Carboxylic Group-Assisted Proton Transfer in Gold-Mediated

Article pubs.acs.org/Organometallics

Carboxylic Group-Assisted Proton Transfer in Gold-Mediated Thiolation of Alkynes Sergey S. Zalesskiy,† Victor N. Khrustalev,‡ Alexandr Yu. Kostukovich,‡ and Valentine P. Ananikov*,† †

N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, Leninsky Prospect, 47, Moscow, 119991, Russia A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilov Street, 28, Moscow, 119991, Russia

‡

S Supporting Information *

ABSTRACT: Combined experimental and theoretical studies revealed a complex mechanistic picture in which the carboxylic group-assisted proton transfer from acetic acid to an alkyne molecule is the key step in the unique gold-mediated alkyne transformation that leads to the formation of gem-disubstituted vinyl gold complexes. The structures of the complexes were unambiguously established using NMR spectroscopy (in solution) and X-ray diffraction (in the solid state). ESI-MS study of the reaction mixture revealed multiple gold-containing complexes and clusters. Investigation of the MS2 fragmentation patterns of the selected ions suggested the involvement of gold acetylides in the transformation. Further treatment of the complexes with protic acid led to the discovery of a novel route for the gold-mediated alkyne hydrothiolation.

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INTRODUCTION In recent decades, gold-catalyzed reactions have become one of the most versatile classes of catalytic transformations.1,2 Considerable progress has been achieved in several types of organic reactions, including nucleophilic additions,3 cycloadditions,4 oxidations,5 nucleophilic substitutions,6 and many other processes.7 Undoubtedly, one of the most prominent directions is reactions involving alkynes.8 The unique affinity of cationic gold complexes to carbon−carbon triple bonds has provided many opportunities for discovering new transformations. Numerous fascinating cyclizations, substitution reactions, and addition reactions have recently been reported.9−13 Equally important is the development of soluble gold species for homogeneous catalysis and the preparation of nanoparticles for heterogeneous catalytic systems.1−14 Although studies of gold-mediated transformations of alkynes have prompted the development of new methods in organic chemistry, the mechanistic picture of these reactions has yet to be elucidated in detail. Recent mechanistic studies have highlighted the important roles of gold carbene (AuCRR′) and gold vinylidene (AuCCRR′) intermediates in metaldriven catalytic cycles.15,16 The AuC species were proposed to play a key role in several important gold-catalyzed synthetic transformations.17 Although these intermediates were hypothesized to be involved in such reactions in many cases, the exact nature of the species was difficult to explore,18 and their presence has not been unambiguously confirmed. The first example of the successful preparation of a stable gold allenylidene was reported by Hashmi et al.19 The prepared complex was stabilized with a bulky NHC ligand attached to the gold (Scheme 1). Dynamic NMR studies determined that © 2015 American Chemical Society

Scheme 1. Preparation of a Stable Gold Allenylidene Complex19

the rotational barrier around the amide bond was approximately 69 kJ/mol, indicating that the gold-propargyl cation resonance structure provided a significant contribution. Detailed theoretical investigations revealed that a decrease in ligand π-acidity results in an increase in gold-to-carbon backdonation, leading to a shortage of Au−C bonds and thus making the allenylidene structure more favorable compared to the propargyl cation.20 The synthetic application of gold allenylidenes was further explored.21 In addition to the considerable attention given to gold vinylidenes as potential reactive intermediates in gold-mediated reactions, gold acetylenides have also recently been proposed to be involved in such transformations.22,23 Because of the possible interconversion of vinylidene and acetylenide, clearly distinguishing between these two species is difficult. There are two conventional methods for obtaining vinylidenes from alkynes: via the [1,2]- or the [1,3]-hydride shift (Scheme 2).24 If the barrier to [1,3]-rearrangement is accessible, both types of species may be present in the reaction mixture, thus making elucidation of the mechanistic picture difficult. Received: March 13, 2015 Published: July 22, 2015 5214

