Axially Chiral P,N-Ligands: Some Recent Twists and Turns - ACS

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Perspective Cite This: ACS Catal. 2018, 8, 624−643

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Axially Chiral P,N-Ligands: Some Recent Twists and Turns Balaji V. Rokade and Patrick J. Guiry* Centre for Synthesis and Chemical Biology (CSCB), School of Chemistry, University College Dublin (UCD), Belfield, Dublin 4, Ireland ABSTRACT: Axially chiral P,N-ligands have emerged as a powerful ligand class over time with significant recent advances in terms of their synthesis, resolution, and applications to a broad range of synthetically important asymmetric transformations. This Perspective describes the evolution of this ligand class from Quinap, the first member of the family, to the most recent UCD-Phim/StackPhim analogues, a journey with many “twists and turns”.

KEYWORDS: asymmetric catalysis, axial chirality, P,N-ligand, metal catalysis, enantioselectivity

1. GENERAL INTRODUCTION The catalytic activity of metal-catalyzed processes originates from the transition metal, whereas the asymmetric induction obtained is due to the influence exerted by the chiral ligands coordinated to the metal. These ligands control the binding of reactants and their subsequent reaction paths through a combination of steric and electronic interactions. The most commonly used donor atoms include phosphorus, nitrogen, oxygen, and sulfur, and these help to electronically tune the metal center. Many of the privileged chiral ligand classes, such as Binap, Binol, etc., possess a C2 axis of symmetry.1,2 C2 symmetry is beneficial, as it is proposed to reduce the number of both possible substrate catalyst conformations and potential reaction pathways.3 However, while many chiral ligands may indeed be C2-symmetric, the two donor atoms may interact differently with the metal in key catalytic transition metal complex intermediates.4 As a result, tuning each donor atom to accommodate a specific purpose in the catalytic cycle has led to ligand desymmetrization becoming a key feature in ligand design and application. The most common approach is to retain the phosphorus donor atom and have nitrogen, oxygen, or sulfur as the second donor atom in the bidentate framework.5,6 In this way, the π-acceptor character of phosphorus can stabilize a metal center in a low oxidation state while the other donor atom can make the metal more susceptible to oxidative addition reactions. This Perspective aims to inform the reader about the most significant axially chiral P,N-ligands reported in the literature and their application in a broad range of metal-catalyzed enantioselective transformations. The ligands will be discussed in the order they have appeared in the literature rather than by the reactions to which their metal complexes have been applied. In this manner, the continued development of ligand architectural design can be more easily monitored. During the 1980s, the Binap ligand developed by Noyori acquired much attention because of its excellent catalytic utility in ruthenium-catalyzed asymmetric hydrogenation.7−9 Con© XXXX American Chemical Society

temporaneously, Hayashi and Kumada successfully applied ferrocene-based P,N-ligands in a range of catalytic asymmetric transformations, in particular for C−C and C−heteroatom couplings,10 thus providing the impetus for preparing a P,N analogue of Binap and closely related structures (Figure 1).11−13

2. QUINAP The first axially chiral P,N-ligand, 1-(2′-diphenylphosphino3′,6′-dimethoxyphenyl)isoquinoline (1), was reported by Brown in 1991 (Figure 2).14 This new class of hybrid ligand generates a six-membered chelate with the metal atom through coordination of the nitrogen and phosphorus atoms. The difference in the electronic natures of the nitrogen and phosphorus donor atoms would give a potentially more labile intermediate that would have different reactivity in asymmetric catalysis. Ligand 1 was found to racemize at ambient temperature, which limits its application in asymmetric catalysis.15 Therefore, it became clear that further structural modifications were required to increase the barrier to rotation around the chiral axis. The key design factor to freeze racemization in the new ligand was to revert to a “binaphthalene-type” backbone, and hence, a naphthalene− isoquinoline combination was chosen. This led to the design, synthesis, and resolution of ligand 2a, named Quinap by analogy to Binap, which was later reported in 1993 (Figure 2).16 2.1. Synthesis and Resolution of Quinap. The original synthesis of Quinap developed by Brown involves Suzuki crosscoupling as a key step for the main aryl−aryl bond formation, and the remaining steps to prepare the racemic ligand were achieved by conventional chemistry (Scheme 1).17 The successful resolution of Quinap occurred when the Pd complexes derived from (R)- or (S)-1′-dimethylamino-1Received: November 3, 2017 Revised: November 30, 2017 Published: December 4, 2017 624

DOI: 10.1021/acscatal.7b03759 ACS Catal. 2018, 8, 624−643

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ACS Catalysis

Figure 1. Axially chiral P,N-ligands.

The same overall method was used to prepare analogues of Quinap in which the phosphine unit was readily modified (Figure 3).19 During the application of Quinap to Pd-catalyzed asymmetric allylic alkylation, NMR spectroscopic and X-ray crystallographic studies showed that the 3-H of the isoquinoline ring had a significant steric effect in determining the enantioselectivity. The 3-H holds a key position in space, leading to crucial ligand−reactant steric interactions that influence the enantioinduction. This observation led to the design principle behind the synthesis of the vaulted analogue Phenap (2h) (Figure 3), which was developed to incorporate additional steric bulk at the 3-position of the isoquinoline ring.20,21 1-Methyl-2-diphenylphosphino-3-(1′-isoquinolyl)indole (2i) was also synthesized by Brown (Figure 3).22 The design feature of this ligand maintained the isoquinoline moiety and changed the naphthyldiphenylphosphine to a 2-diphenylphosphinoindole in an attempt to determine the effect of a varied bite angle on the reactivity and enantioselectivity. Resolution studies showed that ligand 2i racemized easily at room temperature, limiting its application in enantioselective catalysis. Brown’s synthesis of Quinap and its analogues has two inherent disadvantages: the resolution step requires a stoichiometric amount of expensive Pd, and the phosphine is introduced prior to this resolution step, so that, tediously, each new ligand has to be resolved. These limitations prompted

Figure 2. Development of Quinap.

