Rational design of minimal artificial Diels-Alderases based on the copper(II) cation-aromatic pi attractive interaction

Chiral π-Cu(ii)-catalyzed site-, exo/endo-, and enantioselective dearomative (3 + 2) cycloadditions of isoquinolinium ylides with enamides, dienamides, and a trienamide

Here, we report a highly effective dearomative (3 + 2) cycloaddition reaction between isoquinolinium ylides and α,β-enamides, α,β-γ,δ-dienamides, or an α,β-γ,δ-ε,ζ-trienamide, which is catalyzed by a chiral π-Cu(ii) complex (1-10 mol%) and proceeds in a site-selective, exo/endo-selective, and enantioselective manner. The (3 + 2) cycloaddition involving the α,β-enamides proceeds with high exo-selectivity and enantioselectivity. This method is applicable to various substrates including α-substituted, α,β-disubstituted, or β,β-disubstituted α,β-enamides, which are compounds with an intrinsically low reactivity. This method provides synthetic access to pyrroloisoquinoline derivatives with up to three chiral carbon centers, including those featuring fluorine and trifluoromethyl groups, as well as quaternary carbon centers. The (3 + 2) cycloaddition involving α,β-γ,δ-dienamides proceeds with high γ,δ-site-selectivity and enantioselectivity, whereby the exo/endo-selectivity depends on the substrates and ligands. Remarkably, the (3 + 2) cycloaddition of δ-phenyl-α,β-γ,δ-dienamide proceeds with high α,β-site-selectivity, exo-selectivity, and enantioselectivity. In a manner similar to the reaction with the α,β-γ,δ-dienamides, α,β-γ,δ-ε,ζ-trienamide furnishes a (3 + 2) cycloadduct with good ε,ζ-site-selectivity, endo-selectivity, and enantioselectivity.

Tandem Isomerization/α,β-Site-Selective and Enantioselective Addition Reactions of N-(3-Butynoyl)-3,5-dimethylpyrazole Induced by Chiral π-Cu(II) Catalysts

Allenes are important building blocks, and derivatization of products via cycloadditions of allenes could become a powerful strategy for constructing carbocyclic and heterocyclic rings. However, the development of catalytic site-selective and enantioselective cycloaddition reactions of allenes still presents significant challenges. Here, we report chiral π-Cu(II)-complex-catalyzed isomerization of N-(3-butynoyl)-3,5-dimethyl-1H-pyrazole to generate N-allenoylpyrazole in situ and subsequent α,β-site-selective and enantioselective [3 + 2], [4 + 2], or [2 + 2] cycloaddition or conjugate addition reactions. The asymmetric environment created by the intramolecular π-Cu(II) interactions provides the corresponding adducts in moderate to high yield with excellent enantioselectivity. To the best of our knowledge, this is the first successful method for chiral-Lewis-acid-catalyzed tandem isomerization/α,β-site-selective and enantioselective cycloaddition or conjugate addition reactions of latent non-γ-substituted allenoyl derivative.

The effect of anions in the synthesis and structure of pyrazolylamidino copper(II) complexes

