Role of silver salts in palladium-catalyzed arene and heteroarene C-H functionalization reactions

Article abstract of DOI:10.1021/acs.organomet.6b00437

Silver carboxylate salts are widely used as additives in palladium-catalyzed C-H functionalization reactions. However, the role of these silver additives is often not fully understood. This paper describes an investigation of the role of AgOPiv in the stoichiometric activation of C₆F₅H at a well-defined Pd^II complex as well as in the Pd^II-catalyzed oxidative dimerization of 2-alkylthiophenes. Both in situ NMR spectroscopy and H/D exchange studies of the reactions of C₆F₅H implicate a role for AgOPiv in the C-H cleavage event, generating Ag-C₆F₅ as an intermediate. The catalytic studies show similar trends despite the different conditions and substrates, suggesting that AgOPiv promotes a similar metalation of the thiophene in the catalytic transformations. This proposal is supported by DFT calculations, which show energetically feasible pathways for concerted metalation-deprotonation of both 2-methylthiophene and pentafluorobenzene at [Ag(OPiv)]₂. These studies suggest that initial metalation of C-H substrates at Ag^I carboxylates should be considered as a plausible pathway in C-H functionalization reactions involving mixtures of Ag and Pd salts.

Full text of DOI:10.1021/acs.organomet.6b00437

Article
pubs.acs.org/Organometallics
Role of Silver Salts in Palladium-Catalyzed Arene and Heteroarene
C−H Functionalization Reactions
Monica D. Lotz,† Nicole M. Camasso,† Allan J. Canty,*,‡ and Melanie S. Sanford*,†
†Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States
‡School of Physical Sciences, University of Tasmania, Private Bag 75, Hobart, Tasmania 7001, Australia
S
* Supporting Information
ABSTRACT: Silver carboxylate salts are widely used as
additives in palladium-catalyzed C−H functionalization
reactions. However, the role of these silver additives is often
not fully understood. This paper describes an investigation of
the role of AgOPiv in the stoichiometric activation of C6F5H at
a well-defined PdII complex as well as in the PdII-catalyzed
oxidative dimerization of 2-alkylthiophenes. Both in situ NMR
spectroscopy and H/D exchange studies of the reactions of
C6F5H implicate a role for AgOPiv in the C−H cleavage event,
generating Ag−C6F5 as an intermediate. The catalytic studies
show similar trends despite the different conditions and
substrates, suggesting that AgOPiv promotes a similar metalation of the thiophene in the catalytic transformations. This proposal
is supported by DFT calculations, which show energetically feasible pathways for concerted metalation−deprotonation of both 2-
methylthiophene and pentafluorobenzene at [Ag(OPiv)]2. These studies suggest that initial metalation of C−H substrates at AgI
carboxylates should be considered as a plausible pathway in C−H functionalization reactions involving mixtures of Ag and Pd
salts.
catalyzed, AgOPiv-mediated C−H oxidative coupling of 2-
ilver carboxylate and carbonate salts are commonly
Semployed as additives in Pd-catalyzed C−H bond alkylthiophenes.
functionalization reactions.1−4 In many of these systems, the
presence of the silver salt dramatically enhances the reaction
rate and/or yield. However, the mechanistic origin of this
enhancement is often poorly understood. The most commonly
proposed roles for silver carboxylate salts in Pd-catalyzed C−H
functionalization reactions are (1) AgO2CR serving as a source
of carboxylate for the PdII center (often via halide abstraction),
which in turn facilitates carboxylate-assisted C−H cleavage at
PdII (Scheme 1i)1,2 or (2) AgO2CR acting as a terminal oxidant
to (re)generate the active PdII catalyst A (Scheme 1v).1,3
Several other possible roles have also been proposed, including
(3) AgO2CR promoting C−C coupling (Scheme 1iv),4 (4)
AgO2CR reacting with Pd to form bi- or multimetallic Pd−Ag
intermediates that facilitate C−H cleavage at PdII (Scheme
1iii);3a,d,5 and (5) AgO2CR directly promoting C−H cleavage
to form a Ag-arene intermediate (Scheme 1ii).3i,k,6,7 However,
experimental work aimed at establishing which of these
possibilities is operating in a given transformation remains
relatively rare.
