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What happens to the t-butyl cation in the TFA deprotection of a t-butyl ester?
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Mahesh Somasundaram
What happens to the t-butyl cation in the TFA deprotection of a t-butyl ester?
The reaction is, in principle, catalytic in TFA. If t-butyltrifluoroacetate is the by-product, then the catalyst is consumed. If only isobutylene is formed, catalytic is OK. Using excess TFA covers all contingencies. In the initial step, protonation is more likely on the C=O oxygen. Higher electron density.
The reaction is, in principle, catalytic in TFA. If t-butyltrifluoroacetate is the by-product, then the catalyst is consumed. If only isobutylene is formed, catalytic is OK. Using excess TFA covers all contingencies. In the initial step, protonation is more likely on the C=O oxygen. Higher electron density.
Briefly, the released t-butyl carbocation is subsequently deprotonated by the anionic TFA, resulting in the formation of 2-methyl-propene. This regenerates the protonated TFA that is needed to continue the reaction, and thus the TFA is catalytic (i.e. not consumed in the reaction).
Briefly, the released t-butyl carbocation is subsequently deprotonated by the anionic TFA, resulting in the formation of 2-methyl-propene. This regenerates the protonated TFA that is needed to continue the reaction, and thus the TFA is catalytic (i.e. not consumed in the reaction).
The reaction is, in principle, catalytic in TFA. If t-butyltrifluoroacetate is the by-product, then the catalyst is consumed. If only isobutylene is formed, catalytic is OK. Using excess TFA covers all contingencies. In the initial step, protonation is more likely on the C=O oxygen. Higher electron density.
The reaction is, in principle, catalytic in TFA. If t-butyltrifluoroacetate is the by-product, then the catalyst is consumed. If only isobutylene is formed, catalytic is OK. Using excess TFA covers all contingencies. In the initial step, protonation is more likely on the C=O oxygen. Higher electron density.
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I realize this question is old but the mechanism is nicely described here (steps 1-3): http://www.umich.edu/~chemh215/W06HTML/SSG2/ssg6/main_files/Page346.htm
Briefly, the released t-butyl carbocation is subsequently deprotonated by the anionic TFA, resulting in the formation of 2-methyl-propene. This regenerates the protonated TFA that is needed to continue the reaction, and thus the TFA is catalytic (i.e. not consumed in the reaction).
I realize this question is old but the mechanism is nicely described here (steps 1-3): http://www.umich.edu/~chemh215/W06HTML/SSG2/ssg6/main_files/Page346.htm
Briefly, the released t-butyl carbocation is subsequently deprotonated by the anionic TFA, resulting in the formation of 2-methyl-propene. This regenerates the protonated TFA that is needed to continue the reaction, and thus the TFA is catalytic (i.e. not consumed in the reaction).
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