Pd-PEPPSI-IPent: An Active, Sterically Demanding Cross-Coupling Catalyst and its application in the synthesis of tetra-ortho-substituted biaryls

Full text of DOI:10.1002/anie.200805661

Angewandte
Chemie
DOI: 10.1002/anie.200805661
Synthetic Methods
Pd-PEPPSI-IPent: An Active, Sterically Demanding Cross-Coupling
Catalyst and Its Application in the Synthesis of Tetra-Ortho-
Substituted Biaryls**
Michael G. Organ,* Selꢀuk C¸ alimsiz, Mahmoud Sayah, Ka Hou Hoi, and Alan J. Lough
Transition-metal-catalyzed cross-coupling has become one of
the most powerful carbon–carbon bond-forming processes.[1]
Phosphine-based ligands have been investigated most inten-
sively so as to improve catalytic proficiency in these trans-
formations. Recently, a number of monoligated Pd–N-hetero-
cyclic carbene (Pd–NHC) complexes have been prepared and
show high levels of activity in a variety of Pd-catalyzed cross-
coupling reactions as reported by Caddick, Cloke, and co-
workers[2a] Cꢀsar, Bellemin-Laponnaz, and Gade,[2b] Nolan
and co-workers,[2c–f] Beller and co-workers,[2g] Herrmann and
co-workers,[2h] Organ and co-workers,[2i] and others (Figure 1).
The activities of complexes 1–14 have been correlated with
the steric environment around Pd, that is, the bulkier ligands
lead to higher catalytic activity.[3a] For example, we have
observed that yields obtained by using Pd-PEPPSI-IPr (14)
were much greater than those obtained when using
Pd-PEPPSI-IMes (13) in the Negishi, Suzuki–Miyaura,[2i]
Kumada–Tamao–Corriu,[3b] and Buchwald–Hartwig–Yagu-
polꢁskii[3c] amination reactions. Since the s-donor abilities of
iMes and iPr carbenes are similar, sterics are likely to be
involved in the improved performance of the latter.[3d]
We have been using calculations to study the origin of the
increased catalytic activity brought on by the increased steric
bulk on the NHC, and it appears that whereas oxidative
addition is not altered significantly by the sterics, the metal–
metal exchange and the reductive elimination are affected.[3d]
On the basis of this analysis, we set out to compare the
reactivities of sterically demanding analogues 15, 16
(Figure 3), and 17[4] with 14, which is already considered to
be bulky and has been demonstrated to be quite reactive.
Whereas ortho-substituted biaryls are important substruc-
tures of many biologically active compounds and organic
materials,[5] the formation of tetra-ortho-substituted biaryls
under mild conditions remains a challenge.[1] In 1997, Johnson
and Foglesong reported a Suzuki–Miyaura coupling, using
Figure 1. A selection of monoligated palladium–N-heterocyclic carbene
(NHC) complexes used in palladium-catalyzed cross-coupling reac-
[*] Prof. M. G. Organ, Dr. S. C¸alimsiz, M. Sayah, K. H. Hoi
tions.
Department of Chemistry, York University
4700 Keele Street, Toronto, ON, M3J 1P3 (Canada)
Fax: (+1)416-736-5936
E-mail: organ@yorku.ca
[Pd(PPh3)4] in the presence of Na2CO3, to prepare an
Dr. A. J. Lough
Department of Chemistry, University of Toronto
80 St. George Street, Toronto, ON, M5S 3H6 (Canada)
unsymmetrical biaryl in 12% overall yield from an aryl
bromide.[6a] In 2001 and 2002, Fu and co-workers reported
Negishi[6b] and Stille[6c] cross-coupling protocols employing
[**] This work is supported by NSERC (Canada) and the ORDCF
[Pd{P(tBu)3}2]. In each article, the authors have provided an
example for the preparation of a tetra-ortho-substituted
biaryl from an aryl chloride at 1008C. In a series of
publications, Buchwald and co-workers also reported
(Ontario). PEPPSI=Pyridine-Enhanced Precatalyst Preparation
Stabilization and Initiation.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200805661.
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a considerable advancement in the production of sterically
congested biaryls.
We chose the Suzuki–Miyaura coupling to explore the
effect of catalyst bulk on biaryl formation, and used the
reaction between 2-methoxynapthalene bromide (22) and 2,6-
dimethylphenyl boronic acid (23) to optimize the reaction
conditions (Table 1). Since the choice in the base can be
critical,[7] we screened several base/solvent systems . Interest-
ingly, the most widely utilized base, K3PO4, for the synthesis
of tetra-ortho-substituted biaryls led to poor results when 16
was used, even at high temperatures (Table 1, entries 1, 2, and
Table 1: Base and solvent screening for biaryl synthesis using Pd–NHC
complexes.
