Dinuclear gold(i) complexes bearing alkyl-bridged bis(n-heterocyclic carbene) ligands as catalysts for carboxylative cyclization of propargylamine: Synthesis, structure, and kinetic and mechanistic comparison to the mononuclear complex [Au(IPr)Cl]

Article abstract of DOI:10.1021/acs.organomet.0c00404

Eight new dinuclear gold(I) complexes, [Au₂(L)X₂] (1?8), were synthesized using a straightforward synthetic procedure under very mild conditions. The complexes have been characterized by NMR spectroscopy, elemental analysis, and single-crystal X-ray structure analysis. Their catalytic activity was investigated in the carboxylative cyclization of propargylamine (PPA). A superior performance in comparison to [Au(IPr)Cl] (9) was obtained for complexes 1 and 2 having an eight-methylene bridge connecting two NHCs with an arene bearing an isopropyl substituent for X = Cl, Br. This prompted more detailed kinetic and mechanistic studies by FTIR comparing dinuclear complex 2 of X = Cl to complex 9. Fortuitously the FTIR studies allowed monitoring of the formation of the products carbamic acid (CA) and carbamate salt (CS), as well as a key cyclized intermediate first discovered by Ikariya. These data allow additional insight into the mechanism as well as the central role which may be played by Au(I) carbamate formation as a higher energy resting state present in the catalytic cycle. The crystal structures of four of the new complexes and a detailed computational study relevant to the role of carbamic acid (CA) and carbamates in the catalytic cycle are also reported.

Full text of DOI:10.1021/acs.organomet.0c00404

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Article
Dinuclear Gold(I) Complexes Bearing Alkyl-Bridged Bis(N-
heterocyclic carbene) Ligands as Catalysts for Carboxylative
Cyclization of Propargylamine: Synthesis, Structure, and Kinetic and
Mechanistic Comparison to the Mononuclear Complex [Au(IPr)Cl]
Tahani A. C. A. Bayrakdar, Fady Nahra, Jack V. Davis, Mohan M. Gamage, Burjor Captain,*
Manuel Temprado,* Marco Marazzi, Marina Saab, Kristof Van Hecke, Dominic Ormerod,
Carl D. Hoff,* and Steven P. Nolan*
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ABSTRACT: Eight new dinuclear gold(I) complexes, [Au2(L)X2]
(1−8), were synthesized using a straightforward synthetic
procedure under very mild conditions. The complexes have been
characterized by NMR spectroscopy, elemental analysis, and single-
crystal X-ray structure analysis. Their catalytic activity was
investigated in the carboxylative cyclization of propargylamine
(PPA). A superior performance in comparison to [Au(IPr)Cl] (9)
was obtained for complexes 1 and 2 having an eight-methylene
bridge connecting two NHCs with an arene bearing an isopropyl
substituent for X = Cl, Br. This prompted more detailed kinetic
and mechanistic studies by FTIR comparing dinuclear complex 2
of X = Cl to complex 9. Fortuitously the FTIR studies allowed monitoring of the formation of the products carbamic acid (CA) and
carbamate salt (CS), as well as a key cyclized intermediate first discovered by Ikariya. These data allow additional insight into the
mechanism as well as the central role which may be played by Au(I) carbamate formation as a higher energy resting state present in
the catalytic cycle. The crystal structures of four of the new complexes and a detailed computational study relevant to the role of
carbamic acid (CA) and carbamates in the catalytic cycle are also reported.
the hydration of nitriles,8b and the carboxylative cyclization of
INTRODUCTION
■
propargylamines.9 These reactions allow for the production of
The stability and nontoxic nature of gold make this noble
metal a most interesting transition metal to study.1
valuable motifs such as ketones, amides, and oxazolidinone
derivatives. The involvement of water in the hydration of
alkynes as well as in the hydration of nitriles and the
involvement of carbon dioxide (CO2) in the carboxylative
cyclization of propargylamines highlight the sustainability,
stability, reactivity, and atom economy brought about by their
use. On the catalyst synthesis front, this field has received
increased attention with the synthesis of dinuclear gold
complexes, mostly complexes of the dicationic gold(I)
[Au2(NHC-R-NHC)2]2+ type, which consist of two gold
centers that are each strongly coordinated linearly to two
NHCs. Their high stability makes them not quite suited for
Applications of gold chemistry in areas ranging from
homogeneous catalysis2 to medicinal chemistry3 and materials
science4 have enabled important advances in these fields
during the past decade. In particular, the explosion of reactivity
centered around gold(I)−NHC (NHC = N-heterocyclic
carbene) complexes in homogeneous catalysis has simply
been remarkable. The fundamental property rendering metal
complexes bearing NHC ligands of such interest is their unique
electron richness, mainly due to the important carbene−metal
σ-donation.5 The strength of the metal−carbene bond has
been proven to increase the thermal and oxidative stability of
many catalysts by inhibiting decomposition pathways.6 It is
believed that the metal−carbene σ-bond is responsible for the
unique stability of these complexes.2a This ligand family has
been used as supporting ligands in a plethora of transition-
metal-catalyzed reactions such as cross-coupling reactions,
leading to C−C and C−N bond formations.7
Received: June 12, 2020
In the context of gold, organic transformations catalyzed by
gold(I)−NHC complexes include the hydration of alkynes,8
