C-H insertion catalyzed by tetratolylporphyrinato methyliridium via a metal-carbene intermediate

Article abstract of DOI:10.1021/om3005433

C-H insertion reactions between different substrates and diazo reagents were catalyzed by tetratolylporphyrinato methyliridium (Ir(TTP)CH₃). The highest yields were achieved for reactions between the bulky diazo reagent methyl 2-phenyldiazoacet

Full text of DOI:10.1021/om3005433

Article
pubs.acs.org/Organometallics
C−H Insertion Catalyzed by Tetratolylporphyrinato Methyliridium via
a Metal−Carbene Intermediate
Bernie J. Anding, Jakoah Brgoch, Gordon J. Miller, and L. Keith Woo*
Department of Chemistry, Iowa State University, Ames, Iowa 50011-3111, United States
S
* Supporting Information
ABSTRACT: C−H insertion reactions between different
substrates and diazo reagents were catalyzed by tetratolylpor-
phyrinato methyliridium (Ir(TTP)CH3). The highest yields
were achieved for reactions between the bulky diazo reagent
methyl 2-phenyldiazoacetate (MPDA) and substrates contain-
ing electron-rich C−H bonds. An intermediate metal-
loporphyrin complex was identified as a metal−carbene
complex, Ir(TTP)(C[Ph]CO2CH3)(CH3) (4), using 1H
NMR and UV/vis absorption spectroscopy. The presence of 4
was further supported by computationally modeling the
absorption spectra with time-dependent DFT (6-31G(d,p)/
SBKJC basis set, PBE0 functional). Kinetic studies for C−H insertion reactions using different substrates showed substantial
differences in the rate of MPDA consumption, suggesting that carbene transfer is rate-limiting. Furthermore, primary kinetic
isotope effects of 3.7 0.3 and 2.7 0.4 were measured using toluene and cyclohexane, respectively. These data are consistent
with a mechanism that involves direct C−H insertion rather than a radical rebound pathway.
INTRODUCTION
RESULTS AND DISCUSSION
■
■
Selective functionalization of unactivated carbon−hydrogen
bonds is still a great challenge in synthetic chemistry. Insertion
of carbene fragments, generated from diazo reagents, into C−H
bonds using transition metal catalysts provides a promising,
atom-economical method to accomplish this goal. High
selectivities were found using Cu, Ni, and Rh catalyst
complexes for intramolecular C−H insertion reactions.1,2
However, general and selective catalysts for intermolecular
insertions are rare. Among the promising candidates, rhodium-
(III) porphyrin catalysts are particularly appealing because they
have demonstrated unique selectivity for intermolecular
carbene insertion into the primary C−H bonds of simple
aliphatic compounds.3 The most selective catalyst utilized the
bulky tetra(2,4,6-triphenylphenyl)porphyrin (TTPPP).4 In the
presence of Rh(TTPPP)I, the reaction between octane and
methyl 2-phenyldiazoacetate (MPDA) generated insertion
products in 55% yield, with primary insertion favored over
secondary insertion in a ratio of 10.5:1. Recently, intermo-
lecular C−H insertion was also accomplished in high yield and
enantioselectivity using iridium(III) complexes with chiral
N,N′-bis(salicylidene)ethylenediiminato (salen) and D4-sym-
metric Halterman porphyrin ligands.5,6 At low temperatures,
the Ir(III) complexes catalyzed the insertion of alkyl- and
aryldiazoacetates into the C−H bonds of THF and cyclo-
hexadiene in good yields and with 81−99% ee values. These
promising results prompted us to explore further the scope and
mechanism for C−H insertion catalyzed by the iridium(III)
porphyrin complex (5,10,15,20-tetratolylporphyrinato)-
methyliridium (Ir(TTP)CH3).
Optimization studies were carried out in neat cyclohexane with
1.0 mol % Ir(TTP)CH3. Initial reactions with ethyl diazoacetate
(EDA) failed to generate the desired insertion product, 1.
