Cyclization of 2-Alkynyldimethylaniline on Gold (I) Cationic and

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Cyclization of 2‑Alkynyldimethylaniline on Gold(I) Cationic and Neutral Complexes Luca Biasiolo,†,‡ Leonardo Belpassi,‡ Carlo Alberto Gaggioli,§ Alceo Macchioni,‡,§ Francesco Tarantelli,‡,§ Gianluca Ciancaleoni,*,‡,∥ and Daniele Zuccaccia*,†,‡ †

Dipartimento di Chimica, Fisica e Ambiente, Università di Udine, Via Cotonificio 108, I-33100, Udine, Italy Istituto di Scienze e Tecnologie Molecolari del CNR (CNR-ISTM), Università degli Studi di Perugia, Via Elce di Sotto 8, I-06123, Perugia, Italy § Dipartimento di Chimica, Biologia e Biotecnologie, Università degli Studi di Perugia, Via Elce di Sotto 8, I-06123, Perugia, Italy ∥ Departamento de Química, Universidade Federal de Santa Catarina, 88040-900 Florianópolis, SC, Brazil ‡

S Supporting Information *

ABSTRACT: The cyclization reaction of 2-(1-hexynyl)dimethylaniline (S) on gold(I) has been studied by NMR spectroscopy, in order to characterize the ion pair structure of the product, [LAuSc]BF4. The latter is a good model for the catalytic intermediate between the nucleophilic attack and the protodeauration step of a typical gold catalytic cycle. 19F, 1H HOESY NMR results demonstrate that in [LAuSc]BF4, with L being three different ligands, the anion mainly interacts with the ammonium moiety of the substrate, thanks to its formal positive charge, even if the ligand can tune the exact position of the anion. Furthermore, also gold chloride is able to promote the cyclization of S, forming [ClAuSc], which is the first example of a new class of precatalysts with potentially interesting catalytic properties. Preliminary data on its catalytic performances and a detailed DFT characterization of its electronic properties are presented, both of which indicate that Sc behaves as a carbene.

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INTRODUCTION The catalytic hydroamination of unsaturated organic compounds is a very significant reaction in organic chemistry,1 because it yields important fine chemicals and building blocks as functionalized amine, enamine, or imine with 100% atom efficiency. Such a reaction can be efficiently catalyzed by transition metals, either early (as zirconium2 and titanium3) or late (as ruthenium,4 palladium,5 and mercury6), lanthanide7 and actinide complexes.8 In the last years, gold complexes unveiled their potential as catalysts in the hydroamination of alkenes and alkynes,9 as in many other functionalization reaction of unsaturated substrates.10 Initially, it was thought that the mechanism of the reaction involved a three-coordinated gold species, in which the ancillary ligand, the organic substrate, and the amine are coordinated to the metal center,11 but more recent mechanistic studies, both experimental12 and theoretical,13 indicated that the geometry of the intermediate species is anti-periplanar (outer-sphere mechanism14), with the nucleophilic species (NuH) that locates on the opposite side of the unsaturated bond with respect to the σ-coordinated metal. After the nucleophilic attack, in many cases a protodeauration step occurs, in which the proton of the nucleophile takes the place of the gold.15 An experimental paper by Wang and others16 recently explored the ligand effect on the different steps of goldcatalyzed reactions, showing that the rate of the nucleophilic © XXXX American Chemical Society

attack is generally high with electron-withdrawing ligands and low with electron-donating ones. In addition to this, Bertrand and co-workers stated that the cyclization of 2-alkynyl-Nmethylaniline in the presence of [(CAAC)Au(toluene)]B(C6F5)4 (CAAC = cyclic (alkyl)(amino)carbene) is practically instantaneous.12 In order to put these findings on quantitative grounds, we recently published a theoretical study showing how such a relationship can be nontrivial in some cases.17 In fact we confirmed that the nucleophilic attack rate is directly proportional to the ligand acidity, but we also found that if the organic substrate can efficiently delocalize a positive charge, as in the case of a 2-alkynylaniline,17 the reaction barrier is lower for ligands possessing intermediate electron-withdrawing power, with the carbene ligands having the best compromise. In this paper, we experimentally studied the same goldcatalyzed reaction using 2-(1-hexynyl)dimethylaniline (S, Scheme 1) as a substrate and different ligands: a carbene [1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene), L1], an electron-withdrawing phosphine (tris(3,5-bis(trifluoromethyl)phenyl)phosphine, L2), and a phosphite (tris(2,4-di-tertbutylphenyl)phosphite, L3). The products of the cyclization allow us to carry out one of our projects, the ion pair Received: December 28, 2015

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DOI: 10.1021/acs.organomet.5b01030 Organometallics XXXX, XXX, XXX−XXX

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Sc, which are similar to those of a carbene or a strong electrondonating phosphine.

Scheme 1. Reaction of Cyclization for the Substrate S in the Presence of Different Gold Complexes

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RESULTS AND DISCUSSION [LAuSc]BF4 Complexes. The substrate 2-(1-hexynyl)N,N,4-trimethylaniline (S) was prepared by a Sonogashira coupling between 1-hexyne and 2-iodo-N,N,4-trimethylaniline (P2, see Experimental Section for characterization and further details). If AgBF4 is added to an equimolar mixture of S and complex [L1AuCl], the NMR spectrum shows many differences with respect to the unactivated mixture. Indeed, the 13C resonance of carbon atoms 5 and 6 (see Figure 1a for numbering) passes from 79.5 and 95.2 ppm for the free S to 168.3 and 137.1 ppm for the product, respectively. Moreover, carbon 5 shows a longrange scalar correlation not only with H4 but also with H14 (Figure 1b), unequivocally demonstrating that the cyclized product [L1AuSc]BF4 has been formed. As in the case of [(CAAC)Au(toluene)]B(C6F5)4,12 the reaction is too fast to be monitored by NMR spectroscopy, even at the temperature of 240 K. Once the product is formed, it is stable in dry CD2Cl2 at room temperature for at least 3 days. Remarkably, a structure similar to that of Sc can be obtained with an ester functionality, as well.24 The ion pair structure of [L1AuSc]BF4 has been studied by 19 F, 1H HOESY NMR (Figure 1c), revealing that there is an intense contact between the anion and the methyl 14 (Table 1). This is not surprising considering that the nitrogen is an ammonium moiety without any possibility to delocalize the positive charge. Coherently, also the contact with protons 4 (spatially close to the ammonium moiety) is quite intense. Arbitrarily fixing at 1 the normalized intensity of the NOE between the anion and H14, the BF4−/H4 contact results to be 0.83 (Table 1). Unfortunately, the contacts given by the protons 9, 19, and 22 overlap and their separation is not possible. Anyway, the sum of the three intensities is lower than the BF4−/H4 contact (0.84). In addition, protons 15 and 17, which could be useful to assess quantitatively the position of

