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Review

Advances in Versatile Chiral Ligands for Asymmetric Gold Catalysis

by
Yufeng Wu
1,
Hui Yang
2,
Haojie Gao
1,
Xiaoyi Huang
1,
Liyuan Geng
1 and
Rui Zhang
1,*
1
Department of Chemical Engineering and Food Science, Hubei University of Arts and Science, 296 Longzhong Road, Xiangyang 441053, China
2
Hubei Jinhong Chemical Co., Ltd., No. 32, Tianshun Avenue, Yujiahu Industrial Park, Xiangcheng Dist., Xiangyang 441048, China
*
Author to whom correspondence should be addressed.
Catalysts 2023, 13(9), 1294; https://doi.org/10.3390/catal13091294
Submission received: 10 June 2023 / Revised: 23 July 2023 / Accepted: 27 July 2023 / Published: 14 September 2023
(This article belongs to the Special Issue Homogenous and Heterogenous Catalysis in Bioactive Compound Synthesis and Small Molecule Activation)
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Abstract

:
The formation of valuable chiral skeletons through asymmetric gold catalysis has made considerable progress due to the unrivaled affinity of gold complexes with multiple carbon–carbon bonds. The renaissance of chiral ligands in recent decades has enabled the elaborate design of chiral gold complexes, which are of great significance to control chiral formation in these catalytic reactions. Therefore, this review intends to highlight the design and central role of versatile chiral ligands in asymmetric gold catalysis. Specifically, the seminal applications of various chiral ligands with representative examples in various gold-catalyzed asymmetric reactions are comprehensively explored. In addition, the reaction mechanisms are mentioned when the crucial interactions between ligands and activated substrates are introduced. Furthermore, the applications of enantioselective gold catalysis in the construction of chiral functional organic materials and drug molecules are also presented.

1. Introduction

Catalysis is an advanced technology in both the chemical industry and academic research [1,2,3]. Transition-metal catalyzed organic transformations have attracted a great deal of attention for the efficient crack and formations of chemical bonds [4,5,6,7]; therefore, they have been widely applied in the research and development of drugs, pesticides, fine chemicals and other functional materials [8,9,10]. Compared to other transition metals, the d10 electron configuration of gold(I) results in superior π affinity for multiple carbon–carbon bonds (Figure 1a) [11,12,13,14,15]. In addition, the gold center adopted by linear coordination places the ancillary ligand and substrate in opposite positions (Figure 1b). Thus, the conventional chelating mode of the ligands used in catalytic reactions involving other transition metals could not be employed for the design of ligand-modified gold catalysts. Furthermore, the gold(I)-catalyzed carbophilic addition of nucleophiles to carbon–carbon triple bonds proceeds through outer-sphere activation [16,17,18,19,20,21,22,23,24]. These factors make asymmetric modulation in gold(I) catalysis challenging. On the other hand, several gold(III) catalysts have also been reported to be active for such transformations because of their inherent square-planar geometry that promotes the formation of chiral centers.
The practical atom economy and step economy of enantioselective gold catalysis are attributed to its unique catalytic mode for constructing sets of complex molecules and efficient tandem processes, even when dealing with remote substrates and chiral ligand fragments [25,26,27,28,29,30]. Enantioselective gold catalysis has flourished due to its distinct performance. In particular, a series of valuable chiral ligands have been identified, including aryl phosphine ligands bearing the proximal chiral sulfinamide motif, phosphoramidite and phosphonite ligands, phosphine ligands comprising the ferrocene scaffold, bifunctional phosphine ligands, biphosphine ligands, chiral counterion-based ligands derived from chiral phosphoric acids and chiral carbene ligands for gold(I) catalysis, along with unique ligand frameworks for cyclometalated gold(III) catalysis. Notably, these types of chiral ligands and structural modifications are of crucial significance.
In practice, there are several reviews already reported in the literature [31,32,33]. These reviews mainly focus on the latest development of ligands or the defined types of reactions in asymmetric gold catalysis. However, it has been observed that the initial conception of elaborated design for novel ligands can lead to the creation of new asymmetric reactions and further accurate modification for the known ligands. Accordingly, the objective of this review is to stress the key role of chiral ligands in achieving modulation of chiral formation alongside exploring the unprecedented bonding modes and distinct activation efficiencies for multiple carbon–carbon bond-involving reactions. The seminal elaborated design and important progress of chiral ligands are the main focus of the review. To this end, we present a comparison of different ligands in most cases, while the state-of-the-art desired designs for ligands are inevitable considerations in “match” or “mismatch” effects for reactions. There is also a brief mention of reaction mechanisms, which are explained through interactions between ligands and activated substrates. Furthermore, the applications of enantioselective gold catalysis in the construction of chiral functional organic materials and drug molecules are also outlined.

2. Classification of Various Ligands

Various chiral organic ligands have been applied to asymmetric gold catalysis, representing an important part of transition-metal-catalyzed asymmetric transformations. Among them, the phosphine-containing ligands are one of the most common and effective. In addition, non-phosphine ligands, including chiral carbene and cyclometalated ligands, have also been developed by different research groups. To discuss these ligands in an appropriate order, the first Section 2.1, Section 2.2, Section 2.3, Section 2.4, Section 2.5 and Section 2.6 intend to focus on the different types of phosphine ligands. The second Section 2.7 aims to address chiral carbene ligands followed by the third Section 2.8 referring to cyclometalated ligand frameworks.

