Mechanistic understanding of a silver pyridylpyrrolide as a catalyst for 3 + 2 cyclization of a nitrile with diazo ester

Article abstract of DOI:10.1021/om400756t

The trinuclear argentate complex Ag₃(μ₂-3,5- (CF₃)₂PyrPy)₃ (PyrPy = 2,2′- pyridylpyrrolide) catalyzes the 3 + 2 cycloaddition of several NCR (R = Me, Ph, ^tBu) and N₂CHCO₂Et to disubstituted oxazoles, even in the presence of light and air. Structural and theoretical studies imply a three-coordinate silver carbene complex, (3,5-(CF₃) ₂PyrPy)Ag(CHCO₂Et), to be responsible in a stepwise nitrile addition and cyclization step to form the heterocycle.

Full text of DOI:10.1021/om400756t

Article
pubs.acs.org/Organometallics
Mechanistic Understanding of a Silver Pyridylpyrrolide as a Catalyst
for 3 + 2 Cyclization of a Nitrile with Diazo Ester
Jaime A. Flores, Kuntal Pal, Maria E. Carroll,† Maren Pink, Jonathan A. Karty, Daniel J. Mindiola,*,†
and Kenneth G. Caulton*
Department of Chemistry and Molecular Structure Center, Indiana University, Bloomington, Indiana 47405, United States
S
* Supporting Information
ABSTRACT: The trinuclear argentate complex Ag3(μ2-3,5-
(CF3)2PyrPy)3 (PyrPy = 2,2′-pyridylpyrrolide) catalyzes the 3 + 2
cycloaddition of several NCR (R = Me, Ph, tBu) and N2CHCO2Et
to disubstituted oxazoles, even in the presence of light and air.
Structural and theoretical studies imply a three-coordinate silver
carbene complex, (3,5-(CF3)2PyrPy)Ag(CHCO2Et), to be respon-
sible in a stepwise nitrile addition and cyclization step to form the
heterocycle.
The 2,2′-pyridylpyrrolide ligand 3,5-(CF3)2PyrPy12 em-
ive-membered-ring heterocycles such as pyrroles, oxazoles,
Fand oxadiazoles are quintessential building blocks in both
ployed here has an electron-rich pyrrolide moiety but carries
two electron-withdrawing CF3 substituents in the pyrrolide
ring. This ligand scaffold can also enforce a bent geometry for
coinage metals that generally coordinate in a linear fashion.
We have shown that attachment of a cisoid bidentate pyridyl-
pyrrolide ligand to monovalent copper or silver, in the absence
of an additional ligand/Lewis base source, yields an
unprecedented structure in which the pyridylpyrrolide twists
internally to bridge two metals, therefore yielding a trimer,
M3(μ2-3,5-(CF3)2PyrPy)3 (M = Cu, Ag)13−16 Despite the fact
that M3(μ2-3,5-(CF3)2PyrPy)3 forms a trinuclear cluster, we
view it as a possible precursor to the unsaturated and reactive
“bent” M(I) fragment M(3,5-(CF3)2PyrPy),14 when confronted
by some substrate. We report here that a silver analogue to
Cu3(μ2-3,5-(CF3)2PyrPy)3,13 Ag3(μ2-3,5-(CF3)2PyrPy)3,15,16 is
indeed a source of the highly unsaturated argentate fragment
[Ag(3,5-(CF3)2PyrPy)] fragment, together with its catalytic
utility for a practical 3 + 2 cyclization of a nitrile with a diazo
ester, affording some examples of disubstituted oxazoles (eq 1).
natural products and organic synthesis. Some forms of oxazoles are
biomolecules that result from cyclization and oxidation reactions of
serine and threonine nonribosomal peptides; therefore, synthetic
entry to these molecules has gathered enormous interest, given their
role in many biological processes as well as the importance of this
motif in the construction of various natural products.1 For this
reason, an effective synthetic approach to these skeletons, especially
via a catalytic process, would constitute an important advance in
the field. In addition, if an air- and water-stable catalyst could be
obtained readily from easy-to-prepare and -handle starting materials,
such a reaction would appeal from a practical standpoint. There
have been reports of some coinage-metal-based catalysts (Cu(I))
that can promote the coupling of acyl azides and terminal alkynes to
oxazoles,2 while other copper catalysts have been used in tandem
with oxidative cyclization reactions.3 Au(I) catalysts have been
reported for the coupling of alkynes and nitriles under oxidative
conditions (using various N-oxides),4 among other situations.5
Despite these efforts, the only example of a metal-catalyzed 3 + 2
addition of nitrile and a carbene source (derived from a diazo ester
precursor), has been reported using lanternlike Rh2II,II catalysts.6
Other metals such as iron,7 palladium,8 and some lanthanides9 have
been also applied toward the synthesis of substituted oxazoles, but
none derive from a carbene source. Lastly, non-metal-catalyzed and
stoichiometric reactions have been reported in the assembly of
oxazoles,10 but only in the presence of powerful Lewis acids such as
BF3 can these reactions result in the addition of carbenes to nitriles
to form oxazoles.11 To our knowledge, the use of group 11 metals
as catalysts, in particular Ag, in polar 3 + 2 addition reactions to
form oxazoles has not been documented.
