Electron Transfer Mechanism in the Oxidation of Aryl 1-Methyl-1-phenylethyl Sulfides Promoted by Nonheme Iron(IV)-Oxo Complexes: The Rate of the Oxygen Rebound Process

Article abstract of DOI:10.1021/acs.joc.6b02434

The oxidation of aryl 1-methyl-1-phenylethyl sulfides promoted by the nonheme iron(IV)-oxo complexes [(N4Py)Fe^IVO]²⁺ and [(Bn-TPEN)Fe^IVO]²⁺ occurs by an electron transfer-oxygen rebound (ET-OT) mechanism leadi

Full text of DOI:10.1021/acs.joc.6b02434

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Article
Electron Transfer Mechanism in the Oxidation of Aryl 1-
Methyl-1-phenylethyl Sulfides Promoted by Nonheme Iron(IV)-
Oxo Complexes: The Rate of the Oxygen Rebound Process
Alessia Barbieri, Tiziana Del Giacco, Stefano Di Stefano, Osvaldo
Lanzalunga, Andrea Lapi, Marco Mazzonna, and Giorgio Olivo
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.6b02434 • Publication Date (Web): 17 Nov 2016
Downloaded from http://pubs.acs.org on November 21, 2016
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Electron Transfer Mechanism in the Oxidation of Aryl
1ꢀMethylꢀ1ꢀphenylethyl Sulfides Promoted by
Nonheme Iron(IV)ꢀOxo Complexes: The Rate of the
Oxygen Rebound Process
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Alessia Barbieri,a Tiziana Del Giacco,b Stefano Di Stefano,a Osvaldo Lanzalunga,*a Andrea Lapi,a
Marco Mazzonna,a and Giorgio Olivoa
a Dipartimento di Chimica and Istituto CNR di Metodologie ChimicheꢀIMC, Sezione Meccanismi di
Reazione c/o Dipartimento di Chimica, Università degli Studi di Roma “La Sapienza”,
P.le A. Moro 5, 00185 Rome, Italy.
b Dipartimento di Chimica, Biologia e Biotecnologie and Centro di Eccellenza Materiali Innovativi
Nanostrutturati, Università di Perugia, via Elce di sotto 8, 06123 Perugia, Italy.
Email: osvaldo.lanzalunga@uniroma1.it
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if
required according to the journal that you are submitting your paper to)
Abstract: The oxidation of aryl 1ꢀmethylꢀ1ꢀphenylethyl sulfides promoted by the nonhemeꢀ
iron(IV) oxo complexes [(N4Py)FeIV=O]2+ and [(BnꢀTPEN)FeIV=O]2+ occurs by an electron
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transfer ꢀ oxygen rebound (ETꢀOT) mechanism leading to aryl 1ꢀmethylꢀ1ꢀphenylethyl sulfoxides
accompanied by products deriving from CαꢀS fragmentation of sulfide radical cations (2ꢀphenylꢀ2ꢀ
propanol and diaryl disulfides). For the first time the rate constants for the oxygen rebound process
(kOT), which are comprised in the range <0.8×104 sꢀ1 ꢀ 3.5×104 sꢀ1, were determined from the
fragmentation rate constants of the radical cations (kf) and the Sꢀoxidation/fragmentation product
ratios.
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Introduction
The oxidation of sulfides to sulfoxides represents one of the most important processes catalyzed by
heme and nonheme iron enzymes that has attracted a special interest in view of its relevance in
organic synthesis.1 A great number of heme and nonheme iron complexes, able to mimic the
activity of such enzymes, were found to efficiently catalyze the oxidation of sulfides to
sulfoxides.2ꢀ15 Product analysis have been accompanied by several mechanistic studies aimed to
ascertain the role of electron transfer processes in the oxidation of sulfides promoted by the active
species, high valent ironꢀoxo complexes. As a matter of fact, formation of sulfoxide products can
occur either by a direct oxygen transfer or “oxene process” (DOT, Scheme 1, path a) or by a
sequential electron transferꢀoxygen transfer process (ETꢀOT, Scheme 1, paths bꢀc).6ꢀ10,16ꢀ21
Scheme 1. DOT vs ETꢀOT mechanism in the oxidation of sulfides to sulfoxides promoted by high
valent iron(IV)ꢀoxo complexes.
