Cycloaurated Phosphinothioic Amide Complex as a Precursor of Gold

Article pubs.acs.org/Organometallics

Cycloaurated Phosphinothioic Amide Complex as a Precursor of Gold(I) Nanoparticles: Efficient Catalysts for A3 Synthesis of Propargylamines under Solvent-Free Conditions Eva Belmonte Sánchez,† María José Iglesias,† Hajar el Hajjouji,† Laura Roces,‡ Santiago García-Granda,‡ Pedro Villuendas,§ Esteban P. Urriolabeitia,§ and Fernando López Ortiz*,† Á rea de Química Orgánica, Universidad de Almería, Ctra. Sacramento s/n, 04120 Almería, Spain Departamento de Química Física y Analítica, Universidad de Oviedo, C/Julián Clavería 8, 33006 Oviedo, Spain § Instituto de Síntesis Química y Catálisis Homogénea, ISQCH, CSIC-Universidad de Zaragoza, Pedro Cerbuna 12, 50009 Zaragoza, Spain † ‡

S Supporting Information *

ABSTRACT: A C,S-cycloaurated complex based on an orthosubstituted phosphinothioic amide framework has been synthesized in high yield through tin(IV)−Au(III) transmetalation from the corresponding chlorodimethylstannyl derivative. The latter was prepared in a two-step process involving directed ortho lithiation/quenching with Me3SnCl followed by Me/Cl exchange. The tin(IV) and gold(III) complexes have been characterized in solution and in the solid state. In both complexes, the phosphinothioic amide moiety acts as a C−C−P−S pincer ligand with formation of five-membered-ring metalacycles. The use of the gold(III) complex in the amine−aldehyde−alkyne (A3) three-component coupling synthesis of propargylamines showed that the compound is transformed into the Sonogashira-type oalkynylphosphinothioic amide with generation of Au(I) nanoparticles with an average size of around 7 nm. These nanoparticles proved to be excellent catalysts in A3 coupling processes, providing propargylamines in high yields under solvent-free conditions with loadings as low as 0.1 mol % and without the use of additives.

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INTRODUCTION The unique properties of gold make its use in both homogeneous and heterogeneous catalysis a powerful tool in organic synthesis.1 Gold in the oxidation state +1 is the preferred catalyst due to the tendency of Au(III) to undergo reduction, affording metallic gold. Decomposition pathways under the reaction conditions could also occur through the possible incorporation of ligands to the coordination sphere of Au(III). The search for new complexes containing ligands able to tune the electronic and/or steric features around the gold(III) center is, thus, of widespread interest for enhancing the catalytic effectiveness and selectivity. On the other hand, a rational design of the catalyst implies knowing the reaction mechanism. For this purpose, the incorporation of organic fragments to the gold centers may provide useful information by monitoring the course of the reaction through spectroscopic techniques with identification of possible intermediates. Despite this, in most cases activation of C−C multiple bonds is performed using inorganic salts AuX3 (X = Cl, Br).2 However, some of the reactions catalyzed by these salts proceed with low conversion in organic solvents.3 Moreover, AuCl3 is very hygroscopic and acidic, so that its catalytic activity is low in reactions involving acid-sensitive substrates. Au(III) can be stabilized toward reduction by integrating the metal ion into a metallacycle, among which C,N-chelating systems are the most commonly used ligands.4 © XXXX American Chemical Society

A small number of C,X-cycloaurated complexes have been reported in which the X donor site is part of a PX linkage, including complexes with ortho-metalated phosphazenes 1, 2 (X = N),4a,5,6 phosphinic amide 3 (X = O),7 phosphine sulfide 4, phosphonothioic amide 6 (X = S), and phosphine selenide 5 (X = Se) ligands (Figure 1).8 Except for complex 3,7 the general method for synthesizing the cyclometalated aryl gold(III) complexes shown in Figure 1 is transmetalation from the corresponding ortho-mercurated compounds.4a,5,8 However, this method failed to obtain the phosphine oxide analogue of 4 and 5.8 Although the organophosphorus C,X-backbone of these complexes makes the d8 square-planar configuration of Au(III) remarkably stable, their application in catalysis remains largely unexplored. The transformations involving these complexes include the addition of 2-methylfuran and electron-rich arenes to methyl vinyl ketone in the presence of silver salts such as AgOTf and AgBF4,5a,9 intramolecular hydroamination reactions, and the synthesis of propargylamines through the coupling of an alkyne and an amine to a dihalomethane used as the reaction solvent.10 A detailed study of the mechanism of the last process showed that the catalytically active species were Au(I) nanoparticles generated in situ upon reduction of the Au(III) complex.10 We have Received: February 8, 2017

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DOI: 10.1021/acs.organomet.7b00102 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

similar way to that developed for the synthesis of the P(O)−N analogue 3 (Scheme 1).7 Compound 8 is accessible by directed Scheme 1. Synthetic Route for the Preparation of 10

Figure 1. C,X-cycloaurated complexes containing phosphorus in the organic ligand.

ortho lithiation of the N,N-diisopropyl-P,P-diphenylphosphinothioic amide 7 with [n-BuLi·TMEDA] in diethyl ether at 0 °C followed by electrophilic quenching with Me2SnCl2.31 However, this procedure affords 8 in a low yield of 40%. A more efficient alternative consists of a two-step process involving the formation of the o-trimethylstannylated phosphinothioic amide 9 by trapping the ortho anion of 7 with Me3SnCl31 followed by Sn−Me bond cleavage with HCl− Et2O32 to give the desired chlorodimethylstannane 8 in 60% overall yield. The treatment of 8 with 1 equiv of potassium tetrachloroaurate(III) in refluxing acetonitrile for 2 h provided the phosphinothioic amide gold(III) metalacycle 10 with quantitative conversion. Pure 10 was isolated in 71% yield after precipitation in Et2O as a yellow solid that can be stored for months without alteration. The structure of complex 10 is readily identified through the analysis of the mass spectrometry and NMR spectroscopy data. The HRMS (ESI) spectrum shows a peak at m/z 589.0894 amu, consistent with the expected mass of the molecule resulting from the exchange of a chlorine atom by an acetonitrile group ([M − Cl + CH3CN]+calc = 589.0908 amu). This substitution reaction is promoted by the solvent used in the HRMS measurement, as evidenced by the absence of signals of a CH3CN ligand in the 13C NMR spectrum of 10 (see the Supporting Information). Obviously, the exchange of the Me2SnCl group of 8 by the AuCl2 moiety is accompanied by the loss of the corresponding methyl signals and the 119Sn satellites in the 1H and 31P NMR spectra of 10, respectively. The existence of S···Au coordination is supported by the changes observed in the FT-IR spectra for the PS stretching vibration. In neat samples, this absorption decreases in the series 7 (671 cm−1) > 8 (664 cm−1) > 10 (580 cm−1), indicating a weakening of the PS bond due to an increase in the S···M (M = Sn, Au) interaction along that series with a rather abrupt decrease when tin is exchanged with gold. Consequently, the P−N bond is reinforced along the same series of compounds: 7 (970 cm−1) < 8 (977 cm−1) < 10 (1000 cm−1). The effect of the intramolecular PS···Au interaction on the NMR spectra is not as clear as in the analogous PO··· Au coordination in 3. The Sn(IV)−Au(III) transmetalation of 8 led to a slight shielding of the 31P nucleus, Δδ(31P)10−8 = −1.52 ppm, whereas a large deshielding of Δδ(31P) = 35.1 ppm was observed for the phosphinic amide analogues. The