Figure 2. Ligands used in the palladium-catalyzed synthesis of tetra-
substituted biaryls.
Entry
Cat.
Reaction conditions
Conversion [%][a]
1
2
3
4
5
6
7
8
16
16
16
16
16
16
16
16
16
16
16
16
16
16
14
14
14
15
17
PdCl2
PdCl2
–
K3PO4H2O, 708C, toluene
K3PO4, 1008C, toluene
CsF, 708C, toluene
K2CO3, 708C, toluene
K2CO3, 708C, dioxane
KOAc, 608C, toluene
KOtBu, 708C, toluene
KOH, 708C, toluene
K3PO4H2O, 408C, dioxane
Cs2CO3, 408C, dioxane
CsF, 408C, dioxane
KOH, 408C, dioxane
KOH, 658C, dioxane
KOtBu, 658C, tBuOH, 4 ꢀ M.S.
KOH, 408C, dioxane
KOtBu, 658C, tBuOH, 4 ꢀ M.S.
KOH, 658C, dioxane
0
46
0
0
0
0
<2
16
0
9
10
11
12
13
14
15
16
17
18
19
20
21
22
3
15
80
91
90
25
95
41
4
9
0
0
0
KOH, 658C, dioxane
KOH, 658C, dioxane
KOH, 708C, dioxane
KOtBu, 658C, tBuOH, 4 ꢀ M.S.
KOtBu, 658C, tBuOH, 4 ꢀ M.S
[a] Conversion was determined by GC/MS/MS methods using
calibrated internal standard (undecane); reactions were performed in
a
Figure 3. ORTEP representation of the crystal structure of Pd catalyst
16.[14] Hydrogen atoms are omitted for clarity.
duplicate. M.S.=molecular sieves.
Suzuki–Miyaura reactions for the synthesis of tetra-ortho-
substituted biaryls from aryl bromides using hindered biar-
ylphosphine ligands (18 and 19, Figure 2) in conjunction with
[Pd2(dba)3] (dba = dibenzylideneacetone) at 1108C.[6d–f] In
2004, similar biaryls were prepared by Glorius and co-
workers, in which they coupled suitably substituted aryl
chlorides and boronic acids using IBiox12·HOTf (20)/Pd-
(OAc)2 at 1108C.[6g] In 2008, Hoshi, Hagiwara, and co-
workers reported Suzuki–Miyaura couplings employing ruth-
enocenylphosphine R-Phos (21)/[Pd2(dba)3], which led to the
formation of three tetra-ortho-substituted biaryls from aryl
chlorides and boronic acids at 1008C.[6h] The need for
significantly elevated temperatures is a major drawback for
these systems and the development of a catalyst that could
affect such conversions at much lower temperatures would be
9). Pushing forward, we identified two promising base/solvent
systems: KOH in dioxane and KOtBu in tBuOH. By using the
former system at 658C with a slight excess of aryl boronic acid
(1.2 equiv) and 14, the desired product was produced in 41%
yield (Table 1, entry 17); under the same reaction conditions,
16 led to 91% yield of the product (Table 1, entry 13). The
KOtBu/tBuOH conditions (in the presence of the 4 ꢂ
molecular sieves)[8] resulted in excellent conversion with
both catalysts 14 and 16 (Table 1, entries 16 and 14, respec-
tively). When PdCl2 was used under the same reaction
conditions, no product formation was detected (Table 1,
entries 20 and 21). These results confirm that if there is any
palladium black formed from the Pd–NHC catalysts, then the
resulting colloidal Pd is not catalytically active in this
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coupling. Additionally, in the absence of any catalyst product
formation was not observed under the standard reaction
conditions (Table 1, entry 22).
To additionally investigate the intricacies of the model
reaction employing KOH/dioxane, coupling reactions
employing 14 and 16 were monitored at 40 and 708C
(Figure 4). At lower temperature, the activation for both
catalysts appeared to be slow; no product was observed within
the first two hours of the reaction. However, at 708C both
catalysts turned over immediately and the reaction employing
16 was complete within two hours, whereas 14 stalled at
approximately 40% conversion. In both reactions involving
14, unreacted 2-methoxynaphthalene bromide was recovered.
