Photochemical Co-Oxidation of Sulfides and Phosphines with Tris(p-bromophenyl)amine. A Mechanistic Study

Article abstract of DOI:10.1021/acs.joc.8b00913

The photochemistry of tris(p-bromophenyl)amine was investigated in a nitrogen- and oxygen-flushed solution under laser flash photolysis conditions. The detected intermediates were the corresponding amine radical cation ("Magic Blue") and the N-phenyl-4a,4b-dihydrocarbazole radical cation that, under an oxygen atmosphere, is converted to the corresponding hydroperoxyl radical. The role of the last species was supported by the smooth co-oxidation of sulfides to sulfoxides. On the other hand, co-oxidation of nucleophilic triarylphosphines to triarylphosphine oxides arose from an electron transfer between the photogenerated "Magic Blue" and phosphine that prevented the amine cyclization. In this case, intermediate Ar₃POO^?+ was found to play a key role in phosphine oxide formation.

Full text of DOI:10.1021/acs.joc.8b00913

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Article
Photochemical co-oxidation of sulfides and phosphines
with tris(p-bromophenyl)amine. A mechanistic study.
Sergio Mauricio Bonesi, Stefano Protti, and Angelo Albini
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.8b00913 • Publication Date (Web): 25 May 2018
Downloaded from http://pubs.acs.org on May 25, 2018
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Photochemical coꢀoxidation of sulfides and phosphines with tris(pꢀbromophenyl)amine. A
mechanistic study.
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Sergio M. Bonesi*,a,b, Stefano Prottib and Angelo Albinib
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aDepartamento de Química Orgánica,CIHIDECAR – CONICET, 3er Piso, Pabellón 2, Ciudad
Universitaria, FCEyN, University of Buenos Aires, Buenos Aires, 1428, Argentina.
Phone/FAX:+541145763346.
bPhotoGreen Lab, Department of Chemistry, V.leTaramelli 12, 27100 Pavia, Italy.
E-mail: smbonesi@qo.fcen.uba.ar
Table of Content.
HO
e
O
Br
Br
H
Br
Br
Ar3P-OO.+
O2
R2S
N
Ar3P
Ar3P=O
O2
N
Products
Ar
e
Ar
hν
oxidizing
species
e
H
H
R2S=O
+
H2O
Ar3P
Br
Br
Br
Br
N
N
Co-oxidation of
phosphines
Co-oxidation of
sulfides
Ar
Ar
one-electron oxidizing
species
Abstract.
The photochemistry of tris(pꢀbromophenyl)amine was investigated in nitrogenꢀ and oxygenꢀflushed
solution under laser flash photolysis conditions. Intermediates detected were the corresponding
amine radical cation (‘Magic Blue’) and the Nꢀphenylꢀ4a,4bꢀdihydrocarbazole radical cation that,
under an oxygen atmosphere is converted to the corresponding hydroperoxyl radical. The role of the
last species was supported by the smooth coꢀoxidation of sulfides to sulfoxides. On the other hand,
coꢀoxidation of nucleophilic triarylphosphines to triarylphosphine oxides arose from an electron
transfer process between the photogenerated "Magic Blue" and phosphine that prevented the amine
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cyclization. In this case, intermediate Ar3POO•+ was found to play a keyꢀrole in phosphine oxide
formation.
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Introduction.
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Single electron transfer processes (SET, Scheme 1a) have a keyꢀrole ranging from synthetic
chemistry1 to material chemistry.2 However, since the key intermediate contains an unpaired
electron, it is much more reactive than the starting compound, and therefore multiꢀelectron
processes, interference with oxygen and chain reactions often followed spontaneously SET.
Scheme 1. a) Single Electron Transfer (SET) processes; b) SET occurring on triarylamines and
following pathways observed.
In the case of SETꢀbased oxidation processes, organic oxidants, such as the popular 'Magic Blue',3
are often preferred to metal ions for mechanistic investigations since the better control on the
kinetics of the reaction that these reagents allow. In this context, the case of tris(4ꢀ
bromophenyl)amine (1, Chart 1), the precursor of Magic Blue would be of high interest, because in
the past, structurally related, but less oxidizing reagents, diꢀ and triaryl amines were reported to
undergo smooth cyclization to the corresponding carbazoles upon irradiation. The reactions of such
compounds in the presence of nucleophiles under photochemical conditions are also investigated in
detail3 and the competition between different processes may be more informative. Furthermore, the
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photoinduced electrocyclization of stable triarylamine radical cations was deeply described by M. A.
Fox and coꢀworkers.4
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Chart 1. Structure of tri(4ꢀbromophenyl)amine.
For this aim, we thought thus worthwhile to examine in detail the photochemical behavior of the
Magic Blue precursor, viz. tris(pꢀbromophenyl)amine (1), in the presence of nucleophilic additives
such as sulfides and triarylphosphines, oxygen and in solvents of different nature.
Results.
Time-resolved spectroscopic measurements of tris(p-bromophenyl)amine (1) in solution. In order to
obtain mechanistic information on the photochemistry of amine 1 in MeCN at room temperature,
laser flash photolysis studies were carried out. Under nitrogen atmosphere, the initial formation of a
transient with three bands centered at 370, 590 and 695 nm was evidenced (see Fig 1). This
absorption pattern was identical to that of Magic Blue (1.+),5 and evolved in 38 ꢀs into a further
transient (λmax = 750 nm, τ>200 ꢀs) that has been assigned, on the basis of previous studies, to the
dihydrocarbazole 2 (Fig 1).4a,6 A comparison between the absorption spectra of transients 1•+, 2 and
commercial Magic Blue (Figure 2) pointed out that 1•+ is formed after the laser pulse. In Fig. 2a the
decay of the intermediate and its corresponding conversion into 2 are clearly showed and Scheme 2
depicts the observed stepwise process.
