Divergent Synthesis of Carbo- and Heterocycles via Gold-Catalyzed

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Perspective

Divergent Synthesis of Carbo- and Heterocycles via Gold-catalyzed Reactions Yin Wei, and Min Shi ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00048 • Publication Date (Web): 07 Mar 2016 Downloaded from http://pubs.acs.org on March 8, 2016

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ACS Catalysis

Divergent Synthesis of Carbo- and Heterocycles via Gold-Catalyzed Reactions Yin Weia and Min Shi*a a

State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 354 Fenglin Road, Shanghai 200032, P. R. China

ABSTRACT: The latest advances in the field of gold-catalyzed divergent synthesis of carbo- and heterocycles are highlighted in this Perspective. These gold-catalyzed reactions, which can provide structurally novel carbo- and heterocycles, were achieved with good product selectivities, through subtle choice of substrates, catalysts, ligands and counterions. The related reaction mechanisms are also discussed.

KEYWORDS: gold catalysis, carbocycles, heterocycles, cycloisomerization, cycloaddition. INTRODUCTION Carbo- and heterocycles as the core structures exist in a variety of pharmacological agents and natural products.1 Therefore, development of synthetic methods to carbo- and heterocyclic compounds has emerged as an important research field in organic synthesis. Although carbo- and heterocyclic compounds can be accessed through a variety of highly efficient methodologies, more efforts to develop novel reactions using readily accessible starting materials with high product selectivity (chemo-, regio- and stereoselectivity) are still required in current research.2 A series of excellent reviews regarding transition-metalcatalyzed reactions for synthesis of carbo- and heterocyclic compounds have been reported recently.3-8 Among the methods to generate carbo- and heterocyclic compounds via transition-metal-catalyzed reactions, the gold-catalyzed reactions recently emerge as one of efficient methods for the synthesis of useful complex structures and are developing rapidly.3,9 Homogeneous gold catalysis has received remarkable growing research interest in the past decade due to its versatile applications in organic synthesis. Gold salts and complexes, especially gold(I) complexes, are identified as efficient catalysts which can selectively activate the multiple bonds in a variety of hetero- and carbonucleophiles. The properties of gold(I) complexes can be easily adjusted sterically or electronically by varying ligands, consequently altering their reactivity in the activation of alkynes, alkenes, and allenes.10 It is well known that gold complexes containing more σ-donating ligands, such as N-heterocyclic carbenes (NHCs) (1), are less electrophilic than those with phosphine ligands (2, 3) (Figure 1).11 The gold complex having poor σ-donating phosphite ligands (4) represents as the most electrophilic catalyst.

Figure 1. Representative gold catalysts and their relative electrophilicity Alkynes, 1,n-enynes and allenes are the most popular substrates used in current gold-catalyzed transformations. Based on initial gold activation modes, the gold(I)-catalyzed reactions can be classified into several types as shown in Scheme 1, which are proposed by Toste, Zhang, and Hashmi, resectively. The first one is defined as π-activation mode (Scheme 1a), in which the gold catalyst activates the alkyne moiety in substrate by π-coordination, generating an alkenyl gold intermediate, and then accept the attack of nucleophiles. This activation mode is well-established and commonly accepted in gold catalysis. The so-called dual σ, π-activation mode12 was first proposed by Toste and Houk.13 Subsequently, Hashmi14 and Zhang15 independently reported this new dual gold activation mode in gold-catalyzed cyclization of diynes, whereby one alkyne moiety is activated by gold catalyst through π-coordination, while another is activated by gold catalyst through σ-coordination (Scheme 1b).16,17 In 2014, the σ-activation mode was reported by Hashmi’s group.18 In the σ-activation mode, the alkyne moiety of substrate is activated by σ-coordination to the gold catalyst (gold–acetylide), which increases the nucleophilicity of the β-carbon atom (Scheme 1c). These gold activation modes enrich the knowledge of gold chemistry, and are very helpful for further design new gold-catalyzed reactions. Very recently, few examples regarding ligand-controlled gold activation have been reported.19 The gold activation modes can be adjusted by varying catalysts, ligands and reaction conditions, thus the gold-catalyzed reactions provide divergent synthetic routes to access different carbo- and heterocyclic compounds. In early years, Hashmi’s group has demonstrated that the

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reaction selectivity can be switched by subtle tuning the structure of substrate, catalyst or counterion.20 Recently, more research groups reported the examples related to this topic. This Perspective surveys the selected reports in recent two years in the field of homogeneous gold catalysis for divergent synthesis of carbocycles and heterocycles. These reports are desired to construct structurally novel carbo- and heterocycles efficiently, and represent the recent research interests in Gold chemistry. The following issues in this respective will be discussed: (1) accessing different products from the same starting materials by fine tuning substrates, catalysts, ligands and counterions; (2) how to control the reaction with high chemo-, regio- and stereoselectivity; (3) deep understanding of reaction mechanisms.

