Quantitative evaluation of the stability of gem -diaurated species in reactions with nucleophiles

Article abstract of DOI:10.1021/om400083f

The reactivity of diaurated species toward nucleophiles was investigated. The reaction yields vinyl gold species and is described as simple S_N2 ligand exchange at gold. Using suitably strong nucleophiles, equilibrium constants were determined to measure the stability of various diaurated species. On the basis of these equilibrium constants the influence of ligand and the nature of vinyl cores on the stability were analyzed. These results have direct implication for gold catalysis: it was demonstrated that vinyl gold intermediates bind the catalytic LAu⁺ species generally stronger than an alkyne (the substrate) by a factor of 10⁶-10⁹. This demonstrates that the formation of diaurated species from vinyl gold intermediates is thermodynamically favored in a catalytic reaction.

Full text of DOI:10.1021/om400083f

Article
pubs.acs.org/Organometallics
Quantitative Evaluation of the Stability of gem-Diaurated Species in
Reactions with Nucleophiles
Alexander Zhdanko and Martin E. Maier*
Institut fur Organische Chemie, Universitat Tubingen, Auf der Morgenstelle 18, 72076 Tubingen, Germany
̈
̈
̈
̈
S
* Supporting Information
ABSTRACT: The reactivity of diaurated species toward nucleophiles was
investigated. The reaction yields vinyl gold species and is described as simple SN2
ligand exchange at gold. Using suitably strong nucleophiles, equilibrium
constants were determined to measure the stability of various diaurated species.
On the basis of these equilibrium constants the influence of ligand and the nature
of vinyl cores on the stability were analyzed. These results have direct implication for gold catalysis: it was demonstrated that
vinyl gold intermediates bind the catalytic LAu+ species generally stronger than an alkyne (the substrate) by a factor of 106−109.
This demonstrates that the formation of diaurated species from vinyl gold intermediates is thermodynamically favored in a
catalytic reaction.
1. INTRODUCTION
Since 2012 the scientific community witnessed a rapid growth
of publications in the chemistry of gem-diaurated species,
important intermediates in gold catalysis.1,2 At this time,
involvement of this class of compounds has been confirmed for
several catalytic processes and their role in gold catalysis is
currently a major topic of research.3 In our previous study we
provided a convenient route to enol ether derived diaurated
species in a catalytically relevant manner.4 As part of our study
(Table 1; for the ligand abbreviations see Figure 1). The choice
on mechanisms of gold catalysis we report herein on
of these nucleophiles came from the reaction of the diaurated
quantitative evaluation of stability of gem-diaurated species in
species with some ligands from the above series. For example,
PPh3 destroyed all diaurated species while Me2S did not touch
them at all. Species D2, D5, D6, D12, and D14−D16 were
reactions with nucleophiles and provide insights into the
mechanism of these reactions.
used as SbF6− salts prepared in individual states, and in all other
Reversible formation of diaurated species (eq 1) is one of the
central processes in the mechanism of catalytic hydro-
cases the species were obtained from catalysts (Figure 1) and
alkoxylation.5 This reversible reaction is responsible for the
removal of gold from the catalytic cycle because D is an inactive
intermediate, while B is a reactive participant of the living
catalytic cycle. However, the position of this equilibrium or
even whether it is established at all during the catalytic process
has not been investigated. This depends on thermodynamic
stability of diaurated species and on the corresponding reaction
rates, a subject which has not been looked at so far.
manipulated in situ.4 All reactions were performed at constant
26.1 °C temperature, and amounts of equilibrium participants
were determined by integration of NMR spectra. A
representative example is given in Figure 2. Here the CH2O
signals of the vinyl gold and the diaurated species appear at
different chemical shifts and their integrals could be
immediately used for the calculation of Keq. The peak at 2.34
ppm contains the methyl group of 4-picoline but also the
methyl group of the L2AuPic+ salt and an endocyclic methylene
group of vinyl gold complex B5. In the case of a fast LAu+
exchange the molar ratio B/D was determined from the exact
position of the CH2O peak common for these species.
Complete experimental details are given in the Supporting
Information. In these ligand exchange reactions the vinyl gold
species B serves as a ligand and the equilibrium is achieved
through the competition of B and a nucleophile for the
electrophilic LAu+ center. The results are summarized in Table
1.
2. RESULTS AND DISCUSSION
Since thermodynamic stability constants of diaurated species D
(eq 1) were experimentally inaccessible, much useful
information was obtained from reactions with nucleophiles
(eq 2).
