The sulfoxidation reaction catalyzed by CoBr2 complexes, under the magnifying glass of green parameters

Article abstract of DOI:10.1016/j.mcat.2018.05.004

Different cobalt (II) and nickel (II) species were used as catalysts in sulfoxidation reactions giving excellent yields and high chemoselectivity. Among the cobalt (II) species, both γ-cyclodextrin complexes synthesized in ethyl acetate or dichloromethane solvents were very good catalysts, while γCDNiBr₂ synthesized in ethyl acetate was the best of Ni (II) species. Sulfoxidation takes place with high chemoselectivity in the presence of other groups such as aldehyde. Good results were obtained when these reactions were analyzed using green chemistry metrics. Techniques as FT-IR, UV–vis Diffuse Reflectance, colorimetry, TGA, flame atomic absorption, potentiometric halide titration, and elemental analysis were used to characterize novel complexes. The change of solvent in the synthesis of complexes produces positive effects in holistic green metrics.

Full text of DOI:10.1016/j.mcat.2018.05.004

MolecularCatalysis454(2018)44–54
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
The sulfoxidation reaction catalyzed by CoBr2 complexes, under the
magnifying glass of green parameters
⁎
Diana C. Pinilla Peñaa,b, Laura I. Rossia,b,
a
Departamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Ciudad Universitaria, X5000HUA Córdoba, Argentina
Instituto de Investigaciones en Fisicoquímica de Córdoba (INFIQC) – CONICET, Ciudad Universitaria. X5000HUA Córdoba, Argentina
b
A R T I C L E I N F O
A B S T R A C T
Chemical compounds studied in this article:
γ-Cyclodextrin (γCD) (PubChem CID: 86575)
Cobalt (II) bromide [CoBr2] (PubChem CID:
24610)
Nickel (II) bromide (NiBr2) (PubChem CID:
278492)
iron (III) nitrate (Fe(NO3)3) (PubChem CID:
25251)
4-(methylthio)acetophenone, 1 (PubChem CID:
74501)
Different cobalt (II) and nickel (II) species were used as catalysts in sulfoxidation reactions giving excellent yields
and high chemoselectivity. Among the cobalt (II) species, both γ-cyclodextrin complexes synthesized in ethyl
acetate or dichloromethane solvents were very good catalysts, while γCDNiBr2 synthesized in ethyl acetate was
the best of Ni (II) species. Sulfoxidation takes place with high chemoselectivity in the presence of other groups
such as aldehyde. Good results were obtained when these reactions were analyzed using green chemistry me-
trics. Techniques as FT-IR, UV–vis Diffuse Reflectance, colorimetry, TGA, flame atomic absorption, potentio-
metric halide titration, and elemental analysis were used to characterize novel complexes. The change of solvent
in the synthesis of complexes produces positive effects in holistic green metrics.
4-(methylthio)benzaldehyde 2 (PubChem CID:
76985)
4-(methylthio)bromobenzene 3 (PubChem CID:
66037)
(methylthio)bencene 4 (PubChem CID: 7520)
Keywords:
Sulfoxidation
Catalyst
Cobalt complexes
Nickel complexes
Cyclodextrins
1. Introduction
containing drug. It is known that oxidation reactions account for only
4% of the reactions carried out in the pharmaceutical industry [14].
Organosulfur compounds, such as sulfoxides, are very important
synthetic intermediates in organic chemistry [1,2]. These compounds
are valuable in the preparation of pharmaceutically and biologically
relevant materials [3,4,5]. In pharmaceutical research and production,
sulfoxidation is one of the oxidation reactions most commonly found,
achieved with a large variety of reagents, catalysts and oxidants [6,7].
Sulfoxidation reaction is often difficult to carry out in organic synthesis
since other functional groups can be oxidized simultaneously [8].
Furthermore, it is important that catalysts or oxidizing conditions have
a high selectivity to yield sulfoxides, as these can undergo over-
oxidation to sulfones [9,10,11]. Chemoselective transformations are of
central importance [12], especially when several different functional
groups are present in a molecule as in esomeprazole [13], a sulfoxide-
However, one of the reasons why oxidation reactions are not used more
frequently used is due to the fact that the available procedures are not
clean or safe enough [5]. In 2006, ACS GCI pharmaceutical Roundtable
members identified oxidation/epoxidation reactions as an area re-
quiring focused research to advance the principles of green chemistry
and their application in the pharmaceutical industry [15].
In view of continuous interest in the development of synthetic
methods for the selective conversion of sulfides into sulfoxides
[16,17,18,19] and, in the general, in the oxidation of sulfides [20,21],
we are seeking the development of reaction methodologies to achieve
chemoselective sulfoxidation reactions.
From our studies, it was concluded that iron salts made up an ex-
cellent catalytic system for the oxidation of sulfides to sulfoxides
⁎
Corresponding author at: (INFIQC-CONICET) Departamento de Química Orgánica, Facultad de Ciencias Químicas, Universidad Nacional de Córdoba, Ciudad Universitaria,
X5000HUA Córdoba, Argentina.
E-mail addresses: dpinilla@fcq.unc.edu.ar (D.C. Pinilla Peña), lauraros@fcq.unc.edu.ar (L.I. Rossi).
https://doi.org/10.1016/j.mcat.2018.05.004
Received 15 March 2018; Received in revised form 3 May 2018; Accepted 4 May 2018
2468-8231/©2018ElsevierB.V.Allrightsreserved.

D.C. Pinilla Peña, L.I. Rossi
MolecularCatalysis454(2018)44–54
[22,23,24,25]. In addition, we have reported the synthesis, character-
ization and catalytic activity of several cyclodextrins-FeBr3 (CD-Fe)
complexes, these also having a very good catalytic activity in sulfox-
idation reactions [26,27,28].
2. Experimental
2.1. Metal complexes
Cobalt and nickel compounds [29,30] are very important in several
scientific areas, such as materials design [31,32,33], biological pro-
cesses [34,35,36], catalysis [37,38], and organic synthesis [39,40,41].
The structure elucidation of cobalt or nickel complexes [42] largely
benefits all these fields, since the mechanisms of the reactions where
they could be used are better understood when you have detailed
structural information.
2.1.1. Synthesis of metal complexes
Complexes were prepared as previously reported for FeBr3 [26]. For
example, it stirred for 30 min 1 mmol of Co2+ salt in 10 mL of di-
chloromethane or ethyl acetate (colouration could be seen in both
solvents); then solid cyclodextrin was added in 1:1 molar ratio. This
heterogeneous mixture was stirred for 24 h, later the solid was filtered,
washed with DCM or EtAc (3 × 5 mL of the same solvent) and dried
under air. During complex formation, colouration of the white CD
powder in contrast to organic solvent decolouration could be observed.
