[PdA (IPr*) (cinnamyl)Cl]: An efficient pre-catalyst for the preparation of tetra-ortho-substituted biaryls by Suzuki-Miyaura cross-coupling

Article abstract of DOI:10.1002/chem.201104009

The bigger the better: The new well-defined [Pd(IPr*)(cin)Cl] pre-catalyst is described (see scheme). This complex proves to be highly active in the Suzuki-Miyaura cross-coupling for the synthesis of tetra-ortho- substituted biaryls under mild conditions.

Full text of DOI:10.1002/chem.201104009

COMMUNICATION
DOI: 10.1002/chem.201104009
[PdACHTUNGTRENNUNG(IPr*)ACHTUNGTRENNUNG(cinnamyl)Cl]: An Efficient Pre-catalyst for the Preparation of
Tetra-ortho-substituted Biaryls by Suzuki–Miyaura Cross-Coupling**
Anthony Chartoire,[a] Mathieu Lesieur,[a] Laura Falivene,[b] Alexandra M. Z. Slawin,[a]
Luigi Cavallo,[b, c] Catherine S. J. Cazin,*[a] and Steven P. Nolan*[a]
Among the plethora of transition-metal catalyzed cross-
couplings,[1] the Suzuki–Miyaura reaction has emerged as
one of the most powerful methodologies for the assembly of
[2]
À
C C bonds. During the past two decades, significant ef-
forts have been devoted to developing new palladium/ligand
catalyst systems to solve the various limitations of this meth-
odology. In this context, bulky electron-rich tertiary phos-
phines[3] and N-heterocyclic carbenes[4] (NHCs) have dem-
onstrated very high efficacy as ancillary ligands in such
transformations. One of the current main challenges in the
Suzuki–Miyaura reaction remains the preparation of tetra-
ortho-substituted biaryls under mild conditions. Tetra-ortho-
substituted biaryls can be synthesized by using Negishi,[5]
Stille,[6] or Kumada[7] cross-couplings, however, the Suzuki–
Miyaura reaction remains a most desirable methodology to
afford the desired compounds. In 2002, Buchwald and co-
workers were the first to report on such a reaction.[8] The
conditions employed made use of [Pd2ACHTNUTRGNE(NUG dba)3] (dba=diben-
Figure 1. Some NHC-based pre-catalysts and NHC ligands allowing the
preparation of tetra-ortho-substituted biaryls.
zylideneacetone) in combination with a very bulky biaryl
phosphine and K3PO4 as the base, at 1108C in toluene. Fol-
lowing this discovery, other procedures were reported,
which showed the efficiency of very bulky tertiary phos-
phine-modified palladium systems to promote this reac-
tion.[9] Unfortunately, all early protocols suffered from the
same disadvantages, namely high catalyst loadings, the need
for a large excess of phosphine, and elevated temperatures.
In 2004, Glorius and co-workers reported on the use of
a NHC ligand (IBox12; see Figure 1) to promote this chal-
lenging reaction.[10] Nevertheless, high catalyst loadings and
high temperatures were still necessary to achieve the cross-
coupling. More recently, the use of well-defined palladium/
NHC complexes[11] has shown high efficiency in the prepara-
tion of tetra-ortho-substituted biaryls. Indeed, very recently
Organ reported on the use of [Pd-PEPSSI-IPent] (see
Figure 1) at 658C using 2 mol% of the pre-catalyst,[12] and
[a] Dr. A. Chartoire, M. Lesieur, Prof. Dr. A. M. Z. Slawin,
Dr. C. S. J. Cazin, Prof. Dr. S. P. Nolan
EaStCHEM School of Chemistry
University of St Andrews
St Andrews, KY16 9ST (UK)
Fax : (+44)1334-463 808
E-mail: snolan@st-andrews.ac.uk
cc111@st-andrews.ac.uk
Dorta reported the use of [Pd
AHCTUNGTREG(NNNU cin)Cl] (see Figure 1; cinnamyl=cin) at room temperature
ACHTUGTNREN(UNG anti-ACHUTTGNREN(NUGN 2,7)-SICyoctNap)-
also using 2 mol% of pre-catalyst.[13] The latter represents
the first efficient protocol leading to sterically encumbered
biaryls at room temperature, nevertheless it is noteworthy
that the reaction is promoted by the use of the strong base
KOtBu. It appears that the use of very bulky ligands is cru-
cial to efficiently promote the reaction. Indeed, the com-
plexes with more sterically demanding ligands give superior
catalytic performance.
[b] L. Falivene, Prof. Dr. L. Cavallo
Dipartimento di Chimica
Universitꢀ di Salerno
Via Ponte don Melillo, 84084 Fisciano (Italy)
[c] Prof. Dr. L. Cavallo
King Abdullah University of Science and Technology (KAUST)
Chemical and Life Sciences and Engineering
Kaust Catalysis Center, Thuwal 23955-6900 (Saudi Arabia)
Of note, to our knowledge details regarding the synthesis
of IPent have not yet been reported and the synthesis of
[**] IPr*=1,3-bisACHTUNGTRENNUNG(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazo-2-yli-
dene.
(anti-(2,7)-SICyoctNap) involves
Therefore the use of a bulky, easily accessible ligand would
a multistep approach.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201104009.
Chem. Eur. J. 2012, 18, 4517 – 4521
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4517

