I'm not sure why the enamine formed here. The intermediate iminium H should have been more acidic than the benzylic H pulled to form the C=C bond.

What was the actual question asked on the test?

 

[663697]

The intramolecular cyclization in Reaction 2 is accomplished via which mechanistic pathway?


A. Nucleophilic addition of the amine on the aldehyde, followed by dehydration Correct Answer

B. Electrophilic addition of the amine on the aldehyde, followed by dehydration

C. Nucleophilic substitution of the nitrogen by the oxygen

D. Nucleophilic substitution of the oxygen by the nitrogen

 

[77deuce]

The intramolecular cyclization in Reaction 2 is accomplished via which mechanistic pathway?


A. Nucleophilic addition of the amine on the aldehyde, followed by dehydration Correct Answer

B. Electrophilic addition of the amine on the aldehyde, followed by dehydration

C. Nucleophilic substitution of the nitrogen by the oxygen

D. Nucleophilic substitution of the oxygen by the nitrogen

The answer to the question makes sense. Both imine and enamine formation are nucleophilic addition to the carbonyl. The concern about the mechanism that we agree on doesn't really have a bearing on this answer.

If your main concern was with the diagram given and not any of the questions asked, then I wouldn't sweat it too much. It looks like you know your stuff.

 

[77deuce]

The answer to the question makes sense. Both imine and enamine formation are nucleophilic addition to the carbonyl. The concern about the mechanism that we agree on doesn't really have a bearing on this answer.

If your main concern was with the diagram given and not any of the questions asked, then I wouldn't sweat it too much. It looks like you know your stuff.

One more thing. This product may have been favored because it forms an additional aromatic ring. The extra stability gained from the aromatic ring may have been enough to favor the formation of the enamine XD.

 

[663697]

One more thing. This product may have been favored because it forms an additional aromatic ring. The extra stability gained from the aromatic ring may have been enough to favor the formation of the enamine XD.

Yeah the original question was sorta unrelated to my question haha.

I didn't even think about the extra stability provided by the aromatic ring, good point. Perhaps that's why they chose that product

 

[aldol16]

Imines and enamines are tautomers, meaning that they interconvert readily. So if you can make imines and enamines, you'll likely end up making both. Same concept as keto-enol tautomerism. In this case, the enamine form is favored thermodynamically precisely because of the aromatic stabilization of the indole ring.