Study of Intermediates in Iridium–(Phosphoramidite,Olefin)-Catalyzed Enantioselective Allylic Substitution

Recent Advances on the Catalytic Asymmetric Allylic α-Alkylation of Carbonyl Derivatives Using Free Allylic Alcohols

During the last years, the development of more sustainable and straightforward methodologies to minimize the generation of waste organic substances has acquired high importance within synthetic organic chemistry. Therefore, it is not surprising that many efforts are devoted to ameliorating already well-known successful methodologies, that is, the case of the asymmetric allylic allylation reaction of carbonyl compounds. The use of free alcohols as alkylating agents in this transformation represents a step forward in this sense since it minimizes waste production and the substrate manipulation. In this review, we aim to gather the most recent methodologies describing this strategy by paying special attention to the reaction mechanisms, as well as their synthetic applications.

Simultaneous Stereoinvertive and Stereoselective C(sp3)-C(sp3) Cross-Coupling of Boronic Esters and Allylic Carbonates

With increasing interest in constructing more three-dimensional entities, there has been growing interest in cross-coupling reactions that forge C(sp3)-C(sp3) bonds, which leads to additional challenges as it is not just a more difficult bond to construct but issues of stereocontrol also arise. Herein, we report the stereocontrolled cross-coupling of enantioenriched boronic esters with racemic allylic carbonates enabled by iridium catalysis, leading to the formation of C(sp3)-C(sp3) bonds with single or vicinal stereogenic centers. The method shows broad substrate scope, enabling primary, secondary, and even tertiary boronic esters to be employed, and can be used to prepare any of the four possible stereoisomers of a coupled product with vicinal chiral centers. The new method, which combines the simultaneous enantiospecific reaction of a chiral nucleophile with the enantioselective reaction of a chiral electrophile in a single process, offers a solution for stereodivergent cross-coupling of two C(sp3) fragments.

Observation of unusual outer-sphere mechanism using simple alkenes as nucleophiles in allylation chemistry

Transition-metal catalyzed allylic substitution reactions of alkenes are among the most efficient methods for synthesizing diene compounds, driven by the inherent preference for an inner-sphere mechanism. Here, we present a demonstration of an outer-sphere mechanism in Rh-catalyzed allylic substitution reaction of simple alkenes using gem-difluorinated cyclopropanes as allyl surrogates. This unconventional mechanism offers an opportunity for the fluorine recycling of gem-difluorinated cyclopropanes via C - F bond cleavage/reformation, ultimately delivering allylic carbofluorination products. The developed method tolerates a wide range of simple alkenes, providing access to secondary, tertiary fluorides and gem-difluorides with 100% atom economy. DFT calculations reveal that the C - C bond formation goes through an unusual outer-sphere nucleophilic substitution of the alkenes to the allyl-Rh species instead of migration insertion, and the generated carbon cation then forms the C - F bond with tetrafluoroborate as a fluoride shuttle.

Alcohols as Substrates in Transition-Metal-Catalyzed Arylation, Alkylation, and Related Reactions

Alcohols are abundant and attractive feedstock molecules for organic synthesis. Many methods for their functionalization require them to first be converted into a more activated derivative, while recent years have seen a vast increase in the number of complexity-building transformations that directly harness unprotected alcohols. This Review discusses how transition metal catalysis can be used toward this goal. These transformations are broadly classified into three categories. Deoxygenative functionalizations, representing derivatization of the C-O bond, enable the alcohol to act as a leaving group toward the formation of new C-C bonds. Etherifications, characterized by derivatization of the O-H bond, represent classical reactivity that has been modernized to include mild reaction conditions, diverse reaction partners, and high selectivities. Lastly, chain functionalization reactions are described, wherein the alcohol group acts as a mediator in formal C-H functionalization reactions of the alkyl backbone. Each of these three classes of transformation will be discussed in context of intermolecular arylation, alkylation, and related reactions, illustrating how catalysis can enable alcohols to be directly harnessed for organic synthesis.

