Structural and Electronic Control of the Bidentate 1-(2-pyridyl)benzotriazole Ligand in Copper Chemistry with Application to Catalysis in the A3 Coupling Reaction

Article abstract of DOI:10.1002/chem.202004781

The hybrid bidentate 1-(2-pyridyl)benzotriazole (pyb) ligand was introduced into 3d transition metal catalysis. Specifically, [Cu^II(OTf)₂(pyb)₂]?2 CH₃CN (1) enables the synthesis of a wide range of propargylamines by the A³ coupling reaction at room temperature in the absence of additives. Experimental and high-level theoretical calculations suggest that the bridging N atom of the ligand imposes exclusive trans coordination at Cu and allows ligand rotation, while the N atom of the pyridine group modulates charge distribution and flux, and thus orchestrates structural and electronic precatalyst control permitting alkyne binding with simultaneous activation of the C?H bond via a transient Cu^I species.

Full text of DOI:10.1002/chem.202004781

Accepted Article
Accepted Article
Title: Structural and electronic control of 1-(2-pyridyl)benzotriazole
bidentate ligand in copper chemistry with application to catalysis
in the A3 coupling reaction
Authors: Stavroula I. Sampani, Victor Zdorichenko, Jack Devonport,
Gioia Rossini, Matthew C. Leech, Kevin Lam, Brian Cox, Alaa
Abdul-Sada, Alfredo Vargas, and George E. Kostakis
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202004781
Link to VoR: https://doi.org/10.1002/chem.202004781
01/2020

10.1002/chem.202004781
Chemistry - A European Journal
FULL PAPER
Structural and electronic control of 1-(2-pyridyl)benzotriazole
bidentate ligand in copper chemistry with application to catalysis
in the A3 coupling reaction
Stavroula I. Sampani,[a] Victor Zdorichenko,[b] Jack Devonport,[a] Gioia Rossini,[a] Matthew C. Leech,[c]
Kevin Lam,[c] Brian Cox,[a,b] Alaa Abdul-Sada,[a] Alfredo Vargas*[a] and George E. Kostakis*[a]
In memory of Prof. Jim Hanson
[a]
S. I. Sampani, J. Devonport, G. Rossini, Prof. B. Cox, Dr. A. Abdul-Sada, Dr. A. Vargas, Dr. G. E. Kostakis,
Department of Chemistry, School of Life Sciences, University of Sussex, Brighton BN1 9QJ, UK.
E-mail: Alfredo.Vargas@sussex.ac.uk & G.Kostakis@sussex.ac.uk
[b]
[c]
Dr. V. Zdorichenko, Prof.. B. Cox
Photodiversity Ltd c/o Department of Chemistry, School of Life Sciences, University of Sussex, Brighton BN1 9QJ, UK.
Dr. M. C. Leech, Dr. K. Lam,
School of Science, Department of Pharmaceutical Chemical and Environmental Sciences,
University of Greenwich, Central Avenue, Chatham Maritime, ME4 4TB, UK.
Supporting information for this article is given via a link at the end of the document.
Abstract:
We
introduce
the
hybrid
bidentate
1-(2-
understanding of the properties that govern the chemistry of the
1-(2-pyridyl)benzotriazole (pyb) ligand (Scheme 1) and the
resulting coordination complexes. When compared with bpy, pyb
has the following differences: a) additional N atoms that can
participate in H-bonding interactions, b) additional phenyl group
that enforces an electron-rich character of the framework but also
permits participation in stacking interactions, and c) the two,
pyridine and benzotriazole, units are linked via an N atom which
imposes flexibility via the C-N bond, yielding different coordination
behavior when compared with the rigid C-C based bpy ligand.
Steel, in his pioneer work, identified pyb-based compounds to be
more electron-deficient when compared with that of 2,2’-bpy;[10]
however, the use of well-characterized pyb-based complexes as
catalysts or in situ made catalysts containing pyb is almost an
unexplored research field.[11–14]
pyridyl)benzotriazole (pyb) ligand in well-characterized 3d-transition
metal catalysis. Specifically, [Cu(II)(OTf)2(pyb)2]2(CH3CN) (1) enables
the synthesis of a wide range of propargylamines, via the A3 coupling
reaction, at room temperature, in the absence of additives.
