Iodine/DMSO promoted oxidation of benzylic Csp3–H bonds to diketones – A mechanistic investigation

Article abstract of DOI:10.1016/j.tet.2019.130617

This article describes a mechanistic investigation into the I₂/DMSO mediated benzylic C_sp³–H oxidation of an α-methylene ketone. The electron paramagnetic resonance (EPR) spectrum centred at g = 2.0011 supports the involvement of iodine and benzylic radicals, as the α-iodinated compound 2-iodo-1,2-diphenylethanone was isolated as a key reactive intermediate. The oxidation reaction relies, primarily, on DMSO as a source of oxygen in benzil, proven by the reaction of benzyl phenyl ketone with diphenyl sulfoxide (DPSO).

Full text of DOI:10.1016/j.tet.2019.130617

Journal Pre-proof
3
Iodine/DMSO promoted oxidation of benzylic C
mechanistic investigation
–H bonds to diketones – A
sp
Janeeka Jayram, Bheki A. Xulu, Vineet Jeena
PII:
S0040-4020(19)30960-3
DOI:
https://doi.org/10.1016/j.tet.2019.130617
TET 130617
Reference:
To appear in: Tetrahedron
Received Date: 30 July 2019
Revised Date: 11 September 2019
Accepted Date: 13 September 2019
Please cite this article as: Jayram J, Xulu BA, Jeena V, Iodine/DMSO promoted oxidation of
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benzylic C
sp
–H bonds to diketones – A mechanistic investigation, Tetrahedron (2019), doi: https://
doi.org/10.1016/j.tet.2019.130617.
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Iodine/DMSO Promoted Oxidation of Benzylic Csp –H
Bonds to Diketones – a Mechanistic Investigation
Janeeka Jayram, Bheki A. Xulu and Vineet Jeena*
School of Chemistry and Physics, University of KwaZulu-Natal, Scottsville, Pietermaritzburg, 3209, South
Africa.
H
I
O
I2
DMSO
Modified
Kornblum
Oxidation
O
Iodination
O
O

1
3
Iodine/DMSO Promoted Oxidation of Benzylic Csp –H Bonds to
Diketones – a Mechanistic Investigation
Janeeka Jayram, Bheki A. Xulu and Vineet Jeena*
School of Chemistry and Physics, University of KwaZulu-Natal, Scottsville, Pietermaritzburg, 3209,
South Africa.
Jeenav1@ukzn.ac.za
Keywords: Benzylic, DMSO, Iodine, Mechanistic, Oxidation.
Abstract
This article describes a mechanistic investigation into the I2/DMSO mediated benzylic
3
Csp –H oxidation of an α-methylene ketone. The electron paramagnetic resonance (EPR)
spectrum centred at g = 2.0011 supports the involvement of iodine and benzylic radicals, as
the α-iodinated compound 2-iodo-1, 2-diphenylethanone was isolated as a key reactive
intermediate. The oxidation reaction relies, primarily, on DMSO as a source of oxygen in
benzil, proven by the reaction of benzyl phenyl ketone with diphenyl sulfoxide (DPSO).
1. Introduction
3
Conceptually, the direct transformation of Csp –H bonds into carbon–carbon (C–C)
and carbon–heteroatom (C–X, X=N, O, S) bonds offers new methodologies to prepare
synthetically valuable molecules, as these approaches reduce pre-functionalization of starting
3
materials while improving atom/step economy.1 The chemically inert nature of Csp –H bonds,
however, due to: poor acidity, a high bond dissociation energy (BDE), thermodynamic