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subsequently undergoes C-H activation of non-activated aliphatic14c,15a and aromatic14a,14b C-H bonds, and intermolecular C-H bond activation processes (Scheme 2a).14d In the case of 2,3-substituted thiophene derived diynes, a 1,3-diaurated species was initially formed via 6-endo cyclization, subsequently undergoing a gold shift by an aryne–gold transition state to generate a 1,4-diaurated species containing a carbene moiety, followed by C-H insertion (Scheme 2b).21 (a) 5-endo cyclization and C-H activation AuL H

AuL 5-endo cyclization

Au+L H

R

H

dual activation R

R

LAu

AuL

C-H activation R

a)

activation mode Nu

Nu

(b) 6-endo cyclization and C-H activation

well established

AuL

AuL

AuL

H

S

R

dual activation

H

S

S R

LAu

b)

AuL R

6-endo cyclization

Au+L

AuL

activation mode Nu

Nu

AuL

AuL AuL

S

R R

C-H activation

AuL

AuL

AuL

R

Toste

S S

AuL AuL

AuL

Scheme 2. Dual-gold activated cyclization of diynes

AuL Hashmi, Zhang LAu c)

Subsequently, Hashmi’s group presented a gold-catalyzed cycloisomerization of diynes 5 containing a 3,4-substituted thiophene backbone to generate new pentaleno[c]thiophene derivatives 6 bearing a heterocyclic triquinane moiety by 5-endo cyclization and C-H bond activation (Scheme 3). They also investigated the effect of diyne backbone for cyclization selectivity by detailed theoretical studies.22 The stabilities of the 5-endo and 6-endo cyclization products were analyzed using a variety of diynes bearing different backbones; and the crucial potential energy surfaces were investigated through DFT calculations for the selected cases. As for substrates 5 (3,4-thiophene backbone), the cyclization selectivity can be accounted for through the energy difference of two critical transition states for cyclizations. The transition state (TS1) for 5-endo cyclization is more stable than that (TS2) for 6-endo by 4.3 kcal/mol or so (NHC ligand), thus, the 5-endo cyclization product is acquired experimentally. Their calculation results also show that the energy difference between transition states for 5-endo and for 6-endo cyclization decreases with increasing aromatic stabilization of the 6-endo product, making an energetic distinction impossible (Scheme 4). They concluded that the cyclization selectivity is influenced by an electronic effect and not steric effect.

AuL

activation mode

AuL

AuL

LG Hashmi

LG

Scheme 1. Gold activation modes DIVERGENT CONTROL

SYNTHESIS

VIA

SUBSTRATE

Recently, several research groups demonstrated that subtle adjusting the structure of substrates or the substituents on substrates can afford different carbo- and heterocyclic products using gold complex as catalyst. Thus, the wanted products can be synthesized selectively. Hashmi’s group and Zhang’s group independently reported a new activation mold by gold catalyst, in which two gold centers synchronously activate the alkyne moieties of diynes via dual σ, π-activation mode as shown in Scheme 1b. They identified that the backbone of diynes can significantly affect the selectivity towards 5-endo or 6-endo cyclization (Scheme 2).14,15,21 A gold–vinylidene species was first generated in the 5-endo cyclization pathway via the Cβ atom of the gold–acetylide, which


2

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ACS Catalysis

of spirocyclic intermediates 13 and 17 play a key role on the reaction pathways. Further studies could be conducted to account for why subtle changing the structure of substrate has a significant influence on its reaction property. Scheme 3. Cycloisomerization of 3,4-thiophene substituted diynes in the presence of gold catalyst

Scheme 4. Rationalization of cyclization selectivity Our group recently reported a gold-catalyzed intramolecular cycloisomerization of α-yne-furans. Through varying the substituents on the substrates, a variety of cyclic α,β-unsaturated aldehyde or ketone derivatives and nitrogen-containing tricyclic adducts can be produced selectively in moderate to excellent yields under mild conditions.23 Treatment of a series of α-yne-furans 7 with 5 mol% [(IPr)Au(CH3CN)][SbF6] in dry 1,2-dichloroethane (DCE) at 80 oC generated the desired product α,β-unsaturated aldehydes 8 in good to excellent yields (Scheme 5a). Fine tuning the structure of α-yne-furans with increasing the length of carbon chain between the propargylic ester and the furan subunit led to different products. Employing almost same reaction conditions, a series of α-yne-furans 9 underwent the reactions smoothly, however, affording [4C+3C] cycloadducts 10 in good to excellent yields (Scheme 5b). The proposed reaction mechanisms to explain the above experimental results are depicted in Scheme 6. Gold(I) complex first activates the alkyne moiety of 7 to form complex 11, which undergoes 1,2-acyloxy migration to generate gold carbene 12 reversibly. A nucleophilic attack of the α-position of furan generates the spirocyclic intermediate 13, following a C-O cleavage to produce intermediate 14 as Z/E isomeric mixture. The thermodynamically more stable product 8 is finally obtained upon heating. In the case of the formation of [4C+3C] cycloadducts 10, the plausible reaction mechanism is illustrated in Scheme 6b. In the similar manner, an initial activation of 9 to generate the gold complex 15, which undergoes a reversible 1,2-acyloxy migration to obtain gold carbene 16. Subsequently, a nucleophilic attack of the α-position of furan produces the spirocyclic intermediate 17. Instead of C-O cleavage, the spirocyclic intermediate 17 undergoes a cyclization process to afford product 10, and regenerates the gold catalyst. It can be deduced that the stabilities and electronic properties

