Cu-catalyzed Asymmetric Conjugate Addition Research Articles

Cu-Catalyzed Asymmetric Kinetic Boron Conjugate Addition of γ-Substituted α,β-Unsaturated γ-Lactams.

Chiral α,β-unsaturated γ-lactams bearing simple γ- substituents are found in biologically active molecules and natural products, however, their synthesis still remains difficult. Herein, we report an efficient kinetic resolution of γ-substituted α,β-unsaturated γ-lactams via a Cu-catalyzed asymmetric boron conjugate addition, which also leads to the efficient synthesis of chiral β-hydroxy-γ-lactams with β,γ-stereogenic carbon centers. The kinetic resolution proceeded smoothly with a wide range of γ-alkyl or aryl substituted substrates including those bearing aromatic heterocycles and different N-protected substrates in up to 347 of s value. Their highly versatile transformations, synthetic utility in biologically active molecules, and inhibitory activities against cisplatin-sensitive ovarian cancer cell A2780 have also been demonstrated. Differing from the well-known mechanism involving Cu-B species in Cu-catalyzed boron conjugate additions, our mechanistic studies using DFT calculations and experiments indicate that a Lewis acid Cu(I)-catalyzed mechanism is the likely pathway in the catalytic reaction.

Cu-Catalyzed Asymmetric Alkenylation of Enone Diesters with Trialkenylboroxines.

The Cu-catalyzed asymmetric conjugate addition of trialkenylboroxines to enone diesters is reported. This operationally simple and scalable reaction proceeded at room temperature, and a wide range of enone diesters and boroxines were tolerated under the applied reaction conditions. The practical utility of this approach was demonstrated via the formal synthesis of (+)-methylenolactocin. Mechanistic studies revealed that two different catalytic species work synergistically in the reaction.

Reversal of Enantioselectivity in the Conjugate Addition Reaction of Cyclic Enones with the CuOTf/Azolium Catalytic System

Hydroxyamide-functionalized azolium salt (NHC•HI 4) was evaluated for dual enantioselective control in a Cu-catalyzed asymmetric conjugate addition (ACA) reaction. This investigation was based on our previously reported ACA reaction catalyzed using CuOTf combined with NHC•AgI complex 1. It was revealed that the stereocontrol of the catalytic ACA reaction depended on the order of the addition of the substrates. Additionally, the chiral NHC ligand precursors, substrates, the relationship between the catalyst ee (eecat) and product ee (eepro), and halogen counter anion were completely evaluated. These results suggested that the catalytic performance of the CuOTf/4 system was comparable with that of the CuOTf/1 system. Furthermore, to gain knowledge of the Cu species generated using CuOTf and NHC ligand precursor, the reaction of CuOTf with 1 was investigated. Although obtaining the corresponding NHC•CuX species failed, the corresponding NHC•AuCl complex 11 could be synthesized by allowing 1 to react with AuCl•SMe2.

(±)-trans-1,2-Cyclohexanediamine-Based Bis(NHC) Ligand for Cu-Catalyzed Asymmetric Conjugate Addition Reaction

