Highly efficient reduction of sulfoxides with the system borane/oxo-rhenium complexes

Article abstract of DOI:10.1021/om100450a

This work reports the reduction of sulfoxides with boranes, in excellent yields, catalyzed by oxo-rhenium complexes (1 mol %) at room temperature under air atmosphere. The best results were obtained with catecholborane (HBcat) in the catalytic systems HBcat/ReIO₂(PPh₃)₂, HBcat/Re₂O₇, and HBcat/MTO. DFT calculations were performed for the system based on ReIO₂(PPh₃)₂, which was studied in more detail. The reaction starts with the formation of ReIO₂(R₂SO)₂, followed by addition of the first molecule of HBcat and loss of R₂S. In a second step, a second HBcat molecule attacks the Re(VII) intermediate, reducing the metal back to Re(V), while H₂ and catBOBcat are released and the Re catalyst is regenerated, reentering the reaction cycle. The computed reaction barriers are much lower than those for any alternative pathway that could be envisaged.

Full text of DOI:10.1021/om100450a

Organometallics 2010, 29, 5517–5525 5517
DOI: 10.1021/om100450a
Highly Efficient Reduction of Sulfoxides with the System
Borane/Oxo-rhenium Complexes
†
,
‡
§
§
‡
Ana C. Fernandes,* Jos ꢀe A. Fernandes, Carlos C. Rom ~a o, Luis F. Veiros,
,
^
and Maria Jos ꢀe Calhorda*
‡
Centro de Quı ´m ica Estrutural, Complexo I, Instituto Superior T eꢀ cnico, Avenida Rovisco Pais,
§
1
049-001 Lisboa, Portugal, Instituto de Tecnologia Quı´mica e Biol oꢀ gica da Universidade Nova de Lisboa,
Avenida da Rep uꢀ blica, EAN, 2780-157 Oeiras, Portugal, and Departamento de Quı´mica e Bioquı´mica,
CQB, Faculdade de Ci e^ ncias, Universidade de Lisboa, Campo Grande, 1749-016 Lisboa, Portugal
^
Received May 10, 2010
This work reports the reduction of sulfoxides with boranes, in excellent yields, catalyzed by oxo-
rhenium complexes (1 mol %) at room temperature under air atmosphere. The best results were
obtained with catecholborane (HBcat) in the catalytic systems HBcat/ReIO (PPh ) , HBcat/Re O ,
and HBcat/MTO. DFT calculations were performed for the system based on ReIO (PPh ) , which
2
3 2
2
7
2
3 2
was studied in more detail. The reaction starts with the formation of ReIO (R SO) , followed by
2
2
2
addition of the first molecule of HBcat and loss of R S. In a second step, a second HBcat molecule
2
attacks the Re(VII) intermediate, reducing the metal back to Re(V), while H and catBOBcat are
2
released and the Re catalyst is regenerated, reentering the reaction cycle. The computed reaction
barriers are much lower than those for any alternative pathway that could be envisaged.
Introduction
the years, several methods have been developed to reduce
1-29
sulfoxides.
have been used for the deoxygenation of sulfoxides, oxo-
Among the variety of metal complexes that
The reduction (deoxygenation) of sulfoxides to sulfides is a
fundamental reaction in both chemistry and biology. Over
12,13,16
molybdenum complexes
have attracted considerable
interest, because this metal is found in a class of enzymes that
are commonly referred toas mononuclear molybdoenzymes or
oxotransferases, such as dimethyl sulfoxide reductases,
which catalyze similar reactions.
†
Part of the Dietmar Seyferth Festschrift. In honor of Prof. Dietmar
Seyferth for his outstanding contributions as editor of Organometallics.
30-32
*
To whom correspondence should be addressed. (A.C.F.) Tel: 351
18419264. Fax: 351 218464457. E-mail: anacristinafernandes@ist.utl.pt.
M.J.C.) Tel: 351 217500196. Fax: 351 217500088. E-mail: mjc@fc.ul.pt.
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Published on Web 08/23/2010
pubs.acs.org/Organometallics

5
518 Organometallics, Vol. 29, No. 21, 2010
Fernandes et al.
Some rhenium complexes have also been used for the
2
of HBcat (eq 3). The deoxygenation of the sulfoxide was
completed after 1 h 25 min at room temperature. The same
reduction without catalyst required 500 h. This result sug-
gests that high-valent oxo-rhenium complexes could be
appropriate catalysts for the deoxygenation of sulfoxides.
0-26
reduction of sulfoxides.
Very recently, we have demon-
1
7
strated that the catalytic system silane/oxo-rhenium com-
plexes is very efficient for the reduction of sulfoxides with
3
3
high yields and chemoselectivity.
As part of our continuous studies on the activation of
34-36
X-H (X = Si and B) bonds by high-valent oxo-complexes
and the use of these complexes as efficient catalysts in organic
12,13,37-44
reactions,
we also reported the synthesis of the hydrides
PPh ) (O)(I)Re(H)OBcat (2) and (PPh ) (O)(I)Re(H)OBpin
(
(
3
2
3 2
3), from the reaction between catecholborane (HBcat) or
pinacolborane (HBpin) and the high-valent oxo-rhenium com-
36
plex ReIO (PPh ) (1) (eqs 1 and 2).
3 2
2
In this work, we report the reduction of sulfoxides with
boranes catalyzed by a series of high-valent oxo-rhenium(V)
4
9
and -(VII) complexes. A DFT study provides a reaction
mechanism that is consistent with all the experimental
results.
