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Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Variations on the Theme of JohnPhos Gold(I) Catalysts: Arsine and Carbene Complexes with Similar Architectures Javier Carreras,† Ana Pereira,† Margherita Zanini,†,‡ and Antonio M. Echavarren*,†,‡ †

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Institute of Chemical Research of Catalonia (ICIQ), Barcelona Institute of Science and Technology, Av. Països Catalans 16, 43007 Tarragona, Spain ‡ Departament de Química Analítica i Química Orgànica, Universitat Rovira i Virgili, C/Marcel·li Domingo s/n, 43007 Tarragona, Spain S Supporting Information *

ABSTRACT: Arsine and carbene gold(I) complexes with architectures closely related to those of 2-(di-tert-butylphosphino)biphenyl gold(I) complexes have been prepared and structurally characterized. As predicted, 2-(di-tert-butylarsine)biphenyl gold(I) complexes are more electrophilic catalysts in comparison to their phosphine analogues, whereas those based on 4-arylindazole behave similarly to NHC-gold(I) catalysts.

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INTRODUCTION The potential of gold(I) catalysts in the activation of multiple bonds has been exploited to construct a wide range of complex structures1 and natural products.2 The electrophilic activation of alkynes is routinely carried out by using linear dicoordinated chloride gold(I) complexes [LAuCl] (L = ligand) as precatalysts, which are usually activated by chloride abstraction using a silver salt, leading to cationic gold(I) species of the type [LAuL′]+, where L′ is a weakly coordinating ligand, a solvent molecule, or the reactive substrate.3 On the other hand, reaction of complexes [LAuCl] with silver salts AgOTf and AgNTf2 leads to neutral [LAuX] (X = OTf, NTf2) complexes. The properties of gold(I) complexes can be tuned sterically and electronically depending on the ligand, modulating their reactivity in the activation of alkynes, alkenes, or allenes. Complexes bearing bulky and strongly donating ligands are the most used catalysts for the activation of enynes.1o,q,4 Among these, commercially available complexes 1a−d with JohnPhos and related ligands are among the most successful phosphinegold(I) complexes.5,6 Moreover, cationic complexes 2a−c bearing a weakly coordinating ligand such as acetonitrile (or benzonitrile) show analogous catalytic properties but are easier to handle and do not require the use of any chloride abstractor.7 The related neutral complexes 3a−c with a weakly coordinating bis(trifluoromethanesulfonyl)amide ligand show similar reactivity8 (Figure 1). Complex 4 with a bulky phosphite as the ligand exhibits higher electrophilicity and therefore activates less reactive substrates.9 On the other side of the spectrum of reactivity, complexes with electron-donating N-heterocyclic carbene (NHC) ligands of general type 5 are much less electrophilic, which makes these catalysts more selective in many transformations.10 Despite the wide variety of available gold(I) complexes, in many cases the optimal catalysts are the biphenyl phosphine © XXXX American Chemical Society

Figure 1. Gold(I) complexes with dialkyl biphenyl phosphine (1−3), phosphite (4), and N-heterocyclic carbene ligands (5).

derivatives 1−3. The reason for the excellent performance of these complexes in catalysis is not totally well understood. Thus, the interaction between the metal center and the aromatic ring has been shown to be, at most, very weak.6b However, this aromatic ring could still exert a certain degree of stabilization of the carbocationic-like intermediates generated in catalytic transformations of alkynes and other unsaturated molecules. In contrast to that found in other transition-metal complexes, the relation between the donor power of L ligands and the electrophilicity of cationic [LAuL′]+ complexes is not straightforward. This complex bonding phenomenon has been analyzed theoretically in detail in a series of [LAu(CO)]+ complexes with a variety of ligands, including phosphines and NHC ligands.11 Interestingly, in these linear complexes, charge transfer (CT) is very significant for PPh3 (CT = 0.40 e) and Special Issue: In Honor of the Career of Ernesto Carmona Received: April 29, 2018

