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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.


Curtin–Hammitt scenario of redox bicycloisomerization in acetone.
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C8: Curtin–Hammitt scenario of redox bicycloisomerization in acetone.

Mentions: We propose that acetone, being more Lewis basic than THF, has the effect of epimerizing the chiral-at-ruthenium intermediates formed prior to [2 + 2] cycloaddition. The rate of epimerization is much faster than the [2 + 2] cycloaddition, creating a classic Curtin–Hammitt scenario wherein all of the substrate is funneled into the observed enantiomer of product (Scheme 8). Rate k1 is much slower than k2 due to the severe steric hindrance imposed by the ligated chiral sulfoxide, which block alkene coordination. The pendant olefin prefers to approach the carbene anti to the aforementioned sulfoxide, resulting in the observed enantiomer of 53. In THF, the rate of epimerization is significantly slower than the [2 + 2] cycloaddition, which means that the enantiomeric ratios observed in the products are affected more by the initial coordination of the propargyl alcohol.


Chiral cyclopentadienylruthenium sulfoxide catalysts for asymmetric redox bicycloisomerization
Curtin–Hammitt scenario of redox bicycloisomerization in acetone.
© Copyright Policy - Beilstein
Related In: Results  -  Collection

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

C8: Curtin–Hammitt scenario of redox bicycloisomerization in acetone.
Mentions: We propose that acetone, being more Lewis basic than THF, has the effect of epimerizing the chiral-at-ruthenium intermediates formed prior to [2 + 2] cycloaddition. The rate of epimerization is much faster than the [2 + 2] cycloaddition, creating a classic Curtin–Hammitt scenario wherein all of the substrate is funneled into the observed enantiomer of product (Scheme 8). Rate k1 is much slower than k2 due to the severe steric hindrance imposed by the ligated chiral sulfoxide, which block alkene coordination. The pendant olefin prefers to approach the carbene anti to the aforementioned sulfoxide, resulting in the observed enantiomer of 53. In THF, the rate of epimerization is significantly slower than the [2 + 2] cycloaddition, which means that the enantiomeric ratios observed in the products are affected more by the initial coordination of the propargyl alcohol.

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.