Enantioselective Total Synthesis of (-)-Daphenylline

A concise enantioselective total synthesis of (-)-daphenylline, a hexacyclic Daphniphyllum alkaloid with a unique benzene ring, was achieved in 14 steps. The synthesis commences with two chiral stereocenters, C2 and C18, readily installed via Carreira's Ir/amine dual-catalyzed allylation. The allylic bridgehead amine 6 was rapidly prepared through Wickens' photoredox-catalyzed hydrocarboxylation of olefin and CuBr2-catalyzed α-amination of ketone. The tetracycle 4 was formed via Pd-catalyzed reductive Heck reaction or, more concisely, by Krische's Rh-catalyzed reductive 1,6-enyne cyclization. In this synthesis, newly reported Wickens' photoredox-catalyzed hydrocarboxylation was used twice, and Friedel-Crafts acylation thrice.

Iridium/Acid Dual-Catalyzed Enantioselective Aza-ene-type Allylic Alkylation of Nitro Ketene Aminals with Racemic Allylic Alcohols

The enantioselective allylic alkylation of nitro ketene aminals with racemic allylic alcohols was realized by iridium/acid dual catalysis. An allyl group was installed on the α-position of nitro ketene aminals in a branched-selective manner in high efficiency with excellent enantioselectivities (93-99% ee). The protocol was applied to the late-stage modification of neonicotinoid insecticides, which directly furnished a novel neonicotinoid analogue with good insecticidal activity against Aphis craccivora (LC50 = 6.40 mg/L). On the basis of the control experiment, an aza-ene-type allylic alkylation reaction mechanism was proposed.

Evolution of a 'privileged' P-alkene ligand: added planar chirality beats BINOL axial chirality in catalytic asymmetric C-C bond formation

Alkene planar chirality is introduced in the 'privileged' P-alkene phosphoramidite ligand 1. The resulting diastereomeric ligands (pR,R)-5 and (pS,R)-5 form optically pure complexes of Rh(I) and Pd(II), which catalyze conjugate additions of boron C-nucleophiles to enones and allylic alkylations, respectively. In the Rh-catalyzed reaction, the planar chirality of the alkene exerts absolute enantiocontrol over the potent BINOL auxiliary.

Stereochemical Stability of Planar-Chiral Benzazepine Tricyclics: Inversion Energies of P- and S-Alkene Ligands

Inversion barriers ΔG‡ for planar chiral phosphine-alkene and sulfonamide-alkene hybrid ligands based on phenyl-dibenz[b,f]azepine have been determined by density-functional theory calculations. Analysis of the structural and electronic characteristics of the minima and transition states explains the magnitudes of ΔG‡ and the geometrical changes during the inversion process. The steric repulsion caused by bulky substituents attached to the azepine nitrogen atom has a pronounced effect on the ΔG‡ value, explaining, inter alia, the stereochemical stability of the P- and S-alkene ligands when compared to the fluxional parent compound where X = H.

Enantioselective Total Syntheses of the Cephalotaxus Alkaloids (-)-Fortuneicyclidins A and B and (-)-Cephalotine B

Fortuneicyclidins A and B are a pair of recently isolated Cephalotaxus alkaloids with an unprecedented rearranged polycyclic skeleton possessing multiple complex stereocenters and functionalization. In this work, highly stereocontrolled asymmetric total syntheses for title alkaloids were outlined. Key features include an underexplored Ir-catalyzed α-allylation of aldehyde to strategically install a vicinal N-substituted quaternary center and a tertiary stereocenter, Heck and RCM reactions to construct the critical polycyclic framework rapidly, two different tandem oxidation-transannular aldol cyclization processes, one through ozonolysis and another via Swern oxidation, to forge the last ring for fortuneicyclidins A and B, respectively. In this approach, the challenging C-2 hydroxyl group can be installed stereospecifically.