DOI: 10.1021/acs.organomet.5b00210 Organometallics 2015, 34, 5214−5224

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solvent from the reaction mixture afforded colorless crystals of Ph3PAuSPh. Despite the quantitative formation of the target complex, the need for the initial preparation of Ph3PAuMe notably reduced the overall yield (Scheme 3).31 This result motivated us to investigate an alternative method for preparing Ph3PAuSPh. The transformation of interest was performed via a transmetalation approach, and we found that Pb(SPh)2 was an excellent transmetalation agent because of its robustness and its ease of preparation and handling.32 The reaction of Pb(SPh)2 with Ph3PAuCl proceeded in 5 min upon the addition of lead benzenethiolate to a solution of the gold complex in DCM. The color of the suspension turned from yellow to white due to the formation of insoluble PbCl2. The target complex, Ph3PAuSPh, was obtained in pure crystalline form with quantitative yield after filtration and removal of solvent. The observed chemical shift in the 31P NMR spectrum of the isolated complex (40.1 ppm) was in agreement with published data.33 The structure of the complex in the solid state was confirmed by comparing the crystal cell parameters obtained using X-ray diffraction with literature data.33 Note that the present method allows for the preparation of a gold sulfide complex with quantitative yield in a single step because Pb(SPh)2 can be prepared in situ by combining PhSH and Pb(OAc)2. Reaction with Alkynes. Next, we investigated the reactivity of the synthesized gold complex toward alkynes. Notably, Ph3PAuSPh reacted with methyl propiolate in the presence of acetic acid, leading to the formation of products 1− 3 (Scheme 4).34

Scheme 2. Possible Pathways for Metal-Mediated Alkyne Transformations

In the present study, we elucidated plausible vinylidene/ acetylenide pathways within the scope of the Au-mediated hydrothiolation of alkynes. Combined experimental and theoretical studies were conducted on this fascinating transformation to characterize the intermediate vinyl gold complexes. The developed protocol is of considerable importance because further conversion of the synthesized vinyl gold complexes gives rise to a new possibility for adding thiols to alkynes. The catalytic systems for efficient S−H bond addition to alkynes based on Ni,25 Pd,26 and Rh27 catalysts are known, whereas the potential of Au-based catalysts is poorly studied.28

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RESULTS AND DISCUSSION The high reactivity of intermediate organogold complexes often makes isolating or observing such species a rather difficult task. In this study, we developed a novel approach for trapping these intermediates during reactions with alkynes. Important prerequisites for efficient trapping are the presence of a reactive nucleophile tightly bound to the gold center (SPh) and the formation of an easily detectable group (vinyl) as a product. To achieve this goal, the following sequential steps were performed: preparation of gold thiolate, characterization of the prepared complex, reaction with alkynes, and investigation of the formed products. Synthesis of Gold(I) Thiol Complex. In the first step, we prepared a gold(I) phosphine complex, Ph3PAuSPh, that contains a thiolate group as an internal nucleophile. The previously reported methods for preparing gold-chalcogenide complexes included reacting the gold chloride complex Ph3PAuCl with the crude thiol in the presence of base (Et3N,29 KOH30); however, these methods did not provide satisfactory yields. A straightforward method leading to Ph3PAuSPh involves reacting the Ph3PAuMe complex with an equivalent of PhSH without any additives (Scheme 3).

Scheme 4. Formation of Vinyl Sulfides in the Reaction of Ph3PAuSPh and Methyl Propiolatea

a

See Scheme 5 for gold-containing products and mechanism.

The reaction proceeded smoothly at 70 °C, leading to 100% conversion of alkyne in 3 h. Among the products formed, we found three different vinyl sulfides (Scheme 4). The products were easily detected with NMR, and the stereo/regioselectivity of the addition reaction was unambiguously determined through the analysis of spin−spin couplings. Compounds 1, 2, and 3 were formed in 3%, 18%, and 17% yields, respectively. The remainder of the alkyne (∼60%) was converted into unknown product 4, which presented a doublet at 5.99 ppm with J = 6.3 Hz in the 1H NMR spectrum (Figure 1). To identify the structure, the reaction mixture was separated using dry-column vacuum chromatography.35 Crystals of product 4 suitable for X-ray diffraction experiments were grown from a dichloromethane (DCM) solution at −18 °C. The molecular structure of 4 was established on the basis of X-ray diffraction (Figure 2). To our great surprise, complex 4 represented a rare two-coordinate gold derivative containing a σ-attached vinyl moiety. 36 The PPh 3 unit is linearly [178.44(9)°] coordinated to the gold(I) center. The Au1−P1 [2.2773(7) Å] and Au1−C1 [2.049(3) Å] distances are similar to those observed previously in the related complexes, whereas the vinyl C1C2 bond length [1.357(4) Å] is slightly longer. The main P−Au−C(−S)C−C(O)−O−C backbone of 4

Scheme 3. Preparation of Ph3PAuSPh with the Yields of the Isolated Productsa

a

The numbers in parentheses correspond to the overall yield accounting for the synthesis of the precursors, Ph3PAuMe and Pb(SPh)2, respectively.

