Synthesis and structure of trans-bis(1,4-dimesityl-3-methyl-1,2,3-triazol- 5-ylidene)palladium(II) dichloride and diacetate. Suzuki-Miyaura coupling of polybromoarenes with high catalytic turnover efficiencies

Article abstract of DOI:10.3762/bjoc.9.79

trans-Bis(1,4-dimesityl-3-methyl-1,2,3-triazol-5-ylidene)palladium(II) dichloride has been shown to be an excellent catalyst for the multiple Suzuki-Miyaura coupling reactions of polybromoarenes to the corresponding fully substituted polyarylarenes. The reactions proceeded in excellent yields and with high turnover numbers. With 1,4-dibromobenzene the catalyst was found to be active for up to 13 consecutive cycles with a turnover number of 1260. The polyarylarenes were obtained in pure form after crystallization once without recourse to chromatographic purification. The single-crystal X-ray structures of the chloro (1) as well as the corresponding acetato (2) complexes are also reported and compared with the corresponding complexes of 1,4-diphenyl-3-methyl- 1,2,3-triazol-5-ylidene as the ligand.

Full text of DOI:10.3762/bjoc.9.79

Synthesis and structure of trans-bis(1,4-dimesityl-3-
methyl-1,2,3-triazol-5-ylidene)palladium(II) dichloride
and diacetate. Suzuki–Miyaura coupling of poly-
bromoarenes with high catalytic turnover efficiencies
Jeelani Basha Shaik1, Venkatachalam Ramkumar1, Babu Varghese2
and Sethuraman Sankararaman*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2013, 9, 698–704.
1Department of Chemistry, Indian Institute of Technology Madras,
Chennai 600036, India and 2Sophisticated Analytical Instrument
Facility, Indian Institute of Technology Madras, Chennai 600036, India
doi:10.3762/bjoc.9.79
Received: 19 February 2013
Accepted: 19 March 2013
Published: 10 April 2013
Email:
Sethuraman Sankararaman* - sanka@iitm.ac.in
Associate Editor: K. Itami
* Corresponding author
© 2013 Shaik et al; licensee Beilstein-Institut.
License and terms: see end of document.
Keywords:
C–C coupling; N-heterocyclic carbene; palladium; Suzuki–Miyaura
coupling; 1,2,3-triazolylidene
Abstract
trans-Bis(1,4-dimesityl-3-methyl-1,2,3-triazol-5-ylidene)palladium(II) dichloride has been shown to be an excellent catalyst for the
multiple Suzuki–Miyaura coupling reactions of polybromoarenes to the corresponding fully substituted polyarylarenes. The reac-
tions proceeded in excellent yields and with high turnover numbers. With 1,4-dibromobenzene the catalyst was found to be active
for up to 13 consecutive cycles with a turnover number of 1260. The polyarylarenes were obtained in pure form after crystalliza-
tion once without recourse to chromatographic purification. The single-crystal X-ray structures of the chloro (1) as well as the
corresponding acetato (2) complexes are also reported and compared with the corresponding complexes of 1,4-diphenyl-3-methyl-
1,2,3-triazol-5-ylidene as the ligand.
Introduction
Over the past decade N-heterocyclic carbenes (NHCs) have complexes of these versatile ligands have been shown to be
attracted the attention of synthetic and organometallic chemists excellent catalysts for various organic transformations [9-14].
tremendously [1-5]. NHCs have been proven to be useful as Among the various NHCs 1,3-diarylimidazolylidenes are the
organocatalysts in organic synthesis [6-9]. They are excellent most widely studied systems [12-14]. In the past five years
ligands for transition-metal and lanthanide metal chemistry 1,2,3-triazol-5-ylidenes have emerged as promising ligands for
[1,10,11]. Over the past few years they have gradually replaced transition-metal chemistry [15-20]. 1,2,3-Triazol-5-ylidenes
the conventional phosphane ligands. The transition-metal have been termed as abnormal NHCs and mesoionic carbenes
698

Beilstein J. Org. Chem. 2013, 9, 698–704.
