Sulfonato-imino copper(II) complexes: Fast and general Chan-Evans-Lam coupling of amines and anilines

Article abstract of DOI:10.1039/c7dt02260c

Sulfonato-imine copper complexes with either chloride or triflate counteranions were prepared in a one-step reaction followed by anion-exchange. They are highly active in Chan-Evans-Lam couplings under mild conditions with a variety of amines or anilines, in particular with sterically hindered substrates. No optimization of reaction conditions other than time and/or temperature is required.

Full text of DOI:10.1039/c7dt02260c

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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Received 00th January 20xx,
Accepted 00th January 20xx
Sulfonato-imino copper(II) complexes : fast and
general Chan-Evans-Lam coupling of amines and
anilines
DOI: 10.1039/x0xx00000x
www.rsc.org/
V. Hardouin Duparc,a and F. Schapera,*
Sulfonato-imine copper complexes with either chloride or triflate the nature of the base used. Yields and rates can vary
counteranions were prepared in a one-step reaction followed by unpredictably, even for closely related substrates,12 and
anion-exchange. They are highly active in Chan-Evans-Lam reaction conditions have to be adapted accordingly.8 Even
couplings under mild conditions with a variety of amines or then, several substrates show only low or no reactivity.9
anilines, in particular with sterically hindered substrates. No We decided to investigate copper coordination complexes
optimization of reaction conditions other than time and/or with imino-sulfonate ligands (Scheme 1) as potential catalysts
temperature is required.
for Chan-Evans-Lam couplings. The chelating ligand removes
the need for external "base" as ligand and reduces
dependence of catalyst performance on the solvent. The
sulfonate group incorporated in the ligand enables
coordination to boronic acid (see the proposed intermediate in
Scheme 1), which should make the choice of counter-ion less
important. Although several studies reported improved
catalytic performance when nitrogen ligands, such as TMEDA,
were added to Chan-Evans-Lam couplings,21-25 only a very
limited amount of studies used preformed coordination
complexes instead of simple copper salts.26-32 Of these,
[Cu(DMAP)4I]I, recently reported by Phukan, displayed high
activity to a variety of amines and anilines.32
Palladium-catalyzed bond formations are in general without
competition with regard to scope and reactivity, but the
toxicity and high reactivity of trace amounts of palladium
raised concerns, in particular in the pharmaceutical industry.1
The copper-catalyzed Chan-Evans-Lam coupling of boronic
acids with alcohols, amines or other nucleophiles to form
carbon-heteroatom bonds provides an alternative bond-
forming reaction, using milder conditions compared to the
related Ullmann-Goldberg reaction2 or to the analogous
Buchwald-Hartwig coupling using Pd.3, 4 Following the original
work of Chan, Evans and Lam,5-7 a large number of studies
have opened a wide substrate scope and allowed reactions
using catalytic amounts of copper.8-16 With regard to catalyst
performance, copper salts with acetates and or triflate
counterions are often preferable over halogens or perchlorate
anions,17, 18 possibly due to pre-coordination of boronic acid to
the anion.18-20 Chan-Evans-Lam couplings also often require
the addition of base, but its mechanistic implication is unclear
and one of its roles might be that of a ligand activating
dinuclear Cu2(OAc)2.19 For these reasons, Chan-Evans-Lam
couplings using Cu(OAc)2 or other simple copper salts react
strongly to changes in reaction conditions such as solvent and
Scheme 1
N
N
NH2
SO3
CuCl2
N
S
N
S
Cu
Cu
+
Cl
OH2
X
N
R
R'
H
N
O
O
O
HO
O
B
H
O
O
O O
1
N
AgOTf
N
Cu
OTf
N
N
S
O
Cu
X
R(R')NH, O2
S
2
O
O
O
O
O BPh(OH)2
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Chloride complex
Journal Name
1
can be readily prepared in a one-pot
reaction from commercially available starting materials,
following protocols for similar complexes.33 Anion exchange
provided the triflate complex
2 (Scheme 1). The crystal
structure of 2 shows a tetragonally distorted octahedral
coordination geometry around copper (Fig. 1). The triflate
anion is coordinated in the equatorial plane with a Cu-O
distance comparable to that of the sulfonate ligand (1.929(1)
and 2.015(1), respectively). Bridging coordination of the
triflate and sulfonate ligands of neighbouring molecules to the
apical positions (dCu-O = 2.4 – 2.5 Å) leads to formation of a 1D-
coordination polymer along the crystallographic b-axis (Fig.
S1). The increased Lewis acidity of copper due to the weakly
coordinating triflate anion is likely responsible for the
octahedral coordination environment and the slightly shorter
bond distances in
2 when compared to similar Cu complexes
coordination to copper), but variations were relatively small.