Experimental and high-level theoretical calculations suggest that the
bridging N atom of the ligand imposes exclusive trans-coordination at
Cu and allows ligand rotation, while the N atom of the pyridine group
modulates charge distribution and flux, thus orchestrating structural
and electronic pre-catalyst control permitting alkyne binding with
simultaneous activation of the C–H bond via a transient Cu(I) species.
Introduction
Copper is a labile transition metal, plural in oxidation states that
promotes a variety of organic transformations either as a metal
salt, in situ formed or well-characterized complexes.[1,2] In its
dominant oxidation state(II), the d9 electronic configuration
confers elongated or shorten axial axes, known as Jahn and
Teller effect,[3] and depending on the coordinating ligands various
geometries and stereoisomers, i.e. cis – trans for CuX2(N-N)2
where N-N is a bidentate ligand, can be obtained. 2,2-bipyridine
(2,2’-bpy) has been extensively used in coordination chemistry as
a bidentate ligand[4] and in catalysis.[5] The catalytic protocols that
involve in situ blending of bpy, copper salts and substrates
achieve high yields and new products;[6] however, the role of each
component in the catalytic cycle is questioned. Well-characterized
Cu(II) and bpy based complexes have been used as models for
the aerobic oxidation of alcohols[7] and other organic
transformations.[8] In some occasions, well-characterized Cu(II)-
bpy complexes surpass the catalytic performance of isostructural
compounds built with similar N,N’-bidentate ligands. This
differentiation in catalytic efficacy may be a result of electronic
and/or steric effects; however, the formation of different
stereoisomers cannot be ignored. [9]
Scheme 1. The traditional bidentate ligand 2,2’-bipyridine (bpy) and the
bidentate ligand (right) used in this work.
The A3 coupling is a very well known, atom efficient, reaction
that yields propargylamines[15–17] with one molecule of water as a
side product. Methodologies that involve Cu(II) sources for the C–
H activation have been reported,[18–22] however, the emphasis, in
these studies, is given in the final product and achieving excellent
yields, irrespectively of the extreme reaction conditions, i.e.
elevated temperatures, prolonged time, high catalyst loadings.
Knochel’s pioneer work determines the activation of the acetylide
on the coordination sphere of the Cu(I) centre and subsequently
the coupling with the corresponding imine.[23] Besides, copper
reduction may occur in the presence of alkynes, and this process
depends on temperature and concentration.[24]
In this work, we introduce the pyb ligand in well-
characterized 3d-transition metal catalysis and report the
synthesis and characterization of [Cu(II)(OTf)2(pyb)2]2(CH3CN)
The importance of developing the coordination chemistry of
new bidentate ligands and identifying better catalysts is
fundamental. Our groups initiated a combined experimental and
theoretical project to provide an in-depth description and
(1) which enables the synthesis of
a wide range of
1
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propargylamines, via the A3 coupling reaction, at room
temperature, in the absence of additives.
To make the protocol more user-friendly, reactions were
carried out in open air and room temperature. Reactions in
various solvents (Table 1) afforded the anticipated product in
good to moderate yields, and therefore DCM, a non-coordinating
solvent, was chosen from this screening process. Variation of
catalyst loading determines the optimum condition when a 1.5%
loading is applied (Table 2, Entries 1-5), while reactions in less
time cause a significant drop in the yield of 5aaa. (Table 2, Entries
4,6,7) The reaction with copper salts provided similar yields;
however, this was only achieved with higher catalyst loadings
(10%), almost one order of magnitude more loading, which is in
line with previous results (Table 2, Entry 8).[25] Given that the
activation of alkynes is highly dependent on concentration,[26] the
next step was to identify the limits of this catalytic system. Thus,
we performed a reaction in higher concentration (1M) which
yielded 5aaa in moderate yields and identifying that the chosen
0.5M concentration is the optimum (Table 2, Entry 9). This
methodology applies to a variety of primary and secondary
amines, and its scope is extended affording twenty substrates, out
of which three are synthesized and characterized for the first time
(Table 3 and ESI for more details Fig S13-S45).