Previously we established a ligand strength series for LAu+
(MeOH ≪ 3-hexyne < MeCN ≪ Me2S < 2,6-lutidine < 4-
picoline < DMAP < TMTU < PPh3) with binding affinities
covering ∼11 orders of magnitude.6 Now, using 4-picoline,
DMAP, and TMTU as suitably strong nucleophiles we
determined equilibrium constants of ligand exchange reactions
with a number of enol ether derived diaurated species D1−D16
Received: January 30, 2013
Published: March 11, 2013
© 2013 American Chemical Society
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Table 1. Reactions of Diaurated Species with Nucleophiles: Equilibrium Constants
a
All reactions in CDCl3 at 26.1 °C except otherwise noted. Abbreviations: Pic, 4-picoline; DMAP, 4-dimethylaminopyridine; TMTU,
tetramethylthiourea ((Me2N)2CS); Lut, 2,6-lutidine.
Substrate Effect on Stability of Diaurated Species.
First of all, these equilibrium constants allow direct comparison
of the stability of diaurated species bearing different vinyl cores
but having the same ligand or, in other words, establishing the
substrate effect on the stability of diaurated species. In order to
avoid too much steric influence, it seemed better to study these
substrate effects on species bearing nonbulky ligands such as
PMe3 and PPh3. This way it could be established that changing
a propyl to a methyl substituent decreases the stability of PPh3-
based diaurated species by a factor of 2 (compare entries 1 and
4, 2 and 5, and 3 and 6), indicating that with a more electron
rich substituent the stability of diaurated species increases. The
same trend was found for PMe3-based diaurated species
(compare entries 17 and 18). In contrast, installation of a
conjugated π system in the side chain decreases the stability of
the diaurated species rather significantly, be it a simple double
bond or a phenyl substituent (as evidenced from entries 1 and
7, 1 and 8, and 19 and 20). These effects are rather surprising,
since conjugation with a π system should help in stabilization of
a positive charge and should increase the stability. Obviously
this principle does not apply here and other effects must
account for the decrease of stability. We hypothesize that this is
due to an increase in the stability of the vinyl gold species, the
dissociation product. Indeed, in this case there is also a nicely
conjugated π system in vinyl gold (e.g., B3), which gets
disturbed with introduction of a second LAu+ moiety. This
effect might dominate over stabilization in the diaurated
species, making them more prone to dissociation, together with
a small −I effect of the sp2 carbon favoring the same trend.
Introduction of an electron-rich substituent (NMe2) in the
aromatic side chain increases the stability (entries 20 and 21),
while an electron-withdrawing substituent (CN) decreases the
stability dramatically.7 Use of an electron-deficient ligand also
negatively affects the stability (see below).
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more destabilization in the system than the more rigid and flat
alkenyl or phenyl group. The same steric effect must cause an
increase of stability with the change of a propyl substituent to a
smaller methyl group (entries 9 and 10), while in case of the
PPh3 and PMe3 ligands the opposite was true.
Another good illustration of the impact of steric effects is
given by the reaction of sterically hindered alcohol S4. Thus,
when S4 was reacted with catalyst 1 and PrSp (Proton
Sponge), diaurated species D18 smoothly formed, but only
minor formation of diaurated species was observed with 2
(Scheme 1). Instead, the oxonium salt (L2Au)2OH+ was
Scheme 1. Steric Effects Greatly Reduce Stability of
Diaurated Species
Figure 1. Precursors for diaurated species used in the study and
abbreviations of the ligands.
On the basis of this information the following can be
concluded about the influence of a substituent at the enol ether
core on the stability of diaurated species: (1) electron-rich
substituents will increase the stability, while electron-poor
substituents decrease it; (2) any conjugation will decrease the
stability in comparison to nonconjugated systems.
Ligand Effect on Stability of Diaurated Species.
Further, the equilibrium constants disclose ligand effects on
the stability of diaurated species. Thus, among the five ligands
the most stable diaurated species are formed with PPh3 and
PMe3 and the stability decreases in the order PMe3 ≈ PPh3 >
L4 ≈ L5 ≫ L2 > L3. According to this series electron-donor
phosphines favor the formation of diaurated species; however,
in the case of L2 and L3 steric effects obviously predominate
and the stability drastically decreases.
