Chiral N-Heterocyclic Carbene Gold Complexes: Synthesis, Properties

Nov 5, 2012 - Mendez , M.; Munoz , M. P.; Nevado , C.; Cardenas , D. J.; .... David Rendón-Nava , Daniel Mendoza-Espinosa , Guillermo E. Negrón-Silv...
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Chiral N‑Heterocyclic Carbene Gold Complexes: Synthesis, Properties, and Application in Asymmetric Catalysis Dipshikha Banerjee, Andrea K. Buzas, Céline Besnard, and E. Peter Kündig* Department of Organic Chemistry, University of Geneva, 30 Quai Ernest Ansermet, 1211 Geneva 4, Switzerland S Supporting Information *

ABSTRACT: Nine new N-heterocyclic carbene gold chloride complexes (10−18) were synthesized starting from bulky chiral imidazolium salts (1−9) developed in this laboratory. Full characterization of all complexes, including the X-ray structures of gold(I)(1,3-bis((S)-1-(2-methoxyphenyl)-2,2-dimethylpropyl)-1H-imidazol-2(3H)-ylidene) chloride (13) and gold(I)(1,3-bis((S)-2,2-dimethyl-1-(naphthalen-1-yl)propyl)-1H-imidazol-2(3H)-ylidene) chloride (16), is reported. The complexes 10−18 were applied in the methoxycyclization of 1,6-enynes using AgNTf2 as an additive. Synthesis of the N-heterocyclic carbene gold triflimidate (19) was achieved by treating complex 12 with AgNTf2. The complex gold(I)(1,3-bis((R)-1-(2-methoxyphenyl)-2,2-dimethylpropyl)imidazolidin-2-ylidene)(1,1,1-trifluoro-N-(trifluoromethylsulfonyl)methylsulfonamido) (19) was isolated and spectroscopically and structurally (X-ray) characterized.

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INTRODUCTION

of chiral NHC−Au(I) complexes in asymmetric catalysis has been recently reviewed.23 In recent years, our laboratory has developed NHC ligands with chiral N-substituents incorporating bulky alkyl groups and o-substituted aryl rings at the two stereogenic centers (Scheme 1). Among these, ligand precursor 1 gave the best result in the Pd-catalyzed intramolecular α-arylation of amides to give 3,3disubstituted chiral oxindoles,24,25 whereas 5 was best for the synthesis of 3-alkoxy-3-aryl or 3-amino-3-aryl chiral oxindoles.26 More recently, excellent results were obtained using ligand precursor 7 in Pd-catalyzed asymmetric coupling of the C−H bond of an unactivated methylene group, leading to the formation of highly enantioenriched 2-substituted and 2,3disubstituted indolines.27,28 Monodentate ligands were required in this reaction, and the successful application bodes well for Au(I) catalysis. Our hypothesis that the appropriate chiral environment at the reaction center originates from the minimization of allylic strain, which sets the stereocontrol elements of the catalyst in place and brings about high asymmetric induction, was confirmed by the analysis of the solid-state structure of Pd−NHC29,30 and NHC−BH331 complexes. Moreover, highly thermally stable Pd−NHC systems significantly extend the range of successful applications in catalysis.25,27,28,32,33 To have a broader window for these ligands in homogeneous asymmetric catalysis, we explored their scope in asymmetric gold catalysis. In this article, we describe the synthesis of new Au(I) complexes incorporating chiral N-

Homogeneous catalysis by Au(I) complexes has emerged as a powerful synthetic tool for organic synthesis.1−4 The principle of catalysis in the presence of Au(I) relies on the activation of unsaturated bonds, such as carbonyl, imine, and carbon−carbon multiple bonds. Nucleophilic attack is enabled by the soft Lewis acidic cationic gold complexes.1−12 Pioneering work was carried out by Ito, Sawamura, and Hayashi, who reported an asymmetric aldol reaction catalyzed by an in situ generated chiral Au(I) complex with high diastereo- and enantioselectivity.13 This established the first example of an asymmetric goldcatalyzed transformation. Since then, asymmetric gold-catalyzed reactions have been applied to a variety of transformations that provide versatile routes leading to the enantioselective formation of new carbon−carbon or carbon−heteroatom bonds.9,13−16 Au(I)L complexes adopt a linear geometry.14,15 Chiral bidentate ligands, so successful in transition-metal asymmetric catalysis, are thus not suitable for Au(I) catalysis. Moreover, the linear coordination mode of a Au(I) complex places the chiral information carried by the neutral two-electron donor ligand trans to the potential reacting center. Chiral induction elements are, therefore, far from the place where they are needed in the preferred transition state. Nevertheless, different strategies, such as gold complexes of chiral phosphine and phosphoramidite ligands or complexes with chiral counterions, have been developed to bring impressive levels of enantioselectivity for a number of reactions.9,14−16 Successful applications of chiral Au−NHC(I) complexes are still scarce, however. Studies concerning application of chiral Au−NHC(I) catalysts include desymmetrization of diynamide,17,18 cyclization of enynes,19−21 and diboronation of alkenes.22 Application © 2012 American Chemical Society