Of the total mass at the start used, it was recovered between 98% and
100%. When dichloromethane was used, decolouration was complete;
however, in the case of ethyl acetate, light colouration was found (it
corresponds approximately to < 1% initial amount of salt; quantifica-
tion was performed by UV–vis spectroscopy).
Besides, cyclodextrins (CDs) have been extensively used as hosts in
the formation of divers inclusion complexes with organic [43], orga-
nometallic [44,45,46] and inorganic guests [47,48,49]. CDs are cyclic
oligomers of D-glucose, bonded by α-1-4 linkages. They have a toroidal
structure similar to a truncated cone. Their interior behave as a hy-
drophobic cavity, while the 2, 3 and 6 hydroxyl groups of the glucose
units are located at the edges of the truncated cone, defining the hy-
drophilic zones of the oligomer [50]. The inclusion complexes have
been studied, in most cases, using the conventional techniques for so-
lution-state or X-Ray diffraction for crystalline solids [51,52]. The
metal-based CD complex subject was newly reviewed [53,54]. In the
literature, most complexes characterized were prepared in basic solu-
tion [55]; yet, there are a few synthesized in deionized water but not in
basic solution [56], and toluene was used in the synthesis of one
complex [57]. Specifically, natural γ-cyclodextrin (γ-CD) is the less
commonly used CD in industrial products and it is about 5% of global
CD production. Physicochemical properties are well known, natural γ-
CD being the biggest and most soluble of all CD natives [58].
In the previous paper, we have reported studies over transition
metal salt-CD complexes both their synthesis in different organic sol-
vents [26], as their application over sulfoxidation reactions as hetero-
geneous catalysts [28]. The main advantages of this synthesis metho-
dology are its simplicity, the agreement with the aims of the Green
Chemistry Principles, and quantitative yield [59]. Moreover, it should
be noted that these complexes can be stored without any special care
because they are very stable. It should be noted that the complex βCD/
FeBr3, prepared in 1/1 molar ratio and in dichloromethane (DCM) as
synthesis solvent, was a better catalyst, for the oxidation of methyl-
phenylsulfide, than complexes prepared with higher βCD/Fe molar
ratio in other solvents.
2.1.2. Stoichiometry determination
The combination of the data provided by the different analysis is
need to stoichiometry determination of the complex. We could de-
termine the percentages of C, H, Br, O, N and Co or Ni in the sample,
from the data resulting from elemental analysis, potentiometric halogen
titration and flame atomic absorption. The procedure is similar to that
reported in a previous paper [60], with a general formula equal to
A(C48H80O40) ·B (MBr2) ·D(H2O). Only two changes were introduced: D
values from 1 to 20 in 0.5 intervals were used and % Br will be de-
termined as follows (Eq. (1)):
B x 79.9
MW
%Br =
x 100
(1)
We compared the experimental data with the 195 theoretical per-
cent compositions obtained.
2.1.3. Elemental analysis
To realize the measurements, it was used a combustion-gas chro-
matography Perkin Elmer 2400 Series II CHNS. A standard cystine
sample was used to calibrate (Perkin Elmer, 99.99%). All solid com-
plexes were analyzed in triplicate.
2.1.4. Flame atomic absorption
These complexes cannot be characterized by X-Ray diffraction study
of monocrystals since they are amorphous powders. Nevertheless, our
group characterized CDCuX2 complexes of similar nature using many
techniques and analyzing obtained data in an integrated manner [60].
Green Chemistry was introduced with the aim of overcoming health
and environmental problems at the source by developing cleaner che-
mical processes for chemical industry through the design of innovative
and environmentally benign chemical reactions [61,62,63]. Diverse
green metrics have also been approached [64,65,66,67] and used in
Green evaluations over different reactions [68,69,70]. We have re-
ported green metric parameters in previous studies with very good re-
sults (see refs [25] and [56]).
To realize the measurements, it was used a Perkin Elmer Mod. 3110;
nickel complexes were analyzed with air-acetylene flame and cobalt
complexes were analyzed with nitrous oxide-acetylene flame. The
preparation of sample consists of dissolving about 10 mg of complex
with 2 mL of concentrated HCl and 2 mL of concentrated HNO3 for 1 h.
Later, the solution was completed to 5 mL with milliQ water. All sam-
ples were analyzed in triplicate.
2.1.5. Potentiometric halide titration
Potentiometric titration of halide ions was performed with AgNO3
on a pHmeter (model Orion 420A) equipped with a silver (Ag+)/sul-
fide (S2−) selective electrode. The preparation of the sample involved
the dilution of approximately 10 mg of complex in 2 mL of milliQ water.
Afterwards, the solution was transferred to a 25 mL flask. Then, 10 mL
of this solution were taken in a becker, and 5 drops of HNO3 and 5 mL
of miliQ water were added. At last, the solution obtained was titrated
with a solution of AgNO3 with (0.097
titrations were carried out at least three times and all results showed
good agreement with each other. The average value was used.
Thus, we report here that substrates such as 4-(methylthio)acet-
ophenone, 1; 4-(methylthio)benzaldehyde 2; 4-(methylthio)bromo-
benzene 3 and (methylthio)benzene 4 are oxidized with very high
chemoselectivity and in excellent yields (Scheme 1). It should be noted
that highly oxidizable function such as aldehyde remains unaltered
under the reaction conditions used. The sulfoxidation reactions have
been catalyzed by cobalt bromide and γ-cyclodextrin complex synthe-
sized in dichloromethane (DCM) [γCDCoBr2(DCM)] or in ethyl acetate
(EtAc) [γCDCoBr2(EtAc)] or by nickel bromide and γ-cyclodextrin com-
0.002) M concentration. All
plex synthesized in DCM [γCDNiBr2(DCM)] or in EtAc [γCDNiBr2(EtAc)
presented in this paper.
]
2.1.6. Thermal gravimetric analysis
To realize the TGA studies and analyses, it was used a thermo-
gravimetric analyzer equipment Hi-Res Modulated TGA 2950. As from
Finally, all results have been analyzed under the magnifying glass of
green parameters.
experimental
thermograms
it
were
calculated
derivative
45

D.C. Pinilla Peña, L.I. Rossi
MolecularCatalysis454(2018)44–54
Scheme 1. xxx.
Differences smaller than 10 cm−1 were not considered [76].
thermogravimetric analyses (DTGA). All solid complexes were mea-
sured from 25 °C to 500 °C with a heating rate of 10 °C/min, in N2 at-
mosphere.
