Mechanistic studies of alkene isomerization catalyzed by CCC-pincer complexes of iridium

Article abstract of DOI:10.1021/om400786r

Iridium complexes containing CCC-pincer m-phenylene-bridged N-heterocyclic carbene ligands were examined as catalysts for alkene isomerization. Complexes containing either mesityl or adamantyl side groups were found to catalyze the isomerization of a number of alkenes to the internal isomers, including 1-octene, vinylcyclohexane, and allylbenzene. Mechanistic studies indicate a surprising dichotomy, apparently caused by ligand steric effects. For the mesityl-substituted catalyst, several lines of evidence provide strong support for isomerization via an iridium allyl hydride intermediate: (1) H-D crossover experiments indicate that 1,3-hydrogen migration is exclusively intramolecular, (2) the catalyst resting state, a π-allyl hydride species, was isolated and serves as a kinetically competent catalyst, (3) NMR experiments indicate that the π-allyl hydride resting state undergoes reversible C-H reductive elimination that is rapid relative to catalytic turnover, and (4) kinetic studies indicate that the isomerization reaction is first order in substrate and catalyst, consistent with turnover-limiting ligand substitution. H-D crossover experiments for alkene isomerization catalyzed by the adamantyl-substituted complex show selectivity for a 1,3-deuterium shift, as well as the intermolecular transfer of hydrogen. These results are consistent with an insertion/elimination mechanism proceeding selectively through a secondary metal-alkyl or with a π-allyl-type mechanism with an unknown pathway for intermolecular hydrogen crossover.

Full text of DOI:10.1021/om400786r

Article
pubs.acs.org/Organometallics
Mechanistic Studies of Alkene Isomerization Catalyzed by CCC-
Pincer Complexes of Iridium
Spring Melody M. Knapp, Sarah E. Shaner, Daniel Kim, Dimitar Y. Shopov, Jennifer A. Tendler,
David M. Pudalov, and Anthony R. Chianese*
Department of Chemistry, Colgate University, 13 Oak Drive, Hamilton, New York 13346, United States
*
S Supporting Information
ABSTRACT: Iridium complexes containing CCC-pincer m-phenylene-bridged N-heterocyclic carbene ligands were examined as
catalysts for alkene isomerization. Complexes containing either mesityl or adamantyl side groups were found to catalyze the
isomerization of a number of alkenes to the internal isomers, including 1-octene, vinylcyclohexane, and allylbenzene. Mechanistic
studies indicate a surprising dichotomy, apparently caused by ligand steric effects. For the mesityl-substituted catalyst, several
lines of evidence provide strong support for isomerization via an iridium allyl hydride intermediate: (1) H−D crossover
experiments indicate that 1,3-hydrogen migration is exclusively intramolecular, (2) the catalyst resting state, a π-allyl hydride
species, was isolated and serves as a kinetically competent catalyst, (3) NMR experiments indicate that the π-allyl hydride resting
state undergoes reversible C−H reductive elimination that is rapid relative to catalytic turnover, and (4) kinetic studies indicate
that the isomerization reaction is first order in substrate and catalyst, consistent with turnover-limiting ligand substitution. H−D
crossover experiments for alkene isomerization catalyzed by the adamantyl-substituted complex show selectivity for a 1,3-
deuterium shift, as well as the intermolecular transfer of hydrogen. These results are consistent with an insertion/elimination
mechanism proceeding selectively through a secondary metal−alkyl or with a π-allyl-type mechanism with an unknown pathway
for intermolecular hydrogen crossover.
INTRODUCTION
and catalyst lifetime are sensitive to the steric bulk of the
substituents on the phosphines. These PCP-pincer catalysts
show excellent kinetic selectivity for α-olefins, but this
■
4
Selective C−H functionalization of alkanes is of great interest
because it has the potential to provide access to valuable
commodity chemicals, such as alkenes, alcohols, and amines,
from inexpensive petroleum feedstocks. In particular, the
dehydrogenation of linear alkanes through C−H bond
activation can be used to selectively generate α-olefins, a
selectivity is degraded over time by competing catalysis of
4
,5
1
alkene isomerization.
We recently investigated alkane
dehydrogenation using iridium complexes containing mono-
anionic CCC-pincer ligands, where the phosphorus-donor
fragments are replaced with N-heterocyclic carbenes (NHCs;
1
a,f
crucial step in alkane metathesis reactions.
containing PCP-pincer complex ( PCP)IrH is an excep-
The iridium-
6
tBu4
Figure 1). The iridium complexes containing the bulky aryl N-
2
substituents mesityl (1-Mes) and 3,5-di-tert-butylphenyl (1-
dtbp) formed active catalysts for alkane dehydrogenation on
activation with sodium tert-butoxide and were also found to
catalyze the isomerization of the resulting alkenes. Interestingly,
iridium complexes containing the bulky alkyl substituents
adamantyl (1-Ad) and tert-butyl (1-tBu) did not catalyze
tional catalyst for the dehydrogenation of alkanes, achieving
over 1000 turnovers in the transfer dehydrogenation of
cyclooctane; this complex also provided the first reported
2
example of catalyzed acceptorless dehydrogenation of a linear
3
tBu4
alkane. Further work with the related ( PCP)IrH4,
iPr4
tBu3Me
tBu2Me2
(
PCP)IrH , (
PCP)IrH , and (
PCP)IrH com-
4
4
4
plexes showed that the highest rates and greatest turnover
numbers for alkane dehydrogenation could be achieved with
Received: August 6, 2013
Published: January 9, 2014
tBu3Me
(
PCP)IrH , indicating that the rate of dehydrogenation
4
©
2014 American Chemical Society
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kinetically generating the desired E isomers by the sterically
3
20
encumbered nature of the η -allyl intermediate.
As we have previously reported, both 1-Mes and 1-Ad are
active alkene isomerization catalysts on activation with
t
6b
NaO Bu. Of the two, 1-Ad was a more efficient alkene
isomerization catalyst, giving a TON of 487 after 30 min at 60
°
C for the isomerization of 1-octene. In comparison, a reaction
catalyzed by 1-Mes required nearly 24 h at 100 °C to reach a
TON of 435. Notably, 1-Mes catalyzes the dehydrogenation of
alkanes, while 1-Ad does not. As catalysis of alkene
6
Figure 1. Previously reported iridium complexes containing CCC-
pincer ligands.
isomerization limits the selectivity for α-olefins in alkane
dehydrogenation, the rational design of selective alkane
dehydrogenation catalysts will benefit from an understanding
of the available mechanisms for alkene isomerization. Recently,
Goldman, Brookhart, and co-workers described an extensive
study of the mechanism of alkene isomerization catalyzed by
PCP-iridium complexes and concluded that a π-allyl-type
alkane dehydrogenation but were highly active catalysts for
6b
alkene isomerization.
While competing alkene isomerization limits the selectivity of
an alkane dehydrogenation catalyst for the formation of α-
olefins, there are a number of applications in which a highly
active and selective alkene isomerization catalyst is desirable.