because their structures cannot be represented in neutral canon-
ical form [17,19]. Mesoionic NHCs are stronger sigma donors
than the normal NHCs (e.g., imidazolylidenes versus triazolyli-
denes) [21-23]. Hence metal complexes of mesoionic carbene
ligands are expected to show high stability and exceptional
catalytic properties in comparison with their normal NHC coun-
terparts. In particular triazolylidenes with sterically demanding
mesityl and 2,4,6-triisopropylphenyl wingtip groups are catalyt-
ically very active. Polyphenylated arenes are a very important
class of compounds and they find application in the areas of
molecular electronics, organic discotic liquid crystals and
OLEDs [24,25]. One of the ways to approach the synthesis of
these interesting compounds is to carry out multiple C–C
Scheme 1: Synthesis of Pd complexes 1 and 2.
coupling reactions with suitable polybromoarene derivatives. In Crystal structure of complex 1 and 2
multiple C–C coupling reactions one often encounters the for- Although the synthesis of 1 was reported by Fukuzawa [27] the
mation of partially coupled products and incomplete reactions crystal structure of this complex was not reported. Slow evapor-
leading to problematic separation of pure fully substituted com- ation of an acetonitrile solution of 1 yielded single crystals suit-
pounds. Herein we report very clean multiple Suzuki–Miyaura able for X-ray crystallography. Complex 1 crystallized in the
coupling of polybromoarenes. In every case reported herein the tetragonal space group I41/acd. The structure of the complex in
final product, namely fully substituted polyarylarenes, was the crystal clearly showed it to be a mononuclear complex with
isolated in pure form upon single crystallization of the crude two chloro ligands trans with respect to each other, the two tri-
product. These reactions also proceeded with very high turnover azolylidene ligands also trans with respect to each other, and
number. We also report the structures of palladium(II) dichlo- with the palladium in square planar geometry (Figure 2). The
ride complex 1 and diacetate complex 2.
structures of 1 and 3 revealed that the Pd–C and Pd–Cl
distances are comparable in both complexes. However the dihe-
dral angle between the planes containing each of the triazolyl-
idene rings in 3 is zero, i.e., they lie on parallel planes with the
Results and Discussion
Synthesis of Pd complexes
Complexes 1 and 2 were synthesized from the corresponding distance between the planes being 0.305 Å [26]. In the case of 1
silver carbene complex by transmetalation as reported earlier the dihedral angle between the planes is 55.47°. In order to
for the synthesis of bis(1,4-diphenyl-3-methyl-1,2,3-triazol-5- accommodate the bulky mesityl wingtip groups in 1 the carbene
ylidene)palladium(II) derivatives 3 and 4 (Figure 1 and ligands are twisted with respect to the Ccarbene–Pd–Ccarbene axis
Scheme 1), respectively [26,27]. Treatment of 1,4-dimesityl-3- making a dihedral angle of 55.47°. The mesityl rings are also
methyl-1,2,3-triazolium iodide with freshly prepared silver significantly twisted out of plane with respect to the triazolyl-
oxide followed by transmetalation with Pd(Cl)2(CH3CN)2 idene rings in 1 compared to the twist of the phenyl rings in 3.
yielded 1 as a pale yellow solid in 87% (Scheme 1). The acetate
complex 2 was prepared by the transmetalation of the silver
carbene complex with Pd(OAc)2. Addition of Pd(OAc)2 to in
situ generated silver carbene complex yielded 2 in 89% as a
pale yellow solid (Scheme 1).
Figure 2: ORTEP representation of the structure of complex 1 in the
Figure 1: Structure of Pd complexes 1–4.
crystal (35% probability ellipsoids). Hydrogens are omitted for clarity.
699

Beilstein J. Org. Chem. 2013, 9, 698–704.