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Scheme 1. Reaction Mechanism Reported by Lin21
catalysis; however, they have been widely used in electro-
chemical, photophysical, and luminescence studies.10 Other
types of dinuclear gold(I) complexes are complexes based on
bridging diphosphine ligands, which require handling under
inert conditions.11 Air-stable dinuclear gold(I) complexes
bearing a mono-NHC with a bridging (μ-OH) group have
also been extensively studied by Nolan and co-workers.8b,12
These dinuclear gold species were shown to be highly active
catalysts for silver-free and acid-free gold-catalyzed trans-
formations.8b,12 Meanwhile, only a few examples of dinuclear
gold(I) complexes bearing an alkyl bridging two NHC ligands
([Au2(L)X2] where X = Cl, Br and L = bridged bis(NHC))
have been reported in the literature.13 However, the synthetic
procedures leading to their formation present several draw-
backs: (a) the use of a carbene transfer agent via trans-
metalation between bis(Ag-NHC) and a gold precursor,
typically [Au(DMS)Cl] (DMS = dimethyl sulfide), generating
silver waste, (b) the treatment of the highly sensitive
methylene analogue complex [Au2(L)Me2] with a strong
acid (e.g., HCl),13b and (c) the use of a lithium halide additive
(LiBr).13a While developing stable bis(NHC)-modified gold
nanoparticles,14 Crudden and co-workers demonstrated a more
viable methodology using a weak base approach which is
closely related to our previous work on accessing gold-,15
copper-,16 and palladium-based7a,b NHC complexes. Never-
theless, the described protocol was limited to a few examples
and was not optimized. In the pursuit of developing a new
architecture for dinuclear gold(I) complexes bearing alkyl-
bridged bis(NHC) ligands and testing their efficiency in
catalysis, we now report on the straightforward synthesis of
new [Au2(L)X2] complexes. The activity of the synthesized
complexes was investigated in the carboxylative cyclization of
propargylamine. This reaction is of great interest to the
scientific community, as it involves the insertion and fixation of
CO2, which is an abundant greenhouse gas, is inexpensive, and
is a readily available C1 feedstock.17 The use of CO2 as a C1
building block has attracted much attention in recent
years,17,18 as it is one of the most promising alternatives to
phosgene (in certain applications), which is highly toxic.19
During the past decade, several reports dealing with the
capture of CO2 by propargylamines to access valuable
heterocycles have been described using various catalytic
systems.9,20 Studies by Ikariya and co-workers9 have shown
that NHC−mononuclear gold(I) complexes were very
effective catalysts in the carboxylative cyclization of propargyl-
amines. Detailed computational studies of the reaction
mechanism have been reported. The overall mechanism
shown in Scheme 1 below is adapted from the work of Yuan
and Lin.21
complexes experimentally are essential to understand and
design an improved catalyst. This is particularly critical for the
design of dinuclear complexes which are derived from
mononuclear species, for which a mechanistic understanding
is essential. Therefore, in this article we present the first steps
in work aimed at more complete delineation of the catalytic
mechanism for both mononuclear and dinuclear complex
systems.
Seminal work on the mechanism of gold(I)-catalyzed
hydroalkoxylation of alkynes reported by Zhdanko and
Maier22 has highlighted the importance of determining the
resting state of the catalyst and, where possible, isolating and
determining isolated rates of the reaction. The premise that the
catalytic cycle starts as shown in Scheme 1 with binding of
PPA to 9 may be valid in silico but does not correspond to the
experimental conditions, where the catalytic process is itself
much slower than carbonylation of PPA. Even if CO2 is added
after mixing of all reagents, the rate of reaction of CO2 and
PPA producing a mixture of CA and CS in an equilibrium
mixture is much more rapid than the rate of catalysis. In
keeping with the results of Kortunov and co-workers,23 the
concentration of free PPA can be taken as negligible at room
temperature under a CO2 pressure of 1 atm or more. It is also
surprising that the potential role of carbamate complexes in
this catalytic cycle has not received more attention. Toste,
Bergman, and co-workers have shown that [Au(IPr)NR1R2]
rapidly adds CO2 to produce a [Au(IPr)O(OC)NR1R2]
complex,24 and Calderazzo and co-workers25 have determined
the crystal structure of [Au(PPh3)O(OC)NR1R2]. While
stable intermediate complexes of alkynes have been isolated in
several cases and also structurally characterized, these are
always with “noncoordinating” counterions.26 There are a wide
range of carbamate complexes for many metals indicating that
the counterion is not a “noncoordinating” anion.27
A breakthrough achievement in this mechanism was the
isolation and structural characterization, by the Ikariya group,9a
of the IPrAu-vinyl complex labeled IKa in Scheme 1. This was
also shown to be a key intermediate in the catalytic cycle by
both its cleavage by acid to products and its use as a precatalyst
capable of achieving the same rate of reaction as for 9. Key
unanswered questions in the mechanism of this reaction
include why the empirical rate law dP/dt = k [9][PPA]0.75 is
followed, as well as the identity of the resting state of the
catalyst. This experimentally derived reaction order is most
likely the average dependence on temperature of a complex
rate law comprised of a more complex set of elementary steps.