Scheme 5. Gold-catalyzed cycloisomerization of α-yne furans

Scheme 6. Proposed mechanisms for gold-promoted cycloisomerization of α-yne furans Subsequently, our group reported a gold-catalyzed cycloisomerization of aromatic ring tethered vinylidenecyclopropane(VDCP)-enes, which demonstrates a divergent synthetic route for the construction of O-containing fused heterocycles via controllable carbene or non-carbene related processes (Scheme 7).24 Catalyzed by JohnPhosAuCl (10 mol%) and AgNTf2 (10 mol%), the cycloaddition reactions of aromatic ring tethered vinylidenecyclopropane-enes 18 (R = H, F) underwent the reactions smoothly to give product 19 in up to 94% yield. This cycloisomerization reaction was suggested to proceed via a key gold carbene intermediate I. The key gold carbene was proposed to be generated on the basis of so-called amphiphilic strategy. The VDCP moiety in substrate 18 was coordinated with the gold (I) complex to generate species A, which was transformed to afford species B due to the relatively strong nucleophilic nature at C3 position, and subsequently underwent a ring expansion of cyclopropane to form a gold carbene C due to the relatively strong electrophilic nature at C2 position (Scheme


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7). This generation method for gold carbene represents a new amphiphilic strategy to produce gold carbene by a rearrangement of vinylidenecyclopropane. Moreover, it was found that the ortho-substituents on the aromatic ring of substrate 18 significantly affected the reaction mode. Under the same reaction conditions, the substrate 18 (R = Me, Br, Cl, tBu) underwent the non-carbene related process, giving products 20 or 21. The electronic effect of the ortho-substituents in the substrate 18 was proposed, and also supported by DFT calculations, to influence the oxygen atom’s nucleophilicity, leading to the divergent reaction pathways to give products 19 or 20 and 21.

intermediates 26 and 27, or via exo-dig cyclizations to give other two possible spirocyclic indoline intermediates 28 and 29. Spirocyclic indolines intermediates 26 and 28 obtained by α-alkenylation may undergo fragmentation to give iminium ions 30 and 32, respectively. Then, protodeauration and hydrolysis occur to give the ring expansion products 23 and 35. In contrast, β-alkenylation intermediates 27 and 29 can undergo deprotonation at the C1-position by a base to give enamine intermediates 31 and 33, respectively. Then, the subsequent protodeauration occurs, furnishing the spirocyclization products 24 and 34. DFT calculations were conducted to explain the regio- and chemoselectivity. DFT calculations reveal that the exo/endo selectivity of the intramolecular cyclization can be tuned by the electronic nature of the terminal substituent on the alkyne moiety. DFT calculations also show that the intermediates 26 and 28 are interconvertible via [1,5]-alkenyl shift, and the similar situation occurs for the intermediates 27 and 29. The reaction selectivity are influenced by the relative energies of the intramolecular cyclization, [1,5]-alkenyl shift, ring expansion, and spirocyclization processes. The reaction selectivity is under dynamic control if the [1,5]-alkenyl shift is slower than the ring expansion and spirocyclization processes. Otherwise, the chemical outcome is under typical kinetic control and determined by the relative preference of ring expansion versus spirocyclization pathways.

non-carbene related allyl-transfer R' 1

Carbene induced cycloaddition

R

2

JonePhosAuCl (10 mol%) AgNTf2 (10 mol%)

3

X 18

R'

2

3

or X

X

R

20 up to 90% yield

R

3

R = Me, Br, Cl, tBu.

21

X=O

up to 77% yield

R1

R1 Au+L

R = H, F

X I

1 2

DCE, rt, 5 min

Au+L

R

R'

1

X R'

R

R'

X = C, O R1 = Alkyl group

19 up to 94% yield

Amphiphilic strategy 1 2 3 4 AuL+

AuL+

AuL

A B new gold carbene generation process

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C

Scheme 7. Modulable gold-catalyzed cyclization of VDCP-enes and a new gold carbene generation strategy

R1 = alkyl, aryl ring expansion Ph3PAuNTf 2 (1 mol%) MsOH (2.0 equiv)

Yu’s group recently reported that 2-propargyl-β-tetrahydrocarbolines 22 can undergo ring expansion reaction or spirocylization reaction to obtain different products catalyzed by gold complex (Scheme 8).25 Under gold catalysis, the internal alkyne substrate underwent the ring expansion reactions to give the azocinoindole derivatives 23 in excellent yields. As for terminal alkyne substrates, dearomatizing spirocyclization occurred to furnish the spirocyclic indoline products 24 in good to excellent yields with good selectivity. The plausible reaction mechanisms are illustrated in Scheme 9. The reaction starts with the coordination of the alkyne moiety of the substrate to the carbophilic gold catalyst, affording a reactant complex 25. Subsequently, 25 undergoes endo-dig cyclization through the intramolecular nucleophilic attack of the indole moiety to the alkyne moiety, generating two possible spirocyclic indoline

PhMe (0.1 M), rt, 12 h then NaHCO3 (aq) R1

NH R3 N 1

R2 R 23 up to 95% yield

N N

R3

R2 22

JohnPhosAuCl (1 mol%) AgSbF6 (1 mol%) MsOH (2.0 equiv) N + 23

DCM (0.1 M), rt, 12 h then NaHCO3 (aq)