ethylnaphthalene 6 were used, as two easily separated diastereomeric salts 7 were formed. By X-ray crystallography it was shown that the two diastereomeric complexes 7 possess distinct geometries, leading to remarkably distinct solubilities and allowing the resolution to be carried out by fractional crystallization. The diastereomers could then be decomplexed by reaction with a strong ligand such as 1,2-bis(diphenylphosphino)ethane (dppe) to access the enantiopure ligands (Scheme 2). Subsequently, the more economical method of employing half-equivalents was used; here a thermodynamic resolution occurs in which the more stable of the two Pd complexes can be separated by crystallization to access enantiomerically pure free ligand. This route was routinely used to synthesize up to 20 g of each hand of the ligand.18 Scheme 1. Synthesis of rac-Quinap

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ACS Catalysis Scheme 2. Resolution of Quinap

product and also the recovered aryl bromide were produced with high enantiomeric purity, but attempts at phosphinylation of the recovered bromide resulted in loss of enantiomeric purity. Attempts to use the aryl triflate or tosylate precursors were unsuccessful at first because of the high temperature required in the coupling, resulting in racemization of the reactants. However, slow addition of HPPh2 to aryl triflate 12 allowed the cationic addition intermediate to equilibrate before forming the final product in 90% ee with the help of chiral ligand 13. Recrystallization increased the ee to >99.5%. More recently, Lassaletta reported a general approach for the enantioselective synthesis of Quinap that uses a Pd-catalyzed dynamic kinetic C−P cross-coupling between heterobiaryl triflates 12 or nonaflates and trimethylsilylphosphines, with Josiphos-type bidentate phosphine 14 providing the asymmetric induction (Scheme 6).31 This method was applicable for the synthesis of several other axially chiral P,N-ligands such as Quinazolinap and Pinap analogues (vide infra) in good yields and enantioselectivities. 2.2. Applications of Quinap and Phenap. Quinap has continued to be successfully employed in a number of important synthetic transformations, underscoring its position as one of the preeminent axially chiral P,N-ligands. 2.2.1. Rh-Catalyzed Enantioselective Reactions. Hydroboration of Vinyl Arenes. Quinap showed excellent results in the Rh-catalyzed asymmetric hydroboration of vinyl arenes, an important method for the preparation of secondary alcohols. Binap has been applied to this reaction previously, but the reaction required a low temperature of −78 °C to achieve high enantioselectivities of up to 89% ee with p-methoxystyrene. Quinap was shown to be a superior ligand in this reaction, providing high enantioselectivities for the hydroboration of indene, dihydronaphthalene, and styrene (Scheme 7).32−34 However, lower ee values and regioselectivity were observed when Quinap was applied to substrates containing electronpoor substituents. When Phenap was used in the asymmetric

Figure 3. Analogues of Quinap.

other researchers to investigate alternative synthetic routes to Quinap itself and related ligands. The approach by Baker and Sargent demonstrated that (S)-sulfoxide 8 coupled with aryl Grignard reagent 9 through sulfoxide displacement, partially retaining the configuration at the new aryl−aryl bond (Scheme 3).23−26 Unfortunately, this approach was limited by the substantial loss of enantiomeric purity (14% ee). Knochel investigated a similar approach using the chiral sulfoxide intermediate 11, and this avoided the problem of loss of enantiomeric purity.27 The precursor 10 was formed in one step by Negishi coupling of two arene precursors and subsequently converted by reaction with the enantiomerically pure sulfinate into a diastereomeric mixture of sulfoxides 11, which were separated by chromatography. Further phosphinylation and reduction of the individual isomers allowed access to enantiomerically pure (R)- and (S)-Quinap (Scheme 4). It was demonstrated that thermal equilibration of the diastereomeric sulfoxide intermediates was biased in favor of the R isomer over the S isomer, thus simplifying the separation.28 In 2013, Stoltz reported a novel approach for the synthesis of Quinap using a Pd-catalyzed asymmetric phosphination (Scheme 5).29,30 With the key aryl−aryl bond between the naphthyl and quinoline fragments of Quinap already in place, the C−P bond formation was conducted using a dynamic kinetic resolution during the reaction between aryl triflate 12 and diphenylphosphine. The initial work was concerned with finding conditions to produce Quinap with high enantiomeric purity from the corresponding aryl bromide. The phosphine Scheme 3. Baker’s and Sargent’s Approach

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ACS Catalysis Scheme 4. Knochel’s Approach

Scheme 5. Stoltz’s Approach

Scheme 7. Hydroboration of Vinyl Arenes

hydroboration, the highest ee observed was 84% for dihydronaphthalene (Scheme 7).22 Hydroboration/Amination of Vinyl Arenes. Rh-catalyzed asymmetric hydroboration of vinyl arenes was also employed in the synthesis of chiral amines. This reaction involved the asymmetric synthesis of catecholboronate esters from aryl alkenes. In situ alkylation of the ester with MeMgCl followed by electrophilic amination using NH2OSO3H furnished primary amines with complete retention of configuration (Scheme 8).35,36 Secondary (but not tertiary) amines can be easily synthesized when in situ-generated alkylchloramines are employed as the aminating agents. The enantioselectivities are comparable to those observed in H2O2 oxidations, and for secondary amines the yields were also comparable, whereas for primary amines the yields were lower. Diboration of Alkenes. Diboration of alkynes and alkenes has been known since 1993, but in 2003 Morken reported the Rh-catalyzed asymmetric diboration of alkenes using Quinap as the ligand and dicatecholdiboron (B2Cat2) as the boron source (Scheme 9).37−42 High enantioselectivities and diastereoselectivities were reported with excellent yields for the subsequent oxidations to the mainly syn-diol product, as opposed to BINAP, which gave lower diastereoselection in the final product. For transsubstituted alkenes such as trans-stilbene, the reaction furnished the product in 48% yield with 98% ee (Scheme 9a). The reaction appears to be general for trans-alkenes and, unlike the

