Direct arylation catalysis with chloro[8-(dimesitylboryl)quinoline-κN]copper(I)

Article abstract of DOI:10.3762/bjoc.12.272

We report direct arylation of arylhalides with unactivated sp² C-H bonds in benzene and naphthalene using a copper(I) catalyst featuring an ambiphilic ligand, (quinolin-8-yl)dimesitylborane. Direct arylation could be achieved with 0.2 mol % catalyst and 3 equivalents of base (KO(t-Bu)) at 80 °C to afford TON ≈160-190 over 40 hours.

Full text of DOI:10.3762/bjoc.12.272

Direct arylation catalysis with
chloro[8-(dimesitylboryl)quinoline-κN]copper(I)
Sem Raj Tamang and James D. Hoefelmeyer*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2016, 12, 2757–2762.
Department of Chemistry, University of South Dakota, 414 E. Clark
St., Vermillion, SD 57069, USA
doi:10.3762/bjoc.12.272
Received: 22 September 2016
Accepted: 30 November 2016
Published: 15 December 2016
Email:
James D. Hoefelmeyer* - jhoefelm@usd.edu
*
Corresponding author
This article is part of the Thematic Series "C–H
Functionalization/activation in organic synthesis".
Keywords:
catalysis; C–C coupling; C–H activation; copper; direct arylation
Guest Editor: R. Sarpong
©
2016 Tamang and Hoefelmeyer; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
We report direct arylation of arylhalides with unactivated sp2 C–H bonds in benzene and naphthalene using a copper(I) catalyst
featuring an ambiphilic ligand, (quinolin-8-yl)dimesitylborane. Direct arylation could be achieved with 0.2 mol % catalyst and
3
equivalents of base (KO(t-Bu)) at 80 °C to afford TON ≈160–190 over 40 hours.
Introduction
Coupling of aryl C–C bonds is invaluable in organic synthesis, are significant obstacles. The use of directing groups can
and has been the subject of much research with the aim to lower improve the selectivity, but could contribute additional reaction
cost and improve atom efficiency [1-5]. The earliest and most steps and expense. Therefore, C–X/C–H (X = halogen) direct
developed methods involve the reaction of C–M (M = Li, B, arylations are an important avenue of investigation that may
Mg, Si, Sn, Zn) with C–X (X = halide, triflate, tosylate, or aryl- represent a balance between cost, activity, and selectivity
iodide (as Ar2I+)) using Pd-based catalysts. These methods [8-13].
achieve coupling of aryl C–C bonds with high activity and
selectivity; however, the reactants are quite expensive and there The C–X/C–H direct arylation reaction can be achieved with
is significant opportunity to improve the atom efficiency. Addi- X = halogen, triflate, tosylate [14], B(OH)2 [15-20], SnR3 [21],
tionally, there is impetus to utilize less expensive metal cata- Si(OR)3 [22], or with the use of aryliodonium salts [23];
lysts or even metal-free reactions. Recently, there has been wherein halogen atoms represent the most cost-effective and
rapid progress toward the use of C–H bonds as a reactive func- atom-efficient functional group. The reactions typically utilize
tional group [6]. While it would be highly desirable to utilize expensive transition metal catalysts (Pd, Rh, Ir) at high load-
C–H/C–H coupling reactions [7], low reactivity and selectivity ings (typically 10 mol %), and require a large amount of strong
2757

