A Neutral Gold(III)–Boron Transmetalation - Organometallics (ACS

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A Neutral Gold(III)−Boron Transmetalation Manuel Hofer,† Enrique Gomez-Bengoa,‡ and Cristina Nevado*,† †

Department of Chemistry, University of Zurich, Winterthurerstrasse 190, CH-8057 Zurich, Switzerland Dpto. de Quı ́mica Orgánica, Universidad del Paı ́s Vasco, Apt. 1072, 20080 San Sebastián, Spain

‡

S Supporting Information *

ABSTRACT: The occurrence of direct transmetalation between gold(III) and boron species during gold-catalyzed crosscoupling reactions has recently become the subject of intense discussion. In this work, we investigate the transmetalation reaction between discrete, stable gold(III) complexes and boron reagents. Interestingly, electron-rich arylboronic acids remain unreactive under neutral conditions, whereas electron-deficient species undergo transmetalation in a highly efficient manner.

T

heteroatom bonds.11 In contrast to the widespread use of Pd(II)/B transmetalation, the interaction of boron reagents with other late transition metals, particularly gold, has been much less studied.12 Seminal work by Schmidbaur and Fackler demonstrated that aromatic−gold(I)−phosphine complexes could be obtained by reaction of R3PAuX with sodium tetraphenylborate at room temperature.13 A wider reaction scope, both on the ancillary phosphine ligand bound to gold and on the aryl moiety that is transferred, could be achieved with the use of CsCO3 in isopropyl alcohol, as demonstrated by Gray14 and Hashmi.15 Recently, this methodology has been applied to the synthesis of NHC−Au−aromatic compounds by Nolan and co-workers.16 Interestingly, control experiments seem to point toward the formation of a NHC−Au−OH complex rather than a “boronate” intermediate to explain the new C(sp2)−Au bonds, in line with Pd(II)/B transmetalation processes (Scheme 1, top).4−6

he exchange of organic ligands between two metallic species, better known as transmetalation, is one of the key steps in the course of metal-catalyzed cross-coupling reactions. In particular, the transmetalation of boron species with Pd(II) intermediates generated in a preceding oxidative addition step has been studied in detail.1 It is widely recognized that B/ Pd(II) transmetalation is facilitated in the presence of an inorganic base; however, the ultimate nature of the intermediates shaping transmetalation reactions is still a matter of lively discussions.2 Two mechanistic scenarios seem to prevail: the first one relies on the formation of a “boronate” intermediate (upon reaction of the inorganic base and the boronic acid or ester), which then transfers its organic ligand to the in situ generated Pd(II) intermediate.3 In contrast, recent studies by Jutand,4 Hartwig,5 and Schmidt6 suggested the formation of palladium−hydroxo species (generated by reaction of the base with the Pd(II) intermediate) that will then acquire the organic ligand from boron through an associative mechanism. Compelling experimental and computational evidence has been gathered supporting the latter pathway, although the inorganic nature of the base,7 heterogeneity in the reaction,8 and a scarce understanding of the role played by the reaction’s byproducts9 make the generalization of the mechanistic puzzle governing these transformation a challenging task. In the past few decades, gold(I) complexes have arisen as versatile tools to catalyze a wide variety of reactions involving unsaturated moieties, due to their high carbophilic Lewis acidity.10 For most of these transformations, the catalytic cycle is terminated via protonolysis of C−Au(I) bonds to regenerate the active species. However, more recently, the functionalization of C−Au(III) bonds has concealed further attention, as such intermediates could potentially offer an alternative, or at the very least a complementary platform, to Pd(II) intermediates for the efficient formation of C−C and C− © 2014 American Chemical Society

Scheme 1

Received: September 6, 2013 Published: February 5, 2014 1328

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transmetalation under neutral conditions. When the reaction mixture was heated to 150 °C for 9 h, complex 2 could not be detected. Instead, decafluorobiphenyl (3) and Ph3PAuCl were obtained in 89 and 91% yields, respectively (Scheme 2, eq 3). Control experiments showed that heating isolated cis-2 to 150 °C gives 3 in 87% yield (vertical arrow), thus suggesting a reductive elimination on 2 as the likely pathway for the formation of the new C(sp2)−C(sp2) bond.23 These results prompted us to examine the reactivity of gold(III) complexes related to 1. Thus, cis-dichloro(2,4,6trifluorophenyl)(triphenylphosphine)gold(III) (4) was prepared by the reaction of [AuI(2,4,6-C6F3H2)(PPh3)] with PhICl2 in dichloromethane. The structure of 4 could be confirmed by X-ray diffraction analysis (Figure 1).24 A

