Migratory Insertion of Carbenes into Au(III)-C Bonds

Article abstract of DOI:10.1021/jacs.7b11435

Migratory insertion of carbon-based species into transition-metal-carbon bonds is a mechanistic manifold of vast significance: it underlies the Fischer-Tropsch process, Mizoroki-Heck reaction, Ziegler-Natta and analogous late-transition-metal-catalyzed olefin polymerizations, and a number of carbonylative methods for the synthesis of ketones and esters, among others. Although this type of reactivity is well-precedented for most transition metals, gold constitutes a notable exception, with virtually no well-characterized examples known to date. Yet, the complementary reactivity of gold to numerous other transition metals would offer new synthetic opportunities for migratory insertion of carbon-based species into gold-carbon bonds. Here we report the discovery of well-defined Au(III) complexes that participate in rapid migratory insertion of carbenes derived from silyl- or carbonyl-stabilized diazoalkanes into Au-C bonds at temperatures ≥ -40 °C. Through a combined theoretical and experimental approach, key kinetic, thermodynamic, and structural details of this reaction manifold were elucidated. This study paves the way for homogeneous gold-catalyzed processes incorporating carbene migratory insertion steps.

Full text of DOI:10.1021/jacs.7b11435

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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Migratory Insertion of Carbenes into Au(III)−C Bonds
Aleksandr V. Zhukhovitskiy,† Ilia J. Kobylianskii,† Chung-Yeh Wu,‡ and F. Dean Toste*
Department of Chemistry, University of California, Berkeley, California 94720, United States
S
* Supporting Information
ABSTRACT: Migratory insertion of carbon-based species
into transition-metal−carbon bonds is a mechanistic manifold
of vast significance: it underlies the Fischer−Tropsch process,
Mizoroki−Heck reaction, Ziegler−Natta and analogous late-
transition-metal-catalyzed olefin polymerizations, and a
number of carbonylative methods for the synthesis of ketones
and esters, among others. Although this type of reactivity is
well-precedented for most transition metals, gold constitutes a
notable exception, with virtually no well-characterized
examples known to date. Yet, the complementary reactivity
of gold to numerous other transition metals would offer new synthetic opportunities for migratory insertion of carbon-based
species into gold−carbon bonds. Here we report the discovery of well-defined Au(III) complexes that participate in rapid
migratory insertion of carbenes derived from silyl- or carbonyl-stabilized diazoalkanes into Au−C bonds at temperatures ≥ −40
°C. Through a combined theoretical and experimental approach, key kinetic, thermodynamic, and structural details of this
reaction manifold were elucidated. This study paves the way for homogeneous gold-catalyzed processes incorporating carbene
migratory insertion steps.
denes,26−28 insertions of electrophilic species such as SO2 into
1. INTRODUCTION
Au−C bonds,13,29,30 formal carbene insertions into Au−Cl
Migratory insertion of unsaturated carbon species into
bonds,31 and Au-catalyzed generation of carbenes from
transition-metal carbon bonds is an elementary organometallic
diazoalkanes and subsequent carbene transfer reactions.32,33
Concurrently with this work, an independent investigation in our
laboratory into alkyl−CF3 reductive elimination serendipitously
uncovered an example that was proposed to involve migratory
insertion of difluorocarbene into Au−C bonds.9 With only this
example known to date, migratory insertion of carbenes into
Au−C bonds remains virtually unexplored.
transformation that has proven broadly transformative.1 It
features prominently in such name reactions as the Fischer−
Tropsch process,2 Ziegler−Natta polymerization,3 and Mizor-
oki−Heck reaction.4 The former is utilized to synthesize
hydrocarbon fuels and lubricants; the latter two, respectively,
granted us such now-ubiquitous plastics as polyethylene and
polypropylene3 and enabled the production and discovery of
numerous pharmaceuticals and agrochemicals:5 their global
impact on humanity has been recognized with the 1963 and 2010
Nobel Prizes in Chemistry, respectively.
Migratory insertion of unsaturated carbon-based species into
metal−carbon bonds is well-established for most transition
metals.1 Gold is aberrant in this regard: to our knowledge, only
three examples of such reactivity have been reported to date for
gold,6−9 in addition to several formal insertions10−14 that
proceed via alternate mechanisms (Figure 1). Yet the ability to
engage gold in this reactivity manifold is a tantalizing prospect
precisely because the reactivity of organogold species is
frequently complementary to that of analogous organotransi-
tion-metal complexes.9,15−23 For example, the reluctance of
Au(I)-alkyl species to undergo syn-β-hydride elimination23 (a
facile process for Au(III),24 however) or Au−C bond
homolysis25 may enable gold-catalyzed coordination polymer-
ization of unsaturated carbon species.