The structure of 2 was also unambiguously established by and phenylboronic acid (5) as substrates. The reactions were
single-crystal XRD data. Crystals suitable for XRD were grown carried out by using 2 mol % of complex 1 as catalyst and
by slow evaporation of a toluene solution of 2. It belonged to a 4 mol % of PPh3 irrespective of the number of bromines present
monoclinic system with the space group P21/n. The structures in the polybromoarenes (Scheme 2, Table 1). Typically for
of the acetate complexes 2 and 4 are significantly different. polybromoarenes 1.0 to 1.2 equivalents of arylboronic acid and
Complex 4 is binuclear with each of the palladium atoms being 2 equivalents of NaOH per bromine were used. For example, in
part of a pallado cycle formed by insertion to the ortho position the case of 1,4-dibromobenzene (6), 2 equivalents of phenyl-
of one of the N-phenyl groups [26]. The two palladium atoms boronic acid (5) and 4 equivalents of NaOH were used and in
are connected by two bridging acetate ligands. In the case of 2 the case of 1,3,6,8-tetrabromopyrene (18) 4.8 equivalents of
with dimesityl wingtip groups on the triazolylidene ligand the phenylboronic acid (5) and 8 equivalents of NaOH were used.
formation of a pallado cycle is not feasible because the ortho The addition of PPh3 is crucial, and in its absence the reaction
positions are substituted with methyl groups. Secondly, due to mixture turned black, the catalyst was quickly deactivated, and
the bulky nature of the mesityl groups, the formation of a binu- the reactions did not proceed to completion. The reactions were
clear complex with bridging acetate ligands is also infeasible. carried out at 105–110 °C under N2 atmosphere and the course
Complex 2 has a simple mononuclear structure with two of the reaction was followed by TLC. Typically, the reaction of
monodentate acetate ligands each bonded to palladium through polybromoarenes initially showed multiple spots on TLC. Reac-
a single oxygen atom (Figure 3).
tions were carried out for the desired time period until a single
major spot was observed on TLC. Pure polyarylated products
were obtained by a single recrystallization of the crude product
eliminating cumbersome chromatographic separations. Several
polybromoarenes were investigated and the results are summa-
rized in Table 1. In the case of 7 the catalytic activity was tested
for up to 13 cycles by the successive addition of 1,4-dibromo-
benzene (6), phenylboronic acid (5) and NaOH to the same
reaction pot. After 13 cycles of reaction, the catalytic turnover
number (TON) was as high as 1260. TON is defined as the ratio
of the number of moles of product formed to the number of
moles of catalyst used times the number of C–C bonds formed
in the reaction. This is due to the fact that for each C–C bond-
forming reaction one catalytic cycle needs to be completed. It
must be highlighted here that 2,7-di-tert-butyl-4,5,9,10-tetra-
bromopyrene (21) and 4,7,12,15-tetrabromo[2.2]paracyclo-
phane (26) [29] are particularly difficult substrates to undergo
Suzuki–Miyaura coupling in the presence of conventional cata-
lysts such as PdCl2(PPh3)2, Pd(PPh3)4 and Pd(dba)2. The
coupling of 4,7,12,15-tetrabromo[2.2]paracyclophane (26) with
phenylmagnesium bromide in the presence of a
NiCl2(PPh3)2 catalyst has been reported to yield 4,7,12,15-
Figure 3: ORTEP representation of the structure of complex 2 in the
crystal (35% probability ellipsoids). Hydrogens are omitted for clarity.
Suzuki–Miyaura coupling of polybromo-
arenes using complex 1
Complexes of palladium-mesoionic NHCs have been shown to tetraphenyl[2.2]paracyclophane (27) in only 6% [29]. In the
be catalytically much more active than the corresponding present study these substrates underwent four-fold
complexes with normal NHCs (i.e., Pd complexes of 1,2,3-tri- Suzuki–Miyaura coupling smoothly resulting in the formation
azolylidene versus imidazolylidene ligands) in C–C bond- of fully substituted derivatives in near quantitative yields. Inter-
forming coupling reactions [28]. The enhanced catalytic activity estingly, hexabromobenzene (16) [30] and hexabromotriphenyl-
may be due to stronger sigma donor ability of mesoionic NHCs ene 23 [31] also underwent six-fold coupling in a clean manner
compared to normal NHCs. The stronger sigma donor ability of resulting in the formation of hexaphenylbenzene (17) and hexa-
mesoionic NHCs may stabilize intermediates in the catalytic aryltriphenylene 24, respectively, in good yields. Under these
cycle. Complex 1 has been shown by Fukuzawa [27] to be very conditions polychloroarenes did not give a clean reaction, and
active for the coupling of simple aryl chlorides. In this work we the reactions were sluggish in comparison to polybromoarenes.