Indeed, we observed complete conversion of the starting compound to the target complex within minutes at room temperature. In the 1H NMR spectrum, we observed the disappearance of the doublet at 0.5 ppm and the singlet at 3.4 ppm, corresponding to the methyl group of Ph3PAuMe and the thiol group of PhSH, respectively. Slow evaporation of the 5215


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Figure 1. Selected region from the 1H NMR spectrum of the reaction mixture and signal assignments.

Figure 3. 1H−31P HSQC spectrum for the mixture of complexes 4 and 5.

with n-hexyne, we selected alkynes with different electronic effects of the substituents. Surprisingly, considerably different reactivity was observed for the closest analogue: ethyl propiolate. The overall yield of 1+2+3 was 78%, and only 8% and 2% of 4 and 5, respectively, were formed. Internal alkynes, either with electron-donating (diphenylacetylene) or electronwithdrawing (DMAD) substituents, did not afford the desired products.37 Indeed, an important role of the substituent in the alkyne unit was found, which will be further discussed in the mechanistic study (theoretical calculations below). Next, we investigated whether the Ph3PAuSPh thiolate donor can be prepared in situ from PPh3AuCl, Pb(OAc)2, and PhSH, thereby eliminating the need for its preliminary synthesis. Indeed, we observed that the transformation of methyl propiolate can occur in one step starting from a mixture of Pb(OAc)2/PhSH/Ph3PAuCl, yielding the same set of products with the same ratio (Table 1, entries 1 and 3). We found that the presence of acid was essential because no gold-containing products were formed in the absence of AcOH (Table 1, entry 2). Furthermore, without acid, decomposition of Ph3PAuSPh upon heating was observed according to the 31P{1H} NMR spectrum. If Ph3PAuCl was used as the gold(I) precursor, the equivalent of acetic acid released upon the interaction of PhSH with Pb(OAc)2 apparently mediated the further transformation (Table 1, entry 3). Carrying out the reaction without Pb(OAc)2 gave mostly unreacted alkyne without the formation of goldcontaining products (Table 1, entry 4). The extra amount of AcOH introduced into the reaction mixture did not significantly affect the product yields (Table 1, entries 3 and 5). The reaction of methyl propiolate with PhSH and Pb(OAc)2 in the absence of gold complex under the same conditions resulted in 24% conversion of alkyne into a mixture of 1 and 2 with a 1.2:1 ratio. We also tested other acids in the reaction of methylpropiolate with Ph3PAuSPh. It was found that in the presence of HCl, CF3COOH, and TfOH the reaction mixture turns yellow and the source gold complex undergoes decomposition according to the 31P{1H} NMR spectrum. No products were detected in the 1H NMR. The use of butyric and benzoic acids allowed us to obtain the target complexes with yields and ratio similar to the reaction with AcOH. Since inexpensive and readily available AcOH demonstrated a good reaction outcome, we have chosen this system for further studies. The experiments have shown

Figure 2. Molecular structure of product 4 determined by X-ray analysis.

is almost planar (rms deviation is 0.027 Å) and adopts the Econfiguration at the central vinyl CC bond. The cisdisposition of the phenyl ring of the PhS fragment relative to the gold atom [the Au1−C1−S1−C5 torsion angle is −1.9(2)°] is apparently determined by the two types of intramolecular interactions: (i) Au1···π(C5−C6) [Au1···C5 3.181(3) and Au1···C6 3.179(4) Å] and (ii) C28− H28···π(C7−C8) [H28···C7 2.85 and H28···C8 2.82 Å] hydrogen bond. The described intramolecular interactions likely increase the stability of the remarkable arrangement of complex 4. In addition to complex 4, the regioisomeric complex 5 was also discovered in the reaction mixture. Each complex was isolated individually using dry-column vacuum chromatography. The structures of the complexes in solution were confirmed by ESI-MS analysis (m/z measured = 653.0987, calculated = 653.0973, Δ = 2.1 ppm for 4 and m/z measured = 653.0983, calculated = 653.0973, Δ = 1.9 ppm for 5) and 1 H−31P HSQC experiments (Figure 3). Both complexes exhibited unusually strong 4J-coupling between the vinyl proton and phosphorus atom from the Ph3P group, with the coupling constant for the trans-arrangement (10 Hz) of interacting groups being higher than that for the cis-isomer (6.3 Hz). We subsequently tested the discovered transformation on a broader set of alkynes. As long as no reaction was observed 5216