On the other hand, reactions combining [Au(I)] and Selectfluor and an additional metal reagent (B, Si) have been recently discovered.17 A representative example is the oxidative carbo-heterofunctionalization of alkenes in the presence of boronic acids reported by Zhang18 and Toste19 (Scheme 1, bottom). In these transformations, activation of the double bond seems to occur via gold(III) species which are formed in the first step of the catalytic cycle by oxidation of the gold(I) precatalyst with Selectfluor. Next, the intramolecular attack of a N,O-nucleophile affords an alkyl−gold(III) intermediate, which in the presence of aryl boronic acids reacts via transmetalation followed by reductive elimination18 or through a bimolecular reductive elimination to give the new C(sp3)−C(sp2) bonds.19 In the latter case, in the absence of base, the ability to build up a strong B−F interaction in the transition state is key to triggering the formation of a new C−C bond without a proper transmetalation step. While the transmetalations of κ3(C∧N∧C)*Au(OX) complexes with aryl and heteroaryl boronic acids in toluene are known to proceed smoothly,19j-l we decided to study the reactivity of boron reagents with aryl gold(III) dichloride complexes. We aimed not only to learn more about this area of gold reactivity but also to gain knowledge that could guide future gold-catalytic cross-coupling endeavors. Our study commenced with cis-dichloro(pentafluorophenyl)(triphenylphosphine)gold(III) (1), which was prepared in high yield by oxidation of the corresponding gold(I) complex [AuI(C6F5)(PPh3)] in the presence of PhICl2 according to a previously reported procedure.20 We hypothesized that the gold(III) center, electron-deprived by the presence of an electron-deficient aromatic ligand, would favor the transmetalation reaction with neutral boron species. However, the reaction of 1 with phenylboronic acid under neutral conditions returned only unreacted starting material at 25, 90, and 150 °C despite long reaction times (>24 h) (Scheme 2, eq 1). In

Figure 1. X-ray structure of complex 4.

comparative analysis between the X-ray structures of complexes 120a and 424 in terms of distances and dihedral angles showed interesting results. The chloride trans to the aromatic ligand (Cl2) is bound more strongly than the chloride trans to the phosphine ligand (Cl1) in compound 1. In contrast, the opposite trend is observed in compound 4 bearing a trifluorobenzene residue, thus reflecting the less electron deficient character of the trifluoro- vs the pentafluorobenzene ring (Table 1). The reaction of 4 in the presence of phenylboronic acid under neutral conditions afforded, in analogy to the previous case, unreacted starting material even at 150 °C (Table 2, entry 1). In contrast, the reaction with (pentafluorophenyl)boronic acid delivered the corresponding octafluorobiphenyl (5) and Ph3PAuCl in 94 and 92% yields, respectively (Table 2, entry 2) as a result of Au(III)−B transmetalation followed by reductive elimination. Similarly, the reaction with (trifluorophenyl)boronic acid resulted in the formation of the homocoupling product 6 in good yield (Table 2, entry 3). These results also rule out a potential transmetalation between two gold(III) species, with the aryl groups hopping around prior to the reductive elimination step. The reactions of 4 with the corresponding ethylene glycol and pinacol boronic esters did not proceed at all, and only starting materials were recovered from the reaction mixtures, thus showing that the nature of the boron reagent plays a key role in a successful transmetalation process (Table 2, entries 4 and 5). A competition experiment between complexes 1 and 4 in the presence of (C6F5)B(OH)2 was carried out. The reaction of 4 seems to be slightly favored (1:1.4 ratio of 3 to 5; see the Supporting Information for details), which could be related to the longer Au−Cl1 distance in complex 4 in comparison to that in 1 (see Table 1) and/or the easier reductive elimination on the putative bis-aryl gold(III) intermediate preceding the formation of products 3 and 5, respectively.