2.1. Phosphoramidite and Phosphonite Ligands

In the gold(I)-catalyzed [4 + 3] or [4 + 2] cyclization, electron-withdrawing phosphite ligands are believed to fit the excellent activation of the corresponding gold(I) catalysts [34,35]. To construct better chiral modulation for the challenging asymmetric gold(I) catalysis, phosphoramidite ligands have also been incorporated into the catalytic systems owing to their similar electronic properties with phosphites and π–π interactions with substrates resulting from the attenuated flexibility around the gold center as well as the closer chiral information to the new carbon stereocenters. For the enantioselective intramolecular cycloaddition of allene-tethered 1,3-dienes 1, the bicyclic products 2 and 3 can be furnished smoothly, and phosphoramidite ligand L1-ligated gold(I) catalyst displayed superior chemoselectivity (16:1.2) and enantioselectivity (91% enantiomeric excess (ee)) over the chiral bisphophine (S)-(−)-2,2′-bis(diphenylphosphino)-1,1′-binaphthalene (S-BINAP)-coordinated gold(I) analogue (Scheme 1) [36]. The proposed mechanism begins with the activation of an allene through the coordination of a gold complex. This activation leads to a concerted [4 + 3] cycloaddition, resulting in the formation of gold carbene intermediate III. The selectivity between [1,2]-R migration and ring contraction was demonstrated with density functional theory (DFT) calculations. The calculation results indicate that the ring contraction pathway possesses a lower energy barrier than that of the [1,2]-R migration pathway.
Fürstner et al. implemented enantioselective cyclopropanation of styrene 4 catalyzed by a gold(I) catalyst bearing a bulky steric chiral phosphoramidite ligand associated with the 1-(2-hydroxynaphthalen-1-yl)naphthalen-2-ol (BINOL) core [37]. Nevertheless, the performance of structural modifications on this type of ligand is difficult. Accordingly, the authors further developed chiral gold catalysts with phosphoramidites featuring the α,α′-(dimethyl-1,3-dioxolane-4,5-diyl)bis(diphenylmethanol) (TADDOL) backbone (Scheme 2). With a certain adjustment of aryl substitution in the TADDOL part and amine component, L3 was found to be an ideal companion for the gold catalyst in asymmetric [2 + 2] cyclization with a 91% yield and 99% ee value, while inferior results were obtained with the employment of L2 [38]. The crystal structure of L2AuCl suggests that the two phenyl groups from the TADDOL part along with one phenyl ring from the amine part can form a cone-shaped binding pocket surrounding the gold atom. This pocket effectively restricts the flexibility of the gold center and conveys the chirality information to the final product. Another study illustrated that L4 was highly effective to realize the gold-catalyzed enantioselective cycloisomerization of 1,6-enyne 6, leading to the formation of the bicyclic product 7 in 89% yield and 95% ee (Scheme 3) [39]. Despite being an ancillary point to the gold catalysts, this ligand type displays remarkable chiral transformation for π-acidic activation.
In pursuit of greater success, the Alcarazo group adopted an elaborated preparation of chiral cationic ancillary phosphonites and developed asymmetric cyclization of diyne 8 (Scheme 4) [40]. Their design utilized the TADDOL backbone to provide an appropriate chiral pocket for this catalytic process, while the cationic imidazolium unit introduced a positive charge to enhance the catalytic activity of the corresponding gold(I) template, leading to the facile synthesis of chiral helicene 10 with excellent regioselectivity and enantioselectivity when the L6-coordinated gold(I) catalyst was employed. However, L5 was unsatisfactory in this regard. The single-crystal structure of L6AuCl clearly reveals that the gold atom can immerse into a chiral pocket resulting from two CF3-phenyl groups and one mesityl group. Compared with L5AuCl, the formation of a seven-membered ring without the constrains of annulation was observed for L6AuCl, which leads to a closer contact between the gold atom and the three vicinal aryl substituents.
Given the success of TADDOL-type phosphonites in gold(I)-catalyzed enantioselective hydroarylation, the Alcarazo group attempted to construct one helical and two axial stereogenic elements in one molecule using similar ligands (Scheme 5) [41]. They anchored multi-substituted diyne 12 for the attempt of gold(I) catalysts comprising chiral cationic ancillary phosphonites. It was found that L7 led to the simultaneous isolation of 13a and 13b with a remarkable 91% ee value for 13a. However, L8 was not up to the task, resulting in an unsatisfactory 8% ee value for 13a despite exhibiting excellent catalytic activity with a total yield of 90% for 13a and 13b.

2.2. Aryl Phosphine Ligand with a Proximal Chiral Sulfinamide

Apart from the phosphoramidite and phosphonite ligands mentioned above, aryl phosphine ligands bearing one chiral sulfinamide motif have also been developed. As reported by Zhang and coworkers, the in situ formed monogold cationic catalyst generated from digold chloride [(AuCl)2] always showcased a better catalytic activation ability than the pre-prepared monogold cationic catalyst [42,43]. Zhang et al. provided an elaborate design of the MingPhos series (including L9 and L10), where the chiral sulfinamide moiety containing adjustable steric groups was located on the ortho-position of the aryl phosphines (Scheme 6) [44]. The L9/L10-tethered gold(I) complexes performed good activity for the intermolecular asymmetric cycloaddition of 2-(1-alkynyl)-2-alken-1-one 14 and nitrone 15 with the L10-based gold complex exhibiting higher performance than the L9-containing analogue in terms of enantioselectivity. Moreover, a proposed mechanism was presented.
PC-Phos ligands, featuring a bigger chiral cave and bigger retortion of the sulfinamide part to the proximal phosphine part, were later exploited by Zhang et al. (Scheme 7) [45]. As expected, the catalytic system accommodated protected N-allenamide 18 smoothly with a 99% yield and 96% ee. The ligand L11 and a suitable protecting group were crucial for the control of high regioselectivity and enantioselectivity in this catalytic process. The authors proposed an asymmetric induction model based on the structure of L11AuCl and product 18. According to the model, the indole part only attacks the gold-activated allene bond at the Si face. Otherwise, if the attack occurs from the other side, the nucleophilic attack will be blocked by the steric obstacle from the OMe group in phosphine. Additionally, the chirality transfer can be facilitated by the hydrogen bonding interaction between the sulfonyl group and the N–H bond from the amino group, as suggested by the model of the substrates and the chiral PC-Phos ligands.
Recently, MingPhos ligands were introduced into gold(I)-catalyzed intermolecular asymmetric [3 + 2] cycloaddition of N-allenamides 19 and nitrone 20 by Zhang et al. (Scheme 8) [46]. To note, the opposite enantiomers of oxazolidin-2-one 21 or 22 were achieved via the usage of (R,Rs)-L12 or the corresponding (S,Rs)-L12. The interactions between the sulfinamide N−H group and the pentafluorophenyl substituent are responsible for the effective allocation of chirality transfer.