Results and Discussion
The ligand pyb (Fig S1) can be synthesized in one step,[10] and its
reaction with Cu(OTf)2 in a molar ratio 2 : 1, in CH3CN under
aerobic conditions yields [Cu(II)(OTf)2(pyb)2]2(CH3CN) (1) (85%).
We characterized compound 1 by single-crystal x-ray diffraction
(Fig 1), ESI-MS (Fig S2-4), UV-Vis (Fig S5), IR (Fig S6),
thermogravimetric (Fig S7) and elemental analysis and cyclic
voltammetry (Fig 2). Copper adopts a trans geometry and sits in
the centre of an octahedron; the equatorial positions are occupied
by two pyb ligands, while two OTf– anions hold the axial positions.
There is a significant relative difference between the Cu–N and
the Cu-O bonds, which is characteristic of the Jahn-Teller
distortion. Solution studies identify that 1 retains its structure in
solution. The UV-Vis spectrum in a solution of dichloromethane
shows a very broad (550-850 nm) peak with the maximum at
685nm, characteristic of a Cu(II) with Jahn Teller distortion (Fig
S5), while bond valence calculations are in line for a Cu(II)
oxidation state. Besides, the cyclic voltammogram of 1 shows a
reversible one-electron curve (Fig 2).
Table 2. Optimization of Reaction Conditions
Entry
Loading (mol %)
Time (h)
Yield (%)a,b
53c
1
2
3
4
5
6
7
8
9
5
3
2
1.5
1
1.5
1.5
10
1.5
24
24
24
24
24
12
6
74c
95c
93c
53c
52c
30c
24
24
99d
81e
Figure 1. The X-ray structure of compound 1. The CH3CN lattice molecules are
omitted for clarity.
aRelative yield calculated by 1H-NMR based on the remaining 2a and the
intermediate Schiff base derived from 2a and 3a, bReaction conditions:1 (x
mol%), 1.0 mmol aldehyde, 1.1 mmol amine, 1.2 mmol alkyne, molecular sieves
4Å, DCM and concentration 0.5 M based on aldehyde, c25oC and concentration
0.5 M. dReaction with 10 mol% Cu(OTf)2, e25oC and concentration 1 M.
Taking into account that copper salts are less efficient in the
A3 coupling reaction with primary amines, we chose
cyclohexanecarboxaldehyde, aniline and phenylacetylene as
model substrates to evaluate the title reaction in a molar ratio 1 :
1.1 : 1.2.
Table 1. Solvent screening.
Entry
Solvent
Yield
1
2
3
4
5
6
MeOH
EtOH
i-PrOH
CH3CN
DCM
Ethyl acetate
16
22
29
traces
93
47
aRelative yield calculated by 1H-NMR based on the remaining 2a and the
intermediate Schiff base derived from 2a and 3a; bReaction conditions: 1 (1.5
mol%), 1.0 mmol aldehyde, 1.1 mmol amine, 1.2 mmol alkyne, 2 mL solvent,
25°C, 24 hours, concentration 0.5 M based on aldehyde, in the presence of
molecular sieves 4Å.
Figure 2 The cyclic voltammogram of 1 in the presence of phenylacetylene.
Then we performed reactions at elevated temperatures or
microwave conditions as this has been noted in previous Cu(II)
2
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systems.[18–22] Indeed, under heating (Table 4, entry 1) and
microwave (Table 4, entry 2) conditions, the prototype reaction
that affords 5aaa is completed within one hour in excellent yields.
copper centre and the formation of a {Cu(II/I)(pyb)x(Ph-C≡C-H)y}
species.
Table 4. Various experiments to obtain mechanistic evidence.
Table 3. Scope of the reaction with aldehydes, amines and alkynes.