Results and Discussion
Chemical Studies. Initially we investigated the reduction
of the test substrate 4-chlorophenyl sulfoxide with catechol-
borane (HBcat), pinacolborane (HBpin), and borane (BH3
THF), catalyzed by the high-valent oxo-rhenium complexes
3
The structural characterization by X-ray diffraction of 2
and 3 showed that these hydrides are formed by addition of
the B-H bond across the Re-oxo bond without dissociation
of any phosphine or substitution of the iodide ligand. The
synthesis of the novel hydrides 2 and 3 is a clear demon-
stration that the high-valent oxo-rhenium complex ReIO2-
ReIO (PPh ) (1), Re O (4), MTO (5), ReOCl (PPh ) (6),
3 2
and ReOCl (dppm) (7), in order to find the best reaction
3
conditions (Table 1). The progress of the reductions was
1
monitored by thin-layer chromatography and by H NMR.
2
3 2
2
7
3
The results reported in Table 1 show that the most active
catalysts are ReIO (PPh ) and Re O and that the monooxo-
2
3 2
2 7
(
PPh ) activates the B-H bond of boranes. To the best
rhenium complexes ReOCl (PPh ) and ReOCl (dppm) are the
3 3 2 3
3
2
of our knowledge, this is the first example of B-H bond
activation by high-valent metal oxo-rhenium complexes and
overall the first structurally characterized example of this
type of activation reaction. Usually, the activation of bor-
anes reported in the literature involves transition metal
least effective catalysts.
The studies carried out with HBcat showed that the
reductions catalyzed by the complexes ReIO (PPh ) and
Re O were very fast (5 min) at room temperature under air
2
3 2
2
7
atmosphere (Table 1, entries 1 and 4). The deoxygenation
with MTO was much slower since the total reduction of the
sulfoxide was reached only after 25 min at room temperature
(Table 1, entry 7). The reaction catalyzed by the mono-
oxo-rhenium complex ReOCl (PPh ) required significantly
4
5-48
complexes in a low oxidation state.
3
We have also reported the use of the hydride 2 as catalyst
6
in the reduction of 4-chlorophenyl sulfoxide with 2 equiv
3
3 2
(
33) Sousa, S. C. A.; Fernandes, A. C. Tetrahedron Lett. 2009, 50,
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Commun. 2005, 213–214.
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P. M.; Calhorda, M. J. Chem.;Eur. J. 2007, 13, 3934–3941.
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C. C. Dalton Trans. 2008, 6686–6688.
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881–8883.
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72, 60–63.
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53, 96–98.
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009, 74, 6960–6964.
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Lett. 2009, 50, 1407–1410.
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Fernandes, A. C. Organometallics 2009, 28, 6206–6212.
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Commun. (accepted).
longer reaction times, 20 h at room temperature and 4 h
0 min in refluxing THF (Table 1, entries 10 and 11). The
deoxygenation carried out with ReOCl (dppm) afforded the
sulfide in only 48% yield after 20 h at room temperature
(Table 1, entry 13). However at refluxing temperature
the sulfoxide was completely reduced after 7 h (Table 1,
entry 14).
6
3
(
3
(
(
(
8
2
2
2
All the reductions performed with HBpin were com-
pleted (Table 1, entries 2, 5, 8, 12, and 15). Nevertheless,
the reactions of 4-chlorophenyl sulfoxide with HBpin in the
presence of 1 mol % of ReIO (PPh ) , Re O , and MTO
(
(
2
3 2
2
7
(
(Table 1, entries 2, 5, and 8) were much slower than those
carried out with HBcat (Table 1, entries 1, 4, and 7).
Interestingly, the deoxygenation of 4-chlorophenyl sulf-
oxide with BH THF in the presence of 1 mol % of catalysts
(
(
3
3
ReIO (PPh ) , Re O , and MTO gave the sulfide in moder-
7
2
3 2
2
(
ate yields (Table 1, entries 3, 6, and 9).
(
In order to study the scope and the limitations of this
novel methodology, we tested the catalytic systems HBcat/
ReIO (PPh ) , HBcat/Re O , and HBcat/MTO with several
(
(
45) Burkhardt, E. R.; Matos, K. Chem. Rev. 2006, 106, 2617–2650.
46) Vogels, C. M.; Westcott, S. A. Curr. Org. Chem. 2005, 9, 687–
2
3 2
2
7
6
99.
(47) Beletskaya, I.; Pelter, A. Tetrahedron 1997, 53, 4957–5026.
48) Burgess, K.; Ohlmeyer, M. J. Chem. Rev. 1991, 91, 1179–1191.
(49) Parr, R. G.; Yang, W. In Density Functional Theory of Atoms and
Molecules; Oxford University Press: New York, 1989.