2
stability and an unreactive molecular orbital profile, hinder the achievement of a complete
regio-selectivity without requiring multiple synthetic steps.2 Despite selectivity issues in
3
recent decades, direct Csp –H functionalization methods have developed substantially.3 This,
however, often necessitates the use of expensive transition metal catalysts, toxic reagents,
high energy throughput and harsh reaction conditions which result in functional group
incompatibility and limited substrate scope.4
3
Accordingly, Csp –H functionalization reactions, inspired by a rational design of
experimental conditions, leading to significant improvement in both selectivity and
applicability, present an ongoing challenge to researchers engaged in modern synthetic
organic chemistry.5 In context the highly appealing, direct and selective, oxidation of
3
benzylic Csp –H bonds is currently an active area of research, owing to its relevant approach
to synthetically useful arylcarbonyl compounds such as α-diketones which often serve as
important precursors for heterocyclic syntheses, with various attractive methodologies
documented in literature.6
In particular, molecular iodine (I2) is now the most frequently used catalyst for many
organic transformations and, is widely recognized as a replacement for environmentally
unfriendly reagents, owing to its low toxicity, operational simplicity, high stability and
various other user-friendly characteristics.7 Moreover, dimethyl sulfoxide (DMSO) is an
inexpensive and environmentally friendly dipolar aprotic solvent, oxidant and oxygen source
in many organic syntheses.8 Collectively, the I2/DMSO combination is currently a distinct
and complimentary alternative to unsustainable oxidation methodologies, since it has realized
numerous organic transformations.9
3
Accordingly, we have recently reported an I2/DMSO oxidation of the benzylic Csp –H
bonds of benzyl phenyl ketone 1 to afford the corresponding α-diketone 2 in an isolated yield
of 96% (Scheme 1).10 While we were satisfied with the excellent yield, our mechanistic

3
insight into the reaction pathway was minimal. Thus, a significant point of interest, in the
current study, was to provide definitive, mechanistic proof for the oxidation of the α-
methylene ketone to the α-diketone using the I2/DMSO system.
Scheme 1: I2/DMSO oxidation of benzyl phenyl ketone 1 to benzil 2.
2.
Results and Discussion
Our theoretically proposed mechanism was anticipated to proceed via initial
iodination by molecular iodine (I2), followed by a modified Kornblum oxidation in the
presence of dimethyl sulfoxide (DMSO) as presented in Scheme 2. Consequently, a series of
experimental reactions and spectroscopic techniques were undertaken to rationalize each step
in the proposed reaction pathway for the formation of the α-diketone, benzil 2.

4
Scheme 2: Plausible mechanism for the I2/DMSO oxidation of benzyl phenyl ketone 1.
Based on literary studies, the homolytic cleavage of molecular iodine is known to
occur under thermal conditions so as to afford iodine radicals.11 Our mechanistic studies,
therefore, commenced by exploring the formation of iodine radicals in our reaction, under
thermal conditions, using Electron Paramagnetic Resonance (EPR) spectroscopy. Generally,
free radicals are extremely unstable and a highly reactive species with a half-life of the order
10-9 s12 hence butylated hydroxytoluene (BHT) was used as an anti-oxidant to detect the
formation of iodine radicals in our reaction. BHT is a diamagnetic molecule and, accordingly,
no EPR signal was observed in the absence of iodine in our experiments. Subsequently,
molecular iodine and BHT were reacted in DMSO and the mixture heated to 100°C (Scheme
3).
OH
O
tBu
tBu
tBu
tBu
I
I
/DMSO
+ HI + I
100 oC, 15 min
3
4

5
Scheme 3: Formation of oxygen radical from the reaction of I2 and BHT in DMSO.
Following transfer into a flat quartz tube, an EPR signal, consistent with the presence of an
organic radical, was observed (Figure 1). The spectrum is characterized by four intense lines
centered at g = 2.0011 which arises, as BHT quenches iodine radicals through H-atom
transfer and generates a stable BHT radical species. The unpaired electron which is located
on the sterically hindered oxygen atom, and protected by the tertiary butyl groups, is
stabilized by the π-system of the benzene ring. The coupling of the electron to the three
equivalent protons of the 4-methyl group gives rise to the recorded four-line EPR spectrum.13
The EPR spectrum and the g-value for the BHT radical, obtained under our reaction
conditions, are comparable with the experimental data provided in literature.14, 15 This result
therefore correlates with the formation of a phenoxy radical (g = 2.0010-2.0091, RPhO·),16
which subsequently implies the presence of iodine radicals in our reaction.
g = 2.0011
Figure 1: EPR spectrum of a heated I2/BHT sample in DMSO.

