Extending the utility of [Pd(NHC)(cinnamyl)Cl] precatalysts: Direct arylation of heterocycles

Article abstract of DOI:10.3762/bjoc.8.187

The use of [Pd(NHC)(cinnamyl)Cl] precatalysts in the direct arylation of heterocycles has been investigated. Among four different precatalysts, [Pd(SIPr)(cinnamyl)Cl] proved to be the most efficient promoter of the reaction. The C-H functionalization of sulfuror nitrogen-containing heterocycles has been achieved at low catalyst loadings. These catalyst charges range from 0.1 to 0.01 mol % palladium.

Full text of DOI:10.3762/bjoc.8.187

Extending the utility of [Pd(NHC)(cinnamyl)Cl]
precatalysts: Direct arylation of heterocycles
Anthony R. Martin, Anthony Chartoire, Alexandra M. Z. Slawin
and Steven P. Nolan*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 1637–1643.
EaStCHEM School of Chemistry, University of St Andrews, North
Haugh, St Andrews, KY16 9ST, UK
doi:10.3762/bjoc.8.187
Received: 22 June 2012
Email:
Accepted: 24 August 2012
Published: 27 September 2012
Steven P. Nolan* - snolan@st-andrews.ac.uk
* Corresponding author
This article is part of the Thematic Series "C–H Functionalization".
Guest Editor: H. M. L. Davies
Keywords:
C–H functionalization; direct arylation; heterocycles; N-heterocyclic
carbenes; palladium
© 2012 Martin et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
The use of [Pd(NHC)(cinnamyl)Cl] precatalysts in the direct arylation of heterocycles has been investigated. Among four different
precatalysts, [Pd(SIPr)(cinnamyl)Cl] proved to be the most efficient promoter of the reaction. The C–H functionalization of sulfur-
or nitrogen-containing heterocycles has been achieved at low catalyst loadings. These catalyst charges range from 0.1 to
0.01 mol % palladium.
Introduction
As a powerful addition to the classic palladium cross-coupling Despite the efficiency of well-defined palladium catalysts
reactions, C–H bond functionalization has become a growing bearing NHC (N-heterocyclic carbene) ancillary ligands in clas-
field of research over the last few years. The ubiquity of C–H sical cross-coupling reactions, they have rarely been applied to
bonds makes them a convenient and cost-effective anchoring direct arylation procedures [9-16]. Among the family of
position within viable substrates, as no derivatisation to form an [Pd(NHC)] complexes, the [Pd(NHC)(cin)Cl] (cin = cinnamyl)
organometallic reagent is required. Moreover, among the species are known for their ease of activation through the reduc-
plethora of C–H bonds present on a molecule, it is often tion of the metal centre from Pd(II) to Pd(0) [17]. Therefore, we
possible to target one C–H linkage specifically, taking advan- have investigated the use of such precatalysts in the direct aryl-
tage of directing groups or particular catalyst selectivity [1-5]. ation of heteroaromatic compounds in order to compare them to
Thus, heteroaromatic scaffolds, which are a common feature in ligand-free or phosphine-bearing catalytic systems, and in the
biologically relevant compounds and in materials science [6,7] end to see whether the reactivity and application scope of these
can be selectively arylated as the heteroatom can act as an commercially available complexes could be broadened to
intrinsic orientating group [8].
include C–H bond functionalization transformations.
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We now report the activity of the [Pd(NHC)(cin)Cl] complexes tion File 1). Subsequently, 5 was treated with KOt-Bu in dry
1–4 in the direct arylation of heterocycles with NHC ligands THF to generate the corresponding free carbene in situ. The
being SIPr (1,3-bis(2,6-diisopropylphenyl)-4,5-dihydroimi- expected [Pd(IPr*Tol)(cin)Cl] was then obtained in an excellent
dazol-2-ylidene), IPr (1,3-bis(2,6-diisopropylphenyl)imidazol- yield (97%) by a simple fragmentation of the palladium dimer
2-ylidene), IPr* (1,3-bis(2,6-bis(diphenylmethyl)-4- [{Pd(cin)(µ-Cl)}2] using the free carbene solution (Scheme 1).
