A general method for copper-catalyzed arylation of arene C-H bonds

Article abstract of DOI:10.1021/ja805688p

A general method for copper-catalyzed arylation of sp² C-H bonds with pK_a's below 35 has been developed. The method employs aryl halide as the coupling partner, lithium alkoxide or K₃PO₄ base, and DMF, DMPU, or mixed DMF/xylenes solvent. A variety of electron-rich and electron-poor heterocycles such as azoles, caffeine, thiophenes, benzofuran, pyridine oxides, pyridazine, and pyrimidine can be arylated. Furthermore, electron-poor arenes possessing at least two electron-withdrawing groups on a benzene ring can also be arylated. Two arylcopper-phenanthroline complex intermediates were independently synthesized.

Full text of DOI:10.1021/ja805688p

Published on Web 10/15/2008
A General Method for Copper-Catalyzed Arylation of Arene
C-H Bonds
Hien-Quang Do, Rana M. Kashif Khan, and Olafs Daugulis*
Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5003
Received July 21, 2008; E-mail: olafs@uh.edu
Abstract: A general method for copper-catalyzed arylation of sp2 C-H bonds with pKa’s below 35 has
been developed. The method employs aryl halide as the coupling partner, lithium alkoxide or K3PO4 base,
and DMF, DMPU, or mixed DMF/xylenes solvent. A variety of electron-rich and electron-poor heterocycles
such as azoles, caffeine, thiophenes, benzofuran, pyridine oxides, pyridazine, and pyrimidine can be arylated.
Furthermore, electron-poor arenes possessing at least two electron-withdrawing groups on a benzene ring
can also be arylated. Two arylcopper-phenanthroline complex intermediates were independently
synthesized.
1. Introduction
was the first transition metal shown to promote carbon-hydrogen
bond arylation.7 In the past few years palladium-, rhodium-,
and ruthenium-catalyzed sp2 C-H bond arylation has undergone
explosive growth.8 In contrast, only scattered examples of
copper-promoted carbon-hydrogen bond arylation have been
described with most reports dating back to the 1960s and 1970s.9
The majority of the palladium-, rhodium-, or ruthenium-
catalyzed C-H bond functionalization examples involve regi-
oselective arylation of directing-group-containing arenes (Scheme
1A) or electron-rich heterocycles such as azoles or indoles.
Several recent reports describe functionalization of arenes
possessing no conventional directing groups.10 In the latter case
the regioselectivity issues are often unsolved and sometimes
only symmetrical arenes can be employed as the C-H coupling
component due to the possibility of regioisomer formation
(Scheme 1B). Perhaps the only general exception is found in
Compounds containing polyaryl moieties are common among
natural products, pharmaceuticals, and dyes. As a consequence,
regioselective formation of aryl-aryl bonds has attracted
substantial interest over the past century.1 The copper-promoted
biaryl synthesis was pioneered by Ullmann more than a hundred
years ago.2 Until the development of Stille, Suzuki, and Kumada
reactions3 in the 1970s, copper was the only metal widely used
for the formation of aryl-aryl bonds. Recently, copper-catalyzed
cross-coupling reactions are undergoing a resurgence. Efficient
methods for carbon-carbon,4 carbon-nitrogen,5 and carbon-
oxygen6 bond formation have been demonstrated by using
copper complexes. However, copper appears to be underutilized
as a catalyst for C-H bond functionalization even though it
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A R T I C L E S
Do et al.
Scheme 1. Regioselectivity in C-H Bond Functionalization
Scheme 3. Benzyne Mechanism
reported a method for copper-catalyzed heterocycle arylation
by aryl iodides.13a The best results were obtained by employing
lithium tert-butoxide base and relatively acidic heterocycle
substrates such as oxazoles and thiazoles. For less acidic
imidazole and 1,2,4-triazole derivatives a stronger tBuOK base
is required, and the reaction proceeds by a benzyne-type
mechanism.14 Regioisomer mixtures were formed if substituted
aryl halides were used in combination with tBuOK base
(Scheme 3). Additional issues that had to be considered are as
follows. Formation of tert-butyl aryl ether by the reaction of
tert-butoxide bases with aryl iodide was observed, resulting in
decreased conversion to the arylation products. Copper catalyst
was found to be relatively unstable at the temperature required
for the arylation, and thus only fast reactions were successful.