Entry
Time (h)
Atmosphere
Catalyst
Yield (%)a,b
1
2
3
4
5
6
7
8
1
1
1
2
2
2
2
2
Open air
Open air
Open air
N2/Ar
N2
Ar
Ar
Ar
1
1
1
1
1
1
6
95c
96d
96e
98c
65f
68f
Tracesg
Tracesh
Cu(OTf)2
aRelative yield calculated by 1H-NMR based on the remaining 2a and the
intermediate Schiff base of 2a and 3a, bReaction conditions: 1 (1.5 mol%), 1.0
mmol cyclohexanecarboxaldehyde, 1.1 mmol aniline, 1.2 mmol
phenylacetylene, molecular sieves 4Å, solvent DCM 2 mL, concentration 0.5 M,
f
c80oC, dMicrowave conditions 80°C. e(TEMPO, 10mol%), room temperature.
gLoading 2 mol%, room temperature hLoading 1 mol%, room temperature
An attempt to rationalize how the A3 coupling reaction is
promoted by 1, we decided to examine the catalytic efficacy of
compound [Cu(OTf)2(bpy)2] (6).[9,29] (Scheme 2).
Scheme 2. A chemical diagram of 1 and 6, showcasing the trans and cis
geometries.
Isolated yields. Reaction conditions:1.0 mmol aldehyde, 1.1 mmol amine, 1.2
mmol alkyne, molecular sieves (4Å), solvent DCM, concentration 0.5M based
on aldehyde, catalyst loading 1 (1.5 mol%).stirring in open-air at 25°C for 24
hours.
Compound 6 has a cis-octahedral geometry. The catalytic
reactions at room temperature under Ar atmosphere with 6 (2%
loading, Table 4, entry 7) failed to provide the expected product,
and no precipitate or bpy moiety was found in the solution (crude
1H-NMR). Besides, despite compound 6 showing a reversible CV
signal,[9] a titration CV study with phenylacetylene identified a non-
reversible signal (Figure 3). This can be attributed to the fact that
Based on evidence from our recent studies,[27] we
performed a series of reactions to gain mechanistic insights. A
reaction for the synthesis of 5cca, in the presence of a radical trap
(TEMPO, 10mol% based on aldehyde, Table 4, Entry 3), afforded
the desired product (Figure S42), thus excluding the formation of
a radical species. The reactions at elevated temperature, under
N2 or Ar atmosphere, yielded product 5aaa in a similar yield (Table
4, Entry 4), excluding the activation of the catalyst via bonding to
dioxygen.[28] Finally, we performed a reaction under N2 at room
temperature for only two hours, and 5aaa was obtained in a
moderate yield (Table 4, Entry 5). The same reaction was
repeated under Ar atmosphere (Table 4, Entry 6), providing the
anticipated product in a similar yield. This unprecedented result
provides a time-efficient synthetic protocol for the synthesis of
propargylamines, and we will explore its full potential in future
studies. However, to shed light into this very exciting result, we
performed among others cyclic voltammetry titrations, under N2
atmosphere, of 1 in the presence of phenylacetylene (Figure 3).
This study confirms a structural change into the catalytic system
when one equivalent of phenylacetylene is added and could be
explained based on ligand dissociation upon reduction of the
complex
6
is stable and forms
a
very stable
{Cu(II)(bpy)x(HC≡CPh)y}+ species which prevents alkyne
activation. Given the short reaction time (2h), these results
indicate that the stereochemistry of the pre-catalyst is a parameter
that should be taken into account, during the catalytic process,
which is in line with literature evidence.[30,31] Given that even 1
mol% of the catalyst 1 still performs well while the other Cu(II)
salts have been reported to perform poorly at this level (Table 4,
entry 8),[25] this discards a possible full ligand dissociation in 1 but
favours the formation of a stable Cu(I)-complex which possibly is
the active catalytic species. This suggestion can be further
evidenced by the presence of pure ligand peaks in the crude NMR
and LCMS data (Figures S44 and S45). Efforts to trap, monitor,
isolate and characterize this Cu(I)-complex were unsuccessful.