νComplex − νCD = Δν
(4)
2.1.7. UV–vis diffuse reflectance
2.2. Sulfoxidation reaction
BLACK-Comet StellarNet Inc spectrometer equipment was used with
UV–vis optical fibbers suitable for measuring diffuse reflectance and
colorimetry. Spectra were acquired in the range of 200 to 1050 nm
(50000–12500 cm−1). Each solid complex was measured 10 times and
data were averaged. Kubelka-Munk equation was using to para-
meterized the reflectance (R) data obtained [71] (Eq. (2)) and PeakFit
Software was used to deconvoluted the spectra.
1H NMR spectra were carried out in a 400 MHz Bruker Avance II
spectrometer. The solvents used were analytical-grade commercially
available samples used as received. Substrates 4-(methylthio)benzal-
dehyde 1; 4-(methylthio)acetophenone, 2; 4-(methylthio)bromo-
benzene 3 and (methylthio)benzene 4 were obtained from commercial
suppliers. Products were characterized by 1H NMR spectra and found to
be identical to authentic samples. Spectra of compounds are available
in Supplementary Data.
(1 − R)2
F (R) =
(2)
2R
Colorimetric measurements were been using SpectraWiz®
Spectroscopy Software, and with D65 illuminant were obtained CIELab
parameters a, b and L, also averaging 10 measurements [72]1. ΔE, the
color difference,[73] was computed as (Eq. (3))
2.2.1. General procedure
All reactions were carried out in an open reaction tube, with mag-
netic stirring at room temperature. The molar ratios substrate: catalyst:
nitrate were 1.00: 0.10: 0.30. In a typical procedure, the substrate (1
mmol) was dissolved in acetonitrile (3.5 mL) and the cobalt complex
(0.10 mmol) and the iron (III) nitrate (0.30 mmol) were added. The
reaction time was 5 hours. The reaction medium with cyclodextrin
complexes was heterogeneous since complexes are solid, not soluble in
this system. Once the reaction was over, the solution was filtered and
the solid washed several times with dichloromethane or ethyl acetate in
agreement with solvent synthesis of the complex used. After filtration,
products were purified by column chromatography on silica gel 60
(70–230 mesh ASTM), the mobile phase used was 1:1 ethyl ether:
chloroform mixture. The residue was analyzed by different chromato-
graphic and spectroscopic methods. All reactions were conducted in
duplicate. In no case were sulfones detected. The percent enantiomeric
excess (% ee) was calculated as % ee = {([α]observed/[α])x100}, in each
case and when possible, from maximum optical rotation of biblio-
graphy.
½
ΔE = [(L1 − L2)2 + (a1 − a2)2 + (b1 − b2)2]
(3)
2.1.8. FT-IR
To realize the IR spectra, it was used a Nicolet 5SC spectro-
photometer. It was used KBr pellets containing 1% of solid complex.
Spectra were acquired at a resolution of 2 cm−1. The procedure is the
same as that followed in a previous paper (see ref [60]). The pure CD
and the complex presented the main spectral differences in the regions
of 3300–2900 cm−1 (stretching of primary and secondary OH) and
1600–1680 cm−1 (water molecules in the cavity of the CD). Other
changes, less important, were observed to signal of CH bending and OH
in plane bending (1225–1240 cm−1), CO stretching of the secondary
OH (1080–1120 cm−1), CO stretching of the primary OH
(1035–1060 cm−1) and ring vibrations (920–960 cm−1) [74].
The parameter turnover number (TON) (Eq. (5)) and turnover fre-
quency (TOF) (Eq. (6)) were calculated as follows:
When a bathochromic shift in the maximum of the signal corre-
sponding to the stretching of OH is observed, can be due that hydrogen
bonds between CD molecules have weakened. This sould so because to
interactions between CD molecule with the host [75]. The signal at
1640 cm−1 was analysed the same way. The maximum values of these
signals were subtracted from those of the native CD (Eq. (4)).
product mol
catalyst mol
TON =
(5)
(6)
product mol
catalyst mol
TOF =
x h−1
1
The CIELab is considered the most accurate colour model and has a large colour
gamut. This colorimetric method was used as recommended by the Commission
Internationale de l’ Eclairage. The a axis extends from red (+a) to green (-a) and the b
axis from yellow (+b) to blue (-b); lightness (L) axis is perpendicular to (a) and (b) plane
with a value of 0 for black and 100 for white.
2.3. Green metric calculations
The green metrics were calculated using the procedures reported in
46

D.C. Pinilla Peña, L.I. Rossi
MolecularCatalysis454(2018)44–54
the literature. They are defined as follows [77]:
3. Results and discussion
Mass intensity (MI)
In this section, we present and discuss the synthesis and char-
acterization of the Co and Ni complexes, the results obtained in the
sulfoxidation reaction using the complexes as catalysts and Green
Chemistry parameters of all synthesis performed.
Organic solvents used in the synthetic process of complexes were
dichloromethane or ethyl acetate and no water was used. Thus, it is
important to note that Co or Ni salts could not be denatured or hy-
drolysed [81].
In agreement with our previous paper [60], it should be considered
that complexes are made up from CoBr2 or NiBr2 and cyclodextrin
(macrocyclic oligosaccharide); therefore, we expected coordination
between the macrocycle and the metallic centre. In addition, the bro-
mides (counterions) may also form part of the metal coordination
sphere. These complexes were synthesized in two solvents, di-
chloromethane as in our previous studies and ethyl acetate as a very
good alternative of green solvent.
Results obtained from different techniques were analysed in an in-
tegrated way, in order to gather information about the physical che-
mical properties and structure of the complexes, as described below. A
comprehensive description of the procedure is shown, as an example,
for γCD and CoBr2 complex (γCDCoBr2) synthesized in DCM. The other
complexes discussed in this paper were analysed in a similar way. (see
Supplementary Data, Figs. 1S–6S and Table 1S).
Total mass used in a process or process step (g)
=
Mass of product (g)
mass of product
∑
Re action mass efficiency (RME) =
x100
mass of reac tan ts
∑
molecular weight of product
Atom economy (AE) =
molecular weight of reac tan ts
∑
EcoScale [78] and Green Start (GS) [79,80] were calculated using
procedures and critical evaluation [66] reported in the literature.
A
theoretical example of
a
typical calculation follows: 4-
(Methylthio)benzaldehyde (0.1522 g, 1 mmol, FW 152.21) reacts with
iron (III) nitrate nonahydrate (0.1212 g, 0.30 mmol, FW 404.00) and
molecular oxygen (0.016 g, 0.5 mmol, FW 32.00) in the presence of
γCDCoBr2(EtAc) (0.1516 g, 0.10 mmol, FW 1515.87) in acetonitrile
(3.5 mL, 2.751 g) to give 4-(methylsulfinyl)benzaldehyde (FW 168.21)
isolated in 100% yield (1.00 mmol, 0.1682 g). TON = 10; TOF = 2 h−1
.