For example, alkene isomerization is an important step in the
5b
mechanism is operative in those systems. Herein we report an
investigation of the mechanism of alkene isomerization
catalyzed by 1-Ad and 1-Mes. The studies conducted on 1-
Mes include H−D scrambling and crossover experiments,
which indicate that 1,3-hydrogen migration is exclusively
intramolecular. A π-allyl hydride species, believed to be the
catalyst resting state, was observed spectroscopically during
catalysis, was isolated, and serves as a kinetically competent
catalyst. EXSY indicates that this intermediate undergoes fast,
reversible C−H reductive elimination. Finally, kinetic studies
indicate that the isomerization reaction is first order in substrate
and catalyst, consistent with turnover-limiting ligand sub-
stitution. These studies strongly support the conclusion that 1-
Mes catalyzes isomerization via the π-allyl mechanism.
Although limited solubility prevented a complete study on 1-
Ad, H−D scrambling and crossover experiments suggest that it
catalyzes isomerization either via an insertion/elimination
mechanism proceeding selectively through a secondary
metal−alkyl or via a π-allyl-type mechanism with an unknown
pathway for intermolecular hydrogen crossover.
7
Shell Higher Olefins Process, the synthesis of linear amines
8
from internal olefins via hydroaminomethylation, and the
9
industrial synthesis of terpene compounds. Additionally, the
facile preparation of (E)-alkene isomers of allylbenzene
derivatives is of interest to the fragrance and flavoring
1
0
industries. Many different catalysts have been developed to
1
1
facilitate alkene isomerization, including those containing Ti,
1
2
13
14
14a,15
13a,14a,16
17 5b,18
13a,19
Cr, Fe, Co, Ni,
Rh,
Ru, Ir,
Pd,
and
2
0
Pt.
There are two commonly accepted mechanisms of catalyzed
alkene isomerizationthe π-allyl mechanism and the alkyl
21
mechanism (Scheme 1). The π-allyl mechanism starts with
coordination of an olefin to a low-valent metal, followed by
3
oxidative addition of the allylic C−H bond to form an η -allyl
complex with a metal hydride ligand. That metal hydride
undergoes reductive elimination into the terminal position to
generate the isomerized alkene via an overall 1,3-hydride shift.
As shown in Scheme 1b, the alkyl mechanism starts with a
metal hydride complex. Migratory insertion of the coordinated
alkene into the metal hydride forms either a primary or a
secondary metal alkyl complex. In the example shown, the
alkene inserts to form a secondary metal alkyl complex. Finally,
β-H elimination from the internal carbon results in alkene
isomerization via a formal 1,3-hydride shift. Of the two
mechanisms presented here, the π-allyl mechanism has been
proposed to be the pathway with a greater intrinsic bias toward
RESULTS AND DISCUSSION
■
Isomerization of Alkenes. We previously described the
transfer dehydrogenation of n-octane with the sacrificial
hydrogen acceptor norbornene using the precatalysts 1-Mes
t
6b
and 1-Ad, which were activated with NaO Bu at 150 °C.
Transfer dehydrogenation of n-octane with 1-Mes occurred
with 12 turnovers in 20 h, while 1-Ad showed no activity.
When 1-hexene was used as the sacrificial hydrogen acceptor
Scheme 1. Two Mechanisms Proposed for Olefin Isomerization: (a) π-Allyl Mechanism; (b) Alkyl Mechanism
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a
Table 1. Alkene Isomerization Results using 1
a
t
Reactions performed using 1.0 mmol of alkene, 2.0 μmol of catalyst, and 20 μmol of NaO Bu in 2 mL of toluene. Conversions were determined at
1
b
2
4 h by H NMR. T/C/I is the ratio of trans-2-alkene isomer to cis-2-alkene isomer to the remaining internal isomers (3-alkene or 4-alkene, if
possible). The T/C/I ratios given were observed at 24 h. T/C/I ratios observed over the course of the reaction are provided in the Supporting
c
Information. See ref 6b.
under the same reaction conditions, no dehydrogenation was
observed. Instead, 1-Mes catalyzed the isomerization of 1-
hexene to its internal isomers, giving 730 turnovers in 60 min.
Alkene isomerization studies were also conducted using 1-
octene as the substrate. At 100 °C and 0.2 mol % of catalyst
contributing to the decreased isomerization activity of both
catalysts toward allyl phenyl ether. Isomerizations conducted
using 1-Ad showed a slight preference for the thermodynamic
product, generating phenyl propenyl ether in a 1:5.2 E:Z ratio,
while 1-Mes produced a 1:1.4 E:Z ratio of products. Previous
investigations of the isomerization of allyl phenyl ether under
thermodynamic control show that (Z)-phenyl propenyl ether is
t
with 2 mol % of NaO Bu, 1-Mes required 24 h to reach 97%
conversion of 1-octene into its internal isomers. In comparison,
26
1-Ad required only 30 min at 60 °C to reach the same level of
exclusively formed. This same preference for the Z isomer has
20
conversion. When all 1-octene was consumed, 1-Ad continued
to isomerize the 2-octene product into the 3-octene and 4-
octene internal isomers, eventually approaching the thermody-
namic ratio of linear octene isomers. In contrast, after 24 h at
been observed with other catalysts, where a thermodynamic
mixture of products was generated. Given that neither catalyst
generated the thermodynamic mixture of products and that the
E:Z ratio remained constant throughout the reactions, the ratio
of products formed is likely due to kinetic selectivity of the
catalysts.
100 °C, the reaction catalyzed by 1-Mes did not reach a
thermodynamic ratio of octene isomers, indicating that 1-Ad is
more reactive toward both terminal and internal alkenes.
To determine the effect of different functional groups and
substrate size on catalyst activity, further alkene isomerization
studies using 1-Ad and 1-Mes and the alkenes allyl alcohol,
vinylcyclohexane, allyl phenyl ether, and allylbenzene were
conducted (Table 1).
Vinylcyclohexane was isomerized readily to ethylidenecyclo-
hexane by both 1-Mes and 1-Ad (Table 1, entries 5 and 6),
with no further conversion of ethylidenecyclohexane to 1-ethyl-
1-cyclohexene or other cyclohexene isomers. In this case,
isomerization with 1-Mes was faster, which may be due to
reduced steric repulsion between the coordinated alkene and
the bulky N substituents on 1-Mes in comparison to 1-Ad (see
Figure S2 in the Supporting Information for time-course plots
of isomerization).
The catalysts both gave complete conversion of allylbenzene
in 24 h at 100 °C (Table 1, entries 7 and 8). As shown in
Figure 2, 1-Ad gave faster conversion, with 267 turnovers in the
first 15 min of the reaction, while 1-Mes gave only 149
turnovers in the same time period. The E:Z ratio of β-
In comparison to that of 1-octene, both catalysts displayed
minimal activity toward the isomerization of allyl alcohol. When
the reaction was catalyzed by 1-Ad at 80 °C, propionaldehyde
was generated with 5% yield after 24 h. No conversion was
observed when 1-Mes was used. Allyl alcohol is known to
isomerize to the enol and then isomerize (or tautomerize)
1
7b,c,22
further to form propionaldehyde.