Since preliminary studies showed higher catalytic performance
by the silver complex, we report only those here. We present
Received: July 30, 2013
Published: March 24, 2014
© 2014 American Chemical Society
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14.7 (NCCH3), 14.0 (CH3). EI-MS: found 127.06 [M]+, C6H9NO2,
computational studies to assess the role of the Ag+ ion in the
polar cyclization of nitrile and carbene fragment to form the
disubstituted oxazole. Previously we have found that the com-
pound Ag3(μ2-3,5-(CF3)2PyrPy)3 can efficiently catalyze carbene
insertion into the C−H bonds of alkanes (including volatile
examples such as ethane, propane, and butane).15 Likewise, we
have discovered the compound Ag3(μ2-3,5-(CF3)2PyrPy)3 to be a
versatile catalyst, promoting carbene addition across aromatic
calcd 127.06.18 Isolated yield: 70%.
5-Ethoxy-2-phenyl-1,3-oxazole. 1H NMR (25 °C, CD2Cl2): δ 8.07
(d, JHH = 7.5 Hz, 2H, o-Ar), 7.09 (t, JHH = 7.5 Hz, 2H, m-Ar), 7.02
(t, JHH = 7.0 Hz, 1H, p-Ar), 6.09 (s, 1H, HCC), 3.47 (q, JHH = 7.0
Hz, 2H, OCH2), 0.93 (t, JHH = 6.8 Hz, 3H, CH3). 13C NMR (25 °C,
CD2Cl2): δ 160.7 (CCO), 153.1 (NCO), 129.8 (C-Ar), 129.3 (C-
Ar), 128.9 (C-Ar), 126.0 (C-Ar), 101.6 (NCHC), 68.0 (OCH2),
14.7 (CH3). EI-MS: found 189.08 [M]+, C11H11NO2, calcd 189.08.19
Isolated yield: 40%. A second round of catalysis, in an air atmosphere,
was performed by adding more EDA to the solution of Ag trimer in
neat NCPh and confirmed complete conversion to the corresponding
oxazole in another 3 days; a third run was performed, and a similar
outcome was obtained.
CC bonds,16 a process known as the Buchner ring expansion
̈
reaction.17 Here we show how this catalyst system can be
extended to a few examples of cyclization reactions of polar
substrates, but the ultimate goal of this study is to provide some
understanding of the organometallic role of the silver precatalyst 1
in this unique transformation.
1
5-Ethoxy-2-tert-butyl-1,3-oxazole. H NMR (25 °C, CD2Cl2): δ
5.88 (s, 1H, HCC), 3.54 (q, JHH = 7.0 Hz, 2H, OCH2), 1.25 (s, 9H,
C(CH3)3), 0.98 (t, JHH = 7.00, CH3). 13C NMR (25 °C, CD2Cl2): δ
161.8 (NCO), 160.3 (CCO), 99.8 (NCHC), 67.8 (OCH2),
34.1 (C(CH3)3), 28.8 (C(CH3)3), 14.7 (CH3); EI-MS: found 169.11
[M]+, C9H15NO2, calcd 169.11. Isolated yield: 40%. The reaction with
NCtBu was found to be complete after 3 days at room temperature. A
second round of catalysis, in an air atmosphere, was performed by
adding more EDA to the solution of Ag trimer in neat NCtBu and
confirmed complete conversion to the corresponding oxazole in
another 3 days; a third run was performed, and a similar outcome was
obtained.20 Note that C−Cl activation of CH2Cl2 was observed as a
major byproduct when this reaction was performed either in this
solvent or in the presence of light.
EXPERIMENTAL DETAILS
■
General Considerations. All manipulations were carried out in air
(unless otherwise stated). NMR spectra were recorded in CD2Cl2,
C6D6, and CD3CN at 25 °C (1H, 400.11 MHz; 13C, 100.61 MHz; 19F,
376.48 MHz). Proton and carbon chemical shifts are reported in ppm
versus Me4Si, and 19F NMR chemical shifts are referenced relative to
external CF3CO2H. LDI samples were applied to the target in a
volatile solvent that was allowed to evaporate in air, and samples were
irradiated at 355 nm. Ethyl diazoacetate (85% in CH2Cl2), Ag2O,
acetonitrile, benzonitrile, trimethylacetonitrile, diphenyldiazomethane,
trimethylsilyldiazomethane, and dimethyl diazomalonate were purchased
from commercial sources and used as received. H(3,5-(CF3)2PyrPy)12
and compound 115 were synthesized following published procedures. See
the Supporting Information for further details.
