Well-defined, air-stable (NHC)Pd(Allyl)Cl (NHC = N-heterocyclic carbene) catalysts for the arylation of ketones

Article abstract of DOI:10.1021/ol026745m

(formula presented) A number of palladium-N-heterocyclic carbene (NHC) complexes were found to be active catalysts for the arylation of ketones. A large number of substrates, both aryl halides and ketones, are compatible with the reaction conditions. The ketone arylation reactions are achieved with low catalyst loading in short reaction times using aryl chlorides and triflates as reactive partners.

Full text of DOI:10.1021/ol026745m

ORGANIC
LETTERS
2002
Vol. 4, No. 23
4053-4056
Well-Defined, Air-Stable (NHC)Pd(Allyl)Cl
(NHC ) N-Heterocyclic Carbene)
Catalysts for the Arylation of Ketones
Mihai S. Viciu, Romain F. Germaneau, and Steven P. Nolan*
Department of Chemistry, UniVersity of New Orleans, New Orleans, Louisiana 70148
snolan@uno.edu
Received August 16, 2002
ABSTRACT
A number of palladium-N-heterocyclic carbene (NHC) complexes were found to be active catalysts for the arylation of ketones. A large number
of substrates, both aryl halides and ketones, are compatible with the reaction conditions. The ketone arylation reactions are achieved with low
catalyst loading in short reaction times using aryl chlorides and triflates as reactive partners.
The synthesis of R-aryl ketones has received renewed
attention since the discovery of direct coupling between
simple ketones and aryl halides without the use of tin or
silicon intermediates.1 The enolate form of the ketone can
be generated efficiently in situ and acts as a transmetalating
agent in catalytic transformation. The reaction leads to the
formation of new sp2-sp3 bonds and can be conducted
regioselectively.2 With one exception,3 the catalyst supporting
ligands are tertiary phosphines. These ligands are susceptible
to thermal degradation and are often difficult to remove from
products.
Herein, we describe a new class of palladium catalysts
bearing a N-heterocyclic carbene (NHC) ligand (where NHC
is SIPr [N,N′-bis(2,6-diisopropylphenyl)4,5-dihydroimida-
zol)-2-ylidene], IPr [N,N′-bis(2,6-diisopropylphenyl)imida-
zol)-2- ylidene], IMes [N,N′-bis(2,4,6-trimethylphenyl)-
imidazole)-2-ylidene], or ItBu [N,N′-bis tert-butyl-imidazol)-
2-ylidene]), capable of mediating the direct coupling of
readily available ketones with aryl chlorides.
N-Heterocyclic carbenes have proven to be efficient
ligands in numerous coupling reactions mediated by pal-
ladium and nickel.4 The use of bulky substituents on the
imidazole nitrogens and the important σ-donating properties
of the NHC are beneficial to the oxidative-addition and
reductive-elimination steps of the cross-coupling catalytic
cycle.
The ability of the NHC to coordinate metal centers5
strongly makes them excellent candidates for the design of
well-defined catalysts. Recent studies have presented catalytic
systems bearing different numbers (one6 and two7) of ligands
in palladium-mediated cross-coupling reactions. Jutand and
(4) For reviews see: (a) Herrmann W. A Angew. Chem., Int. Ed. 2002,
41, 1290-1309. (b) Hillier, A. C.; Nolan, S. P. Platinum Metals ReV. 2002,
46, 50-64. (c) Hillier, A. C.; Grasa, G. A.; Viciu, M. S.; Lee, H. M.; Yang,
C.; Nolan, S. P. J. Organomet. Chem. 2002, 653, 69-82. (d) Jafarpour, L.;
Nolan, S. P. AdV. Organomet. Chem. 2000, 46, 181-222. (e) Weskamp,
T.; Bohm, V. P. W.; Herrmann, W. A. J. Organomet. Chem. 2000, 600,
12-22.
(5) (a) Voges, M. H.; Rømming, C.; Tilset, M. Organometallics 1999,
18, 529-533. (b) Herrmann, W. A.; Kocher, C. Angew. Chem., Int. Ed.
Engl. 1997, 36, 2163-2187.
(6) (a) Jackstell, R.; Gomez A.; Frisch, A.; Selvakumar, K.; Zapf, A.;
Klein, H.; Spannenberg, A.; Rottger, D.; Briel, O.; Karch, R.; Beller, M.,
Angew. Chem., Int. Ed. 2002, 41, 986-989. (b) Lee, H.-M.; Nolan, S. P.
Org. Lett. 2000, 2, 2053-2055. (c) Grasa, G. A.; Nolan, S. P. Org. Lett.
2001, 3, 119-122. (d) Zhang, C.; Huang, J.; Trudell, M. T.; Nolan, S. P.
J. Org. Chem. 1999, 64, 3804-3805.
(1) (a) Hamann, B. C.; Hartwig, J. F. J. Am. Chem. Soc. 1997, 119,
12382-12383. (b) Palucki, M.; Buchwald, S. L. J. Am. Chem. Soc. 1997,
119, 11108-11109. (c) Satoh, T.; Kawamura, Y.; Miura, M.; Nomura, M.
Angew. Chem., Int. Ed. Engl. 1997, 36, 1740-1742. (d) Terao, Y.; Fukuoka,
Y.; Satoh, T.; Miura, M.; Nomura, M. Tetrahedron Lett. 2002, 43, 101-
104. (e) Fox, J. M.; Huang, X.; Chieffi, A.; Buchwald, S. L. J. Am. Chem.
Soc. 2000, 122, 1360-1370.
(2) (a) Ahman, J.; Wolfe, J. P.; Troutman, M. V.; Palucki, M.; Buchwald,
S. L. J. Am. Chem. Soc. 1998, 120, 1918-1919. (b) Hamada, T.; Chieffi,
A.; Ahman, J.; Buchwald, S. L. J. Am. Chem. Soc. 2002, 124, 1261-1268.
(3) Semmelhack, M. F.; Chong, B. P.; Stauffer, R. D.; Rogerson, T. D.;
Chong, A.; Jones, L. D. J. Am. Chem. Soc. 1975, 97, 2507-2516.
10.1021/ol026745m CCC: $22.00 © 2002 American Chemical Society
Published on Web 10/16/2002