A number of iridium
2
3
CF3
24
catalysts, including the pincer complexes Ir( PCP) and
tBu4
25
Ir( POCOP), have been shown to either catalytically or
stoichiometrically decarbonylate aldehydes. Decarbonylation of
propionaldehyde may occur with 1-Ad and 1-Mes, contributing
to the very low activity of these catalysts toward allyl alcohol.
No attempt was made to determine the fate of the iridium in
our reactions employing the allyl alcohol. Both catalysts were
more active for isomerization of other substrates; therefore,
reactions with allyl alcohol were not pursued further.
Both complexes catalyze the isomerization of allyl phenyl
ether (Table 1, entries 3 and 4), although these reactions were
much slower and required higher temperatures in comparison
to the other alkenes investigated. Theoretical investigations
with other catalysts have indicated that the oxygen of allyl
ethers or allyl alcohols coordinates to the catalyst during
Figure 2. Isomerization of allylbenzene catalyzed by 1-Mes (◆) or 1-
Ad ( ) at 100 °C. The reaction was performed with 1.0 mmol of
■
1
7b
t
isomerization.
Competitive coordination of the ether over
alkene, 2.0 μmol of catalyst, and 20 μmol of NaO Bu in 2 mL of
the alkene would inhibit alkene isomerization and may be
toluene.
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Scheme 2. Expected Products of the Deuterium Scrambling Experiment with d -Allylbiphenyl: (a) Sole Product of the π-Allyl
2
Mechanism; (b) Some Products of the Alkyl Mechanism
methylstyrene products generated by both catalysts remains
essentially constant at 95:5 E:Z throughout the reaction (see
Table S2 in the Supporting Information). Thermodynamic
studies have previously been conducted on the isomerization of
allylbenzene to its β-methylstyrene isomers: the thermody-
namic mixture at 100 °C is composed of 0.3% allylbenzene,
deuterium label in the allylic position may undergo a net 1,3-
migration or a net 1,2-migration. In practice, catalysts operating
via the alkyl mechanism typically give extensive scrambling of
13a,29
deuterium labels due to reversible alkene insertion.
Labeling studies employing a substrate deuterated in the
29
allylic position, such as allyl-1,1-d2 alcohol or 3-ethyl-1-
27
13c
4.9% cis-β-methylstyrene, and 94.9% trans-β-methylstyrene,
pentene-3-d1,
have been used previously to examine the
indicating that both catalysts are forming the thermodynamic
mechanism of isomerization of other alkene isomerization
catalysts. Because of the excellent activity of both 1-Mes and 1-
Ad toward allylbenzene, as well as the good selectivity of both
28
ratio of β-methylstyrene products. Because of the high activity
of both 1-Mes and 1-Ad toward allylbenzene isomerization and
the predominant formation of a single product, further
mechanistic studies conducted with both catalysts were done
using allylbenzene and its derivatives.
catalysts for the trans product, 4-(2-propenyl-1,1-d )-1,1′-
2
biphenyl (henceforth called d -allylbiphenyl) was used as the
2
substrate of choice. If isomerization proceeds via the π-allyl
H/D Scrambling and Crossover Experiments with 1-
Ad. Hydrogen−deuterium scrambling studies and hydrogen−
deuterium crossover experiments are known to generate
different and distinguishable products, depending on the
mechanism of isomerization; therefore, these experiments
serve as an initial indicator of which of the two accepted
mechanism, d -allylbiphenyl is expected to isomerize exclusively
2
via a 1,3-deuterium shift into 4-(1-propenyl-1,3-d )-1,1′-
2
biphenyl (henceforth called 1,3-d -1-propenylbiphenyl)
2
(Scheme 2a). A mixture of deuterated 1-propenylbiphenyl
isomerization products is likely if isomerization occurs via the
alkyl mechanism, because isomerization can occur via both a
1,2-deuterium shift and a 1,3-deuterium shift (Scheme 2b).
13c,29
isomerization mechanisms is in operation.
The hydride
shift that occurs during alkene isomerization following the π-
In a typical hydrogen−deuterium scrambling experiment, d -
2
allyl mechanism is completely intramolecular, and isomerization
allylbiphenyl was combined with 0.8 mol % of the precatalyst 1-
13c
t
occurs via a 1,3-hydride shift.
The alkyl mechanism, in
Ad and 8 mol % of NaO Bu, and the isomerization reaction was
contrast, starts with a metal hydride complex; thus, hydride
transfer is intermolecular. Additionally, this mechanism may
involve insertion of the alkene into the metal hydride via both
Markovnikov and anti-Markovnikov addition, so that a
monitored via NMR spectroscopy at 80 °C (Figure 3). Under
these conditions, d -allylbiphenyl was completely converted
2
into a mixture of 1-propenylbiphenyl isomers within 2 h.
1
Following the reaction by H NMR spectroscopy, the peaks
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Figure 3. (a) Isomerization of d -allylbiphenyl to d -1-propenylbiphenyl products catalyzed by 1-Ad. The reaction was performed with 0.5 M d -
2
2
2
t
13
allylbiphenyl, 4 mM (0.8 mol %) catalyst, and 40 mM NaO Bu in 0.44 mL of d -benzene. C NMR spectra were collected at room temperature after
8% conversion was observed by H NMR spectroscopy. (b) C{ H} NMR spectrum with C−H NOE enhancement. (c) Quantitative C{ H}
NMR spectrum of the propenyl carbons (C1, C2, and C3) for the products of the isomerization reaction catalyzed by 1-Ad. The heights of C1 are
6
1
13
1
13
1
9
equivalent in both spectra.
Figure 4. 13C NMR of the products of the alkene isomerization reaction with 1-Ad and (a) allylbenzene, (b) d -allylbiphenyl, and (c) a 1:1 mixture
2
of allylbenzene and d -allylbiphenyl.
2
corresponding to d -allylbiphenyl at 6.00 and 5.14 ppm
disappeared, and the product peaks appeared at 6.20 and
spectroscopy confirmed that a mixture of products formed,
where overlapping methyl carbon peaks are observed between
17.4 and 18.7 ppm (Figure 3). These signals correspond to four
different products based on the C−D splitting pattern. The
most downfield signal is a singlet, indicating complete
2
1.80 ppm. No signal was observed at 6.45 ppm, indicating
complete deuteration of the internal vinylic position (C1). No
deuterium was incorporated into the vinylic C2 carbon,
indicating that all H−D scrambling is occurring via a 1,3-
deuterium shift, which is consistent with a π-allyl mechanism.
protiation of the methylene carbon (−CH ). A 1:1:1 triplet,
3
1:2:3:2:1 quintet, and 1:3:6:7:6:3:1 septet are also present,
1
However, the H NMR resonance corresponding to the methyl
corresponding to single (−CH D), double (−CHD ), and
2
2
group at 1.80 ppm appeared as a mixture of a doublet and a
doublet of 1:1:1 triplets, indicating the product contains a
complete (−CD ) deuteration of the methylene carbon. This
3
product distribution requires H−D crossover between
13
mixture of CH D and CH at the C3 position. C NMR
substrates, which is indicative of the alkyl mechanism. To
2
3
4
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Figure 5. 13C NMR spectra of the products of the alkene isomerization reaction with 1-Mes and (a) allylbenzene, (b) d -allylbiphenyl, and (c) a 1:1
2
mixture of allylbenzene and d -allylbiphenyl.
