Indium(I)-Catalyzed Asymmetric Allylation, Crotylation, and α-Chloroallylation of Hydrazones with Rare Constitutional and High Configurational Selectivities

Amine adducts of triallylborane as highly reactive allylborating agents for Cu(I)-catalyzed allylation of chiral sulfinylimines

The implementation of selective catalytic processes with highly active reagents is an attractive strategy that meets the modern principles of sustainable development of chemistry. In the current study, we for the first time describe the method and general principles of Cu(I)-catalyzed allylation of imines with amine adducts of allylic triorganoboranes. Triallylborane is an extremely reactive compound and cannot be used for the catalytic allylation of imines, whereas its amine adducts are ideal substrates for catalysis. The structure of the amine fragment successfully balances the safety, selectivity and stability of the allylboron reagent, allowing it to demonstrate high activity in catalytic allylation reactions, exceeding many times any known allylboranes. The obtained results are supported by quantitative kinetics data and DFT calculations. The catalytic efficacy of the system was demonstrated on model sulfinylimines (23 examples). High diastereoselectivity up to >99% was achieved, including for the gram-scale synthesis of 2-hydroxyphenyl-derivatives. Taking into account the high reactivity and unsurpassed atom-economy of amine adducts of triallylborane (AAT), they can be considered as prospective allylation reagents with Cu(I) and other appropriate metallocatalysts.

Bi(OAc)3/Chiral Phosphoric Acid-Catalyzed Enantioselective 1,2- and Formal 1,4-Allylation Reaction of β,γ-Unsaturated α-Ketoesters

A highly efficient asymmetric 1,2-allylation reaction of β,γ-unsaturated α-ketoesters was realized by using a Bi(OAc)₃/chiral phosphoric acid catalyst system under mild conditions. Meanwhile, using this combined strategy of enantioselective 1,2-allylation and subsequent anionic oxy-Cope rearrangement, the asymmetric formal 1,4-allylation reaction was achieved by a one-pot process. These reactions offer rapid access to an array of homoallylic tertiary alcohols and γ-allyl-α-ketoesters with good yields and excellent enantioselectivities. Density functional theory calculations were conducted to interpret the high enantioselectivity.

Diastereo- and Enantioselective Synthesis of 3-Allyl-3-hydroxyoxindoles via Allylation of Isatins

A highly diastereo- and enantioselective allylation of isatins with 3-substituted allylboronic compounds was achieved by the chiral N,N'-dioxide/Lu(OTf)₃ complex. This approach provides an efficient route to useful enantioenriched 3-allyl-3-hydroxyoxindoles with adjacent tetrasubstituted tertiary or tetrasubstituted quaternary stereogenic centers. Density functional theory calculations were performed to understand the different diastereoselection between the background reaction and catalytic process. Moreover, allylation with potassium allyltrifluoroborate was accomplished by the chiral N,N'-dioxide/In(OTf)₃ complex. The synthetic utility was demonstrated by further transformation of the product to a tetrahydrofuranyl oxindole derivative.

Copper(I)-catalyzed asymmetric 1,6-conjugate allylation

Catalytic asymmetric conjugate allylation of unsaturated carbonyl compounds is usually difficult to achieve, as 1,2-addition proceeds dominantly and high asymmetric induction is a challenging task. Herein, we disclose a copper(I)-NHC complex catalyzed asymmetric 1,6-conjugate allylation of 2,2-dimethyl-6-alkenyl-4H-1,3-dioxin-4-ones. The phenolic hydroxyl group in NHC ligands is found to be pivotal to obtain the desired products. Both aryl group and alkyl group at δ-position are well tolerated with the corresponding products generated in moderate to high yields and high enantioselectivity. Moreover, both 2-substituted and 3-substituted allylboronates serve as acceptable allylation reagents. At last, the synthetic utility of the products is demonstrated in several transformations by means of the versatile terminal olefin and dioxinone groups.

Catalytic asymmetric conjugate allylation of unsaturated carbonyl compounds is usually difficult to achieve due to competing 1,2-addition. Here, the authors report a copper(I)-catalyzed asymmetric 1,6-conjugate allylation of 2,2-dimethyl-6-alkenyl-4H-1,3-dioxin-4-ones with good scope and high enantioselectivity.