co-workers8 have recently described palladium/phosphine
systems that benefit from the presence of a single bulky
tertiary phosphine ligand in the coordination sphere of
palladium. These results, along with our early metal/ligand
ratio optimization studies in palladium-mediated cross-
coupling reactions, encouraged the design of catalysts bearing
one NHC ligand and the examination of reactivity displayed
by such complexes in various cross-coupling reactions.9
Recently, we observed that reaction of NHC with [(η3-
allyl)Pd(Cl)]2 led, with high yields, to the formation of
monomeric species with the general formula (NHC)Pd(allyl)-
Cl.10 1H NMR and single-crystal X-ray structures confirmed
the η3 coordination of the allyl fragment and the distorted
square planar geometry of the complex around the palladium
center (Scheme 1).
We investigated the use of (NHC)Pd(allyl)Cl complexes
in cross-coupling of aryl halides and ketone enolates.
Propiophenone and chlorobenzene were chosen as substrates
for optimization studies. Propiophenone has the advantage
of only having one available site for deprotonation. A strong
base was required in order to abstract the acidic proton and
to generate the enolate form of the ketone. NaOtBu, 1.1
mmol, was found to be the most convenient and affordable
base. The role of the base is twofold: to generate active Pd-
NHC species and to deprotonate the ketone. No aryl-tert-
butyl ether was detected by GC, indicating alkoxide anion
attack to be occurring exclusively at the ketone and not on
an oxidative adduct.14 Hartwig and co-workers highlighted
the need for the ketone to be in the enolate form to prevent
side reactions.15 In the present cases, an excess of base led
to an increased amount of biarylated ketone. We concluded
that monoarylated product is the subject of a second
deprotonation due to the enhanced acidity of the methine
proton.
Scheme 1. Synthesis of (SIPr)Pd(allyl)Cl
Table 1. Catalyst Effect on R-Arylation of Propiophenonea
catalyst
yield(%)
(SIPr)Pd(allyl)Cl (1)
(IAd)Pd(allyl)Cl (2)
(IPr)Pd(allyl)Cl (3)
(IMes)Pd(allyl)Cl (4)
(ItBu)Pd(allyl)Cl (5)
100
95
95
93
99
The formal 16-electron configuration at the palladium
center confers air-stability to the complexes. It is well-known
in the literature that nucleophilic attack on the allyl moiety
by a base, which should generate active species, is a very
plausible catalyst activation step.11 This active “NHC-Pd”
species would then be able to oxidatively add aryl halides
or pseudohalides. Beller and co-workers have also proposed
the intermediacy of such a species in the palladium-mediated
telomerization of butadiene.6a The product of this allylic
attack by base, allyl tert-butyl ether, was unambiguously
a Conditions: 1 mol % (NHC)Pd(ally)Cl, 1.1 mmol of NaOtBu, THF,
70 °C, 1 h. GC yields are averages of two runs.
A survey of catalyst activity (Table 1) proved that steric
properties of the ligand are important. The most effective
catalyst was found to be (SIPr)Pd(allyl)Cl (1), although it is
apparent that at 70 °C, all entries are very efficient catalysts
for ketone arylation. The use of 1 leads to complete
conversion of the substrates in 1 h at 70 °C. Other catalysts
are slightly less active, and differences in reactivity, although
not that significant at 70 °C, are notable at lower tempera-
tures. For example, the difference in activity, at 50 °C in 1
h, between (SIPr)Pd(allyl)Cl (1) and its unsaturated relative
(IPr)Pd(allyl)Cl (3) is noteworthy: 1 led to 97% conversion,
while 3 yielded only 51% of the desired product. The reasons
behind the difference in reactivity are not obvious at this
time since only slight differences exist between steric and
electronic properties of the saturated and unsaturated ligand
1
characterized by H NMR spectroscopy.12 Its formation is
nearly quantitative based on the amount of palladium
precursor. The trapping of a (NHC)Pd-PR3 species13 also
confirmed the reduction of (NHC)Pd(allyl)Cl complexes to
a (NHC)Pd species in the presence of base.
(7) (a) Huang, J.; Grasa, G. A.; Nolan, S. P. Org. Lett. 1999, 1, 1307-
1309. (b) Gsto¨ttmayr, C. W. K.; Bo¨hm, V. P. W.; Herdtweck, E.; Grosche,
M.; Herrmann W. A. Angew. Chem., Int. Ed. 2002, 41, 1363-1365.
(8) (a) Galardon, E.; Ramdeehul, S.; Brown, J. M.; Cowley, A.; Hii, K.
K.; Jutand, A. Angew. Chem., Int. Ed. 2002, 41, 1760-1763. (b) McGuiness,
D.; Cavell, K. J. Organometallics 2000, 19, 741-748.
(9) Viciu, M. S.; Kissling, R. M.; Stevens, E. D.; Nolan, S. P. Org. Lett.
2002, 4, 2229-2231.
(10) Viciu, M. S.; Germaneau, R. F.; Navarro-Fernandez, O.; Stevens,
E. D.; Nolan, S. P. Manuscript in preparation.
(11) (a) Vedernikov, A. N.; Sayakhov, M. D.; Solomonov, B. N.
MendeleeV Comm. 1997, 5, 205-206. (b) Stanton, S. A.; Felman, S. W.;
Parkurst, C. S.; Godleski, S. A. J. Am. Chem. Soc. 1983, 105, 1964-1969.
(12) The allyl ether could also be generated via halide replacement and
reductive-elimination of the allyl alkoxy fragments on palladium. At this
point, both routes are viewed as possible.
(13) Such species have been fully characterized. See: Titcomb, L. R.;
Caddick, S.; Cloke, F. G. N.; Wilson, D. J.; McKerrecher, D. Chem.
Commun. 2001, 1388-1389.
(14) A reviewer suggested that it may be possible that the Pd(aryl)(OtBu)
complex forms but reacts with free ketone to form the enolate or enolate
complex faster than it does C-O bond-forming reductive elimination. This
possibility also exists, and we thank the reviewer for his alternative
explanation.
(15) (a) Kawatsura, M.; Hartwig, J. F. J. Am. Chem. Soc, 1999, 121,
1473-1478. (b) Hartwig has recently reported the use of a NHC palladium
system generated in situ in reactions with esters: Lee, S.; Beare, N. A.;
Hartwig, J. F. J. Am. Chem. Soc. 2001, 123, 8410-8411.
4054
Org. Lett., Vol. 4, No. 23, 2002

