Kinetics and Mechanism of Isocyanide-Promoted Carbene Insertion into the Aryl Substituent of an N-Heterocyclic Carbene Ligand in Ruthenium-Based Metathesis Catalysts

Article abstract of DOI:10.1021/acs.organomet.7b00342

In situ IR spectroscopy was used to study the kinetics of addition of L = alkyl and aryl isocyanides to the Grubbs second-generation carbene complex Ru(H₂IMes)(CHPh)(PCy₃)Cl₂ (H₂IMes = 1,3-dimesityl-4,5-dihydroi

Full text of DOI:10.1021/acs.organomet.7b00342

Article
pubs.acs.org/Organometallics
Kinetics and Mechanism of Isocyanide-Promoted Carbene Insertion
into the Aryl Substituent of an N‑Heterocyclic Carbene Ligand in
Ruthenium-Based Metathesis Catalysts
Justin R. Griffiths, Elan J. Hofman, Jerome B. Keister,* and Steven T. Diver*
Department of Chemistry, University at Buffalo, the State University of New York, Buffalo, New York 14260-3000, United States
S
* Supporting Information
ABSTRACT: In situ IR spectroscopy was used to study the kinetics
of addition of L = alkyl and aryl isocyanides to the Grubbs second-
generation carbene complex Ru(H2IMes)(CHPh)(PCy3)Cl2
(H2IMes = 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene), which
triggers carbene insertion into an aromatic ring of the N-heterocyclic
carbene supporting ligand, forming Ru{1-mesityl-3-(7′-Ph-2′,4′,6′-
trimethylcycloheptatrienyl)-4,5-dihydroimidazol-2-ylidene}-
L2(PCy3)Cl2. The rate law was determined to be first order in
isocyanide concentration and first order in carbene complex concen-
tration. For various isocyanides CNR the rate increases as R = tert-
butyl ≪ cyclohexyl < n-octyl < CH2Ph ≈ CH2CO2Me ≈ CH2SO2-
C6H4-4-Me < C6H4-4-OMe < C6H4-4-Cl. The proposed mechanism involves reversible addition of isocyanide followed by
rate-determining, irreversible carbene insertion and subsequent, rapid addition of the second isocyanide. The carbene insertion
is accelerated by the electrophilicity of the carbene, which is enhanced due to ligand binding by isocyanides with lower σ-donor/
π-acceptor ratios.
The close proximity of the aromatic ring and the electrophilic
carbene could lead to unwanted reactions between propagating
metal alkylidenes and the aromatic ligand. For instance, the
Grubbs group has identified a benzylic CH insertion product as
a minor byproduct in the synthesis of complex 3.10 Interaction
between the electrophilic carbene and the aromatic ring of the
N-heterocyclic carbene supporting ligand could lead to carbene
insertion into the aromatic ring system. A ligand-promoted
insertion of a carbene into an aromatic tetraaza macrocycle was
reported previously by Floriani et al.11
We previously reported a novel carbene insertion reaction
involving the Grubbs’ second-generation complex 3, which is
initiated by coordination of carbon monoxide or isocyanide
ligands (Scheme 1a).12 The full paper12b described a variety of
different ruthenium carbene complexes such as 2−4, which
undergo the ligand-promoted Buchner reaction, and studied
the ability of different added ligands to trigger the reaction.
The earliest work studied the effect of carbon monoxide, but
our more comprehensive study described other ligands that
gave this unique chemistry, including phosphites and iso-
cyanides. The resulting coordination complexes were structur-
ally characterized. On binding, we speculated that the π-acceptor
ligand dramatically changed the electronic properties of the metal,
causing the carbene to become electrophilic. A mechanism was
proposed,12a and subsequent DFT investigation by Cavallo13
lent support to the proposed mechanism and found an
INTRODUCTION
■
The development of functional group tolerant ruthenium-based
alkene metathesis catalysts (Figure 1) has led to numerous
applications in organic and materials synthesis.1 In 1999, the
Grubbs group reported the “second-generation” ruthenium
carbene complex 3, bearing an N-heterocyclic carbene (NHC)
ligand, 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene (H2IMes).2
Due to the special properties of the NHC ligand, complex 3
was found to be a superior catalyst for alkene metathesis in
comparison to its predecessor, benzylidene 1.3 Use of 3 in
organic synthesis applications led to an expanded scope of
both the alkene and ene-yne metathesis reactions. Recently,
phosphine-free catalysts 4,4 5,5 and 66 have been developed,
which has provided a wider repertoire of reactive alkenes in
cross-metathesis applications. Catalysts 3 and 4 are the most
widely used ruthenium carbenes in organic synthesis appli-
cations and are commercially available.
Generally, the N-heterocyclic carbene ligand of the cata-
lysts 2−6 plays an important role as a supporting ligand in
metathesis reactions.7 The N-heterocyclic carbene is stabilizing
and electron-donating. The strong σ-donating properties of the
N-heterocyclic carbene contribute electron density to the metal,
which serves to stabilize the reactive intermediates in alkene
metathesis. In the crystal structure of the H2IMes-liganded
ruthenium carbene, one mesityl group drapes over the car-
bene.8 Molecular orbital interactions between the carbene frag-
ment and the aromatic ring are evident in the second-gener-
ation Grubbs complex and are strengthened on binding of
a strong π-acceptor ligand, such as CO or an isocyanide.9
Received: May 3, 2017
© XXXX American Chemical Society
A
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Figure 1. Grubbs’ ruthenium carbene complexes 1−6.