Bis(NHC) ligand precursors, L1, based on trans-1,2-diaminocyclohexane were designed and synthesized. To introduce chirality at the hydroxyamide side arm on the NHC of L1, a chiral β-amino alcohol, such as enantiopure leucinol, was used. Cu-catalyzed asymmetric conjugate addition reactions of cyclic and acyclic enones with Et2Zn were selected to evaluate the performance of L1 as a chiral ligand. For the reaction of cyclic enone, a combination of [bis(trimethylsilyl)acetylene]-(hexafluoroacetylacetonato)copper(I) (Cu(hfacac)(btmsa)) with a (±)-trans-1,2-cyclohexanediamine-based bis(NHC) ligand precursor, (rac; S,S)-L1, which was prepared from (S)-leucinol, was the most effective. Thus, treating 2-cyclohexen-1-one (3) with Et2Zn in the presence of catalytic amounts of Cu(hfacac)(btmsa) and (rac; S,S)-L1 afforded (R)-3-ethylcyclohexanone ((R)-4) with 97% ee. Similarly, use of (rac; R,R)-L1, which was prepared from (R)-leucinol, produced (S)-4 with 97% ee. Conversely, for the asymmetric 1,4-addition reaction of the acyclic enone, optically pure (−)-trans-1,2-cyclohexanediamine-based bis(NHC) ligand precursor, (R,R; S,S)-L1, worked efficiently. For example, 3-nonen-2-one (5) was reacted with Et2Zn using the CuOAc/(R,R; S,S)-L1 catalytic system to afford (R)-4-ethylnonan-2-one ((R)-6) with 90% ee. Furthermore, initially changing the counterion of the Cu precatalyst between an OAc and a ClO4 ligand on the metal reversed the facial selectivity of the approach of the substrates. Thus, the conjugate addition reaction of 5 with Et2Zn using the Cu(ClO4)2/(R,R; S,S)-L1 catalytic system, afforded (S)-6 with 75% ee.

DFT Study of the Strong Solvent Effects in the Cu‐Catalyzed Asymmetric Conjugate Addition Reaction

Significant solvent effects on enantioselection exist in the reaction of Cu‐catalyzed asymmetric conjugate addition of alkylzinc reagents to α,β‐unsaturated carbonyl compounds. Results of our DFT study suggest that solvent effects on enantioselection can be caused by structural changes in the stereodetermining Transition States (TS). We have seen how unconventional hydrogen bonding of the nucleophilic solvent results in a change of the computed optical yield comparable to that caused by well‐known coordination of the solvent to the electrophilic metal center.

Inversions in asymmetric conjugate addition reaction of cyclic enones catalyzed by the Cu/NHC-AgX system: Factors affecting the stereoselective formation of both enantiomers

A switchable enantioselectivity was achieved in a Cu-catalyzed asymmetric conjugate addition (ACA) reaction. The ethylene-bridged, hydroxyamide-functionalized NHC-AgI complex, readily accessible from a chiral β-amino alcohol, was found to be a versatile chiral ligand precursor for dual enantioselective control. The stereocontrol of the catalytic ACA reaction depended on the order of addition of the substrates. To a THF solution containing CuOTf (6 mol %), NHC-AgI complex (1b, R1 = Et, R2 = Bn, 4 mol %), and 2-cyclohexen-1-one (3) was added Et2Zn to yield (R)-3-ethylcyclohexanone ((R)-4) in 74% enantioselectivity (method A). In stark contrast, when 3 was added as a last component to a mixture of CuOTf (4 mol %), 1b (10 mol %), and Et2Zn in THF (method B), the ACA reaction afforded (S)-4 with 87% ee. In order to obtain an insight into various aspects of the present ACA reaction, the chiral NHC ligand precursors, substrates, and NHC skeletons were fully evaluated. A study of the relationship between the catalyst ee (eecat) and the product ee (eepro) revealed a chiral amplification phenomenon. Moreover, it was found that the ACA reaction using method A was not affected by the halide ion on the NHC-AgX complex, whereas the halide ion on the silver complex showed a significant influence on the enantiocontrol of the ACA reaction using method B. In particular, a Cu/NHC/I catalytic ratio of 1/1/2 was needed for a successful ACA reaction using method B. According to these factors affecting the reversal of enantioselectivity in the ACA reaction, a possible reaction pathway was proposed.