RESULTS AND DISCUSSION
■
We initially focused on a stoichiometric model system to
explore the role of Ag carboxylates in Pd-mediated C−H
cleavage. We set two criteria for this model reaction. First, we
sought an aryl−H substrate and a palladium precursor that
would react to yield a stable, isolable Pd−σ-aryl product.8 This
could enable us to selectively investigate the C−H cleavage
step. Second, we sought an aryl−H substrate that was relevant
to catalysis. These criteria led us to the reaction of
(dtbpy)Pd(OAc)2 (1; dtbpy = 4,4′-di-tert-butyl-2,2′-bipyridine)
with C6F5H, a substrate that has been widely used in Pd-
catalyzed, Ag-mediated cross-coupling reactions.1f
Our initial studies showed that the reaction of 1 with C6F5H
in the presence of certain Ag additives affords a mixture of the
PdII−pentafluorophenyl complexes 2 and 3 (Table 1). These
PdII products are stable and do not undergo C−C bond-
forming reductive elimination over 24 h at 60 °C. We
conducted a detailed investigation of the impact of the Ag
salt structure on the conversion of 1 to 2/3. As shown in Table
1, entry 1, heating complex 1 with C6F5H in THF at 60 °C for
In this paper, we report studies aimed at unraveling the role
of AgO2CR in the stoichiometric activation of C6F5H. We
provide spectroscopic evidence that AgOPiv reacts with C6F5H
to afford a Ag−C6F5 complex that then undergoes trans-
metalation to Pd. In addition, we use experimental and DFT
studies to demonstrate that the findings from this stoichio-
metric activation of C6F5H are potentially relevant to a Pd-
Special Issue: Hydrocarbon Chemistry: Activation and Beyond
Received: June 1, 2016
© XXXX American Chemical Society
A
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Ag2CO3, AgOAc, and AgOTFA afforded mixtures of 2 and 3
under otherwise identical conditions (entries 3−5). In contrast,
Ag salts bearing weakly basic anionic ligands such as
tetrafluoroborate and hexafluorophosphate provided ≤1% of
the organometallic products. The presence of Ag appears to be
important for achieving high yields of 2/3 under these
conditions. For example, other M2CO3 and MOPiv salts (M
= Li, Na, Cs, NMe4) afforded a maximum of 9% of 2 and 2% of
3 under otherwise identical conditions (entries 8 and 9).
These initial results allow us to rule out several potential
roles for AgOPiv in this specific transformation. This
transformation is a redox-neutral process, and complex 3 is
inert to reductive elimination. This eliminates contributions
from the possibilities given by Scheme 1iv,v in this system. The
low reactivity of other metal pivalate salts suggests that AgOPiv
is not simply serving as a pivalate source (Scheme 1i). Thus, we
hypothesized that the AgOPiv plays a role in the C−H cleavage
step by forming either a Pd/Ag bimetallic or a Ag−C6F5
intermediate (Scheme 1ii,iii).
Scheme 1. Potential Roles of Silver in C−H
Functionalization Reactions
In an effort to observe one or both of these intermediates, we
next monitored the reaction of C6F5H with 1/AgOPiv via 19F
NMR spectroscopy. After 30 min at 60 °C, compound 4 was
detected based on a diagnostic signal at approximately −105
ppm (Figure 1a). Compound 4 was consumed over the course
Table 1. Effect of Additives on the Reaction of 1 with C6F5H
a
To Afford 2/3
yield, %
entry
additive
2
3
1
2
3
4
5
6
7
8
9
none
<1
<1
21
12
60
<1
<1
<1
88
39
30
19
1
Figure 1. 19F NMR spectra (between −95 and −140 ppm): (a)
reaction of 1, AgOPiv, and C6F5H after 0.5 h at 60 °C; (b) reaction of
AgOPiv and C6F5H after 1 h at 60 °C; (c) authentic sample of Ag−
C6F5 (synthesized from AgOTFA and LiC6F5).