Instead, the only observed products resulted from dimerization
of the diazo reagent to afford diethyl fumarate and diethyl
maleate, 2 (Table 1). Attempts to decrease dimerization by
lowering the reaction temperature failed, which was due in part
to the low solubility of Ir(TTP)CH3 in cyclohexane. Changing
the diazo reagent to MPDA, which is more resistant to
dimerization, afforded C−H insertion products in low yields at
room temperature. The major product of this reaction, azine 3,
resulted from MPDA dimerization. The structure of the Z,Z-
isomer of 3 was verified by an analysis of single-crystal X-ray
diffraction data with metrical parameters that matched
published molecular structures.7,8 High yields of 1 were
achieved using elevated temperatures and dropwise addition
of MPDA. Increasing the catalyst loading to 5.0 mol % also
significantly increased the yield of product 1, but such a high
catalyst loading is not practical given the cost of Ir(TTP)CH3.
Attempts to catalyze C−H insertion with Ir(TTP)Cl(CO)
under these conditions yielded trace amounts of 1 and
incomplete MPDA conversion.
The optimized method was extended to several other
substrates (Table 2). Treating Ir(TTP)CH3 with MPDA in
refluxing pentane afforded azine 3 in nearly quantitative yields.
With the higher boiling octane, C−H insertion products were
Received: June 15, 2012
Published: July 30, 2012
© 2012 American Chemical Society
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the selectivity was modest, Ir(TTP)CH3 was more selective for
primary insertion than that for the previously reported
Rh(TTP)I.9 Extending this method to toluene and cyclo-
hexadiene gave good yields of the products corresponding to
benzylic and allylic C−H insertion, respectively. Products
corresponding to vinyl or aryl C−H insertion were not
detected. Furthermore, unlike reactions with Fe(TPP)Cl, there
Table 1. Optimization of Cyclohexane C−H Insertion Using
a
Ir(TPP)CH3
were no signs of competitive cyclopropanation or Buchner
̈
additions.10 THF was also a suitable substrate for C−H
insertion. In this case, the products corresponding to α-C−H
insertion were isolated in 75% yield with a moderate selectivity
for the anti-isomer. No β-insertion products or ring-opened
ethers were identified. In all cases, as the reaction temperature
was decreased, yields of the insertion products decreased, the
formation of azine 3 increased, and catalyst decomposition was
more evident. This effect was more pronounced with octane
and toluene than with THF and cyclohexadiene. Additionally,
trace water contamination, specifically with THF, generated
considerable amounts of the competitive O−H insertion
product, methyl 2-hydroxy-2-phenylacetate.
Addition of MPDA in the above reactions, specifically those
with cyclohexane, octane, and toluene, resulted in a temporary
color change from orange to greenish-brown, indicating the
presence of an observable intermediate. This intermediate was
investigated by UV/vis absorption spectroscopy for the reaction
between a cyclohexane solution of Ir(TTP)CH3 and MPDA
over three consecutive additions of the diazo reagent. The Soret
band of Ir(TTP)CH3 was observed at 404 nm. After the first
addition of MPDA (5.7 equivalents), the peak at 404 nm was
immediately consumed and three new bands were formed at
375, 417, and 443 nm (Figure 1). The intermediate bands
e
e
diazo reagent
EDA
temperature (°C)
yield 1 (%)
yield dimers (%)
0
0
0
100
31
68
14
3
b
MPDA
trace
28
74
94
MPDA
80
80
80
c
MPDA
cd
,
MPDA
a
Conditions: Ir(TTP)CH3 (1.0 mol %) and diazo reagent (0.30
mmol) in cyclohexane (4.5 mL). Unreacted MPDA remained (24%
determined by NMR). Dropwise addition of MPDA. Using 5.0 mol
% catalyst. Isolated yields. Determined by NMR.
b
c
d
e
e
Table 2. C−H Insertion of Various Substrates Using MPDA
e
and Ir(TPP)CH3
Figure 1. Absorption spectra for the treatment of Ir(TTP)CH3 with
5.6 equivalents of MPDA. The selected spectra depicts Ir(TTP)CH3
before MPDA addition (solid), the intermediate at 210 s after MPDA
addition (dashed), and the return to Ir(TTP)CH3 300 s after MPDA
addition (dotted).