(

Article
Organometallics, Vol. 29, No. 21, 2010 5519
a
Table 1. Reduction of 4-Chlorophenyl Sulfoxide with Boranes Catalyzed by High-Valent Oxo-rhenium Complexes
b
entry
catalyst
borane
time
conversion (%)
1
2
3
4
5
6
7
8
9
ReIO
ReIO
ReIO
2
2
2
7
7
7
(PPh
(PPh
(PPh
3
)
3
)
3
)
2
2
2
HBcat
HBpin
BH
5 min
65 min
20 h
5 min
35 min
20 h
25 min
60 min
20 h
100
100
43
100
100
43
100
100
48
3
Re
2
Re
2
Re
2
O
O
O
HBcat
HBpin
BH
3
MTO
MTO
MTO
HBcat
HBpin
BH
3
10
11
12
13
14
15
ReOCl
ReOCl
ReOCl
ReOCl
ReOCl
ReOCl
3
3
3
3
3
3
(PPh
(PPh
(PPh
(dppm)
(dppm)
(dppm)
3
)
)
)
2
2
2
HBcat
HBcat
HBpin
HBcat
HBcat
HBpin
20 h
100
100
100
48
100
100
3
3
4.5 h
20 h
20 h
7 h
20 h
a
All reactions were carried out with 1.0 mmol of sulfoxide and 2.0 mmol of borane at rt, except entries 11 and 14, which were carried out at reflux.
Conversion was determined by H NMR.
b
1
Table 2. Reduction of Sulfoxides with the System
a
HBcat/Oxo-rhenium Complex
The catalytic activity of Re O was also explored in the
2 7
reduction of a series of sulfoxides with HBcat at room tem-
perature under air (Table 2, entries 2, 5, 8, 11, 14, and 17).
The system HBcat/Re O (1 mol %) is highly effective for the
2
7
deoxygenation of sulfoxides, giving the sulfides in a few
minutes (5-15 min) with excellent yields and with tolerance
of halo, ester, and nitro groups (Table 2, entries 2, 11, and 17).
The results obtained with this catalytic system are remarkably
similar to those obtained with the system HBcat/ReIO (PPh )
2
3 2
over the whole range of substrates.
Finally, we investigated the reduction of sulfoxides with
the system HBcat/MTO (1 mol %) (Table 2, entries 3, 6, 9,
1
2, 15, and 18). From the analysis of these results, we con-
cluded that this catalytic system is also very effective, reach-
ing total conversion of sulfoxides to the corresponding sul-
fides in excellent yields. However, the reduction of diphenyl
sulfoxides (Table 2, entries 3, 6, and 9) required more time
then the similar deoxygenations with the systems HBcat/
ReIO (PPh ) and HBcat/Re O . The functional groups
2
3 2
2
7
halo, ester, and nitro were also well tolerated under the
reaction conditions of the catalytic system HBcat/MTO
(Table 2, entries 3, 12, and 18).
The previous data show that several combinations of high-
valent oxo-rhenium complexes and boranes are very effici-
ent catalysts for the room-temperature deoxygenation of
mono- and diaryl sulfoxides with different functionaliza-
tions. Catecholborane is clearly the most efficient borane, in
V
VII
combination with Re O and Re O complexes producing
2
a
3
All reactions were carried out with 1.0 mmol of sulfoxide, 2.0 mmol
of HBcat, and 1 mol % of catalyst. Isolated yield.
b
catalysts that are rather more active than the one obtained
with the mono-oxo complex ReOCl (PPh ) . Interestingly,
the later complex has been shown to be an efficient catalyst
3
3 2
sulfoxides in THF at room temperature under air (Table 2).
The reductions with the system HBcat/ReIO (PPh ) were
for sulfoxide deoxygenation when combined with PPh as
3
2
3 2
2
3
completed within a few minutes (5-20 min) with excellent
yields (Table 2, entries 1, 4, 7, 10, 13, and 16). This system
showed good chemoselectivity with tolerance of some func-
tional groups such as halo, ester, and nitro (Table 2, entries 1,
oxygen sink.
In spite of the differences between the catalyst precursors
ReIO (PPh ) and Re O , both HBcat/ReIO (PPh ) and
HBcat/Re O systems are equally reactive toward all sub-
2
3 2
2
7
2
3 2
2
7
1
0, and 16).
The reusability of the ReIO (PPh ) catalyst was studied
strates examined. The similarity is so high that it suggests the
presence of very similar active species. At first glimpse it is
not immediately obvious how this can happen, and therefore
we tried to gain more insight into the mechanism of this
reduction, using the catalytic system HBcat/ReIO (PPh )
2
3 2
using phenyl methyl sulfoxide as substrate. After the first cycle,
more fresh substrate and HBcat were added to the reaction
mixture, but only 35% conversion was achieved after 7 h.
2
3 2

5
520 Organometallics, Vol. 29, No. 21, 2010
Fernandes et al.
(1 mol %) because it provides more handles for spectroscopic
Chart 1. Competent Catalytic Precursor Sulfoxide Complexes
monitoring of the reactions. As described in the experimental
procedure, the catalytic sulfoxide deoxygenation is carried
out by treating the catalyst with sulfoxide in a 1:100 molar
ratio and adding the borane to start the catalytic reaction.
When 4-chlorophenyl sulfoxide was treated with 2 equiv of
HBcat without ReIO (PPh ) , only a trace of deoxygenated
2
3 2
product was formed. When 4-chlorophenyl sulfoxide was
treated with 1 mol % of ReIO (PPh ) without HBcat, at
2
3 2
room temperature under air, a color change took place, but
only a trace of deoxygenated product was formed. A similar
color change is observed during the reduction of 4-methyl-
phenyl sulfoxide with the system HBcat/ReIO (PPh )
velocity of the catalytic reduction, which decreases in the
order HBcat > HBpin . H B THF
3
3
The deoxygenation of 4-chlorophenyl sulfoxide with
HBcat catalyzed by the hydride 2, reported in our previous
2
3 2
3
6
(
1 mol %). The initially violet solution of ReIO (PPh ) in
2
work (eq 3), required 1 h 25 min at room temperature under
nitrogen atmosphere. This is much longer than the 5 min
3 2
THF turns to yellow upon addition of the sulfoxide and then
changes to green and violet-brown after addition of HBcat.