6
Since the iodine radical was detected using EPR spectroscopy, this indicates that the
oxidation reaction is initiated by the thermal homolytic cleavage of molecular iodine
supporting a radical mediated mechanism. The presence of iodine radicals in the reaction
infers the formation of a benzylic radical on benzyl phenyl ketone 1. Consequently, an iodine
radical assisted proton abstraction from the α-methylene position of 1 was proposed to
generate benzylic radical A. As a result, we aimed to trap the benzylic radical using the spin
trap (2, 2, 6, 6-tetramethylpiperidin-1-yl)oxyl (TEMPO) and, under the standard reaction
conditions, benzyl phenyl ketone, iodine and the radical inhibitor TEMPO were added to
DMSO and heated to 100 °C for 2 h, affording the capture product 5 in 46% yield (Scheme
4).
Scheme 4: Reaction of benzyl phenyl ketone 1 with TEMPO to afford 1,2-diphenyl-2-((2, 2,
6, 6-tetramethylpiperidin-1-yl)oxy)ethanone 5.
The singlet at 5.92 ppm17 in the NMR spectrum corresponds to the α-proton (C–H) of
1, 2-diphenyl-2-((2, 2, 6, 6-tetramethylpiperidin-1-yl)oxy)ethanone 5 (Figure 2), indicating
the formation of the C–O bond between the benzylic radical of intermediate A and the
oxygen radical of TEMPO.

7
ppm
11 10
9
8
7
6
5
4
3
2
1
0
Figure 2: NMR spectrum of 1,2-diphenyl-2-((2, 2, 6, 6-tetramethylpiperidin-1-
yl)oxy)ethanone 5.
While the TEMPO trapped reaction in Scheme 4 theoretically supports the formation
of A, in which the radical is located on the α-position of 1, this experiment does not
necessarily lead to this conclusion. It has been reported that in the presence of a halogen co-
catalyst,18 TEMPO is readily oxidized to the corresponding N-oxoammonium cation as
shown in Scheme 5.
Scheme 5: Iodine catalyzed N-oxoammonium cation generation.
Additionally, it is known that N-oxoammonium cations, upon heating, react with
enolizable ketones to generate the α-TEMPO ketone 5 (Scheme 6). This is the same end
product as though the α-benzylic radical A was hypothetically trapped by TEMPO itself. It
was, however, noted that the conversion to the N-oxoammonium salt is significantly

8
influenced by the pH of the applied reaction media and the addition of an aqueous solution of
base is necessary to achieve the required conversion.19
Scheme 6: N-oxoammonium catalyzed oxidation of enolizable 6.
Thus, in order to support the involvement of iodine and benzylic radicals in our
reaction, supplementary studies were undertaken to rule out the formation of the N-
oxoammonium salt. This was achieved by monitoring the TEMPO radical using EPR
spectroscopy. Initially, molecular iodine and TEMPO were reacted in DMSO under the
optimized reaction conditions to afford the EPR spectrum, displayed in Figure 3. An intense
triplet signature is ascribed to the stable free radical of TEMPO,20 indicating that TEMPO
was not oxidized by molecular iodine upon heating within the elapsed time of the experiment.
Figure 3: EPR spectrum for the reaction of iodine and TEMPO in DMSO.

9
The reaction was repeated with the addition of an aqueous solution of sodium
bicarbonate (NaHCO3), followed by heating for 2 hours and analyzed using EPR
spectroscopy. This resulted in the disappearance of the characteristic TEMPO signal,
indicating the formation of the N-oxoammonium cation (Figure 4). Consequently, these
experiments support the proposed reaction mechanism presented in Scheme 2 in which the
reaction proceeds via a radical pathway and indicates that the capture product 5 originates
from the formation of A, rather than by reaction of the enolizable ketone and N-
oxoammonium salt.
Figure 4: EPR spectrum for the reaction of iodine and TEMPO with aqueous NaHCO3.
Next, owing to the involvement of iodine and benzylic radicals in our reaction, an α-
iodinated species B was predicted to be the reactive intermediate in the oxidation of 1 to
afford benzil 2. As a result, we sought to isolate the α-iodinated intermediate from the
reaction by varying time and temperature (Table 1), since the optimized reaction conditions
afford complete oxidation of benzyl phenyl ketone 1 to benzil 2 (Table 1, entry 1). Despite
numerous attempts of varying reaction conditions, we did not achieve any success in isolating

