Role of electron transfer processes in the oxidation of aryl sulfides catalyzed by nonheme iron complexes

Article abstract of DOI:10.1080/10426507.2016.1255623

The oxidation of a series of aryl 1-methyl-1-phenylethyl sulfides with H₂O₂catalyzed by the two tetradentate nonheme-iron complexes [(PDP)Fe^II(SbF₆)₂] and [(BPMCN)Fe^II(OTf)₂] occurs by an electron transfer-oxygen transfer (ET/OT) mechanism as supported by the observation of products deriving from fragmentation of the corresponding radical cations in association with S-oxidation products (sulfoxides).

Full text of DOI:10.1080/10426507.2016.1255623

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
Role of electron transfer processes in the
oxidation of aryl sulfides catalyzed by nonheme
iron complexes
Alessia Barbieri, Stefano Di Stefano, Osvaldo Lanzalunga, Andrea Lapi,
Marco Mazzonna & Giorgio Olivo
To cite this article: Alessia Barbieri, Stefano Di Stefano, Osvaldo Lanzalunga, Andrea Lapi,
Marco Mazzonna & Giorgio Olivo (2017) Role of electron transfer processes in the oxidation of aryl
sulfides catalyzed by nonheme iron complexes, Phosphorus, Sulfur, and Silicon and the Related
Elements, 192:2, 241-244, DOI: 10.1080/10426507.2016.1255623
To link to this article: http://dx.doi.org/10.1080/10426507.2016.1255623
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Date: 27 May 2017, At: 05:53

PHOSPHORUS, SULFUR, AND SILICON
, VOL. , NO. , –

http://dx.doi.org/./..
Role of electron transfer processes in the oxidation of aryl sulfides catalyzed
by nonheme iron complexes
Alessia Barbieri, Stefano Di Stefano, Osvaldo Lanzalunga, Andrea Lapi, Marco Mazzonna, and Giorgio Olivo
Dipartimento di Chimica, Sapienza Università di Roma and Istituto CNR di Metodologie Chimiche (IMC-CNR), Sezione Meccanismi di Reazione,
c/o Dipartimento di Chimica, Sapienza Università di Roma, Rome, Italy
ABSTRACT
ARTICLE HISTORY
The oxidation of a series of aryl 1-methyl-1-phenylethyl sulfides with H O catalyzed by the two tetradentate
Received  October 
Accepted  October 
2
2
II
II
nonheme-iron complexes [(PDP)Fe (SbF ) ] and [(BPMCN)Fe (OTf) ] occurs by an electron transfer-oxygen
6
2
2
transfer (ET/OT) mechanism as supported by the observation of products deriving from fragmentation of
the corresponding radical cations in association with S-oxidation products (sulfoxides).
KEYWORDS
Oxidation; sulfide; electron
transfer; nonheme iron
complexes
GRAPHICAL ABSTRACT
Introduction
accompanied by fragmentation products that unequivocally
demonstrated the occurrence of an ET-OT mechanism.1
In the oxidation of aryl 1-methyl-1-phenylethyl sulfides, the
exclusive Cα–S bond cleavage of the corresponding radical
cations allowed us to estimate the rate constant for the oxygen
rebound process (kOT).13
2,13
The oxidation of sulfides catalyzed by high-valent iron-oxo
species in heme and nonheme iron enzymes and their synthetic
models, has been the subject of intensive mechanistic investi-
gation in the last decades in light of its biological and synthetic
relevance.1 In this context, we have recently investigated the
role of electron transfer processes in the S-oxidation of aryl sul-
fides with iodosobenzene, PhIO, catalyzed by the pentadentate
–11
In order to gain more information on the role of ET pro-
cesses in the reactions promoted by nonheme iron complexes,
we have extended the mechanistic analysis to the oxidation of
aryl 1-methyl-1-phenylethyl sulfides with H2O2 catalyzed by
II 2+
II 2+ 12,13
iron complexes [(N4Py)Fe ] and [(Bn-TPEN)Fe ]
.
The S-oxidation of sulfides promoted by the active species,
II
two tetradentate nonheme-iron complexes [(PDP)Fe (SbF6)2]
IV
2+
the iron(IV)-oxo complexes [(N4Py)Fe (O)]
and [(Bn-
II
and [(BPMCN)Fe (OTf)2], where nonheme iron(V)-oxo com-
IV
2+
TPEN)Fe (O)] , can occur either by direct oxygen atom
transfer (DOT, Scheme 1, path a) or by sequential electron
transfer/oxygen rebound mechanism (ET-OT, Scheme 1, path
b-c).1
plexes have been proposed as the active species involved in the
oxidative processes.2
7–29
4–20
Product analysis in the oxidation of aryl diphenylmethyl sul-
fides and aryl 1-methyl-1-phenylethyl sulfides (Figure 1), rep-
Results and discussion
resents a useful tool to distinguish the DOT and ET-OT path- Oxidations of aryl 1-methyl-1-phenylethyl sulfides (1, X=,
ways since the corresponding radical cations undergo very fast OCH3; 2, X= CH3; 3, X= H; 4, X= Cl) by H2O2 catalyzed
II
II
fragmentation processes, involving either the Cα–H or Cα–S by [(PDP)Fe (SbF6)2] and [(BPMCN)Fe (OTf)2] were car-
bond cleavage (Scheme 1, path d).1
4,21–26
In the oxidations ried out by addition of the oxidant (60µmol) to a solution
IV
2+
promoted by the iron(IV)-oxo complexes [(N4Py)Fe (O)]
of the substrate (60µmol) and the iron complex (1.5µmol)
IV
2+
and [(Bn-TPEN)Fe (O)] formation of aryl sulfoxides was in CH3CN (600µL) stirred at 0°C for 6 min. Reaction
CONTACT Osvaldo Lanzalunga
IMC-CNR), Sezione Meccanismi di Reazione, c/o Dipartimento di Chimica, Sapienza Università di Roma, P.le A. Moro , I- Rome, Italy.
 Taylor & Francis Group, LLC
osvaldo.lanzalunga@uniroma.it
Dipartimento di Chimica, Sapienza Università di Roma andIstituto CNR di MetodologieChimiche
(
©