It is obvious that ReIO (PPh ) reacts with the sulfoxide, and
necessary to complete the same deoxygenation using ReIO
(PPh as catalyst. In agreement with this, when the catalytic
reduction of 4-chlorophenyl sulfoxide is started by the addi-
tion of HBcat to ReIO (PPh , producing the hydride 2,
-
2
)
3 2
2
3 2
the species that is formed reacts with the HBcat to gene-
rate the deoxygenated products. This was confirmed by the
study of the reaction between ReIO (PPh ) and 2 equiv of
)
3 2
2
followed by addition of the sulfoxide, complete deoxygena-
tion requires 1 h at room temperature under air. We will
specifically address this point later (see Scheme 3).
The two preceding examples plainly show that the phos-
phine hydrides like 2 or 3, which result from the activation of
2
3 2
31
1
4
-methylphenyl sulfoxide using H and P NMR in C D .
6 6
The resonances of the aromatic protons of the free sulfoxide
δ 7.51 and 6.78 ppm) change to δ 7.30 and 6.80 ppm upon
addition of ReIO (PPh ) . Likewise, the singlet correspond-
(
2
3 2
ing to the CH group was displaced from δ 1.87 ppm in the
the B-H bond across the RedO bond of ReIO
not, per se, very efficient deoxygenating agents.
In agreement with the chemistry of Re in coordinating
solvents, the species existing in THF solution is [cis-(THF)
2 3 2
(PPh ) , are
3
3
1
free sulfoxide to δ 1.97 ppm in the mixture. The P NMR
measurements reveal that the resonance corresponding to
the PPh ligands in ReIO (PPh ) (δ 4.0 ppm) disappears
O
2 7
-
2
3
2
3 2
5
0
after the addition of 4-methylphenyl sulfoxide. Two new
peaks appear at δ -5.2 and 25.4 ppm, and they were assigned
to free PPh and OPPh . These results show that both PPh
3
O
3
Re-O-ReO
oxide, it is quite likely that the similar species [cis-(R
Re-O-ReO ] (9) becomes dominant and reduction of the
3
]. Upon addition of a large excess of sulf-
SO) -
2
2
O
3
3
3
3
ligands in the catalyst precursor are displaced by the large
excess of sulfoxide most likely to form ReIO (R SO) (8). In
sulfoxides should start from here upon reaction with the
boranes. From the preceding information we propose that
the catalytic reactions start from the sulfoxide complexes
8, 9 (9 ), and 10 depicted in Chart 1.
We provided evidence for the formation of 8 from the
2
2
2
spite of several attempts, we were unable to isolate any
example of a pure complex 8, for a more complete structural
characterization, from the reaction between ReIO (PPh )
0
2
3 2
and 2 equiv of sulfoxide.
Concomitantly with PPh displacement, some oxidation
precursor PPh analogue ReIO (PPh ) under the reaction
3 2 3 2
conditions, and the other ones are predictable from the
known coordination chemistry of the highly electrophilic
3
of the free phosphine is taking place, as indicated by the
formation of some free OPPh . This is not unexpected
VII
50
Re trioxides Re O and MeReO (MTO). The reaction
2 7 3
3
because the oxygen transfer between R SO and PPh cata-
3
proceeds by attack of the borane on these complexes. Two
borane equivalents are required to lead the reaction to
2
2
lyzed by RedO complexes has been reported long ago. This
3
could suggest that the deoxygenation of the sulfoxides in our
catalytic system was being effected by PPh with concomitant
formation of OPPh as final oxygen sink. However, this would
3
completion. On the other hand, the detection of H by GC
2
at the end of the reduction suggests further reaction with another
molecule of HBcat, affording catBOBcat and hydrogen.
3
4
DFT Studies. DFT calculations (Gaussian 03; see
9
51
require at least a stoichiometric amount of PPh relative to the
3
initial sulfoxide. Since in our catalytic system the sulfoxide is
present in ca. 50-fold excess against the phosphine, the deox-
ygenation of the sulfoxide by PPh , catalyzed by ReIO (PPh ) ,
details in the Experimental Section) were performed on the
3
2
3 2
(
50) Rom ~a o, C. C.; K u€ hn, F. E.; Herrmann, W. A. Chem. Rev. 1997,
would require the regeneration of PPh from OPPh , eventually
3
97, 3197–3246.
3
by the action of the borane. However, neither the reaction of
OPPh with 2 equiv of HBcat without catalyst nor the reaction
(51) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;
Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.;
Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson,
G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.;
Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai,
H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo,
C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin,
A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma,
K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.;
Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.;
Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.;
Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe,
M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.;
Pople, J. A. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT,
2004.
3
of OPPh with 2 equiv of HBcat catalyzed by ReIO (PPh )
3 2
3
2
(1 mol %) led to reduction of OPPh after 1 h at room
temperature. We can conclude that the regeneration of PPh3
3
by reduction of OPPh with HBcat does not occur in our system
3
and that PPh is not the main reducing agent of the sulfoxide.
3
The effect of the amount of borane was also examined
in the deoxygenation of 4-chlorophenyl sulfoxide with the
catalyst ReIO (PPh ) . When this reaction was carried out
2
3 2
with 1.2 equiv of HBcat, only 6% of the sulfide was obtained.