2
obtain data more reflective of the relative concentrations of
between iridium allyl hydride intermediates, occurring by an
unknown, possibly base-mediated pathway. We are aware of
only one example of an alkene isomerization catalyst where
1
3
1
31
these products, a quantitative C{ H} NMR spectrum was
collected to eliminate any NOE contribution to the 13C NMR
signal (Figure 3c).
To confirm that H−D crossover between substrates occurs
with 1-Ad as the catalyst, a reaction was conducted using
both H−D crossover between substrates occurred and a
30c
selective 1,3-deuterium shift was observed. To explain their
results, the authors of that study proposed that catalysis
proceeded via a modified π-allyl mechanism, where the pathway
for crossover was not specified.
equimolar amounts of allylbenzene and d -allylbiphenyl with
2
t
0
.4 mol % of precatalyst 1-Ad and 4 mol % of NaO Bu, and the
1
13
isomerization reaction was monitored via H and C NMR
spectroscopy. As would be expected on the basis of the above
observations, H−D crossover between the substrates was
observed, although the catalyst maintained its selectivity for a
Attempts were made to isolate and characterize the catalyst
resting state to gain a better understanding of the mechanism of
the reaction. In these attempts, a hydride species was observed
1
in the H NMR spectrum. However, this species was not well
1
,3-deuterium shift. As seen in Figure 4, the β-methylstyrene
characterized; therefore, it is unknown if it is a resting state of
the catalytic cycle. Additionally, the catalyst was not completely
soluble for the entirety of the reaction, despite the low catalyst
loading. These experimental challenges prohibited a more
thorough kinetic characterization of the reaction using 1-Ad;
thus, it is difficult to gain a more thorough understanding of the
mechanism by which this catalyst operates.
product was partially deuterated at the methyl position, and no
deuterium was incorporated into any of the other alkenyl
carbons. The 1-propenylbiphenyl product shows complete
deuteration at the internal alkene position (C1) and partial
protiation at the methyl (C3) position.
The results of the crossover experiment catalyzed by 1-Ad
provide evidence that is not fully consistent with what is
typically observed for either the π-allyl mechanism or the alkyl
mechanism and does not allow us to definitively determine the
mechanism of isomerization. There are several examples in the
literature of catalysts that operate via the alkyl mechanism;
typically, deuterium labeling studies generated products of a
H/D Scrambling and Crossover Experiments with 1-
Mes. The same H/D scrambling experiments were conducted
with the catalyst 1-Mes. In these experiments, d -allylbiphenyl
2
was completely converted into d -1-propenylbiphenyl within 15
2
h at 80 °C, as indicated by the disappearance of the reactant
peaks at 6.00 and 5.14 ppm and the appearance of the product
1
1
,2-deuterium shift and a 1,3-deuterium shift with H−D
peaks at 6.19 and 1.80 ppm in the H NMR spectrum. Again,
1
3a,19d,29,30
crossover between substrates.
Catalysts that operate
no peak was observed at 6.45 ppm, indicating complete
deuteration of the internal vinyl position (C1). The positions of
via the π-allyl mechanism generated exclusively the products of
a 1,3-deuterium shift
substrates.
13c
13
without H−D crossover between
each deuterium in the product were confirmed using C NMR
5
b
spectroscopy, where the peaks at 131.3 and 18.6 ppm both
appeared as 1:1:1 triplets, indicating that each carbon contains
one deuterium (Figure 5b). No deuterium incorporation was
observed for the peak at 125.6 ppm, indicating that an exclusive
1,3-deuterium shift occurred, consistent with the π-allyl
mechanism.
The observed product distribution during isomerization with
-Ad may be rationalized with modifications to either
1
mechanism. If the alkyl mechanism is operative here, the
selectivity for 1,3-deuterium migration over 1,2-migration may
be explained by selective insertion of the alkene substrate into
Ir−H (or Ir−D) to give a secondary metal alkyl (Scheme 2b,
left pathway), with negligible insertion to give a primary metal
alkyl (Scheme 2b, right pathway). Alternatively, the selectivity
of 1-Ad for the product with a 1,3-deuterium shift is consistent
with the π-allyl mechanism, although this mechanism does not
produce the intermolecular hydrogen crossover observed. In
this case, the observed crossover may be due to H−D exchange
The intramolecular nature of hydrogen transfer catalyzed by
1-Mes was confirmed with a hydrogen−deuterium crossover
experiment. Equimolar amounts of allylbenzene and d -
2
allylbiphenyl were added to a solution containing 0.4 mol %
t
of the precatalyst 1-Mes and 4 mol % of NaO Bu, and the
isomerization reaction was monitored via NMR. No deuterium
crossover between allylbenzene and d -allylbiphenyl occurred,
2
4
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and the only products observed were the normal isomerization
was repeated using 2-Mes (0.5 mM) as the catalyst, and the
isomerization reaction occurred at the same rate. These kinetic
results, combined with the observation of only 2-Mes via NMR
during isomerization, indicate that 2-Mes is kinetically
competent as a catalytic intermediate.
1
3
products, as shown in the C NMR spectrum (Figure 5c).
These results suggest that 1-Mes catalyzes alkene isomerization
following the π-allyl mechanism.
3
Observation of an Ir(III) η -Allyl Hydride Catalyst
Resting State. The H/D scrambling experiments and
crossover experiments indicate that alkene isomerization with
Further kinetic studies were done using 2-Mes, which
showed that the isomerization reaction has first-order depend-
ence on both [allylbenzene] (Figure 8) and [2-Mes] (Figure
9), giving a second-order rate constant (k) of 0.177 ± 0.009
1
1
-Mes likely proceeds via the π-allyl mechanism. Treatment of
t
-Mes with allylbenzene (20 equiv) and 10 equiv of NaO Bu or
3
−1 −1
32
NaOMe in benzene at 80 °C generated an Ir(III) η -allyl
hydride species (2-Mes) cleanly (Scheme 4). The H NMR
M
s
(rate = k[allylbenzene][2-Mes]).
1
Proposed Mechanism of Isomerization with 1-Mes.
On the basis of the results of the studies done with 1-Mes and
2-Mes above, the mechanism of isomerization shown in
Scheme 5 is proposed: (a) 1-Mes undergoes dehydrohaloge-
3
Scheme 4. Synthesis of the η -Allyl Complex 2-Mes
t
nation by NaO Bu or NaOMe to generate an Ir(I)
2
intermediate, which reacts with 1-alkene to generate the η -1-
alkene complex; (b) fast oxidative addition of an allylic C−H
3
bond occurs, followed by rotation of the η -allyl ligand to
generate the resting state of the catalyst, (c) 2-Mes is produced;
2
(
(
d) reductive elimination generates the η -2-alkene complex;
e) 2-alkene is substituted for 1-alkene, starting the cycle again.