Elucidation of those elementary steps experimentally as well as
identification of the possible resting state and intermediate
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Dinuclear Gold(I)
Complexes. As shown in Scheme 2, the reaction of the ligand
(L·2HX) with [Au(DMS)Cl] (DMS = dimethyl sulfide) in the
presence of K2CO3 at 60 °C in acetone afforded the desired
complexes [Au2(L)X2] (1−8) in 40−120 min. The products
B
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ppm), highlighting a trans effect induced by the halogen
ligands. This is in accordance with previous observations on
the trans effect of halogen anionic ligands.29 This effect is still
relatively smaller than that brought on by the N substitution as
we clearly see when comparing the relative 13C chemical shifts
of 2 (173 ppm) vs 1 (176 ppm) and 2 (173 ppm) vs 6 (166
ppm).
Scheme 2. Synthesis of Dinuclear Gold(I) Complexes
In addition, NMR analysis indicated that both gold centers
have the same coordination environment, supporting a
symmetrical coordination motif. Single-crystal X-ray diffraction
analysis confirmed the expected structures of the dinuclear
gold(I) complexes. Single crystals of complexes 1 and 3 were
grown by slow diffusion of n-hexane into a saturated
chloroform solution of the corresponding complex, and those
for complexes 5 and 6 were grown by slow diffusion of n-
pentane into a dichloromethane solution of the corresponding
complex (Figure 1). Selected bond lengths and angles are
given in Table 1.30 The structural parameters of the bromide
complex exhibit only a slight difference in comparison to the
chloride analogues. As shown in Figure 1, the geometry at each
gold center of the gold(I) dinuclear complexes is linear with
Ccarbene−Au−X angles close to 180°: complex 1, 177.6(4)° for
Ccarbene−Au−Br1 and 174.0(3)° for Ccarbene−Au−Br2, which is
in good agreement with the analogue of the gold(I) dinuclear
bromide complex reported in the literature;31 complex 3,
177.95(9)° for Ccarbene−Au1−Cl1; complex 5, 178.28(8)° for
both gold centers Ccarbene−Au1−Cl1; complex 6, 178.84(8)°
for Ccarbene−Au1−Cl1 for both gold centers. The Au1−Cl1
bond lengths in complexes 3, 5, and 6 are 2.2831(9),
2.2848(8), and 2.2859(7) Å, respectively, which are metrical
parameters comparable to those reported in the literature for
mononuclear gold−NHC chloride complexes.32
were isolated as off-white solids in good yields (50−84%). The
identity and composition of the complexes were confirmed by
NMR spectroscopy and elemental analysis and unambiguously
determined via single-crystal X-ray diffraction analysis. All of
these complexes are stable toward air and moisture. In the 13
C
NMR spectra, the carbene signals of 1−5 were found in the
range of 171−176 ppm and those of 6−8 appeared at 166, 171,
and 168 ppm, respectively, which are resonances analogous to
those of previously reported dinuclear complexes13a as well as
to the mononuclear gold complexes [Au(NHC)Cl].15,28
By a comparison of the structural features of complexes 1
and 2, which only differ in their anionic ligand Br vs Cl, their
1H NMR spectra are very similar, although a slight difference
was observed in the 13C NMR chemical shifts of the
corresponding carbenes. A more downfield shift of the carbene
signal was noticed for 1, bearing a bromide ligand (176.2
ppm), in comparison to 2, which bears a chloride ligand (172.7
Furthermore, the Au1−Br1 bond length of 2.4 Å is in
accordance with the gold(I) dinuclear bromide analogue
reported.31 The Au−Ccarbene bond lengths are in agreement
with those reported in the literature for mononuclear
[Au(NHC)Cl]32 and [Au(IPr)Br]33 and for the dinuclear
gold(I) dinuclear bromide complexes.13a,31 The purity of all
Figure 1. Ball and stick representations of complexes 1, 3, 5, and 6 with hydrogen atoms omitted for clarity.
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Table 1. Selected Bond Distances (Å) and Angles (deg) for X-ray Structures of 1, 3, 5, and 6
1
3
5
6
Au−X
2.4088(15) (Au1−Br1)
2.4021(14) (Au2−Br2)
1.998(13) (Au1−C1)
1.979(13) (Au2−C24)
177.6(4) (C1−Au1−Br1)
174.0(3) (C24−Au2−Br2)
2.2831(9) (Au1−Cl1)
2.2848(8) (Au1−Cl1)
2.2859(7) (Au1−Cl1)
Au−Ccarbene
Ccarbene−Au−X
1.989(4) (Au1−C1)
1.979(3) (Au1−C1)
1.992(3) (Au1−C1)
177.95(9) (C1−Au1−Cl1)
178.28(8) (C1−Au1−Cl1)
178.84(8) (C1−Au1−Cl1)
the synthesized complexes was confirmed by elemental
analysis.