Six new pyrazolylamidino Cu(II) complexes are synthesized directly from the reactions of Cu(X)₂ salts (X = ClO₄^-, BF₄^-, or Cl^-) and pyrazole (pzH) in nitrile solution (RCN, R = Me or Et) at 298 K via the metal-mediated coupling of RCN with pzH: [Cu(HNC(R)pz)₂(X)₂] (X = ClO₄^- or BF₄^-, R = Me, 1 or 7 and Et, 2 or 8, respectively) and dichloro Cu(II) complexes [Cu₂Cl₂(μ-Cl)₂(HNC(Me)pz)₂] (3) and [CuCl₂(HNC(Et)pz)] (4). Four more new complexes, [Cu₂(μ-Cl)₂(HNC(Me)pz)₂(pzH)₂][X]₂ (X = ClO₄^-, 5 and BF₄^-, 9) and [Cu₂(μ-Cl)₂(HNC(Et)pz)₂(pzH)₂(X)₂] (X = ClO₄^-, 6 and BF₄^-, 10), are obtained indirectly from the anion substitution reaction with Cl^- ions in 1 and 7, and 2 and 8, respectively. All complexes are characterized by EA, FTIR, UV-vis and EPR spectroscopy and X-ray crystallographic analyses. HNC(Et)pz or pzH is unobserved in both the nitrile-exchange reaction of 2 to d₆-1 and the anion-substitution reaction of 2 to d₆-5 in the CD₃CN solution. The ¹H NMR results reveal that the pzH-RCN coupling is intramolecular and reversible on a Cu(II) center. The crystal structures of these complexes show diverse supramolecular assemblies through imino NH⋯anion hydrogen bonds and pyrazolylamidino pz-pz (π⋯π) and pz-Cu(II) (π⋯metal) interactions. EPR results suggest weak magnetic couplings between Cu(II) centers in the polynuclear Cu(II) complexes. The yield and rate of the formation of 1 are higher in the reaction of Cu(ClO₄)₂ with a 4-fold molar excess of pzH compared with a 2-fold excess, indicating that [Cu(pzH)₄]²⁺ is the more active species for pzH-RCN coupling. The highest rate for the formation of 1 is achieved when [Cu(pzH)₄(ClO₄)₂] is used in MeCN solution. Thus, a plausible synthetic path for synthesizing pyrazolylamidino Cu(II) complexes is established. An intermediate species, [Cu(HNC(Me)pz)₂(pzH)₂][ClO₄]₂ (1a), is proposed for the synthetic process based on spectroscopic studies and DFT calculations. The reaction of [Cu(pzH)₄X₂] (X = ClO₄^-, Cl^-, NO₃^-, or BF₄^-) in MeCN solution suggests that the lability of coordinated anions upon nitrile substitution affects the rate of the formation of bis-pyrazolylamidino Cu(II) complexes.

Enantioselective aromatic Claisen rearrangement of allyl 2-naphthyl ethers catalyzed by π-Cu(ii) complexes

The first catalytic enantioselective aromatic Claisen rearrangement of allyl 2-naphthyl ethers using 5-10 mol% of π-copper(ii) complexes is reported. A Cu(OTf)₂ complex with an l-α-homoalanine amide ligand gave (S)-products in up to 92% ee. Conversely, a Cu(OSO₂C₄F₉)₂ complex with an l-tert-leucine amide ligand gave (R)-products in up to 76% ee. Density-functional-theory (DFT) calculations suggest that these Claisen rearrangements proceed stepwise via tight-ion-pair intermediates, and that (S)- and (R)-products are enantioselectively obtained via the staggered transition states for the cleavage of the C-O bond, which is the rate-determining step.

Chiral π-Cu(II) Catalysts for the Enantioselective α-Amination of N-Acyl-3,5-dimethylpyrazoles

We report the highly enantioselective α-amination of N-acyl-3,5-dimethylpyrazoles with dialkyl azodicarboxylates, catalyzed by in situ generated π-Cu(II) complexes that consist of Cu(OTf)₂ and N-(5H-dibenzo[a,d][7]annulen-5-yl)-l-alanine-derived amides, to give the corresponding products as d-α-amino acid derivatives (up to >99% yield and 99% ee). The site-selectivity and enantioselectivity can be satisfactorily explained by the coordination of dialkyl azodicarboxylate with π-Cu(II) complex. The synthetic potential of this one-pot transformation to the α-amino ester is also described.

Copper-catalyzed aerobic oxidative radical alkoxycyclization of tryptamines to access 3-alkoxypyrroloindolines

We report a copper-catalyzed alkoxycyclization of tryptamine derivatives with O₂ as the sole oxidant, leading to a variety of C3a-alkoxypyrroloindolines in good yields with high diastereoselectivities. This reaction involves an interesting double catalytic cycle in which copper-catalyzed carboamination cyclization is favored to form the C-3 radical pyrrolidinoindoline intermediate, then a copper-catalytic radical alkoxylation reaction proceeds smoothly.