be desirable to provide the synthetic community access to
a catalyst that can achieve high catalytic performances. Re-
cently, Marko and co-workers described the synthesis of
IPr* (IPr*=1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)-
imidazo-2-ylidene), one of the bulkiest NHC ligands to
date.[14] As a continuation of our work using palladium cin-
namyl complexes in cross-coupling reactions,[4b,c,15] we now
explore the combination of IPr* with the palladium cinnam-
yl moiety. Herein, we describe the synthesis of [Pd
ACHTUNGTRNE(NUNG IPr*)-
ACHTUNGTRENNUNG
plex to prepare tetra-ortho-substituted biaryls under mild
conditions.
The preparation of complex 1 was straightforward starting
from IPr*·HCl. The free carbene (IPr*) was generated by
deprotonation of IPr*·HCl using a slight excess of KOtBu in
THF at room temperature for 4 h.[16] The free N-heterocyclic
carbene, IPr*, was coordinated by adding the palladium cin-
namyl dimer [{Pd
ACHTUNGTRENNUNG(cin)ACHTUNGTREN(NGUN m-Cl)}2] to afford the desired complex
1 in excellent yield (Scheme 1, 95%). Compound 1 is both
Figure 2. Molecular structure of 1. Hydrogen atoms have been omitted
for clarity. Selected bond lengths [ꢂ] and angles [8]: Pd1–C1 2.038(6),
Pd1–C72 2.110(7), Pd1–C74 2.155(11), Pd1–C73 2.270(10), Pd1–Cl1
2.3455(18); C1-Pd1-C72 103.0(2), C72-Pd1-C74 69.2(2), C74-Pd1-Cl1
95.7(3), C1-Pd1-Cl1 91.61(17).
99%). The use of chlorides or bromides does not affect the
reactivity (Table 1, entry 1 vs. 2, entry 9 vs. 10, entry 15 vs.
16). This result suggests that the oxidative addition of the
halide to the active palladium species is not the rate-limiting
step. In some cases, and particularly when the 2,3,5,6-tetra-
methylbenzene boronic acid is used, a slight increase of tem-
perature to 658C is necessary to achieve complete reactions
in acceptable times (Table 1, entries 4, 7, 12, 14, 19). It is
noteworthy that in those cases an increase of the catalyst
loading (2 mol%) does not modify significantly the conver-
sion at room temperature, the main issue being the competi-
tive and parasitic hydrodeboronation reaction.
Scheme 1. Synthesis of [PdACHTUNRGTENN(UG IPr*)ACHUTTGNREN(NUGN cinnamyl)Cl] 1.
air and moisture stable and crystals suitable for X-ray dif-
fraction were grown by slow diffusion of hexane into a satu-
rated C6D6 solution of the complex (Figure 2).[17] The Pd–C1
bond length (2.038 (6) ꢂ) is comparable to those found in
other [Pd
spite the steric bulk of the NHC ligand.
To probe the reactivity of [Pd(IPr*)(cin)Cl] (1) in a chal-
ACHTUNGTRENNUNG(NHC)ACHTUNGTRENNUNG
(cin)Cl] congeners (see Table 2),[4b,13] de-
A
ACHTUNGTRENNUNG
lenging Suzuki–Miyaura cross-coupling involving sterically
hindered substrates at room temperature, the reaction in-
volving 2-chloromesitylene and 2,6-dimethylbenzene boronic
acid was selected as test reaction. After optimization of the
reaction conditions,[18] best results were obtained by using
KOH as the base in 1,2-dimethoxyethane (DME), using
only 1 mol% of the pre-catalyst 1. Under these conditions,
the expected biaryl was obtained in an excellent 95% isolat-
ed yield (Table 1, entry 1). This procedure represents one of
the mildest reactions described in the literature. Indeed, if
we directly compare this result with the recent work of
Dorta and co-workers,[13] our protocol uses only 1 mol% of
pre-catalyst, and KOH is a milder and cheaper base than
KOtBu. To further explore the potential of the new proce-
dure, the reactivity of various aryl chlorides and bromides
with several boronic acids was investigated. Indeed, phenyl,
naphthyl or heteroaryl halides can be coupled with phenyl
or naphthyl boronic acids, overall in very high yields (67–
To evaluate the bulkiness of IPr*,[19] the percent buried
volume (%VBur) of IPr* was calculated and compared to
that computed for other NHCs.[20] With a %VBur of 44.6%,
IPr* is the bulkiest NHC ligand so far reported for catalyti-
cally active [PdACTHNUGRTENNUG(NHC)ACHTUGNTREN(NUNG cin)Cl] complexes (Table 2). This
result supports the hypothesis earlier stated that the prepa-
ration of tetra-ortho-substituted biaryls requires very bulky
ligands.
To better understand the reactivity of 1, the steric map-
ping of the metal center was undertaken using the %VBur
concept that was calculated in four different regions around
the metal center.[13,21] The calculation shows how the ligand
adapts its shape to the metal environment. Figure 3 shows
that IPr* is completely twisted around the palladium atom.
In fact, one quadrant is very bulky (%VBur top left=
76.1%), one is moderately bulky (%VBur bottom right=
44.9%), and two quadrants present much lower steric hin-
drance (%VBur top right=31.5% and %VBur bottom left=
4518
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ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2012, 18, 4517 – 4521