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Scheme 2. Formation of transients 1•+ and 2 in nitrogen equilibrated media.
On the other hand, in oxygen equilibrated solution the endꢀofꢀpulse spectrum showed an additional
feature at 550 nm, then evolving to give the same end spectrum as under nitrogen, where the
absorption spectra of transients 1•+ and 2 were detected (Fig 1b). Figure 2b shows the decay trace of
1•+ and the formation of trace of transient 2 recorded after the laser pulse under oxygen atmosphere.
The quantum yield values for formation of the two intermediates along with their lifetime were
measured under both atmospheres, by using benzophenone in benzene as the reference (Table 1).
0.016
0.012
0.008
0.004
0.000
0.020
0.015
0.010
0.005
0.000
(a)
(b)
400
500
600
700
800
400
500
600
700
800
λ / nm
λ / nm
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2.0
1.5
1.0
0.5
0.0
(c)
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700
800
λ / nm
Figure 1. Timeꢀresolved absorption spectra of transients of trisꢀ(pꢀbromophenyl)amine (1, 0.3 mM)
recorded during 200 ꢀs after laser pulse (355 nm) in MeCN under (a) nitrogen and (b) oxygen
atmospheres. (c) Absorption spectra of commercial Magic Blue (solid line), photogenerated 1•+
recorded at 10 ꢀs after the pulse (dashed line) and transient 2 recorded at 90 ꢀs after the pulse
(dotted line).
(a)
(b)
0.02
0.01
0.00
0.02
0.01
0.00
695 nm
740 nm
705 nm
750 nm
0
50
100
t / ꢀs
150
200
0
50
100
150
200
t / ꢀs
Figure 2. Decay traces of transients from tris(pꢀbromophenyl)amine (0.3 mM) recorded after laser
pulse (355 nm) in MeCN under (a) nitrogen and (b) oxygen atmospheres.
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The fleeting band located at 550 nm in the timeꢀresolved absorption spectrum of tris(pꢀ
bromophenyl)amine 1 (see fig. 1(b)) was assigned to the chargeꢀseparated complex between 1 and
molecular oxygen which is energetically favorable process in polar solvents as it was early proposed
by Wilkinson7 and in recently reported literature.8 Scheme 3 shows the formation of the chargeꢀ
transfer complex that is converted to the tris(pꢀbromophenyl)amine radical cation (1.+) and
superoxide ion and Figure 3 depicts the decay trace of the chargeꢀtransfer complex in MeCN under
O2 atmosphere.
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3
Br
N
*
Br
N
Br
τ
ET = 17.9 ꢀs
kET = 5.6x104 sꢀ1
δ
.ꢀ
3O2
δ
O2
+
+
O2
N
Br
Br
Br
Br
Br
Br
(1.+; λabs = 695 nm)
Charge-Transfer Complex
Laser Pulse
Solvent, O2
φISC
λ
abs = 550 nm
3O2
Br
τB = 2.2 ꢀs
kB = 4.5x105 sꢀ1
N
Br
Br
1
Scheme 3. Formation of a chargeꢀtransfer complex and transient 1•+ in oxygen equilibrated media.
0.025
λ = 550 nm
0.020
0.015
0.010
0.005
0.000
-0.005
-10
0
10
20
30
40
t / ꢀs
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Figure 3. Decay trace of charge transfer complex recorded at 550 nm after excitation with a laser
pulse (355 nm) of a solution of trisꢀ(pꢀbromophenyl)amine (0.3 mM) in MeCN under oxygen
atmosphere.
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The chargeꢀtransfer complex trace shown in fig. 3 fitted with a biꢀexponential decay and two halfꢀ
lifetime decays were obtained, viz. 2.2 ꢀs and 17.9 ꢀs, respectively. The evolution of the chargeꢀ
transfer complex involved the reversion pathway to tris(pꢀbromophenyl)amine (1) and molecular
oxygen with a rate constant kB of 4.5x105 sꢀ1 and an electronꢀtransfer pathway with a rate constant
kET of 5.6x104 sꢀ1 to provide tris(pꢀbromophenyl)amine radical cation (1.+) and superoxide ion (see
Scheme 3).
Noteworthy, under O2 atmosphere the quantum yield values for the formation of 1•+ and 2 were
halved highlighting the interaction between molecular oxygen and 1 in its triplet excited state to
produce singlet oxygen. Therefore, this competitive pathway diminishes the formation of both of the
ensuing intermediates, 1•+ and 2. A similar oxygen effect was observed during the irradiation of
triphenylamine in different solvents and was attributed to the quenching of the triplet excited state of
triphenylamine by molecular oxygen producing singlet oxygen with φꢁ = 0.63.6 In this case, the
corresponding intermediate dihydrocarbazole, which is equivalent to intermediate 2 is formed in a
lesser extent in oxygen atmosphere than in nitrogen atmosphere.