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In this context we have recently investigated the oxidation of a series of aryl diphenylmethyl
sulfides by the nonheme iron(IV)ꢀoxo complex, [(N4Py)FeIV(O)]2+ [N4Py = N,N-bis(2ꢀ
pyridylmethyl)ꢀNꢀbis(2ꢀpyridyl)methylamine] in CH3CN.22 Product analysis revealed that formation
of aryl diphenylmethyl sulfoxides was accompanied by fragmentation products (diphenylmethanol,
benzophenone and diaryl disulfides) that unequivocally demonstrated an ETꢀOT mechanism.
Accordingly, these products originate from the fast fragmentation processes of radical cations,
involving both CꢀH and CꢀS bond cleavage as indicated in Scheme 2.23ꢀ28
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2+
O
N
IV
Fe
N
S
+
N
N
N
X
[(N4Py)FeIV=O]2+
X= OCH3, CH3, H, CF3
ET
O
.
+
+ [(N4Py)FeII]2+
S
S
OT
+ [(N4Py)FeIII=O]+
X
X
C-S
Fragmentation
Proton Loss
.
+
S
.
S
+ [(N4Py)FeIII-OH]2+
+
X
X
Scheme 2. Oxygen rebound (OT) vs fragmentation of sulfide radical cations generated by oxidation
of aryl diphenylmethyl sulfides with [(N4Py)FeIV(O)]2+.
On the basis of the effect of aryl ring substituents on the product distribution, we provided strong
evidence that fragmentation of radical cations competes with the oxygen rebound process (Scheme
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2). It has to be remarked that while the oxidation of aryl diphenylmethyl sulfides by
[(N4Py)FeIV(O)]2+ proceeds by an ETꢀOT mechanism, thioanisole oxidation by the same nonheme
ironꢀoxo complex, occurs by a DOT process.9,16
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When an ETꢀOT mechanism is operating, it is possible to estimate the rate of the oxygen rebound
step from the fragmentation rate constants of the sulfide radical cations and the product distribution
(fragmentation products vs sulfoxides). For instance, this strategy was adopted for the oxidation of
aryl benzyl sulfides with H2O2 catalyzed by horseradish peroxidase.8 Though aryl diphenylmethyl
sulfides represent suitable substrates to test the intermediacy of radical cations in biomimetic
oxidation processes,6 the occurrence of both CꢀH and CꢀS bond cleavage represents a major
drawback when the rate of the oxygen rebound process has to be determined. Since the rates of CꢀH
bond cleavage in sulfide radical cations are dependent on the nature of the deprotonating base, it
was not possible to directly compare the fragmentation rates of radical cation in the biomimetic
oxidation with those determined by laser flash photolysis (LFP) analysis of the photochemical
oxidation of the same substrates carried out in the presence of Nꢀmethoxyphenanthridinium
− 22
hexafluorophosphate (MeOP+PF6 ) due to the different bases involved in the CꢀH bond cleavage
processes ([(N4Py)FeIII=O]+ and phenanthridine, respectively). Accordingly, in the biomimetic
oxidation promoted by [(N4Py)FeIV=O]2+ a higher contribution of the CꢀS bond cleavage to the
overall fragmentation process was observed whereas CꢀH bond cleavage prevailed in the
photochemical process.22
To this purpose, in order to determine the rate of the oxygen rebound process from reduced
nonheme ironꢀoxo complexes to aryl sulfide radical cations (Scheme 1, path c), we have
investigated the oxidation of a series of aryl 1ꢀmethylꢀ1ꢀphenylethyl sulfides (1ꢀ4) promoted by two
nonheme iron(IV)ꢀoxo complexes: [(N4Py)FeIV=O]2+ and [(BnꢀTPEN)FeIV=O]2+ [BnꢀTPEN = Nꢀ
benzylꢀN,N’,N’ꢀtris(2ꢀpyridylmethyl)ꢀ1,2ꢀdiaminoethane)] (Figure 1). Since no cleavable benzylic
CꢀH bonds are present in the α position with respect to the S atom, fragmentation of radical cations
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1+•ꢀ4+• exclusively involves the cleavage of CαꢀS bond, as observed in the photochemical oxidations
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6
−
of aryl 1ꢀmethylꢀ1ꢀarylethyl sulfides with MeOP+PF6 and 9,10ꢀdicyanoanthracene/O2 systems.26,28
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Figure 1. Aryl 1ꢀmethylꢀ1ꢀphenylethyl sulfides 1ꢀ4 and nonheme iron(IV)ꢀoxo complexes
[(N4Py)FeIV(O)]2+ and [(BnꢀTPEN)FeIV=O]2+.
