Mechanism of efficient anti-markovnikov olefin hydroarylation catalyzed by homogeneous Ir(iii) complexes

Article abstract of DOI:10.1039/c0gc00330a

The mechanism of the hydroarylation reaction between unactivated olefins (ethylene, propylene, and styrene) and benzene catalyzed by [(R)Ir(μ-acac-O, O,C³)-(acac-O,O)₂]₂ and [R-Ir(acac-O,O) ₂(L)] (R = acetylacetonato, CH₃, CH₂CH ₃, Ph, or CH₂CH₂Ph, and L = H₂O or pyridine) Ir(iii) complexes was studied by experimental methods. The system is selective for generating the anti-Markovnikov product of linear alkylarenes (61:39 for benzene + propylene and 98:2 for benzene + styrene). The reaction mechanism was found to follow a rate law with first-order dependence on benzene and catalyst, but a non-linear dependence on olefin. ¹³C-labelling studies with CH₃¹³CH₂-Ir-Py showed that reversible β-hydride elimination is facile, but unproductive, giving exclusively saturated alkylarene products. The migration of the ¹³C-label from the α to β-positions was found to be slower than the C-H activation of benzene (and thus formation of ethane and Ph-d ₅-Ir-Py). Kinetic analysis under steady state conditions gave a ratio of the rate constants for CH activation and β-hydride elimination (k _CH: k_β) of ～0.5. The comparable magnitude of these rates suggests a common rate determining transition state/intermediate, which has been shown previously with B3LYP density functional theory (DFT) calculations. Overall, the mechanism of hydroarylation proceeds through a series of pre-equilibrium dissociative steps involving rupture of the dinuclear species or the loss of L from Ph-Ir-L to the solvento, 16-electron species, Ph-Ir(acac-O,O)₂-Sol (where Sol refers to coordinated solvent). This species then undergoes trans to cis isomerization of the acetylacetonato ligand to yield the pseudo octahedral species cis-Ph-Ir-Sol, which is followed by olefin insertion (the regioselective and rate determining step), and then activation of the C-H bond of an incoming benzene to generate the product and regenerate the catalyst.

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PAPER
Mechanism of efficient anti-Markovnikov olefin hydroarylation catalyzed
by homogeneous Ir(III) complexes†
Gaurav Bhalla,a Steven M. Bischof,b Somesh K. Ganesh,a,b Xiang Yang Liu,a C. J. Jones,a
Andrey Borzenko,a William J. Tenn, III,a Daniel H. Ess,b,c Brian G. Hashiguchi,b Kapil S. Lokare,b
Chin Hin Leung,b Jonas Oxgaard,c William A. Goddard, III*c and Roy A. Periana*a,b
Received 15th July 2010, Accepted 28th October 2010
DOI: 10.1039/c0gc00330a
The mechanism of the hydroarylation reaction between unactivated olefins (ethylene, propylene,
and styrene) and benzene catalyzed by [(R)Ir(m-acac-O,O,C3)-(acac-O,O)2]2 and
[R-Ir(acac-O,O)2(L)] (R = acetylacetonato, CH3, CH2CH3, Ph, or CH2CH2Ph, and L = H2O or
pyridine) Ir(III) complexes was studied by experimental methods. The system is selective for
generating the anti-Markovnikov product of linear alkylarenes (61 : 39 for benzene + propylene
and 98 : 2 for benzene + styrene). The reaction mechanism was found to follow a rate law with
first-order dependence on benzene and catalyst, but a non-linear dependence on olefin.
13C-labelling studies with CH313CH2-Ir-Py showed that reversible b-hydride elimination is facile,
but unproductive, giving exclusively saturated alkylarene products. The migration of the 13C-label
from the a to b-positions was found to be slower than the C–H activation of benzene (and thus
formation of ethane and Ph-d5-Ir-Py). Kinetic analysis under steady state conditions gave a ratio
of the rate constants for CH activation and b-hydride elimination (kCH: kb) of ~0.5. The
comparable magnitude of these rates suggests a common rate determining transition
state/intermediate, which has been shown previously with B3LYP density functional theory
(DFT) calculations. Overall, the mechanism of hydroarylation proceeds through a series of
pre-equilibrium dissociative steps involving rupture of the dinuclear species or the loss of L from
Ph-Ir-L to the solvento, 16-electron species, Ph-Ir(acac-O,O)2-Sol (where Sol refers to coordinated
solvent). This species then undergoes trans to cis isomerization of the acetylacetonato ligand to
yield the pseudo octahedral species cis-Ph-Ir-Sol, which is followed by olefin insertion (the
regioselective and rate determining step), and then activation of the C–H bond of an incoming
benzene to generate the product and regenerate the catalyst.
starting materials and several additional synthetic steps both
Introduction
of which generate excessive and hazardous halogenated waste.
The coupling reaction of arenes and olefins to generate
In addition, regioselectivity in these reactions is often difficult
alkylarenes continues to be both an important C–C bond
to predict and control because it is determined by the stabil-
forming reaction and route to functionalize C–H bonds.1 Clas-
ity of the carbocation intermediate.3 Therefore, development
sically, straight-chain and branched-chain alkylbenzenes are
of alternative catalysts not based on obligatory carbocation
generated by a Friedel–Crafts acylation followed by reduction
intermediates that control regio- and stereoselectivity with toler-
or electrophilic substitution of an arene (Scheme 1a and c,
ance to a broad range of substrates and functional groups would
respectively).2 This requires the use of halogenated hydrocarbon
be advantageous. This suggests that strategies based on using the
C–H bond activation reaction4 followed by selective C–C bond
functionalization (Scheme 1b) would minimize halogenated
aThe Loker Hydrocarbon Institute, Department of Chemistry, University
waste and unnecessary multistep procedures while still allowing
of Southern California, Los Angeles, California, 90089.
access to these useful functionalized products.
bThe Scripps Energy Laboratories, Department of Chemistry, The
The most practical approach to generate the formal
Markovnikov (branched) hydroarylation addition was devel-
oped by Murai and co-workers using chelation-assistance to
directly react olefins with acylheteroaromatics catalyzed by Ru.5
Due to the availability of a coordinating N or O, it is believed
these systems operate via chelation assisted C–H activation
Scripps Research Institute, Jupiter, Florida, 33458, USA.
E-mail: rperiana@scripps.edu
cMaterials and Process Simulation Center, Division of Chemistry and
Chemical Engineering, California Institute of Technology, Pasadena,
California, 91125. E-mail: wag@wag.caltech.edu
† Electronic supplementary information (ESI) available: Kinetic experi-
ments. See DOI: 10.1039/c0gc00330a
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Whereas more electron-rich metal/ligand systems also yield
higher linear/branch ratios. In addition, the effect of sterics
versus electronics on the linear to branched ratio cannot be
generalized and is dependent on each specific system.