Table 1. Measured Quantum Yields (Φ) for the formation of intermediates 1•+ and 2 under the
examined conditionsa
Transients
1•+
Atmosphere Conc. / M
1.24x10ꢀ6
λ
abs / nm
φ
τ / ꢀs
695
N2
0.158 39
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O2
N2
O2
8.29x10ꢀ7
3.53x10ꢀ7
2.17x10ꢀ7
0.077
750
0.066 > 200
0.030
5
8
9
a
Measured in MeCN after laser pulse (355 nm); ε(1•+) = 19000 Mꢀ1 sꢀ1; ε(2) = 30000 Mꢀ1 sꢀ1. The
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initial absorbance of 1 was fixed at 0.40. Actinometer: solution of benzophenone in benzene under
N2 atmosphere (A(355 nm) = 0.37); ε(ketyl radical) = 7220 Mꢀ1 cmꢀ1.
Further, the decay traces of transient 1•+ at 695 nm were also measured in MeCN after a laser pulse
(355 nm) under different conditions (Figure 4) with the aim to determine both the rate constants of
cyclization (kcyc) and backꢀelectron transfer (kBET, see Scheme 2). The obtained results were
collected in Table 2. Noteworthy, the traces of 1•+ show a nicely biꢀexponential decay fitting with r2
values higher than 0.998 independent from the atmosphere used. As can be seen in Table 2 τcyc and
kcyc remain almost constant for all the atmospheres used and also in an airꢀequilibrated solution
while a noticeable dependence on the atmosphere was seen for the backꢀelectron transfer process
(τBET and kBET), particularly in the cases of airꢀequilibrated and oxygen atmosphere. Likewise, a
similar behavior was observed for the concentration of 1•+ at zero time after the laser pulse which
was attributed to the quenching of the triplet excited state of 1 by molecular oxygen.
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0.14
0.12
0.10
0.08
0.06
0.04
0.02
0.00
-0.02
N
2
N O
2
Air
O
2
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0
50
100
t / ꢀs
150
200
Figure 4. Normalized decay traces of transients from tris(pꢀbromophenyl)amine (1, 0.3 mM)
recorded after laser pulse (355 nm) in MeCN under: N2 (blue line); N2O (black line); Air (red line)
and O2 (violet line) atmospheres.
Table 2. Rate constants, lifetimes and concentration of tris(pꢀbromophenyl)amine radical cation
(1•+) at the endꢀofꢀpulse.a
Atmosphere
kcyc / sꢀ1
2.5x104
2.5x104
2.6x104
2.7x104
kBET / Mꢀ1 sꢀ1
2.2x1010
2.1x1010
1.9x1010
1.4x1010
Conc. / M
5.93x10ꢀ6
5.93x10ꢀ6
4.39x10ꢀ6
3.29x10ꢀ6
τ
cyc / ꢀs
τ
BET / ꢀs
N2
N2O
Air
O2
40.0
3.7
40.0
3.7
39.2
5.4
37.0
6.4
a Measured in MeCN after laser pulse (266 nm); ε(1+·) = 19000 Mꢀ1 sꢀ1. The initial absorbance of the
solutions was fixed at 0.64. The error of τ1 is ± 0.2 and for τ2 is ± 0.1.
Due to the biꢀexponential decay fitting of 1.+, the consumption of transient 1.+ can be written as eq. 1
highlighting the unimolecular electrocyclization step and the bimolecular backꢀelectron transfer
process, respectively, where the oneꢀejected electron after the laser pulse is the solvated electron and
is designed as eꢀ.
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The conversion of 1•+ to 2 (see Scheme 2) involves a first order decay, and the kcyc can be calculated
straightforward from the reciprocal of the halfꢀlifetime (τcycꢀ1) under all the conditions examined and
the data thus obtained are shown in Table 2. On the other hand, back electron transfer (BET) of 1•+
to tris(pꢀbromophenyl)amine (see Scheme 2 and eq. (1)) can be represented by kBET. From the slopes
of the linear regression of the reciprocal of the concentration of transient 1•+ vs. time plots the
bimolecular rate constants, kBET, of 1010 Mꢀ1 sꢀ1 were obtained under all the conditions examined (see
Table 2).
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Co-oxidation of sulfides and phosphines. The oxidation activity of the photogenerated intermediates,
namely 1.+ and 2, was tested by irradiating solutions of 1 in the presence of some substrates as the
electron donors such as sulfides and phosphines (see Scheme 4). Under such conditions, 1 was
consumed, while the additives were coꢀoxidized selectively to the corresponding sulfoxides and
phosphine oxides. In the case of benzyl ethyl sulfide, however, significant amounts of benzaldehyde
were also formed. The obtained results have been summarized in Tables 3 and 4.
Sulfides (including Ph2S that is inert towards singlet oxygen9) were coꢀoxidized with a rate value
ranging from 0.003 and 0.244 ꢀmol minꢀ1 while the ratio sulfoxidation/electrocyclic cyclization was
lower than unity (0.06 to 0.80) in all of the solvents investigated, with the only exception of benzyl
ethyl sulfide in MeCN/H2O 9/1 mixture. Nucleophilic triarylphosphines were also coꢀoxidized with
a phosphine oxidation/electrocyclic cyclization ratio ranging from ca. 26/1 to 5/1 under irradiation
of amine 1 in acetonitrile.
