Iron-Catalyzed Cross-Coupling Reactions of Arylmagnesium Reagents with Aryl Chlorides and Tosylates: Influence of Ligand Structural Parameters and Identification of a General N-Heterocyclic Carbene Ligand

Article abstract of DOI:10.1021/acs.organomet.7b00180

A systematic evaluation of N-heterocyclic carbene ligands in the iron-catalyzed cross-coupling reactions of aryl chlorides and arylmagnesium reagents is performed. There is no clear correlation between the donor strength of the N-heterocyclic carbene and the reaction outcome. Instead, the highest yields of the desired biaryl product are obtained with sterically demanding ligands possessing large %V_bur values. Through this study, SIPrNap has been identified as an efficient and general ligand for the coupling of both aryl chlorides and tosylates.

Full text of DOI:10.1021/acs.organomet.7b00180

Article
pubs.acs.org/Organometallics
Iron-Catalyzed Cross-Coupling Reactions of Arylmagnesium
Reagents with Aryl Chlorides and Tosylates: Influence of Ligand
Structural Parameters and Identification of a General N‑Heterocyclic
Carbene Ligand
Wenqin Wu,†,§ Qiaoqiao Teng,‡,§ Yi-Yuan Chua,† Han Vinh Huynh,*,‡ and Hung A. Duong*,†
†Institute of Chemical and Engineering Sciences (ICES), Agency for Science, Technology and Research (A*STAR), 8, Biomedical
Grove, Neuros, #07-01, Singapore 138665, Republic of Singapore
‡Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Republic of Singapore
S
* Supporting Information
ABSTRACT: A systematic evaluation of N-heterocyclic
carbene ligands in the iron-catalyzed cross-coupling reactions
of aryl chlorides and arylmagnesium reagents is performed.
There is no clear correlation between the donor strength of the
N-heterocyclic carbene and the reaction outcome. Instead, the
highest yields of the desired biaryl product are obtained with
sterically demanding ligands possessing large %Vbur values.
Through this study, SIPrNap has been identified as an efficient
and general ligand for the coupling of both aryl chlorides and
tosylates.
Scheme 1. Reported Iron Catalyst Systems for the Cross-
Couplings of Aryl Chlorides and Tosylates with Aryl
Grignard Reagents5,7,8
INTRODUCTION
■
Palladium- and nickel-catalyzed cross-coupling reactions are
widely employed in organic synthesis.1 Recently, there have
been considerable efforts toward employing iron catalysts as a
replacement due to the crustal abundance, inexpensiveness, and
low toxicity profile of this element.2 The iron-catalyzed
Kumada reaction for aryl−aryl coupling is of particular interest,
since biaryls are prevalent in fine chemicals, agrochemicals,
pharmaceuticals, and materials.3 However, it is difficult to
control this type of coupling, as arylmagnesium reagents can
undergo self-coupling rather readily under iron catalysis.4
In a seminal report, Nakamura et al. disclosed an iron
fluoride/N-heterocyclic carbene (NHC) catalyst system for the
selective cross-coupling of aryl chlorides with aryl Grignard
reagents.5 Presumably, fluoride strongly coordinates to FeII and
stabilizes the iron center against reduction by the arylmagne-
sium reagent, ultimately suppressing the homocoupling
pathway.5b We subsequently established that an iron(III)
alkoxide/NHC is also highly effective for the cross-coupling
reaction.6 Recently, we realized that a combination of Fe(OTf)2
and NHC displayed a high efficiency, even surpassing that of
the iron fluoride system in some cases.7 SIPr (1,3-bis(2,6-
diisopropylphenyl)imidazolidin-2-ylidene) was the best ligand
among the NHCs surveyed (Scheme 1a). For the cross-
couplings of C−O-based aryl electrophiles, the unsaturated
NHC analogue IPr (1,3-bis(2,6-diisopropylphenyl)imidazolin-
2-ylidene) was, however, superior to SIPr (Scheme 1b).7,8
Interestingly, the iron triflate system seems more tolerant
toward structural modifications of the NHC ligand in
comparison to the iron fluoride system. Whereas FeF3/IPr
performed poorly in the reaction of chlorobenzene, a good
yield of the biaryl product could still be obtained with
Fe(OTf)2/IPr (Scheme 1a). Similar observations could be
made for FeF3/SIPr vs Fe(OTf)2/SIPr in the coupling of C−O-
based electrophiles (Scheme 1b). In our efforts to gain more
insights into the factors governing the coupling selectivity, we
have performed a systematic evaluation of a series of NHC
ligands with varied steric and electronic profiles in the
Fe(OTf)2-catalyzed reaction.9 During this investigation, we
Received: March 8, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.7b00180
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also discovered an efficient NHC ligand for the coupling of
both aryl chlorides and tosylates. We report herein our findings.