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DOI: 10.1021/acs.organomet.8b00276 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics JohnPhos (CT = 0.35 e) and low for NHC ligands such as IPr (CT = 0.26 e). In general, back-donation from the L ligands is very important in [LAuI-S]+ (S = alkene, alkyne) complexes, whereas the donation component is essentially constant even for ligands with different charges (chloride vs phosphines and NHC ligands).12 There are only a few examples of gold(I) complexes13 or other metals14 with an architecture similar to that of complexes 1−3. With the aim of contributing to the understanding of the ligand effects in gold(I) catalysis, we decided to explore other related gold(I) complexes with steric environments closely similar to that of JohnPhos but with very different electronic properties. Thus, we prepared the first examples of 2-(di-tertbutylarsine)biphenyl gold(I) complexes (Figure 2). Addition-

The structures of new arsine gold(I) complexes 7−10 were determined by X-ray diffraction (Figure 3). The As−Au bond

Figure 2. New gold(I) complexes with 2-(di-tert-butylarsine)biphenyl and 4-arylindazole ligand analogues of Johnphos Au(I) complex 1b.

Figure 3. Structures of complexes 7 (a), 8 (b), 9 (c), and 10 (d) as ORTEP plots (ellipsoids set at 50% probability). Hydrogen atoms, solvate molecules, and anions are omitted for clarity.

ally, we also prepared in a simple manner a series of new 4arylindazole gold(I) complexes with highly donating ligands. Here we describe the synthesis, structure, and reactivity of both types of complexes in homogeneous gold(I) catalysis.

length in complex 7 is longer (2.352 Å) than the corresponding P−Au bond in the analogous JohnPhos gold(I) complexes 1b, 2b,6 and 3b16 (2.24−2.25 Å) (Table 1), in

RESULTS AND DISCUSSION Synthesis and Structure of Arsine Gold(I) Complexes. We first synthesized the 2-(di-tert-butylarsine)biphenyl ligand 6 in two steps and overall 48% yield following a modified procedure for the synthesis of the analogous JohnPhos ligand15 by reaction of the Grignard reagent of 2-bromobiphenyl with tBu2AsCl (Scheme 1). The structure of dialkylarylarsine 6 was

Table 1. Selected Distances (Å) and Angles (deg) for 7−10 and 1b−3b

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bond length Au−X complex (X = P, As) a

1b 2ba 3bc 7 8 9 10

Scheme 1. Synthesis of Biphenylarsine Gold(I) Complexes 7−10

2.254(3) 2.2466(2) 2.2445(9) 2.3519(7) 2.332(5) 2.3400(5) 2.3457(4)

Au−Cl,N

Au···Cipso

bond angle X−Au−Cl,N (X = P, As)

2.303(4) 2.0338(9) 2.113(3) 2.295(2) 2.103(3) 2.028(3) 2.045(2)

3.165b 3.024(1) 3.187(3) 3.247(7) 3.329(3) 3.154(3) 3.133(2)

172.56(14) 174.43(3) 170.76(8) 175.29(5) 170.99(8) 174.81(9) 173.98(6)

a

Reference 15. bThe standard deviation of the calculated Au−Cipso distance could not be determined due to the applied constraints of the coordinated atoms involved. cReference 13.

agreement with that previously reported for arsine gold complexes.18−20 Similar bond lengths were observed in complexes 8−10 and JohnPhos gold(I) ligands (Table 1). The Au−Cl or Au−N bond lengths in arsine complexes 7−10 are shorter than those in the JohnPhos complexes 1b−3b. The As−Au−L angles for complexes 7−10 are in the 170−176° range, similar to those observed in analogous phosphine complexes 1b−3b (Table 1). Chloride-bridged dinuclear gold(I) complex 11 was prepared in quantitative yield by mixing 7 with 0.5 equiv of AgSbF6 in CH2Cl2 as a noncoordinating solvent (Scheme 2), in a manner similar to that observed before for the analogous phosphine complexes.16 The Au−Au distance (3.44 Å) and Au−Cl−Au angle (94.1°) for 11 are similar to those of the analogous dinuclear gold(I) complex [(JohnPhosAu)2Cl](SbF6) (Au−Au = 3.48 Å, Au−Cl−Au = 94.7°).16

