Cu(I) mediated hydrogen borrowing strategy for the α-alkylation of aryl ketones with aryl alcohols

Article abstract of DOI:10.1007/s00706-021-02735-5

Abstract: New triazolium Schiff bases (TSBs) were synthesised via a simple and high throughput process. The new salts were successfully characterised. When reacted with Cu(CH₃CN)₄PF₆, the TSB salts formed mononuclear triazole Schiff base copper(I) complexes and dinuclear complexes that were also characterised. The?copper complexes were generated in situ (mixtures of TSB salts with Cu(CH₃CN)₄PF₆) and applied as homogeneous catalysts for the C–C coupling of a variety of aryl ketones with aryl alcohols, from which?significant reactivity was observed. Reaction conditions were optimised, and the results indicate that the catalyst systems are very robust. A catalyst concentration of 10?mol% efficiently and selectively catalysed the α-alkylation of methyl phenyl ketone and its derivatives to afford up to 94% yield of 1,3-diphenylpropan-1-one and its analogues. The process is adaptable with analogues of acetophenone and benzyl alcohol bearing various regulating substituents tolerated. Graphic abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s00706-021-02735-5

Monatshefte für Chemie - Chemical Monthly (2021) 152:275–285
https://doi.org/10.1007/s00706-021-02735-5
ORIGINAL PAPER
Cu(I) mediated hydrogen borrowing strategy for the α‑alkylation
of aryl ketones with aryl alcohols
Nasir S. Lawal1 · Halliru Ibrahim1 · Muhammad D. Bala1
Received: 15 October 2020 / Accepted: 9 January 2021 / Published online: 2 February 2021
© Springer-Verlag GmbH Austria, part of Springer Nature 2021
Abstract
New triazolium Schif bases (TSBs) were synthesised via a simple and high throughput process. The new salts were success-
fully characterised. When reacted with Cu(CH3CN)4PF6, the TSB salts formed mononuclear triazole Schif base copper(I)
complexes and dinuclear complexes that were also characterised. The copper complexes were generated in situ (mixtures
of TSB salts with Cu(CH3CN)4PF6) and applied as homogeneous catalysts for the C–C coupling of a variety of aryl ketones
with aryl alcohols, from which signifcant reactivity was observed. Reaction conditions were optimised, and the results
indicate that the catalyst systems are very robust. A catalyst concentration of 10 mol% efciently and selectively catalysed
the α-alkylation of methyl phenyl ketone and its derivatives to aford up to 94% yield of 1,3-diphenylpropan-1-one and its
analogues. The process is adaptable with analogues of acetophenone and benzyl alcohol bearing various regulating sub-
stituents tolerated.
Graphic abstract
Keywords C–C coupling · α-Alkylation · Copper complex · Schif base · 1,2,3-Triazolium salt
reactions [1]. Recently, homogeneous catalysis-driven C–C
coupling via the α-alkylation of ketones with alcohols, has
become an attractive and environmentally sustainable way
of achieving saturated C–C bond formation in organic syn-
thesis. The metal-centred catalyst activated coupling reac-
tion is mechanistically characterized by hydride abstraction
(from the alcohol) and donation to the derived chalcone [2],
a process that is widely considered unique due to its excel-
lent atom efciency and greenness with H2O as the only
by-product.
Introduction
Carbon–carbon (C–C) bond formation is a very important
transformation in organic synthesis; as it provides a viable
route to the functionalization, chain extension, and even
dipole inversion of organic molecules. Various routes are
available for achieving the C–C coupling reaction. However,
traditional reaction methods of C–C bond coupling sufer
from the generation of unfavourable organic waste from side
The α-alkylation of ketones with alcohols has tradition-
ally been conducted with the aid of catalyst systems based
on expensive heavy metals such as iridium [3–5], ruthenium
[6–11], or rhodium [12–15]. And, only a handful of catalytic
systems based on environmentally benign earth-abundant
metal complexes have been reported to date. These include
*
Muhammad D. Bala
bala@ukzn.ac.za
1
School of Chemistry and Physics, University of KwaZulu-
Natal, Westville Campus, Private Bag X54001,
Durban 4000, South Africa
Vol.:(0123456789)
1 3

276
N. S. Lawal et al.
homogeneous systems based on Fe [16, 17], Mn [18], and
Co [19]. Also, some heterogeneous catalysts such as CuHT
(HT=hydrotalcites) [20], NiNP (NP=nanoparticles) [21],
and Ag/Mo oxides [22] have also been reported for C–C
coupling reactions via the α-alkylation of ketones with alco-
hols. Due to a shortage of catalyst systems that are both
selective and high yielding for this important chemical trans-
formation, a recent promising paper [23] that reported the
β-alkylation of secondary alcohols with primary alcohols
catalysed by homogeneous Cu-NHT (NHT=N-heterocyclic
thiolates) complexes serve as an inspiration for this study.
