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Chiral cyclopentadienylruthenium sulfoxide catalysts for asymmetric redox bicycloisomerization

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ABSTRACT

1: A full account of our efforts toward an asymmetric redox bicycloisomerization reaction is presented in this article. Cyclopentadienylruthenium (CpRu) complexes containing tethered chiral sulfoxides were synthesized via an oxidative [3 + 2] cycloaddition reaction between an alkyne and an allylruthenium complex. Sulfoxide complex containing a p-anisole moiety on its sulfoxide proved to be the most efficient and selective catalyst for the asymmetric redox bicycloisomerization of 1,6- and 1,7-enynes. This complex was used to synthesize a broad array of [3.1.0] and [4.1.0] bicycles. Sulfonamide- and phosphoramidate-containing products could be deprotected under reducing conditions. Catalysis performed with enantiomerically enriched propargyl alcohols revealed a matched/mismatched effect that was strongly dependent on the nature of the solvent.

No MeSH data available.


(a) Mechanism for the redox biscycloisomerization reaction. (b) Ruthenium catalyst containing a tethered chiral sulfoxide. (c) Possible diastereomeric complexes formed from alcohol coordination.
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Figure 1: (a) Mechanism for the redox biscycloisomerization reaction. (b) Ruthenium catalyst containing a tethered chiral sulfoxide. (c) Possible diastereomeric complexes formed from alcohol coordination.

Mentions: While there had been reports of utilizing chiral ruthenium complexes for asymmetric catalysis prior to our studies [30–40], there had previously been no reported examples of asymmetric ruthenium-catalyzed cycloisomerization reactions in the literature. In 2011, our research group disclosed the ruthenium-catalyzed redox bicycloisomerization of 1,6- and 1,7-enynes to construct structurally complex [3.1.0] and [4.1.0] bicycles containing vicinal, quaternary all-carbon stereocenters [41] (Fig. 1). The proposed mechanism of this fascinating reaction involves chloride abstraction of the ruthenium catalyst by indium(III) triflate and phosphine ligand dissociation. The propargyl alcohol then coordinates to the now coordinatively unsaturated cyclopentadienylruthenium (CpRu) fragment in a bidentate fashion and undergoes a redox isomerization reaction wherein the carbinol proton performs a 1,2-hydride shift. The resulting vinylruthenium intermediate can be seen as a resonance structure of a ruthenium carbene, which coordinates to a pendant alkene, performs a [2 + 2] cycloaddition to form a ruthenacyclobutane, and reductively eliminates to generate the bicyclic product.


Chiral cyclopentadienylruthenium sulfoxide catalysts for asymmetric redox bicycloisomerization
(a) Mechanism for the redox biscycloisomerization reaction. (b) Ruthenium catalyst containing a tethered chiral sulfoxide. (c) Possible diastereomeric complexes formed from alcohol coordination.
© Copyright Policy - Beilstein
Related In: Results  -  Collection

License 1 - License 2
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getmorefigures.php?uid=PMC4979649&req=5

Figure 1: (a) Mechanism for the redox biscycloisomerization reaction. (b) Ruthenium catalyst containing a tethered chiral sulfoxide. (c) Possible diastereomeric complexes formed from alcohol coordination.
Mentions: While there had been reports of utilizing chiral ruthenium complexes for asymmetric catalysis prior to our studies [30–40], there had previously been no reported examples of asymmetric ruthenium-catalyzed cycloisomerization reactions in the literature. In 2011, our research group disclosed the ruthenium-catalyzed redox bicycloisomerization of 1,6- and 1,7-enynes to construct structurally complex [3.1.0] and [4.1.0] bicycles containing vicinal, quaternary all-carbon stereocenters [41] (Fig. 1). The proposed mechanism of this fascinating reaction involves chloride abstraction of the ruthenium catalyst by indium(III) triflate and phosphine ligand dissociation. The propargyl alcohol then coordinates to the now coordinatively unsaturated cyclopentadienylruthenium (CpRu) fragment in a bidentate fashion and undergoes a redox isomerization reaction wherein the carbinol proton performs a 1,2-hydride shift. The resulting vinylruthenium intermediate can be seen as a resonance structure of a ruthenium carbene, which coordinates to a pendant alkene, performs a [2 + 2] cycloaddition to form a ruthenacyclobutane, and reductively eliminates to generate the bicyclic product.

View Article: PubMed Central - HTML - PubMed

ABSTRACT

1: A full account of our efforts toward an asymmetric redox bicycloisomerization reaction is presented in this article. Cyclopentadienylruthenium (CpRu) complexes containing tethered chiral sulfoxides were synthesized via an oxidative [3 + 2] cycloaddition reaction between an alkyne and an allylruthenium complex. Sulfoxide complex containing a p-anisole moiety on its sulfoxide proved to be the most efficient and selective catalyst for the asymmetric redox bicycloisomerization of 1,6- and 1,7-enynes. This complex was used to synthesize a broad array of [3.1.0] and [4.1.0] bicycles. Sulfonamide- and phosphoramidate-containing products could be deprotected under reducing conditions. Catalysis performed with enantiomerically enriched propargyl alcohols revealed a matched/mismatched effect that was strongly dependent on the nature of the solvent.

No MeSH data available.