1
Ethyl 2,3-Dichloropropionate. H NMR (25 °C, CDCl3): δ 4.48
(dd, JHH = 8.8 Hz, 5.4 Hz, 1H), 4.37 (q, JHH = 7.1 Hz, 2H), 4.03 (dd,
JHH = 11.2 Hz, 8.8 Hz, 1H), 3.86 (dd, JHH = 11.2 Hz, 5.2 Hz, 1H), 1.40
(t, JHH = 7.2 Hz, 3H).21
Synthesis of Complex 2. Complex 1 (0.01g, 0.026 mmol Ag)
was dissolved in acetonitrile (1 mL), and ethyl diazoacetate (3 μL,
0.026 mmol) was added at room temperature. After 6 h of stirring a
white solid precipitated; total conversion was observed within 12 h.
Crystals as transparent rectangles were obtained from slow evap-
oration in a 3/1 benzene/dichloromethane solution at room tem-
perature. The title compound is air and light stable as a solid and in
solution for more than 1 month. Isolated yield: 95%. Mp: 172−178 °C
NMR Spectroscopic Study of the Reaction of 1 with Diazo
Compounds. In a J. Young NMR tube [Ag(3,5-(CF3)2PyrPy)]3
(0.01 g, 0.026 mmol Ag) was dissolved in 0.5 mL of CD2Cl2. To
this solution was added the diazo compound (0.026 mmol), and the
mixture was agitated for 10 min. 19F NMR spectra of the resulting
solutions were then obtained immediately at different temperatures.22
Computational Details. All calculations were carried out using
DFT as implemented in the Jaguar 7.723 suite of ab initio quantum
chemistry programs. Geometry optimizations were performed with the
B3LYP24−27 functional and the 6-31G** basis set. Ag was represented
using the Los Alamos LACVP basis,28,29 which includes relativistic
effective core potentials. The energies of the optimized structures were
reevaluated by additional single-point calculations on each optimized
geometry using Dunning’s correlation consistent triple-ζ basis set30 cc-
pVTZ(-f) that includes a double set of polarization functions. For Ag,
we used a modified version of LACVP, designated as LACV3P, in
which the exponents were deconstructed to match the effective core
potential with triple-ζ quality. Solvation energies were evaluated by a
self-consistent reaction field31−33 (SCRF) approach based on accurate
numerical solutions of the Poisson−Boltzmann equation. In the results
reported, solvation calculations were carried out with the 6-31G**/
LACVP basis at the optimized gas-phase geometry employing the
dielectric constant of ε = 9.08 and solvent dichloromethane. Analytical
vibrational frequencies within the harmonic approximation were
computed with the 6-31G**/LACVP basis to confirm proper
convergence to well-defined minima or saddle points on the potential
energy surface.
1
dec. H NMR (25 °C, CD2Cl2): δ 8.26 (apparent doublet, J = 4 Hz,
1H, H-pyridyl), 7.84 (m, 2H, H-pyridyl), 7.26 (t, J = 7 Hz, 1H,
H-pyridyl), 6.90 (s, 1H, H-pyrrolide), 6.04 (s, 1H, H-4-oxazole), 4.13
(q, J = 7 Hz, 2H, OCH2), 2.44 (s, 3H, CH3-2-oxazole), 1.42 (t, J = 7
Hz, 3H, OCH2CH3) . 19F NMR (25 °C, CD2Cl2): δ −53.84 (s, 3F),
−60.40 (s, 3F) . 13C NMR (25 °C, CD2Cl2): δ 160.42, 155.07,
153.50, 150.44, 138.89, 123.90, 122.59, 111.19, 100.23 (CO2), 69.35
(OCH2), 15.13 (CH3), 14.84 (CH3). MS (LDI, positive): found
512.97 [L107Ag(oxazole)]+, C17H14107AgF6N3O2, calcd 513.00; found
514.97 [L109Ag(oxazole)]+, C17H14109AgF6N3O2, calcd 515.00.
Catalytic Reactions with Nitriles and EDA To Form
Disubstituted 1,3-Oxazoles. A 10 mg amount of complex 1 (5%
of catalyst based on monomer Ag(μ2-3,5-(CF3)2PyrPy), 0.026 mmol)
was dissolved in 3 mL of neat nitrile, then EDA (64 μL, 0.520 mmol)
was added with a syringe at once, and the reaction was monitored over
time by GC-MS or 1H NMR spectroscopy. The products were isolated
by evaporation of the solvent. For 5-ethoxy-2-methyl-1,3-oxazole, pure
product was obtained upon removal of solvents. For 5-ethoxy-2-
phenyl-1,3-oxazole and 5-ethoxy-2-tert-butyl-1,3-oxazole the products
were purified by silica gel column chromatography, with a 1/1 mixture
of petroleum ether and diethyl ether as eluent. Yields were calculated
on the basis of integration of 1H NMR spectra containing hexa-
methylbenzene as an internal standard. The procedure does not
require protection from light, except in the case of trimethylacetoni-
trile; in the presence of light, that reaction mixture yields catalyst
decomposition with precipitation of a silver mirror on the walls of the
reaction vessel.