The most practical approach for the generation of anti-
Markovnikov hydroarylation products was initially reported by
Periana, Matsumoto, and co-workers in 2000.14 The reaction
entailed C–H bond activation of benzene followed by olefin in-
sertion to generate C–C functionalized products with selectivity
towards the straight chain alkylbenzene (Scheme 1b).
The hydroarylation reaction was catalyzed by an O-donor,
dinuclear, IrIII complex based on the bis-acac motif, [Acac-C–
Ir]2. Throughout this paper, unless otherwise specified, acac-O,O
is the O-bound, acetylacetonato or 2,4-pentanedione ligand. To
further simplify discussion, the four-coordinate (acac-O,O)2IrIII
motif is abbreviated as, Ir, and trans and cis refer to the
orientation of the two non-spectator ligands on this motif. Thus,
the abbreviation, trans-R-Ir-L, is understood to be the trans-
(acac-O,O)2IrIII(L)(R) complex with the non-spectator ligands,
R and L, in a trans orientation (and the two acac-O,O ligands
are in a single plane). Conversely, cis-R-Ir-L is understood to
be cis-(acac-O,O)2IrIII(L)(R) where the non-spectator ligands,
R and L, are in a cis orientation (and the two acac-O,O
ligands are not in a single plane).15,16 The related mononuclear
species, (acac-O,O)2Ir(R)(L) (where R = acac, CH3, CH2CH3,
Ph, CH2CH2Ph), also catalyzes rapid benzene-solvent H/D
exchange. Previously reported experimental and theoretical
studies revealed several important conclusions: 1) the dinuclear
and mononuclear complexes follow the same mechanistic steps;
2) initiation involves either dissociation of the dinuclear complex
or ligand loss from the mononuclear species to generate a
reactive 5-coordinate intermediate; 3) C–H activation involves
the 5-coordinate species, requiring trans to cis isomerization of
the acac ligand; and 4) the C–H activation step is not the rate
determining step.
Given the importance and potential broad utility of hydroary-
lation reactions at reducing halogenated waste and direct access
to straight chain alkylbenzenes,17 we have begun a systematic
study to understand the reactivity and selectivity of the bis-
(acac-O,O)Ir motif in order to design more active catalysts.18
We have recently reported a bis-tropolonato IrIII analogue which
shows a higher rate of C–H activation and comparable rates
of hydroarylation.19 Herein, we present a detailed experimental
study of hydroarylation catalyzed by O-donor IrIII complexes
to expand our understanding of this system and answer the
following questions:
Scheme 1 Comparison of (a) Friedel–Crafts acylation followed by
Clemmenson reduction, (b) hydroarylation of olefins, and (c) Friedel–
Crafts alkylation.
by initial coordination of the N or O group followed by
directed intramolecular C–H activation. More recently, similar
O-atom chelation assistance was observed using Re, Rh, and
Au complexes.6 The Goldman group has shown that Ir(PCP)
pincer complexes selectively add the ortho C–H bonds of
nitrobenzene and acetophenone to olefins; however this was
found to be the result of thermodynamic control and not
chelation assistance.7 Other groups have utilized intramolecular
reactions with alkenes tethered to aryl substrates to facilitate a
ring-closing hydroarylation reaction.8 Currently, Markovnikov
hydroarylation catalytic systems have been limited to metals such
as Pt, Pd, Ir, Au, and Ru.9
There are limitations to the route developed by
Murai and Goldman because they yield only the branched
(Markovnikov) products, and access to the linear (anti-
Markovnikov) products are vital to a synthetic toolbox. In
addition, conventional techniques (Scheme 1a or c) require the
use of multi-step processes that produce large quantities of
halogen-containing waste. More recently, we and others have
developed a variety of systems based on Ru, Rh, Ir, and Pt that
generate the preferred anti-Markovnikov selectivity in a single
step reaction.10
Computational studies of such systems have revealed a
delicate balance between the two steps of hydroarylation: 1)
olefin insertion into the coordinated Ir-phenyl and 2) C–H
activation of the arene.11 It was determined that these two
steps often have an inverse relationship and the energy barrier
for one step adversely affects the other. By studying several
computational analogues of our bis-(acac-O,O)Ir motif and
Gunnoe’s Tp-Ru(Ph)(CH3CN)(CO)12 system (which reacts via
the same mechanism), it was concluded that the barriers for C–
H activation and olefin insertion are dependent on the energy
of the metal d-orbitals. Thus, the barrier for olefin insertion
increases with higher energy d-orbitals, while the barrier for C–
H activation correspondingly decreases. Similar computational
studies were conducted with selected systems that favor anti-
Markovnikov products to reveal that both sterically bulky
and electron-deficient olefins appear to favor linear products.13
(1) Is the observed anti-Markovnikov regioselectivity due to
thermodynamic or kinetic control?
(2) Is the active catalyst a dinuclear or mononuclear species?
(3) What are the reaction orders of the substrates?
(4) Why are no b-hydride elimination products observed?
(5) What is the rate-determining step and complete mecha-
nism for hydroarylation?
Results and discussion
The (acac-O,O)2Ir motif (Fig. 1) has been well established as
both a C–H activation and hydroarylation catalyst.11,13,14,15,16 For
C–H activation, the IrIII system has been shown to activate
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number (TON) versus time for the hydroarylation reaction
between propylene and benzene using Ph-Ir-Py.20 The linear
relationship between TON and time shows that the catalyst is
◦
active and thermally stable at 180 C over 4 h. This showcases
the impressive thermal stability of the O-donor acac ligands.
Mononuclear or dinuclear catalyst
In the initial report of hydroarylation by the (acac-O,O)2Ir(III)
complexes, we utilized the dinuclear complex, [Acac-C–Ir]2.14,16
Subsequently, we found that the mononuclear complexes (Ph-
Ir-L, Acac-C–Ir-L where L = H2O, Py, etc.) are also active
hydroarylation catalysts. While this suggests that the active
catalyst is a mononuclear (acac-O,O)2Ir(R)(L) complex, it could
be speculated that the active catalyst could be a dinuclear Ir
species or operating via a bimolecular pathway.
Fig. 1 Acetylacetonato based Ir(III) O-donor complexes studied.
the C–H bonds of a wide variety of sp2 and sp3 hydrocarbons
both stoichiometrically and catalytically. The motif can tolerate
a wide array of substrates and yields predominantly linear
alkylarenes as a hydroarylation catalyst. Both mononuclear
and dinuclear (acac-O,O)2IrIII complexes have been shown to
generate a mononuclear cis-(acac-O,O)2IrIII species in situ. The
most active hydroarylation catalyst previously reported was Ph-
Ir–H2O, with a turnover frequency (TOF) of 1.3 ¥ 10-2 s-1.20
Reactions of Ph-Ir–H2O in benzene with propylene and styrene
led to a ratio of linear:branched products of 61 : 39 (benzene +
propylene) and 98 : 2 (benzene + styrene). Due to the unique
selectivity towards linear products, lack of rearrangements,
and the lack of b-hydride elimination products, we were led
to examine the mechanism for the hydroarylation reaction in
greater detail.