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P{1H} NMR spectroscopy. These experiments revealed the crucial role of the proton source in the system and motivated a mechanistic study. Mechanistic Study. Three different mechanisms can be proposed for the metal-catalyzed addition reaction of thiols to alkynes.38 Anti-Markovnikov-type adducts 1 and 2 may be accessible via metal-free radical or nucleophilic pathways, as well as via the metal-mediated transformation. The formation of product 3 requires a metal-mediated transformation.38 To determine the involvement of the metal center in the C−S bond formation, we conducted the reaction of methyl propiolate and PhSH in the absence of gold. Indeed, product 3 was not formed under the investigated conditions. The metal-mediated hydrothiolation of alkynes catalyzed by Pd, Ni, and Rh complexes involves either intramolecular insertion into the metal−sulfur bond or external attack of a sulfur nucleophile (Scheme 5).39 However, a different reactivity may be expected for gold complexes. It has been proposed that the vast majority of gold-catalyzed additions to carbon−carbon multiple bonds proceed via the attack of an external nucleophile to the gold-coordinated π-complex.40−42 Nevertheless, we may assume that both pathwaysinsertion and external attackmay be involved in the formation of the products. It is not possible to distinguish between the pathways based on the overall transformation (Scheme 5) because the same products, 1−3, would be obtained. However, the pathways can be distinguished by considering the structures of the metal intermediates and by performing deuteriumlabeling experiments (Scheme 5). Alkyne insertion into the gold−sulfur bond should involve intermediate complexes with syn-geometry, followed by the formation of products 1 and 3′ after protonolysis. External nucleophilic attack of PhS− to the coordinated triple bond should proceed via anti-adducts, finally affording products 2 and 3. We subsequently performed the experiment using deuterated alkyne. Analysis of the obtained 2H NMR spectra demonstrated deuterium label scrambling, leading to an almost uniform distribution of deuterium among all possible products (1−3 and 1′−3′). Surprisingly, the reaction of protonated alkyne with

Table 1. Outcome of the Gold-Mediated Hydrothiolation Depending on the Type of Metal Precursor and Addition of Acetic Acid

a

All reagents were used in equimolar quantities (0.05 mmol). Determined by 1H NMR (the yields < 5% should be considered as trace amounts; estimated accuracy of NMR measurements ±2%). Reaction conditions: 3 h, 70 °C, DCM.

b

that mild carboxylic acids are required to mediate the transformation (as shown for the acetic, butyric, and benzoic acids in the present study). To determine the influence of the transmetalation agent, we performed additional experiments using AcONa and AcOAg. The results indicated that the use of silver or sodium acetate also resulted in the formation of products 1−5. The use of AcONa afforded almost the same conversion of alkyne and product ratio as the reaction with Pb(OAc)2, whereas the yields of 4 and 5 in the presence of AcOAg were lower (18% and 3%, respectively).37 For all acetates tested, the in situ formation of Ph3PAuSPh upon interaction with Ph3PAuCl was confirmed via

Scheme 5. Different Mechanisms for Gold-Mediated C−S Bond Formation

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proposed as intermediates in gold-catalyzed reactions. To estimate the possibility of either vinylidenes or acetylides being involved in the transformation, we performed DFT calculations on the formation of these intermediates from alkyne and Ph3PAuSPh. Quantum chemical investigations demonstrated that the formation of both vinylidene and acetylide are endothermic processes; however, the formation of the latter is strongly preferred (energy difference of 13.3 kcal/mol). On the basis of this result, we focused our attention on investigating the gold acetylide-mediated pathway. At the first step of the reaction, the thiolate gold complex reacts with methyl propiolate, yielding gold acetylide III via transition state TSI−III (Figure 4). Although this process yields the favored gold acetylide, the reaction barrier is relatively high (ΔE⧧ = 38.9 kcal/mol, ΔE = 2.6 kcal/mol; I → TSI−III → III). Keeping in mind the crucial role of acid revealed in the experiment, we also investigated the possibility of AcOH being involved in the formation of acetylide. The interaction of AcOH with Me3PAuSPh resulted in the formation of gold acetate complex II accompanied by the elimination of free PhSH (ΔE⧧ = 13.7 kcal/mol, ΔE = 4.9 kcal/mol; I → TSI−II → II). The next step, i.e., the reaction of Me3PAuOAc with methyl propiolate, yielded the same gold(I) acetylide III and involved overcoming the relatively lower barrier (ΔE⧧ = 19.7 kcal/mol, ΔE = −2.3 kcal/mol; II → TSII−III → III) compared to the pathway involving transition state TSI−III. In the next step, we calculated energy profiles for the addition of PhSH to gold(I) acetylides leading to the formation of product 4 (Figure 4). The association of the gold acetylide-