reported that propargylamines were efficiently synthesized in the three-component coupling of an aldehyde, an amine, and an acetylene (A3 coupling) in the presence of 1−3 mol % of the phosphinic amide Au(III) complex 3 in acetonitrile as solvent.7 We surmised that a C,S-cycloaurated complex analogous to 3 could provide insight into the effect of the X-donor site in this transformation and into the reaction mechanism. Propargylamines are important building blocks in organic synthesis11 for the preparation of a variety of products, and these moieties are also present in a large number of natural products12 and compounds of pharmaceutical interest.13 The one-pot A3 coupling synthesis of these compounds14 may be improved by using solvent-free conditions.15 Recently, various solvent-free synthesis of propargylamines based on alkyne C− H activation by metal catalysts such as lithium,16 rhenium,17 silver,18 copper,19,20 nickel,21 zinc,22 manganese,23 iron,24 indium,25 and some polymetallic systems26 have been reported. For gold, the preparation of propargylamines via solvent-free A3 processes is limited to the use of AuBr327 and silica-supported Au0 nanoparticles.28 The good biocompatibility of gold provides an additional advantage to the eco-friendly synthesis of propargylamines using this metal as catalyst.29 Nicholson et al. found that the catalytic activity of C,Xcycloaurated systems 1, 2, 4, and 5 in the alkylation of 2methylfuran depends on the nature of the heteroatom X = N, S, Se, with the greatest reactivity being observed for the chalcogenide derivatives.9 On the basis of these features, we decided to investigate the synthesis and reactivity of the sulfur analogue of 3. Here we report the results obtained in the preparation and structural characterization, in solution and the solid state, of the new phosphinothioic amide C,S-cycloaurated complex 10. We also report its application as a highly active precatalyst that originates Au(I) nanoparticles, which proved to be very active in the solventless synthesis of propargylamines through an A3 coupling process.

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RESULTS AND DISCUSSION Synthesis of the Phosphinothioic Amide Gold(III) Complex 10. The cycloaurated phosphinothioic amide complex 10 has been prepared via Sn(IV)−Au(III) transmetalation30 from the o-chlorodimethylstannyl derivative 8 in a B


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Organometallics

longer than that observed for the PO derivative (2.505(1) Å), indicating a larger trans effect of sulfur vs that of oxygen. Compound 10 crystallizes in the triclinic space group P1̅. The asymmetric unit contains two unique molecules, one of which is shown in Figure 3. Selected structural parameters for

shielding effect of the Sn(IV)/Au(III) exchange on the ipso carbon bonded to the metal in the C,S chelate, Δδ(CAu)10−8 = −4.1 ppm, is almost half that found in the C,O chelate, Δδ(CAu) = −7.51 ppm. It is known that chemical shifts depend on a number of factors that may oppose each other. In polar species, shielding/deshielding effects associated with positive/ negative charges may be counterbalanced by electric field effects caused by those charges. The different trends observed in the NMR spectra of 8/10 with respect to the corresponding phosphinic amide derivatives may be assigned to changes in the charge distribution in the metalacycles. Solid-State Characterization of Complexes 8 and 10. Crystals of both complexes suitable for X-ray diffraction analysis were obtained through recrystallization from CH3CN−CHCl3 mixtures at room temperature for 2 days. 8 crystallizes in the orthorhombic space group Pbca. The molecular structure is shown in Figure 2, and selected crystal

Figure 3. ORTEP drawing of one of the unique molecules in the asymmetric unit of 10 (molecule B). Ellipsoids are drawn at 50% probability.

Table 1. Selected Structural Parameters for 10 molecule A P1−S1 P1−N1 P1−C1 Au1−C2 Au1−Cl2 Au1−Cl1 Au1−S1

Figure 2. ORTEP drawing of 8. Ellipsoids are drawn at 50% probability. Selected bond distances (Å) and angles (deg) with estimated standard deviations: Sn1−C8 2.127(3), Sn1−C7 2.129(3), Sn1−C1 2.155(3), Sn1−Cl1 2.5228(8), Sn1−S1 2.7974(8), P1−S1 1.9846(10); C8−Sn1−C7 123.04(12), C8−Sn1−C1 122.05(11), C1− Sn1−C7 113.82(11), S1−Sn1−Cl1 178.75(2), Cl1−Sn1−C7 91.06(9), Cl1−Sn1−C8 93.30(9), Cl1−Sn1−C1 96.07(7).

Cl2−Au1−Cl1 S1−Au1−Cl1 S1−Au1−Cl2 C2−Au1−Cl1 C2−Au1−Cl2 C2−Au1−S1 P1−S1−Au1

data are given in Table S1 in the Supporting Information. The crystal structure of 8 is very similar to that of the phosphinic amide analogue. The tin atom in 8 adopts a distorted-trigonalbipyramidal geometry (tbp) showing a difference between the equatorial and apical bonds33 of ΔΣθ = Σθec − Σθax = 78.47°. The apical positions are occupied by the sulfur and chlorine atoms, leading to a S1−Sn1−Cl1 bond angle of 178.75(2)°, slightly larger than that found in the PO analogue (176.06(4)°),7 whereas the deviation of the metal from the centroid of 0.147 Å is lower than that in the phosphinic amide derivative (0.177 Å).34 Coordination between sulfur and tin is reflected in the S1··· Sn1 distance of 2.7974(8) Å, shorter than the sum of the van der Waals radii of both atoms (3.970 Å), and the elongation of the P1−S1 bond (1.985(1) Å) with respect to the PS linkage of uncomplexed phosphinothioic amides (average distance of 1.944 Å). 35 The same behavior is observed in the triorganostannyl hexafluorophosphate {4-t-Bu-2-[P(O)(O-iPr)2]-6-[P(S)Ph2]-C6H2}SnPh2+PF6− (P−S distance 2.006(1) Å), the only example described to date where a sulfur atom of a phosphine sulfide moiety interacts with a tin atom in the ortho position.36 The Sn1−Cl1 distance of 2.5228(8) Å is notably

Bond Distances (Å) 2.0472(18) 1.626(4) 1.792(5) 2.043(5) 2.3045(13) 2.3596(14) 2.3009(12) Bond Angles (deg) 90.58(5) 88.17(5) 177.86(5) 177.37(14) 91.97(14) 89.31(14) 94.17(6)

molecule B 2.0455(16) 1.633(4) 1.789(5) 2.049(4) 2.3070(15) 2.3506(13) 2.3018(15) 89.87(6) 86.37(5) 176.24(5) 176.91(15) 92.92(14) 90.83(14) 98.90(6)

both molecules are given in Table 1. 10 consists of a fivemembered-ring metalacycle that adopts a slightly distorted envelope conformation with the sulfur atom puckered out the plane defined by the other four atoms of the heterocycle (torsion angles for molecules A/B: P1−C1−C2−Au1 3.6(6)/ 0.5(6)°, C1−C2−Au1−S1 −27.6(4)/−15.7(4)°, and C2−C1− P1−S1 = 26.6(4)/17.5(4)°). The geometry around the gold atom is essentially square planar with bond angles ranging from 88.17(5)/86.37(5)° (angle S1−Au1−Cl1) to 91.97(14)/ 92.92(14)° (angle C2A−Au1−Cl2) for molecules A/B. The comparison of the structurally related cycloaurated complexes 3, 4, 6, and 10 provides some trends regarding the geometry of the metallacycle. Thus, the ligands containing the PS group show very similar bite angles and P−S−Au angles of about 90 and 95°, respectively. The corresponding angles in the PO complex 3 deviate significantly from these values C


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Organometallics Table 2. A3 Coupling Reactions Catalyzed by 10a

entry

R1

R2

amine

n (mol %)

solvent

time (h)

11

conversn (%)b

c

Ph Ph Ph 4-ClC6H4 Ph Ph Ph Ph Ph Ph Ph Ph Ph 4-ClC6H4 4-MeOC6H4 Ph Ph Ph Ph Ph Ph Ph 4-ClC6H4 Ph Ph

Ph Ph Ph Ph TMS Ph TMS TMS TMS TMS TMS TMS Ph TMS TMS CH3(CH2)3 4-FC6H4 3-(HCC)C6H4 TMS Ph Ph TMS TMS TMS TMS

Pip Pip Pip Pip Pip Mor Mor Pip Pip Pip Pip Pip Pip Pip Pip Pip Pip Pip Mor Mor MePro MePro MePro Pip Pip

3 1 1d 3 3 3 3 3 3 3 1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1h 0.1i

CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN H2O H2Oe

6 12 12 6 6 6 6 6 6 6 6 8 8 8 14 8 16 8 8 14 14 6 6 6 8

a a a b c d e c c c c c a f g h i j e d k l m c c

95 74 62 71 97 96 98 66 85 99 97 99 94 >99 90 42 94 91 93 98 91 (95)f 95 (99)f 87 (97)f 99 90

1 2 3 4 5c 6 7 8 9c 10c 11 12 13 14 15 16 17 18 19 20 21 22g 23g 24 25

a Conditions: 10/aldehyde/amine/alkyne = n/1/1/1.5. bDetermined by 1H NMR analysis of the crude product. c31P NMR measurements were carried out, showing that 13 and in some cases 7 were obtained as byproducts (see text) originating from 10. d1 mol % of AgOTf added. e Deoxygenated H2O was used as solvent. fValues in parentheses indicate diastereomeric excesses. g2 equiv of amine was used. h3 was used as catalyst. i AuCl was used as catalyst.

