Development and outlook of chiral carbene–gold(I) complexes catalyzed asymmetric reactions

Tetrahedron Letters 55 (2014) 577–584

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Digest Paper

Development and outlook of chiral carbene–gold(I) complexes catalyzed asymmetric reactions Peng Gu a, Qin Xu a,⇑, Min Shi a,b,⇑ a Key Laboratory for Advanced Materials and Institute of Fine Chemicals, School of Chemistry & Molecular Engineering, East China University of Science and Technology, 130 Mei-Long Road, Shanghai 200237, China b State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road, Shanghai 200032, China

a r t i c l e

i n f o

Article history: Received 15 August 2013 Revised 24 September 2013 Accepted 8 October 2013 Available online 4 December 2013 Keywords: Chiral carbene–gold(I) complexes Asymmetric reactions N-Heterocyclic carbene ligands Nitrogen acyclic carbene ligands Hydrogen bonded heterocyclic carbene ligands

a b s t r a c t Great progress has been made in developing of homogeneous Au-catalyzed reactions in the past decade. The unique versatility and efficiency of gold complexes have been obtained including carbene ligated gold complexes. Due to the special linear coordination mode of gold(I) complex, Au catalyzed asymmetric reactions have become a huge challenge. Chiral carbene–gold complexes also have been applied in asymmetric reactions. Major breakthrough in this field has been obtained by the Toste group recently and more are expected in the future. This digest, by highlighting recent works, aims to make further progress in this fascinating research field. Ó 2013 Elsevier Ltd. All rights reserved.

The first decade of the new century witnessed the rapid growth of homogeneous gold catalysis.1 Due to the character as soft carbophilic Lewis acid toward C–C multiple bonds, the homogeneous gold catalysis has been successfully used as the one important kind of enyne reaction catalysts. This high carbophilicity translates to a low oxophilic character and a labile carbon–metal bond, efficiently affording high turnover for the gold catalysts. Besides, the gold catalysts are inert from air oxidation due to the high oxidation potential between gold(I) and gold(III). So, compared with other transition metal catalysts, gold complexes could catalyze reactions under mild conditions (air or moisture), giving high efficiency, wide functional group compatibility, and outstanding chemoselectivity. However, excepting its high efficiency in enyne and allene activation, the homogeneous gold catalysts also exhibit some limitations like many other transition metal complexes. Firstly, the gold catalysts show poor thermostability due to disproportionation of the metal cation (Scheme 1) as well as the easy reduction of the gold catalyst caused by the substrate and the intermediate or the product even at the room temperature.2 Most of gold catalyzed reactions need relatively more catalyst loading, leading to the poor TON of these reactions.3 Secondly, the coordination mode of gold(I) complexes causes much difficulty in asymmetric transformations.4 The gold(I) complexes usually have two coordination sites with a

180° bond angle. The chiral information carried by the neutral two-electron donor ligands is opposite far away from the potential reacting center (Scheme 2). Carbene ligands including heterocyclic carbene and heteroacyclic carbene ligands, have attracted much interest in organic and organometallic chemistry due to their unique properties and extensive applications.5 Carbene ligands are one kind of stronger r-donor and weaker p-acceptor ligands than phosphine ligands,6 and most of metal complexes ligated with carbene ligands are often air/moisture stable, which makes the use of these carbene– metal complexes more convenient.7 Based on the above advantages, the design and synthesis of new gold–carbene complexes have AuIL

heat

Scheme 1. Disproportionation of AuI.

AuI X substrates

X = I, Br, Cl etc.

R1 AuI

R2

R4

R3

R1

R2

Au

⇑ Corresponding authors. Tel.: +86 21 64252995; fax: +86 21 64166128 (Q.X.); tel.: +86 21 54925137; fax: +86 21 64166128 (M.S.). E-mail addresses: [email protected] (Q. Xu), [email protected] (M. Shi). 0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tetlet.2013.10.036

Au 0

AuIL2 +

.

I

R

4

R1 R2 Au I Y Y = N, O R1 AuI

R3

Scheme 2. Coordination modes of AuI.

R2

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P. Gu et al. / Tetrahedron Letters 55 (2014) 577–584

always been the hot topic in catalysis research. Teles, Herrmann, Raubenheimer, Hashmi and Nolan’s groups have made the significant contributions in the early period.8 Notably, carbene– gold(I) complexes can catalyze reactions with dramatically different reactivity and selectivity with the same reactions catalyzed by the phosphine–gold(I) complexes.9 But as compared to phosphine–ligated gold(I) complexes which have made quite significant progress in gold catalyzed asymmetric reactions,10 the asymmetric reactions catalyzed by chiral carbene–ligated gold(I) complexes are still rare and inefficient. Inspiringly, some excellent results catalyzed by chiral carbene–gold(I) complexes recently have been obtained by using special strategies or unique chiral ligands. Herein, we focus on recent progress achieved in asymmetric reactions catalyzed by carbene–gold(I) complexes. Asymmetric cyclization of 1,6-enynes: Alkynes are the most popular substrates in homogeneous gold catalysis. The electrophilic activation of alkynes by gold complexes with a variety of nucleophiles has been considered as a powerful tool for the synthesis of organic cyclic compounds.11 Recent reports include asymmetric cycloisomerization of enynes and the desymmetrization of dialkynes in enantioselective carbene–ligated gold(I) catalysis. In 2010, Tomioka and co-workers reported chiral C2-symmetric N-heterocyclic carbene (NHC)–gold(I) complexes for the cycloisomerization of 1,6-enynes to the corresponding cyclopentane derivatives.12 It was the first example of carbene–ligated gold(I) complexes catalyzed asymmetric reactions. NHC–AuCl complex G5 bearing a bis(2,5-bismethylphenyl)methyl substituent was found to be the most effective catalyst, giving the chiral cyclopentanes in excellent yields but with moderate enantioselectivities (up to 56% or 59% ee) (Scheme 3). According to this work, the N-substituted groups have a significant influence on the enantioselectivity of the reaction. The X-ray structures of carbene–gold(I) complexes G1 and G2 showed that the aryl group on the nitrogen atom was fixed to the direction against the Au–Cl bond, these structures led to the absence of a stereocontrolling group around the Au coordination site. Based on above theory, the N-substituent should include a diarylmethyl group, one of the aryl group could be fixed