Bi(III)-Catalyzed Enantioselective Allylation Reactions of Ketimines

Chiral homoallylic amines not only are found in pharmaceutically relevant compounds but also serve as versatile building blocks for chemical synthesis. However, catalytic allylation of ketimines with allylboronates, an attractive approach to synthesize chiral homoallylic amine scaffolds remain scarce. Herein, we develop a highly enantioselective allylation of isatin-derived ketimines with boron allylation reagents catalyzed by a Bi(OAc)₃-chiral phosphoric acid catalyst system. The reactions are remarkably efficient and mild, most of which were completed in less than an hour at room temperature with only 1/2 mol% (Bi(OAc)₃/CPA) catalyst loading. A wide range of chiral 3-allyl 3-aminooxindoles were obtained in excellent yields and enantioselectivities. The synthetic utility was demonstrated by efficient formal synthesis of (+)-AG-041R and (−)-psychotriasine. Preliminary mechanism was studied by control experiments and theoretical calculations.

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Asymmetric allylation of ketimines

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Bi(OAc)₃-chiral phosphoric acid catalyst

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Downstream synthetic transformations

Delayed catalyst function enables direct enantioselective conversion of nitriles to NH2-amines

Accessing enantiomerically enriched amines often demands oxidation-state adjustments, protection and deprotection processes, and purification procedures that increase cost and waste, limiting applicability. When diastereomers can be formed, one isomer is attainable. Here, we show that nitriles, largely viewed as insufficiently reactive, can be transformed directly to multifunctional unprotected homoallylic amines by enantioselective addition of a carbon-based nucleophile and diastereodivergent reduction of the resulting ketimine. Successful implementation requires that competing copper-based catalysts be present simultaneously and that the slower-forming and less reactive one engages first. This challenge was addressed by incorporation of a nonproductive side cycle, fueled selectively by inexpensive additives, to delay the function of the more active catalyst. The utility of this approach is highlighted by its application to the efficient preparation of the anticancer agent (+)-tangutorine.

Acyclic Remote 1,5- and 1,4,5-Stereocontrol in the Catalytic Stereoselective Reactions of β-Lactams with Aldehydes: The Effect of the N-Methylimidazole Ligand

The application of the N-methylimidazole ( N-MI) ligand in the Pd(0)/InI-promoted allylations of aldehydes with β-lactam-derived organoindiums enables the reaction of azetidin-2-ones with diversely substituted allyl moieties, inert under previously reported conditions. As a result, allylations and crotylations of a variety of aromatic and aliphatic aldehydes with previously unavailable chiral ε-amido-allylindiums bearing α-, β-, or γ-substituted allyl fragments were developed. The reactions occur under thermodynamic control with a highly efficient remote 1,5- or 1,4,5-stereocontrol to afford a diversity of (3 Z)-2,5- anti-2,6- syn- or (3 Z)-2,5- syn-2,6- anti-substituted enediols, aminoalcohols, and homoallylic alcohols in moderate to high yields and with an excellent diastereoselectivity. A detailed study on the effect of the β-lactam and aldehyde structures and chirality on the yield and stereochemistry in the products was carried out.

Stereoselective Homocrotylation of Aldehydes: Enantioselective Synthesis of Allylic-Substituted Z/ E-Alkenes

Cyclopropanated allyl- and crotylboron reagents participate in homoallylation and homocrotylation reactions that enable enantioselective access to motifs that otherwise require many steps to synthesize. In this study, we investigated the effect of substituents α- to boron, predicted either to counteract or reinforce the 1,3- selectivity of the parent reagents. We then investigated the transformation of the substituted homocrotylation products in intramolecular photocycloadditions to produce stereochemically complex natural-product-like scaffolds, finding that flow conditions enhanced the regioselectivity and yield.

Catalytic asymmetric propargyl- and allylboration of hydrazonoesters: a metal-free approach to sterically encumbered chiral α-amino acid derivatives

Metal-free asymmetric catalysis for the synthesis of sterically encumbered amino-acids was developed using allyl- and allenylboronic acid reagents.

Enantioselective indium(i)-catalyzed [4 + 2] annulation of alkoxyallenes and β,γ-unsaturated α-keto esters

An indium(i)–chiral phosphoric acid complex was found to catalyze the enantioselective [4 + 2] annulation reaction of β,γ-unsaturated α-keto esters with alkoxyallenes, affording cyclic O,O-acetals in good yields and with high regio- and stereo-selectivities.

Mechanism and Stereoselectivity of the BINOL-Catalyzed Allylboration of Skatoles

Density functional theory calculations have been performed to investigate the binaphthol-catalyzed allylboration of skatoles. The high stereoselectivity observed for the reaction is reproduced well by the calculations and was found to be mainly a result of steric repulsions in the corresponding Zimmerman-Traxler transition states. The role of the additive MeOH in enhancing the stereoselectivity was also investigated and is suggested to promote the formation of less reactive allylboronic ester intermediates, thereby suppressing the formation of allylboroxine species, which undergo the facile racemic background reaction.

Iron-catalyzed boration of allylic esters: an efficient approach to allylic boronates

The first general iron-catalyzed boration of allylic esters has been developed.