Scheme 1. Buchner Carbene Insertion Promoted by Carbon Monoxide or Isocyanides12,18
unexpected intermediate. Though the Buchner reaction was
found to be very fast for CO and isocyanides, no detailed
kinetic data were previously obtained for the ligand-promoted
Buchner reaction.
carbene reactivity and may provide a decomposition route
for ruthenium carbenes during alkene metathesis. Ethylene is
produced during the alkene metathesis of 1-alkenes and is
frequently used to promote difficult ene-yne metatheses using 3
and other carbenes.17 Despite the fact that ethylene has donor−
acceptor properties similar to those of CO, it does not induce
the Buchner insertion. Do the steric properties of the π-acceptor
ligand influence its ability to promote the Buchner insertion?
On the basis of these considerations, we were interested
in studying the rate of the Buchner reaction using different
isocyanide π-acceptor ligands. Since the ligand properties of the
isocyanide are key in flipping the carbene reactivity, we focused
our investigation on the isocyanide ligands. We wanted to
evaluate the key ligand attributes which affect the rate of this
process. In this contribution, we report a study of the kinetics of
the insertion and propose a mechanism for the net process
shown in Scheme 1b.
The Buchner reaction has emerged as a useful tool to stop
metathesis reactions and to aid in the removal of the
organometallic product. The second-generation Grubbs com-
plex 3 may still be present at the conclusion of a metathesis
reaction, and other ruthenium carbenes may be present as well.
It is useful to quench these intermediates so that undesired
reactions (e.g., polymerization) do not occur during product
isolation. The addition of a ligand such as an isocyanide leads to
the Buchner reaction, rapidly destroying metathesis-active
carbene species. As such, the ligand-promoted Buchner reaction
provides a useful method to quench metathesis activity and to
remove ruthenium from the product mixture when the pro-
moting ligand is a polar potassium isocyanoacetate14 or a silica
gel supported isocyanide.15 Grela and co-workers found that
amine-containing isocyanide ligands are also useful as reagents
for the removal of ruthenium.16 Though it is known to be fast
on a practical level, is the quench fast enough to arrest reactive
metal carbene intermediates?
RESULTS
■
In situ IR spectroscopy was used to monitor the net Buchner
reaction via the relatively strong NC stretches in the
2300−1900 cm−1 region. Treatment of a solution of complex 3
with 2.0 equiv of 4-chlorophenyl isocyanide resulted in a
rapid color change from red to light orange-yellow due to 7a.
The ligand-promoted Buchner reaction is also important
because it both provides insight into a fundamental aspect of
B
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Figure 2. IR spectra during reaction of 4-chlorophenyl isocyanide (0.02 M) with 3 (0.01 M), at 0 °C.
Figure 2 shows the IR spectral changes during the reaction.
Disappearance of the absorption at 2129 cm−1 due to free
isocyanide is accompanied by increases in absorptions at 2060
and 2005 cm−1 due to the Buchner insertion product. No peaks
attributable to intermediates are observed. The isosbestic
point at 2124 cm−1 indicates that only reactants and a single
product are present at any time.
where x is the concentration of isocyanide consumed at time t,
A0 is the concentration of isocyanide at time 0, and B0 is the
concentration of complex 3 at time 0. The linearity (Figure 4)
Since we are monitoring the reaction via the change in
absorbance of the isocyanide reactant, pseudo-first-order
isocyanide conditions (large excess of isocyanide) cannot be
used. Therefore, data analysis was done at a stoichiometric 2:1
molar ratio of isocyanide to carbene complex. Because of the
2:1 reaction stoichiometry, the disappearance of the isocyanide
is twice as fast as the decrease in concentration of 3. Under
stoichiometric conditions, plots of [CNR]−1 vs time are linear
(Figure 3), indicating that the reaction is second order overall,
Figure 4. First-order plot for 4-chlorophenyl isocyanide with 3 at 0 °C,
0.01 M isocyanide, and 0.01 M 3.
indicates that the rate law is first order in complex 3 as well
as isocyanide. At 0 °C the second-order rate constant is
2.8(0.3) M−1 s−1.
The rate was unaffected by the presence of excess PCy3.
At 0.1 M PCy3, 0.01 M 3, and 0.02 M 4-chlorophenyl isocyanide
the second-order rate constant was 2.7 M−1 s−1, the same as that
above within experimental error. This is as expected from the
kinetics of dissociative phosphine exchange for 3 and other com-
plexes, previously reported by Grubbs et al.3a On the basis of
their data, the extrapolated initial rate of PCy3 dissociation (0.01 M
◦
Ru complex) at 0 C is 1 × 10−8 M s−1, in comparison to the
rate of 5.6 × 10−4 M s−1 for this Buchner reaction. Therefore,
phosphine dissociation does not precede the Buchner reaction.
Activation parameters were determined from measurements
at temperatures from 0 to −30 °C. An Eyring plot yielded ΔH⧧
as +34.6(2.9) kJ/mol (+8.27 kcal/mol) and ΔS⧧ as −109(11)
J/(K mol) (−26.1 eu). The negative entropy of activation is as
expected for a second-order process. In contrast, ΔS⧧ for PCy3
dissociation from 3 is +54 J/(K mol) (+13 eu).3a The observed
activation parameters support an associative process as the rate-
limiting step.
Figure 3. Second-order plot for 4-chlorophenyl isocyanide with 3 at
0 °C, 0.02 M isocyanide, and 0.01 M 3.
either first order in [CNR] and first order in ruthenium carbene
or second order in [CNR].
To establish the order in [3], a 1:1 molar ratio was used.
In this case, the integrated rate equation for a rate law first
order in each reactant is given by
Effects of Isocyanide Substituents. Having determined
the rate law for 4-chlorophenyl isocyanide, we next wanted
to compare the rates for various isocyanide substituents.
