Dataset Information

Stereogenic-at-metal Zn- and Al-based N-heterocyclic carbene (NHC) complexes as bifunctional catalysts in Cu-free enantioselective allylic alkylations.

ABSTRACT: Investigations detailed herein demonstrate the ability of chiral bidentate N-heterocyclic carbenes to promote directly-without the need for a Cu salt-site- and enantioselective C-C bond forming reactions. Within this context, catalytic allylic alkylations of various allylic phosphates with dialkylzinc and trialkylaluminum reagents, performed with chiral bidentate imidazolinium salts and in the absence of a Cu salt, are described. The Cu-free transformations deliver products bearing tertiary or (all-carbon) quaternary stereogenic centers with exceptional site- (>98:<2 S(N)2':S(N)2) and high enantioselectivity [up to 97.5:2.5 enantiomer ratio (er)]. A chiral Zn-based N-heterocyclic carbene (NHC) complex, which serves as the catalyst for the enantioselective allylic alkylation reactions, is isolated and fully characterized. Spectroscopic and X-ray crystallographic studies indicate that the sulfonate group within the stereogenic-at-Zn bidentate complexes coordinates syn to the proximal phenyl substituent of the NHC backbone (vs the initially expected anti). Investigations regarding a related NHC-Al complex reveal similar structural attributes. The studies outlined provide a plausible rationale regarding the mechanism of Cu-free allylic alkylation reactions that involve dialkylzinc or trialkylaluminum reagents as well as the previously reported alkylmagnesium halides. On the basis of the research disclosed, it is proposed that bidentate metal-carbene complexes can serve as effective bifunctional catalysts, a critical attribute that differentiates such NHC-based complexes from the corresponding monodentate variants.

SUBMITTER: Lee Y

PROVIDER: S-EPMC2761703 | biostudies-literature | 2009 Aug

REPOSITORIES: biostudies-literature

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Stereogenic-at-metal Zn- and Al-based N-heterocyclic carbene (NHC) complexes as bifunctional catalysts in Cu-free enantioselective allylic alkylations.

Lee Yunmi Y Li Bo B Hoveyda Amir H AH

Journal of the American Chemical Society 20090801 32

Investigations detailed herein demonstrate the ability of chiral bidentate N-heterocyclic carbenes to promote directly-without the need for a Cu salt-site- and enantioselective C-C bond forming reactions. Within this context, catalytic allylic alkylations of various allylic phosphates with dialkylzinc and trialkylaluminum reagents, performed with chiral bidentate imidazolinium salts and in the absence of a Cu salt, are described. The Cu-free transformations deliver products bearing tertiary or ( ...[more]

PMID: 19630406

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Project description:Installing quaternary stereogenic carbon is an arduous task of contemporary importance in the domain of asymmetric catalysis. To this end, an asymmetric allylic alkylation of α,α-disubstituted aldehydes by using allyl benzoate in the presence of Wilkinson's catalyst [Rh(Cl)(PPh3)3], (R)-BINOL-P(OMe) as the external ligand, and LiHMDS as the base has been reported to offer high enantioselectivity. The mechanistic details of this important reaction remain vague, which prompted us to undertake a detailed density functional theory (SMD(THF)/B3LYP-D3) investigation on the nature of the potential active catalyst, energetic features of the catalytic cycle, and the origin of high enantioselectivity. We note that a chloride displacement from the native Rh-phosphine [Rh(Cl)(PPh3)3] by BINOL-P(OMe) phosphite and an ensuing MeCl elimination can result in the in situ formation of a Rh-phosphonate [Rh(BINOL-P[double bond, length as m-dash]O)(PPh3)3]. A superior energetic span (δE) noted with such a Rh-phosphonate suggests that it is likely to serve as an active catalyst. The uptake of allyl benzoate by the active catalyst followed by the turnover determining C-O bond oxidative addition furnishes a Rh-π-allyl intermediate, which upon interception by (Z)-Li-enolate (derived from α,α-disubstituted aldehyde) in the enantiocontrolling C-C bond generates a quaternary stereogenic center. The addition of the re prochiral face of the (Z)-Li-enolate to the Rh-bound allyl moiety leading to the R enantiomer of the product is found to be 2.4 kcal mol-1 more preferred over the addition through its si face. The origin of the stereochemical preference for the re face addition is traced to improved noncovalent interactions (NCIs) and less distortion in the enantiocontrolling C-C bond formation transition state than that in the si face addition. Computed enantioselectivity (96%) is in very good agreement with the experimental value (92%), so is the overall activation barrier (δE of 17.1 kcal mol-1), which is in conformity with room temperature reaction conditions.

Dataset Information

Stereogenic-at-metal Zn- and Al-based N-heterocyclic carbene (NHC) complexes as bifunctional catalysts in Cu-free enantioselective allylic alkylations.

Publications

Stereogenic-at-metal Zn- and Al-based N-heterocyclic carbene (NHC) complexes as bifunctional catalysts in Cu-free enantioselective allylic alkylations.

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