Rh/Quinap-catalyzed hydroboration, does not require the presence of an aromatic group for high reactivity or enantioselectivity (Scheme 9b). For trisubstituted alkenes, the enantioselectivites were excellent but the yields were low (Scheme 9c), while monosubstituted and cis-substituted alkenes gave products with low enantioselectivities (Scheme 9d). 2.2.2. Pd-Catalyzed Enantioselective Reactions. Allylic Alkylations. Quinap and Phenap were successfully applied to the Pd-catalyzed asymmetric allylic alkylation of 1,3-diphenyl-2propenyl acetate with the sodium salt of dimethyl malonate (Scheme 10). After extensive optimization, it was found that the best enantioselectivity of 98% with 95% yield was achieved using Quinap,20 whereas the highest ee observed with Phenap was 95%.21 Dynamic Kinetic Asymmetric Buchwald−Hartwig Amination and Alkynylation. More recently, Lassaletta reported a new application of Quinap in the Pd-catalyzed dynamic kinetic asymmetric Buchwald−Hartwig amination and alkynylation of racemic heterobiaryl electrophiles.43 An efficient approach for the asymmetric synthesis of IAN-type N,N-ligands was reported that was based on dynamic kinetic asymmetric Buchwald−Hartwig amination of racemic heterobiaryl electrophiles (Scheme 11). The products were obtained in high yields with good to excellent enantioselectivities using a Pd/(S)-

Scheme 6. Lassaletta’s Approach

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ACS Catalysis Scheme 8. Hydroboration/Amination of Vinyl Arenes

Scheme 9. Diboration of Alkenes

Scheme 10. Allylic Alkylations

Scheme 11. Dynamic Kinetic Asymmetric Buchwald− Hartwig Amination

Scheme 12. Dynamic Kinetic Asymmetric Pd-Catalyzed Alkynylation

Quinap combination. Reactivity and structural studies of neutral and cationic oxidative addition intermediates supported a dynamic kinetic asymmetric amination mechanism based on the labilization of the stereogenic axis in the latter. Mechanistic studies also suggested that coordination of the amine to the Pd center is the stereodetermining step. Lassaletta applied a similar approach to the dynamic kinetic Pd-catalyzed alkynylation of racemic heterobiaryl nonaflates (Scheme 12).44 Excellent enantioselectivities were achieved using a Pd/(S)-Quinap combination under the reaction conditions along with a broad substrate scope and functional

group tolerance. The newly installed alkynyl group was readily transformed to access novel axially chiral bidentate ligands. 2.2.3. Cu-Catalyzed Enantioselective Reactions. Addition of Alkynes to Enamines. Knochel applied (R)-Quinap in the Cu-catalyzed reaction between terminal alkynes and enamines to afford propargylic amines in high yields with good enantioseletivities (Scheme 13).45 The reaction showed broad substrate scope and occurred under mild reaction conditions. Addition of Alkynes to Imines. Knochel reported an improved synthesis of propargylic amines via three-component coupling between aldehydes, amines, and alkynes (A3). This reaction afforded propargylic amine products in high yields with 628


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mized at room temperature, indicating that larger groups were required at the 2-position. On the basis of this observation, Guiry initiated a study of the synthesis of a new axially chiral phosphinamine ligand class, headed by Quinazolinap (22a), which would permit variation of the steric bulk at the 2-position of the quinazoline ring (equivalent to the 3-position in Quinap) (Figure 4).55 Initially Quinazolinap with the 2-phenyl substituent was investigated, as the naphthalene−quinazoline backbone would be inert to racemization. Furthermore, the incorporation of the quinazoline ring over the isoquinoline ring facilitated investigation of the reduced basicity of the donor nitrogen. The presence of the second donor nitrogen in quinazoline has the effect of lowering the pKa of the donor nitrogen compared with isoquinoline (5.1 vs 3.3). With electronic and steric variation, the ability of this ligand class to induce enantioselectivity in a range of metalcatalyzed transformations could be studied relative to Quinap. 3.1. Synthesis and Resolution of Quinazolinap. In the synthesis of Quinazolinap, one of the key steps, i.e., aryl−aryl bond formation, was achieved by Suzuki coupling between 4chloro-2-phenylquinazoline (23) and 2-methoxy-1-naphthylboronic acid (4) in high yield (83%) using Pd(PPh3)4. Demethylation of the methyl ether 24 gave the free naphthol, and triflation led to the formation of aryl triflate 25. Nicatalyzed C−P coupling (Cai’s method)56 of 25 produced racemic Quinazolinap 22a in a moderate yield of 74% (Scheme 20).55 While Quinazolinap has the advantage of allowing variation of the steric bulk at the 2-position relative to Quinap, a drawback is the need to resolve it using a stoichiometric amount of a chiral Pd complex. The reaction of racemic Quinazolinap analogues 22b−d with the o-palladated derivatives of (R)-dimethyl(1-(1-naphthyl)ethyl)amine was carried out because of its prior success in the resolution of Quinap and Phenap. Treatment of racemic ligands 22 with chiral Pd salt 6 in dry degassed methanol or dichloromethane (if solubility issues were encountered) formed the required diastereomeric complexes (Schemes 21 and 22).57 In cases where R is small (H, Me, i-Pr), the diastereomers were formed as their hexafluorophosphate salts (26) and separated by fractional crystallization. Upon further study of the formed Pd complexes by X-ray crystallography, the ligands were found to be bound to Pd in a bidentate fashion (Scheme 21). When R is large (Ph, t-Bu, Bn, Ad), the diastereomers formed could be resolved as their chloride salts 27 (Scheme 22). Fractional crystallization of the Pd-bound complexes using a solvent system such as chloroform/diethyl ether or butanone/ diethyl ether was applied successfully in the separation of the diastereomeric mixtures. In the case of 2-benzyl Quinazolinap analogue 22f, the difference in the solubilities of the diastereomers in nonpolar solvents such as diethyl ether led to the formation of insoluble precipitates, allowing facile separation of the mixture. Decomplexation of the corresponding diastereomeric Pd− ligand complexes (26 and 27) to give the enantiopure ligands