The amount of catalyst was not used in the calculations since it is
recoverable by filtration and can be reused. For AE, reagents in catalytic
quantities and catalysts are not considered in the calculation. The score
of principles in GS are listed in supplementary data. Principles P4 and
P11 of GS are not assessed here as they correspond to an industrial
evaluation.
With the integrated analysis of obtained results in elemental ana-
lysis (EA), flame atomic absorption and potentiometric titration, it was
determined that, for γCDCoBr2, the molecular formula is DH2O
%(C48H80O40)·(CoBr2). Thus, there is a 1:1 relationship between γ-CD
and CoBr2. The value of D is 4.9, when it is calculated from TGA ana-
lysis, while EA data yield a value of 4.0 (see Supplementary Data,
Table 1S for native γ-CD and other complexes).
Mass Intensity = [(0.1522 + 0.1212 + 0.016 + 2.751)/0.1682] = 18.1
Reaction Mass Efficiency = [0.1682/(0.1522 + 0.1212+ 0.016)] x 100
= 58.1%
Atom Economy = [168.21/(152.21 + 32.00)] x 100 = 91.3%
EcoScale: 100 − Σ penalty points = 81
3.1. Thermogravimetric analysis
The TGA and DTGA analysis of γCD native agree with data reported
[82], and they are considerably different with from those of the com-
plex (see Fig. 1).
Parameters of Quality
Penalty points assigned
It can be observed in the DTGA plots that around 53 °C a peak ap-
pears in both compounds. Also γCD has a small peak at 116 °C that
appears at 130 °C in the complex. These peaks are attributed, in general,
to the release of hydration water molecules (hwm)[83]. However, the
fact that the elimination of some water molecules takes place at high
temperature has been considered as evidence that they are coordinated
Yield
0
3
5
1
0
10
19
Price of reaction components
Safety
Temperature/Time (Energy)
Technical Setup
Work-Up/Purification
Σ penalty points
Green Start: GSAI % (Green Star Area Index) = 86.0
Fig. 1. TGA and DTGA of γCDCoBr2(DCM) complex (solid lines) and γCD (dotted
line).
47

D.C. Pinilla Peña, L.I. Rossi
MolecularCatalysis454(2018)44–54
Table 1
Thermal analysis results.
b
c
e
d
Native or Complex
Thwm (°C) Tdec (°C) Td 50%d %mr
TDTGApeaks (°C)
γCDa
(°C)
γCD*
62 and 109 267
306
306
329
297
10.65 53 - 116 - 295
10.54 63 - 105 - 303
20.23 54 - 130 - 242
10.66 33 - 65 - 150 -
226 - 434
γCD**
50 and 97
54
274
206
187
γCDCoBr2 (DCM)*
γCDCoBr2 (EtAc)**
32 and 64
γCDNiBr2 (DCM)*
γCDNiBr2 (EtAc)**
48 and 112 187
71 180
356
310
30.94 60 - 108 - 213 -
239
6.49
71 - 215 - 328 -
384
dTDTGApeaks
(DTGA) plots
: maximum values in differential thermogravimetric analysis
a
Two batches of CD were used; the complexes synthesized with each of them
are indicated by * or **.
b
Thwm: Temperature of mass loss corresponding to hydration water mole-
Fig. 2. UV–vis Diffuse Reflectance (DR) of γCDCoBr2(DCM). Dashed lines show
cules.
c
the deconvolution of the bands.
Tdec: Initial temperature of macrocyclic decomposition step.
Td50%: Decomposition temperature of 50% of the initial mass.
%mr: Percentage of remaining mass.
d
e
d orbital into high and low energy orbitals when transition metal is
attach with ligands to form a coordination complex; it is characteristic
of both metal and ligand. Δ0 values and ligand-to-metal charge transfer
(LMCT) were analyzed by UV–vis diffuse reflectance measurements
[87].
to the metal centre [84]. On the other hand, there is no regularity in the
behaviour of the different complexes above this temperature (see
Table 1).
Since Co2+ has a d7 configuration, three transitions should be ex-
pected in the Uv–vis of the complex [88]. As these three bands are close
in wavenumber values, it is necessary the deconvolution of the spectra
with three Gaussians. The UV–vis diffuse reflectance of the
γCDCoBr2(DCM) powder is showed into Fig. 2, as well as the bands re-
sulting from the deconvolution of the spectra (the maximum values are
listed in Table 2).
The decomposition profiles of the complexes is also very different
from that of native CD. While the native CD shows only one clear
thermal transition at 306 °C, the macrocycle degradation in γCDCoBr2 is
carried out in several steps. Metallic centre is able to coordinate the
smaller fragments generated from initial carbohydrates, so they are
eliminated at different temperatures [85]. All complexes showed a si-
milar behaviour (see Table 1 and Supplementary Data, Figs. 1S, 3S, 5S).
From different parameters, i.e., the initial temperature of macro-
cyclic decomposition step (Tdec), decomposition temperature of 50% of
the initial mass (Td50%), and the temperature value of the transitions in
the DTGA, it can be discussed the thermal stability of the complexes. As
shown in Table 1, in all cases, the initiation of the decomposition event
is carried out at a temperature lower than that of the γCD since the
complexes synthesized in EtAc show lower temperature. On the other
hand, the Td50% values of the complexes are higher than the native CD,
except for γCDCoBr2(EtAc). Hence, it can be concluded that the presence
of Co or Ni salts accelerates the thermal decomposition of the macro-
cycle and the solvent used in the synthesis could have some participa-
tion in the stability of solid complexes.
Δo is determined from the maximum of the highest intensity signal
with A value of 10586 cm−1, consistent with a distorted octahedral
geometry. The observed bands have been assigned to the transition in
agreement with the bibliography. Complexes presented here showed no
ligand-to-metal charge transfer (LMCT) transition (see also spectra of
other complexes, Figs. 2S, 4S, 6S and Table 2) [88,89].
It was not possible to determine a preferred position for the metal
centre being that the only coordinating atoms in CDs are the primary or
secondary OH groups (at the rim) and the glycosidic oxygens (oxygens
located in the cavity). Insomuch as its mobility is constrained in a
confined space and the interaction may be more efficient. Nevertheless,
it may be assumed that a higher orbital overlap is mostly favoured
when the guest is more included [90].