Regiodivergent and Diastereoselective CuH-Catalyzed Allylation of Imines with Terminal Allenes

A copper-catalyzed, chemoselective hydrometalation process enables the use of simple allenes as allylmetal nucleophile surrogates in imine allylation reactions. By modulating the nitrogen-protecting group, either highly branched- or linear-selective addition can be achieved from the same allene. Both reactions exhibit excellent diastereoselectivity and broad functional-group tolerance. Preliminary results indicate that good enantioselectivity can also be achieved in the linear-selective reaction. Finally, a mechanistic model for the regiodivergence is proposed on the basis of density functional theory calculations.

Catalytic Asymmetric Allylboration of Indoles and Dihydroisoquinolines with Allylboronic Acids: Stereodivergent Synthesis of up to Three Contiguous Stereocenters

The catalytic asymmetric allylboration of cyclic imines with γ,γ‐disubstituted allylboronic acids provides products with adjacent stereocenters in high yield and stereoselectivity. Various electrophiles, including 3,4‐dihydroisoquinolines and indoles, were prenylated in a fully stereodivergent fashion by switching the E/Z geometry of the allylboronate and/or the enantiomer of the BINOL catalyst. 3‐Methylindole provided products with three adjacent stereocenters with high stereoselectivity in one synthetic operation.

Enantioselective Generation of Adjacent Stereocenters in a Copper-Catalyzed Three-Component Coupling of Imines, Allenes, and Diboranes

A highly enantio- and diastereoselective copper-catalyzed three-component coupling affords the first general synthesis of homoallylic amines bearing adjacent stereocenters from achiral starting materials. The method utilizes a commercially available NHC ligand and copper source, operates at ambient temperature, couples readily available simple imines, allenes, and diboranes, and yields high-value homoallylic amines that exhibit versatile amino, alkenyl, and boryl units.

Lewis Acid Catalyzed Borotropic Shifts in the Design of Diastereo- and Enantioselective γ-Additions of Allylboron Moieties to Aldimines

Catalytic allylboron additions to aldimines are presented for which small amounts of Zn(OMe)2 serve as the co-catalyst to accelerate allyl exchange and 1,3-borotropic shift processes. Low-yielding and moderately α- and diastereoselective reactions are thus turned into highly efficient γ-, diastereo-, and enantioselective transformations that exhibit considerable scope.

Enantioselective Decarboxylative Arylation of α-Amino Acids via the Merger of Photoredox and Nickel Catalysis

An asymmetric decarboxylative Csp(3)-Csp(2) cross-coupling has been achieved via the synergistic merger of photoredox and nickel catalysis. This mild, operationally simple protocol transforms a wide variety of naturally abundant α-amino acids and readily available aryl halides into valuable chiral benzylic amines in high enantiomeric excess, thereby producing motifs found in pharmacologically active agents.

Copper-catalyzed asymmetric allylation of chiral N-tert-butanesulfinyl imines: dual stereocontrol with nearly perfect diastereoselectivity

Copper-catalyzed asymmetric allylation of chiral N-tert-butanesulfinyl imines has been described.

The isomerization of allylrhodium intermediates in the rhodium-catalyzed nucleophilic allylation of cyclic imines

Allylrhodium species generated from potassium allyltrifluoroborates can undergo isomerization by 1,4-rhodium(I) migration to give more complex isomers, which then react with cyclic imines to provide products with up to three new stereochemical elements. High enantioselectivities are obtained using chiral diene-rhodium complexes.

An efficient, practical, and enantioselective method for synthesis of homoallenylamides catalyzed by an aminoalcohol-derived, boron-based catalyst

A practical catalytic method for enantioselective addition of an allene unit to aldimines is disclosed. Transformations are promoted by an in-situ-generated B-based catalyst that is derived from a simple, robust, and readily accessible (in multigram quantities) chiral aminoalcohol. A range of aryl-, heteroaryl-, and alkyl-substituted homoallenylamides can be obtained in 66–91% yield and 84:16 to >99:1 enantiomeric ratio through reactions performed at ambient temperature and in the presence of 0.1–3.0 mol% of the chiral catalyst and a commercially available allenylboron reagent. The catalytic protocol does not require strict anhydrous conditions, can be performed on gram scale, and promotes highly selective addition of an allenyl unit (vs a propargyl group). The utility of the approach is demonstrated through development of succinct approaches to syntheses of anisomycin and epi-cytoxazone.

Catalytic asymmetric allylation of carbonyl compounds and imines with allylic boronates

Enantioselective allylation is a highly used organic reaction to prepare chiral homoallylic alcohols and amines, which serve as important building blocks in the synthesis of a variety of natural products and pharmaceuticals.