methylphenyl)imidazol-2-ylidene) and IPr*Tol (1,3-bis(2,6-
bis(di-p-tolylmethyl)-4-methylphenyl)imidazol-2-ylidene) The newly synthesized complex 4 was unequivocally charac-
(Figure 1). Complexes 1 and 2 are commercially available and terised by X-ray diffraction [24] (Figure 2, Supporting Informa-
have proven to be highly efficient in Suzuki–Miyaura coupling tion File 2 and Supporting Information File 3) after suitable
and Buchwald–Hartwig amination reactions [17-20]. We have crystals were grown from slow diffusion of hexane in
also evaluated the recently reported [Pd(IPr*)(cin)Cl] (3), which dichloromethane. Based on this crystal structure, the percentage
has shown potency in Suzuki–Miyaura couplings [21] and buried volume (%VBur) of the IPr*Tol ancillary ligand was
Buchwald–Hartwig N-arylations [22] even with challenging determined by using the “SambVca” web application [25] and
substrates. To complete this study and to examine the effect of compared to complexes 1–3 (Table 1) [21]. IPr*Tol featured a
bulky ligands about the metal centre, we have synthesised a %VBur in the same range as IPr* (+0.4% difference). SIPr and
new complex [Pd(IPr*Tol)(cin)Cl] (4), which is a IPr* congener. IPr have been reported as less hindered ligands with %VBur of
37.0 and 36.7, respectively. The length of the Pd–C1 bond in 4
Results and Discussion
The study begins with the preparation of the palladium com-
was also examined and is close to the one observed in 3.
plex 4. Following the strategy recently reported by Markó [23], With complexes 1–4 in hand, their catalytic activity towards the
we were successful in the synthesis of the IPr*Tol·HCl imida- direct arylation of heteroaromatic compounds was evaluated.
zolium salt 5 in a 53% overall yield (see Supporting Informa- For this purpose, the arylation of benzothiophene (6) with
Figure 1: [Pd(NHC)(cin)Cl] catalysts examined in direct arylation.
Scheme 1: Synthesis of [Pd(IPr*Tol)(cin)Cl] (4).
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Table 2: Catalyst screening for the direct arylation of benzothiophene
(6).
Catalyst
Conversion (%)a
[Pd(SIPr)(cin)Cl] (1)
[Pd(IPr)(cin)Cl] (2)
[Pd(IPr*)(cin)Cl] (3)
[Pd(IPr*Tol)(cin)Cl] (4)
76
50
8
49
aConversion of the starting material into C–H arylated product deter-
mined by GC, [6] = 0.3 M.
Figure 2: Molecular structure of 4. H atoms were omitted for clarity.
Selected bond lengths (Å) and angles (°): Pd1–C1 2.034(0), Pd1–Cl1
2.352(5), Pd1–C85 2.132(8), Pd1–C86 2.119(7), Pd1–C87 2.226(6);
C1–Pd1-C85 102.9(5), C85–Pd1–C87 71.2(6), C87–Pd1–Cl1 93.3(8),
Cl1–Pd1–C1 91.8(6).
Table 1: Comparison of the %VBur and d(Pd–C1) in the
[Pd(NHC)(cin)Cl] family.
Figure 3: Previously reported catalytic systems in the direct arylation
of benzothiophene (6).
NHC
%VBura
Pd–C1 (Å)
SIPr
IPr
37.0
36.7
44.6
45.0
2.025(7)
2.041(9)
2.038(6)
2.034(0)
to efficiently activate the [Pd(NHC)(cin)Cl] precatalysts [17].
DMA was selected as the solvent and the reaction was
conducted at 140 °C.
IPr*
IPr*Tol
a%VBur calculated for a 2.00 Å Pd–C1 length.
This survey showed that 1 is the most efficient precatalyst under
these reaction conditions with 76% conversion of the starting
4-bromotoluene (7) was selected as a benchmark reaction material. Precatalysts 2 and 4 exhibited closely related activity,
(Table 2). This C–H functionalization, initially described by with 50 and 49% conversion, respectively. However, complex 3
Ohta [26], was then reported by Bhanage and Mori using gave relatively poor conversion of the benzothiophene (6).
2–10 mol % of well-defined palladium catalysts [27,28]
(Figure 3). Alternatively, Fagnou and Kappe proposed a Thus, selecting 1 as the best precatalyst, the use of other
Pd/phosphine system involving 1–2 mol % of palladium and solvents, bases and additives was evaluated to optimize the
2–4 mol % of phosphine [29,30], but no example of this reac- reaction (see the Supporting Information File 1). From this opti-
tion involving a well-defined [Pd(NHC)] complex has been mization study, it was found that 0.1 mol % of 1 with K2CO3 in
described. However, it is noteworthy that variously substituted DMA as solvent at 140 °C in the presence of a catalytic amount
benzothiophene cores have been extensively studied in the of pivalic acid (30 mol %) generated the best reaction condi-
direct arylation process [4,31-37].