Ph3PAuSPh in the presence of deuterated acetic acid resulted in the formation of 4 with 82% deuterium incorporation. The third possible combination of products1′ and 2′ may arise from protonolysis of the C−Au bond in complexes 5 and 4, respectively. To confirm this assumption, we performed NMR monitoring of the protonolysis of isolated gold complexes 4 and 5 with deuterated acid (Scheme 6). We Scheme 6. Protonolysis of 4 and 5 Leading to Products 1 and 2

achieved complete conversion of 4 and 5 to deuterated products 2 and 1 in 24 h at 100 °C with 10 equiv of acetic acid. However, such harsh conditions indicate that the protonolysis of 4 and 5 is unlikely to provide a significant contribution to the overall yields of 1′ and 2′. Mechanistic considerations suggest that neither nucleophilic attack nor insertion will lead to the gem-disubstituted complex 4 or 5. Thus, another mechanistic picture should be employed to rationalize their formation. As discussed above (see Introduction), gold vinylidenes or gold acetylides are often

Figure 4. Energy profile for the formation of 4 from alkyne via carboxyl-assisted proton transfer. 5218


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Figure 5. Key gold-containing ions found in the ESI-MS spectrum of the reaction mixture.

between PhSH and AcOH results in the high degree of deuterated 4 observed experimentally in the reaction with AcOH-d4. The overall mechanism clearly highlights the crucial role of the acid in the system (Figure 4). An important role of the substituent in the alkyne unit is evident from the nature of the transition states TSIV−V and TSV−VI, in which the COOR group is directly involved in the reaction pathway. Even the size of the substituent R in the COOR group may have a significant influence, as evident from the optimized structures (Figure 4). Thus, the calculated mechanistic pathway is in excellent agreement with the results of the experimental studies. Despite all of our attempts, we were not able to isolate gold acetylides from the reaction mixture nor detect them via NMR spectroscopy. Remarkably, electrospray ionization mass-spectrometry (ESI-MS)45 made it possible to enrich the mechanistic study and to obtain additional insights into the nature of the intermediate gold complexes. ESI-MS analysis of the reaction mixture revealed the broad scope of gold-containing ions, and the key results are summarized in Figure 5. A gold atom with two coordinated phosphine ligands is a well-known stable ion detected as a high-intensity signal in the mass spectrum (m/z = 721.1517). A gold-alkyne complex was detected at m/z = 805.1706 and may involve two structural possibilities: either a gold(III) allene complex or a gold(I) πcomplex with an alkyne (Figure 5). In the MS2 spectrum of this ion, the most intense fragment ions exhibited m/z = 721 ((Ph3P)2Au+) and m/z = 459 (Ph3PAu+), corresponding to the loss of a neutral alkyne molecule, which is in favor of the gold(I) π-complex.46 Note that a gold complex with alkyne and thiophenolate units was also detected in the ESI-MS study (m/ z = 913.1728). It was not possible to perform MS2 experiments in this case due to the low intensity of the detected signal.

PhSH system with acid was an exothermic step with an energy gain of ΔE = 5.2 kcal/mol (III → IV). The succeeding rearrangement occurred via proton transfer and led to the formation of vinyl gold intermediate V (ΔE⧧ = 35.7 kcal/mol, ΔE = 16.7 kcal/mol; IV → TSIV−V → V). The rotation of the carboxylate group around the C−C(OH)OMe bond and proton movement was calculated to be a facile step with a relatively small barrier and highly exothermic energy change (ΔE⧧ = 14.4 kcal/mol, ΔE = −42.4 kcal/mol; V → TSV−VI → VI). Although the formation of gold acetylide III was an endothermic step (ΔE = 2.6 kcal/mol; I → III), the overall transformation leading to product 4 was a highly exothermic process that provided the necessary driving force for the reaction (ΔE = −28.3 kcal/mol; I → VI). It should be pointed out that described theoretical calculations may have some limitations with respect to the model system, theory level, and effect of solvent. Previously it was shown that model systems calculated at the DFT level may not give precise activation barriers; however relative barrier heights are reproduced correctly, as were shown for goldcontaining systems.43,44 Comparison of relative barrier heights was made in the present study to estimate accessibility of different reaction pathways, and the calculations have shown reliable agreement with our findings. It is of interest to combine the results of experimental and theoretical studies. During the IV → V conversion, the proton from PhSH is bound by the oxygen atom from the carboxyl group, and the proton from acid is transferred to the carbon atom of the vinylic fragment. After the flip of the carboxylic fragment (V → VI), the proton originating from PhSH is transferred to the acetic acid anion. Such proton interchange 5219


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Figure 6. MS2 dissociation pathways for gold acetylide with m/z = 1263 (laboratory frame collision energy −20 eV).