characterization of species modeling catalytic intermediates of typical gold(I)-catalyzed reactions. In fact, in 2009 we published the first 19F, 1H HOESY NMR study on the ion pair of a cationic olefin−gold complex,18 which is the intermediate species before the nucleophilic attack step (intermediate 1),19 opening the way to a systematic combined NMR/DFT study of [LAuUHC]BF4 (UHC = unsaturated hydrocarbon) ion pairs.20 Since the cyclized form of S on the metal center, [LAuSc]BF4 (Scheme 1), can be intercepted, as already demonstrated by Bertrand and coworkers,12 we are now able to model and characterize also the ion pair structure of the intermediate species between the nucleophilic attack and the protodeauration step (intermediate 2). Such studies are important for the complete rationalization of the mechanism.21 In fact, as we recently demonstrated, in some cases a direct correlation between the anion/cation relative orientation and the catalytic performance exists.22,23 Among the results of our work, we would like to underline that S cyclizes also when coordinated to gold(I) chloride, giving the complex [ScAuCl] as product, with potentially interesting catalytic properties. Preliminary catalytic data in combination with a detailed theoretical analysis are also presented, with the aim to characterize the electronic properties of the new ligand

Figure 1. (a) Numbering of atoms in [L1AuSc]BF4. (b) Two sections of the 13C, 1H HMBC spectrum of [L1AuSc]BF4 in CD2Cl2 (298 K). (c) Two sections of the 19F, 1H HOESY NMR spectrum of [L1AuSc]BF4 in CD2Cl2 (298 K). B


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nitrogen. It is important to remember that in the case of [L1AuUHC]BF4 or [L1AuPPh3]BF4 the anion mainly locates close to the backbone of L1.18,20b,d,25 Evidently, only a strong interaction, like the ammonium/anion one, is able to modify such an orientation. The same reaction has been performed using L2 and L3 ligands (Scheme 1), but in all the cases the cyclization reaction is too fast to be monitored, even at low temperature. For this reason, here we are not able to verify our previous theoretical predictions.17 We recall that the DFT-computed activation barriers were always relatively low, spanning from 5 to 14 kcal/ mol for CAAC and CH3O−, respectively, with phosphines presenting values of 5−8 kcal/mol. However, all the products are stable in solution, and their ion pair structure can be studied (Figure 2). Also in the case of [L2AuSc]BF4, the BF4−/H14 contact is the most intense one, and its intensity can be taken as reference (1.00). Here the BF4−/H9 contact does not overlap with other peaks and its intensity results to be 0.83. On the other hand, the BF4−/H16 contact, normalized by the number of equivalent nuclei of the PArF, is 0.33 (Table 2). Noteworthy, also in the case of [L2AuNAC]BF4, where NAC = [(tert-butylamino)(pyrrolidin-1-yl)methylidene], the intensity of the BF4−/H16 contact is one-third with respect to the BF4−/NH one,25 whereas for [L2AuUHC]BF4, the BF4−/H16 contact is by far the most intense one.20a The comparison of the interionic structures of [L2AuNAC]BF4 and [L2AuSc]BF4 suggests that the ammonium moiety of Sc has about the same ability to attract the anion of NAC ligands containing only one NH group. The BF4−/H12 contact, which was undetectable in [L1AuSc]BF4, now has a relative intensity of 0.21. The spatial proximity of the anion and H12 is a consequence of the interaction between H16 and the anion.

Table 1. Relative (I) and Normalized (I/f) NOE Intensities for [L1AuSc]BF4, Determined Arbitrarily Fixing at 1 the Normalized Intensity of the NOE between the Anion and H14a signal

I

I/f

1 2/17 3/15 13 4 16 14 12 10 9/19/22 23

0.04 0.34 0.56 n.d.c 0.46 n.d.c 1.00 n.d.c 0.03 0.89 0.27

0.06 0.26b 0.43b n.d.c 0.83 n.d.c 1.00 n.d.c 0.10 0.84b 0.43

The scaling factor f is equal to (nH × nF)/(nH + nF), where n is the number of magnetically equivalent nuclei. bWhen different peaks overlap, the f factor is computed assuming that all the overlapping protons contributed equally to the NOE contact, which is not necessarily true. cPeak intensity comparable with experimental noise.

a

the anion with respect to the cation, overlap with protons 3 and 2, respectively, giving relative intensities of 0.43 and 0.26, respectively. Finally, we can note that the BF4−/H16 contact is absent. As demonstrated for similar [L1AuUHC]BF4 complexes,18,20b,d if the anion is close to the carbene backbone, it always interacts with H16. These facts indicate that the interaction between the anion and the backbone of the carbene is very weak, probably absent, and the ion pair structure of [L1AuSc]BF4 can be described as in Figure 1a, with the anion electrostatically kept close to the

Figure 2. Two sections of the 19F, 1H HOESY NMR spectrum of [L2AuSc]BF4 (left) and [L3AuSc]BF4 (right) in CD2Cl2 (298 K). C


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(Scheme 2) with an excellent purity and free THT as byproduct. [ClAuSc] can be easily isolated and was stable at room temperature, even in the presence of light, moisture, or oxygen.

Table 2. Relative (I) and Normalized (I/f) NOE Intensities for [L2AuSc]BF4 and [L3AuSc]BF4, Determined Arbitrarily Fixing at 1 the Normalized Intensity of the NOE between the Anion and H14a signal 1 2 3 13 4 14 10 12 9 16 18 1 22 2 21 3 13 4 14 12 10/19 17/20 9

I [L2AuSc]BF4 0.07 0.07 0.07 0.07 0.23 1.00 n.d.c 0.07 0.28 0.35 0.05 [L3AuSc]BF4 0.06 0.16 0.06 0.13 0.08 0.02 0.18 1.00 0.04 0.11 0.07 0.23

I/f

Scheme 2. Reaction of Cyclization for the Substrate S in the Presence of a Neutral Gold Complex