endured for approximately 5 minutes before reverting back to
the single Soret peak of Ir(TTP)CH3. The conversion between
Ir(TTP)CH3 and intermediate bands occurred too rapidly to
observe isosbestic behavior. Furthermore, catalyst solubility
seemed to increase throughout the reaction. Addition of a
second portion of MPDA resulted in similar behavior, except
that the band at 417 nm was larger throughout the reaction
relative to other peaks (Figure S7). The resulting equilibrated
solution displayed a significant shoulder at 417 nm, which
seemed to form at the expense of Ir(TTP)CH3. Finally, with
a
b
c
Isolated yields. Yield determined by NMR. C1:C2+C3:C4,
d
e
normalized by the number of hydrogen atoms. anti:syn. Conditions:
A solution of MPDA (0.12 mmol) and substrate (1.0 mL) was added
dropwise to a refluxing solution of Ir(TTP)CH3 (1.0 mol %) and
substrate (4.5 mL) over the course of 2 h.
isolated in 37% yield. Insertion occurred preferentially into
secondary (C2 and C3, 18%) carbons, but a considerable
amount of primary insertion (7%) was also observed. Although
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the third addition of MPDA, the band at 417 nm was dominant
and persisted indefinitely (Figure S8). This resulting metal-
loporphyrin was unreactive with further additions of MPDA
and prolonged heating, indicating it is a product of catalyst
decomposition. The catalyst TON was between 11.4 and 17.1
under these conditions.
In analogy with the mechanism proposed for iridium
porphyrin-catalyzed cyclopropanation,11 the observed inter-
mediate was believed to be the metal−carbene complex
Ir(TTP)(C[Ph]CO2CH3)(CH3), 4. To validate that the
changes observed in the absorption spectra arose from the
formation of a carbene complex, time-dependent density
functional theory (TD-DFT) was used to calculate the
absorption spectra for both Ir(TTP)CH3 and 4. All calculations
were carried out using GAMESS12−15 with a mixed basis set,
including 6-31G(d,p) for the main group elements, the SBJKC
effective core potential for the iridium atom,16−18 and the
hybrid functional PBE0,19,20 which has been shown to
reasonably represent the electronic structure of porphyrin
systems.21,22 Accordingly, structural optimizations via DFT
calculations provided reasonable ground-state geometries
(optimized structures and selected metrical parameters
provided in the Supporting Information). For example, the
calculated geometrical parameters for Ir(TTP)CH3 are
consistent with the reported crystallographic structure of
Ir(TTP)CH3·H2O.23 Specifically, the calculated Ir−CH3 bond
length, 2.037 Å, agrees with the reported distance of 2.059(11)
Å. Upon coordination of the carbene ligand, the Ir−CH3 bond
in 4 lengthened to 2.123 Å due to the trans influence exerted by
the carbene ligand.24 The calculated Ir−C carbene bond length
was 2.045 Å, which is slightly long compared to the metal−
carbene complexes of group 8 metalloporphyrins. Previous
studies of similar systems report M−C bond lengths ranging
from 1.767(3) to 2.035(2) Å.24−26 Considering the high
carbene transfer reactivity of iridium porphyrin complexes
compared to that of group 8 metal complexes,6,11 a slightly
elongated Ir−C carbene bond is reasonable. Coordination of
the carbene ligand also caused noticeable distortion to the
porphyrin core. The out-of-plane displacement (Doop) and in-
plane displacement (Dip) were calculated by normal-coordinate
structure decomposition (NSD) analysis (Table S2).27,28
Figure 2. TD-DFT absorption spectra for Ir(TTP)CH3 (bold) and 4
(dotted).
spectra, providing additional evidence in support of a metal−
carbene intermediate.