242
A. BARBIERI ET AL.
IV
+
IV
+
Scheme . Mechanism of oxidation of aryl sulfides promoted by the iron(IV)-oxo complexes[(NPy)Fe (O)] and [(Bn-TPEN)Fe (O)] .
in Table 1 and in Table 2 for the oxidations catalyzed by
II
II
[
(PDP)Fe (SbF6)2] and [(BPMCN)Fe (OTf)2], respectively.
The formation of fragmentation products in the oxidation
of aryl 1-methyl-1-phenylethyl sulfides 1–4 is a clear indi-
cation of the occurrence of an ET process from the sul-
V
2+
V
2+
fides to [(PDP)Fe =O] and [(BPMCN)Fe =O] (Scheme 2,
path a). As previously reported for the oxidations of the
Figure . Aryl diphenylmethyl sulfides and aryl -methyl--phenylethyl sulfides.
IV
2+
same substrates promoted by [(N4Py)Fe =O] and [(Bn-
IV
2+ 13
TPEN)Fe =O] , 2-phenyl- 2-propanol and diaryl disulfides
1
–
are formed after C S bond cleavage of radical cations produc-
ing the 2-phenyl-2-propyl cation and the arylsulfenyl radical
products were identified by H NMR analyses (comparison with
authentic specimens). 2-Phenyl-2-propanol and diaryl disul-
fides were the main products accompanied by minor amounts
of aryl 1-methyl-1-phenylethyl sulfoxides. Aryl 1-methyl-1-
phenylethyl sulfones were also observed in trace amounts
(
Scheme 2, path b). The cation leads to 2-phenyl-2-propanol
by reaction with traces of water present in CH3CN, while aryl-
21–26
sulfenyl radicals dimerize to diaryl disulfides.
Fragmenta-
+
•
+•
tion of radical cations 1 -4 occurs in competition with the
oxygen rebound process from the reduced iron(IV)-oxo com-
plexes to the radical cations leading to sulfoxides (OT, Scheme 2,
path c). In accordance with the presence of the two competi-
(
<1%). No products were formed in the absence of H2O2,
while small amounts of aryl sulfoxides (<1%, referred to the
amount of oxidant) were observed in the oxidation with H2O2 in
II
the absence of the nonheme complexes [(PDP)Fe (SbF6)2] and
+
•
+•
II
–
tive decay pathways for 1 -4 (C S fragmentation and oxygen
rebound), a regular increase of the 2-phenyl-2-propanol / aryl
sulfoxide product ratios (Tables 1–2) are observed by increasing
the electron withdrawing effect of the aryl substituents.
[(BPMCN)Fe (OTf)2].
Yields of the fragmentation product 2-phenyl-2-propanol
and of the S-oxidation products aryl 1-methyl-1-phenylethyl
sulfoxides, referred to the amount of oxidant, are reported
II
a
Table . Products and yields in the oxidation of aryl -methyl--phenylethyl sulfides (1–4) with H O catalyzed by [(PDP)Fe (SbF ) ] in CH CN at °C.