Carboxylative Cyclization of a Propargylamine. The
catalytic activity of the dinuclear gold complexes 1−8 was
tested in a manner analogous to that used by Ikariya and co-
workers in the carboxylative cyclization of propargylamine.9a
Data in Table 2 summarize results for the activity of 2 with
leads to a reduction to an 11% reduction in yield to 79% from
90% for entries 2 and 3. This reaction was performed, however,
at a higher absolute concentration of PPA2.50 M versus 1.25
M for all of the other reactions. As discussed later, more
concentrated solutions may lead to reduction in the activity,
possibly due to inhibition by the oxazolidinone product 11.
Entries 5−8 probed the role of solvent on the catalytic
activity of the linker complex. There was a modest reduction in
conversion in going from MeOH (Table 2, entry 3) to EtOH
(Table 2, entry 5) and a further reduction in i-PrOH (Table 2,
entry 6). Somewhat surprisingly, some activity was observed in
H2O (Table 2, entry 7) but little activity in DMSO (Table 2,
entry 8). This highlights the importance of H bonds with the
solvent.21 Entries 9 and 10 show that 2 gave yields of 87% at
room temperature over 15 h but the yield approached 99%
when that time was extended to 24 h. This would appear to
show little beneficial effect of an increase in temperature on the
rate by complex 2, since the change in yield from 90% to 87%
is in the range of experimental error. Moreover, the activity of
9 after 24 h at room temperature was significantly less, with a
yield of only 75% in comparison to 2 (Table 2, entry 10 vs 11).
The comparisons between 2 and 9 and the solvent studies in
Table 3 were extended in order to probe the role of the linker
structure and its effect on yield. All reactions were carried out
for 15 h to allow comparison to 9. These data are summarized
in Table 3. Lower yields in comparison to 9 were obtained
Table 2. [Au]-Catalyzed Carboxylative Cyclization of PPA
a
10
entry
[Au] precatalyst (mol %)
solvent
temp (°C) yield (%)
1
2
3
9 (2)
2 (2)
2 (1)
2 (0.5)
2 (1)
2 (1)
2 (1)
2 (1)
2 (1)
2 (1)
9 (2)
MeOH
MeOH
MeOH
MeOH
EtOH
i-PrOH
H2O
DMSO
MeOH
MeOH
MeOH
40
40
40
40
40
40
40
40
rt
81
90
90
79
82
62
45
4
b
4
5
6
7
8
9
87
∼99
75
c
10
11
rt
rt
c
Table 3. [Au]-Catalyzed Carboxylative Cyclization of PPA
a
a
Conditions unless specified otherwise: reactions carried out with
10
PPA 10 (0.5 mmol) in MeOH (0.4 mL) under 1 atm of CO2 for 15 h.
The mol % of the catalyst used is relative to PPA 10. Yields were
determined by 1H NMR using 1,3,5-trimethoxybenzene as an internal
b
c
standard. 0.2 mL of MeOH used. Reaction time 24 h.
CO2 in the carboxylative cyclization of N-benzylbut-2-yn-1-
amine (PPA 10) to afford the desired 3-benzyl-5-vinyl-
oxazolidin-2-one (11).34 Reactions were carried out in
methanol at 40 °C under a CO2 atmosphere (CO2 balloon,
1 atm) using various catalyst loadings. As was reported earlier
for 9, only one stereoisomer is obtained (Z configuration at the
double bond), as confirmed by NMR analysis.9a,35
Entries 1−4 of Table 2, referring to reactions done in
MeOH at 40 °C, provide comparisons of the performance of
the catalyst between 2 and 9 as a function of the substrate to
catalyst ratio. Entry 1 shows an 81% conversion rate in 15 h for
9, which is compared to the literature value of 83% under
comparable conditions. The data for 2 appear to show a
somewhat higher conversion in this time period of 90% for
both 2 and 1 mol % (Table 2, entries 2 and 3). It should be
noted that, since 2 contains two Au centers per mole, entry 3
provides the best comparison to entry 1, since it reflects the
activity per mole of Au. Entry 4 warrants additional comment.
It appears to show that reduction to 0.5 mol % of the catalyst
c
entry
Au catalyst
yield (%)
1
2
3
4
5
6
7
8
1 {IPr′, 8, Br}
2 {IPr′, 8, Cl}
3 {IPr′, 6, Cl}
4 {IPr′, 4, Cl}
5 {IMes′, 8, Cl}
6 {IAd′, 8, Cl}
7 {IMe′, 8, Br}
8 {IMe′, 8, Cl}
9 {Au(IPr)Cl}
87
87
79
76
47
62
47
47
63
b
9
a
Reaction conditions unless specified otherwise: Reactions were
carried out with PPA 10 (0.5 mmol, 1 equiv) and a catalyst loading of
1 mol % (0.01 equiv) in methanol (0.4 mL) under a CO2 atmosphere
for 15 h at room temperature. The yields were determined by H
NMR using 1,3,5-trimethoxybenzene as an internal standard.