We hypothesize that adapting a similar green process; the
C–C coupling of ketones with alcohols with improvements
in product selectivity could be achieved with appropriately
designed ligands and complexes as catalysts. In this respect,
we herein report the synthesis of new triazolium Schif bases
(TSBs), and for the frst time, the application of their in situ
generated Cu(I)-TSB complexes as catalysts for the selective
C–C coupling of ketones with alcohols.
All the copper complexes are soluble in acetonitrile, THF,
methanol, and DMSO. They gradually decomposed on
exposure to air and moisture. Their gradual decomposi-
tion upon exposure to air made it practically challenging
to obtain full spectroscopic characterization data, which
is common for highly reactive and unstable Cu(I) com-
plexes [31].
However, the NMR data yielded important evidence
on the coordination environments of the ligands to the
copper metal centres. For instance, a comparative analy-
1
sis of the H NMR data of ligand 6 (Fig. 1, top) and its
corresponding copper complex 11 (Fig. 1, bottom) shows
that only the imine nitrogen (Cu–N) was coordinated to
the copper in 11. This is evidenced by the significant far
downfield shift of the imine proton (a, Fig. 1, from 8.3
to 10 ppm) while the triazolium C5-proton (b, Fig. 1)
remained virtually unaffected by the coordination at
8.9 ppm. There is also a corresponding downfield shift
of the imine carbon from 160 to 191 ppm (Fig. 2). Litera-
ture reports have observed similar trend when Schiff base
ligands coordinate to metal centres, which results in the
deshielding of the coordinated imine (HC=N) protons in
the 1H NMR data as we earlier noted [32–36].
Results and discussion
Synthesis and characterization
The FT-IR spectra of the salts showed characteristic
Schif base imine C=N stretches at 1642–1692 cm−1. This
was observed to slightly shift with coordination of the
imine to Cu in all the complexes. The shifts in frequencies
to higher wavenumbers point to coordination to the metal
centres, as shown in Scheme 1 for complexes 7–11. Simi-
larly, broad absorption bands around 3200 cm−1 in the FT-IR
spectra of the Cu-TSB complexes are due to absorbed mois-
ture, an inevitable characteristic of the highly hygroscopic
compounds. The complexes remain stable in dry acetoni-
trile solution for a few days, but dried samples gradually
decomposed upon storage. This also made it difcult to
obtain acceptable CHN analyses data or suitable crystals
for single-crystal X-ray difraction despite several attempts
[31]. However, based on the spectroscopic evidence avail-
able, 2-coordinate linear geometries are proposed for all
the complexes in the solid-state (Scheme 1). Essentially,
in all the complexes, the ligands coordinated to the copper
The general synthetic procedure to the title triazole Schif
base compounds is presented in Scheme 1, of which details
on compounds 1, 2, and 6 have been reported elsewhere
[24, 25]. The properties shown by all the new compounds;
3–5 are similar to that reported for 2 and consistent with
previous reports [26–28]. Formation of each TSB was con-
frmed by the disappearance of the aldehyde singlet pro-
ton at 10 ppm and the appearance of distinct singlet signals
between 8.30–8.89 ppm and 8.25–8.37 ppm in their proton
NMR spectra assigned to the C-5 triazolium and imine pro-
tons, respectively. NMR spectra of all the salts gave aromatic
and aliphatic signals within the expected regions [29]. In the
13C NMR spectra, the characteristic imine carbon resonance
appeared as expected downfeld at 160–162 ppm. Further
characterization of the TSB salts was achieved with IR and
MS analysis. Attempts to obtain microanalytical data were
unsuccessful due to varying moisture contents, as they eas-
ily absorbed atmospheric moisture once exposed or during
handling [30].
metal via the N-donor atom(s) of the Schif base Cu–N(SB)
,
with no evidence of Cu–C(carbene) binding via the triazole
N-heterocycle.
The copper complexes were synthesized by the reac-
tion of each respective TSB salt with Cu(CH3CN)4PF6.
Formation of the copper complexes was confirmed by
proton NMR spectroscopy via the significant downfield
shifts of the imine proton signals that appear at circa
10 ppm in their 1H NMR spectra. This is accompanied by
corresponding far downfield shifts of the imine carbon
(CH=N) signals (circa 191 ppm) in their 13C NMR spec-
tra as compared to that of the triazolium salts (Figs. 1, 2).
C–C coupling of aryl ketones with benzyl alcohol
The homogenous C–C coupling of aryl ketones and pri-
mary aromatic alcohols has mostly been achieved by the
use of acetophenone and benzyl alcohol as starting materi-
als with Ir, Ru, or Rh metal complexes as catalysts under
basic conditions (usually KOH) in solvents such as diox-
ane or toluene at 80–120 °C for 4–24 h. One of the mildest
1 3

Cu(I) mediated hydrogen borrowing strategy for the α-alkylation of aryl ketones with aryl…
277
Scheme 1
conditions reported for this transformation was by Donohoe
and coworkers on a Rh complex used for the alkylation of
valerophenone at temperatures as low as 65 °C over a period
of 48 h [12]. However, in the case of earth-abundant metal
(Fe, Co, or Mn) complex based catalyst systems, the mild-
est conditions were 110–140 °C for not less than 22 h [16,
18, 19]. Hence, we set out in this project to develop new
Cu-based catalysts capable of operating under mild reac-
tion conditions. To achieve this objective, we began with
the choice of acetophenone and benzyl alcohol as model
substrates and used milder THF as a solvent to substitute
for the higher boiling and environmentally harsher dioxane
or toluene utilized by previous researchers [12, 16, 18, 19].