The Cu-catalyzed asymmetric conjugate addition with chiral diphosphite ligands derived from D-(-)-tartaric acid

ABSTRACTA series of diphosphite ligands, which were derived from D-(-)-tartaric acid, have been synthesized and successfully applied in the Cu-catalyzed asymmetric conjugate addition of organozincs to enones. There was a synergic effect between the stereogenic centers of the D-(-)-tartaric acid skeleton and the axially H8-binaphthyl moieties of ligand 2c. And ligand 2c shows a comparative catalytic performance to ligand 1-N-benzylpyrrolidine-3,4-bis[(S)-1,1′-H8-binaphthyl-2,2′-diyl]phosphite-L-tartaric acid1d derived from L-(+)-tartaric acid. Therefore, for cyclic enones, both enantiomers of the addition products can be obtained in high enantioselectivity (ees up to 96%) by simply selecting ligands 1d or 2c derived from D-(-)-tartaric acid or L-(+)-tartaric acid. Moreover, the sense of enantiodiscrimination of the products was mainly determined by the configuration of the binaphthyl phosphite moieties.

Synthesis of novel tartaric acid-derived chiral phosphite ligands and their application in the Cu-catalyzed conjugate addition of diethylzinc to cyclic enones

Five novel tropos (3R,4R)- and/or (3S,4S)-N-benzyltartarimide-derived biphenylphosphite ligands were synthesized and applied in the Cu-catalyzed asymmetric conjugate addition of diethylzinc to cyclic enones with up to 75% e.e. Compared with the reported ligand 1-N-benzylpyrrolidine-3,4-bis[(R)-1,1’-binaphthyl-2,2’-diyl]phosphite-L-tartaric acid, the issue that L-(+)-tartaric acid backbone and (R)-binaphthyl showed strong matched/mismatched character was solved with these tropos ligands. It was found that the enantioselectivity was mainly controlled by the absolute configuration of N-benzyltartarimide backbone, and both enantiomers of the addition products can be obtained by simply changing the configuration of N-benzyltartarimide substituent.

Cu-Catalyzed Asymmetric Conjugate Addition of Dialkylzincs to Enones Using a (±)-trans-1,2-Cyclohexanediamine-Based Bis(NHC) Derived from l-Leucinol

A hydroxyamide-functionalized azolium salt as the precursor of a (±)-trans-1,2-cyclohexanediamine-based bis(NHC) ­ligand was designed and synthesized from readily accessible l -leucinol. The combination of a Cu salt with this chiral ligand precursor promoted the asymmetric conjugate addition of Et2Zn to 2-cyclohexen-1-one at room temperature without the need for temperature control to afford the corresponding 1,4-adduct with up to 95% ee.

Cu-Catalyzed Asymmetric Conjugate Addition of Dialkylzincs to Enones Using a (±)-trans-1,2-Cyclohexanediamine-Based Bis(NHC) Derived from l-Leucinol

A hydroxyamide-functionalized azolium salt as the precursor of a (±)-<i>trans</i>-1,2-cyclohexanediamine-based bis(NHC) ­ligand was designed and synthesized from readily accessible <sc>l</sc>-leucinol. The combination of a Cu salt with this chiral ligand precursor promoted the asymmetric conjugate addition of Et₂Zn to 2-cyclohexen-1-one at room temperature without the need for temperature control to afford the corresponding 1,4-adduct with up to 95% ee.

C2-symmetric functionalized azolium salt from serine ester for Cu-catalyzed asymmetric conjugate addition reaction

C2-symmetric ester-amide functionalized azolium salt was synthesized from readily available α-amino ester such as L-serine methyl ester. The combination of a Cu salt and the chiral azolium salt promoted the asymmetric conjugate addition reaction of enones with dialkylzincs. Thus, treatment of acyclic enone such as chalcone with Et2Zn afforded the corresponding 1,4-adduct with up to 85% ee. An excellent ee value of 93% was obtained when 3-nonen-2-one was reacted with Et2Zn. The present catalytic system was found to be useful for the 1,4-addition reaction of cyclic enone. For example, the reaction of 2-cyclohepten-1-one with Et2Zn produced (R)-3-ethylcycloheptanone with 80% ee.