We have previously reported a facile conversion of these
dinuclear complexes to stable mononuclear complexes when
treated with coordinating ligands L (where L = pyridine),16e
which is inconsistent with dinuclear complexes as the stable,
active catalysts. Furthermore, study of the labile [CH3-Ir]2
complex by 1H and 13C NMR revealed that dissociation to stable
5-coordinate square pyramidal complexes (or 6-coordinate sol-
vento complexes) occurs with an activation parameter (DG‡
)
298 K
of 14.1 0.5 kcal mol-1, which is lower than the DG‡ barrier for
hydroarylation.16e
We have previously proposed a mechanism for the hydroary-
lation reaction involving benzene and an olefin using the (acac-
O,O)2IrIII motif as the catalyst (Scheme 2).16e The mechanism en-
tails the dinuclear complex, [L-Ir]2 or the mononuclear complex,
R-Ir-L, disassociating into a coordinatively unsaturated species
to undergo a trans to cis isomerization followed by benzene
C–H activation to generate a cis-iridium phenyl intermediate.
The olefin then rapidly coordinates to the open coordination
site to form cis-Ph-Ir-Ol (Ol = olefin). Alkene insertion into
the iridium phenyl bond followed by coordination of benzene
yields cis-PhCH2CH2-Ir-g2(C6H6). C–H bond activation by an
oxidative hydrogen migration (OHM) mechanism releases the
product and regenerates the iridium phenyl intermediate.11,13,15
Examination of the predicted rate laws for a mononuclear
catalytic species versus a dinuclear catalytic species generated
in situ shows concentration dependence on L (Scheme 3). If we
presume that the active catalyst is a dinuclear complex (M-M)
and that it is in equilibrium with the catalyst precursor, M-
L, then the rate law will be proportional to [M-L]2/[L]2 under
steady state conditions. This is reasonable if the equilibrium
constant for the formation of M-M or the amount of L formed
from dissociation of M-L are small. Consistent with these
assumptions, VT-NMR analysis of a C6D6 solution of Ph-Ir-
Py at 100 ◦C showed no detectable free pyridine or dinuclear
complexes. These results are also consistent with previous
theoretical calculations that indicate that both reactions are
endoergic with a DG > 5 kcal mol-1.11,13,15
Catalyst stability
If the catalyst is the dinuclear complex M-M, plotting the TOF
vs. [1/L2] should yield a straight line. On the other hand, if the
active catalyst is the mononuclear species “M” generated by the
loss of L from added M-L, then plotting TOF vs. [1/L] should
yield a straight line at constant [M-L]. Analysis of reactions
charged with varying amounts of pyridine (2–10 eq.) under
standard pseudo first order conditions21 at 180 ◦C for 30 min in a
temperature controlled oil bath, yields a linear correlation (R2 =
0.997) when 1/(equivalents of pyridine) is plotted against TOF
as shown in Fig. 3. The linear correlation for Ph-Ir-Py strongly
indicates a mononuclear catalyst as the active species. The system
is also inhibited by pyridine suggesting a coordination catalysis
mechanism.
Initially, we examined if the reaction system was stable over
reaction times >1 h, as previously reported reactions were
carried out for <1 h.14,16 Fig. 2 shows a plot of turnover
Reaction order of substrates
Next, we wanted to examine the order of the various reactants
(benzene, catalyst, and olefin) present during reaction. The
reaction order on benzene was determined using Ph-Ir-Py as
the catalyst in cyclohexane. Control reactions have shown that
Fig. 2 Time-dependent hydroarylation of propylene using Ph-Ir-Py as
the catalyst.
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Scheme 2 Catalytic cycle for the hydroarylation reaction using [L-Ir]2 or R-Ir-L.
cyclohexane does not react under hydroarylation conditions and
therefore can be used as an inert solvent for examining the order
of benzene. A 10 mL glass Schlenk bomb flask fitted with a
resealable Teflon valve and a magnetic stir bar was charged
with dry, distilled benzene (0.1 to 1 mL), styrene (0.6 mL)
and 0.1 mol% of c◦atalyst from a stock solution. The solution
was heated to 180 C for 30 min in a well stirred, temperature
controlled oil bath. Analysis of the reaction mixture by GC-MS
and referenced to the internal standard, yields the data shown in
Fig. 4. A linear relationship exists between benzene concentra-
tion and TOF, which is expected for first-order dependence on
benzene.
The hydroarylation of olefins to generate mono-alkylarenes is
typically carried out at ~200 ◦C with a 5–10 mM catalyst loading
in 1 mL of neat arene (which acts as both reactant and solvent)
and 10–20 mol% olefin. Studies of olefin concentration (with
styrene as the olefin) under these conditions show a complicated
dependence (Fig. 5) on concentration. The initial increase in
the TOF is consistent with a first order dependence on olefin;
however, higher concentrations of olefin lead to a drastic drop
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Scheme 3 TOF dependence on [L] for dinuclear and mononuclear
complexes.
Fig. 5 Kinetic dependence of hydroarylation on olefin concentration.
Fig. 3 TOF vs. 1/equivalents of pyridine catalyzed by Ph-Ir-Py.
Scheme 4 Proposed rate law for Ph-Ir-Py.
and hydroarylation show first order dependence on the catalyst
(acac-O,O)2Ir(R)(L).16 As can be seen from the data there is
an inverse dependence on L and a first order dependence on
benzene and olefin at low pressures.
Fig. 4 Kinetic dependence of hydroarylation on benzene concentration
using Ph-Ir-Py.
Thermodynamic vs. kinetic control
in catalytic rate. We previously had observed inhibition by high
concentrations of olefin.15,16 Likely, high olefin concentrations
leads to a ground-state stabilization effect by binding to the
Ir center and thus effectively blocking arene binding that is
required for catalysis.
As the concentration of free L during catalysis could not be
easily determined, we carried out reactions in the presence of
excess L (L0 > 1 eq. of added catalyst) as this allows the simplifi-
cation that L = L0 if we assume that the pre-equilibrium term, K1,
is small. Based on the proposed mechanism (Scheme 2)11,13,15,16
and as can be seen from the simplified rate law in Scheme 4, a
first order dependence on the concentrations of catalyst, olefin
and benzene is predicted along with an inverse dependence
on L. We have previously shown that both C–H activation
A key feature of the (acac-O,O)2IrIII catalysts is the preference
for anti-Markovnikov regioselectivity in olefin hydroarylation
to give straight-chain (linear) alkylbenzenes. However, based
on DFT calculations,11,13,15 the 40 : 60 ratio of iso- to n-propyl
benzene can be expected based on the thermodyanmic heats
of formation of the products (see Supporting Information†).22
Therefore, we wanted to examine whether the reaction is under
thermodynamic or kinetic control.
