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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.

No MeSH data available.


Proposed catalytic cycle.The formal [3+2] cycloaddition of 1a with 2a using the catalytic triad Ni(OAc)2, (R,R)-4e and iPrNH2.
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f5: Proposed catalytic cycle.The formal [3+2] cycloaddition of 1a with 2a using the catalytic triad Ni(OAc)2, (R,R)-4e and iPrNH2.

Mentions: Structural analyses of I in the solid and solution states (Figs 2, 3, 4), as well as their catalytic activity differences (Table 2), suggested that the monomeric species should predominantly control the stereo-discrimination process (Fig. 5). We assume that the exposed nature of the d-orbital allows it to interact with α-ketoester 1a as a Lewis acid, making the ketoester susceptible to deprotonation by the acetate ligand acting as a Brønsted base, leading to the formation of (Z)-Ni(II)–enolate that contains the five-membered chelating ring (Fig. 5, step (i)). Based on the X-ray (Fig. 2b) and EDD analyses of I (Fig. 3), we propose a Ni(II)–enolate, in which the carbonyl group in the ester in 1 coordinates to Ni(II) at the pseudoapical position. A new perspective in the transient Ni(II)–enolate is its coordination pattern in the octahedral structure; the Ni(II)–enolate reported by Evans78 occupies the same plane with the chiral diamine. The crystallographic evidence that the N–H functionality at the equatorial position in Ni(II) complex I can contribute to activating the Lewis base (Fig. 2b) suggests that the subsequent H-bonding activation of the nitrone 2a (refs 34, 44, 45) would be a key driving force for fixing the two reaction components in close proximity (Fig. 5, step (ii)), thereby enhancing the reaction rate with high diastereo- and enantioselectivity (Fig. 5, step (iii)). In the proposed model, the centrochiral Ni(II) directly activates the α-ketoester 1a with the ligand-enabled H-bonding activation of the nitrone 2a. The model presented herein can explain the obtained absolute stereochemistry of 3aa.


Naked d -orbital in a centrochiral Ni(II) complex as a catalyst for asymmetric [3 + 2] cycloaddition
Proposed catalytic cycle.The formal [3+2] cycloaddition of 1a with 2a using the catalytic triad Ni(OAc)2, (R,R)-4e and iPrNH2.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5384211&req=5

f5: Proposed catalytic cycle.The formal [3+2] cycloaddition of 1a with 2a using the catalytic triad Ni(OAc)2, (R,R)-4e and iPrNH2.
Mentions: Structural analyses of I in the solid and solution states (Figs 2, 3, 4), as well as their catalytic activity differences (Table 2), suggested that the monomeric species should predominantly control the stereo-discrimination process (Fig. 5). We assume that the exposed nature of the d-orbital allows it to interact with α-ketoester 1a as a Lewis acid, making the ketoester susceptible to deprotonation by the acetate ligand acting as a Brønsted base, leading to the formation of (Z)-Ni(II)–enolate that contains the five-membered chelating ring (Fig. 5, step (i)). Based on the X-ray (Fig. 2b) and EDD analyses of I (Fig. 3), we propose a Ni(II)–enolate, in which the carbonyl group in the ester in 1 coordinates to Ni(II) at the pseudoapical position. A new perspective in the transient Ni(II)–enolate is its coordination pattern in the octahedral structure; the Ni(II)–enolate reported by Evans78 occupies the same plane with the chiral diamine. The crystallographic evidence that the N–H functionality at the equatorial position in Ni(II) complex I can contribute to activating the Lewis base (Fig. 2b) suggests that the subsequent H-bonding activation of the nitrone 2a (refs 34, 44, 45) would be a key driving force for fixing the two reaction components in close proximity (Fig. 5, step (ii)), thereby enhancing the reaction rate with high diastereo- and enantioselectivity (Fig. 5, step (iii)). In the proposed model, the centrochiral Ni(II) directly activates the α-ketoester 1a with the ligand-enabled H-bonding activation of the nitrone 2a. The model presented herein can explain the obtained absolute stereochemistry of 3aa.

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.