We reasoned that employing a phenanthroline ligand as
described by Buchwald and co-workers6a should allow for a
more efficient heterocycle arylation by stabilizing the copper
catalyst and facilitating the halide displacement step. Replac-
ing tBuOK with a weaker lithium alkoxide or K3PO4 base
should shut down the benzyne mechanism thus ensuring
arylation regioselectivity. For less reactive substrates employ-
ing hindered Et3COLi base instead of tBuOLi should be
beneficial by slowing the nucleophilic substitution of aryl
iodide while not influencing the arylation rate. We were
pleased to discover that addition of a phenanthroline ligand
allows lithium tert-butoxide base to be used for a less acidic
heterocycle arylation avoiding the problems associated with
the benzyne mechanism. Additionally, the modified reaction
conditions allow for the arylation of heterocycles that were
not reactive under our previous conditions (Table 1). It is
possible to employ K3PO4 base in the arylation of the most
acidic heterocycles such as benzothiazole (entry 1). Caffeine
and N-methyl-1,2,4-triazole can be arylated by using tBuOLi
base (entries 2 and 3). Previously, tBuOK was required for
the arylation of those substrates.13a For the least acidic
heterocycles, hindered Et3COLi base is required for optimal
results. Arylation of N-methylimidazole (entry 4), thiophenes
(entries 5, 6, 10, and 11), N-phenylpyrazole (entry 7),
benzofuran (entry 9) can be accomplished in good yields.
Reaction of 2-chlorothiophene with 2-iodotoluene afforded
only the o-tolylated heterocycle (entry 10). If the benzyne
mechanism was operative, either isomer mixture or m-isomer
would be formed. Arylation of 2-chlorothiophene with
3-iodotoluene afforded only the m-tolylated isomer (entry 11)
in contrast with the previous results obtained by employing
tBuOK base (Scheme 3). Furans and N-substituted indoles
were found to be unreactive under any conditions tried, while
heterocycles possessing acidic N-H bonds were arylated on
the nitrogen as reported by Buchwald.5e The following DMSO
pKa’s of heterocycle C-H bonds have been reported:
Scheme 2. Copper-Catalyzed Arylation
recent elegant work by Fagnou who showed that fluorinated
arenes can be regioselectively arylated by aryl halides under
palladium catalysis.11 The regioselectivity is imparted by the
acidification of the C-H bonds by ortho-fluorine substituents
(Scheme 1B, FG ) F). Thus, two issues that need to be solved
are apparent. First, regioselectivity of arylation is often prob-
lematic unless the coupling C-H component contains a directing
group. Second, expensive transition metals such as palladium,
rhodium, and ruthenium are routinely employed as arylation
catalysts. Cheap copper and iron complexes are only rarely used
in noncarbene C-H bond functionalization chemistry.12
We have recently disclosed a method for copper-catalyzed
arylation of C-H bonds in electron-poor and electron-rich
heterocycles as well as polyfluorobenzenes.13 The reactions
proceed by initial deprotonation of a relatively acidic sp2 C-H
bond by an alkali metal base (or tBuOCu) followed by
transmetalation and coupling with an aryl or vinyl halide
(Scheme 2). Even 1,4-difluorobenzene can be arylated, although
the efficiency is low, presumably due to insufficient acidity. If
the pKa of the C-H bond is the major factor determining the
arylation efficiency, the copper-catalyzed cross-coupling method
should be very general.
We report here a general method for copper-catalyzed, highly
regioselective arylation and alkenylation of electron-rich and
electron-poor heterocycles as well as benzenes possessing at
least two electron-withdrawing groups. Mechanistic investiga-
tions of the arylation process are also described.
2. Results
2.1. Arylation of Electron-Rich Heterocycles. Our initial
attempts were directed toward developing optimized conditions
for electron-rich heterocycle arylation. We have recently
(11) (a) Lafrance, M.; Rowley, C. N.; Woo, T. K.; Fagnou, K. J. Am. Chem.
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Lett. 2006, 8, 5097.
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J. Am. Chem. Soc. 2008, 130, 5858. (c) Zhang, Y.; Li, C.-J. Angew.