2.3. Phosphine Ligands Comprising the Ferrocene Scaffold

Phosphine ligands bearing the ferrocene backbone are a typical class of chiral ligands used for enantioselective gold(I) catalysis due to their easy accessibility and possibility to modulate chiral formation. Hayashi and colleagues reported the successful cooperation between chiral bisphosphines with the ferrocene backbone decorated with an amino group and a gold complex, in situ generating an chiral gold species for the asymmetric aldol reaction of aldehyde 23 and isocyanoacetate 24 (Scheme 9) [47]. The diastereoselectivity and enantioselectivity depend on the selection of a bulky aldehyde and a suitable ligand. Furthermore, the L13-tethered gold template provided divergent chiral oxazoline derivative 25, while the similar ligand (L14) did not efficiently catalyze the process.
Owing to the outer-sphere catalytic mode between a gold(I)-activated carbon–carbon triple bond and a nucleophile, the intermolecular asymmetric [2 + 2] cycloaddition of terminal alkynes 27 with alkenes 28 is challenging to conduct in gold(I) catalysis. High-throughput methods, including the screening of 90 chiral ligands, were employed to accelerate the formation of chiral cyclobutene via gold(I) catalysis (Scheme 10) [48]. Non-C2 symmetric bisphosphine ligand L15 with the ferrocene skeleton was found to be an excellent promoter for the corresponding gold(I) complex, providing catalytic reactivity under the accurate ratio of the gold(I) complex to NaBArF4 in a 1:1 ratio.
In light of the chiral JohnPhos-type scaffold, the Echavarren group synthesized a set of planar chiral monodentate 1,3-disubstituted ferrocenyl phosphines for the assembly of a sterogenic gold(I) complex (Scheme 11) [31]. It was suggested that the ancillary bulky adamantyl in ferrocene was required for the excellent performance of L16 in the desired [4 + 2] cyclization [49]. From the DFT calculations, the T-shaped π–π interaction between 3,5-(CF3)2-aryl from the phosphine ligand and an aryl group from the substrate as well as the Si face interaction of an alkenyl group with an alkynyl group meet the lowest energy for the chirality transfer.

2.4. Bifunctional Phosphine Ligand

Benefitting from enol chemistry that the α-C−H bond of a carbonyl could be clearly attenuated by the activation from an acid (Lewis acid or protic acid) to the carbonyl group, the Zhang group developed bifunctional phosphine ligands for gold(I)-catalyzed asymmetric reactions [50]. In such reactions, a Lewis acid acts as a ‘pull’, and a weak base functions as a ‘push’. The weak base Et3N (pKa in DMSO, 9.0) can be employed to remove α-H of a carbonyl or an imine group (pKa in DMSO ∼16–30) [51]. Without the utilization of a strong base, the reactions could accommodate more base-sensitive substances (Scheme 12A) [52,53]. From the initial design, it is anticipated that the α-H of a C−C triple bond, i.e., a propargylic proton (estimated pKa in DMSO, >30) [54,55], could be removed by a weak base with the ‘pull’ of a gold(I) catalyst to an alkynyl group (Scheme 12B,C). As a typical example, biphenyl 2-ylphosphines with remote tertiary amino groups for gold(I) catalysis were conceived by Zhang and coworkers (Scheme 13) [52]. As expected, the ligated tertiary amino group could play the role of ‘push’ for the removal of the propargylic proton to initiate new allene reactions, conjugated alkene reactions and aldol-type reactions in alkyne chemistry. With the introduction of asymmetric elements into the ligand, chiral products could be effectively achieved. As per the principle, the authors successfully converted racemic β,γ-butenolides 32 into chiral α,β-butenolides 33 promoted by chiral bifunctional phosphine ligand (S-L17)-ligated gold(I) catalysts (Scheme 13) [56]. In contrast to the JohnPhos-tethered gold(I) catalyst exhibiting 6% product yield and no ee value, the (S)-L17-tethered gold(I) catalyst showed excellent asymmetric catalysis for the γ-protonation process (99% yield and 99% ee).
The bifunctional phosphine ligand (S)-L17 features a fluxional biphenyl axis and contains a remote tertiary amino group possessing a vicinal chiral group to remove the α-H of an alkynyl group or a carbonyl group (Scheme 14). Though the biphenyl motif is fluxional, the chiral group can shield the amino group from one conformer to conduct the deprotonation step, which displays the chiral modulation for the process. Moreover, the cooperative ‘pull’ and ‘push’ of the bifunctional phosphine ligand-ligated gold catalysts were rationalized. Upon gold activation to the carbonyl group, the α-H is anticipated to be more acidic, allowing it to be removed by a weak base such as a tertiary amino group. Consequently, the metal enolate generated from soft enolization and the chiral ammonium cation constitute a chiral ion pair. Eventually, the chiral protonation process becomes possible, catering to the principle of soft deprotonation and chiral protonation for the asymmetric isomerization into α,β-butenolides 33 with high enantioselectivity from β,γ-butenolides 32.
The Zhang group achieved the implementation of a reaction from propargylic sulfonamides into chiral polysubstituted pyrroles via the cooperative catalysis of gold(I) and chiral bifunctional phosphine ligand (S)-L17 (Scheme 15) [57]. The innate diastereoselectivity of this reaction and the ‘matched’ geometry of the (S)-L17-associated gold(I) catalyst with substrates contributed to an excellent diastereo ratio (d.r.) with two chiral centers (2-methyl-5-cyclohexyl pyrrole derivative(36a–36c). The key procedure is the formation of allenes generated from the asymmetric isomerization of sulfonamides. Notably, the asymmetric reaction could tolerate high reaction temperature despite the routine pattern of low temperature to achieve higher enantioselectivity. However, the desired product was obtained in poor yield and low enantioselectivity when (R)-L17 was exposed to the catalytic system.
Additionally, the Zhang group reported that the asymmetric version of ynolates 37 to chiral α-Allyl-α,β-butenolides 38 was achieved with the aid of a (S)-L17-ligated gold catalyst (Scheme 16) [58]. As described, in this tandem reaction, the ‘push’ and ‘pull’ processes were involved several times for the generation of an allene intermediate, a furan intermediate and the isomerization product. According to their reports, no ee value was detected for the desired product if a racemic L18 ligand was introduced instead of (S)-L17, despite the excellent yield obtained [59].
Later, the same group reported gold(I)-catalyzed enantioselective dearomatization of phenol 39 to drug-valuable chiral spirocyclic enone products 40, employing the strategy of gold(I)-chiral ligand cooperation (Scheme 17) [60]. The distal phosphonate presented in the binaphthyl framework L19 displayed better catalytic performance compared to the JohnPhos ligand. It is believed that the H-bond interaction of the phosphate with the hydroxyl group in phenol is the crucial step that dictates asymmetric selectivity and accelerates the dearomatization/cyclization process.