⎧
⎨
⎩
⎫
⎬
⎭
⎧
⎨
⎩
⎫
⎬
⎭
(2B0 − x)
2B0
ln
= (2B0 − A0)kt + ln
(A − x)
A
0
0
C
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Scheme 2. Buchner Insertion Involving the Hoveyda Complex 412b
Figure 5. Cyclohexyl isocyanide promoted Buchner reaction of 3, shown as absorbance vs time and tracked using in situ IR. IR bands: cyclohexyl
isocyanide at 2145 cm−1, intermediate 7g at 2100 cm−1, and product 8g at 2185 cm−1. Conditions: 0.01 M 3, 0.2 M CyNC, CH2Cl2, 25 °C.
Isocyanides are useful for quenching metathesis reactions,
and knowledge of relative rates will guide in the choice of
quenching ligand. Reactions of 3 with 4-methoxyphenyl iso-
cyanide, toluenesulfonylmethyl isocyanide (TosMIC), methyl
isocyanoacetate, benzyl isocyanide, n-octyl isocyanide, cyclo-
hexyl isocyanide, and tert-butyl isocyanide were kinetically
evaluated. For all isocyanides the initial product of the reaction
is the bis(isocyanide) Buchner insertion product 7 (Scheme 1b).
At 0 °C with alkyl isocyanides, or with aryl isocyanides at all
temperatures, 7 is the only product observed. However, for
slower Buchner reactions or at higher reaction temperatures,
additional ligand substitution by unreacted isocyanide occurs.
This converts the initially formed bis(isocyanide) product 7
into the tris(isocyanide) complex 8 and releases free PCy3.
Tris(isocyanide) Buchner products have been previously
prepared by reaction of Hoveyda−Blechert carbene 4 with 3
equiv of 4-chlorophenyl isocyanide (Scheme 2).12b The pro-
duct exists as a mixture of conformational (cycloheptatrienyl
ring) isomers, and the coordination geometry has been estab-
lished by X-ray crystallography. A third isocyanide ligand
displaced the coordinating ether ligand present in the chelate 4.
For alkyl isocyanides, the initial bis(isocyanide) intermediate
7 can be observed. For cyclohexyl isocyanide, the disappearance
of free isocyanide was accompanied by the appearance of two
new absorptions at 2062 and 2100 cm−1, due to the expected
bis(isocyanide) Buchner insertion product. However, this com-
plex is unstable and, with time, the tris(isocyanide) complex 8
is formed and free tricyclohexylphosphine is observed in the 31P
NMR spectrum of the product mixture. We were unable to
isolate and fully characterize the bis(isocyanide) complex, and a
low yield of the tris(isocyanide) was obtained at the 2:1 (CNR
to Ru complex) stoichiometry used in the kinetic experiments.
Figure 5 shows the change in absorbances at 2145 cm−1 due to
cyclohexyl isocyanide, at 2100 cm−1 due to the bis(isocyanide)
Buchner insertion product 7g, and at 2185 cm−1 due to the
tris(isocyanide) complex 8g (3:1 isocyanide to Ru molar ratio)
during a reaction at 25 °C. Similar results were obtained for
n-octyl isocyanide (data not shown).
Ligand substitution of isocyanide for PCy3 prior to the
Buchner insertion must also be considered as a possible mech-
anism for formation of 8. Grubbs et al. previously reported the
kinetics of dissociative phosphine exchange for 3 and other
complexes.3a On the basis of their data, the extrapolated initial
rate of PCy3 dissociation (0.01 M Ru complex) at 25 °C is
7 × 10−7 M s−1 and at 0 °C, 1 × 10−8 M s−1. The slowest
measured isocyanide-promoted Buchner reactions had initial
rates of 2.4 × 10−5 M s−1 (cyclohexyl isocyanide) at 25 ◦C and
8.2 × 10−6 M s−1 (n-octyl isocyanide) at 0 °C. Therefore, in all
these cases the initial rate of the Buchner reaction is at least
10 times faster than the limiting rate for PCy3 dissociation. This
is in accord with the kinetic data above in the presence and
absence of added PCy3. However, higher temperatures should
favor the dissociative PCy3 process over the associative iso-
cyanide-promoted reaction, and so there may be conditions
under which the rate of Buchner reaction with 3 will be slower
than isocyanide substitution for PCy3; in such a case we expect
the reaction with a second equivalent of isocyanide would
generate a Buchner insertion, to be followed by coordination of
a third isocyanide to give the same tris(isocyanide) product.
D
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factor into the faster rate of the Buchner reaction; on the basis
of activation parameters reported by Grubbs et al.,3a the rate
constant of 1.5 × 10−10 s−1 at −78 °C for PPh3 dissociative
substitution would predict a half-life of 8 × 107 min. Clearly,
the rate of isocyanide-promoted Buchner insertion is faster than
that of PPh3 ligand dissociation.
Table 1. Rate Constants for Reaction of 3 with Isocyanides
CNR
isocyanide
CNC6H4-4-Cl
rate constant (M−1 s−1
)
temp (°C)
2.8(0.3)
0.0
−10.0
−20.0
−30.0
0.0
1.18(0.10)
0.82(0.15)
0.37(0.07)
1.42(0.12)
0.67(0.03)
0.63(0.08)
0.57(0.04)
0.041(0.001)
0.381(0.006)
0.121(0.005)
DISCUSSION
■
CNC6H4-4-OMe
CNCH2SO2C6H4-4-Me
CNCH2CO2Me
CNCH2Ph
Mechanism of Ligand-Induced Buchner Insertion.
Metal-catalyzed diazoalkane decompositions in the presence
of arenes and alkenes have shown synthetic utility for the prep-
aration of both cycloheptatrienes and cyclopropanes.19 Metal
carbenoid additions to carbon−carbon π bonds typically
involve very electrophilic carbenes. Ruthenium carbenes used
in alkene metathesis are much less electrophilic and typically do
not form cyclopropanation products.