10
the target α-iodinated intermediate. On the contrary, the reaction proceeded to deliver the α-
diketone 2 or unreacted benzyl phenyl ketone 1 (Table 1, entry 2). We attempted to further
decrease the reaction temperature so as to decrease the rate of oxidation, however, only
quantitative yields of benzyl phenyl ketone 1 was obtained (Table 1, entries 3-4).
Table 1: Varying reaction conditions for isolation of the α-iodinated intermediate.a
Entry
Time (h)
Temp (°C)
Yield (%)
B
1
2
1b
2
2
100
25
n. d.
90
n. d.
96
24
24
24
n. d.
10
3
0
100
100
n. d.
n. d.
n. d.
4
– 78
n. d.
a Determined by 1H NMR. b Isolated yield.
As a result, the failure to isolate the α-iodinated reactive intermediate from the
reaction is reasoned by solvation effects, as well as the instability and reactivity of the C–I
bond in dipolar aprotic solvents:21 (i) Carbon–iodine (C–I) bonds are easily cleaved
thermally, or photo-chemically, due to the lower bond dissociation energy (BDE), 56.5 kcal
mol-1, as compared to other carbon–halogen bonds. (ii) Molecular iodine is least dependent
upon hydrogen bond stabilization with dipolar aprotic solvents. Its nucleophilicity thus
decreases, allowing for immediate attack by DMSO on the α–carbon of B. (iii) The sulfoxide

11
moiety of DMSO has an electronic charge, residing on both the oxygen and sulfur atoms,
which increases its nucleophilicity for interaction with an electrophile such as B. (iv) The
corresponding iodide ion (I–) distributes, more effectively, the negative charge that it has
obtained, making it a highly reactive leaving group in nucleophilic displacements. Benzylic
iodates are therefore difficult to observe and isolate, if formed in situ, in the presence of a
strong nucleophilic solvent such as DMSO.
As a result, the utility of alternate reaction conditions were explored and literary
studies disclosed that benzylic iodates have successfully been synthesized and isolated via
oxidation in weak, nucleophilic protic solvents, in good to excellent yields.22 Using this
approach, the oxidation was performed in a series of weak nucleophilic solvents (Table 2),
rather than DMSO, under the optimized reaction conditions to provide the α-iodinated
intermediate, 2-iodo-1, 2-diphenylethanone B. Initially, benzyl phenyl ketone 1 was heated to
100°C in tetrahydrofuran (THF) or dichloroethane (DCE) for 2 hours, however, the reaction
failed to produce the α-iodinated intermediate B (Table 1, entries 1-2). We, subsequently,
changed the solvent to acetonitrile (CH3CN) and iso-propanol (i-PrOH), however, only a
minor amount of iodo-intermediate was detected (Table 1, entries 3-4). However, when
benzyl phenyl ketone 1 and iodine were reacted in ethanol, under the optimized reaction
conditions, the target α-iodinated intermediate B was isolated in a yield of 26% (Table 2,
entry 5).

12
Table 2: Oxidation of benzyl phenyl ketone 1 in weak nucleophilic solvents.a
Entry
Solvent
Yield (%)
1
B
n. d.
n. d.
9
1
2
Tetrahydrofuran (THF)
Dichloroethane (DCE)
Acetonitrile (CH3CN)
Iso-propanol (i-PrOH)
Ethanol (EtOH)
100
100
91
3
4
90
10
5b
74
26
a Determined by 1H-NMR. b Isolated yield.
Analysis of the NMR spectra, provided in Figure 5, shows the singlet at 6.65 ppm23
corresponding to the α-proton of B, identifying the iodo-ketone as a key intermediate in the
oxidation process, thus further supporting the mechanism proposed in Scheme 2.