(angle C2−Au1−O1 of 86.6(3)°, angle Au1−O1−P1 of 117.0(3)°). The puckering of the metalacycle in 10 is comparable to that of the phosphine sulfide 4 (torsion angles C1−C2−Au1−S1/C2−C1−P1−S1 of −22.2(3)/26.7(4)°),8 with compounds 3 and 6 showing the flattest and the most twisted five-membered-ring conformations, respectively (see Table S2 in the Supporting Information for selected torsion angles). The PS bond distance of 2.0464 Å (average of two molecules) is the longest in the series of known ortho-aurated PS derivatives 4 (2.0324(16) Å) < 6 (average of two molecules of 2.0427 Å) < 10.8 This elongation of the PS bond upon binding to the gold(III) atom is in agreement with the large decrease in the PS absorption frequency observed in the FT-IR spectrum in comparison with those in the tin derivative 8 and the unsubstituted phosphinothioic amide 7. In contrast, the bond distances involving the gold atom of 10 are shorter than the corresponding distances reported for 4 and 6 (see Table 1 and Table S2). The largest differences are found for the S···Au distances (cf. 2.3009(12)/2.3018(15) Å for 10, 2.3073(11) Å for 4, and 2.3108(14)/2.3064(13) Å for 6) and the Au−Cl bonds (cf. Au1−Cl2/Au1−Cl1 lengths of

2.3045(13)/2.3596(14) and 2.3070(15)/2.3506(13) Å for molecules A/B of 10, with 2.3094(12)/2.3535(11) Å in 4 and 2.3178(13)/2.3700(13) and 2.3205(13)/2.3715(12) Å for molecules A and B of 6). These features indicate that the coordination properties of the C,S-bidentate ligand of 10 involving a phosphinothioic amide group are not intermediate between those of 4 and 6 containing a phosphine sulfide and phosphonothioic amide moieties, respectively. The chelating ability of the ligand of 10 seems to be slightly stronger. On the other hand, the Au1−Cl2 bond length of 10 is notably longer than that in the PO analogue 3 (2.243(2) Å), which supports the greater trans influence of sulfur with respect to oxygen.5a,b,7 Catalytic Activity of Cycloaurated Complex 10 in A3 Coupling Reactions. As mentioned above, propargylamines are important compounds in organic synthesis.11 We have previously reported about the effectiveness of the C,Ocycloaurated complex 3 (Scheme 1) as a catalyst for the preparation of propargylamines through three-component coupling (A3) reactions.7 The structural analysis of complexes 3 and 10 revealed some differences associated with the gold··· chalcogenide interaction. It has been noted that C,XD


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Organometallics

11a with 94% conversion in 8 h (Table 2, entry 13). In contrast to the reaction in CH3CN as solvent (entry 4), the electronwithdrawing effect of the chlorine of p-chlorobenzaldehyde increased the reaction rate notably, so that quantitative conversion to 11f was achieved in 8 h (entry 14). Expectedly, the electron-donating group of p-methoxybenzaldehyde slows the reaction significantly, as evidenced by the formation of 11g in 90% conversion in 14 h (entry 15, conversion of 85% in 8 h of reaction). Aliphatic alkynes such as hex-1-yne also undergo the coupling, albeit with notably lower efficiency (conversion of 11h of 42% in 8 h, entry 16), in agreement with the known poor performance of aliphatic alkynes in the A3 synthesis of propargylamines. Electron-withdrawing groups in the alkyne also produced a decrease in the reaction rate. The use of 4-fluorophenylacetylene as alkyne provided 11i with a conversion of 94% in 16 h (Table 2, entry 17). In the equimolecular reaction of benzaldehyde, 1,3-diethynylbenzene, and piperidine in the presence of 0.1 mol % of 10 at 60 °C over 8 h only the monopropargylamine 11j was formed (91% conversion, entry 18). Morpholine proved to be slightly less reactive than piperidine. The coupling of this amine with benzaldehyde and trimethylsilylacetylene over 8 h furnished 11e with 93% conversion (entry 19), whereas the analogous reaction using phenylacetylene as the alkyne required a longer reaction time of 14 h to form the propargylamine 11d with a conversion of 98% (entry 20). Chiral propargylic amines 11k−m were synthesized in high conversions and excellent diastereoselectivities using (S)-2(methoxymethyl)pyrrolidine as a chiral reagent (Table 2, entries 21−23). Slight modifications of the standard procedure were introduced to achieve performances similar to those of the achiral derivatives. Thus, 11k was obtained in a conversion of 91% (de of 95%) after 14 h of reaction (entry 21). The coupling reactions involving trimethylsilylacetylene with benzaldehyde and p-chlorobenzaldehyde proceeded more efficiently to give 11l,m, respectively when 2 equiv of amine was used (6 h, entries 22 and 23). To complete the study, two additional experiments were carried out. For the sake of comparison, we carried out the synthesis of 11c using 0.1 mol % of 3 as catalyst. The 1H NMR spectrum of the crude reaction mixture showed that the A3 coupling was completed in 6 h (Table 2, entry 24), which indicates that the performance of the C,O-cycloaurated complex is slightly superior to that of the C,S analogue 10. It has been reported that AuCl (1 mol %, 100 °C, 12 h) is an efficient catalyst for the three-component synthesis of propargylic amines in water.3 Under solvent-free conditions, the reaction of benzaldehyde, trimethylsilylacetylene, and piperidine in the presence of 0.1 mol % of AuCl at 60 °C over 8 h afforded 11c in 90% conversion: i.e., lower than that achieved with the cycloaurated complexes 3 and 10 (entry 25). From the previous results we can conclude that the thiophosphinamidic gold(III) complex 10 catalyzes very efficiently the A3 coupling reaction for the synthesis of propargylamines under very mild, solvent-free conditions, with results comparable to those for phosphinamidic gold(III) complex 3. Catalyst loading can be reduced to 0.1 mol % without decreasing the activity, and no additives are required. Substituent effects are as expected. Electron-withdrawing groups on aryl aldehydes accelerate the reaction, and trimethylsilylacetylene displayed the highest reactivity among the alkynes tested.