2.5. Biphosphine Ligands

Apart from the above monophosphine ligands, biphosphine ligands have also been developed. Thus, this section intends to present the development of these biphosphine ligands and their applications in gold-catalyzed asymmetric reactions. Although the linear coordination of a ligand with the gold(I) complex generates a single coordination site from the metal center, the asymmetric bisphosphine ligands have been successfully applied to multiple carbon–carbon bond activation reactions. The arguable Au–Au interaction is believed to play a crucial role in modulating chiral transfer due to its subtle tortuosity for the linear coordination way.
The Echavarren group first discovered the potential of a bisphosphine ligand (L20) in the gold(I)-catalyzed enantioselective tandem cycloisomerization and hydromethoxylation process (Scheme 18) [61]. Remarkably, the addition of a catalytic amount of a silver salt suggested that the monocationic Au species promoted the transformation. The reaction occurred as the electron-rich alkenyl part acted as a nucleophile to attack the chiral gold-activated alkynyl group. After the gold complex feedbacked one pair of electrons to the double bond, the alkenyl group combined with an unstable cation for the formation of a gold carbene species. Following the ring-opening and protodeauration process, the chiral alkoxylation product could be given.
In the same year, the Toste group documented cyclopropantion reactions of alkyne 43 and aryl alkenes 44 using (R)-DTBM-SEGPHOS (L21) as the optimal bisphosphine ligand, affording the desired products 45 with high diasteroselectivity (>20:1) and enantioselectivity (76–94% ee) [62]. In this catalytic process, reactive gold carbene species were involved after a Rautenstrach rearrangement process. The proposed mechanism suggests that substituents from alkenes will intrinsically interact with the ligated gold complex (IIb), which disfavors the formation of trans-substituted cyclopropanes. Thus, the cis-selectivity was easily offered in most cases. However, the sterically less hindered BINAP ligand could not efficiently induce the transfer of chirality (Scheme 19), as observed from the ligand screening data.
Additionally, bisphosphine-containing gold complexes were also applicable to asymmetric alkoxylation of allenes, as illustrated by Widenhoefer and coworkers (Scheme 20) [63]. Specifically, bisphosphine ligand L22 was smoothly accommodated with allene 46, providing (R)-4,4-diphenyl-2-vinyltetrahydrofuran 47 with a moderate yield and high enantioselectivity (67% yield, 93% ee).
Shin and coworkers applied a chiral bisphosphine ligand (L23) derived from the SegPhos backbone to a gold-catalyzed enantioselective intermolecular [4 + 2] cycloaddition of propiolate 48 with substituted alkene 49 (Scheme 21) [64]. α, β-Unsaturated δ-lactone 50 was detected as the main product along with several diene side-products generated from metathesis and conjugate addition. Furthermore, the use of 1,1,2,2-Cl4-ethane as a solvent and the addition of sodium dodecyl sulfate (SDS) as a surfactant were significant for enhancing chemoselectivity and mediating chiral transfer.

2.6. Chiral Counterions Derived from Chiral Phosphoric Acids

The phosphine-containing organic ligands are summarized from Section 2.1 to Section 2.5. In this section, a unique type of phosphine ligand, referred to as chiral phosphoric acid-derived chiral counterion ligands, is discussed. The development of asymmetric gold catalysis via chiral ion pairs between cationic gold catalysts and chiral counterions is a successful strategy to confer to the enantioselectivity of gold-catalyzed reactions [65]. The Toste group explored the counterion strategy in gold catalysis for the intramolecular asymmetric hydroalkoxylation of allenes [66]. To achieve high enantioselectivity in the reactions, the “matched” effect of a phosphine-associated gold catalyst with a chiral silver salt should be considered (Scheme 22). Specifically, the cooperation of bis(chlorogold) bis(diphenylphosphino)methane [dppm(AuCl)2] with Ag-L24 prompted the asymmetric cyclization of allenol 51 to tetrahydrofuran 52 with excellent yield and enantioselectivity. On the contrary, inferior results were provided when Ph3PAuCl was utilized instead of dppm(AuCl)2.
On the other hand, the chiral counterion induction approach could be extended to enantioselective cyclization of alkynyl hydroxylamine 53 (Scheme 23) [67]. The ‘matched’ system of dppm(AuCl)2 with Ag-L25 could induce efficient chirality mediation for the formation of vinyl isoxazolidine 54. As an alternative, if Ag-L25 was displaced with Ag-L26, the enantioselectivity of the reaction decreased dramatically.