0.09 0.12 0.12 0.09 0.40 1.00 n.d.c 0.21 0.83 0.33 0.07

The zwitterionic26 nature of the complex, with the positive charge formally localized on the NMe2 moiety and the negative charge formally on the gold, has been verified dissolving [ClAuSc] in CD2Cl2 and adding tetrabutylammonium tetrafluoroborate (TBABF4) in solution. The 19F, 1H HOESY NMR spectrum of the resulting mixture shows that the anion interacts not only with the TBA+ but also with the N-methyl groups 14 (Figure 3). In agreement with these results, the visual inspection of the Coulomb potential for [ClAuSc′] (in Sc′ the butyl chain has been simplified with a methyl group), mapped on an electronic isodensity surface, reveals that the Sc′ site of the complex presents an extensive region where the potential is attractive (high values, blue-colored, Figure 3), while, on the opposite side, the AuCl metal fragment is strongly repulsive (low values, red-colored in Figure 3). The nitrogen is not directly visible, buried as it is in the steric hindrance of the surrounding groups, but the effect of the positive charge is evident on the protons of the two methyl groups. Moreover, [ClAuSc] can be further activated with a silver salt. The abstraction of the chlorine atom by AgBF4 in the presence of a second equivalent of S should lead to the formation of [ScAuSc]BF4, but any effort to isolate such a species was unsuccessful, always leading to the formation of metallic gold. It can be supposed that two carbon ligands donate too much electronic density to the gold, making the complex too unstable to be isolated. Anyway, the reaction between [ClAuSc] and a silver salt gives a cationic species with potentially interesting catalytic properties.27 In order to explore this point, [ClAuSc] has been tested as a catalyst for the methoxylation of 3-hexyne,28 the cyclization of N-(prop-2-yn-yl)benzamide to 2-phenyl-5-vinylidene-2-oxazoline,29 and the cyclization of 2-(1-hexynyl)aniline (S2, see Scheme 3).30 A typical catalytic run was performed by mixing the substrate in the presence of the catalyst (1 mol %) and AgOTf (1 mol %) at 30 °C in CDCl3 (Experimental Section). Under these conditions, [ClAuSc] resulted to be inactive for the methoxylation of 3-hexyne and poorly active for the cyclization of N-(prop-2-ynyl)benzamide (conversion of 9% after 2 h). In particular, for the former, the catalyst seems to be unstable in the catalytic conditions, forming a black, unidentified precipitate immediately after the addition of the silver salt. Since in both the cases the medium is slightly acidic, it is likely that in these conditions the Sc−Au bond is cleaved, giving the unstable [Au(substrate)]+ species, which rapidly decomposes to metallic Au(0). On the other hand, [ClAuSc] was found to be moderately active for the cyclization of 2-(1-hexynyl)aniline, in which the

0.08 0.11 0.11 0.13 0.14 0.04 0.31 1.00 0.12 0.12b 0.07b 0.69

The scaling factor f is equal to (nH × nF)/(nH + nF), where n is the number of magnetically equivalent nuclei (H or F). bWhen different peaks overlap, the f factor is computed assuming that all the overlapping protons contributed equally to the NOE contact, which is not necessarily true. cPeak intensity comparable with experimental noise. a

Finally, also in the case of [L3AuSc]BF4, the BF4−/H14 contact is the most intense one (relative intensity 1.00). Remarkable contacts can be measured between the anion and H9, H4, and H3 (relative intensities 0.69, 0.31, and 0.14, respectively, Figure 2 and Table 2), confirming the anion presence on the back of Sc. All the contacts between the anion and L3 (H17−H22) are very weak and below 0.16 (Table 2). The anion interactions with H9 and H4 are quite intense (0.69 and 0.31, respectively), as they are spatially close to H14. The low ability of L3 to interact with the anion was evident also in the case of [L3AuUHC]BF4.20c In summary, the 19F, 1H HOESY NMR data presented here reveal that the ion pair structure of the product of the nucleophilic attack is similar to all the tested ligands, with the anion mainly close to the heteroatom carrying a formal positive charge. Only a very acidic proton (like the H16 of L2) can tune such a structure, partially shifting the anion to a different position (above the gold, 25%). [ClAuSc]. In our recent paper, we theoretically predicted that AuCl promotes the cyclization of S,17 with one of the highest activation energies (about 11 kcal/mol). For this reason, we added [(THT)AuCl] (THT = tetrahydrothiophene) to a solution of S in CD2Cl2 at 248 K. Also in this case, the cyclization was too fast to be monitored, giving directly and quantitatively the complex [ClAuSc] D


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Figure 3. Section of the 19F, 1H HOESY NMR spectrum of a solution of CD2Cl2 at 298 K containing [ClAuSc] (10 mM) and TBABF4 (6 mM). In the lower left corner, the Coulomb potential of [Sc′AuCl] mapped on an electronic isodensity surface (ρ = 0.007 e/Å3, Coulomb potential in au) is shown.

moiety, as the 19F, 1H HOESY NMR spectra demonstrate (see previous section); consequently its participation in the protodeauration step is likely negligible, and other nucleophile species (another molecule of the product or reactant, for instance13a,b,33 or water clusters13d) can replace the anion as a proton shuttle and assist the protodeauration. In other words, if we want to use the catalytic performance of [ClAuSc] to characterize the electronic properties of the ligand Sc, it is important to compare it with a catalyst with a similar anion/cation orientation. Considering all the gold complexes for which the interionic structure has been characterized to date,18,20,22 the most promising complex is [NACAu]+, where NAC = [(tert-butylamino)(diisopropylamino)methylidene], in which the −NH moiety establishes a hydrogen bond with the anion, as well as the ammonium moiety in [ScAu]+, and keeps the anion far from the catalytic site.22 As a confirmation, the similarity between Sc and NAC emerged also in the previous section. The use of [NACAuCl]/AgOTf in the catalysis depicted in Scheme 3 gives a conversion of 43% after 120 min, with an initial rate of 31.8 h−1 (Figure 4). This value is very similar to that obtained for [ClAuSc], allowing us to conclude that, under the same conditions of anion/cation relative position, Sc and NAC behave similarly in catalysis and, likely, have similar electronic properties (see next section). It is interesting to note that, differently from [ScAuCl], [NACAuCl] is active in the methoxylation of 3-hexyne (conversion of 92% after 2 h) and in the cyclization of N-(prop-2-ynyl)benzamide, and this is probably due to the higher stability of the carbene−gold bond in slightly acidic conditions. DFT Studies. Finally, the electronic properties of the Sc′ ligand can be characterized by DFT calculations. In particular, our investigation focused on (i) characterizing the electronic structure of the Sc′ ligand in terms of donor/acceptor capability within the framework of the Dewar−Chatt−Duncanson (DCD)34 bonding model, comparing the results with those obtained for the NAC ligand, and (ii) extracting information on the electronic structure of the Sc′Au+ metal fragment through the analysis of simple spectroscopic observables of a coordinated carbonyl or ethyne (e.g., systems such as [Sc′Au(C2H2)]+ and [Sc′AuCO]+).