Received: October 3, 2012 Published: November 5, 2012 8348

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Scheme 1. Bulky Chiral NHC Ligands Developed in Our Laboratory

Scheme 2. Synthesis of Chiral NHC−Silver Complexes

Scheme 3. Synthesis of Chiral NHC−Gold Complexes

and NMR analysis of the NHC silver halide complex turned out to be inconvenient due to the formation of a mixture of silver mono- and bis-carbene complexes (Scheme 2).35,36 For this reason, the one-pot synthesis of NHC−gold chlorides was attempted. Treatment of the imidazolium salt with silver oxide in CH2Cl2 at r.t. generated the NHC−Ag intermediate from which NHC was transmetalled to Au by adding [Au(Cl)(SMe2)], leading to the formation of the desired Au(Cl)(NHC) complex.37,38 Filtration over a short pad of Celite, followed by recrystallization from CH2Cl2/pentane, afforded the complexes as off-white or yellow solids. This method proved to be quite general, and a large number of Au−

heterocyclic carbene ligands and their application in methoxycyclization of 1,6-enynes.

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RESULTS AND DISCUSSION

Synthesis of Chiral NHC−Gold Complexes. We first attempted to synthesize Au−NHC complexes by trapping the free carbene, generated from the parent imidazolium salts by different bases (such as tBuLi, tBuOK, NaH), with [Au(Cl)(SMe2)].34 Results obtained were not satisfactory in terms of yields of the Au−NHC complexes. We, therefore, opted for an alternative protocol where an equimolar amount of [Au(Cl)(SMe2)] is reacted with a Ag(I)−NHC complex.34 Isolation 8349

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NHC complexes were synthesized in excellent yields from their corresponding imidazolium salts (Scheme 3). Complexes 10−18 are stable in the presence of air and moisture and can be stored in a refrigerator in closed vessels for extended periods without decomposition. The complexes were characterized by their 1H and 13C NMR, IR, and ESI-MS spectra. NMR Spectroscopic Studies. In the 1H NMR spectra of complexes 10−18, the characteristic signal for the protons at the stereogenic centers resonates in the range of 6.94−5.78 ppm. In the cases of complexes 10, 12, and 17 derived from the dihydroimidazolium salts, the heterocyclic ring protons appeared between 3.76 and 3.16 ppm as two sets of multiplets. On the other hand, the two olefinic imidazole protons of complexes 11, 13−16, and 18 resonate in the range of 7.47− 6.88 ppm. The singlet corresponding to the tBu group in complexes 10−13, and 15−18 appeared between 1.14 and 1.01 ppm. The iPr group in complex 14 appeared as a doublet at 0.89 and 0.81 ppm. The diagnostic signal for such types of complexes in the 13C NMR spectra is the carbene carbon atom bound to the metal gold. Complexes 11, 13−16,and 18 exhibit 13C NMR resonances for the carbene carbon at 171.9−176.4 ppm, whereas complexes 10, 12, and 17 with a saturated NHC backbone display downfield-shifted resonances at 199.8−197.3 ppm.34 Furthermore, by analyzing the proton carbon correlation spectra, that is, HMBC, a prominent 3J(C,H) coupling between the carbene carbon and the imidazole ring protons and the protons at the stereogenic centers was observed. In all the cases, the Ccarbene resonances are shifted markedly downfield when compared with those of the imidazolium precursors (Table 1).38

Crystals of complexes 13 and 16 suitable for X-ray diffraction analysis were obtained by slow diffusion of n-hexane into a solution of the complexes in CH2Cl2 (Figure 1 and Table 2). Table 2. Characteristic Bond Lengths and Bond Angles of (S,S) 13 and (S,S) 16

1 2 3 4 5 6 7 8 9 a

10 11 12 13 14 15 16 17 18

Au−CNHC (Å)

Au−Cl (Å)

CNHC−Au−Cl (deg)

(S,S) 13 (S,S) 16

2.024(9) 1.985(4)

2.301(3) 2.278(1)

176.6(3) 179.6(1)