The energy components have been computed with the follow-
ing protocol. The free energy in solution phase G(sol) has been
calculated as
G(sol) = G(gas) + G(solv)
(2)
5-Ethoxy-2-methyl-1,3-oxazole. 1H NMR (25 °C, CD2Cl2): δ 5.91
(s, 1H, HCC), 3.47 (q, JHH = 7.0 Hz, 2H, OCH2), 1.95 (s, 3H, 
CCH3), 0.92 (t, JHH = 7.3 Hz, 3H, CH3). 13C NMR (25 °C, CD2Cl2):
δ 160.5 (CCO), 152.2 (NCO), 99.9 (NCHC), 67.9 (OCH2),
G(gas) = H(gas) − TS(gas)
(3)
H(gas) = E(SCF) + ZPE
(4)
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Scheme 2. Proposed Mechanism of Formation of 2,5-
Disubstituted Oxazoles via a Lewis Acid Promoted 3 + 2
Cyclization
ΔE(SCF) =
E(SCF for products) −
E(SCF for reactants)
∑
∑
(5)
ΔG(sol) =
G(sol for products) −
G(sol for reactants)
∑
∑
(6)
where G(gas) is the free energy in the gas phase, G(solv) is the free
energy of solvation as computed using the continuum solvation model,
H(gas) is the enthalpy in the gas phase, T is the temperature (in K),
S(gas) is the entropy in the gas phase, E(SCF) is the self-consistent
field energy (i.e. “raw” electronic energy as computed from the SCF
procedure), and ZPE is the zero-point energy. Note that by entropy
here we refer specifically to the vibrational/rotational/translational
entropy of the solute(s); the entropy of the solvent is incorporated
implicitly in the continuum solvation model.
To locate transition states, the potential energy surface was first
explored approximately using the linear synchronous transit (LST)34
method, followed by a quadratic synchronous transit (QST)35 search
using the LST geometry as an initial guess. IRC (intrinsic reaction
coordinate) scans were performed to verify the convergence of the
transition states to the appropriate reactants and products.
compounds such as TiF4 have been used in similar cyclization
reactions,36 but in each case the product retention on the metal
in the reaction prevents catalytic turnover. Mechanistic experi-
ments were in best agreement with electrophilic activation of
the nitrile, followed by nucleophilic attack of an intact diazo
ester carbonyl oxygen on the nitrile carbon, followed by N2
loss and cyclization (C/N bond formation), as suggested in
Scheme 2.36 Notably, no free or coordinated carbene transient
was invoked in this type of transformation.36
For the silver pyridylpyrrolide system it was found that
trimer 1 reacts readily with MeCN, either 3/1 MeCN/Ag3(μ2-
3,5-(CF3)2PyrPy)3 or neat MeCN (or CD3CN), to form a new
species consistent with a monomeric adduct, (3,5-(CF3)2-
RESULTS AND DISCUSSION
■
Synthesis and Characterization of the Silver Trimer
Ag3(μ2-3,5-(CF3)2PyrPy)3 (1). Previously we reported the syn-
thesis of the silver complex Ag3(μ2-3,5-(CF3)2PyrPy)3 (1), by re-
action of Ag2O with H(3,5-(CF3)2PyrPy)3 in MeCN (Scheme 1).15
1
PyrPy)Ag(NCMe) (A); this shows simpler H and 19F NMR
spectra than does the trimer in CD2Cl2, indicating that nitrile
does break up the trimer into a discrete monomer (Scheme 3).
Scheme 1. Synthesis of Complex 1 from Ag2O and H(3,5-
(CF3)2PyrPy)3 in MeCN
Scheme 3. Generation of the MeCN Adduct (3,5-
(CF3)2PyrPy)Ag(NCMe) (A) and Subsequent 3 + 2
Cyclization To Yield the 2,5-Disubstituted Oxazole Silver(I)
Adduct 2
Trimer 1 is quite Lewis acidic and it is necessary to dissolve the
crude product in toluene and then evaporate all volatiles to
completely remove any water or adventitious Lewis base from
coordination to silver. As a solid complex 1 is both air and light
stable at room temperature for several weeks; dichloromethane
solutions are stable for at least 2 days.15
The molecular structure of 1 has been established by a
single-crystal X-ray diffraction study and showed a non-C3-
symmetric trimer analogous to (but not crystallographically
isomorphous with) that of the copper analogue.13 A detailed
description of the molecular structure of 1 has been published
recently.15 A larger expansion of the metal/metal distances in
the silver complex compared with copper renders the silver
centers more accessible to external reagents. The ease in pre-
paration of 1 also makes it an ideal catalyst to study insertion
reactions.
