Published: August 23, 2011

r 2011 American Chemical Society 14293 dx.doi.org/10.1021/ja2012627 | J. Am. Chem. Soc. 2011, 133, 14293–14300

ARTICLE

pubs.acs.org/JACS

Two Metals Are Better Than One in the Gold Catalyzed OxidativeHeteroarylation of AlkenesEkaterina Tkatchouk,‡ Neal P. Mankad,† Diego Benitez,‡ William A. Goddard, III,*,‡ andF. Dean Toste*,†

‡Materials and Process Simulation Center, California Institute of Technology, Pasadena, California 91125, United States†Department of Chemistry, University of California, Berkeley, California 94720, United States

bS Supporting Information

’ INTRODUCTION

During the past few decades, group 10 transition metalcatalyzed cross couplings have become indispensable in organicsynthesis.1 This has been enabled by the rich redox reactivity ofPd, Pt, and Ni with organic molecules. In contrast, whilehomogeneous gold catalysis has emerged as one of the mostactive fields of organometallic chemistry,2 reports of transfor-mations based on overall redox neutral gold(I) activation ofπ-bonds toward nucleophilic attack have almost exclusivelydominated the recent literature.3 Developments by Hashmi,4

Zhang,5 Toste,6 and others7 have increased the scope of thealready rich chemistry of Au(I) by developing oxidative trans-formations proposed to involve Au(I)/Au(III) cycles. In thisregard, the gold-catalyzed oxidative heteroarylation of alkeneshas attracted attention as a versatile transformation that mayresemble traditional cross-coupling reactions. The originalreactivity paradigm has been expanded to include variousnucleophiles (oxy and aminoarylations), coupling partners(arylboronic acids and arylsilanes), and π-bonds (alkenes andalkynes). Moreover, both intra- and intermolecular variantshave been described. Despite the significant progress recentlyachieved in the development of gold-catalyzed oxidative cou-pling reactions, there remain considerable uncertainties aboutthe mechanism.

The few experimental reports on the oxidative heteroarylationof alkenes propose two distinct conflicting mechanisms. Zhang

proposed as the first steps in the mechanism either oxidation ofAu(I) to Au(III) by Selectfluor followed by arylboronic acidtransmetalation, or transmetalation followed by oxidation toform an aryl Au(III) complex. Both pathways produce a highoxidation state arylgold(III) intermediate that coordinates andactivates the alkene substrate toward heteroauration. This pro-posed intermediate undergoes nucleophilic alkene addition,followed by traditional reductive elimination to regenerate theAu(I) catalyst. On the basis of Zhang’s proposed transmetala-tion/reductive elimination mechanism, a similar mechanismcould be envisioned (Scheme 1b) for the gold-catalyzed hetero-arylation reaction. However, on the basis of experimental ob-servations and preliminary computational evidence, we proposeda bimolecular reductive elimination step that does not involvetransmetalation of the arylboronic acid coupling partner(Scheme 1a). To help resolve this and other discrepancies, wereport herein a detailed computational and experimental investigationon the three main mechanistic stages of oxidative heteroarylation:oxidation, heteroauration, and reductive elimination.

’RESULTS AND DISCUSSION

Oxidation. DFT Analysis. During our ongoing mechanisticstudies on reactions using LAuX as catalyst and Selectfluor as

Received: February 10, 2011

ABSTRACT:We present a detailed study of the mechanism for oxidative heteroarylation,based on DFT calculations and experimental observations. We propose binuclearAu(II)�Au(II) complexes to be key intermediates in the mechanism for gold catalyzedoxidative heteroarylation. The reaction is thought to proceed via a gold redox cycleinvolving initial oxidation of Au(I) to binuclear Au(II)�Au(II) complexes by Selectfluor,followed by heteroauration and reductive elimination. While it is tempting to invoke atransmetalation/reductive elimination mechanism similar to that proposed for othertransition metal complexes, experimental and DFT studies suggest that the key C�Cbond forming reaction occurs via a bimolecular reductive elimination process (devoid oftransmetalation). In addition, the stereochemistry of the elimination step was determinedexperimentally to proceed with complete retention. Ligand and halide effects played an important role in the development andoptimization of the catalyst; our data provides an explanation for the ligand effects observed experimentally, useful for future catalystdevelopment. Cyclic voltammetry data is presented that supports redox synergy of the Au 3 3 3Au aurophilic interaction. Themonometallic reductive elimination frommononuclear Au(III) complexes is also studied fromwhich we can predict a∼15 kcal/moladvantage for bimetallic reductive elimination.

