The document discusses the Meyer-Schuster rearrangement and Rupe rearrangement, which are acid-catalyzed rearrangements of propargylic alcohols. The Meyer-Schuster rearrangement involves a [1,3]-shift of the hydroxyl group to form α,β-unsaturated aldehydes or ketones. The Rupe rearrangement is similar but involves a [1,2]-shift to form α,β-unsaturated ketones. Both reactions can be catalyzed by protic or Lewis acids. The products differ depending on whether the alkyne is terminal or disubstituted.

1. BY
Dr. Gurumeet.C.Wadhawa
DEPARTMENT OF CHEMISTRY
K. B. P. College,Vashi,Navimumbai
MEYER-SCHUSTER AND RUPE
REARRANGEMENT

2. In 1922, K.H. Meyer and K. Schuster reported that the attempted
conversion of 1,1,3-triphenyl-2-propynol to the corresponding ethyl
ether with concentrated sulfuric acid and ethanol afforded 1,3,3-
triphenyl propenone, an α,β- unsaturated ketone.
The authors showed that the use of other reagents such as acetic
anhydride and acetyl chloride also brought about the same reaction.
A few years later, H. Rupe and co-workers investigated the acid-
catalyzed
rearrangement of a large number of α-acetylenic (propargylic)
alcohols. The acid-catalyzed isomerization of secondary and tertiary
propargylic alcohols, via a [1,3]-shift of the hydroxyl group, to the
corresponding α,β- unsaturated aldehydes or ketones is known as the
Meyer-Schuster rearrangement.

7. Meyer-Schuster rearrangement. The general features of this
transformation are: 1) when the substrate contains a terminal alkyne,
the product is an aldehyde, whereas substrates containing
disubstituted alkynes yield ketones;
2) the substrates, 2° or 3° propargylic alcohols, may not have a
proton
at their α-position so that the initial propargylic cation can isomerize
to an allenyl cation, which provides the product carbonyl compound;
3) the rearrangement can be catalyzed by both protic and Lewis
acids under anhydrous or aqueous conditions.

8. The related acid-catalyzed rearrangement of tertiary propargylic alcohols, via a
formal [1,2]-shift of the hydroxyl group, yielding the corresponding α,β-
unsaturated ketones is called the Rupe rearrangement. The most important
features of this reaction are:
1) the product is always the α,β-unsaturated ketone regardless of the
substitution of the triple bond;
2) the substrates are tertiary propargylic alcohols that have hydrogen atoms
available
at their α-position;
3) most often strong protic acids mixed with alcohol solvents are used to bring
about the rearrangement, but certain Lewis acid such as mercury(II)-salts and
even dehydrating agents (SOCl2, P2O5, etc.) were shown to be effective;
4) the nature of the acid catalyst does not affect the course of the
rearrangement.

9. disadvantages of the above two rearrangements are: 1) certain substrates may
give rise to a mixture of Rupe and Meyer-Schuster rearrangement products;
2) low yields are observed when the product (especially aldehydes) undergoes
self-condensation, or is readily oxidized under the reaction conditions;
3) acid-sensitive functionalities in the substrate may give undesired
elimination products; and
4) the initial propargylic cation occasionally undergoes Wagner-Meerwein or
Nametkin rearrangement.

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