Asymmetric allylation of sulfonyl imines catalyzed by in situ generated Cu(ii) complexes of chiral amino alcohol based Schiff bases

We have developed a catalytic route for enantioselective synthesis of homoallyl amines through Cu(ii)-Schiff base catalyzed allylation of aryl N-sulfonylimines.

Simple organic molecules as catalysts for enantioselective synthesis of amines and alcohols

The discovery of catalysts that can be used to synthesize complex organic compounds by enantioselective transformations is central to advances in the life sciences; for this reason, many chemists aim to discover catalysts that allow for preparation of chiral molecules as predominantly one mirror-image isomer. The ideal catalyst should not contain precious elements and should bring reactions to completion in a few hours through operationally simple procedures. Here we introduce a set of small organic molecules that can catalyse reactions of unsaturated organoboron reagents with imines and carbonyls; the products of the reactions are enantiomerically pure amines and alcohols, which might serve as intermediates in the preparation of biologically active molecules. A distinguishing feature of this catalyst class is the presence of a 'key' proton embedded within their structure. Catalysts are derived from the abundant amino acid valine and are prepared in large quantities in four steps with inexpensive reagents. Reactions are scalable, do not demand stringent conditions, and can be performed with as little as 0.25 mole per cent catalyst in less than six hours at room temperature to generate products in more than 85 per cent yield and ≥97:3 enantiomeric ratio. The efficiency, selectivity and operational simplicity of the transformations and the range of boron-based reagents are expected to render this advance important for future progress in syntheses of amines and alcohols, which are useful in chemistry, biology and medicine.

Boron-based pronucleophiles in catalytic (asymmetric) C(sp3)–allyl cross-couplings

Allylic and allenyl boronates or boranes were uncovered as suitable pronucleophiles in catalytic C–C bond formations with C(sp3) electrophiles such as O,O-acetals and N,O-aminals or ethers and carbohydrates. These transformations were most efficiently catalyzed by In(I) triflate. Importantly, chiral counteranion-directed, catalytic asymmetric allylation and allenylation of N,O-aminals was developed by employing a catalyst system composed of In(I) chloride and a chiral silver 2,2'-dihydroxy-1,1'-binaphthalene (BINOL)-phosphate.

Low-oxidation state indium-catalyzed C-C bond formation

The development of innovative metal catalysis for selective bond formation is an important task in organic chemistry. The group 13 metal indium is appealing for catalysis because indium-based reagents are minimally toxic, selective, and tolerant toward various functional groups. Among elements in this group, the most stable oxidation state is typically +3, but in molecules with larger group 13 atoms, the chemistry of the +1 oxidation state is also important. The use of indium(III) compounds in organic synthesis has been well-established as Lewis acid catalysts including asymmetric versions thereof. In contrast, only sporadic examples of the use of indium(I) as a stoichiometric reagent have been reported: to the best of our knowledge, our investigations represent the first synthetic method that uses a catalytic amount of indium(I). Depending on the nature of the ligand or the counteranion to which it is coordinated, indium(I) can act as both a Lewis acid and a Lewis base because it has both vacant p orbitals and a lone pair of electrons. This potential ambiphilicity may offer unique reactivity and unusual selectivity in synthesis and may have significant implications for catalysis, particularly for dual catalytic processes. We envisioned that indium(I) could be employed as a metallic Lewis base catalyst to activate Lewis acidic boron-based pronucleophiles for selective bond formation with suitable electrophiles. Alternatively, indium(I) could serve as an ambiphilic catalyst that activates both reagents at a single center. In this Account, we describe the development of low-oxidation state indium catalysts for carbon-carbon bond formation between boron-based pronucleophiles and various electrophiles. We discovered that indium(I) iodide was an excellent catalyst for α-selective allylations of C(sp(2)) electrophiles such as ketones and hydrazones. Using a combination of this low-oxidation state indium compound and a chiral semicorrin ligand, we developed catalytic highly enantioselective allylation, crotylation, and α-chloroallylation reactions of hydrazones. These transformations proceeded with rare constitutional selectivities and remarkable diastereoselectivities. Furthermore, indium(I) triflate served as the most effective catalyst for allylations and propargylations of C(sp(3)) electrophiles such as O,O-acetals, N,O-aminals, and ethers, and we applied this methodology to carbohydrate chemistry. In addition, a catalyst system composed of indium(I) chloride and a chiral silver BINOL-phosphate facilitated the highly enantioselective allylation and allenylation of N,O-aminals. Overall, these discoveries demonstrate the versatility, efficiency, and sensitivity of low-oxidation state indium catalysts in organic synthesis.