Scheme 13. Addition of Alkynes to Enamines

excellent ee without the use of a preformed enamine (Scheme 14).46,47 Li reported the application of Quinap to a cross-dehydrogenative coupling (CDC) of alkynes and tetrahydroisoquinolines by combining Cu and photoredox catalysis.48 A ligand screen of bisoxazolines, bisphosphines, and P,N-ligands showed that Quinap was the optimal ligand for the reaction with CuBr and [Ir(ppy)2(dtbbpy)]PF6 as the optimal photoredox catalyst to provide the products with excellent enantioselectivities and yields (Scheme 15). β-Borylation of α,β-Unsaturated Esters. The β-borylation of α,β-unsaturated esters (e.g., ethyl 2-butenoate) using CuCl/ Quinap was recently reported with an excellent conversion and enantioselectivities of up to 72% for the borylated product (Scheme 16).49 2.2.4. Ag-Catalyzed 1,3-Dipolar Cycloaddition. Quinap has been shown to be an excellent ligand in the Ag-catalyzed reaction of azomethine ylides with electron-poor alkenes, providing pyrrolidine products with up to four new stereogenic centers. While the basic reaction had previously been discovered,50 Schreiber greatly expanded the substrate scope using glycine-derived ylides and acrylates. The use of Quinap provided superior results compared with a wide range of ligands tested (Scheme 17).51 More recently, Reisman reported the synthesis of pyrrolizidines using a double dipolar cycloaddition in a one-pot process delivering up to five new stereocenters, with Ag complexes of Quinap inducing up to 91% ee (Scheme 18).52 2.2.5. Ni-Catalyzed Asymmetric Allene Cycloaddition. A novel enantioselective Ni-catalyzed annulation between 1,2,3,4benzothiatriazine-1,1(2H)-dioxides 18 and allenes 19 to provide products 20 was discovered by Murakami (Scheme 19).53 Of the ligands screened, Quinap proved to be the most effective, providing the products in high yields with high regioand enantioselectivities.

3. QUINAZOLINAP 1 H NMR spectroscopic studies of Pd−Quinap complexes in Pd-catalyzed allylic alkylation showed that the 3-position affects the ligand−reactant steric interactions. On the basis of this design principle, Guiry investigated a Quinap analogue named Pyrazinap (21) based on a substituted pyrazine backbone (Figure 4).54 The application of this ligand would allow investigation of the effect of the 3,6-dimethylpyrazine ring in enantioinduction. Initially it was proposed that interaction of the 2-methyl substituent and the naphthyl ring would prevent rotation and provide axial chirality. Unfortunately, 21 raceScheme 14. Addition of Alkynes to Imines

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ACS Catalysis Scheme 15. Cross-Dehydrogenative Coupling of Alkynes and Tetrahydroisoquinolines

Scheme 16. β-Borylation of α,β-Unsaturated Esters

ities (51% ee). Ligand 22o with an additional 7-chloro substituent afforded the borylated product in 100% conversion with 43% ee.

4. PYPHOS Chan developed the pyridine-based P,N-ligand PyPhos (28) (Figure 6).65 4.1. Synthesis and Resolution of PyPhos. The initial synthesis of PyPhos was attempted from the enantiopure pyridylphenol (Scheme 26).66a The optically pure pyridylphenol was converted to its triflate in excellent yield without any racemization. The phosphine moiety was installed by reacting the triflate with triphenylphosphine under Pd catalysis at 110− 115 °C.66b,c Nevertheless, the phosphine ligand 28 was found to be a racemic mixture. Since no phosphination occurred below 110 °C and other phosphination methods proved to be ineffective,67 the synthesis of enantiopure PyPhos from its corresponding optically active triflate precursor was not possible. Hence, racemic PyPhos was prepared from racemic pyridylphenol (Scheme 26). Racemic PyPhos was resolved through diastereomeric complexation with chiral Pd complex 6. The hexafluorophosphate salts of the diastereomers (29) were found to be separable by fractional recrystallization, whereas the corresponding chloride salts were difficult to recrystallize (Scheme 27). 4.2. Application of PyPhos in Hydroboration of Vinyl Arenes. The cationic Rh complex of PyPhos was applied in the enantioselective hydroboration of vinyl arenes (Scheme 28).65 The products were formed with excellent regioselectivity and good to high enantioselectivity. It was observed that the ee values were dependent on the electronic properties of parasubstituted styrenes.

(22) was carried out using dppe in dichloromethane (Scheme 21 and 22).57 Electronic as well as steric variation of chiral ligands has also been shown to play a role in enantioinduction. Therefore, various analogues 22b−r with steric, electronic, and postresolution modification were synthesized and applied in asymmetric catalysis (Figure 5).58−62 3.2. Applications of Quinazolinap Ligands in Asymmetric Catalysis. 3.2.1. Rh-Catalyzed Asymmetric Hydroboration of Vinyl Arenes. As previously noted, enantioselective hydroboration of styrenes is an important transformation. The original work by Hayashi had some excellent results for Rh complexes of Binap. However, low levels of enantioselectivity and conversion were obtained with more sterically demanding vinyl arenes, such as β-methylstyrene, stilbene, and indene. Rh complexes of Quinazolinap ligands showed excellent activities, providing the products with excellent conversions, good to high regioselectivities, and the highest enantioselectivities for several styrene derivatives (Scheme 23).61,63,64 The 2-methyl Quinazolinap analogue 22d was found to be the best ligand, furnishing the corresponding alcohols in good yields with high regioselectivities and excellent enantioselectivities (81−97% for most of the styrene substrates and 99.5% for indene). 3.2.2. Pd-Catalyzed Asymmetric Allylic Alkylation. Quinazolinap ligands were also employed in Pd-catalyzed allylic asymmetric alkylation (Scheme 24).58,61,62 The Quinazolinap ligands containing a 2-cyclobutyl ring (22h) or a 2-adamantyl ring (22g) furnished the allylated product in 95% conversion with 89% ee and 49% ee, respectively. Ligands containing a 2-iPr substituent and different aryl group on phosphorus (e.g., 22m and 22n) yielded product in 100% conversion with 92% ee and 76% conversion with 94% ee, respectively. 3.2.3. Cu-Catalyzed Asymmetric β-Borylation of α,βUnsaturated Esters. To expand the scope of the Quinazolinap ligand class, they have also been applied in Cu-catalyzed βborylation of conjugated esters.49 A variety of Quinazolinap ligands were screened, and the conversions were moderate to excellent, whereas the enantioselectivities were found to be moderate (Scheme 25). The Quinazolinap ligand containing a 2-i-Pr substituent (22c) furnished the borylated products with good conversions (up to 100%) but moderate enantioselectiv-