Inspired by the well-documented polymerization of carbenes
derived from α-diazocarbonyl compounds through migratory
insertion at Rh(I) and Rh(III)34−40 and Pd0 and Pd(II)41−53 and
oligomerization mediated by Cu0 powder54 and boranes,55 we
wondered if analogous transformations could be mediated by
organogold complexes. Intriguingly, more than 60 years ago,
heterogeneous gold catalysts were first noted to convert
diazomethane and higher diazoalkanes (though not diazocar-
bonyl compounds) into linear poly(alkylidene)s.56,57 Remark-
ably, among all the polymerization-competent heterogeneous
metal catalysts, only in the case of gold was some stereoregularity
observed in the produced polymers.57 (Among the homoge-
neous ones, only Rh(I)/Rh(III) produces highly syndiotactic
poly(alkylidenes).34−36,38,39,58) More remarkably still, to date,
the mechanism of this gold-catalyzed transformation remains ill-
defined.59 Thus, as we set out to investigate carbene migratory
insertion into Au−C bonds of well-defined homogeneous
We were particularly intrigued at the outset of this venture, by
the complete absence of precedent for migratory insertion of
carbenes into Au−C bonds, despite numerous reports of
spectroscopically observable or even isolable gold alkyli-
Received: October 26, 2017
© XXXX American Chemical Society
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Figure 1. Precedents and current work.
Table 1. Initial Optimization of Reaction Conditions
a
b
1H NMR yields. See SI for cif file and general X-ray crystallography information.
B
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Figure 2. Substrate scope. NMR yields are shown. Synthesis of pure reference samples for the observed products is described in the SI.
organogold species, we conjectured that our findings might also
offer some insight into this long-standing question.
scope studies, we noticed that differences in water content in
dichloromethane (DCM)-d2 (as well as other solvents) led to a
variation in the yield of 2, as well as the relative content of side-
products. A systematic investigation of this “water effect”
revealed that about two equivalents of water (relative to 1)
were optimal (Table 1). Augmented water content led to a
gradual reduction in yield of 2 and generally increased formation
of biphenyl and side-product 3 (Supplementary Figure S3)
formed via water-promoted proto-de-auration; reduced water
content below two equivalents led to a steep drop in the yield of 2
due to formation of biphenylene and 5 upon chloride abstraction
(Supplementary Figure S3). Furthermore, biphenylene reductive
elimination was rather rapid: in the presence of 1,3,5-
trimethoxybenzene (TMB), a competitive π-donor, exchange
with biphenylene was found to be approximately first-order in
1,3,5-trimethoxybenzene, which indicates that the rate-determin-
ing step occurs after reductive elimination (Supplementary
Figure S4). NMR spectroscopy61 and X-ray crystallography of
the cationic Au(III) intermediate formed in the presence of
excess water identified it as aquo complex63 6 (Table 1).
Exposure of 6 to EDA in DCM-d2 indeed generated 2. These
results confirm the critical role of water as a ligand, which
stabilizes the intermediate cationic Au(III) species from
reductive elimination, but, when present in large excess, also
promotes competitive proto-de-auration to generate biphenyl
and 3. Notably, 3 was not formed through degradation of 2:
degradation of a mass-differentiated analogue of 2 (methyl ester
instead of ethyl ester) was not observed by GC-MS despite the
formation of 3 (Supplementary Figure S5). Observation of 3,
therefore, indicates that the product of migratory insertion can be
intercepted prior to reductive elimination.
2. RESULTS AND DISCUSSION
In particular, we envisioned that an organogold(III) complex
with a readily available coordination site would be well-suited for
diazoalkane coordination, gold-alkylidene generation, and
migratory insertion of the carbene into a proximal Au−C
bond.33,60 Our group recently reported that an example of such a
complex derived from 1 by chloride abstraction was notable for
its room-temperature persistence and Lewis acidity.61 Indeed,
treatment of 1 (8.3 μM in CD2Cl2) with silver bis-
(trifluoromethanesulfonyl)imide (AgNTf2), followed by expo-
sure to ethyl diazoacetate (EDA) at room temperature, led to
rapid generation of fluorene derivative 2 as the major product
(Table 1). We hypothesized that 2 formed via Csp2−Csp3
reductive elimination subsequent to carbene migratory insertion.