Enantioselective Total Synthesis of the Cephalotaxus Alkaloid (-)-Cephalotine A

Cephalotine A, a recently isolated Cephalotaxus alkaloid, was first synthesized enantioselectively through three critical reactions. SmI2 -mediated radical cyclization of lactone and aldehyde to forge the final ring system, Chang's iridium-catalyzed C-H amidation to construct pyrrolidone stereoselectively, and Carreria's dual Ir/amine catalyzed allylation to install the vicinal tertiary stereocenters.

Iridium-Catalyzed Asymmetric Difunctionalization of C-C σ-Bonds Enabled by Ring-Strained Boronate Complexes

Enantioenriched organoboron intermediates are important building blocks in organic synthesis and drug discovery. Recently, transition metal-catalyzed enantioselective 1,2-metalate rearrangements of alkenylboronates have emerged as an attractive protocol to access these valuable reagents by installing two different carbon fragments across C═C π-bonds. Herein, we report the development of an iridium-catalyzed asymmetric allylation-induced 1,2-metalate rearrangement of bicyclo[1.1.0]butyl (BCB) boronate complexes enabled by strain release, which allows asymmetric difunctionalization of C-C σ-bonds, including dicarbonation and carboboration. This protocol provides a variety of enantioenriched three-dimensional 1,1,3-trisubstituted cyclobutane products bearing a boronic ester that can be readily derivatized. Notably, the reaction gives trans diastereoisomers that result from an anti-addition across the C-C σ-bond, which is in contrast to the syn-additions observed for reactions promoted by Pd^II-aryl complexes and other electrophiles in our previous works. The diastereoselectivity has been rationalized based on a combination of experimental data and density functional theory calculations, which suggest that the BCB boronate complexes are highly nucleophilic and react via early transition states with low activation barriers.

Iridium-Catalyzed Diastereo- and Enantioselective [4 + 1] Cycloaddition of Hydroxyallyl Anilines with Sulfoxonium Ylides

We present here an iridium-catalyzed diastereo- and enantioselective [4 + 1] cycloaddition reaction of hydroxyallyl anilines with sulfoxonium ylides under mild reaction conditions, leading to 3-vinyl indolines in moderate to good yields with excellent enantioselectivities. Control experiments disclosed a plausible reaction mechanism.

Allenyl Carbonate as a Butadiene Surrogate in Cobalt-Catalyzed Crotylation of Aldehydes

Allenyl carbonate was used as a 1,3-butadiene surrogate to develop a photocatalytically sustainable protocol for cobalt-catalyzed crotylation of aldehydes. The developed method tolerated a wide range of aromatic and aliphatic aldehydes with retention of functional groups under mild conditions and produced good-to-excellent yields of crotylated secondary alcohols. Based on preliminary mechanistic studies and literature precedents, a plausible mechanism is proposed.

Iridium-Catalyzed Chemo-, Diastereo-, and Enantioselective Allyl-Allyl Coupling: Accessing All Four Stereoisomers of (E)-1-Boryl-Substituted 1,5-Dienes by Chirality Pairing

Here, we report a highly chemo-, diastereo-, and enantioselective allyl-allyl coupling between branched allyl alcohols and α-silyl-substituted allylboronate esters, catalyzed by a chiral iridium complex. The α-silyl-substituted allylboronate esters can be chemoselectively coupled with allyl electrophiles, affording a diverse set of enantioenriched (E)-1-boryl-substituted 1,5-dienes in good yields, with excellent stereoselectivity. By permuting the chiral iridium catalysts and the substrates, we efficiently and selectively obtained all four stereoisomers bearing two consecutive chiral centers. Mechanistic studies via density functional theory calculations revealed the origins of the diastereo- and chemoselectivities, indicating the pivotal roles of the steric interaction, the β-silicon effect, and a rapid desilylation process. Additional synthetic modifications for preparing a variety of enantioenriched compounds containing contiguous chiral centers are also included.

