Formation of Oxazoles from Elusive Gold(I) α-Oxocarbenes: A Mechanistic Study

Article abstract of DOI:10.1002/chem.201601634

The gold(I) catalyzed reaction between phenylacetylene, pyridine N-oxide and acetonitrile leading, via a putative gold-α-oxocarbene intermediate, towards an oxazole product has been investigated. A novel mass spectrometric method called “delayed reactant

Full text of DOI:10.1002/chem.201601634

DOI: 10.1002/chem.201601634
Full Paper
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Gold Catalysis |Hot Paper|
Formation of Oxazoles from Elusive Gold(I) a-Oxocarbenes:
A Mechanistic Study
[a]
ˇ
ˇ
Jirꢀ Schulz, Juraj Jasꢀk, Andrew Gray, and Jana Roithovꢁ*
Abstract: The gold(I) catalyzed reaction between phenylace-
tylene, pyridine N-oxide and acetonitrile leading, via a puta-
tive gold-a-oxocarbene intermediate, towards an oxazole
product has been investigated. A novel mass spectrometric
method called “delayed reactant labeling” is used to track
consecutive and parallel reactions. It clearly shows that the
intramolecular formation of a pyridine adduct of gold-a-oxo-
carbene is in competition with the formation of the oxazole
product. The reaction mechanism most probably corre-
sponds to competition between acetonitrile and pyridine in
an almost barrierless reaction with putative gold-a-oxocar-
bene within the solvent cage. The detected ionic species
have been characterized by helium tagging infrared photo-
dissociation spectroscopy.
(Int+ in Figure 1). A question remains, whether the elusive
gold-a-oxocarbenes exist as short-lived intermediates within
a solvent cage or whether the reaction proceeds as an SN2 or
SN2’ substitution[10] of pyridine by the other nucleophile (possi-
ble pathways to the grey panel in Figure 1).
Introduction
One of the prominent properties of gold(I) complexes is their
ability to activate alkynes for nucleophilic additions.[1,2] It was
recently shown that this mode of reactivity can also be used
for reactions of alkynes with oxidizing agents such as pyridine
N-oxide or its derivatives.[3,4] The reaction formally leads to
gold-a-oxocarbenes that can be utilized in subsequent reac-
tions.[5] The first intermolecular reaction of gold-a-oxocarbenes
was coupling with nitriles leading to oxazole rings
(Scheme 1).[6,7]
Mass spectrometry connected with electrospray ionization
(ESI-MS) is a useful tool for detection of ionic reaction inter-
mediates and for the investigation of reaction mechanisms.[11]
Contrary to other methods, ESI-MS has great sensitivity even
for very low abundant ionic species. These species can be sep-
arated from the reaction mixture to the gas phase where their
structure and reactivity can be probed either in collisional[11] or
spectroscopic experiments.[12,13] This merit is however associat-
ed with a generally nonlinear response to the concentration of
the detected species in the sampled solution.[14] The concentra-
tion of the species Figure 1 can be determined by the use of
isotopically labeled internal standards.[15]
We have recently introduced a method for investigation of
reaction kinetics that we refer to as delayed reactant label-
ing.[16] The method consists in running a reaction with one (or
more) of the reactants being both unlabeled and isotopically
labeled. The key trick is addition of the labeled (or unlabeled)
reactant with a certain time-delay. Monitoring of such a reac-
tion mixture with ESI-MS then reveals pairs of peaks corre-
sponding to the unlabeled and labeled reactants, intermedi-
ates and products. The mutual time evolution of the pairs of
peaks reflects the kinetics of formation of these ions (or neu-
trals that are detected as ions) in solution. This method helps
to overcome problems arising from the non-linear ESI response
to the concentration of the given species in solution, because
the concentrations of the given ions are determined only rela-
tive to their isotopically labeled analogues. Here, we will dem-
onstrate the utility of this approach to distinguish consecutive
and parallel reactions.