The bond lengths of Au−CNHC and Au−Cl perfectly correlate with those of the literature known NHC−gold chloride complexes, and the two ligands are coordinated in a linear gometry.38 We previously pointed out the role of minimization of allylic strain in setting the stereocontrol elements in place in the NHC−Pd and NHC−borane complexes.29−31 The same structural features were observed in the Au(Cl)(NHC) complexes. Thus, the smallest substituent (H) at the stereogenic center is coplanar with the Au−CNHC bond. This fixes the stereogenic center. The aryl plane is fixed by an alignment of the aryl o-substituent (o-OMe group in the case of complex 13 and the second aromatic ring of the naphthyl moiety in the case of complex 16) with the C−H bond of the stereogenic center. This fixes the aryl plane in space.29−31 Isolation of Chiral NHC Gold(I) Triflimidate. Part of this report concerns application of Au(Cl)(NHC) complexes in asymmetric catalysis, which will be discussed at a later stage. We have chosen a bis-(trifluoromethanesulfonyl)imidate moiety (NTf2−) as a labile ligand in this regard. The gold triflimidate complex (R,R) 19 was isolated as an off-white solid by treating complex (R,R) 12 with AgNTf2 in CH2Cl2 at r.t., as shown in Scheme 4.

Table 1. Chemical Shift of Carbene Carbon in 13C NMR Spectra in CDCl3 entry complex

Au(Cl)(NHC)

Scheme 4. Synthesis of Chiral Au(NTf2)(NHC) Complex

δC (Au(Cl)(NHC)) (ppm)

δC (NHC·HI) (ppm)

ΔC (ppm)

197.3 174.7 199.6 176.2 171.3 174.7 175.5 199.8 176.4

159.3 137.7 158.9 138.1 136.8 136.1 138.4 159.2 138.6

38.0 37.0 40.7 38.1 34.5 38.6 37.1 40.6 37.8

a

Triflimidate 19 was characterized by NMR, IR, and ESI-MS spectra. In agreement with literature precedence, the 13C NMR spectrum of the C atom of the NHC ligand in the Au−Cl complex 12 experienced a downfield shift in Au−NTf2 complex 19 from 199.6 to 193.6 ppm.39 Crystals of complex 19 suitable

ΔC = δC (Au(Cl)(NHC)) − δC (NHC·HI).

Figure 1. X-ray structure of Au(Cl)(NHC) complexes (a) (S,S) 13 and (b) (S,S) 16. 8350

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for X-ray analysis were obtained by slow diffusion of n-hexane into a solution of the complex in CH2Cl2 (Figure 2).

Table 3. Methoxycyclization of Enyne 20:

entry

cat.

time (h)

yield (%)

ee (%)

1 2 3 4 5 6 7 8 9

(S,S) 10 (S,S) 11 (R,R) 12 (S,S) 13 (R,R) 14 (R,R) 15 (S,S) 16 (R,R) 17 (R,R) 18

48 24 72 24 24 24 24 36 24

98 96 92 90 85 88 98 94 90

−59 −54 59 −67 7 17 −43 64 72

enantioselectivities in the range of 54−59% (Table 3, entries 1−3). Asymmetric induction increased to 67% when complex 13 was used (Table 3, entry 4). Complex 14, which has a less bulky iPr group instead of a tBu group at the stereogenic center and thus lacks rigidity in conformation, afforded the product with only 7% ee (Table 3, entry 5). This again proved the importance of bulky groups at the ligand’s stereogenic center.29 Complex 15 with a β-naphthyl substituent gave cyclopentane 21 in 17% ee, whereas 21 was obtained in 43% enantiopurity when complex 16 with an α-naphthyl substituent was used (Table 3, entries 6 and 7). Complex 17 rendered the product 21 in 64% ee (Table 3, entry 8), and the highest ee of 72% was obtained when catalyst 18 was employed (Table 3, entry 9). Generally, the chiral gold complexes derived from the dihydroimidazolium salts were less reactive compared to their unsaturated analogues (Table 3, entry 1 vs 2, entry 3 vs 4, and entry 8 vs 9). Chiral gold complexes with an unsaturated NHC backbone delivered the cyclopentane product in higher enantioselectivity compared to their saturated analogues (Table 3, entry 3 vs 4 and entry 8 vs 9) with one exception in the case of complexes 10 and 11.

Figure 2. X-ray structure of Au(NTf2)(NHC) complex (R,R) 19.