confirmed by X-ray diffraction. Gold chloride complex 7 was prepared by reaction of 6 with [Au(SMe2)Cl] in 86% yield. Reaction of 7 with silver bis(trifluoromethanesulfonyl)amide gave neutral gold complex 8, following the procedure described for the analogous JohnPhos complex.16 In addition, acetonitrile and benzonitrile cationic complexes 9 and 10 were obtained in quantitative yields from 7 using silver hexafluoroantimonate in the presence of an excess of the corresponding nitrile.17 B


Article

Organometallics

phosphine complex has a higher Lewis acidity in comparison to its arsine analogue. In order to more clearly assess the differences between arsine ligand 6 and JohnPhos, we decided to compare the corresponding gold(I) π-alkene complexes. Complexes with simple alkenes were synthesized by treating gold(I) complex 7 with 1.1 equiv of AgSbF6 in the presence of an excess of alkene in dry CH 2 Cl 2 following the procedure reported by Widenhoefer23 (Scheme 3). Interestingly, in the case of both

Scheme 2. Synthesis and Structure of Chloride-Bridged Dinuclear Gold(I) Complex 11a

Scheme 3. Synthesis and Structure of Gold(I) π-Alkene Complexes 12 and 13a

a

An ORTEP plot (50% thermal ellipsoids) is given. Hydrogen atoms, solvate molecules, and anions are omitted for clarity.

The substitution of PPh3 by AsPh3 in the case of Ni0 and RhI carbonyl complexes has only minor effects on the νCO frequency (Ni0, 2069 cm−1 for PPh3 vs 2068 cm−1 for AsPh3; RhI, 1979 cm−1 for PPh3 vs 1975 cm−1 for AsPh3).21 However, the Rh−Cl bond distances in trans-[RhCl(CO)(L)2] complexes are shorter for L = AsPh3 (2.354(2) Å) than for L = PPh3 (2.382(1) Å, which suggests a lower donor ability of the arsine ligand. As shown before, in our case, the Au−Cl and Au−N bond lengths in arsine complexes 7−10 are in all cases shorter than those of their phosphine analogues (Table 1), which agrees with the lower donor capacity of the arsines. However, as has been observed before,11 rationalizing ligand effects in linear gold(I) is far from trivial. We decided to apply the Gutmann−Beckett method to determine the relative Lewis acidity of the gold(I) complexes by adding an excess of phosphine oxides R3PO to CH2Cl2 solutions of gold(I) complexes, measuring the chemical shift differences between free and coordinated phosphite (Table 2).22 Surprisingly, no significant differences were observed for the biphenylphosphine and arsine complexes (Table 2, entries 1−6), which indicates that both types of complexes have similar Lewis acidities. The comparison between [Au(PPh3)Cl] and [Au(AsPh3)Cl] (Table 2, entries 7−10) actually suggests that the

a

An ORTEP plot (ellipsoids set at 50% probability) is given. Hydrogen atoms, solvate molecules, and anions are omitted for clarity.