Role of Noncovalent Interactions in Inducing High Enantioselectivity in an Alcohol Reductive Deoxygenation Reaction Involving a Planar Carbocationic Intermediate

The involvement of planar carbocation intermediates is generally considered undesirable in asymmetric catalysis due to the difficulty in gaining facial control and their intrinsic stability issues. Recently, suitably designed chiral catalyst(s) have enabled a guided approach of nucleophiles to one of the prochiral faces of carbocations affording high enantiocontrol. Herein, we present the vital mechanistic insights from our comprehensive density functional theory (B3LYP-D3) study on a chiral Ir-phosphoramidite-catalyzed asymmetric reductive deoxygenation of racemic tertiary α-substituted allenylic alcohols. The catalytic transformation relies on the synergistic action of a phosphoramidite-modified Ir catalyst and Bi(OTf)₃, first leading to the formation of an Ir-π-allenyl carbocation intermediate through a turn-over-determining S_N1 ionization, followed by a face-selective hydride transfer from a Hantzsch ester analogue to yield an enantioenriched product. Bi(OTf)₃ was found to promote a significant number of ionic interactions as well as noncovalent interactions (NCIs) with the catalyst and the substrates (allenylic alcohol and Hantzsch ester), thus providing access to a lower energy route as compared to the pathways devoid of Bi(OTf)₃. In the nucleophilic addition, the chiral induction was found to depend on the number and efficacy of such key NCIs. The curious case of reversal of enantioselectivity, when the α-substituent of the allenyl alcohol is changed from methyl to cyclopropyl, was identified to originate from a change in mechanism from an enantioconvergent pathway (α-methyl) to a dynamic kinetic asymmetric transformation (α-cyclopropyl). These molecular insights could lead to newer strategies to tame tertiary carbocations in enantioselective reactions using suitable combinations of catalysts and additives.

Development of Immobilized Carreira (Phosphoramidite, Olefin) Ligands and Application in Iridium-Catalyzed Asymmetric Allylic Amination

A family of polystyrene-supported (phosphoramidite, olefin) ligands L1-L4, based on the original design by Defieber and Carreira, has been developed and applied in iridium-catalyzed asymmetric allylic amination of unmasked allylic alcohols (27 examples, up to 99% ee). Among them, functional resins L1 and L4 exhibit important advantages such as easy preparation, ligand recyclability, and easy handling for sequential use. As a distinctive advantage, the catalytic use of the iridium complexes of L1 and L4 allows the straightforward reuse of a high percentage of the expensive iridium metal involved in the complexes, which is not achievable under homogeneous conditions with the corresponding monomeric complexes.

Ir-Catalyzed Asymmetric Cascade Allylation/Spiroketalization Reaction for Stereoselective Synthesis of Oxazoline-Spiroketals

An asymmetric cascade allylation/spiroketalization reaction between 2-(1-hydroxyallyl)phenols and 5-methyleneoxazolines is accomplished by using a chiral Ir(I) catalyst derived from commercially available iridium precursor and the Carreira ligand. This protocol furnishes a class of structurally novel and unique oxazoline-spiroketals in up to 86% yield, >99% ee and >20:1 dr. Moreover, control experiments reveal that 4,4-disubstitution on 5-methyleneoxazolines is necessary to avoid the aromatization and for the spiroketalization to occur. On the basis of this, a plausible reaction mechanism is illustrated.

Secondary Phosphine Sulfide-Enabled Iridium-Catalyzed Asymmetric Allylic Substitution

An iridium-catalyzed asymmetric synthesis of branched allylic phosphine compounds under mild conditions is reported. Products bearing various functional groups can be synthesized with excellent stereoselectivity (up to 99.9 % ee) and regioselectivity. The employment of phosphine sulfides with relatively low deactivation capacity against metal catalysts is crucial for the success of this reaction.