The intermediate carbene complex was also investigated by
1H NMR. Addition of MPDA (ca. 5 equivalents) to a solution
of Ir(TTP)CH3 in benzene-d6 resulted in the complete
formation of a new metalloporphyrin complex, as indicated
by the clean singlets for the porphyrin β-pyrrole and Ir−CH3
protons at 8.83 and −4.80 ppm, respectively (Figure S10). The
Ir−CH3 protons were shifted significantly downfield from those
of Ir(TTP)CH3 (−5.92 ppm). Furthermore, the appearance of
new phenyl proton signals [6.55 (t, J = 7.6 Hz, 1H), 5.92 (t, J =
7.6 Hz, 2H), 3.93 (d, J = 7.6 Hz, 2H)] and a new methyl singlet
at 2.19 ppm (3H) suggested that the carbene fragment derived
from MPDA was coordinated to iridium; the carbene signals
were strongly shifted upfield due to the porphyrin ring current
effect. After 20 h, the major remaining porphyrin species was
Ir(TTP)CH3. MPDA had been completely consumed, forming
azine 3 as the predominant product. Despite numerous efforts,
including attempts with chromium(III) acetylacetonate, a signal
corresponding to the putative carbene carbon of the
intermediate was not located by 13C NMR. Difficulty locating
the carbene signal for MPDA is documented,31 and coupling to
iridium, a spin-active nuclei, would further reduce the carbene
signal-to-noise ratio. Unfortunately, numerous attempts to
isolate the intermediate, including lyophilization from benzene,
also failed.
Results for complex 4 showed Doop = 0.5154 and Dip
=
0.1430. The most prominent distortions were ruffling (B1u) and
doming (A1g) out-of-plane displacements. Deformations were
A kinetic study of the reaction between 4.9 equivalents of
MPDA and 1.0 equivalent of Ir(TTP)CH3 at 300.0 K revealed
first-order consumption of MPDA with a rate constant k = 8.0
× 0.3 s−1. In the presence of up to 55.3 equivalents of toluene
or cyclohexane, the rate of MPDA consumption was
unchanged, and the major product was azine 3. Apparently, a
second equivalent of MPDA was more effective at trapping the
intermediate than toluene or cyclohexane. In contrast, reactions
run in the presence of 11.1 equivalents of cyclohexadiene
afforded primarily C−H insertion products and completely
consumed MPDA in less than 60 s, demonstrating that the rate
of MPDA consumption is dependent on the nature of the
substrate.
The above data support a catalytic cycle involving a metal−
carbene intermediate (Scheme 1), analogous to related C−H
insertion and cyclopropanation systems.2,11,24,25,32 Coordina-
tion of MPDA to Ir(TTP)CH3 initially generates a diazonium
complex, which forms a metal−carbene complex, 4, after loss of
dinitrogen. While the observed intermediate may conceivably
considerably less significant for Ir(TTP)CH3, which had Doop
=
0.1071 and Dip = 0.2598. In general, the distortions exhibited by
Ir(TTP)CH3 and 4 are relatively small compared to other
metalloporphyrin systems.29
The calculated electronic spectra of Ir(TTP)CH3 and 4 are
shown in Figure 2. The main absorption band of Ir(TTP)CH3,
arising from a π to π* transition, occurred as one large peak
centered at 371 nm, which was consistent with the
experimentally observed transition at 404 nm. The calculated
spectrum of 4 showed π to π* transitions at 357, 366, and 427
nm. Similar spectral changes were reported for other six-
coordinate alkyl Ir porphyrin complexes with neutral donor
ligands trans to the alkyl group.30 The resolution of the
experimental spectrum likely coalesces the bands at 357 and
366 nm into one peak. Thus, the calculated spectrum is in good
agreement with the observed peaks of the intermediate complex
at 375 and 445 nm. Overall, TD-DFT modeling of Ir(TTP)-
CH3 and 4 adequately reproduced the observed absorption
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residual proton resonances of deuterated solvents (δ, ppm: CDCl3,
7.26; C6D6, 7.16). Ratios for kinetic isotope data were determined