Changing the PPh3 ligand to a more bulky one (L2, L3) not
only generally reduces the stability but also might cause some
transpositions in the aforementioned trends of the substrate
effects. Thus, in some cases a conjugated π system in the side
chain decreases the stability as before (compare Table 1, entries
9 and 11 and 9 and 12), but in others the stability increased
(entries 13 and 14). This must be caused by a steric effect
which becomes more and more pronounced as the ligand gets
larger. Indeed, a free-rotating propyl side chain now brings
formed as a major gold product resulting from residual water in
the system.8 When the reaction was repeated under essentially
water-free conditions in the presence of CaH2, the new species
D19 could be generated. Not surprisingly, D19 appears to be
highly unstable toward nucleophilic attack and readily reverts
back to (L2Au)2OH+ upon simple manipulation of the reaction
mixture in open air. Obviously, installation of two methyl
groups in close proximity to the bulky diaurated core drastically
reduced the stability of D19 in comparison to the less crowded
situation for D18 with the PPh3 ligand. It appears that in
sterically hindered diaurated species the steric effects naturally
become more and more important and in many cases may
dominate over electronic effects.
Figure 2. Representative NMR spectrum of the reaction mixture in CDCl3 for determination of the equilibrium constant for D5/B5 (entry 9, Table
1).
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However, since the affinities of various LAu+ species for the
spectator ligands are also slightly different, rigorous comparison
(in terms of thermodynamic stability constants) is not possible
without an auxiliary equilibrium constant (3). Such a constant
was also experimentally inaccessible. In order to evaluate the
ligand effect in a situation closely analogous to catalytic
conditions, we determined an auxiliary constant for MeCN (4).
With this value it was possible to compare equilibria (5) and
(6) theoretically. The ratio of the constants is now related to
the experimentally established picoline constants (7) and (8)
by the simple expression (9). For example, this way it could be
established that diaurated species D1 is 1500 times more stable
than D5 (in chloroform). That is the impact of the bulky
ligand!
Figure 3. General ligand strength series showing thermodynamic
binding affinities of the key catalytic intermediates to a LAu+ unit.
Some key equilibrium constants are given for ligand exchange at
L2Au+.
substrate! Obviously vinyl gold species represent the strongest
binding partners among all species in the whole hydro-
alkoxylation reaction mixture, and this binding to a LAu+ unit is
remarkably strong. It can be also suggested that the
corresponding alkene derived diaurated species should be
generally weaker than the corresponding enol ether derived
analogues by 2−3 orders of magnitude.
Kaux
Ph3PAuPic+ + L2Au+ HoooI Ph3PAu+ + L2AuPic+
(3)
Kinetic Aspects. Unfortunately, the determination of
equilibrium constants in reactions with nucleophiles was not
possible in a number of cases due to kinetic reasons (slow
reaction). For example, an equilibrium of D5 with 2,6-lutidine
is not established even after 2 h reaction time. Some other
examples are given in Scheme 2. Thus, diaurated species D20−
Ph3PAuPic+ + L2AuNCMe+
HoooooooooI Ph3PAuNCMe+ + L2AuPic+
Kaux=0.37
(4)
K1
RAuPPh3 + Ph3PAuNCMe+ ⇌ R(AuPPh3)2 + MeCN
+
(5)
Scheme 2. Slowly Reacting Diaurated Species
K2
+
RAuL2 + L2AuNCMe+ ⇌ R(AuL2)2 + MeCN
(6)
K′
+
RAuPPh3 + Ph3PAuPic+ ⇌ R(AuPPh3)2 + Pic
(7)
K″
+
RAuL2 + L2AuPic+ ⇌ R(AuL2)2 + Pic
(8)
K′
K″Kaux
2 × 104
36 × 0.37
K1
K2
=
=
= 1500
(9)
Since the auxiliary constant (3) for different LAu+ can be
assumed to be generally around 1, since it is an equilibrium
between species of similar nature, direct comparison of
equilibria with picoline can be used to roughly compare
thermodynamic stabilities of diaurated species bearing different
ligands. It can be suggested that PPh3-based species should be
generally 1000 times stronger than with L2. Given the
inhibitory effect of diaurated species in catalysis, this constitutes
one of the reasons why catalyst 2 is far more efficient than 1 in
many reactions in this study and in the literature.9
Comparison with the Previously Known Aryl Diau-
rated Species. Finally, we synthesized one example of the
well-known aryl diaurated species D17 in order to compare the
stability of this compound with those of our enol ether derived
species.10 As can be concluded from Table 1, entries 1 and 23,
installation of an enol ether moiety in place of an aryl ring
increases the stability of diaurated species by a factor of 10000!