At the beginning of this catalytic process, it was necessary
to determine if any of the in situ generated metal complexes
obtained by mixing equimolar quantities of the ligand pre-
cursor salt 2 and Cu(CH3CN)4PF6 is any good as a catalyst
system and to compare its efciency to that of an isolated
and characterized metal complex bearing the same ligand.
Hence, the catalytic activity of complex 7 and its in situ gen-
erated equivalent were frst trialed on acetophenone (12a)
and benzyl alcohol (13a) (Table 1). It is interesting to note
1 3

278
N. S. Lawal et al.
Fig. 1 Comparative analysis of the 1H NMR spectra of the triazolium Schif base 6 (red), and the corresponding triazole imino Cu(I) complex 11
(blue)
Fig. 2 Comparative analysis of the 13C NMR spectra of compounds 6 (red) and 11 (blue)
that both the in situ generated catalyst and the isolated com-
plex 7 (Table 1, entries 1, 2) gave quite similar activities
(45% vs. 43%) and also both methods exclusively yielded
the desired 1,3-diphenylpropan-1-one product 14a with no
evidence of any base promoted chalcone formation. Hence-
forth, in keeping with the overall objective of simplifying
the catalytic process, for the remainder of this study only
in situ generated catalysts were utilized. It is important to
emphasize that in situ generated species have been widely
employed in homogeneous catalysis as viable alternatives
to isolated molecular species. This is due to the advantages
they ofer that include their ease of preparation, potential
stability in the catalytic medium (this is especially impor-
tant for unstable complexes such as those in this study) and
cost-efectiveness through the elimination of often laborious
and time-consuming catalyst synthesis and isolation proce-
dures. Specifcally, in situ generated Cu catalyst systems
have been reported for use in many important homogeneous
1 3

Cu(I) mediated hydrogen borrowing strategy for the α-alkylation of aryl ketones with aryl…
279
Table 1 Optimization of
reaction conditions
Entry
1b
Catalyst
Base
Solvent
THF
Isolated yield /%a
KOH
43
45
39
7
2
3
2:Cu
2:Cu
KOH
THF
THF
KOtBu
4
5
6
2:Cu
2:Cu
2:Cu
K3PO4
K2CO3
Et3N
THF
THF
THF
21
-
-
7
Cu(NCCH3)4PF6
KOH
THF
THF
THF
THF
THF
THF
THF
31
33
24c
56
72
94
64
48
64
87
66
72
79
8
KOH
2
9
None
KOH
10
11
12
13
14
15d
16
17
18
19
2:Cu (4 mol%)
2:Cu (8 mol%)
2:Cu (10 mol%)
2:Cu
KOH
KOH
KOH
KOH (1 mmol)
2:Cu
KOH (0.5 mmol) THF
2:Cu
KOH
KOH
KOH
KOH
KOH
Neat
THF
THF
THF
THF
3:Cu
4:Cu
5:Cu
6:Cu
In all entries except otherwise mentioned: 12a (1.0 mmol), 13a (1.5 mmol), catalyst (2 mol% in situ gen-
erated from mixing equimolar quantities of 2–6 and Cu(CH3CN)4PF6), base (KOH, 2.0 mmol), 3 cm3
solvent, 75 °C
aAverage of two runs
bIsolated copper complex
cNo added catalyst; the yield is 100% chalcone
dBenzyl alcohol as the solvent
transformations that include for the transfer hydrogenation
base activated coupling reactions and Cu catalyzed Ullman
reactions [29, 39]. Investigation of the infuence of the base
on the catalytic process (entries 2–6) indicates that more
exotic and organic bases are less efective than KOH with
lowered (KOtBu to K3PO4) or zero isolated product yields
(K2CO3 and trimethylamine). Obviously, in the absence of
a catalyst system (i.e. absence of both the Cu(I) salt and
of unsaturated substrates [37], coupling reactions [38], and
cyclohexane oxidation reactions [31].
The choice of KOH as the base for the optimization tri-
als is because it is readily available, cheap and is commonly
used in reactions that require a simple basic medium. It is
also readily used as an auxiliary in transfer hydrogenation,
1 3

280
N. S. Lawal et al.
ligand precursor salt), a base promoted chalcone formation
was observed (24% isolated yield of chalcone). On the other
hand, a signifcant drop in isolated yield of the desired prod-
uct was observed when either only the metal precursor salt
Cu(NCCH3)4PF6 or the ligand precursor salt 2 was used in
isolation (entries 7 and 8, respectively).
structural backbones were used to evaluate the overall ef-
ciency and generality of the in situ catalysis based on 7. The
obtained results show that the system is highly efcient for
catalyzing the coupling of various substituted aryl ketones
with benzyl alcohols to achieve the required α-alkylated
ketone products in moderate to excellent isolated yields.