The Au−CNHC bond length was found to be 1.937(1) Å, slightly shorter than that in complex 13 (2.024(9) Å) and complex 16 (1.985(4) Å), and the Au−NNTf2 (2.140(1) Å) bond is also slightly shorter than the Au−Cl bond in complex 13 (2.301(3) Å) and complex 16 (2.278(1) Å).38,39 Linear coordination at Au(I) is reflected in the bond angle CNHC−Au− NNTf2 = 180.0(13). These data perfectly correlate with the literature known NHC−gold triflimidates.39 In complex 19, minimization of the allylic strain forces coplanarity of the hydrogen atoms at the stereogenic centers with the CNHC−Au bond. Furthermore, o-OMe groups on the phenyl rings become coplanar with the C(stereogenic center)−H bond.29−31 Chiral NHC−Gold Catalyzed Asymmetric Cyclization of 1,6-Enynes. To benchmark these new chiral gold complexes 10−18, intramolecular methoxycylization of 1,6enyne 20 in methanol was chosen as a model reaction.11 Treatment of the Au(Cl)(NHC) complexes with AgNTf2 generated in situ the active cationic NHC−gold species Au(NHC)(NTf2). The choice of counterion NTf2− is justified by the fact that the corresponding cationic complexes exhibit enhanced alkynophilicity in polar solvents, such as methanol.39,40 AgNTf2 was added at r.t. to a solution of Au(Cl)(NHC) in MeOH. After stirring the mixture for 5 min, the enyne 20 was added. Activation of the triple bond, followed by the intramolecular cyclization and nucleophilic attack by MeOH, resulted in the cyclized product 21. Using 2 mol % catalyst in MeOH at r.t., only traces of product formation was observed after 6−7 days of reaction. Increasing the catalyst loading to 10 mol % also did not afford satisfactory conversion of the starting enyne 20. This was due to the poor solubility of 20 in MeOH. After thorough studies, we found that the use of “Au(Cl)(NHC) (5 mol %), AgNTf2 (5 mol %), MeOH (5 equiv) in CH2Cl2 at r.t.” was optimal for this reaction. The previously mentioned chiral NHC gold complexes (10−18) were tested under these conditions (Table 3). In all cases, the cyclopentane 21 was obtained in high yields (Table 3, entries 1−9). Complexes 10−12 produced 21 with

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CONCLUSION The synthesis of a series of new chiral gold NHC complexes, their structural and spectroscopic characteristics, and an application in asymmetric methoxycyclization of 1,6-enynes are reported. The enantioselectivities obtained are moderate, although they represent the best results obtained in an enantioselective transformation by using a chiral gold NHC complex in this reaction. Crystal structures of the NHC gold chlorides as well as NHC gold triflimidate complexes demonstrated the importance of the avoidance of allylic strain in the arrangement of the ligands’ substituents in space. Inspection of the X-ray structure suggests that ligands with more extended aryl groups at the stereogenic centers may lead to more selective catalyst systems.

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EXPERIMENTAL DETAILS

General Methods. Experiments were carried out under a nitrogen atmosphere using standard Schlenk techniques. Solvents were dried with a Solvtek system (filtration over alumina). Methanol was dried over magnesium. All commercially available compounds were used as received. Deuterated solvents were obtained from Cambridge Isotope Laboratories. NMR spectra: Bruker AMX 500 and AMX 400 spectrometers at ambient probe temperature. IR spectra: PerkinElmer Spectrum One spectrometer. Melting points: Büchi 510 apparatus. 8351