AgOPiv
Ag2CO3
AgOAc
AgOTFA
AgBF4
AgPF6
<1
of the reaction, with concomitant generation of products 2 and
3. As shown in Figure 1b, this same intermediate was formed
when C6F5H and AgOPiv were combined in THF at 60 °C in
the absence of Pd. In contrast, no new 19F NMR signals were
observed in the reaction of C6F5H with CsOPiv under
analogous conditions.
b
MOPiv
up to 1
up to 9
up to 1
up to 2
c
M2CO3
a
Conditions: 1 (0.025 mmol), additive (0.075 mmol), C6F5H (1.8
mmol) in THF at 60 °C for 18 h. Yields are based on 1 serving as the
limiting reagent. M = Na, Cs, NMe4. M = Li, Na, Cs.
b
c
To confirm the assignment of 4 as a Ag−C6F5 complex, we
independently synthesized this Ag complex from LiC6F5 and
AgOTFA according to a literature procedure.10 As shown in
Figure 1c, the 19F NMR spectrum of this independently
prepared Ag−C6F5 matches that of compound 4 formed in situ
from AgOPiv/C6F5H.11 Additionally, the treatment of complex
18 h afforded ≤1% conversion to the organometallic products 2
and 3.9 Under otherwise identical conditions, the addition of a
Ag carboxylate or carbonate salt resulted in a dramatic
enhancement in conversion. AgOPiv provided a high yield of
the bis(pentafluorophenyl)−PdII complex 3 (entry 2), while
B
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Article
1 with 3 equiv of an authentic sample of Ag−C6F5 produced 3
in 91% yield.12,13
Table 2. 1-Catalyzed Oxidative Coupling of 2-
Methylthiophene and 2-Trifluoromethylthiophene with
We next studied H/D exchange between C6F5H and D2O (5
equiv relative to C6F5H) catalyzed by MOPiv.1f,3i,k,6 The ratio
of C6F5H to MOPiv in this experiment was the same as that in
the reactions in Table 1. As shown in eq 1, analysis by 2H NMR
Various Additives
a
b
entry
R
additive
yield of 5, %
TON
1
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CF3
CF3
CF3
CF3
CF3
CF3
CF3
none
<1
5.0
1.9
1.5
<1
<1
2
AgOPiv
Ag2CO3
AgOAc
AgF
50
19
15
7
3
4
5
6
AgPF6
MOPiv
none
<2
c
7
<1
8
<1
6.1
1.7
1.9
2.9
<1
spectroscopy showed 63% deuterium incorporation in reactions
with AgOPiv under these conditions. In contrast, 9% and <1%
deuterium incorporation were detected with CsOPiv and
NMe4OPiv, respectively, under otherwise identical condi-
tions.14 Moreover, no deuterium exchange was observed
when complex 1 was subjected to the reaction conditions in
the absence of a MOPiv additive.
Collectively, these data provide evidence that AgOPiv and
C6F5H react to generate Ag−C6F5, which can undergo
transmetalation at Pd to form products 2/3.15,16 Notably,
Nolan proposed a similar Ag−arene intermediate in the
Au(IPr)(OH)/Ag2O-catalyzed carboxylation of 1,3,5-trifluor-
obenzene.6a Furthermore, similar intermediates have been
proposed in a number of Pd-catalyzed C6F5H cross-coupling
reactions.1f,3i,k,6 However, this is a rare example of the
spectroscopic observation of an organometallic Ag intermediate
in such transformations.
9
AgOPiv
Ag2CO3
AgOAc
AgF
61
17
19
29
<2
10
11
12
13
14
AgPF6
MOPiv
c
<1
a
Yields determined on the basis of AgI, which serves as the limiting
b
reagent and as a 2e− oxidant. TON of 5 reported on the basis of Pd
complex 1. M = Na, Cs, Me4N.
c
Notably, analogous results were obtained using 2-trifluorome-
thylthiophene as a substrate (Table 2, entries 8−14). In this
case, the AgOPiv conditions afforded the dimer in 61% yield
(TON = 6.1; entry 9).
Third, as shown in eq 2, AgOPiv was found to promote H/D
exchange at the 5-position of both 2-methylthiophene and 2-
We next sought to probe the catalytic relevance of these
findings. Our initial studies focused on the 1-catalyzed oxidative
dimerization of C6F5H17 in the presence of AgOPiv. However,
the stability of Pd intermediate 3 precluded product formation
under all of the conditions examined. As such, we next
examined the 1-catalyzed oxidative dimerization of 2-methyl-
thiophene and 2-trifluoromethylthiophene (Table 2). Notably,
the Pd-catalyzed C−H functionalization of thiophenes is well-
precedented;18,19 furthermore, silver carboxylates and other Ag
salts are commonly used as additives in these systems.18,19 In
the literature, the role of the silver salts is primarily proposed to
involve regeneration of the active catalyst by oxidation of Pd0 to
PdII (Scheme 1v).