L2 vs L10, L4 vs L12, and L7 vs L14). To probe the possibility
of mesoionic (abnormal) carbene formation from bulky,
unsaturated NHCs (e.g., IPr) through competing deprotona-
tion of the C4/5 protons,10 we also tested the IPrMe·HCl salt
(L19).11 However, the catalytic reaction gave a result similar to
that obtained with IPr.
RESULTS AND DISCUSSION
■
A reaction of chlorobenzene 1a with p-tolylmagnesium
bromide 2a was studied in the presence of Fe(OTf)2 and
different NHCs generated in situ from the corresponding salt
precursors and NaOtBu (Table 1).
Since the degree of backbone unsaturation also correlates to
the stereoelectronic properties of NHCs, an assessment of their
donor strength and steric bulk is relevant. In this regard, it must
be mentioned that the commonly used Tolman electronic
parameter (TEP values) cannot discern saturated versus
unsaturated NHCs.12a,c Thus, the donor strength examinations
were attempted by using Huynh’s method,12 whereby the
Table 1. Influence of NHCs on the Iron-Catalyzed Cross-
a
Coupling of 1a and 2a
i
influence of the NHC on the Pr2-bimy (1,3-diisopropylbenzi-
midazolin-2-ylidene) reporter signal in trans-[PdBr2(iPr2-bimy)-
(NHC)] complexes is measured by 13C NMR spectroscopy. A
i
greater downfield shift of the Pr2-bimy carbene signal (HEP)
would indicate a stronger donor.
The preparation of the complex probes is summarized in
Scheme 2 and follows two methods (see the Supporting
Scheme 2. Preparation of Complex Probes for HEP and %
Vbur Determination
a
The amounts of 1a, 3a, and 3a′ were determined by GC using
dodecane as an internal standard. The yield of 3a′ was calculated on
the basis of 2a.
Notably, the counteranions of the NHC precursors affected
the reaction yields considerably. The chloride salts were more
effective than the corresponding tetrafluoroborate salts in all
cases (e.g., L1 vs L7 or L9 vs L14). 1H NMR measurements of
0.0025 M solutions of SIPr·HCl (L1) and SIPr·HBF4 (L7) in
CDCl3 indeed revealed that the C2−H proton in L1 (8.27
ppm) is more downfield than that in L7 (7.67 ppm). This
observation led us to question whether the counterion of NHC
precursors has an influence on the acidity of the carbenic
proton and, hence, the in situ generation of free carbenes. We
thus performed the reaction of 1a and 2a in the presence of
Fe(OTf)2/SIPr·HBF4 and different bases (Table 2). Notably,
KOtBu led to an improvement in the yield of 3a to 93% (entry
1).
Information for experimental details). For azolium bromides, a
one-pot procedure involving silver carbene transfer with
concurrent bridge cleavage of the dipalladium complex
[PdBr2(iPr2-bimy)]2 (I) was sufficient (method A). The
syntheses of the complex probes using azolium tetrafluor-
oborates required method B. Here, the NBu4[PdBr3(iPr2-
bimy)] (II) had to be prepared first prior to mixing with the
azolium BF4 salt and Ag2O. II was prepared by mixing dimer I
with TBAB and provides a bromide anion for AgI precipitation,
which drives the reaction.
The complex probes for ligands L1−L18 are all air-stable
compounds and well soluble in CDCl3, which facilitates
handling and HEP determination. Single crystals suitable for
X-ray diffraction were also obtained for most of them, and the
molecular structures of two representatives are depicted in
Figure 1.
The availability of solid-state molecular structures of this
series of trans-[PdBr2(iPr2-bimy)(NHC)] complexes allows for
the determination of the respective NHC percent buried
volume (%Vbur) via SambVca2 calculations for proper
comparisons.13 Table 3 summarizes HEP and %Vbur values
for the relevant NHCs. For completeness, %Vbur values
reported in the literature are given in parentheses, where
available. However, it must be noted that these were obtained
using different metal systems, making a direct comparison
difficult.
In general, NHCs with a saturated backbone displayed a
higher activity than the unsaturated counterparts (i.e., L1 vs L9,
Table 2. Effects of Bases on the Iron-Catalyzed Cross-
Coupling of 1a and 2a Employing Fe(OTf)2/SIPr·HBF4
a
a
a
ab
,
entry
base
KOtBu
conversn (%)
3a (%)
3a′ (%)
1
2
3
4
100
100
85
93
85
65
10
10
5
KH
NaH
13
40
KHMDS
47
a
Determined by GC using dodecane as an internal standard.