Chem., Int. Ed. 2006, 45, 1949. (d) Chen, X.; Hao, X.-S.; Goodhue,
C. E.; Yu, J.-Q. J. Am. Chem. Soc. 2006, 128, 6790. (e) Uemura, T.;
Imoto, S.; Chatani, N. Chem. Lett. 2006, 35, 842. (f) Li, Z.; Cao, L.;
Li, C.-J. Angew. Chem., Int. Ed. 2007, 46, 6505. (g) Phipps, R. J.;
Grimster, N. P.; Gaunt, M. J. J. Am. Chem. Soc. 2008, 130, 8172.
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Do, H.-Q.; Daugulis, O. J. Am. Chem. Soc. 2008, 130, 1128.
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Copper-Catalyzed Arylation
A R T I C L E S
Table 1. Electron-Rich Heterocycle Arylationa
a Copper(I) iodide (0.1 mmol), phenanthroline (0.1 mmol), aryl halide (1-3 mmol), heterocycle (1-2 mmol), base (1.7-3 mmol), DMF or DMPU
solvent (0.5-0.6 mL). Yields are isolated yields. See the Supporting Information for details.
N-alkylindoles, ∼37; furan, 35; N-methylimidazole, 33.15 It
can be concluded that copper-catalyzed electron-rich hetero-
cycle arylation is successful for compounds possessing pKa’s
below 35.
2.2. Arylation of Electron-Poor Heterocycles. We have
previously reported one example of copper-catalyzed electron-
poor heterocycle arylation.13a If the mechanistic consider-
ations presented in Scheme 2 are correct, arylation of
electron-poor heterocycles with C-H bond DMSO pKa’s
below 35 should be feasible. Gratifyingly, conditions devel-
oped for electron-rich heterocycle arylation worked well also
in this case (Table 2). While most pyridines are not reactive,
more acidic pyridine oxides can be arylated by using either
tBuOLi or K3PO4 base (entries 1-5). 2-Iodopyridine is
incompatible with alkoxides due to the formation of 2-tert-
butoxypyridine under the reaction conditions, and K3PO4 base
has to be used (entry 2). 2-Methylpyridine oxide is also
reactive, but the yield is diminished compared to other
substrates, presumably due to acidic benzylic protons de-
creasing the effective concentration of the arylcopper inter-
mediate. 2-Phenylpyridine oxide is efficiently arylated by
substituted aryl iodides, and the products are obtained in
excellent yield (entries 4 and 5). More interestingly, py-
ridazine can be arylated in a good yield (entry 6). A four-
step synthesis of 4-phenylpyridazine has been reported.16 In
contrast, direct arylation methodology allows synthesis of
this compound in a single step from commercially available
starting materials. Pyrimidine is arylated in low yield,
(15) (a) Shen, K.; Fu, Y.; Li, J.-N.; Liu, L.; Guo, Q.-X. Tetrahedron 2007,
63, 1568. (b) Bordwell, F. G. Acc. Chem. Res. 1988, 21, 456.
(16) Helm, M. D.; Moore, J. E.; Plant, A.; Harrity, J. P. A. Angew. Chem.,
Int. Ed. 2005, 44, 3889.
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A R T I C L E S
Do et al.
Table 2. Electron-Poor Heterocycle Arylationa
a Copper(I) iodide (0.1 mmol), phenanthroline (0.1 mmol), aryl halide (1-2 mmol), heterocycle (1-2 mmol), base (1.7-2 mmol), DMF or DMPU
solvent (0.5-0.6 mL). Yields are isolated yields. See the Supporting Information for details. b 2,6-Diphenylpyridine oxide also isolated (20%).
presumably due to insufficient acidity (pKa ) 37;15 entry 7).
As expected, the most acidic positions in pyrimidine and
pyridazine are arylated.15a Cyanidine and 1,2,3-triazine
decompose under the reaction conditions.