methylencyclohexane and 2,3-dimethyl-2-butene, instead of the direct formation of the π-complex with the double bond, complexes 12 and 13 of the corresponding isomers arising from a formal 1,3-hydrogen shift in the alkene were obtained as the only products in quantitative yields. The structures of 12 and 13 were confirmed by X-ray diffraction.23 We hypothesized that the double-bond isomerization takes place outside the coordination sphere of the metal by a Brønsted acid catalyzed pathway.24 In support of this hypothesis, treatment of arsine complex 9 with an excess of methylenecyclohexane in dry CH2Cl2 under silver-free conditions leads to the formation of complex 14 without isomerization of the double bond, demonstrating that the silver salt is fundamental for the in situ generation of the Brønsted acid (Scheme 4). With 2,5-dimethylhexa-1,5-diene as the starting material, the corresponding dicationic digold(I) complex 15 was prepared. In the reaction of the same diene with 7 and AgSbF6, a mixture of oligomerization products was obtained, supporting the hypothesis of the involvement of a Brønsted acid in the reaction. However, under these reaction conditions we never observed the formation of the nonisomerized complex with 2,3-dimethyl-2-butene, probably due to steric reasons.24 The Widenhoefer group reported that the reaction of JohnPhos gold(I) complex 1b with excess alkenes in the presence of AgSbF6 under anhydrous conditions leads to the πalkene complexes without isomerization of the starting alkenes.23 However, isomerization was observed when the reactions were performed with 1b and AgSbF6 in wet CH2Cl2 (Scheme 5). The structures of the complexes 16a,b were confirmed by X-ray diffraction.

Table 2. 31P Chemical Shift Differences upon Coordination of Phosphine Oxides to Arsine and Phosphine Gold(I) Complexes

complex

T (°C)

R3PO

δ coord R3PO

Δδ

1b 7a 2b 9 3b 7 [Au(PPh3)Cl]a [Au(AsPh3)Cl]a [Au(PPh3)Cl]a [Au(AsPh3)Cl]a

25 25 −50 −50 −50 −50 25 25 25 25

POPh2Me POPh2Me POPh2Me POPh2Me POPh2Me POPh2Me POPh2Me POPh2Me POEt3 POEt3

44.8 43.5 56.6 56.9 56.6 56.9 48.6 39.5 65.6 59.4

13.2 11.8 24.3 24.6 24.3 24.6 16.9 8.3 11.3 5.1

entry 1 2 3 4 5 6 7 8 9 10

a

a

1 equiv of AgSbF6. C


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Organometallics Scheme 4. Synthesis and Structure of Gold(I) π-Alkene Complexes 14 and 15

respectively. These structural data, as well as those found for 7−10 (Table 1), demonstrate the higher electrophilic character of the arsine−gold frame in comparison to the phosphine counterpart (Table 3). Table 3. Selected X-ray Diffraction Distances (Å) for Complexes 12, 13, and 16a,b bond length

a

complex

Au−X (X = P, As)

Au−C1a

Au−C2b

12 13 16a 16b

2.375(2) 2.3844(3) 2.9414(18) 2.306(8)

2.22(3) 2.197(4) 2.239(9) 2.24(5)

2.32(3) 2.310(6) 2.351(11) 2.35(5)

Less substituted sp2 carbon. bMore substituted sp2 carbon.

The synthesis of the two Ag(I) complexes 18 and 19 with ligand 6 that are isoleptic with respect to 9 and 14 was also performed by using essentially the same synthetic procedures (Scheme 7).

Scheme 5. Synthesis of Gold(I) π-Alkene Complexes 16a,b

Scheme 7. Synthesis of Bisphenylarsine Silver(I) Complexes

The structure of 18 was determined by X-ray diffraction (Figure 4). As already observed with analogous phosphine complexes,6b the As−Ag−N angle in 18 deviates more from linearity than that in the gold(I) complex 9 (168.2° vs 174.8°).

The 1,3-hydrogen migration can be induced on the preformed complex with both JohnPhos and arsine ligands in the presence of an exogenous Brønsted acid such as TfOH as the catalyst.25 Considering these and other related results,25 we propose that, in the case of methylencyclohexane, the initial πcomplex 17 undergoes ligand exchange with triflate anion to form neutral complex I (Scheme 6). The free alkene then Scheme 6. Mechanism of the Formal 1,3-Hydrogen Migration in the π-Complexes in the Presence of a Brønsted Acid

Figure 4. Structure of complex 18 as an ORTEP plot (ellipsoids set at 50% probability). Selected bond lengths (Å) and angle (deg): Ag−As = 2.444(4), Ag−N = 2.135(3), Ag−Cipso = 3.222; As−Ag−N = 168.18(9).