Catalyst loading of 2 mol % (0.02 equiv). In braces are given the
functional group of the ligand, the length of the carbon linker, and the
halide ligand, respectively.
1
b
c
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with complexes bearing an eight-carbon linker with the less
sterically crowded Me (Table 3, entries 7 and 8)- and Mes-
substituted ligands (Table 3, entry 5). The IAd′ functionalized
ligand was intermediate (Table 3, entry 6), and the IPr′-based
systems were found to be the most active in comparison to 9
and the other functionalized systems (Table 3, entries 1−4 vs
entries 5−9). There did not appear to be any effect of the
halide ion (Cl− or Br−). In terms of the length of the linker
chain, a comparison of entries 2−4 shows a small decrease in
activity for the shorter chains.
It was hoped that this work would reveal synergistic activity
between the two metal centers, in keeping with recent
literature reports for related linked systems.8b,36 In comparison
to the reactivity of 9, an increase in reactivity was observed for
2 in comparison to 9 as discussed in the previous section. The
overall yield and rate of the reaction depend on a number of
key steps in the reaction mechanism, as well as possible
deactivating steps such as formation of inactive complexes
which reduce the amount of Au that actively participates in
catalysis.
A number of factors contribute to the catalytic activity and
the lower performance of complexes 5−8 in comparison to 9.
Speculation regarding possible reasons for this is unproductive.
More complete kinetic data on specific steps in the catalytic
cycle may not only provide insight into the possible reasons for
this but also point the way to finding optimal conditions and
design to speed up this process. For that reason, the authors
have begun a more detailed probe of the factors involved in the
individual reaction steps in an attempt to determine possible
reasons for this on the basis of quantitative kinetic data.
FTIR Investigation of Initial Rates of Reaction for 2
and 9. The bulk yield studies described above give insight into
the catalytic activity over a fixed period of time. Additional
insight may be gained from studies of the initial rates of
reaction with respect to the reaction mechanism. This is a
critical factor in determining the maximum possible rate a
catalytic system might be capable of achieving in the absence
or removal of possible deactivating steps that may occur later
in the reaction over time. In addition to possible catalyst
deactivation, changes in the solution properties that may occur
as the reactant is converted into the product may alter the
kinetics of some of the steps as well.
It is well to keep in mind that PPA is a base which on
exposure to CO2 forms a mixture of CA and CS that is
temperature-, pressure-, and concentration-dependent.23 Dur-
ing the cyclization the N−H bond of PPA is converted to a C−
H bond in the oxazolidinone product. The oxazolidinone
product, in addition to being a weak base, contains an olefin
capable of coordinating to Au as well. The changes in the
solution itself during the catalytic process in terms of its acidity
and ionic activity are significant. This presents a rather unique
and challenging environment in comparison to processes such
as olefin hydrogenation or metathesis, where relatively little
change in the nature of the solution occurs as part and parcel
of the catalytic reaction.
Scheme 3. CA/CS Equilibrium
the Supporting Information. These data support that the ratio
of CS to CA, as expected, will increase with an increasing
concentration of PPA (see Scheme 3). The data are in
agreement with recently reported NMR studies of Kortunov
and co-workers.23
Reaction trajectories for the reaction between 9 or the linker
complex 2 with the equilibrium mixture of CS and CA
obtained after exposing PPA 10 to CO2 were studied and
compared using techniques described in the Supporting
Information. The primary target in this work was a
determination of the maximum initial rate dependence in an
attempt to find a practical model for the rate of the reaction.
Long-term behavior of the catalytic system as well as other
more detailed aspects of the reaction mechanism will be
discussed in a separate contribution.37 Data for the initial rates
of the reaction are shown in Figure 2.
Figure 2. Plot of [11] versus time for the first 400 min of the reaction
for 2 (right) and 9 (left). For tabulated data of rates versus
concentration see the Supporting Information.
In addition to a ca. 50% faster initial rate as shown in Figure
2, the advantage of using 2 over 9 is maintained throughout the
course of the reaction, as shown in Figure 3. While a direct
comparison of kinetic data at 21 °C cannot be directly
transferred to the yield data shown in Table 2 (entries 10 and
11), they are in accordance with the data in Figures 2 and 3 in
The starting point for these studies was the spectroscopic
investigation of the CA/CS equilibrium as a function of
substrate concentration. There is rapid uptake of CO2 by
EtOH solutions of PPA, and full equilibrium between CA and
CS is achieved in less than 10−15 min according to Scheme 3.