MeO2C

MeO2C

NHC-AuCl, AgSbF6

MeO2C

MeO2C

MeOH, rt

2

1

Ph N

Ph R1

N

OMe Au MeO Cl G1, 5% ee

Ph

Ph

Au Cl G2, 4% ee MeO2C MeO2C 3

Ph

Ph R2

N

N

R1 R1

Au Cl

R2

Ph N

R1

R2

1

N

OMe

R2 2

G3, R = R = H, 8% ee G4, R1 = Me, R2 = H, 32% ee G5, R1 = R2 = Me, 56% ee

G5, AgSbF6

MeO2C

MeOH, rt

MeO2C

by the p–p stacking interaction with aryl group on the chiral carbon. Moreover, a free aryl group could overlay the Au–Cl bond and increase more steric hindrance. The results of this work perfectly confirmed the above speculation. Tomioka’s work seems to create a new strategy of goldcatalyzed asymmetric reactions in spite of the moderate enantioselectivity. Unfortunately as for these mono-NHC gold(I) complexes, in which chiral center is existing at C4 and C5, the reaction center is far away from the chiral environment. Due to the constant rotation of these single bonds, the chiral transference is extremely weakened.13 More special strategies or chiral ligands are required to provide spatial arrangements of the opposite site of the gold cation. In the same period, asymmetric reactions catalyzed by chiral 2,20 -bis(diphenylphosphino)-1,10 -binaphthalene (BINAP) and derivatives ligated gold(I) complexes have made a huge progress with excellent enantioselectivities. The strategy of gold(I) complexes based on 1,10 -binaphthyl framework seems more reasonable, because the sterically hindered groups and chiral environment are more close. More importantly, the sterically hindered groups might be able to locate in the same plane with the reaction site (Fig. 1). In 2011, Shi’s group reported a new class of mono- and bisgold(I) complexes ligated with axially chiral NHC ligands based on 1,10 -binaphthyl scaffold.14,15 The diverse types of NHC–gold(I) complexes with different sterically hindered groups and coordinated halogen atoms were obtained conveniently. More interestingly, the species of the sterically hindered groups became extremely rich from normal aryl or alkyl groups to amine, imine, and amide groups which had potential ability to fix to the substrates by hydrogen bonding. Typical mono- and bis-NHC gold(I) complexes based on 1,10 -binaphthyl scaffold are shown in Figure 2. Different kinds of asymmetric reactions were chosen to test the catalytic activities of these chiral NHC–gold(I) complexes. Firstly, Shi’s group tested the catalytic activities of these new chiral NHC–gold(I) complexes by simple cycloisomerization of 1,6-enynes like Tomioka’s work, excepting they chose dry AcOH as the nucleophile (Scheme 4).14 Within the carried out investigation, the sterically hindered mono-gold(I) complex (S)-G14, having a pyrrolidin-1-yl group, was found to be the best catalyst in this reaction, affording the cyclopentane derivative 6 in 99% yield and up to 59% ee. Unfortunately, the expecting bis-gold(I) complex (R)-G6 showed poor ability of chiral induction in the reaction with 25% ee. In the same work, these axially chiral NHC–gold(I) complexes were also successfully applied in the asymmetric oxidative rearrangement of 1,6-enynes, using Ph2SO as oxidant, giving the corresponding aldehyde products in good to excellent yields and up to 70% ee. The sterically less hindered mono-gold(I) complex (S)-G9, having an acetamide group, showed the outstanding activity in the reaction. This work develops a brand-new type of chiral NHC–Au complex and the best enantioselectivity obtained from the reaction is 70% ee which is the best result for NHC–gold(I) complex catalyzed transformations at that time (Scheme 4). Although there are many shortages, such as the enantioselectivities are moderate and the scope of the substrates is quite limited. this paper is still inspiring and beneficial to the future development of efficient chiral NHC–gold(I) complexes.

R1

5 4

1

Ph OMe

4 59% ee

Scheme 3. Enantioselective cycloisomerization of 1,6-enynes with Tomioka’s chiral [NHC(AuCl)].

R3 N

R2 3

2

Au X A

N

N R4

N R Au

B

Figure 1. The chiral introduction mode of NHC–Au complexes.