Results and Discussion
For product analysis of the biomimetic oxidations, aryl 1ꢀmethylꢀ1ꢀphenylethyl sulfides (1ꢀ4) (50
µmol) were added to a solution of the iron(IV)ꢀoxo complexes prepared by oxidation of
[(N4Py)FeII]2+ or [(BnꢀTPEN)FeII]2+ (2.5 µmol) with PhIO18 (12.5 µmol) in CH3CN (0.5 mL) and
stirred at 0oC for 90 min and 30 min respectively. The exclusive reaction products, identified by
GC, GCꢀMS, HPLC and 1H NMR analyses (comparison with authentic specimens), were 2ꢀphenylꢀ
2ꢀpropanol, diaryl disulfides, aryl 1ꢀmethylꢀ1ꢀphenylethyl sulfoxides. In all cases material recovery
was satisfactory (>95%). No products were formed in the absence of PhIO while small amounts of
aryl 1ꢀmethylꢀ1ꢀphenylethyl sulfoxides (< 1 %, referred to the amount of oxidant) were observed in
the oxidation with PhIO in the absence of the nonheme complexes [(N4Py)FeII]2+ or [(Bnꢀ
TPEN)FeII]2+.
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Products and yields, referred to the amount of oxidant, are reported in Table 1 and in Table 2 for the
oxidations promoted by [(N4Py)FeIV(O)]2+ and [(BnꢀTPEN)FeIV=O]2+, respectively.
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Table 1. Products and Yields in the Oxidation of Aryl 1ꢀMethylꢀ1ꢀphenylethyl Sulfides (1ꢀ4) by
[(N4Py)FeIV=O]2+ in CH3CN at 0°C.a
Substrate
Products (Yields %)b
Ratio
[OH]/[SO]
1 X=OCH3
2 X=CH3
3 X=H
31±2
39±2
40±4
48±5
14±1
15±1
16±2
22±2
64±5
44±4
0.48
0.89
1.6
25±2
4 X=CF3
8.4±0.5
5.7
a
Iodosylbenzene (12.5 µmol), [(N4Py)FeII(OTf)2] (2.5 µmol) and aryl 1ꢀmethylꢀ1ꢀphenylethyl sulfides (50 µmol) in
CH3CN (500 µL). b Yields (mol %) refer to the amount of oxidant PhIO.
Table 2. Products and Yields in the Oxidation of Aryl 1ꢀMethylꢀ1ꢀphenylethyl Sulfides (1ꢀ4) by
[(BnꢀTPEN)FeIV=O]2+ in CH3CN at 0°C.a
Products (Yields %)b
Substrate
Ratio
[OH]/[SO]
1 X=OCH3
2 X=CH3
3 X=H
56±4
62±5
64±6
70±5
27±2
25±2
28±4
27±3
44±4
33±2
18±2
10±1
1.3
1.9
3.6
7.0
4 X=CF3
a
Iodosylbenzene (12.5 µmol), [(BnꢀTPEN)FeII(OTf)2] (2.5 µmol) and aryl 1ꢀmethylꢀ1ꢀphenylethyl sulfides (50 µmol)
in CH3CN (500 µL). b Yields (mol %) refer to the amount of oxidant PhIO.