cycloaurated complexes derived from phosphine sulfide 4 and phosphine selenide 5 (Figure 1) showed a higher catalytic activity than the isoelectronic phosphazenyl derivatives 1 and 2.9 This difference in reactivity has been associated with the higher trans effect of the softer chalcogenide donors in comparison to the nitrogen atom, which would facilitate the dissociation of the chloride trans to the X atom of the chelating system. In order to ascertain the influence of C,O vs C,S chelation of Au(III) in catalysis, we examined the catalytic activity of the thiophosphinamidic gold(III) complex 10 in the A3 process. In addition, the issue of the real catalytic species was also investigated. First, we undertook the synthesis of propargylamines under the catalysis of 10 using the procedure described for the analogous reaction catalyzed by 3. Thus, 1 equiv of benzaldehyde was allowed to react with 1 equiv of piperidine/morpholine and 1.5 equiv of phenylacetylene/ trimethylsilylacetylene in the presence of 3 mol % of 10 using acetonitrile as solvent at 60 °C for 6 h under a nitrogen atmosphere to afford propargylamines 11. Reactions proceeded with excellent conversions (Table 2, entries 1 and 5−7), comparable to those reported for the phosphinic amide gold(III) complex 3.7 The decrease in conversion when the catalyst loading was reduced to 1 mol % (entry 2) was similar to that found for the C,O-cycloaurated complex. However, in the case of 10 the addition of AgOTf did not improve the reaction rate. Instead, a slight decrease of the conversion to 62% was observed (entry 3). Conversion was notably lower with p-chlorobenzaldehyde (71%, entry 4). The reusability of the catalyst was tested for the synthesis of 11c (entry 5), showing that the yield is decreased to 78% after four runs. The search for more efficient and environmentally friendly synthetic methods represents a fundamental target in organic chemistry. In this context, very good results for the goldcatalyzed A3 coupling under mild conditions using water as solvent have been reported in the literature.3,37 We have explored the feasibility of the coupling of benzaldehyde, trimethylsilylacetylene, and piperidine in the presence of 3 mol % of 10 at 60 °C using water as solvent. Under such conditions, after 6 h of reaction, a 66% yield of propargylamine 11c was obtained (Table 2, entry 8). When water was deoxygenated prior to its use as solvent, conversion increased up to 85% (entry 9), indicating that oxygen exerts a negative effect on the catalyst. Going further with our search for a greener reaction, the previous results encouraged us to test the coupling under solvent-free conditions.15 To ascertain the catalytic activity of 10 in solventless reactions, we achieved the synthesis of 11c by heating benzaldehyde (1.97 mmol), piperidine (1.97 mmol), and trimethylsilylacetylene (2.91 mmol) in the presence of 3 mol % of 10 at 60 °C for 6 h under a nitrogen atmosphere. The 1H NMR spectrum of the crude reaction mixture showed the quantitative formation of 11c (Table 2, entry 10). Encouraged by this result, we evaluated the effect of the catalyst loading on the product yield, as this is a crucial issue in reactions catalyzed by noble metals.38 Repeating the reaction using 1 mol % of 10 as catalyst afforded 11c in 97% yield (entry 11). The best results were obtained by decreasing the catalyst loading to 0.1 mol % and heating the reaction mixture for 8 h at 60 °C. Under such conditions, propargylamine 11c was formed quantitatively (entry 12). With optimized reaction conditions available, we extended the study to the coupling of a variety of aldehydes, amines, and alkynes. Phenylacetylene reacted slightly more slowly to give E


Article

Organometallics Reaction Mechanism. A few phosphorus-containing cycloaurated complexes consisting of C,N-Au(III) chelates have been shown to catalyze a number of reactions.5a,9,10 The identification of the catalytically active species in these processes remains a challenge due to the low loadings of gold complexes used.9 In the study above, it has been assumed that complex 10 acts as a catalyst for the A3 coupling reactions. However, some features indicate that 10 undergoes a transformation in situ to the true catalyst. Thus, in the synthesis of propargylamines performed using a loading of 3 mol % of 10, a very small amount of a fine black powder is present in the reaction medium at the end of the process. Moreover, the 31P NMR spectra of the crude reaction mixtures corresponding to entries 1 (11a, solvent CH3CN), 5 (11c, solvent CH3CN), 9 (11c, solvent H2O), and 10 (11c, solventfree) in Table 2 showed the complete disappearance of 10 (δ 70.96 ppm). In the spectrum of the coupling reaction providing 11a using acetonitrile as solvent (entry 1) only a signal at δ 62.8 ppm was detected (Figure 4a). Interestingly, the 31P NMR

type products 13a,b (Figure 4 and Scheme 2). These assignments were confirmed by an alternative synthesis through Scheme 2. Synthesis of 13 Following a Sonogashira Procedure

a palladium(II)−copper(I)-catalyzed Sonogashira coupling between o-iodophosphinothioic amide 12a31 and trimethylsilylacetylene or phenylacetylene (Scheme 2).39 Compounds 13a,b were fully characterized on the basis of HRMS, FT-IR, and NMR spectroscopy data. The A3 synthesis of 11c using the phosphinic amide gold(III) complex 3 followed the same pattern. The process using 3 mol % of 3 was monitored through 1H and 31P NMR spectroscopy. After 15 min of reaction at 60 °C the 1H NMR spectrum showed the formation of propargylamine 11c in a conversion of 25%. The 31 P NMR spectrum evidenced that the phosphinic amide ligand of complex 3 (δ 72.2 ppm) was quantitatively converted into the Sonogashira product 13c (δ 31.30 ppm, Scheme 2). Subsequent measurements of 1H NMR spectra after 3 and 7 h of reaction provided conversions of 71% and 87%, respectively (the reaction in the NMR tube in the absence of stirring proceeds at a slightly lower rate than that on the laboratory scale; cf. Table 2, entry 24). No additional changes were observed in the respective 31P NMR spectra. For the phosphinothioic amide derivative, the structure of 13c was confirmed through an alternative synthesis via cross-coupling of o-iodophosphinic amide 12b40 with trimethylsilylacetylene (Scheme 2).39 Monitoring through 31P NMR spectroscopy the formation of 11c under solvent-free conditions using 0.1 mol % of 10 also showed that after ca. 10 min of reaction the only organophosphorus species present in the crude mixture were compounds 7 and 13b in almost equimolar amounts (Figure S1 in the Supporting Information). This implies that the detachment of the metal from the organic ligand in 10 takes place from the very beginning of the reaction. Gold(III) species have been proposed as intermediates in the cross-coupling of terminal alkynes and arylboronic acids catalyzed by Ph3PAuCl in the presence of the strong oxidant Selectfluor.41 However, Corma and co-workers showed that in the reaction between terminal alkynes and aryl halides using homogeneous and hetereogenous gold(III) complexes as catalysts only low yields of bis-alkyne homocoupling products were obtained.42 Notwithstanding, Minghetti, Stoccoro, and coworkers43 found that the C,N,N-cyclometalated complex 14 undergoes reductive elimination upon treatment with 2 equiv of triphenylphosphine, leading to [Au(PPh3)2][PF6] 15 and the unsymmetrical acetylene 16 (Scheme 3). This transformation involving an Au(III) complex provides support for the formation of compounds 13 via a Sonogashira-type crosscoupling from the C,O- and C,S-cycloaurated complexes 3 and 10, respectively.

Figure 4. 31P NMR (121.49 MHz) spectra of crude reaction mixtures corresponding to the synthesis of 11a in CH3CN (a), 11c in CH3CN (b), 11c in H2O (c), and 11c without solvent (d) using 3 mol % of 10.

spectra of the synthesis of 11c consisted of two signals at δ 63.6 and 62.8 ppm in a ratio of 30:70 for the reactions in acetonitrile and in the absence of solvent (Figure 4b,d), whereas in water as solvent the ratio of these species was 80:20 (Figure 4c). The decrease of about 8 ppm in the δP of these species supports the loss of the metallic fragment from the ortho position of the phosphinothioic amide, as coordination of the sulfur atom to gold(III) causes the deshielding of the 31P. The signal at δ 63.6 ppm corresponds to the phosphinothioic amide 7,31 whereas the new signal at δ 62.8 ppm was assigned to the SonogashiraF