2.7. Chiral Carbene Ligand

Apart from the above phosphine-containing organic ligands, chiral carbene ligands have also been developed for asymmetric gold catalysis. As the distinct performance of gold(I)-carbene complexes in terms of chemoselectivity and regioselectivity in carbophilic activation, chiral carbene ligand-involving gold(I) catalysts were also elaborately designed to conduct a set of asymmetric transformations [68]. Considering the repertoire of an electron-rich group in offering the stability of an allene intermediate, Toste and coworkers conceived that, compared to phosphite-tethered gold complexes, the more electron-donating carbene-coordinated gold(I) catalysts could facilitate 6-endo-trig cyclization of propargyl ester 55 in a more efficient manner (Scheme 24) [69]. Undoubtedly, incorporation of different substituents into the ligand skeleton is crucial for the effective enantioselectivity control. As observed, ligand L28 bearing a 4-CF3 group was identified as an excellent auxiliary for achieving 85% yield and 91% ee, while the unsubstituted ligand (L27) triggered an inferior result.
Subsequently, Slaughter and coworkers developed impressive monodentate acyclic diaminocarbene ligands (L29, L30) for the enantioselective gold(I) catalysis based on the comprehension of the chiral (carbene)gold template (Scheme 25) [70]. The designed monodentate gold(I) complexes were introduced into tandem cyclization and nucleophilic addition of ortho-alkynylbenzaldehyde 57. Although L29 mismatched the catalytic system with a detrimental result, the modified ligand (L30) was found to be an excellent promoter for the formation of sterogenic 1H-isochromene 58 (70% yield, 99% ee). It appeared that the secondary interactions generated from the electrostatic attraction between the decorated amino group and suitable substrate promoted the efficient construction of the chiral motif. The crystal structure of L30AuCl reveals that the rotation of the gold complex with the substrates is restricted due to the interaction between the gold atom and the aryl ring (3,5-(CF3)2-Ph in binaphthyl skeleton) as well as the π–π stacking between a phenyl (in the amino part) and a naphthyl unit. This restriction facilitates chirality control during the cyclization process.
Acyclic diaminocarbene gold(I) complexes were further applied to the asymmetric hydroazidation and hydroamination of allenes 59 by the Toste group (Scheme 26) [71]. The introduction of carbene ligand L31 resulted in excellent yield and ee values, whereas the other explored ligands failed to dictate the chirality formation. It is noteworthy that, with L31 as a ligand, opposite conformers can be obtained only by using different nucleophiles (TMSN3 for the formation of 60 and H2NBoc for the generation of 61).
The Toste group subsequently challenged enantio-induction of tandem [3,3]-sigmatropic rearrangement and [2 + 2]-cyclization of propiolate 62 containing an indolyl group with the aid of computation (Scheme 27) [72]. The bulky L32-tethered gold(I) complex was identified as the optimized catalyst for chirality transformation through the construction of three chiral carbon centers. The computational studies suggested that the intramolecular H-bond formation from vicinal amino NH and O facilitated the mediation of chirality achievement.

2.8. Cyclometalated X-Y (X = C, O; Y = C, O) Ligand Frameworks

Enantioselective reactions with gold(I) catalysis have been extensively developed [73,74,75], as also illustrated in all the above examples. However, the linear geometry of gold(I) catalysts restricts its ability to modulate chirality formation (Scheme 28). On the other hand, the square-planar geometry of gold(III) catalysts introducing cyclometalated ligand frameworks is expected to avoid the restriction owing to the potential mediation from lithered chiral ligand proximal to the reactive center. However, the unstable risk of gold(III) complex 64 for reduction to gold(I) or unreactive state of traditional gold(III) catalyst 65 impeded its application in enantioenriched catalysis [76,77,78].
To address the above-mentioned limitations of traditional gold(III) catalysts, the Toste group developed a cyclometalated C−C ligand framework to stabilize the gold(III) cationic species [79]. This framework possesses powerful catalytic activity while maintaining a stable structure that facilitates effective reaction turnover (Scheme 29). A chiral N-heterocyclic carbene (NHC) ligand was positioned in the square vicinal to the reaction center to induce chiral formation. The resulting elaborated chiral NHC-tethered gold(III) catalysts displayed varied activity and divergent enantioselectivity in the cycloisomerization of enynes 66. Notably, (R,R)-gold(III)(biphenyl)L33Cl selectively facilitated the conversion of the (R)-enyne substrate into bicyclic adducts 67 in moderate conversion (44%) and good ee value (85%), whereas the gold(III) catalyst bearing L34 showcased a sluggish result (4% conversion, −2% ee) [80]. Meanwhile, (S)-enyne substrate 68 was provided along with the enantioconvergent cyclization via kinetic resolution. As described above, the reaction is initiated by the effective coordination of the gold complex with the triple bond of the enyne substrates, leading to the formation of complex I. Following this, a concerted cycloaddition took place, resulting in the generation of gold carbenes (II and II’). Subsequently, the bicyclic scaffold is furnished with the migration of α–H and the release of the catalyst from the system.
The square-planar chiral gold(III) catalysts were further applied to the asymmetric cycloaddition of 2,4-hexadienals 69 and cyclopentadiene 70 to synthesize valuable chiral bicycles 71 (Scheme 30) [81]. Different NHC ligands were prepared for the formation of chiral gold(III) catalysts. As observed, 1-naphthyl substituted NHC (L36) demonstrated superior chiral induction compared with the 2-naphthyl counterpart (L35), which suggests that a sterically bulky ligand is beneficial for this catalytic transformation.
Subsequently, Wong and coworkers synthesized O,O-chelated cyclometalated oxazoline gold(III) catalysts [78]. When the chiral O,O-chelated backbone was introduced into the gold(III) complexes, the combination of gold(III) catalyst (74 or 75) and L-camphorsulfonic acid (L-CSA) effectively initiated the carboalkoxylation reaction of alkyne 72 for the formation of 3-methoxyindanone 73 (Scheme 31) [82]. From the proposed transition state, the steric hindrance from the 2,4,6-trimethylphenyl group in the oxazoline part would be significant. This hindrance creates clear repulsion towards the substrate in the unfavored TSB, which contributed significantly to the improvement of enantioselectivity (75% ee for 74 vs. 90% ee for 75).