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P. Gu et al. / Tetrahedron Letters 55 (2014) 577–584

R1

N

N

Au X Au X N R1

(R)

N

N

N

R1 Au

(S)

I

NHR2

O

N

N

Au 3

N R

I

N

(Sa,S)-G12 (Sa,R)-G13

N

N

∗

N Boc

N

N

Au

Au HO I

N

I

Ph

R2 (S)-G18

(S)-G14: R2, R3 = (CH2)4 (S)-G15: R2 = Me; R3 = Me (S)-G16: R2 = Bn; R3 = H (S)-G17: R2 = Bn; R3 = Bn

I

NH

(S)-G9: R1 = Me; R2 = Ac (S)-G10: R1 = Me; R2 = Bz (S)-G11: R1 = Me; R2 = Boc

(R)-G6: R1 = Me, X = I (R)-G7: R1 = Bn, X = Br (R)-G8: R1 = Bn, X = Cl

N

N Au

(S)-G19

Figure 2. Typical NHC–Au complexes of Shi’s work.

(S)-G14, AgSbF6

Ts N

AcOH, DCE, 0 oC, 24 h 5

Ph

Ts N

gold intermediate was not protonated by normal methanol or acids, but reacted with indole at C3 position as a Friedel–Crafts reaction (Fig. 3). After examination of NHC–gold(I) complexes, Ag salts and solvents, (S)-G15, and AgBF4 in DCE were found as the best catalytic system, and the highest enantioselectivity as 66% ee was obtained by using the N-methyl-5-bromoindole as substrate (Scheme 5). Undeniably, this work showed common problems in NHC–gold catalyzed asymmetric reactions such as moderate enantioselectivities and the limitation of substrates. On the positive side, this work develops a new method to synthesize chiral indole derivatives catalyzed by NHC–gold(I) complexes. Besides, this work creatively attempts to use more sterically hindered (Ph3C)P(O)(OAg)2 as a counter ion, although this catalytic system showed poor catalytic activity, it also exhibits a different idea to introduce sterically hindered groups to the reactive site. Asymmetric addition to alkynes or allenes: The preparation of saturated and unsaturated heterocyclic structures is of particular importance given the plethora of substances incorporating such

H Ph

H OAc 6 99% yield, -59% ee CHO

(S)-G9, AgSbF6, Ph2SO

Bs N 7

PhCl, 4A MS, 10 oC, 12 h

Bs N

Ph

H 8 99% yield, 70% ee

Ph

Scheme 4. Enantioselective cycloisomerization of 1,6-enynes with Shi’s chiral [NHC(AuCl)].

In 2011, Shi and co-workers extended the application of their chiral NHC–gold(I) complexes by gold-catalyzed asymmetric cyclization and Friedel–Crafts reactions of 1,6-enynes with indole derivatives.15 In this work, after cyclization of 1,6-enynes, the ionic

LAu H+ Ts N

AuL Ts N

ts af

r -C el d e (3)

H Ph H

E H

N Ph

Ts N

H Ph

N

N

12

i

Fr LAu Ts N

LAu

LAu

Ts N

Ph

O=SR2

O

O

Ts N

SR2 Ph

(2)

A

Ph

B

H

D LAu

- SR2

Ts N

H

Ph

11

H

(1) AcOH

Ts N

LAu

Ph H

OAc

H+ Ts N

C

H Ph H

OAc

Figure 3. Proposed mechanism of Shi’s work.

Ts N

6

H Ph H

OAc

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P. Gu et al. / Tetrahedron Letters 55 (2014) 577–584

Br (S)-G15, AgSbF6

Ts N

+ N Ph

5

Br Ph

Ts N

DCE, 25 oC, 24 h

H

9

H N

10

86% yield, 13:1dr, -59% ee Scheme 5. Enantioselective Friedel–Crafts/cyclization of 1,6-enynes with Shi’s chiral [NHC(AuCl)].

structural motives. A significant number of methods for the efficient preparation of five- and six-membered rings are known. Thanks to the efficient activation of C–C multiple bonds by homogenous gold(I) complexes,16 gold-catalyzed asymmetric hydroamination and hydroxylation have been considered as a useful method to build up chiral heterocyclic structures in recent years. The C–C multiple bonds activated by homogenous gold complexes can be attacked by various O-or N-nucleophiles.17 In 2009, Czekelius and co-workers reported a gold-catalyzed endo-cyclization of 1,4-diynes to seven-membered heterocycles.18 At the begining of this work, similar diynols and diynamides were chosen to react with Au(PCy3)Cl and AgBF4 to give racemic sevenmembered rings. At the end of this work, two kinds of chiral gold(I) complexes were chosen to catalyze the enantioselective desymmetrization of diynamides to oxazepine derivatives 14. The gold(I) complex ligated with Herrmann’s optically active mono-carbene ligand G20 showed good reactivity in 66% yield but poor enantioselectivity with 17% ee. Comparatively, the chiral [(AuCl)2MeOBIPHEP] complex G21 provided the resulting oxazepine derivative 14 with a large improvement in enantioselectivity (60% ee) but poor yield (23%), presumably due to the limited catalyst lifetime (Scheme 6). According to the above work, NHC–gold(I) complex G20 showed better catalytic activity than phosphine–gold(I) complex G21 due to the stability of NHC–gold complexes. The challenge of this asymmetric reaction catalyzed by chiral NHC–gold complexes is focused on how to improve the enantioselectivity. In 2011, Czekelius and co-workers reported the similar class of NHC–gold(I) complexes based on the tetrahydroisoquinoline backbone catalyzed enantioselective desymmetrization of diynamides. They focused on the optimization of these NHC–gold catalysts by adding different sterically hindered groups to improve the enantioselectivities of the reaction.19 The synthesized different types of NHC–gold catalysts are shown in Figure 4. These NHC–gold(I) complexes were successfully applied in the enantioselective desymmetrization of diynamides. The best result was obtained by NHC–gold(I) complex G25 bearing a sterically