Determined on the basis of 2a.
b
B
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(Figure 2). In this “stereoelectronic map”, notable clustering of
the best-performing carbenes was observed in the upper left
Figure 1. Molecular structures of trans-[PdBr2(iPr2-bimy)(SIAd)] and
trans-[PdBr2(iPr2-bimy)(IPrMe)]. Hydrogen atoms are omitted for
clarity.
Table 3. Huynh’s Electronic Parameter (HEP) and %Vbur for
NHC Ligands L1−L18 in trans-[PdBr2(iPr2-bimy)(NHC)]
Complexes
Figure 2. Plot of HEP versus %Vbur. Yields obtained for 3a are given in
parentheses.
entry
ligand
HEP (ppm)
%Vbur(lit.)
L1/L7
L2
SIPr
177.6
177.6
178.6
177.8
178.2
178.7
177.5
177.2
177.6
177.1
178.9
180.6
181.2
− (30)14
34.3 (27)14
36.7
area, where NHCs of moderate donor strength and increased
steric bulk are located.
Encouraged by these findings, we further evaluated NHCs
possessing large reported %Vbur values (Table 4). IPr* (L20)
SIMes
SItBu
SIAd
SIPrtBu
SIPrPh
IPr
L3/L8
L4
36.2
L5
36.1
L6
35.3
Table 4. Influence of Bulky NHCs on the Iron-Catalyzed
Cross-Coupling of 1a and 2a
L9/L14
L10
34.4 (29)14
33.6 (26)14
35.4 (37)14
36.0 (37)14
29.9
IMes
ItBu
L11/L15
L12/L16
L13
IAd
IPrMe
IiPr
L17
− (27)15
L18
ICy
31.1 (27)15
Consistent with chemical intuition, the HEP values obtained
show that saturated NHCs are generally more strongly
donating than their unsaturated counterparts (Table 3),
which could contribute to their superior activities. However,
among saturated NHCs, L3 and L4 do not perform better than
the slightly weaker L1 ligand. Furthermore, unsaturated ligands
such as L17 and L18 with strong donicity are very poor ligands
in the iron-catalyzed reaction.
There is, however, a more pronounced trend in the influence
of steric bulk on the cross-coupling performance. Saturated
NHCs have larger %Vbur values in comparison to their
unsaturated counterparts with identical wingtips due to larger
N−C−N angles. Indeed, a comparison of %Vbur values reveals
that the most sterically demanding ligands also give rise to the
best catalysts (Table 3), while small differences in the electron-
donating ability are less important in this type of catalysis.
These bulky NHCs may provide improved steric protection for
the active iron center and/or facilitate reductive elimination in a
“metalate mechanism” as suggested by Nakamura et al.5 In
agreement with this notion, we observed that the catalytic
performance and selectivity are much more responsive to steric
alterations. NHCs with bulky N substituents such as Dipp, tBu,
and Ad and their mixed variations (L1/L7, L3−L5, L9, L11,
and L12) in general afford a high coupling selectivity of 3a.
Even the replacement of only one wingtip group with a smaller
one such as Ph and Me led to very poor yields for 3a (e.g., L6
and L13).
a
Determined by GC using dodecane as an internal standard.
Determined on the basis of 2a.
b
and IHept (L21), which were previously shown to be superior
to IPr in Pd-catalyzed C−C and C−N cross-coupling
reactions,16 only resulted in low yields of 3a, indicating that
an increase in the size of the substituents does not always
translate into a gain in catalytic activity. These may be even too
bulky, blocking incoming substrates. For IPr*, deprotonation at
the benzylic C−H bond may occur under the highly basic
conditions of the Kumada reaction, hindering the formation of
the desired iron/NHC complex. Notably, a family of easily
For a better visualization of the trends observed, a plot of
HEP and %Vbur based on our PdII complex probes was prepared
C
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accessible and stable NHC salts with N-naphthyl substituents
developed by Dorta et al.17 displayed a steric trend similar to
the N-phenyl NHC series in the iron-catalyzed reaction, as the
yields of 3a increased in the order SINap (L22, 6%), SIMeNap
(L23, 75%), and SIPrNap (L24, > 99%). As reported by Dorta
et al., the %Vbur value of SIPrNap is somewhat larger than that
of SIPr.17a,18
Table 6. Cross-Coupling of Aryl Tosylates and
Arylmagnesium Reagents Using Fe(OTf)2/SIPrNap·HCl/
a
NaOtBu
The promising results obtained with SIPrNap prompted us
to explore the substrate scope of Fe(OTf)2/SIPrNap catalyst
system. Under the optimized conditions employing 3 mol %
Fe(OTf)2, 3 mol % of SIPrNap, and 3 mol % of NaOtBu, 3a
was isolated in 98% yield. These conditions were also amenable
to the preparation of other biaryls (Table 5). Both electron-rich
Table 5. Cross-Coupling of Aryl Chlorides and
Arylmagnesium Reagents Using Fe(OTf)2/SIPrNap·HCl/
a
NaOtBu
a
Conditions: 3 mol % of Fe(OTf)2, 9 mol % of SIPrNap·HCl, 9 mol %
b
of NaOtBu, THF, 60 °C. Determined by GC using dodecane as an
internal standard.