Scheme 1. The first type of an intermolecular reaction of gold-a-oxocar-
benes.
We have recently shown that the gold-a-oxocarbenes do
not exist as free long-lived species in solution, but are formed
as adducts in an intramolecular reaction with the leaving
group from the oxidant, that is, in the form of gold-a-oxocar-
benoids.[8,9] Hence, in the reaction with pyridine N-oxide, pyri-
dine adducts of the formal gold-a-oxocarbenes are formed
ˇ
[a] Dr. J. Schulz, Dr. J. Jasꢀk, Dr. A. Gray, Prof. J. Roithovꢁ
Department of Organic Chemistry,
Faculty of Science, Charles University in Prague
Hlavova 2030/8, 128 43 Prague (Czech Republic)
E-mail: roithova@natur.cuni.cz
Supporting information and the ORCID identification number for the corre-
sponding author of this article can be found under http://dx.doi.org/
10.1002/chem.201601634.
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Figure 1. Possible pathways in the [(IPr)Au]+ catalyzed reaction between phenylacetylene, pyridine N-oxide and acetonitrile. The species detected by ESI-MS
are in light grey panels denoted by labels in italics.
tected by means of mass spectrometry. The dependence of
Results
helium complex depletion on the photon wavenumber pro-
The reaction mechanism will be studied for the
[(IPr)Au(CH3CN)]BF4 catalyzed oxidation of phenylacetylene
using pyridine N-oxide as oxidant and acetonitrile as the com-
peting nucleophile in dichloromethane solvent. We have
chosen the terminal alkyne because it is the typical substrate
in this reaction and because the oxidation is stereoselective
avoiding the problem of the formation of two isomeric prod-
ucts. The method of choice is electrospray ionization mass
spectrometry (ESI-MS)[11] that can detect a number of relevant
complexes including Int+, P1+, P2+ and P3+ (Figure 1). The
initial reaction step leads to the formation of a pyridinium-oxy
vinylgold intermediate. In the absence of other suitable nucle-
ophiles, this adduct intramolecularly rearranges to the gold-a-
oxocarbenoid Int+ that can undergo either protodeauration to
form the P1+ product or a coupling with another molecule of
alkyne leading to P2+. Formation of the products P1+ and
P2+ can also be observed in the presence of acetonitrile, but
its concentration has to be low. If acetonitrile is used as sol-
vent, their formation is completely suppressed and only Int+
and P3+ are observed.
vides the IRPD spectrum.
We have applied this technique to structurally assign the
ions with m/z corresponding to the Int+ (m/z 782) and P3+
(m/z 744) ions. Figure 2 shows the helium tagging IRPD spec-
trum of the ions with m/z 782 that contains three dominant
peaks in the finger-print region at 1223 cmÀ1, 1485 cmÀ1, and
1682 cmÀ1 and a number of peaks at the spectral region above
2700 cmÀ1. The bands at high wavenumbers and the band at
1485 cmÀ1 are associated with the IPr ligand and can be found
in all spectra of ions containing the gold atom coordinated to
this ligand.[8] The bands at 1223 cmÀ1 and 1682 cmÀ1 are char-
acteristic for the gold-a-oxocarbenoid. The 1223 cmÀ1 band is
assigned to the vibration of the bond between the phenyl ring
and the carbonyl group (CPhÀCCO) and the band at 1682 cmÀ1
corresponds to the carbonyl stretching vibration (cf. Fig-
ure 2b). These bands would not occur if the structure of the
ions would correspond to the initial pyridine N-oxide adduct
(cf. Figure 2c).
