Exceptionally fast carbon-carbon bond reductive elimination from gold(III)

Article abstract of DOI:10.1038/nchem.1822

Reductive elimination of carbon-carbon bonds occurs in numerous metal-catalysed reactions. This process is well documented for a variety of transition metal complexes. However, carbon-carbon bond reductive elimination from a limited number of Au(III) complexes has been shown to be a slow and prohibitive process that generally requires elevated temperatures. Herein we show that oxidation of a series of mono- and bimetallic Au(I) aryl complexes at low temperature generates observable Au(III) and Au(II) intermediates. We also show that aryl-aryl bond reductive elimination from these oxidized species is not only among the fastest observed for any transition metal, but is also mechanistically distinct from previously studied alkyl-alkyl and aryl-alkyl reductive eliminations from Au(III).

Full text of DOI:10.1038/nchem.1822

ARTICLES
PUBLISHED ONLINE: 22 DECEMBER 2013 | DOI: 10.1038/NCHEM.1822
Exceptionally fast carbon–carbon bond reductive
elimination from gold(III)
William J. Wolf1†, Matthew S. Winston1† and F. Dean Toste1
*
Reductive elimination of carbon–carbon bonds occurs in numerous metal-catalysed reactions. This process is well
documented for a variety of transition metal complexes. However, carbon–carbon bond reductive elimination from a limited
number of Au(III) complexes has been shown to be a slow and prohibitive process that generally requires elevated
temperatures. Herein we show that oxidation of a series of mono- and bimetallic Au(I) aryl complexes at low temperature
generates observable Au(III) and Au(II) intermediates. We also show that aryl–aryl bond reductive elimination from these
oxidized species is not only among the fastest observed for any transition metal, but is also mechanistically distinct from
previously studied alkyl–alkyl and aryl–alkyl reductive eliminations from Au(III).
eductive elimination of C–H, C–C and C–X bonds is a key step Therefore, although non-A-frame systems may be more appropriate
in many metal-catalysed reactions1. These processes have been for mechanistic studies that ultimately inform catalysis, only one
R
studied extensively for nickel2–6, palladium7–15 and platinum16–22
,
non-A-frame bis(aryl) bimetallic Au(II) complex has been pre-
yet relatively little is known about the reductive elimination from pared38; as a result of the strongly electron-withdrawing perfluori-
gold, which is markedly more stable towards air and water. nated aryl ligands, biaryl reductive elimination does not occur at
In fact, since Kochi23–25 and Tobias’s26 seminal investigations of room temperature, and reductive elimination at elevated tempera-
high-temperature dialkylgold(III) reductive eliminations over tures was not investigated.
35 years ago, fundamental studies of C–C bond reductive
Current studies of the fundamental chemistry at oxidized gold
coupling from oxidized gold centres remain exceedingly rare. centres are essential to establishing new modes of gold reactivity,
Previous studies have shown that cis-EtMe2Au(PPh3)24 and particularly those that involve redox cycling40. Herein we report
[cis-(CH3)2Au(PPh3)2](PF6)26 are stable at room temperature our preparation of several monometallic and non-A-frame bimetal-
and undergo alkyl–alkyl bond reductive elimination at 70 8C lic bis(aryl) gold(I) complexes that allow kinetic analysis of C–C
(kobs ≈ 1025–1023 s21), and cis-(CH3)2AuCl(PPh3)25 reductively bond-forming reductive eliminations from oxidized species.
eliminates ethane slowly at 40 8C (kobs ≈ 1027 s21). These processes Contrary to Kochi’s pioneering report, we show that C–C bond
are inverse first-order in PPh3 and are severely inhibited by even reductive elimination at Au(III) is not necessarily a ‘disfavoured’41,42
small amounts of free ligand. Mechanistic analysis suggests that process, but is among the fastest C–C bond-forming reductive elim-
reductive elimination does not occur directly from the four-coordi- inations (between 250 and 210 8C) reported for any transition
nate complex, but rather from a high-energy T-shaped intermediate metal complex. In one case, we calculated the fastest C–C bond
formed by slow, reversible phosphine dissociation23. Since these reductive elimination recorded to date. We show that biaryl reduc-
reports, the field has not only witnessed the advent and maturation tive elimination at monometallic Au(III) proceeds via an unexpected
of now-ubiquitous palladium-catalysed cross-coupling methods, mechanism, and report the mechanism of biaryl reductive elimin-
but the development of homogeneous redox-neutral gold catalysis ation from non-A-frame bimetallic gold complexes.