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Organometallics

water, whereas the maximum rate in THF as solvent was found for NPs with a particle diameter of 20 nm.45 Moreover, for cationic gold NPs supported on ZrO2 and CeO2, no correlation between the size of the particle and the performance of the catalysts in water as solvent was observed.46 The TEM images of the AuNPs formed under solvent-free conditions showed a pattern similar to that found for the reaction in solution with a narrow size distribution of about 7 nm diameter (Figure 5b and Figure S4 in the Supporting Information).47 However, in this case, no immobilizing support was detected. Interestingly, the use of AuNPs isolated from either the reaction in acetonitrile as solvent or without a solvent as catalysts in the A3 synthesis of 11c afforded the product in very low conversions (39/21% for the reaction in acetonitrile/ solventless). The XPS spectrum of the AuNPs generated in the synthesis of 11c after one and two catalytic runs consisted of one doublet at 84.8 (84.9 for the second run) and 88.5 (88.6 for the second run) eV (Figure 6 and Figures S5 and S6 in the Supporting

Scheme 3. Synthesis of Unsymmetrical Acetylene 16 via Reductive Elimination of the Gold(III) Complex 14

The above results indicate that, after all the reagents are mixed, complexes 3 and 10 undergo a transformation where the phosphorus-containing C,O-/C,S-chelating ligand leaves the coordination sphere of gold via reductive elimination and C(sp2)−C(sp) cross-coupling. The metal is detected as a black powder precipitated in the reaction medium when the synthesis is performed in the presence of 3 mol % of 10. In order to get insight into the composition of the gold species formed that act as the true catalyst, we achieved the characterization of the black powder using transmission electron microscopy (TEM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) methods. First of all, we analyzed the solid precipitated in the synthesis of 11c after 6 h of reaction (one cycle of catalysis) using acetonitrile as solvent. The TEM image showed highly dispersed spherical gold nanoparticles (AuNPs) with a homogeneous size of around 7 nm diameter (Figure 5a).

Figure 6. XPS deconvolution spectra of Au nanoparticles formed in the A3 synthesis of propargylamine 11c in the presence of 3 mol % of 10 in acetonitrile as solvent.

Information) for the Au 4f lines Au 4f7/2 and Au 4f5/2, respectively, corresponding to monovalent Au(I).10,42a,48 However, for the isolated AuNPs the binding energies for the Au 4f components appeared at 83.8 and 87.4 eV, indicating that the gold ions have been reduced to metallic gold (Figure S7 in the Supporting Information).42a,48 Only one contribution of gold was found in each of the three samples. In addition, the presence of silicon, oxygen, and nitrogen has also been detected in the samples measured from the crude reaction. These components may come from the matrix in which the AuNPs are immobilized. Additional experiments aimed at getting insight into the nature of the stabilizing agent of the AuINPs proved to be unsuccessful. The FT-IR spectrum of freshly prepared AuNPs measured as a suspension in Nujol showed no absorptions attributable to an acetylenic moiety. Heating the AuNPs overnight in the presence of PPh3 in CD3CN under reflux failed to produce soluble species. The 1H and 31P NMR spectra of the solution showed only the signals corresponding to free phosphine. Furthermore, when the AuNPs were treated with 1 equiv of AgBF4 over 6 h at 130 °C, no AgCl precipitate was formed and the 11B and 19F NMR spectra consisted of the expected signals for AgBF4. On the basis of the above analysis, it can be concluded that the catalytically active species in the synthesis of propargylamines 11 are Au(I) nanoparticles with an average size of about 7 nm formed by elimination of the organic moiety of

Figure 5. TEM images of AuNPs formed in the A3 synthesis of 11c in the presence of (a) 3 mol % of 10 after one catalysis cycle in acetonitrile as solvent and (b) 0.1 mol % of 10 under solvent-free conditions.

There also exist some very small AuNPs, with an average size of between 1.5 and 2.5 nm. After a second catalytic process the AuNPs grew slightly in size (2−3/7−9 nm diameter for the smallest/biggest AuNPs, respectively; Figure S2a in the Supporting Information). In both samples, the AuNPs appeared immobilized on an amorphous matrix. The SEM images of freshly prepared AuNPs showed the presence of gold nanoparticles embedded into the matrix together with a crystalline material assigned to piperidine chlorhydrate (Figure S3 in the Supporting Information). The AuNPs were separated from the crude reaction mixture through centrifugation and gently washed with CH3CN in the open air. The TEM analysis of the AuNPs isolated revealed that they formed agglomerates of nanoparticles with an average size of around 9−10 nm (Figure S2b). The effect of the particle size in the A3 coupling reaction catalyzed by AuNPs is unclear. High catalytic activity of AuNPs with gold in the metallic state has been reported for particles smaller than 5 nm44 when the reaction is carried out in G


Organometallics

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CONCLUSIONS In summary, the C,S-cyclometalated gold(III) complex 10 based on an ortho-substituted phosphinothioic amide backbone has been prepared via tin(IV)−gold(III) transmetalation from the corresponding o-chlorodimethylstannyl derivative 8 in high yield. The organotin and organogold molecules have been structurally characterized in solution and in the solid state. Both complexes exist as five-membered metalacycles in which the phosphinothioic amide moiety acts as a C,S-chelate ligand. The geometrical parameters around the gold atom indicate that the sulfur−gold interaction in 10 is stronger than that in the isoelectronic C,S-cycloaurated phosphine sulfide and phosphonothioic amide analogues. The new gold(III) complex acts as a precatalyst in the synthesis of propargylamines via A3 coupling processes in solution (acetonitrile and water) and under solvent-free conditions. In the presence of an amine and an acetylene complex 10 affords the Sonogashira-type product 13 and Au(I) nanoparticles. The latter are highly active catalysts in the three-component coupling reaction, promoting the formation of a variety of propargylamines in high yields under solvent-free conditions using very small loadings (0.1 mol %) without the use of silver salts as activators. The AuNPs generated from 10 are among the most active catalysts in analogous multicomponent reactions.

cycloaurated 10 that acts as the precatalyst. The homogeneous catalysis of the A3 coupling process by Au(I)/Au(III) salts is well-documented.3,37a,49 For gold nanoparticles, efficient procedures using gold in the metallic state14a,44,45 and Au(III) species immobilized onto a variety of supporting materials have been described.14b,37b,46,50 Very few investigations involving AuNPs with the gold in oxidation state +1 have been reported for this multicomponent transformation. Au(I) nanoparticles have been considered inefficient catalysts in comparison with Au(III) species for the synthesis of propargylamines via A3 coupling reactions.44a However, in PbS-Au nanocomposites containing Au(0) and Au(I) nanoparticles, greater catalytic activity in water as solvent was assigned to the cationic species.51 Propargylamines have been prepared via the threecomponent coupling of an alkyne, an amine, and a dihalomethane (solvent) catalyzed by Au(I) nanoparticles.10 Similarly to the behavior of complex 10, the AuNPs were formed in situ by reduction of a gold(III) cycloaurated phosphazenyl complex. To the best of our knowledge, there are no precedents for the catalysis by Au(I) nanoparticles in A3 coupling reactions under solvent-free conditions. A tentative reaction mechanism for the synthesis of propargylamines 11 through the condensation of an aldehyde, an acetylene, and an amine involving the gold(III) complex 10 is shown in Scheme 4.