1
Several side- and byproducts were also identified by H NMR
spectroscopy and gas chromatography-mass spectrometry (GC-
MS), including biphenylene, biphenyl, insertion/proto-deaura-
tion product 3, diethyl fumarate and maleate, and complex 4,
produced via formal insertion of EDA into the Au−N bond of 562
(see Supplementary Figures S1 and S2).
2.1. Survey of Conditions and Substrates. Examination
of multiple Ag(I) salts utilized for Cl− abstraction indicated that a
weakly coordinating anion was necessary to promote the
−
formation of 2, with NTf2 affording the highest yield of 2. By
the same token, 1 exhibited no background reactivity toward
EDA (Table 1). Yet, Ag+ was not required to bring about the
observed reactivity: anion metathesis with NaBArF24 in the
presence of EDA similarly promoted the formation of 2, albeit
more slowly (the exact time scale depended on EDA content).
These observations supported the proposed mechanism that
enlisted Au(III) for carbene formation from EDA, followed by
migratory insertion and reductive elimination to generate 2.
The nature of the solvent was also critical to the observed
transformation. The use of coordinating solvents such as
methanol-d4, acetone-d6, or acetonitrile-d3 proved detrimental
in this context (Table 1). On the other hand, weakly coordinating
solvents such as benzene, toluene, and chlorinated alkanes
supported the formation of 2. During the course of the solvent
The reactivity observed for EDA extended to several other
diazoalkanes, albeit with equal or diminished yield of the
corresponding fluorene derivatives. Notably, 7 was not formed at
all (diazoalkane hydrolysis was observed instead: see Supple-
mentary Figure S6), and disubstituted α-diazocarbonyl com-
pounds afforded only trace fluorene products 8 and 9, detected
by GC-MS, and none of the expected products 10 and 11 (Figure
2, Supplementary Figures S7−S10). On the other hand,
increased steric bulk of tert-butyl diazoacetate and the electron-
withdrawing nature of the ester moiety in 2,2,2-trifluoroethyl
C
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Figure 3. Influence of the ligands on the yield of products 2 and 2′. When product formation was only observed by GC-MS, we designate the yield as
“trace”.
diazoacetate were well-tolerated, leading to comparable yields
(relative to EDA) of corresponding fluorene derivatives 12 and
13. Migratory insertion reactivity was also observed for
representative diazoketone and silyl diazoalkane, albeit with
lower yields of corresponding products (14 and 15 in Figure 2)
compared to EDA. Representative azide and isocyanide
alternate nitrene and carbene surrogatesessentially did not
participate in migratory insertion (see 16 and 17 in Figure 2).
Hence, given its reactivity and relative structural simplicity, EDA
was selected as the optimal substrate for further investigations of
carbene migratory insertion at Au(III).
2.2. Investigation of Ligand Effects. With several key
parameters of the transformation optimized and prevalent side-
reactions identified for complex 1, the influence of the ligands in
complexes 1 and 18−30 on the outcome of the transformation,
e.g., the yield of products 2 or 2′, was the next critical
consideration to address (Figure 3). AgNTf2 and NaBArF24 were
both evaluated: the former generated the cationic Au(III)
quantitatively prior to EDA addition; the latter generated it
concurrently with its consumption by reaction with EDA.
Saturation of the N-heterocyclic carbene (NHC) backbone
(complex 18) or reduction in the NHC buried volume64−66
(complexes 19 and 20) led to a precipitous drop in the yield of 2
and an increased relative production of biphenyl and/or
biphenylene, 3, and other side-products (Supplementary Figure
S11). Steric shielding of the Au center evidently reduces side-
reactions. Yet, despite added steric demand in the case of 21, the
yield of 2 was substantially diminished compared to 1: we
speculate that the phenyl groups of the ligand may irreversibly
intercept the carbene intermediate prior to migratory insertion.
Complexes with phosphorus-based ligands (22−24) with
widely varying steric bulk and electronics led to low yields of 2.