5. PINAP As previously stated, one of the main disadvantages in the synthesis of Quinap, Quinazolinap, and PyPhos ligands is the need for a stoichiometric amount of a chiral Pd salt for their resolution. Also, for every structural modification in the ligand structure, as shown in the synthesis of Phenap and variants of Quinazolinap, the resolution procedure must also be modified. In 2004, Carreira reported the synthesis of a new axially chiral P,N-ligand, Pinap (30), which incorporates a covalently bound chiral element that allows diastereoisomers to be separated by column chromatography or crystallization without the need to use chiral Pd salts for resolution (Figure 7).68

Scheme 17. 1,3-Dipolar Cycloaddition

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ACS Catalysis Scheme 18. Double Dipolar Cycloaddition

Scheme 19. Allene Cycloaddition

5.2. Application of Pinap in Asymmetric Catalysis. 5.2.1. Rh-Catalyzed Hydroboration of Vinyl Arenes. With the synthesis of Pinap as an alternative to Quinap/Quinazolinap, Carreira reported initial results for reactions in which Quinap and Quinazolinap were the most successful. Pinap was applied to the asymmetric hydroboration of vinyl arenes, and the results were comparable to those with Quinap/Quinazolinap in the majority of cases. In case of the cationic Rh complex formed with (R,S)-30a, the electronics of the aryl substrate exerted little influence on the ee value of the product (Scheme 30).68 5.2.2. Ag-Catalyzed Azomethine Cycloaddition with Acrylates. Carreira also applied Pinap to Ag-catalyzed azomethine cycloaddition with acrylates (Scheme 31).68 Pinap in combination with AgOAc afforded the cyclized products in high yields with excellent enantioselectivities that were comparable to the results provided by Quinap. 5.2.3. Cu-Catalyzed Enantioselective Reactions. Alkyne Conjugate Addition to Meldrum’s Acid Derivatives. Carreira reported the Cu-catalyzed asymmetric conjugate addition of alkynes to Meldrum’s acid derivatives using Pinap ligands in aqueous media (Scheme 32).70 The use of Meldrum’s acid derivatives is attractive because the products can be converted to enantioenriched β-alkynyl acids. An initial chiral ligand screen of this reaction showed that many of the phosphine ligands (Josiphos, Binap, Monophos) as well as N,N-ligands gave low enantioinduction, with the highest enantioselectivity

Figure 4. Development of Quinazolinap.

5.1. Synthesis of Pinap. The ligand backbone 31 can be conveniently accessed by oxidative Friedel−Crafts coupling of the dichlorophthalazine with 2-naphthol (Scheme 29). The reaction is selective for the 1-chloro position, and no secondary coupling at the 4-position is observed. Reaction of heteroaryl chloride 31 with (R)-phenylethanol provides the diastereomeric aryl ethers, which upon triflation furnish 32. On the other hand, triflation of 31 followed by reaction with (R)-(+)-αphenylethylamine furnishes 33. Finally, phosphination of 32 and 33, using the same methodology as in the synthesis of Quinap and Quinazolinap, furnished Pinap ligands 30a and 30b, respectively, as diastereomeric mixtures that were then easily separated by column chromatography.68 During the catalytic applications of the original Pinap ligands, various analogues (30c−g) were also synthesized and in some cases found to be better than the parent ligands (Figure 8).69 Scheme 20. Synthesis of rac-Quinazolinap

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ACS Catalysis Scheme 21. Resolution of Quinazolinap Analogues

Scheme 22. Resolution of Quinazolinap Analogues

Figure 5. Quinazolinap ligand series.


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ACS Catalysis Scheme 24. Allylic Alkylation

Scheme 25. β-Borylation of α,β-Unsaturated Esters

amine. The ligand (R,Sa)-30b provided the propargylamine in 84% yield with 98% ee, while the ligand (R,Ra)-30b gave a yield of 82% with 99% ee. Similarly, Ma has also applied Pinap ligand (R,Ra)-30b in the synthesis of chiral propargylic amines using A3 coupling (Scheme 34).71 While the use of trimethylsilylacetylene as the alkyne component has been shown to provide high ee values in this reaction, the use of aromatic aldehydes still provides inferior ee values compared with aliphatic aldehydes. With pyrrolidine as the amine component, 2-methylbut-3-yn-2-ol as the alkyne component, and a range of aromatic aldehydes, the propargylic amine products can be produced with high ee values of 91−99%. More recently, Ma developed a Cu-catalyzed threecomponent tandem reaction for the synthesis of (E)-Nallylpyrroles via [1,5]-hydride transfer (Scheme 35).72 Reaction of the propargylic alcohol, aldehyde, and 3-pyrroline with CuBr/(R,Ra)-30b in toluene afforded the corresponding chiral propargylic amine with 97% ee. Subsequent transformation of the crude chiral propargylic amine was carried out in the same solvent in the presence of CuCl, affording the chiral (E)-Nallylpyrrole in 63% yield with 97% ee. Following mechanistic studies, it was postulated that the unsaturated cyclic dialkylamine acts as the hydrogen donor. Alkynylation of Tetrahydroisoquinolines. Ma developed a novel α-alkynylation of tetrahydroisoquinolines with aldehydes and terminal alkynes using a Cu/(R,R)-30b combination (Scheme 36).73a This reaction tolerates a wide range of functional groups at the terminal alkyne with varied substrate