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Scheme 4. Suggested Mechanism for the Formation of Au(I)NPs and the Catalyzed A3 Coupling Synthesis of Propargylamines 11

EXPERIMENTAL SECTION

Materials and Methods. All reactions were carried out under an inert atmosphere, in previously dried Schlenks. Et2O and toluene came from a Pure Solv 400-4-MD system. CH3CN was distilled in the presence of P2O5 and degassed before use. The rest of the solvents were used without any treatment. N,N,N′,N′-Tetramethylethylendiamine (TMEDA), triethylamine, benzaldehyde, piperidine, morpholine, phenylacetylene, and trimethylsilylacetylene were obtained through commercial suppliers and purified by distillation before use. The rest of the reagents were commercially purchased and used without further purification. Concentrations of the organolithium bases were tested through titration before their use, employing cyclooctadiene (COD) as an internal standard.53 Compounds 7,31 9,31 12a,31 and 12b40 were synthesized as previously reported. Compounds 8,31 11a,3 11b,3 11c,54 11d,55 11e,55 11h,51 and 11i51 were described previously. All new compounds were characterized on the basis of their spectroscopic data (IR, NMR) and high-resolution mass spectra. Elemental analysis of complex 10 is also included. NMR spectra were obtained on a Bruker Avance III HD 300 (1H, 300.13 MHz; 13C, 75.47 MHz; 31P, 121.49 MHz) and Bruker Avance III HD 500 (1H, 500.13 MHz; 13C, 125.76 MHz; 31P, 202.46 MHz). Chemical shifts are given in ppm using tetramethylsilane (TMS) for 1H and 13C and 85% H3PO4 for 31P as internal standards. Unless otherwise stated, 1H, 1 H{31P}, and 31P NMR spectra were acquired from all crude reaction mixtures, in CDCl3 as solvent. Diastereoselectivities were determined by integration of the 1H NMR spectra of the crude reaction mixtures. The following abbreviations are used to indicate the multiplicity of signals: s, singlet; d, doublet; t, triplet; q, quartet; sep, septet. Elemental analyses were determined on an ELEMENTAR Vario Micro CHNS analyzer. IR spectra were recorded using a Bruker Alpha FTIR instrument and registered in KBr tablets or using an attenuated total reflectance (ATR) module. High-resolution mass spectra (HRMS) were recorded on an Agilent Technologies LC/MSD-TOF and HP 1100 MSD spectrometer using electrospray ionization. Melting points were recorded on a Büchi B-540 capillary melting point apparatus and are uncorrected. TEM and XPS Measurements. The size and distribution of AuNPs formed in the reactions in acetonitrile as solvent using 3 mol % of 10 were studied by transmission electron microscopy (TEM) using JEOL-2000 FXII equipment working at 200 kV. Samples were dispersed in ethanol in an ultrasound bath for 10 min, and a drop of

First, amine-driven nucleophilic displacement of a chloride ion from cycloaurated complex 10 by alkyne would afford the monoalkynyl gold(III) complex I and the corresponding amine chlorhydrate. This intermediate species I may undergo hydrolysis or reductive elimination, leading to phosphinothioic amide 7 or the unsymmetric acetylene 13, respectively, together with Au(I) nanoparticles. These nanoparticles would be the catalytically active species of the A3 coupling reaction. They may activate the alkyne C−H bond through coordination to the carbon−carbon triple bond52 (intermediate complex II) to generate the alkynylgold complex III, which upon addition to the iminium ion IV proceeding from the in situ reaction between the aldehyde and the secondary amine would provide propargylamine 11 and regenerate the catalyst for a new cycle. H


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Organometallics the solution containing the nanoparticles was placed onto a copper grid coated with carbon film. TEM measurements of the AuNPs formed in the solvent-free A3 coupling using 0.1% of 10 were recorded on a JEOL-2100 TEM instrument operating at 200 kV fitted with an Orius SC 200 Model 830 (Gatan Inc.) camera. Samples were prepared by placing ca. 15 μL of the solution onto a Formvar- and carboncoated copper grid. The samples were allowed to dry and introduced in the instrument. For the statistical particle size analysis, 524 nanoparticles were analyzed using the measurement tools of the Digital Micrograph v2.31.734.0 (Gatan Microscopy Suite) software package. Data processing and statistical studies were performed with Excel Office 2010. X-ray photoelectron spectroscopy (XPS) was carried out on an ESCAPlus Omicron spectrometer using a monochromated Mg X-ray source (1253.6 eV). The binding energy scale was calibrated by setting the C 1s transition at 284.7 eV. Data were analyzed using the Casa XPS software package. Crystallography. Single-crystal X-ray diffraction data were collected on a Bruker AXS Smart Apex diffractometer for compound 8 and on an Oxford Diffraction Xcalibur Gemini R instrument using graphite-monochromated Mo Kα radiation for 10 (λ = 0.71069 Å). The crystal of 8/10 was kept at 100/123.15 K during data collection. Using Olex2,56 the structures were solved with the Superflip57 and olex2.solve58 structure solution programs for 8 and 10, respectively, using charge flipping and refined with the ShelXL59 refinement package using least-squares minimization. CCDC 1531490 for 8 and 1530698 for 10 contain supplementary crystallographic data for this paper. P-(2-(Chlorodimethylstannyl)phenyl)-N,N-diisopropyl-Pphenylphosphinothioic Amide (8). Although the synthesis of 8 has been described previously,31 we used a different method for its preparation. 9 (700 mg, 1.46 mmol) was dissolved in toluene (40 mL), and a solution of hydrogen chloride (1.88 mL of a 0.84 M solution in diethyl ether, 1.59 mmol) was added at room temperature. The reaction mixture was stirred for 15 min and then was concentrated under vacuum. The resulting yellowish oil was washed with diethyl ether, filtered, and dried under vacuum. This sequence was repeated twice. Isolated yield: 75% (548 mg, 1.09 mmol). Crystallization was carried out in CH3CN−CHCl3 mixtures, affording suitable crystals for X-ray diffraction. NMR data were in agreement with those reported previously.31 (2-((Diisopropylamino)(phenyl)phosphorothioyl)phenyl)gold(III) Chloride (10). To a solution of 8 (297 mg, 0.59 mmol) in CH3CN (15 mL) was added K[AuCl4] (224 mg, 0.59 mmol). The reaction mixture was stirred at 90 °C for 2 h, and then the solution was cooled, filtered, and concentrated under vacuum. The resulting yellow solid was washed with diethyl ether, filtered, and dried under vacuum. This sequence was repeated twice. Isolated yield: 71% (247 mg, 0.42 mmol). 10 was crystallized in CH3CN−CHCl3 mixtures, affording suitable crystals for X-ray diffraction. Anal. Calcd for C18H23AuCl2NPS: C, 37.00; H, 3.97; N, 2.40; S, 5.49. Found: C, 36.11; H, 3.88; N, 2.34; S, 5.43. Mp: 227 °C dec. IR (KBr, ν cm−1): 580 (PS). 1H NMR (300.13 MHz, CDCl3): δ 1.23 (d, 6H, J 6.9 Hz, CH3), 1.31 (d, 6H, J 6.9 Hz, CH3), 3.71 (dsep, 2H, JPH 17.8, J 6.9 Hz, CH), 7.31−7.45 (m, 3H, ArH), 7.60−7.64 (m, 2H, ArH), 7.70−7.74 (m, 1H, ArH), 8.10−8.15 (m, 2H, ArH), 8.45−8.48 (m, 1H, ArH). 13 C NMR (75.47 MHz, CDCl3): δ 23.17 (d, CH3, JPC 2.9 Hz), 23.51 (d, CH3, JPC 2.7 Hz), 50.37 (d, CH, JPC 3.7 Hz), 127.23 (d, Cipso, JPC 101.3 Hz), 127.31 (d, CAr, JPC 12.0 Hz), 129.45 (d, CAr, JPC 13.6 Hz), 130.97 (d, CAr, JPC 11.1 Hz), 133.49 (d, CAr, JPC 11.9 Hz), 134.46 (d, CAr, JPC 3.3 Hz), 134.70 (d, CAr, JPC 3.3 Hz), 136.57 (d, CAr, JPC 16.5 Hz), 138.14 (d, Cipso, JPC 119.9 Hz), 145.19 (d, Cipso, JPC 23.9 Hz). 31P NMR (121.49 MHz, CDCl3): δ 70.98. HRMS (ESI): m/z calculated C20H26AuClN2PS (M − 35 + CH3CN) 589.0908, found 589.0894. General Procedure for the Gold-Catalyzed Three-Component Coupling. Benzaldehyde (200 μL, 1.97 mmol) was dissolved in acetonitrile (5 mL), and then piperidine (195 μL, 1.97 mmol), 10 (34 mg, 0.059 mmol), and phenylacetylene (325 μL, 2.95 mmol) were added at room temperature. The reaction mixture was heated at 60 °C for 6 h, after which time the solution was cooled and the solvent