Besides, the absence of bulky groups in the pyridine or
benzotriazole moieties, that would contribute steric effects and
3
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possibly prevent two different ligands from coordinating to the
Cu(I) centre, prevented us from isolating and characterizing this
species in reactions with Cu(I) salts; this possibility will be
explored in future studies.
Scheme 3. A proposed mechanism, highlighting the two interlinked paths,
dashed and blue lines. The blue/dashed arrows present the shared part of these
two paths.
Additionally, we wanted to investigate, by carrying out
theoretical calculations, some aspects of the chemistry of the
compounds and look at the possible catalytic driving forces that
support the proposed experimentbased mechanism. At the
OLYP/ZORA/TZP level of theory in the gas-phase, the structure
of 1 presents a non-planar geometry concerning the pyb ligands
(Fig. S9), showcasing conformational lability through rotation
around the central C–N bond; a feature not offered by bpy in 6.
Such flexibility should thus allow dispersion interactions with
neighbouring
atoms;
furthermore,
this
should
allow
conformational changes to accommodate excess charge and/or
spin density during the activation process. Indeed, upon OTf–
abstraction, the two ligands show a slightly non-equivalent
deviation from planarity which points to one critical asset; these
neutral ligands can act as efficient charge sinks given the
extended  system and the ability to modulate conjugation
through the C-N rotation, thus affording to accommodate charge
density upon change of oxidation state or of any reductive
process. This can be depicted for instance by considering the
Figure 3 (upper) The cyclic voltammogram of 1 and 6. (lower) The cyclic
voltamogram of 1 and 6 in the presence of phenylacetylene
molecular
electrostatic
potential
on
the
calculated
[Cu(I)(OTf)(pyb)2]+ and [Cu(II)(OTf)(pyb)2] (Fig 4); the metal
centre is relatively unchanged whereas the ligands can get highly
charged with high accumulation on the unbound nitrogen.
Based on all these observations and bearing in mind that a)
Cu should simultaneously bind both nucleophile (phenylacetylide)
and electrophile (imine),[23] b) the benzotriazole entity can act as
an acid or a weak base[32,33] and possibly form radicals,[32] c) the
reaction proceeds in the absence of additives, and d) previous
[15–22]
mechanisms based on Cu(I) or Cu(II) sources
we propose
the catalytic mechanism shown in Scheme 3. This mechanism
has two but interlinked pathways. The external path (highlighted
in dashed lines) involves the following steps: one OTf–
dissociation, alkyne binding, a copper reduction via a Single
Electron Transfer (SET) process,[34,35] ligand dissociation, OTf–
dissociation and acetylide activation, reaction with the imine and
catalyst regeneration. The internal pathway (highlighted in blue)
initiates when the {Cu(I)(OTf)(pyb)} species is formed in the first
path and discards the reduction-oxidation process of the Cu
centre but incorporates the triflate ion for the regeneration of the
catalyst and alkyne activation purposes. Taking into account that
the reduction of Cu(II) to Cu(I) by alkynes is a prolonged
procedure,[26] we consider the formation of the Cu(I)-intermediate
as the rate-determining step under ambient conditions (24h for
completion), while under inert atmosphere the catalytic cycle can
be achieved in almost 2 hours.