3 + 2 Cyclization Reactions Catalyzed by Complex
Ag3(μ2-3,5-(CF3)2PyrPy)3 (1) To Form Oxazoles. Previous
work11 has shown Lewis acid acceleration of the cyclization
between a nitrile and a diazo ester, via the stoichiometric use
BF3 (Lewis acid = LA). Under such a scenario the reaction is
complete within minutes at 25 °C (Scheme 2). Attempts at
catalytic application of BF3 failed due to competing fluorination of
the diazo reagent. Likewise, highly Lewis acidic transition-metal
Intermediate A is stable in MeCN against deposition of silver
metal over 24 h at 25 °C even without protection from light. If
1 equiv of MeCN per Ag(I) is added to a solution of 1 in C6D6,
the MeCN chemical shift is shifted by over 1 ppm from
the value of free MeCN in benzene, clearly confirming adduct
formation. Addition of EDA (ethyl diazoacetate, Scheme 3) to
an MeCN solution of this monomeric adduct shows no sig-
nificant change in 1H or 19F NMR spectra of either EDA or that
of the adduct A, but within 5 min new resonances for the
CH(CO2Et) moiety of the coordinated oxazole grow in. The
formation of “carbene dimers”, (olefins) such as the ethyl esters
of maleate and fumarate, are not observed, a factor promising
for catalysis. Reaction (1/1) of 1 with EDA in MeCN at 25 °C
slowly (12 h) produces a white precipitate which exhibits all
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NMR spectroscopic signals of a coordinate pyridylpyrrolide
ligand, together with OEt and CH resonances derived from
EDA, in a 1/1 mole ratio. Single crystals grown from benzene/
dichloromethane identified this as an oxazole Ag(I) adduct,
(3,5-(CF3)2PyrPy)Ag(κ1N-5-ethoxy-2-methyl-1,3-oxazole) (2),
resulting from N2 loss with the EDA carbene fragment cyclized
to the nitrile triple bond in a 3 + 2 addition mode (Figure 1).
Figure 1. Structural drawing (50% probability thermal ellipsoids) of
the non-hydrogen atoms of complex 2, showing selected atom
labeling. Selected structural parameters (bond distances in Å and
angles in deg): Ag1−N3, 2.117(2); Ag1−N2, 2.157(2); Ag1−N1,
2.419(2); N3−Ag1−N2, 161.42(8); N3−Ag1−N1, 125.78(8); N2−
Ag1−N1, 72.80(8); C12−N3−Ag1, 125.94(18); C14−N3−Ag1,
127.37(16).
Figure 2. (top) Structural drawing of 2 showing intermolecular H··F
bonds (50% thermal ellipsoid probability). (bottom) Argentophilic
interactions observed in the packing of 2.
The structure of 2 reveals that the three-coordinate silver center
has a T shape, since one of the N−Ag−N angles deviates from
an ideal 120° (N3−Ag1−N2, 161.42(8)°; N3−Ag1−N1,
125.78(8)°).37 The resulting oxazole heterocycle is coordinated
to the Ag(I) ion via the nitrogen atom, is rigorously coplanar
with the pyridylpyrrolide ligand, and is formed without any
substituent migration. There are some intermolecular CH···FC
contacts to the ortho pyridyl hydrogens (in the distance range
currently discussed as weak hydrogen bonds; Figure 2, top),38 as
well as an Ag/Ag distance between the stacked planar molecules of
3.44 Å (in the range of those currently described as “argentophilic
interactions”,39 but they are much longer than that in trimer 1)
seem to contribute to the good tendency of this compound to
crystallize from MeCN, with efficient planar stacking (Figure 2,
bottom). As shown in Figure 1, the methyl group (from MeCN) is
syn to the CF3 group of the pyridylpyrrolide.
Table 1. Catalytic 3 + 2 Cyclization Reactions of Nitriles and
a
Diazoalkyl Ester
The clean formation of complex 2 suggested that a catalytic
reaction could be performed with an excess of EDA, given
the labile nature of both the oxazole and the nitrile adducts.
Accordingly, 2,5-disubstituted oxazole formation at 25 °C can
be run catalytically, in MeCN and using 5 mol % of precatalyst
1, with the reaction being highly selective for oxazole in 70%
yield (Table 1, entry 1). In the reaction there is complete
conversion of EDA and negligible formation of “carbene dimer”
products fumarate and maleate esters or the corresponding
azine, RHCNNCHR.40 Since the oxazole complex can be
isolated in neat MeCN, a slow step in catalysis must be
replacement of oxazole by MeCN. This preference is apparently
the reason BF3 promotion of oxazole formation is stoichio-
metric, and not catalytic in boron. Other Lewis acids have been
shown to catalyze this reaction, even boron or aluminum Lewis
acids, where carbene complexes are unlikely to be involved.36
a
The reaction was performed using neat nitrile and 85% EDA
(solution in CH2Cl2).