Iridium-catalyzed enantioselective alkynylation and kinetic resolution of alkyl allylic alcohols

Herein, we report an efficient kinetic resolution of alkyl allylic alcohols enabled by an iridium-catalyzed enantioselective alkynylation of alkyl allylic alcohols with potassium alkynyltrifluoroborates. A wide range of chiral 1,4-enynes bearing various functional groups and unreacted enantioenriched allylic alcohols were obtained with excellent enantioselectivities and high kinetic resolution performance (s-factor up to 922). Additionally, this method is particularly effective for preparing some useful optically pure alkyl allylic alcohols, such as the key components towards the synthesis of prostaglandins and naturally occurring matsutakeols, which are difficult to access via other asymmetric reactions. Mechanistic studies revealed that the efficient kinetic resolution might be due to the significant distinction of the η ²-coordination between the (R)- and (S)-allylic alcohols with the iridium/(phosphoramidite, olefin) complex.

Ir and NHC Dual Chiral Synergetic Catalysis: Mechanism and Stereoselectivity in γ-Butyrolactone Formation

Cooperative dual catalysis is a powerful strategy for achieving unique reactivity by combining catalysts with orthogonal modes of action. This approach allows for independent control of the absolute and relative stereochemistry of the product. Despite its potential utility, the combination of N-heterocyclic carbene (NHC) organocatalysis and transition metal catalysis has remained a formidable challenge as NHCs readily coordinate metal centers. This characteristic also makes it difficult to rationalize or predict the stereochemical outcomes of these reactions. Herein, we use quantum mechanical calculations to investigate formation of γ-butyrolactones from aldehydes and allyl cyclic carbonates by means of an NHC organocatalyst and an iridium catalyst. Stereoconvergent activation of the racemic allyl cyclic carbonate forms an Ir-π-allyl intermediate and activation of an unsaturated aldehyde forms an NHC enolate, the latter of which is rate-limiting. Union of the two fragments leads to stereodetermining C-C bond formation and ultimately ring closure to generate the product lactone. Notably, CO2 loss occurs after formation of the C-C bond and Et3NH+ plays a key role in stabilizing carboxylate intermediates and in facilitating proton transfer to form the NHC enolate. The computed pathways agree with the experimental findings in terms of the absolute configuration, the enantiomer excess, and the different diastereomers seen with the (R)- and (S)-spiro-phosphoramidite combined with the NHC catalyst. Calculations reveal the lowest energy pathway includes both an NHC ligand and a phosphoramidite ligand on the iridium center. However, the stereochemical features of this Ir-bound NHC were found to not contribute to the selectivity of the process.

Enantioselective and Diastereodivergent Allylation of Propargylic C-H Bonds

An iridium-catalyzed stereoselective coupling of allylic ethers and alkynes to generate 3,4-substituted 1,5-enynes is reported. Under optimized conditions, the coupling products are formed with excellent regio-, diastereo-, and enantioselectivities, and the protocol is functional group tolerant. Moreover, we report conditions that allow the reaction to proceed with complete reversal of diastereoselectivity. Mechanistic studies are consistent with an unprecedented dual role for the iridium catalyst, enabling the propargylic deprotonation of the alkyne through π-coordination, as well as the generation of a π-allyl species from the allylic ether starting material.

Mechanistic Basis for the Iridium-Catalyzed Enantioselective Allylation of Alkenyl Boronates

Iridium(phosphoramidite) complexes catalyze an enantio- and diastereoselective three-component coupling reaction of alkenyl boronic esters, organolithium reagents, and secondary allylic carbonates. The reaction proceeds through an allylation-induced 1,2-metalate shift of the alkenyl boronate to form non-adjacent stereocenters. Mechanistic investigations outline the overall catalytic cycle and reveal trends in reactivity and selectivity. Analysis of relative stereochemistry in products derived from a variety of 1,1-disubtituted alkenyl boronates provides insight into the transition state of the addition and indicates a concerted pathway. Kinetic analysis of the reaction revealed the kinetic order dependence in boronate, the catalyst, and both the slow- and fast-reacting enantiomer of allylic carbonate as well as the turnover-limiting step of the reaction. Determination of nucleophile-specific parameters N and s_N for alkenyl boronate complexes enabled comparison to other classes of nucleophiles. DFT calculations indicate the addition of the alkenyl boronate to the cationic Ir(π-allyl) intermediate and the 1,2-metalate shift occur in a concerted mechanism. The stereoselectivity is determined by ligand-substrate steric repulsions and dispersion interactions in the syn addition transition state. Hammett studies supported the computational results with regard to electronic trends observed with both aryl-derived alkenyl boronates and aryl carbonates.