demonstrate that complex 1 is an excellent catalyst for multiple In addition, unlike the polybromo derivatives, the poly-
Suzuki–Miyaura coupling reactions of polybromoarenes. Reac- chloroarene derivatives of pyrene, triphenylene and [2,2]para-
tion conditions were optimized using 1,4-dibromobenzene (6) cyclophane are not readily available.
700

Beilstein J. Org. Chem. 2013, 9, 698–704.
Scheme 2: Multiple Suzuki–Miyaura coupling of polybromoarenes using complex 1.
Table 1: Multiple Suzuki–Miyaura coupling of polybromoarenes with arylboronic acids by using complex 1a.
Product
(% Yield, TONb)
Entry
Polybromoarene
ArB(OH)2
Time (h)
1
PhB(OH)2 (5)
3
6
7
(97, 1260 after 13 cyclesc)
PhB(OH)2 (5)
4-CF3C6H4B(OH)2 (8)
3.5
3
2
9
Ar = Ph, 10 (94, 141)
Ar = 4-CF3C6H4, 11 (94, 141)
3
4
PhB(OH)2 (5)
PhB(OH)2 (5)
9
7
12
14
13 (95, 193)
15 (97, 194)
5
PhB(OH)2 (5)
15
16
18
17 (59, 178)
PhB(OH)2 (5)
4-CF3C6H4B(OH)2 (8)
8
6
6
Ar = Ph, 19 (97, 194)
Ar = 4-CF3C6H4, 20 (94, 188)
7
PhB(OH)2 (5)
9.5
21
22 (95, 190)
701

Beilstein J. Org. Chem. 2013, 9, 698–704.
Table 1: Multiple Suzuki–Miyaura coupling of polybromoarenes with arylboronic acids by using complex 1a. (continued)
PhB(OH)2 (5)
4-CF3C6H4B(OH)2 (8)
7.5
7
8
Ar = Ph, 24 (99, 297)
Ar = 4-CF3C6H4, 25 (95, 285)
23
PhB(OH)2 (5)
4-CF3C6H4B(OH)2 (8)
10
10
9
Ar = Ph, 27 (93, 187)
Ar = 4-CF3C6H4, 28 (88, 176)
26
a100 mg scale bromoarene, 2 mol % catalyst 1, 4 mol % PPh3, 1.2 equiv ArB(OH)2 per bromine atom, 2 equiv NaOH per bromine; bTON = (moles of
product formed × number of bromine atoms substituted)/(moles of catalyst used); csubstrates (100 mg bromoarene, 1.2 equiv ArB(OH)2 and 2 equiv
NaOH) were added to the same pot up to 13 cycles after completion of the previous cycle.
Conclusion
tion was stirred at room temperature in the dark for 8 h. The
In conclusion, the dichloro complex 1 has been shown to be silver carbene complex thus generated was not isolated. It was
catalytically very active for multiple Suzuki–Miyaura coupling directly treated with Pd(CH3CN)2Cl2 (83 mg, 0.34 mmol,
of polybromoarenes, and several otherwise difficult to treat 0.6 equiv) and stirred for 8 h. The reaction mixture was passed
polybromo substrates have been efficiently converted to the through a bed of celite, and then removal of CH2Cl2 gave com-
corresponding polyaryl derivatives by using this catalyst. We plex 1 as a pale yellow solid (396 mg, 10.49 mmol) in 87%
also report the structural characterization of dichloro (1) and yield. Crystals of 1 suitable for single-crystal diffraction were
diacetate (2) complexes. Comparison of the structures of 1 and grown by slow evaporation of a solution of 1 in acetonitrile.
