What Is Rupe Rearrangement?
The history of the Rupe rearrangement is inextricably linked with the Meyer-Schuster rearrangement. In 1922, Kurt H. Meyer and Kurt Schuster reported the conversion of propargylic alcohols bearing either secondary or tertiary carbinol centers into α,β‑unsaturated aldehydes (terminal alkynes) or ketones (internal alkynes) under acidic conditions. A few years later, Rupe made a key observation while studying tertiary terminal α‑alkylacetylenic alcohols: under similar conditions, the expected α,β‑unsaturated aldehyde did not form. Instead, the product was identified as the corresponding α,β‑unsaturated methyl ketone, a transformation that proceeds via an enyne intermediate.
Thus, the Rupe rearrangement is defined as the acid‑catalyzed transformation of a tertiary propargylic alcohol bearing a terminal acetylenic group (i.e., with the structure R2C(OH)–C≡CH) into an α,β‑unsaturated methyl ketone through an enyne intermediate.
- Reagents: Brønsted acids (formic acid, phosphoric acid, P2O5–MeSO3H) or solid acids (strongly acidic cation-exchange resins such as Dowex‑50, zeolites); also near‑critical water with additives (e.g., NaHSO4) or platinum carbonyl complexes.
- Reactants: Tertiary propargylic alcohols bearing a terminal alkyne group (R2C(OH)–C≡CH).
- Products: α,β‑Unsaturated methyl ketones (R2C=CH–C(=O)CH3). The carbonyl group is at the original alkyne terminus (C‑1 position).
- Reaction Type: Acid‑catalyzed isomerization (formal 1,2‑shift); atom‑economic rearrangement.
- Related Reactions:
– Meyer-Schuster rearrangement (competitive pathway for tertiary propargylic alcohols with terminal alkynes; gives α,β‑unsaturated aldehydes instead of methyl ketones).
– Divinyl ketone cyclopentannulation (Rupe rearrangement integrated with Nazarov cyclization).
Fig 1. Rupe rearrangement reaction and its mechanism. [1]
Mechanism of Rupe Rearrangement
The Classical View
For many years, the mechanistic details of the Rupe rearrangement were considered less firmly established than those of the Meyer-Schuster rearrangement. Classical textbooks described both rearrangements as acid‑catalyzed isomerizations of α‑acetylenic alcohols, with the structural nature of the starting alcohol being the primary determinant of the product outcome. In the Meyer-Schuster pathway, a formal 1,3‑shift of the hydroxyl group takes place, leading to the α,β‑unsaturated carbonyl compound via an allenol intermediate. For the Rupe rearrangement, the reaction proceeds by way of an enyne intermediate (enol‑alkyne) followed by water addition to furnish the methyl ketone framework.
Modern Mechanistic Understanding
Deeper insight has emerged from computational studies. A seminal DFT investigation by Yamabe and co‑workers examined the Rupe and Meyer-Schuster rearrangements for the model substrates 2‑methylbut‑3‑yn‑2‑ol (R = Me) and 1,1‑diphenylprop‑2‑yn‑1‑ol (R = Ph). The key findings are especially informative:
- The Rupe rearrangement was found to be much more favorable than the Meyer-Schuster rearrangement when R = Me (i.e., when the substrate possesses alkyl substituents).
- Conversely, for the diphenyl‑substituted substrate (R = Ph), the Meyer-Schuster rearrangement is highly favorable, proceeding with a very small activation energy.
Crucially, the computational analysis revealed that neither rearrangement proceeds via a discrete free carbenium ion intermediate. Instead, the calculated transition‑state geometries are carbonium‑ion‑like, and the rearrangements may proceed through an intermolecular proton relay along hydrogen‑bond chains. [2]
Comparison with the Meyer-Schuster Rearrangement
The Rupe and Meyer-Schuster rearrangements share the same starting material class but differ fundamentally in product outcome and mechanistic pathway. The table below summarizes the key distinctions:
| Feature | Meyer-Schuster | Rupe |
| Alcohol type | Secondary or tertiary | Tertiary only |
| Alkyne type | Terminal → aldehyde; Internal → ketone | Terminal only |
| Product | α,β‑Unsaturated aldehyde/ketone (carbonyl at C‑3) | α,β‑Unsaturated methyl ketone (carbonyl at C‑1) |
| Intermediate | Allenol | Enyne |
| Substrate preference | Secondary alcohols; tertiary alcohols with aryl groups | Tertiary alcohols with alkyl groups |
| Typical catalysts | Metal‑catalyzed (Au, Ru, Sc, Fe) or Brønsted acids | Brønsted acids; solid acids; Pt complexes |
| Stereochemical outcome | Usually (E)‑isomer | Typically (E)‑isomer for enone |
Application Examples of Rupe Rearrangement
- Total Synthesis of Natural Products: The Rupe rearrangement has found prominent use in the assembly of natural product frameworks. The reaction is emerging as a method of choice for homologation of carbonyl compounds into α,β‑unsaturated aldehydes and ketones, particularly useful for the synthesis of many hindered ketones that are not readily accessible by conventional aldol or Wittig approaches. Cadierno and co‑workers have emphasized that both the Meyer–Schuster and Rupe rearrangements are emerging as the methods of choice for the homologation of carbonyl compounds, because they offer atom‑economical and straightforward access to these valuable synthons. [3]
- Pharmaceutical Process Development: During scale‑up studies for a new steroidal drug, Weinmann and colleagues encountered the need to perform a Rupe rearrangement on a very large scale in the pilot plant. The traditional formic acid procedure was not safely adaptable to the pilot plant environment. The alternative approach—using strongly acidic cation‑exchange resins—proved to be remarkably efficient. The reaction was successfully scaled to 64 kg, and the procedure was applied to several other steroids on a laboratory scale. [4]
Fig 2. Synthetic examples via Rupe rearrangement reaction.
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References
- Jie Jack Li. Name Reactions-A Collection of Detailed Mechanisms and Synthetic Applications, Fourth Edition, 2014, 529-530.
- Yamabe, Shinichi, et al. Journal of Chemical Theory and Computation 2.5 (2006): 1379-1387.
- Cadierno, Victorio, et al. Dalton Transactions 39.17 (2010): 4015-4031.
- Weinmann, Hilmar, et al. Organic process research & development 6.3 (2002): 216-219.
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