Asymmetric Synthesis with N‐Heterocyclic Carbenes. Application to the Copper‐Catalyzed Conjugate Addition

Abstract

Chiral N‐heterocyclic carbenes are good ligands for the copper‐catalyzed conjugate addition of dialkylzinc reagents to various Michael acceptors. With as little as 4% of chiral ligand, the enantioselectivity may reach 93% on cycloheptenone.

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).…”

Transition Metal-Catalyzed Reactions Using N-Heterocyclic Carbene Ligands (Besides Pd- and Ru-Catalyzed Reactions)

“…30) [65]. In this study, several Ag/carbene complexes (20)(21)(22)(23)(24) were tested and found to produce significantly higher ee values than those in previous studies.…”

Some insights into the gold-catalysed A 3 -coupling reaction

“…Reaction of the imidazolium chlorides 7 and 8 with Ag 2 O, following the procedure reported by Alexakis et al, 45 cleanly afforded the Ag(I) complexes 1,3-bis-(1(S)-1-phenyl-ethyl)-imidazolin-2-ylidene silver chloride 9 and 1,3-bis-(1(S)-1-naphthyl-ethyl)-imidazolin-2-ylidene silver chloride 10 respectively. Finally, transmetallation from Ag(I)-NHC complexes to a gold(I) centre was effected by reaction of these complexes with ClAu(THT) 47 (where THT = tetrahydrothiophene), 48 and afforded analytically pure samples of 11 and 12 after a single recrystallisation from CH 2 Cl 2 /hexane.…”

“…The chiral, cyclometallated, complex 4, and 5 were prepared according to Scheme 1 starting from readily available 43 The chiral imidazolium salts 6, 7 and 8 which were required for the preparation of the gold NHC complexes 11 and 12 were synthesised by the 'one-pot' procedures reported by Alexakis et al 45 and Hermann and co-workers (Scheme 2). 46 Hence, reaction of either (S)-(-)-α-methylbenzylamine or (S)-(-)-1-(1-naphthyl)ethylamine with glyoxal and paraformaldehyde in the presence of either HCl or HBF 4 at 40 °C in toluene overnight afforded the desired salts.…”