N

spirocyclization

R2 24

R1 = H

R3

up to 97% yield (24/23 > 20:1)

Scheme 8. Gold-catalyzed ring expansion and spirocyclization of 2-propargyl-β-tetrahydrocarbolines


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ACS Catalysis

Scheme 9. Proposed mechanisms 2-propargyl-β-tetrahydrocarbolines

for

gold-catalyzed

ring

expansion

and

spirocyclization

of

The gold carbene 40 without carbocation-stabilizing phenyl groups alternatively prefers to rearrange to another tertiary carbocation 43. Ring cleavage of the cyclobutane then generates the diene product 38 (Path 2, Scheme 10a). As for formation of product 39, the gold carbene intermediate 40 is also generated, which undergoes addition of the aryl moiety to the gold carbene to give a highly strained tetracyclic intermediate 44, probably due to steric hindance. The tetracyclic intermediate 44 relieves strain by cyclopropane ring cleavage and formation of a tertiary carbocation 45. Concomitant protodeauration and aromatization yields the corresponding tricycles 39 (Scheme 10b). Further detailed experimental and theoretical mechanistic studies will be more interesting, which will not only provide solid evidence for proposed mechanisms, but also serve as a guideline for the rational prediction of new transformations.

In comparison with a lot of transformations of 1,6-enynes, the reactions of easily accessible homologous 1,7-enynes under gold catalysis are not investigated enough. Kumar and Waldmann recently reported divergent skeletal rearrangement of 1,7-enynes catalyzed by gold catalyst.26 Due to electronic and steric effects, differently substituted 1,7-enynes 36 undergo the reactions by different carbocations generated from a common gold carbene intermediate to yield novel exocyclic allenes 37 or dienes 38, and tricyclic hexahydro-anthracenes 39 through a novel formal dehydrogenative Diels-Alder reaction (Scheme 10). The proposed mechanisms for formation of different products are also depicted in Scheme 10. A key gold carbene intermediate 40 is first generated via 6-exo-dig cyclization. A sufficient nucleophilicity of the aryl group promots generation of stable benzylic carbocation 41, which may undergo a 1,5-hydride shift leading to tertiary carbocation 42. Finally, the allene product 37 is produced after deauration of the gold complex (Path 1, Scheme 10a).


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Scheme 10. The gold-promoted divergent transformations of 1,7-enyne rearrangements and plausible reaction mechanisms The gold-catalyzed cycloaddition has shown its power to access a variety of structurally novel carbocycles and heterocycles, however, there are still challenges in terms of the cycloaddition mode and the enantioselectivity.27 Zhang’s group recently28 reported an asymmetric [2+2] and a [4+2]-cycloaddition of 3-styrylindoles to N-allenamides catalyzed by gold(I)/chiral phosphoramidite complexes, affording synthetically valuable, optically active substituted cyclobutanes and tetrahydrocarbazoles in high yields with excellent enantioselectivities. They discovered that the cycloaddition modes were strongly influenced by the electronic character of the N-substituent 3-styrylindoles. This is an excellent example of a significant substituent effect in adjusting the cycloaddition mode with high chemo-, regio- and enantioselectivity in enantioselective gold catalysis. As for a series of 3-styrylindoles 46 having electron-donating groups as N-protecting groups, [2+2] cycloadditions of 3-styrylindoles 46 to N-allenamides 47 occurred to give cyclobutanes 48 in good yields with high enantioselectivities; in contrast, a series of 3-styrylindoles 46 having electron-withdrawing groups as N-protecting groups underwent [4+2] cycloadditions to afford tetrahydrocarbazoles 49 in good yields with excellent enantioselectivities (Scheme 11). DFT calculations were conducted to explain the reason of this dramatic substituent effect on the cycloaddition mode in gold catalysis. DFT calculations indicate that the [2+2] reaction pathway is more favored thermodynamically and kinetically with respect to 3-styrylindoles 46 having electron-donating groups as N-protecting groups. In contrast, the [4+2]

reaction pathway is more favored thermodynamically and kinetically with respect to 3-styrylindoles 46 having electron-withdrawing groups as N-protecting groups.

Scheme 11. Strong substituent effects in gold-catalyzed asymmetric [2+2] vs [4+2] cycloadditions of 3-styrylindoles to N-allenamides DIVERGENT SYNTHESIS VIA CATALYST CONTROL The air and moisture stability of gold complexes currently make them particular appealing catalysts for effective synthesis of carbo- and heterocyclic compounds. Moreover, the relativistic effect observed for gold probably results in the particular catalytic properties and activities of gold complexes in reactions. Using the same substrates, the gold-catalyzed reaction can give the products which are