(Scheme 7, eq 1). Although the equilibrium of this reaction is largely shifted back to the Ph3PAuSPh complex, the presence of

The signal arising from complexes 4 and 5 was observed as a medium-intensity peak (m/z = 653.0980). Notably, in addition to this signal, another ion with m/z = 1111.1480 was detected. The mass difference between these ions corresponds to the one Ph3PAu group, and we may assume that the ion with m/z = 1111.1480 belongs to gem-diaurated compounds (Figure 5). The gem-diaurated species have been discussed in the literature and are occasionally proposed to represent the resting states of gold-driven reactions.47 The structure of the compound corresponding to the molecular formula with m/z = 1263.2194 may also be visualized as a σ,π-bound digold acetylide (Figure 5). To evaluate the proposed structure, we performed a dedicated MS2 experiment. The fragmentation pattern for m/z = 1263.2194 strongly supported the presence of the acetylide fragment. In the MS2 spectrum, we observed the loss of neutral gold acetylide followed by the appearance of an ion with m/z = 721, corresponding to (Ph3P)2Au+ species (Figure 6). In this case the collision energy required to activate fragmentation was significantly lower. This observation is consistent with the fact that the π-bond in the alkyne-gold complex may be significantly stronger than that in the acetylide complex due to the high degree of electron density transfer from the triple bond to the σ-bound gold atom in the latter.48 A series of unique Au3 and Au4 clusters were detected in the ESI-MS spectrum with m/z = 1409.1452, 1569.1947, 1595.1954, and 1875.1757 (Figure 5). However, other than the ESI-MS study, we did not observe other evidence for the involvement of gem-diaurated compounds and gold clusters in the reaction because it was not possible to detect them using other experimental methods. At present, we cannot clearly state whether the observed Au2, Au3, and Au4 species were present in the solution or were generated in the ESI source during the mass spectrometry measurements. The generation of metal clusters during the measurements is a known drawback of the electrospray ionization method.45 Further studies should be conducted to obtain more insights into the nature of these fascinating gold species.

Scheme 7. Summary of Carboxylate-Mediated Transformations in the Studied Reaction

a small amount of gold acetate in the mixture would allow the next transformation step to proceed (Scheme 7, eq 2). The attack of the benzenethiol on the gold acetylide center leads to the formation of vinyl gold complexes (Scheme 7, eq 3). Surprisingly, the acid played a key role in the system, which acted as a proton transfer agent in each step. Note that despite the regular use of various metal acetates as well as acetic acid in catalytic reactions, their mechanistic role often remains unclear. We anticipate more attention to be directed toward these factors in gold-mediated organic transformations. As a highlighting example, the key role of carboxylate has been demonstrated in the Pd-catalyzed reactions involving alkynes.49 The participation of the carboxylate group in the catalytic reaction provided flexible and tunable control over the direction of the chemical transformation and reaction selectivity.

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EXPERIMENTAL SECTION

General Procedures. Unless otherwise noted, all reactions were conducted in PTFE screw-capped tubes equipped with a magnetic stirrer bar, and heating was performed in a temperature-controlled oil bath using a magnetic stirrer. All reactions were performed under air. All reagents were purchased from commercial sources and used as received (checked by 1H and 31P NMR spectroscopy prior to use). NMR yields were calculated based on the 1H and 31P{1H} NMR spectra; the isolated yields were calculated after purification. CDCl3 (99.8%) for NMR spectroscopy was obtained from Deutero GmbH and stored over molecular sieves (4 Å) and silver foil. LC-MS-grade solvents (MeCN and DCM) for ESI-MS experiments were ordered from Merck and used as received. All samples (solutions in MeCN or

CONCLUSIONS In summary, we have developed a unique gold-mediated pathway to access 1,1-disubstituted gold complexes. Protonolysis of the Au−C bond in these complexes leads to the formation of vinyl sulfides. The combined experimental and theoretical study revealed a crucial role of the carboxylate group in the investigated reactions, and the following selected mechanistic steps deserve a special note. The dissociation of the Ph3PAuSPh complex leads to the formation of gold acetate 5220