For instance, electron-rich acetophenone derivatives, such as
4-methoxy-, 4-nitro-, 4-methyl-, and 2-methylacetophenone
reacted smoothly with benzyl alcohol and 4-chlorobenzyl
alcohol to form a series of α-alkylated ketones with good to
excellent isolated yields (66–86%). Similarly, 4-aminoace-
tophenone was tolerated due to the high selectivity of the
catalyst system, since there is the possibility of the amino
group reacting with the aldehyde group formed in situ to
give a Schif base product, but that was not observed. How-
ever, an exception was recorded for 4-chlorobenzyl alcohol,
as the chalcone product was obtained in an excellent yield
(92%; Table 2, 14l). Besides, electron-defcient acetophe-
nones were also alkylated to their corresponding desired
products with good isolated yields. Also, 2-Phenylaceto-
phenone was not tolerated due to steric hindrance. Cyclic
and alkyl ketones were however not tolerated. These obser-
vations were attributed to ring strain in the former and aro-
matic stability in the latter cases [40, 41]. Simple alkanols
like ethanol or octanol as well as other primary and ter-
tiary alcohols were not tolerated by the catalyst system,
as all attempts led to the isolation of only the substrates,
even with an extended reaction time of 3 days. This was
not exactly unexpected as cyclic and aliphatic alcohols have
been reported to portray poor reactivity toward α-alkylation
with ketones [15].
A gradual increase in catalyst concentration (entries
10–12) from 4 mol%, 8 mol% through to 10 mol% led to
steady improvements (56%, 72% and 94%, respectively)
in isolated yields of 14a. On the other hand, an attempt to
improve the efciency of the base only resulted in lower
isolated yields at reduced KOH concentrations (entries
13, 14). This observation is anticipated, as the base is the
auxiliary species in the catalyst system responsible for the
initial abstraction of a proton from the methyl ketone. The
formed enolate eventually condenses with the in situ gener-
ated aldehyde (from the benzyl alcohol) to yield the desired
product, hence the use of an excess of the base is standard
in these reactions [16, 19]. The result presented in entry 15,
is our attempt to utilize some of the reported advantages of
highly concentrated green reactions by eliminating the need
for a solvent. However, only a modest yield of the 14a was
recorded which helps to establish the positive infuence of
the THF in catalyst generation and ensuring its homogene-
ity with the substrates, leading to high conversions to the
desired product.
Further optimization trials using the same reaction con-
ditions but varying the ligand precursor salt 3–6 for the
generation of the in situ catalyst led to moderate to good
isolated yields of 14a (entries 16–19). Additional azolium
functionality on the ligand framework as in 4, led to the
lowest yield (66%, entry 17), which was in part due to its
relatively lower solubility in the solvent THF. Likewise, the
bifunctional 1,2,3-triazolium Schif base compounds 5 and
6 gave lower yields when compared to 2 (entries 18, 19)
which could be attributed to steric hindrance due to their
bulky multifunctional ligand frameworks. The main difer-
ence between ligand precursor salts 3–6 and the more active
precursor 2 is the additional potentially N-chelating pyridyl
moiety on the imino Schif base backbone of 2, which played
a role in stabilizing the metal centre as a hemilabile N-donor.
Thus, the optimized reaction conditions were found to be
refuxing a mixture of in situ generated copper complex 7
(10 mol% each of Cu(CH3CN)4PF6 and 2) as the catalyst,
the substrates (acetophenone, 1 mol equiv.; benzyl alcohol,
1.5 mol equiv.), and 2 equivalents of KOH (base, 2 mol
equiv.) in THF at refux for 24 h.
Proposed reaction mechanism
The hydrogen auto-transfer (or hydrogen borrowing) mecha-
nism is generally accepted as the most plausible path for the
α-alkylation of ketones with alcohols [2]. The activities of
the catalyst systems reported herein are rationalized based
on the steps presented in Scheme 2. Thus, as an example,
benzyl alcohol (A) is frst dehydrogenated to benzaldehyde
(B) by the Cu(I) catalyst pre-formed in situ to form the pro-
tonated Cu(II)–H intermediate. Subsequent Aldol condensa-
tion with the methyl ketone promoted by the base produces
the α,β-unsaturated ketone (C), which is then hydrogenated
to the desired α-alkylated ketone (D) by the Cu(II)–H inter-
mediate hence completing the cycle. Along with the metal,
the imine N-coordinated TSB ligand serves to provide a
stable metal–ligand cooperative environment involving the
Cu(II)–H intermediate that provides the needed steric and
electronic control of product yield and selectivity as pro-
posed by Tan et al. [23]. This is justifed by the scope and
selectivity of substrates presented herein.