Synthesis and Structure of Indazole Gold(I) Complexes. We also decided to target 4-arylindazoles as electronically modular ligands with a related JohnPhos architecture. Only three examples of indazole gold(I) complexes have been reported,26,27 and their catalytic properties were only examined in one instance for the hydration of alkynes.26c For the synthesis of these complexes, we commenced with the Suzuki coupling of 4-bromo-1-methyl1H-indazole (20) to introduce aryl groups with different electronic properties28 (Scheme 8). Nucleophilic aromatic substitution was used to prepare the polyfluorinated tolyl derivative 21e in good yield. Quaternization of the indazole

undergoes a Brønsted acid catalyzed isomerization to form 1methyl-1-cycloxene, which reacts with I to form the thermodynamically more stable complex 16a. In the case of reaction with 2,3-dimethylbut-2-ene, the less hindered complexes 13 and 16b are obtained, since in the initial complexes two methyl groups point toward the phenyl ring of the ligand, causing significant steric congestion. In all of the complexes with an isomerized double bond, the alkene is bound asymmetrically to the metal center, with the less substituted carbon closer to gold. The Csp2−gold(I) distances are shorter for arsine complexes 12 and 13 in comparison to those found for phosphine complexes 16a,b, D


Article

Organometallics Scheme 8. Preparation of 4-Arylindazole Gold(I) Complexes 23a−e and 24b,c

proceeded in moderate to good yields using diethyl sulfate in toluene at reflux, followed by treatment with an aqueous NaBF4 solution (65−84%). The formation of salts 22·BF4a−e was confirmed by their 1H NMR spectra, which show the characteristic downfield signal for the hydrogen at C-3. Anion exchange by chloride gave salts 22·Cla−e in excellent yields by treatment with Amberlyte 402 IRA Cl−.29 The structures of salts 22·BF4b,c and 22·Clb were confirmed by X-ray diffraction. The synthesis of the gold(I) complexes was accomplished using the silver−carbene transfer methodology.30 Indazolium salts 22·Cla−e reacted with Ag2O, and the resulting silver complexes were directly transmetalated with [Au(SMe2)Cl] to give complexes 23a−e in excellent yields (81−94%) (Scheme 8). Cationic gold(I) complexes 24b,c were synthesized using AgSbF6 and benzonitrile as a weakly coordinating ligand. Complexes 23a−e and 24b were characterized by X-ray diffraction (Figure 5). The expected nearly linear arrangement around gold was observed with C(3)−Au−Cl bond angles from 173.4 to 178.4° (Table 4). The Au−Cl distances ranged from 2.291 to 2.318 Å and those of Au−C(3) from 1.982 to 1.990 Å, in agreement with those previously reported for indazole gold complexes.26 Notably, complex 23e revealed significant differences in the Au−Cipso distance, which is approximately 0.2−0.3 Å shorter than those for the rest of the indazole complexes. The distance observed in 23e (3.245 Å) is closer to the reported data for JohnPhosAuCl (3.165 Å).6 The angle between the indazole and the aromatic ring is also very different in this complex, with a value close to 90°, instead of the 50−55° observed for the rest of the arylindazole gold(I) complexes. 13 C NMR spectroscopy has been used for the study of the properties of N-heterocyclic carbenes and their corresponding complexes,31 and a relation between the chemical shift of the carbenic carbon and the acidity of the complex has been suggested32 (Table 5). In the case of our complexes, no significant differences were observed in complexes 23a−e, which display chemical shifts between 170.0 and 168.4 ppm, slightly upfield with respect to the values reported for the known complexes 25 and 26, in similar solvents (CD2Cl2 vs CDCl3, entries 6 and 7, Table 5).26b,c

Figure 5. Structures of the complexes 23a (a), 23b (b), 23c (c), 23d (d), 23e (e) and 24b (f) as ORTEP plots (ellipsoids set at 50% probability). Hydrogen atoms, solvate molecules, and anions are omitted for clarity.