0.0
0.0
0.0
CN(CH2)7CH3
0.0
25.0
25.0
CN-cyclohexyl
After establishing that the additions of alkyl isocyanides cause
the same Buchner insertions in all cases, we determined the
rates (Table 1). For those reactions which gave a mixture of
bis- and tris(isocyanide) products, we used the initial rates
(where the contribution of the ligand substitution was small)
and assumed the same rate law to determine rate constants.
At 0 °C, the rate constants decreased in the order 4-chlorophenyl
isocyanide (2.8(0.2) M−1 s−1) > 4-methoxyphenyl isocyanide
(1.42(0.12) M−1 s−1) > TsCH2NC (0.67(0.08) M−1 s−1) ≈
methyl isocyanoacetate (0.63(0.08) M−1 s−1) ≈ benzyl isocyanide
(0.57(0.04) M−1 s−1) > n-octyl isocyanide (0.041(0.001) M−1 s−1).
At this temperature more hindered alkyl isocyanides reacted
too slowly for convenient kinetic analysis; therefore, we used
25 °C for comparison. At 25 °C, the rate constants decreased as
n-octyl isocyanide (0.381(0.006) M−1 s−1) > cyclohexyl
isocyanide (0.121(0.005) M−1 s−1) > tert-butyl isocyanide
(slow). Clearly, both steric and electronic properties affect the
rate of reaction. The rate is accelerated for electron-with-
drawing substituents and retarded by increasing steric bulk.
Effect of Variation in PR3 for Grubbs’ Complexes.
In order to determine whether the phosphine ligand influences
carbene insertion, the triphenylphosphine analogue of 3,
Ru(H2IMes)(CHPh)(PPh3)Cl2, was studied for comparison.
Triphenylphosphine is a weaker σ donor than the more elec-
tron rich Cy3P present in Grubbs complex 3. The reaction
with 4-chlorophenyl isocyanide was too fast to be measured
(less than 1 min) even at −78 °C. This result does not address
the question of steric vs electronic differentiation, since PPh3 is
both smaller and less electron donating than PCy3. Although
PPh3 dissociation is faster than PCy3 dissociation, this does not
The mechanism of the ligand-induced Buchner reaction
observed in the Grubbs second-generation ligand environment
is believed to follow a cyclopropanation pathway due to an elec-
trophilic carbene. DFT calculations have been used to probe
the energetics of CO-induced Buchner insertion. Cavallo et al.
calculated the energies of intermediates in CO-promoted Buchner
insertion for Ru(H2IMes)Cl2(CH2)L (L = CO, PMe3).13 They
also calculated free energies for coordination of L1 = CO,
PMe3 and other Lewis bases to Ru(H2IMes)Cl2(CH2)L2
(L2 = PMe3, py, PF3, CO). For example, ΔG for coordination to
Ru(H2IMes)Cl2(CH2)(PMe3) ranges from −17.7 kcal/mol
for methyl isocyanide to +3.6 kcal/mol for PMe3. For
phosphine binding, steric effects obviously would be much
more significant for the PCy3 analogue, which would result in
less favorable coordination free energies. Cavallo’s DFT
calculations suggest that the binding of the π-acceptor ligand
occurs at the open apical position, trans to the benzylidene
moiety. The calculations also led to the proposal that the role of
the π acid is not due to lowering the energy barrier for carbene
attack on the mesityl ring but instead serves to stabilize the
metallacycle 13 (Scheme 3) prior to opening to give the
norcaradienyl ring expansion product.
Comparisons of rates for Buchner insertions of 3 induced by
reactions with isosteric 4-chlorophenyl and 4-methoxyphenyl
isocyanides and nearly isoelectronic octyl and cyclohexyl iso-
cyanides demonstrate that both electronic and steric properties
of the isocyanide affect the rate. The origin of the steric effect
is obvious from the crystal structure8 of 3, as the vacant
coordination site is highly congested, sandwiched between a
a
Scheme 3. Proposed Mechanism for CNR-Promoted Buchner Insertion
a
Intermediate 13 was proposed by Cavallo et al. on the basis of DFT calculations.13
E
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mesityl group and a cyclohexyl group up to a distance of ca. 7 Å
from the ruthenium atom. The minimum distance between a
methyl hydrogen of the mesityl and the 4-methylene hydrogen
of the nearest cyclohexyl group is 2.4 Å, a distance within
van der Waals contact. Accommodation of an isocyanide
requires these groups to separate such that, in the crystal
structure of Buchner product 7a,12b the mesityl methyl group is
3.24 Å from the closest atom of the 4-chlorophenyl group.
Clearly the apical coordination site is constrained and most
accessible to slender ligands.
The origin of the electronic effect is less clear. Typical
Hammett parameter sets related to inductive effects rank
from electron donating to electron withdrawing: alkyl < benzyl
< aryl < CH2CO2R. Figure 6 is a Hammett plot of ln k
Figure 7. Plot of ln k for CNR vs δ for the equatorial carbonyls of
Cr(CO)5(CNR). R: 1, 4-ClC6H5; 2, 4-MeOC6H5; 3, CH2Ts; 4,
CH2CO2Me; 5, Bn; 6, octyl. The trend line does not include 3
(CNCH2Ts).
thickness of CH2Ts, access to the apical coordination site is
sterically impeded, resulting in a decelerated Buchner inser-
tion. Therefore, we suggest that steric effects make the reaction
slower for CNCH2Ts than is predicted on the basis of
electronic effects, as revealed by the δCO values.