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oxidant, our group identified several key mechanistic clues(eqs 1�7). The failure to produce pyrrolidine B (eq 1) fromalkylgold(I) complexAwith phenylboronic acid and Selectfluoris consistent with oxidation preceding aminoauration. Further-more, during catalyst optimization, we found binuclear catalystdppm(AuBr)2 [dppm = bis(diphenylphosphanyl)methane] toexhibit superior performance compared to the mononuclearPPh3AuBr (81% vs 47% yield). We speculated that a bimetalliccatalytic process8 might be operative especially considering thesignificant literature reports on binuclear Au(II) and Au(III)complexes9 and the recent reports by Ritter on the advantagesof bimetallic reductive elimination in Pd catalyzed aromaticC�H oxidation. To investigate this, we performed calculationson the oxidative aminoarylation reaction using the binuclearcatalyst.

We studied three potential oxidation pathways depending onthe molecularity of the reaction (Figure 1, Scheme 2), all leadingto a common intermediate presumed to lead to nucleophilicaminoauration. We envisioned initial 2-electron 1-center oxida-tion of LAu(I)X to LAu(III)XF(Solv)+ by Selectfluor followedby either coordination of the normally more abundant substrate(Scheme 2a) or interaction with LAu(I)X (Scheme 2b). Wedeem these two pathways as monometallic oxidations that lead toa binuclear complex. We also considered a possible bimetallicoxidation (2-electron 2-center) pathway characterized bya concerted oxidation of a weakly bound Au(I) dimer.10

Au(I)�Au(I) interactions or “aurophilic bonding”,11 especiallyin complexes with bidentate ligands10,12 (e.g., dppm) may favorthe bimetallic oxidation pathway (Scheme 2c). We were able tofind transition structures leading to the formation of a binucleargold complex with a gold�gold bond for processes wheresubstrate is bound to Au (Figure 1a, Scheme 2a), and withoutbound substrate (Figure 1b, Scheme 2b). Our calculations usingthe M06 functional and LACV3P**++(2f) basis set suggest thatmonometallic oxidation with binuclear substrate coordination(Scheme 2b) pathway is competitive with the bimetallic path-way (Scheme 2c). Given the challenge of computing accurateentropies,13 especially in cases with a change in the numberof particles, we propose that, for the case of a monodentatephosphine ligand, the monometallic binuclear pathway couldbe operative. On the other hand, the use of a bidentatephosphine, especially one that brings the gold atoms in closeproximity (such as dppm), should favor the bimetallic oxida-tion pathway due to the minimal entropic cost. Notably, thebinuclear complex is the lowest point in the enthalpy surface,and thus the pathway to its formation could be determined bythe concentration of species.Cyclic Voltammetry. The superior performance of digold

catalysts over monogold catalysts for so-called Au(I)/Au(III)catalysis6 is particularly interesting given recent proposalsby Ritter and co-workers that cooperative “redox synergy” ofAg 3 3 3Ag and Pd 3 3 3 Pd interactions qualitatively modulatesredox potentials and thereby facilitates oxidative catalysis.8 Wetherefore sought to measure quantitatively how Au 3 3 3Au inter-actions modulate redox potentials in catalytically relevant Au

Scheme 1. Proposed Pathways for the Oxidative Heteroarylation of Alkenesa

a (a) Proposed bimolecular reductive elimination pathway, and (b) transmetalation/unimolecular reductive elimination pathway.

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complexes. To accomplish this, we obtained cyclic voltammo-grams of complexes 15�20.

The monogold complexes 15 and 16 provided benchmarkredox potentials in the absence of Au 3 3 3Au interactions. Anoxidative scan of 15 revealed two irreversible, 1-electron oxida-tions at approximately 1.48 and 1.98 V,14 corresponding toAu(II)/Au(I) and Au(III)/Au(II) redox events, respectively(see Supporting Information). These oxidative peaks wereabsent for the fully oxidized 16, which instead exhibited anirreversible, 2-electron Au(III)/Au(I) reduction at�0.69 V witha corresponding reoxidation at 0.72 V (Figure 2b).The known mixed-valent digold complex 17 has been shown

to lack any significant Au 3 3 3Au interaction in solution (based onUV�vis) or in the solid state (based on X-ray crystallography).15