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O
S
R2S
R
R
5
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8
Ph2S(O)
PhS(O)Me
Ph2S
pꢀMeOC6H4SMe
PhSMe
PhCH2SEt
+
PhCHO
pꢀMeOC6H4S(O)Me
PhCH2SEt
Br
9
Ar3P
hn (366 nm)
Solvent, O2
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Ar3P=O
N
Me
Me
Br
Br
1
P
P=O
Me
Me
3
3
Me
P
Me
P=O
Ph3P
Ph3P=O
3
3
Me
Me
Scheme 4. Coꢀoxidation of sulfides (upper path) and triarylphosphines (lower path) in the presence
of 1.
Table 3. Rate of coꢀoxidation of sulfides upon irradiation in the presence of 1.a
Rate of reaction (ꢀmol.minꢀ1)
Substrate
Ph2S
Solvent
MeCN
Sulfoxide 1b
0.003 0.051
Other products
MeCN/H2O 9/1 0.005
0.041
0.049
0.026
0.20
TFE
0.033
0.008
0.024
DCM
MeCN
PhSMe
MeCN/H2O 9/1 0.091
0.31
TFE
0.193
0.012
0.026
0.24
DCM
0.05
pꢀMeOC6H4SMe MeCN
0.085
0.239
MeCN/H2O 9/1 0.118
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TFE
0.240
0.005
0.088
0.234
0.030
0.065
0.150
0.320
0.024
DCM
MeCN
PhCH2SEt
PhCHO, 0.010
PhCHO, 0.024
PhCHO, 0.030
PhCHO, 0.013
MeCN/H2O 9/1 0.184
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TFE
0.244
0.093
DCM
a 5x10ꢀ3 M solution of 1 irradiated in the presence of the chosen sulfide (10ꢀ2 M) at 310 nm (10 x 15
W lamps).b Rate of consumption of 1.
Table 4. Rate of coꢀoxidation of triarylphosphines upon irradiation in the presence of 1 in MeCN.a
Rate of coꢀoxidation (ꢀmol.minꢀ1)
Substrate
Ph3P
Phosphine oxide 1b
2.91
2.75
0.11
(oꢀMeC6H4)3P
0.31
0.91
(2,4,6ꢀMe3C6H2)3P 4.13
5x10ꢀ3 M solution of 1 irradiated in the presence of the chosen phosphine (1.1x10ꢀ2 M) at 366 nm
a
b
(High pressure Hg lamp provided with an interference Schott filter at 366 nm). Rate of
consumption of 1.
Blank experiments were also performed. Thus, irradiation of oxygenated solutions of sulfides and
phosphines in all the solvents used in the absence of tris(pꢀbromophenyl)amine (1) did not provide
any sulfoxides or phosphine oxides. Likewise, experiments performed under N2 atmosphere but in
the presence of tris(pꢀbromophenyl)amine (1) did not give any oxide.
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Interaction of sulfides and triphenylphosphine with photogenerated transients. Timeꢀresolved
spectroscopy was applied to solutions of 1 in the presence of selected sulfides and
triphenylphosphine. Diphenyl sulfide, thioanisole and benzyl ethyl sulfide caused no change on the
previously observed transients. On the other hand, pꢀmethoxythioanisole and triphenylphosphine
quenched the radical cation 1•+ as shown in Fig. 5.
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30000
25000
20000
15000
10000
5000
0
0.000
0.002
0.004
0.006
0.008
0.010
0.012
[Q] / M
Figure 5. Quenching of 1•+ with increasing concentration of pꢀmethoxythioanisole (□) and of
triphenyl phosphine (○). Straight lines are the best linear regression fittings.
In the case of pꢀmethoxythioanisole and triphenylphosphine, the observed firstꢀorder decay constant
(kobs) of 1•+ depends linearly on the quencher concentration as it is shown in eq. 3.
Plotting kobs vs. the quencher concentration [Q] resulted in a linear correlation (see Figure 5). The
corresponding bimolecular rate constants were obtained from the slope of the linear regressions, viz
kpMeO = 9.7x105 Mꢀ1sꢀ1for pꢀmethoxythioanisole and kPh3P = 2.1x106 Mꢀ1 sꢀ1 for triphenylphosphine.
Discussion.
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Analysis of the intermediates formed during irradiation of tris(p-bromophenyl)amine (1). The above
data show that excitation of 1 with a laser pulse (355 nm) generates two key intermediates in both
N2ꢀ and in O2ꢀsaturated solution. However, one of these species reacts efficiently with molecular
oxygen, as is shown in Scheme 5. Photoionization of 1 forms radical cation 1•+ (path (a)). Under
nitrogen atmosphere, electrocyclization to radical cation 2 occurs (kcyc = 2.5x104 sꢀ1 in acetonitrile;
path (b)) in competition with electron recombination (kBET = 2.2x1010 Mꢀ1.sꢀ1 in acetonitrile; path
(b’)). Ensuing an electron transfer pathway (path(c)) intermediate, 2 reverts to amine 1 in
competition with formation of products (path (d)). Irradiation of di(tri)arylamines can be counted
among the many mild conversions into carbazoles.6,10,11 Indeed, the photochemical preparation of
carbazoles under visible light in the presence of a copperꢀbased sensitizer has been reported7g and
formation of triarylamine radicalꢀcation followed by cyclization to dihydrocarbazole intermediate
that in turn provides a substituted carbazole, was also proposed.11f However, when amine 1 was
irradiated under visible light photoredox catalysis conditions, no tribromocarbazole was observed,
and only Nꢀphenylcarbazole was obtained in low yield (> 16 %).11f
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O
O
3O2
H
H
Br
Br
Ar3N + 1O2
3Ο2
.ꢀ
O2
φꢁ
3O2
H
N
H
eꢀ (solvent)
Br
Br
(I)
Ar
path (e)
kcyc
hν
3Ar3N*
(31*)
Ar3N.+
(1.+)
φISC
N
path (f) e
path (a)
kBET
path (b)
HO
H
kd
(2)
Ar3N
(1)
Ar
O
path (d)
path (b')
krev
path (c)
Br
Br
Ar3N
eꢀ (solvent)
Products
N
Ar3N+—
Ar
(II)
Ar3N = (pꢀBrC6H4)3N
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Scheme 5. Proposed reaction mechanism of the photocyclization of 1. (Filled arrows: nitrogen
4
5
atmosphere; dashed arrows: oxygen atmosphere).