Thus, the aryl diaurated species Ar(AuL)2+ should be generally
much less stable than the enol ether derived analogues. Even
dimethyl sulfide can be used to cause reversible dissociation of
this weak species (entry 25).
D23 reacted with picoline slowly within hours. Reactions with
stronger nucleophiles (TMTU and Et4N+Cl−) occurred
considerably more quickly. However, in the case of D23
reaction even with Cl− (a strong and unhindered nucleophile)
is slow (66% conversion in 4 h), which is unprecedented for
diaurated species. In all these slow cases liberation of vinyl gold
is accompanied by subsequent slow protodeauration, taking
place at the cost of water in the system (11), so that
equilibrium constants (10) cannot be determined. In contrast,
all vinyl gold products generated from D20−D23 are kinetically
reactive, like all other vinyl gold species from Table 1, reacting
immediately already with the weak acid PrSpH+OTf−. Thus,
addition of PrSpH+OTf− to the mixture of D23 and Et4N+Cl−
immediately removes all vinyl gold but cannot speed up
decomposition of diaurated species themselves (12). This
further proves the kinetic inertness of this species.
Outlook. In our previous report on the coordination
chemistry of gold catalysts we established a ligand strength
series for a number of ligands covering 11 orders of
magnitude.6 Now we can supplement the series with our new
data on binding affinities of vinyl gold to a LAu+ unit (Figure
3). Using the intermediate equilibrium constants it could
thereby be estimated that enol ether derived vinyl gold binds
LAu+ generally 106−109 times more strongly than the alkyne
k1
+
R(AuL)2 + Nu HooI RAuL + LAuNu+
k−1
(10)
(11)
k2
RAuL + H2O → RH + LAuOH
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PrSpH+
second-order kinetics with the rate constant k = 0.27
L·mmol−1·h−1, as evidenced from the linear graph
+
R(AuL)2 + Cl− ⎯⎯⎯→ [RAuL] ⎯⎯⎯⎯⎯⎯→⎯ RH
(12)
slow
fast
ln([D] + [T]0 − [D]0) − ln[D]
The aforementioned examples point out that diaurated
species derived from the sterically hindered IPr ligand are the
most kinetically inert among all other diaurated species in this
study (despite the fact that they should be the least stable
thermodynamically). The reactivity also depends on the vinyl
core (Scheme 2). Thus, 5-endo and 6-endo diaurated species are
generally more reactive than 5-exo or 6-exo species. There are
also ligand effects on the reactivity. Thus, PPh3-derived
diaurated species are the most kinetically reactive (but at the
same time thermodynamically stable), which is evident from
y =
= kt + const
[T]0 − [D]0
where [D]0 and [T]0 are starting concentrations of the
reactants (derivation of this equation is given in the Supporting
Information). Importantly, the reverse reaction did not have
any impact because the equilibrium theoretically could be
prognosticated to be reached only at >99% conversion.12 Thus,
the entire process can be accurately described as an irreversible
one-way reaction (Scheme 3).
To further demonstrate that the reaction is not influenced by
the reverse process and does not involve predissociation of D21
to free vinyl gold species B21, the reaction kinetics were
performed in the presence of acid additives at various
concentrations (Scheme 4). In comparison with the previous
+
the fact that R(AuPPh3)2 /RAuPPh3 mixtures always exhibit a
single set of sharp signals in proton NMR spectra, indicating
fast exchange of the Ph3PAu+ unit.11 A similar situation is
observed for DTBP and L5 based species, but the rate of
exchange is less, which is evident from the broader shape of the
+
signals. In the case of L2 and L3, the corresponding R(AuL)2 /
Scheme 4. Second-Order Kinetics of Reaction of D21 with
TMTU in the Presence of an Acid Additive
RAuL mixtures always exhibited separate sets of signals,
indicating a slow mode of LAu+ exchange (despite the fact
that these diaurated species are thermodynamically less stable).
Slow reactions of D20 and D21 with TMTU were used to
conveniently study the reaction kinetics and reaction
mechanism.