eliminated under vacuum. Yields were determined by integration of the 1H NMR spectra of the crude reaction mixtures. 1-(1-(4-Chlorophenyl)-3-(trimethylsilyl)prop-2-ynyl)piperidine (11f). Prepared according to the general procedure: 4-chlorobenzaldehyde (0.277 g, 1.97 mmol), piperidine (195 μL, 1.97 mmol), 10 (1.15 mg, 1.97 × 10−3 mmol), and trimethylsilylacetylene (409 μL, 2.95 mmol) afford the title compound as a colorless oil in 62% (0.368 g, 1.21 mmol) yield after column chromatography on silica gel (6% EtOAc in hexanes). IR (KBr, ν cm−1): 2163 (CC). 1H NMR (300.13 MHz, CDCl3): δ 0.28 (s, 9H), 1.44−1.49 (m, 2H), 1.56−1.63 (m, 4H), 2.46−2.49 (m, 4H), 4.58 (s, 1H), 7.32−7.35 (m, 2H, ArH), 7.54−7.57 (m, 2H, ArH). 13C NMR (75.47 MHz, CDCl3): δ 0.25 (CH3), 24.43 (CH2), 26.16 (CH2), 50.42 (CH2), 61.85 (CH), 92.69 (C′), 101.43 (C′), 128.12 (CAr), 129.73 (CAr), 133.13 (Cipso), 137.07 (Cipso). HRMS (ESI): m/z calculated C17H24ClNSi (M + H) 306.1445, found 306.1440. 1-(1-(4-Methoxyphenyl)-3-(trimethylsilyl)prop-2-ynyl)piperidine (11g). Prepared according to the general procedure: 4-methoxybenzaldehyde (240 μL, 1.97 mmol), piperidine (195 μL, 1.97 mmol), 10 (1.15 mg, 1.97 × 10−3 mmol), and trimethylsilylacetylene (409 μL, 2.95 mmol) afforded the title compound as a colorless oil in 64% (0.374 g, 1.24 mmol) yield after column chromatography on silica gel (2% EtOAc in hexanes). IR (KBr, ν cm−1): 2162 (CC), 1248 (s, C− O). 1H NMR (300.13 MHz, CDCl3): δ 0.27 (s, 9H), 1.43−1.49 (m, 2H), 1.55−1.66 (m, 4H), 2.46−2.48 (m, 4H), 3.83 (s, 3H), 4.56 (s, 1H), 6.89−6.91 (m, 2H, ArH), 7.48−7.51 (m, 2H, ArH). 13C NMR (75.47 MHz, CDCl3): δ 0.28 (CH3), 24.51 (CH2), 26.17 (CH2), 50.36 (CH2), 55.22 (CH3), 61.19 (CH), 91.87 (C′), 102.49 (C′), 113.32 (CAr), 129.58 (CAr), 130.46 (Cipso), 158.94 (Cipso). HRMS (ESI): m/ z calculated C18H27NOSi (M + H) 302.1940, found 302.1947. 1-(3-(3-Ethynylphenyl)-1-phenylprop-2-ynyl)piperidine (11j). Prepared according to the general procedure: benzaldehyde (100 μL, 0.99 mmol), piperidine (98 μL, 0.99 mmol), 10 (0.58 mg, 9.9 × 10−4 mmol), and 1,3-diethynylbenzene (197 μL, 1.48 mmol) afforded the title compound as a colorless oil in 60% (0.177 g, 0.60 mmol) yield after column chromatography on silica gel (2% EtOAc in hexanes). IR (ATR, ν cm−1): 3297 (C−H). 1H NMR (300.13 MHz, CDCl3): δ 1.46−1.49 (m, 2H), 1.60−1.65 (m, 4H), 2.56−2.60 (m, 4H), 3.12 (s, 1H), 4.82 (s, 1H), 7.29−7.42 (m, 4H, ArH), 7.46−7.53 (m, 2H, ArH), 7.64−7.68 (m, 3H, ArH). 13C NMR (75.47 MHz, CDCl3): δ 24.43 (CH2), 26.19 (CH2), 50.73 (CH2), 62.37 (CH), 77.75 (C′), 82.86 (C′), 86.86 (C′), 87.00 (C′), 122.39 (Cipso), 123.70 (Cipso), 127.53 (CAr), 128.11 (CAr), 128.40 (CAr), 128.49 (CAr), 131.63 (CAr), 132.10 (CAr), 135.35 (CAr), 138.40 (Cipso). HRMS (ESI): m/z calculated C22H21N (M + H) 300.1752, found 300.1754. (S)-2-(Methoxymethyl)-1-(1-phenyl-3-(trimethylsilyl)prop-2-yn-2yl)pyrrolidine (11l). Prepared according to the general procedure: benzaldehyde (100 μL, 0.99 mmol), (S)-2-(methoxymethyl)pyrrolidine (240 μL, 1.98 mmol), 10 (0.58 mg, 9.9 × 10−4 mmol), and trimethylsilylacetylene (204 μL, 1.48 mmol) afforded the title compound as a colorless oil in 52% (0.154 g, 0.51 mmol) yield after column chromatography on silica gel (3% EtOAc in hexanes). IR (ATR, ν cm−1): 2161 (CC). 1H NMR (300.13 MHz, CDCl3) δ: 0.26 (s, 9H), 1.59−1.75 (m, 3H), 1.89−1.98 (m, 1H), 2.49−2.53 (m, 1H), 2.63−2.71 (m, 1H), 3.20−3.28 (m, 1H), 3.39−3.54 (m, 2H), 3.43 (s, 3H), 5.11 (s, 1H), 7.25−7.38 (m, 3H, ArH), 7.58−7.61 (m, 2H, ArH). 13C NMR (75.47 MHz, CDCl3): δ 0.32 (CH3), 23.21 (CH2), 28.76 (CH2), 47.76 (CH2), 57.70 (CH), 59.16 (CH), 60.03 (CH3), 77.02 (CH2), 91.79 (C′), 102.66 (C′), 127.25 (CAr), 128.10 (CAr), 128.15 (CAr), 139.47 (Cipso). HRMS (ESI): m/z calculated C18H27NOSi (M + H) 302.1940, found 302.1941. (S)-1-(1-(4-Chlorophenyl)-3-(trimethylsilyl)prop-2-ynyl)-2(methoxymethyl)pyrrolidine (11m). Prepared according to the general procedure: 4-chlorobenzaldehyde (138 mg, 0.99 mmol), (S)2-(methoxymethyl)pyrrolidine (240 μL, 1.98 mmol), 10 (0.58 mg, 9.9 × 10−4 mmol), and trimethylsilylacetylene (204 μL, 1.48 mmol) afforded the title compound as a colorless oil in 57% (0.190 g, 0.57 mmol) yield after column chromatography on silica gel (5% EtOAc in hexanes). IR (ATR, ν cm−1): 2162 (CC). 1H NMR (500.13 MHz, CDCl3): δ 0.25 (s, 9H), 1.58−1.75 (m, 3H), 1.89−1.97 (m, 1H), I


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Organometallics

2.5 Hz), 131.08 (d, CAr, JPC 2.9 Hz), 131.54 (d, CAr, JPC 10.8 Hz), 133.71 (d, CAr, JPC 8.0 Hz), 135.01 (d, Cipso, JPC 127.7 Hz), 135.17 (d, CAr, JPC 9.7 Hz), 135.76 (d, Cipso, JPC 116.1 Hz). 31P NMR (CDCl3, 121.50 MHz): δ 31.01. HRMS (ESI): m/z calculated C23H32NOPSi (M + H) 398.2069, found 398.2079 (M+1).