However, slower rates and side products from halogenations
by other catalysts or from solvent render complex 1 overall a
superior catalyst. In MeCN, the catalysis is complete in 12 h
at 60 °C, and no carbene dimers are produced under standard
conditions, giving a turnover number (TON) of 14. At the end
of a catalytic run, only one set of resonances is detected, indi-
cating that all of the catalyst has converted to one species in the
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catalytic cycle. Our catalytic studies were intentionally limited
to 25 °C in order to extend catalyst life and also to optimize
selectivity for a single product, but under these conditions this
transformation is slow to reach completion. In general, and
even at high temperatures, only negligible formation of prod-
ucts from carbene insertion into the C−Cl bond was
observed.41 It should be noted that an attempt to conduct
the same transformation using simply AgOTf as a catalyst
resulted in ∼10% conversion to product (60 °C, 12 h)
accompanied by formation of silver mirror even when the
reaction was performed in darkness.
also found to be facilitated by the ability of the PyrPy ligand
to easily twist from planarity around the inter-ring C−C
bond (∼4 kcal/mol).13,14
The 3 + 2 cyclization reactions can be narrowly expanded to
other groups on the nitrile. For example, reactions under the
above standard conditions in PhCN as solvent are faster than
those in MeCN (90% of EDA is consumed in 12 h at room
temperature) to give 5-ethoxy-2-phenyl-1,3-oxazole (Table 1,
entry 2).43 However, further heating for several days provides
full conversion to product. Analysis of a stoichiometric reac-
tion by 1H and 19F NMR spectroscopy (Ag/EDA ratio of 1/1)
showed complete conversion in less than 1 h, to give the
coordinated oxazole (3,5-(CF3)2PyrPy)Ag(κ1N-5-ethoxy-2-
phenyl-1,3-oxazole), therefore implying that the rate-determin-
ing step (RDS) in the catalytic cycle is most likely product
release: nitrile or Lewis base coordination displacing oxazole to
close the cycle. The faster reaction observed with PhCN is
therefore most likely due to faster product release, which we
attribute to steric conflict between the oxazole phenyl
substituent and the Ag(3,5-(CF3)2PyrPy) scaffold. Addition of
a second charge of EDA to this solution again gave complete
conversion of added EDA, as did a third charge of EDA to the
resulting solution. Therefore, our case is likely an example of a
living catalyst. We have also found that repetitive batch studies
conducted in an air atmosphere do not affect our yields;
therefore, catalyst 1 does not appear to be affected by normal
atmospheric constituents.
In an attempt to detect the intermediate (e.g., a silver
carbene complex such as (3,5-(CF3)2PyrPy)Ag(CHCO2Et)
(C)) formed simply from 1 and EDA (i.e., in the absence of
MeCN), these were combined (1/3 mole ratio) in CD2Cl2.
The mixture showed an immediate color change from yellow to
red and only one small broad additional resonance by 19F NMR
spectroscopy at 25 °C. If 19F NMR spectra of this solution are
recorded with decreasing temperature, one can observe not
only decoalescence of the six resonances for the trimer 1
but also the sharpening of two resonances due to one new
compound, which we assign to be an adduct, (3,5-
(CF3)2PyrPy)Ag(EDA) (B), presumably monomeric in metal.
The ratio of reagent to product in this ∼0.05 M solution
at −50 °C is 6/1 by intensity, hence a 2/1 mole ratio. Variable-
1
temperature H NMR spectra of this same solution shows
analogous equilibrium (partial) conversion of 1 to adduct,
including the prominent appearance of signals due to free EDA.
The unique CH resonance of EDA is broad at room tem-
perature and decoalesces below ∼0 °C into two resonances in a
60/40 ratio apparently due to two conformers of the ester
group. Attempts with other diazoalkane reagents such as
N2CPh2 give only slightly higher equilibrium conversions to
adduct, even at −50 °C (established by 19F NMR spectrosco-
py). The fact that partial conversion to adduct is detected with
N2CPh2 suggests that the carbonyl group is not essential to
binding and is thus not necessarily a critical donor to silver.
Using another diazoalkane derivative such as N2CH(SiMe3)
evinced greater (but not 100%) conversion to an adduct, but
the new complex contained six 19F chemical shifts (all in a 1/1
ratio intensity), indicative of a product where the trimer
structure remains intact in the adduct. We therefore conclude
that EDA can effect at most partial conversion of catalyst
precursor onto the catalytic cycle. Although EDA could bind via
its terminal nitrogen, the C-bound adduct is thermodynamically
more stable and competent along the reaction coordinate.16,42
We questioned whether these adducts contained an intact
diazoalkane or if nitrogen had already been expelled.
Accordingly, we attempted to use the equilibrium conversion
established above to answer this question. First, complex 1 was
contacted with EDA (1/3 mole ratio) for 30 min, to form the
equilibrium mixture of adduct and reactants. After this time
volatiles were then removed at 25 °C, followed by addition of
fresh CD2Cl2 (19F and 1H NMR). The NMR spectra revealed a
complete reformation to 1, which rules out a nitrogen-loss
carbene as the equilibrium participant (adduct identity) without
any evidence for carbene dimers being formed (the olefins).