H N

O

Ts

O

chiral gold catalyst AgBF4

N Ts

toluene, rt 14

13

H

N AuCl

H

N

G20 yield = 66%, ee = 17%

MeO MeO

Ar Ar P AuCl

tBu OMe Ar =

P AuCl Ar Ar

tBu

G21 yield = 23%, ee = 60%

Scheme 6. Enantioselective gold catalyzed intramolecular hydroamination of diynamides.

highly encumbered 3,5-bis(2,4,6-triisopropylphenyl)phenyl group. Treatment of diynamides 13 with gold complex G25 and AgBF4 in toluene led to the formation of product 14 in 77% yield with 51% ee (Scheme 7). Interestingly, the less sterically hindered 3,5-bistrifluoromethylphenyl substituted NHC–gold(I) complex G30 also showed quite impressive catalytic activity with 45% ee. The X-ray of complex G30 shows that the trifluoromethyl groups at the ligand substituents are in close proximity in the crystal state, presumably by a stabilizing interaction similar to interactions in fluorous phases. In 2010, Espinet and co-workers reported a new series of chiral HBHC (hydrogen bonded heterocyclic carbenes) and NAC (nitrogen acyclic carbenes) ligands with their mononuclear and binuclear gold complexes.20 The synthetic strategy of HBHC and NAC gold complexes has been used widely, because an obvious advantage of this kind of complex is that they are prepared by the nucleophilic attack of amines to isocyanide gold complexes. The systematic series of chiral carbene–gold(I) complexes could be obtained by simply combining achiral or chiralamines and isocyno-gold complexes. The isocyanide ligands are easy to obtain from chiral amines. Following this methodology, the authors synthesized several chiral mononuclear or binuclear HBHC or NAC gold(I) complexes. The synthetic route and structures of these chiral carbene–gold complexes are shown in Figure 5. The author remarkably introduced some sterically hindered groups to 3,30 -positions in the 1,10 -binaphthyl scaffold. The synthesis of 3,30 -substituted BINAM derivatives is difficult and inefficient. It is the reason that the applications of BINAM and their derivatives are quite few than BINAP and their derivatives. This work shows a new way to introduce some sterically hindered groups to the chiral carbene ligands based on 1,10 -binaphthyl scaffold. Then the authors tested the catalytic activities and enantioleselectivities of these chiral carbene–gold(I) complexes by gold -catalyzed intramolecular asymmetric hydroxylation of allene 15. The chiral HBHC ligated gold complex G33 was found to be the best catalyst. Treatment of c-hydroxyallene 15 with catalytic mixture of chiral bis-gold complex G33 and AgOTs in toluene gave the desired 2-vinyl tetrahydrofuran 16 in 95% yield with 22% ee (Scheme 8). The authors also used chiral bis-phosphine (S-DTMB-MeOBIPHEP) gold(I) complexes to test the same reaction, affording the product 16 in 73% yield and 90% ee. Although the enantioselectivity of carbene–gold(I) catalyzed reaction is still poor, some highlights can be found in this work. A new series of chiral acyclic carbene ligands and their gold(I) complexes have been obtained. Introducing chiral backbones and structural-electronic variations becomes easier in the synthesis of the NAC ligands, which can offer more opportunities to get more effective carbene–gold(I) catalysts. In 2011, Shi’s group reported a new class of chrial mono-NHC gold complexes with 1,10 -biphenyl scaffold.21 This work originated from their previous work about mono-NHC gold(I) complexes based on 1,10 -binaphthyl scaffold. According to literatures, the dihedral angles of biaryl structures have a huge influence on the structural and electronic properties of the biaryl scaffold. Thus, the NHC–gold(I) complexes based on 1,10 -biphenyl scaffold were synthesized to get more efficient gold catalysts. The asymmetric intramolecular hydroamination of N-allenyl sulfonamides has been chosen to test the catalytic activities and enantioselectivities of these new NHC–gold(I) complexes. The best result was obtained by employing NHC–gold complex G40 with sterically more hindered 1-adamantanecarbonyl amide group. The corresponding 2-vinylpyrrolidine 18 was achieved in 47% yield with 44% ee (Scheme 9). Other NHC–gold catalysts showed extremely poor enantioselectivities. Overall, not many improvements could be obtained from this work. Much more new strategies are strongly needed in the field of chiral carbene–gold(I) complexes catalyzed asymmetric reactions.

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OMe

Ph R R= H

N AuCl

H

N

Ph

G22 R

R = H G28 R = Ph G29

G23

G24

OMe

iPr tBu iPr

CF3

F3 C

tBu

CF3

iPr iPr CF3

tBu CF3 iPr G25

G30

iPr

G26

G27

tBu

CF3

Figure 4. Structures of Czekelius’ chiral NHC–gold complexes

H N

O

OH Ts

chiral gold catalyst AgBF4

O

N Ts

Ph

toluene, rt

N

Cl Au

H N

N HN H N

NH Au Cl

R

NH

Cl Au

Cl Au C N N

HN

N N H R

R = Ph, G34 R = -(CH2)4-, G35

R = Ph, G31 R = Napth, G32 R1 N R1 NHR12

R

R N C Au Cl

NHC-Au G40 AgClO4

Ph

Ph

.