Indeed, an oxidative scan for 17 revealed two irreversible,1-electron oxidation events with potentials similar to those forcomplex 15 (1.48 and 1.96 V for Au(II)Au(III)/Au(I)Au(III)and Au(III)Au(III)/Au(II)Au(III), respectively), confirmingthat the distant presence of a coordinatively saturated Au(III)center has negligible effect on the oxidation potential of theAu(I) center in 17 (see Supporting Information for overlay).Interestingly, the irreversible, 2-electron reduction for 17 wasobserved at a less negative potential relative to that of 16 byapproximately 160 mV (Figure 2b), suggesting that Au 3 3 3Auinteractions become important upon reduction of the Au(III)center in 17.Though complexes similar to the digold(I) complex 18 exhibit

aurophilic Au(I) 3 3 3Au(I) interactions in solution (based onEXAFS),16 18 itself has been characterized both with andwithouta short Au 3 3 3Au contact in the solid state depending on thecrystallization method.17 Thus, the Au(I) 3 3 3Au(I) interactionmay be of the same order of magnitude as crystal packing forces.

In this context, we sought to determine whether this weakaurophilic bonding measurably affects redox potentials. Thecyclic voltammogram of 18 featured two irreversible, 1-electronoxidations that were shifted cathodically from complexes 15 and17 by approximately 140 mV (Figure 2a). A correspondingrereduction peak (that was absent during reductive scans) wasobserved at �0.45 V, approximately where reduction of 17occurred (Figure 2a). The 140 mV cathodic shift in oxidationpotential of 18 relative to 15 and 17, as well as the 160mV anodicshift in reduction potential of 17 relative to 16, are to ourknowledge the first quantitative measurements evaluating theextent that aurophilic interactions modulate redox potentials.The observed trend in redox potentials is fully consistentwith early observations by Fackler and others that aurophilicAu(I)/Au(I) complexes are able to activate organohalide sub-strates that are inert toward mononuclear Au(I) complexes,through a bimetallic oxidative addition driven by Au(II)/Au(II)bond formation.18 This concept has catalytic relevance given thatthe first step in the proposed heteroarylation catalytic cyclerepresents a 2-electron oxidation of the catalyst, and that theturnover step represents a 2-electron reduction of the catalyst.We next determined that this concept holds for carbene-

ligated gold complexes as well as for phosphine-ligated goldcomplexes. The digold(I) complexDIMes(AuCl)2 (20, DIMes = 1,10-di(mesityl)-3,30-methylenediimidazol-2,20-diylidene) was synthe-sized by acidolysis of DIMes(AuMe)2 with HCl. The crystalstructures of both DIMes(AuMe)2 and 20 are provided in theSupporting Information; neither complex provided evidence ofshort Au 3 3 3Au contacts in the solid state. Nonetheless, both 20and its monogold analogue 19 exhibited irreversible, 2-electronoxidation events by cyclic voltammetry, with the 2-electronoxidation of 19 occurring at a higher potential (1.96 V) thanthat of 20 (1.64 V).19 Thus, though the aurophilic interactionproved unobservable by X-ray crystallography in the case of 20,our results suggest the existence of a weak and fluxional interac-tion in solution with a measurable impact on redox potential.This causes digold(I) complex 20 to be oxidized at a milderpotential than monogold complex 19. The compiled electro-chemistry results presented herein are summarized in Table 1.Lastly, we note that DFT calculations using the M06 functionaland LACV3P**++(2f) basis set correctly reproduce the observedtrend in redox potentials and predict a difference of 0.26 V in theoxidation potentials of complexes 19 and 20 (see SupportingInformation), which can be viewed as a validation of this DFTmethod for analyzing bimetallic gold complexes.Aminoauration. Binuclear intermediates also appear to be

key to aminoauration. For example, our calculations showed thatusing [Ph3PAu]

+, aminoauration does not lead to a stable cyclized

Figure 1. Transition structures 3, 5, and 9, including selected interatomic distances (Å) for the oxidative formation of binuclear Au species.