However, in the presence of 2 equiv of HBcat, the reduction
of sulfoxide was complete. Besides the amount, the nature of
the borane used has a very pronounced influence on the

Article
Organometallics, Vol. 29, No. 21, 2010 5521
Figure 2. Pathway for the loss of R S from ReHIO(R SO) -
2
2
2
(
2 2
OBcat) (C) with formation of ReHIO (R SO)(OBcat) (D). ΔG
-
1
in kcal mol , relative to the pair of reagents, 8 þ HBcat;
Figure 1. Pathway for the reaction between the catalyst ReIO2-
barriers in italics.
(
(
8
R
2
SO)
OBcat) (C). ΔG in kcal mol , relative to the pair of reagents,
þ HBcat; barriers in italics.
2 2 2
(8) and HBcat to form the hydride ReHIO(R SO) -
-
1
3
(eqs 2 and 3 above), while OBcat is trans to the oxo ligand.
˚
The Re-O(B) distance is 1.988 A, very close to the 1.932
and 1.985 A values observed in the analogous distances in
2 and 3.
˚
basis of the experimental evidence described above, namely,
the formation of ReIO (R SO) from ReIO (PPh ) . This
2
2
2
2
3 2
catalyst was selected because more studies were performed
using it and it displays very good performance in terms of
conversion and speed of reaction. Me SO was used as a
The loss of thioether from intermediate C takes place
-1
easily, with a barrier of 7.2 kcal mol , and a new species
ReHIO (R SO)(OBcat) (D) is formed (Figure 2). In the
2
2
2
model for the substrate. The fact that the hydride ReHIO-
(PPh ) (OBcat) (2) was isolated from the precursor and
transition state, the S
˚ ˚
1.572 A in C to 1.843 A.
3 3 O distance has increased from
3
3
2
also exhibited catalytic activity led us to look for a hydride
intermediate. Also, hydrides were shown to be formed
when high-valent oxo-rhenium or molybdenum complexes
activated Si-H or H-H bonds. The mechanism should
account for the need to use 2 equiv of borane and the
The three steps described above show a low-energy path-
way that leads from the catalyst 8 to the formation of the
thioether and a new intermediate (D). In D, the metal has
been oxidized to Re(VII), owing to the loss of R S. The
2
former neutral R SO ligand has given rise to the dianionic
2
5
2
formation of both H and catBOBcat.
2
The reaction pathway of the catalyst ReIO (R SO) (8)
2
oxo ligand, formally changing the metal oxidation state from
V to VII. All the previous intermediates (B and C) were
Re(V) complexes.
2
2
with the first molecule of HBcat to yield the hydride ReHIO-
R SO) (OBcat) (C) is shown in Figure 1. The energies
(
A relevant question concerns the order of the reactions.
Would it be possible for complex 8 first to lose R S and then
2
2
-1
shown throughout this work (kcal mol ) include solvent
correction (THF) obtained with the PCM model (see com-
putational details). The reaction takes place in two steps
2
react with HBcat to reach exactly the same intermediate?
This alternative was examined, and the corresponding path-
way is depicted in Figure 3.
-
1
with very small activation barriers (1.8 and 3.3 kcal mol
,
respectively), giving rise to stable species. The reaction is
The direct loss of R S from the catalyst can occur with a
2
-
1
-1
0
highly exergonic (31.7 kcal mol ). In the first step, the
borane adds to one oxide, forming a new B-O bond. The
environment of boron thus changes from planar to tetrahe-
dral. In the transition state TS , the B O distance is
small barrier (4.6 kcal mol ), and the species formed (B )
can activate the B-H bond in HBcat, although with a
-
1
relatively large barrier (15.9 kcal mol ). The S O distance
3 3
3
˚
in the transition state TS8B is 1.760 A, even shorter than in
0
the corresponding TS of the mechanism shown in Figure 1.
8
B
3 3 3
˚
still very long (3.387 A) and the HBcat molecule is planar.
Except for the RedO bond, which disappeared when HBcat
approached, B is structurally very close to 8, with very
similar bond lengths and angles. In the second step, the H(B)
is transferred from boron to rhenium, with the boron envi-
ronment becoming again planar and the rhenium octahedral.
The hydride is trans to iodide, as observed in the X-ray
determined structures of the related hydride complexes 2 and
0
Interestingly, B can activate the B-H bond in only one step,
with simultaneous formation of the new B-O bond and the
Re-H bond. In this transition state (TS
0
), the new B-O
bond is almost formed (1.547 A), being rather close to the
B D
˚
˚
final distance (1.337 A). The H atom, on the other hand, is
˚
bridging between B and Re (1.268 and 2.227 A, for B
H
and Re H, respectively). This pathway is less likely to
3
3 3
3
3 3
occur than the previous one (Figures 1 and 2), because the
barriers are significantly higher, despite the same initial and
final states. The discrepancy of the energy values of inter-
mediate D presented in Figures 2 and 3 arises from a different
(52) (a) Kennedy-Smith, J. J.; Nolin, K. A.; Gunterman, H. P.; Toste,
F. D. J. Am. Chem. Soc. 2003, 125, 4056–4057. (b) Nolin, K. A.; Krumper,
J. R.; Puth, M. D.; Bergman, R. G.; Toste, F. D. J. Am. Chem. Soc. 2007, 129,
1
4684–14696. (c) Reis, P. M.; Costa, P. J.; Rom ~a o, C. C.; Fernandes, J. A.;
Calhorda, M. J.; Royo, B. Dalton Trans. 2008, 1727–1733.