6

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ACS Catalysis

(a) Gold catalyzed cycloisomerization

different from the products obtained via other transition-metal catalyzed reactions. Our group recently developed intramolecular cycloisomerizations of nitrogen- or carbon-tethered indolylcyclopropenes promoted by gold and silver, affording biologically and pharmaceutically interesting azepino[4,5-b]indole and spirocyclic [indoline-3,4’-piperidine] derivatives (Scheme 12).29 Using gold complex as a catalyst, a series of indolylcyclopropenes 50 underwent cycloisomerization to provide azepino- [4,5-b]indole products 51 in moderate to good yields. Switched to silver catalyst (AgOTf), another type of cycloisomerization reactions took place to afford spirocyclic [indoline-3,4’-piperidine] products 52 in moderate to good yields. Having done deuterium-labeling experiments and control experiments, plausible mechanisms are proposed as shown in Scheme 13. For gold-catalyzed transformation, coordination of gold to the ester moiety and the double bond of the cyclopropene in [D]-50a produces intermediate 52, which undergoes a Friedel-Crafts reaction at the C2 position of the indole moiety to form intermediate 53. After aromatization and protodeauration, the expected product [D]-51a containing infused seven-membered heterocycle is obtained stereospecifically (Scheme 13a). In the case of silver-catalyzed transformation of [D]-50a, two alternative reaction pathways are proposed. The major one is that the silver catalyst is attacked by the C3 position of the indole moiety to generate a sp3 C-Ag species 54, which undergoes a syn addition to the double bond of cyclopropene to afford intermediate 55. After protodemetalation, the cis-product [D]-52aa is obtained. Alternatively, the minor reaction pathway is suggested to proceed as the gold-catalyzed reaction. In the similar manner, the intermediate 56 is generated via the coordination of silver and the ester moiety and the double bond of cyclopropene in [D]-50a, which subsequently undergoes a Friedel-Crafts reaction at the C3 position of indole moiety to give intermediate 57. The trans product [D]-52aa’ is finally acquired after deprotonation and protodeargentation (Scheme 13b).

N Ts

3

CO2Et

2

N1 H

OEt

N Ts

AuL

O

D

N H

[D]-50a

D

52

Au L

NTs

NTs OEt N HD 53

N HD

AuL AuL

O

CO2Et

[D]-51a

(b) Silver catalyzed cycloisomerization CO2Et D

NTs Ag

major

EtO2C D

EtO2C NTs

D

Ag

[D]-50a

Ag

54 minor

N H

N 55 H

OEt

O

NTs

Ag

Ag

O

OEt

Ag NTs

D

N [D]-52aa EtO2C NTs

D

Ag

D

NTs

3

N H

2

N1 56 H

N [D]-52aa'

57

Scheme 13. Plausible reaction mechanisms based on deuterium-labeling experiments Subsequently, we achieved a efficient stereodivergent and regioselective synthesis of an indole-fused heterocycles bearing several contiguous stereocenters by formal cycloadditions of indolyl-allenes in the presence of gold catalyst or platinum catalyst.30 The [3+2] and [2+2] cycloadditions of indolyl-allenes 58 can be controlled by tuning catalysts, affording indole-fused heterocycles 59, 60, and 61. The quaternary carbon stereocenter in products 59 and 60 can be constructed via [3+2] cycloaddition reactions, and controlled by using Pt or Au catalysts, respectively (Scheme 14a and b). In addition, the exo-type [2+2] cycloadduct 61 was obtained by employing [IPrAuCl]/AgNTf2 catalytic system (Scheme 14c). After investigations by control experiments and isotopic labeling experiments, plausible reaction mechanisms are outlined in Scheme 15. In the case of [3+2] cycloaddition, a metallo-carbon intermediate 62 in the presence of metal catalyst via reversible metalation at the C3 position of indole moiety. The following cis-addition step cannot take place by using gold catalyst, probably due to the gold catalytic system are sterically hindered. In contrast, the cis-addition step can occur by using PtCl2 as catalyst, further furnishing heterocyclic intermediate 63. A Pt-carbene intermediate 64 is subsequently formed, which undergoes a 1,2-hydride migration and then regeneration of catalyst to afford product 59. The coordination of gold complex to the allene moiety of substrate to yield intermediate 62′ is suggested as a key step for gold-catalyzed reactions. The C3 position of indole moiety nucleophilic attacks the Au-activated allene moiety in 62′, furnishing the alkenyl gold (I) complex 63′, which may undergo divergent paths to provide different products. In Path a, the expected adduct 60 is produced through a

Scheme 12. Gold catalyzed indolylcyclopropenes vs silver catalyzed indolylcyclopropenes


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sequential cyclization, hydride migration and elimination process via Au-carbene intermediate 64′ and cationic intermediate 65′, which is similar to Pt catalysis. Probably due to the different E and Z configurations of alkenyl metal intermediates 63 and 63′ resulted in the stereo-divergence of the quaternary carbon stereocenter in products 59 and 60. In Path b, the alkenyl gold intermediate 63′alternatively undergoes a cyclization to deliver a formal [2+2] cycloaddition product 61. Moreover, the ligands in the gold complex, the steric and electronic characters of the R1 and R2 substituents were also considered to have effects on the product selectivities towards 60 and 61, however, detailed explanations still need further experimental and theoretical studies.

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sulfonamides 66 afford substituted pyrroles 67 in good to excellent yields; however, the use of AuCl/AgSbF6 containing a silver co-catalyst furnishes substituted dihydropyridines 68 as products (Scheme 16). The reaction mechanisms for cycloisomerization reactions of N-propargyl-N-vinyl sulfonamides are proposed as illustration in Scheme 17. Using the substrate 66a as a representative example, a metal-mediated propargyl-Claisen rearrangement initially takes place to form β-allenyl imine 69, which undergoes gold-mediated 5-exo-dig cyclization and then follows a subsequent aromatisation via hydrogen shift to produce 2-sulfonylmethyl pyrrole 67a. Dihydropyridine 68a is suggested to generate through the aza-triene intermediate 70 and subsequent 6π-aza-electrocyclisation.