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intermediate. Relaxed coordinate scans of the cyclization reactionusing [Ph3PAu]

+, exhibited an uphill process with no minimumfor the cyclized intermediate (see Supporting Information). Thismay seem counterintuitive since aminoauration has been de-scribed before;20 however, the substrates that cyclize successfullyinvolve stronger nucleophiles than amides. Because ionization bysolvation (heterolysis) of the gold halide LAuCl is unfavorable(predicted to be endothermic by∼30 kcal/mol in acetonitrile),21

we envision that a mononuclear mechanism would involve oxida-tion of LAuX, followed by exothermic (by 12.9 kcal/mol) coor-dination of the resulting gold(III) complex with the alkene.Subsequent intramolecular base-assisted (amine originating fromthe reduction byproduct of Selectfluor) addition of the amidenucleophile then would lead to a cyclized phosphinegold(III)intermediate.On the other hand, bimetallic aminoauration was indeed

found to be a downhill process according to DFT (Scheme 3).A transition state for the aminoauration pathway was locatedusing the binuclear catalysts with PPh3 and dppm ligands. Wefind the aminoauration barrier using both PPh3 and dppmligands to be∼8 kcal/mol (8.8 kcal/mol for PPh3 and 7.7 kcal/molfor dppm).Reductive Elimination. DFT Study. Probably, the largest dis-

crepancy between the proposed mechanisms is transmetalation.Zhang initially proposed a mechanism involving arylboronic acidtransmetalation (before or after oxidation) based on the reactivitythat boronates display in analogous to d10 metal-catalyzed crosscoupling reactions.22 We were unable to observe transmetalation of

arylboronic acids, arylsilanols, and aryltrimethylsilanes with LAuXunder relevant noncatalytic reaction conditions (eqs 2 and 3).6

However, these tests do not rule out a reversible process with a Keq

that greatly favors reactants. Thus, transmetalation was tested undercatalytic conditions by subjecting substrate and active catalystPPh3AuPh to Selectfluor in acetonitrile (eq 4). The lack of reactivityin the absence of an exogenous arylboronic acid, together withthe recovered reactivity upon addition of PhB(OH)2 (eq 5) arguesagainst a pathway involving direct transmetalation. Notably, itwas observed that the transferred aryl group originates from thearylboronic acid and not from the phenylgold species (eq 6),conclusively establishing the absence of transmetalation under thepresent reaction conditions. We investigated these observationscomputationally; relaxed coordinate scans for transmetalation toAu(III) fluoride species were not able to locate a viable pathway andexhibited high energies (>80 kcal) without formation of a Au-Phbond. These data suggest that transmetalation to Au(I) and Au(III)species within the catalytic cycle of oxidative heteroarylation is notoperative.Given the absence of direct transmetalation, we posited that

C-aryl bond formation resulting from traditional reductiveelimination should also be inoperative.23 We therefore focusedon investigating likely C�C bond formation steps. Consideringthe exothermicity provided by the formation of a B�F (or Si�F)bond, we examined the minimum energy pathway by decreasingthe distance between arylboronic acid and Au(III) fluoridespecies. Relaxed coordinate scans exhibited concerted reductiveelimination with formation of both C�C and B�F bonds,

Scheme 2. Mechanistic Hypotheses for Oxidation Leading to a Substrate Coordinated Binuclear Complex

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effectively achieving regeneration of the catalyst and productdemetalation in one step. We found a concerted but asynchro-nous five-membered cyclic transition structure (Figure 3a), inwhich the B�F bond is formed prior to the C�C bond. Thepolarized Au�F bond and the B�F interaction are key forreductive elimination.Stereochemical Analysis.To further characterize the nature of

C�C reductive elimination, we sought to determine the stereo-chemical course of this reaction. Stereochemical analyses of theoverall catalytic aminoarylation reaction have been performedboth by Zhang5b and by Toste,6b but different conclusionsregarding the stereochemical course of C�C bond formationwere reached depending on whether syn- or anti-aminoaurationwas assumed. To gain further insight, herein we focus now onexploiting our previously reported ability to isolate6c alkylgold(III)

fluoride complexes [(IPr)Au(R)F2] (IPr = 1,3-bis(2,6-di-iso-propylphenyl)imidazol-2-ylidene) and thus study C�C reductiveelimination as a stoichiometric transformation. In particular, wechose to examine the coupling of PhB(OH)2 with [(IPr)Au-(R)F2] bearing the neo-hexyl-d2 substituent (i.e., R = syn- or anti-CHDCHDtBu), whose relative stereochemistry can be easilymonitored by measuring the vicinal 3JHH coupling in routine1H NMR experiments (Scheme 4).24

Reaction of syn-tBuCHDCHDB(OH)2 (3JHH = 4.2 Hz) with

(IPr)AuOH25 resulted in clean transmetalation with inversion ofstereochemistry, producing anti-tBuCHDCHDAu(IPr) (3JHH =11.2 Hz) in good yield. Because alkyl transfer from boron togold is unprecedented,26 connectivity of the unlabeled analoguetBuCH2CH2Au(IPr) was established by X-ray crystallography(see Supporting Information). Toour knowledge, the stereochemistry

Scheme 3. Mechanistic Hypothesis for Aminoauration and Bimetallic Reductive Eliminationa

a Free energies at 298 K (ΔG298K) at M06/LACV3P**++(2f) level of theory.