substituents and an additional electron-withdrawing group
(entry 11). For 1,4-difluorobenzene arylation, hindered
Et3COLi base is required. Previously we were unable to
efficiently arylate such compounds by using tBuOLi base due
to formation of tBuOAr byproduct. Penta- and tetrachlo-
robenzenes can be phenylated in excellent yield (entries 12
and 13). Less acidic 1,3-dichlorobenzene is regioselectively
phenylated in an acceptable 43% yield by employing Et3COLi
base (entry 14). If tBuOLi base was used, the arylation product
was isolated in only 18% yield. 1,3-Dinitrobenzene and
3-nitrobenzonitrile are also reactive affording the arylation
products in moderate yields (entries 15 and 16). The latter two
arenes are slowly decomposed by the base, and thus only the
most reactive aryl iodides can be used. High arylation yield
and absence of cyclized products for entry 6 suggest that an
2.3. Arylation of Electron-Poor Benzenes. We have recently
disclosed preliminary results showing that polyfluorobenzene
derivatives can be arylated and alkenylated under copper
catalysis.13b Both aryl iodide and bromide reagents can be
employed. The reactivity parallels the acidity of C-H bonds,
with the most acidic C-H bonds, those flanked by two C-F
bonds, arylated most efficiently. The arylation of C-H bonds
that are not flanked by two C-F bonds was inefficient, and
only 10% yield was obtained in the reaction of 4-iodotoluene
with 1,2,3,4-tetrafluorobenzene. Since introduction of electron-
withdrawing substituents in the aromatic ring is expected to
decrease the pKa of C-H bonds, we reasoned that arylation
of a variety of other electron-deficient arenes should be
possible. The improved conditions for arylation of electron-
rich heterocycles were successfully applied to the arylation
of electron-deficient arenes (Table 3). Pentafluorobenzene and
tetrafluoroarenes can be arylated by aryl iodides (entries 2
and 6) as well as aryl bromides (entries 1, 7, 8). Even some
heteroaryl chlorides can be used (entries 3 and 4), although
for 2-pyridyl chloride a 150 °C reaction temperature is
required. Alkenylation is also possible (entries 5 and 9).
Potassium phosphate can be used as a base if an arene
contains more than two fluorine substituents or two fluorine
S
RN1 mechanism is unlikely.17 Recent data obtained by Hartwig
and co-workers argue against intermediacy of aryl radicals for
copper-catalyzed C-N bond formation reactions.18 The fol-
lowing limitations have been observed. If aryl bromides are used
in combination with lithium alkoxide bases, low yields of
arylation products are obtained. Low conversions (<5%) are
(17) (a) Beckwith, A. L. J.; Gara, W. B. J. Chem. Soc., Perkin Trans. 2
1975, 7, 795. (b) Branchi, B.; Galli, C.; Gentili, P. Eur. J. Org. Chem.
2002, 2844.
(18) Tye, J. W.; Weng, Z.; Johns, A. M.; Incarvito, C. D.; Hartwig, J. F.
J. Am. Chem. Soc. 2008, 130, 9971.
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Copper-Catalyzed Arylation
A R T I C L E S
Table 3. Arylation of Electron-Poor Arenesa
Figure 1. ORTEP view of 2. Selected interatomic distances (Å) and angles
(deg): Cu-C(13) ) 1.932(2), Cu-N(1) ) 2.0720(18), Cu-N(2) )
2.0949(19),Cu-Cu)2.5770(6),C(13)-Cu-N(1))135.68(9),C(13)-Cu-N(2)
) 132.69(8).
obtained in the arylation of fluorobenzene, nitrobenzene, and
R,R,R-trifluorotoluene.
2.4. Mechanistic Considerations. As shown in Scheme 2, the
arylation reaction can be divided into three parts: metalation,
transmetalation with copper halide, and reaction of arylcopper
with a haloarene. Metalation and reaction of arylcopper with
haloarene steps will be discussed in more detail.
2.4.1. Metalation Step. One can expect that the metalation
step may be facilitated by coordination of copper species to
Lewis basic heteroatoms of the substrate. However, base-
promoted H/D exchange in polyfluoroarenes as well as electron-
rich and electron-poor heterocycles occurs with the same
efficiency in both the presence or absence of CuI (Scheme 4).
Consequently, the acidity of substrate determines the position
and efficiency of metalation even though the substrates belong
to different classes of compounds and some of them possess
heteroatoms capable of coordinating transition metals. Copper
tert-butoxide is a competent metallating reagent under the
arylation conditions (Scheme 4B) complicating the mechanistic
situation. The lifetime of aryllithium and arylpotassium species
must be short since significant amounts of benzyne-derived
products are not formed from polyfluoro- or polychloroarenes
under catalytic or H/D exchange conditions.
The rate of metalation/demetalation relative to subsequent
reaction steps also has been considered (Scheme 5). If ben-
zothiophene is arylated under the usual reaction conditions but
with added tBuOD, incorporation of deuterium in the unreacted
starting material is observed. The protonation of aryllithium and/
or arylcopper intermediates by relatively weak tert-butanol acid
is competitive with the arylation step.