Thus, the kinetics of reaction of D21 with TMTU was
investigated in CD2Cl2 under water-free conditions in the
presence of CaH2 (Scheme 3). In a completely closed and dry
system the appearing vinyl gold species B21 remained stable
during the entire course of the reaction. Correspondingly, the
reaction system was free of any side processes, including
protodeauration. The reaction cleanly displayed general
Scheme 3. Second-Order Kinetics of Reaction of D21 with
TMTU
case here one should consider the expenditure of an additional
1 equiv of TMTU to immediately bind the gold liberated upon
protodeauration of the vinyl gold intermediate. The whole
reaction sequence now consists of slow liberation of the vinyl
gold species B21 and subsequent fast protodeauration with
formation of additional L2AuTMTU+. In complete accordance
with our expectations, the reaction nicely followed general
second-order kinetics. Importantly, the kinetics was independ-
ent not only on the acid concentration but also on the acid
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strength (two differently strong acids were used: PrSpH+OTf−
and tBu2PyH+OTf−), which is consistent with the rate-limiting
vinyl gold formation step. The rate constant k = 0.27 0.01 L
mmol−1 h−1 determined in this series is equal to that obtained
in the previous experiment above, which further demonstrates
the independence of the reaction of any acid.
stability, as do electron donor ligands at gold. Any conjugation
of the enol ether residue with a simple double bond or an aryl
residue will decrease the stability. Steric factors also play a
crucial role in the stability of diaurated species and can readily
dominate over the electronic factors. Thus, diaurated species
derived from the very bulky ligands L2, L3, and IPr will be
considerably less stable than the PPh3- or PMe3-derived
analogues. Additionally, we have found that the rate of reaction
with nucleophiles drastically depends on steric factors. While
diaurated species bearing small or nonrigid ligands such as
PMe3, PPh3, L4, and L5 always reacted quickly with suitably
strong nucleophiles, some species bearing very bulky ligands
reacted very sluggishly, even if the reaction was thermodynami-
cally favored. This is a consequence of the mechanism of the
reaction, which was found to occur as an associative SN2
process.
The results presented herein have direct implications for gold
catalysis, because they demonstrate that the formation of
diaurated species from vinyl gold intermediates generally is
thermodynamically very favored but will strongly depend on
the catalyst and substrate used. Also, the kinetic reactivity of
diaurated species can vary significantly depending on the steric
accessibility of gold: sterically hindered diaurated species are
less reactive and at the same time less stable than their less
hindered analogues.
Analogously the rate constant k = 3.0 L mmol−1 h−1 for
reaction of D20 with TMTU was found. Therefore, D20 reacts
with TMTU 11 times more quickly than D21, which reflects
the easier accessibility of gold in D20 toward nucleophilic
attack. This can be associated with both the reduced steric
hindrance (methyl substituent vs ethyl) and the higher rigidity
and flat character of the five-membered ring in comparison to
the six-membered ring.
These experiments unambiguously point to an associative
SN2 mechanism for the reaction of diaurated species with
nucleophiles. In order to establish the thermodynamic
parameters of this reaction, we performed a series of
experiments at various temperatures (−10.2, −4.1, 4.9, 15,
26, and 36.9 °C). From this, the Arrhenius activation energy EA
for the reaction of D21 with TMTU was found to be 60.1 kJ
mol−1. Analysis of the Eyring equation gave the following
activation parameters for the reaction: ΔH⧧ = 57.7 kJ mol−1
and ΔS⧧ = −73.6 J mol−1 K−1. The negative value of ΔS⧧ is
consistent with an associative process.
It can be also concluded that formation/dissociation of
diaurated species in a catalytic cycle should be considered as an
associative SN2 process with any suitable molecule acting as a
nucleophile (solvent, substrate, product, water, MeCN, etc.).
That reaction of diaurated species with nucleophiles occurs
as an associative process additionally follows from a number of
observations. We mentioned that the equilibrium in the
reaction of D5 with Lut is established much more slowly
than with Pic. This fact unambiguously points to an associative
SN2 mechanism for the reaction. Indeed, if the reaction
followed a dissociative SN1-like route, the dependence of the
overall reaction rate on the nature of the nucleophile would not
be so pronounced (if at all).13 Similar nucleophile effects are
revealed in reactions of Scheme 2 and the aforementioned
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, tables, and figures giving complete experimental
procedures and detailed NMR spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
+
comparisons of R(AuL)2 /RAuL exchange rates. Indeed, if the
reaction followed a dissociative SN1 mechanism, the least stable
L2 and L3 derived diaurated species should dissociate more
easily and undergo fast exchange, which is not the case. It
therefore can be generally accepted that the reaction of
diaurated species with nucleophiles occurs via an associative
SN2 nucleophilic substitution at gold (Scheme 5). It is known
that gold complexes undergo ligand exchange by an associative
SN2 mechanism, and our experiments established that diaurated
species are not an exception.14
AUTHOR INFORMATION
Corresponding Author
*E-mail for M.E.M.: martin.e.maier@uni-tuebingen.de.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank Dr. K. Eichele and Institut fur Anorganische Chemie
for allowing us to use their NMR spectrometer, Dr. D. Wistuba
for HRMS analysis, and Dr. M. Kramer for the assistance with
the various temperature experiments. Financial support by the
■
̈
Scheme 5. SN2 Mechanism of the Reaction of Diaurated
Species with Nucleophiles
state of Baden-Wurttemberg is greatly acknowledged.