MECHANISM
■
The experimentally determined rate law is consistent with three
mechanisms:
(a) rate-limiting and irreversible addition of one isocyanide,
with fast follow-up steps
(b) rate-limiting addition of one isocyanide with concerted
benzylidene addition to the mesityl group and fast
follow-up steps
(c) reversible coordination of one isocyanide, followed by
rate-limiting benzylidene addition to the mesityl group,
with fast follow-up steps
Figure 6. Plot of ln k for CNR vs σi for R: 1, 4-ClC6H5; 2,
4-MeOC6H5; 3, CH2Ts; 4, CH2CO2Me; 5, Bn; 6, octyl. The plot shows
no correlation between Hammett σi and the rate of Buchner insertion.
for Buchner insertion vs σi.20 Scatter can be seen in the
plot, and no correlation between these parameters is evident.
Other Taft parameters such as σp do not show improved cor-
relations.
A property which may be more relevant to the Buchner
insertion is the σ-donor to π-acceptor ratio of the isocyanide as
a ligand to a transition metal. Ample precedent exists for use of
the 13C chemical shifts of the CO ligands of Ni(CO)3L or
Cr(CO)5L complexes as a measure of this ratio for L.21 Better
σ donor/π acceptor ligands L cause deshielding of the carbonyl
ligands, resulting in downfield chemical shifts.
Two previous studies have reported Cr(CO)5(isocyanide)
properties vs 13C NMR shifts.22 A significant compilation of
isocyanides of varying electronic and steric demand can be
found in the work of Figueroa et al.22c Figure 7 shows the
correlation of ln k for Buchner insertion vs chemical shift for
the equatorial carbonyls of Cr(CO)5(CNR).23,21 An increase
in reaction rate correlates with more upfield chemical shifts.
Except for CNCH2Ts, the correlation is good. The exception
for CNCH2Ts is surprising. Hammett parameters for induc-
tive effects for CH2Ts and CH2CO2Me are quite similar, as
are the rate constants for Buchner insertion, but the chemical
shifts for Cr(CO)5(CNCH2Ts) and Cr(CO)5(CNCH2CO2Me)
are very different. The steric properties are difficult to quantify,
but clearly CNCH2Ts should be larger than CNCH2CO2Me.
Molecular modeling of isocyanides in conformations such
that the substituents fill the minimum distance perpendicular to
the NC vector reveals the following van der Waals thicknesses:
4-ClC6H5 ≈ 4-MeOC6H5, 3.4 Å; CH2CO2Me = Bn = octyl,
4.2 Å; cyclohexyl, 5.25 Å; CH2Ts, 5.4 Å. Due to the greater
All three would be expected to display decreasing rates with
increasingly sterically hindered isocyanides, as is observed.
However, mechanisms a and b would be expected to display
decreasing rates for less nucleophilic isocyanides, the opposite
of the observed behavior for aryl vs primary alkyl and methyl
isocyanoacetate vs primary alkyl. This suggests that the
electrophilicity of the carbene, as influenced by prior isocyanide
coordination, is the determining factor in the rate-determining
step. Thus, we favor mechanism c (Scheme 3). Addition of
isocyanide is proposed to be reversible; most probably addition
is trans to the carbene, forming an 18-electron ruthenium
carbene species 12. In this case, the observed second-order
rate constant for the reaction contains the equilibrium constant
k12/k21 for isocyanide coordination (which must be small, since
no intermediate complex is observable in the IR spectrum) and
the rate-limiting constant k23. The equilibrium constant should
be proportional to the isocyanide ligand properties: it would
increase with increased nucleophilicity of CNR and decrease
with increasing steric hindrance. However, the step k23 should
be relatively insensitive to CNR steric bulk and strongly
affected by alkylidene electrophilicity resulting from the trans
isocyanide. Due to the π back-bonding to the isocyanide, the
ruthenium center no longer is able to stabilize the π orbital of
the carbenic carbon of the benzylidene. This lack of
stabilization weakens the ruthenium−carbene bond, making
the carbene more electrophilic.24 Cyclopropanation of the prox-
imal mesityl moiety of the N-heterocyclic carbene generates
16-electron norcaradiene intermediate 14, which is followed by
a second isocyanide addition and electrocyclic ring opening to
F
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provide the cycloheptatrienyl product 7. Cavallo’s ipso
metallocyclic intermediate 1313 lies between 12 and 14.
Strongly σ donating ligands are not adequate to trigger the
Buchner insertion. The ligand-induced Buchner reaction
requires a ligand that possesses strong σ-donor and π-acceptor
properties. σ donation results in metal binding but is not itself
sufficient to cause the Buchner insertion. For instance, pyridine
can add to the metal, as seen in the 18-electron pyridine solvate
6, but pyridine does not cause a Buchner reaction. The
presence of σ donors that are not π acceptors induces a differ-
ent decomposition pathway which is mechanistically distinct
from the Buchner reaction. Grubbs et al. reported that
decomposition of the methylidene complex 8 in the presence
of excess pyridine forms MePCy3+ and Ru(H2IMes)(py)3Cl2.25
More recently, Fogg et al. have studied Lewis base promoted
decomposition in greater detail.26
The initial binding at the open apical position of the
ruthenium carbene complex requires a small ligand. In this
study, sterically hindered alkyl isocyanides gave slower Buchner
reactions in comparison to linear (i.e., normal) alkyl iso-
cyanides. It may be inferred that highly congested aromatic
isocyanides will react more slowly that the aryl isocyanides
studied here due to poor initial binding to access intermediate
12. Previously, a similar trend was observed for phosphites.
The key switching step that alters the electronic nature of the
carbene occurs at the stage of intermediate 12 in Scheme 3.
The steric size of the binding ligand and its σ-donor/π-acceptor
properties are key in inducing the Buchner insertion.