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On the other hand, under aerobic conditions amine 1 in its triplet excited state sensitizes singlet
oxygen thus halving the formation of intermediates 1•+ and, as a consequence, 2, as can be judged
from the quantum yields obtained under oxygen atmosphere (see Table 1 and compare these values
with those obtained in nitrogen atmosphere). Photoionization of triplet amine (1) forms 1•+ and the
ejected electron was trapped, at least in part, by molecular oxygen giving superoxide ion with a
bimolecular rate constant of 2x1010 Mꢀ1 sꢀ1.12 Additionally, a backꢀelectron transfer process (path(b’)
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in Scheme 5) can occur between 1•+ and the ejected electron with a bimolecular rate constant (kBET
)
of 1.4x1010 Mꢀ1 sꢀ1. Cyclization (path (b)) afforded 2 that reacted with molecular oxygen and
provided 4ꢀperoxyl radical (I) (path (e)). Ensuing intramolecular hydrogen atom transfer led to
intermediate (II) (path (f)) and both intermediates are, at least in part, potential oxidizing agents that
are formed during irradiation of amine 1 under oxygen atmosphere. Superoxide ion then further
reacted with adventitious water present in the organic media and led to ROS species, including OH•,
HOO• and H2O2.
Under airꢀequilibrated conditions a similar photochemical behavior was observed for 1 in MeCN,
where both 1.+ and 2 were detected. Transient 1•+ decays competitively by two pathways, backꢀ
electron transfer to amine 1 (path (b’) in Scheme 4, kBET = 1.9x1010 Mꢀ1 sꢀ1) and electrocyclization to
2 (path (b) in Scheme 4, kcyc = 2.6 x 104 sꢀ1).
Nitrous oxide is a good quencher of solvated electron and has been often used under pulse radiolysis
and laser flash photolysis.13 However, bubbling N2O to a solution of 1 in MeCN no effect on the
transient intermediates was observed, the competitive rate constants kcyc and kBET exhibiting the
same values under N2O as well as under N2 atmospheres (see Table 3). This is due to the fact that
chemical reaction occurs at the geminate pair level (1•+….eꢀ) and no solvated electron (eꢀsolv) is
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formed. Furthermore, comparison of the rate constant for the reaction of N2O with the electron
(9x109 Mꢀ1 sꢀ1)13g and the BET reaction of 1.+ (2.1 x 1010 Mꢀ1 sꢀ1) account for the spectroscopic
behavior observed.
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As concerning oxygen trapping by 2, it was surmised that some intermediates generated during the
irradiation may similarly oxidize other compounds under mild conditions.
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Photo co-oxidation of sulfides (Ph2S; PhSMe and PhCH2SEt) in the tris(p-bromophenyl)amine/O2
system. Steadyꢀstate experiments where amine 1 was irradiated in the presence of a range of
sulfides, the latter are coꢀoxidized to sulfoxides at a rate dependent on their structure and the solvent
used (rate values ranging from 0.003 to 0.244 ꢀmol sꢀ1) while amine 1 is consumed efficiently.
Further, the rate ratio of sulfoxide versus consumption of 1 was <1 (see Table 3) a fact that we
attributed to the intermediacy of one (or more) common intermediate responsible for both
cyclization and coꢀoxidation of sulfides. It was pointed out that oxidant species such as singlet
oxygen and superoxide ion are formed during the irradiation of amine 1 under oxygen atmosphere
(see Scheme 5). However, we can state that the coꢀoxidation reaction is not a result of a singlet
oxygen reaction as brought about the following evidences: (1) sulfones are known to always
accompany the sulfoxides in the singlet oxygen oxidation of sulfides (not observed in that case); (2)
benzyl sulfides undergo both sulfoxidation and oxidative cleavage to benzaldehyde in protic
media,14 oxidation takes place also on diphenyl sulfides, that is usually nonꢀreactive towards singlet
oxygen. As for superoxide ion, under the present conditions is not involved in the coꢀoxidation of
sulfides, but reduces the radicalꢀcation 1•+ to 1 through a backꢀelectron transfer process. Therefore,
according to our results and to the literature data, we assigned to the hydroperoxide intermediate (II)
the role of the oxidant species that is able to transfer an oxygen atom to the sulfides to afford the
corresponding sulfoxides and a new intermediate (III). The latter underwent dehydration to furnish
products (see Scheme 5). Intermediate II also underwent aromatization by losing hydrogen
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peroxide, a process that has been previously observed to occur at room temperature with a rather
slow radical chain mechanism15. It is noteworthy to mention that dihydrocarbazole aromatization
(path (e)) and donor oxidation (path (c)) were coupled and the result depended on the nature of the
sulfide. Thus, diphenyl sulfide was coꢀoxidized with a lower efficiency than thioanisole or than
benzyl ethyl sulfide as can be observed in Table 3.
