New N‐Heterocyclic Carbene Ligand and Its Application in Asymmetric Nickel‐Catalyzed Aldehyde/Alkyne Reductive Couplings.

Abstract

Alcohols P 0110New N-Heterocyclic Carbene Ligand and Its Application in Asymmetric Nickel--Catalyzed Aldehyde/Alkyne Reductive Couplings. -Various functional groups are tolerated in the procedure. Generally, the trialkylsilyl group is regioselectively installed at the newly formed hydroxyl group. In the case of substrate (IId), the regioselectivity in the coupling with (VI) is reversed with ligand (Ib). The methodology is also applied to the asymmetric macrocyclization of ynal (IX). In this case, the regioselection is reversed in comparison to intermolecular examples. -(CHAULAGAIN, M. R.; SORMUNEN, G. J.; MONTGOMERY*, J.; J. Am. Chem. Soc. 129 (2007) 31, 9568-9569; Dep. Chem., Univ. Mich., Ann Arbor, MI 48109, USA; Eng.) -Bartels 50-059

Unsymmetric N-Aryl Substituent Effects on Chiral NHC-Cu: Enantioselectivity and Reactivity Enhancement by Ortho-H and Syn-Configuration

“…A mixture of three diastereomeric catalysts (17,33:50) was obtained as shown in the related literature. 20,27,31 On the other hand, just <2% 2a−c e.e. change and nearly exactly the same borylation yield were noted by replacing the ortho-H on L1 with D, suggesting that the H/D just behaved the same and ortho-CH oxidative addition is unlikely involved.…”

“…48 This preference is contrary to the common expectation based on the steric model at first sight, especially when compared to those employed C 2 -symmetric designs 2 and in chiral NHC-Ni cases with the same Y-ring. 20,27 Yet, it can be explained by other competing steric repulsions in the syn-/anti-catalysts under different tilted angles (Scheme 2). Indeed, the tilted N-aryl on the syn-catalyst adopts a configuration that can accommodate the apparently more bulky o-Cy and the core's Ph sitting on the same side (torsion angle ∼100°).…”

“…On the other hand, highly enantioselective B-substituted quaternary center synthesis is still challenging. Chiral NHC/CuCl, 26,27,29,30,33 bipyridines/ Cu(OH) 2 /AcOH, 2°diamine/CuPF 6 / i PrOH, 43 (R,R)-Qui-noxP*/CuPF 6 , and NHC organocatalyst 44,45 are excellent catalysts for those based on acyclic conjugated carbonyl, linear dialkyl-substituted, and cyclic enones. Nevertheless, the use of electron-rich β-(hetero)aryl over Ph often resulted in an obvious e.e.…”

“…23,24 Early approaches mostly adopt a C 2 -symmetric chiral NHC core design 21,25 and use different N-aryl side arms to relay the chirality information to the substrate. However, the use of unsymmetric 2,6-disubstituted N-aryls often resulted in a set of three isomers in the catalyst synthesis 26,27 (either diastereomeric or atropisomeric) due to the N-aryl rings with high rotational barriers. Separation of those isomers by different techniques is required to give high e.e.…”

Organosilanes in Metal-Catalyzed, Enantioselective Reductions

“…Montgomery et al reported that a Ni-catalyzed triethylsilane asymmetric reductive coupling of an aldehyde and an alkyne produced the triethylsilylated allylic alcohol with good regioselectivity, yield and ee values (Scheme 143). 136 The aryl substituent of NHC ligand L81 was critical for high enantioselectivity. Regioselectivity was influenced by the two alkyne substituents; an alkyne with an aryl and alkyl group was optimal, while examples with two small alkyl groups exhibited poor regioselectivity.…”