Figure 4. The molecular electrostaic potential (MEP) of [Cu(I)(OTf)(pyb)2]+ (left)
and [Cu(II)(OTf)(pyb)2] (right)
In terms of electronic structure, an inspection of molecular
orbitals of (MO) 1 (Fig. 5) shows the presence of doubly
degenerate levels which per se points to spontaneous symmetry
lowering with concomitant lowering in energy, hence providing a
potential towards reactivity. Moreover, the diagram indicates a
low-lying unoccupied beta (spin down) MO. Upon removal of one
OTf– (1a) for instance, the degenerate levels split (see Fig. S10),
but even in the lowered symmetry, the characteristics of this
unoccupied level is conserved. That is, given the unhindered
4
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quasi-planar geometry and the alignment of in-phase p(N)-d(Cu)-
p(N) as the main components spanning this unoccupied MO,
binding to electron-rich molecules should thus be facilitated (see
Fig. S10). Indeed, comparing the levels of 1 with those of the
reduced [Cu(II)(OTf)(pyb)2(HC≡CPh)]+ shows the expected
electronic structure, i.e. the highest unoccupied spin-down level
gets occupied, the degeneracies are lifted, and the intercalation
of levels centred on the ligands is conserved (see Fig. S11). This
latter observation further supports the idea of the active role
played by the ligands upon change of oxidation state. The stability
We report the first example of a Cu(II) complex built from pyb that
efficiently promotes the synthesis of propargylamines at room
temperature through the A3 coupling reaction. The Nbridging atom of
this hybrid ligand imposes exclusive trans-coordination at Cu and
allows ligand rotation, while the overall construct of the ligand, in
particular, the presence of the Npyridine atom modulates charge
distribution and flux, thus orchestrating structural and electronic
pre-catalyst control permitting alkyne binding with simultaneous
activation of the C–H bond through an in situ catalytic active
[Cu(I)(OTf)(pyb)] species. This notion is not feasible in the cis-
[Cu(II)(OTf)2(bpy)2] indicating that the stereochemistry of the pre-
catalyst and the nature of the N,N’-bidentate ligand are
parameters to be taken into account when designing such
catalysts. The results presented herein shall pave the way for
future discoveries and explorations in Coordination Chemistry, for
bidentate ligands, and Catalysis, for reactions requiring substrate
activation.
gained
upon
reduction
is
enormous,
the
[Cu(II)(OTf)(pyb)2(HC≡CPh)]+ to [Cu(I)(OTf)(pyb)2(HC≡CPh)]
transformation is associated to a free Gibbs energy amounting to
135 kcal/mol in favour of the Cu(I) species. Such a large energy
barrier is mainly due to the structural rearrangement of the
ligands. Incidentally, upon removal of the alkyne, the energy
difference in terms of full electronic energy is increased by 6
kcal/mol, hinting to a SET mechanism. Detailed calculations
regarding the SET mechanism are beyond the scope of the
current manuscript and will be communicated in the future.
Finally, it can be noticed that in the [Cu(II)(OTf)(pyb)2(HC≡CPh)]+
the OTf– is not displaced and the metal centre adopts a see-saw
geometry; on the other hand, in the [Cu(I)(OTf)(pyb)2(HC≡CPh)]
intermediate, the OTf– is displaced, leaving a tetrahedral
[Cu(I)(pyb)2]+, the lost of the OTf– is associated with an increase
in charge is aided by an additional stabilization induced by the two
flanking ligands through dispersive interaction as suggested by
the geometry. These observations highly suggest that the
reaction proceeds through alkyne capture and hydrogen
abstraction proceed uniquely with Cu(I) and is not feasible with
Cu(II). We assessed the gas-phase viability of some of the
species in the mechanism (see Fig. S12), and given the strong
influence of the environment, a full mechanism study without
explicit solvent molecules would be merely conjectural.
Experimental Section
Synthesis and characterization of 1-(2-Pyridyl)benzotriazole
The ligand 1-(2-Pyridyl)benzotriazole (pyb) was synthesized by
two different methods (i) and (ii), appropriate for synthesis in large
and small scale respectively: Method (i): Reflux under N2 using a
round-bottomed flask equipped with a magnetic stirbar. The round
bottom flask was charged with 3.15 g (20.0 mmol) of 2-
bromopyridine, 12 g (86.2 mmol) K2CO3, 0.5 g (3 mmol) KI and
4.76g (40.0 mmol) benzotriazole in 20 mL of acetone. The flask
was heated at reflux for 8 hours. The reaction mixture was
dissolved in ethyl acetate (250mL), washed with cold 10% KOH
(2 x 150 mL), dried MgSO4 and evaporated to afford the pure
product as a white solid which was recrystallized from H2O/EtOH.
Yield = 4.65 g (79%) based on 2-bromopyridine.
