Asymmetric Petasis Borono-Mannich Allylation Reactions Catalyzed by Chiral Biphenols

Asymmetric organocatalytic synthesis of chiral homoallylic amines

In recent decades, the chiral allylation of imines emerged as a key methodology in the synthesis of alkaloids and natural products with 4-, 5- and 6-membered cyclic amine motifs. Initially reliant on stoichiometric reagents, synthetic chemists predominantly used N-substituted chiral imines, organometallic chiral reagents and achiral reagents with an equimolar chiral controller. However, recent years have witnessed the rise of asymmetric transition-metal catalysts and, importantly, organocatalytic allylation, reshaping the landscape of modern synthetic chemistry. This review explores the latest developments in the asymmetric allylation of imines, encompassing cutting-edge advances in hydrogen-bond catalysis and non-classical approaches. Furthermore, practical examples showcasing the application of these innovative methodologies in total synthesis are presented.

Heterogeneous Single-Atom Zinc on Nitrogen-Doped Carbon Catalyzed Electrochemical Allylation of Imines

Organometallic reagents are effective for carbon-carbon bond formation; however, consumption of stoichiometric amounts of metals is problematic. We developed electrochemical allylation reactions of imines catalyzed by nitrogen-doped carbon-supported single-atom zinc, which were fixed on a cathode to afford a range of homoallylic amines efficiently. The system could suppress generation of metallic waste, and the catalyst electrode showed advantages over bulk zinc in terms of activity and robustness. An electrochemical flow reaction was also successfully performed to produce the homoallylic amine continuously with minimum amounts of waste.

Palladium-catalyzed asymmetric three-component reaction between glyoxylic acid, sulfonamides and arylboronic acids for the synthesis of α-arylglycine derivatives

A palladium-catalyzed asymmetric three-component synthesis of α-arylglycine derivatives starting from glyoxylic acid, sulfonamides and arylboronic acids is reported. This novel, operationally simple method offers access to the α-arylglycine scaffold in good yields and enantioselectivities. The utilization of α tailored catalyst system enables the enantioselective synthesis of the desired α-arylglycines despite a fast racemic background reaction. The obtained products can be directly employed as building blocks in peptide synthesis.

The Chelate Effect Rationalizes Observed Rate Acceleration and Enantioselectivity in BINOL-Catalyzed Petasis Reactions

Density functional theory (DFT) calculations afforded insight into the origin of the experimentally observed reaction rate acceleration (≥500 fold) and enantioselectivity (≥99 % ee) of 1,1'-bi-2-naphthol- (BINOL-) catalyzed three-component Petasis reactions . BINOL accelerates the rate determining step by forming a B^IV chelate, which involves the loss of water from the hemiaminal moiety to generate an iminium intermediate. Subsequent vinyl group transfer from B^IV to the iminium carbon affords the enantiomerically enriched product and a cyclic trigonal B(III)BINOL complex, which rapidly releases the BINOL allowing it to re-enter the catalytic cycle. In the transition state of the vinyl transfer step, C-H-O hydrogen bonding between the iminium C-H and O of (R)-BINOL directs the vinyl group addition to the Re-face of the iminium carbon. This mechanism explains both the rate acceleration and high enantioselectivity of the stereo determining step.

Diastereo- and regioselective petasis aryl and allyl boration of ninhydrins towards synthesis of functionalized indene-diones and dihydrobenzoindeno-oxazin-ones

Petasis aryl and allyl borations were accomplished using substituted ninhydrins, boronic acids or 2-allyl-4,4,5,5-tetramethyl-1,3,2-dioxaborolane and 1,2-aminophenols in Hexafluoroisopropanol (HFIP) without any catalysts to synthesize different aryl and allyl derivatives of ninhydrins. The nature of substitution in the boronic acids and 1,2-amino phenols was the key factor in determining the diastereo-regioselectivity and the type of product distributions. The products were isolated and characterized by HMBC, HSQC, ¹H, ¹³C NMR experiments and X-ray single crystallographic analysis. A probable reaction pathway involves in situ formation of acyclic and cyclic ninhydrin-amino alcohol adducts, with the positioned hydroxyl group determining the stereo-regioselective outcome via tetracoordinated boron intermediates. A metal free diastereo- and regioselective Petasis aryl and allyl boration of ninhydrins.

Efficient access to general α-tertiary amines via water-accelerated organocatalytic multicomponent allylation

A tetrasubstituted carbon atom connected by three sp³ or sp²-carbons with single nitrogen, i.e., the α-tertiary amine (ATA) functional group, is an essential structure of diverse naturally occurring alkaloids and pharmaceuticals. The synthetic approach toward ATA structures is intricate, therefore, a straightforward catalytic method has remained a substantial challenge. Here we show an efficient water-accelerated organocatalytic method to directly access ATA incorporating homoallylic amine structures by exploiting readily accessible general ketones as useful starting material. The synergistic action of a hydrophobic Brønsted acid in combination with a squaramide hydrogen-bonding donor under aqueous condition enabled the facile formation of the desired moiety. The developed exceptionally mild but powerful system facilitated a broad substrate scope, and enabled efficient multi-gram scalability.