Adopting the optimum reaction conditions presented
above, the scope and limitations of the catalyzed α-alkylation
reaction were further studied with the results presented in
Table 2.
A variety of substituted ketone and alcohol substrates
bearing electronic and steric regulating groups on their
1 3

Cu(I) mediated hydrogen borrowing strategy for the α-alkylation of aryl ketones with aryl…
281
Table 2 α-Alkylation of
acetophenone derivatives with
various alcohols
Product
14a
14b
14c
R1
H
R2
H
Yield /%
94
77
79
76
66
85
88
82
86
73
79
4-F
H
4-Br
4-I
H
H
14d
14e
4-CH3O
4-NH2
4-CH3
2-CH3
H
H
H
14f
H
14g
14h
14i
H
4-Cl
4-Cl
H
4-F
14j
4-NO2
14k
92
14l
4-CH3
4-CH3O
4-NO2
4-Cl
4-Cl
4-Cl
77
65
82
14m
14n
14o
In all entries except otherwise noted, reaction conditions: 12 (1.0 mmol), 13 (1.5 mmol), catalyst
(10 mol% in situ generated copper complex: mixture of Cu(CH3CN)4PF6 with 2), base (KOH,
2.0 mmol), solvent (3 cm3 THF), refux, 24 h. Isolated yields of 14, average of two runs
Conclusion
Experimental
In this report, we have reported for the frst time, a new set
of triazolium Schif bases and fully characterized them by
spectroscopic techniques. Combinations of the salts with
a copper precursor salt [Cu(CH3CN)4PF6] were tested as
in situ catalysts for the α-alkylation of a series of func-
tionalized aryl ketones with aryl alcohols. The catalyst
systems proved to be simple to prepare, mild and cost-
efective, with high isolated yields and selectivity to the
1,3-diphenylpropan-1-one analogue products.
All substrates and reactants were purchased from Sigma-
Aldrich and used as received. All the solvents (acetoni-
trile, tetrahydrofuran, methanol, and diethyl ether) were
purchased from Merck and purifed using a commercially
available MBraun MB-SP Series solvent purifcation system
equipped with activated alumina columns. The methods of
Lal and Diez-Gonzalez [42] was adopted in the prepara-
tion of the triazole synthon 4-(2-formylphenyl)-3-methyl-
1-propyl-1H-1,2,3-triazol-3-ium iodide (1, Scheme 1).
1 3

282
Scheme 2
N. S. Lawal et al.
(3, C21H25F6N4P) The procedure is similar to that reported
for 2 [24, 25]. The quantities used are: 0.357 g 1 (1 mmol)
and 0.242 g 2-phenylethanamine (2 mmol). The salt was
1
obtained as a yellow viscous oil. Yield 0.41 g (86%); H
NMR (CD3CN, 400 MHz): δ = 1.05 (3H, t, J = 7.39 Hz),
2.05 (2H, m, J = 7.25 Hz), 2.80 (2H, t, J = 7.11 Hz), 3.73
(2H, t, J=3.55 Hz), 3.79 (3H, s), 4.56 (2H, t, J=7.07 Hz),
7.17 (2H, d, J = 7.29 Hz), 7.23 (1H, t, J = 7.31 Hz), 7.31
(2H, t, J = 7.34 Hz), 7.50 (1H, d, J = 7.54 Hz), 7.69 (1H,
t, J = 3.25 Hz), 7.79 (1H, t, J = 3.24 Hz), 7.85 (1H, d,
J=7.67 Hz), 8.27 (1H, s, CH=N, imine), 8.30 (1H, s, tria-
zolium) ppm; 13C NMR (CD3CN, 100.6 MHz): δ=10.42,
23.24, 37.07, 38.14, 55.84, 62.52, 120.65, 126.67, 128.92,
129.01, 129.27, 131.17, 132.16, 132.16, 132.67, 132.88,
136.12 (C5-triazolium), 140.40, 143.21, 160.52 (CH=N,
imine) ppm; IR (ATR): v=2972 (C–H, sp2), 1692 (C=N),
D
A
C
B
1587 (C=C), 1137 (N=N), 832 (PF−), 771 (Ar C–H) cm−1;
6
HRMS: m/z calcd for C21H25N4+ 333.2079, found 333.2088.