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Scheme 6. Proposed mechanism for the coꢀoxidation of sulfides during the photocyclization of 1.
Photo co-oxidation of p-methoxythioanisole in the tris(p-bromophenyl)amine/O2 system. The photo
coꢀoxidation of pꢀmethoxythioanisole with tris(pꢀbromophenyl)amine in the presence of O2 deserves
some comments and additional analysis. pꢀMethoxythioanisole sulfoxide could arise from two
alternative sequence of pathways: (1) an electron transfer process followed by oxidation with
molecular oxygen involving the sequence from path (f) to path (h) (see Scheme 7) and (2) oxygen
donation from intermediate (II) to pꢀmethoxythioanisole (path (c) in Scheme 7).
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Scheme 7. Proposed mechanism for the coꢀoxidation of pꢀmethoxythioanisole during the
photocyclization of 1.
The electron transfer process (path (f) in Scheme 7) occurs efficiently because tris(pꢀ
bromophenyl)amine radical cation (1•+) has an oxidation potential of +1.16 V vs. SCE16 and can thus
oxidize pꢀmethoxythioanisole (Eox= +1.13 V vs. SCE)17 providing the corresponding radicalꢀcation
(see Scheme 7). Also, this process is a thermodynamically feasible process by ꢀ0.03 V. On the other
hand, laser flash spectroscopy showed that pꢀmethoxythioanisole quenches 1•+ with a bimolecular
rate constants kpMeO = 9.7x105 Mꢀ1 sꢀ1 (see Fig. 4) giving the pꢀmethoxythioanisole radicalꢀcation.
Once the pꢀmethoxythioanisole radicalꢀcation was formed, reaction with molecular oxygen in the
presence of a further molecule of sulfide gave the electrophilic radicalꢀcation intermediate IV (path
(g)). Then, this last intermediate reacted with the sulfide and superoxide ion to give finally two
molecules of the sulfoxides (path (h)) completing the sequence pathways from path (f) to path (h).
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On the other hand, we pointed out above that in the photoreaction of tris(pꢀbromophenyl)amine
under O2 atmosphere, superoxide ion was also formed (see Scheme 5). A possible reaction between
superoxide ion and sulfide radicalꢀcation could provide a thiodioxirane, an electrophilic
intermediate, as proposed by Baciocchi18. However, we have earlier demonstrated19 that dialkyl and
diaryl sulfide radicalꢀcations react efficiently with molecular oxygen to give a loose dipolar complex
(R2SꢀOꢀO•+) that further reacts with another molecule of sulfide and gives a strongly bound
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•+
electrophilic intermediate (R2SꢀOꢀOꢀSR2 ) rather than a thiadioxirane intermediate. The strongly
bound electrophilic intermediate was assigned to intermediate (IV) which is shown in Scheme 7.
•ꢀ
Then, we can conclude that O2 has a role, but does not add to the sulfide radicalꢀcation. A
reasonable alternative is that superoxide ion operates as electron carrier in the reduction of the
intermediate IV (ArMeSꢀOꢀOꢀSArMe•+) as shown in path (h) (Scheme 7).
It is noteworthy to mention that the oxygen donating sequence pathways illustrated from path (a) to
path (d) in Scheme 7 providing pꢀmethoxythioanisole sulfoxide cannot be ruled out. In fact, the
cyclization process of tris(pꢀbromophenyl)amine radical cation (1•+) competes in a lesser extent with
the oneꢀelectron transfer process between 1•+ and pꢀmethoxythioanisole (path (f) in Scheme 7).
Comparing the rate constant values of both competitive processes, kcyc = 2.5x104 sꢀ1 and kpꢀMeO
=
9.7x105 Mꢀ1 sꢀ1 respectively, it is clearly that the main reaction pathway in the formation of pꢀ
methoxythioanisole sulfoxide is the oneꢀelectron transfer oxidation process (path (f)) over the
cyclization process of the tris(pꢀbromophenyl)amine radical cation (1•+).
Photo co-oxidation of phosphines. Phosphines were stronger oxygen acceptor and, as resumed in
Table 4 were coꢀoxidized efficiently upon irradiation of amine 1 in the presence of molecular
oxygen with a rate ranging from 2.75 to 4.13 ꢀmol sꢀ1, while 1 is consumed smoothly. Further, the
rate ratio of phosphine oxidation versus consumption of 1 grows from ca. 5/1 to 26/1 indicating, at
least, that two common oxidant intermediates are responsible for the coꢀoxidation of phosphines
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which inhibit significantly the consumption of amine 1. On the basis of steadyꢀstate and laser flash
4
5
photolysis measurements the proposed reaction mechanism is shown in Scheme 8.
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8
9
HO
O
H
Br
Br
path (e)
•
O
path (d)
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O
Products
H
H
N
Br
Br
(II)
e
Ar
H2O2
Br
N+
Ar
path (b)
•
(I)
O
Ar'3P
H
H
Br
Ar'3P=O
3O2
H
path (a)
N+
(III)
Ar
H
Ar'3P
Br
Br
path (c)
N+
Ar
Ar'3P=O
(2)
Ar3N
3O2
kcyc
Ar'3P
Ar'3PꢀOO.+
Ar'3P.+
path (g)
Ar3N.+
(1.+)
Br
Br
h
path (f)
Ar'3P
.ꢀ
O2
3O2
path (h)
N
Ar'3PꢀO.+
path (i)
3Ar3N*
Ar'3P=O
isc
Ar'3P=O
(1; Ar3N)
Br
Scheme 8. Proposed mechanism for the coꢀoxidation of triarylphosphines during the
photocyclization of 1.