To shed some light onto the poorer performance of 14, we
conducted some control experiments. At 708C, the addition
of an additional 2 mol% of 14 after two hours did not improve
the yield. However, the addition of an extra 1.2 equivalents of
2,6-dimethylphenyl boronic acid (with or without an extra
2 mol% 14) after two hours led to a 10% improvement in the
overall yield (51%). It can be inferred from these results that
at 708C 14 remained active after
Figure 4. Effect of the temperature and reaction time on the model
Suzuki–Miyaura reaction utilizing Pd-PEPPSI complexes. *: 16 at
708C, *: 16 at 408C, !: 14 at 708C, !: 14 at 408C.
dered biaryls from aryl chlorides under very mild reaction
conditions. The examples in Table 4 illustrate that the
conditions are quite tolerant of sensitive functional groups.
Two final experimental observations are of note: a) the use of
the S-Phos (19)/Pd catalyst system to couple chlorides under
two hours, but the boronic acid was
completely consumed to give the
Table 2: Hindered Suzuki–Miyaura couplings with aryl bromides.
corresponding undesired phenol,
which was observed in the GC
analysis.
After obtaining these promising
early results, 14 and 16 were eval-
uated in the coupling of a variety of
hindered aryl bromides (Table 2). A
tetrasubstituted heterobiaryl was
Entry
1
ArBr
(OH)2BAr’
Product
14 Yield [%][a]
16 Yield [%][a]
87[b]
69[b]
successfully
synthesized
using
2
3
4
44[c]
–
–
88[c]
(45% with 15)
(45% with 17)
KOH/dioxane (Procedure A) with
14 (Table 2, entry 1), although with
other substrates this base/solvent
system resulted in significant
phenol formation.[9] This side reac-
tion was avoided by using KtOBu/
tBuOH in the presence of 4 ꢂ
molecular sieves (Procedure B),
which effectively produced a vari-
ety of biaryls. Whereas these con-
ditions were good for 16, they were
less so for 14. That phenol forma-
tion is base dependent suggests that
transmetalation is rate limiting.
Additional investigation of hin-
dered aryl chlorides was particu-
larly productive (Table 3). Chlor-
ides, which are sluggish toward
oxidative addition, coupled not
only with higher yields than the
corresponding bromides, but the
product mixtures were also gener-
ally much cleaner. By using 16 at
658C (except in the case of entries 5
and 9 in Table 3) and the KOtBu/
tBuOH system, we were able to
5
6
7
30[c]
<2[c]
42[e]
75[c]
80[c]
82[e]
90[b]
8
9
10
41[b]
95[c]
–
91[c] (80)[d]
50[f]
11
64[c]
90[c]
[a] Yields of isolated products are the average of two runs. [b] Procedure A: KOH (0.75 mmol), dioxane
(1 mL). [c] Procedure B: KOtBu (0.75 mmol), tBuOH (1 mL), 4 ꢀ M.S. [d] 2.1 mmol scale and 16
(1.2 mol%) [e] Procedure C: KOtBu (0.75 mmol), tBuOH (0.7 mL), dioxane (0.3 mL), 4 ꢀ M.S. [f] KOH
synthesize a striking array of hin- (0.75 mmol), dioxane (1 mL), 16 (10 mol%), RT.
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Table 3: Hindered Suzuki–Miyaura couplings with aryl chlorides.
Entry
1
ArCl
(OH)2BAr’
Product
14 Yield [%][a]
16 Yield [%][a]
8[b]
59[b]
2
3
<2[c]
70[c] (74)[d]
32[c]
78[c]
4
5
34c]
0[c,e]
0[c]
49[c]
65[c,e]
61[c]
6
7
8
95[c]
47[c]
–
(32% using
19/[Pd2(dba)3])[c]
0[c]
95[c]
80[c,e]
88[c]
9
0[c,e]
10
0[c]
[a] Yields of isolated products are the average of two runs. [b] Procedure A: KOH (0.75 mmol), dioxane (1 mL) . [c] Procedure B: KOtBu (0.75 mmol),
tBuOH (1 mL), 4 ꢀ M.S. [d] 0.5 mmol scale. [e] Reaction was performed at 808C.
identical reaction conditions to those used for 16 led to
significantly lower yields (e.g., Table 3, entry 7),[10] and b) 16
is active enough to produce a highly hindered tetra-ortho-
substituted biaryl by using the Suzuki–Miyaura reaction at
room temperature (e.g., Table 2, entry 10).[11]
We have demonstrated previously that a dramatic reduc-
tion in catalyst performance can simply result from removing
a methyl group from both isopropyl units on one side of the
iPr catalyst 14.[12] Examining entries 13, 17, 18, and 19 in
Table 1, and entries 2–4 in Table 2, shows a similar sensitivity
to changing not only the bulk, but the nature of the bulk that
surrounds Pd. From the analysis of these data relating to the
structures of catalysts 14, 15, 16, and 17, several things are
essential for high catalytic activity with Pd–NHC complexes:
1) branching at the first carbon atom of the ortho-alkyl
substituent on the aryl ring (e.g., 15 versus 16) is necessary,
2) increasing the bulk (provided point 1 above is satisfied)
improves catalyst performance (e.g., 14 versus 16), and
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21, 5470 – 5472; d) M. S. Viciu, R. A. Kelly, E. D. Stevens, F.