as well. The shortage of mechanistic insight of processes at Au(III)
may be due, in part, to synthetic challenges in accessing appropriate Results and discussion
Au(III) models, and to the high oxidation potential of Au(I) that tra- Biaryl reductive elimination from a mononuclear gold complex.
ditionally precludes Au(I)/Au(III) redox cycling27.
cis-Ph3PAu(4-F-C6H4)2Cl (3) was synthesized on partial oxidation
Electronic cooperation between metal centres is well established of Ph3PAu(4-F-C6H4) (1) with PhICl2 at 278 8C, followed by fast
in redox processes. Hence, bimetallic gold complexes may be useful transmetallation from the remaining 1 to Ph3PAu(4-F-C6H4)Cl2
systems for studying the fundamental chemistry of oxidized organo- (2) (Fig. 1). The reaction is unaffected by equimolar or excess
gold species. In the bimetallic core, each Au(I) can formally undergo amounts of oxidant, which indicates that transmetallation from 1
a one-electron oxidation, which results in two d9 metal centres with to 2 is faster than oxidation of 1. Although 3 cannot be isolated,
unpaired electrons capable of making a Au(II)–Au(II) s bond. the cis-aryl relationship is supported by 19F NMR (two 1:1
Indeed, Fackler28–34 and Laguna35–38 have shown that dihalides singlets at 2118.4, 2119.5 ppm) and 31P NMR (one singlet at
and alkyl halides oxidize A-frame bimetallic Au(I) complexes to 27.9 ppm) spectroscopy.
access Au(II) species, yet reductive elimination processes from
In light of Kochi’s findings, we expected biaryl reductive elimin-
Au(II) have not been investigated rigorously. In fact, well-behaved ation from 3 to require heating via a mechanism that involved phos-
reductive eliminations of C–C bonds from bimetallic Au(II) phine dissociation. However, to our surprise, 3 was immediately
species are incredibly rare39. Owing to the preference for linearity consumed at room temperature (20 8C), and quantitatively gener-
by Au(I), A-frame bimetallic Au(I) complexes lack available coordi- ated 4,4′-difluorobiphenyl and (Ph3P)AuCl. At 252 8C, aryl–aryl
nation sites and may only coordinate nucleophiles on oxidation. reductive elimination from 3 could be monitored by 19F NMR
1Department of Chemistry, University of California, Berkeley, California 94720, USA; †These authors contributed equally to this work.
e-mail: fdtoste@berkeley.edu
*
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1822
ARTICLES
pseudo-first order rate constant is (6.2+0.5) × 1023 s21. No new
19F and 31P NMR signals were observed under these conditions. In
contrast, neither Bu4NCl nor radical traps, such as 9,10-dihydroan-
thracene, affect reductive elimination of 3; furthermore, the rate
remains unchanged in the presence of excess oxidant (PhICl2 acts
as a phosphine scavenger), which precludes the scenario in which
small amounts of PPh3 liberated from 3 induce reductive elimination.
These observations are consistent with two operative pathways to
biaryl: one independent of added PPh3 and another that involves
phosphine coordination. In the absence of added phosphine, reduc-
tive elimination occurs directly from the square-planar complex 3.
In the presence of excess PPh3, 3 probably undergoes associative
ligand exchange to generate cation 4, which rapidly reductively elim-
inates biaryl. This process is first order in PPh3 (Fig. 3b). As 4 is unob-
servable by NMR spectroscopy, reductive elimination from this
complex must be substantially faster than that from 3, given its
bulkier steric environment and cationic charge. When 10 equiv. of
PPh3 and 40 equiv. of Bu4NCl are added, the rate remains unchanged
with respect to that of the phosphine-accelerated reaction, which
Ph PAu(4-F-C H )
2
3
6 4
1
–196 °C → –78 °C
PhICl2
CD2Cl2
Fast
Ph3PAu(4-F-C6H4)
+
Cl
Ph3P
Au
– PPh3AuCl
Fast
Ph3P
Cl
F
F
Au
3
Cl
F
2
PPh3
–52 °C
k = 0.019 0.001 s–1 M–1
–52 °C
PPh3AuCl
+
+
F
PPh3
_
Ph3P
Au
Cl
–PPh3
k ≥ 0.22 s–1
F
F
4
F
suggests that reductive elimination from
4 is substantially
faster than the reverse ligand exchange. At 252 8C, association of
Figure 1 | Oxidation of monometallic Au(I) and biaryl reductive elimination
from Au(III). A CD2Cl2 solution of Ph3PAu(4-F-C6H4) (1) undergoes fast
partial oxidation by PhICl2 on thawing at low temperature to afford 2, which
reacts rapidly with the remaining 1 to generate a cis-diaryl Au(III) species (3).