Figure 6. PyPhos ligand.

being 25% ee. The initially reported Pinap ligands (30a and 30b) produced moderate enantioselectivities (up to 80% ee) and yields (up to 58%). However, further structural variation of a glycine-derived chiral auxiliary led to Pinap ligands (Figure 8) that gave increased ee values.69 The diethyl variant, (R,Ra)-30d produced the highest enantioselectivity (94%) but with only moderate conversion of 40%. Addition of a methoxy group to the naphthyl moiety increased the reactivity of the ligand, as (R,Ra)-30g furnished the product in 94% yield with 95% ee. Interestingly, diastereomeric (R,Sa)-30g furnished the product with lower ee (8%) in 44% isolated yield, providing evidence for the critical role of the chiral amine group (Scheme 32). A3 Coupling. Following Knochel’s success with the application of Quinap to the A3 coupling, Pinap was also tested in the same reaction (Scheme 33).68 The initial screen delivered excellent ee values (90−99%), with the best result using isobutyraldehyde, trimethylsilylacetylene, and dibenzylScheme 26. Synthesis of rac-PyPhos

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ACS Catalysis Scheme 27. Resolution of PyPhos

to construct planar-chiral quinilinoferrocenes (Scheme 38).75 A new catalytic system, Pd(OAc)2/(R,Sa)-30a, provided an efficient route to the construction of planar-chiral quinilinoferrocenes from the starting 2-halophenyl ferrocenecarboxylic amides.


6. QUINAZOX In an attempt to prepare Quinazolinap ligands that could be resolved without the need to form chiral Pd complexes, Guiry reported the synthesis of the Quinazox ligands 31a and 31b (Figure 9).76 The Quinazox ligands incorporate the parent biaryl backbone of Quinazolinap (22) but also the privileged ligand scaffold 2-oxazoline. The tridentate P,N,N-ligands 31a and 31b combine the electronically disparate soft phosphorus and hard nitrogen atoms while also containing a fused 6,6chelation model for metal binding. 6.1. Synthesis of Quinazox. The synthesis of ligand 31 started with Suzuki−Miyaura cross-coupling between 2methoxy-1-naphthylboronic acid (4) and aryl chloride 32 in the presence of Pd(PPh3)4, furnishing biaryl 33 in excellent yield (Scheme 39).76 Subjecting alkene 33 to OsO4-mediated dihydroxylation gave the corresponding diol, which was then transformed via oxidative cleavage with H5IO6 to afford aldehyde 34 in excellent yield. Subsequent reaction with hydroxylamine furnished the oxime, which upon dehydration using Ac2O afforded nitrile 35 in 96% yield. Nitrile 35 was demethylated with AlBr3/NaI, and subsequent treatment with Tf2O gave triflate 36. Nickel-catalyzed cross-coupling between triflate 36 and Ph2PH led smoothly to phosphine 37 in good yield.

scope, giving excellent enantioselectivities and yields. The application of this methodology was shown in the synthesis of precursors to the natural products (+)-crispine A and (+)-dysoxyline with an excellent ee of 98%. By means of a similar strategy, various naturally occurring alkaloids were also synthesized.73b−d Alkynylation of Nitrogen-Containing Heterocycles. Arndtsen reported the application of Pinap ligands in the asymmetric alkynylation of nitrogen-containing heterocycles such as pyridine, quinoline, and isoquinoline (Scheme 37).74 During initial ligand screening, Quinap gave 49% ee for the alkynylation of pyridine. Further ligand screens using Pinap ligand (R,Sa)30h increased the ee to 84% in a yield of 72%. 5.2.4. Enantioselective Approach to Planar-Chiral Quinilinoferrocenes. More recently, Gu reported a Pd-catalyzed asymmetric intramolecular Cp−H bond functionalization/ cyclization reaction of 2-halophenyl ferrocenecarboxylic amides

Figure 7. Pinap ligands. 634


Perspective

ACS Catalysis Scheme 29. Synthesis of Pinap

(S,Sa)-31a and (S,Sa)-31b were unequivocally established by single-crystal X-ray diffraction analyses.76 6.2. Application of Quinazox in Pd-Catalyzed Asymmetric Allylic Alkylation. Quinazox ligands 31a and 31b, which are analogues of Quinazolinap containing the oxazoline moiety, were applied in Pd-catalyzed allylic alkylation and afforded the product in 95% conversion with 81% ee and 60% ee, respectively (Scheme 41).76

7. STACKPHOS The axially chiral ligands described thus far in this Perspective are all based on a six-membered heterocyclic motif. One attempt was made to incorporate an indole possessing a PPh2 group (2i), but that ligand was determined to be too stereolabile for catalytic asymmetric applications.22 Very recently, Aponick reported the elegant synthesis of the new axially chiral P,N-ligand StackPhos (39) containing a fivemembered imidazole moiety (Figure 10).77 Racemization about the chiral axis was inhibited because of a beneficial π−π interaction between the electron-poor pentafluorophenyl group on the nonligating nitrogen and the electron-rich naphthalene ring. This provided a unique P,N-ligand with modified bite and dihedral angles. 7.1. Synthesis and Resolution of StackPhos Ligands. Racemic ligand 39 was synthesized from readily available 2hydroxy-1-naphthaldehyde (40) (Scheme 42).77 The core of the ligand, 41, was synthesized by condensation of 40 with benzil and ammonium acetate. Later, protection of the free hydroxy group followed by pentafluorobenzylation afforded 42, which upon TBS deprotection and triflation generated the aryl triflate. Finally, the phosphine moiety was installed by Nicatalyzed C−P cross-coupling to produce racemic ligand 39.

Figure 8. Pinap family.