Another important factor influencing the rate and effective-
ness of the Buchner reaction is the electronic environment of
the metal. Our previous study found that electron-rich metal
carbenes such as Fischer carbenes do not undergo the Buchner
reaction, whereas π conjugation (such as that found in vinyl
carbene 2) is permitted.12b In addition, the nature of the
carbene ligand coordinated trans to the NHC ligand is signifi-
cant, as it contributes or withdraws electron density from the
metal, which modulates or increases chemical reactivity toward
the Buchner reaction. The interplay of the combined electronic
effects arising from the carbene substituent combined with that
of the trans donor ligand (such as Cy3P) requires further study.
This kinetic study was focused on the most commonly used
Grubbs carbenes, but new metal carbene complexes continue to
be discovered or developed to perform specific duties. Future
studies on these ruthenium carbenes and postulated inter-
mediates in alkene and ene-yne metathesis are ongoing in these
laboratories.
• The rate is accelerated by electron-withdrawing isocyanide
substituents and retarded by increasing steric bulk.
• Our observations led to a proposed mechanism which
involves reversible coordination of a single isocyanide
ligand, followed by rate-determining carbene migration
to the mesityl group.
• A single isocyanide binding is likely to be sufficient to
trigger the Buchner reaction.
Because isocyanides are powerful quenching reagents for
metathesis reactions and have seen use as metal scavengers,
these data have important implications for selection of a
quenching agent and for future materials synthesis. For the
fastest quenching rates, aromatic isocyanides offer the best
choice. In addition, many aromatic isocyanides are commer-
cially available. For sequesteration of metal through multi-
valency, an alkyl isocyanide may offer a better choice due to the
potential for three binding ligands. Synthesis and evaluation of
new materials as quenching and scavenging agents are the focus
of continuing studies in our laboratories.
EXPERIMENTAL SECTION
■
General Information. Unless otherwise stated, reactions were
conducted with oven-dried glassware under an atmosphere of
nitrogen. Solvent (dichloromethane) was passed through alumina
(Anhydrous Engineering solvent purifier) and stored under nitrogen.
The Grubbs second-generation catalyst 3 and Hoveyda-II complex 4
were obtained from Materia Inc. (Pasadena, CA) and used as received.
Ru(H2IMes)(CHPh)(PPh3)Cl2 was also synthesized from 3 according
to the literature. All isocyanides were either purchased (4-chlorophenyl
isocyanide, cyclohexyl isocyanide, benzyl isocyanide, TsCH2NC) or
synthesized by previously reported literature procedures (4-methoxy-
phenyl isocyanide,27 octyl isocyanide,28 methyl isocyanoacetate14) and
purified prior to use by distillation or column chromatography with
EtOAc/hexanes. All ruthenium carbene and isocyanide stock solutions
were stored under nitrogen and used immediately after preparation.
Flash chromatography was carried out on untreated silica gel 60 from
Sorbtech Technologies Inc. (230−400 mesh) under air pressure. Thin-
layer chromatography (TLC) was performed on glass-backed silica
plates (F254, 250 μm thickness, EMD Millipore) and visualized with
UV light, phosphomolybdic acid, iodine, or potassium permanganate.
IR kinetics were obtained using a ReactIR iC10 instrument equipped
with a K4 conduit and a SiComp sensor (2.5 cm × 10 cm) running iC
IR software. Preparative TLC was carried out using glass-backed silica
plates (F254, 1 mm thickness) and visualized with UV light. 1H NMR
spectra were recorded at 300, 400, or 500 MHz, proton-decoupled 13C
NMR spectra were recorded at 75, 100, or 125 MHz using Varian
Mercury 300, Inova 400, and Inova 500 instruments, and proton-
decoupled 31P NMR spectra were recorded at 121 MHz on a Varian
1
Inova 400 spectometer. H NMR chemical shifts are reported in ppm
relative to the solvent used (chloroform-d, 1H 7.26 ppm, 13C 77 ppm;
CONCLUSIONS
1
1
benzene-d6, H 7.15 ppm, 13C 128.06 ppm; dichloromethane-d2, H
7.15 ppm), and 31P was referenced using H3PO4 as an external
reference. Infrared spectra were recorded using a PerkinElmer Spec-
trum Two FTIR-ATR instrument. Mass analysis was performed on a
Bruker SolariX 12T FTMS instrument using electrospray ionization
with acetonitrile or dichloromethane as solvent.
■
In conclusion, the rate of isocyanide-promoted Buchner reac-
tions depends on a balance of σ-donor/π-acceptor properties of
the isocyanide ligand. Aryl and alkyl isocyanides were studied,
where aryl isocyanides gave the fastest Buchner reactions. The
reaction is an associative process where the initial binding of
isocyanide occurs, showing first-order rate dependence on iso-
cyanide and the Ru carbene complex. Aryl isocyanides yielded
products with two isocyanide ligands. Slender alkyl isocyanides,
on the other hand, gave an additional substitution reaction,
replacing the tricyclohexylphosphine ligand, giving a tris-
(isocyanide) coordination complex.
Kinetic Procedures: Buchner Reaction. Buchner insertion
kinetics were tracked using a ReactIR iC10 instrument equipped
with a K4 conduit and a SiComp sensor (2.5 cm × 10 cm) running iC
IR software. The disappearance of isocyanide was tracked using the
NC stretch in the region 2300−2000 cm−1. Temperature control
was achieved using a water bath regulated by a Hake C1 thermostat.
Reactions were performed using a 50 mL oven-dried flask with an
internally threaded glass adapter sealed with a PTFE bushing and
Viton O-ring around the probe. The flask was equipped with a
magnetic stirrer, sealed with a rubber septum, and cooled under a
nitrogen atmosphere.