The parts of the catalytic cycle most strongly supported by
experimental evidence are steps c−e, as follows: (c) the identity
of 2-Mes was confirmed by NMR spectroscopy and X-ray
crystallography studies and is confirmed to be a part of the
catalytic cycle by showing chemical and kinetic competence in
comparison to the precatalyst 1-Mes; (d) exchange spectros-
copy shows a fast equilibrium between the resting state and an
spectrum for 2-Mes includes a hydride resonance at −14.7 ppm
and four signals for the cinnamyl ligand at 5.01, 3.56, 2.05, and
1
.30 ppm. Additionally, six separate signals are observed for the
mesityl methyl groups, reflecting the loss of C symmetry upon
s
formation of the allyl complex. This species is generated quickly
and is observed throughout the catalytic reaction. No other
2
η -2-alkene complex, which is likely an intermediate before the
1
iridium species is observed in the H NMR spectrum.
turnover-limiting step of the reaction; (e) the first-order
dependence of the reaction on both the alkene and the catalyst
indicates that the alkene substitution step proceeds via
associative ligand displacement and is likely the turnover-
limiting step of the reaction. It should be noted that these
studies were conducted using benzylic alkenes; thus, the
mechanism of isomerization may be different for isomerization
of alkyl-substituted alkenes.
An exchange spectroscopy (EXSY) experiment conducted at
2
5 °C reveals that the hydride ligand exchanges with the
terminal CH hydrogens on the allyl ligand. This is consistent
2
3
2
with a rapid equilibrium between the η -allyl and the η -alkenyl
complexes, where the alkene is at the internal position (Figure
6). No exchange was observed between the hydride and the
A similar mechanism was proposed by Goldman and co-
workers for alkyl-substituted alkene isomerization catalyzed by
tBu4
tBu4
5b
(
PCP)IrH and ( POCOP)IrH2. In contrast to our
2
2
observations with 1-Mes, the PCP-Ir systems have an η -1-
alkene resting state similar to (a) in Scheme 5. A similar π-allyl
intermediate (c) was synthesized independently, however this
intermediate was only observed spectroscopically at low
temperatures and was found to convert to the catalyst resting
state upon warming. DFT calculations confirmed that alkene
isomerization proceeds via π-allyl intermediates in the PCP-Ir
systems. Interestingly, the calculations show that the π-allyl
3
Figure 6. Equilibrium between the η -allyl complex 2-Mes catalyst
2
resting state and the internal η -alkenyl complex.
internal allyl hydrogens. Treatment of 1-Mes with 20 equiv of
trans-β-methylstyrene and 10 equiv of NaO Bu in benzene at 80
C also generated 2-Mes, indicating that the π-allyl complex
can be formed using either the terminal alkene reagents or the
internal alkene products.
The solution structure of the π-allyl complex was confirmed
in the solid state using X-ray crystallography (Figure 7). An
ORTEP diagram of 2-Mes shows that the terminal allylic
carbon is trans to Caryl in the CCC-pincer backbone. The allyl
ligand extends over the pincer backbone and away from the
bulky mesityl groups. Although it was not observed in the
crystal structure, the hydride ligand is likely positioned cis to
the terminal allyl carbon, and cis to each pincer carbon.
Kinetics of Catalytic Isomerization with 1-Mes. A
kinetic study of the isomerization of allylbenzene was
conducted using either 1-Mes or 2-Mes as the catalyst with
t
°
2
intermediate is preferentially formed by dissociation of the η -
bound alkene to give a σ complex of the allylic C−H bond,
which then undergoes oxidative addition. Our data do not
distinguish between this mechanism and a direct addition of the
allylic C−H bond without alkene dissociation.
SUMMARY
■
The CCC-pincer iridium complexes 1-Mes and 1-Ad were
investigated as catalysts for alkene isomerization. Both
complexes are active alkene isomerization catalysts for a
number of substrates, with the exception of allyl alcohol. The
mechanism of isomerization of both catalysts was investigated
using H−D scrambling and crossover experiments. Further
mechanistic studies were conducted with 1-Mes, for which the
catalyst resting state was synthesized and isolated. It was
d -toluene as the NMR solvent. Addition of allylbenzene (100
mM) to 1-Mes (0.5 mM) and NaO Bu (5 mM) generated β-
methylstyrene with 98% yield in 12.8 h at 100 °C. The reaction
8
t
4
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Organometallics
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Figure 7. Crystal structure of 2-Mes, showing 50% probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected metric data (bond lengths
in Å and angles in deg): Ir(1)−C(8), 2.042(4); Ir(1)−C(5), 1.982(4); Ir(1)−C(2), 2.019(5); Ir(1)−C(44), 2.270(5); Ir(1)−C(45), 2.172(5);
Ir(1)−C(46), 2.303(6); C(2)−Ir(1)−C(5), 78.00(18); C(2)−Ir(1)−C(8), 155.03(17); C(5)−Ir(1)−C(8), 77.35(18); C(5)−Ir(1)−C(46),
112.7(2); C(5)−Ir(1)−C(44), 177.82(19).
Figure 8. First-order kinetics plot for the isomerization of allylbenzene to β-methylstyrene catalyzed by 2-Mes at 100 °C. The reaction was
performed with 0.996 M allylbenzene and 0.25−0.99 mM (0.25−1.0 mol %) catalyst in 1.0 mL of d -toluene: (a) 0.5 mol % of 2-Mes, k = (8.35 ±
8
obs
−5
−1
−5 −1
−5 −1
0
.04) × 10 s ; (b) 0.5 mol % of 2-Mes, k = (8.47 ± 0.05) × 10 s ; (c) 1.0 mol % of 2-Mes, k = (18.47 ± 0.13) × 10 s ; (d) 0.25 mol %
obs obs
5
−
−1
of 2-Mes, kobs = (2.84 ± 0.02) × 10 s . Data were plotted to 4 half-lives or 12.8 h, whichever came first.
determined that the size of the pincer side group drastically
affected the mechanism of isomerization: the mesityl-
substituted catalyst proceeds via the π-allyl mechanism, while
the adamantyl-substituted catalyst operates via a distinct
mechanism that is not unambiguously determined by the
available data.
activated alumina, using an MBraun Solvent Purification System. Flash
chromatography using solvent gradients was performed using a
6
b
6b
33
Combiflash RF system. 1-Mes, 1-Ad, and allylbiphenyl were
prepared as previously described. Procedures for the preparation of d2-
allylbiphenyl can be found in the Supporting Information. No
isomerization reactions were conducted in the absence of ambient
light. cis-β-Methylstyrene was stored at −32 °C in the dark until use.
Instrumentation. Nuclear magnetic resonance spectra were
EXPERIMENTAL SECTION
■
1
13
recorded on a Bruker 400 MHz ( H, 400 MHz; C, 100 MHz)
spectrometer and referenced to the residual solvent resonance (δ in
parts per million and J in Hz). Elemental analyses were performed by
Robertson Microlit, Madison, NJ.