Scheme 2. Proposed Catalytic Cycle for the R-Arylation of
Table 2. Palladium-Mediated R-Arylation of Ketones
Ketones Using a (NHC)Pd(allyl)Cl System
pair.16 To investigate the scope and limitations of the
palladium-allyl system, complex 1 was used as a catalyst.
A closer examination of the proposed catalytic cycle
(Scheme 2) raises the question of possible â-hydrogen
elimination from a tricoordinated species after a transmeta-
lating step.
The availability of a vacant site on a species generated
after oxidative addition could possibly facilitate â-hydrogen
elimination. This reaction route appears to be quite sensitive
to steric factors. We observed the formation of benzene in
6% yield when (IMes)Pd(allyl)Cl (4) was used as catalyst,
but no benzene formation was observed when more sterically
demanding catalysts were used.
A survey of reactivity of aryl and heteroaromatic halides
with various alkyl-alkyl or aryl-alkyl ketones under
catalytic conditions is provided in Table 2.
Operationally, the ketone was added to a mixture of
catalyst, base, and solvent in order to induce the formation
of the enolate. The aryl halides were injected last.
We focused our study on the use of aryl chlorides as
reactive partners.17 As a general trend of reactivity, aryl
bromides are more reactive than chloride counterparts, but
the amount of side-products is higher. The reactions of
propiophenone with chloro- and bromobenzene reach comple-
tion within 1 h, but in the latter case, 10% benzene was
observed. The presence of an aryl group R to the carbonyl
minimizes the formation of multiarylated products, but it is
not clear if either electronic or steric factors are essential.
The reaction times are little or not affected by the electronics
of substituents on the aryl halides. Unhindered dialkyl
a Conditions: 1, 1 mol %; THF, 4 mL; NaOtBu, 1.05 mmol; ketone, 1
mmol; aryl halide, 1 mmol; NMR yields, 95% pure compounds. Isolated
yields reported in brackets. b Base (2 mmol) and aryl chloride.
ketones have a tendency to react with more than 1 equiv of
aryl halides. The reaction of chlorobenzene and 3-butanone
gave an 80:20% mixture of mono- and diarylated products.
By using 2 equiv of aryl halides per equivalent of 3-butanone,
we were able to obtain pure diarylated product in less than
15 min. It was concluded that since the reaction was fairly
rapid, base could deprotonate the product, thereby generating
the second enolate, which would be involved in the subse-
quent reaction. We investigated the use of hindered substrates
as a way to minimize the formation of diarylated byproducts.
The results of coupling between hindered aryl halides and
ketones are presented in Table 3.
The amount of diarylated and dehalogenated byproducts
was effectively suppressed. One notable exception is the
reaction of 2-bromomesitylene and propiophenone that led
to 11% mesitylene and 80% isolated coupling product.
To test the generality of the catalytic system, a number of
triflates were tested as substrates (Table 4). In initial attempts,
using the reaction conditions described for the aryl chlorides,
very poor or no yields of the desired coupling products were
obtained. Only starting materials were observed under these
conditions with no other product formed. However, changing
the solvent from THF to toluene leads to facile arylation
product formation in short reaction times. The reactions
(16) (a) Hillier, A. C.; Yong, B. S.; Sommer, W.; Petersen, J. L.; Nolan,
S. P. Organometallics, manuscript submitted. (b) Bourissou, D.; Guerret,
O.; Gabbai, F. P.; Bertrand, G. Chem. ReV. 2000, 100, 39-92.
(17) For Pd-phosphine complexes able to mediate this transformation
with aryl chlorides, see: Ehrentraut, A.; Zapf, A.; Beller, M. AdV. Synth.
Catal. 2002, 344, 209-217.
Org. Lett., Vol. 4, No. 23, 2002
4055