Also, the triazolium Schiff bases (E)-3-methyl-1-pro-
pyl-4-[2-[[[2-(pyridin-2-yl)ethyl]imino]methyl]phenyl]-
1H-1,2,3-triazol-3-ium hexafluorophosphate(V) (2) and
4,4′-[[(1E)-[butane-1,4-diylbis(azanylylidene)]bis(methanylylidene)]-
bis(2,1-phenylene)]bis(3-methyl-1-propyl-1H-1,2,3-triazol-
3-ium) hexafuorophosphate(V) (6) were prepared following
our methods [24, 25]. Unless otherwise stated, all syntheses
were performed under a dinitrogen atmosphere using stand-
ard Schlenk line techniques. NMR spectra were recorded on
a Bruker Avance 400 MHz spectrometer operated at ambient
temperature with δ values reported in ppm and referenced
to Me4Si as the internal standard for both 1H and 13C NMR
data. Infrared spectra were recorded on a Perkin Elmer uni-
versal ATR Spectrum 100 FT-IR spectrophotometer. Mass
spectrometry and elemental analysis (where applicable)
were recorded on a Waters Micromass LCT Premier TOF
MS-ES+ and ThermoScientifc Flash2000 Elemental Ana-
lyzer, respectively.
(E)‑3‑Methyl‑4‑[2‑[[[2‑(3‑methyl‑1H‑imidazol‑3‑ium‑1‑yl)‑
ethyl]imino]methyl]phenyl]‑1‑propyl‑1H‑1,2,3‑triazol‑3‑ium
hexafuorophosphate(V) (4, C19H26F12N6P2) To a dry 20 cm3
Schlenk tube charged with excess anhydrous MgSO4
(0.602 g, 5 mmol) was added 0.357 g 1 (1 mmol), and
excess DCM solution of in situ liberated 2-aminoethylbro-
mide (0.496 g, 4 mmol). The mixture was then stirred at
room temperature for 12 h. At the end of the reaction time,
the contents of the tube were fltered under vacuum, and
the fltrate concentrated to give a crude TSB salt bearing
an ethyl bromide substituent on the N-atom. This was then
further refuxed with 0.164 g methylimidazole (2 mmol) in
acetonitrile for 12 h to aford an intermediate precursor to
4 bearing bromide counterion to the imidazolium moiety.
Subsequent anion metathesis with KPF6 to exchange the Br−
for PF6− followed by purifcation with dry ether (3×20 cm3)
gave the diazolium hexafourophosphate salt 4 as a yellow
1
oil. Yield 0.51 g (81%); H NMR (CD3CN, 400 MHz):
General procedure for the synthesis of the TSB salts
δ=1.06 (3H, t, J=7.29 Hz), 2.09 (2H, m, J=4.36 Hz), 3.73
(6H, s), 3.88 (2H, s), 3.94 (2H, s), 4.67 (2H, t, J=7.14 Hz),
7.40 (1H, s), 7.44 (1H, s), 7.56 (1H, d, J = 2.85 Hz), 7.73
(1H, t, J=3.29 Hz), 7.80 (1H, t, J=3.29 Hz), 8.02 (1H, d,
J=3.89 Hz), 8.37 (1H, s, CH=N, imine), 8.69 (1H, s, tria-
zolium), 8.89 (1H, s, imidazolium) ppm; 13C NMR (CD3CN,
100.6 MHz): δ=10.41, 23.16, 36.60, 38.52, 50.48, 56.04,
59.58, 121.73, 123.22, 124.05, 129.97, 131.02, 131.84,
132.44, 133.10, 134.91, 135.62 (C5-triazolium), 136.99
(C2-imidazolyl), 141.96, 162.92 (CH=N, imine) ppm; IR
(ATR): v = 2970 (C–H, sp2), 1643 (C=N), 1522 (C=C),
The Schif base-functionalized triazolium salts were syn-
thesized by the adaptation of published procedures [43].
Hence, to a 20 cm3 Schlenk tube equipped with a stirrer
bar and condenser was added methanol solution of 0.357 g
1 (1 mmol). The solution was then refuxed for 4 h with the
respective equivalents of either mono- (for salts 2, 3, and
4) or diamines (for salts 5 and 6) as shown in Scheme 1.
Aforded crude products were then allowed to cool to room
temperature and after washing with dry ethyl acetate and
diethyl ether (3×20 cm3), yielded the corresponding tria-
zolium Schif bases.
1168 (N=N), 832 (PF−), 751 (Ar C–H) cm−1; LRMS: m/z
6
calcd for C19H26N622+K 188.2211, found 188.1953.
(E)‑3‑Methyl‑4‑[2‑[(phenethylimino)methyl]phenyl]‑
1‑propyl‑1H‑1,2,3‑triazol‑3‑ium hexafluorophosphate(V)
1 3

Cu(I) mediated hydrogen borrowing strategy for the α-alkylation of aryl ketones with aryl…
283
4,4′‑[[(1E)‑[Ethane‑1,2‑diylbis(azanylylidene)]bis‑
(methany‑lylidene)]bis(2,1‑phenylene)]bis(3‑methyl‑1‑
propyl‑1H‑1,2,3‑triazol‑3‑ium) hexafluorophosphate(V)
(5, C28H36F12N8P2) The reaction of 2 mol equivalents of 1
(0.714 g, 2 mmol), with 1 mol equivalent of 1,2-diaminoeth-
ane (0.060 g, 1 mmol) in refuxing MeOH for 4 h yielded
crude 5. The resulting crude product was then concen-
trated and thoroughly purifed with dry ether (3×25 cm3).