Materials and Methods. Unless stated otherwise, all manipu-
lations were carried out in either an Ar-filled MBraun Labmaster 130
glovebox or on a Schlenk line using Ar. Solvents were generally
purified by sparging with argon and passing through columns of
4
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Synthesis of the Iridium Complex 2-Mes with β-Methylstyr-
ene. 1-Mes (2 mg, 2.4 μmol) and sodium tert-butoxide (2.3 mg, 24
μmol) were combined with 1 mL of C D in an oven-dried vial in the
6
6
glovebox. β-Methylstyrene (6.4 μL, 49 μmol) was added to the
reaction mixture, which was then heated to 75 °C with stirring for 1.5
h. The reaction mixture was cooled to room temperature and then
1
transferred to a J. Young tube. The H NMR spectrum obtained
contains peaks corresponding solely to 2-Mes (as prepared with
allylbenzene) and β-methylstyrene.
Isomerization of Allylbenzene with 1-Mes and 1-Ad. In an
argon-filled glovebox, a vial was charged with a stirbar, the iridium
catalyst (1.96 μmol), allylbenzene (132 μL, 996 μmol), sodium tert-
butoxide (1.9 mg, 19.7 μmol), and 2.0 mL of toluene. The vial was
capped and heated in the glovebox while the contents were stirred. At
the time points indicated, aliquots were removed for analysis. A 50 μL
Figure 9. Second-order kinetics plot of the observed first-order rate
constant vs catalyst concentration for the isomerization of allylbenzene
to β-methylstyrene catalyzed by 2-Mes at 100 °C. The plot gives a
aliquot was transferred to 700 μL of CDCl . The progress of the
3
1
reaction was monitored by H NMR spectroscopy by observing the
−1
−1
disappearance of the allylbenzene resonances at 5.82 (m, 1H), 4.93
second-order rate constant of 0.177 ± 0.009 M s .
(m, 2H), and 3.17 ppm (d, 2H) and the appearance of the β-
methylstyrene resonances at 6.26 (m, 1H), 5.98 (m, 1H), and 1.66
ppm (d, 3H). Separately, a 50 μL aliquot was added to 1.5 mL of ethyl
acetate, filtered, and analyzed by GC-MS to determine the ratios of
internal isomers formed.
Isomerization of Vinylcyclohexane with 1-Mes and 1-Ad. In
an argon-filled glovebox, a vial was charged with a stirbar, the iridium
catalyst (1.96 μmol), vinylcyclohexane (137 μL, 1.0 mmol), sodium
tert-butoxide (1.9 mg, 19.7 μmol), and 2.0 mL of toluene. The vial was
capped and heated in the glovebox while the contents were stirred. At
the time points indicated, aliquots were removed for analysis. A 50 μL
Synthesis of the Iridium Complex 2-Mes with Allylbenzene.
-Mes (24.7 mg, 0.030 mmol) and sodium methoxide (17.7 mg, 0.328
1
mmol) were combined with 5 mL of benzene in an oven-dried
scintillation vial in the glovebox. Allylbenzene (80.4 μL, 0.607 mmol)
was added to the reaction mixture, which was then heated to 80 °C
with stirring for 1 h. The reaction mixture was cooled to room
temperature and then filtered through a 0.45 μm PTFE Whatman filter
to obtain a clear yellow solution. The volatiles were removed under
reduced pressure to obtain a yellow solid. The solid was extracted with
aliquot was transferred to 700 μL of CDCl . The progress of the
3
1
2
0 mL of pentane, and the solution was filtered. The solid was washed
reaction was monitored by H NMR spectroscopy by observing the
with additional pentane until the filtrate became clear. The pentane
was removed under vacuum to yield a gummy yellow solid. Benzene
was added to dissolve the solid, and then this solution was placed in
the freezer until solid. Sublimation of the benzene yielded the product
disappearance of the vinylcyclohexane resonance at 5.85 ppm (m, 1H)
and the appearance of the ethylidenecyclohexane resonance at 5.20
ppm (q, 1H). Separately, a 50 μL aliquot was added to 1.5 mL of ethyl
acetate, filtered, and analyzed by GC-MS.
Isomerization of Allyl Phenyl Ether with 1-Mes and 1-Ad. In
an argon-filled glovebox, a vial was charged with a stirbar, the iridium
catalyst (1.96 μmol), allyl phenyl ether (137 μL, 998 μmol), sodium
tert-butoxide (1.9 mg, 19.7 μmol), and 2.0 mL of toluene. The vial was
capped and heated in the glovebox while the contents were stirred. At
the time points indicated, aliquots were removed for analysis. A 50 μL
1
as a powdery yellow solid. Yield: 15.3 mg, 0.018 mmol, 59%. H NMR
3
3
(
7
=
6
1
2
C D ): δ 8.02 (d, J = 8.5 Hz, 1H), 7.85 (d, J = 8.2 Hz, 1H),
6 6 HH HH
3
3
3
.74 (d, J = 3.1 Hz, 1H), 7.72 (d, J = 3.5 Hz, 1H), 7.36 (t, J
7.8 Hz, 1H), 7.07 (t, J = 8.0 Hz, 1H), 7.02 (t, J = 8.0 Hz, 1H),
HH
HH
HH
3
3
HH
HH
3
3
.93 (t, J = 7.6 Hz, 1H), 6.89 (t, J = 7.6 Hz, 1H), 6.82−6.97 (m,
HH
HH
H), 6.78 (s, 1H), 6.74 (s, 1H), 6.71−6.68 (m, 2H), 6.68−6.64 (m,
3
H), 6.64−6.57 (m, 4H), 5.01 (td, J =10.2 Hz, 7.2 Hz, 1H), 3.56
aliquot was transferred to 700 μL of CDCl . The progress of the
HH
3
3
3
1
(
d, J =10.2 Hz, 1H), 2.19 (s, 3H), 2.17 (s, 3H), 2.05 (d, J = 7.2
reaction was monitored by H NMR spectroscopy by observing the
HH
HH
Hz, 1H), 2.04 (s, 3H), 2.03 (s, 3H), 2.02 (s, 3H), 1.94 (s, 3H), 1.30
disappearance of the allyl phenyl ether resonance at 4.64 ppm (dt, 2H)
and the appearance of the cis-1-propenyloxybenzene resonance at 1.85
ppm (d, 3H) and the trans-1-propenyloxybenzene resonance at 1.79
ppm (d, 3H). Separately, a 50 μL aliquot was added to 1.5 mL of ethyl
acetate, filtered, and analyzed by GC-MS to determine the ratios of
internal isomers formed.