13
H2O
ppm
10
9
8
7
6
5
4
3
2
1
Figure 5: NMR spectrum of α-iodinated intermediate 2-iodo-1, 2-diphenylethanone B.
A supplementary study was performed in which the α-iodinated intermediate was
heated in DMSO, under the optimized reaction conditions, to afford benzil in 97% yield thus
adding further credence that B is the key intermediate in our reaction (Scheme 7).
Scheme 7: Oxidation of α-iodinated intermediate B to afford benzil 2.

14
This result supports the effect of solvent (solvation) on the reaction pathway in terms
of the nucleophilicity of the oxidant (solvent) and isolation of the highly reactive
intermediate, B, in the I2/DMSO oxidation of the α-methylene group of benzyl phenyl ketone.
Having completed the isolation of the reactive intermediate 2-iodo-1, 2-
diphenylethanone from the reaction, we turned our attention to determining the source of
oxygen in the reaction. Accordingly, there are three potential oxygen sources in the reaction
system: molecular oxygen in the air, a trace amount of water in the solvent DMSO and
DMSO itself. The reaction proceeded well under a nitrogen atmosphere, affording 2 in 95%
yield and indicating that oxygen from the air does not participate in the reaction (Scheme 8,
a). When the reaction was performed in a 1:1 toluene/water biphasic media, the oxidation
product 2 was isolated in 16% yield, indicating that water plays a minor role in the oxidation
reaction to afford 2 (Scheme 8, b). Finally, the reaction of benzyl phenyl ketone 1 in
anhydrous DMSO, under a nitrogen atmosphere, afforded the diketone in 98% yield (Scheme
8, c).
Scheme 8: Control experiments determining oxygen source in benzil 2.

15
This result suggests that the major source of oxygen in the diketone originates from
DMSO and, in order to support this result, we aimed to isolate the reductive product of the
sulfoxide moiety. The reductive product of DMSO is dimethyl sulfide (DMS) which is
difficult to isolate and spectroscopically analyze; hence we turned our attention to the use of
diphenyl sulfoxide (DPSO)24 as the source of oxygen, since its reductive product, diphenyl
sulfide (DPS), can be isolated and analyzed using NMR spectroscopy. Thus, the reaction of
benzyl phenyl ketone and diphenyl sulfoxide (DPSO) in dioxane afforded benzil 2 and DPS 8
in 92% and 79% yield, respectively, with a mole ratio of approximately 1:1 (Scheme 9).
≈
Scheme 9: Oxidation of benzyl phenyl ketone with DPSO under the optimized
reaction conditions.
This indicates that one molecule of DMSO reacts with one molecule of benzyl phenyl
ketone and the second oxygen atom in benzil originates from the sulfoxide. Accordingly, the
oxidation of 1 is not solely a DMSO catalyzed reaction, since the catalytic water still exists
but relies, primarily, on DMSO to react with the α-iodinated intermediate to afford the
diketone benzil 2.

16
3.
Conclusions
3
In conclusion, this research provides insight into the mechanism of the benzylic Csp –
H oxidation of an α-methylene ketone using an I2/DMSO system. The proposed reaction
mechanism was proven to proceed through: iodine and benzylic radicals, an α-iodinated
intermediate, 2-iodo-1, 2-diphenylethanone, and oxidation via DMSO (the major source of
oxygen in benzil). Each key intermediate and reaction step was proven using isolation and
spectroscopic techniques: EPR spectroscopy, NMR analysis, the judicious choice of radical
spin traps and experimental conditions to support our proposed oxidation reaction. This
3
study, therefore, provides much needed insight into the benzylic Csp –H oxidation of an α-
methylene bond to afford synthetically useful α-diketones.
4.
Experimental Details
1
All reagents were purchased without further purification. All H and 13C Nuclear
Magnetic Resonance (NMR) spectra were recorded on a Bruker Advance III spectrometer
operating at 400 MHz. Chemical shifts (δ) were reported in ppm using the Dimethyl
Sulfoxide-d6 (DMSO-d6) residual peak (δ 2.50) or Chloroform (CDCl3) residual peak (δ
1
7.26) for H NMR. Chemical shifts of 13C NMR were reported, relative to DMSO-d6 (δ
39.51) or CDCl3 (δ 77.0). The following abbreviations were used to describe peak splitting
patterns when appropriate: br = broad, s = singlet, d = doublet, t = triplet, q = quartet, m =
multiplet. Coupling constants, J, were reported in Hertz unit (Hz). High-resolution/Low-
resolution electron-spray ionization (ESI) mass spectra were recorded on a time-of-flight
(TOF) micromass spectrometer. Infra-Red (IR) spectra were recorded on Carey 630 FTIR.
Absorption maxima are expressed in wavenumbers (cm-1). Melting points were determined
using Kofler hot-stage melting apparatus. EPR measurements were conducted using a Bruker
EMX Ultra X spectrometer.