On the other hand, substitution of the NHC ligand in 1 (IPr)
with a cyclic (alkyl)(amino)carbene (cAAC),67,68 which is both
more σ-donating and simultaneously more π-accepting68 (i.e.,
complex 25), afforded the highest yield of 2 among the tested
Au(III) complexes. However, the use of NaBArF24 was crucial to
prevent premature reductive elimination of biphenylene
(Supplementary Figure S12). We reasoned that the strong π-
accepting ability of the cAAC promotes reductive elimination,69
leading to 2 (or biphenylene) in preference to proto-de-auration
and other side-reactions. Indeed, cAAC−gold(I) complexes are
known to be more resistant to oxidation compared to gold(I)
complexes of imidazolin-2-ylidenes.70
We anticipated that replacement of the 2,2′-biphenyl ligand
with 4,5-phenanthryl would constrain the geometry of the phenyl
rings to render reductive elimination energetically unfeasible
prior to migratory insertion. As expected, treatment of 26 with
AgNTf2 led to no observable reductive elimination, even under
rigorously anhydrous conditions. Moreover, the yield of 2′
D
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Figure 4. (A) Structure determination of 26·AgNTf2 in solution via variable-temperature (VT) NMR. Relevant resonances, integration, and scalar
coupling constants are shown. Inset in bottom spectrum: Structure of 26·Ag+ obtained via density functional theory (DFT) (MO6-L/cc-PVDZ,
Au:SDD(2f,g); vacuum, −NTf2 was omitted; color scheme: H = not shown; C = gray; N = dark blue; Cl = green; Au = yellow; and Ag = light blue. (B)
Full electrospray ionization (ESI) mass spectrum of 26·AgNTf2 in CD2Cl2 (8.3 × 10−5 M) (top), simulated and measured isotope patterns (middle),
and collision-induced dissociation experiment of the ion 26·Ag+ (bottom).
considerably improved compared to the yield of 2 in the case of 1,
with byproduct formation suppressed for both AgNTf2 and
NaBArF24 (Supplementary Figure S13).
transformations are proposed to be analogous to the previously
reported biaryl homocoupling at Au(III), which proceeds via
transmetalation of an aryl group to Au(III) and extremely rapid
Csp2−Csp2 reductive elimination.72 These results were
instructive: ligand modification is, indeed, a powerful approach
to shape the topography of the entire reaction manifold; raising or
lowering one barrier may render relevant new, previously
inaccessible pathways.
2.3. Experimental and Theoretical Mechanistic Stud-
ies. The results presented above are consistent with the
proposed mechanism of migratory insertion at a cationic Au(III)
center followed by reductive elimination.
It stands to mention that multiple carbene insertions prior to
reductive elimination were not observed in any of the cases
examined above. Such a mechanistic scenario is desirable, as it
would open the door to homogeneous gold-catalyzed carbene
polymerization and offer entry to a new class of ester-laden cyclic
compounds. To that end, and as captured in compounds 27−30,
we explored two ligand-centric tactics to suppress reductive
elimination: (1) interception of tricoordinate Au(III) via
reversible intramolecular chelation and (2) stronger σ-donation
and weaker π-acceptance of the L-type ligand. To address (1), we
replaced each of the iPr groups on the NHC with “MeO”, capable
of weakly coordinating to the cationic Au(III); indeed, slower
formation of 2′ was realized, but products of multiple insertions
were not observed. To address (2), we synthesized 29 and 30,
which possess the most σ-donating and least π-accepting of the
tested carbene ligands: a mesoionic (or abnormal) carbene66,71
(aNHC), better regarded as an ylide. Compared to 1 and 26,
complexes 29 and 30 afforded significantly reduced yields of 2/
2′, but also no evidence of multiple carbene insertion. Instead, in
the presence of NaBArF24, both 29 and 30 led to species whose
m/z and fragmentation pattern were consistent with products of
BArF24/biphenyl (or phenanthryl) cross-coupling (Supplemen-
tary Figure S14). The mechanisms of these cross-coupling
However, to rule out alternate pathways and provide a more
detailed mechanistic description, we co-opted a combination of
variable-temperature (VT) NMR and computational analysis on
the combination of complex 26, AgNTf2, and EDA. To begin
with, we must address the nature of the Au(III) species generated
by treatment with AgNTf2. In the presence of water, abstraction
of chloride gives rise to AgCl(s) and an aquo complex analogous
to 6. However, under rigorously anhydrous conditions, another
species is exclusively formed (Figure 4) without concomitant
AgCl precipitation. Evidently, instead of the expected AgCl(s)
formation,73 AgNTf2 is solubilized by interaction with the
Au(III) complex. 19F NMR analysis (Supplementary Figure S15)
−
suggested that the NTf2 anion in the latter scenario is largely
dissociated from Ag+: the resonance corresponding to NTf2− is
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1
Figure 5. (A) Transformation monitored by 1H NMR spectroscopy. (B) H NMR spectra, which illustrate the concurrent consumption of 31 and
formation of 2′ and 5 and, later, 4 at 233.2 K. (C) Linearized plots of VT NMR kinetics data at various initial concentrations of 31 utilized to quantify kobs
and τ1/2
.