Reductive elimination from 3 affords 4,4′-difluorobiphenyl. Associative ligand
exchange of 3 with excess PPh3 generates cationic 4, which undergoes very
fast biaryl reductive elimination.
PPh3 (k ¼ 0.019+0.001 s21
M
21) and reductive elimination from 4
(k ≥ 0.22 s21, see Supplementary Information) are remarkably facile
processes. We cannot definitively discount the possibility of reductive
elimination from a five-coordinate intermediate after coordination of
PPh3, but this scenario is improbable. Presumably, alkyl Au(III)
complexes should behave similarly in the presence of excess PPh3,
but their reductive eliminations are, instead, drastically slowed. Also,
although Yamamoto has demonstrated that
a cis-arylmethyl
(kobs ¼ (1.5+0.1) × 1024 s21) (Fig. 2); at 223 8C, reductive elimin- nickel(II) complex with a chelating bis(phosphine) undergoes
ation is 100 times faster (kobs ¼ 0.015+0.001 s21). The observed accelerated reductive elimination in the presence of PPh3 via a five-
rate constant remains unchanged over a range of concentrations, coordinate species3, comparisons with complex 3 are imperfect
as well as in the presence of 10 equiv. of either Bu4NCl or because of the potentially labile Au–Cl bond.
Ph3PAuCl, which indicates a unimolecular process that is first
To our knowledge, reductive eliminations from diarylgold(III) com-
order in 3.
plexes 3 and 4 are among the fastest observed C–C bond-forming
That reductive elimination is substantially more facile from 3 reductive couplings at any transition metal centre at temperatures
than from alkylgold(III) complexes may not be attributed simply below 220 8C. For comparison, reported rate constants for reductive
to faster coupling of aryl–aryl relative to alkyl–alkyl bonds9, but elimination of diarylplatinum(II)19 and diarylpalladium(II)12 com-
suggests fundamentally different rate-determining steps or mecha- plexes are between 1025 to 1023 s21 at 95 and 50 8C, respectively,
nisms. Contrary to that established for alkyl reductive elimination although palladium-catalysed Kumada–Corriu couplings can be
of Au(III), excess PPh3 increases the observed rate of reductive elimin- achieved at temperatures as low as 265 8C (ref. 43). In some cases,
ation (Fig. 3). In the presence of 10 equiv. of PPh3 at 252 8C, the rate diarylplatinum(II) systems only reductively eliminate biaryl at
of reductive elimination from 3 increases by an order of magni- temperatures above 100 8C (ref. 19). Reductive elimination rate
tude (kobs ¼ (1.2+0.2) × 1023 s21); with 40 equiv., the observed constants of ꢀ0.1 s21 have also been reported for nickel-catalysed
a
35 min
30 min
25 min
20 min
15 min
10 min
b
0.6
0.5
0.4
0.3
0.2
0.1
0.0
y = –0.018775 + 0.00015076x
R = 0.99493
5 min
0 min
3
F
F
0
1,000
2,000
3,000
4,000
–114.5 –115.5 –116.5 –117.5 –118.5 –119.5 –120.5 –121.5 –122.5
ppm
Time (s)
Figure 2 | Kinetics of decay of cis-(Ph3P)Au(4-F-C6H4)2Cl (3). a, First-order decay of 3 monitored by 19F NMR (470 MHz, CD2Cl2) at 232 8C over a period
of 35 minutes. b, A natural log plot of [3]/[3]t¼0 indicative of a first-order process. Error bars represent the inherent error in quantifying 19F NMR
signal intensity.