17
4.1 General procedure for α-methylene oxidation with I2/DMSO to afford benzil (2).
Benzyl phenyl ketone (1.0 mmol, 0.196 g)) and iodine (0.5 mmol, 0.126 g) were mixed
in a round bottomed flask with 1 mL DMSO and heated to 100 °C for 2h. After cooling,
a Na2S2O3/water solution was added to the reaction mixture, extracted with
dichloromethane and dried over anhydrous MgSO4. Removal of the solvent, under
vacuum, afforded the crude product which was purified by column chromatography
using 5:1 hexane:ethyl acetate.
Benzil 2 (0.202 g, 96%) was obtained as a yellow solid: Mp 94-96 °C; ʋmaz
(neat, cm-1): 3064, 1655, 1590, 1449, 1208. 1H NMR (400 MHz, CDCl3): 7.95-7.93 (m, 4H),
7.83-7.78 (m, 2H), 7.66-7.62 (m, 4H). 13C NMR (400 MHz, CDCl3): 129.5, 129.5, 132.2,
135.5, 194.8. GC-MS (m/z): 210.0 (10), 105.0 (100).25
4.2 General procedure for synthesis of 1, 2-diphenyl-2-((2, 2, 6, 6-tetramethylpiperidin-
1-yl)oxy)ethanone (5). Benzyl phenyl ketone (1.0 mmol, 0.196 g), TEMPO (1.0 mmol,
0.156 g) and iodine (0.5 mmol, 0.126 g) were mixed in a round bottomed flask with 1 mL
DMSO and heated to 100 °C for 2h. After cooling, a Na2S2O3/water solution was added
to the reaction mixture, extracted with dichloromethane, and dried over anhydrous
MgSO4. Removal of the solvent under vacuum, afforded the crude product which was
purified by column chromatography using 9:1 hexane:ethyl acetate
.

18
1, 2-Diphenyl-2-((2, 2, 6, 6-tetramethylpiperidin-1-yl)oxy)ethanone
5 (0.163 g, 46%) was obtained as a white solid: Mp 229-231 °C; ʋmaz (neat, cm-1): 3068,
1
2922, 2852, 1667, 1596, 1446, 1262, 1042; H NMR (400 MHz, CDCl3): 8.01-7.99 (m, 2H),
7.43-7.39 (m, 3H), 7.34-7.30 (m, 2H), 7.22-7.18 (m, 2H), 7.14-7.10 (m, 1H), 5.92 (s, 1H),
1.38-1.37 (m, 6H), 1.24-1.11 (m, 6H), 0.92 (m, 3H), 0.73 (m, 3H); 13C NMR (400 MHz,
CDCl3): 198.3, 137.8, 135.3, 132.9, 129.3, 128.3, 127.5, 127.2, 93.5, 60.0, 59.8, 40.3, 33.6,
33.3, 20.3, 20.2, 17.0; HRMS (ESI-TOF) m/z: [M + H]+ Calcd for C23H30NO2 352.2276;
found 352.2277.17
4.3 General procedure for synthesis of α-iodinated Intermediate 2-Iodo-1, 2-
diphenylethanone (B). Benzyl phenyl ketone (1.0 mmol, 0.196 g) and iodine (0.5 mmol,
0.126 g) were mixed in a round bottomed flask with 1 mL ethanol and heated to 100 °C
for 2h. After cooling, the solvent was removed under vacuum, affording the crude
product which was purified by column chromatography using 9:1 hexane:ethyl acetate.
2-Iodo-1, 2-diphenylethanone B (0.085 g, 26%) was obtained as a yellow
1
solid: Mp 92-93 °C; ʋmaz (neat, cm-1): 3056, 1670, 1210, 746; H NMR (400 MHz, CDCl3):
8.06-8.04 (m, 2H), 7.66-7.58 (m, 3H), 7.51-7.47 (m, 2H), 7.39-7.30 (m, 3H), 6.65 (s, 1H);
13C NMR (400 MHz, CDCl3): 27.8, 128.7, 128.8, 128.9, 129.0, 129.5, 133.6, 133.7, 137.4,