We were grateful to note that addition of external base was
indeed not necessary. To the contrary, its presence even in
catalytic amounts drastically reduced activity (Table S1), in
agreement with the mechanistic interpretation that nitrogen
bases employed in Chan-Evans-Lam couplings act as a ligands.
Presence of water was tolerated and addition of molecular
with chloride or carboxylate counteranions, which show
square-pyramidal coordination geometries without bridging
sulfonate groups.33-37 It should be noted that the formation of
the coordination polymer strongly resembles the interaction of
2
with boronic acid proposed in Scheme 1: bridging
coordination of the sulfonate group to a Lewis-acidic centre
(Cu or B, Fig. 1) and interaction of an anionic group on this
sieves did not improve the reaction (Table S1). Complex
slightly more active than with 98% and 86% conversion,
respectively, after 320 min (Table S1). Kinetic studies showed
that this was mainly due to a longer induction period for (40
min vs. 10 min for ), after which the reaction proceeded with
very similar rates (kobs
) = 0.37(3) h–1, kobs ) = 0.49(1) h–1, Fig.
2 was
centre with copper (triflate or Ph). Chloride complex
1 shows
1
square-pyramidal coordination geometry (τ = 0.1, Fig. S2) and
is very similar to that of the related complex without the 3-
methyl group, in which water is replaced by a µ-chloride
ligand.34
1
2
(1
(2
S3). Similar rates, but different induction periods indicate that
the anion most likely dissociates from copper and does not
further participate in the reaction mechanism. The presence of
the chelating sulfonate ligand thus eliminated the requirement
of an acetate or triflate counteranion, catalytic performance
was good in a variety of solvents and no additional base/ligand
was needed. Together with the tolerance towards water, this
removed the main variables which typically need to be
optimized in Chan-Evans-Lam couplings and which required
different protocols even for substrates as closely related as
amines and anilines.38, 39 Complex
2 could thus be applied in
base-free Chan-Evans-Lam couplings without any optimization
of reaction conditions and the arylation of anilines, primary
and secondary amines proceeded with quantitative conversion
(Chart 1, isolated yields >85%) at room temperature and in 12
h reaction time.
Reactivity studies. Given the good reactivity towards amines
and anilines in general, the reactivity of different substrates
was investigated in more detail with particular attention
Figure 1. X-ray structure of 2. Thermal ellipsoids are drawn at 50% probability.
Hydrogen atoms and co-crystallized dichloromethane were omitted for clarity. Selected
bond distances (Å) and angles (°): Cu-N1 = 2.0261(13), Cu-N2 = 1.967(1), Cu-O1 =
1.929(1), Cu-O4 = 2.015(1), Cu-O2= 2.379(1), Cu-O4 = 2.488(1), N1-Cu-N2 = 81.89(5),
N2-Cu-O4 = 93.58(5), O1-Cu-O4 = 91.38(5), N1-Cu-O1 = 94.25(5), X-Cu-O2 = 82.18(4)-
94.18(5).
towards substrates reported to be less reactive. Complex
2
was chosen for these studies, since its shorter induction time
facilitates reactivity comparisons at short reaction times.
Poorer nucleophiles are generally more difficult to react in
Chan-Evans-Lam couplings,30, 39-41 but para-fluoro, -bromo or -
iodo anilines reacted only slightly slower than unsubstituted
Catalytic performance. Complex
1 catalyses the Chan-Evans-
Lam coupling of aniline with phenyl boronic acid in a variety of
solvents (Table S1). Increasing polarity led to higher activity
(until deactivation in acetonitrile, probably by solvent
aniline (Chart 2,
A
-
D
). While para-phenoxy aniline reacted
), anilines carrying alkoxy
readily at room temperature (
E
substituents in para-position surprisingly only yielded starting
2 | J. Name., 2012, 00, 1-3
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Amines react in general faster and easier than anilines. Primary
amines typically reached full conversion in less than 2 h at
ambient temperature. Catalytic activity was investigated in
more detail with octylamine (N) and full conversion could be
achieved in 30 min with a reduced catalyst loading of 0.5
mol%. The tolerance of water in Chan-Evans-Lam couplings
with
2
allowed the use of an aqueous solution of methylamine,
). To the best of
which readily reacted in 1 h to MeN(Ph)H (
O
our knowledge, no copper-catalyzed coupling of boronic acids
with methylamine has been reported. Prolonged reaction
times led in this case to the appearance of the double coupling
product, MeNPh2 (O'). The second coupling is, however,
notably slower and formation of MeNPh2 can be avoided by
controlling reaction time or reducing the excess of phenyl
boronic acid. With 2.5 equiv of phenyl boronic acid, full
conversion to MeNPh2 (O') was observed after 12 h. The same
product was obtained from coupling of N-methyl aniline.