For the ions with m/z 744, again a gold-a-oxocarbenoid
structure with acetonitrile can be suggested (Figure 3c). This
structure would be revealed by analogous bands as detected
for the ions above (CPhÀCCO and C=O vibrations). The recorded
helium tagging IRPD spectrum (Figure 3a) does not contain
these characteristic a-oxocarbenoid bands and instead reveals
several vibrations characteristic for the oxazole structure: C=N
stretch at 1576 cmÀ1, C=C stretch at 1636 cmÀ1, H-C-N bending
at 1157 cmÀ1, and CÀH stretch at 3155 cmÀ1 (Figure 3b). The
detected ions thus correspond to the gold-tagged oxazole
products.
Helium tagging infrared photodissociation
(IRPD) spectroscopy
The structure of the detected ions was established based on
helium tagging IRPD spectroscopy,[12,17,18] the method provides
infrared spectra of isolated ions in the gas phase.[19] The mass-
selected ions are stored in an ion trap at 3 K, where they form
loosely bound complexes with helium atoms. Helium can be
considered as an innocent tag as it has a negligible effect on
the structure as well as the IR spectrum of the parent ions.[20,21]
Upon absorption of an IR photon, the internal energy of the
given complex increases which results in the detachment of
the helium atom. The absorption of the IR photon by the ion
is thus translated to the change of its m/z ratio that can be de-
Delayed reactant labeling
As outlined above, the kinetics of the formation of gold-a-oxo-
carbenoid (detected as m/z 782) and oxazole (detected as m/
z 744) has been be investigated by delayed reactant labeling.
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Figure 2. a) Helium tagging IRPD spectrum of the ions with m/z 782. b,c) Theoretical spectra of possible isomers of the [(IPr)Au(PhCCH)(PyO)]+ complex.
Figure 3. a) Helium tagging IRPD spectrum of ions with m/z 744. b,c) Theoretical spectra of possible isomers of the [(IPr)Au(PhCCH,O)(CH3CN)]+ complex.
To this end, the reaction mixture of phenylacetylene, pyridine
N-oxide and the [(IPr)Au(CH3CN)]BF4 catalyst in CH2Cl2 was left
to react for 1 hour, then a freshly mixed acetonitrile solution
with equivalent amounts of D6-phenylacetylene, pyridine N-
oxide and catalyst was added. The whole mixture was moni-
tored by ESI-MS (Figure 4). During the time delay, a certain
concentration of all detected gold complexes was built up—
except the oxazole product, which is only formed after the
time delay, when acetonitrile is added to the mixture. As ex-
pected, the mutual time evolution of the individual signals
shows that the relative concentration of the labeled complexes
(curves are shown in grey) is increasing. Looking at the com-
plexes formed between the catalyst and phenylacetylene, we
can state that the formation of digold acetylides
[(PhCC)Au2(IPr)2]+ is quasi-irreversible as the relative concentra-
tion of the unlabeled complexes decreases only very slowly
and prevails even in the end of the experiment, when the con-
centration of the unlabeled PhCCH in the reaction mixture is
lower than that of D5-PhCCD as evidenced from other signals.
The establishment of the steady-state concentration of the
mono-gold complexes of phenylacetylene (Figure 4 g) is rela-
tively slow. We ascribe this to the fact that most of the gold
cations are bound in acetylides or other gold complexes. The
kinetics changes to a fast steady-state establishment as soon
as an acid is added to the reaction mixture (see the kinetic
modeling in the Supporting Information). The kinetics associat-
ed with the formation of the pyridine gold-a-oxocarbenoids is
predictable (Figure 4c). It reflects the initial high concentration
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Figure 4. Delayed reactant labeling: Firstly, a mixture of [(IPrAu)(CH3CN)]BF4 (0.5 mmol), PhCCH (10 mmol) and PyO (10 mmol) in DCM (0.5 mL) was stirred for
1 hour. Then, a freshly prepared solution of [(IPrAu)(CH3CN)]BF4 (0.5 mmol), D5-PhCCD (10 mmol) and PyO (10 mmol) in CH3CN (0.5 mL) was added. The whole
reaction mixture was then monitored by ESI-MS. a,b) ESI-MS spectra obtained at the monitoring time of 10 min and 2 h, respectively. c–h) Time evolution of
the unlabeled and labeled signals corresponding to the denoted intermediates and products in the ESI-MS spectra (the sum of the signal intensities of the
given labeled and unlabeled ions is set to 1).
of the carbenoids formed during the time delay. After labeling,
the relative concentrations reflect the formation as well as sub-
sequent reactions of both, labeled and unlabeled carbenoids.