The α-tertiary amine functional group is an essential structure of diverse naturally occurring alkaloids and pharmaceuticals. Here the authors show an efficient water-accelerated organocatalytic method to access α-tertiary amines incorporating homoallylic amine structures by exploiting ketones as useful starting material.

Synthesis of syn- and enantioenriched anti-β-amino alcohols by highly diastereoselective borono-Mannich allylation reactions

A highly diastereoselective method for the synthesis of syn-β-amino alcohols and enantioenriched anti-β-amino alcohols has been developed involving α-hydroxyl aldehydes and chiral α-phenylaminoxyaldehydes or α-benzoyloxyaldehydes, respectively in Petasis borono-Mannich allylation reactions. This study broadens the scope and utility of the Petasis reaction to include pinacol allylboronate and highlights its unique reactivity and stereochemical outcomes.

Developing a Methodology for Catalytic Asymmetric Crotylation of Aldehydes

Asymmetric crotylation has firmly earned a place among the set of valuable synthetic tools for stereoselective construction of carbon skeletons. For a long time the field was heavily dominated by reagents bearing stoichiometric chiral auxiliaries, but now catalytic methods are gradually taking center stage, and the area continues to develop rapidly. This account focuses primarily on preformed organometallic reagents based on silicon and, to some extent, boron. It narrates our endeavors to design new and efficient chiral Lewis base catalysts for the asymmetric addition of crotyl(trichloro)silanes to aldehydes. It also covers the development of a novel protocol for kinetic resolution of racemic secondary allylboronates to give enantio- and diastereomerically enriched linear homoallylic alcohols. As a separate topic, cross-crotylation of aldehydes by using enantiopure branched homoallylic alcohols as a source of crotyl groups is discussed. Finally, the synthetic credentials of the developed methodology are illustrated by total syntheses of marine natural products, in which crotylation plays a key role in setting up stereogenic centers.

1 Introduction

2 Pyridine N-Oxides as Lewis Base Catalysts

3 Bipyridine N,N′-Dioxides as Lewis Base Catalysts

4 Chiral Allylating Reagents

5 Synthetic Applications

6 Concluding Remarks

Enantioselective, Catalytic Multicomponent Synthesis of Homoallylic Amines Enabled by Hydrogen-Bonding and Dispersive Interactions

We report a one-step catalytic, enantioselective method for the preparation of homoallylic N-Boc amines directly from acetals. Reactive iminium ion intermediates are generated in situ through the combination of an acetal, a chiral thiourea catalyst, trialkylsilyl triflate, and N-Boc carbamate and are subsequently trapped by a variety of allylsilane nucleophiles. No homoallylic ether byproducts are detected, consistent with allylation of the iminium intermediate being highly favored over allylation of the intermediate oxocarbenium ion. Acetals derived from aromatic aldehydes possessing a variety of functional groups and substitution patterns yield homoallylic amines with excellent levels of enantiomeric enrichment. Experimental and computational data are consistent with an anchoring hydrogen-bond interaction between the protioiminium ion and the amide of the catalyst in the enantiodetermining transition state, and with stereodifferentiation achieved through specific noncovalent interactions (NCIs) with the catalyst pyrenyl moiety. Evidence is provided that the key NCI in the major pathway is a π-stacking interaction, contrasting with the cation-π interactions invoked in previously studied reactions promoted by the same family of aryl-pyrrolidino-H-bond-donor catalysts.

Asymmetric Petasis reaction for the synthesis of chiral α- and β-butadienyl amines

The Petasis reaction using (1S,2R)-1-amino-2-indanol as the substrate and an activator to construct α- and β-butadienyl amines in optically pure forms was realized, which are otherwise difficult to prepare. The reactions feature a metal-free nature, broad substrate scope, complete regioselectivities (γ-selectivity of pinacol homoallenyl- and isoprenylboronates), and high to excellent chirality induction (up to >20 : 1 dr). The favored nucleophilic addition across the Si-face of the imine intermediate was explained using DFT calculations of the six-membered chair-like transition state.

Petasis adducts of tryptanthrin – synthesis, biological activity evaluation and druglikeness assessment

The first example of a tryptanthrin-based Petasis multicomponent reaction is reported, with one of the new derivatives showing moderate fungicidal activity.