Also the remaining mass (%mr) was analysed. This parameter can
also shed light into the complex structure (see Table 1). In two com-
plexes synthesized in DCM, the remaining mass was higher than in the
γCD, whereas that in those synthesized in EtAc the remaining mass was
equal to or minor than native CD. Since no solvent remained in the solid
complexes, these results might indicate that in complexes synthesized
in DCM the coordination of the oxygens of the host with the metal is
stronger than that in complexes synthesized in EtAc. This may be due to
interaction and/or competition of oxygen atoms of EtAc by the metallic
centre during the synthetic process.
When we compared the thermal behaviour of the complexes, we
observed that it was different from that of other complexes reported in
the literature [86], while this behaviour has some likeness with our
previous report on Cu complexes [60]. The dissimilar synthetic pro-
cesses, that could lead to compounds with different structures, can be
responsible of these discrepancies [56].
The bands in the UV–vis are broad with a considerable value of area,
it suggest that several isomers of the complex may be present in the
powder. γ-CD is a big ligand with multiple coordinating atoms, then it
can be possible than complexes with different geometry are be present.
For example, three different ligands (oxygen from water molecules,
halide, oxygens form γ-CD) with identical stoichiometry, may be in a
distinct coordination position (equatorial, axial or both) and result in
Co2+ atoms with different environments, which may cause a broad-
ening in the UV–vis bands. All these was discussed in a previous paper
with Cu2+ as metallic centre [60].
So all complexes show a distorted octahedral geometry as it can be
inferred from data showed in Table 2.
In Addition, we compared the Δo values obtained for complexes of
γCD synthesized in different solvents, and it can be said that there are
differences in transitions 4T1g→4T1g(4P) for Co complexes and
3
3A2g→3T1g(3F) and A2g →3T1g(3P) for Ni complexes, in γCDNiBr2(DCM)
the last transition is missing.
The colorimetric measurements are be used in scientific fields such
as synthesis and characterization of materials [91] or nanopigments
[92] among others; however, when organometallic compounds are
3.2. UV-vis diffuse reflectance
It is called Δ0 to the difference in energy generated by the split of the
48

D.C. Pinilla Peña, L.I. Rossi
MolecularCatalysis454(2018)44–54
Table 2
Spectroscopic data of the complexes.
Complex
(cm−1) ; transition
Color (CIE L*a*b*)
L
Δ0
4T1g
→
4T2g(4F)
4T1g→4A2g(4F)
4T1g→4T1g(4P)
a
b
ΔE
γCDCoBr2(DCM)
γCDCoBr2(EtAc)
10586
10722
17574
17431
23899
25285
82.32
77.15
-2.91
-5.35
-14.34
-13.78
5.72
3A2g
9732
9996
→
3T2g(3F)
3A2g→3T1g(3F)
18749
17464
3A2g →3T1g(3P)
–
27283
γCDNiBr2(DCM)
γCDNiBr2(EtAc)
93.35
95.36
-6.97
-8.62
11.12
18.30
7.64
The analysis of the data shows that colorimetric values are highly
dependent on the metallic centre. From the differences in the L, a* and
b* parameters, is possible say that the colorimetric values can be used
as a fingerprint for these CD complexes. This allows the identification of
each one by a simple procedure. Looking at the L, a* and b* values for
each Co complexes in Table 2, it can be determined that the complexes
do not match in colour. These values show that γCDCoBr2(DCM) is
lighter, less green, and more blue than γCDCoBr2(EtAc). If we put the
values of ΔL* = +5.17, Δa* = −2.44, and Δb* = −0.56 into the color
difference equation, it can be determined that the total colour differ-
ence between the two complexes is 5.72. The same analysis can be
made for Ni complexes. It is remarkable that the highest absolute values
of the CIELab parameters were obtained in Ni complexes.
3.3. IR spectroscopy
Fig. 3. a* and b* parameters of the CIELab space of the different complexes.
Also, we analysed IR spectra. For this, the regions of
3300–2900 cm−1, corresponding to OH of the γ-CD (1st and 2nd OHrim
)
characterized, this technique is rarely used [93]. The same happens
with the CIE L*a*b* parameters, these have been used to characterize of
ceramic glazes [94]; to monitor oxidation in raw meat [95]; or to find
relationships between solvatochromism and chromaticity [96] but
seldom for organometallic complexes.
The colorimetric data of the complexes are summarized in Table 2.
Whereas that in Fig. 3 are showed the a* b* chromaticity diagram of the
CIELAB color space. For parameter a, the negative direction of the
horizontal axis indicates a green component; the positive direction in-
dicates a red component. For parameter b, the positive direction of the
vertical axis indicates a yellow component while the negative indicates
a blue one. On the other hand, the chromaticity is obtained of the ab-
solute value of the distance from the origin of coordinates to the col-
orimetric value. This parameter expresses the intensity of the colour,
the greater the distance, the higher the chromaticity (Fig. 4).
and 1600–1680 cm−1 corresponding to water molecules present in the
cavity of the γ-CD (H2Ocavity) have been chosen to discuss.
When the differences in FT-IR spectra are considered (Table 3), it
may be observed that only the OH signal is significantly modified. But
this fact occurs with a different relative magnitude. It shows than me-
tallic centre interacts with the OH at the rims of the CD, altering the
formation of inter and/or intramolecular H bond. However, no differ-
ences were observed in the signal of H2O from the cavity were ob-
served, evidencing that the metal salt does not present a certain degree
of inclusion. In addition, the modification in OH signal produced by Ni
complexes was lower than that in Co complexes.
3.4. Sulfoxidation reaction
On the basis of our previous studies, we chose four sulfides with
different reactivity in agree with Hammett relation; these values are 4-
(methylthio)acetophenone, 1, σ = +0.50; 4-(methylthio)benzaldehyde
2, σ = +0.42; 4-(methylthio)bromobenzene 3, σ = +0.23 and (me-
thylthio)benzene 4, σ = 0.00 [97]; the amount of co-oxidant was also
increased from 0.1 to 0.3 mmol with respect to 1 mmol substrate. The
objective of these changes was to search enantio and chemoselectivity
conditions on the base of a previous study (see ref. [23]). On the other
hand, the use as catalysts of complexes of γCD synthesized in different
Table 3
Differences in the FT-IR bands analysed for all the compounds synthesized.