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Carom), 129.3 (s, Carom), 128.7 (s, Carom), 126.6 (s, Carom), 119.9 (s, Carom), 119.4 (s, Carom), 111.8 (s, Colefinic), 65.5 (s, CH), 56.0 (s, OCH3), 36.2 (s, C(CH3)3), 28.2 (s, C(CH3)3). IR (neat, cm−1): 2961, 1601, 1490, 1461, 1426, 1368, 1290, 1241, 1162, 1109, 1028, 915, 854, 734. HRMS (ESI): calcd. for C27H36N2O2Au ([M − Cl]+), 617.2436; found, 617.2426. Gold(I)(1,3-bis((R)-1-(2-methoxyphenyl)-2-methylpropyl)imidazolidin-2-ylidene) Chloride (14). Following the general procedure and using 8 (52 mg, 0.1 mmol, 1 equiv), Ag2O (11.6 mg, 0.05 mmol, 0.5 equiv), and [Au(Cl)(SMe2)] (29.7 mg, 0.1 mmol, 1 equiv), complex 14 was obtained as a white solid in 96% yield (60 1 mg). mp 276−278 °C. [α]25 D = −116.7 (c = 0.43 in CH2Cl2). H NMR (400 MHz, CDCl3): δ 7.49 (d, J = 7.1 Hz, 2H, CHarom), 7.30−7.28 (m, 2H, CHarom), 6.97−6.95 (m, 4H, CHarom + CHolefinic), 6.86 (d, J = 8.2 Hz, 2H, CHarom), 5.78 (d, J = 11.4 Hz, 2H, CH), 3.83 (s, 6H, OCH3), 2.71 (sept., J = 11.4, 6.4 Hz, 2H, CH(CH3)2), 0.89 (d, J = 6.4 Hz, 6H, CH(CH3)2), 0.81 (d, J = 6.4 Hz, 6H, CH(CH3)2). 13C NMR (100 MHz, CDCl3): δ 171.7 (s, NCN), 157.7 (s, Carom), 129.6 (s, Carom), 128.4 (s, Carom), 126.5 (s, Carom), 120.5 (s, Carom), 118.4 (s, Carom), 111.1 (s, Colefinic), 66.3 (s, CH), 55.2 (s, OCH3), 30.9 (s, CH(CH3)2), 20.1 (s, CH(CH3)2), 20.0 (s, CH(CH3)2). IR (neat, cm−1): 2969, 1602, 1493, 1465, 1245, 1172, 1050, 1030, 749. HRMS (ESI): calcd. for C25H32N2O2Au ([M − Cl]+), 589.2123; found, 589.2130. Gold(I)(1,3-bis((R)-2,2-dimethyl-1-(naphthalen-2-yl)propyl)1H-imidazol-2(3H)-ylidene) Chloride (15). Following the general procedure and using 6 (59 mg, 0.1 mmol, 1 equiv), Ag2O (11.6 mg, 0.05 mmol, 0.5 equiv), and [Au(Cl)(SMe2)] (29.7 mg, 0.1 mmol, 1 equiv), complex 15 was obtained as a pale orange solid in quant. yield (69 mg). mp 167−170 °C (dec.). [α]25 D = −141.8 (c = 0.53 in CH2Cl2). 1H NMR (500 MHz, CDCl3): δ 7.96 (bs, 2H, CHarom), 7.90−7.83 (m, 6H, CHarom), 7.58 (dd, J = 1.4, 6.8 Hz, 2H, CHarom), 7.54−7.51 (m, 4H, CHarom), 7.47 (s, 2H, CHolefinic), 6.14 (s, 2H, CH), 1.07 (s, 18H, C(CH3)3). 