The ease of electron-deficient arene metalation demonstrated
in this work may have other synthetic implications since strong
alkyllithium bases and cryogenic conditions are not required.
For substrates possessing DMSO pKa’s below 27, even K3PO4
base is an efficient metallating agent.
2.4.2. Arylcopper Reaction with Haloarene Step. Several
competition experiments were undertaken to determine the
relative reactivities of aryl iodides and arenes. The intermediate
arylcopper species were identified by NMR as well as inde-
pendently synthesized.
2.4.2.1. Relative Reactivities. Competition between arylation
of pentafluorobenzene and tetrafluorobenzene by 4-iodotoluene
results in preferential functionalization of pentafluorobenzene
(Scheme 6). This result may be explained by the higher
concentration of arylmetal intermediate for the more acidic
pentafluorobenzene.
The reactivities of electron-rich and electron-poor aryl halides
were compared by reacting a mixture of 4-trifluoromethylha-
lobenzene and 4-halotoluene with pentafluorobenzene (Scheme
7). A 4/1 product ratio was observed favoring trifluorometh-
a Copper(I) iodide (0.1 mmol), phenanthroline (0.1 mmol), halide
(1-2 mmol), arene (1-3 mmol), base (1.7-4 mmol), DMF, DMPU, or
DMF/xylenes solvent (0.5-0.8 mL). Yields are isolated yields. See the
Supporting Information for details. b Copper(I) iodide (0.15 mmol),
phenanthroline (0.15 mmol), halide (1 mmol), arene (3 mmol), base (4
mmol). c tBuOLi base.
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A R T I C L E S
Do et al.
Scheme 4. H/D Exchange Experiments
Scheme 5. Deuteration under Reaction Conditions
Scheme 6. Competition Between Pentafluorobenzene and Tetrafluorobenzene
ylphenylation for both Hal ) I and Br. Thus, electron-deficient
aryl halides are more reactive as reported earlier.9b
iodide, potassium phosphate, pentafluorobenzene, and phenan-
throline in DMF under the conditions of the catalytic process
affords complex 1 as determined by 19F NMR of the crude
reaction mixture. The complex reacts with aryl iodides producing
cross-coupled biaryls. An analogous 4-methoxy-2,3,5,6-tetra-
fluorophenylcopper-phenanthroline complex 2 was prepared
as dark rust-colored crystals by reacting tBuOCu with 2,3,5,6-
tetrafluoroanisole followed by addition of phenanthroline ligand.
The complex is stable in the solid state under inert atmosphere
at -20 °C; however, slow decomposition is observed in a
2.4.2.2. Arylcopper Intermediates. We independently syn-
thesized one of the presumed arylation intermediates, penta-
fluorophenylcopper-phenanthroline complex 1 (Scheme 8). It
exists as a moisture- and temperature-sensitive dark orange solid
that is either insoluble or poorly soluble in most common organic
solvents. The connectivity was verified by X-ray crystal-
lography; however, it was not possible to fully refine the
structure due to twinning of the crystals. The reaction of copper
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Copper-Catalyzed Arylation
A R T I C L E S
Scheme 7. Comparison of Aryl Halide Reactivity
Scheme 8. Polyfluorophenylcopper-Phenanthroline Complexes
CH2Cl2 solution. It is sparingly soluble in most organic solvents
and can be recrystallized from dichloromethane at -30 °C. The
structure of 2 was verified by single-crystal X-ray diffraction
analysis. The ORTEP diagram of 2 is shown in Figure 1. The
complex exists as a dimer in the solid state with a Cu-Cu
distance of 2.5570(6) Å that is shorter than the van der Waals
radii sum of 2.80 Å signifying a Cu-Cu bonding interaction.19
Copper assumes a distorted tetrahedral geometry with a
C(13)-Cu-N(1) angle of 135.68(9)°. As expected, phenan-
throline complexes to Cu in a bidentate fashion with a Cu-N(1)
distance of 2.0720(18) Å and a Cu-N(2) distance of 2.0949(19)
Å. The copper-C(aryl) bond length is 1.932(2) Å, which is
slightly shortened compared to the corresponding Cu-C
distance in tetrameric pentafluorophenylcopper (1.962(2) to
2.007(2) Å).20 However, the Cu-C distance in the penta-
fluorophenylcopper-pyridine complex is shorter at 1.8913(17)
Å.21 No arylcopper-phenanthroline complexes appear to have
been crystalographically characterized. Isomeric tolylcopp-
er-phenanthroline complexes are known.22
Furthermore, electron-poor arenes possessing at least two
electron-withdrawing groups on the benzene ring can also be
arylated. Unusual regioselectivity has been achieved allowing
arylation of the most hindered position. This method supple-
ments the well-known C-H activation/borylation methodology23
where functionalization usually occurs at the least hindered
position. Additionally, the copper-catalyzed arylation methodol-
ogy is complementary to existing lithiation/boronation/cross-
coupling methods and in some cases may offer advantages with
regards to the number of synthetic steps and functional group
tolerance.24
4. Experimental Section
General Procedure for Coupling Reactions. Outside the
glovebox a 1-dram vial equipped with a magnetic stir bar was
charged with haloarene, phenanthroline (10 mol%), substrate, and
solvent (DMF or a 1/1 mixture of DMF and xylenes). If anhydrous
DMPU was used, the reaction was set up inside the glovebox. The
vial was flushed with argon, capped, and placed inside a glovebox.