Native or Complex γCDa
OH (cm−1
)
(cm−1
)
H2O (cm−1
)
(cm−1
)
Δ0
Δ0
γCD*
3384
3364
3380
3396
3370
3371
–
-20
-4
–
-26
-25
1646
1643
1644
1647
1643
1643
–
γCDCoBr2(DCM)
*
*
-3
-2
–
-3
-4
γCDNiBr2(DCM)
γCD**
γCDCoBr2(EtAc)**
γCDNiBr2(EtAc)**
a
Two batches of CD were used. The complexes synthesized with each of
them are indicated by * or **.
Fig. 4. Chromaticity value of all the compounds synthesized.
49

D.C. Pinilla Peña, L.I. Rossi
MolecularCatalysis454(2018)44–54
Table 4
Table 5
Oxidation reaction of 4-(methylthio)acetophenone to 4-(methylsulfinyl) acet-
Oxidation reaction of 4-(methylthio)benzaldehyde to 4-(methylsulfinyl) ben-
ophenone with γCDCoBr2 complexes or CoBr2 as catalysts.a
zaldehyde with γCDCoBr2 complexes or CoBr2 as catalysts.a
Entry Catalystb
Substratec Yieldc (%)d
[polarimetry]e
TON TOF
Entry Catalystb
Substratec Yieldc (%)d
[polarimetry]e
TON
TOF
g
g
1
2
3
4
5
6
–
100
100
100
100
100
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
1
2
3
4
5
– (29.3%oxid)f
CoBr2
80
20 (16.6)
–
–
–
(0.57)
–
–
–
(0.11)
–
–
–
f
h
CoBr2
100
100
100
9
f
h
h
γCDCoBr2(DCM)
γCDCoBr2(EtAc)
CoBr2
γCDCoBr2(DCM)
γCDCoBr2(EtAc)
CoBr2
f
–
91 (80.4)
9.59
1.92
f
γCDCoBr2(DCM) (10.2%
100 (76.7) [-0.90
7.50 1.50
(8.4%cat)
cat)g
(1.00)h]
6
7
γCDCoBr2(DCM)
–
100 (92.7) [+0.57 11.86
(0.26)i]
2.37
1.67
f
7
γCDCoBr2(EtAc) (10.1%
–
100 (79.1) [-0.33
(0.36)h]
7.82 1.56
(7.8%cat)
cat)g
γCDCoBr2(EtAc)
1
99 (74.5) [+0.34
8.33
f
(7.8%cat)
(0.18)i]
a
Solvent acetonitrile, co-oxidant Fe(NO3)3·9 H2O, ratio substrate: catalyst:
a
Solvent acetonitrile, co-oxidant Fe(NO3)3·9 H2O, ratio substrate: catalyst:
co-oxidant 1.00: 0.10: 0.30 mol, at room temperature with stirring, under air,
reaction time 5 h.
co-oxidant 1.00: 0.10: 0.30 mol, at room temperature with stirring, under air,
reaction time 5 h.
b
Catalysts: γCDCoBr2 = complex (γ-cyclodextrin)CoBr2; (DMC) di-
b
Catalysts: γCDCoBr2 = complex (γ-cyclodextrin)CoBr2; (DMC) di-
chloromethane; (EtAc) ethyl acetate.
c
Percent substrate or product recovered by filtration, determined by 1H
chloromethane; (EtAc) ethyl acetate.
Percent substrate or product recovered by filtration, determined by 1H
c
NMR.
d
NMR.
Percent yield of oxidation product after isolation and purification.
d
e
Specific optical rotation [α]2D1 [mm−1x (g/100 mL)−1
] measured in
Percent yield of oxidation product after isolation and purification.
e
Specific optical rotation [α]2D1 [mm−1x (g/100 mL)−1
] measured in
acetone (C 2% at 21 °C λ = 589 nm).
f
acetone (C 2% at 21 °C λ = 589 nm).
Without co-oxidant Fe(NO3)3·9H2O.
f
g
% catalyst (cat) or co-oxidant (oxid) with respect to the amount of substrate
used in the experiment reported. This information is necessary for green cal-
culations.
% catalyst (cat) with respect to the amount of substrate used in the ex-
periment reported. This information is necessary for green calculations.
h
Percent enantiomeric excess (% ee) [98].
g
The value was calculated in relation to oxidant mol used.
Without co-oxidant Fe(NO3)3·9 H2O.
Percent enantiomeric excess (% ee) [99].
h
solvents (dichloromethane and ethyl acetate) and/or different metallic
centre Co or Ni has allowed us to analyze the greenness of our reaction
conditions.
i
Table 6
Compound 1 was oxidized with high yield and chemoselectivity
using γCDCoBr2(DCM) or γCDCoBr2(EtAc). As can be observed in Table 4,
we obtained 4-(methylsulfinyl) acetophenone 1a with excellent yields
when complexes of CoBr2 were used, entries 6 and 7, whereas, CoBr2
salt did not catalyze sulfoxidation reaction, entry 5. Entries 1–4 in
Table 4 show than without the catalyst or without co-oxidant, there is
no reaction. While all the reactions gave almost quantitative yield, the
amount of isolated product was not the same since the work-up is in
some cases more complicated than in others. No efforts were made to
optimize the isolation procedures. Fe(NO3)3·9H2O and CoBr2 are so-
luble in the reaction system; this fact hinders the isolation from the
reaction product with an important loss of the same.
Oxidation reaction of 4-(methylthio)bromobenzene to 4-(methylsulfinyl) bro-
mobenzene with γCDCoBr2 complexes or CoBr2 as catalysts.a
Entry Catalystb
Substratec Yieldc (%)d
[polarimetry]e
TON
TOF
1
2
3
4
5
6
– (31.5%oxid)f
CoBr2
γCDCoBr2(DCM)
γCDCoBr2(EtAc)
CoBr2 (10.0%cat)
29
71 (38.3)
–
–
–
49 (29.9)
95 (56.8)
[-0.21(0.20; S)i]
100 (61.2)
[-0.27(0.26; S)i]
(1.22)g (0.24)g
h
100
100
100
51
–
–
h
h
–
–
–
–
f
2.99
6.23
0.60
1.25
γCDCoBr2(DCM)
5
f
(9.1%cat)
7
γCDCoBr2(EtAc)
–-
6.33
1.27
f
(9.7%cat)
On the other hand, the sulfoxidation catalyzed was very efficient
over compound 2 and it was good over compound 3, Tables 5 and 6.
Under the same reaction conditions and during the same time, 5 h,
sulfide oxidation was observed in the presence of co-oxidant alone. This
is possible due to the greater availability of electronic density on sulfur
atom in compounds 2, 3, and principally compound 4, Table 7.
The results reported in Table 7 show the inhibition of sulfoxidation
reaction in the presence of CoBr2 salt and complexes γCDCoBr2(DCM)
and γCDCoBr2(EtAc), entries 5, 6 and 7. This effect was observed also in
compounds 3 when CoBr2 was used as a catalyst, Table 6 entry 5.