Of particular interest is the ratio of CA to CS, both of which
may play a role in the kinetics of reactions within the catalytic
cycle. Representative equilibrated spectra at 21 °C are given in
Figure 3. Comparison of rate for the first 10000 min of the reactions
of complexes 2 and 9 under the same concentration conditions (0.52
M in PPA 10, 0.0072 M in Au, at 21 °C, 2 atm of CO2 in EtOH).
E
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terms of both initial and long-term catalytic behavior in this
system.
Estimation of the Approach of the Turnover
Frequency (TOF) to a Limiting Value for 2 and 9 at 21
°C in EtOH. The variation of the initial rate of production of
11 as a function of the initial concentration of PPA 10 shown
in Figure 2 is nonlinear for both 9 and 2, as is shown in the
plot of turnover frequency (TOF) versus [PPA 10] in Figure 4.
Figure 5. Concentration of IKa as a function of time during the first
∼400 min of reaction.
The rate of achieving a steady state in IKa for 9 increases
with increasing initial concentration of PPA 10. Except for the
lowest PPA 10 initial concentration of 0.0536 M, which has an
induction period of ∼50 min, all other species approach a
nearly constant concentration of IKa within ∼25 min following
the ∼5 min period, while the fully dissolved solid catalyst
reduces this to about 20 min. There is a sharper rise in [IKa]
for more concentrated solutions. The more concentrated
solutions also display a slightly downward trend in [IKa]
immediately following the induction period. There is, however,
no observed decrease in the rate of production of 11, as can be
seen in Figure 2. The decrease is small, and other factors might
serve to compensate for this. A more detailed kinetic study of
the relationship of the [IKa] to product production is in
progress.
Since the total Au concentration in these studies ranges from
0.0072 0.0004 M in all cases, we can conclude that in this
reaction more than half of the Au is not present as IKa. The
approximate steady-state behavior in IKa concentration does
not remain for long periods of time. It should be kept in mind
that when [IKa] is approximately constant and yet the product
is being produced at a significant rate that it is being constantly
replenished, also at a more or less constant rate from a “resting
state” carbamate complex as discussed in the following section.
Diversion of Au out of the active loop, either by re-formation
of 9 as discussed below or by formation of other inert
complexes as has been noted by others,24,25 may occur over
time. In addition, the rate of decay of IKa concentration
appears to increase with an increase in the oxazolidinone 11
production and a mechanism of product inhibition may also be
at play in this system.
Computational Study of “Resting State” of Complex.
As discussed above, the mechanism of the carboxylative
cyclization of propargylamines using 9 as catalyst has been
previously examined via computational studies (see Scheme
1).21 However, in that study, a carbamate-bound species
similar to the [Au(PPh3)O(OC)NR1R2] complex previously
reported25 was not described and the amine-bound complex
present in the first step of the cycle (see Scheme 1) is not
consistent with the experimental conditions (vide supra).
Therefore, in the current work, several aspects of the
mechanism were reanalyzed by means of DFT calculations
(see the Supporting Information for computational details).
Calculations were performed using a truncated complex in
which the N-heterocyclic carbene ligand present in the Au
complexes was modeled with the simpler IMe. In addition, the
Figure 4. Plot of initial turnover frequency (min−1) versus initial
[PPA 10] for 2 (orange squares) and 9 (green circles). For a data
table and additional information see the Supporting Information.
The two curves appear to be approaching limiting rates. As
shown in the Supporting Information, plots of ln{TOFmax
−
TOF} versus [PPA 10] are linear with TOFmax = 0.06 min−1
for 2 and 0.0335 min−1 for 9. This corresponds to a turnover
time of ca. 17 min for 2 versus ca. 30 min for 9. These data
provide approximate upper boundary limits, and the value for 2
indicates that the reaction is approximately 1.8 times faster
than that when 9 is used under the stated reaction conditions.
These data are in agreement with the time period to reach
steady-state levels in [IKa] as discussed in the following
section. Work to further analyze these data in terms of the
fundamental steps of the catalytic cycle is in progress.
Near-Steady-State Concentration of IKa for 9 during
the First 400 min of Reaction. In addition to providing
kinetic data, the FTIR methods used in this study also allowed
quantitative detection of the key intermediate IKa (see Scheme
1). In order to determine the extinction coefficient of this
complex, we have prepared IKa in pure form following the
Ikariya procedure.9a The concentration may be followed with
reasonable accuracy for ∼40 turnovers, at which time
computer subtraction of the oxazolidinone product (which
has interfering bands) results in signal to noise problems. This
key intermediate provides further insight into the mechanism
and has been the subject of continued investigation.21 As
shown in Figure 5, there is an initial rise in [IKa], which may
be viewed as an induction period despite the fact that it occurs
concurrently with the production of product 11 as shown in
Figure 2. Following that, [IKa] either stays approximately
constant or changes only slowly with the reaction progress. An
average [IKa] can therefore be used to estimate rate constants
during this time with errors <10%.
The time to achieve a nearly constant concentration in IKa
and its approximate steady-state level depend on the starting
concentration of PPA 10 as shown in Figure 5.