Synthesis of Tetra-ortho-substituted Biaryls by Cross-Coupling
Table 1. Preparation of tetra-ortho-substituted biaryls.[a]
COMMUNICATION
Entry
Ar–Ar’
X
Time[b]
[h]
Conversion[c]
Yield[d] [%]
/
Entry
Ar–Ar’
X
Time[b]
[h]
Conversion[c]
Yield[d] [%]
/
1
2
Cl
Br
20
18
>99/95
>99/91
11
Br
Br
20
20
20/–
91/84
12[e]
3
Cl
Cl
20
20
38/–
>99/87
13
Br
Br
18
20
31/–
81/75
4[e]
14[e]
15
16
Cl
Br
17
18
>99/93
>99/99
5
Cl
22
>99/90
6
Cl
Cl
20
20
41/–
>99/85
17
Br
18
>99/85
7[e]
18
Br
Br
20
20
48/–
71/67
8
Br
18
>99/95
19[e]
9
10
Cl
Br
17
16
>99/99
98/92
20
Br
20
90/87
[a] Reagents and conditions: ArX (0.5 mmol), Ar’B(OH)2 (1.0 mmol), KOH (1.5 mmol), 1 (1 mol%), DME (1.0 mL), 258C, time. [b] Reaction times
have not been optimized. [c] Conversion to coupling product based on starting material determined by GC. [d] Isolated yields after chromatography on
silica gel. [e] Performed at 658C.
Table 2. Comparison of 1 with [PdACTHNUGRTNENUG(NHC)ACHTUNGTRNE(NUGN cin)Cl] analogues.
lyst. The ligand synthesis has previously been reported and
is amenable to scaleup. The palladium complex has shown
excellent catalytic activity in the Suzuki–Miyaura cross-cou-
pling for the preparation of tetra-ortho-substituted biaryls
(67–99%). The conditions used to achieve the reaction are
among the mildest described in the literature, using low cat-
alyst loadings (1 mol%) and a relatively mild and inexpen-
sive base (KOH) at room temperature or with gentle heat-
ing (658C). Further studies examining the potential of 1 in
related cross-coupling reactions are ongoing in our laborato-
ries.
IPr
SIPr
anti-(2,7)-SICyoctNap
IPr*
Pd–C1 [ꢂ]
% VBur
2.041(9)
36.7
2.025(7)
37.0
2.034(4)
42.0
2.038(6)
44.6
[a]
[a] % VBur calculated for Pd–C1=2.00 ꢂ.
27.3%). In comparison, the space IPr occupies around the
metal center appears more homogeneously distributed
(%VBur from 27.8 to 44.6). Finally, if we compare IPr* with
the previously described anti-(2,7)-SICyoctNap,[13] we can
see that the difference in bulk between the most hindered
quadrant is much more significant for IPr*. The less hin-
dered faces are probably the route of approach of substrate
Experimental Section
favoring the formation of the [PdACHTUNTRGENN(GU NHC)(Ar)ACHTUNGTERN(NUGN Ar’)] inter-
mediate.[13] The reductive elimination and the formation of
products might be highly influenced by the presence of
highly bulky quadrants around palladium. The high reactivi-
ty observed under mild conditions can probably be ex-
plained by the very bulky, yet flexible nature of the IPr*
ligand.
Preparation of [PdACTHNUGTRENNUG(IPr*)ACHTUNTGREN(NUGN cinnamyl)Cl] (1): In a glovebox, in a 500 mL
round bottom flask equipped with a magnetic stirring bar were added
IPr*·HCl (2.08 g, 2.2 mmol) and KOtBu (0.28 g, 2.4 mmol) in THF
(160 mL). The reaction mixture was stirred at room temperature for 4 h
and then [{PdACHTNUTRGENNGU(cinnamyl)ACHTUTGNREN(NUGN m-Cl)}2] (0.512 g, 1 mmol) was added in THF
(40 mL). The reaction mixture was then stirred overnight at room tem-
perature. After this time, outside the glovebox, THF was evaporated, the
crude product was dissolved in CH2Cl2, filtered on a pad of silica covered
with celite, and eluted with CH2Cl2. After evaporation of the solvents,
In summary, we have developed an easy and efficient
access to the new well-defined [PdACTHUNRGTNNEUG(IPr*)ACHTUNGTERN(NUGN cin)Cl] pre-cata-
Chem. Eur. J. 2012, 18, 4517 – 4521
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4519