Removal of all volatiles aforded the bis-triazolium hex-
afuorophosphate salt 5 as a yellow oil. Yield 0.72 g (93%);
1H NMR (CD3CN, 400 MHz): δ=1.02 (6H, t, J=7.42 Hz),
2.05 (4H, m, J=7.25 Hz), 3.59 (4H, s), 3.89 (6H, s), 4.58
(4H, t, J = 7.09 Hz), 7.55 (2H, d, J = 6.93 Hz), 7.71 (2H,
t, J = 3.25 Hz), 7.81 (2H, t, J = 3.28 Hz), 7.93 (2H, d,
J=7.71 Hz), 8.35 (1H, s, CH=N, imine), 8.54 (1H, s, tria-
zolium) ppm; 13C NMR (CD3CN, 100.6 MHz): δ=10.43,
23.26, 38.38, 55.83, 61.59, 120.88, 129.38, 131.36, 131.95,
132.71, 132.90, 135.98 (C5-triazolium), 142.91, 161.17
(CH=N, imine) ppm; IR (ATR): v=2981 (C–H, sp2), 1644
(C=N), 1607 (C=C), 1137 (N=N), 832 (PF−), 771 (Ar C–H)
6
cm−1; MS (ESI+): m/z (%)=333 (100, [Ligand]+).
(E)‑3‑Methyl‑4‑[2‑[(phenethylimino)methyl]phenyl]‑1‑
propyl‑1H‑1,2,3‑triazol‑3‑ium copper(I) hexafluoro‑
phosphate(V) ((Cu‑3)PF6, 8, C42H50CuF6N8P) The complex
was synthesized following a procedure similar to 7 with
the following quantities of materials: 0.479 g compound 3
(1 mmol) and 0.186 g Cu(CH3CN)4PF6 (0.5 mmol). Gummy
dark green liquid. Yield 0.253 g (58%); 1H NMR (CD3CN,
400 MHz): δ=1.04 (2×3H, t, J=7.42 Hz), 1.83 (2×2H, t,
J=3.19 Hz), 2.08 (2×2H, m, J=7.20 Hz), 3.67 (2×2H, t,
J=6.40 Hz), 3.95 (2×3H, s), 4.61 (2×2H, t, J=7.04 Hz),
7.62 (2×1H, d, J=3.90 Hz), 7.91 (2×1H, d, J=3.18 Hz),
7.93 (2×1H, d, J=3.28 Hz), 7.98 (2×4H, m, J=3.22 Hz),
8.20 (2 × 1H, d, J = 2.91 Hz), 8.36 (2 × 1H, s, triazole),
10.00 (2 × 1H, s, HC=N imine) ppm; 13C NMR (CD3CN,
100.6 MHz): δ = 9.38, 22.24, 24.91, 37.47, 55.14, 66.96,
120.65, 128.77, 132.22, 132.52, 133.96, 134.17, 134.57 (C5
triazole), 140.86, 191.87 (HC=N imine) ppm; IR (ATR):
v=3342 (O–H), 2931 (C–H, sp2), 1695 (C=N), 1603 (C=C),
(C=N), 1588 (C=C), 1174 (N=N), 852 (PF−), 771 (Ar C–H)
6
cm−1; HRMS: m/z calcd for C28H36F6N8P+ 629.2705, found
629.2687.
1169 (N=N), 822 (PF−), 767 (Ar C–H) cm−1; MS (ESI+):
6
m/z (%)=396 (0.5, [Cu+Ligand]+), 334 (24, [Ligand]+).
General synthetic procedure to the synthesis
of the copper complexes
(E)‑3‑Methyl‑4‑[2‑[[[2‑(3‑methyl‑1H‑imidazol‑3‑ium‑1‑yl)‑
ethyl]imino]methyl]phenyl]‑1‑propyl‑1H‑1,2,3‑triazol‑3‑ium
acetonitrilecopper(I) hexafuorophosphate(V) ((Cu‑4)PF6,
9, C21H30CuF6N7P) The complex was synthesized follow-
ing a procedure similar to 7 with the following quantities
of materials: 0.314 g compound 4 (0.5 mmol) and 0.186 g
Cu(CH3CN)4PF6 (0.5 mmol). Gummy light green liquid;
yield 0.194 g (66%); 1H NMR (CD3CN, 400 MHz): δ=1.05
(3H, s br), 2.08 (2H, s br), 2.59 (4H, s br), 3.88 (3H, s br),
3.95 (3H, s br), 4.62 (2H, s br), 7.45 (2H, s, br), 7.63 (1H,
s, br), 7.97 (2H, s br), 8.19 (1H, s, br), 8.37 (1H, s br, tri-
azole), 8.51 (1H, s br, imidazole), 9.99 (1H, s br, HC=N
imine) ppm; 13C NMR (CD3CN, 100.6 MHz): δ = 9.33,
22.16, 37.41, 55.10, 124.02, 128.72, 132.17, 132.46, 133.88,
134.12 (C5 triazole), 134.52 (C2 imidazole), 140.81, 191.82
(HC=N imine) ppm; IR (ATR): v = 3167 (O–H), 2971
(C–H, sp2), 1762 (C=N), 1716 (C=C), 1173 (N=N), 832
A typical method for the synthesis of the triazole Schif base
[Cu-2]PF6, complex 7, is described for the Cu(I) compounds.