Table 3. R-Arylation of Ketones Using Hindered Aryl
Table 4. R-Arylation of Ketones Using Aryl Triflates
Halidesa
a Reaction conditions: aryl triflates, 1 mmol; ketone, 1.1 mmol; 60 °C;
NaOtBu, 100 mg; THF, 4 mL; isolated yields. b GC yield. All yields are
averages of two runs.
makes this class of catalyst suitable for a large variety of
cross-coupling reactions. The catalyst is very efficient in
coupling aryl chlorides, bromides, or triflates with ketones,
in terms of reaction temperature, reaction time, and catalyst
loading. The regioselective reactions using catalysts based
on chiral N-heterocyclic carbenes are currently under inves-
tigation.
a Conditions: 1, 1 mol%; THF, 4 mL; NaOtBu, 1.05 mmol; ketone, 1
mmol; aryl halide, 1 mmol; NMR yields, 95% pure compounds. Isolated
yields are reported in brackets.
conducted in toluene with aryl chlorides are kinetically
slower than those performed in THF. The generality of the
present catalytic system is presently being investigated on a
larger number of aryl triflates and halides. The origin of the
surprising solvent effect is also being examined.
Acknowledgment. We gratefully acknowledge financial
support from the National Science Foundation, the donors
of the Petroleum Research Fund, administered by the
American Chemical Society, and the Louisiana Board of
Regents.
In summary, a general and efficient protocol for coupling
of aryl halides (and triflates) with ketones has been devel-
oped. The catalyst is part of a new generation of air-stable,
well-defined palladium N-heterocyclic carbene systems. The
activation of the catalyst is achieved by an alkoxide base
acting as a nucleophile. The generality of the activation step
Supporting Information Available: Experimental details
for the synthesis of (SIPr)Pd(allyl)Cl, the catalysis protocol,
and the product isolation procedure. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL026745M
4056
Org. Lett., Vol. 4, No. 23, 2002