The formation of the products P1+ and P2+ can be mod-
eled as consequent reactions of the pyridine gold-a-oxocarbe-
noids (Figure 4d and e). It is not further discussed here, be-
cause the formation of these products is suppressed when an
acid is added to the reaction mixture (as is done in synthetic
applications). The most important result is shown in Figure 4 h.
It shows that the kinetics of the oxazole formation is not con-
nected with that of the carbenoid. Oxazole is only formed after
the addition of acetonitrile with the labeled reaction mixture. If
oxazole were formed by the subsequent reaction of the pyri-
dine carbenoid, the formation of the unlabeled oxazole would
have to prevail in the beginning of the experiment. Instead,
the signals reflect the higher concentration of the labeled D5-
PhCCD in the solution (unlabeled PhCCH is reacting for
a longer time, therefore its concentration is lower). This clearly
shows that oxazole is not formed by SN2 substitution of the
pyridine carbenoid and it is rather formed in competition to
the carbenoid formation.
the gold acetylides and gold-a-carbenoids. The experiments
were therefore designed so that the kinetics was observed
after the concentration of the gold catalyst reached a certain
equilibrium level (for details of experiments and reaction mix-
ture composition, see the Supporting Information). Numerical
kinetic modeling of the obtained data (data sets together with
the kinetic modeling can be found in the Supporting Informa-
tion) allows estimation of the relative ratio of the rate con-
stants for the parallel formations of pyridine carbenoids and
oxazoles. In reaction with 100 molar excess of acetonitrile to
pyridine N-oxide the formation of oxazole is about 20–60 times
faster.
Density functional theory (DFT) calculations
Finally, we investigated the alternative reaction pathways with
DFT calculations (Figure 5). Structure optimization was per-
formed using the polarized continuum model and pure aceto-
nitrile was considered as solvent. The initial addition of pyri-
dine N-oxide to the gold-activated alkyne proceeds via an
energy barrier of 8.7 kcalmolÀ1 (TS1/2) and leads to the pri-
mary pyridine N-oxide adduct (2). The pyridine molecule can
be eliminated via a 12.5 kcalmolÀ1 (TS2/3) barrier. Theoretically,
the dissociation leads to a complex of carbene 3 and pyridine
(3·Py). The complex can either spontaneously dissociate to
We have also performed experiments for a quantitative eval-
uation. The reaction kinetics is complicated by the fact that
the concentration of the free gold catalysts changes drastically
during the first minutes of the reaction by its incorporation to
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Figure 5. DFT (mPW1PW91/cc-pVTZ:cc-pVTZ-pp//mPW1PW91/cc-pVDZ:LANL2DZ) potential energy surface considering acetonitrile as solvent (via the PCM
model). The energies are Gibbs free energies (enthalpies are given in brackets) with free solvation energies at 298 K in kcalmolÀ1 and are related to the
energy of carbenoid 4. All geometries and energies can be found in the Supporting Information (Table S2). The structures of transition states are shown in
ball-and-sticks models, the grey shapes show the IPr ligands at the gold atoms. The dotted lines show the formation/annihilation of bonds within the transi-
tion structure. The geometry of TS3/5 was optimized in the gas phase (see Theoretical Details and the Supporting Information).
yield the elusive gold(I) a-oxocarbene (3) or can collapse to
form pyridine gold(I) a-oxocarbenoid (4) in a highly exothermic
reaction. The structure of the gold(I) a-oxocarbene is partially
stabilized by an interaction between the oxygen atom and the
carbene carbon atom (cf. the geometry in the frame in
Figure 5). We have shown in our previous study that gold(I) a-
oxocarbenes are prone to undergo rearrangement to their re-
spective ketenes.[8] The energy barrier for such rearrangement
is 7.0 kcalmolÀ1.
acetonitrile is the solvent in this reaction, the entropic penalty
is overestimated. Thus, although the calculations suggest that
the SN2’ pathway will not be competitive with the dissociation-
association mechanism, we should not exclude it.