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From the data reported in Tables 1ꢀ2, it can be readily noted that higher yields of oxidation products
are observed in the oxidations promoted by [(BnꢀTPEN)FeIV=O]2+ with respect to those promoted
by [(N4Py)FeIV(O)]2+ in accordance with the higher oxidizing ability of the former iron(IV)ꢀoxo
complex as also found in the oxidation of paraꢀsubstituted thioanisoles.18 Moreover a regular
increase of the total yields of oxidation products are observed with both the ironꢀoxo complexes by
increasing the electron releasing power of the aryl substituents, in accordance with the electrophilic
nature of the oxidizing species [(N4Py)FeIV=O]2+ and [(BnꢀTPEN)FeIV=O]2+.18 As previously
observed in the oxidation of aryl diphenylmethyl sulfide by the [(N4Py)FeIV=O]2+,22 the Sꢀ
oxygenation products (sulfoxides) are accompanied by significant amounts of fragmentation
products (2ꢀphenylꢀ2ꢀpropanol and diaryl disulfides). Thus also the oxidation of 1ꢀ4 promoted by
[(N4Py)FeIV=O]2+ and [(BnꢀTPEN)FeIV=O]2+ involves an ET process with formation of radical
cations 1+•ꢀ4+•. CαꢀS bond cleavage of 1+•ꢀ4+• leads to the 2ꢀphenylꢀ2ꢀpropyl cation and an
arylsulfenyl radical as described in path a of Scheme 3. The cation leads to 2ꢀphenylꢀ2ꢀpropanol by
reaction with traces of water present in CH3CN,29 while arylsulfenyl radicals dimerize to diaryl
disulfides.26ꢀ28,32
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CH3
S
+ [(L)FeII]2+
X
5
O
CH3
6
7
(b)
kOT
8
9
CH3
CH3
[(L)FeIV=O]2+
.
+
S
X + [(L)FeIII=O]+
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S
X
CH3
CH3
X= OCH3, CH3, H, CF3
(a)
kf
.
CH3
CH3
L = N4Py, Bn-TPEN
S
+
X
x 2
+H2O, -H+
CH3
OH
S
S
X
X
CH3
Scheme 3. Fragmentation vs oxygen rebound processes in the oxidation of aryl 1ꢀmethylꢀ1ꢀ
phenylethyl sulfides (1ꢀ4) by [(N4Py)FeIV=O]2+ and [(BnꢀTPEN)FeIV=O]2+ in CH3CN.
Fragmentation of radical cations 1+•ꢀ4+• occurs in competition with the oxygen rebound process
from the reduced iron(III)ꢀoxo complexes to the radical cations leading to sulfoxides (OT, Scheme
3, path b).22 In accordance with the presence of the two competitive decay pathways for 1+•ꢀ4+•
(oxygen rebound and CαꢀS fragmentation), a regular increase of the 2ꢀphenylꢀ2ꢀpropanol / aryl 2ꢀ
phenylꢀ2ꢀpropyl sulfoxide product ratios (Tables 1ꢀ2) is observed on increasing the electron
withdrawing (EW) effect of the aryl substituents as shown in Figure 2.
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Figure 2. Yields (% referred to the amount of oxidant) of aryl 1ꢀmethylꢀ1ꢀphenylethyl sulfoxides
and 2ꢀphenylꢀ2ꢀpropanol in the oxidation of aryl 1ꢀmethylꢀ1ꢀphenylethyl sulfides 1ꢀ4 by (a)
[(N4Py)FeIV=O]2+ and (b) [(BnꢀTPEN)FeIV=O]2+ in CH3CN at 0°C.
Such increase of the relative amount of the CαꢀS fragmentation products observed by increasing the
EW power of the X substituents, results from the enhancement of fragmentation rate constants of
the radical cations on going from 1+• to 4+•, as already reported for the oxidation of aryl
diphenylmethyl sulfides by the [(N4Py)FeIV=O]2+.22 The rate of the CαꢀS bond cleavage of a series
of aryl 1ꢀmethylꢀ1ꢀarylethyl sulfide radical cations, including 1+• and 3+•, have been previously
determined by laser flash photolysis (LFP) in the photochemical oxidation of the sulfides carried
ꢀ
out in the presence of MeOP+PF6 in CH3CN at 25°C.26,33 In the same way we have now determined
the rate constants for fragmentation of 2+•ꢀ4+• (kf) at 0°C by fitting the exponential decays at the
maximum absorption wavelengths (see experimental section, Table 3 and Figures S5ꢀS7 in the
Supporting Information).