Catalytic Studies. To determine the electrophilicity of the new gold(I) complexes 7−9 and 23a−e, we investigated their catalytic potential in the context of 1,6-enyne activation, focusing initially on intramolecular transformations. First, we examined the [4 + 2] cycloaddition reaction of phenylsubstituted 1,6-enyne 27a to form tricyclic derivative 285,33 (Table 6). For this transformation, arsine complexes provide excellent results, comparable to those reported with phosphine ligands.33 Neutral complex 7, in combination with NaBArF, gave tricyclic product 28 in excellent yield after 6 h (Table 6, entry 1). Similar good results were obtained with neutral (8) or cationic (9) complexes (Table 6, entries 2 and 3). For 4-arylindazole-gold(I) complexes, we observed a significant difference between dimethoxyphenyl- and phenylsubstituted complexes 23a,b, which gave product 28 in moderate yields after relatively long reaction times (Table 6, entries 4 and 5), and those substituted with electron-poor aromatic rings, particularly the fluorinated complexes (Table 6, entries 6−8), which gave results similar to those for phosphines or arsine ligands regarding reaction times and yield.34 We also tested the new catalysts in the intramolecular cyclopropanation of dienyne 27b to afford tetracycle 29 or the product of single-cleavage rearrangement35 (Table 7). Using arsine-gold(I) complex 9 at −40 °C, 29 was obtained as the main product together with triene 30 (Table 7, entry 1). When the temperature was increased to 23 °C, the reaction was fast and less selective, leading to a 57:43 mixture of 29 and 30 (Table 7, entry 2). The selective formation of tetracycle 29 was E


Article

Organometallics Table 4. Selected X-ray Diffraction Distances (Å) and Angles (deg) for 23a−e bond length

angle

complex

Au−Cl

Au−C(3)

Au···Cipso

C(3)−Au−Cl

C−C−C−C

23a 23b 23c 23d 23e

2.293(2) 2.3184(17) 2.2905(6) 2.2934(14) 2.2986(7)

1.982(7) 1.988(8) 1.988(2) 1.983(5) 1.990(2)

3.586(7) 3.463(9) 3.539(2) 3.458(6) 3.245(2)

176.5(2) 178.4(2) 178.41(7) 175.79(16) 173.42(7)

52.3(9) 55.8(9) 50.4(3) 51.1(8) 84.0(3)

Table 5. Chemical Shift of Carbene Carbon for Indazole Gold(I) Complexes

entry

complex

Ar

δc(Au−C)

solvent

1 2 3 4 5 6 7

23a 23b 23c 23d 23e 25a 26b

3,5-MeOC6H3 C6H5 3,5-CF3C6H3 2-NO2C6H4 3-CF3C6F4

170.0 170.0 169.3 169.1 168.4 172.6 173.7

CD2Cl2 CD2Cl2 CD2Cl2 CD2Cl2 CD2Cl2 CDCl3 CDCl3

a

Table 7. Gold(I)-Catalyzed Cyclization of Dienyne 27b

1 2 3 4 5 6 7 8

7/NaBAr 8 9 23a/AgSbF6 23b/AgSbF6 23c/AgSbF6 23d/AgSbF6 23e/AgSbF6

time (h)

yield (%)b

6 1 1 7 7 1.5 1.25 1

95 86 93 55 60 89 73 87

time (h)

1 2 3 4 5 6 7

9 9 23a/AgSbF6 23b/AgSbF6 23c/AgSbF6 23d/AgSbF6 23e/AgSbF6

−40 23 −40 −40 −40 −40 −40

24 0.5 1 1 1 1 1

yield (%)b 84 92 70 92 89 55 52

(88:12) (57:43) (100:0) (100:0) (100:0) (100:0) (100:0)

See the Supporting Information for detailed reaction conditions. Yields determined by 1H NMR using diphenylmethane as internal standard for entries 1 and 2; isolated yields for entries 3−7. Product ratios are given in parentheses.

