Theo Nash
Propargyl alcohols are useful intermediates for organic synthesis, containing both an alcohol and a carbon-carbon triple bond that can undergo a variety of reactions. Gold-catalyzed rearrangement of propargyl alcohols is an important experiment in organic chemistry, despite not being commonly featured in second-year organic chemistry courses. The air-stable and water-insensitive nature of gold(I) catalysts enables a variety of synthetic transformations with a simple reaction setup.
| Characteristics | Values |
|---|---|
| Experiment type | Gold-catalyzed rearrangement |
| Experiment focus | Propargyl alcohols |
| Experiment aim | Determining isomeric ratios |
| Catalyst | Gold |
| Gold type | Cationic gold(I) |
| Reactant | Propargylic alcohol |
| Reactant properties | Contains alcohol and carbon-carbon triple bond |
| Reactant conversion | α,α-diiodo-β-hydroxyketones |
| Reactant conversion | α,α-dichloro-β-hydroxyketones |
| Reactant conversion | 3-hydroxyketones |
| Reactant conversion | 3-aminoketones |
| Reactant conversion | 1,3-amino alcohols |
| Reactant conversion | Enantiomers |
| Reactant conversion | Enones |
| Reactant conversion | Aromatic heterocycles |
| Reactant conversion | β-aminoketones |
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What You'll Learn
Gold-catalysed hydroamination of propargylic alcohols
Propargylic alcohols are useful intermediates for organic synthesis, as they contain both an alcohol and a carbon-carbon triple bond. These compounds can undergo a variety of catalytic transformations, including conversion into aromatic heterocycles and the substitution of the alcohol group with nucleophiles.
One such transformation is the gold-catalysed hydroamination of propargylic alcohols, which provides access to three different products from the same starting materials. This process involves the hydroamination of propargylic alcohols with anilines, resulting in 3-hydroxy imines with complete regioselectivity. These 3-hydroxy imines can then be reduced to 1,3-amino alcohols with high syn selectivity. Alternatively, using a catalytic quantity of aniline, 3-hydroxyketones can be obtained directly from propargylic alcohols. Further manipulation of reaction conditions allows for the selective formation of 3-aminoketones through a rearrangement/hydroamination pathway.
The utility of this process was demonstrated in the one-pot synthesis of N-arylpyrrolidines and N-arylpiperidines. Additionally, the Meyer-Schuster rearrangement of propargylic alcohols into enones provides an effective alternative to the olefination of carbonyl compounds with phosphorus reagents.
In terms of specific reactions, the combination of a gold(I) catalyst and potassium carbonate has been used to mediate the addition of phenols to propargylic alcohols/amines, resulting in phenyl enol ethers. This reaction is chemo-, regio-, and stereoselective and exhibits excellent functional group tolerance.
Furthermore, the gold-catalysed dihalohydration of propargylic alcohols has been studied, with the regioselective conversion of these alcohols into α,α-diiodo-β-hydroxyketones achieved in the presence of a gold catalyst and N-iodosuccinimide.
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Meyer-Schuster rearrangement
The Meyer–Schuster rearrangement is an acid-catalyzed rearrangement of secondary and tertiary propargyl alcohols to α,β-unsaturated ketones if the alkyne group is internal and α,β-unsaturated aldehydes if the alkyne group is terminal. The reaction is named after Kurt Meyer and Kurt Schuster.
The Meyer–Schuster rearrangement proceeds by three major steps:
- The rapid protonation of oxygen
- The slow, rate-determining step comprising the 1,3-shift of the protonated hydroxy group
- The keto-enol tautomerism followed by rapid deprotonation.
The formation of the unsaturated carbonyl compound is irreversible. Solvent caging is proposed to stabilize the transition state. The Meyer–Schuster rearrangement has been used in a variety of applications, including the conversion of ω-alkynyl-ω-carbinol lactams into enamides using catalytic PTSA and the synthesis of α,β-unsaturated thioesters from γ-sulfur-substituted propargyl alcohols.
One of the most interesting applications is the synthesis of a part of paclitaxel in a diastereomerically-selective way that leads only to the E-alkene. The Meyer–Schuster rearrangement has also been used in the synthesis of taxol, with a 70% yield (91% when the byproduct was converted to the Meyer-Schuster product in another step).
The major challenge associated with the Meyer–Schuster reaction is selectively promoting the desired rearrangement over the myriad of other reaction pathways available to propargyl alcohols. The Meyer–Schuster rearrangement of propargylic alcohols into enones provides an extremely effective alternative to the olefination of carbonyl compounds with phosphorus reagents.
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Propargyl alcohol's reaction with strong acid
Propargyl alcohols are useful intermediates for organic synthesis, as they contain both an alcohol and a carbon-carbon triple bond, which can undergo a variety of reactions. In an experiment, propargylic alcohol was reacted with a gold catalyst and N-iodosuccinimide (NIS) in PhMe at room temperature. This reaction resulted in the regioselective conversion of propargylic alcohols into α,α-diiodo-β-hydroxyketones.
Propargyl alcohol is highly flammable and can cause severe health hazards. It is a central nervous system depressant and may be fatal if absorbed through the skin or inhaled. It can also cause severe irritation to the mucous membranes, upper respiratory tract, eyes, and skin. Mixtures of propargyl alcohols with concentrated sulfuric acid and strong hydrogen peroxide can cause explosions.