2
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0.0070
0.0056
0.0042
0.0028
0.0014
0.0000
a
3.0
2.5
2.0
1.5
1.0
0.5
0.0
b
40 equiv. PPh3
30 equiv. PPh3
20 equiv. PPh
10 equiv. PPh3
0 equiv. PPh3
y = 6.495 ×10–6 + 0.01891x
R = 0.99698
0
200
400
600
800
1,000
0.0
0.1
0.2
0.3
0.4
Time (s)
[PPh3] (M)
Figure 3 | Unexpected phosphine acceleration on the decay of 3 at 252 8C. a, The rate of decay of 3 (slope of the natural log plot) increases with [PPh3].
b, A linear relationship between [PPh3] and kobs for the decay of 3 in the presence of excess PPh3 (pseudo-first-order conditions) indicates that ligand
exchange from 3 to 4 is first order in PPh3. Error bars represent the inherent error in interpreting 19F NMR signal intensity (a) or the standard deviation over
three separate experiments (b).
oxidative homocouplings at 235 8C (ref. 6), although the presumed benzoyl peroxide34. Furthermore, electrochemical studies and
diarylnickel(II) intermediate was not observed. Indeed, for a third- density functional theory calculations suggest that aurophilic
row metal, the rates of aryl–aryl reductive elimination from Au(III) interactions reduce the oxidation potential of bimetallic Au(I)
are particularly impressive, outcompeting even C–C bond coupling species relative to that of mononuclear complexes27.
in alkynylarylpalladium(IV) complexes44 (k ≈ 1023 s21 at 235 8C).
To assess the role of metal–metal bonding in the C–C bond
An Eyring analysis over a 29 8C range reveals that the reductive reductive elimination of oxidized gold, we prepared a bimetallic
elimination from 3 not only has an unusually small enthalpic Au(I) complex stabilized by a bis(diphenylphosphinoamine)
barrier, but a small entropic contribution to the activation energy ligand, Ph2P–NR–PPh2 (PNP)46,47. X-ray diffraction analysis of
as well (DH‡ ¼ 17.2+0.2 kcal mol21, DS‡ ¼ 2.0+0.8 e.u., see PNP[Au(4-F-C6H4)]2 (5, R¼CH3) indicates an aurophilic inter-
Supplementary Information); these kinetic parameters suggest a action (Au(I)–Au(I) distance ¼ 3.0357(2) Å) that is probably
transition state that resembles 3 in which the Au–C(sp2) bonds are conserved in solution (see Supplementary Information). Laguna
mostly conserved.
has shown that direct oxidation of PNP[Au(C6F5)]2 (R¼H) with
It is not immediately obvious why aryl and chloride ligands dras- Cl2 generates the symmetric bimetallic Au(II) complex PNP
tically change the mechanism of C–C bond reductive elimination at [Au(C6F5)Cl]2, which is stable at room temperature; crystallo-
Au(III). Generally, the barrier to aryl–aryl reductive elimination is graphic analysis established a Au(II)–Au(II) bond (2.576(2) Å) and
lower than that for alkyl–alkyl reductive elimination8, and the a trans relationship between chloride ligands38.
weaker s-donating ability of aryl and chloride ligands relative to
The analogous low-temperature oxidation of 5 with PhICl2
that of alkyls may increase the barrier to phosphine dissociation. generates the symmetric bimetallic Au(II) complex 6 (19F NMR
The notion that ‘Au(III) and Pt(II) catalysts show similar reactivity’42 and 31P NMR singlets at 2120.4 ppm and 83.5 ppm, respectively),
is not necessarily true when redox processes are involved: at greater which undergoes slow reductive elimination at temperatures
than 90 8C, Pt(II) biaryl complexes undergo reductive elimination at below 230 8C (Fig. 4). At 223 8C, 6 undergoes first-order
rates comparable to those observed for Au(III) at approximately decay (k ¼ (1.6+0.3) × 1024 s21) with concomitant formation of
250 8C. The reason for this discrepancy in rates may lie in pro- 4,4′-difluorobiphenyl. Without observing a mixed-valent inter-
nounced relativistic effects, which result in Au(I) having a strong mediate, we cannot distinguish kinetically between a pathway that
preference for linearity over other transition metals42. As the involves rearrangement and reductive elimination, and one that
product of reductive elimination from a square planar complex is involves bimetallic reductive elimination via a four-centred
necessarily two-coordinate, and the transition state is partway to lin- transition state48. However, Laguna has reported that similar
earity, the activation energy of reductive elimination of Au(III) PNP-supported binuclear perfluoroarylgold(II) complexes rearrange
to Au(I) may be less than that for other transition metals. This over several hours to mixed-valent Au(I)/Au(III) species38. Given the
is reasonable given the early transition state to reductive fast aryl transmetallation and reductive elimination of mononuclear
elimination of 3.