• Isocyanide-induced Buchner carbene insertion proceeds
with a rate law first order in isocyanide and first-order in
carbene complex.
G
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Article
General Kinetic Procedure. Reactions were carried out by trans-
ferring a desired amount of the isocyanide stock solution (2.8 mL)
using a gastight syringe. The solution was then equilibrated at the
desired temperature (0−25 °C) for 10 min under a nitrogen
atmosphere. Then a volume of ruthenium carbene stock solution
(0.2 mL) was injected into the reaction flask (total volume 3 mL).
Consumption of isocyanide was followed in situ by the decreasing IR
absorbance of the isocyanide functional group. All reactions were run
in duplicate using the same stock solutions; additional runs were also
performed using new stock solutions in order to assess the repro-
ducibility of the experiments. The obtained data were treated with
error limits at the 95% confidence interval using Microsoft Excel.
Confirmation of the product was achieved by addition of mesitylene
(0.33 equiv) with CDCl3, and the product mixture was analyzed by
1H NMR. Varying the isocyanide stock solutions and multiple runs
gave the kinetic data reported in Tables S1−S3 in the Supporting
Information.
Characterization of Buchner Insertion Products. In general,
the isocyanide-promoted Buchner reactions were clean and afforded
the insertion products 7 and 8 cleanly. In most cases, the complexes
could be purified by flash chromatography, and purity was assessed in
all cases by 1H NMR. Over time, acquisition of carbon-13 data showed
evidence of decomposition occurring in solution over a period of
hours. Evidence of decomposition precluded satisfactory combustion
analysis for new complexes except for 7b and 8d.
Complex 8d.
Rf = 0.13 (30% EtOAc/1% NEt3/hexanes). The purity was >93%
1
1
based on H NMR. H NMR (300 MHz, CD2Cl2, ppm): δ 7.54 (d,
3J = 7.4 Hz, 2 H, aromatic), 7.18−7.43 (m, 16 H, aromatic), 7.09 (t,
3J = 6.9 Hz1 H, aromatic), 7.01 (t, 3J = 6.9 Hz, 2 H, aromatic), 6.75 (s,
1 H, mesityl CH), 6.70 (s, 1 H, mesityl CH), 5.70 (s, 1 H, vinylic
CHCHT), 5.62 (s, 1 H, vinylic CHCHT), and 5.00−4.70 (m, 3 H, ben-
zylic methine and CH2Ph), 4.53 (s, 2 H, CH2Ph), 4.32 (br s, 2 H,
CH2Ph), 3.64−3.55 (m, 2 H, NCH2CH2N), 3.42−3.37 (m, 1 H,
NCH2CH2N), 3.14−3.09 (m, 1 H, NCH2CH2N), 2.44 (s, 6 H, mesityl
CH3), 2.13 (s, 3 H, CH3), 2.12 (s, 3 H, CH3), 2.06 (s, 3 H, CH3), 1.76
(s, 3 H, CH3). IR (CH2Cl2): 2144 and 2200 cm−1. Low-resolution MS
(ESI+, m/z): molecular ion calculated for M+ − Cl [C52H53N5RuCl]+
884.30, found 884.31. Anal. Calcd for C52H53N5Cl2Ru: C, 67.89; H,
5.81; N, 7.61; Cl, 7.71. Found: C, 67.55; H, 5.95; N, 7.25; Cl, 7.40.
Complex 8f.
Complex 7a.
The product was isolated as a bright yellow solid (23 mg, 64% yield
after column chromatography (10−30% EtOAc/hexanes). Rf = 0.19
1
1
(30% EtOAc/hexanes). The purity was >97% based on H NMR. H
NMR (500 MHz, CDCl3, ppm): δ 7.51 (d, 3J = 7.5 Hz, 2 H, o-phenyl
CH), 7.23 (t, J = 7.0 Hz, 2 H, m-phenyl CH), 7.20−7.17 (m, 1 H,
The data for samples prepared were identical with spectroscopic data
previously reported in the literature.4 Methine protons in the cyclo-
heptatriene substructure are particularly diagnostic. Select H NMR
(300 MHz, CDCl3, ppm): δ 5.61 (s, 1 H, vinylic CHCHT), 5.41 (s, 1 H,
vinylic CHCHT) and 5.04 (s, 1 H, benzylic methine). IR (CH2Cl2):
2120 (s), 2060 and 2015 (s) cm−1.
3
1
p-phenyl CH), 6.89 (s, 2 H, mesityl CH), 5.75 (s, 1 H, vinylic CHCHT),
5.63 (s, 1 H, vinylic CHCHT), 4.83 (s, 1 H, benzylic methine), 3.64−
3.50 (m, 6 H), 3.39−3.32 (m, 1 H), 3.11−3.02 (m, 3 H), 2.43 (s, 3 H,
mesityl CH3), 2.42, (s, 3 H, mesityl CH3), 2.24 (s, 3 H, mesityl CH3),
2.15 (s, 3 H, vinylic CH3), 2.06 (s, 3 H, vinylic CH3), 1.82 (s, 3 H,
vinylic CH3), 1.73 (pentet, 3J = 7.7 Hz, 4 H, aliphatic), 1.55 (br s, 3 H,
aliphatic), 1.43−1.33 (m, 6 H, aliphatic), 1.32−1.18 (m, 26 H,
aliphatic), 0.89−0.85 (m, 9 H, alkyl CH3). IR (CH2Cl2): 2125 and
2145 (s) cm−1. Low-resolution MS (ESI+, m/z): molecular ion calcul-
ated for M+ − Cl [C55H83N5RuCl]+ 950.54, found 950.54. Due to the
instability of this compound, combustion analysis was not obtained.
Complex 8g.
Complex 7b.
