Unexpected outcomes of the oxidation of (pentafluorophenyl) triphenylphosphanegold(I)

Article abstract of DOI:10.1002/ejic.201100956

Redox gold(I)/gold(III) catalytic cycles have been proposed as the productive mechanistic manifold in a number of gold mediated C-C and C-X bond forming reactions. These transformations rely on the use of stoichiometric oxidants such as iodonium salts and

Full text of DOI:10.1002/ejic.201100956

SHORT COMMUNICATION
DOI: 10.1002/ejic.201100956
Unexpected Outcomes of the Oxidation of (Pentafluorophenyl)triphenyl-
phosphanegold(I)
Manuel Hofer[a] and Cristina Nevado*[a]
Keywords: Gold / Redox / Oxidation / Ligand transfer / Iodonium salts / Ion pairs
Redox gold(I)/gold(III) catalytic cycles have been proposed
as the productive mechanistic manifold in a number of gold
PhI(OAc)2 afforded cis-[Au(C6F5)2Cl(PPh3)] (3), which
seemed to be produced by oxidation of the starting complex
mediated C–C and C–X bond forming reactions. These trans- followed by a gold(I)/gold(III) transmetalation process or li-
formations rely on the use of stoichiometric oxidants such as
iodonium salts and Selectfluor. We have studied the oxi-
dation of electron deficient [Au(C6F5)(PPh3)] (1) in the pres-
ence of such oxidizing agents. The reaction of 1 with
gand exchange. The reaction of 1 with Selectfluor afforded
an unprecedented gold(I)–gold(III) complex [Au(PPh3)2]-
[Au(C6F5)4] (7).
AsPh3) into YAuX2L.[12] In addition, interesting transfor-
mations occur when bis(pentafluorophenyl)thallium(III)
halides were used as stoichiometric oxidants. Royo and La-
guna[13] have reported the oxidation of YAuL with (C6F5)2-
TlX (X = Cl, Br, I), which occurred with concomitant
transfer of the organic perfluorinated ligand to the gold
center to give cis-(C6F5)2AuYL (L = PPh3, AsPh3; Y = Cl,
Br, I) and TlX. Interestingly, [Au(C6F5)PPh3] (1) did not
react under these conditions and also proved to be inert
towards oxidation with Cl2 or Br2. The lack of reactivity of
this electron deficient gold(I) complex fostered our interest
in its oxidation chemistry.
Introduction
The redox chemistry of gold has recently regained the
interest of the organometallic community due to the pre-
sumed ability of AuI/AuIII redox catalytic cycles to perform
synthetically useful transformations.[1] Following pioneer-
ing stoichiometric examples reported by Benett,[2]
Lippert,[3] and Hashmi,[4] Wegner et al. have shown that bis-
coumarins were efficiently formed from arylpropionic esters
via cyclization and dimerization in the presence of catalytic
amounts of HAuCl4 with tert-butyl hydroperoxide as the
stoichiometric oxidant.[5] The same catalyst with PhI-
(OAc)2 as the oxidant has been shown to trigger general
oxidative homocoupling of arenes.[6] Zhang et al. have re-
ported the gold-catalyzed dimerization of propargylic acet-
ates with Selectfluor.[7] Beyond dimerization processes, nu-
merous transformations that involve C–C and C–X bond
formation have also been devised, which assume the in situ
oxidation of gold(I) to gold(III) species in the presence of
an external oxidant. Alkynylation reactions,[8] difunctionali-
zation of unactivated alkenes,[9] and cross-coupling with B
and Si reagents[10] have been recently added to the toolbox
of organic chemists.
Results and Discussion
We began our study with oxidants that are commonly
used in the gold-catalyzed cross-coupling reactions de-
scribed above, such as hypervalent iodine reagents[14] and
Selectfluor.[15] In this context, the formation of dichloro-
gold(III) complexes by oxidation of (NHC)AuY (NHC =
N-heterocyclic carbene; Y = Cl, C6H5, C6F5) in the pres-
ence of PhI(Cl)2 as the oxidant has already been re-
ported,[16] and PhI(OAc)2 has been used to oxidize palladi-
um(II) complexes to stable diacetoxypalladium(IV) spe-
cies.[17]
Although the synthetic application of gold redox cata-
lytic cycles is an emerging field, the stoichiometric two-elec-
tron oxidation of linear gold(I) salts to the corresponding
square planar gold(III) complexes is well precedented.[11]
Thus, an excess of halogen (X2 = Cl2, Br2, I2) at room tem-
perature or TlCl3 in boiling chloroform have been used to
transform YAuL (Y = C6Z5, Cl; Z = H, Cl, F; L = PPh3,
The reaction of 1 with PhI(Cl)2 at room temperature in
CCl4 afforded cis-dichloro(pentafluorophenyl)triphenyl-
phosphanegold(III) (2) in 93% yield (Scheme 1). The cis-
[a] University of Zürich, Organic Chemistry Institute,
Winterthurerstrasse 190, 8057 Zürich, Switzerland
Fax: +41-44-6356812
E-mail: nevado@oci.uzh.ch
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201100956.
Scheme 1. Oxidation of 1 with PhICl2.
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Inorg. Chem. 2012, 1338–1341
1338