Functionalized BINOL-mono-PHOS for Multinuclear Cu-Catalysts in Asymmetric Conjugate Addition of Organozinc Reagents

Abstract Functionalization of BINOL-mono-PHOS achieved the Cu-catalyzed highly asymmetric conjugate addition of organozinc reagents to enones. The incorporation of a bulky hydroxy group at the 3′-position of BINOL-mono-PHOS dramatically improved the yield and enantioselectivity. The present novel BINOL-mono-PHOS ligands are effective in the Cu-catalyzed asymmetric conjugate addition of organozinc reagents in both acyclic enones and cyclohexenone.

キラルなアゾリウム塩による銅触媒不斉共役付加反応

The Cu-catalyzed 1,4-addition of organometallic reagents to α,β-unsaturated compounds is a powerful method for the efficient construction of carbon-carbon bonds. In the past decade, catalytic asymmetric conjugate addition reactions with high enantioselectivities have been successfully achieved by using a variety of chiral ligands. Among the growing number of the chiral ligands, N-heterocyclic carbenes (NHCs) have been recognized as useful ligands. One of the most attractive features that make NHCs a very interesting class of ligands is the easy access to their ligand precursors (mostly azolium salts). The azolium salts are very easy to functionalize often by simple reactions that allows the design of a wide variety of NHC ligands and the rapid elaboration of NHC ligand libraries. In this review article, the Cu-catalyzed asymmetric conjugate addition reactions by employing easy accessible and highly tunable functionalized azolium salts are summarized.

Chiral phosphite ligands derived from l-(+)-tartaric acid: synthesis and application in the Cu-catalyzed 1,4-conjugate addition of organozincs to cyclic enones

A new series of chiral phosphite ligands derived from readily available l-(+)-tartaric acid and binaphthol derivatives were designed and prepared in convenient synthetic routes. These ligands were successfully employed in the Cu-catalyzed asymmetric conjugate addition of organozincs to cyclic enones. The ligands bearing (R)- and (S)-binaphthol and/or 3,3′-bis(trimethylsilyl)-2,2′-dihydroxy-1,1′-binaphthol afforded both enantiomers of the product in high yields and good enantioselectivities (up to 95% ee). The chiral organocopper reagent did exhibit high enantioselectivities for cyclic enones bearing different ring sizes. The stereochemically matched combination of l-(+)-tartaric acid backbone and (R)-binaphthyl in ligand 1-N-benzylpyrrolidine-3,4-bis[(R)-1,1′-binaphthyl-2,2′-diyl]phosphite-l-tartaric acid was essential to afford 95% (R) ee for 3-ethylcyclopentanone, 95% ee (S) for 3-ethyl-cyclohexanone, and 63% ee (R) for 3-ethylcycloheptanone in ether as solvent, using (CuOTf)2·C6H6 as a catalytic precursor. Moreover, the sense of the enantio discrimination of the products was mainly determined by the configuration of the binaphthyl phosphite moieties.

Multinuclear Cu-Catalyzed Asymmetric Conjugate Addition

Multinuclear Cu-Catalyzed Asymmetric Conjugate Addition

Hydroxy-amide Functionalized Azolium Salts for Cu-Catalyzed Asymmetric Conjugate Addition: Stereocontrol Based on Ligand Structure and Copper Precatalyst