-
1
scale. The energy in Figure 2 (-42.3 kcal mol ) is relative to

5
522 Organometallics, Vol. 29, No. 21, 2010
Fernandes et al.
the pair of reactants, 8 and HBcat, interacting weakly, while
approaches the oxygen in the B-O-Re bond of D. At the
end of this reaction, involving intermediates E and F, both
H and the catBOBcat species have been released, while
-1
the energy of D in Figure 3 (-45.9 kcal mol ) refers to the
isolated reagents. Thus, the energy difference (3.6 kcal
2
-
1
mol ) represents the stabilization of the pair of reagents,
a new intermediate, G, has been formed. Notice the small
barriers involved in the three steps leading from D to G
8 þ HBcat, when they approach each other, relative to the
two separated molecules.
At this stage of the reaction, one molecule of R S has been
2
released by the catalyst, another sulfoxide remains coordi-
nated, and it is known that a second HBcat molecule is
involved in the reaction. Therefore, the reaction between D
and HBcat was addressed. The pathway shown in Figure 4
depicts the lowest energy route for this reaction, and the
most interesting feature is that the new HBcat molecule
-
1
(between 0.9 and 4.3 kcal mol ). The boron atom of the
incoming HBcat becomes tetrahedral in E, but otherwise this
species is very similar to D. The coordinated sulfoxide has
rotated in order to accommodate the bulky HBcat molecule.
Then this HBcat molecule rotates, so that the H(B) ap-
proaches the H(Re). In the transition state (TSEF), the
˚
B-H bond has elongated to 1.335 A and the Re-H to
˚
1.830 A, while the H H distance of 1.046 A is character-
˚
3
3 3
istic of a weak H-H bond. This allows for an easy loss of
both H and catBOBcat from this transition state. Indeed,
H was detected by gas chromatography at the end of
2
2
the reduction. During this process, Re(VII) has been
brought back to Re(V) by the second HBcat molecule,
but the catalytic cycle has not yet closed. Intermediate G
can either lose another R S molecule or regenerate the
2
catalyst by reacting with another R SO substrate molecule.
2
A closer look at D suggests that HBcat might attack
another oxygen, since there are two oxo groups bound to
Re and they are nonequivalent. Two different products can
be obtained, as shown in Figure 5.
The immediate conclusion is that, although there is only
one transition state, these two alternatives require much
higher energies than the first pathway (Figure 4), namely,
-1
2
5.9 or 23.1 against 4.3 kcal mol . The products are very
stable, but, considering the unlikeness of overcoming these
huge barriers, when competitive pathways are available, we
did not even attempt to pursue the study of the reaction from
0
H or H . Nevertheless, these two species are interesting. The
B-H activation ends with the Bcat fragment attached to the
former oxo group and the H transferred to the other O, so
that in H the Bcat is stabilized by a hydrogen bond with the
neighbor O-H. Even more interesting is the formation of
Figure 3. Alternative pathway for the reaction between the
catalyst ReIO
2 2 2 2
(R SO) (8) and HBcat losing R S and forming
-
1
the intermediate ReHIO (R SO)(OBcat) (D). ΔG in kcal mol
,
2
2
0
relative to 8; barriers in italics.
a coordinated dihydrogen molecule in H . The H from the
Figure 4. Pathway for the reaction between the intermediate ReHIO
-
2
(R
2 2
SO)(OBcat) (D) and HBcat affording H and catBOBcat and
1
forming the intermediate ReIO (R SO) (G). ΔG in kcal mol , relative to D þ HBcat; barriers in italics.
2
2

Article
Organometallics, Vol. 29, No. 21, 2010 5523
Scheme 1. Catalytic Cycle for the Reduction of One R
Molecule
2
SdO
Figure 5. Alternative pathway for the reaction between the
intermediate ReHIO (R SO)(OBcat) (D) and HBcat forming
2
2
0
-1
intermediate H or H . ΔG in kcal mol , relative to D þ HBcat;
barriers in italics.
Scheme 2. Entrance of Re(VII) Catalysts MeReO (MTO) and
3
2 7
Re O in the Catalytic Cycle
Having analyzed these alternative reaction paths, only the
recovery of the catalyst remains, although one may still
consider the possibility of R S loss from G. These reaction
2
paths are represented in Figure 6.
The reaction with another substrate molecule (top of
Figure 6) is accompanied by a lower barrier (almost 10 times
smaller than the barrier needed to lose thioether), suggest-
ing that the catalyst will be regenerated. Interestingly, only one
molecule of R S results from a catalyst (8) containing two
2
coordinated sulfoxides, and, thus, one of the initial R SO
2
Figure 6. Pathways for the reaction between the intermediate
ligands in 8 remains coordinated during the entire mechanism.
This mechanism, summarized in Scheme 1, requires 2
equiv of HBcat, which will in the end give rise to catBOBcat
ReIO
ΔG in kcal mol , relative to G þ R
bottom); barriers in italics.
2
(R
2
SO) (G) and R
2
SO (top) or loss of R
2
S (bottom).
SO (top) or to isolated G
-
1
2
(
and H . The Re(V) complex is oxidized to Re(VII) after
2
second HBcat migrates to the hydride. The final H-H bond
length is 0.824 A, only slightly elongated from the 0.744 A
calculated for free H2.
losing R S from the hydride complex C at the end of the first
2
part of the reaction, and the reaction with the second HBcat
produces the electrons needed to reduce it back to Re(V).