Organocatalytic Synthesis of α-Trifluoromethyl Allylboronic Acids by Enantioselective 1,2-Borotropic Migration

Chiral α-substituted allylboronic acids were synthesized by asymmetric homologation of alkenylboronic acids using CF₃/TMS-diazomethanes in the presence of BINOL catalyst and ethanol. The chiral α-substituted allylboronic acids were reacted with aldehydes or oxidized to alcohols in situ with a high degree of chirality transfer. The oxygen-sensitive allylboronic acids can be purified via their isolated diaminonaphthalene (DanH)-protected derivatives. The highly reactive purified allylboronic acids reacted in a self-catalyzed reaction at room temperature with ketones, imines, and indoles to give congested trifluoromethylated homoallylic alcohols/amines with up to three contiguous stereocenters.

Bi(OAc)3/chiral phosphoric acid catalyzed enantioselective allylation of seven-membered cyclic imines, dibenzo[b,f][1,4]oxazepines

An efficient asymmetric allylation reaction of allylboronates with seven-membered cyclic imines, dibenzo[b,f][1,4]oxazepines, is described. The reaction, which is catalyzed by a Bi(OAc)₃/CPA system, gives a range of chiral nitrogen-containing heterocycle structures in high yields and with good enantioselectivities. The conversion of these products to nitrogen-containing heterocycles is also demonstrated.

Enantioselective Catalytic Allylation of Acyclic Ketiminoesters: Synthesis of α-Fully-Substituted Amino Esters

We report the first direct catalytic enantioselective allylation of acyclic α-ketiminoesters to afford α-allyl-α-aryl and α-allyl-α-trifluoromethyl amino esters in excellent isolated yield (91-99%) and with high optical purity (90-99+% ee). The allylation proceeds on a gram scale with 5-10 mol % of indium(I) iodide and commercially available BOX-type ligands. The allylated products are easily converted to enantiomerically enriched α-substituted proline derivatives.

Reactivity and Synthetic Applications of Multicomponent Petasis Reactions

The Petasis boron-Mannich reaction, simply referred to as the Petasis reaction, is a powerful multicomponent coupling reaction of a boronic acid, an amine, and a carbonyl derivative. Highly functionalized amines with multiple stereogenic centers can be efficiently accessed via the Petasis reaction with high levels of both diastereoselectivity and enantioselectivity. By drawing attention to examples reported in the past 8 years, this Review demonstrates the breadth of the reactivity and synthetic applications of Petasis reactions in several frontiers: the expansion of the substrate scope in the classic three-component process; nonclassic Petasis reactions with additional components; Petasis-type reactions with noncanonical substrates, mechanism, and products; new asymmetric versions assisted by chiral catalysts; combinations with a secondary or tertiary transformation in a cascade- or sequence-specific manner to access structurally complex, natural-product-like heterocycles; and the synthesis of polyhydroxy alkaloids and biologically interesting molecules.

Enantioselective Strecker and Allylation Reactions with Aldimines Catalyzed by Chiral Oxazaborolidinium Ions

Chiral oxazaborolidinium ion (COBI)-catalyzed enantioselective nucleophilic addition reactions of aldimines using tributyltin cyanide and allyltributylstannane have been developed. Various α-aminonitriles and homoallylic amines were synthesized in high yield (up to 98%) with high to excellent enantioselectivity (up to 99% ee). A rational mechanistic model for the complex of COBI and aldimine is provided to account for these enantioselective reactions.

Development of bifunctional organocatalysts and application to asymmetric total synthesis of naucleofficine I and II

The proline-type organocatalysts has been efficiently employed to catalyze a wide range of asymmetric transformations; however, there are still many synthetically useful and challenging transformations that remain unachievable in an asymmetric fashion. Herein, a chiral bifunctional organocatalyst with a spirocyclic pyrrolidine backbone-derived containing fluoro-alkyl and aryl sulfonamide functionalities, are designed, prepared, and examined in the asymmetric Mannich/acylation/Wittig reaction sequence of 3,4-dihydro-β-carboline with acetaldehyde, acyl halides, and Wittig reagents. As a result, the spirocyclic pyrrolidine trifluoromethanesulfonamide catalyst can facilitate this versatile sequence as demonstrated by 18 examples displaying excellent enantioselectivity (up to 94% ee), as well as moderate to good yields (up to 54% over 3 steps). As a practical application, the asymmetric total synthesis of naucleofficine I (1a) and II (1b) in ten steps have been accomplished.

Natural products often contain complex N-fused polycyclic structures with multiple substituents and stereocentres. Here, the authors developed a bifunctional organocatalyst that is instrumental in obtaining such structures and applied it to the total synthesis of naucleofficine I and II in 6 steps.

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

1,2-Amino alcohol-dependent Petasis allylboration for racemic and chiral homoallylamines

Asymmetric Petasis allylboration among an aldehyde, 1,2-amino alcohol and pinacolallylboronate has been established at room temperature without any catalysts.