19
192.3; HRMS (ESI-TOF) m/z: [M + Na]+ Calcd for C14H11IONa+ 344.9752; found
344.9745.23
4.4 General procedure for α-methylene oxidation by diphenyl sulfoxide (DPSO) to
afford diphenyl sulfide (DPS) (8). Benzyl phenyl ketone (1.0 mmol, 0.196 g), diphenyl
sulfoxide (2.0 mmol, 0.404 g) and iodine (0.5 mmol, 0.126 g) were mixed in a round
bottomed flask with 1 mL dioxane and heated to 100 °C for 2h. After cooling, a Na2S-
2O3/water solution was added to the reaction mixture, extracted with dichloromethane
and dried over anhydrous MgSO4. Removal of the solvent, under vacuum, afforded the
crude product which was purified by column chromatography using 9:1 hexane:ethyl
acetate to afford benzil 2 (0.194 g, 92%) as a yellow solid and diphenyl sulfide 7.
Diphenyl sulfide 8 (0.147 g, 79%) was obtained as a clear liquid: Mp
60-62 °C; ʋmaz (neat, cm-1): 3056, 1578, 1474, 1438, 1023; 1H NMR (400 MHz, DMSO-d6):
7.2-7.41 (m, 10H); 13C NMR (400 MHz, DMSO-d6): 127.4, 129.5, 130.7, 134.8; HRMS
(ESI-TOF) m/z: [M]+ Calcd for C12H10S 186.0503; found 186.0507.26
5.
Acknowledgements
This work was supported by the National Research Foundation (NRF) of South Africa
through a postgraduate bursary (JJ) and a Thuthuka Research Grant (VJ) (Grant Number:
TTK180410319052). We would like to thank Njabulo Mbatha for his assistance with the EPR
analysis.

20
6.
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09 September 2019
Subject: Highlights for Iodine/DMSO Promoted Oxidation of Benzylic Csp3–H Bonds to
Diketones – a Mechanistic Investigation
Dear Sir/Madam
•
•
•
A mechanistic investigation into a benzylic C-H oxidation to diketones, using
an Iodine/DMSO system, is reported.
A cocktail of techniques such as NMR, EPR, radical traps and a surrogate
nucleophile was used.
Key intermediates were detected spectroscopically or isolated to confirm
the proposed mechanism.
Kind Regards
_________________________________________
Dr Vineet Jeena
Lecturer: Organic Chemistry
University of KwaZulu-Natal
School of Chemistry & Physics
College of Agriculture, Engineering and Science
Postal Address: Private Bag X01, Scottsville, 3209 South Africa
Telephone: +27 (0) 33 260 5327 Email: Jeenav1@ukzn.ac.za

Pietermaritzburg
South Africa
+(27) 033 260 5327
Jeenav1@ukzn.ac.za
School of Chemistry & Physics
College of Agriculture, Engineering and Science
Postal Address: Private Bag X01, Scottsville, 3209 South Africa
Telephone: +27 (0) 33 260 5327 Email: Jeenav1@ukzn.ac.za