Overall, our data indicate that the adduct contains intact EDA,
in accord with intermediate B, and that EDA is bound weakly
enough to be removed merely by vacuum. Indeed, the
regeneration of 1 by evaporation of the solvent from a solution
containing a three-coordinated Ag(PyrPy) complex is not only
feasible, as is evident from the synthesis of 1 (vide supra), but is
Reaction under standard conditions (in the dark) in neat
tBuCN shows catalytic production of the 2-tBu-substituted
oxazole (Table 1, entry 3), which was again faster in com-
parison to the catalytic reaction of 1 using MeCN. Likewise, no
dimer coupling products were observed in the reaction mixture.
The oxazole was identified by both GC/MS in combination
1
with H and 13C NMR spectroscopy. The same catalytic test,
but performed with ordinary room light, not only produced a
lower yield of oxazole, with precipitation of Ag by formation of
a mirror on the walls of the reaction vessel, but also showed
formation of the fumarate and maleate esters, a broad array of
less volatile products, and unconsumed EDA; the latter led to
some C−Cl insertion product,41 proving that the catalyst was
still active when CH2Cl2 diluent was added prior to GC/MS
analysis.
Proposed Mechanism and Computational Studies.
Scheme 4 depicts our proposed catalytic 3 + 2 addition reaction
involving precatalyst 1, nitrile RCN, and EDA. On the basis of
the fact that nitrile MeCN readily forms a new species with 1,
namely A, in addition to oxazole forming monomer 2, we
propose that nitrile or Lewis base (LB) first breaks trimer 1 to
form the adduct A, (3,5-(CF3)2PyrPy)Ag(LB). Breaking of the
trimer could also be triggered by other potential Lewis bases
such as EDA, and not nitrile alone. This process mostly
involves a pre-equilibrium scenario. Displacement of the nitrile
or other Lewis bases by EDA forms intermediate B, which then
undergoes N2 extrusion to form carbene C, a previously
discussed species relevant to C−H insertion chemistry.15 As
suggested in Scheme 4, the role of nitrile could be 3-fold. Not
only could it participate in activation of precatalyst 1 to form a
monomeric species but it could also be a substrate in the
addition step to C to yield the oxazole adduct 2 and also could
displace the oxazole to close the catalytic cycle by re-forming
the adduct (3,5-(CF3)2PyrPy)Ag(NCR). As in the formation of
B, the latter step might also involve a pre-equilibrium process,
and displacement of oxazole might also be promoted by excess
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EDA. (Note: we favor the nitrile as the Lewis base since it is
used as the solvent medium; however, coordination of EDA is
also plausible.)
Scheme 4. Proposed Catalytic Formation of Disubstituted
Oxazoles Promoted by Complex 1 and the Reagents RCN
and EDA
a
Our simplistic proposed cycle shown in Scheme 4 excludes
critical steps such as the binding mode of EDA to the fragment
(3,5-(CF3)2PyrPy)Ag or the role of carbene C in the addition
of nitrile to form an oxazole-based ligand. For these reasons, we
examined the above transformation using computational
studies. Specifically, theoretical studies for the above reaction
coordinate could allow us to predict whether N2 extrusion or
exchange steps are rate-determining and what intrinsic details
could be important in the C−O and N−C bond forming
processes in C → 2.
As expected, the reaction of EDA with MeCN to form N2
and oxazole is an exergonic reaction with ΔG(solv) = −24.7
kcal/mol. Figure 3 depicts our computed reaction profile for a
silver-catalyzed 3 + 2 cycloaddition of a :CHCO2Et fragment
with MeCN. Binding of MeCN to 1 breaks the trimer to give
the three-coordinate silver monomer A, which is nearly iso-
thermal with its predecessor. The computed structure of A
suggests this species to possess a highly distorted T-shaped
structure whereby the nitrile coordinates trans to the pyrrolide
nitrogen (a similar structure was obtained by X-ray crystallo-
graphy for the Cu(I) analogue).14 The predicted geometry of A
closely resembles that of oxazole adduct 2. Displacement of
MeCN by EDA results in formation of B, which is 6.5 kcal/mol
higher in energy than A. Binding of EDA to the (3,5-
(CF3)2PyrPy)Ag fragment does not occur via the diazo nitrogen
a
For simplicity, we exclude equilibrium arrows in the catalytic cycle. It
is also quite possible that EDA reacts with 1 to form B directly.
Figure 3. Computed reaction profile for the catalytic cycloaddition of a carbene (:CHCO2Et) and a nitrile (MeCN) by precatalyst 1 to form an
oxazole. The 2,2′-pyridylpyrrolide has been abbreviated as the monoanionic chelate ligand N1N2. *The transition states for initial formation of A, B,
and D cannot be located on the electronic surface, as these are dominated by the translational entropy change.
