Application of N-heterocyclic carbene–Cu(I) complexes as catalysts in organic synthesis: a review

• Nosheen Beig,
• Varsha Goyal and
• Raj K. Bansal

Beilstein J. Org. Chem. 2023, 19, 1408–1442, doi:10.3762/bjoc.19.102

• ]. Later in 2003, Buchwald, Sadighi and Jurkauskas [47] succeeded in the application of [(IPr)CuCl] as NHC–Cu(I) complex to catalyze the conjugate reduction of α,β-unsaturated carbonyl compounds. In the decade following these initial reports, the field has blossomed and NHC–Cu(I) complexes have been

Tosylhydrazine-promoted self-conjugate reduction–Michael/aldol reaction of 3-phenacylideneoxindoles towards dispirocyclopentanebisoxindole derivatives

• Sayan Pramanik and
• Chhanda Mukhopadhyay

Beilstein J. Org. Chem. 2022, 18, 469–478, doi:10.3762/bjoc.18.49

• Sayan Pramanik Chhanda Mukhopadhyay Department of Chemistry, University of Calcutta, 92 APC Road, Kolkata-700009, India 10.3762/bjoc.18.49 Abstract An efficient tosylhydrazine-mediated conjugate reduction of 3-phenacylideneoxindole and sequential Michael/intramolecular aldol reaction is reported

• and operational simplicity through one pot reaction. Keywords: chemoselective conjugate reduction; dispirocyclopentanebisoxindole scaffolds; metal-free; one-pot operation; reductive cyclization; Introduction There is a vast demand of the structurally complex spirooxindole scaffold which is an

• important functional group transformation for the synthesis of heterocyclic and carbocyclic building blocks and reactive intermediates. Besides the use of various reducing agents, it is observed that tosylhydrazine develops the transition-metal-free and highly chemoselective conjugate reduction of α,β

Copper-catalyzed enantioselective conjugate reduction of α,β-unsaturated esters with chiral phenol–carbene ligands

• Shohei Mimura,
• Sho Mizushima,
• Yohei Shimizu and
• Masaya Sawamura

Beilstein J. Org. Chem. 2020, 16, 537–543, doi:10.3762/bjoc.16.50

• , Sapporo, Hokkaido 001-0021, Japan 10.3762/bjoc.16.50 Abstract A chiral phenol–NHC ligand enabled the copper-catalyzed enantioselective conjugate reduction of α,β-unsaturated esters. The phenol moiety of the chiral NHC ligand played a critical role in producing the enantiomerically enriched products. The

• catalyst worked well for various (Z)-isomer substrates. Opposite enantiomers were obtained from (Z)- and (E)-isomers, with a higher enantiomeric excess from the (Z)-isomer. Keywords: catalyst; chiral NHC; conjugate reduction; copper catalysis; enantioselective reaction; Introduction Since the leading

• generated copper hydride in situ, has successfully been utilized for enantioselective reactions with β,β-disubstituted α,β-unsaturated carbonyl compounds [4][5][6][7][8][9][10][11]. The pioneering work of Buchwald and co-workers on the enantioselective conjugate reduction of α,β-unsaturated esters using a

Phosphazene-catalyzed desymmetrization of cyclohexadienones by dithiane addition

• Matthew A. Horwitz,
• Elisabetta Massolo and
• Jeffrey S. Johnson

Beilstein J. Org. Chem. 2017, 13, 762–767, doi:10.3762/bjoc.13.75

• developed an acyl anion addition promoted by N-heterocyclic carbenes (NHC) that furnished bicyclic furanones via Stetter addition [21]; later, the You group developed an extension of this theme using the same catalytic manifold [22]. More recently, the Corey group has enabled the enantioselective conjugate

• reduction of prochiral cyclohexadienones using copper hydride generated in situ [23]. Inspired by these advances, we sought to develop an alternative and complementary method invoking the dithiane moiety as an established and easily accessible glyoxylate anion surrogate [24][25][26][27][28][29]. This would

Copper-catalyzed cascade reactions of α,β-unsaturated esters with keto esters

• Zhengning Li,
• Chongnian Wang and
• Zengchang Li

Beilstein J. Org. Chem. 2015, 11, 213–218, doi:10.3762/bjoc.11.23

• aldolization followed by a lactonization. This method provides a facile approach to prepare γ-carboxymethyl-γ-lactones and δ-carboxymethyl-δ-lactones under mild reaction conditions. Keywords: aldol addition; cascade reaction; catalysis; conjugate reduction; copper; lactonization; Introduction Paraconic acid

• conjugate reduction of the α,β-unsaturated diester with newly generated copper hydride, followed by aldol reaction to yield the key intermediate alkoxide A, which is subjected to further lactonization to form the lactone. Lam’s group has furnished a cobalt-catalyzed conjugate reductive aldolization

• of γ-carboxy-γ-lactones via a copper-catalyzed cascade reaction. Considering that the reported conjugate addition–aldolization–lactonization cascade reactions proceed via the key intermediate A in Figure 1, we envisioned that the conjugate reduction of a methacrylate and the following aldol reaction

Olefin cross metathesis based de novo synthesis of a partially protected L-amicetose and a fully protected L-cinerulose derivative

• Bernd Schmidt and
• Sylvia Hauke

Beilstein J. Org. Chem. 2014, 10, 1023–1031, doi:10.3762/bjoc.10.102

• successfully for the conjugate reduction of a related enoate [35], failed completely in this case and resulted only in the isolation of unreacted starting material. For these reasons we resumed to a hydrogenation catalyzed by Pd/C, in spite of the well-known capricious nature of these transformations [64

Systematic investigations on the reduction of 4-aryl-4-oxoesters to 1-aryl-1,4-butanediols with methanolic sodium borohydride

• Subrata Kumar Chaudhuri,
• Manabendra Saha,
• Amit Saha and
• Sanjay Bhar

Beilstein J. Org. Chem. 2010, 6, 748–755, doi:10.3762/bjoc.6.94

• equiv) was employed, we obtained the corresponding γ-hydroxy-trans-α,β-enoic ester 10 from 1g. γ-Hydroxy-α,β-acetylenic esters have been reported [26] to undergo conjugate reduction of the triple bond with NaBH4 at low temperature (−34 °C) to give the corresponding γ-hydroxy-α,β-alkenoic esters, where

• the conjugate reduction does not proceed beyond the double bond. However, we have observed conjugate reduction of γ-hydroxy-α,β-alkenoic esters with methanolic NaBH4 (4 equiv) at 30 °C during the transformation of 10 to 2a. Conjugate reduction here might be explained by the following plausible

• mechanistic scheme (Figure 1) where a mixed alkenyloxy alkoxy borohydride is initially formed by the reaction of 10 with sodium borohydride followed by conjugate reduction of olefinic linkage by intramolecular hydride attack to produce saturated 4-hydroxyester, which subsequently cyclizes to yield 9 and then