DCM, rt

Ph

NCbz

18 47% yield, 44% ee

R = H, R1 = Me, G36 R = H, R1 = iPr, G37 R = Ph, R1 = Me, G38 R = Ph, R1 = iPr, G39

Cl Au NH

R

R HN Au Cl

N

MeO MeO

∗

Cl Au C N

NHCbz

Cl

H N N

Ph

17

G33

N H

Scheme 8. Enantioselective hydroalkoxylation of allene with chiral carbene–gold(I) complexes.

Au

R

Cl Au

Ph 16 yield up to 95% ee up to 22%

14 ee up to 51%

Scheme 7. Asymmetric NHC–gold catalyzed hydroamination of diynamides.

H N

Ph

15

13

N

.

Ph

O

carbene-gold, AgOTs toluene, rt

R1 N R1

Figure 5. Structures of chiral NAC- or HBHC–gold complexes.

In 2011, Slaughter and co-workers reported an enantioselective alkynylbenzaldehyde cyclization catalyzed by chiral NAC–gold(I) complexes.22 They synthesized a new series of mono-carbene gold(I) complexes based on 1,10 -binaphthyl scaffold. The substituents at the other 2-position on 1,10 -binaphthyl backbone are substituted with aryl groups. Interestingly, the X-ray structure of 3,5-bistrifluoromethylphenyl substituted gold(I) complex G41

NH

N Au I

O G40 Scheme 9. Enantioselective hydroamination of allene with chiral carbene–gold(I) complexes.

showed that there was a weak interaction between the aryl group and the gold atom, as the distance of the aryl ring centroid and the Au atom is 0.2 Å shorter than other distances of these gold complexes. After testing all the carbene–gold(I) catalysts in the same reactions, the gold complex G41 and G42 containing weak Au–arene interactions showed excellent enantioselectivities with 99% ee, whereas other gold complexes exhibited relatively poor activities with the ees ranging from 8% to 61%. For example, treatment of alkynylbenzaldehyd 19 and n-octanol with a catalytic mixture of chiral carbene–gold complex G41 and LiNTf2 in DCE afforded the corresponding product 20 in 86% yield with 99% ee (Scheme 10). The toleration of these substrates catalyzed by carbene–gold complexes was excellent since all the products could be obtained with good yields and excellent enantioselectivities. The successful key secret of this work is the weak interactions between aryl groups and the gold atoms. The interaction could efficiently decrease the distance between the sterically hindered group and the reaction site. Moreover, this distance has a

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P. Gu et al. / Tetrahedron Letters 55 (2014) 577–584

O H

19

n-C8 H17 OH

+

OC 8 H17

NHC-Au G41 LiNTf 2

O

DCE, rt

Ph 20

Ph R N H

87% yield, 99% ee N R Au

Cl CF 3

CF3 R = isopropyl, G41 R = (S)-MePhCH, G42 Scheme 10. Enantioselective alkynylbenzaldehyde cyclization by chiral carbene– gold(I) complexes.

significant influence on the transference of chirality. This work makes a huge improvement on carbene–gold(I) complexes catalyzed asymmetric reactions. Asymmetric reactions of propargyl esters: The chiral vinyl substituted cycle unit can be found in many naturally occurring products with interesting properties. They have further been used as versatile substrates for a number of useful chemical transformations. Among of the diverse synthetic ways to get vinyl substituted rings, chiral gold complexes catalyzed propargyl esters reactions have been investigated as an efficient and atom-economic method. This catalytic cycle always begins with a gold catalyzed acyl migration of terminal propargyl esters, and the generated gold vinyl carbenoids will be nucleophilic attacked by nucleophiles. The final cyclic products will be obtained by further transformations of these intermediates. According to the previous literature, asymmetric reactions of propargyl esters catalyzed by homogeneous gold(I) complexes ligated with chiral phosphine ligands have obtained quite impressed improvements with excellent yields and enantioselectivities.23–25 Comparatively, the chiral carbene–gold(I) complexes catalyzed asymmetric reactions of propargyl esters are rare. In this digest, we have described the Espinet’s work about the synthesis of chiral gold complexes ligated with HBHC and NAC ligands in 2010. In that work, they also chose gold-catalyzed asymmetric cyclopropanation to test the catalytic activities and enantioselectivities of these carbene–gold(I) complexes.20 For example, treatment of propargyl ester 21 and styrene with a mixture of chiral carbene–gold complex G36 and AgSbF6 provided vinyl substituted cyclopropane 22 in 72% yield with 24% ee (Scheme 11). This was the best result they could obtain from these carbene–gold complexes catalyzed reactions. They also examined the same reaction catalyzed by gold complex ligated with chiral bis-phosphine ligand (R-DTMB-SEGPHOS), giving the cyclopropane derivate 22 in 70% yield with 81% ee. This work extends the applied range of carbene–gold(I) complexes catalyzed asymmetric reactions. It also shows the limitation of carbene–gold complexes with generally poor enantioselectivities. In 2011, Toste and co-workers reported chiral HBHC–gold(I) complexes catalyzed dynamic kinetic asymmetric reaction of propargyl esters.26 Initially, the propargyl ester 23 was treated