in the iron-catalyzed couplings of both aryl chlorides and
tosylates is rather remarkable.
CONCLUSION
■
In conclusion, a systematic evaluation of NHC structural
parameters in the Fe(OTf)2/NHC-catalyzed cross-coupling
reaction of chlorobenzene 1a and p-tolylmagnesium bromide
2a revealed a strong dependence of the reaction outcome on
the bulk of N substituents, the degree of unsaturation of the
NHC backbone, and the nature of the counterions of the
carbene precursors. Using HEP and %Vbur, the stereoelectronic
properties of the respective NHCs were evaluated. While there
was no clear correlation between the reaction outcome and
NHC donor strengths, a pronounced steric effect could be
observed. Sterically demanding ligands possessing large %Vbur
values gave rise to the highest yields of the desired biaryl
product. We also discovered that SIPrNap featuring bulky N-
naphthyl substituents is a general and efficient ligand for the
coupling of both aryl chlorides and tosylates with aryl Grignard
reagents.
a
Conditions: 3 mol % of Fe(OTf)2, 3 mol % of SIPrNap·HCl, 3 mol %
b
of NaOtBu, THF, 60 °C. Determined by GC.
and electron-deficient substrates, including aryl chlorides and
aryl Grignard reagents, were coupled in good to excellent
yields. 2-Chlorotoluene 1d reacted with 2a under the iron-
catalyzed conditions, giving 3d in 87% yield whereas 2-chloro-
1,3-dimethylbenzen 1e was unreactive. The ortho,ortho′-
disubstituted biaryl 3f was obtained in a good yield. Attempts
to prepare tri-ortho-substituted biaryls resulted in only a poor
yield of 3g, even at a higher catalyst loading (ca. 5 mol %).
We previously found that Fe(OTf)2/IPr·HCl/NaOtBu is a
highly efficient catalyst system for the coupling of aryl tosylates
and aryl magnesium reagents.7 For example, phenyl tosylate 4a
reacted with 4-anisylmagnesium bromide 2b to give 3i in 95%
isolated yield. On the other hand, SIPr and other bulky
unsaturated NHCs (i.e., IAd and ItBu) gave inferior perform-
ance (Table 6). Notably, SIPrNap was also highly active in this
reaction, affording 3i in 93% yields (Table 6). Indeed, SIPrNap
is comparable to IPr in the coupling of a range of aryl tosylates
and Grignard reagents (Table 6).7 In comparison to N-phenyl
NHCs (i.e., IPr and SIPr), the generality of SIPrNap as a ligand
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00180.
■
S
Full experimental procedures, compound characteriza-
tion, and NMR spectra (PDF)
Accession Codes
CCDC 1521203−1521210 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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(12) (a) Huynh, H. V.; Han, Y.; Jothibasu, R.; Yang, J. A.
Organometallics 2009, 28, 5395−5404. (b) Guo, S.; Sivaram, H.;
Yuan, D.; Huynh, H. V. Organometallics 2013, 32, 3685−3696.
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(13) (a) Poater, A.; Cosenza, B.; Correa, A.; Giudice, S.; Ragone, F.;
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(b) See also www.molnac.unisa.it/OMtools/sambvca2.0.
(14) Values based on [RuCl(Cp*)(NHC)] complexes with a M−C
distance of 2.00 Å from: Cavallo, L.; Correa, A.; Costabile, C.;
Jacobson, H. J. Organomet. Chem. 2005, 690, 5407−5413.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail for H.V.H.: chmhhv@nus.edu.sg.
*E-mail for H.A.D.: duong_hung@ices.a-star.edu.sg.
ORCID
■
Han Vinh Huynh: 0000-0003-4460-6066
Author Contributions
§W.W. and Q.T. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
The financial support for this work was provided by “GSK-EDB
Singapore Partnership for Green and Sustainable Manufactur-
ing” (WBS R-143-000-580-592), the National University of
Singapore, and the Institute of Chemical and Engineering
Sciences (ICES), Agency for Science, Technology and Research
(A*STAR), Republic of Singapore.
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