Scheme 2

contrast, the reaction of 1 with (pentafluorophenyl)boronic acid at 150 °C for 2 h allowed us to isolate cis-chlorobis(pentafluorophenyl)(triphenylphosphine)gold(III) (cis-2) in 73% yield,20a,21 thus proving that transmetalation of discrete gold(III) species with electron-deficient boronic acids can occur under neutral conditions (Scheme 2, eq 2). The reaction also returned minor amounts of decafluorobiphenyl (3) and pentafluorobenzene as a result of reductive elimination on 2 and protodeborylation of the boron reagent under the reaction conditions, respectively.22 To the best of our knowledge, this result is one of the first examples of efficient gold(III)−B 1329

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Table 1. Comparison of Complexes 1, 4, 7, and 8

bond (Å)/angle (deg) entry Au−Cl1 Au−Cl2 Au−C Cl1−Au−C

1 2 3 4

1

4

7

8

2.3230(9) 2.3167(9) 2.029(3) 88.58(11)

2.335(1) 2.343(1) 2.016(5) 87.2(1)

2.3464(7) 2.3241(8) 2.034(3) 81.47(8)

2.2710(6) 2.2806(6) 2.045(2) 87.04(7)

transmetalation while the trans arrangement of the chloride ligands eliminates the possibility of the competitive reductive elimination observed for 7 (eq 5). A comparison between the structural parameters of cis-[AuIII(C6F5)Cl2(PPh3)] (1) and cis[AuIII(C6F5)Cl2(PtBu3)] (7) is enlightening: the key Cl1−Au− C(sp2) dihedral angle is reduced in the latter (88.58 vs 81.47°) because of the large cone angle of P(tBu)3, thus explaining the facile reductive elimination in 7 to form a new C(sp2)−Cl bond (see Table 1). In addition, when the structural parameters of [AuIII(C6F5)Cl2(PPh3)] (1) were compared with those of [AuIII(C6F5)Cl2(IPr)] (8), decreasing bond angles were found. The high steric demand of the ancillary ligand reduces the sum of the angle of the other ligands and thus hinders transmetalation, in the order PtBu3 > IPr > PPh3. The Au−Cl bonds in 8, corresponding to 2.271 and 2.281 Å, respectively, seem to be stronger than those found in 1 (2.323, 2.317 Å), which together with the steric bulk of the NHC ligand might explain its lack of reactivity toward transmetalation. We decided to gain further insight in these transformations through DFT calculations. The transmetalations of 1 with both phenyl- and (pentafluorophenyl)boronic acid, are exothermic processes, as shown in Figure 2. A direct transmetalation

Table 2. Transmetalation Studies on Complex 4

entry

R2, X

1 2 3 4 5

Ph, H C6F5, H 2,4,6-C6F3H2, H C6F5, −CH2CH2− C6F5, −C(Me)2C(Me)2−

5a,b

6a,b

Ph3PAuClc

65d

92 91

94

a

The reactions were carried out using 4 (1 equiv) and R2-B(OH)2 (1 equiv). bYields determined by GC using dodecane as the internal standard. cIsolated yield after column chromatography on silica gel. d Complete conversion of the boronic acid was observed due to the formation of C6F3H3 as a side product.

Finally, we decided to explore the effect of the ancillary ligand on gold in this transmetalation reaction. For this purpose we selected cis-dichloro(pentafluorophenyl)(tri-tertbutylphosphine)gold(III) (7)20b and trans-dichloro[N,N-bis(2,6-diisopropylphenyl)imidazol-2-yl](pentafluorophenyl)gold(III) (8)20b (Scheme 3). We first tested their reactivity in the Scheme 3

Figure 2. Computational study on the transmetalation of 1. Activation energies are given in kcal/mol.