using a Finnegan Magnum GC-MS fitted with a HP-5 column (30 m
× 0.25 μm) and a time-of-flight mass analyzer. Column chromatog-
raphy was performed using silica gel (40−63 μm) purchased from
Sorbent Technologies. Characterization data for the C−H insertion
products were previously reported.4,5,39
Scheme 1. Proposed Catalytic Cycle for C−H Insertion of
Hydrocarbons Using MPDA and Ir(TTP)CH3
Computational Details. The ground-state geometries of Ir-
(TTP)CH3, MPDA, and 4 were determined without any symmetry
constraints using the program GAMESS.12−15 Exchange and
correlation were treated by the hybrid density functional PBE0 and
included a mixed basis set. All main group elements were calculated
with the 6-31G(d,p) basis set, while an effective core potential
(SBKJC) was utilized for Ir. The electronic absorption spectra were
calculated by the TD-DFT method implemented within the GAMESS
code. The effects of hexane as a solvent were handled using the
polarizable continuum model.40−43
General C−H Insertion Procedure. A CH2Cl2 stock solution of
Ir(TTP)CH3 (216 μL from a 5.34 × 10−3 M stock solution, 1.15
μmol) was transferred to a side arm, round-bottom flask and taken to
dryness under a nitrogen stream. Substrate (4.5 mL) was added and
the solution was taken to reflux. Meanwhile, a solution of MPDA
(0.115 mmol) and substrate (1.0 mL) was prepared in a dry round-
bottom flask. The MPDA solution was added dropwise to the
Ir(TTP)CH3 solution over the course of 2 h using a syringe pump.
After MPDA addition and after the color reverted back to orange,
volatiles were removed in vacuo. Products were isolated by column
chromatography on silica gel using hexanes and ethyl acetate (40:1) as
the eluent system.
Observation of Complex 4 by UV/Vis Absorption Spectros-
copy. Ir(TTP)CH3 (0.0768 μmol) from a 3.84 × 10−3 M CH2Cl2
stock solution was taken to dryness in a 1 mm path length cuvette and
then dissolved in cyclohexane (0.3 mL). A UV/vis spectrum of the
starting material was collected at this point. Then, MPDA from a
cyclohexane stock solution (0.011 M, 40 μL, 0.44 μmol) was added,
and a UV/vis spectrum was obtained every 30 s until equilibrium was
reached (ca. 6 min). This process was repeated two more times until
only an unreactive metalloporphyrin complex, with a Soret peak at 417
nm, remained.
be the diazonium complex,33 UV/vis absorption spectroscopy,
TD-DFT, and kinetic studies all support carbene complex 4 as
the observed intermediate. Furthermore, an NN stretch
corresponding to a diazonium intermediate was not observed
by IR spectroscopy. After carbene formation, subsequent
nucleophilic attack at the carbene carbon with a C−H bond
results in product formation. Insight into the mechanism of
carbene transfer was gained from kinetic isotope studies.
Separate competition reactions between toluene/toluene-d8
and cyclohexane/cyclohexane-d12 revealed kinetic isotope
effects (KIE) of 3.7 0.3 and 2.7 0.4, respectively. These
values support a mechanism involving direct C−H insertion
rather than a radical rebound pathway, which exhibits a larger
KIE, typically between 7 and 12.9 Direct C−H insertion may be
most accurately described as a concerted, nonsynchronous
hydride transfer and C−C bond formation.34,35
Observation of 4 by 1H NMR Spectroscopy. Solid Ir(TTP)CH3
(5.7 mg, 6.5 μmol) was transferred into a medium-walled NMR tube
and dissolved in ca. 0.5 mL of C6D6. The solution was treated with
MPDA (3.5 μL, 22.3 μmol); spectra were acquired immediately and
after 20 h. 1H NMR (C6D6, 400 MHz): δ 8.83 (s, 8H), 8.14 (d, J = 7.6
Hz, 4H), 7.93 (m, 4H, overlapping with a signal from azine 3), 7.31 (d,
J = 7.6 Hz, 4H), 7.18 (d, J = 7.6 Hz, 4H, obsured by the residual
solvent signal), 6.55 (t, J = 7.6 Hz, 1H), 5.92 (t, J = 7.6 Hz, 2H), 3.93
(d, J = 7.6 Hz, 2H), 2.38 (s, 12H), 2.19 (s, 3H), and −4.80 (s, 3H).