tions. Under these optimized parameters, a second precatalyst
screening was performed. As shown in Table 3, better activity
Initial screening of precatalysts 1–4 was performed with a was observed for precatalysts 1 and 2, which have smaller
2 mol % loading, by using KOt-Bu as the base, which is known ligands when compared to the NHCs in 3 and 4. This result
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Beilstein J. Org. Chem. 2012, 8, 1637–1643.
suggests a strong dependence of the activity on the steric prop-
erties of the NHC ligand. Moreover, the small difference
between 1 and 2 underlines the fact that the difference in the
σ-donation properties of the NHC ligands [38-41] is not likely
to play a crucial role in the catalytic activity.
Table 3: Catalyst screening under optimised conditions.
In comparison with the previously mentioned methodologies to
perform this C–H functionalization [27-30], the catalyst loading
can be decreased by at least 10-fold without drastically
affecting the yield (Table 4, entry 1). Using the optimized reac-
tion conditions, we examined the scope and the limitations of
this catalytic system using various aryl bromides and hetero-
cycles (Table 4). It appeared that the sterics of the aryl bromide
had almost no impact on the reaction. Indeed, para-, meta- and
ortho- substituted aryl bromides could be employed to arylate 6
in good yields. (Table 4, entries 1–3, 77–89%). However, ortho-
disubstituted aryl bromide, such as bromomesitylene appeared
Catalyst
Conversion (%)a
[Pd(SIPr)(cin)Cl] (1)
[Pd(IPr)(cin)Cl] (2)
[Pd(IPr*)(cin)Cl] (3)
[Pd(IPr*Tol)(cin)Cl] (4)
80
75
57
58
aConversion of the starting material into C–H arylated product deter-
mined by GC, [6] = 0.3 M.
Table 4: Palladium-NHC catalysed direct arylation of heterocycles with arylbromides.
Entrya
Heterocycles
Products
R
Yieldb
1c
2c
3c
4c
5c
6c
7c
4-Me, 8a
3-Me, 8b
2-Me, 8c
4-OMe, 8d
4-Cl, 8e
89%
80%
77%
70%
49%
53%
37%
6
8
4-F, 8f
4-CHO, 8g
8
4-Me, 10a
2-Me, 10b
4-F, 10c
85%
83%
52%
9
10
9
10
12
11
12
13
4-Me, 12a
4-OMe, 12b
4-F, 12c
90%
75%
57%
11
14
74%
69%
53%
76%
4-Me, 14a
15d
16d
17d
4-OMe, 14b
4-F, 14c
13
14
aUnless noted, reactions were performed on 0.6 mmol scale with: Heterocycle (1 equiv), aryl bromide (1 equiv), [Pd(SIPr)(cin)Cl] (0.1 mol %), PivOH
(30 mol %), K2CO3 (1.5 equiv), DMA (2 mL), 140 °C. bIsolated yields, average of two independent runs. c6 (1.2 equiv). d[Pd(SIPr)(cin)Cl]
(0.01 mol %).
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Beilstein J. Org. Chem. 2012, 8, 1637–1643.
to be too sterically demanding and led to no conversion (data pyridine. C–H functionalization of such heterocycles was
not shown). Concerning the electronic properties of the aryl performed in moderate to good yields by using only
bromide, electron-withdrawing (EWG) and electron-donating 0.1–0.01 mol % of precatalyst. This study highlights the fact
groups (EDG) were tolerated, although the presence of EWGs that [Pd(NHC)(cin)Cl] complexes are multipurpose precata-
resulted in decreased yields (Table 4, entries 4–6, 49–70%). The lysts as they may be utilised in various cross-coupling and, now,
substrate 4-bromobenzaldehyde was also successfully involved C–H-bond-functionalization reactions.
in the direct arylation of 6. Despite its electron-withdrawing
Experimental
nature as well as its high reactivity, the expected biaryl was
General procedure for the direct arylation of
heterocycles
obtained in moderate yield (Table 4, entry 7). The limits of the
scope were determined by switching from benzothiophene (6)
to the more sterically demanding 3-methylbenzothiophene (9)
(Table 4, entries 8–10). Closely related reactivity was observed
for 6 and 9, as these were arylated in comparable yields
(Table 4, entry 1 vs 8, 3 vs 9 and 6 vs 10).