13C NMR (125 MHz, CDCl3): δ 174.7 (s, NCN), 134.8 (s, Carom), 133.2 (s, Carom), 133.0 (s, Carom), 128.8 (s, Carom), 128.6 (s, Carom), 128.4 (s, Carom), 127.7 (s, Carom), 126.8 (s, Carom), 126.7 (s, Carom), 126.4 (s, Carom), 119.8 (s, Colefinic), 75.5 (s, CH), 36.3 (s, C(CH3)3), 28.4 (s, C(CH3)3). IR (neat, cm−1): 2961, 1599, 1058, 1475, 1424, 1402, 1370, 1224, 1171, 1126, 858, 740. HRMS (ESI): calcd. for C33H36N2Au ([M − Cl]+), 657.2538; found, 657.2543. Gold(I)(1,3-bis((S)-2,2-dimethyl-1-(naphthalen-1-yl)propyl)1H-imidazol-2(3H)-ylidene) Chloride (16). Following the general procedure and using 7 (59 mg, 0.1 mmol, 1 equiv), Ag2O (11.6 mg, 0.05 mmol, 0.5 equiv), and [Au(Cl)(SMe2)] (29.7 mg, 0.1 mmol, 1 equiv), complex 16 was obtained as a white solid in 85% yield (59 1 mg). mp 270 °C (dec.). [α]25 D = +35.7 (c = 0.5 in CH2Cl2). H NMR (400 MHz, CDCl3): δ 8.79 (d, 2H, J = 8.7 Hz, CHarom), 7.88−7.82 (m, 6H, CHarom), 7.69 (t, 2H, J = 7.8 Hz, CHarom), 7.54 (t, 2H, J = 7.5 Hz, CHarom), 7.48 (t, 2H, J = 7.8 Hz, CHarom), 7.21 (s, 2H, CHolefinic), 6.94 (s, 2H, CH), 1.10 (s, 18H, C(CH3)3). 13C NMR (100 MHz, CDCl3): δ 175.5 (s, NCN), 134.3 (s, Carom), 133.6 (s, Carom), 132.3 (s, Carom), 129.5 (s, Carom), 129.1 (s, Carom), 127.7 (s, Carom), 126.4 (s, Carom), 125.8 (s, Carom), 124.4 (s, Carom), 124.2 (s, Carom), 119.8 (s, Colefinic), 67.5 (s, CH), 37.0 (s, C(CH3)3), 28.9 (s, C(CH3)3). IR (neat, cm−1): 2962, 1599, 1512, 1475, 1404, 1368, 1219, 1191, 1162, 1114, 1031, 771. HRMS (ESI): calcd. for C33H36N2Au ([M − Cl]+), 657.2431; found, 657.2427. Gold(I)(1,3-bis((R)-1-(4-methoxybiphenyl-3-yl)-2,2dimethylpropyl)imidazolidin-2-ylidene) Chloride (17). Following the general procedure and using 5 (105 mg, 0.15 mmol, 1 equiv), Ag2O (17.4 mg, 0.075 mmol, 0.5 equiv), and [Au(Cl)(SMe2)] (44.5 mg, 0.15 mmol, 1 equiv), complex 17 was obtained as an orange yellow solid in 87% yield (105 mg). mp 115 °C (dec.). [α]25 D = +111.3 (c = 0.6 in CH2Cl2). 1H NMR (400 MHz, CDCl3): δ 7.64 (d, J = 1.8 Hz, 2H, CHarom), 7.54−7.51 (m, 5H, CHarom), 7.45−7.42 (m, 4H, CHarom), 7.34−7.31 (m, 3H, CHarom), 7.02 (d, J = 6.9 Hz, 2H, CHolefinic), 6.41 (s, 2H, CH), 3.95 (s, 6H, OCH3), 3.74−3.71 (m, 2H, CH2), 3.35−3.32 (m, 2H, CH2), 1.14 (s, 18H, C(CH3)3). 13C NMR (100 MHz, CDCl3): δ 199.8 (s, NCN), 157.8 (s, Carom), 141.0 (s, Carom), 132.6 (s, Carom), 129.0 (s, Carom), 128.3 (s, Carom), 127.8 (s,