3
13
(
d, J = 10.9 Hz, 1H), −14.75 (s, 1H) C NMR (C D ): δ 183.0,
HH
6
6
1
1
1
1
1
79.6, 156.7, 145.5, 144.5, 139.1, 138.9, 138.3, 138.1, 137.0, 136.9,
35.8, 135.3, 133.2, 132.8, 132.7, 132.6, 130.3, 130.2, 129.3, 129.2,
27.9, 126.7, 123.4, 123.3, 123.0, 122.5, 122.4, 119.8, 111.6, 111.0,
10.4, 110.0, 108.4, 107.8, 93.6, 55.5, 21.0, 20.9, 20.1, 20.0, 19.7, 18.5,
8.1. Anal. Calcd for C H N Ir: C, 65.94; H, 5.06; N, 6.54. Found:
47
43
4
General Procedure for Allylbenzene Isomerization with 2-
Mes. In an argon-filled glovebox, 2-Mes (1.9 mg, 0.002 mmol) was
C, 65.72; H, 5.08; N, 6.39.
a
Scheme 5. Proposed Mechanism of Isomerization using 1-Mes or 2-Mes as the Catalyst
a
Mesityl groups and the pincer backbone are omitted for clarity.
4
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Organometallics
Article
dissolved in 1.0 mL of d -toluene to form a 2.2 mM stock solution. In a
1-Ad include 1,3-d -1-propenylbiphenyl, 1-d -1-propenylbiphenyl, and
8
2
1
1
13
J. Young tube, allylbenzene (13.2 μL, 0.1 mmol) and d -toluene (537−
3-d -β-methylstyrene. The H and C NMR peaks observed for 3-d1-
8
1
8
1
74 μL) were added to the 2-Mes stock solution (113−450 μL, 0.25−
.0 mol %). The J. Young tube was inserted into a preheated and
β-methylstyrene were the same as the resonances observed with β-
1
methylstyrene except for those listed here. 3-d -β-methylstyrene: H
1
1:1:1
3
2
13
preshimmed NMR set to 100 °C. The progress of the reaction was
NMR (C D ) δ 1.65 (d t, J = 6.5 Hz, J = 2.0 Hz, 2H); C
6 6 HH DH
1
1:1:1
1
monitored by H NMR spectroscopy by observing the disappearance
NMR (C D ) δ 18.2 ( t, J = 19.2 Hz).
6 6 CD
of the allylbenzene resonances at 5.82 (m, 1H), 4.93 (m, 2H), and
Isomerization of cis-β-Methylstyrene with 1-Ad. In an argon-
filled glovebox, a vial was charged with a stirbar, 1-Ad (2.04 μmol), cis-
β-methylstyrene (33.1 μL, 255 μmol), sodium tert-butoxide (1.96 mg,
3.17 ppm (d, 2H) and the appearance of the β-methylstyrene
resonances at 6.26 (m, 1H), 5.98 (m, 1H), and 1.66 ppm (d, 3H).
Isomerization of Allylbiphenyl or d -Allylbiphenyl with 1-
20.3 μmol), and 0.44 mL of d -benzene. The vial was capped and
2
6
Mes and 1-Ad. In an argon-filled glovebox, a vial was charged with a
heated in the glovebox at 80 °C while the contents were stirred. At 15,
30, and 45 min and 1, 2, and 24 h, an 80 μL aliquot was transferred to
stirbar, the iridium catalyst (1.98 μmol), d -allylbiphenyl (55.5 μL, 253
2
μmol), sodium tert-butoxide (1.9 mg, 20 μmol), and 0.44 mL of C D .
6
6
450 μL of d -benzene. The progress of the reaction was monitored by
6
The vial was capped and heated in the glovebox while the contents
were stirred, and aliquots were removed periodically for analysis. A 50
μL aliquot was transferred to 700 μL of C D in an NMR tube. The
1
H NMR spectroscopy by observing the disappearance of the cis-β-
methylstyrene resonance at 1.81 ppm (dd, 3H) and the appearance of
the trans-β-methylstyrene resonance at 1.75 ppm (dd, 3H).
Control Isomerization of cis-β-Methylstyrene. In an argon-
filled glovebox, a vial was charged with a stirbar, cis-β-methylstyrene
6
6
1
progress of the reaction was monitored by H NMR spectroscopy by
observing the disappearance of the allylbiphenyl resonances at 5.85
(
m, 1H), 4.97 (m, 2H), and 3.20 ppm (d, 2H) and the appearance of
the 1-propenylbiphenyl resonances at 6.34 (m, 1H), 6.05 (m, 1H), and
.69 ppm (d, 3H). The spectroscopic data are in agreement with
(33.1 μL, 255 μmol), sodium tert-butoxide (1.96 mg, 20.3 μmol), and
0
.44 mL of d -benzene. The vial was capped and heated in the
6
1
34
glovebox at 80 °C while the contents were stirred. At 15, 30, and 45
reported values. When d -allylbiphenyl was used, no peaks were
2
min and 1, 2, and 24 h, an 80 μL aliquot was transferred to 450 μL of
observed at 3.20 or 6.34 ppm. Isomerization with 1-Mes generated
1
d -benzene. The progress of the reaction was monitored by H NMR
1
6
1
7
1
(
1
,3-d -1-propenylbiphenyl: H NMR (C D ) δ 7.52−7.42 (m, 4H),
2
6
6
spectroscopy by observing the disappearance of the cis-β-methylstyr-
ene resonance at 1.81 ppm (dd, 3H) and the appearance of the trans-
β-methylstyrene resonance at 1.75 ppm (dd, 3H). Even after 24 h, no
isomerization was observed.
3
.30−7.20 (m, 4H), 7.17−7.12 (m, 1H), 6.07 (br t, J = 7.1 Hz,
HH
H), 1.68 (d1 t, J = 6.7 Hz, J = 2.0 Hz, 2H); C NMR
:1:1
3
2
13
HH
DH
1
:1:1
1
C D ) δ 141.4, 140.0, 137.4, 131.0 ( t, J = 23.1 Hz), 129.1,
6 6
CD
:1:1
27.6, 127.4, 127.3, 126.8, 125.5, 18.3 ( t, 1JCD = 19.4 Hz).
1
Competition Isomerization of cis-β-Methylstyrene and
Isomerization with 1-Ad generated 1,3-d -1-propenylbiphenyl, 1-d -1-
2
1
Allylbenzene or d -Allylbiphenyl with 1-Ad. In an argon-filled
1
13
2
propenylbiphenyl, and 1,3,3-d -1-propenylbiphenyl. The H and
C
3
glovebox, a vial was charged with a stirbar, 1-Ad (2.04 μmol), cis-β-
methylstyrene (48.9 μL, 377 μmol), allylbenzene (33.8 μL, 255 μmol)
NMR peaks observed were the same for each compound except for
those listed here. 1-d -1-propenylbiphenyl: H NMR (C D ) δ 1.68 (d,
JHH = 6.6 Hz, 2H); C NMR (C D ): δ 18.6 (s). 1,3,3-d -1-
1
1
6
6
or d -allylbiphenyl (55.5 μL, 255 μmol), sodium tert-butoxide (1.96
3
13
2
6
6
3
mg, 20.3 μmol), and 0.44 mL of d -benzene. The vial was capped and
13
1:2:3:2:1
1
6
propenylbiphenyl: C NMR (C D ) δ 18.0 (
quintet, J = 19.1
6
6
CD
heated in the glovebox at 80 °C while the contents were stirred. At 0,
Hz).