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Organometallics DCM) for the ESI-MS experiments were prepared in 1.5 mL Eppendorf tubes. All plastic disposables (Eppendorf tubes and tips) used for sample preparation were washed with MeCN or DCM prior to use. The solvents used for column chromatography were purified according to well-known published procedures. Quantum chemical calculations were conducted using the Gaussian 09 package.50 The geometry optimizations and frequency calculations for the ground and transition states were performed using B3LYP/Gen (the LanL2DZ basis set was used for Au atoms, and the 6-31++G(d) basis set was used for C, H, O, P, S atoms) level of theory with no symmetry constraints. For the studied structures, frequency analysis (298.15 K and 1 atm) was performed to characterize the nature of the stationary points. Transitions states were confirmed with IRC (intrinsic reaction coordinate) calculations. NMR Experiments. All NMR measurements were performed using a three-channel Bruker DRX 500 spectrometer equipped with a 5 mm BBI probehead operating at 500.1, 202.5, and 125.8 MHz for 1 H, 31P, and 13C, respectively. Unless otherwise noted, all samples for the NMR experiments were prepared in CDCl3, and spectra were acquired directly after the end of the reaction. The 1H chemical shifts are reported relative to TMS. The 13C chemical shifts are reported relative to the corresponding solvent signals used as an internal reference. The 31P chemical shifts are reported relative to external 85% H3PO4/H2O. The spectra were processed on a Windows workstation using the TOPSPIN 2.1 software package. 1 H−31P HSQC Experiment. The 2D gs-HSQC experiment was performed utilizing the standard hsqcetgpsisp2 pulse program. The spectral widths were 4006 and 13 244 Hz for the F2 and F1 dimensions, respectively, with 1024 points acquired in the F2 domain. Acquisition was performed in 140 F1 increments, collecting 8 scans for each data point with 4 dummy scans before the start of acquisition. The relaxation delay was set to 1 s. The refocusing delay was optimized for direct coupling J(P−H) = 200 Hz. The obtained raw data were zero filled to a 1024 × 1024 square matrix, and quadratic sine multiplication with line broadening factors of 0.3 and 1.00 was applied in F2 and F1 domains, respectively, prior to Fourier transformation. X-ray Crystal Structure Determination of Ph3PAuC(SPh)CCOOMe (4). Data were collected on a Bruker SMART APEX-II CCD diffractometer (λ(Mo Kα) radiation, graphite monochromator, ω and φ scan mode) and corrected for absorption using the SADABS program.51 For details, see the Supporting Information. The structure was solved using direct methods and refined using the full-matrix leastsquares technique on F2 with anisotropic displacement parameters for non-hydrogen atoms. The absolute structure of 4 was objectively determined by the refinement of the Flack parameter, which equaled 0.006(4). All hydrogen atoms were placed in the calculated positions and refined within the riding model with fixed isotropic displacement parameters (Uiso(H) = 1.5Ueq(C) for the CH3 groups and Uiso(H) = 1.2Ueq(C) for the other groups). All calculations were performed using the SHELXTL program.52 Crystallographic data for 4 have been deposited with the Cambridge Crystallographic Data Center. CCDC 966036 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 1223 336033; e-mail: [email protected] or www.ccdc.cam.ac.uk). ESI-MS Measurements. High-resolution mass spectra were recorded on a Bruker maXis Q-TOF instrument (Bruker Daltonik GmbH, Bremen, Germany) equipped with an electrospray ionization (ESI) ion source. The measurements were performed in a positive (+MS; +MS/MS) ion mode (HV capillary: 4500 V; HV end plate offset: −500 V) with a scan range of m/z 50−3000. External calibration of the mass spectrometer was performed using an electrospray calibrant solution (Fluka). A direct syringe injection was used for all of the analyzed solutions in MeCN or DCM (flow rate: 3 μL/min). Nitrogen was used as the nebulizer gas (0.4 bar), dry gas (4.0 L/min), and collision gas for all of the MS/MS analyses and experiments; the dry temperature was set at 180 °C. All of the