Figure 6. Potential energy diagram for the primary reactivity of the cationic complex 32 (black) and for the second consecutive insertion of EDA (in
blue) at the given level of DFT. Counterion was excluded from the computational model to reduce the complexity of the system for computational
expediency. See Computations section in the SI for further details.
1.1 ppm upfield of AgNTf2 and only 0.3 ppm downfield of
broaden and shift upfield (Figure 4A). These observations point
to the reversible coordination of Ag+ to both the phenanthrene
and the Cl− anion of complex 26. Mass spectrometry analysis
m/z, isotope pattern, and MS/MScorroborated the formation
−
nBu4N+NTf2 .
Furthermore, at −40 °C, the 1H NMR resonances
corresponding to a portion of the 4,5-phenanthryl moiety
F
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of a coordination complex between 26 and Ag+ (Figure 4B).
Density functional theory (DFT) computations further
supported this notion: in fact, one could regard the newly
formed bimetallic complex 26·AgNTf2 as simultaneously a
coordination and π-complex between AgCl and the T-shaped
cationic Au(III) species (Figure 4).73,74
and analogous to that of carbene 33, the π-accepting ability of the
NHC ligand is expected to be a critical parameter for outcome
selection. We anticipate that with further ligand design to
suppress reductive elimination other decomposition pathways
will enable multiple insertions at Au(III).
3. CONCLUSION
Addition of EDA at −80 °C led to partial conversion of 26·
AgNTf2 to 31 (EDA complex and a diazonium salt) during the
course of ∼2 h, with no detectable formation of 2′
(Supplementary Figure S16). However, at 233.2 K (−40 °C),
consumption of 31 proceeded with first-order kinetics and a rate
constant of (4.9 0.4) × 10−4 s−1 (τ1/2 = 24 2 min, Figure 5).
The concentration-independent first-order behavior and absence
of an induction period supported the proposed mechanism,
wherein the carbene is generated at the Au(III) site via N2
dissociation, as opposed to alternate bimolecular pathways.
Furthermore, concomitant generation of 2′ was observed, but no
intermediates were detected, although side-processes were
observed that accounted for the moderate yield of 2′. Hence,
we can draw the following conclusions: (1) migratory insertion
of carbenes into Au−C bonds is not only feasible but also rapid
even at −40 °C, and (2) Csp3−Csp2 reductive elimination from
Au(III) is faster than N2 loss followed by migratory insertion, but
is competitive with such side-reactions as proto-de-auration.
DFT modeling of the reaction energy profile (Figure 6, see
Computations section in the SI) provided a mechanistic picture
that is consistent with the observed reactivity. Cationic Au(III)
complex 32 accessible via halide abstraction coordinates water or
EDA to form the corresponding adducts 6′ and 31 (ΔH = −10.7
or −6.6 kcal/mol, respectively). The latter is capable of N2
elimination to form a carbene intermediate 33 (ΔH = 16.5 kcal/
mol); the transition state leading to 33 was not located,
indicating that ΔH⧧ ≈ ΔH in this case (see Supplementary
Figure S17). Note that similar ΔH⧧ values have been previously
computed for N2 elimination from IPrCu(EDA)+ and IPrAg-
(EDA)+ (16.1 and 18.3 kcal/mol, respectively).75 Carbene 33
has a minute energy barrier (ΔH⧧ = 2.4 kcal/mol) and a large
enthalpic driving force (ΔH = −66.2 kcal/mol) for migratory
insertion to afford 34. Note that the transition state for the
migratory insertion is an early one, energetically and structurally
similar to carbene 33. Consequently, N2 extrusion and migratory
insertion in this case are best regarded as coupled processes,
which, in combination, constitute the rate-limiting step en route
to 2′, with a combined ΔH⧧ ≈ 18.9 kcal/mol. This value is
consistent with the observed rapid consumption of 31 even at
−40 °C.