In most cases and in agreement with TON − TOF values reported in
this paper, the better catalyst was γCDCoBr2(DCM), entries 6 in Tables 4,
5 and 7. In no case was obtained a good enantiomeric excess.
On the other hand, γCDNiBr2(EtAc) was the best Nickel catalysts,
entry 7 in Table 8, in agreement with TON − TOF values reported here.
The γCDNiBr2(DCM) complex did not catalyze the sulfoxidation reaction.
As in the case of Co Complexes, a good enantiomeric excess was not
obtained.
a
Solvent acetonitrile, co-oxidant Fe(NO3)3·9 H2O, ratio substrate: catalyst:
co-oxidant 1.00: 0.10: 0.30 mol, at room temperature with stirring, under air,
reaction time 5 h.
b
Catalysts: γCDCoBr2 = complex (γ-cyclodextrin)CoBr2; (DMC) di-
chloromethane; (EtAc) ethyl acetate.
c
Percent substrate or product recovered by filtration, determined by 1H
NMR.
d
Percent yield of oxidation products after isolation and purification.
e
Specific optical rotation [α]2D1 [mm−1x(g/100 mL)−1] measured in acetone
(C 2% at 21 °C λ = 589 nm).
f
% catalyst (cat) or co-oxidant (oxid) with respect to the amount of substrate
used in the experiment reported. This information is necessary for green cal-
culations.
g
The value was calculated in relation to oxidant mol used.
Without co-oxidant Fe(NO3)3·9 H2O.
Percent enantiomeric excess (% ee) [100].
h
i
The best result of each parameter to the different substrates is in bol.
AE data showed that sulfoxidation reaction studied here with mo-
lecular oxygen present in air as oxidant, 30% co-oxidant and catalyst, is
a good reaction. For GSAI%, the complexes synthesized in ethyl acetate
3.5. Green evaluation
The results obtained in green evaluation are summarized in Table 9.
50

D.C. Pinilla Peña, L.I. Rossi
MolecularCatalysis454(2018)44–54
Table 7
is reported in reference [23]) but it was obtained chemoselectivity, the
changes that could be made to improve the values in the green para-
meters, using γCD complexes as catalysts, would be: to decrease the
amount of solvent, co-oxidant and catalyst used in the reaction. This
would improve MI and RME parameters. If both workup and purifica-
tion are improved, EcoScale values will also increase.
Oxidation reaction of 4-(methylthio)benzene to 4-(methylsulfinyl)benzene with
γCDCoBr2 complexes or CoBr2 as catalysts.a
Entry Catalystb
Substratec Yieldc (%)d
[polarimetry]e
TON
TOF
1
2
3
4
5
6
– (26.2%oxid)f
–
100 (62.7)
(5.92)g (1.18)g
–
–
–
4.52
15.58
h
CoBr2
γCDCoBr2(DCM)
100
100
100
24
7
–
–
–
–
–
–
0.90
3.12
h
3.6. General Comments
h
γCDCoBr2(EtAc)
CoBr2 (10.1%cat)f
γCDCoBr2(DCM)
(5.2%cat)f
76 (45.6)
93 (81.5) [-0.22
(0.15; S)i]
81 (68.4) [-0.14
(0.09; S)i]
In our previous study with Cu(II) as metallic centre (see ref. [58]),
we showed that the thermal stability of complexes depends markedly
on the nature of both neutral and anionic ligands when all complexes
were synthesized in the same solvent.
7
γCDCoBr2(EtAc)
19
13.50
2.70
(5.1%cat)f
In this case, by comparing the values of Td50%, it can be said that
the presence of CoBr2 or NiBr2 modifies the stability of macrocycle;
however, the effect over the complexes of solvent used in the synthetic
process is more important.
Td50% is frequently used to compare the thermal stability of solids.
In our complexes, these values are useful since important differences
between Td50% were found for the complexes between them and the
γCD.
The decomposition starts at a lower temperature in the complexes,
this is observed in the analyses by DTGA curves. Between them, frag-
ments of complexes synthesized in DCM are slowly liberated probably
due to remain coordinated with the metal centre. So, the carbohydrate
is thermally destabilized by the presence of a metallic salt.
Besides, in the percentage of remaining mass is evidenced the effect
of salt coordination and the synthetic solvent since it is high in
γCDNiBr2(DCM) decomposition (30.94%) or low in γCDNiBr2(EtAc)
(6.49%).
a
Solvent acetonitrile, co-oxidant Fe(NO3)3·9 H2O, ratio substrate: catalyst:
co-oxidant 1.00: 0.10: 0.30 mol, at room temperature with stirring, under air,
reaction time 5 h.
b
Catalysts: γCDCoBr2 = complex (γ-cyclodextrin)CoBr2; (DMC) di-
chloromethane; (EtAc) ethyl acetate.
c
Percent substrate or product recovered by filtration, determined by 1H
NMR.
d
Percent yield of oxidation product after isolation and purification.
e
Specific optical rotation [α]2D1 [mm−1x (g/100 mL)−1
] measured in
acetone (C 2% at 21 °C λ = 589 nm).
f
% catalyst (cat) or co-oxidant (oxid) with respect to the amount of substrate
used in the experiment reported. This information is necessary for green cal-
culations.
g
The value was calculated in relation to oxidant mol used.
Without co-oxidant Fe(NO3)3·9 H2O.
Percent enantiomeric excess (% ee) [101,102].
h
i
All the results indicate that the structure of complexes is determined
not only by the metal coordination with γ-cyclodextrin but also by the
solvent used in the synthesis itself. The difference in color, ΔE, (> 5)
shows that complexes synthesized in DCM and EtAc solvents are dif-
ferent.
Table 8
Oxidation reaction of 4-(methylthio)benzaldehyde to 4-(methylsulfinyl) ben-
zaldehyde with γCDNiBr2 complexes or NiBr2 as catalysts.a
Entry Catalystb
Substratec Yieldc (%)d
[polarimetry]e
TON
TOF
The different thermal stability helps to understand the behaviour of
complexes as catalysts, where γCDNiBr2(DCM) is the least active.