S. P. Nolan, C. S. J. Cazin et al.
analyses. Umicore AG is thanked for its generous gifts of materials.
S.P.N. is a Royal Society Wolfson Research Merit Award holder. C.S.J.C.
acknowledges the Royal Society for a University Research Fellowship.
Keywords: biaryls
·
cross-coupling
·
N-heterocyclic
carbenes · palladium
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Figure 3. Mapping of the sterics for [PdACTHUNRGTNNEUG(NHC)CAHTUNGTRENN(UGN cin)Cl] complexes with %
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H
Cin), 2.56 (d, J=8.4 Hz, 1H; HCin), 2.25 (s, 6H; CH3), 1.16 ppm (d, J=
12.6 Hz, 1H; HCin); 13C {1H} NMR (100 MHz, CDCl3): d=182.6 (NCN),
144.6 (CAr), 143.8 (CAr), 141.4 (CAr), 140.6 (CAr), 138.4 (CAr), 137.8 (CAr),
135.9 (CAr), 130.6 (CHAr), 130.3 (CHAr), 129.3 (CHAr), 128.7 (CHAr),
128.4 (CHAr), 128.2 (CHAr), 127.7 (CHAr), 127.2 (CHAr), 126.5 (CHAr),
126.4 (CHAr), 123.5 (CHIm), 109.0 (CCin), 91.3 (CCin), 51.5 (CH), 47.4
(CCin), 22.0 ppm (CH3); elemental analysis calcd (%) for C78H65ClN2Pd:
C 79.92, H 5.59, N 2.39; found: C 80.09, H 5.46, N 2.29.
General procedure for the preparation of the tetra-ortho-substituted biar-
yls: In a glovebox, in a vial equipped with a stirring bar and sealed with
a screw cap fitted with a septum, were added KOH (88 mg, 1.5 mmol),
the boronic acid (1.0 mmol), the aryl halide (0.5 mmol), and finally a solu-
tion of 1 in DME (5.85 mg, 1 mL, 1 mol%). Outside the glovebox, the re-
action mixture was then stirred (800 rpm) at room temperature or 658C
for 16–22 h. Then the solution was cooled, quenched with water (5 mL),
and the aqueous layer was extracted with CH2Cl2 (3ꢃ10 mL). The com-
bined organic layers were dried over MgSO4 and the solvents were
evaporated in vacuo. The crude product was finally purified by flash
chromatography on silica gel.
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Acknowledgements
We gratefully acknowledge the EC for funding through the seventh
framework programme SYNFLOW. We thank the EPSCR National
Mass Spectrometry Service Center in Swansea for mass spectrometric
4520
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Chem. Eur. J. 2012, 18, 4517 – 4521

Synthesis of Tetra-ortho-substituted Biaryls by Cross-Coupling
COMMUNICATION
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[18] See Supporting Information for more information about optimiza-
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[20] The bond length between the Pd and the central carbon atom of the
NHC has been fixed at 2.00 ꢂ in those calculations, to compare effi-
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for more details about those calculations.
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Received: December 21, 2011
Published online: March 13, 2012
Chem. Eur. J. 2012, 18, 4517 – 4521
ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4521