Finally, the oxazoline subunit was installed by heating racemic nitrile 37 with chiral amino alcohols 38 in PhCl in the presence of Cd(OAc)2. Oxazolines 31a and 31b were formed in moderate yield as 1:1 mixtures of atropdiastereomers (Scheme 40). Interestingly, each pair of isomers proved to be separable by column chromatography, and the structures of ligands 635


Perspective

ACS Catalysis Scheme 31. Cycloaddition with Acrylates

Scheme 32. Alkyne Conjugate Addition to Meldrum’s Acid Derivatives

Scheme 33. A3 Coupling with Aliphatic Aldehydes

Scheme 34. A3 Coupling with Aromatic Aldehydes

Scheme 35. A3 Coupling Followed by [1,5]-Hydride Transfer

Scheme 36. Alkynylation of Tetrahydroisoquinolines

complex (R)-6 was used for resolution, the π-stacking interaction conferred greater stability on the R,Ra diastereomer over the R,Sa diastereomer, and hence, the diastereomer (R,Ra)43 could be fully isolated. The π-stacking interaction was confirmed by X-ray crystallography. Interestingly, when the unsubstituted phenyl ring was used instead of the fluorinated phenyl ring, the ligand was found to be configurationally unstable.

It was found that the racemic ligand 39 could be converted to a single enantiomer in a two-step process. Treatment of rac-39 with Pd complex 6 and KPF6 in refluxing acetone for 24 h provided 43 in 81% yield as a single diastereomer, as confirmed by X-ray crystallography (Scheme 43).77 This diastereomeric complex, after decomplexation with dppe, generated the free ligand 39a in high yield with 98% ee. Ligand 39 has a low energy barrier to rotation and therefore will racemize at the temperature applied during the resolution step. When Pd 636


Perspective

ACS Catalysis Scheme 37. Alkynylation of Quinoline

lectivities. The reaction time was drastically reduced from 4 days to 24 h using the StackPhos ligand compared with other axially chiral ligands such as Quinap and Pinap.46,47 Aponick applied the StackPhos ligand in the challenging Cucatalyzed enantioselective synthesis of amino skipped diynes (Scheme 45).81 Despite the challenges, such as minimal difference in the reactant substituent and potential reactivity issues, a Cu/StackPhos combination proved to be extremely useful for this A3 coupling. This catalytic system affords amino skipped diynes in high yields and in a highly enantioselective manner (up to 94% ee) under very mild conditions while tolerating an exceptionally broad substrate scope. 7.2.2. Quinoline Alkynylation. StackPhos was later utilized in a highly enantioselective Cu-catalyzed alkynylation of quinolinium salts (Scheme 46).79 This three-component reaction between a quinoline, ethyl chloroformate, and a terminal alkyne employs a Cu/StackPhos combination and affords the desired products in high yields with excellent ee values. The reaction offers excellent substrate scope and tolerates a wide range of functional groups with respect to both the quinoline and the alkyne. The scope of the reaction was shown in the syntheses of the tetrahydroquinoline alkaloids (+)-galipinine, (+)-cuspareine, and (−)-angustureine. StackPhos was also successfully applied in a catalytic enantioselective total synthesis of (−)-martinellic acid (Scheme 47).80 The key step includes a Cu-catalyzed enantioselective alkynylation of an allyl carbonate-substituted quinoline. This route provides the most concise enantioselective synthesis of (−)-martinellic acid to date.

Scheme 38. Pd-catalyzed Asymmetric Cp−H Functionalization

Figure 9. Quinazox family.

During the catalytic applications of StackPhos ligand 39a, various analogues were also synthesized and in some cases found to be better than the parent ligand (Figure 11).78 7.2. Application of StackPhos in Asymmetric Cu Catalysis. 7.2.1. A3 Coupling. The first application of StackPhos was in the Cu-catalyzed A3 coupling between isobutyraldehyde, dibenzylamine, and trimethylsilylacetylene (Scheme 44).77 The Cu complexes of StackPhos ligand (S)-39a showed excellent results for aliphatic and aromatic aldehydes, furnishing the propargylamines in high yields and enantioseScheme 39. Synthesis of the Quinazox Backbone

637


Perspective

ACS Catalysis Scheme 40. Synthesis of Quinazox Ligands

Scheme 41. Allylic Alkylation

that were later used in an enantioselective synthesis of the preclinical agent OPC 51803.

8. UCD-PHIM/STACKPHIM In spite of the excellent properties shown by Quinap, Quinozolinap, PyPhos, and StackPhos ligands, they suffer from a serious drawback: the need for a resolution step using a stoichiometric or substoichiometric amount of expensive chiral Pd amine complex in order to access enantiomerically pure ligands. Moreover, resolution of the racemic ligand, which includes fractional crystallization of diastereomeric Pd complexes, has been found to be challenging. Therefore, it somewhat limits the further application of this class of ligands in asymmetric catalysis. On the other hand, Pinap and Quinazox have a built-in resolution unit that potentially allows for the easy separation of the atropisomeric diastereomers by chromatography or crystallization. On the basis of this idea, Guiry and Aponick independently reported a new class of phosphinoimidazoline (Phim) ligands

Figure 10. StackPhos ligand.