We want to stress that the profile of the potential energy
surface is influenced by the choice of theoretical model, solva-
tion model, and thermochemical parameters. Consideration of
Gibbs energies leads to entropic disfavoring of the association
channels (see Figure S4 and S5 in the Supporting Information).
As mentioned above, the energy barriers for the reactions with
acetonitrile are too high as it is a solvent molecule. In addition,
the association can be favored by a better treatment of disper-
sion interactions within the theoretical model. The energies
presented here should be treated as a pointer to understand
reactivity, not as absolute values. The energies shown in
Figure 5 were obtained at the mPW1PW91/cc-pVTZ level of
theory (see the Experimental Section). We have chosen this
functional, because we have shown in past that it provides IR
spectra that are in very good agreement with experiment and
thus correctly describes the structure for this type of com-
plexes. In order to demonstrate the effect of the method, we
have recalculated the potential energy surface at the M06/
def2TZVPP[22] level of theory using the SMD solvation model.[23]
This approach leads to a Gibbs energy profile where the for-
mation of free carbene 3 is less favored and the barriers for its
coupling with pyridine and acetonitrile are only on the order
of 2 kcalmolÀ1 (Figure S6 in the Supporting Information).
Gold(I) a-oxocarbene 3 can couple either with pyridine or
acetonitrile. The energy barrier for the reaction with acetoni-
trile (TS3/5) is lower (6.2 kcalmolÀ1) than for coupling with pyr-
idine (9.4 kcalmolÀ1) but both couplings are rather exothermic.
Formation of the acetonitrile gold(I) a-oxocarbenoid (5) is
16.0 kcalmolÀ1 less energetically favorable than that of the pyr-
idine adduct. The acetonitrile adduct 5 smoothly rearranges
via a 5.8 kcalmolÀ1 energy barrier (TS5/6) and forms the final,
highly favored oxazole product.
The suggested SN2 substitution of pyridine has an energy
barrier of 55.8 kcalmolÀ1 (TS4/5) and therefore will not be effi-
cient. The transition state TS2/5 for the simultaneous pyridine
elimination from the oxygen atom of the pyridine N-oxide
adduct 2 and acetonitrile addition to the carbon atom, that is,
the SN2’ mechanism, lies 4.5 kcalmolÀ1 above TS2/3 for the un-
imolecular pyridine elimination. The additional energy demand
of the concerted SN2’ mechanism is mostly down to entropic
effects (see the potential energy surface constructed from the
enthalpies, Figure S4 and S5 in the Supporting Information). As
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Discussion
Conclusion
The combined experimental and theoretical investigation pre-
sented here explains why the gold-a-oxocarbene intermediates
are not formed as long-lived species in gold-catalyzed oxida-
tions of alkynes. The reaction barriers calculated for the cou-
pling between gold-a-oxocarbene and nucleophiles are very
low and the coupling is highly exothermic. For the oxidation
with pyridine N-oxide, the energy barrier for the coupling be-
tween the gold-a-oxocarbene and pyridine is in fact on the
order of the energy required for the formation of their encoun-
ter complex. Hence, if no more reactive or better located (in-
tramolecular) nucleophile is available, the complex [3·Py]
formed upon the elimination of pyridine from the primary oxy-
pyridinium adduct will collapse to form pyridine gold-a-oxo-
carbenoid. This explains, why we observe the formation of pyr-
idine gold(I) a-oxocarbenoid in the intramolecular reaction.