Thus, we have demonstrated herein the first examples of
migratory insertion of carbenes derived from diazoalkanes into
Au−C bonds. We identified a number of reaction pathways that
occurred prior to and post-migratory insertion (e.g., reductive
elimination and proto-de-auration) and optimized the con-
ditions for the formation of products 2 and 2′.
We determined that the bulky NHC ligand in 1 and 26 (IPr)
and the more π-accepting cAAC ligand in 25 and 28 lead to the
highest yields of 2/2′, presumably through promotion of
reductive elimination after migratory insertion in favor of other
processes; furthermore, installation of the 4,5-phenanthryl ligand
inhibited many of these side-processes. Mechanistic analysis
revealed AgCl ligation to the T-shaped cationic Au(III) species
and π-complexation with a portion of the phenanthryl ligand,
which refined our conception of the chloride abstraction step
with Ag+ reagents. Furthermore, N2 elimination/migratory
insertion was determined to proceed with first-order kinetics
and a half-life of 24 2 min at −40 °C; alternative bimolecular
pathways for carbene generation and transfer were ruled out.
Lastly, DFT computations supported the proposed mechanism
for the formation of 2′ and, by analogy, other carbene insertion
products reported herein. Looking ahead, these computations
point to the feasibility of EDA polymerization at a gold center;
further ligand scaffold design is a promising approach to achieve
this goal. Hence, these studies demonstrate for the first time the
scope, viability, and mechanistic details of carbene migratory
insertion into Au−C bonds, an elementary organometallic
transformation with fundamental significance for gold chemistry
and beyond.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/jacs.7b11435.
■
S
Materials and methods, synthetic and characterization
procedures, supplementary figures, and spectral data
(PDF)
Coordinates for DFT-computed structures (XYZ)
Crystallographic data for gold complexes 4, 6, and
The fate of intermediate 34 merits careful consideration: it can
be consumed through a number of competing processes, two of
which have been computed and depicted in Figure 6. One is
reductive elimination to yield π-complex 35 (ΔH⧧ = 3.0 kcal/
mol) and ultimately 2′ and 36 after dissociation; the other is
coordination of a second equivalent of EDA (37) followed by a
second round of N2 dissociation (38) and migratory insertion
(39) (for 37 → 39, ΔH⧧ = 21.5 kcal/mol). The latter is a
desirable pathway for poly(alkylidene) synthesis, as described
above. A crucial realization is that the system in consideration is
in a Curtin−Hammett scenario (37 → 39 via 38/TS3 vs 34 → 35
via TS2), where 34 and 37 are likely in equilibrium, and the
reaction outcome is dictated by the difference in transition state
energies leading to the respective products. This difference, in
this case, is 11.6 kcal/mol, which explains the absence of multiple
carbene migratory insertions in this system (vide supra). Given
that the electronic structure of TS3 is similar to that of carbene 38
IPrAuSbF6-(TMB) (CIF)
(CIF)
(CIF)
AUTHOR INFORMATION
Corresponding Author
*fdtoste@berkeley.edu
ORCID
Aleksandr V. Zhukhovitskiy: 0000-0002-3873-4179
F. Dean Toste: 0000-0001-8018-2198
■
Present Address
‡Novartis Institutes for Biomedical Research, Cambridge,
Massachusetts 02139, United States.
Author Contributions
†A. V. Zhukhovitskiy and I. J. Kobylianskii contributed equally.
G
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(27) Harris, R.; Widenhoefer, R. Chem. Soc. Rev. 2016, 45, 4533−4551.
(28) Sarria Toro, J. M.; García-Morales, C.; Raducan, M.; Smirnova, E.
S.; Echavarren, A. M. Angew. Chem. 2017, 129, 1885−1889.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the National Institutes of Health (R35 GM118190) for
support of this work. A.V.Z. is a Merck Fellow of the Life Sciences
Research Foundation. I.J.K. is a Swiss National Science
Foundation postdoctoral fellow. We thank C. Hong and C.
Winslow for X-ray crystallography, E. Kreimer for elemental
analysis, Z. Zhou and R. Nichiporuk for assistance with HRMS,
and Prof. K. Vikse for assistance with ESI-MS of 26·AgNTf2. This
work made use of the UC Berkeley Catalysis Center, as well as
the College of Chemistry and QB-3 Institute NMR facilities.
DFT studies were conducted at the Molecular Graphics and
Computation Facility, funded by NIH grant S10OD023532.
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