Table 4: Functional group tolerance using 16.
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Fang, Tetrahedron 2005, 61, 9723 – 9735.
Entry
1
X
Product
Yield [%][a]
51[b]
Br
2
3
Cl
Cl
quant.[c]
90[b] (89)[c]
4
5
Br
Cl
85[d]
70[e]
[4] Complexes 15–17 were prepared using the protocol outlined in
reference [2i]; experimental details and characterization are
provided in the Supporting Information.
[5] J. Hassan, M. Svignon, C. Gozzi, E. Schulz, M. Lemaire, Chem.
Rev. 2002, 102, 1359 – 1469.
[a] Yields of isolated products are the average of two runs. [b] Procedure
A: Ar’B(OH)2 (0.5 mmol), KOH (0.75 mmol), dioxane (1 mL), 658C.
[c] Procedure B: Ar’B(OH)2 (0.5 mmol), KOtBu (0.75 mmol), tBuOH
(1 mL), 4 ꢀ M.S., 658C. [d] Ar’B(OH)2 (0.3 mmol), KOtBu (0.375 mmol),
iPrOH (1 mL), 4 ꢀ M.S., 308C. [e] Ar’B(OH)2 (0.3 mmol), KOtBu
(0.375 mmol), tBuOH (1 mL), 4 ꢀ M.S., 308C.
[6] a) M. G. Johnson, R. J. Foglesong, Tetrahedron Lett. 1997, 38,
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S. L. Buchwald, J. Am. Chem. Soc. 2002, 124, 1162 – 1163;
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[7] F. Bellina, A. Carpita, R. Rossi, Synthesis 2004, 2419 – 2440.
[8] As previously reported, the use of strictly anhydrous conditions
was found to be essential to minimize the competing proto-
deboronation of the aryl boronic acid. For more information
please see reference [6g] and A. N. Cammidge, K. V. L. Crꢀpy, J.
Org. Chem. 2003, 68, 6832 – 6835.
3) conformational flexibility in the alkyl substituent is essen-
tial (16 versus 17).
Pd-PEPPSI-IPent (16) proved to be an excellent catalyst
for the Suzuki–Miyaura cross-coupling of sterically hindered
aryl bromides/chlorides with aryl boronic acids to form bulky
tetra-ortho-substituted biaryls in good yields under mild
reaction conditions. Ortho substituents, such as methyl,
primary alkyl, phenyl, fluorine, and alkoxy groups are
accommodated. These results, in addition to our earlier
reports, suggest that for effective cross-coupling reactions, a
sterically demanding yet conformationally flexible environ-
ment, also referred to as the “flexible steric bulk”,[6g,13] in the
vicinity of the metal center is essential. Additional inves-
tigation of the use of Pd-PEPPSI-IPent (16) in various
challenging cross-coupling reactions is currently underway.
[9] K. W. Anderson, T. Ikawa, R. E. Tundel, S. L. Buchwald, J. Am.
Chem. Soc. 2006, 128, 10694– 10695.
[10] When the identical conditions were used to couple analogous
bromide substrates, identical yields (ca. 90%) were obtained
using S-Phos (19)/Pd and 16.
[11] To the best of our knowledge, there exists only one example of a
Suzuki–Miyaura tetra-ortho-substituted biaryl produced at room
temperature. However, this coupling employed an activated
dimethoxyphenyl boronic acid derivative. See: C. Song, Y. Ma,
Q. Chai, C. Ma, W. Jiang, M. B. Andrus, Tetrahedron 2005, 61,
7438 – 7446.
[12] N. Hadei, E. A. B. Kantchev, C. J. OꢁBrien, M. G. Organ, J. Org.
Chem. 2005, 70, 8503 – 8507.
[13] G. Altenhoff, R. Goddard, C. W. Lehmann, F. Glorius, Angew.
Chem. 2003, 115, 3818 – 3821; Angew. Chem. Int. Ed. 2003, 42,
3690 – 3693.
Received: November 19, 2008
Revised: January 9, 2009
Published online: February 16, 2009
Keywords: biaryls · carbenes · cross-coupling · ligands ·
.
palladium
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Reactions, 2nd ed., Wiley-VCH, Weinheim, 2004.
[14] CCDC 718904 (16) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
[2] a) L. R. Titcomb, S. Caddick, F. G. N. Cloke, D. J. Wilson, D.
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