̈
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D.; Adduci, L. L.; Gagne, M. R. Angew. Chem., Int. Ed. 2012, 51, 2452−
́
M. R.; Widenhoefer, R.
́
2456. (h) Heckler, J. E.; Zeller, M.; Hunter, A. D.; Gray, T. G. Angew.
Chem., Int. Ed. 2012, 51, 5924−5928. (i) Hashmi, A. S. K.; Wieteck,
M.; Braun, I.; Rudolph, M.; Rominger, F. Angew. Chem., Int. Ed. 2012,
51, 10633−10637. (j) Hansmann, M. M.; Rudolph, M.; Rominger, F.;
Hashmi, A. S. K. Angew. Chem., Int. Ed. 2013, 52, 2593−2598. (k) For
́ ́
a recent highlight about gem-diaurated species, see: Gomez-Suarez, A.;
Nolan, S. P. Angew. Chem., Int. Ed. 2012, 51, 8156−8159.
(4) Zhdanko, A.; Maier, M. E. Chem. Eur. J. 2013, DOI: 10.1002/
chem.201204491.
(5) Seidel, G.; Lehmann, C. W.; Furstner, A. Angew. Chem., Int. Ed.
̈
2010, 49, 8466−8470. See also ref 3f.
(6) Zhdanko, A.; Strobele, M.; Maier, M. E. Chem. Eur. J. 2012, 18,
̈
14732−14744.
(7) The corresponding equilibrium constant for the CN-containing
species could not be determined, because the species already
decomposed upon addition of K2CO3 during preparation of the
reaction mixture for determination of the equilibrium constant (see
the Supporting Information). This fact points to the high instability of
the species.
(8) Complexes L2AuNu+ bearing a weak ligand (e.g., MeCN) are
known to react with PrSp and H2O to displace the Nu: 2 L2AuNu+ +
PrSp + H2O → (L2Au)2OH+ + PrSpH+ + Nu.6 Nu should be weaker
than Me2S for this reaction to occur.
(9) The superiority of catalyst 2 over 1 can be seen in the following
examples: (a) Nieto-Oberhuber, C.; Lop
Am. Chem. Soc. 2005, 127, 6178−6179. (b) Nieto-Oberhuber, C.;
Munoz, M. P.; Lopez, S.; Jimenez-Nunez, E.; Nevado, C.; Herrero-
ez, E.; Raducan, M.; Echavarren, A. M. Chem. Eur. J. 2006, 12,
1677−1693; 2008, 14, 5096 (corrigendum). (c) Aponick, A.; Li, C.-Y.;
Palmes, J. A. Org. Lett. 2009, 11, 121−124. (d) Barabe, F.; Betournay,
́
ez, S.; Echavarren, A. M. J.
́
́
́
̃
̃
Gom
́
́
́
G.; Bellavance, G.; Barriault, L. Org. Lett. 2009, 11, 4236−4238.
(10) Nesmeyanov, A. N.; Perevalova, E. G.; Grandberg, K. I.;
Lemenovskii, D. A.; Baukova, T. V.; Afanassova, O. B. J. Organomet.
Chem. 1974, 65, 131−144.
(11) The same fast exchange was previously observed in
3g
+
Ar(AuPPh3)2 /ArAuPPh3 systems.
(12) The prognosis for the equilibrium is described in the Supporting
Information.
(13) Assume a fast dissociation preequilibrium with the subsequent
reaction with a nucleophile:
Nu
+
R(AuL)2 ⇌ RAuL + LAu+ ⎯⎯→ LAuNu
The difference in reactivity of Pic and Lut with LAu+ unit is not so
high as to cause such a dramatic difference in the reactivity of D5. The
equilibrium constant for the exchange L2AuLut+/Pic is 1.7 and is
established immediately,6 indicating that any LAu+ would react almost
2006
dx.doi.org/10.1021/om400083f | Organometallics 2013, 32, 2000−2006