Table 6. [4 + 2] Cycloaddition of 27a To Form 28 with Different Gold(I) Complexes

F

T (°C)

b

Reference 26c. Reference 26b.

catalysta

catalysta

a

b

entry

entry

Table 8. Gold(I)-Catalyzed Cyclization of 1,6-Enyne 27c

entry c

1 2d 3e 4 5 6

catalysta F

7/NaBAr 7/NaBArF 7/NaBArF 8 9 24b

time (h) 0.25 1 1 0.5 0.25 0.5

yield (%)b 92 73 70 87 96 24

(92:7:1) (97:0:3) (97:0:3) (39:58:3) (97:0:3) (42:51:7)

a

See the Supporting Information for detailed reaction conditions. Yields determined by 1H NMR spectroscopy using diphenylmethane as internal standard. Product ratios are given in parentheses. c Conditions A: see text. dConditions B: see text. eConditions C: see text. b

a

See the Supporting Information for detailed reaction conditions. b Yields determined by 1H NMR using diphenylmethane as internal standard for entries 1−4; isolated yields for entries 5−9.

achieved in high yields when the reaction was performed at −40 °C with the less electrophilic indazole-gold complexes 23a−c (Table 7, entries 3−5). In the case of the more electrophilic catalysts 23d,e, 29 was again selectively formed, although the yields were lower (Table 7, entries 6 and 7). To further delineate the reactivity of our new gold(I) complexes, the cycloisomerization of 1,6-enyne 27c35−37 was examined (Table 8). First, we examined the reaction with neutral arsine complex 7 under three different conditions: (A) in situ generation of the catalyst from 7 in the presence of the substrate (Table 8, entry 1), (B) in situ generation of the catalyst followed by the addition of the substrate (Table 8, entry 2), and (C) filtration of the catalyst through Celite after its generation in situ followed by the addition of the substrate (Table 8, entry 3). The fastest reaction and highest yield of

diene 31 were achieved under conditions A, in line with what we have observed before for phosphine gold(I) complexes.16 Presumably, when precatalyst 7 is activated in situ in the presence of the substrate (conditions A), formation of the less reactive chloride-bridged dinuclear complex 11 is minimized. A similar result was obtained using cationic complex 9, which does not require activation by a silver salt (Table 8, entry 5). Using neutral bistriflimide complex 8, a significant amount of the product of double-bond isomerization, 32, was obtained (Table 8, entry 4). Indazole gold complex 24b led to poor results in terms of both reactivity and selectivity (Table 8, entry 6). Our study was extended to intermolecular reactions between different nucleophiles and 1,6-enynes or alkynes.38 We F


Article

Organometallics explored first the reaction of 1,6-enyne 27d with indole (Table 9). This reaction has been studied by our group39 and others40

for 16 h in aqueous methanol using just 1 mol % of catalyst (Table 10, entries 1 and 2). Two unsymmetrical alkynes could be efficiently converted to the corresponding mixture of ketones (Table 10, entries 3 and 4). Under these reaction conditions, indazole-gold(I) complexes suffered decomposition.