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but instead by the γ carbon, presumably given the canonical
form NN+C−HCO2Et. The binding mode of EDA to
silver(I) and N2 extrusion to form the carbene C has been
discussed elsewhere, and we will focus our attention on sub-
sequent steps.15 Given the electrophilic nature of the carbene
carbon in C, MeCN binds to this site to form a remarkably
stable Lewis base adduct of the silver carbene species, the
intermediate (3,5-(CF3)2PyrPy)Ag{CH(NCMe)CO2Et} (D).
This conversion is analogous to that shown by some elec-
trophilic “Fischer-type” carbene complexes.44 Binding of the
nitrile to carbene carbon in C greatly stabilizes this species by
19.0 kcal/mol. As expected for a σ donor, Figure 4 shows how
binding of NCMe to the electrophilic carbene results in
negligible change in the nitrile NC bond (1.156 Å) from that of
free NCMe (1.159 Å).45 Note (Figure 4) that the C1/N5/C3
electrophile that activates the nitrile carbon to nucleophilic
attack is the carbene carbon, not silver, but silver (and CO2R)
is an EWG which enhances the carbene electrophilicity.
We have demonstrated by the use of a silver complex, which
is stable to water and light, that there are significant advantages
of this methodology from previous studies, since the prod-
ucts arising from carbene dimers are essentially absent. This
feature eliminates the usage of a controlled slow addition
of EDA via a syringe pump as well as the dilution of the
reagents using copious amount of solvent. Although our silver
catalyst is sometimes living, such a system also reduces the
necessity to rigorously exclude water or light from the reactor, a
factor which is ideal for larger scale synthesis. In summary, it
appears that the electrophilic character of the putative complex
[Ag(3,5-(CF3)2PyrPy)] is responsible for the catalytic capa-
bility of the system we report here, and sensitivity to light is not
excessive; therefore, the catalyst is practical as well as robust to
atmospheric O2 and adventitious water. In addition to carbene
insertion into C−H bonds and CC bonds, complex 1 is a
versatile reagent and can also promote 3 + 2 cycloaddition
chemistry presumably via a transient, three-coordinate silver
carbene reagent.
ASSOCIATED CONTENT
* Supporting Information
■
S
Tables, figures, and a CIF file giving computational details such
as computed structures and coordinates, spectral data, and
information on the X-ray crystal structure analysis of
compound 2. This material is available free of charge via the
Internet at http://pubs.acs.org.
Figure 4. Computed structures for transition state D-TS (left) and
intermediate E (right). Distances are reported in Å and angles in deg.
For D-TS: Ag−C1, 2.252; C1−C2, 1.471; C2−O1, 1.242; C1−N5,
1.403; N5−C3, 1.192; O1−C3, 2.205; N1−Ag−C1, 149.3; N2−Ag−
C1, 139.5. For E: Ag−C1, 2.359; C1−C2, 1.389; C2−O1, 1.354; C1−
N5, 1.412; N5−C3, 1.288; O1−C3, 1.392; N1−Ag−C1, 153.1; N2−
Ag−C1, 134.7.
AUTHOR INFORMATION
Corresponding Authors
*E-mail for D.J.M.: mindiola@sas.upenn.edu.
■
*E-mail for K.G.C.: caulton@indiana.edu.
unit in D-TS is already nonlinear, hence developing electro-
philicity at C3, which is needed for cyclization. The silver ion
facilitates electron flow in the 3 + 2 addition step involving
D-TS, and its computed structure is shown in Figure 4. In D-
TS, there is little C−O bond formation (O1−C3 van der Waals
distance of 3.22 Å), therefore implying a relatively early
transition state with respect to formation of E. There is no
change in the connectivity of silver with substrate in these steps,
but silver serves as an electron-withdrawing substituent on the
carbenoid carbon; that is its catalytic role. Carbon−oxygen
bond formation between the nitrile carbon and ester CO
group results in an overall concerted ring-closing step to form
the C-bound oxazole (3,5-(CF3)2PyrPy)Ag{C(H)NC(Et)OC-
(OEt)} (E), traversing a small barrier of 13.4 kcal/mol. The
computed structure of E is best described as a C-bound oxazole
bound to Ag(I) whereby a single C−O bond bond has formed
(1.39 Å). From E, a nearly barrierless 1,2-silver migration
results in the more thermodynamically favorable N-bound
oxazole isomer 2.
Present Address
†University of Pennsylvania, Department of Chemistry, 231 S.
34 Street, Philadelphia, PA 19104, United States.
Author Contributions
The manuscript was written through contributions of all
authors.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Indiana University Bloomington for financial support
of this research through a Faculty Research Support Program
(FRSP). Financial support of this research was also provided by
the Chemical Sciences, Geosciences, and Biosciences Division,
Office of Basic Energy Science, Office of Science, U.S. Depart-
ment of Energy (DE-FG02-07ER15893 to D.J.M.).
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In the sense that it is the nucleophilic substituent pendant on
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