Scheme 3

high concentration of S2 makes the medium basic rather than acidic. The formation of 2-butyl-1H-indole (Sc2) was monitored by NMR spectroscopy (see Experimental Section for further details and Figure 4), giving a conversion of 54%

Figure 4. Cyclization of S2 catalyzed by [LAuCl]/AgOTf in CDCl3.

after 2 h, with an initial rate of 39.6 h−1 after 0.5 h. Notably, the catalyst remains active until complete conversion, after 13 h (Figure S1, Supporting Information). In this catalysis, the rate-determining step (RDS) is likely the protodeauration step, whose rate depends on the electronic properties of the ligand.16 Nevertheless, since the protodeauration is usually assisted by the anion,31 the nature of the anion and the relative anion/cation orientation can be crucial in tuning the catalytic rate,32 as demonstrated also for the nucleophilic attack.22 In the case of the catalyst [AuSc]+, the anion is kept far from the catalytic site by the ammonium E


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Figure 5. Contribution to deformation density of the four most significant NOCV pairs for [Sc′AuCl], with fragments being Sc′ and AuCl. Isodensity surfaces (±0.001 e/au3) are superimposed to the molecular structure of the complex. Red surfaces (negative values) identify charge depletion regions; purple surfaces (positive values) identify accumulation regions. At the bottom: the related CDFs. Black dots indicate the z position of the atomic nuclei, with CSc′ representing the position of the carbon of Sc′ pointing out the Au atom. A dashed vertical line marks the boundary between the AuCl and the Sc′ fragments (see Computational Details for the definition).

Noteworthy, Δρ2 and Δρ4 are clearly related to the π-backdonation with the expected in- and out-of-plane contributions (π∥ and π⊥, respectively), while Δρ3 represents a back-donation component of σ-symmetry (σback). The latter component has been recently observed in a strictly related system, NHCAgCl.38 Quantitative information about these charge fluxes can be obtained by evaluating the corresponding charge displacement functions Δqk(z) (Figure 5, lower panel).39 We recall here for the reader’s convenience that, at a given point z, the chargedisplacement function (CDF) gives the exact amount of charge that moves across a plane perpendicular to the integration axis joining the two fragments (AuCl and Sc′) and passing through the z point. A positive CDF value corresponds to a charge flow from right to left (i.e., AuCl ← Sc′), while a negative value corresponds to a charge flow in the opposite direction (i.e., AuCl → Sc′). The CDF values at the inter-fragment boundary give the quantification of the different charge transfers: the CT for the σ-donation (CTσd) amounts to 0.37 e, the π-backdonation components, disentagled in in- and out-of-plane (CT) contributions, are 0.03 and 0.09 e, respectively, and the σ-backdonation (CTσback) is 0.04 e. In order to have a comparison with the electronic properties of the carbene ligand that we have used in catalysis (NAC, Figure 4), we applied the NOCV-CD analysis to the [NACAuCl] system. The results (Figure S2, Supporting Information) show that the two ligands, Sc′ and NAC, present similar bonding components when they interact with the AuCl fragment. In particular the Sc′ ligand is characterized by a slightly larger CTσd (0.37 vs 0.35 e for Sc′ and NAC, respectively; see Table S1 in the Supporting Information), while all the other components, including CTσback, give the same values. These findings support the similarities between the bonding properties of Sc′ and those of NAC, in agreement with the catalytic data. The electronic properties of Sc′ can be framed in a more general view through the analysis of the corresponding gold complexes bearing ethylene and carbon monoxide. Recently, we have demonstrated that properly chosen experimental observables can provide information about the DCD components of a coordination bond.40,41 For instance, measuring (or computing) the properties of a coordinated ethyne42 or carbonyl43,44 moiety gives a reliable estimation of the electronic properties of the ligand−metal fragment in terms of σ/π acidity. We have collected a large data set precisely for gold(I) complexes of formula [LAu(C2H2)]+ and [LAuCO]+, and here we apply the same methodology to the [Sc′Au]+ metal fragment. For these reasons, we worked out the structural information (geometries and harmonic frequencies) for the complexes [Sc′Au(C2H2)]+ and [Sc′AuCO]+ (Figure 6).

Δρ1 contribution presents a region of charge accumulation mainly located at the AuCl site, while a region of charge depletion is located at the Sc′ site, the overall rearrangement is characterized by a clear cylindrical symmetry, and we can unambiguously ascribe this component to σ-donation (σd) of the DCD model. The other electronic rearrangements (Δρ2, Δρ3, and Δρ4) go in the opposite direction (i.e., with a flux going from the metal to the ligand), and, although the charge rearrangements are of different symmetries, the metal fragment appears constantly characterized by a decrease of charge.

Figure 6. DFT-optimized geometries of [Sc′Au(C2H2)]+ and [Sc′AuCO]+. Distances are in angstroms; angles in degrees.

The DCD components of the Sc′−AuCl bond can be disentangled and computed through the natural orbitals for chemical valence35-charge displacement36 (NOCV-CD) method, as recently proposed by some of us.37 Such an approach is based on the analysis of the electron density rearrangement (Δρ) occurring upon the formation of the adduct (the [Sc′AuCl] complex in this case) from two molecular fragments (Sc′ and [AuCl]), taking as a reference the occupied orbitals of fragments, suitably orthogonalized to each other and renormalized (for details, see Computational Details and ref 37). The total Δρ is decomposed in contributions coming from the different NOCV pairs (Δρ′k) that can be ascribed to a DCD component on the basis of their local symmetry. The result of this decomposition is reported in Figure 5, upper panel. By a simple visual inspection, we can see that the