1
5, 30, and 45 min and 1, 2, and 24 h, an 80 μL aliquot was transferred
General Procedure for Allylbiphenyl or d -Allylbiphenyl
Isomerization with 2-Mes. In an argon-filled glovebox, 2-Mes (1.9
2
to 450 μL of d -benzene. The progress of the reaction was monitored
6
1
by H NMR spectroscopy by observing the disappearance of the
mg, 0.002 mmol) was dissolved in 1.0 mL of d -toluene to form a 2.2
8
allylbenzene resonance at 3.28 ppm (d, 2H) and the cis-β-
methylstyrene resonance at 1.81 ppm (dd, 3H) and the appearance
of the trans-β-methylstyrene resonance at 1.75 ppm (dd, 3H).
EXSY Experiments. In an argon-filled glovebox, a vial was charged
with a stirbar, 1-Mes (8.0 mg, 9.8 μmol), allylbenzene (26.4 μL, 199
μmol), sodium tert-butoxide (9.4 mg, 97.8 μmol), and 2.0 mL of
benzene. The reaction mixture was heated to 80 °C with stirring for 1
h, and then the solvent was removed under vacuum. The yellow solid
was dissolved in C D and added to a J. Young tube. Formation of the
mM stock solution. In a J. Young tube, either allylbiphenyl or d2-
allylbiphenyl (20.4 mg, 0.10 mmol) was combined with 0.23 mL of the
2
-Mes stock solution (0.51 μmol, 0.5 mol %) and d -toluene to make a
8
1
.0 mL solution. The J. Young tube was inserted into a preheated and
preshimmed NMR spectrometer set to 100 °C. The progress of the
reaction was monitored by H NMR spectroscopy by observing the
1
disappearance of the allylbiphenyl resonances at 5.85 (m, 1H), 4.97
(
m, 2H), and 3.20 ppm (d, 2H) and the appearance of the 4-
propenylbiphenyl resonances at 6.34 (m, 1H), 6.05 (m, 1H), and 1.69
6
6
1
2
-Mes was confirmed by H NMR, and then a 2D NOESY experiment
ppm (d, 3H). The spectroscopic data are in agreement with reported
34
was conducted at 25 °C.
values. When d -allylbiphenyl was used, no peaks were observed at
2
X-ray Structure Determination of 2-Mes. X-ray-quality crystals
3
.20 or 6.34 ppm.
Crossover Experiments. In an argon-filled glovebox, a vial was
t
were grown by adding 2-Mes (synthesized using NaO Bu) to pentane
until it was just dissolved and then cooling to −32 °C. After 2 months,
X-ray-quality crystals were present in the sample. Structure
determinations were performed on an Oxford Diffraction Gemini-R
diffractometer, using Cu Kα radiation. Single crystals were mounted
on Hampton Research Cryoloops using Paratone-N oil. Unit cell
determination, data collection and reduction, and absorption
charged with a stirbar, the iridium catalyst (1.98 μmol), allylbenzene
33.5 μL, 253 μmol), d -allylbiphenyl (55.5 μL, 253 μmol), sodium
(
2
tert-butoxide (1.9 mg, 20 μmol), and 0.44 mL of C D . The vial was
6
6
capped and heated in the glovebox while the contents were stirred, and
aliquots were removed periodically for analysis. A 50 μL aliquot was
transferred to 700 μL of C D in an NMR tube. The progress of the
6
6
35
1
correction were performed using the CrysAlisPro software package.3
reaction was monitored by H NMR spectroscopy by observing the
disappearance of the allylbenzene resonances at 5.82 (m, 1H), 4.93
6
Direct methods structure solution was accomplished using SIR92,
(
m, 2H), and 3.17 ppm (d, 2H) and the d -allylbiphenyl resonances at
and full-matrix least-squares refinement was carried out using
2
37
5.85 (m, 1H) and 4.97 ppm (m, 2H) and the appearance of the β-
CRYSTALS. All non-hydrogen atoms were refined anisotropically.
Unless otherwise noted, hydrogen atoms were placed in calculated
positions, and their positions were initially refined using distance and
angle restraints. All hydrogen positions were fixed in place for the final
refinement cycles. Highly disordered solvent was present in the unit
cell; correction for this residual density was performed using the
methylstyrene resonances at 6.26 (m, 1H), 5.98 (m, 1H), and 1.66
ppm (d, 3H) and the 4-propenylbiphenyl resonances at 6.34 (m, 1H),
6
.05 (m, 1H), and 1.69 ppm (d, 3H). At the end of the reaction, a 13
C
NMR spectrum was acquired, and the position of the deuteriums in
the products was determined by observing the β-methylstyrene
resonances at 131.6, 125.5, and 18.5 ppm and the 4-propenylbiphenyl
resonances at 131.0, 125.4, and 18.3 ppm. Isomerization with 1-Mes
3
8
option SQUEEZE in the program package PLATON. A total of 213
electrons per unit cell were removed, from a total potential solvent-
3
generated the normal products β-methylstyrene and 1,3-d -1-
accessible void of 1396 Å . In the early stages of refinement, minor
2
propenylbiphenyl. The products observed from isomerization with
disorder was observed in the allyl fragment. Attempts to include this
4
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disorder in the model did not produce improved results. The iridium-
bound hydride was not located in the difference map.
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ASSOCIATED CONTENT
Supporting Information
A CIF file and text, tables, and figures giving crystallographic
data for 2-Mes, complete experimental procedures and NMR
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*
S
spectra for the synthesis of d -allylbiphenyl, time course plots
2
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for the isomerization of vinylcyclohexane and allylphenyl ether,
time course data for the E:Z ratio of products observed during
isomerization of allyl phenyl ether and allylbenzene, time
course data for competition experiments on cis-β-methylstyrene
and allylbenzene isomerization catalyzed by 1-Ad, detailed
(
b) Lim, H. J.; Smith, C. R.; RajanBabu, T. V. J. Org. Chem. 2009, 74,
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565−4572.
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1
13
́
NMR assignments for 2-Mes, and full H and C NMR spectra
from the isomerization reaction with allylbiphenyl. This
material is available free of charge via the Internet at http://
pubs.acs.org.
Chem. 2006, 245, 200−209. (b) Varela-Alvarez, A.; Sordo, J. A.;
Piedra, E.; Nebra, N.; Cadierno, V.; Gimeno, J. Chem. Eur. J. 2011, 17,
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AUTHOR INFORMATION
Corresponding Author
E-mail for A.R.C.: achianese@colgate.edu.
■
4
2
627−4629. (b) Kim, I. S.; Dong, G. R.; Jung, Y. H. J. Org. Chem.
007, 72, 5424−5426. (c) Canovese, L.; Santo, C.; Visentin, F.
*
Notes
Organometallics 2008, 27, 3577−3581. (d) Spallek, M. J.; Stockinger,
The authors declare no competing financial interest.
S.; Goddard, R.; Trapp, O. Adv. Synth. Catal. 2012, 354, 1466−1480.
(
20) Scarso, A.; Colladon, M.; Sgarbossa, P.; Santo, C.; Michelin, R.
ACKNOWLEDGMENTS
We thank the National Science Foundation (Grant No. CHE-
057792) for financial support.
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