Kinetic Resolution of Secondary Allyl Boronates and Their Application in the Synthesis of Homoallylic Amines

Highly enantioenriched, chromatographically-stable secondary allyl boronates featuring a 1,1,2,2-tetraethyl-1,2-ethanediol fragment (Epin) were obtained by kinetic resolution of their racemic mixtures. The Epin group at boron considerably improved stability of allyl boronates allowing them to be readily isolated by chromatography on silica. The resolved reagents were applied in stereoselective synthesis of homoallylic amines with an internal double bond employing unprotected imines formed in situ from aldehydes and ammonia. The reactions proceeded with an excellent transfer of chirality.

Chiral Diol-Based Organocatalysts in Enantioselective Reactions

Organocatalysis has emerged as a powerful synthetic tool in organic chemistry in the last few decades. Among various classes of organocatalysis, chiral diol-based scaffolds, such as BINOLs, VANOLs, and tartaric acid derivatives, have been widely used to induce enantioselectivity due to the ability of the hydroxyls to coordinate with the Lewis acidic sites of reagents or substrates and create a chiral environment for the transformation. In this review, we will discuss the applications of these diol-based catalysts in different types of reactions, including the scopes of reactions and the modes of catalyst activation. In general, the axially chiral aryl diol BINOL and VANOL derivatives serve as the most competent catalyst for most examples, but examples of exclusive success using other scaffolds, herein, suggests that they should not be overlooked. Lastly, the examples, to date, are mainly from tartrate and biaryl diol catalysts, suggesting that innovation may be available from new diol scaffolds.

Adducts of Triallylborane with Ammonia and Aliphatic Amines as Stoichiometric Allylating Agents for Aminoallylation Reaction of Carbonyl Compounds

Triallylborane-amines adducts are effective stoichiometric allylating agents in the aminoallylation reaction of carbonyl compounds in methanol. Moreover, copper-catalyzed diastereoselective allylation of Ellman's imine was achieved with triallylborane-methylamine adduct.

Carbohydrate/DBU Cocatalyzed Alkene Diboration: Mechanistic Insight Provides Enhanced Catalytic Efficiency and Substrate Scope

A mechanistic investigation of the carbohydrate/DBU cocatalyzed enantioselective diboration of alkenes is presented. These studies provide an understanding of the origin of stereoselectivity and also reveal a strategy for enhancing reactivity and broadening the substrate scope.

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.

Neighboring phenolic group-activated gem-difluoroallylboration of imines for the catalyst-free synthesis of gem-difluorohomoallylamines

An unprecedented imine allylation with pinacol gem-difluoroallylborates for the synthesis of a wide range of racemic gem-difluorohomoallylamine derivatives under catalyst-free conditions was enabled by a neighboring phenolic group in an N-protecting group.

Asymmetric Traceless Petasis Borono-Mannich Reactions of Enals: Reductive Transposition of Allylic Diazenes

The traceless Petasis borono-Mannich reaction of enals, sulfonylhydrazines, and allylboronates, catalyzed by chiral biphenols, results in an asymmetric reductive transposition of the in situ generated allylic diazene. Acyclic 1,4-diene products bearing either alkyl- or aryl-substituted benzylic stereocenters are afforded in excellent yields and enantiomeric ratios of up to 99:1. The use of crotylboronates in the reaction results in concomitant formation of two stereocenters in either a 1,4-syn or anti relationship from the corresponding E- or Z-crotylboronate used in the reaction. The use of β-monosubstituted enals in the asymmetric traceless Petasis borono-Mannich reaction of crotylboronates installs tertiary methyl-bearing stereocenters in good yields and high enantioselectivities.

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.

Enantioselective Synthesis of Allenes by Catalytic Traceless Petasis Reactions

Allenes are useful functional groups in synthesis as a result of their inherent chemical properties and established reactivity patterns. One property of chemical bonding renders 1,3-substituted allenes chiral, making them attractive targets for asymmetric synthesis. While there are many enantioselective methods to synthesize chiral allenes from chiral starting materials, fewer methods exist to directly synthesize enantioenriched chiral allenes from achiral precursors. We report here an asymmetric boronate addition to sulfonyl hydrazones catalyzed by chiral biphenols to access enantioenriched allenes in a traceless Petasis reaction. The resulting Mannich product from nucleophilic addition eliminates sulfinic acid, yielding a propargylic diazene that performs an alkyne walk to afford the allene. Two enantioselective approaches have been developed; alkynyl boronates add to glycolaldehyde imine to afford allylic hydroxyl allenes, and allyl boronates add to alkynyl imines to form 1,3-alkenyl allenes. In both cases, the products are obtained in high yields and enantioselectivities.