Kinetic Measurement Experiments. A CH2Cl2 stock solution of
Ir(TTP)CH3 (400 μL, 4.26 × 10−3 M, 1.70 μmol) was transferred to a
medium-walled NMR tube and taken to dryness at 45 °C. In a
glovebox, the tube was loaded with triphenylmethane as an internal
standard (9.96 μmol) and substrate (ranging from 0 to 94 μmol) and
diluted to a total volume of 420 μL with C6D6. The tube was fitted
with a septum and taken to the NMR spectrometer, where the
temperature was equilibrated to 300.0 K prior to diazo addition. Then,
MPDA (8.40 μmol) from a 0.240 M, room-temperature C6D6 stock
solution was added, and reaction progress was monitored at 60 s
intervals.
Procedure for Kinetic Isotope Studies. A CH2Cl2 stock solution
of Ir(TTP)CH3 (100 μL from a 3.83 × 10−3 M stock solution, 0.383
μmol) was transferred to a round-bottom flask fitted with a side arm
and taken to dryness under a nitrogen stream. Ir(TTP)CH3 was
dissolved in 1.8 mL of a premixed solution of substrate (1.0 mL) and
deuterated substrate (1.0 mL), where the substrates are either toluene
or cyclohexane. In a separate round-bottom flask, MPDA was
dissolved in the remaining 0.2 mL of the mixed substrate solution.
Then, to a refluxing solution of Ir(TTP)CH3, the MPDA solution was
added manually dropwise. After each drop, the color of the solution
was allowed to revert back to orange before the next drop of MPDA
CONCLUSIONS
■
In summary, C−H insertion catalyzed by Ir(TTP)CH3 was
examined using different substrates and diazo reagents. The
highest yields were found using MPDA and substrates with
electron-rich C−H bonds, such as THF and cyclohexadiene.
Mechanistic studies and spectroscopic examination of the
reaction intermediate suggest that C−H insertion proceeds
through a metal−carbene intermediate and that carbene
transfer is the rate-limiting step.
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were performed
under a dry nitrogen atmosphere. Most substrates were dried over 4
Å molecular sieves, freeze−pump−thawed, and run through a plug of
activated alumina under a glovebox atmosphere. THF was
deoxygenated and dried by passage through columns of reduced
copper and alumina. Ir(TTP)CH3 and MPDA were prepared
according to previously reported methods.36−38 Absorption spectra
were collected using a Hewlett-Packard/Agilent Technologies 8453
UV−vis spectrophotometer. General NMR spectra were collected
using Varian VXR 300 MHz and Varian VXR 400 MHz spectrometers,
and kinetic measurements were done using a Bruker DRX 400 MHz
spectrometer. 1H NMR peak positions were referenced against
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solution was added. Once MPDA addition was complete and the color
returned to orange, the product mixture was analyzed by GC-MS.
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Eur. J. 2004, 10, 3486.
ASSOCIATED CONTENT
* Supporting Information
Optimized structures of MPDA, Ir(TTP)CH3, and complex 4,
normal-coordinate structure decomposition analysis, experi-
mental and calculated absorption spectra, 1H NMR spectrum of
complex 4, and kinetics data. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
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AUTHOR INFORMATION
Corresponding Author
*E-mail: kwoo@iastate.edu.
Notes
■
(34) Nakamura, E.; Yoshikai, N.; Yamanaka, M. J. Am. Chem. Soc.
2002, 124, 7181.
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
NSF grant CHE-0809901 and a GAANN fellowship (B.J.A.)
provided partial financial support for this work. J.B. and G.J.M.
thank the NSF, Materials World Network (DMR 08-06507),
for generous financial support.
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