OPiv + 21

NHC-Au G36, AgSbF6 Ph

MeNO2, rt

Ph PivO 22 72% yield, 24% ee

Scheme 11. Gold-catalyzed enantioselective cyclopropanation of styrene with propargyl pivaloate.

with two kinds of gold complexes ligated with common chiral bis-phosphine ligands in the presence of sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaBArF), affording the corresponding product 24 with moderate enantioselectivities. Electron-rich phosphoramidite-gold complex showed poor enantioselectivity in this reaction. Then they turned their attention to more electron-donating carbene ligands. Firstly mono-carbene ligated gold(I) complex G3 with NaBArF was chosen as the catalyst in the reaction, giving the desired product 24 with low enantioselectivity (7% ee). Inspired by the success with 3,30 -midified BINOLderived phosphoramidite ligands in gold catalysis and the Espinet’s work, Toste and co-workers chose the chiral gold complexes ligated with HBHC (hydrogen bonded heterocyclic carbenes) based on 3,30 -functionalization of BINAM as the catalysts. The substituted phenyl groups at the 3,30 -positions of the BINAM scaffold had a huge influence on the enantioselectivities of the reaction with a range from 7% ee to 91% ee. According to this work, only 4-substitution of the 3,30 -aryl group could improve the enantioselectivities of the reaction. The best result was obtained by using 3,30 -(4-trifluoromethyl)phenyl substituted carbene–gold(I) complex G43 and AgOTf as catalyst in the solution of CDCl3 at 0 °C (Scheme 12). The toleration of these substrates catalyzed by carbene–gold complexes was excellent because all of the corresponding products could be obtained in excellent yields and enantioselectivities. It is the first time that the carbene ligated gold(I) complexes can exhibit excellent catalytic activities and enantioselectivities even better than chiral phosphine ligands ligated gold complexes. The significant challenges in the field of carbene–gold(I) catalyzed asymmetric reactions such as low enantioselectivities and limitation of the substrates seem to be solved in this reaction. The structural properties of these new axially carbene–gold(I) complexes are the keys to make this remarkable improvement. The 3,30 substituted groups of 1,10 -binaphthyl scaffold have important effect on enantioselectivities of asymmetric reactions. These phenomena have been confirmed by the many successful examples of gold(I) complex ligated with chiral phosphine and phosphoramidite ligands.27 Due to the development of the synthesis of 3,30 substituted BINAM, more efficient gold(I) complexes ligated with different types of carbene ligands based on 3,30 -substituted BINAM will be synthesized in the near future. Asymmetric gold-catalyzed hydrogenation reaction: A growing trend in the pharmaceutical industry is directly to market with chiral drugs in enantiomerically pure form to provide desired positive effects in humans. One of the most important ways to introduce chiral carbon centers is asymmetric hydrogenations.28 Many efficient transition-metal complexes are successfully employed in the industrial process using iridium, rhodium, palladium, and ruthenium complexes.28 Hydrogenation catalyzed by gold(0) nanoparticles supported by other materials have been reported several times.29 In 2005, Ujaque and co-workers first reported the homogeneous and heterogeneous gold(III) complexes catalyzed hydrogenations of diethyl itaconate.30 However, asymmetric hydrogenation of unsaturated bonds catalyzed by gold(I) complexes has been reported rarely. In 2010, Sánchez and co-workers reported the synthesis of chiral bis-carbene ligands based on a dioxolane backbone.31 Next, the same authors synthesized these gold(I), palladium(II) and rhodium(I) complexes ligated with their chiral carbene ligands. These chiral ligated transition metal complexes were tested in the asymmetric hydrogenations of (E)-diethyl succinate derivatives. The bis-carbene gold(I) complexes showed good catalytic activities and enantioselectivities with TOF up to 2000 and ee up to 95%. For example, when the benzylidene substituted succinate derivative 25 was treated with gold complex G44 (0.5 mol %) under the 4 atm pressure of H2 in ethanol, the corresponding hydrogenation product 26 could be obtained in the TOF

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OR'

OPiv

O

NHC-Au G43, AgOTf

R'

Cl Au

OPiv

CDCl3, 0 °C R

R N H N

Ar

Ar

HN R

23

24 ee = 83% - 99%

NH Au

N H N R

Cl

G43 R = 4-CF3C6H4

Scheme 12. Gold-catalyzed enantioselective synthesis of chromenyl pivaloate.

bis-NHC-Au G44 H2 (4 atm) ethanol, 40 °C

EtO2C 25

CO2Et

26 O

N

CO2Et

1250 TOF, 90% ee

O

N

N Ar

∗

EtO2C

Au Au Cl Cl

N Ar

G44 Ar = 2,4,6-trimethylphenyl

Scheme 13. Gold-catalyzed enantioselective hydrogenation of vinyl substituted succinate esters.