F


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Organometallics In the optimized geometry of [Sc′Au(C2H2)]+ the ethyne is perpendicular to the plane of Sc and bent, due to the interaction with the gold, with a deviation from linearity (Δθ) of 12.5°. Similarly, the CC stretching frequency (Δν) is 128.4 cm−1 lower than that of free ethyne. According to a previous study,42 Δθ and Δν are strictly related to the DCD components of the bond between [LAu]+ and C2H2 through a system of equations (see Computational Details). In such a system, Δθ and Δν are expressed as a linear combination of the σ-donation (CTdon), π-back-donation (CTback) and an electrostatic contribution (Δθelect and Δνelect, generally small; see Computational Details and ref 37). Knowing Δθ, Δν, Δθelect, and Δνelect, the DCD components CTdon and CTback can be calculated and are found to be 0.26 (C2H2 → [Sc′Au]+) and −0.13 e ([Sc′Au]+ → C2H2), respectively. Noteworthy, these values are in remarkable agreement with those we can derive by the NOCV-CD analysis (Figure S3 and Table S2, Supporting Information). In particular, summing up all the NOCV-CD contributions relative to the DCD σ-donation component (Δρ1 and Δρ3), the value at the interfragment boundary is 0.26 e. On the other hand, the sum of Δρ2 and Δρ4, relative to the DCD π-backdonation component, gives a back-donation of −0.15 e at the boundary. Comparing these values with the literature, the [Sc′Au]+−ethyne bond is found to be similar to the bond between [NHCAu]+ (NHC = 1,3-dimethylimidazol-2-ylidene) and ethyne, for which CTdon = 0.25 e and CTback = −0.12 e,41,45 with a slightly lower interaction energy between the fragments (−37.9 and −40.5 kcal/mol for [Sc′Au(C2H2)]+ and [NHCAu(C2H2)]+, respectively). In [Sc′AuCO]+, the C−O bond length (rCO) is 1.137 Å and the CO stretching frequency (νCO) is 2117 cm−1. Taking as reference the free CO (rfree‑CO = 1.137 Å and νfree‑CO = 2106 cm−1, computed values), ΔrCO and ΔνCO result to be null and 11 cm−1, respectively. In metal carbonyl complexes, the value of ΔrCO (and, less reliably because of mode coupling, ΔνCO) quantifies to an excellent extent the π-back-donation components on both sides of the gold.43,44 In the case of [Sc′AuCO]+ the values of ΔrCO indicate that [Sc′Au]+ → CO is 0.22 e and Sc′ ← [AuCO]+ is 0.01 e. Similar values can be obtained when L is a carbene or a very-electron-donating phosphine, such as tricyclohexylphosphine.43,44 Also in this case the NOCV-CD approach gives essentially the same values as those derived from the experimental observables of the carbon monoxide (Figures S4 and S5 and Tables S3 and S4, Supporting Information). Finally, the geometry of the complexes between [NACAu]+ (the metal fragment whose catalytic activity is similar to that of [Sc′Au]+) and our probes (carbon monoxide and ethyne) has been optimized. The CO distortion, ΔrCO and ΔνCO, in [NACAuCO]+ result to be 0.002 Å and 22 cm−1, while in [NACAuC2H2]+ the ethyne presents values of 12.5° and 123.2 cm−1 respectively for Δθ and Δν. As expected, these values are indeed very close to those found above in the related complexes bearing [Sc′Au]+.

the proton, and their ion pair structure has been carefully determined. In all the cases, the anion locates almost exclusively close to the ammonium moiety, because of the positive charge of the latter. Only in the case of [L2AuSc]BF4, the presence of a very acidic proton is able to finely tune the anion/cation position, partially shifting the BF4− anion to a different location. In real catalytic intermediates, the nitrogen bears at least one hydrogen; therefore an additional hydrogen bond is expected, but it would only reinforce the main ion pair structure found for the model compounds. This fact has an important catalytic implication: we already showed that when the nucleophilic attack is the RDS, ligands with the same electronic properties but inducing different ion pair structures show different catalytic properties.22 On the contrary, our data seem to indicate that when the proton shuttle is the RDS, the ion pair structure is similar to all the ligands and only the electronic properties of the ligand can influence the catalytic rate.16,17 The cyclization of S resulted to be feasible also on gold chloride, giving a stable precatalyst. The latter, if activated with a silver salt, is an active catalyst for the cyclization of 2-(1hexynyl)aniline, but not for the methoxylation of 3-hexyne and cyclization of N-(prop-2-ynyl)benzamide, likely due to the instability of the Au−Sc bond in slightly acidic conditions. The catalytic performances of [ScAuCl] have been compared with those of another precatalyst having a similar ion pair structure and bearing a carbene as ligand [NACAuCl]. The similarity of the results indicates that the electronic properties of Sc are similar to those of a carbene. On the other hand, the presence of localized charge on the ligand could give the complex interesting solubility and ion pair properties, making Sc a promising candidate for further optimization. Finally, DFT studies on [Sc′AuCl] and [NACAuCl] confirmed that the two ligands have similar electronic properties, in terms of the DCD components of the L− [AuCl] bond, computed through the NOCV-CD method. For a more general characterization, the spectroscopic properties of [Sc′AuCO]+ and [Sc′Au(C2H2)]+ have been studied too, allowing us to compare Sc′ with a wide selection of ligands and to declare that the electronic properties of Sc′ are similar to those of a carbene or a strong electron-donating phosphine. From the methodologic point of view, we underline that the DCD components can be evaluated directly, through the recently proposed NOCV-CD method, or indirectly, through the properties of the coordinated moiety (CO or C2H2), with the same results, demonstrating the consistency of the different methods.

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EXPERIMENTAL SECTION

General Procedures. Solvents, HAuCl4, tetrahydrothiophene, chloro[tris(2,4-di-tert-butylphenyl)phosphite]gold (L3AuCl), all the ligands, and organic reactants were purchased from Ricci Chimica and Sigma-Aldrich. AgBF4 was charged in a Schlenk flask and stored under a nitrogen atmosphere at −20 °C. 2-(1-Hexynyl)aniline (S2),12 (THT)AuCl, 46 chloro[1,3-bis(2,6-diisopropylphenyl)imidazol-2ylidene]gold(I) (L1AuCl),20 [NACAuCl],22 and chloro[tris(3,5-bis(trifluoromethyl)phenyl)phosphine]gold (L2AuCl)20 were synthesized according to the literature methods. All the new compounds were characterized in solution by 1H, 13C, 19 F, and 31P NMR spectroscopies. One- and two-dimensional 1H, 13C, 19 F, and 31P NMR spectra were measured on a DRX Avance 400 spectrometer equipped with a QNP probe or on a Bruker Avance III HD 400 spectrometer equipped with a Smartprobe. Synthesis of 2-Iodo-4-methylaniline (P1). 2-Iodo-4-methylaniline was prepared according to a literature procedure.47 Into a 100 mL

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CONCLUSIONS In this work, we concluded that the cyclization of 2alkynyldimethylaniline (S) on gold complexes [LAu]+/0, even if always too fast to be monitored, allowed the isolation of the cyclized product, [LAuSc]BF4. Such species are structurally similar to the catalytic intermediate just before the protodeauration step, with a methyl group on the nitrogen instead of G