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Table 3 shows that fragmentation rates regularly increase regularly from 1+• to 4+• in the same way
as the 2ꢀphenylꢀ2ꢀpropanol / aryl 1ꢀmethylꢀ1ꢀphenylethyl sulfoxide molar ratios increase going
from 1 to 4 as reported in Tables 1 and 2.
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Table 3. Maximum Absorption Wavelengths and CαꢀS Fragmentation Rate Constants (kf) of Aryl 1ꢀ
Methylꢀ1ꢀphenylethyl Sulfide Radical Cations (1+•ꢀ4+•) Generated by Photooxidation of 1ꢀ4
Sensitized by MeOP+PF6 (λexc=355nm) and Oxygen Rebound Rate Constants (kOT) for the
ꢀ
Oxidation of 1ꢀ4 by [(N4Py)FeIV=O]2+ and [(BnꢀTPEN)FeIV=O]2+ in CH3CN at 0°C.
kOT(104 sꢀ1)b
kf(104 sꢀ1)a
λMAX
[(N4Py)FeIV=O]2+ [(BnꢀTPEN)FeIV=O]2+
1 X=OCH3
2 X=CH3
3 X=H
570
550
530
510
< 1
3.0
3.4
6.7
< 2.1
3.5
< 0.8
1.6
2.0
1.0
4 X=CF3
1.2
0.9
ꢀ
a From LFP experiments in N2ꢀsaturated CH3CN, [sulfide]=1.0×10ꢀ2 M, [MeOP+PF6 ]=1.6×10ꢀ4 M. b Calculated from eq
1.
Since the radical cations 1+•ꢀ4+• produced in the oxidations of 1ꢀ4 by [(N4Py)FeIV=O]2+ and
[(BnꢀTPEN)FeIV=O]2+ undergo partitioning between the oxygen rebound and the CαꢀS bond
fragmentation shown in Scheme 3, it is possible to estimate the rate of the oxygen rebound step
(kOT) from the overall fragmentation rate (kf) of the radical cations and the ratio of the yields of the
sulfoxides and the fragmentation product 2ꢀphenylꢀ2ꢀpropanol according to eq 1 (see Tables 1ꢀ2).34
The values of kOT are reported in the last column of Table 3.
kOT = kf [ArSOC(CH3)2C6H5]/[C6H5C(CH3)2OH]
(1)
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Interestingly, the kOT values with all the sulfides and both the nonheme iron(IV)ꢀoxo complexes are
of the same order of magnitude (see Table 3), thus supporting a common ETꢀOT oxidation
mechanism involving a competition between the oxygen rebound (OT) and CαꢀS fragmentation of
radical cations 1+•ꢀ4+•. The lower kOT values found with [(BnꢀTPEN)FeIII=O]+ can be reasonably
explained on the basis of a higher steric hindrance of the BnꢀTPEN with respect to the N4Py iron
ligand. Finally, it has to be noted that the kOT values are slightly higher than those determined in the
oxidation of aryl benzyl sulfides with H2O2 catalyzed by horseradish peroxidase,8 where the OT
process involves the reaction of aryl sulfide radical cations with an heme iron(IV)ꢀoxo complexes.
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Conclusions
The ETꢀOT mechanism previously proposed for the oxidation of aryl diphenylmethyl sulfides by
[(N4Py)FeIV=O]2+ is a rather general process which also operates with both tꢀalkyl aryl sulfides,
such as aryl 1ꢀmethylꢀ1ꢀphenylethyl sulfides, and other nonheme iron(IV)ꢀoxo complexes like
[(BnꢀTPEN)FeIV=O]2+. The rate constant for the oxygen rebound step has been determined for the
first time to range between <0.8×104 and 3.5×104 sꢀ1, with values very close to those measured for
the heme enzyme horseradish peroxidase. The dependence of the rate constants of the oxygen
rebound process on the ligand and sulfide structures will be investigated in future studies with the
analysis of the oxidation of a more extended series of tꢀalkyl aryl sulfides promoted by different
nonheme iron(IV)ꢀoxo complexes.
