γ-Selective allylic substitution reaction with Grignard reagents catalyzed by copper N -heterocyclic carbene complexes and its application to enantioselective synthesis

Abstract

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Copper–NHC complexes in catalysis

“…However, further development of the ligand structures should enable catalyst-based control of the reaction, including enantioselectivity and regioselectivity. Okamoto reported a novel procedure for this transformation using copper-NHC complexes as well as an asymmetric version using chiral ligands [29]. Interestingly the steric bulk of the NHC affects the regioselectivity and the rate of the reaction.…”

1,4‐Dichloro‐ and 1,4‐Dibromo‐2‐butenes as Substrates for Cu‐Catalyzed Asymmetric Allylic Substitution

“…Other research groups have reported that a cis isomer yields a product with significant loss of enantioselectivity because of steric or other energetically competitive interactions between the catalyst and the substrate. [10] Two exceptions to this observation were reported by Okamoto and co-workers [11] in the addition of Grignard reagents to various allylic silylated ethers using chiral diaminocarbene catalysts, [12] and by Gennari and co-workers in the desymmetrization of meso cyclic allylic bisphosphates. [13] Table 1 summarizes the comparative results for the asymmetric addition of cyclohexylmagnesium chloride to the trans and cis isomers of 1,4-dichloro-2-butene (1 and 2, respectively), promoted by a small range of biphenol- [14] and binaphtol-based [6b, 15] phosphoramidite ligands (Scheme 1).…”

Copper-catalyzed asymmetric allylic substitution reactions with organozinc and Grignard reagents

“…Modular peptide-based ligands developed by Hoveyda et al [18,19] provide highly effective catalysts for a number of different substrates using organozinc compounds; these methods also provide access to quaternary centers. More recently, it has been shown by Okamoto [20] that chiral nonracemic N-heterocyclic carbene (NHC) ligands promote substitutions by Grignard reagents. Hoveyda [21] has illustrated that bidentate NHC ligands allow highly efficient and selective substitution with dialkyl zinc reagents.…”