As can be seen in Scheme 8, irradiation of 1 gave amine radical cation 1•+ and superoxide ion. Two
competitive pathways occur: (1) electrocyclization to provide intermediate 2 and (2) oneꢀelectron
oxidation of triarylphosphine to furnish the corresponding radical cation. The first path takes place
with a rate constant of 2.5x104 sꢀ1 while the second pathway (path (f) in Scheme 8) with a rate
constant of 2.1 x 106 Mꢀ1 sꢀ1. Following the electrocyclization pathway, intermediate 2 reacted with
molecular oxygen to form I and, after intramolecular hydrogen abstraction, II. Both species are
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potential oxygenꢀdonating species that are able to transfer oxygen to the phosphines. It appears
reasonable that phosphines, which are strong nucleophiles, were able to trap intermediate (I)
efficiently in competition with intramolecular hydrogen abstraction (path (b) vs path (d)). Indeed,
the reactivity of peroxy radicals with phosphines has been previously demonstrated.21 Thus, I
oxidized the phosphine to the corresponding phosphine oxides (path (b)) the PꢀO bond formation
operating as the driving force of the process. Finally, intermediate (III) transferred a second oxygen
atom to phosphines (path (c)) and radical cation 2 was formed. Thus, two oxygen atoms were
sequentially transferred to the phosphine and led back to 2 completing the oxidation cycle. This
reaction pathway was early proposed when triphenylamine was used instead of tris(pꢀ
bromophenyl)amine.6 The oxidation of phosphines inhibits the conversion of intermediate (II) to
products in agreement with the experimental evidences indicating that path (b) is an efficient
process that competes with path (d).
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On the other hand, an alternative oxidation pathway of phosphine may involve the direct oneꢀ
electron transfer between 1•+ and Ar3P (path (f) in Scheme 8) to give triarylphosphine radical cation
(Ar3P•+) as it was clearly confirmed by laser flash photolysis experiments and thermodynamic
calculations. In fact, triphenylphosphine quenched 1•+ with a bimolecular rate constant (kPh3P) of 2.1
x 106 Mꢀ1 sꢀ1 (see Fig.5) and the oneꢀelectron transfer process is thermodynamically feasible (
ꢁG ꢀ
0.13 V), since 1•+ has an oxidation potential of +1.16 V (vs. SCE), whereas the oxidation potential of
triphenylphosphine is +1.00 V (vs. SCE).20. Trapping of generated Ar3P•+ with molecular oxygen
•+
occurred producing peroxyl radical cation Ar3PO2 (path (g)), a reaction pathway that has been
previously observed under pulse radiolysis and under laser flash photolysis experiments22. Reaction
of this intermediate with another molecule of Ar3P and ensuing mesolytic cleavage yields
triarylphosphine oxide and radical cation Ar3PO•+ (path (h)). Finally, the last intermediate reacted
with superoxide ion giving another molecule of Ar3PO and molecular oxygen (path (i)).
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An active role of singlet oxygen which is formed during the oxidative photoreaction was excluded
because the reaction between Ar3P•+ and molecular oxygen is faster than expected. In fact,
comparing the bimolecular rate constant between Ph3P and singlet oxygen (kr = 1.6 x 107 Mꢀ1 sꢀ1)23
and the bimolecular rate constant between Ph3P.+ and molecular oxygen (k2 = 1.6 x 1010 Mꢀ1 sꢀ1 for
DCA/biphenyl and k2 = 3.4 x 1010 Mꢀ1 sꢀ1 for NMQ+ in MeCN, respectively)22e is clearly that singlet
oxygen sensitization cannot compete efficiently with path (g) (see Scheme 8). Furthermore,
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•+
formation of intermediate Ar3PO2 was proposed for path (g) instead of a phosphodioxirane
intermediate (Ph3PO2)23 as recently demonstrated in the photosensitized electronꢀtransfer oxidation
of triphenylphosphine with DCA and NMQ+ in different organic solvents.22e Therefore, we excluded
the reaction of singlet oxygen with arylphosphines from the reaction mechanism as a competitive
oxidative pathway.
As it was pointed out for the coꢀoxidation of sulfides, superoxide ion is not involved in the oxidation
.ꢀ
pathway and coupling of Ph3P•+ with superoxide ion has been excluded. Therefore, the role of O2 is
to operate as an electron carrier in the reduction of intermediate Ph3PO•+, as is depicted in path (i)
(see Scheme 8).
We can conclude that two competitive oxidation pathways occur, oxygen transfer from
intermediates (I) and (III) to Ar3P (paths (b) and (c), respectively) and subsequent oxidation of
triarylphosphine radical cation (paths (h) and (i)). However, intermediates (I) and (III) arise from
the electrocyclization of 1•+ with a rate constant that is lower than the rate constant of the electronꢀ
transfer process between 1•+ and Ar3P. Therefore, the last competitive pathway prevails significantly
over the first oxidation pathway because inhibition of the consumption of amine 1 in the presence of
Ar3P was clearly described.
Conclusions.