Experimental Section
Instruments and General Methods: Oxidation products were identified by comparison of their
GC and/or GC/MS retention times with those of authentic compounds. GC analyses were carried
out on a gas chromatograph equipped with a capillary methylsilicone column (30 m x 0.25 mm x 25
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µm). GCꢀMS analyses were performed with a mass detector (EI at 70eV) coupled with a gas
chromatograph equipped with a melted silica capillary column (30 m x 0.2 mm x 25 µm) covered
with a methylsilicone film (5% phenylsilicone, OV5). HPLC analyses were performed on a
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chromatograph equipped with a C18 column (core shell 5 ꢀm, 25 x 4.6 mm). NMR spectra were
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recorded on a 300 MHz spectrometer and were internally referenced to the residual proton solvent
signal. Laser flash photolysis experiments were carried out with a laser kinetic spectrometer
providing 8 ns pulses using the third harmonic (355 nm) of a Qꢀswitched Nd:YAG laser. The laser
energy was adjusted to ≤10 mJ/pulse through the use of the appropriate filter. A 3.5 mL Suprasil
quartz cell (10 mm × 10 mm) was used for all experiments.
Materials. All commercial chemicals employed were of the highest purity available and used
without further purification. CH3CN (spectrophotometric grade) was distilled over CaH2 under
argon prior to use. Nꢀmethoxyphenanthridinium hexafluorophosphate was prepared according to a
literature procedure.35 Iodosylbenzene was prepared by a literature method and stored at 0°C under
inert atmosphere.36 (N4Py)Fe(OTf)2 and (BnꢀTPEN)Fe(OTf)2 were prepared by metallation of the
ligand N4Py37 and BnꢀTPEN38 with Fe(OTf)2 according to a literature method.39 [(N4Py)FeIV=O]
and [(BnꢀTPEN) FeIV=O] were prepared by reacting Fe(N4Py)(OTf)2 and Fe(BnꢀTPEN)(OTf)2 with
excess solid PhIO.39 Aryl 1ꢀmethylꢀ1ꢀphenylethyl sulfides 1ꢀ3 were prepared by acid catalyzed
reaction of 4ꢀXꢀthiophenols with 2ꢀphenylꢀ2ꢀpropanol according to a literature method40 and
characterized according to the data reported in the literature.26,41 4ꢀTrifluoromethylphenyl 1ꢀmethylꢀ
1ꢀphenylethyl sulfide 4 was prepared by a procedure reported in the literature42 with some
modification. A solution of 500 mg (2.8 mmol) of 4ꢀtrifluoromethylthiophenol in 0.8 mL of dry
diethyl ether was added to phenylmagnesium bromide prepared from 420 mg (2.7 mmol) of
bromobenzene and 68 mg (2.8 mmol) of magnesium in 5 ml of dry diethyl ether. The mixture was
refluxed for 15 min, then 456 mg (2.3 mmol) of 2ꢀphenylꢀ2ꢀpropyl bromide was added. The
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resulting mixture was refluxed for 3.5 h. A standard aqueous workup followed by purification by
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silica gel column chromatography with hexane as eluent gave 450 mg (67 %) of 4.
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4. White solid, mp 35ꢀ36 °C. 1H NMR (300 MHz, CDCl3): δ 1.75 (s, 6H), 7.19ꢀ7.36 (m, 5H), 7.41ꢀ
7.49 (m, 4H) (Figure S1). 13C NMR (75 MHz, CDCl3): δ 30.4, 52.3, 119.2ꢀ130.9 (q, J = 270 Hz),
125.5ꢀ125.6 (q, J = 3.7 Hz), 127.1, 127.4, 128.7, 130.4ꢀ131.3 (q, J = 1.1 Hz), 136.1, 138.6ꢀ138.7 (q,
J = 1.4 Hz), 146.5 (Figure S2). EIꢀMS (70 eV) m/z (rel. intensity): 77 (7), 91 (35), 103 (9), 115 (5),
119 (100), 178 (9), 296 (M+,1). Anal. Calcd. for C16H15F3S: C, 64.85; H, 5.10; S, 10.82. Found: C,
65.04; H, 5.07; S, 10.95.