of 1250 with 90% ee (Scheme 13). The rate of hydrogenation would decrease rapidly if chlorine was replaced by OPNB (4-nitrobenzoate), probably due to the increased steric hindrance. In this work, the sterically hindered groups play an important role in this reaction. In general, the sterically more hindered groups in gold complexes can improve the enantioselectivities of hydrogenations, but decrease the reaction rates. The proper size of sterically hindered groups in catalysts should be carefully selected to balance the enantioselectivity and the reaction rate. Overall, this is an inspiring work and the authors give us more choices to apply chiral carbene–gold(I) complexes in some useful and practical industrial reactions. Potential feasible strategies: After researching and combining these previous literatures in the field of gold-catalyzed asymmetric reactions in the recent years, we now put forward five strategies that probably can make improvements in the carbene–gold complex catalyzed reactions. Au(III) strategy: Au(III) catalysts show the similar reactivity with Au(I) catalysts in many reactions, but they exhibit quite different properties in the asymmetric transformations. Compared to the gold(I) catalyst’s linear coordination mode, the gold(III) complexes usually have four coordination sites and exist as square planar complexes. With four possible coordination sites around the metal center, the gold(III) complex seems to be a reasonable choice for asymmetric gold catalysts. It should be easy to build the needed sterical control around the reaction center as chelated coordination mode. The structures of these chelated ligated gold complexes are more stable sterically because the rotations of carbon–gold bonds have been blocked out. Many non-chiral NHC–gold(III) complexes have been synthesized by different ways.32 Although the reports involving gold(III) complexes catalyzed asymmetric reactions are rare,33,34 it is still a possible strategy to make progress in goldcatalyzed asymmetric reactions. Weak gold(I)-arene interaction strategy: According to Slaughter’s work, the secondary interactions of the ligand help to overcome the inherent difficulty of achieving a chiral environment at gold(I)

center.35–37 These weak gold(I)–arene interactions lead to the formation of analogous chelated coordination mode, impairing the free rotation around carbon–gold(I) bond and enhancing the steric hindrance around the gold(I) atom. Nitrogen acyclic carbene stragety: The best results of carbene– gold(I) complexes catalyzed asymmetric reactions are obtained by nitrogen acyclic carbene ligands ligated gold(I) complexes. The bond angles of N–C–N are 116–121° which are much wider than normal N-heterocyclic carbenes.38,39 This wide bond angles can place chiral substituents closer to the metal, indirectly improving the asymmetric induction. 3,30 -Substituted BINAM backbone strategy: The gold(I) complexes ligated with 3,30 -substituted BINOL-derived phosphine and phosphoramidite ligands have successfully catalyzed many asymmetric gold-catalyzed reactions with excellent yields and enantioselectivities. The substituted aryl groups have a strong p–p stacking interaction with the aryl groups from the phosphine ligand. These twisted interacted aryl groups create a huge sterically hindered environment around the gold(I) atom. This finding has been provided to be an efficient way to increase the enantioselectivities of gold(I)catalyzed asymmetric reactions. On the basis of Maruoka group’s excellent contribution to the synthesis of 3,30 -substituted BINAM,40 the gold(I) complexes ligated with 3,30 -modified BINAM-derived carbene ligands will be one kind of reasonable method to explore novel chiral carbene–gold(I) complexes and their new applications. Chiral counterion stragety: In the recent years, many chemists focused their attention to introduce diverse sterically hindered substituents to the ligands used to coordinate with gold(I). In some special works, the enantioselectivities were found to be significantly dependent upon the counteranion employed. In 2007, Toste’s group reported the first example of gold(I) catalyzed asymmetric reactions based on chiral counter anions.41 The interactions between the achiral cationic gold(I) metal center and the chiral counteranion could also induce chirality efficiently. This finding gives us an alternative choice in asymmetric gold catalysis: if changing the chiral substituent in the carbene ligand does not work, introducing chiral counteranion with achiral carbene ligands probably can be another choice. Conclusions The concept of asymmetric catalysis in the presence of gold(I) complex has been the hottest and most challenged field since the seminal work published by Ito, Sawamura and Hayashi in 1986.42 Carbene ligands as one kind of important chiral ligands have been introduced into the gold-catalyzed asymmetric reactions recently. Due to the ability of stronger r-donor and weaker p-acceptor, the carbene–gold(I) complexes exhibit more stabilized and electronrich properties than chiral phosphine–gold(I) complexes. Even thoughmost of the chiral carbene–gold(I) complexes show poorer catalytic activities and enantioselectities, the studies of chiral carbene–gold(I) catalysts and their application in asymmetric reactions are still hot topics in the field of homogeneous gold

