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S.C.A. Sousa et al. / Catalysis Communications 40 (2013) 134–138
Table 3
Oxidation of alcohols catalyzed by ReOCl3(PPh3)2 in different solventsa.
Entry
Solvent
Temperature
Yield A/B (%)b
1
2
3
4
5
6
7
Toluene
Toluene
Benzene
Dichloromethane
Chloroform
Acetonitrile
THF
Reflux
r.t.
93/94 (5)c
59/93 (36)c
79/82 (9)c
Reflux
Reflux
Reflux
Reflux
Reflux
65/82 (30)c (8)d
56/77 (42)c (15)d
27/75 (68)c
5/80 (83)c
a
All reactions were carried out with 1.0 mmol of sulfoxide and 1.0 mmol of alcohol.
Isolated yield.
Alcohol recovered.
Sulfone formed.
b
c
d
4-chlorobenzaldehyde, and 4-cyanobenzaldehyde were obtained
in 76–93% yields (Table 4, entries 1–7). Another interesting
result was the selective oxidation of cinnamyl alcohol in 79%
yield without affecting the double bond (Table 4, entry 8). In con-
trast, the oxidation of 2-phenylethanol only produced the
phenylacetaldehyde in low yield (44%) (Table 4, entry 9).
The catalytic activity of the system bis(4-chlorophenyl) sulfoxide/
ReOCl3(PPh3)2 was also investigated in the oxidation of several second-
ary alcohols. Excellent yields of ketones were also obtained in the oxida-
tion of the alcohols 1-(4-nitrophenyl)ethanol and 1-(4-bromophenyl)
ethanol (93% and 92%) (Table 4, entries 10 and 11). The oxidation of
1-(4-cyanophenyl)ethanol and 1-(4-iodophenyl)ethanol produced the
corresponding ketones in 76% and 62% yields, respectively (Table 4,
entries 12 and 13). The reactions of 1-(4-bromophenyl)ethanol and
1-(4-cyanophenyl)ethanol (Table 4, entries 11 and 12) were carried
out at a lower temperature (80 °C), since the boiling points of
1-(4-bromophenyl)ethanol and 4-acetylbenzonitrile are very simi-
lar to the reflux temperature of toluene.
compound such as oxalyl chloride, which is toxic, a powerful irritant,
and an expensive substance.
The mechanism for the oxidation of alcohols using the system of
bis(4-chlorophenyl) sulfoxide/ReOCl3(PPh3)2 (10 mol%), should in-
volve the coordination of the sulfoxide to the rhenium with the for-
mation of the complex ReOCl3(SR2)(OPPh3) as reported by other
authors [22–24]. The addition of the alcohol to this species should re-
sult in the oxidation of the alcohol to the corresponding carbonyl
compound with the liberation of a molecule of water and sulfide.
Further mechanistic studies and new applications of this methodol-
ogy to the oxidation of other substrates are now under investigation in
our group.
4. Conclusion
In conclusion, we have demonstrated that the oxo-rhenium com-
plex ReOCl3(PPh3)2 is a good catalyst for the oxidation of primary and
secondary alcohols to the corresponding aldehydes and ketones, using
a sulfoxide as oxidant agent. The catalytic system bis(4-chlorophenyl)
sulfoxide/ReOCl3(PPh3)2 is also selective for the oxidation of primary
alcohols to the corresponding aldehydes with no further oxidation to
acids.
Finally, the oxidation of the secondary diaromatic alcohols
(4-nitrophenyl)(phenyl)methanol and bis(4-chlorophenyl)methanol
afforded the corresponding ketones in 85% and 40% yields, respectively
(Table 4, entries 14 and 15). For all of the reactions, the sulfide was iso-
lated in high yields (82–96%).
The results obtained demonstrate that the catalytic system bis(4-
chlorophenyl) sulfoxide/ReOCl3(PPh3)2 is efficient for the oxidation
of primary and secondary alcohols, and highly selective for the oxidation
of primary alcohols to the corresponding aldehydes with no further oxi-
dation to acids, which is of extreme importance for the industry. Compar-
ison between the catalytic systems bis(4-chlorophenyl) sulfoxide/
ReOCl3(PPh3)2 and di-p-tolyl sulfoxide/HReO4, used in our previous
work [19], shows that both systems give similar yields of aldehydes,
but the oxidations with HReO4 require less amount of catalyst. Nev-
ertheless, the system bis(4-chlorophenyl) sulfoxide/ReOCl3(PPh3)2
proved to be also efficient for the oxidation of secondary alcohols to
the corresponding ketones, in contrast to low activity of the system
di-p-tolyl sulfoxide/HReO4 [19] in the oxidation of secondary alcohols.
This methodology is also an excellent alternative to the Swern
oxidation since it does not use DMSO as oxidant agent, avoiding the
production of the volatile and unpleasant dimethyl sulfide. Another
advantage of this method is that the bis(4-chlorophenyl) sulfide,
obtained as by-product in high isolated yields, can be used as sub-
strate in other reactions or can be oxidized and reused in this proce-
dure. This novel method does not require the addition of any other
This practical method shows some advantages compared to other
methods for the oxidation of alcohols: 1) use of a sulfoxide (other than
DMSO) as oxidant agent; 2) does not require the addition of any other re-
agents; 3) applicability to a variety of alcohols; 4) use of commercial and
easy-to-handle catalysts; 5) high isolated yields of carbonyl compounds;
6) selective oxidation of primary alcohols to aldehydes; 7) high isolated
yields of the sulfide that can be used in other reactions or oxidized and
reused in this method; and 8) simple experimental operation. All of
these features make this method an attractive and useful alternative for
the oxidation of primary and secondary alcohols.
Acknowledgment
This research was supported by FCT through project PTDC/
QUI-QUI/110080/2009. S.C.A. Sousa (SFRH/BD/63471/2009), J.R.
Bernardo (SFRH/BD/90659/2012) and P.R. Florindo thank FCT for
the grants. The authors thank the project PEst-OE/QUI/UI0100/
2013 and the Portuguese NMR Network (IST-UTL Center) for pro-
viding access to the NMR facilities.