The product was isolated as an orange solid (50 mg, 70% yield) after
recrystallization from CH2Cl2/pentane. The purity was >98% based on
1H NMR. 1H NMR (500 MHz, CDCl3, ppm): δ 7.49 (d, 3J = 8.0 Hz,
2 H, o-phenyl CH), 7.30 (d, 3J = 8.5 Hz, 2 H, m-Ar CH), 7.21 (t, 3J =
7.5 Hz, 2 H, m-phenyl CH), 7.16 (d, 3J = 7.0 Hz, 1 H, p-phenyl CH),
3
The product was isolated as a yellow solid (10 mg, 33% yield) by
6.95 (d, J = 8.5 Hz, 2 H, m-Ar CH), 6.83−6.78 (m, 4 H, o-Ar CH),
column chromatography (5% EtOAc/3% NEt3/hexanes). Rf = 0.31
6.64 (s, 1 H, mesityl CH), 6.60 (s, 1 H, mesityl CH), 5.94 (s, 1 H,
vinylic CHCHT), 5.41 (s, 1 H, vinylic CHCHT), 5.12 (s, 1 H, benzylic
methine), 3.82 (s, 6 H, OCH3), 3.55−3.38 (m, 3 H, NCH2CH2N),
3.18 (m, 1 H, NCH2CH2N), 2.53 (s, 3 H, mesityl CH3), 2.49 (s, 3 H,
mesityl CH3), 2.22 (s, 3 H, vinyl CH3), 2.12 (m, 6 H, vinyl CH3),
2.00−1.85 (m, 6 H), 1.67−1.5 (m, 19 H), 1.47 (s, 3 H), 1.21 (m, 4 H),
0.94 (m, 6 H). IR (CH2Cl2): 2070, 2130 (s), and 2040 (s) cm−1.
Low-resolution MS (ESI+, m/z): molecular ion calculated for M+ − Cl
[C62H79N4O2PRuCl]+ 1079.47, found 1079.50. Anal. Calcd for
C62H79Cl2N4O2PRu: C, 66.77; H, 7.14; N, 5.02; Cl, 6.36. Found: C,
66.41; H, 7.10; N, 5.04; Cl, 6.25.
1
(30% EtOAc/1% NEt3/hexanes). The purity was >97% based on H
NMR. 1H NMR (500 MHz, CDCl3, ppm): δ 7.54 (d, 3J = 6.5 Hz, 2 H,
3
o-phenyl CH), 7.24 (t, J = 7.0 Hz, 2 H, m-phenyl CH), 7.19−7.17
(m, 1 H, p-phenyl CH), 6.90 (s, 1 H, mesityl CH), 6.88 (s, 1 H,
mesityl CH), 5.75 (s, 1 H, vinylic CHCHT), 5.62 (s, 1 H, vinylic
CHCHT), 4.94 (s, 1 H, benzylic methine), 3.84−3.79 (m, 1 H, cyclo-
hexyl CH), 3.58−3.49 (m, 2 H, NCH2CH2N), 3.37−3.30 (m, 1 H,
NCH2CH2N), 3.08−3.04 (m, 1 H, NCH2CH2N), 2.43 (s, 3 H, mesityl
CH3), 2.41 (s, 3 H, mesityl CH3), 2.25 (s, 3 H, mesityl CH3), 2.14
(s, 3 H, vinylic CH3), 2.05 (s, 3 H, vinylic CH3), 1.99−1.93 (m, 3 H,
H
DOI: 10.1021/acs.organomet.7b00342
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Article
aliphatic), 1.83−1.40 (m, 20 H, aliphatic), 1.27 (m, 10 H, aliphatic).
IR (CH2Cl2): 2100, 2135, and 2185 cm−1. Low-resolution MS (ESI+,
m/z): molecular ion calculated for M+ − Cl [C49H65N5RuCl]+ 860.40,
found 860.40. Due to the instability of this compound, combustion
analysis was not obtained.
assistance and Materia, Inc. (Pasadena, CA), for supplying
Grubbs’ catalyst.
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3
1H NMR (500 MHz, CDCl3, ppm): δ 7.49 (d, J = 7.0 Hz, 2 H,
o-phenyl CH), 7.24 (t, 3J = 7.5 Hz, 2 H, m-phenyl CH), 7.20−7.17 (m,
1 H, p-phenyl CH), 6.88 (s, 1 H, mesityl CH), 6.87 (s, 1 H, mesityl
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1 H, NCH2CH2N), 2.45 (s, 3 H, mesityl CH3), 2.43 (s, 3 H, mesityl
CH3), 2.52 (s, 3 H, mesityl CH3), 2.14 (s, 3 H, vinylic CH3), 2.08 (s,
3 H, vinylic CH3), 1.82 (s, 3 H, vinylic CH3). IR (CH2Cl2): 2100 (s),
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mass spectrometry or combustion analysis was obtained.
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■
S
* Supporting Information
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Diver, S. T. J. Am. Chem. Soc. 2005, 127, 15702−15703. (b) Galan, B.
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The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.organomet.7b00342.
Details of the kinetic derivations, tables of data, and
characterization of (OC)5Cr(isocyanide) complexes (PDF)
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AUTHOR INFORMATION
■
(15) French, J. M.; Caras, C. A.; Diver, S. T. Org. Lett. 2013, 15,
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Corresponding Authors
*E-mail for J.B.K.: keister@buffalo.edu.
*E-mail for S.T.D.: diver@buffalo.edu.
ORCID
Jerome B. Keister: 0000-0001-5376-9399
Steven T. Diver: 0000-0003-2840-6726
Notes
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ACKNOWLEDGMENTS
■
This work was supported by the NSF (CHE-601206, to S.T.D.
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