Increased steric hindrance typically overcomes the increase in
nucleophilicity and tertiary alkyl amines show very low
activities in Chan-Evans-Lam couplings.38 Couplings of tert-
butyl amine have been reported with (isolated) yields of 28% –
60% using reaction temperatures of 40 – 80 °C and 20 – 24 h
reaction time.50-52 Coupling of tert-butyl amine with phenyl
boronic acid using
and full conversion is achieved at room temperature in 1 h,
even at lowered catalyst loadings of 0.5 mol% ( ).
Cyclic secondary amines, such as piperidine ( ) and pyrrolidine
), undergo Chan-Evans-Lam couplings readily,5 and full
conversion is also obtained with at room temperature in 2 h.
2 proceeds with drastically higher activity
P
Q
(
R
2
Low reactivity of acyclic secondary amines, on the other hand,
has been reported as a "major restriction" in copper-catalyzed
coupling reactions.9 Reactions with Cu(OAc)2 as a catalyst
typically require longer reaction times at elevated temperature
and yields vary between traces to 60%.39, 41, 45, 50, 51, 53-56 Using
(Xantphos)Cu(OtBu) as catalyst, Lalic obtained iPr2NAr from
benzoyloxy diisopropylamine in 94% conversion at 60 °C.48
Phukan reported yields of 83% – 94% in 5 – 10 min for the
coupling of N-methyl-benzylamine to various arylboronic acids
using 2 mol% of a highly active [(DMAP)4CuI]I catalyst.32 Using
Chart 2. Conversion to product (determined by GC-MS vs. internal calibrated standard)
for the coupling of nitrogen nucleophiles with phenyl boronic acid.
material (F-I). Since formation of diphenyl side product was
likewise suppressed in these reactions, electron-rich anilines
probably coordinate to either copper or boron in an
unproductive off-cycle complex. Dissociation of these
complexes would be more favourable at higher temperatures
and full conversion can be achieved by conducting the reaction
at 50 °C (
F
-I
).‡ In the case of para-tert-butyl aniline, the
2
, dibutylamine showed full conversion to nBu2NPh in 10 min
at room temperature at reduced catalyst loadings of 0.5% ( ).
Pyrazol ( ) and imidazole ( ) could also be successfully
reaction yielded a 1:2 mixture of the coupling product (
J
) and
S
4,4'-di(tert-butyl)azobenzene (J'). Cu-catalyzed formation of
azabenzenes by aerial oxidation of anilines is well
established,42, 43 but has not been reported as a side reaction
in Chan-Evans-Lam couplings. At 50 °C, aza-benzene formation
was reduced to 10%, but could not be suppressed completely.
Chan-Evans-Lam couplings of sterically demanding ortho-
disubstituted anilines are typically difficult32, 38, 39, 44-48 and only
T
U
coupled at 50 °C, using the same reaction conditions employed
for amines and anilines. Ortho-amino phenol, amino ethanol
and ortho-amino pyridine did not react or only sluggishly (
probably due to formation of phenyl boronic esters which
were identified by GC-MS for ortho-aminophenol (V' V").
Addition of glycol, of B(OH)3 or both, did not change the
reaction outcome. Picolylamine ( ) was coupled successfully,
but was the only alkylamine which required heating.
Consequently, para-amino pyridine Z1 and para-amino
V-X),
,
19
45, 46, 48
few optimized protocols report yields above 80%.39,
Y
Complex
2 coupled 2,4,6-dimethylaniline (K) with only slightly
lower activity than aniline. 2,6-Diisopropyl aniline reacted
more slowly and only partial conversion of 51% was observed
(
)
phenol (Z2), which cannot form chelated boronic esters, were
coupled quantitatively at 50 °C. Using Cu(OAc)2 catalyst,
couplings of para-amino phenol were reported with up to 75%
(isolated) yield, but required carefully optimized reaction
conditions (dioxane, 90 °C, 12 h, CsCO3, benzoic acid).57 As
even after 12 h at 50 °C (L). We are aware of only one other
Cu-catalyzed coupling of diisopropylaniline with arylboronic
acid.49 If one iPr-substituent is removed, coupling proceeds
readily at room temperature (M).
with 2, ortho-aminophenols could not be coupled.
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‡ Heated reactions were conducted under oxygen atmosphere.
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to significant evaporation of amine and/or to reduced yields due
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Sulfonato-imino copper(II) complexes : fast and
general Chan-Evans-Lam coupling of amines and
anilines
V. Hardouin Duparc and F. Schaper
Table of Contents
One size fits all: a simple coordination complex allows Chans-Evans-Lam arylations of a wide variety of
amines and anilines without optimization of reaction conditions