After some optimization, we identified conditions for the 1-
catalyzed oxidative dimerization of 2-methylthiophene to afford
product 5 in 50% yield (TON = 5.0; Table 2, entry 2). Silver
additives proved critical for this transformation (entries 2−5),
with AgOPiv leading to the highest yield/TON (entry 2). The
AgOPiv serves as a terminal oxidant (Scheme 1v), as indicated
by the formation of a silver mirror at the end of the reaction.
However, several pieces of evidence implicate an additional role
for AgOPiv in this system. First, soluble AgI salts bearing
noncoordinating counterions (e.g., AgPF6) proved ineffective
for promoting this transformation (entry 6), despite the fact
that they should be competent oxidants for Pd0.20 Second, the
use of other metal pivalate salts (e.g., Na, Cs, or NMe4OPiv)
resulted in <1 turnover of the oxidative dimerization product
(entry 7). While these other metal pivalates cannot oxidize Pd0
to PdII, if they operate in an otherwise similar fashion to
AgOPiv, they should be capable of promoting 1 turnover of 1.
trifluoromethylthiophene.21 This H/D exchange occurs in the
absence of palladium, under conditions otherwise analogous to
the catalytic oxidative dimerization reaction (1.0 mmol of
thiophene, 0.30 mmol of AgOPiv in dioxane for 18 h at 100 °C,
but with 20 mmol of added D2O). In contrast, <1% deuterium
incorporation was observed with CsOPiv or NMe4OPiv under
these conditions.22,23
C
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A variety of experiments were conducted in an effort to
spectroscopically observe the proposed Ag−thiophene inter-
mediate (6 with R = CF3; eq 2) via 19F NMR spectroscopy.8
However, these experiments were inconclusive, possibly due to
the low equilibrium population and/or the instability of 6 at the
temperatures required to form this species. In view of the
inability to experimentally observe putative intermediate 6, we
turned to DFT to probe the feasibility of accessing this (or
related) species, in conjunction with analogous computation for
pentafluorobenzene reactivity as a model system.24,25 In
considering potential mechanisms for metalation at Ag, we
noted that AgI carboxylates typically exist as dimers containing
a weak argentophilic interaction.26,27 For example, in the solid
state, AgOAc exhibits linear “AgO2” coordination, with a Ag···
Ag interaction as well as weak axial coordination from
neighboring molecules to give the approximately planar motif
“O··Ag(AcO-μ-O,O′)2Ag··O”.26a We hypothesized that in
dioxane and THF solutions the solvent would likely serve as
an axial ligand. These considerations are related to those
adopted in studies of metalation at bimetallic Pd/Ag system-
s.5a,28
Our calculations confirm that dimeric structures bearing two
axial dioxane ligands (d) are lower in energy than the dimer
with free axial coordination site(s) (ΔG = −4.4 kcal/mol). In
contrast, THF has little influence, as [AgOPiv]2·THF has a
value of −0.3 kcal/mol and [AgOPiv]2·2THF of +0.2 kcal/mol.
Although these small energy differences are influenced by
estimated values for corrections to the translational contribu-
tion of entropy changes,29 the computed lowest energy ground
state species were used as the reference energy for reaction
profiles. Overall, DFT shows that monomeric AgOPiv is
unlikely to be the reactive species in these transformations.30
Transition structures were obtained by employing the
concerted metalation−deprotonation (CMD) model31 applied
to binuclear four-coordinate AgI cores. As shown in Figure 2, a
variety of related transition structures were found, and these
can generally be grouped into categories a and b. Category a
transition states show metalation at the silver atom that is not
coordinated by the carboxylate group that participates in CMD.
In contrast, those in category b show CMD occurring at a
single AgI center. The eight structures shown in Figure 2 are
comprised of four closely related pairs, e.g. TS3_thi and
TS4_thi, differing only in the number of coordinated solvent
molecules. Each pair exhibits ΔG⧧ values within 0.4−2.0 kcal/
mol. In light of the high relative concentration of solvent and,
in some cases, the presence of weak Ag···solvent interactions
(e.g., 2.746 Å in TS2_thi), it is difficult to definitively select
between these species for a preferred pathway in each pair.
Notably, TS3_thi and TS4_thi show a possible role for the
sulfur atom of 2-methylthiophene.