To this mixture was added CuI (10 mol %) and base (1.7-4.0
equiv). The sealed vial was taken out of the glovebox, stirred at
room temperature for 5 min, and placed in a preheated oil bath.
After the completion of the reaction, the mixture was cooled to
room temperature and diluted with ethyl acetate (50 mL). The
resulting solution was washed with brine (15 mL), dried over
anhydrous MgSO4, and concentrated under vacuum to a volume
of about 1 mL. The mixture containing the product was subjected
to column chromatography on silica gel (hexanes followed by
appropriate solvent to elute the products). After concentrating the
fractions containing the product, the residue was dried under reduced
pressure to yield a pure product.
3. Summary
A general method for copper-catalyzed arylation of sp2 C-H
bonds possessing DMSO pKa’s below 35 has been developed.
The choice of base is dependent on the acidity of the C-H
bond to be arylated. For comparatively acidic C-H bonds with
a pKa below 27, K3PO4 base may be employed. If the substrates
are less acidic (pKa 27-35), a stronger lithium alkoxide base
is required. A variety of electron-rich and electron-poor
heterocycles such as azoles, caffeine, thiophenes, benzofuran,
pyridine oxides, pyridazine, and pyrimidine can be arylated.
(19) (a) Bondi, A. J. Phys. Chem. 1964, 68, 441. (b) Carvajal, M. A.;
Alvarez, S.; Novoa, J. J. Chem.sEur. J. 2004, 10, 2117.
(20) Sundararaman, A.; Lalancette, R. A.; Zakharov, L. N.; Rheingold,
A. L.; Ja¨kle, F. Organometallics 2003, 22, 3526.
(23) (a) Ishiyama, T.; Takagi, J.; Ishida, K.; Miyaura, N.; Anastasi, N. R.;
Hartwig, J. F. J. Am. Chem. Soc. 2002, 124, 390. (b) Cho, J.-Y.; Tse,
M. K.; Holmes, D.; Maleczka Jr., R. E.; Smith III, M. R. Science
2002, 295, 305.
(21) Sundararaman, A.; Zakharov, L. N.; Rheingold, A. L.; Ja¨kle, F. Chem.
Commun. 2005, 1708.
(24) (a) Anctil, E. J.-G.; Snieckus, V. J. Organomet. Chem. 2002, 653,
150. (b) Snieckus, V. Chem. ReV. 1990, 90, 879.
(22) Camus, A.; Marsich, N. J. Organomet. Chem. 1970, 21, 249.
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A R T I C L E S
Do et al.
Acknowledgment. We thank the Welch Foundation (Grant No.
E-1571), National Institute of General Medical Sciences (Grant No.
R01GM077635), A. P. Sloan Foundation, Camille and Henry Dreyfus
Foundation, and Norman Hackerman Advanced Research Program for
supporting this research. We thank Dr. James Korp for collecting and
solving the X-ray structure of 2, and Dr. Ashok Krishnaswami (JEOL
USA, Peabody MA) for acquiring a 13C spectrum of 1.
Supporting Information Available: Detailed experimental
procedures, characterization data for new compounds, and
X-ray crystallography data for 2. This material is available
free of charge via the Internet at http://pubs.acs.org.
JA805688P
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15192 J. AM. CHEM. SOC. VOL. 130, NO. 45, 2008