3. Application of Asymmetric Gold Catalysis

Axially chiral biaryl compounds exert important roles in natural product synthesis. The asymmetric hydroalkenylation strategy for the synthesis of axially chiral N-hetero-biaryl compound 77 was first achieved by Tanaka and coworkers utilizing chiral bisphosphine L37 as the ligand for asymmetric gold catalysis (Scheme 32). This methodology, though achieving a moderate ee value, opened the avenue for the enantioselective formation of various axially chiral polyaryl molecules.
Axially chiral phosphines have been demonstrated to be an important class of chiral ligands. Aiming at the chiral non-C2-symmetric BINOL ligand-directed synthesis, the Alcarazo group developed the asymmetric arylation of naphthyl alkynone 78 into enantiopure 1,1′-binaphthalene-2,3′-diol 79 (90% yield, 95% ee) with the assistance of the phosphonite L38-tethered gold(I) catalyst (Scheme 33). To ensure higher catalytic activity of the gold complex, the positively charged heterocyclic rest was cooperated into the ligand, which had a strong electron-withdrawing character [83]. After several steps, a new axially chiral phosphine 80 with a 92% ee value was provided, which could be used for the formation of new chiral gold compounds 81.
Helicenes have been extensively studied as functional organic materials due to their potential photophysical properties. In the asymmetric gold-catalyzed arylation of tetraalkynes 82 (Scheme 34), (S)-BINAP was introduced to achieve the desired S-shaped double azahelicenes [84]. The tandem control for chiral formation posed challenges for diasteroselectivity and enantioselectivity. The desired enantio-pure (+)-83 and separable (rac)-83 and (meso)-83 could be smoothly produced. Gratifyingly, the desired azahelicenes 84 with enhanced circularly polarized luminescence were readily given after another two steps.
Chiral 3-aminochromane is an important precursor used to synthesize medicinally relevant chiral serotonin 5-HT7 receptor [85,86] or MALT1 inhibitor [87]. For the effective construction of this skeleton, the Patil group developed a gold(I)/gold(III) catalytic system for asymmetric 1,2-difunctionalization of alkenes in 2022 (Scheme 35) [88]. To tackle the challenging conflict between linear dicoordination in gold(I) and square-planar tetracoordination in gold(III), the authors introduced a hard Lewis base part (an amino group) adjacent to a sterically bulky soft phosphine group. Gold(III) is anticipated to form rigid geometry due to the combination of the hard amino group and the soft phosphine group simultaneously, while only the soft phosphine can construct coordination with gold(I). The Ad-ChetPhos-tethered gold(I) complex was exposed to the iodoaryl alkene 85 to yield 3-methoxychromane 86 in high yield and enantioselectivity (up to 88% yield and 99% ee). After an additional three steps, chiral 3-aminochromane 87 was smoothly achieved.
The prenylated stibenoid scaffolds have attracted the research interest of synthetic chemists due to their potential pharmaceutical applications. The Carexanes, which were isolated from the leaves of Carex distachya, demonstrated considerable good antimicrobial and antioxidant activity [89]. Recently, the Carexane core was efficiently prepared using bifunctional phosphines with a dual H-bond donor group (urea) for the gold catalyst by the Echavarren group (Scheme 36). To achieve this, the novel catalytic system was utilized for the effective asymmetric 5-exo-dig or 6-exo-dig cycloisomerization of 1,6-enynes 88. The bicyclic alcohol 89 was obtained in 77% yield and 85% ee, and it could be further transformed into other valuable molecules (69% yield and 93:7 er for 90; 88% yield and 92:8 er for 91) [90]. The chiral benzoate 90 and diene core 91 of the Carexane natural products offer considerable pharmaceutical potential.

4. Conclusions and Perspectives

In this review, a variety of chiral ligands are presented and their applications in asymmetric gold catalysis are demonstrated. These ligands are divided into different types, including aryl phosphine ligands bearing the proximal chiral sulfinamide motif, phosphoramidite and phosphonite ligands, phosphine ligands comprising the ferrocene scaffold, bifunctional phosphine ligands, biphosphine ligands, chiral counterion-based ligands derived from chiral phosphoric acids and chiral carbene ligands for Gold(I) catalysis along with unique ligand frameworks for cyclometalated gold(III) catalysis. The pivotal roles of the versatile ligands are comprehensively emphasized and the representative ligand-involving catalytic mechanisms are briefly discussed. Moreover, the potential applications of these gold-involved asymmetric reactions are further illustrated by the preparation of valuable functional molecules.
From the above discussions, significant progress has been made in the rapidly growing field of asymmetric gold catalysis. Nevertheless, several challenging problems and limitations remain, as described in the following aspects.
(1) Although the combination of photoredox catalysis and gold catalysis has been utilized to construct structurally diverse molecules, the direct construction of chiral molecules through this strategy has yet to be accomplished. Thus, designing chiral Gold photocatalysts for enantioselective transformations should be explored in future research.
(2) Gold(I) catalysts have been investigated more extensively than gold(III) catalysts. Thus, conducting detailed mechanistic studies utilizing gold(III) catalysts could provide a better understanding of the precise functions of gold and ligands in these transformations, potentially leading to broader applications of gold(III) catalysts.
(3) Apart from the intermolecular cycloaddition reactions, most of the asymmetric transformations have been achieved in an intramolecular manner. Thus, the exploration of various intermolecular reactions in the future may be another hot research topic in gold catalysis.
(4) The gold loadings for most reported reactions in the literature are between 5 and 10 mol%, which are relatively high and may restrict the applications of these techniques in industry. As a solution, designing new ligands capable of better binding to both gold catalysts and substrates may help alleviate this problem.