For the insertion to be under thermodynamic control, it is
required that a facile pathway be available for the intramolec-
ular interconversion of the n-propyl and isopropyl benzene
isomers under the conditions required for hydroarylation re-
action. Importantly, such alkylarene isomerizations must not be
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Table 1 Comparison of the thermodynamic and experimental ratio
for hydroarylation with various olefins where the expected thermo-
dynamic ratio is based on relative free energies via DFT using
B3LYP/LACV3P++**//LACVP** (see Supporting Information†)
Table 2 Stoichiometric products after treatment of Ph-Ir-Py with R–
CH CH2, where R = C6H5, CH3, or C4H9
R group
Linear/L
Branched (B)
L:B Ratio
Benzene (%)
CH3
Ph
C4H9
40
39
50
27
1
25
61 : 39
98 : 2
69 : 31
33
60
25
Expected thermo-
dynamic ratio
Observed thermo-
dynamic ratio
Olefin
Linear
Branched
Linear
Branched
Propylene
Styrene
1-Hexene
Isobutylene
75
97
87
96
35
3
13
4
61
98
69
82
39
2
31
18
gioselectivity. To investigate this step, we examined the stoichio-
metric reaction of Ph-Ir-Py with various olefins and determined
a hydrogen donor is required for the generation of alkylarenes.
In the proposed catalytic reaction mechanism, this is provided
by the arene co-reactant in the C–H activation step, or from
the olefin via vinylic C–H bond activation. However, since the
C–H activation step is much faster than insertion, mesitylene
was used as a solvent (rather than benzene) to allow for
only stoichiometric reactivity. Control experiments showed that
olefin hydroarylation reaction between benzene and propylene
catalyzed by Ph-Ir-Py is uneffected by added mesitylene. NMR
spectra of these reactions show that the organometallic complex
after the reaction is either Mes-Ir-L or vinyl-Ir-L, the latter being
the major product.16d,24
accompanied by intermolecular trans-alkylation (reactions that
transfer alkyl groups between different arenes), because in
that case, the predicted thermodynamic products would be
expected to be poly-alkylarenes (e.g. di-n-propylbenzene or
di-iso-propylbenzene), which are not observed by GC-MS or
NMR analysis. To test the possibility that a selective intramolec-
ular alkylarene isomerization catalyst is generated, isopropyl
benzene was monitored under the hydroarylation conditions
(i.e., in the presence of benzene and the catalyst, Ph-Ir-Py)
to detetermine if rearrangement occurs to yield the thermody-
namically more stable n-propyl benzene isomer (eqn (1)). GC-
MS analysis of the resulting reaction mixture showed that the
isopropyl benzene remains unchanged and no n-propyl benzene
was formed (Table 1). To further mimic reaction conditions, the
reaction was repeated in the presence of ethylene to rule out
the possibility that the potential active catalyst is generated only
in the presence of olefins. Under these conditions, while ethyl
benzene is observed (indicating that an active hydroarylation cat-
alyst is present) no rearrangement of isopropyl benzene occurs
and n-propyl benzene is not observed. These experiments rule
out the possibility that the anti-Markovnikov selectivity stems
from thermodynamic control during the hydroarylation reaction
and suggest that the similarity between the thermodynamic
and the observed product distribution with propylene is merely
coincidental. This leads to the conclusion that the reaction
operates under kinetic control.
The stoichiometric reaction was studied using Ph-Ir-Py (15
mmol) with olefins such as propylene, ethylene, styrene and 1-
◦
hexene in liquid mesitylene at 180 C for 20 min. The gas and
liquid phases were analyzed by GC-MS to identify and quantify
the reaction products. The solvent was then removed and the
non-volatile reaction products were dissolved in CDCl3 and
analyzed by NMR (Table 2). Addition of styrene (R = Ph) to Ph-
Ir-Py in mesitylene yields 40% of the hydroarylation products,
dihydrostilbene and 2,2-diphenylethane in a 98 : 2 ratio, along
with 60% benzene (the total yield of linear + branched + benzene
is 100% with respect to Ph-Ir-Py). Similarly, addition of 1-hexene
(R = C4H9) leads to the formation of 1-phenyl hexane and 2-
phenylhexane ina69 : 31ratio, alongwithfree benzene. Identical
regioselectivity was observed when cis-Ph-Ir-Py was used in lieu
of Ph-Ir-Py.
In addition to hydroarylation products, a substantial amount
of benzene is generated along with derivatives of the correspond-
ing vinyl-Ir-L complex. Benzene should be the product of the
microscopic reverse C–H transfer step:
(1)
Ph-Ir-Py + CH2 CHR → cis-Ph-Ir-(CH2 CHR) + Py
(2)
→ cis-(CH2 CR)-IR-C6H6 + Py → (CH2 CR)-Ir-Py
+ C6H6
It is interesting to note that the same regioisomers are
observed in the well documented Heck reaction.23 The common
step of both the Heck and hydroarylation reactions is the
insertion of the olefin into a M–Ph bond. Based on our
theoretical results, electronic and steric factors both contribute
to controlling the reaction selectivity.11
During the catalytic cycle, this transformation is expected to
be reversible and would yield the olefin intermediate, cis-Ph-Ir-
Ol. However, in the stoichiometric reaction, the concentration
of benzene is sufficiently low to trap the vinyl intermediate.
From these results, there are two important observations.
First, the stoichiometric reactions of Ph-Ir-Py with olefins results
in the same ratios of branched to linear alkylarene products as
those observed in olefin hydroarylation catalysis. This suggests
The olefin insertion step with the Ph–Ir olefin intermediate,
cis-Ph-Ir-Ol, is proposed to control the anti-Markovnikov re-
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Scheme 5 Reversible versus irreversible b-hydride elimination.
that our catalyst structure is maintained in the catalytic cycle,
and no induction period is required for catalysis. Second, the
presence of both the hydroarylated product and benzene in the
non-complete stoichiometric reactions shows that both C–H
activation and insertion occurs in this system. However, the
observed relative rates of (linear + branched) vs. benzene does
not allow us to determine relative rates of insertion vs. C–H
activation.
reversible, unproductive b-hydride elimination reactions occur.
Previous DFT calculations15 have suggested that reversible b-
hydride elimination is favorable, but that such reactions are
reversible and unproductive. The calculated barrier for disso-
ciative loss of olefin from the saturated, 6-coordinate hydride
intermediates is higher than the barrier for the C–H activation
step (Scheme 4).