2 with that of the corresponding (1,4-diphenyl-3-methyl-1,2,3- Complex 1 decomposed at 258–260 °C without melting.
triazol-5-ylidene)palladium(II) complexes 3 and 4 is presented. 1H NMR (400 MHz, CDCl3) δ 6.95 (s, 8H) 3.72 (s, 6H), 2.45
The structures of the diacetate complexes 2 and 4 are signifi- and 2.44 (two overlapping singlets, 12H), 1.98 and 1.97 (two
cantly different in that 4 is a cyclopalladated dinuclear complex overlapping singlets, 24H); 13C NMR (100 MHz, CDCl3) δ
with acetate bridges whereas 2 is a mononuclear complex with 162.8 (Ccarbene), 143.8, 139.7, 139.0, 138.8, 136.1, 135.9,
monodentate acetate ligands. We predict that these catalysts 128.8, 128.3, 124.3, 35.7, 21.6, 21.5, 21.1, 18.9; ESIMS: m/z
could be potentially useful in polymer chemistry for the syn- 815 along with the expected isotope peaks. HRMS
thesis of polyaryl polymers that are important in OLEDs and (ESI–QTOF): m/z calcd for C42H50N6Cl2Pd 815.2587, found
molecular electronics applications.
815.2574.
Experimental
Synthesis of complex 2
1,4-Dimesityl-3-methyl-1,2,3-triazolium iodide was prepared The silver carbene complex generated as described above was
from the corresponding triazole, which in turn was prepared by subsequently treated with Pd(OAc)2 (75 mg, 0.34 mmol,
the click reaction of mesitylacetylene and mesitylazide 0.6 equiv) and stirred for 8 h. The reaction mixture was passed
according to the literature [27]. Complex 1 was synthesized in a through a bed of celite, and then dichloromethane was removed
similar manner as reported previously [27].
under vacuum to give complex 2 as a pale yellow solid
(430 mg, 0.5 mmol) in 89%.Crystals of 2 suitable for single
crystal diffraction were grown by slow evaporation of a solu-
Synthesis of complex 1 [27]
1,4-Dimesityl-3-methyl-1,2,3-triazolium iodide (250 mg, tion of 2 in toluene. Complex 2 decomposed at 149–152 °C
0.56 mmol) was treated with freshly prepared silver oxide without melting. 1H NMR (400 MHz, CDCl3) δ 7.07–6.87 (m,
(78 mg, 0.34 mmol, 0.6 equiv) in CH2Cl2 (12 mL). The solu- 8H), 3.85–3.68 (m, 6H), 2.42–2.40 (m, 12H), 2.14–2.13 (m,
702

Beilstein J. Org. Chem. 2013, 9, 698–704.
6H), 2.04–1.96 (m, 18H), 1.25 (s, 6H); 13C NMR (100 MHz, 270 Hz); HRMS (ESI–QTOF): m/z calcd for C60H31F18
CDCl3) δ 181.5, 162.8, 143.7, 140.9, 140.7, 139.6, 139.39, 1093.2138, found 1093.2100.
139.3, 138.8, 138.7, 135.8, 135.7, 135.2, 135.0, 130.0, 129.4,
129.0, 128.7, 128.2, 36.4, 35.8, 35.7, 23.3, 21.5, 21.48, 21.43, 4,7,12,15-Tetrakis(4-trifluoromethylphenyl)[2.2]paracyclo-
21.0, 20.0, 18.8, 17.4; ESIMS: m/z 863 along with the expected phane (28): Prepared from 4,7,12,15-tetrabromo[2.2]paracyclo-
isotope peaks. HRMS (ESI–QTOF): m/z calcd for phane (26, 100 mg, 0.19 mmol), complex 1 (3 mg, 2 mol %),
C46H56N6O4Pd 863.3476, found 863.3491.