F
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benzyl group present in PPA 10 was also replaced by a methyl
group for simplicity. A schematic Gibbs energy diagram for the
formation of the key intermediate IKa is shown in Figure 6.
extent on the way to the transition state and not to be a
“resting state” of the complex.
The second major irreversible step in the catalytic
carboxylative cyclization of propargylamines is protonation of
IKa, as shown in Scheme 4. The key to establishing levels of
Scheme 4. Possible Pathways for IKa Protonation
Figure 6. Gibbs free energy diagram computed at the PBE0-
D3(BJ)(PCM,CH3OH)/Def2-TZVP level of theory for IKa for-
mation. R = −CH2CCCH3.
The transformation of the alkyne-bound complex Int2 into the
IKa intermediate is strictly analogous to what was previously
reported by Lin and co-workers.21 However, the displacement
of chloride by the CS produced by reaction of CO2 with PPA
(see Scheme 3), formation of the carbamate-bound complex
Int1, and its successive isomerization to yield the active Int2
intermediate are proposed and computed in this work for the
first time. The optimized structure of Int1 in Figure 6 is in
reasonable agreement with the reported structure for [Au-
(PPh3)O(OC)NR1R2].25 Good agreement is also observed
among the optimized structures obtained for [Au(IMe)Cl],
Int2, TS2, and IKa and those previously reported by Lin and
co-workers.21
As can be seen in Figure 5, generation of IKa is achieved
during an induction period, as observed experimentally, in
which [Au(IMe)Cl] establishes an unfavorable equilibrium
yielding [Au(IMe)O(OC)N(CH3)R] (Int1). This first step
of the reaction is uphill by ∼7 kcal/mol in terms of Gibbs
energy at 298 K. However, due to the large excess of
carbamate, some population on the order of ∼2% for 1 M PPA
may be estimated (see the Supporting Information). In spite of
many attempts, the transition state for displacement of chloride
by the CS was not located. However, NMR studies at low
temperature (−60 °C) of associative ligand substitution on the
Au(I) complexes indicate that this equilibrium is approached
rapidly even at low temperatures.38
Isomerization of the carbamate-bound Int1 to the alkyne-
bound Int2, as shown in Figure 6, is computed to have
ΔG(298 K) = +10.2 kcal/mol, making the acetylene complex
+17.6 kcal/mol above [Au(IMe)Cl] at 298 K. This high value
is in keeping with the computational results of Lin.21 The
transition state computed for formation of the acetylene
complex is located 24.4 kcal/mol above 9 and 17.0 kcal/mol
above Int1. The computed barrier for the acetylene complex to
go forward to IKa is 6.4 kcal/mol. Accordingly, significant
accumulation of the acetylene complex Int2 would appear to
be disfavored relative to the carbamate.
IKa relatively rapidly involves exothermic oxazolidinone
generation by reaction of IKa, which may be accomplished
by any of the potent acids present in the media: the
ammonium chloride salt formed after ligand displacement of
the [Au(IMe)Cl] catalyst (Scheme 4, step A), the CA
(Scheme 4, step B) and the ammonium CS (Scheme 4, step
C). Thus, the H+ activity in solution involved in the acid
cleavage may be viewed as a slowly changing variable that
depends on all species present in solution. In all cases,
protonation of IKa is an exergonic process with ΔG°(298 K)
values of −17.1, −10.9, and −9.7 kcal/mol, respectively, for
steps A−C of Scheme 4, providing the driving force for the
entire transformation.
In step A of Scheme 4, the ammonium Cl− salt, which is the
most thermodynamically potent but less concentrated acid in
the reaction medium, would produce the oxazolidinone
product, PPA, and [Au(IMe)Cl]. This, in effect, would return
the catalyst to its thermodynamically most stable form and
would require an additional induction period, at a slower rate
(see Figure 6 and Scheme 4). In steps B and C of Scheme 4,
the proton sources are respectively the CA and the CS that
coexist in equilibrium (see Scheme 3). The advantage of these
two later steps is that, in addition to oxazolidinone formation,
they do not return the Au center to the most thermodynami-
cally stable complex in this system [Au(IMe)Cl], but to
[Au(IMe)O(OC)N(CH3)R] (Int1) with the benefit of ∼7
kcal/mol lower activation with respect to IKa formation, in
comparison to the transformation starting from [Au(IMe)Cl]
(see Figure 6). This is an exciting opportunity for catalyst
design which we are now exploring.
CONCLUSION
■
A straightforward procedure for the synthesis of several new
dinuclear gold(I) complexes bearing bridged bis(NHC)
ligands has been achieved in air and under mild conditions.