[α]D values were recorded on a Perkin-Elmer 241 polarimeter using a quartz cell (l = 10 cm) and a Na high-pressure lamp (λ = 589 nm). General Procedure for the Synthesis of Au(Cl)(NHC) Complexes 10−18. To a solution of the imidazolium iodide (0.1 mmol, 1 equiv) in CH2Cl2 (3 mL) was added Ag2O (0.05 mmol, 0.5 equiv), and the mixture was stirred at r.t. in the absence of light. The suspension became clear; that is, no more solid Ag2O was observed after stirring for 5−6 h. A solution of [Au(Cl)(SMe2)] (0.1 mmol, 1 equiv) in CH2Cl2 (1 mL) was then added dropwise. The resulted reaction mixture was stirred overnight, and then the solution was filtered through a short pad of Celite and washed with CH2Cl2. The filtrate was partially evaporated, and addition of excess pentane resulted in the precipitation of the complexes as off-white or yellow solids. Gold(I)(1,3-bis((S)-2,2-dimethyl-1-o-tolylpropyl)imidazolidin-2-ylidene) Chloride (10). Following the general procedure and using 3 (260 mg, 0.5 mmol, 1 equiv), Ag2O (58 mg, 0.25 mmol, 0.5 equiv), and [Au(Cl)(SMe2)] (150 mg, 0.5 mmol, 1 equiv), complex 10 was obtained as an off-white solid in 83% yield (260 mg). mp 85−87 °C (dec.). [α]25 D = −67.8 (c = 0.84 in CH2Cl2). 1 H NMR (400 MHz, CDCl3): δ 7.46−7.44 (d, J = 7.6 Hz, 2H, CHarom), 7.28−7.17 (m, 6H, CHarom), 5.96 (s, 2H, CH), 3.77−3.66 (m, 2H, CH2), 3.30−3.21 (m, 2H, CH2), 2.64 (s, 6H, CH3), 1.10 (s, 18H, C(CH3)3). 13C NMR (100 MHz, CDCl3): δ 197.3 (s, NCN), 138.9 (s, Carom), 134.3 (s, Carom), 132.0 (s, Carom), 128.6 (s, Carom), 128.1 (s, Carom), 125.3 (s, Colefinic), 67.4 (s, CH), 47.9 (s, CH2), 37.3 (s, C(CH3)3), 29.1 (s, C(CH3)3), 22.4 (s, CH3). IR (neat, cm−1): 2958, 1162, 1478, 1444, 1366, 1262, 1229, 1080, 1030, 790. HRMS (ESI): calcd. for C27H38N2Au ([M − Cl]+), 587.2695; found, 587.2678. Gold(I)(1,3-bis((S)-2,2-dimethyl-1-o-tolylpropyl)-1H-imidazol-2(3H)-ylidene) Chloride (11). Following the general procedure and using 1 (52 mg, 0.1 mmol, 1 equiv), Ag2O (11.6 mg, 0.05 mmol, 0.5 equiv), and [Au(Cl)(SMe2)] (29.7 mg, 0.1 mmol, 1 equiv), complex 11 was obtained as pale yellow solid in quant. yield (62 mg). 1 mp 244 °C. [α]25 D = −92.3 (c = 0.5 in CH2Cl2). H NMR (400 MHz, CDCl3): δ 7.57−7.55 (m, CHarom), 7.32 (s, 2H, CHolefinic), 7.23−7.19 (m, 6H, CHarom), 6.28 (s, 2H, CH), 2.69 (s, 6H, CH3), 1.06 (s, 18H, C(CH3)3). 13C NMR (100 MHz, CDCl3): δ 174.7 (s, NCN), 138.5 (s, Carom), 136.0 (s, Carom), 131.9 (s, Carom), 128.3 (s, Carom), 127.7 (s, Carom), 125.9 (s, Carom), 119.7 (s, Colefinic), 68.9 (s, CH), 37.0 (s, C(CH3)3), 28.5 (s, C(CH3)3), 22.2 (s, CH3). IR (neat, cm−1): 3142, 2963, 1478, 1424, 1260, 1169, 1549, 1089, 1016, 850, 797. HRMS (ESI): calcd. for C27H36N2Au ([M − Cl]+), 585.2538; found, 585.2523. Gold(I)(1,3-bis((R)-1-(2-methoxyphenyl)-2,2dimethylpropyl)imidazolidin-2-ylidene) Chloride (12). Following the general procedure and using 4 (110 mg, 0.2 mmol, 1 equiv), Ag2O (23.2 mg, 0.1 mmol, 0.5 equiv), and [Au(Cl)(SMe2)] (59.3 mg, 0.2 mmol, 1 equiv), complex 12 was obtained as pale yellow solid in 92% yield (120.5 mg). mp 270 °C (dec.). [α]25 D = +26.9 (c = 0.51 in CH2Cl2). 1H NMR (400 MHz, CDCl3): δ 7.43 (d, 2H, J = 7.2 Hz, CHarom), 7.32−7.28 (m, 2H, CHarom), 6.95−6.89 (m, 4H, CHarom), 6.33 (s, 2H, CH), 3.89 (s, 6H, OCH3), 3.70−3.59 (m, 2H, CH2), 3.29−3.16 (m, 2H, CH2), 1.08 (s, 18H, C(CH3)3). 13C NMR (100 MHz, CDCl3): δ 199.6 (s, NCN), 158.2 (s, Carom), 129.4 (s, Carom), 129.1 (s, Carom), 125.2 (s, Carom), 119.4 (s, Carom), 111.6 (s, Carom), 63.6 (s, CH), 55.8 (s, OCH3), 47.6 (s, CH2), 36.4 (s, C(CH3)3), 28.8 (s, C(CH3)3). IR (neat, cm−1): 2957, 1661, 1600, 1489, 1460, 1438, 1263, 1238, 1107, 10256, 785. HRMS (ESI): calcd. for C27H38N2O2Au ([M − Cl]+), 619.2593; found, 619.2602. Gold(I)(1,3-bis((S)-1-(2-methoxyphenyl)-2,2-dimethylpropyl)-1H-imidazol-2(3H)-ylidene) Chloride (13). Following the general procedure and using 2 (46.8 mg, 0.085 mmol, 1 equiv), Ag2O (9.9 mg, 0.043 mmol, 0.5 equiv), and [Au(Cl)(SMe2)] (25.3 mg, 0.085 mmol, 1 equiv), complex 13 was obtained as an off-white solid in 98% yield (54.4 mg). mp 180−182 °C. [α]25 D = −63.3 (c = 1.03 in CH2Cl2). 1H NMR (500 MHz, CDCl3): δ 7.42 (dd, J = 1.2, 6.4 Hz, 2H), 7.28−7.24 (m, 4H, CHarom), 6.91−6.88 (m, 4H, CHarom + CHolefinic), 6.63 (s, 2H, CH), 3.88 (s, 6H, OCH3), 1.01 (s, 18H, C(CH3)3). 13C NMR (125 MHz, CDCl3): δ 176.2 (s, NCN), 157.8 (s, 8352