For the metalation of 2-methylthiophene, the transition
structures TS1_thi and TS2_thi are favored over TS3_thi and
TS4_thi by ∼5 kcal/mol, and complete reaction profiles
involving TS1_thi and TS2_thi are shown in Figure 3. The
four pentafluorobenzene transition structures have similar
activation barriers (ΔG⧧ = 24.8−25.5 kcal/mol), and reaction
profiles for these together with those for TS3_thi and TS4_thi
(all supported by intrinsic reaction coordinate (IRC) analysis
and potential energy scans) are provided as Supporting
Information.32 Gaussview diagrams for TS2_thi (a representa-
tive category a transition structure) together with TS3_C6F5 (a
representative category b transition structure) are shown in
Figure 4.
Figure 2. Potential transition structures for concerted metalation−
deprotonation (CMD) of 2-methylthiophene and pentafluorobenzene
at binuclear silver centers. Category a involves metalation across two
AgI centers. Category b involves metalation at a single AgI center.
Energies ΔG⧧ (ΔH⧧) in kcal/mol are with respect to [AgOPiv]2·THF.
Abbreviations: d = dioxane; THF = tetrahydrofuran.
For 2-methylthiophene, the formation of an O··H interaction
and a weak η2 interaction in IIIA,B leads directly to transition
structures followed by dissociation of pivalic acid to give VA,B
(Figure 3). In these profiles, the weak Ag···Ag interactions are
computed to cover the range of 2.684−2.805 Å. The shortest
distances are observed for intermediates VA (2.794 Å) and VB
(2.684 Å), where the 2-methylthienyl groups are participating
in a bridging three-center−two-electron interaction (Ag−C
2.153−2.235 Å).
Computed structures for the initial products arising from the
eight transition structures in Figure 2 are shown as VA,B in
Figure 3 and VI−VIII in Figure 5. AgI species are expected to
be labile, and a representative μ-C6F5 structure (IX) is shown
for comparison with VA,B. We also note the similarity of
products VA,B and IX to crystallographically characterized
polymeric AgI pentafluorophenyl complexes containing μ-C6F5
groups.33b,34
For 2-methylthiophene the highest calculated barrier for the
process illustrated in Figure 3 (23.0 kcal/mol) should be
experimentally feasible under the catalytic oxidative coupling
conditions (100 °C in dioxane). In addition, these calculations
D
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Figure 3. Energy profile for the reaction of silver pivalate with 2-methylthiophene in dioxane (d) via transition structures TS1_thi and TS2_thi
showing selected distances (Å). Energies ΔG (ΔH) are given in kcal/mol. Additional data for TS2_thi are presented with Figure 4. Abbreviations: d
= dioxane; THF = tetrahydrofuran.
Figure 5. Computed products from the four transition structures
shown in Figure 2, together with a structure calculated for a μ-C6F5
species (IX). Energies ΔG (ΔH) in kcal/mol are with respect to
[AgOPiv]2·THF.
Figure 4. Examples of computed TS structures for the metalation of
(i) 2-methylthiophene (TS2_thi) via a category a transition state and
(ii) pentafluorobenzene (TS3_C6F5) via a category b transition state.
and in the catalytic oxidative dimerization of 2-methylthiophene
and 2-trifluoromethylthiophene catalyzed by (dtbpy)Pd(OAc)2.
Both in situ NMR spectroscopy and H/D exchange studies of
the stoichiometric reactions implicate a role for AgOPiv in the
Selected bond distances are shown in Å.
demonstrate that the metalated Ag−2-methylthiophene inter-
mediate VB is 14.4 kcal/mol uphill relative to I and 7.0 kcal/
mol higher than the analogous Ag−C6F5 intermediate IX.35
This energy difference is consistent with our inability to detect
this species experimentally. Transmetalation to PdII is then
expected to be a low-energy process involving VA,B or related
intermediates, on the basis of reports of related facile
transmetalation from AgI to PdII.36,37
C−H cleavage event, generating Ag−C6F5 as an intermediate.
The catalytic studies show similar trends despite the different
conditions and substrates, suggesting that AgOPiv promotes a
similar metalation of 2-alkylthiophenes in the catalytic trans-
formations. This proposal is supported by DFT calculations,
which show energetically feasible pathways for concerted
metalation−deprotonation of both 2-methylthiophene and
pentafluorobenzene at [Ag(OPiv)]2. These studies suggest
that initial metalation of C−H substrates at AgI carboxylates
should be considered as a plausible pathway in C−H
In summary, this paper describes a study of the role of
AgOPiv salts in the stoichiometric C−H activation of C6F5H
E
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functionalization reactions involving mixtures of Ag carbox-
ylates and Pd salts.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00437.