Beilstein J. Org. Chem. 2016, 12, 2757–2762.
base (typically 3 equivalents KO(t-Bu)). There has been some
success using inexpensive first-row transition metals (Fe
[
24,25], Co [26-28], Ni [29-33], and Cu [34-37]) or an alumi-
num-based metal-organic framework [38], and there are several
reports of metal-free direct arylation reactions in which the
reaction is promoted with 2–3 equivalents KO(t-Bu) and (typi-
cally) 10–30 mol % of an additive [39-48].
The use of ambiphilic molecules as ligands for transition metals
has given rise to an important new class of catalysts [49]. In
previous work from our laboratory, we prepared the intramolec-
ular frustrated Lewis pair 8-quinolyldimesitylborane (1) and its
complexes with Cu(I), Ag(I), and Pd(II) [50]. Recently, we
Scheme 1: Coordination of Cu(I) with the ambiphilic ligand 1 to form
the catalyst 2.
reported the Pd(II) complex catalyzed Heck-type C–C coupling Results and Discussion
[
51]. The observation may implicate reductive elimination The results of direct arylation catalysis are summarized in
and oxidative addition can cycle repeatedly on the palladium Table 1. We performed the reaction of iodobenzene with
center coordinated to 1. With this in mind, we sought to utilize benzene/DMF (10:1 v/v) at 80 °C using a catalyst loading of
the Cu(I) complex, chloro[8-(dimesitylboryl)quinoline- 2 mol % of 2 and 30 equivalents KO(t-Bu) (Table 1, entry 1).
κN]copper(I) (2, Scheme 1), and investigate its performance as After 10 hours the yield of biphenyl was 94% according to
a catalyst for direct arylation reactions. We note that compound GC–MS analysis. A control reaction (Table 1, entry 4) of iodo-
2
consists of the ambiphilic ligand 1 coordinated to Cu(I) via benzene with benzene/DMF (10:1 v/v) using 2 mol % Cu(I)Br
nitrogen atom coordination and an η3-BCC interaction [52]. and 30 equivalents KO(t-Bu) gave 5% yield of biphenyl after
Herein we report the use of the preformed catalyst 2 and 10 hours. Reactions performed in the absence of catalyst, using
demonstrate good activity for C–X/C–H direct arylation reac- 3 or 30 equivalents KO(t-Bu) gave no reaction (Table 1, entries
tions.
5 and 6). While the high yield of the direct arylation suggests
Table 1: Coupling reaction of aryl halides with benzene. Reactions were carried out with 4 mL of benzene and 0.4 mL of DMF.
entry
catalyst
cat. (mol %)
KO(t-Bu) (equiv)
X
Ra
time (h)
yield (%)
1
2
3
4
5
6
7
8
9
2
2
30
30
30
30
30
3
I
I
H
p-OMe
p-NO2
H
10
20
20
10
10
10
10
40
40
40
40
40
40
40
40
40
40
40
94
77
<1
5
2
2
2
2
I
CuBr
2
I
none
0
I
H
0
none
2
0
I
H
0
2
0
I
H
0
2
0.5
0.5
0.5
0.3
0.3
0.3
0.3
0.3
0.3
0.2
0.2
3
I
p-OMe
H
70
85
52
53
36
25
0
2
3
I
1
0
1
2
3
4
5
6
7
8
2
3
I
p-CH3
o-OMe
H
1
1
1
1
1
1
1
1
2
3
I
2
3
I
2
3
I
p-CH3
p-OH
m-Cl
m-CH3
p-OMe
Hb
2
3
I
2
3
Br
Br
I
4
2
3
8
2
3
33
43
2
3
I
aFunctional group on the aryl halide. bReaction of naphthalene (2 g) with iodobenzene and 0.4 mL DMF.
2758