7.2.3. Alkyne Conjugate Addition to Meldrum’s Acid Derivatives. Very recently, StackPhos was utilized by Aponick in a Cu-catalyzed enantioselective conjugate alkynylation of Meldrum’s acid acceptors (Scheme 48).78 Different StackPhos ligands (3b−f) were prepared during this study. It was found that Me-StackPhos (39e) along with Cu(OAc)2 proved to be an excellent combination for this transformation. The application of the reaction was shown in the synthesis of highly useful chiral β-alkynyl Meldrum’s acid building blocks Scheme 42. Synthesis of rac-StackPhos

638


Perspective

ACS Catalysis Scheme 43. Resolution of StackPhos

showed excellent activities in the reaction (ee values up to 98.1% and yields up to 98%). The catalyst loading for the reaction can be reduced to 1 mol % (1:1.2 Cu:Phim ratio) without much effect on the yield or ee. During catalytic activity, it is observed that the axial chirality element dominates over the central chirality. This observation was examined by synthesizing corresponding Phim ligand lacking axial chirality.82 Aponick employed ligand 45 in the Cu-catalyzed enantioselective synthesis of the C2-aminoalkyl five-membered heterocycle motif (Scheme 51).83 This two-step alkynylation/ cyclization sequence is highly convergent and modular. As can be seen, all of the above-discussed axially chiral P,Nligands (Quinap, Quinazolinap, PyPhos, Pinap, Quinazox, StackPhos, and UCD-Phim/StackPhim) are based on nitrogen-containing heterocyclic components. They were successfully employed in a variety of asymmetric metal-catalyzed reactions. Similarly, another category of axially chiral P,Nligands, the MAP ligands 50 (Scheme 52), were reported in 1998.84−89 These ligands were applied in Pd-catalyzed asymmetric allylic alkylation,90−94 Suzuki cross-coupling95 and Cu-catalyzed diethylzinc addition.88

Figure 11. StackPhos family.

44 and 45 possessing axial chirality (Figure 12).82,83 It was proposed that the chiral imidazoline in the Phim ligand class could act as a built-in resolution unit and that its nonligating nitrogen would carry the pentafluorobenzyl group, which could π-stack with the electron-rich naphthalene core to stabilize the axial chirality. This strategy allowed for the synthesis of a novel Phim ligand possessing axial chirality with a modified bite and dihedral angle. 8.1. Synthesis of the UCD-Phim Ligand. Guiry reported the synthesis of ligand 44 in five straightforward steps starting from commercially available 2-methoxy-1-naphthaldehyde (46) (Scheme 49).82 The key steps were the synthesis of imidazoline 47 by N-bromosuccinimide (NBS)-mediated cyclization of (1S,2S)-(−)-1,2-diphenylethylenediamine with aldehyde 46 and installation of the phosphine moiety by Ni-catalyzed C− P cross-coupling. This ligand can be accessed easily in enantiopure form either by fractional crystallization or column chromatography of the corresponding atropdiastereomeric mixture. 8.2. Applications of UCD-Phim/StackPhim. Guiry applied this new Phim ligand in a Cu-catalyzed enantioselective A3 coupling (Scheme 50).82 Cu complexes of this ligand

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CONCLUSIONS Since the initial report of Quinap by Brown, significant efforts have been made to expand and apply metal complexes of axially chiral P,N-ligands in a range of catalytic asymmetric transformations. Over time there has been an enhancement of the routes employed for their synthesis, with a number of improvements made to reduce the number of steps required. In addition, methods for their resolution have been improved, from the amount of the chiral Pd complex required to even its elimination from the resolution process altogether. Stoltz and

Scheme 44. Cu-Catalyzed A3 Coupling

639


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ACS Catalysis Scheme 45. Synthesis of Amino Skipped Diynes via A3 Coupling

Scheme 46. Quinoline Alkynylation and Its Applications

Scheme 47. Enantioselective Total Synthesis of (−)-Martinellic Acid

Scheme 48. Conjugate Addition of Alkynes to Meldrum’s Acid Derivatives

ligands. The Quinazolinap ligands are an example of how changes in the pKa value of the donor nitrogen atom can have completely different effects on the enantioselectivities obtained in asymmetric catalysis. The development of the StackPhos ligands by Aponick has been the most exciting recent development, as the use of the pentafluorobenzyl unit for πstacking is a novel ligand design feature. The Pinap, Quinazox, and UCD-Phim/StackPhim ligands are examples of P,N-ligands that possess a built-in resolution unit and, as a result, can be accessed in atropisomerically pure form without the need for resolution. This ligand class resembles much work in

Figure 12. UCD-Phim/StackPhim ligands.

Lassaletta have reported significant advances in P,N-ligand synthesis and resolution for Quinap, Quinazolinap, and Pinap 640


Perspective

ACS Catalysis Scheme 49. Synthesis of UCD-Phim

Scheme 50. Cu-Catalyzed A3 Coupling

Scheme 51. Cu-Catalyzed Imine Alkynylation/Cyclization

chiral P,N-ligands is a highly active field of research. Considering the unique metal-coordinating abilities of these ligands, further exciting developments are expected in this area in years to come. Therefore, it will be interesting to continue to monitor the literature for more “twists and turns” of this ligand class in synthetically important transformations.

Scheme 52. Other P,N-Ligands

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AUTHOR INFORMATION

Corresponding Author

asymmetric catalysis that requires strict matching of ligands and substrates, with no one ligand being capable of high levels of asymmetric induction across a range of reactions and substrates. Of the various catalytic asymmetric reactions studied with these ligands, the following observations can be made: (1) Quinap and Quinazolinap were the optimal ligands in Rh-catalyzed hydroboration of alkenes and Pd-catalyzed allylic alkylations; (2) Quinap showed excellent enantioselectivities in Pdcatalyzed dynamic kinetic amination and alkynylation and also in Ni-catalyzed allene cycloaddition; (3) Quinap, Pinap, StackPhos, and UCD-Phim/StackPhim all induced high levels of enantioselectivity in the A3 coupling reaction, with the last two being most efficient; (5) Quinap and Pinap were excellent in the 1,3-dipolar cycloaddition reaction; and (6) Pinap and StackPhos were also applied in the Cu-catalyzed alkynylation of quinolines and Meldrum’s acid acceptors, with StackPhos being more effective. The MAP-type ligands have also been successfully applied in a series of catalytic asymmetric transformations. The examples given in this Perspective highlight that the design, synthesis, and application of axially

*E-mail: [email protected]. ORCID

Balaji V. Rokade: 0000-0001-6878-4751 Patrick J. Guiry: 0000-0002-2612-8569 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS B.V.R. thanks the Irish Research Council (IRC) for the award of a postdoctoral fellowship (GOIPD/2015/453). REFERENCES

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Axially Chiral P,N-Ligands: Some Recent Twists and Turns - ACS

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