Propargyl alcohol is an acetylenic alcohol, which is an important group of film-forming corrosion inhibitors for carbon steel in hydrochloric acid (HCl). The degradation mechanism of propargyl alcohol in HCl has been studied using proton nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR) techniques. The reaction between propargyl alcohol and HCl results in the formation of water-soluble products such as 1-hydroxypropan-2-one, 2-chloroprop-2-en-1-ol, 2,2-dichloropropan-1-ol, and 2-chloropropane-1,2-diol.
In summary, propargyl alcohols can undergo a variety of reactions, including those catalysed by gold and strong acids. However, due to the hazardous nature of propargyl alcohol, caution must be exercised when handling this substance.
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Gold as a transition metal in organic synthesis
Gold is an increasingly prominent transition metal in organic synthesis. Transition metal catalysis has contributed to the discovery of novel methodologies and the preparation of natural products, as well as new opportunities to increase the chemical space in drug discovery programs. Gold's role in organic synthesis is partly due to its stability against oxidation and its variety in morphology, for instance, gold cluster materials. Gold has been shown to be effective in low-temperature carbon monoxide (CO) oxidation and acetylene hydrochlorination to vinyl chlorides. Gold-catalyzed reactions fall into two major categories: heterogeneous catalysis, including catalysts by gold nanoparticles (e.g., Au/TiO2) and thiol-monolayer gold surfaces, and catalysts on alumina support, including alumina supported Au/CeO2. These catalysts are used for industrially important processes like the oxidation of alcohols, CO oxidation, and various selective hydrogenation reactions (e.g., butadiene to butene).
Homogeneous catalysis with gold, on the other hand, uses simple or ligand-bound gold(I) or gold(III) compounds that are soluble in organic solvents and is used for the synthesis of fine chemicals in organic chemistry. Gold(I) complexes are 2-coordinate, linear, diamagnetic, 14-electron species that exist as adducts LAuR with a ligand L, such as a triphenylphosphine or an isocyanide. Gold(I) can also exist as the aurate M [AuR2], where the cation is usually fitted with a complexing agent to improve stability. Gold(III) complexes, on the other hand, have lower stability under catalytic conditions, but simple AuCl3 has been found to be an efficient catalyst in some cases, such as in the Hashmi-reported AuCl3-catalyzed alkyne/furan Diels–Alder reaction. Gold catalyses many organic transformations, usually carbon-carbon bond formation from Au(I) and C-X (X = O, N) bond formation from the Au(III) state due to the ion's harder Lewis acidity.
Gold's use in organic synthesis has been fuelled by several key attributes, including exceptional functional group tolerance, superior biocompatibility, more stability against bleaching and poisoning, high atom economy, and distinct reactivity profiles compared to other transition metal catalysts. Gold catalysts have been particularly useful in the regioselective conversion of propargylic alcohols into previously unreported α,α-diiodo-β-hydroxyketones in the presence of a gold catalyst. The Meyer–Schuster rearrangement of propargylic alcohols into enones is another important reaction, providing an effective alternative to the olefination of carbonyl compounds with phosphorus reagents. Gold-catalyzed reactions of propargylic alcohols have been targeted at second-year organic chemistry students to introduce them to the growing importance of gold in organic synthesis.
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Gold-catalysed transformations of propargyl amines
Propargyl alcohols and propargyl amines are useful building blocks for the synthesis of heterocycles and complex molecules. The development of homogeneous gold catalysis has greatly benefited organic synthesis.
Another example of gold-catalysed transformations of propargyl amines is the work of Zhang and colleagues, who synthesized O- and N-containing 4-membered rings. They based their synthesis on the propensity of α-oxo gold carbenes, generated from alkynes by Au(I)-catalyzed intermolecular oxidation. This reaction produced oxetan-3-ones from readily available propargylic alcohols, carried out under air using pyridine N-oxides as oxidants and catalytic amounts.
Furthermore, propargyl amines have been used in the Ugi reaction and post-Ugi transformations, which allow for the synthesis of diverse and complex molecules through simple and sustainable processes. For instance, Peshkov, Pereshivko, and their team reported a post-Ugi cationic gold-catalysed alkyne activation for the synthesis of polycyclic indole-fused frameworks.
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Frequently asked questions
Propargyl alcohols are useful intermediates for organic synthesis, containing both an alcohol and a carbon-carbon triple bond. In a gold-catalyzed rearrangement experiment, propargyl alcohols can undergo a variety of reactions, including conversion into enones, aromatic heterocycles, and nucleophiles.
One example is the Meyer-Schuster rearrangement, where propargylic alcohol is reacted with a gold catalyst and N-iodosuccinimide (NIS) to form α-haloenones. Another example is the regioselective conversion of propargylic alcohols into α,α-diiodo-β-hydroxyketones, also using a gold catalyst and NIS.
Gold, specifically homogeneous gold catalysts, has become an increasingly prominent transition metal in organic synthesis due to its stability and versatility. Gold catalysts enable a variety of synthetic transformations with a simple reaction setup and can promote reactions that may not occur in their absence.
One technique is to use MeOH, which increases the concentration of MeOH and allows for an accelerated nucleophilic attack on the OH group of propargyl alcohol. Additionally, catalytic quantities of aniline can be used to obtain 3-hydroxyketones directly from propargylic alcohols.
Yes, there are non-catalyzed reactions of propargylic alcohols that can be explored, as well as the use of other catalysts such as silver(I) in similar reactions. Additionally, the gold-catalyzed hydroamination of propargylic alcohols is a related experiment that provides controlled access to different products from the same starting materials.