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Laser Flash Photolysis Studies. LFP experiments were carried out with a laser kinetic
spectrometer using the third harmonic (355 nm) of a Qꢀswitched Nd:YAG laser, delivering 8 ns
pulses. The laser energy was adjusted to
≤
10 mJ/pulse by the use of the appropriate filter. A 3.5 mL
Suprasil quartz cell (10 mm
× 10 mm) was used in all the experiments. N2ꢀsaturated CH3CN
ꢀ
solutions of MeOP+PF6 (1.6 × 10ꢀ4 M) and aryl 1ꢀmethylꢀ1ꢀphenylethyl sulfides (1.0×10ꢀ2 M) were
employed under magnetic stirring. The transient spectra of the MeOP+/2 and MeOP+/4 systems (for
the transient spectra of MeOP+/1 and MeOP+/3 systems see ref. 26) were obtained at 25 ± 0.5 °C by
a pointꢀtoꢀpoint technique, monitoring the change of absorbance (ꢁA) after the laser flash at
intervals of 10 nm over the spectral range of 390−650 nm (see Figures S3ꢀS4 in the Supporting
Information). The firstꢀorder rate constants for the fragmentation of radical cations 2+•−4+• (kf) were
obtained by fitting the absorbance decay data at the maximum absorption wavelengths (510−570
nm) to the exponential equation (see Figures S5ꢀS7 in the Supporting Information). The rate
constants kf refer to 0 ± 0.5 °C. Five to seven kinetic runs were performed with each solutions and
the rate constants averaged. The error estimated on the rate constants was ±10%.
Product Analysis of the Oxidation of 1−4 by [(N4Py)FeIV=O]2+. [(N4Py)FeIV=O] was generated
in situ at room temperature by adding iodosylbenzene (12.5 ꢂmol) to a stirred solution of
[(N4Py)FeII(OTf)2] (2.5 ꢂmol) in acetonitrile (500 ꢂL).39 The mixture was vigorously stirred at
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room temperature for 15 min, then the solution was cooled down at 0 °C and 50 ꢂmol of aryl 1ꢀ
methylꢀ1ꢀphenylethyl sulfides (1−4) were added. After 90 min of vigorous stirring 25 ꢂmol of a
Na2S2O5 aqueous solution and the internal standard were added. The mixture was extracted with
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Et2O, the organic layer was dried over Na2SO4 and analyzed by HPLC, GC and H NMR. All
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products formed (2ꢀphenylꢀ2ꢀpropanol, aryl disulfides, aryl 1ꢀmethylꢀ1ꢀphenylethyl sulfoxides)
were identified by comparison with authentic specimens. No products were observed in blank
experiments carried out in the absence of the oxidant. Only small amounts of aryl 1ꢀmethylꢀ1ꢀ
phenylethyl sulfoxides (<1% refer to the amount of oxidant PhIO) were observed in the oxidation
with PhIO in the absence of the complex [(N4Py)FeII(OTf)2].
Product Analysis of the Oxidation of 1−4 by [(BnꢀTPEN)FeIV=O]2+. [(BnꢀTPEN)FeIV=O] was
generated in situ at 0 °C by adding iodosylbenzene (12.5 ꢂmol) to a stirred solution of [(Bnꢀ
TPEN)FeII(OTf)2] (2.5 ꢂmol) in acetonitrile (500 ꢂL).39 The mixture was vigorously stirred for 5
min at 0 °C, then 50 ꢂmol of aryl 1ꢀmethylꢀ1ꢀphenylethyl sulfides (1−4) were added. After 30 min
of vigorous stirring 25 ꢂmol of a Na2S2O5 aqueous solution and the internal standard were added.
The mixture was extracted with Et2O, the organic layer was dried over Na2SO4 and analyzed by
1
HPLC, GC and H NMR. Reaction products were identified as reported above for the oxidation
promoted by [(N4Py)FeIV=O]. No products were observed in blank experiments carried out in the
absence of the oxidant. Only small amounts of aryl 1ꢀmethylꢀ1ꢀphenylethyl sulfoxides (<1% refer
to the amount of oxidant PhIO) were observed in the oxidation with PhIO in the absence of the
complex [(BnꢀTPEN)FeII(OTf)2].
Acknowledgements. Thanks are due to the Ministero dell’Istruzione, dell'Università e della Ricerca
(MIUR) for financial support and to the CIRCC, Interuniversity Consortium of Chemical Catalysis
and Reactivity. We thank Prof. M. Bietti and Dr. M. Salamone for the use of a LFP equipment.