involving the exchange of the chloride ligand from Au(III) to the boron atom and aryl transfer from boron onto the metal center was studied. As shown in Figure 2, this mechanism appears to be highly unlikely. Not only are the activation energies for the corresponding transition states I and II abnormally high19b (>50 kcal/mol) but also the reactivity trend is against the experimental observations (57.8 kcal/mol for transmetalation of 1 with C6F5B(OH)2 vs 54.1 kcal/mol for the analogous transmetalation with C6H5B(OH)2). These results are not surprising as, in a classical transmetalation mechanism, the organic fragment on the boron reagent plays a role as nucleophile when being transferred to the metal center. Thus,

presence of phenylboronic acid. As in the previous cases, only unreacted starting material was observed in the case of complex 8. In contrast, the reaction of 7 delivered, after only 1 h, pentafluorochlorobenzene and [(tPBu3)AuICl] as a result of the reductive elimination in the starting gold complex under the reaction conditions (eq 4). The reaction of 7 with C6F5B(OH)2 afforded an outcome similar to that shown in eq 4, which emphasized the greater ability of this complex to undergo reductive elimination prior to transmetalation with the electron-deficient boron species.25 The reaction of 8 with C6F5B(OH)2 returned the starting complex unreacted, thus showcasing that the bulkiness of the NHC ligand blocks the 1330

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the presence of five fluorine atoms on the benzene ring would strongly compromise the nucleophilic character of the organic ligand to be transferred. A different mechanism in which the electrophilicity, rather than the nucleophilicity, on boron plays a key role must thus govern these transformations. The electrophilicity values (ω) obtained from the HOMO and LUMO energies on the boron reagents show that (pentafluorophenyl)boronic acid is a better electrophile (1.58 eV) in comparison to phenylboronic acid (1.10 eV).26 Thus, an explanation of the higher reactivity observed for electron-deficient boronic acids could stem from a stepwise mechanism in which the first, rate-determining step involves the activation of the gold by abstraction of a chloride ligand by the electrophilic boron, which would thus act as a Lewis acid. This possibility seems to be plausible on the basis of the equilibrium shown in Figure 3. Although it is well

dinates. This material is available free of charge via the Internet at http://pubs.acs.org.

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*E-mail for C.N.: [email protected]. Notes

The authors declare no competing financial interest. Biography

Cristina Nevado graduated in chemistry at the Autónoma University of Madrid in 2000. In October 2004 she received her Ph.D. in organic chemistry from the same university, working with Prof. Antonio M. Echavarren in late-transition-metal-catalyzed reactions. After a postdoctoral stay in the group of Prof. Alois Fürstner at the MaxPlanck-Institut für Kohlenforschung (Germany), she joined the University of Zürich as an Assistant Professor in May 2007. In 2011, Cristina was awarded the Chemical Society Reviews Emerging Investigator Award and the Thieme Chemistry Journal Award in recognition of her contributions in the field of synthetic organic chemistry. In 2012 she received an ERC Junior Investigator grant and has been awarded the Werner Prize of the Swiss Chemical Society. In 2013 she became Full Professor at the Organic Chemistry Institute of the University of Zürich. Rooted in the wide area of organic chemistry, her research program is focused on complex chemical synthesis and new organometallic reactions.

Figure 3. Electrophilic mechanism for Au(III)−B transmetalation.

recognized that the electrophilicity of boron, and thus its ability to accept anions such as chloride, is reduced in the presence of donating (hydroxy or alkoxy) substituents, the corresponding B−Cl-bonded complexes could be localized as zwitterionic intermediates III and IV. Upon chloride abstraction, the OH groups on the boronic acid coordinate to gold to stabilize the electrophilic metal center. The energy values show that the formation of the B−Cl bond is favored by 1.6 kcal/mol in the perfluorinated reagent in comparison to that in the phenylboronic acid. It is thus reasonable to propose that intermediates of type III precede the migration of the aryl ligand from boron to gold, although the exact nature of this process or alternative reaction mechanisms cannot be conclusively proposed at this stage.27 We report here the first examples of direct gold−boron transmetalation for aryl gold(III) dichlorides, which have been proposed as likely intermediates in gold(I)/gold(III) catalytic cycles.19j−l Discrete arene−gold(III) complexes transmetalate with electron-deficient boronic acids in the absence of base to give the corresponding diaryl−gold(III) complexes. Unlike gold(III) hydroxo complexes, under our conditions gold(III) chloride complexes proved unfortunately unreactive toward electron-rich boronic acids.