Article

Organometallics round-bottom flask were placed 4-methylaniline (2.27 g, 21.15 mmol) and NaHCO3 (2.67 g, 31.7 mmol) in 30 mL of water. The mixture was stirred and cooled to 10−15 °C. Then, powdered and resublimed iodine (5.37 g, 21.15 mmol) was introduced in 1 g portions. The reaction mixture was stirred overnight at room temperature. The organic phase was extracted from the reaction mixture with diethyl ether. The extracts were combined and dried with MgSO4. The mixture was concentrated, and then the product was precipitated and washed with cold pentane, obtaining a brown solid: yield 3.92 g (79.6%). Spectroscopic data are consistent with those obtained in the literature.12 Synthesis of 2-Iodo-N,N,4-trimethylaniline (P2). 2-Iodo-N,N,4trimethylaniline was prepared according to a literature procedure.48 To a solution of P1 (1.8 g, 8 mmol) and iodomethane (1.5 mL, 24 mmol) in DMF (10 mL) was added K2CO3 (2.23 g, 16 mmol). The resulting mixture was stirred at room temperature for 48 h. Water (10 mL) was added to the reaction mixture. The resulting solution was extracted with diethyl ether (3 × 10 mL). The organic layers were combined and washed with water to remove any remaining DMF and dried over anhydrous MgSO4. The solvent was removed under vacuum, and the residue was purified by flash column chromatography on silica gel using ethyl acetate/pentane (1:5−1:1) as eluent. We obtain the product as dark brown oil; yield 1.6 g (75%). Anal. Calcd for C9H12IN (261.10): C, 41.40; H, 4.63; I, 48.60; N, 5.36. Found: C, 41.39; H, 4.65; N, 5.38. 1H NMR (200 MHz, CDCl3, J in Hz): δ = 7.67 (s, 1H), 7.09 (d, J3HH = 1.3, 1H), 6.98 (d, J3HH = 8.1, 1H), 2.72 (s, 6H), 2.25 (s, 3H). 13C{1H} NMR (CD3Cl, 50 MHz): δ = 152.30 (s), 140.26 (s), 134.66 (s), 129.59 (s), 119.90 (s), 97.28 (s), 45.02 (s), 20.00 (s). Synthesis of 2-(1-Hexynyl)-N,N,4-trimethylaniline (S). 2-(1-Hexynyl)-N,N,4-trimethylaniline was prepared according to a literature procedure.12 To a mixture of Pd(OAc)2 (2 mol %, 52.44 mg), CuI (3 mol %, 66.73 mg), and PPh3 (5 mol %, 153.18 mg) in CH3CN (6 mL) were successively added a solution of P2 (3.05 g, 11.68 mmol) in CH3CN (8 mL), Et3N (4.06 mL, 29.20 mmol), and 1-hexyne (2.01 mL, 17.52 mmol). The reaction mixture was stirred overnight a room temperature. Then a saturated aqueous solution of NH4Cl was added, and the mixture was extracted with Et2O (3 × 15 mL). The combined organic layers were washed with brine and dried over MgSO4. Solvent was then removed under reduced pressure, and the residue was purified by column chromatography (silica gel, hexane/ethyl acetate = 95:5) to afford the product as a brown liquid; yield 1.61 g (63.8%). Anal. Calcd for C15H21N (215.33): C, 83.67; H, 9.83; N, 6.50. Found: C, 83.68; H, 9.86; N, 6.51. 1H NMR (CD3Cl, 200 MHz, J in Hz): δ = 7.23 (d, J = 2.1, 1H, H9), 7.03 (dd, J = 8.3, 1.6, 1H, H12), 6.83 (d, J3HH = 8.3, 1H, H10), 2.91 (s, 6H, H14), 2.52 (t, J3HH = 6.9, 2H, H4), 2.27 (s, 3H, H13), 1.77−1.42 (m, 4H, H2,3), 0.99 (t, J3HH = 7.2, 3H, H1). 13 C{1H} NMR (50 MHz, CDCl3): δ = 152.21 (s), 134.54 (s), 130.00 (s), 128.88 (s), 116.82 (s), 116.56 (s), 95.20 (s), 43.61 (s), 30.80 (s), 21.94 (s), 20.07 (s), 19.41 (s), 13.49 (s). Synthesis of 2-(1-Hexynyl)aniline (S2). 2-(1-Hexynyl)aniline was prepared according to a literature procedure.12 To a mixture of Pd(OAc)2 (2 mol %, 42.5 mg), CuI (3 mol %, 52.0 mg), and PPh3 (5 mol %, 121.0 mg) in CH3CN (6 mL) were successively added a solution of 2-iodoaniline (2003 mg, 9.15 mmol) in CH3CN (5 mL), Et3N (3.18 mL, 22.88 mmol), and 1-hexyne (1.58 mL, 13.75 mmol). The reaction mixture was stirred overnight at room temperature. Then saturated aqueous NH4Cl was added, and the mixture was extracted with Et2O (3 × 10 mL). The combined organic layers were washed with brine and dried over MgSO4. Solvent was then removed under reduced pressure, and the residue was purified by column chromatography (silica gel, pentane/ethyl acetate = 3:1) to afford the product as a brown liquid; yield 1.51 g (95%). Anal. Calcd for C12H15N (173.25): C, 83.19; H, 8.73; N, 8.08. Found: C, 83.20; H, 8.75; N, 8.09. 1H NMR (CD3Cl, 200 MHz, J in Hz): δ = 7.29−7.19 (m, 1H), 7.13−7.00 (m, 1H), 6.66 (td, J3HH = 7.7, 0.9, 2H), 4.16 (s, 2H), 2.47 (t, J3HH = 6.9, 2H), 1.71−1.40 (m, 4H), 0.95 (t, J3HH = 7.2, 3H). 13C{1H} NMR (CD2Cl2, 50 MHz): δ = 148.28 (s), 132.20 (s), 129.13 (s), 117.88 (s), 114.29 (s), 109.06 (s), 96.13 (s), 77.13 (s), 31.45 (s), 22.44 (s), 19.59 (s), 13.77 (s).