Method (ii): A 5 ml sealed vessel equipped with magnetic stir bar
was charged with 0.32 g (2.0 mmol) 2-bromopyridine and 0.48 g
(4.0 mmol) benzotriazole and was exposed to microwave
irradiation at 160˚C for 3 hours, under solvent-free, non-inert
conditions, according to previously reported protocol.[36] After
reaction completion, the cooled to ambient temperature mixture
was, diluted in 1 mL DCM and purified by flash column
chromatography. The product is isolated through silica gel using
a diethyl ether/petroleum ether (fraction 40-60) mixture as the
gradient eluent (01:99-30:70 v/v). The pyridyl-benzotriazole was
obtained as a white solid. Isolated yield = 5.06g (86%) based on
2-bromopyridine. 1H NMR (600 MHz, Chloroform-d) δ 8.66 (dt, J
= 8.4, 1.0 Hz, 1H), 8.64 – 8.60 (m, 1H), 8.31 (dt, J = 8.4, 0.9 Hz,
1H), 8.12 (dt, J = 8.4, 0.9 Hz, 1H), 7.94 (ddd, J = 8.4, 7.4, 1.8 Hz,
1H), 7.61 (ddd, J = 8.1, 6.9, 1.0 Hz, 1H), 7.46 (ddd, J = 8.0, 6.9,
1.0 Hz, 1H), 7.33 (ddd, J = 7.4, 4.8, 0.9 Hz, 1H), 1.54 (d, J = 0.6
Hz, 2H). MS (LCMS) m/z: ([M + H]+) calcd for C11H9N4, 197.2;
found, 197.1, Rt = 2.64 min.
Synthesis and characterization of 1.
[Cu(II)L2(OTf)2]·2(CH3CN) (1) was synthesized by the following
procedure: L (0.2 mmol, 39 mg) and Cu(OTf)2 (0.1 mmol, 36 mg)
were added in acetonitrile (10ml), using a round-bottomed flask
equipped with a magnetic stirbar. The resulting mixture was
stirred for 1 hour. The solution mixture was filtered, and the green
filtrate was then collected and underwent slow evaporation,
forming green block-shaped crystals after four days. Yield =
75mg, 85% based on CuII. Elemental analysis for
Figure 5. Molecular orbital levels of 1.
Conclusion
5
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[11] Z. Xu, D. S. Wang, X. Yu, Y. Yang, D. Wang, Adv. Synth. Catal. 2017,
359, 3332–3340.
CuC28H28F6N10O6S2: C 39.93, H 3.35, N 16.63; found C 40.01, H
3.31, N 16.66. ESI-FTMS of (1) in methanol m/z: ([M]+) calcd for
[CuC23H16N8F3O3S]+, 604.0314; found, 604.0338.
[12] R. Huang, Y. Yang, D. S. Wang, L. Zhang, D. Wang, Org. Chem. Front.
2018, 5, 203–209.
X-Ray crystallography
[13] Q. Wu, L. Pan, G. Du, C. Zhang, D. Wang, Org. Chem. Front. 2018, 5,
2668–2675.
The precipitate of 1 was zcrystallized by slow evaporation in
CH3CN solution. Data for 1 were obtained at the University of
Sussex by use of an Agilent Xcalibur Eos Gemini Ultra
diffractometer with CCD plate detector under a flow of nitrogen
gas at 173(2) K using Cu Kα radiation (λ = 1.54184Å). CRYSALIS
CCD and RED software was used respectively for data collection
and processing. Reflection intensities were corrected for
absorption by the multi-scan method. The crystal structure was
then refined on Fo2 by full-matrix least-squares refinements using
SHELXL.[37] Geometric/crystallographic calculations were
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1
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6
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10.1002/chem.202004781
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[Cu(II)(OTf)2(pyb)2]2(CH3CN) (1), pyb is 1-(2-pyridyl)benzotriazole, enables the synthesis of a wide range of propargylamines, via the
A3 coupling reaction, at room temperature and absence of additives. This hybrid N,N’-bidentate ligand imposes exclusive trans-
coordination at Cu, can rotate and can modulate charge distribution and flux, thus orchestrating structural and electronic pre-catalyst
control permitting activation of the C–H bond.
7
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