The reactions reported here, from the green chemistry point of view,
follow several of its principles and have very good green metrics. It is
also important to note that the green evaluation has been performed
from the synthesis of the catalysts used.
f
1
2
3
4
5
6
– (28.8%oxid)
NiBr2
γCDNiBr2(DCM)
γCDNiBr2(EtAc)
80
20 (15.3)
–
–
–
87 (66.0)
20 (14.6) [+0.15
(0.02)i]
(0.53)g (0.11)g
h
100
100
100
13
–
–
h
–
–
h
–
–
f
NiBr2 (9.1%cat)
7.28
1.48
1.46
0.30
γCDNiBr2(DCM)
80
f
(9.9%cat)
7
γCDNiBr2(EtAc)
4
96 (87.9) [+0.23
10.58
2.12
4. Conclusion
f
(8.3%cat)
(0.08)i]
a
We were determined that CoBr2 and NiBr2 form stable complexes
with γCD in dichloromethane or ethyl acetate as synthesis solvents. The
synthetic methodology is simple and yield complexes with 1:1 (γCD:
CoBr2 or NiBr2) stoichiometry. These complexes also contains water
molecules. After comparison of their TGA analysis, UV–vis and IR
spectra, we could observed and determined that the structure of the
complexes obtained in several repetitive syntheses was reproductible.
We could determine the stoichiometry and the predominant inter-
actions and some structural information of the Co2+ or Ni2+ complexes
in different solvents. For this, the integration of analytical data obtained
by several techniques (EA, TGA, UV–vis and FT-IR) and the standar-
dized method for the study of metal complexes was used [60]. Through
reflectance measurements, complexes showed have octahedral co-
ordination geometry, but with varying degrees of distortion. CIELab
colorimetric data showed the differences between the complexes syn-
thesized. This methodology is easily applied and is very useful for the
rapid characterization of complexes in their second synthesis. It should
be noted that, despite the great utility of the last two techniques dis-
cussed, we have found little literature on its use in the characterization
of catalysts.
Solvent acetonitrile, co-oxidant Fe(NO3)3·9 H2O, ratio substrate: catalyst:
co-oxidant 1.00: 0.10: 0.30 mol, at room temperature with stirring, under air,
reaction time 5 h.
b
Catalysts: γCDNiBr2 = complex (γ-cyclodextrin)NiBr2; (DMC) di-
chloromethane; (EtAc) ethyl acetate.
c
Percent substrate or product recovered by filtration, determinate by 1H
NMR.
d
Percent yield of oxidation product after isolation and purification.
e
Specific optical rotation [α]2D1 [mm−1x (g/100 mL)−1
] measured in
acetone (C 2% at 21 °C λ = 589 nm).
f
% catalyst (cat) or co-oxidant (oxid) with respect to the amount of substrate
used in the experiment reported. This information is necessary for green cal-
culations.
g
The value was calculated in relation to oxidant mol used.
Without co-oxidant Fe(NO3)3·9 H2O.
Percent enantiomeric excess (% ee) [99].
h
i
(a green solvent) showed the best performance. The other parameters
are highly dependent on the yield and mass reactant used.
In general, the γCD complexes showed the best green results with all
substrates.
We report herein a green and efficient method for the selective
oxidation of sulfides to sulfoxides under very mild heterogeneous
Since enantiomeric excess was not obtained (sought based on what
51

D.C. Pinilla Peña, L.I. Rossi
MolecularCatalysis454(2018)44–54
Table 9
Green parameters of sulfoxidation reactions.
Entry
Catalyst
GSAI%
Yield%a (%oxid)b
EcoScale
MI
RME
AE
Sulfoxidation of 4-(methylthio)acetophenone to 4-(methylsulfinyl)acetophenone.
1
2
γCDCoBr2(DCM)
γCDCoBr2(EtAc)
77.5
76.7
(24.3%oxid)
79.1
69.4
51.0
45.9
91.9
91.9
86.0
70.6
58.4
43.8
(28.8%oxid)
Sulfoxidation of 4-(methylthio)benzaldehyde to 4-(methylsulfinyl)benzaldehyde.
3
4
5
6
–
65.0
62.50
77.5
86.0
16.6
(29.3%oxid)
80.4
(26.2%oxid)
92.7
(26.6%oxid)
74.5
39.3
71.2
77.4
68.3
101.6
18.9
15.5
21.7
9.8
91.3
91.3
91.3
91.3
CoBr2
49.8
57.4
43.6
γCDCoBr2(DCM)
γCDCoBr2(EtAc)
(30.1%oxid)
Sulfoxidation of 4-(methylthio)bromobenzene to 4-(methylsulfinyl)bromobenzene.
7
–
65.0
62.5
77.5
86.0
38.3
(31.5%oxid)
29.9
(33.7%oxid)
56.8
(28.8%oxid)
61.2
50.2
46.0
59.4
61.6
57.3
73.4
35.4
34.6
15.5
11.8
23.9
25.5
93.2
93.2
93.2
93.2
8
CoBr2
9
γCDCoBr2(DCM)
γCDCoBr2(EtAc)
10
(29.4%oxid)
Sulfoxidation of 4-(methylthio)benzene to 4-(methylsulfinyl)benzene.
11
12
13
14
–
65.0
62.5
77.5
86.0
62.7
(26.2%oxid)
45.6
(31.8%oxid)
81.5
(16.2%oxid)
68.4
62.4
53.8
71.8
65.2
28.7
47.1
14.2
16.5
36.1
23.8
57.8
47.7
89.8
89.8
89.8
89.8
CoBr2
γCDCoBr2(DCM)
γCDCoBr2(EtAc)
(17.0%oxid)
Sulfoxidation of 4-(methylthio)benzaldehyde to 4-(methylsulfinyl)benzaldehyde.
15
16
17
18
–
65.0
62.5
77.5
86.0
15.3
(28.8%oxid)
66.0
(28.5%oxid)
14.6
(33.4%oxid)
87.9
38.7
64.0
38.3
75.0
108.8
24.8
9.1
91.3
91.3
91.3
91.3
NiBr2
39.4
8.1
γCDNiBr2(DCM)
γCDNiBr2(EtAc)
121.5
17.1
54.7
(26.0%oxid)
a
b
Percent yield of oxidation product after isolation and purification.
% co-oxidant (oxid) with respect to the amount of substrate used in the experiment reported. This information is necessary for green calculations.
conditions with high yields and excellent chemoselectivity. This sul-
foxidation method could have scope or be applicable in molecules with
more complex structures containing aryl sulfides with electron-with-
drawing groups. A good comparison of complexes of γCD was also made
as catalysts.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.mcat.2018.05.004.
The type of green evaluation carried out in this study allows us to
project changes to improve the work methodology. It allows evaluating,
for example, if it is more beneficial to change to a greener solvent that
produces less effective catalysts or to use a less environmentally
friendly solvent that can be recycled in the synthesis of the same
complex. Undoubtedly, a holistic and global analysis allows to better
understanding of the chemical reactions and processes involved.
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