Metal alkyls that possess b-C–H bonds, but do not react
to generate olefins, could be useful in a variety of catalytic
reactions (polymerization, hydroarylation, etc.). To further the
understanding about the selectivity towards alkylarenes, the a-
13C-labelled complex, CH313CH2-Ir-Py, was synthesized by a
route analagous to CH3-Ir-Py and the C–H activation with C6D6
was examined.16d The ethyl-Ir complex was chosen because it
would not show a steric or electronic bias to possible [1,2]-Ir-
carbon rearrangements from reversible b-hydride elimination
reactions. CH313CH2-Ir-Py was synthesized using 13C-labelled
Et2Hg. As shown in Scheme 6, if reversible b-hydride elimination
does occur with CH313CH2-Ir-Py, this would lead to migration
of the 13C-label from the a to the b-position and formation of the
13CH3CH2-Ir-Py regioisomer. Indeed, carrying out this reaction
in C6D6 led to two regioisomers of ethane: 13CH3CH2D and
13CH2DCH3 (formed by C–D activation of the C6D6 solvent and
loss of 13C-ethane). The progress of the C–H activation reaction
b-Hydride elimination
It is well known that metal-alkyl complexes possessing b-C–H
bonds are susceptible to facile b-hydride elimination reactions.25
During hydroarylation reactions using the (acac-O,O)2Ir motif
no olefin products are observed during post reaction analysis.16
Additionally, analysis of the liquid and gas phases of the stoi-
chiometric C–H activation reactions between both PhCH2CH2-
Ir-Py and CH3CH2-Ir-Py with benzene, by NMR and GC-MS
showed that no olefinic products (such as styrene or ethylene)
or Ir bound olefins were formed.14,16 Olefin products would be
expected from irreversible b-hydride elimination reactions from
the coordinatively unsaturated intermediates cis-PhCH2CH2-Ir
and cis-CH3CH2-Ir. The observation that only PhCH2CH2D is
quantitatively formed from the stoichiometric CH activation of
C6D6 with PhCH2CH2-Ir-Py is only negative evidence towards
b-hydride elimination reactions occurring. Complete lack of
olefinic products could suggest that b-hydride elimination
reactions either do not occur or are reversible and unproductive
(Scheme 5). Given these plausible possibilities, the selective
formation of PhCH2CH2D from the C–H activation of C6D6
with PhCH2CH2–Ir-Py does not rule out the possibility that
1
was monitored by 13C{ H} (with sufficiently long relaxation
delay to afford accurate integration of the 13C resonances) and
1H NMR as shown in Fig. 6. The study of the reaction progress
shows evidence for the migration of the 13C label of CH313CH2-
Ir-Py (~-9 ppm) from the a to the b-position, resulting in
a formation of the b-13C regio-isotopomer, 13CH3CH2-Ir-Py
(~18 ppm).
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Scheme 6 Possible products expected from heating CH313CH2-Ir-Py in C6D6 to generate ethane by C–H activation showcasing reversible but
unproductive b-hydride elimination.
label of CH313CH2-Ir-Py. Importantly, the lack of formation of
equimolar amounts of CH313CH2-Ir-Py and 13CH3CH2-Ir-Py as
ethane is lost with concomitant C–H activation of C6D6, strongly
(3)
indicates that the a to b-migration of the 13C-label is slower than
both the C–H activation of benzene and the formation of ethane
with Ph-d5-Ir-Py. This result is confirmed by analysis of the
dissolved ethane produced from arene C–H activation. Thus,
both regioisomers of mono-2H,13C-ethane (~8 ppm composed
Fig. 6 also shows that a steady state concentration of the b-
isomer, 13CH3CH2-Ir-Py (~18 ppm), is generated but the concen-
tration is substantially lower than the CH313CH2-Ir-Py present
(-9 ppm). This indicates that reversible b-hydride elimination
does occur and accounts for the a to b-migration of the 13C-
2
of a singlet from 13CH3CH2D and a 1 : 1 : 1 triplet from H-13C
coupling in 13CH2DCH3) are generated from the C–H activation
of C6D6 as shown in eqn (2). Simulation of this pattern readily
shows that the predominant ethane product is 13CH2DCH3
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Fig. 6 Time dependent 13C NMR spectra for the reaction of CH313CH2-Ir-Py (-9 ppm) with C6D6 to form 13CH3CH2-Ir-Py (18 ppm) and two
regioisomers of ethane (8 ppm).
with ~16 mol% of 13CH3CH2D and that this ratio is essentially
constant over the course of the reaction. Analyses by 1H NMR
also confirms (on the basis of the acac resonances) that Ph-d5-
Ir-Py is the only new (acac-O,O)2Ir product formed after loss of
ethane.
Importantly, rate measurements of the migration of the 13C-
label in CH313CH2-Ir-Py and/or the relative ratio of the two
regioisomers of ethane could both be expected to provide infor-
mation on the relative rates of reversible b-hydride elimination
versus benzene C–H activation. This was further examined
through the data gathered from NMR experiments (Fig. 6).
An assumption that the formation of 13CH3CH2-Ir-Py is under
steady-state conditions was applied to the kinetic analysis. It is
assumed that C–H activation operates via a bimolecular pathway
based on the concentration of benzene. b-hydride elimination
is observed via 13C migration of the ethane label suggesting
a unimolecular reaction pathway. Comparison of the ratio of
rate constants for the C–H activation to b-hydride elimination
Scheme 7 Kinetic scheme for C–H activation and a to b 13C-migration.
(kCH : kb) reactions gives a ratio of 13CH2DCH3 to 13CH3CH2D
of ~0.5 (Scheme 7).
are also the thermodynamically most stable products.27 This
comparison highlights an important characteristic of the more
electronegative O-donor complexes. Unlike the more electron
rich compounds, the O-donor acac system avoids a thermo-
dynamic sink of irreversible olefin binding by reducing the
electron density on the Ir and allows for reversible olefin binding.
The comparable rate constants for C–H activation and b-
hydride elimination would seem to suggest a common rate
determining intermediate for both processes, as was seen for
the C–H activation and the trans-cis isomerization reactions of
(acac-O,O)2Ir complexes.16e The intermediate is most likely, cis-
R-Ir-Sol, where R = CH3CH2- in this case. Indeed, previous
theoretical calculations have shown that the formation of the
cis-Et-Ir-Sol intermediate is rate determining and the rates for
C–H activation and hydroarylation are equivalent.11,13,15
These findings should be contrasted to the related but more
electron rich IrIII complex, [Cp*(PMe3)IrR(OTf)], which under-
goes irreversible b-hydride elimination to form very stable olefin
metal hydrides that inhibit catalysis.26 Another closely related
molecule, TpIr(olefin)2, has been shown to transform into
IrIII vinyl hydride isomers, [TpIr(CH CH2)H(C2H4)], which
Proposed mechanism
The proposed mechanism of the hydroarylation reaction cat-
alyzed by the O-donor, (acac-O,O)2IrIII complexes is shown in
Scheme 2. As previously reported, the trans complexes are the
kinetic products that are isolated during synthesis, which upon
heating lead to the quantitative formation of the thermody-
namically stable cis isomers when trapped with excess L such
as pyridine.16 The reaction mechanism is proposed to proceed
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via three key steps: 1) a pre-equilibrium, or isomerization
step(s), that generates the “active catalyst”, independent of
which R-Ir-L complex is used to catalyze the reaction, 2) a rate
determining olefin insertion step, and 3) a C–H activation step.