Author Contributions

Conceptualization, writing-original draft preparation Y.W. and H.Y.; writing—review and editing H.G., X.H. and L.G.; Supervision, project administration, funding acquisition, review and editing R.Z. All authors have read and agreed to the published version of the manuscript.

Funding

We are grateful for the Natural Science Foundation of Hubei Province of China (Grant Nos. 2022CFB779) and the financial support of Hubei University of Art and Science (qdf2023006), Foundation of Scientific and Technological Project of Xiangyang-area of Agriculture and Country (No. 202719290).

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 13 01294 g001
Figure 1. The superior π affinity mode (a) and the linear coordination format (b).
Figure 1. The superior π affinity mode (a) and the linear coordination format (b).
Catalysts 13 01294 g001
Catalysts 13 01294 sch001
Scheme 1. The chiral phosphoramidite-tethered gold(I)-catalyzed asymmetric cycloaddition.
Scheme 1. The chiral phosphoramidite-tethered gold(I)-catalyzed asymmetric cycloaddition.
Catalysts 13 01294 sch001
Catalysts 13 01294 sch002
Scheme 2. The application of TADDOL-containing ligands L2 and L3 in asymmetric gold catalysis.
Scheme 2. The application of TADDOL-containing ligands L2 and L3 in asymmetric gold catalysis.
Catalysts 13 01294 sch002
Catalysts 13 01294 sch003
Scheme 3. The application of TADDOL-containing ligand L4 in asymmetric gold catalysis.
Scheme 3. The application of TADDOL-containing ligand L4 in asymmetric gold catalysis.
Catalysts 13 01294 sch003
Catalysts 13 01294 sch004
Scheme 4. Chiral cationic ancillary phosphonites applied in gold(I) catalysis.
Scheme 4. Chiral cationic ancillary phosphonites applied in gold(I) catalysis.
Catalysts 13 01294 sch004
Catalysts 13 01294 sch005
Scheme 5. Chiral cationic ancillary phosphonites applied in gold(I) catalysis.
Scheme 5. Chiral cationic ancillary phosphonites applied in gold(I) catalysis.
Catalysts 13 01294 sch005
Catalysts 13 01294 sch006
Scheme 6. Asymmetric synthesis of chiral α-Allyl-α,β-butenolides.
Scheme 6. Asymmetric synthesis of chiral α-Allyl-α,β-butenolides.
Catalysts 13 01294 sch006
Catalysts 13 01294 sch007
Scheme 7. PC-Phos ligands ancillary to gold complex for the asymmetric arylation.
Scheme 7. PC-Phos ligands ancillary to gold complex for the asymmetric arylation.
Catalysts 13 01294 sch007
Catalysts 13 01294 sch008
Scheme 8. MingPhos ligands ancillary to gold complexes for asymmetric arylation.
Scheme 8. MingPhos ligands ancillary to gold complexes for asymmetric arylation.
Catalysts 13 01294 sch008
Catalysts 13 01294 sch009
Scheme 9. Phosphine ligands possessing the ferrocene backbone for asymmetric gold catalysis.
Scheme 9. Phosphine ligands possessing the ferrocene backbone for asymmetric gold catalysis.
Catalysts 13 01294 sch009
Catalysts 13 01294 sch010
Scheme 10. Ferrocenyl bisphosphine-ligated gold catalyzed asymmetric [2 + 2] cycloaddition.
Scheme 10. Ferrocenyl bisphosphine-ligated gold catalyzed asymmetric [2 + 2] cycloaddition.
Catalysts 13 01294 sch010
Catalysts 13 01294 sch011
Scheme 11. Design of planar chiral phosphines for the asymmetric [4 + 2] cyclization.
Scheme 11. Design of planar chiral phosphines for the asymmetric [4 + 2] cyclization.
Catalysts 13 01294 sch011
Catalysts 13 01294 sch012
Scheme 12. The idea of enol chemistry (A) and the inspiration for the propargyl chemistry (B) and designed approach for bifunctional phosphine ligands in asymmetric gold catalysis (C).
Scheme 12. The idea of enol chemistry (A) and the inspiration for the propargyl chemistry (B) and designed approach for bifunctional phosphine ligands in asymmetric gold catalysis (C).
Catalysts 13 01294 sch012
Catalysts 13 01294 sch013
Scheme 13. Enantioselective isomerization of β,γ-butenolides to chiral α,β-butenolides.
Scheme 13. Enantioselective isomerization of β,γ-butenolides to chiral α,β-butenolides.
Catalysts 13 01294 sch013
Catalysts 13 01294 sch014
Scheme 14. Enantioselective isomerization of β,γ-butenolides to chiral α,β-butenolides.
Scheme 14. Enantioselective isomerization of β,γ-butenolides to chiral α,β-butenolides.
Catalysts 13 01294 sch014
Catalysts 13 01294 sch015
Scheme 15. Enantioselective isomerization of β,γ-butenolides to chiral α,β-butenolides.
Scheme 15. Enantioselective isomerization of β,γ-butenolides to chiral α,β-butenolides.
Catalysts 13 01294 sch015
Catalysts 13 01294 sch016
Scheme 16. Asymmetric synthesis of chiral α-Allyl-α,β-butenolides.
Scheme 16. Asymmetric synthesis of chiral α-Allyl-α,β-butenolides.
Catalysts 13 01294 sch016
Catalysts 13 01294 sch017
Scheme 17. Asymmetric synthesis of chiral α-Allyl-α,β-butenolides.
Scheme 17. Asymmetric synthesis of chiral α-Allyl-α,β-butenolides.
Catalysts 13 01294 sch017