Chiral Amines via Enantioselective π-Allyliridium-C,O-Benzoate-Catalyzed Allylic Alkylation: Student Training via Industrial-Academic Collaboration

ConspectusCyclometalated π-allyliridium-C,O-benzoate complexes discovered in the Krische laboratory display unique amphiphilic properties, catalyzing both nucleophilic carbonyl allylation and electrophilic allylation of diverse amines as well as nitronates. Given the importance of chiral amines in FDA-approved small-molecule drugs, a collaboration with medicinal chemists at Genentech that included on-site graduate student internships was undertaken to explore and expand the scope of π-allyliridium-C,O-benzoate-catalyzed allylic amination and related processes. As described in this Account, our collective experimental studies have unlocked asymmetric allylic aminations of exceptionally broad utility and scope. Specifically, using racemic branched alkyl-substituted allylic acetate proelectrophiles, primary and secondary aliphatic or aromatic amines, including indoles, engage in highly regio- and enantioselective allylic amination. Additionally, unactivated nitronates were found to be competent nucleophilic partners for regio- and enantioselective allylic alkylation, enabling entry to β-stereogenic α-quaternary primary amines. Notably, these π-allyliridium-C,O-benzoate-catalyzed allylic substitutions, which display complete branched regioselectivity in reactions of alkyl-substituted allyl electrophiles, complement the scope of corresponding iridium phosphoramidite-catalyzed allylic aminations, which require aryl-substituted allyl electrophiles to promote high levels of branched regioselectivity. Computational, kinetic, ESI-CID-MS, and isotopic labeling studies were undertaken to understand the mechanism of these processes, including the origins of regio- and enantioselectivity. Isotopic labeling studies suggest that C-N bond formation occurs through outer-sphere addition to the π-allyl. DFT calculations corroborate C-N bond formation via outer-sphere addition and suggest that early transition states and distinct trans effects of diastereomeric chiral-at-iridium π-allyl complexes render the reaction less sensitive to steric effects, accounting for complete levels of branched regioselectivity in reactions of hindered amine and nitronate nucleophiles. Reaction progress kinetic analysis (RPKA) reveals a zero-order dependence on allyl acetate, a first-order dependence on the catalyst, and a fractional-order dependence on the amine. As corroborated by ESI-CID-MS analysis, the 0.4 kinetic order dependence on the amine may reflect the intervention of cesium-bridged amine dimers, which dissociate to form monomeric cesium amide nucleophiles. Hence, the requirement of cesium carbonate (vs lower alkali metal carbonates) in these processes may reside in cesium's capacity for Lewis acid-enhanced Brønsted acidification of the amine pronucleophile. Beyond the development of catalytic processes for the synthesis of novel chiral amines, the present research was conducted by graduate students who benefited from career development experiences associated with training in both academic and industrial laboratories.

Enantioselective Synthesis of Spiroketals and Spiroaminals via Gold and Iridium Sequential Catalysis

The enantioselective cascade reaction between racemic 2-(1-hydroxyallyl)phenols and alkynols/alkynamides was realized by using a gold and iridium sequential catalytic system. In this procedure, the in situ generated exocyclic vinyl ethers or enamides undergo the asymmetric allylation/spiroketalization with π-ally-Ir amphiphilic species, which provides an efficient and straightforward access to spiroketals and spiroaminals with excellent enantioselectivities. Moreover, racemic 2-(1-hydroxyallyl)anilines were also suitable in this reaction along with a kinetic resolution process, affording enantioenriched spiroaminals and 2-(1-hydroxyallyl)anilines in good yields. The synthetic utility of this method has been demonstrated by efficient enantioselective synthesis of the analogue of Paecilospirone.