All complexes were investigated as catalysts in the carbox-
ylative cyclization of N-benzylbut-2-yn-1-amine (PPA 10). The
best activity at room temperature was obtained using
complexes bearing a more sterically hindered phenyl-based R
On the basis of computational work, the distribution of
stable Au species is thought to consist of 9, Int1, IKa, and
other minor species which may develop over time. The
acetylene complex is viewed as being populated to a minor
G
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Article
group (R = 2,6-diisopropylphenyl) in complexes 1−4 in
comparison to 9, with complexes 1 and 2 exhibiting the highest
activity, affording the desired 3-benzyl-5-vinyloxazolidin-2-one
(11) in excellent yields. By a comparison of the reaction
kinetics using 2 and 9, at higher concentrations 2 appears to be
faster by 1.8 times than 9. In addition, over longer periods, a
decrease in the rate of the reaction was observed with 9 in
comparison to 2, which indicates that the latter is less
susceptible to deactivation. Moreover, mechanistic and DFT
studies of a monogold model system revealed that the two rate-
limiting steps which may each contribute to the total time
needed to achieve one turnover are the formation of IKa from
either 9 or [Au(IPr)O(OC)NR1R2] and the catalyst
cleavage of IKa by acid, which may occur by CA, CS, or
PPA·HCl (Scheme 4). The induction period is proposed to
occur while the relatively unreactive 9 is converted to the
carbamate complex [Au(IPr)O(OC)NR1R2], in a thermo-
dynamically uphill reaction that is coupled with product
formation when IKa is cleaved by CA. The more potent but
less concentrated acid PPA·HCl may also react with IKa to
generate the product, but it re-forms 9 which must repeat the
induction period process if it is to re-form the carbamate gold
complex [Au(IPr)O(OC)NR1R2], which the authors believe
is a resting-state complex in this system. Additional
mechanistic studies are in progress to further understand the
activity and identify the possible intermediates for the linker
complex 2 in this reaction. Due to the importance of carbon
dioxide utilization as a source for chemical and pharmaceutical
feedstocks, experimental and computational work in this
direction are in progress in our laboratories.
Belgium; orcid.org/0000-0001-9024-2035;
Email: Steven.Nolan@UGent.be
Authors
Tahani A. C. A. Bayrakdar − Department of Chemistry and
Centre for Sustainable Chemistry, Ghent University, 9000
Ghent, Belgium; orcid.org/0000-0003-3406-0485
Fady Nahra − Department of Chemistry and Centre for
Sustainable Chemistry, Ghent University, 9000 Ghent, Belgium;
VITO (Flemish Institute for Technological Research),
Separation and Conversion Technology, B-2400 Mol, Belgium;
orcid.org/0000-0002-1115-9725
Jack V. Davis − Department of Chemistry, University of Miami,
Coral Gables, Florida 33146, United States
Mohan M. Gamage − Department of Chemistry, University of
Miami, Coral Gables, Florida 33146, United States
́
́
́
Marco Marazzi − Departamento de Quimica Analitica, Quimica
́
́
́
́
́
Fisica e Ingeniera Quimica; Instituto de Investigacion Quimica
"Andres M. del Rio" (IQAR), Universidad de Alcala,
Universidad de Alcala, 28805 Madrid, Spain
́
́
́
́
Marina Saab − Department of Chemistry and Centre for
Sustainable Chemistry, Ghent University, 9000 Ghent, Belgium
Kristof Van Hecke − Department of Chemistry and Centre for
Sustainable Chemistry, Ghent University, 9000 Ghent,
Belgium; orcid.org/0000-0002-2455-8856
Dominic Ormerod − VITO (Flemish Institute for
Technological Research), Separation and Conversion
Technology, B-2400 Mol, Belgium; orcid.org/0000-0001-
6305-3095
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00404
ASSOCIATED CONTENT
■
sı
Author Contributions
All authors have given approval to the final version of the
manuscript.
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00404.
Funding
Additional experimental data, synthetic procedures,
characterization of compounds, and DFT calculations
(PDF)
Financial support for this work was provided by VITO
(Flemish Institute for Technological Research), the Ministerio
de Economia y Competitividad, Spain (CTQ2016-80600-P),
the Department of Energy (DOE-BES-DE-SC0019456), the
Hercules Foundation (to K.V.H. project AUGE/11/029), and
VLAIO (SBO project CO2PERATE to S.P.N.).
́
Accession Codes
CCDC 1977714 and 1978031−1978034 contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/data_request/
cif, or by emailing data_request@ccdc.cam.ac.uk, or by
contacting The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the funding agencies noted for
support as well as the Special Research Fund (BOF) of Ghent
University (starting and project grants to S.P.N. and project
01N03217 to K.V.H. and M.S.). Umicore AG is acknowledged
for generous gifts of materials. Dedicated to the memory of
Professor Edwin D. Stevens.
AUTHOR INFORMATION
Corresponding Authors
Burjor Captain − Department of Chemistry, University of
Miami, Coral Gables, Florida 33146, United States;
orcid.org/0000-0002-7272-5655; Email: captain@
miami.edu
■
́
́
Manuel Temprado − Departamento de Quimica Analitica,
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(or from the Cambridge Crystallographic Data Centre, 12, Union
J
https://dx.doi.org/10.1021/acs.organomet.0c00404
Organometallics XXXX, XXX, XXX−XXX