Catalysts 13 01294 sch018
Scheme 18. Enantioselective bisphosphine-tethered gold(I) catalysis.
Scheme 18. Enantioselective bisphosphine-tethered gold(I) catalysis.
Catalysts 13 01294 sch018
Catalysts 13 01294 sch019
Scheme 19. Asymmetric cyclopropanation of alkynes and aryl alkenes.
Scheme 19. Asymmetric cyclopropanation of alkynes and aryl alkenes.
Catalysts 13 01294 sch019
Catalysts 13 01294 sch020
Scheme 20. Gold(I)-catalyzed asymmetric alkoxylation of allenes.
Scheme 20. Gold(I)-catalyzed asymmetric alkoxylation of allenes.
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Catalysts 13 01294 sch021
Scheme 21. Dimeric gold catalyst-promoted enantioselective [4 + 2] cycloaddition.
Scheme 21. Dimeric gold catalyst-promoted enantioselective [4 + 2] cycloaddition.
Catalysts 13 01294 sch021
Catalysts 13 01294 sch022
Scheme 22. Chiral counterion-induced asymmetric cyclization of allenol 51.
Scheme 22. Chiral counterion-induced asymmetric cyclization of allenol 51.
Catalysts 13 01294 sch022
Catalysts 13 01294 sch023
Scheme 23. Gold(I)-catalyzed asymmetric formation of vinyl isoxazolidines.
Scheme 23. Gold(I)-catalyzed asymmetric formation of vinyl isoxazolidines.
Catalysts 13 01294 sch023
Catalysts 13 01294 sch024
Scheme 24. Asymmetric [3 + 3] reaction catalyzed by (acyclic diaminocarbene)-gold(I) complexes.
Scheme 24. Asymmetric [3 + 3] reaction catalyzed by (acyclic diaminocarbene)-gold(I) complexes.
Catalysts 13 01294 sch024
Catalysts 13 01294 sch025
Scheme 25. Tandem cyclization reaction catalyzed by (acyclic diaminocarbene)gold(I) complexes.
Scheme 25. Tandem cyclization reaction catalyzed by (acyclic diaminocarbene)gold(I) complexes.
Catalysts 13 01294 sch025
Catalysts 13 01294 sch026
Scheme 26. Chiral carbene-tethered gold catalyzed nucleophilic addition of allenes.
Scheme 26. Chiral carbene-tethered gold catalyzed nucleophilic addition of allenes.
Catalysts 13 01294 sch026
Catalysts 13 01294 sch027
Scheme 27. (Acyclic diaminocarbene)gold(I) complexes for challenging chirality formation.
Scheme 27. (Acyclic diaminocarbene)gold(I) complexes for challenging chirality formation.
Catalysts 13 01294 sch027
Catalysts 13 01294 sch028
Scheme 28. The innate difference of gold(I) and gold(III) catalytic mode.
Scheme 28. The innate difference of gold(I) and gold(III) catalytic mode.
Catalysts 13 01294 sch028
Catalysts 13 01294 sch029
Scheme 29. The chiral gold(III)-catalyzed asymmetric cycloisomerization of enynes.
Scheme 29. The chiral gold(III)-catalyzed asymmetric cycloisomerization of enynes.
Catalysts 13 01294 sch029
Catalysts 13 01294 sch030
Scheme 30. The chiral gold(III)-catalyzed enantioselective Diels–Alder reaction.
Scheme 30. The chiral gold(III)-catalyzed enantioselective Diels–Alder reaction.
Catalysts 13 01294 sch030
Catalysts 13 01294 sch031
Scheme 31. Asymmetric O,O-chelated cyclometalated oxazoline gold(III) catalysis.
Scheme 31. Asymmetric O,O-chelated cyclometalated oxazoline gold(III) catalysis.
Catalysts 13 01294 sch031
Catalysts 13 01294 sch032
Scheme 32. Bisphosphine-tethered gold catalyzed asymmetric hydroalkenylation.
Scheme 32. Bisphosphine-tethered gold catalyzed asymmetric hydroalkenylation.
Catalysts 13 01294 sch032
Catalysts 13 01294 sch033
Scheme 33. Atropselective synthesis of non-C2-symmetric BINOL.
Scheme 33. Atropselective synthesis of non-C2-symmetric BINOL.
Catalysts 13 01294 sch033
Catalysts 13 01294 sch034
Scheme 34. Gold-(S)-BINAP complex promoted enantioselective arylation of alkynes.
Scheme 34. Gold-(S)-BINAP complex promoted enantioselective arylation of alkynes.
Catalysts 13 01294 sch034
Catalysts 13 01294 sch035
Scheme 35. Redox gold(I)/gold(III) for asymmetric 1,2-difunctionalization of alkenes.
Scheme 35. Redox gold(I)/gold(III) for asymmetric 1,2-difunctionalization of alkenes.
Catalysts 13 01294 sch035
Catalysts 13 01294 sch036
Scheme 36. Synthesis of chiral core of the carexane asymmetric gold catalysis.
Scheme 36. Synthesis of chiral core of the carexane asymmetric gold catalysis.
Catalysts 13 01294 sch036
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MDPI and ACS Style

Wu, Y.; Yang, H.; Gao, H.; Huang, X.; Geng, L.; Zhang, R. Advances in Versatile Chiral Ligands for Asymmetric Gold Catalysis. Catalysts 2023, 13, 1294. https://doi.org/10.3390/catal13091294

AMA Style

Wu Y, Yang H, Gao H, Huang X, Geng L, Zhang R. Advances in Versatile Chiral Ligands for Asymmetric Gold Catalysis. Catalysts. 2023; 13(9):1294. https://doi.org/10.3390/catal13091294

Chicago/Turabian Style

Wu, Yufeng, Hui Yang, Haojie Gao, Xiaoyi Huang, Liyuan Geng, and Rui Zhang. 2023. "Advances in Versatile Chiral Ligands for Asymmetric Gold Catalysis" Catalysts 13, no. 9: 1294. https://doi.org/10.3390/catal13091294

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