Scheme 14. Catalyst-controlled cycloaddition of indolyl allenes R1

Scheme 16. Divergent synthesis of 2-sulfonylmethyl pyrroles and dihydropyridines controlled by catalysts

PtCl2

R1

R1 [M] M

cis-addition

R2

N

PtCl2

R2

N

NTs

R2 TsN

N

NTs

58

63

62

R1

R1

R2

N 65

59

R1

R2 NTs

N

NTs

64

R1 AuL

R1 b AuL R2 Path b

AuL

R2

N

PtCl2

R2

NTs

N

R1

PtCl2

PtCl2

a

R2

N

NTs

NTs

58

61

Scheme 17. Proposed mechanisms for cycloisomerisations of aza-enynes catalyzed by Au or Au-Ag catalysts

NTs

N

63'

62'

Path a R1

R1 R2 N 60

NTs

AuL

AuL

R1

H

NTs

N 65'

DIVERGENT SYNTHESIS VIA LIGAND CONTROL As aforementioned, the gold-activated modes have several different types. Through subtle choice of ligands having different electronic properties, the gold-activated modes can be tuned using the same substrates. Medio-Simón’s group demonstrated the competition of between π- and dual σ,π-gold activation modes of terminal alkynes, which can be switched by the ligands in gold complexes.19a They have reported that either 6-exo-dig or 5-endo-dig gold-mediated cyclization of 1-(o-ethynylaryl)ureas 71 can take place to give 4-methylene-3,4-quinazolin-2-ones 72 or indoles 73, depending on the choice of ligands (Scheme 18). They

AuL R2

R2 N

NTs 64'

Scheme 15. Proposed reaction mechanisms for Au- and Pt-catalyzed cycloaddition of indolyl allenes Very recently, Menon’s group reported that the cycloisomerization reactions of N-propargyl-N-vinyl sulfonamides can afford different products through using different gold catalytic systems.31 Using (JhonPhos)Au(CH3CN)SbF6 as a catalyst, the cycloisomerization reactions of N-propargyl-N-vinyl


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isolated and identified several aurated reaction intermediates involved in the heterocyclization of 71 through NMR spectroscopy, X-ray diffraction, or MALDI spectrometry. These experimental results gave solid evidences for that using IPr as the ligand is helpful to promote the π activation of terminal alkynes, whereas employing P(tBu)3 as the ligand favors the dual σ,π-activation mode.

N 72

N H

Ph O

[IPrAu] (5 mol%)

O

DMF, 60 oC -activation mode

71

N H

N H

Ph

DMF, 60 oC 73

N CONHPh

-activation mode

[Au] R

More recently, our group also demonstrated that the gold activation modes can be tuned through ligands-control.19b In the presence of [Ph3PAuOTf] as the catalyst, propargylic thioureas 74 underwent the N-cyclization, furnishing the N-cyclization products 75 in good to excellent yields; switched to [Me4tBuXPhosAuNTf2] as gold catalyst, the substrates 74 underwent the S-cyclization, affording the S-cyclization products 76 in good to excellent yields (Scheme 19). Through a series of NMR spectroscopy, deuterium labeling experiments, kinetic experiments, the competitive gold activation modes occurred during the reactions via dual σ,π-activation intermediate 77 or π-activation intermediate 78 were identified (Scheme 19).

N-1 attack 5-endo-dig [(tBu)3PAu]+ (5 mol%)

N-3 attack 6-exo-dig +

Scheme 18. Ligand-controlled gold-catalyzed reaction of urea

[Au] H

R

[Au]

Scheme 19. Gold-catalyzed intramolecular heterocyclization of propargylic thioureas controlled by ligands proposed that the steric hinderance between the phosphine ligands in catalyst A and catalyst B and the substrate may account for the experimental product selectivities. The key gold-coordinated intermediate 82 is suggested to form during the reaction. Using catalyst A having JohnPhos ligand, the intermediate 82 undergoes 6-endo-dig cyclization to generate the gold intermediate 83 that leads to form the cycloheptenone product 80 selectively (Scheme 21, path a). However, in the case of reactions mediated by catalyst B having Me4tBuXPhos ligand, the steric hindrance is increased between the ligand and the terminal alkene moiety in vinyl gold intermediate 83. Thus, generating spirocyclic [4.4]nonenyl gold species 84 via the 5-exo-dig cyclization is more favorable in this case (Scheme 21, path b), which prefers to afford the hexenone product 81.