recorded spectra were processed using the Bruker Data Analysis 4.0 software package. Synthesis of Ph3PAuSPh by Transmetalation. First, 0.05 g (2.5 × 10−5 mol) of Pb(SPh)2 was placed in a 25 mL round-bottom flask, followed by the addition of 3 mL of dichloromethane. To the resulting yellow suspension was added 0.0426 g (5 × 10−5 mol) of Ph3PAuCl under stirring. After 5 min at room temperature, the color of the suspension changed to white. The reaction mixture was filtered through silica, and the solvent was removed on a rotary evaporator. Product: white solid, 0.056 g (98%). The procedure was adapted from ref 32. 1 H NMR (500.13 MHz; CDCl3; δ, ppm): 7.64 (d, J = 7.9 Hz; 2H; SPh), 7.53 (m; 9H; PPh3), 7.46 (m; 6H; PPh3), 7.06 (t, J = 7.6 Hz; 2H; SPh), 6.93 (t, J = 7.6 Hz; 1H; SPh). 31P{1H} NMR (202.5 MHz; CDCl3; δ, ppm): 40.1. Stoichiometric Reaction of Ph3PAuCl, Pb(OAc)2, PhSH, and Methyl Propiolate. A total of 0.5561 g (1.1 × 10−3 mol) of Ph3PAuCl was placed in a screw-capped tube and dissolved in 10 mL of dichloromethane, followed by the addition of 100 μL (1.1 × 10−3 mol) of methyl propiolate and 0.2312 g (0.56 × 10−3 mol) of Pb(OAc)2 to the solution. A white suspension was formed. Then, 0.1212 g (1.1 × 10−3 mol) of benzenethiol was added to this suspension under stirring. Upon the addition of benzenethiol, the color of the suspension rapidly changed from white to yellow, indicating the formation of Pb(SPh)2. Stirring was continued for an additional 5 min until the color of the suspension returned to white due to the formation of Ph3PAuSPh. The resulting reaction mixture was sealed and heated at 70 °C for 3 h under stirring. The reaction mixture was separated by dry-column vacuum flash chromatography with gradient elution.35 Product 4 was eluted with hexanes/ethyl acetate = 2:1. Product 5 was eluted with hexanes/ethyl acetate = 1.5:1. Methyl (2E)-3-(Phenylthio)-3-[(triphenylphosphine)gold]acrylate (4). 1H NMR (500.1 MHz; CDCl3; δ, ppm): 7.70−7.79 (m, 2H), 7.12−7.58 (m, 15H), 7.02−7.11 (m, 3H), 5.99 (d, J = 6.3 Hz, 1H), 3.73 (m, 3H). 13C{1H} NMR (125.8 MHz; CDCl3; δ, ppm): 206.27 (d, J = 117.22 Hz), 166.28 (d, J = 9.06 Hz), 142.21, 134.51, 134.26, 134.13, 132.02, 131.40, 129.99 (d, J = 52.82 Hz), 129.48, 129.37, 129.12, 129.01, 128.54, 127.64, 116.93, 50.78. 31P{1H} NMR (202.5 MHz; CDCl3; δ, ppm): 40.22. HRMS (ESI): calculated for C28H25O2PSAu [M + H+] = 653.0973, found 653.0987 (Δ = 2.1 ppm). Anal. Calcd for C28H25O2PSAu: C, 51.54; H, 3.71. Found: C, 51.53; H, 3.76. Methyl (2Z)-3-(Phenylthio)-3-[(triphenylphosphine)gold]acrylate (5). 1H NMR (500.1 MHz; CDCl3; δ, ppm): 7.14−7.63 (m, 20H), 6.29 (d, J = 10 Hz, 1H), 3.61 (m, 3H). 13C{1H} NMR (125.8 MHz; CDCl3; δ, ppm) 199.32 (d, J = 114.20 Hz), 169.51 (d, J = 3.11 Hz), 135.10, 134.65, 134.50, 134.35, 131.92, 131.36, 130.57 (d, J = 52.99 Hz), 129.47, 129.35, 129.22, 129.11, 128.48, 117.78, 50.81. 31P{1H} NMR (202.5 MHz; CDCl3; δ, ppm): 41.13. HRMS (ESI): calculated for C28H25O2PSAu [M + H+] = 653.0973, found 653.0983 (Δ = 1.9 ppm). Anal. Calcd for C28H25O2PSAu: C, 51.54; H, 3.71. Found: C, 51.48; H, 3.81. Protonolysis of the Ph3PAuC(SPh)CCOOMe Complexes. A mixture of complexes 4 and 5 (9 mg), 0.06 mL of AcOH-d4, and 0.6 mL of solvent (acetone-d6) were placed into a screw-capped NMR tube. The reaction mixture was heated at 100 °C until complete disappearance of signals of source complexes in the 1H NMR spectrum was observed. The total reaction time was 23.5 h. The decrease in the integral intensities of signals of complexes 4 and 5 in 1H NMR (5.99 and 6.29 ppm, respectively) was followed by an increase in the intensities of signals corresponding to deuterated alkenes in the 2H NMR spectrum (5.65 and 5.92 ppm). Note that the reaction can also be conducted in more convenient solvents. Acetone-d6 was chosen for the purpose of the spectral study to avoid overlapping of solvent signals with the signals of compounds of interest in 2H NMR. 5221


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ASSOCIATED CONTENT

S Supporting Information *

1

H and 31P{1H} NMR spectra for complexes 4 and 5. Detailed crystallographic data for complex 4. XYZ files for all structures calculated. Additional experimental data for reaction with different alkynes and test of different transmetalation agents. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00210.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+7) 499 1355328. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge support from the Russian Science Foundation (RSF grant 14-50-00126). The authors thank Dr. Levon L. Khemchyan for recording the mass spectra.

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REFERENCES

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