Oxidation of (Pentafluorophenyl)triphenylphosphanegold(I)
isomer was found exclusively, which was confirmed by sin- However, at this stage, none of these mechanisms can be
gle-crystal X-ray structural analysis.[18]
ruled out, and the formation of 3 can be explained by the
In contrast, the reaction of 1 with PhI(OAc)2 at room simultaneous interplay of both mechanisms.[25]
temperature returned unreacted starting material. When the
reaction was performed in 1,2-dichloroethane at 93 °C, cis-
chloro(bispentafluorophenyl)triphenylphosphanegold(III)
(3) was isolated in moderate yield (Scheme 2). The structure
of 3 was confirmed by single-crystal X-ray structural analy-
sis.[19]
Scheme 3. Possible mechanism for the formation of 3.
Scheme 2. Oxidation of 1 with PhI(OAc)2.
The reaction of 1 with Selectfluor in 1,2-dichloroethane
also delivered an unexpected result: a gold(I)–gold(III)
complex, [Au(PPh3)2][Au(C6F5)4] (7), was isolated in 25%
yield and its structure was confirmed by single-crystal X-
ray structural analysis (Scheme 4). This compound, which,
to the best of our knowledge, has not been previously re-
ported,[26] might stem from an oxidation reaction of 1 fol-
lowed by a ligand exchange process, which highlights the
instability of the potential fluorinated gold(III) species in
the absence of electron-donor ligands.[27]
The formation of 3 from 1 was intriguing, and we de-
1
cided to monitor the reaction by H, 31P, and 19F NMR
spectroscopy to understand the mechanism by which 3 is
formed.[20] Experiments performed in 1,2-dichloroethane at
93 °C showed an increasing concentration of 3 after 2 h ac-
cording to 31P NMR spectroscopy. In parallel, a linear de-
crease of the starting material 1 was observed (Figure S4,
Supporting Information). Several intermediates were also
identified by 31P NMR spectroscopy. Thus, an increasing
concentration of [Au(PPh3)2]+ (46 ppm) was ob-
served,[10c,21] which decreased after three hours. Addition-
ally, Ph3PO and/or [AuCl(Ph3P)2][21b] were responsible for
the signal detected at 29 ppm. A third signal with complex
multiplicity at 30 ppm was also observed, which could not
be assigned to previously described species. A second ex-
periment was designed that used equivalent amounts of 1
and [AuCl(PPh3)] (4) as starting materials to evaluate the
effect of the anionic ligand on the reaction intermediates
and products (Figure S5). The formation of 3 was faster in
this case than in the experiment described above, and 3 was
detected after only one hour. Compound 1 and the cationic
intermediate [Au(PPh3)2]+ were also converted more
quickly in this case. Based on these observations, two dif-
ferent mechanisms can be proposed for the formation of 3
(Scheme 3). Pathway A involves the dissociation of 1 to give
[Au(PPh3)2]+ followed by oxidation to give [AuX4]– (5, X =
OAc or Cl if solvent activation occurs).[22] Alternatively, di-
rect oxidation of 1 by PhI(OAc)2 forms cis-[Au(C6F5)-
(PPh3)X2] (6, X = OAc and/or Cl), which could be responsi-
ble for the signal observed at 30 ppm. In both cases, 5 or 6
would undergo a gold(III)/gold(I) transmetalation or a li-
gand exchange process with 1[7,23] to yield 3 selectively.[24]
The dissociation of the anionic ligand in the first step of
the reaction and the oxidation of [Au(PPh3)2]+ to 5 have
been described previously.[21a,21b] Surprisingly, the redox
potential for [Au(PPh3)2]+ is more negative than that of
neutral species such as [AuCl(PPh3)]. Additionally, the ac-
celerated formation of 3 in the reaction carried out in the
presence of [AuCl(PPh3)] and the fact that no new interme-
diates were detected in this reaction seems to indicate that
dissociated species might be responsible for the formation
Scheme 4. Oxidation of 1 with Selectfluor.
Conclusions
A study of the oxidation of the electron deficient gold(I)
complex [Au(C6F5)(PPh3)] in the presence of strong oxi-
dants commonly used in redox gold(I)/gold(III) catalytic
cycles has been carried out. The reaction in the presence of
PhI(OAc)2 afforded cis-[Au(C6F5)2Cl(PPh3)], which is likely
to be a result of ligand dissociation followed by oxidation
and ligand exchange or transmetalation with the starting
gold(I) complex. In analogy, the reaction in the presence of
Selectfluor delivers a new gold(I)–gold(III) ion pair:
[Au(PPh3)2][Au(C6F5)4].
Experimental Section
General: NMR spectra were recorded with AV2 400 or AV2
500 MHz Bruker spectrometers. High-resolution electrospray ion-
ization MS was performed with a Finnigan MAT900 (Thermo Fin-
nigan) doublefocusing magnetic sector mass spectrometer. Unless
otherwise stated, all reagents were used as received except for iodo-
benzene diacetate, which was recrystallized from an aqueous solu-
tion of acetic acid (5 m) and dried overnight under vacuum. Sol-
vents were purchased of HPLC quality, degassed by purging thor-
oughly with nitrogen, and dried with activated molecular sieves. All
reactions were performed in anhydrous conditions under nitrogen
of the productive gold(III) intermediates in this process. in sealed Schlenk flasks.
Eur. J. Inorg. Chem. 2012, 1338–1341
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
1339