Fine-tuning the steric property of the ligand in the gold complex can significantly affect the product selectivity sometimes. Chan’s group presented a good example for this case.32 They prepared tricyclic bridged heptenones and hexenones through double cycloisomerization of 1,11-dien-3,9-diyne benzoates 79 catalyzed by gold complex (Scheme 20). Using the [MeCNAu(JohnPhos)]+SbF6− catalytic system, 1,3-acyloxy migration/metallo-Nazarov cyclization/1,6-enyne addition/Cope rearrangement of the substrate sequentially took place, selectively generating the bridged heptenone adduct 80 in moderate to good yields. However, switching the catalyst to [MeCNAu(Me4tBuXPhos)]+SbF6− including steric hindered ligand resulted in formation of 1,11-dien-3,9-diyne benzoates 79, which undergoes a more rapid tandem 1,3-acyloxy migration/metallo-Nazarov cyclization/[4+2]-cyclization process to deliver the bridged hexenone derivative 81 in moderate to good yields. Detailed mechanistic studies have not been done yet. They


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was obtained as main product. In contrast, the corresponding regiospecific C(3)-functionalization product 87 was mainly accessed when the air-tolerant (ArO)3PAuCl/AgTFA was employed (Scheme 22). They proposed that the regiodivergent profile relies on the different coordination attitude/basic character of TFA− with respect to OTf−. They also envisioned the establishment of strong hydrogen bond interactions between the anion and the N(1)−H indole site, resulting in the nucleophilic activation of the hetereoaromatic nucleus (mainly at the C(3)-position, 89) and concomitant “protection” of the N(1) site.

tBu tBu P Au NCMe

SbF6

cat. A i) cat. A (10 mol%), (CH2Cl)2, 4Å MS, rt, 72 h ii) K2CO3 (0.5 equiv), MeOH/THF (1:1), rt, 12 h

R2

O R1 R2

OPNB

80 up to 73% yield

R1

i) cat. B (10 mol%), (CH2Cl)2, 4Å MS, 80 oC, 72 h ii) K2CO3 (0.5 equiv), MeOH/THF (1:1), rt, 12 h

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O R1 R2

tBu tBu Me

SbF6

P Au NCMe iPr

81 up to 78% yield

iPr

Me Me

iPr Me cat. B

Scheme 20. Product selectivity controlled by ligands in gold-catalyzed double cycloisomerization of 1,11-dien-3,9-diyne benzoates

Scheme 22. Counterion effect in gold-catalyzed reactions of 2,3-disubstitued indoles with allenamides

CONCLUSIONS The recent reports on developing new gold-catalyzed reactions for the efficient construction of elegant carboand heterocycles are highlighted in this Perspective. These studies demonstrated a series of divergent synthesis of different carbo- and heterocyclic products from the same starting materials by fine-tuning the substrates or subtle choice of catalysts, ligands and counterions. These research results enrich the knowledge of gold chemistry. It also can be concluded that the gold-catalyzed transformations are complicated processes, and the product selectivities can be affected by many factors such as substrates, catalysts, ligands, counterions and other reaction conditions. Although some experimental and theoretical mechanistic investigations have been conducted to account for the current experimental results, further studies are still required for how to quantify product selectivity in gold-catalyzed transformations and to guideline the rational predictions for new transformations. In the future, more efforts to develop efficient reactions with high selectivity to construct biologically and synthetically interesting cyclic compounds are still demanded; moreover, there are a lot of challenges and opportunities for the future researches dealing with fascinating transformations involving enantioselective gold catalysis.

Scheme 21. Rationalization of the origin of product selectivity DIVERGENT SYNTHESIS VIA COUNTERION CONTROL The counterion effect in the gold catalysis is still far from being fully rationalized. Bandini and co-workers recently reviewed some leading examples of counterion-controlled gold catalysis.33 They also reported an excellent example for a dramatic counterion effect on the site-selective functionalization of 2,3-disubstitued indoles 85 with allenamides 86 to give densely functionalized indolenine cores 87.34 The overall regiochemistry of the condensation was significantly influenced by the nature of the gold counterion. Employing (ArO)3PAuCl/AgOTf catalytic system, the compound 88

AUTHOR INFORMATION Corresponding Author * [email protected]


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Notes The authors declare no competing financial interest.