Unfortunately, we have been unable to synthesize or isolate the
active catalytic species as they are believed to be uphill from the
trans ground state and readily collapse back to the trans-R-Ir-L
species upon isolation attempts.
An alternative mechanism to C–H activation is the alkyla-
tion of arenes using Lewis acid catalysts. The (acac-O,O)2IrIII
complexes are considered “soft” Lewis acids,1 especially given
the recent precedent for the use of transition metal complexes
such as Au, Pt and Pd as general Lewis acids for catalyzing
reactions between alkenes/alkynes and arenes.28 However, the
central observation that the hydroarylation reaction catalyzed
by the (acac-O,O)2IrIII complexes generate alkylarenes with anti-
Markovnikov regioselectivity would tend to rule out mecha-
nisms occurring via carbocation intermediates. Several other
observations also have indicated that a carbocation intermediate
is likely not involved:
1) Alkylarenes are more reactive than the parent arenes toward
alkylation in Lewis acid catalyzed reactions. Alkyl benzenes
are less reactive when using (acac-O,O)2Ir(R)(L) catalysts.
For example, hydroarylation of ethylene with benzene shows
approximately double the TON and TOF as compared to
ethylbenzene.16a
2) Lewis acid catalyzed reactions are typically inhibited by
1 eq. of water. However, the (acac-O,O)2IrIII catalysts are not
inhibited by the addition of several equivalents of water.16
3) Lewis acids readily catalyze the exchange of alkyl
groups between alkylarenes. However, the (acac-O,O)2IrIII com-
plexes do not catalyze reactions of diethylbenzene with ben-
zene to generate mono-ethyl benzene under hydroarylation
conditions.
4) Reaction rates of Lewis acid catalyzed reactions strongly
correlate with the strength of the Lewis acid. However, while
IrCl3 could be expected to be a stronger Lewis acid than
the (acac-O,O)2IrIII complexes, IrCl3 does not catalyze the
hydroarylation reaction under identical conditions.
5) The observation that other Ir catalysts show no hydroary-
lation activity or the same product selectively strongly suggests
a unique catalytic activity for the (acac-O,O)2IrIII complexes.16
As can be seen in Scheme 2, it is proposed that all the various
R-Ir-L as well as dinuclear ([R-Ir]2) complexes are catalyst
precursors that in a series of pre-equilibrium steps (that can
involve ligand loss, trans-cis isomerization, arene C–H activation
and olefin coordination) lead to the same active catalyst, cis-
Ph-Ir-Ol, independent of the starting complex. Mechanisms
involving C–H activation16 and insertion29 are typically inner-
sphere reactions and as such require a vacant coordination site
on the metal for coordination of the substrate.
The ligand “L” of the O-donor (acac-O,O)2Ir(R)(L) com-
plexes significantly influences the rate of the catalytic hydroary-
lation reaction. Strongly donating ligands, such as pyridine
severely inhibit the catalysis; whereas, labile ligands, such as
CH3OH and H2O, are weaker inhibitors. The TOFs have been
correlated to the calculated relative energies of these complexes,
and a linear correlation between the relative energy of the ground
states and TOFs was found.15
We have previously demonstrated that when L = pyridine,
exchangeis facile (e.g. exchangeoffree pyridine withcoordinated
pyridine between Ph-Ir-Py and CH3-Ir-Py is rapid at room
temperature and independent of the concentration of added
free pyridine, as expected for a dissociative process in a pre-
equilibrium step). The exchange was also examined with the
cis-Ph-Ir-Py complex. Pyridine exchange with cis-Ph-Ir-Py is
much slower (160 ◦C vs. RT) than that of its trans analog, which
is consistent with theoretical predictions.11
To generate the proposed active catalyst cis-Ph-Ir-Ol, olefin
coordination is required. This most likely occurs in a manner
similar to conversion of trans-Ph-Ir-Py to cis-Ph-Ir-Py, i.e. by
ligand dissociation, cis-trans isomerization and the coordination
of the olefin. However, while olefin complexes can be observed by
in situ NMR spectroscopy, attempts at generating and isolating
such olefin complexes with various trans-R-Ir-L complexes
failed, presumably due to instability. NMR analysis showed
that addition of ethylene to the dinuclear complex, [Acac-Ir]2,
leads to a new mononuclear complex containing coordinated
ethylene. However, all attempts at isolating this complex have
failed. The instability of the olefin complex(es) is consistent
with the lability of the Py and H2O complexes to substitution
at room temperature. Ethylene does not coordinate as strongly
with (acac-O,O)2Ir(R)(L) as other p-acids (such as Py and CO),
preventing isolation of an olefin complex. Furthermore, our
theoretical calculations showed that the five coordinate cis-Ph-
Ir species is higher in energy than the insertion transition state,
suggesting that the complex undergoes further reaction under
any conditions that allow for its generation.15
Since there is rapid scrambling of mixtures of benzene
and deuterobenzene (relative to the hydroarylation reaction,
see below), a kinetic isotope effect could not be obtained
under hydroarylation conditions using mixtures of C6H6 and
C6D6. However, comparison of the absolute rates of catalytic
hydroarylation reactions under the same conditions (styrene and
Ph-Ir-Py) in separate experiments with C6H6 and C6D6 showed
no kinetic isotope effect. This result is not surprising given
that olefin insertion is the rate determining step and not C–H
activation as been shown previously.13 Carrying out the catalysis
with ethylene and C6D6 also shows no kinetic isotope effect
(KIE), and yields as the major product C6D5-CH2-CH2D (as
identified by 13C NMR spectroscopy). Under these conditions,
only a low level of deuterium is incorporated into unreacted
ethylene as seen by GC-MS spectrometry. The possibility of a
small secondary KIE is possible; however, we do not see this
effect.