M. Hofer, C. Nevado
SHORT COMMUNICATION
cis-[Au(C6F5)(PPh3)Cl2] (2): To a mixture of [Au(C6F5)(PPh3)] (1,
37.6 mg, 0.06 mmol) and iodobenzene dichloride (18.1 mg,
0.066 mmol) was added CCl4 (3.8 mL). The mixture was stirred at
room temperature for 1 h. The solvent was evaporated under re-
duced pressure to ca. 0.2 mL, and the product was precipitated by
the addition of hexane. The product was separated to give 2 as a
white solid. Yield: 93% (39.0 mg, 0.056 mmol). 1H NMR
(400 MHz, CD2Cl2): δ = 7.72–7.36 (m, 15 H) ppm. 13C NMR
(125 MHz, CD2Cl2): δ = 143.7 (dm, J = 235.0 Hz), 140.4 (dm, J =
244.0 Hz), 138.0 (dm, J = 240.0 Hz), 134.9 (d, J = 10.7 Hz), 134.2
(d, J = 3.0 Hz), 129.8 (d, J = 12.5 Hz), 123.7 (d, J = 69.8 Hz)
ppm. 31P NMR (162 MHz, CD2Cl2): δ = 35.64 (s) ppm. 19F NMR
(376 MHz, CD2Cl2): δ = –123.48 to –124.35 (m), –156.90 (t, J =
19.8 Hz), –159.92 to –160.91 (m) ppm. MS (EI): m/z = 626.0 [M –
Cl2]+.
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0.02 mmol) and iodobenzene diacetate (7.0 mg, 0.022 mmol) was
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solid. Yield: 38% (2.8 mg, 0.0034 mmol). 1H NMR (400 MHz,
CD2Cl2): δ = 7.63–7.40 (m, 15 H) ppm. 13C NMR (125 MHz,
CD2Cl2): δ = 145.0 (dm, J = 235.0 Hz), 143.5 (dm, J = 236.0 Hz),
139.8 (dm, J = 250.0 Hz), 137.6 (dm, J = 252.0 Hz), 134.3 (d, J =
10.5 Hz), 132.7 (d, J = 3.0 Hz), 129.2 (d, J = 12.0 Hz), 124.3 (d, J
= 59.8 Hz), 117.5 (dt, J = 150.0, 43.0 Hz), 109.9 (t, J = 39.0 Hz)
ppm. 31P NMR (162 MHz, CD2Cl2): δ = 28.30 (t, J = 12.3 Hz)
ppm. 19F NMR (376 MHz, CD2Cl2): δ = –122.07 to –122.46 (m),
–122.99 to –123.59 (m), –157.31 (t, J = 19.8 Hz), –157.58 (t, J =
19.8 Hz), –160.70 to –161.02 (m), –161.71 to –162.08 (m) ppm. MS
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[Au(PPh3)2][Au(C6F5)4] (7): To a mixture of 1 (50.1 mg, 0.08 mmol)
and Selectfluor® (62.3 mg, 0.18 mmol) was added 1,2-dichloro-
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138.3 (dtm, J = 245.0, 14.0 Hz), 137.0 (dm, J = 250.0 Hz), 134.1
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30.0 Hz), 116.0 (t, J = 48.0 Hz) ppm. 31P NMR (162 MHz,
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Acknowledgments
The Organic Chemistry Institute of the University of Zürich is ac-
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Eur. J. Inorg. Chem. 2012, 1338–1341

Oxidation of (Pentafluorophenyl)triphenylphosphanegold(I)
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Received: September 8, 2011
Published Online: November 28, 2011
Eur. J. Inorg. Chem. 2012, 1338–1341
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
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