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AUTHOR INFORMATION

Corresponding Author

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ACKNOWLEDGMENTS The European Research Council (ERC Starting grant agreement no. 307948) and the Organic Chemistry Institute of the University of Zurich are kindly acknowledged for financial support. Prof. Anthony Linden is kindly acknowledged for the X-ray crystal structure determinations and the SGI/IZO-SGIker UPV/EHU and Schrödinger (UZH) for allocation of computational resources.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Text, figures, tables, and CIF files giving synthetic and computational details, crystallographic data, and XYZ coor1331

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Flower, K. R.; Pritchard, R. G.; Brisdon, A. K.; Quayle, P. Dalton Trans. 2011, 40, 11696−11697. (20) (a) Hofer, M.; Nevado, C. Eur. J. Inorg. Chem. 2012, 9, 1338− 1341. (b) Hofer, M.; Nevado, C. Tetrahedron 2013, 69, 5751−5757. (c) Gaillard, S.; Slawin, A. M. Z.; Bonura, A. T.; Stevens, E. D.; Nolan, S. P. Organometallics 2010, 29, 394−402. (d) Pažický, M.; Loos, A.; Ferreira, M. J.; Serra, D.; Vinokurov, N.; Rominger, F.; Jäkel, C.; Hashmi, A. S. K.; Limbach, M. Organometallics 2010, 29, 4448−4458. (21) Uson, R.; Laguna, A.; Pardo, J. Synth. React. Inorg. Met.-Org. Chem. 1974, 4, 499−513. (22) Upon heating to 150 °C, the decomposition of (pentafluorophenyl)boronic acid to pentafluorobenzene was clearly observed. Thus, we favor the idea of a protodeborylation to explain the formation of pentafluorobenzene in the reaction mixture in comparison to protodeauration, as no significant amount of C6F5H was observed from 1 (see Scheme 2, eq 1) under similar conditions. (23) For reductive elimination on gold(III) bis-aryl species, see: (a) Wolf, W. J.; Winston, M. S.; Toste, F. D. Nat. Chem. 2014, 6, 159− 164 For previous studies on gold(III) bis-alkyl species, see:. (b) Komiya, S.; Kochi, J. K. J. Am. Chem. Soc. 1976, 98, 7599−7609. (c) Komiya, S.; Albright, T. A.; Hoffmann, R.; Kochi, J. K. J. Am. Chem. Soc. 1976, 98, 7255−7265. (d) Hashmi, A. S. K.; Blanco, M. C.; Fischer, D.; Bats, J. W. Eur. J. Org. Chem. 2006, 1387−1389. (e) Ghidiu, M. J.; Pistner, A. J.; Yap, G. P. A.; Lutterman, D. A.; Rosenthal, J. Organometallics 2013, 32, 5016−5029. For studies on reductive elimination for other transition metals, see: (f) Roy, A. H.; Hartwig, J. J. Am. Chem. Soc. 2001, 123, 1232−1233. (24) CCDC 952781 (4) contains supplementary crystallographic data for this paper. Crystallographic data for 1, cis-2, 7, and 8 have been previously reported. See ref 20a and also CCDC 843007 and 842899 for 1 and cis-2, respectively. See ref 20b and CCDC-920666 and CCDC-921424 for 7 and 8, respectively. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. (25) C6F5H stemming from competitive protodeuration and/or protodeborylation process was also detected. (26) (a) Parr, R. G.; von Szentpaly, L.; Liu, S. J. Am. Chem. Soc. 1999, 121, 1922−1924. (b) Gomez-Bengoa, E.; Helm, M. D.; Plant, A.; Harrity, J. P. A. J. Am. Chem. Soc. 2007, 129, 2691−2699. (27) The reactions summarized in Scheme 2, eqs 2 and 3, were repeated in the presence of BHT without significant variation in the reaction outcome. Reactions in the presence in TEMPO showed a decreased efficiency, since both the boronic acid and the starting gold complex 1 undergo decomposition upon stirring in the presence of TEMPO at high temperatures.

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NOTE ADDED AFTER ASAP PUBLICATION In the version of this paper that was published on February 5, 2014, there was an inadvertent omission of a relevant precedent by Bochmann and co-workers describing examples of transmetalation of gold(III) hydroxide with boronic acids. In the version of this paper that appears as of February 19, 2014, the appropriate references have been added as refs 19j−l. In addition, we now cite this previous work in the main text and in our conclusions at the end of the paper. We apologize for this unintentional omission.

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