 

Substrate
Products (Yields %)b
Ratio[OH]/[SO]
1
X=OCH
.



.
.
.
.
.




2
3
4
X=CH

X=H
X=Cl
a
II
H O ( μmol), [(PDP)Fe (SbF ) ] (. μmol) and aryl -methyl--phenylethyl sulfides ( μmol) in CH CN ( μL) at °C in  min.


 

b
Yields (mol %) refer to the amount of oxidant.
II
b
Table . Products and yields in the oxidation of aryl -methyl--phenylethyl sulfides (1–4) with H O catalyzed by [(BPMCN)Fe (OTf) ] in CH CN at °C.




Substrate
Products (Yields %)b
Ratio[OH]/[SO]
1
X=OCH




.
.
.
.
.



2
3
4
X=CH

X=H
X=Cl

a
II
H O ( μmol), [(PDP)Fe (SbF ) ] (. μmol) and aryl -methyl--phenylethyl sulfides ( μmol) in CH CN ( μL) at °C in  min.


 

b
Yields (mol %) refer to the amount of oxidant.

PHOSPHORUS, SULFUR, AND SILICON
243
II
II
Figure . Tetradentate nonheme-iron complexes [(PDP)Fe (SbF ) ] and [(BPMCN)Fe (OTf) ].



Scheme . Competition between C–S fragmentation and oxygen rebound in the oxidation of aryl -methyl--phenylethyl sulfides 1–4 promoted by the iron(V)-oxo
V
+
V
+
.
complexes[(PDP)Fe =O] and [(BPMCN)Fe =O]
Table . C–S Fragmentation rate constants (k ) of aryl -methyl--phenylethyl sulfide radical cations (1 •-4 ) and oxygen rebound rate constants (kOT) for the oxidation
of 1–4 by [(PDP)Fe =O] and [(BPMCN)Fe =O]
+
+•
f
V
+
V
+
.
kOT( s
−
)

k ( s
f
− a
V
+
V
+
)
[(PDP)Fe =O]
[(BPMCN)Fe =O]
1
X=OCH
<
.
.
.
<.
.
.
<.
.
.
.

2
3
4
X=CH

X=H
X=Cl
.
a
From LFP experiments, see ref. .
V
2+
It is interesting to note that [(BPMCN)Fe =O] resulted Conclusions
V
2+
a better oxidant with respect [(PDP)Fe =O] leading to sig-
The oxidations of aryl 1-methyl-1-phenylethyl sulfides with
nificantly higher yields of the fragmentation product, 2-phenyl-
H2O2 catalyzed by the tetradentate nonheme iron complexes
2
-propanol. The higher 2-phenyl-2-propanol / aryl sulfox-
II
II
[
(PDP)Fe (SbF6)2] and [(BPMCN)Fe (OTf)2] involve an elec-
ide product ratios observed in the oxidations promoted by
tron transfer-oxygen rebound mechanism as previously pro-
posed for the oxidations of the same substrates promoted
V
2+
[
(BPMCN)Fe =O] (see Tables 1–2) might be rationalized on
the basis of a minor efficiency of the oxygen rebound process
IV
2+
by nonheme iron(IV)-oxo complexes [(N4Py)Fe =O] and
IV
+
from the reduced iron(IV) oxo complex [(BPMCN)Fe =O]
IV
2+
[
(Bn-TPEN)Fe =O] . The role of the ET process in the S-
IV
+
with respect to [(PDP)Fe =O] . The rate constants of the oxy-
gen rebound step (kOT) can be calculated from the fragmenta-
tion rate (kf) of the radical cations, previously determined by
laser flash photolysis experiments of the photochemical oxi-
dations of 1–4 by N-methoxyphenanthridinium ion,13 and the
ratio of the yields of the sulfoxides and the fragmentation prod-
uct 2-phenyl-2-propanol by applying the following equation:
oxidations of a more extended series of aryl sulfides catalyzed
by other nonheme iron complexes will be investigated in future
studies.
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