(E)‑3‑Methyl‑1‑propyl‑4‑[2‑[(pyridin‑2‑ylimino)methyl]phenyl]‑
1H‑1,2,3‑triazol‑3‑ium copper(I) hexafuorophosphate(V)
((Cu‑2)PF6, 7, C40H48CuF6N10P) Into a 20 cm3 Schlenk tube
with 4 Å molecular sieves, 1 mmol of compound 2 (0.478 g)
and 0.5 equivalent of Cu(CH3CN)4PF6 (0.186 g, 0.5 mmol)
in 3 cm3 of dry acetonitrile solution were added and stirred at
room temperate for 6 h under nitrogen. The resulting mixture
was fltered through a short pad of Celite. The solvent was
then removed in vacuo and the mixture was washed with dry
ether (10 cm3 ×2). Removal of all volatiles yielded 0.354 g
(81%) of complex 7 as a gummy light green liquid. 1H NMR
(CD3CN, 400 MHz): δ=1.06 (2×3H, t, J=7.37 Hz), 2.25
(2 × 2H, m, J = 7.58 Hz), 3.20 (2H, t, J = 3.21 Hz), 3.43
(2H, s), 3.97 (2×3H, s), 4.61 (2×2H, t, J=7.02 Hz), 7.33
(2×3H, m, J=7.11 Hz), 7.41 (2×2H, d, J=7.45 Hz), 7.65
(2 × 1H, d, J = 7.36 Hz), 7.98 (2 × 2H, m, J = 2.82 Hz),
8.22 (2 × 1H, d, J = 7.19 Hz), 8.39 (2 × 1H, s, triazole),
10.02 (2 × 1H, s, HC=N imine) ppm; 13C NMR (CD3CN,
100.6 MHz): δ=10.32, 23.16, 33.28, 41.78, 56.10, 121.58,
1227.79, 129.44, 129.48, 129.73, 133.16, 133.47, 134.94,
135.11, 135.53 (C5 triazole), 141.82, 192.84 (HC=N imine)
ppm; IR (ATR): v = 3416 (O–H), 2972 (C–H, sp2), 1692
(PF−), 771 (Ar C–H) cm−1; MS (ESI+): m/z (%) = 483 (1,
6
[M+CH3CN+H]+), 172 (8, [Ligand+2H]2+).
4, 4′‑[[(1E)‑[Ethane ‑1, 2‑ diylbis(azanylylidene)]‑
bis‑(methanylylidene)]bis(2,1‑phenylene)]bis(3‑methyl‑1‑
propyl‑1H‑1,2,3‑triazol‑3‑ium) acetonitrilecopper(I)
h e x a f l u o r o p h o s p h a t e ( V ) ( ( C u 2 ‑ 5 ) 2 P F 6 , 1 0 ,
C32H42Cu2F12N10P2) The complex was synthesized follow-
ing a procedure similar to 7 with the following quantities
of materials: 0.387 g compound 5 (0.5 mmol) and 0.372 g
Cu(CH3CN)4PF6 (1 mmol). Light green solid; yield 0.191 g
1 3

284
N. S. Lawal et al.
(39%); m.p.: 112 °C; 1H NMR (CD3CN, 400 MHz): δ=1.05
(6H, t, J=7.42 Hz), 1.97 (4H, t, J=2.46 Hz), 2.08 (4H, q,
J = 7.24 Hz), 3.95 (6H, s), 4.62 (4H, t, J= 7.06 Hz), 7.63
(2H, d, J = 7.12 Hz), 7.94 (2H, t, J = 3.29 Hz), 7.98 (2H,
t, J=2.91 Hz), 8.20 (2H, d, J=2.93 Hz), 8.37 (2H, s, tria-
zole), 10.00 (2H, s, HC=N imine) ppm; 13C NMR (CD3CN,
100.6 MHz): δ=9.33, 22.21, 37.50, 55.12, 120.64, 128.73,
128.78, 132.21, 132.51, 133.94, 134.16, 134.55 (C5 tria-
zole), 140.84, 191.86 (HC = N imine) ppm; IR (ATR):
v=3351 (O–H), 2920 (C–H, sp2), 1614 (C=N), 1455 (C–H),
Supplementary Information The online version contains
supplementary material available at https://doi.org/10.1007/s0070
6-021-02735-5.
Acknowledgements We acknowledge fnancial support from the Uni-
versity of KwaZulu-Natal, ESKOM (TESP 2019) and the NRF. NSL
thanks Ahmadu Bello University for a paid study fellowship.
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