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Notes

Carom), 127.0 (s, Carom), 126.9 (s, Carom), 125.5 (s, Carom), 111.9 (s, Carom), 63.6 (s, CH), 56.0 (s, OCH3), 47.7 (s, CH2), 36.4 (s, C(CH3)3), 28.9 (s, C(CH3)3). IR (neat, cm−1): 2953, 1668, 1486, 1241, 1023, 819, 788, 772, 664. HRMS (ESI): calcd. for C39H49N3O2Au ([(M − Cl) + NH3]+), 788.3484; found, 788.3495. Gold(I)(1,3-bis((R)-1-(4-methoxybiphenyl-3-yl)-2,2-dimethylpropyl)-2,3-dihydro-1H-imidazol-2-yl) Chloride (18). Following the general procedure and using 9 (70 mg, 0.1 mmol, 1 equiv), Ag2O (11.6 mg, 0.05 mmol, 0.5 equiv), and [Au(Cl)(SMe2)] (29.7 mg, 0.1 mmol, 1 equiv), complex 18 was obtained as an orange yellow solid in 92% yield (74 mg). mp 115−119 °C (dec.). [α]25 D = +148.5 (c = 1.175 in CH2Cl2). 1H NMR (400 MHz, CDCl3): δ 7.63 (s, 2H, CHarom), 7.51−7.42 (m, 10H, CHarom), 7.35−7.32 (m, 4H, CHarom), 7.00 (d, J = 8.4, 2H, CHolefinic), 6.71 (s, 2H, CH), 3.96 (s, 6H, OCH3), 1.08 (s, 18H, C(CH3)3). 13C NMR (100 MHz, CDCl3): δ 176.4 (s, NCN), 157.3 (s, Carom), 141.0 (s, Carom), 133.1 (s, Carom), 129.0 (s, Carom), 128.1 (s, Carom), 127.5 (s, Carom), 127.0 (s, Carom), 126.9 (s, Carom), 119.5 (s, Carom), 112.1 (s, Colefinic), 65.5 (s, CH), 56.3 (s, OCH3), 36.3 (s, C(CH3)3), 28.3(s, C(CH3)3). IR (neat, cm−1): 2964, 1609, 1484, 1250, 1023, 880, 820, 732, 697. HRMS (ESI): calcd. for C39H47N3O2Au ([(M − Cl) + NH3]+), 786.3328; found, 786.3339. Gold(I)(1,3-bis((R)-1-(2-methoxyphenyl)-2,2dimethylpropyl)imidazolidin-2-ylidene)(1,1,1-trifluoro-N(trifluoromethylsulfonyl)methylsulfonamido) (19). To a solution of (R,R) 12 (72 mg, 0.11 mmol, 1 equiv) in CH2Cl2 (1.6 mL) was added AgNTf2 (42.7 mg, 0.1 mmol, 1 equiv). The solution was stirred for 30 min in the absence of light and then filtered over a short pad of Celite impregnated with CH2Cl2. The filtrate was concentrated under vacuum. The crude material was then crystallized from CH2Cl2/ pentane to get the desired complex as a pale yellow solid in 80% yield (79 mg). mp 167−170 °C (dec.). [α]25 D = −1.7 (c = 0.63 in CH2Cl2). 1 H NMR (400 MHz, CDCl3): δ 7.48 (dd, 1H, J = 1.5, 7.7 Hz), 7.35− 7.30 (m, 2H), 6.98−6.92 (m, 4H), 6.14 (s, 2H), 3.86 (s, 6H), 3.68− 3.59 (m, 2H), 3.27−3.14 (m, 2H), 1.10 (s, 18H). 13C NMR (100 MHz, CDCl3): δ 193.6 (s, NCN), 158.4 (s, Carom), 129.6 (s, Carom), 129.5 (s, Carom), 124.2 (s, Carom), 119.5 (s, Carom), 111.6 (s, Carom), 64.2 (s, CH), 55.5 (s, OCH3), 47.7 (s, CH2), 36.5 (s, C(CH3)3), 28.7 (s, C(CH3)3). IR (neat, cm−1): 2962, 1602, 1491, 1453, 1396, 1243, 1187, 1134, 1023, 796, 754. HRMS (ESI): calcd. for C27H38N2O2Au ([M − C2F6NO4S2]+), 619.2593; found, 619.2565. General Method for the Methoxycyclization of 1,6-enynes 20. To a solution of the corresponding Au(Cl)(NHC) complex (0.05 equiv) in CH2Cl2 (2 mL) was added AgNTf2 (0.05 equiv), and the mixture was stirred for 15 min at r.t. in the absence of light. MeOH (5 equiv) was added, followed by the addition of the substrate 1,6-enyne 20 (1 equiv), and the mixture was stirred at r.t. for the time indicated in Table 3. After the reaction was finished, the mixture was filtered over a plug of Celite and washed with CH2Cl2, and the solvents were removed under vacuum. The crude material was purified by f.c. on silica gel using cyclohexane/AcOEt (80/20 to 70/30) as eluent to give 21; spectroscopic analyses were in agreement with a previous literature report.41 1H NMR (400 MHz, CDCl3): δ 8.14−8.09 (m, 4H), 7.74− 7.67 (m, 2H), 7.63−7.59 (m, 2H), 7.57−7.53 (m, 2H), 7.34−7.31 (m, 4H), 7.24−7.20 (m, 1H), 6.41 (s, 1H), 3.77 (dd, J = 8.2, 6.6 Hz, 1H), 3.69 (ddd, J = 15.9, 3.0, 1.8 Hz, 1H), 2.94 (s, 3H), 2.89 (d, J = 6.2 Hz, 1H), 2.82 (dd, J = 8.8, 1.5 Hz, 1H), 2.53 (d, J = 15.7 Hz, 1H), 0.89 (s, 3H), 0.80 (s, 3H).

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the University of Geneva and the Swiss National Science Foundation for financial support.

ASSOCIATED CONTENT

S Supporting Information *

1 H and 13C NMR spectra of complexes 10−18 and 19 and the CIF files giving X-ray crystal structures of 13, 16, and 19. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

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*E-mail: peter.Kü[email protected]. 8353

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