˚
˚

5
524 Organometallics, Vol. 29, No. 21, 2010
Fernandes et al.
Scheme 3. Competition between Phosphine Substitution by
Sulfoxide and Hydride Formation
waste of borane, since 2 equiv are needed. The proposed
reaction mechanism, based on the results of a detailed DFT
computational study, accounts for the formation of catBOB-
cat and H as residues, while one molecule of sulfoxide per
2
complex is reduced, despite the presence of two coordi-
nated sulfoxides in the catalytically active species. The first
HBcat reacts with this active species, giving rise to a hydride
complex, which loses R S more easily than the parent
2
species. The remaining intermediate is a Re(VII) complex
that is reconverted in the initial Re(V) catalyst, after reacting
with the second HBcat molecule and producing catBOBcat
and H2.
These results extend the scope of the use of oxo-rhenium(V)
and -(VII) complexes as effective catalysts for reduction
reactions and open a new area of catalysis for these com-
plexes, since reactions with boranes are among the most
important synthetic methodologies in organic chemistry. We
believe that this procedure will present a useful and attrac-
tive alternative to the existing methods for the reduction
of sulfoxides to sulfides. Other synthetic applications and
further mechanistic studies of these mild and highly efficient
catalytic systems are still under investigation in our group.
When the catalyst is MTO, a Re(VII) complex, addition of
R SdO yields complex 10 (Scheme 2). HBcat adds in a
2
Experimental Section
similar way to one RedO bond, forming directly an analo-
gue of D where Me replaces I, and the reaction proceeds.
THF was dried and distilled prior to use over Na/benzophe-
none under a nitrogen atmosphere. Boranes and oxo-rhenium
complexes were obtained from Aldrich. Flash chromatography
0
Re O can also add R SdO to afford complex 9 , where
2
7
2
ReO plays the same role as Me in 10. Therefore, the pro-
4
1
was performed on MN Kieselgel 60 M 230-400 mesh. H NMR
and C NMR spectra were measured on a Bruker Avance III
posed catalytic cycle applies to Re(VII) catalysts, although
the starting point is a different one (D instead of 8). It should
13
400 MHz spectrometer. Chemical shifts are reported in parts per
million (ppm) downfield from an internal standard.
0
also be noticed that addition of HBcat to 9 or to 10 is similar
0
to the same reaction involving intermediate B (the second
0
General Procedure for the Reduction of Sulfoxides with the
System HBcat/Oxo-rhenium Complex. To a solution of catalyst
(1 mol %) in THF (3 mL) was added the sulfoxide (1.0 mmol)
and the solution of HBcat in THF (2.0 mmol). The reaction
mixture was stirred under an air atmosphere (the temperatures
and the reaction times are indicated in Table 2), and the progress
step of the profile in Figure 3), but with Me (10) or ReO (9 )
4
0
in place of I (B ).
Another open question concerns the role of the hyd-
ride (PPh ) (O)(I)Re(H)OBcat, 2. As discussed above, this
3
2
hydride also catalyzes sulfoxide reduction, but the reaction
takes much longer, suggesting that in normal reaction con-
ditions (sulfoxide added before catecholborane) the active
species is not 2, but a different one. The energetics of these
reactions are shown in Scheme 3. Replacing a phosphine by a
sulfoxide is an unfavorable process, but is easier to achieve
1
of the reaction was monitored by TLC and H NMR. Upon
completion, the reaction mixture was evaporated and purified
by silica gel column chromatography with the appropriate
mixture of n-hexane and ethyl acetate to afford the sulfides,
which are all known compounds.
4-Chlorophenyl Sulfide. The spectroscopic data are similar
): δ 7.22
-1
1
to the commercial sulfide. H NMR (400 MHz, CDCl
starting from ReO I(PPh ) (1) (27.5 kcal mol ) than from
2
3
13
3 2
-
1
the hydride 2 (31.4 kcal mol ) and probably driven by the
high concentration of sulfoxide compared with the con-
centration of the catalyst precursor. On the other hand, both
complexes 1 and 8 react easily with HBcat to yield the
hydrides 2 and C. Thus, in the normal course of the reaction,
when sulfoxide is added first, followed by addition of HBcat,
complex 8 will form preferentially and the hydride C will be
the catalyst. Only in the absence of sulfoxide will hydride 2
appear. If substrate is then added, it will also catalyze the
reaction.
(d, 2H, J = 9.0 Hz), 7.18 (d, 2H, J = 9.0 Hz) ppm. C NMR
101 MHz, CDCl ): δ 134.1, 133.6, 132.5, 129.6 ppm. Anal.
(
3
Calcd for C12
6.30; H, 3.05; S, 12.36.
Phenyl Sulfide. The spectroscopic data are similar to the
8 2
H Cl S: C, 56.48; H, 3.16; S, 12.57. Found: C,
5
1
commercial sulfide. H NMR (400 MHz, CDCl ): δ 7.28-7.16
3
13
m, 10 H) ppm. C NMR (101 MHz, CDCl
(
3
): δ 135.9, 131.1,
1
17.21. Found: C, 77.28; H, 5.26; S, 17.16.
29.3, 127.1 ppm. Anal. Calcd for C12 10S: C, 77.37; H, 5.41; S,
H
4-Methylphenyl Sulfide. The spectroscopic data are similar
1
to the commercial sulfide. H NMR (400 MHz, CDCl
3
): δ 7.27
d, 2H, J = 8.0 Hz), 7.14 (d, 2H, J = 8.4 Hz), 2.36 (s, 3H) ppm.