2.46−2.50 (m, 1H), 2.60−2.65 (m, 1H), 3.19−3.24 (m, 1H), 3.41 (dd, 1H, J 9.4 Hz, J 5.6 Hz), 3.42 (s, 3H), 3.49 (dd, 1H, J 9.4 Hz, J 6.0 Hz), 5.09 (s, 1H), 7.29−7.31 (m, 2H, ArH), 7.52−7.54 (m, 2H, ArH). 13C NMR (125.76 MHz, CDCl3): δ 0.25 (CH3), 23.19 (CH2), 28.63 (CH2), 47.66 (CH2), 57.08 (CH), 59.14 (CH), 59.95 (CH3), 77.07 (CH2), 92.25 (C′), 102.06 (C′), 128.17 (CAr), 129.48 (CAr), 132.92 (Cipso), 138.12 (Cipso). HRMS (ESI): m/z calculated C18H26ClNOSi (M + H) 336.1551, found 336.1548. Monitoring of the Catalysis Reaction by NMR. In a dried 5 mm NMR tube with a THF-d8 internal capillary were placed benzaldehyde (200 μL, 1.97 mmol), trimethylsilylacetylene (409 μL, 2.95 mmol), 10 (1.15 mg, 1.97 × 10−3 mmol), and piperidine (195 μL, 1.97 mmol). The tube was manually shaken before it was inserted into the spectrometer probe, preheated to 60 °C. 1H and 31P NMR spectra were periodically measured. General Procedure for the Sonogashira Reaction.39 In a dried Schlenk were placed 12a (125 mg, 0.28 mmol), Et3N (0.85 mL, 6.10 mmol), trimethylsilylacetylene (47 μL, 0.34 mmol), PdCl2(PPh3)2 (4 mg, 5.70 × 10−3 mmol), and CuI (0.54 mg, 2.84 × 10−3 mmol). The mixture was heated to 55 °C for 2 h (12 h for 13b) and then was poured into water, extracted with CH2Cl2 (2 × 20 mL), washed with HCl 1 M (1 × 20 mL), and dried over anhydrous Na2SO4. It was then filtered over Celite and concentrated at reduced pressure. N,N-Diisopropyl-P-phenyl-P-(2-phenylethynyl)phenyl)phosphinothioic Amide (13a). Yellow solid, 61% yield after column chromatography purification on silica gel (6% EtOAc in hexanes as eluent). Mp: 106.9−107.2 °C. IR (ATR, ν cm−1): 691 (PS), 2219 (CC). 1H NMR (300.13 MHz, CDCl3): δ 1.24 (d, 6H, J 6.8 Hz, CH3), 1.40 (d, 6H, J 6.8 Hz, CH3), 3.84 (dsep, 2H, JPH 18.0 Hz, J 6.8 Hz, CH), 7.08−7.12 (m, 2H, ArH), 7.19−7.26 (m, 3H, ArH), 7.37− 7.40 (m, 3H, ArH), 7.46−7.50 (m, 2H, ArH), 7.62−7.66 (m, 1H, ArH), 7.98−8.05 (m, 2H, ArH), 8.20−8.29 (m, 1H, ArH). 13C NMR (75.47 MHz, CDCl3): δ 23.38 (d, CH3, JPC 2.3 Hz), 23.51 (d, CH3, JPC 2.9 Hz), 48.87 (d, CH, JPC 4.7 Hz), 89.30 (d, C′, JPC 5.4 Hz), 98.18 (s, C′), 122.95 (s, Cipso), 125.80 (d, Cipso, J 7.9 Hz), 127.67 (d, CAr, JPC 12.8 Hz), 127.77 (d, CAr, JPC 11.3 Hz), 127.85 (s, CAr), 128.18 (s, CAr), 130.51 (s, CAr), 131.22 (s, CAr), 131.54 (d, JPC 11.3 Hz, CAr), 132.68 (d, JPC 10.7 Hz, CAr), 134.84 (d, JPC 9.4 Hz, CAr), 136.32 (d, Cipso, JPC 101.7 Hz), 136.55 (d, Cipso, JPC 99.8 Hz). 31P NMR (121.49 MHz, CDCl3): δ 63.03. MS (ESI): m/z calculated C26H28NPS (M + H) 418, found 418. N,N-Diisopropyl-P-phenyl-P-(2-((trimethylsilyl)ethynyl)phenyl)phosphinothioic Amide (13b). Yellow solid, 72% yield after column chromatography purification on silica gel (2% EtOAc in hexanes as eluent). Mp: 105.7−106.5 °C. IR (ATR, ν cm−1): 692 (PS), 2154 (CC). 1H NMR (300.13 MHz, CDCl3): δ 0.00 (s, 9H, CH3), 1.26 (d, 6H, J 6.8 Hz, CH3), 1.37 (d, 6H, J 6.8 Hz, CH3), 3.82 (dsep, 2H, JPH 16.8 Hz, J 6.8 Hz, CH), 7.38−7.50 (m, 5H, ArH), 7.55−7.60 (m, 1H, ArH), 7.90−7.99 (m, 2H, ArH), 8.31−8.39 (m, 1H, ArH). 13C NMR (75.47 MHz, CDCl3): δ −0.64 (s, CH3), 23.31 (d, CH3, JPC 2.4 Hz), 23.40 (d, CH3, JPC 2.4 Hz), 48.84 (d, CH, JPC 4.8 Hz), 103.73 (s, C′), 103.86 (d, C′, JPC 4.8 Hz), 125.27 (d, Cipso, JPC 7.7 Hz), 127.82 (d, CAr, JPC 13.6 Hz), 128.05 (d, CAr, JPC 12.6 Hz), 130.37 (d, CAr, JPC 2.8 Hz), 130.57 (d, CAr, JPC 3.3 Hz), 131.36 (d, CAr, JPC 11.4 Hz), 133.06 (d, CAr, JPC 11.1 Hz), 135.72 (d, CAr, JPC 9.4 Hz), 136.45 (d, Cipso, JPC 101.7 Hz), 136.51 (d, Cipso, JPC 100.3 Hz). 31P NMR (121.49 MHz, CDCl3): δ 62.72. HRMS (ESI): m/z calculated C23H32NPSSi (M + H) 414.1841, found 414.1829 (M + 1). N,N-Diisopropyl-P-phenyl-P-(2-((trimethylsilyl)ethynyl)phenyl)phosphinic Amide (13c). Brown solid, 55% yield after column chromatography purification on silica gel (50% EtOAc in hexanes as eluent). Mp: 104.1 °C. IR (KBr, ν cm−1): 1179 (PO), 2158 (C C). 1H NMR (CDCl3, 300.13 MHz): δ −0.03 (s, 9H, CH3), 1.23 (d, 6H, J 6.7 Hz, CH3), 1.27 (d, 6H, J 6.7 Hz, CH3), 3.63 (dsep, 2H, JPH 16.6 Hz, J 6.7 Hz, CH), 7.36−7.49 (m, 5H, ArH), 7.53−7.57 (m, 1H, ArH), 7.78−7.85 (m, 2H, ArH), 8.16−8.23 (m, 1H, ArH). 13C NMR (CDCl3, 75.47 MHz): δ 0.58 (s, CH3), 22.79 (d, CH3, JPC 1.9 Hz), 23.26 (d, CH3, JPC 2.2 Hz), 46.81 (d, CH, JPC 5.0 Hz), 101.44 (s, C′), 104.18 (d, C′, JPC 4.6 Hz), 125.41 (d, Cipso, JPC 7.5 Hz), 127.91 (d, CAr, JPC 13.1 Hz), 128.11 (d, CAr, JPC 11.3 Hz), 130.61 (d, CAr, JPC

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00102. TEM images and XPS spectra of gold nanoparticles, NMR spectra of all new compounds, and ORTEP diagrams and crystallographic data for compounds 8 and 10 (PDF) Accession Codes

CCDC 1530698 and 1531490 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for F.L.O.: fl[email protected]. ORCID

Fernando López Ortiz: 0000-0003-1786-0157 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the MINECO and FEDER program for financial support (projects CTQ2011-27705 and CTQ2014-57157-P). E.B.S. thanks the MICINN for a Ph.D. fellowship. E.P.U. thanks the Gobierno de Aragón-FEDER (Spain, group E97) for financial support.

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REFERENCES

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