In a glovebox, a vial containing a stirring bar was charged with
K2CO3 (124 mg, 0.9 mmol, 1.5 equiv) and pivalic acid
(0.18 mmol, 18 mg, 30 mol %), and sealed with a screw cap
fitted with a septum. The heterocycle (0.6 mmol, 1.0 equiv)
and/or the arylbromide (0.6 mmol, 1.0 equiv) were added at this
point if in solid form, and DMA (1.9 mL) was poured into the
vial. Outside of the glovebox, the heterocycle and/or the aryl
bromide were added at this point if in liquid form. Finally,
[Pd(SIPr)(cin)Cl] (1) was added as a 0.06 M solution in DMA
(0.6–6 µmol, 10–100 µL, 0.01–0.1 mol %), and the vial was
heated to 140 °C for 16 h. The solution was then cooled down
to room temperature, diluted with 40 mL of ethyl acetate, and
washed with water (2 × 20 mL) and brine (20 mL). The organic
layer was dried over MgSO4, filtered and concentrated in
vacuo. The crude residue was finally purified by either tritura-
tion in pentane (if not soluble) or silica-gel column chromatog-
raphy using pentane as the eluent.
A more challenging heterocycle, 2-methylthiophene (11), was
investigated. Simple thiophene rings are known to be less reac-
tive in C–H functionalization reactions [42]; nevertheless, 11
was successfully arylated in moderate to good yields, depending
on the electronic properties of the bromobenzene substituents
(Table 4, entries 11–13, 57–90%). Electron rich 4-methoxy-
bromobenzene reacted more efficiently than the electron poor
4-fluorobromobenzene. An opposite effect of the electronics
was observed by Doucet et al. in their ligandless procedure at
low catalyst loadings [43,44]. This is surely due to the nature of
the catalyst and thus offers complementary direct arylation
methods for thiophene derivatives.
To complete the study, experiments were performed at lower
catalyst loading using imidazopyridine (13). This class of sub-
strate has recently been involved, by Doucet et al. [45], in direct
arylation with a catalytic charge of Pd(OAc)2 ranging from 0.1
to 0.01 mol %. In our case, comparable yields were obtained
when the catalyst loading was decreased from 0.1 to
0.01 mol %, highlighting the high efficiency of the catalytic
system (Table 4, entries 14 and 15). Following the same trend
as reported by Doucet [45], a better reactivity was observed
with bromobenzenes substituted with EWGs compared to with
EDGs (Table 4, entries 16 and 17).
Supporting Information
Supporting Information File 1
Synthesis and characterization of complex 4; compound
characterization data for all the direct arylated products and
copies of their 1H and 13C NMR spectra.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-8-187-S1.pdf]
Supporting Information File 2
CIF-Check for compound 4.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-8-187-S2.pdf]
Conclusion
In summary, we report here the synthesis and characterization
of a new member of the [Pd(NHC)(cin)Cl] family,
[Pd(IPr*Tol)(cin)Cl]. The catalytic activity of this family of
complexes was surveyed in the direct arylation of heterocycles.
The bulkiness of the NHC ligand appears to play a major role in
the catalytic efficiency, whereas the σ-donation properties
(within the small electronic space examined) have little influ-
Supporting Information File 3
Crystal structure data for compound 4.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-8-187-S3.cif]
ence. Among the four complexes, [Pd(SIPr)(cin)Cl] exhibited Acknowledgements
the highest catalytic efficiency and was investigated for the We gratefully acknowledge the EPSRC and the ERC
arylation of various benzothiophenes, thiophene and imidazo- (Advanced Researcher Award-FUNCAT) for funding. We
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Beilstein J. Org. Chem. 2012, 8, 1637–1643.
24.CCDC-887349 contains the crystallographic data for 4. These data can
be obtained free of charge from The Cambridge Crystallographic Data
Centre via http://www.ccdc.cam.ac.uk/data_request/cif.
25.Poater, A.; Cosenza, B.; Correa, A.; Giudice, S.; Ragone, F.;
Scarano, V.; Cavallo, L. Eur. J. Inorg. Chem. 2009, 1759–1766.
doi:10.1002/ejic.200801160
thank the EPSRC National Mass Spectrometry Service Center
in Swansea for mass spectrometric analyses. Umicore AG is
acknowledged for its generous gifts of material. S.P.N. is a
Royal Society Wolfson Research Merit Award holder.
26.Ohta, A.; Akita, Y.; Ohkuwa, T.; Chiba, M.; Fukunaga, R.; Miyafuji, A.;
Nakata, T.; Tani, N.; Aoyagi, Y. Heterocycles 1990, 31, 1951–1958.
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