4-trifluoromethylphenylboronic acid (8, 174 mg, 0.92 mmol),
NaOH (61 mg, 1.53 mmol), PPh3 (2 mg, 4 mol %). Yield
132 mg, 88%; colorless solid, mp 224 °C. 1H NMR (500 MHz,
CDCl3) δ 7.69–7.67 (m, 8H), 7.49–7.47 (m, 8H), 6.86 (s, 4H);
3.52 and 2.83 (AA’BB’ pattern, 8H); 13C NMR (125 MHz,
CDCl3) δ 143.7, 139.5, 137.5, 132.6, 129.4 (q, J = 32.6 Hz),
129.2, 124.3 (q, J = 270 Hz), 33.4; HRMS (ESI–QTOF): m/z
calcd for C44H29F12 785.2078, found 785.2077.
Representative procedures for
Suzuki–Miyaura coupling of polybromo-
arenes:
Synthesis of 1,3,6,8-tetraphenylpyrene (19): A mixture of
1,3,6,8-tetrabromopyrene (18, 100 mg, 0.193 mmol), phenyl-
boronic acid (5, 113 mg, 0.93 mmol, 4.8 equiv), complex 1
(3 mg, 2 mol %), triphenylphosphine (4 mg, 4 mol %), NaOH
(62 mg, 1.54 mmol, 8 equiv) was heated under reflux in 4 mL
of 1,4-dioxane under nitrogen atmosphere. The reaction was
completed in 8 h (monitored by TLC). The reaction mixture
was diluted with water (10 mL) and extracted with CH2Cl2 (3 ×
5 mL). The organic layer was dried over sodium sulfate and
filtered, and then the solvent was removed under vacuum. The
crude product was recrystallized from CH2Cl2 to give 1,3,6,8-
tetraphenylpyrene (19, 95 mg, 0.19 mmol) in 97% yield as a
lime-yellow solid, mp: 264 °C. 1H NMR (400 MHz, CDCl3) δ
8.17 (s, 4H), 8.01 (s, 2H), 7.68–7.66 (m, 8H), 7.55–7.52 (m,
8H), 7.48–7.44 (m, 4H); 13C NMR (100 MHz, CDCl3) δ 141.2,
137.4, 130.8, 129.6, 128.5, 128.2, 127.4, 126.1, 125.4; HRMS
(ESI–QTOF): m/z calcd for C40H27 507.2113, found 507.2094.
Supporting Information
Supporting Information File 1
Spectroscopic characterization data of compounds 8, 10,
11, 13, 15, 17, 22, 24 and 27.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-79-S1.pdf]
Supporting Information File 2
CIF file for complex 1.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-79-S2.cif]
Supporting Information File 3
CIF file for complex 2.
1,3,6,8-Tetrakis(4-trifluoromethylphenyl)pyrene (20):
Prepared from 1,3,6,8-tetrabromopyrene (18, 100 mg,
0.19 mmol), complex 1 (3 mg, 2 mol %), 4-trifluo-
romethylphenylboronic acid (8, 176 mg, 0.93 mmol), NaOH
(62 mg, 1.54 mmol), PPh3 (2 mg, 4 mol %). Yield 142 mg,
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-9-79-S3.cif]
94%; colorless solid, mp 231–232 °C (lit. 231 °C) [32]. Acknowledgements
1H NMR (500 MHz, CDCl3) δ 8.13 (s, 4H), 7.99 (s, 2H), We thank CSIR and DST, New Delhi for financial support, the
7.83–7.77 (AA’BB’ pattern, 16H); 13C NMR (125 MHz, Department of Chemistry, and SAIF, IIT Madras for infrastruc-
CDCl3) δ 144.3, 136.4, 131.0, 130.0 (q, J = 31.8 Hz), 129.4, ture. JBS thanks CSIR for a fellowship.
128.5, 125.9, 125.7, 125.6 (q, J = 2.5 Hz), 124.4 (q, J =
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