A series of hydroxy-amide functionalized azolium salts have been designed and synthesized for Cu-catalyzed asymmetric conjugate addition reaction. The (CH(2))(2)-bridged hydroxy-amide functionalized azolium ligand precursors 2, in addition to the previously reported CH(2)-bridged azolium salts 1, have been prepared from readily available enantiopure β-amino alcohols. The combination of a Cu species with 1 or 2 efficiently promoted the 1,4-addition reaction of cyclic enones with dialkylzincs. For example, the reaction of 2-cyclohepten-1-one (17) with Bu(2)Zn in the presence of catalytic amounts of Cu(OTf)(2) and 1 gave (S)-3-butylcycloheptanone (20) in 99% yield and 96% ee. On the other hand, when the reaction was carried out under the influence of Cu(OTf)(2) combined with 2, (R)-20 in preference to (S)-20 was obtained in 98% yield and 80% ee. In this manner, the enantioselecvity was switched by controlling the structure of chiral ligand. Additionally, the reversal of enantioselectivity was also achieved by changing the Cu precatalyst from Cu(OTf)(2) to Cu(acac)(2) with the same ligand. The combination of Cu(acac)(2) with CH(2)-bridged azolium salt 1 in the reaction of 17 with Bu(2)Zn led to formation of (R)-20 as a major product in 55% yield and 80% ee. This result was in contrast to the Cu(OTf)(2)/1 catalytic system, where the 1,4-adduct with opposite configuration was obtained. Moreover, use of the Cu(acac)(2)/2 catalytic system produced (S)-20, while (R)-20 was formed by the Cu(OTf)(2)/2 catalytic system. Thus, it was found that either varying the linker of the chiral ligands or changing the counterion of Cu species between a OTf and acac ligand initially on the metal led to dual enantioselective control in the 1,4-addition reaction.

ヒドロキシ-アミド置換アゾリウム塩存在下、非環状エノンへのジアルキル亜鉛の銅触媒不斉共役付加反応

ヒドロキシ-アミド置換アゾリウム塩存在下、非環状エノンへのジアルキル亜鉛の銅触媒不斉共役付加反応

Copper-Catalyzed Asymmetric 1,4-Addition of Alkenyl Alanes to N-Substituted-2-3-dehydro-4-piperidones

Readily available vinyl alanes are used in the Cu-catalyzed asymmetric conjugate addition reaction to N-substituted-2-3-dehydro-4-piperidones. The enhanced reactivity of recently developed and easily prepared phosphine amine ligands in combination with inexpensive Cu(II)naphtenate (CuNp) allows the introduction of a great variety of alkenyl, alkyl, and aryl aluminums in high enantioselectivity.

Cu-Catalyzed Asymmetric Conjugate Addition of Glycine Derivatives

Key words asymmetric catalysis - copper - conjugate addition - chiral pyrrolidines

Ligand design for dual enantioselective control in Cu-catalyzed asymmetric conjugate addition of R2Zn to cyclic enone

A new chiral N-heterocyclic carbene (NHC) ligand derived from a natural α-aminoester has been designed and synthesized. The coupling of N-methylbenzimidazole with an α-chloroacetamide derivative, which was prepared from chloroacetyl chloride and (S)-serine methyl ester, gave the corresponding ester/amide-functionalized azolium compound 20. The reaction of 2-cyclohexen-1-one (17) with Et2Zn in the presence of catalytic amounts of Cu(OTf)2 and 20 produced (R)-3-ethylcyclohexanone (18) as a major product. In contrast, the enantioselective conjugate addition (ECA) reaction catalyzed by Cu(OTf)2 under the influence of a hydroxy-amide-functionalized azolium compound 15, which was derived from (S)-tert-leucinol, produced (S)-18 in preference to (R)-18. A series of azolium salts were synthesized from (S)-serine esters, and the reaction conditions for the ECA reaction were optimized to produce (R)-18 with 69% ee. The best results were obtained in the case of the reaction of 4,4-dimethyl-2-cyclohexen-1-one (34) with Et2Zn catalyzed by Cu(OTf)2 in combination with azolium compounds. When the reaction of 34 with Et2Zn was carried out in the presence of catalytic amounts of Cu(OTf)2 and 20, (S)-3-ethyl-4,4-dimethylcyclohexanone (35) was obtained with 97% ee, whereas the ECA reaction under the influence of hydroxy-amide-functionalized azolium 15 afforded (R)-35 with >99% ee. In this manner, the reversal of enantioselectivity was achieved by controlling the structure of chiral ligands.