Comparative hydroarylation studies between trans-Ph-Ir-Py
and cis-Ph-Ir-Py were carried-out at 180 ◦C, and it was
determined that the trans isomer is 4.0 0.4 times more active
than the cis isomer, cis-Ph-Ir-Py. The TOF of the trans complex
decreases over time, most likely because it is converted to the
more thermodynamically stable cis form. However, it is possible
that the active catalyst may be generated by loss of the acac
ligands. Several observations indicate this is unlikely: 1) the
catalyst is long lived (see Fig. 2); 2) the resting state of the catalyst
after >50 turn-overs (with ethylene) is Ph-Ir-Py; 3) the O-donor
(acac-O,O)2IrIII complexes are thermally stable to exchange in
the presence of acac-H and protic acids such as CH3CO2H and
CF3CO2H;16f 4) trans-Ph-Ir-Pyis thermallystable inbenzeneand
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cleanly undergoes C–H activation and olefin insertion reactions
to generate alkylarenes. Furthermore, our reported theoretical
results showed that loss of an acac group costs ~50 kcal mol-1, i.e.
significantly more than the activation energy of the reaction.30
These observations strongly suggest that the (acac-O,O)2IrIII
motif is part of the “active catalyst.”
fever if inhaled or swallowed. Chronic exposure may cause
irreversible central nervous system damage, sensitization, weight
loss, immunological disease and other serious effects. Work with
these dangerously toxic compounds must not begin before a full
assessment of the risks have been made and suitable protocols
established including reading and understanding available safety
information (MSDS).33
[(CH3-13CH2)Ir(O,O-acac)2]2 ([CH313CH2-Ir]2). The synthe-
sis was analogous to a previously reported procedure.16 A flask
was loaded with Acac-C-Ir-H2O (97 mg, 0.19 mmol) and 13C
enriched diethylmercury (630 mg, 0.24 mmol). The crude reac-
tion mixture was developed on a preparatory silica TLC plate
with THF:ether (1 : 1) as an eluent and extracted with CH2Cl2.
After pumping off the solvent, the solid was redeveloped on a
preparatory silica TLC plate using 1 : 1 : 2 THF–CH2Cl2:hexane.
The orange band Rf = 0.87 was scraped off and extracted with
CH2Cl2 and THF. The solvent was pumped off to yield an orange
powder (0.0410 g, 51% yield). Characterization data matches
with a previously reported sample of the non-labeled analog.
Conclusion
The O-donor R-Ir-L and [R-Ir]2 complexes are active and stable
catalysts for the hydroarylation of unactivated olefins and arenes.
This reaction is selective to the linear alkylarenes and generates
saturated products only, although it undergoes reversible and
unproductive b-hydrogen elimination as determined by labeling
studies. The hydroarylation reactions studied are under kinetic
control. The catalyst is a mononuclear species and stable at high
temperatures. A key factor is that this reaction leads in one step,
directly to alkyl arenes without the production of halogentated
waste while avoiding multistep procedures. Mechanistic studies
suggest that the mechanism involves pre-equilibrium steps,
followed by insertion of olefin, and finally C–H activation of
benzene to yield the linear alkyl benzene product and regenerate
the catalyst.
1H NMR (CD3OD): d 5.47 (s, 2H, acac-C3H), 2.83 (dq, 1JCH
=
128.5, 3JHH = 7.7, -13CH2–Ir), 1.76 (s, 12H, acac-CH3), 0.198 (m,
3H, CH3-13CH2–Ir) 13C { H} few scans (CD3OD): d -17.59 (s,
1
-13CH2–Ir).
[Ir(O,O-acac)2(13CH2CH3)(Py)] (CH313CH2-Ir-Py). A flask
was loaded with [CH313CH2-Ir]2 (100 mg, 0.119 mmol) in CHCl3
and pyridine (1 mL, 12.3 mmol) following a previously reported
prep to give 115 mg (> 95%) of title compound.16 1H NMR
(C6D6): d 8.69 (d, 2H, o-Py), 6.84 (t, 1H, p-Py), 6.56 (t, 2H,
Experimental Section
All manipulations were carried out using glovebox and high
vacuum line techniques under an inert atmosphere of N2 or
argon. Benzene, benzene-d6, toluene-d8 and THF were purified
by vacuum transfer from sodium benzophenone ketyl. CD2Cl2
and pyridine were dried by vacuum transfer from CaH2.
Synthetic work involving iridium complexes was carried out
in an inert atmosphere in spite of the air stability of the
complexes. Reagent-grade chemicals and solvents were used
as is and purchased from Aldrich or Strem. IrCl3·H2O was
purchased from Strem or Pressure Chemical. CH313CH2I was
purchased from Cambridge Isotopes Inc. and was used as
received. Complexes R-Ir-L, [R-Ir]2 and cis-R-Ir-L,14,16,31 and
diethylmercury32 were prepared as described in the literature.
Elemental analyses were done by Desert Analytics laboratory
(now Columbia Analytical Services), Tucson, Arizona. Liquid
phases of the reaction mixtures were analyzed with a Shimadzu
GC-MS QP5000 (ver. 2) equipped with cross-linked methyl
silicone gum capillary column, DB5. Gas measurements were
performed using a GasPro column. The retention times of the
products were confirmed by comparison to authentic samples.
NMR spectra were obtained on a Bruker AC-250 (250.134 MHz
for 1H and 62.902 MHz for 13C), Bruker AM-360 (360.138 MHz
1
m-Py), 5.10 (s, 2H, acac-C3H), 3.47 (dq, 2H, JCH = 125.3, -
13CH2–Ir), 1.60 (s, 12H, acac-CH3), 1.23 (m, 3H, CH3-13CH2–Ir).
1
13C { H} NMR (C6D6): d 182.69 (acac C O), 149.57 (o-py),
136.32 (p-py), 124.38 (m-py), 102.76 (acac-CH), 26.66 (acac-
CH3), 15.99 (d, 1JCC = 34, CH3), -10.56 (-13CH2–Ir).
Cis-[Ir(O,O-acac)2(Ph)(Py)] (cis-Ph-Ir-Py). In addition to
the method for the synthesis of cis-Ph-Ir-Py we reported
previously,16a a second route is available. A 5 mL thick-walled
glass tube equipped with a resealable Teflon valve and a magnetic
stir bar was charged with 2.5 mL of benzene containing Ph-Ir-Py
(10 mg, 0.02 mmol). The tube was heated for 100 h in a well-
stirred oil bath maintained at 180 ◦C. The autoclave was cooled
thereafter, and the solvent was removed in vacuo and the solid
obtained was washed with cold methanol to yield the complex
in >90% yield.
Reaction procedure for the olefin arylation. A 3 mL stainless
steel autoclave, equipped with a glass insert, and a magnetic stir
bar was charged with 1 mL of distilled benzene and 3–5 mg
(5 mmol, ~0.1 mol%) of catalyst (unless otherwise mentioned).