Beilstein J. Org. Chem. 2016, 12, 2757–2762.
that the product cannot be exclusively obtained from homocou- begin the cycle anew. The mechanism explains the metal-free
pling of iodobenzene, we performed additional experiments to direct arylation catalysis, but there has been some question
demonstrate a heterocoupling pathway. The coupling of about the initiation step. As KO(t-Bu) does not have sufficient
benzene with p-iodoanisole at 2 mol % loading of catalyst 2 reducing power to generate ArI•−, it has been proposed that ini-
was evaluated (Table 1, entry 2). We were pleased to find that tiation could arise from the reaction of ArI and KO(t-Bu) to
the reaction yielded 4-methoxybiphenyl according to GC–MS form benzyne or that organic electron donors form in situ with
analysis, and the isolated yield after flash column chromatogra- suitable additives under the basic conditions of the reaction [58-
phy was 77%. Encouraged by these findings, we attempted the 60]. For example, observation of faster direct arylation in the
reaction with lower catalyst loadings and 3 equivalents presence of DMF led to proposals of deprotonated DMF as one
KO(t-Bu). The reaction of iodobenzene with benzene with cata- electron donor [61] or forming a dibasic -enediol in situ as an
lyst loading of 0.5 mol % gave biphenyl in isolated yield of electron donor [62].
8
5% (Table 1, entry 9). The reaction of p-iodoanisole with
benzene using 0.2 or 0.3 mol % of catalyst 2 gave 4-methoxy- While the SET mechanism has drawn much interest, our obser-
biphenyl in isolated yields of 33% and 53%, respectively, which vation of large rate enhancement upon addition of the
indicate a TON ≈190 (Table 1, entries 11 and 17). The reaction preformed catalyst 2 may be better described with a meta-
of p-iodotoluene and benzene (Table 1, entries 10 and 13) with lation–deprotonation step followed by oxidative addition/reduc-
3
equivalents KO(t-Bu) gave 4-methylbiphenyl in yields of 25% tive elimination (Scheme 2) [63]. Thus, t-BuO− substitutes the
and 52% using 0.3 and 0.5 mol % of catalyst 2, respectively halogen ligand on 2, followed by deprotonation–metalation of
(
(
TON ≈170 for the latter). The reactions of p-iodophenol benzene, followed by oxidative addition of the arylhalide, and
Table 1, entry 14) or p-nitroiodobenzene (Table 1, entry 3) finally reductive elimination of the biaryl. The molecule 2 may
with benzene were very sluggish with only trace quantities be uniquely suited for this pathway. The electron-rich Cu(I) in
<1% yield) of products. The coupling of m-bromotoluene and the (L-Z)Cu(OR) intermediate may be well-stabilized by boron
m-bromochlorobenzene with benzene proceeded more slowly as a Z-type ligand, and the mesityl groups surrounding boron
Table 1, entries 15 and 16). The yields of 3-methylbiphenyl may favor the association of arenes for the deprotonation–meta-
and 3-chlorobiphenyl at catalyst loading of 0.3 mol % and lation and the oxidative addition steps. Wang et al. proposed
equivalents KO(t-Bu) were 8% and 4%, respectively. In all of concerted metalation–deprotonation via a sigma bond metathe-
the reactions, we note that metallic precipitates did not form, sis involving a cyclic 4-membered transition state, followed by
(
(
3
and the solutions did not become dark.
oxidative addition of ArI to Cu(I) [63]. Oxidative addition of
arylhalide to Cu(I) produces Cu(III) intermediates, for which
Activation of the sp2 C–H bonds in naphthalene was possible as there is substantial evidence [64-67]. However, under catalytic
well. The reaction of iodobenzene in neat naphthalene at 85 °C conditions, there is no requirement that the copper catalyst pass
using 0.2 mol % catalyst and 3 equivalents KO(t-Bu) gave a through an intermediate with a formal oxidation state of +3,
≈
2:1 ratio of 1-phenylnaphthalene and 2-phenylnaphthalene in a which may undoubtedly have a large activation energy. Rather,
total yield of 43% (Table 1, entry 18). The observed substitu- a concerted pathway through a 4-membered transition state will
tion ratio is typical of substitutions on naphthalene under kinetic have less localization of charge. A concerted process for cou-
control that tend to favor the alpha C–H bonds due better reso- pling of nucleophiles with arylhalides on copper centers have
nance stabilization effects [53]. Importantly, we note the alpha been proposed by Bacon [68] and were elaborated by Litvak
protons are slightly more acidic than the beta protons [54].
[69] who proposed SET within the 4-membered transition state.
It is noteworthy that modern DFT calculations [63] also
The mechanism of Cu catalyzed coupling reactions and, more produce cyclic transition states.
specifically, direct arylation have been the subject of intense
interest. Mechanistic models appear to diverge along those Conclusion
favoring oxidative addition/reductive elimination via Cu(I)/ In conclusion, we observe direct arylation reactions (C–X/C–H;
Cu(III) versus proposals favoring a single electron transfer X = halogen) catalyzed by a Cu(I) center stabilized by an
(
SET) pathway [55,56].
ambiphilic ligand. The activation of stable sp2 C–H bonds in
benzene and naphthalene occurs as a result of the catalysis. We
In the base-promoted SET mechanism [57], electron donation to favor a mechanism involving 4-membered cyclic transition
ArI leads to a short-lived radical anion ArI•− that decomposes to states for metalation–deprotonation followed by concerted oxi-
I− and Ar•. The Ar radical undergoes homolytic aromatic substi- dative addition/reductive elimination. The scope of reactivity,
tution with benzene to form a biaryl radical, and deprotonation including functional group tolerance on the reactants, types of
gives a biaryl radical anion that transfers one electron to ArI to C–H bonds that can be activated, selectivity of C–H bond acti-
2759

Beilstein J. Org. Chem. 2016, 12, 2757–2762.
Scheme 2: Proposed mechanism of direct arylation catalyzed by 2 (X = Cl/I; Ar = aryl).
vation, further optimization studies, and new catalyst design
Supporting Information
will be topics for further study. The initial results are very
encouraging in that an inexpensive copper catalyst at low
loading exhibited good activity for the reaction. The observa-
tion of high activity using a preformed copper catalyst may
assist in the development of new catalysts.
Supporting Information File 1
NMR spectra and GC–MS data of the products.
http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-12-272-S1.pdf]
[
Experimental
General. Compounds 1 and 2 were prepared according to the Acknowledgements
literature [50]. All organic reagents and solvents were obtained This work was made possible through funds from the National
from commercial sources and used without further purification. Science Foundation grant nos. EPS-0554609 (single crystal
A GCMS-QP2010SE gas chromatograph–mass spectrometer X-ray diffractometer and glovebox), CHE-1229035 (400 MHz
(
Shimadzu Corp., Kyoto, Japan) was used for GC–MS analyses. NMR spectrometer), and DGE-0903685 (graduate student
NMR spectra were recorded on an Avance 400 MHz spectrom- support).
eter (Bruker, Billerica, MA, USA).
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