(
1
3
C NMR (101 MHz, CDCl ): δ 137.0, 132.8, 131.2, 130.0, 21.2
Conclusions
3
7 8
ppm. Anal. Calcd for C H S: C, 67.69; H, 6.49; S, 25.82. Found:
C, 67.50; H, 6.27; S, 25.67.
Methyl 2-(Phenylthio)acetate. The spectroscopic data are
In conclusion, we have developed a facile and very efficient
method for the reduction of sulfoxides with high yields
and good chemoselectivity, under mild conditions. Other
remarkable advantages of this methodology include low
catalyst loading (1 mol %), fast reaction times, clean reac-
tions, and stability of the catalysts toward air and moisture,
allowing the reaction to be carried out under an air atmo-
sphere. Among the disadvantages, one might mention the
1
similar to the commercial sulfide. H NMR (400 MHz, CDCl ):
3
δ 7.33 (d, 2 H, J = 9.0 Hz), 7.26-7.16 (m, 3H), 3.65 (s, 3H), 3.58
13
(
3
s, 2H) ppm. C NMR (101 MHz, CDCl ): δ 170.1, 135.1,
1
C, 59.32; H, 5.53; S, 17.59. Found: C, 59.19; H, 5.40; S, 17.39.
29.9, 129.1, 126.9, 52.4, 36.5 ppm. Anal. Calcd for C H O S:
9 10 2
Methyl Phenyl Sulfide. The spectroscopic data are similar
1
to the commercial sulfide. H NMR (400 MHz, CDCl
3
): δ

Article
Organometallics, Vol. 29, No. 21, 2010 5525
1
3
7
.25-7.18 (m, 4 H), 7.09-7.07 (m, 1H), 2.41 (s, 3H) ppm.
NMR (101 MHz, CDCl ): δ 138.4, 129.0, 126.8, 125.2, 16.0 ppm.
Anal. Calcd for C H S: C, 67.69; H, 6.49; S, 25.82. Found: C,
C
The energy profiles reported result from single-point energy
calculations using a VTZP basis set (basis b2) and the geome-
tries optimized at the PBE1PBE/b1 level. Basis b2 consisted of
3
7
8
6
0
6
7.50; H, 6.35; S, 25.71.
-Nitrothioanisole. The spectroscopic data are similar to the
a Stuttgart/Dresden ECP basis set with an added f-polariza-
tion function for Re and a d-polarization function for I,
5
6
57
4
commercial sulfide. H NMR (400 MHz, CDCl
1
61
3
): δ 8.15 (d,
and standard 6-311þþG(d,p) for the remaining elements.
1
3
2
H, J = 7.7 Hz), 7.31 (d, 2H, J = 8.4 Hz), 2.57 (s, 3H) ppm.
C
Solvent (dichloromethane) effects were considered in the
PBE1PBE/b2//PBE1PBE/b1 energy calculations using the
polarizable continuum model (PCM) initially devised by
NMR (101 MHz, CDCl
ppm. Anal. Calcd for C
1
3
): δ 148.9, 144.8, 125.0, 123.9, 14.9
NO S: C, 49.69; H, 4.17; N, 8.28; S,
8.95. Found: C, 49.58; H, 4.03; N, 8.01; S, 18.84.
DFT Calculations. All calculations were performed using the
Gaussian 03 software package and the PBE1PBE functional,
without symmetry constraints. That functional uses a hybrid
generalized gradient approximation (GGA), including 25%
H
7 7
2
6
2
63
Tomasi and co-workers as implemented in Gaussian 03.
The molecular cavity was based on the united atom topologi-
cal model applied on UAHF radii, optimized for the HF/
6-31G(d) level. Structural representations were obtained with
5
1
6
4
Chemcraft.
5
3
49
mixture of Hartree-Fock exchange with DFT exchange-
correlation functional, given by Perdew, Burke, and Ernzerhof
Acknowledgment. This research was supported by
FCT through project PTDC/QUI/71741/2006. J.A.F.
thanks FCT for a postdoctoral grant (SFRH/BPD/
23461/2005). We wish to acknowledge M. C. Almeida
and Dr A. Coelho for providing data from the Elemental
Analysis Service at ITQB. The NMR spectrometers are
part of the National NMR Network and were purchased
in the framework of the National Programme for Scien-
tific Re-equipment, contract REDE/1517/RMN/2005,
with funds from POCI 2010 (FEDER) and Funda c- ~a o
para a Ci ^e ncia e a Tecnologia (FCT).
54
(PBE). The optimized geometries were obtained with a VDZP
basis set (basis b1) consisting of the LanL2DZ basis set
5
5
5
6
augmented with an f-polarization function for Re and a
5
d-polarization function for I, and a standard 6-31G(d,p)
7
58
for the remaining elements. Transition-state optimizations were
performed with the synchronous transit-guided quasi-Newton
5
9
method (STQN) developed by Schlegel et al. Frequency
calculations were performed to confirm the nature of the sta-
tionary points, yielding one imaginary frequency for the transition
states and none for the minima. Each transition state was further
confirmed by following its vibrational mode downhill on both sides
and obtaining the minima presented on the energy profiles.
Supporting Information Available: Atomic coordinates for all
optimized species. This material is available free of charge via
the Internet at http://pubs.acs.org.
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