The reactor was degassed with nitrogen, pressurized with 0.96
MPa of propylene with an extra 2.96 MPa of nitrogen. The
autoclave was heated for 30 min in a well stirred heating bath
maintained at 180 ◦C. The liquid phase was sampled and
the product yields were determined by GC-MS using methyl
cyclohexane as an internal standard, which was added after the
reaction.
1
for H and 90.566 MHz for 13C), or on a Varian Mercury 400
1
(400.151 MHz for H and 100.631 MHz for 13C) spectrometer.
Chemical shifts are given in ppm relative to TMS or to residual
solvent proton resonances. All carbon resonances are singlets
unless otherwise mentioned.
Warning! Organomercury compounds are highly toxic! There
is a danger of cumulative effects. These compounds may
cause serious and irreversible effects on skin contact. These
compounds may be fatal if absorbed through the skin - even
small amounts, such as a single drop, may cause serious
injury or potentially be fatal. They may cause metal fume
Insertion reactions of olefins with Ph-Ir-Py. A 3 mL stainless
steel autoclave, equipped with a glass insert and a magnetic stir
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bar was charged with 1 mL of distilled mesitylene and 15 mg of
Ph-Ir-Py. The autoclave was heated at 180 ◦C for 10 min. after
adding the olefin. The liquid phase was sampled and the product
yields were determined by GC-MS using methyl cyclohexane as
an internal standard, which was added after the reaction. The
added amount of olefin is as follows: 2 MPa of ethylene, 0.96
MPa of propylene with an extra 2 MPa of nitrogen, 0.25 mL of
styrene with 2 MPa of nitrogen and 0.25 mL of 1-hexene with 2
MPa of nitrogen.
Acknowledgements
We gratefully acknowledge financial support of this research by
the Chevron Corporation, the University of Southern Califor-
nia, The Scripps Research Institute, and the Center for Catalytic
Hydrocarbon Functionalization, a DOE Energy Frontier Re-
search Center (DOE DE-SC000-1298) for financial support. We
also thank Dr William Schinski for helpful discussions during
the preparation of this manuscript.
Reaction procedure for substrate dependence (olefin).
A
Notes and references
10 mL glass Schlenk flaskfitted witharesealableTeflon valve was
equipped with a magnetic stir bar and charged with dry, distilled
benzene (1 mL) and 3–5 mg (5 mmol, ~0.1 mol%) of catalyst
from a stock solution. To it was added varied amount of styrene
(typically 0.1 to 1 mL) and 20 mL of methylcyclohexane, added as
an internal standard. The valve was closed and the flask heated
to 180 ◦C for 30 min. in a well stirred, temperature controlled oil
bath. The liquid phase was sampled and the product yields were
determined by GC-MS by comparison to the internal standard.
1 (a) G. A. Olah, Friedel–Crafts and Related Reactions. Vol. I. General,
Aspects.; Vol. II. Alkylation, and Related Reactions. Part 1.; Part
2.; 1964, 1031 pp.; 658 pp.; 703 pp. Wiley-Interscience: New York;
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2 (a) M. B. Smith and J. March, March’s Advanced Organic Chemistry,
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Reaction procedure for substrate dependence (benzene).
A
10 mL glass Schlenk flask fitted with a resealable Teflon valve
and a magnetic stir bar was charged with dry, distilled styrene
(0.6 mL) and 3–5 mg (5 mmol, ~0.1 mol%) of catalyst from a
stock solution. Benzene (0.1 to 1 mL) and cyclohexane (0.1 to
1 mL) were added to the flask. An internal standard of 20 mL of
methylcyclohexane was also added to the flask. The valve was
´
3 G. A. Olah and A. Molna´r, Hydrocarbon Chemistry, 2nd ed.; Wiley-
Interscience: New York, 2003; 229–233.
◦
4 We define the C–H activation reaction to be a coordination reaction
that proceeds without the involvement of free radicals, carbocations,
or carbanions to generate discrete M-R intermediates (a) A. E. Shilov
and G. B. Shul’pin, Chem. Rev., 1997, 97, 2879; (b) B. A. Arndtsen,
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Ziatdinov, J. Mol. Catal. A: Chem., 2004, 220, 7 and citations therein;
(d) B. L. Conley, W. J. Tenn, III, K. J. H. Young, S. K. Ganesh, S. K.
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Goddard, III and R. A. Periana, J. Mol. Catal. A: Chem., 2006, 251,
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5 (a) S. Murai, F. Kakiuchi, S. Sekine, Y. Tanaka, A. Kamatani, M.
Sonoda and N. Chatani, Nature, 1993, 366, 529; (b) S. Murai, N.
Chatani and F. Kakiuchi, Pure Appl. Chem., 1997, 69, 589; (c) X.
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6 (a) M. T. Wentzel, V. J. Reddy, T. K. Hyster and C. J. Douglas, Angew.
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closed and the solution heated to 180 C for 30 min. in a well
stirred, temperature controlled oil bath. The liquid phase was
sampled and the product yields were determined by GC-MS by
comparison to the internal standard.
Reaction procedure for catalyst (Ph-Ir-Py) dependence.
A
10 mL glass Schlenk flask fitted with a resealable Teflon valve
and a magnetic stir bar was charged with dry, distilled benzene
(1 mL) and styrene (0.6 mL). Varying amounts of Ph-Ir-Py
(2–10 mg) were added as well as 1 eq. of pyridine to each
flask. A 20 mL aliquot of methylcyclohexane was added as an
internal standard. The valve was closed and the solution heated
to 180 ◦C for 30 min in a well stirred, temperature controlled oil
bath. The liquid phase was sampled and the product yields were
determined by GC-MS by comparison to the internal standard.
Computational methodology
7 (a) K. Krogh-Jespersen, M. Czerw, K. Zhu, B. Singh, M.
Kanzelberger, N. Darji, P. D. Achord, K. B. Renkema and A. S.
Goldman, J. Am. Chem. Soc., 2002, 124, 10797; (b) X. Zhang, M.
Kanzelberger, T. J. Emge and A. S. Goldman, J. Am. Chem. Soc.,
2004, 126, 13192.
All calculations were performed using the B3LYP hybrid
DFT functional as implemented by the Jaguar 7.0 program
package.34–36 This functional has provided good agreement with
experiment for (acac-O,O)2Ir(III)(R)(L) complexes and other
reaction profiles of transition metal containing compounds.37,38
Iridium atoms were described using the Wadt and Hay39 core-
valence (relativistic) effective core potential in the LACVP basis
set for geometry optimizations and single point energies with
the LACVP**++ basis set.40,41 All energies reported are DH(298
K) = DE + zero point energy correction + solvation correction.
Relative energies on the DH(298 K) surface are expected to
be accurate to within 3 kcal mol-1 for stable intermediates, and
within 5 kcal mol-1 for transition structures. We also note that to
be consistent with past computational efforts, the methyl groups
on the acac ligands were replaced with hydrogens.11
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