A tryptophan-based copper(ii) coordination polymer: catalytic activity towards Suzuki–Miyaura cross-coupling reactions

An unusual tryptophan coordination mode with copper(ii) wherein l-tryp is coordinated through the carboxylate and amine groups. A heterogeneous catalyst for the Suzuki cross-coupling reaction with ∼98% yield under normal reaction conditions.

Enantio- and Site-Selective α-Fluorination of N-Acyl 3,5-Dimethylpyrazoles Catalyzed by Chiral π-CuII Complexes

Catalytic enantioselective α-fluorination reactions of carbonyl compounds are among the most powerful and efficient synthetic methods for constructing optically active α-fluorinated carbonyl compounds. Nevertheless, α-fluorination of α-nonbranched carboxylic acid derivatives is still a big challenge because of relatively high pK_a values of their α-hydrogen atoms and difficulty of subsequent synthetic transformation without epimerization. Herein we show that chiral copper(II) complexes of 3-(2-naphthyl)-l-alanine-derived amides are highly effective catalysts for the enantio- and site-selective α-fluorination of N-(α-arylacetyl) and N-(α-alkylacetyl) 3,5-dimethylpyrazoles. The substrate scope of the transformation is very broad (25 examples including a quaternary α-fluorinated α-amino acid derivative). α-Fluorinated products were converted into the corresponding esters, secondary amides, tertiary amides, ketones, and alcohols with almost no epimerization in high yield.

Enantioselective [1,3] O-to-C rearrangement: dearomatization of alkyl 2-allyloxy/benzyloxy-1/3-naphthoates catalyzed by a chiral π-Cu(ii) complex

An unprecedented catalytic asymmetric [1,3] O-to-C rearrangement of alkyl 2-allyloxy/benzyloxy-1/3-naphthoates was realized under the catalysis of a chiral π-Cu(ii) complex (1-10 mol%). This dearomatization strategy provides facile access to highly functionalized β-naphthalenone derivatives bearing an all-carbon quaternary stereogenic center in high yield with excellent enantioselectivity. The π-cation interaction between the aromatic substituent of the ligand and the Cu(ii) center was proved by X-ray diffraction analysis and shown to be crucial for enantioselective control. Further preliminary mechanistic studies suggest that this intramolecular reaction proceeds through a contact ion pair intermediate.

Zinc(II)-Catalyzed Asymmetric Diels-Alder Reaction of ( E)-1-Phenyl Dienes with β,γ-Unsaturated α-Ketoesters

A highly regio-, diastereo-, and enantioselective Diels-Alder reaction of β,γ-unsaturated α-ketoesters with ( E)-1-phenyl dienes has been accomplished by using a stable and easily available chiral N,N'-dioxide/zinc(II) complex as catalyst. Only one isomer of the corresponding cyclohexenes with three chiral centers was obtained in good to excellent yields with excellent ee values under mild reaction conditions. The configurations of the product and chiral N,N'-dioxide/zinc(II) catalyst were identified by X-ray diffraction analysis. In addition, a possible catalytic model was proposed to explain the origin of the asymmetric induction.

Tunable regiodivergent phosphine-catalyzed [3 + 2] cycloaddition of alkynones and trifluoroacetyl phenylamides

The regiodivergent phosphine-catalyzed [3 + 2] cycloaddition of 2-(2,2,2-trifluoroacetyl)phenylamides and alkynones have been achieved through adjusting the acidities of substrates, changing reaction temperature and addition of protic additive H₂O. The detailed mechanism was systematically studied.