Table 9. Gold(I)-Catalyzed Addition of Indole to 1,6-Enyne 27d

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entry

catalysta

1 2 3 4 5 6

9 23a/AgSbF6 23b/AgSbF6 23c/AgSbF6 23d/AgSbF6 23e/AgSbF6

CONCLUSIONS We have reported the synthesis and characterization of a series of arsine and carbene gold(I) complexes structurally related to a 2-(di-tert-butylphosphino)biphenyl gold(I) complex. In comparison to the biphenylphosphine complexes, the arsine analogues are more electrophilic but have similar Lewis acidity. We additionally synthesized a series of arsine gold(I) π-alkene complexes. Under conditions in which Brønsted acids are generated in situ, the initial alkenes undergo isomerization by formal 1,3-H shifts to finally form gold(I) complexes with the most stable alkene isomers. In the 4-arylindazole series, the new complexes show structural features in line with previously reported indazole-containing complexes, although complex 23e revealed significant differences, having an architecture closer to JohnPhosAuCl in comparison to the rest of the series. Additionally, we performed several catalytic studies with the two new families of complexes. As expected from structural studies, the arsine series are more electrophilic, whereas the 4arylindazole family shows a behavior similar to that of NHCgold(I) complexes.

yield (%)b 95 21 42 73 41 61

(69:31) (20:80) (25:75) (40:60) (33:67) (75:25)

a

See the Supporting Information for detailed reaction conditions. Yields determined by 1H NMR using 1,3,5-trimethoxybenzene as internal standard for entry 1; isolated yields for entries 2−6. Product ratios are given in parentheses. b

as a test of the electrophilicity of gold(I) catalysts. Two different adducts can be obtained in this reaction: product 34 from attack of indole at the cyclopropane of the key reaction intermediate, which is favored by the more electrophilic catalysts, or 35 from attack at the gold(I) carbene carbon, favored by the less electrophilic catalysts.39 As expected, an electrophilic complex such as cationic arsine-gold(I) 9 favors formation of 34 (Table 9, entry 1), whereas indazole gold(I) complexes 23a−d led preferentially to 35 (Table 9, entries 2− 5). Interestingly, complex 23e essentially gives the same result (Table 9, entry 6) previously obtained with JohnPhos complex 2b (74% yield, 80:20).39a Finally, we focused on the hydration of alkynes, one of the most studied reactions in gold catalysis.41 Among the many experimental procedures, the general procedure reported by Nolan using low catalyst loadings of a [(NHC)Au(I)]-based complex stands out because no acidic promoters are required.42 With this in mind, we decided to study the catalytic activity of our newly prepared complexes in the hydration of different alkynes under Brønsted acid free conditions (Table 10). Hence, cationic arsine complex 9 provided excellent results in the hydration of 1-octyne and diphenylacetylene when the reaction was carried out at 70 °C

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00276. All procedures and characterization data for new compounds (PDF) Accession Codes

CCDC 1839790−1839811 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for A.M.E.: [email protected].

Table 10. Gold(I)-Catalyzed Hydration of Alkynes with Complex 9

ORCID

Antonio M. Echavarren: 0000-0001-6808-3007 Notes

entry

alkyne

yield (%)

1 2 3 4

1-octyne diphenylacetylene 2-hexyne 1-phenyl-1-butyne

99a 99a 96 (45:55)b,c 99 (43:57)b,d

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This article is dedicated to Professor Ernesto Carmona on the occasion of his 70th birthday. We thank Agencia Estatal de Investigación (AEI)/FEDER, UE (CTQ2016-75960-P), Severo Ochoa Excellence Acreditation 2014-2018 (SEV-20130319 and La Caixa-Severo Ochoa predoctoral fellowship to M.Z.), the European Research Council (Advanced Grant No. 321066), the AGAUR (2017 SGR 1257 and Beatriu de Pinós Postdoctoral Fellowship to J.C.), and CERCA Program/

a

Yields were determined by GC-FID using a single-point calibration method and diphenylmethane as internal standard. bYields were determined by 1H NMR using diphenylmethane as internal standard. c Ratio 2-hexanone:3-hexanone. dRatio 1-phenyl-2-butanone:butyrophenone. G


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Organometallics Generalitat de Catalunya for financial support. We also thank the ICIQ X-ray Diffraction unit.

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