Figure 2. Cyclic voltammetry data: (a) oxidative scans for complexes 17 (solid) and 18 (dashed); (b) reductive scans for complexes 16 (solid) and 17(dashed). Conditions: CH2Cl2 solvent, 0.1 M Bu4NPF6, 100 mV/s scan rate.

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of B-to-Au transmetalation has not been studied previously.The stereochemistry of B-to-Pd transmetalation has beenstudied previously by analysis of catalytic reaction products,with inversion of stereochemistry27 during transmetala-tion being postulated less commonly than retention ofstereochemistry.28

In order to test the validity of our experimental design, wechose to examine the stereochemical course of C�I reductiveelimination from Au(III). Scott et al. have shown29 that reactionof (IPr)AuMe with I2 results in oxidative addition to formtrans-(IPr)AuMeI2 as a transient intermediate, followed byreductive elimination of MeI. A detailed kinetics analysis in thatreport indicated inner-sphere C�I reductive elimination ratherthan SN2-like attack of outer-sphere I

� on a cationicmethylgold(III)intermediate. Thus, C�I reductive elimination is expected toproceed with retention of stereochemistry. Indeed, we found thatreaction of anti-tBuCHDCHDAu(IPr) with I2 resulted in quanti-tative conversion to anti-tBuCHDCHDI (3JHH = 12.8 Hz).Wehave previously shown that oxidationof (IPr)AuMewithXeF2

produces cis-(IPr)AuMeF2, which reacts with PhB(OH)2 to pro-duce toluene (eq 7).6c In order to test the stereochemical courseof this C�C reductive elimination process, we oxidized anti-tBuCHDCHDAu(IPr) with XeF2 to generate cis-[anti-tBuCHD-CHDAuF2(IPr)].

19F NMR properties of this oxidized intermediatewere consistent with those previously reported for cis-(IPr)AuMeF2.

Subsequent reaction with PhB(OH)2 proceeded with retention ofstereochemistry, producing anti-tBuCHDCHDPh (3JHH = 12.4Hz) as the sole C�C coupling product (Scheme 4). Analogousresults were obtained by performing the C�C coupling with syn-tBuCHDCHDAu(IPr) (3JHH = 5.4 Hz) to yield syn-tBuCHD-CHDPh (3JHH = 5.0 Hz).Electronic Structure and Halide Effect. Given the importance

of the concerted formation of the B�F (or Si�F) bond in thecross coupling step, we expect that the more polarized the Au�Fwill lower the barrier for the reductive carbon�carbon bondformation. We observe this structurally with calculated bonddistances and electronically with NBO analyses (Table 2).30 TheAu�F bond distance for the intermediate that precedes reductiveelimination increases from 2.02 Å for the PPh3 mononuclearcomplex, to 2.07 Å for the PPh3 binuclear complex. Electro-nically, the natural charge on F atom is �0.70 e for the mono-nuclear complex and �0.81 e for the binuclear complex. Thiseffect has been observed before as a structural trans effect andtrans influence that is amplified by the Au�Au bonding system.31

In addition, it is expected that a bromide ligand further polarizesthe trans Au�F bond relative to a chloride. Indeed, we find thatthe Au�F bond is elongated by ∼0.01 Å with respect to the Clcomplex (Table 2). Moreover, the charge on fluorine is 0.01emore negative in natural charge in the F�Au�Au�Br complexcompared to the F�Au�Au�Cl tetrad. The natural atomicorbital bond orders follow this trend; the Au�F natural bondorder decreases from 0.233 for F�Au�Cl to 0.218 for F�Au�Brto 0.169 for F�Au�Au�Cl to 0.160 for F�Au�Au�Br. Takentogether, these observations suggest that the bromide ligandincreases the polarization of the Au�F bond, which is key for thereductive elimination to occur; this hypothesis is in agreement withthe higher activity of the dppm/bromide catalyst. The molecularorbitals for the intermediate that undergoes reductive eliminationsupport this view(Figure 4).TheHOMOis bonding forC�Au�Auand antibonding for F�Au and Au�Cl. Using NBO analyses, wealso characterized the Au�Au bond as a polarized interactionthat involves the 5dz2 orbital with partial 6s (∼7%) character onthe donor Aub atom to an empty orbital on Aua that is mostly 6s

Table 1. Summary of Electrochemistry Data

complex Eox for Au(I) (V)a Ered for Au(III) (V)

a

15 1.48b /

16 / �0.69

17 1.48b �0.53

18 1.34b /

19 1.96c /

20 1.64c /aReferenced to FeCp2

+/FeCp2.b Potential of first 1-electron oxidation.

c Potential of 2-electron oxidation.