Pyridine gold-a-oxocarbenoids cannot be considered as
masked gold-a-oxocarbenes, because their formation is irrever-
sible and the pyridine molecule does not serve as a leaving
group in an SN2 type reaction. This result is clearly evident
from delayed reactant labeling studies. The formation of pyri-
dine gold-a-oxocarbenoids can be avoided by increasing the
energy barrier for the coupling reaction and decreasing the
exoergicity of the formation of gold-a-oxocarbenoid. Both ef-
fects are achieved by using sterically hindered analogues of
pyridine N-oxide in synthesis. Such an approach favors parallel
reactions with other nucleophiles. In extreme cases, even reac-
tions with otherwise non-reactive solvent molecules can be
more favored.[24]
In conclusion, we have presented the application of delayed
reactant labeling for the investigation of gold(I) catalyzed oxi-
dation of alkynes followed by coupling with acetonitrile to
form oxazole. We have previously shown that for oxidation
using pyridine N-oxide, the putative gold(I) a-oxocarbene in-
termediates undergo intramolecular coupling with pyridine to
form the corresponding carbenoid. Here, we show that pyri-
dine gold(I) a-oxocarbenoids do not undergo an SN2 reaction
with acetonitrile to form oxazole. The coupling with acetoni-
trile proceeds prior to the coupling with pyridine probably via
elimination of pyridine and trapping of gold(I) a-oxocarbene
within the solvent cage. The reactions with nitriles have to be
done with a large molar excess otherwise the formation of un-
desired carbenoids will prevail. Alternatively, the barrier for the
formation of the carbenoids has to be increased by the use of
sterically demanding and less nucleophilic derivatives of pyri-
dine N-oxide as oxidants. Usually, a combination of both ap-
proaches is used in synthesis.[27]
Experimental Section
Reaction mixtures for helium tagging infrared photodissociation
(IRPD) spectroscopy measurements were prepared by dissolving
phenylacetylene (20 mmol), pyridine N-oxide (20 mmol) and gold
complex [Au(IPr)(CH3CN)]BF4 (1 mmol) in the mixture of CH2Cl2
(0.9 mL) and CH3CN (0.1 mL). For the delayed reactant labeling ex-
periments, phenylacetylene (10 mmol), pyridine N-oxide (10 mmol)
and gold complex [Au(IPr)(CH3CN)]BF4 (0.5 mmol) were mixed in
CH2Cl2 (0.5 mL) and the reaction mixture was stirred for 1 hour.
Then it was mixed with a freshly prepared solution of phenylacety-
We note in passing that we have also performed the experi-
ments with 1-phenylpropyne in order to have data for the oxi-
dation reaction of an internal alkyne. The results are qualita-
tively the same. The helium tagging IRPD spectra (Figure S1 in
the Supporting Information) again show that we detect the rel-
evant pyridine gold-a-oxocarbenoid ions and oxazole com-
plexes by mass spectrometry. Delayed reactant labeling clearly
shows that the carbenoid and oxazole complexes are formed
in parallel reactions (Figure S2 and S3 in the Supporting Infor-
mation). We have also tested reaction mixtures where 3,5-di-
bromopyridine N-oxide was used as an oxidant with qualita-
tively the same results.
lene-d6
(10 mmol),
pyridine
N-oxide
(10 mmol)
and
[Au(IPr)(CH3CN)]BF4 (0.5 mmol) in CH3CN (0.5 mL).
ESI-MS spectra: Experiments were performed with a Finnigan LCQ
Deca ion trap mass spectrometer.[28] The mass spectrometer is
equipped with a conventional ESI source consisting of a spray unit
that is followed by a heated capillary, a first set of and two transfer
octopoles. Generated ions are stored within a Paul ion trap and
then sequentially ejected from the ion trap to an electron multipli-
er in order to record the mass spectrum.