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The photochemical coꢀoxidation of sulfides and phosphines examined in the present paper take
place, at least, through two distinctly reaction mechanism, depending on the nucleophilicity of the
substrates used. In any case, irradiation of tris(pꢀbromophenyl)amine (1) gives the corresponding
radical cation 1•+. The intermediate can undergo electrocyclization to 2 that in turn reacts with
molecular oxygen to provide peroxyl (I) and hydroperoxyl (II) radicals, successively. These last two
intermediates are potential oxidants and react with a mechanism that depends on the substrate used.
Thus, sulfides are oxidized by hydroperoxyl (II), while phosphines are oxidized by peroxyl (I) due
to the strong nucleophilicity of these substrates in comparison with sulfides. Further, consumption of
amine 1 is observed in the presence of sulfides while in the presence of phosphines is significantly
inhibited. On the other hand, for the case of pꢀmethoxythioanisole and of triarylphosphines a oneꢀ
electron transfer process between 1•+ and these substrates operates efficiently (path (f) in both
Schemes 7 and 8) which is supported by laser flash photolysis experiments. Then, reaction of pꢀ
methoxythioanisole and triarylphosphines radical cations with molecular oxygen gave intermediates
R2SOO•+ and Ar3POO•+, respectively, that finally were converted into the corresponding oxides
(path (h) in Scheme 5 for sulfides and paths (h) and (i) in Scheme 8 for phosphines). Singlet oxygen
sensitization of sulfides and phosphines was excluded from the reaction mechanism as a competitive
oxidation pathway because reaction of triarylphosphine radical cation with molecular oxygen is
faster than singlet oxygen sensitization (compare rate constant of singlet oxygen k = 1,6x107 Mꢀ1 sꢀ1
with rate constant of Ph3P•+ with molecular oxygen k = 1,6x1010 Mꢀ1 sꢀ1) and the product distribution
obtained for sulfides under our experimental conditions are not those expected for a singlet oxygen
sensitization. In fact, under singlet oxygen sensitization, Ph2S is unreactive against to singlet oxygen
while benzyl ethyl sulfide gives benzaldehyde and the corresponding sulfoxide and sulfone.14a,b
Furthermore, thioanisole and pꢀmethoxythioanisole give the corresponding sulfoxides and sulfones
as the main products under singlet oxygen sensitization.14c Finally, the role of superoxide ion which
is formed during irradiation of 1 under oxygen atmosphere, operates as an electron carrier that it is
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involved, for sulfides, in path (h) (see Schemes 6 and 7) and for phosphines in path (i) (see Scheme
8) and does not react directly with sulfides and phosphines radical cations, respectively.19, 22e In
conclusion, the photoionization of Magic Blue precursor (amine 1) is a versatile tool to promote
oxidation of organic substrates.
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Experimental details.
Materials. Tris(pꢀbromophenyl)amine, thioanisole, pꢀmethoxythioanisole, diphenyl sulfide,
triphenylphosphine, triphenylphosphine oxide, tris(2ꢀmethylphenyl)phosphine, tris(2,4,6ꢀ
trimethylphenyl)phosphine, diphenyl sulfoxide, methyl phenyl sulfoxide, pꢀmethoxyphenyl methyl
sulfoxide, benzaldehyde and benzophenone were commercial products. Sulfoxides and
triarylphosphine oxides used as reference compounds were prepared by reported procedures.24 The
benzyl ethyl sulfide25 and the corresponding sulfoxide26 were prepared by reported procedures.
Coꢀoxidation reactions.The coꢀoxidation experiments were carried out by using 5.0 x 10ꢀ3 M of 1
in the presence of 0.01 M solutions of the chosen sulfide or triarylphosphine in different solvents
(2,2,2ꢀtrifluoroethanol, acetonitrile, acetonitrile water mixture and dichloromethane). The solutions
were contained in rubber stoppered, 1 cm diameter quartz tubes and a stream of dry oxygen
saturated with the appropriate solvent was passed into the solution through a needle during 10
minutes in the dark. The quartz tubes were exposed to ten phosphorꢀcoated 15 W lamps (Rayonet)
emitting at 366 nm. The products were determined by GC on the basis of calibration curves in the
presence of cyclododecane as the internal standard.
Timeꢀresolved laser flash spectroscopy. Nanoꢀtoꢀmicrosecond transient absorption experiments
were performed using a nanosecond laser flash photolysis apparatus equipped with a 20 Hz
Nd:YAG laser (20 ns, 1 mJ at 355 nm and at 266 nm) and a 150 W Xe flash lamp as the probe light.
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Samples were placed in a quartz cell (10x10 mm2 section) at a concentration adjusted to obtain an
OD value of 1.0 at 355 nm. Nonꢀlinear fitting procedures by the leastꢀsquares method and χ2 and
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distribution of residuals were used to judge the goodness of fit. Quantum yield of formation (φ, see
Table 1) 1•+ and 2 were measured by comparison of the zero time intensity of the signal of the
transients with that of the ketyl radical formed after flash excitation (355 nm) of a solution (in
benzene) of benzophenone.
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Acknowledgments.
S. M. B. is a research member of CONICET (Argentinean National Research Council).
Supporting Information.
Determination of the kinetic data of transient decay traces. Determination of the rate data of
1
1
products formation. HꢀNMR and 13CꢀNMR spectra of compound 1. H spectra of compound 1
irradiated in MeCN (reaction mixture) This material is available free of charge via the Internet at
http://pubs.acs.org.
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