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catalysis. The successful works from Toste’s and Slaughter’s groups give a significant inspiration and encouragement to the scientists who are working in this field. At the end of this digest, we present five strategies which can probably improve the applications of chiral carbene–gold complexes based on a clear understanding of these catalysts. We believe that more and more efficient chiral carbene–gold complexes will be obtained in the future. Acknowledgments We thank the Shanghai Municipal Committee of Science and Technology (11JC1402600), the National Basic Research Program of China (973)-2010CB833302, the Fundamental Research Funds for the Central Universities, and the National Natural Science Foundation of China for financial support (21072206, 20472096, 20872162, 20672127, 21102166, 21121062 and 20732008). References and notes 1. For recent reviews: (a) Corma, A.; Leyva-Pérez, A.; Sabater, M. J. Chem. Rev. 2011, 111, 1657; (b) Krause, N.; Winter, C. Chem. Rev. 2011, 111, 1994; (c) Shapiro, N.; Toste, F. D. Synlett 2010, 675; (d) Gorin, D. J.; Sherry, B. D.; Toste, F. D. Chem. Rev. 2008, 108, 3351; (e) Hashmi, A. S. K.; Hutchings, C. J. Angew. Chem., Int. Ed. 2006, 45, 7896. 2. (a) Hashmi, A. S. K.; Schwarz, L.; Choi, J. H.; Trost, T. M. Angew. Chem., Int. Ed. 2000, 39, 2285; (b) Hashmi, A. S. K.; Trost, T. M.; Bats, J. W. J. Am. Chem. Soc. 2000, 122, 11553; (c) Hashmi, A. S. K.; Blanco, M. C.; Fischer, D.; Bats, J. W. Eur. J. Org. Chem. 2006, 12, 1387. 3. Some excellent turnover numbers have been reported for some simple goldcatalyzed reactions recently, see: (a) Jaimes, M. C. B.; Böhling, C. R. N.; SerranoBecerra, J. M.; Hashmi, A. S. K. Angew. Chem., Int. Ed. 2013, 52, 7963; (b) Hashmi, A. S. K. Science 2012, 338, 1434. 4. For recent reviews: (a) Bongers, N.; Krause, N. Angew. Chem. 2008, 120, 2208. Angew. Chem. Int. Ed. 2008, 47, 2178; (b) Widenhoefer, R. A. Chem. Eur. J. 2008, 14, 5382; (c) Hashmi, A. S. K. Nature 2007, 449, 292. 5. For the latest developments of NHC–gold(I) complexes, see: (a) Würtz, S.; Glorius, F. Acc. Chem. Rev. 2008, 12, 1370; (b) Díez-González, S.; Marion, N.; Nolan, S. P. Chem. Rev. 2009, 109, 3612; (c) Hashmi, A. S. K.; Lothschütz, C.; Böhling, C.; Hengst, T.; Hubbert, C.; Romingera, F. Adv. Synth. Catal. 2010, 352, 3001; (d) Hashmi, A. S. K.; Lothschütz, C.; Graf, K.; Häffner, T.; Schuster, A.; Romingera, F. Adv. Synth. Catal. 2011, 353, 1407; (e) Hashmi, A. S. K.; Riedel, D.; Rudolph, M.; Rominger, F.; Oese, T. Eur. J. Org. Chem. 2012, 18, 3827. 6. Lee, M. T.; Hu, C. H. Organometallics 2004, 23, 976. 7. (a) Bugaut, X.; Liu, F.; Glorius, F. J. Am. Chem. Soc. 2011, 133, 8130; (b) Dabrowski, J. A.; Gao, F.; Hoveyda, A. H. J. Am. Chem. Soc. 2011, 133, 4778. 8. (a) Teles, J. H.; Brode, S.; Chabanas, M. Angew. Chem., Int. Ed. 1998, 37, 1415; (b) Deetlefs, M. H.; Raubenheimer, G.; Esterhuysen, M. W. Catal. Today 2002, 72, 29; (c) Schneider, S. K.; Herrmann, W. A.; Herdtweck, E. Z. Anorg. Allg. Chem. 2003, 629, 2363; (d) Boogaerts, I. I. F.; Nolan, S. P. J. Am. Chem. Soc. 2010, 132, 8858; (e) de Fremont, P.; Marion, N.; Nolan, S. P. J. Organomet. Chem. 2009, 694, 551. 9. (a) Lopez, S.; Herrero-Gomez, E.; Perez-Galan, P.; Nieto-Oberhuber, C.; Echavarren, A. M. Angew. Chem., Int. Ed. 2006, 45, 6029; (b) Shapiro, N. D.; Toste, F. D. J. Am. Chem. Soc. 2007, 129, 4160; (c) Benitez, D.; Shapiro, N. D.; Tkatchouk, E.; Wang, Y.; Goddard, W. A., III; Toste, F. D. Nat. Chem. 2009, 1, 482; (d) Döpp, R.; Lothschütz, C.; Wurm, T.; Pernpointner, M.; Keller, S.; Rominger, F.; Hashmi, A. S. K. Organometallics 2011, 30, 5894. 10. (a) Melhado, A. D.; Amarante, G. W.; Wang, Z. J.; Luparia, M.; Toste, F. D. J. Am. Chem. Soc. 2011, 133, 3517; (b) LaLonde, R. L.; Wang, Z. J.; Mba, M.; Lackner, A. D.; Toste, F. D. Angew. Chem., Int. Ed. 2010, 49, 598; (c) Liu, F.; Qian, D. Y.; Zhao, X. L.; Zhang, J. L. Angew. Chem., Int. Ed. 2010, 49, 6669; (d) Kleinbeck, F.; Toste, F. D. J. Am. Chem. Soc. 2009, 131, 9178; (e) Zhang, Z.; Lee, S. D.; Widenhoefer, R. A. J. Am. Chem. Soc. 2009, 131, 5372. 11. (a) MuÇoz, M. P.; Adrio, J.; Carretero, J. C.; Echavarren, A. M. Organometallics 2005, 24, 1293; (b) Chao, C.; Vitale, P. Y.; Genet, J. P.; Michelet, V. Chem. Eur. J. 2009, 15, 1319; (c) Chao, C.; Beltrami, D.; Toullec, P. Y.; Genet, J. P.; Michelet, V. Chem. Commun. 2009, 6988.

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