Au(III), as well as the scarcity of reported binuclear reductive
elimination, we favour a mechanism that involves isomerization of
Biaryl reductive elimination from bimetallic gold complexes. The 6 to mixed-valent species 7, which then undergoes fast, unobserv-
unexpected redox behaviour of mononuclear Au(III) species led us able reductive elimination. The kinetic parameters for the rate-
to investigate analogous reactivity of bimetallic gold complexes. limiting isomerization of 6 to 7 (DH‡ ¼ 16.9+0.4 kcal mol21
;
Others have reported that bimetallic complexes with three-atom DS‡ ¼ 27.8+1.6 e.u.) are reflective of a low-barrier process with
linkers do not undergo two-electron oxidation at a single Au(I), a small entropic penalty (see Supplementary Information). This step
but instead undergo formal one-electron oxidation at two Au(I) is first order in 6 and unaffected by excess Bu4NCl. Complex 6 also
centres to generate a species stabilized by a Au(II)–Au(II) bond28–38
.
oxidizes excess ligand, presumably via chloronium transfer; therefore,
Electronic cooperation between metal centres in bimetallic phosphine dissociation is probably not a prerequisite for either iso-
complexes offers a lower barrier oxidation pathway relative to a merization or reductive elimination, because PNP(AuCl)2 is formed
two-electron oxidation at a single metal centre45, which allows, for quantitatively when no excess ligand is added, and no decomposition
example, oxidation of dicationic A-frame Au(I) complexes by of liberated ligand (PNP þ 6) is observed. These results are consistent
oxidants typically unreactive towards monocationic Au(I), such as with a mechanism that involves intramolecular aryl transfer from 6 to
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1822
ARTICLES
X
X
a
0.014
Ph2P
Au
PPh2
Au
Ph2P
PPh2
Cl
PhICl2
CD2Cl2
F
Cl Au
Au
0.012
0.010
0.008
0.006
0.004
0.002
0.000
–196 °C → –78 °C
F
Fast
F
F
F
F
5 (X = NMe)
8 (X = CH2)
6 (X = NMe)
9 (X = CH2)
Au(I)/Au(III) (10)
6 (–23 °C):
9 (–52 °C):
k = (1.6 0.3) × 10–4 s–1 k = (4.6 0.1) × 10–4 s–1
Au(II)/Au(II) (9)
F
Cl
PNP(AuCl)2 (X = NMe)
or
dppm(AuCl)2 (X = CH2)
7 (–23 °C):
Fast
Au
0
1,000
2,000
3,000
4,000
X
Ph2P
Au
Cl
P
Ph2
Time (s)
F
10 (–52 °C):
k = 3.9 × 10–4 s–1
+
b
F
F
9
10
7 (X = NMe)
10 (X = CH2)
F
F
Figure 4 | Oxidation of bimetallic Au(I)/Au(I) complexes. This is under
kinetic control, and affords Au(II)/Au(II) intermediates that isomerize to
mixed-valent Au(I)/Au(III) species and reductively eliminate biaryl
–114.5 –115.5 –116.5 –117.5 –118.5 –119.5 –120.5 –121.5 –122.5
ppm
from a single metal centre. Solutions of PNP[Au(4-F-C6H4)]2 (5) and
dppm[Au(4-F-C6H4)]2 (8) undergo fast oxidation by PhICl2 on thawing to
generate intermediates stabilized by intramolecular Au(II)–Au(II) bonds.
Complex 6 undergoes isomerization more slowly than 9, perhaps because of
decreased ligand flexibility. As isomerization for PNP-supported complexes is
rate-limiting, reductive elimination from 7 is not observable. In contrast, the
rates of isomerization and reductive elimination of dppm-supported
complexes are comparable, which allows kinetic analysis of both steps to
preclude the possibility of a bimetallic reductive elimination directly from 9.