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ACKNOWLEDGMENT This work was financially supported by the National Basic Research Program of China (973)-2015CB856603, and the National Natural Science Foundation of China (21572052, 21421091, 21372250, 21361140350, 21372241, 21302203, 20672127, 21121062, 20732008 and 20472096). REFERENCES (1) (a) Kirsch, S. F. Org. Biomol. Chem. 2006, 4, 2076-2080. (b) Hou, X.-L.; Yang, Z.; Yeung, K.-S.; Wong, H. N. C. in Progress in Heterocyclic Chemistry, Vol. 19, Pergamon, Oxford, 2008, 176-207. (c) Katritzky, A. R.; Ramsden, C. A.; Scriven, E. F. V.; Taylor, R. J. K. Comprehensive Heterocyclic Chemistry III, Elsevier, Oxford, 2008. (d) Majumdar, K. C.; Chattopadhyay, S. K. Heterocycles in Natural Product Synthesis, Wiley-VCH, Weinheim, 2011. (2) Gulevich, A. V.; Dudnik, A. S.; Chernyak, N.; Gevorgyan, V. Chem. Rev. 2013, 113, 3084-3213. (3) For general reviews on gold-catalyzed transformations, see: (a) Hashmi, A. S. K. Chem. Rev. 2007, 107, 3180-3211. (b) Muzart, J. Tetrahedron 2008, 64, 5815-5849. (b) Li, Z.; Brouwer, C.; He, C. Chem. Rev. 2008, 108, 3239-3265. (c) Shen, H. C. Tetrahedron 2008, 64, 3885-3903. (d) Shen, H. C. Tetrahedron 2008, 64, 7847-7870. (e) Wang, S.; Zhang, G.; Zhang, L. Synlett 2010, 692-706. (f) Hashmi, A. S. K. Angew. Chem. Int. Ed. 2010, 49, 5232-5241. (g) Shapiro, N. D.; Toste, F. D. Synlett 2010, 675-691. (h) Corma, A.; Leyva-Perez, A.; Sabater, M. J. Chem. Rev. 2011, 111, 1657-1712. (i) Krause, N.; Winter, C. Chem. Rev. 2011, 111, 1994-2009. (j) Garayalde, D.; Nevado, C. Beilstein J. Org. Chem. 2011, 7, 767-780. (k) Rudolph, M.; Hashmi, A. S. K. Chem. Commun. 2011, 47, 6536-6544. (l) Qian, D.; Zhang, J. Chem. Rec. 2014, 14, 280-302. (m) Qian, D.; Zhang, J. Chem. Soc. Rev. 2015, 44, 677-698. (n) Dorel, R.; Echavarren, A. M. Chem. Rev. 2015, 115, 9028-9072. (4) For selected recent reviews on palladium-catalyzed transformations and synthesis of heterocycles, see: (a) Beccalli, E. M.; Broggini, G.; Martinelli, M.; Sottocornola, S. Chem. Rev. 2007, 107, 5318-5365. (b) Majumdar, K. C.; Chattopadhyay, B.; K. Maji, P.; K. Chattopadhyay, S; Samanta, S. Heterocycles 2010, 81, 517-584. (c) Majumdar, K. C.; Chattopadhyay, B.; K. Maji, P.; K. Chattopadhyay, S; Samanta, S. Heterocycles 2010, 81, 795-866. (5) For general reviews on rhodium-catalyzed transformation, see: (a) Davies, H. M. L.; Hedley, S. J. Chem. Soc. Rev. 2007, 36, 1109-1119. (b) Davies, H. M. L.; Lian, Y. Acc. Chem. Res. 2012, 45, 923-935. (c) Padwa, A. Chem. Soc. Rev. 2009, 38, 3072–3081. (6) For general reviews on copper-catalyzed transformations and synthesis of heterocycles, see: (a)


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(c) Brown, T. J.; Weber, D.; Gagné, M. R.; Widenhoefer, R. A. J. Am. Chem. Soc. 2012, 134, 9134-9137. (d) Vachhani, D. D.; Galli, M.; Jacobs, J. Van Meervelt, L.; Van der Eycken, E. V. Chem. Commun. 2013, 49, 7171–7173. (e) Grirrane, A.; Garcia, H.; Corma, A.; Ylvarez, E. Chem. Eur. J. 2013, 19, 12239–12244. (f) Braun, I.; Asiri, A. M.; Hashmi, A. S. K. ACS Catal. 2013, 3, 1902-1907. (17) (a) Hooper, T. N.; Green, M.; Russell, C. A. Chem. Commun. 2010, 46, 2313-2315. (b) Brown, T. J.; Widenhoefer, R. A. Organometallics 2011, 30, 6003-6009. (18) Bucher, J.; Wurm, T.; Nalivela, K. S.; Rudolph, M.; Rominger, F.; Hashmi, A. S. K. Angew. Chem. Int. Ed. 2014, 53, 3854–3858. (19) (a) Gimeno, A.; Cuenca, A. B.; Suarez-Pantiga, S.; Arellano, C. R. D.; Medio-Simón, M. Asensio, G. Chem. Eur. J. 2014, 20, 683-688. (b) Jiang, Y.; Wei, Y.; Tang, X.-Y.; Shi, M. Chem. Eur. J. 2015, 21, 7675-7681. (20) For early year’s examples, see: (a) Hashmi, A. S. K.; Rudolph, M.; Siehl, H.-U., Tanaka, M.; Bats, J. W.; Frey, W. Chem. Eur. J. 2008, 14, 3703-3708. (b) Hashmi, A. S. K.; Rudolph, M.; Huck, J.; Frey, W.; Bats, J. W.; Hamzić, M. Angew. Chem. Int. Ed. 2009, 48, 5848-5852. (c) Hashmi, A. S. K.; Schuster, A. M.; Rominger, F. Angew. Chem. Int. Ed. 2009, 48, 8247-8249. (d) Hashmi, A. S. K.; Schuster, A. M.; Gaillard, S.; Cavallo, L.; Poater, A.; Nolan, S. P. Organometallics 2011, 30, 6328-6337. (21) (a) Hashmi, A. S. K.; Wieteck, M.; Braun, I.; Rudolph, M.; Rominger, F. Angew. Chem. Int. Ed. 2012, 51, 10633–10637. (22) Hansmann, M. M.; Tšupova, S.; Rudolph, M.; Rominger, F.; Hashmi, A. S. K. Chem. Eur. J. 2014, 20, 2215-2223. (23) (a) Yang, J.-M.; Tang, X.-Y.; Shi, M. Chem. Eur. J.


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Divergent Synthesis of Carbo- and Heterocycles via Gold-Catalyzed

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