Ir-Catalyzed Asymmetric Allylic Alkylation of Dialkyl Malonates Enabling the Construction of Enantioenriched All-Carbon Quaternary Centers

An enantioselective iridium-catalyzed allylic alkylation of malonates with trisubstituted allylic electrophiles to form all-carbon quaternary stereocenters is reported. This reaction proceeds at ambient temperature and enables the preparation of a wide range of enantioenriched products in up to 93% yield and 97% ee. The quaternary products can be readily converted to several valuable building blocks such as vicinal quaternary products and β-quaternary acids.

Regio- and Enantioselective γ-Allylic Alkylation of In Situ-Generated Free Dienolates via Scandium/Iridium Dual Catalysis

An unprecedented asymmetric γ-allylic alkylation of free dienolates via Sc/Ir dual catalysis is reported, which affords a range of synthetically versatile γ-allylic crotonaldehydes in high efficiency with excellent chemo-, regio-, and enantioselectivities. The dienolates bearing no essential auxiliary groups were generated in situ by scandium triflate-mediated Meinwald rearrangement of vinyloxiranes atom-economically. With the assistance of computational density functional theory calculations, a Sc/Ir bimetallic catalytic cycle was proposed to illustrate the reaction mechanism.

Catalytic, Asymmetric Total Synthesis of (+)-α-, (+)-β-, (+)-γ-, and (-)-δ-Lycorane

The first collectively asymmetric total synthesis of all members of lycorane, including (+)-α, (+)-β, (+)-γ, and (-)-δ, in a catalytic manner has been achieved. The cornerstone of this synthesis features an asymmetric, stereodivergent Ir/amine dual catalytic α-allylation of 2-phthalimidoacetaldehyde.

Recent advances in iridium-catalyzed enantioselective allylic substitution using phosphoramidite-alkene ligands

This minireview describes the recent progress of iridium-catalyzed enantioselective allylic substitution using phosphoramidite-alkene ligands realizing highly enantioselective carbon–carbon and carbon–heteroatom bond formation.

Iridium-catalyzed C3-selective asymmetric allylation of 7-azaindoles with secondary allylic alcohols

The development of efficient synthetic methods of 7-azaindoles has been desired due to the useful biological activities and physical properties. We report the first example of the iridium-catalyzed C3-selective asymmetric allylation of 7-azaindoles with racemic secondary allylic alcohols to give only branched allylation products in good to high yields with high enantioselectivity (up to >99.5% ee). Allylic alcohols and 7-azaindoles with a variety of functional groups including halogen and heteroaromatic groups are compatible with the reaction conditions. Furthermore, transformations of the obtained allylation products are demonstrated without a significant loss of enantiomeric excess.

Kinetic Resolution of Spiroindolines through Ir-Catalyzed Asymmetric Allylative Ring-Opening Reaction

Kinetic resolution of racemic spiroindolines with s factors of ≤15200 has been developed to access enantiomerically enriched indole-annulated medium-sized lactams and spiroindolines through Ir-catalyzed asymmetric allylative ring-opening reaction. Density functional theory calculations support the idea that the accurate discrimination of two spiroindoline enantiomers by (η³-allyl)-iridium(III) species and the perfect central-to-axial chirality conversion during C-C bond fragmentation ensure the stereoselective formation of two contiguous stereogenic centers and one axis in the medium-sized lactams.