Helium tagging IRPD spectra: The IRPD spectra were measured
with the ISORI instrument.[17] The ions were generated by ESI, mass
selected by the first quadrupole and transferred by an octopole to-
wards a cryo-cooled wire quadrupole ion trap operated at 3 K with
a 10 Hz trapping cycle. The ions were trapped with a helium buffer
gas pulse (30 ms long). During their interaction with helium, about
10% of the trapped ions were transformed to helium tagged com-
plexes. After an 85 ms time delay, the ion cloud was irradiated by
a photon pulse (8 ns) generated in an optical parametric oscillator
(OPO) operating at 10 Hz frequency. At 90 ms, the exit electrode of
the trap was opened, the ions were mass-analysed by the second
quadrupole, and their number (Ni) was determined by a Daly type
detector operated in ion-counting mode. In the following cycle the
light from the OPO was blocked by a mechanical shutter, giving
the number of unirradiated ions (N0). The IRPD spectra are con-
structed as the wavenumber dependence of (1 - Ni/N0).
All these results are in perfect agreement with the facts
known from the synthetic development of the reactions based
on the concept of gold-a-oxocarbenes, nevertheless these
need to be considered as elusive intermediates. While the reac-
tivity of phenylacetylene shown here can be perfectly under-
stood by invoking the formation of the respective gold-a-oxo-
carbene, it does not exist as an isolated intermediate. The ob-
servation of an isolated gold-a-oxocarbene remains a challenge
for future. It will require sufficiently high energy barriers for
the formation of carbenoids with all components in the reac-
tion mixture (might be assured by bulky ligands at the gold
catalyst)[25] and the putative gold-a-oxocarbene will have to be
stabilized to avoid rearrangement to the ketene (or enone)[26]
along with all other intramolecular reactions.
DFT calculations: Calculations have been performed using the
mPW1PW91 density functional[29] and the polarized continuum
model (PCM) for the approximation of the solvation effect[30] of
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acetonitrile as implemented in the Gaussian 09 program pack-
age.[31] The geometry optimizations and Hessian matrix calculations
(for the confirmation of the nature of all stationary points and ob-
taining thermochemical corrections and IR spectra) were done
with the LanL2DZ basis set for the gold atoms and the cc-pVDZ
basis set for the remaining elements. The geometry of TS3/5 had
to be optimized in the gas phase using an ultrafine grid, because
the optimization in acetonitrile led always to the geometry of car-
benoid 5. The total energies of all structures were then refined by
single-point calculations using the basis set cc-pVTZ-pp for gold
and cc-pVTZ for the remaining atoms considering solvation by ace-
tonitrile (PCM). Reported energies refer to the Gibbis energies at
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ed for by a correction of (1.9 Dn) kcalmolÀ1. The predicted gas
phase IR spectra were convoluted with a Gaussian function with
a full width at half maximum (fwhm) of 5 cmÀ1. The computed fre-
quencies were scaled by a factor of 0.965 in the finger-print region
and that of 0.942 in the CÀH and NÀH stretching vibrations region
to obtain optimal match between the experimental IRMPD and
theoretical spectra.[32,33]
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Keywords: carbenes · gold catalysis · mass spectrometry ·
reaction intermediates · reaction mechanism
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Received: April 15, 2016
Published online on && &&, 0000
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FULL PAPER
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Gold Catalysis
ˇ
J. Schulz, J. Jasꢀk, A. Gray, J. Roithovꢁ*
&& – &&
Formation of Oxazoles from Elusive
Gold(I) a-Oxocarbenes: A Mechanistic
Study
A new approach: The mechanism of
oxazole formation in a gold mediated
oxidation reaction of terminal alkynes is
investigated by several means. A new
method for the determination of con-
secutive and parallel reactions is pre-
sented. It is shown that intermolecular
formation of pyridine carbenoid is in
competition with oxazole formation
(see scheme).
Chem. Eur. J. 2016, 22, 1 – 9
www.chemeurj.org
9
ꢂ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
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These are not the final page numbers! ÞÞ