Figure 3. Transition structures for the bimolecular reductive elimination from (a) mononuclear PPh3Au, (b) binuclear [PPh3Au]2, and (c) binuclear(dppm)Au2 complexes.

Scheme 4. Stereochemistry of C�C Reductive Elimination

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character (∼87%) with partial 5dz2 character (∼11%). Thismolecular orbital (HOMO-54) is shown in Figure 4.

’CONCLUSIONS

In summary, based on combining both experimental observa-tions and computational investigations, we propose a mechanismfor gold-catalyzed heteroarylation reactions that is composedof threemain steps: (1) oxidation of Au(I) by Selectfluor, (2) activa-tion of the alkene followed by nucleophilic addition, and (3) bi-molecular reductive elimination. Our calculations show that

access to binuclear Au(II)�Au(II) intermediates lowers thebarriers for all three of these steps, supported by our cyclicvoltammetry data. In accord with our experimental findings, ourcomputational studies indicate that the Au�F bond plays a keyrole in the bimolecular reductive elimination. Thus, we concludethat the spectator halide (Br vs Cl) in the gold catalysts impactsreactivity by changing the nature of the Au�F bond.

An overall proposed catalytic cycle based on all experimentaland computational data presented herein is shown in Scheme 5for the catalyst dppm(AuBr)2. Facile oxidation of the aurophilicAu(I)Au(I) catalyst with Selectfluor is partially driven by Au-(II)�Au(II) σ-bond formation, yielding cationic [dppm(AuBr2)-(AuF)]+. Heteroauration then occurs in an anti sense to give aneutral alkylgold species. Concerted bimolecular reductive elim-ination with an arylboronic acid coupling partner then occurs withretention of stereochemistry to give a catalytic process with overallinversion of stereochemistry as has been noted before.5b,c,6b

’ASSOCIATED CONTENT

bS Supporting Information. Synthetic details, computationaldetails, crystallographic data, and XYZ coordinates. This material isavailable free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Authorwag@wag.caltech.edu; fdtoste@berkeley.edu

’ACKNOWLEDGMENT

This material is based upon work supported as part of the Centerfor Catalytic Hydrocarbon Functionalization, an Energy FrontierResearch Center funded by the U.S. Department of Energy, Officeof Science, Office of Basic Energy Sciences under Award NumberDE-SC0001298. Computational facilities were funded by grantsfrom ARO�DURIP and ONR-DURIP. N.P.M. was supportedby an NIH Kirchstein-NRSA postdoctoral fellowship. Prof. ChrisChang provided access to a potentiostat for cyclic voltammetrymeasurements.

’REFERENCES

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Figure 4. Selected molecular orbitals for key intermediate 12-dppm[(dppm)(AuCl)2F(Cyclized Substrate)].

Scheme 5. Revised Mechanism for Heteroarylation of Al-kenes Catalyzed by dppm(AuBr)2

Table 2. Selected Bond Distances, Natural Bond Orders andNatural Atomic Charges of Mononuclear[(PPh3)(AuX)F(Cyclized Substrate)]and Binuclear AuIntermediates [(PPh3)2(AuX)2F(Cyclized Substrate)]

F�Au�Cl F�Au�Br F�Aua�Aub�Cl F�Aua�Aub�Br

Bond Distances and Natural Bond Orders

Au�F 1.98 Å 1.99 Å 2.06 Å 2.07 Å

(0.233) (0.218) (0.175) (0.160)

Au�X 2.30 Å 2.48 Å 2.49 Å 2.67 Å

(0.418) (0.394) (0.260) (0.228)

Au�Au � � 2.64 Å 2.66 Å

(0.232) (0.236)

Natural Atomic Charges

Aua +0.905 +0.850 +0.819 +0.806

Aub / / +0.615 +0.510

X �0.446 �0.354 �0.545 �0.561

F �0.667 �0.675 �0.732 �0.729

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