Figure 5 | Monitoring oxidation of dppm[Au(4-F-C6H4)]2 (8) to
Au(II)/Au(II) intermediate 8, isomerization to Au(I)/Au(III) species 10 and
reductive elimination of 4,4′-difluorobiphenyl. a, Reaction profile for the
isomerization of 9 to 10 and reductive elimination of biaryl from 10 at
252 8C, monitored by 19F NMR. b, Selected region of 19F NMR spectrum
(470 MHz, CD2Cl2) of the oxidation of 8 at 252 8C. Complex 8 was fully
consumed. The equivalent fluorides of Au(II)/Au(II) species 9 are clearly
visible at 2120.3 ppm, as are the inequivalent fluorides in Au(I)/Au(III)
species 10 at 2118.2 and 2118.6 ppm. Error bars represent the inherent
error in interpreting 19F NMR signal intensity.
generate 7, which then directly reductively eliminates to PNP(AuCl)2
and 4,4′-difluorobiphenyl.
We reasoned that N ꢁ P p-donation in PNP-type ligands results
in a barrier to N–P bond rotation that becomes significant at low
temperature49. A more flexible ligand, such as 1,1-bis(diphenylpho- oxidation of dppp[Au(4-F-C6H4)]2 (11) by PhICl2 is mixed-valent
sphino)methane (dppm), could presumably facilitate intramolecu- species 13 (two 1:1 19F NMR singlets at 2118.3 and 2118.6 ppm
lar aryl transfer from a bimetallic Au(II) intermediate, which and two 1:1 31P NMR singlets at 27.3 and 21.7 ppm), presumably
results in
a lower barrier to isomerization. Oxidation of formed via fast isomerization of 12 (Fig. 6). Even at 280 8C, no
dppm[Au(4-F-C6H4)2]2 (8) at 278 8C by PhICl2 affords symmetric signals consistent with a symmetric bimetallic Au(II) species were
bimetallic Au(II) species 9 (19F NMR and 31P NMR singlets at observed, suggestive that a mixed-valent species is formed directly
2120.3 ppm and 218.2 ppm, respectively), which is consumed on oxidation. On warming to 252 8C, 13 undergoes first-order
faster than the analogous PNP complex 6 (Fig. 4). Indeed, at reductive elimination to dppp(AuCl)2 and 4,4′-difluorobiphenyl
252 8C, 9 undergoes first-order decay, with the fast appearance (k ¼ (3.0+0.1) × 1025 s21). Hence, the kinetic profiles of oxidation
and slow consumption of mixed-valent Au(I)/Au(III) complex 10, and reductive elimination of bimetallic complexes bearing
and the appearance of 4,4′-difluorobiphenyl at approximately the linkers that restrict metal–metal interactions resemble those of
same rate as the disappearance of 9 (Fig. 5). Kinetic modelling mononuclear species.
precludes any scenario in which a bimetallic reductive elimination
Importantly, these studies reveal that access to a kinetic bimetal-
occurs directly from 9. Instead, 9 probably undergoes irreversible lic Au(II) product of oxidation does not necessarily compromise the
isomerization to 10 (k ¼ (4.6+0.1) × 1024 s21), which then rate of reductive elimination from Au(III); in fact, the rate of
reductively eliminates 4,4′-difluorobiphenyl at a comparable rate reductive elimination from dppm-supported 10 is about an
(k ¼ 3.9 × 1024 s21). At 230 8C, both reactions are immeasurably order of magnitude faster than that of 13, perhaps because of
fast. Although PNP-supported Au(I)/Au(III) species 7 could not increased steric crowding as a result of a shorter linker, aurophilic
be observed, these findings support a similar reductive elimination interactions between the gold centres in 10 or a combination
mechanism of PNP-supported complexes that involves the interme- thereof. Even Au(II)/Au(II) ꢁ Au(I)/Au(III) isomerization for
diacy of such mixed-valent species, as proposed above.
PNP- and dppm-supported complexes is a facile process at temp-
Using the dppm-supported system discussed above as a bench- eratures below 0 8C.