Synergistic Ir/Cu Catalysis for Asymmetric Allylic Alkylation of Oxindoles: Enantio- and Diastereoselective Construction of Quaternary and Tertiary Stereocenters

3,3-Disubstituted oxindoles bearing quaternary and tertiary stereogenic centers are privileged structural motifs, which widely exist in pharmaceutical and natural products. Herein, a highly regio-, enantio-, and diastereoselective allylic alkylation of 3-alkyl oxindoles through synergistic iridium and copper catalysis is described, which provides a series of 3,3-disubstituted oxindole derivatives containing adjacent quaternary and tertiary stereogenic centers in excellent yields, enantiomeric excess, and diastereomeric ratio (for 30 examples, up to 97 % yield, >99 % ee, and >20 : 1 dr). This method provides exclusive branched selectivity, excellent enantio- and diastereoselectivities, and good functional compatibility. Control experiments suggested that the chiral copper catalyst is required for achieving high reactivities and diastereoselectivities under mild reaction conditions.

Iridium-Catalyzed Asymmetric Allylic Alkylation of Deconjugated Butyrolactams

Compared with the ever-growing list of nonprochiral nucleophiles in Ir-catalyzed asymmetric allylic substitution reactions, prochiral nucleophiles are less studied. We present a new prochiral nucleophile, namely, deconjugated butyrolactam, for Ir-catalyzed asymmetric allylic alkylation (AAA). This reaction provides access to α-allyl deconjugated butyrolactams with a moderate to good dr and an excellent er. This is the first AAA reaction of deconjugated butyrolactams. Allyl transposition through Cope rearrangement appears to proceed stereospecifically to form γ-allyl conjugated butyrolactams.

Iridium-Catalyzed Asymmetric Cascade Allylation/Pictet-Spengler Cyclization Reaction for the Enantioselective Synthesis of 1,3,4-Trisubstituted Tetrahydroisoquinolines

An iridium-catalyzed trifluoroacetic acid-promoted asymmetric cascade allylation/Pictet-Spengler cyclization reaction of azomethine ylides with aromatic allylic alcohols is reported. This protocol provides a facile and scalable method for the construction of 1,3,4-trisubstituted tetrahydroisoquinolines containing two stereogenic centers in good yields (up to 96%) with generally excellent diastereo- and enantioselectivities (up to >20:1 dr and >99% ee). Furthermore, a series of aromatic heterocycle-fused piperidines were also obtained with excellent enantiocontrol by this methodology.

Enantioselective Allylation of Alkenyl Boronates Promotes a 1,2-Metalate Rearrangement with 1,3-Diastereocontrol

Alkenyl boronates add to Ir(π-allyl) intermediates with high enantioselectivity. A 1,2-metalate shift forms a second C-C bond and sets a 1,3-stereochemical relationship. The three-component coupling provides tertiary boronic esters that can undergo multiple additional functionalizations. An extension to trisubstituted olefins sets three contiguous stereocenters.

Iridium-catalyzed enantioselective olefinic C(sp2)-H allylic alkylation

The first iridium-catalyzed enantioselective olefinic C(sp2)-H allylic alkylation is developed in cooperation with Lewis base catalysis. This reaction, catalyzed by cinchonidine and an in situ generated cyclometalated Ir(i)/phosphoramidite complex, makes use of the latent enolate character of an α,β-unsaturated carbonyl compound, namely coumalate ester, to introduce an allyl group at its α-position in a branched-selective manner in moderate to good yield with good to excellent enantioselectivities (up to 98 : 2 er).

Iridium-Catalyzed Diastereo- and Enantioselective [4 + 3] Cycloaddition of 4-Indolyl Allylic Alcohols with Azomethine Ylides

An unprecedented iridium-catalyzed asymmetric [4 + 3] cycloaddition of racemic 4-indolyl allylic alcohols with azomethine ylides is reported. The ability of acid promoter zinc triflate to perform multiple roles is the key factor for the success of this strategy. This method provides scalable and efficient access to biologically important azepino[3,4,5-cd] indoles in good yields with generally excellent diastereo- and enantioselectivities (up to >20:1 dr and >99% ee). Mild reaction conditions, easily accessible substrates and chiral catalyst, and broad substrate scope highlight the practicality of this methodology.

Practical allylation with unactivated allylic alcohols under mild conditions

A practical palladium/calcium catalytic system was developed for dehydrative allylation with unactivated allylic alcohols.