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Supporting Information Available: 1H NMR and 13C NMR spectra of 4ꢀtrifluoromethylphenyl 1ꢀ
methylꢀ1ꢀphenylethyl sulfide (4); timeꢀresolved absorption spectra and decay kinetics after LFP of
the MeOP+/2ꢀ4 systems in CH3CN; This material is available free of charge via the Internet at
http://pubs.acs.org.
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References and notes
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Oae, S.; Watanabe, Y.; Fujimori, K. Tetrahedron Lett. 1982, 23, 1189.
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Park, J.; Morimoto, Y.; Lee, Y.ꢀM.; Nam, W.; Fukuzumi, S. J. Am. Chem. Soc. 2011, 133,
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Park, J.; Morimoto, Y.; Lee, Y.ꢀM.; Nam, W. Fukuzumi, S. J. Am. Chem. Soc. 2012, 134,
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Park, J.; Lee, J.; Suh, Y.; Kim, J.; Nam, W. J. Am. Chem. Soc. 2006, 128, 2630.
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Barbieri, A.; De Carlo Chimienti, R.; Del Giacco, T.; Di Stefano, S.; Lanzalunga, O.; Lapi,
A.; Mazzonna, M.; Olivo, G.; Salamone M. J. Org. Chem. 2016, 81, 2513.
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(a) Lanzalunga, O.; Lapi, A.; J. Sulfur Chem. 2011, 33, 101. (b) Baciocchi, E.; Del Giacco,
T.; Giombolini, P.; Lanzalunga, O. Tetrahedron 2006, 62, 6566. (c) Glass, R. S. Top. Curr. Chem.
1999, 205, 1.
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Adam, W.; Argüello, J. E.; Peñéñory, A. B. J. Org. Chem. 1998, 63, 3905.
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25.
Del Giacco, T.; Lanzalunga, O.; Mazzonna, M.; Mencarelli, P. J. Org. Chem. 2012, 77,
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1843.
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26.
Baciocchi, E.; Bettoni, M.; Del Giacco, T.; Lanzalunga, O.; Mazzonna, M.; Mencarelli, P. J.
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Formation of alcohols by reaction of relatively stable tertiary carbocations such as the 2ꢀ
phenylꢀ2ꢀpropyl cation with water in acetonitrile (ca. 0.01 M in the spectrophotometric grade
CH3CN) was reported in the photosensitized oxidation of aryl 1ꢀmethylꢀ1ꢀphenylethyl sulfides (refs.
26ꢀ28). Acetamides deriving from the Ritter reaction with the solvent CH3CN were instead formed
from less stable and less selective carbocations such as the tꢀbutyl and benzyl cations (refs.
28,30,31).
30.
Baciocchi, E.; Del Giacco, T.; Lanzalunga, O.; Mencarelli, P.; Procacci, B. J. Org. Chem.
2008, 73, 5675.
31.
Cavattoni, T.; Del Giacco, T.; Lanzalunga, O.; Mazzonna, M.; Mencarelli, P.; J. Org. Chem.
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32.
The yields of diaryl disulfides are less than half of those of 2ꢀphenylꢀ2ꢀpropanol probably
due to a further oxidation of the disulfides under the reaction conditions.
33.
The fragmentation rate constant of 1+• was too low to be determined in the LFP experiments
and only an upper limit could be given (Ref. 26).
34.
It was not possible to calculate kOT for 1+• since the fragmentation rate constant was too low
to be determined in the LFP experiments, see Table 3.
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Duelund, L.; Hazell, R.; McKenzie, C. J.; Nielsen, L. P.; Toftlund, H. J. Chem. Soc. Dalton
Trans 2001, 152.
39.
Kaizer, J.; Klinker, E. J.; Oh, N. Y.; Rohde, J.ꢀU.; Song, W. J.; Stubna, A.; Kim, J.; Münck,
E.; Nam, W.; Que, L. Jr. J. Am. Chem. Soc. 2004, 126, 472.
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Guo, Y.; Jenks, W. S. J. Org. Chem. 1997, 62, 857.
Creary, X. J. Org. Chem. 1985, 50, 5080.
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