Experimental and spectral details for all new compounds
and all reactions reported as well as DFT energy
parameters, Cartesian coordinates of all optimized
structures, and Gaussview diagrams for species not
shown above (PDF)
Crystallographic data for 3 (CIF)
Crystallographic data for S-1 (CIF)
(8) For related studies of thiophene C−H activation at PtII, see:
Sugie, A.; Furukawa, H.; Suzaki, Y.; Osakada, K.; Akita, M.; Monguchi,
D.; Mori, A. Bull. Chem. Soc. Jpn. 2009, 82, 555.
(9) An authentic sample of 3 was synthesized from the reaction of
4,4′-di-tert-butyl-2,2′-bipyridine (dtbpy) with (1,4-cyclooctadiene)
Pd(C6F5)2. An X-ray crystal structure was obtained along with
characterization by 1H, 13C, and 19F NMR spectroscopy to confirm the
structure of 3.
(10) Miller, W. T.; Sun, K. K. J. Am. Chem. Soc. 1970, 92, 6985.
(11) Ag-C6F5 likely exists as a polymer. (a) Ibad, M. F.; Schulz, A.;
Villinger, A. Organometallics 2015, 34, 3893. (b) Kuprat, M.; Lehmann,
M.; Schulz, A.; Villinger, A. Organometallics 2010, 29, 1421.
(12) The modest solubility of Ag−C6F5 precluded quantitative
monitoring of this reaction using NMR spectroscopy.
(13) Importantly, the observation of Ag−C6F5 does not rule out an
additional possible role for dimeric Pd−Ag intermediates in the C−H
cleavage process.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail for A.J.C.: Allan.Canty@utas.edu.au.
*E-mail for M.S.S.: mssanfor@umich.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the NSF under the CCI Center for
Enabling New Technologies through Catalysis (CENTC)
Phase II Renewal, CHE-1205189, and the Australian Research
Council. N.M.C. acknowledges the ACS Division of Organic
Chemistry for a graduate fellowship. We acknowledge Dr. Jeff
W. Kampf for X-ray crystallographic analysis of 3 and S-1 as
well as funding from NSF Grant CHE-0840456 for X-ray
instrumentation.
(14) For a complete list of H/D exchange studies with C6F5H, see
the Supporting Information.
(15) For examples of related metalations at AuI, see: (a) Lu, P.;
Boorman, T. C.; Slawin, A. M. Z.; Larrosa, I. J. Am. Chem. Soc. 2010,
132, 5580. (b) de Haro, T.; Nevado, C. Synthesis 2011, 2011, 2530.
(c) Gaillard, S.; Cazin, C. S. J.; Nolan, S. P. Acc. Chem. Res. 2012, 45,
778.
(16) For examples of related metalations at Cu, see ref 1b and: Suess,
A. M.; Ertem, M. Z.; Cramer, C. J.; Stahl, S. S. J. Am. Chem. Soc. 2013,
135, 9797.
(17) Chen, F.; Feng, Z.; He, C. Y.; Wang, H. Y.; Guo, Y. L.; Zhang,
X. Org. Lett. 2012, 14, 1176.
(18) Reviews: (a) Mori, A.; Sugie, A. Bull. Chem. Soc. Jpn. 2008, 81,
548. (b) Shibahara, F.; Murai, T. Asian J. Org. Chem. 2013, 2, 624.
(c) Okamoto, K.; Zhang, J.; Housekeeper, J. B.; Marder, S. R.;
Luscombe, C. K. Macromolecules 2013, 46, 8059.
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(20) For a complete list of Ag additives screened under the reaction
conditions, see the Supporting Information.
(21) These reactions were conducted on the benchtop using reagent
grade dioxane in order to mimic the catalytic conditions. As such,
significant quantities of residual H2O are present, which is expected to
dilute the extent of H/D exchange.
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(22) Ag2CO3, AgOAc, and AgF all afforded ≤1% deuterium
incorporation.
(23) No C−C coupling was detected during H/D exchange.