Synthesis and Characterization of [L1AuSc]BF4. A 6.6 mg amount of AgBF4 (0.034 mmol) was loaded into a Schlenk vessel under a nitrogen atmosphere. A solution containing 18.6 mg of L1AuCl (0.03 mmol) and 6.4 mg of S (0.03 mmol) dissolved in 1 mL of dry CD2Cl2 was added. The resulting mixture was stirred for 20 min, then filtered directly in a screw-cap NMR tube. 1H NMR (400.13 MHz, CD2Cl2, 298 K, J in Hz): δ = 7.76 (t, 2H, J3HH = 7.7, H23), 7.58 (d, 1H, J3HH = 7.3, H9), 7.57 (s, 2H, H19), 7.53 (d, 4H, J3HH = 7.7, H22), 7.28 (d, 1H, J3HH = 7.3, H10), 6.83 (s, 1H, H12), 3.32 (s, 6H, H14), 2.71 (sept, 4H, J3HH = 7.0, H16), 2.48 (m, 2H, H4), 2.36 (s, 3H, H13), 1.48 (m, 2H, H3), 1.43 (d, 12H, J3HH = 6.9, H15), 1.33 (d, 12H, J3HH = 6.9, H17), 1.31 (m, 2H, H2), 0.88 (t, 3H, J3HH = 7.4, H1). 13C{1H} NMR (100.5 MHz, CD2Cl2, 298 K): δ = 190.1 (s, C18), 168.3 (s, C5), 147.1 (s, C21), 145.6 (s, C8), 141.8 (s, C11), 140.6 (s, C7), 137.1 (s, C6), 133.9 (s, C20), 130.3 (s, C23), 128.7 (s, C10), 126.6 (s, C12), 124.5 (s, C19), 123.7 (s, C22), 116.3 (s, C9), 52.4 (s, C14), 34.7 (s, C3), 29.2 (s, C16), 26.2 (s, C4), 24.6 (s, C15), 23.8 (s, C17), 22.7 (s, C2), 21.0 (s, C13), 13.1 (s, C1). 19F NMR (376.42 MHz, CD2Cl2, 298 K): δ = −152.60 (br, 10BF4), −152.64 (br, 11BF4). Synthesis and Characterization of [L2AuSc]BF4. A 6.6 mg amount of AgBF4 (0.034 mmol) was loaded into a Schlenk vessel under a nitrogen atmosphere. A solution containing 27 mg of L2AuCl (0.03 mmol) and 6.4 mg of S (0.03 mmol) dissolved in 1 mL of dry CD2Cl2 was added. The resulting mixture was stirred for 20 min, then filtered directly in a screw-cap NMR tube. 1H NMR (400.13 MHz, CD2Cl2, 298 K, J in Hz): δ = 8.30 (d, 3H, H18), 8.15 (d, J3HP = 12.0, H16), 7.71 (d, 1H, J3HH = 8.1, H9), 7.52 (s, 1H, H12), 7.35 (d, 1H, J3HH = 8.1, H10), 3.46 (s, 6H, H14), 2.73 (t, 2H, J3HH = 7.9, H4), 2.49 (s, 3H, H13), 2.05 (quintet, 2H, J3HH = 7.5, H3), 1.52 (sextet, 2H, J3HH = 7.5, H2), 0.85 (s, 3H, H1). 13C{1H} NMR (100.5 MHz, CD2Cl2, 298 K, J in Hz): δ = 163.1 (br, C5), 152.8 (d, J2CP = 108, C6), 145.9 (s, C8), 142.0. (s, C11), 140.6 (s, C7), 134.0 (d, J2CP = 16.0, C16), 133.8 (quartet of d, J3CP = 11.4, J2CF = 35.3, C17), 130.1 (d, J1CP = 53.1, C15), 128.8 (s, C10), 127.6 (br s, C18), 126.9 (s, C12), 122.3 (quartet, J1CF = 273.6, C19), 116.1 (s, C9), 52.0 (s, C14), 34.2 (s, C3), 25.6 (s, C4), 22.5 (s, C2), 21.0 (s, C13), 13.3 (s, C1). 19F NMR (376.42 MHz, CD2Cl2, 298 K): δ = −63.43 (s, m-CF3), −152.41 (br, 10 BF4), −152.45 (br, 11BF4). 31P{1H} NMR (161.97 MHz, CD2Cl2, 298 K): δ = 47.56 (s). Synthesis and Characterization of [L3AuSc]BF4. A 6.6 mg amount of AgBF4 (0.034 mmol) was loaded into a Schlenk vessel under a nitrogen atmosphere. A solution containing 26.4 mg of L3AuCl (0.03 mmol) and 6.4 mg of S (0.03 mmol) dissolved in 1 mL of dry CD2Cl2 was added. The resulting mixture was stirred for 20 min, then filtered directly in a screw-cap NMR tube. 1H NMR (400.13 MHz, CD2Cl2, 298 K, J in Hz): δ = 7.65 (d, 1H, J3HH = 8.2, H9), 7.56 (s, 3H, H17), 7.55 (dd, 3H, J3HH = 8.5, J4HP = 1.3, H20), 7.27 (m, 1H, H10), 7.26 (dd, 3H, J3HH = 8.5, J5HP = 2.5, H19), 7.16 (s, 1H, H12), 3.37 (s, 6H, H14), 2.56 (t, 2H, J3HH = 7.7, H4), 2.44 (s, 3H, H13), 1.85 (quintet, 2H, J3HH = 7.5, H3), 1.54 (s, 27H, H21), 1.40 (m, 2H, H2), 1.33 (s, 27H, H22), 0.88 (t, 3H, J3HH = 7.3, H1). 13C{1H} NMR (100.5 MHz, CD2Cl2, 298 K, J in Hz): δ = 161.6 (d, J3CP = 14.0, C5), 154.5 (d, J2CP = 171.1, C6), 148.7 (s, C18), 147.4 (d, J2CP = 5.4, C7), 146.0 (d, J4CP = 6.5, C8), 141.2 (s, C11), 139.4 (d, J3CP = 6.2, C16), 128.2 (s, C10), 126.6 (s, C12), 125.8 (s, C17), 125.3 (d, J2CP = 186.6, C15), 123.8 (s, C19), 118.9 (d, J3CP = 9.2, C20), 116.0 (s, C9), 51.5 (s, C14), 35.1 (s, C24), 34.6 (s, C23), 33.5 (s, C3), 31.1 (s, C22), 30.3 (s, C21), 25.3 (s, C4), 22.5 (s, C2), 21.3 (s, C13), 13.5 (s, C1). 19F NMR (376.42 MHz, CD2Cl2, 298 K): δ = −152.09 (br, 10BF4), −152.13 (br, 11BF4). 31 1 P{ H} NMR (161.97 MHz, CD2Cl2, 298 K): δ = 136.37 (s). Synthesis and Characterization of [ClAuS]. A solution containing 30.0 mg of (THT)AuCl (0.094 mmol) and 20.1 mg of S (0.094 mmol) dissolved in 5 mL of dry CH2Cl2 was stirred for 20 min, then filtered on a Celite pad. The volume of the solution was reduced to 2 mL, and 10 mL of hexane was added to precipitate the product as a white powder, which was filtered, washed with hexane (3 × 3 mL), and dried under reduced pressure. Yield: 37.8 mg (90%). 1H NMR (400.13 MHz, CD2Cl2, 298 K, J in Hz): δ = 7.62 (s, 1H, H12), 7.41 (d, 1H, J3HH = 8.1, H9), 7.21 (dd, 1H, J3HH = 8.2, J4HH = 1.1, H10), 3.31 (s, 6H, H14), 2.55 (m, 2H, C4), 2.45 (s, 3H, H13), 2.09 (m, 2H, H3), H


Article

Organometallics

where Δθ is the deviation of ethyne from linearity (in degrees), Δν is the difference between the stretching frequency of the triple bond in the complex and the value for the free ethyne (1991 cm−1), and Δθelect and Δνelect are the same quantities due to the electrostatic effect. The latter has been calculated by computing the distortion and the frequency of the ethyne placed in the field generated by point charges replacing the [ScAu]+ atoms, of value equal to the computed atomic charges in the adduct.42 The changes in electron density encountered upon bond formation were analyzed by means of the charge-displacement function36 Δq(z), defined as in eq 2:

1.54 (sestet, J3HH = 7.5, H2), 1.03 (t, J3HH = 7.3, H1). 13C{1H} NMR (100.5 MHz, CD2Cl2, 298 K): δ = 154.1 (s, C5), 145.6 (s, C8), 144.1 (s, C6), 143.2 (s, C7), 141.3 (s, C11), 128.4 (s, C12), 127.3 (s, C10), 114.4 (s, C9), 51.5 (s, C14), 33.1 (s, C3), 26.0 (s, C4), 22.8 (s, C2), 21.1 (s, C13), 13.6 (s, C1). Anal. Calcd for C15H21AuClN (447.75): C, 40.24; H, 4.73; N, 3.13. Found: C, 40.01; H, 4.99; N, 3.08. Catalysis. A. Methoxylation of 3-Hexyne. 3-Hexyne (100 μL, 0.88 mmol) and CH3OH (143 μL, 3.52 mmol) were poured into a vial and mixed with CDCl3 (500 μL) containing 5 μL of TMS. The solution was then introduced into a 5 mm NMR tube previously charged with 1:1 LAuCl/AgOTf (1 mol %). The mixture was briefly vigorously shaken, and the time count was started simultaneously. The progress of the reaction was monitored by 1H NMR at 30 °C. Conversion was calculated from the integral intensities of the corresponding signals (conversion [%] = (nacetal + nketone)/(n3‑hexyne) × 100). B. Cycloisomerization of N-(Prop-2-ynyl)benzamide to 2-Phenyl5-vinylidene-2-oxazoline. N-(Prop-2-yn-1-yl)benzamide (80 mg, 0.5 mmol) was dissolved in CDCl3 (500 μL) containing 5 μL of TMS. The solution was then transferred into a 5 mm NMR tube previously charged with 1:1 LAuCl/AgOTf (1 mol %), and the mixture was vigorously shaken. The time count was started simultaneously. The progress of the reaction was monitored by 1H NMR at 30 °C. Conversion was calculated from the integral intensities of the vinylidene signals (conversion[%] = (noxazole)/(npropagylamide) × 100). C. Cyclization of S2 to 2-Butyl-1H-indole. S2 (43 mg, 0.25 mmol) was dissolved in CDCl3 (500 μL) containing 5 μL of TMS. The solution was then transferred into a 5 mm NMR tube previously charged with 1:1 LAuCl/AgOTf (1 mol %), and the mixture was vigorously shaken. The time count was started simultaneously. The progress of the reaction was monitored by 1H NMR at 30 °C. Conversion was calculated from the integral intensities of the corresponding signals (conversion[%] = (nindole)/(naniline) × 100) (Table 3).

∞

Δq(z) =

L

time

conv [%]b

TOFic

A

Sc NAC Sc NAC Sc NAC

120 120 120 120 120 120

92 9 51 54 43

96.0 4.8 37.2 39.6 31.8

B C

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(2)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b01030. Additional catalytic data; additional NOCV data (PDF) XYZ coordinates of optimized complexes (XYZ)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the Ministero dell’Istruzione dell’Università e della Ricerca (MIUR, Rome, Italy) through the FIRB-futuro in ricerca (RBFR1022UQ, Novel Au(I)-based molecular catalysts: from know-how to know-why, “AuCat”) program.

Catalysis conditions: 30 °C, substrate (A: 0.88, B: 0.50, C: 0.25 mmol), MeOH (A: 4 equiv, B = C: 0 equiv), 1:1 LAuCl/AgOTf (1 mol %) in CDCl3 (A: 400 μL, B = C: 500 μL). bConversions and TOFi were determined by 1H NMR spectroscopy as an average of three runs. cTOFi = (nproduct/ncatalyst)/60 min. a

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⎧ Δθ = 8|CTdon| + 58|CTback | + Δθelect ⎨ ⎩ Δv = − 193|CTdon| − 528|CTback | + Δνelect

REFERENCES

(1) (a) Pohlki, F.; Doye, S. Chem. Soc. Rev. 2003, 32, 104. (b) Severin, R.; Doye, S. Chem. Soc. Rev. 2007, 36, 1407. (c) Müller, T. E.; Beller, M. Chem. Rev. 1998, 98, 675. (2) Walsh, P. J.; Baranger, A. M.; Bergman, R. G. J. Am. Chem. Soc. 1992, 114, 1708. (3) McGrane, P. L.; Jensen, M.; Livinghouse, T. J. Am. Chem. Soc. 1992, 114, 5459. (4) Uchimaru, Y. Chem. Commun. 1999, 1133. (5) Kadota, I.; Shibuya, A.; Lutete, L. M.; Yamamoto, Y. J. Org. Chem. 1999, 64, 4570. (6) Barluenga, J.; Aznar, F.; Liz, R.; Rodes, R. J. Chem. Soc., Perkin Trans. 1 1980, 1, 2732. (7) Li, Y.; Marks, T. J. Organometallics 1996, 15, 3770. (8) Haskel, A.; Straub, T.; Eisen, M. S. Organometallics 1996, 15, 3773. (9) (a) Widenhoefer, R. A.; Han, X. Q. Eur. J. Org. Chem. 2006, 20, 4555. (b) Goodwin, J. A.; Aponick, A. Chem. Commun. 2015, 58, 8730. (10) (a) Hashmi, A. S. K. Chem. Rev. 2007, 107, 3180. (b) Bandini, M. Chem. Soc. Rev. 2011, 40, 1358. (c) Boorman, T. C.; Larrosa, I.

Computational Details. All calculations were carried out using density functional theory employing Becke’s exchange functional49 plus the Lee−Yang−Parr correlation functional50 (BLYP) as implemented in the ADF package (v.2012).51 A triple-ζ basis set with two polarization functions was used on all atoms (TZ2P) with a small frozen core. Relativistic effects were included by means of a zeroth-order regular approximation (ZORA) Hamiltonian.52−54 The choice of the computational details guarantees that the results are comparable with our old papers.43−45 The Cartesian coordinates of all optimized structures used in this work are reported as Supporting Information. All complexes and relative fragments are closed-shell systems and show only positive frequencies. The DCD components CTdon and CTback can be derived from the following system of equations42 (eq 1): ⎪

⎪

z

where Δρ(x, y, z) is the difference between the electron densities of the complex and the occupied orbitals of the fragments, suitably orthogonalized to each other and renormalized. For the decomposition of Δρ(x, y, z), see ref 37. In order to quantify the charge transfer upon the bond formation, it is useful to fix a plausible boundary separating the fragments in the complexes. We choose the isodensity value representing the point on the z-axis at which equal-valued isodensity surfaces of the isolated fragments are tangent.

Table 3. Catalytic Performances of LAuCl in the Activation of Various Alkynesa reaction

∞

∫−∞ dx ∫−∞ dy ∫−∞ dz′Δρ(x , y , z′)

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