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Naked d -orbital in a centrochiral Ni(II) complex as a catalyst for asymmetric [3 + 2] cycloaddition

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ABSTRACT

Chiral metal catalysts have been widely applied to asymmetric transformations. However, the electronic structure of the catalyst and how it contributes to the activation of the substrate is seldom investigated. Here, we report an empirical approach for providing insights into the catalytic activation process in the distorted Ni(II)-catalysed asymmetric [3+2] cycloaddition of α-ketoesters. We quantitatively characterize the bonding nature of the catalyst by means of electron density distribution analysis, showing that the distortion around the Ni(II) centre makes the dz2 orbital partially ‘naked', wherein the labile acetate ligand is coordinated with electrostatic interaction. The electron-deficient dz2 orbital and the acetate act together to deprotonate the α-ketoester, generating the (Λ)-Ni(II)–enolate. The solid and solution state analyses, together with theoretical calculations, strongly link the electronic structure of the centrochiral octahedral Ni(II) complex and its catalytic activity, depicting a cooperative mechanism of enolate binding and outer sphere hydrogen-bonding activation.

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Scope of Ni(II)-catalysed [3+2] cycloaddition.*Yields are for isolated materials. †d.r. values were determined from 1H NMR spectra of crude mixture. ‡Run for 48 h. ¶2.0 equiv. of 2a was used. §Run at a concentration of 1.0 M. #10 mol% of metal complex and 3.0 equiv. of 2a and Et3N (10 mol%) were used. **10 mol% of metal complex and 3.0 equiv. of 2a were used.
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f6: Scope of Ni(II)-catalysed [3+2] cycloaddition.*Yields are for isolated materials. †d.r. values were determined from 1H NMR spectra of crude mixture. ‡Run for 48 h. ¶2.0 equiv. of 2a was used. §Run at a concentration of 1.0 M. #10 mol% of metal complex and 3.0 equiv. of 2a and Et3N (10 mol%) were used. **10 mol% of metal complex and 3.0 equiv. of 2a were used.

Mentions: The scope of the formal IED [3+2] cycloaddition using the catalytic triad of Ni(OAc)2, (R,R)-4e and iPrNH2 was examined (Fig. 6). α-Ketoesters 1 bearing various substituents on the aromatic moiety as well as longer alkyl chain substrates served as substrates, giving the corresponding [3+2] adducts 3. Substrate 1h, bearing the sterically demanding 1-adamantyl group can also participate in the catalytic reaction, affording the corresponding isoxazolidine 3ha in 74% yield, with 25/1 d.r. and 94% e.e. The terminal olefin in 1i remains intact in the reaction of 2a, giving 3ia in 77% yield, 35/1 d.r. and 88% e.e. Substituted (E)-nitrones (3ab, 3ac and 3ad) were also applicable in the developed catalytic system, and comparable reactivity and selectivity were obtained by slightly tuning the reaction conditions.


Naked d -orbital in a centrochiral Ni(II) complex as a catalyst for asymmetric [3 + 2] cycloaddition
Scope of Ni(II)-catalysed [3+2] cycloaddition.*Yields are for isolated materials. †d.r. values were determined from 1H NMR spectra of crude mixture. ‡Run for 48 h. ¶2.0 equiv. of 2a was used. §Run at a concentration of 1.0 M. #10 mol% of metal complex and 3.0 equiv. of 2a and Et3N (10 mol%) were used. **10 mol% of metal complex and 3.0 equiv. of 2a were used.
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Related In: Results  -  Collection

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getmorefigures.php?uid=PMC5384211&req=5

f6: Scope of Ni(II)-catalysed [3+2] cycloaddition.*Yields are for isolated materials. †d.r. values were determined from 1H NMR spectra of crude mixture. ‡Run for 48 h. ¶2.0 equiv. of 2a was used. §Run at a concentration of 1.0 M. #10 mol% of metal complex and 3.0 equiv. of 2a and Et3N (10 mol%) were used. **10 mol% of metal complex and 3.0 equiv. of 2a were used.
Mentions: The scope of the formal IED [3+2] cycloaddition using the catalytic triad of Ni(OAc)2, (R,R)-4e and iPrNH2 was examined (Fig. 6). α-Ketoesters 1 bearing various substituents on the aromatic moiety as well as longer alkyl chain substrates served as substrates, giving the corresponding [3+2] adducts 3. Substrate 1h, bearing the sterically demanding 1-adamantyl group can also participate in the catalytic reaction, affording the corresponding isoxazolidine 3ha in 74% yield, with 25/1 d.r. and 94% e.e. The terminal olefin in 1i remains intact in the reaction of 2a, giving 3ia in 77% yield, 35/1 d.r. and 88% e.e. Substituted (E)-nitrones (3ab, 3ac and 3ad) were also applicable in the developed catalytic system, and comparable reactivity and selectivity were obtained by slightly tuning the reaction conditions.

View Article: PubMed Central - PubMed

ABSTRACT

Chiral metal catalysts have been widely applied to asymmetric transformations. However, the electronic structure of the catalyst and how it contributes to the activation of the substrate is seldom investigated. Here, we report an empirical approach for providing insights into the catalytic activation process in the distorted Ni(II)-catalysed asymmetric [3+2] cycloaddition of α-ketoesters. We quantitatively characterize the bonding nature of the catalyst by means of electron density distribution analysis, showing that the distortion around the Ni(II) centre makes the dz2 orbital partially ‘naked', wherein the labile acetate ligand is coordinated with electrostatic interaction. The electron-deficient dz2 orbital and the acetate act together to deprotonate the α-ketoester, generating the (Λ)-Ni(II)–enolate. The solid and solution state analyses, together with theoretical calculations, strongly link the electronic structure of the centrochiral octahedral Ni(II) complex and its catalytic activity, depicting a cooperative mechanism of enolate binding and outer sphere hydrogen-bonding activation.

No MeSH data available.