(24) Frisch, M. J., et al. Gaussian 09, revision A.02; Gaussian, Inc.,
Wallingford, CT, 2009.
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DOI: 10.1021/acs.organomet.6b00437
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(25) Computation employed the B3LYP functional for geometry
optimization with dioxane or tetrahydrofuran as solvent utilizing the
SDD basis set on Ag and the 6-31G(d) basis set for other atoms.
Single-point energy calculations for all structures at the M06 level
employed the quadruple-ξ valence def2-QZVP basis set on Ag along
with the corresponding ECP and the 6-311+G(2d,p) basis set on other
atoms. See the Supporting Information for further details.
(26) (a) Olson, L. P.; Whitcomb, D. R.; Rajeswaran, M.; Blanton, T.
N.; Stwertka, B. J. Chem. Mater. 2006, 18, 1667. (b) Paramanov, S. E.;
Mychlo, E. V.; Troyanov, S. I.; Kuzmina, N. P. Russ. J. Inorg. Chem.
2000, 45, 1852.
(27) Schmidbaur, H.; Schier, A. Angew. Chem., Int. Ed. 2015, 54, 746.
(28) Yang, Y.-F.; Cheng, G.-J.; Liu, P.; Leow, D.; Sun, T.-Y.; Chen,
P.; Zhang, X.; Yu, J.-Q.; Wu, Y.-D.; Houk, K. N. J. Am. Chem. Soc.
2014, 136, 344.
(29) Mammen, M.; Shakhnovich, E. I.; Deutch, J. M.; Whitesides, G.
M. J. Org. Chem. 1998, 63, 3821.
(30) A mononuclear mechanism with a transition structure similar to
TS3_thi proceeds with a prohibitively high activation energy (59.3
kcal/mol relative to I). As such, we deem this pathway unlikely. See
the Supporting Information for full details.
(31) (a) Gorelsky, S. I.; Lapointe, D.; Fagnou, K. J. Am. Chem. Soc.
2008, 130, 10848. (b) Lapointe, D.; Fagnou, K. Chem. Lett. 2010, 39,
1118. (c) Gorelsky, S. I.; Lapointe, D.; Fagnou, K. J. Org. Chem. 2012,
77, 658. (d) Guihaume, J.; Clot, E.; Eisenstein, O.; Perutz, R. N.
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111, 1315. (f) Topczewski, J. T.; Sanford, M. S. Chem. Sci. 2015, 6, 70.
(g) Gorelsky, S. I. Coord. Chem. Rev. 2013, 257, 153.
(32) Computation for acetate analogues of the transition structures
shown in Figure 2 provided trends identical with those for OPiv,
illustrating discrimination between 2-methylthiophene species (ΔG⧧ =
23.0, 22.7, 28.3, and 27.6 kcal/mol for the analogues of TS1−4_thi)
and a narrow range of values for C6F5H (ΔG = 25.8, 25.2, 26.7, and
24.9 kcal/mol for the analogues of TS1−4_C6F5) (see the Supporting
Information for full details). We preliminarily hypothesize that the
lower observed reactivity of AgOAc versus AgOPiv in these systems is
due to the lower solubility of the former.
(33) Ag−C6F5 likely exists as a polymer. (a) Ibad, M. F.; Schulz, A.;
Villinger, A. Organometallics 2015, 34, 3893. (b) Kuprat, M.; Lehmann,
M.; Schulz, A.; Villinger, A. Organometallics 2010, 29, 1421.
(34) Structure IX resembles the reported solid-state structure of Ag−
C6F5 when it is crystallized from acetonitrile. The acetonitrile adduct is
polymeric ((··Ag··Ag··)n 2.802(1) Å) with C6F5 forming three-center−
two-electron bridges (i.e., [Ag(μ-C6F5) (NCMe)Ag]n).
(35) A separate estimation of this energy difference, using
optimizations for all species in the same solvent (dioxane) applied
to VA + C6F5H = IX + 2-methylthiophene, provides an energy
difference of 6.5 kcal/mol.
(36) Alkynyl transfer at −30 °C: Meana, I.; Espinet, P.; Alben
́
iz, A. C.
Organometallics 2014, 33, 1.
(37) Carbene transfer at room temperature: McGuinness, D. S.;
Cavell, K. J. Organometallics 2000, 19, 741.
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DOI: 10.1021/acs.organomet.6b00437
Organometallics XXXX, XXX, XXX−XXX