mark, we probed the behaviour of a bimetallic system with a
five-atom linker that could perturb intramolecular metal–metal Conclusion
interactions. We envisioned that a longer linker (such as 1,3-bis(diphe- We uncovered several unexpected properties of oxidized gold that
nylphosphino)propane (dppp)) between gold atoms might preclude are contrary to established ideas. First, C–C bond reductive elimin-
formation of a seven-membered metallacyclic Au(II)–Au(II) inter- ation from Au(III) does not necessarily require phosphine dis-
mediate on oxidation. Indeed, the only product of low-temperature sociation. By avoiding this process, biaryl reductive eliminations
4
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NATURE CHEMISTRY DOI: 10.1038/NCHEM.1822
ARTICLES
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Au
PPh2
Ph2P
Au
Ph2P
Au
PhICl2
CDCl2
Cl Au Cl
Fast
–196 °C → –78 °C
Fast
F
F
F
F
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12
PPh2
Cl Au
Ph2P
F
F
Au
Cl
–52 °C
k = (3.0 0.1) × 10–5 s–1
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13
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Figure 6 | Five-atom linker dppp discourages formation of Au(II)/Au(II)
intermediates on oxidation of bimetallic dppp[Au(4-F-C6H4)]2 (11).
Solutions of 11 undergo fast oxidation by PhICl2 on thawing to afford directly
the unobservable Au(I)/Au(III) complex 12, which isomerizes to 13.
Reductive elimination from 13 affords 4,4′-difluorobiphenyl.
from Au(III) are remarkably fast, and occur as low as 252 8C.
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of bimetallic Au(I) complexes may be reduced, which allows access
to symmetric intermediates with Au(II)–Au(II) bonds. When stabil-
´
´
and the role of coupling additives. A DFT study supported by experiment. J. Am.
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15. Racowski, J. M., Dick, A. R. & Sanford, M. S. Detailed study of C–O and C–C
ized by dppm, these complexes undergo fast isomerization and
bond-forming reductive elimination from stable C2N2O2-ligated palladium(IV)
reductive elimination at very low temperatures; when stabilized by
more rigid PNP-type ligands, these reactions may be arrested,
although at slightly higher temperatures isomerization and reduc-
tive elimination become facile.
The stoichiometric behaviour of the complexes reported in this
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sociation is not required for C–C bond formation may slow or 18. Stahl, S. S., Labinger, J. A. & Bercaw, J. E. Formation and reductive elimination of
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19. Shekhar, S. & Hartwig, J. F. Distinct electronic effects on reductive eliminations
entirely preclude deleterious ligand oxidation by excess oxidant
under conditions relevant to catalysis. Efforts to develop gold-cata-
lysed transformations using relatively weak oxidants compatible
of symmetrical and unsymmetrical bis-aryl platinum complexes. J. Am. Chem.
with substrates should focus on bimetallic Au(I) precatalysts with
three-atom linkers, such as PNP or dppm. Given that transmetal-
lation to Au(I) can be achieved at temperatures as low as 278 8C
(ref. 50), and that Au(I) oxidation, isomerization and reductive
elimination are facile processes below 0 8C, catalysis may be
achieved at temperatures low enough to avoid unwanted side
reactions between oxidant and substrate. Investigations to probe
ligand and electronic effects on the reductive elimination at
Au(III), as well as the stoichiometric reactivity of bimetallic Au(II)
complexes, are ongoing to further our nascent understanding of
the fundamental chemistry of oxidized gold.
Soc. 126, 13016–13027 (2004).
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26. Kuch, P. L. & Tobias, R. S. Synthesis of cationic dialkylgold(III) complexes: nature
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Received 15 August 2013; accepted 13 November 2013;
published online 22 December 2013
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Acknowledgements
This research was supported in part by a grant from the National Institutes of Health (NIH)
General Medical Sciences (RO1 GM073932). We thank the National Science Foundation
(predoctoral fellowship to W.J.W., grant no. DGE 1106400) and NIH (postdoctoral
fellowship to M.S.W., grant no. F32 GM103238-02) for funding. We thank R. G. Bergman,
N. P. Mankad and M. D. Levin for helpful discussions, and A. DiPasquale for
crystallographic assistance.
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M.S.W. and W.J.W. conceived, designed and performed the experiments, analysed the data
and wrote the manuscript; M.S.W. derived kinetic models; W.J.W. performed the
crystallographic analysis. F.D.T. supervised the project.
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Additional information
Supplementary information and chemical compound information are available in the
online version of the paper. Reprints and permissions information is available online at
www.nature.com/reprints. Correspondence and requests for materials should be
addressed to F.D.T.
41. Soriano, E. & Marco-Contelles, J. Computational Mechanisms of Au and Pt
Catalyzed Reactions (Topics in Current Chemistry 302, Springer, 2011).
Competing financial interests
The authors declare no competing financial interests.
6
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