The Cation-π Interaction in Small-Molecule Catalysis

Catalysis by small molecules (≤1000 Da, 10(-9)  m) that are capable of binding and activating substrates through attractive, noncovalent interactions has emerged as an important approach in organic and organometallic chemistry. While the canonical noncovalent interactions, including hydrogen bonding, ion pairing, and π stacking, have become mainstays of catalyst design, the cation-π interaction has been comparatively underutilized in this context since its discovery in the 1980s. However, like a hydrogen bond, the cation-π interaction exhibits a typical binding affinity of several kcal mol(-1) with substantial directionality. These properties render it attractive as a design element for the development of small-molecule catalysts, and in recent years, the catalysis community has begun to take advantage of these features, drawing inspiration from pioneering research in molecular recognition and structural biology. This Review surveys the burgeoning application of the cation-π interaction in catalysis.

Highly enantioselective construction of carbazole derivatives via [4+2] cycloaddition of silyloxyvinylindoles and β,γ-unsaturated α-ketoesters

A highly enantioselective [4+2] cycloaddition between silyloxyvinylindoles and β,γ-unsaturated α-ketoesters was realized by an avaliable chiral N,N′-dioxide/yttrium triflate complex.

N,N′-Dioxide/nickel(ii)-catalyzed asymmetric Diels–Alder reaction of cyclopentadiene with 2,3-dioxopyrrolidines and 2-alkenoyl pyridines

A chiral N,N′-dioxide/Ni(OTf)₂ complex-catalyzed asymmetric Diels–Alder reaction of cyclopentadiene with 2,3-dioxopyrrolidines and 2-alkenoyl pyridines has been achieved, leading to the corresponding chiral bridged compounds in up to 97% yield, 95 : 5 dr and 97% ee.

Enantioselective 1,3-dipolar cycloaddition of azomethine imines with propioloylpyrazoles induced by chiral π-cation catalysts

We developed 1,3-dipolar cycloadditions of azomethine imines with propioloylpyrazoles catalyzed by a chiral copper(II) complex of 3-(2-naphthyl)-l-alanine amide. The asymmetric environment created by intramolecular π-cation interaction and the N-alkyl group of the chiral ligand gives the corresponding adducts in high yields with excellent enantioselectivity. This is the first successful method for the catalytic enantioselective 1,3-dipolar cycloaddition of azomethine imines with internal alkyne derivatives to give fully substituted pyrazolines.

Predicting the cation–π binding of substituted benzenes: energy decomposition calculations and the development of a cation–π substituent constant

This work proposes a new substituent constant,Π⁺, to describe cation–π binding using computational methods at the MP2(full)/6-311++G** level of theory with Symmetry Adapted Perturbation Theory (SAPT) calculations on selected cation–π complexes.

Catalytic asymmetric β,γ activation of α,β-unsaturated γ-butyrolactams: direct approach to β,γ-functionalized dihydropyranopyrrolidin-2-ones

Skeleton in the closet: The title reaction enables the development of the first catalytic β,γ-selective Diels-Alder [4+2] annulation of α,β-unsaturated γ-butyrolactams (see scheme; Boc=tert-butoxycarbonyl, Ts=4-toluenesulfonyl). This process provides a direct method for the enantioselective construction of bi- or tricyclic dihydropyranopyrrolidin-2-one skeletons in only one step.

Enantioselective synthesis of tryptophan derivatives by a tandem Friedel-Crafts conjugate addition/asymmetric protonation reaction

The tandem Friedel-Crafts conjugate addition/asymmetric protonation reaction between 2-substituted indoles and methyl 2-acetamidoacrylate is reported. The reaction is catalyzed by (R)-3,3'-dibromo-BINOL in the presence of stoichiometric SnCl(4), and is the first example of a tandem conjugate addition/asymmetric protonation reaction using a BINOL·SnCl(4) complex as the catalyst. A range of indoles furnished synthetic tryptophan derivatives in good yields and high levels of enantioselectivity, even on a preparative scale. The convergent nature of this transformation should lend itself to the preparation of unnatural tryptophan derivatives for use in a broad array of synthetic and biological applications.