On the mechanism of the synergistic oxidation of saturated hydrocarbons and hydrogen sulfide under Gif conditions

Article abstract of DOI:10.1039/a800552d

It is demonstrated by a study of cyclohexane phenylselenation that the synergistic oxidation of cyclohexane with H₂S and O₂ does not involve carbon or oxygen radicals.

Full text of DOI:10.1039/a800552d

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On the mechanism of the synergistic oxidation of saturated hydrocarbons and
hydrogen sulfide under Gif conditions
Derek H. R. Barton*† and Tingsheng Li*
Department of Chemistry, Texas A & M University, PO Box 300012, College Station, TX 77842, USA
It is demonstrated by a study of cyclohexane phenyl-
selenation that the synergistic oxidation of cyclohexane with
acid was no longer needed. The phenylselenocyclohexane was
then produced in quantitative yield (entries 3, 4 and 5).
We consider that phenyselenocyclohexane must be produced
H
2
S and O
2
does not involve carbon or oxygen radicals.
5
by the same type of mechanism that we proposed before
Recently we reported a new reaction in which saturated
(Scheme 1).
In our first publication on the H
1
1
hydrocarbons were oxidized synergistically with H
2
S and O
2
.
2
S–O
2
reaction we did not
This unusual reaction is an extension of Gif chemistry that
should be of industrial importance. We now present further
evidence on the mechanism of this reaction.
comment on the mechanism except to classify it as ‘Gif
chemistry’. We suggested that the oxidant was superoxide and
II
III
that this reacted with Fe to furnish the species Fe –OOH. The
2
III
The bond strength in the H–SH bond is 90.5 (±1.1) kcal
species can also be produced by displacement on Fe with
21
1
2
mol . Thus any secondary or primary carbon radical would be
immediately reduced by H-atom transfer. When we photolyzed
the Barton ester of adamantane-1-carboxylic acid in the
H
2
O
2
. Without hydrocarbon this has t ca. 45 min. Upon
addition of hydrocarbon it is rapidly inserted into the Fe–C
bond, which slowly (t ca. 45 min) affords ketone or, with
1
2
9
presence of H
mantane. Similarly, oxidation of adamantane in the presence of
S gave a normalized secondary/tertiary ratio of about 1. This
2
S it was converted quantitatively into ada-
iodide, rapidly gives the iodide of the hydrocarbon. In order to
explain why the selective functionalization of saturated hydro-
H
2
carbons takes place in the presence of H
2 2 3
S, Ph S, Ph P,
is three times as great as normal and corresponds to the
reduction of tertiary radicals back to hydrocarbon. Since there
are no secondary radicals, secondary position oxidation pro-
ceeded normally.
Cyclohexane gave a mixture of cyclohexanone and cyclohex-
(MeO) P, PhSH and even PhSeH, we proposed in 1992 that it
3
3
was the contact of a relatively inert iron species with the
hydrocarbon which created the active iron species, which then
reacted immediately with the hydrocarbon. We cited the agostic
effect10 as an explanation. It is the reactivity of PhSeH,
5
anol. The latter was produced by reduction of the intermediary
produced by PhSeSePh and Bu
3
P, that gives real substance to
4
IV
V
hydroperoxide by H
2
S.
our proposal. We now suggest that Fe and Fe are only
produced at the moment of contact with the hydrocarbon and
that these species react instantly with their hydrocarbon
activator. We offer the same explanation for the KIE of close to
1 (see above).
The formation of alcohol and ketone at the same time enabled
us to determine the kinetic isotope effect (KIE) for alcohol
formation. It was of the order 1.1–1.2.
Recently5 we showed that phenylselenation of saturated
6a
III
V
hydrocarbons involves PhSeH, an excellent trap for radi-
Thus the Fe –Fe manifold can be modified according to
6
b
6a
0
cals. In fact the earlier work involved reduction with Zn –
Scheme 2. The H
reduction of Fe to Fe .
The alternative route for phenylselenation is from Fe and
2
S–O
2
2
chemistry then takes place by H S
II
V
IV
Fe catalyst. Methylation showed that all the PhSeSePh had
been reduced to PhSeH. This was, in fact, a proof of the non-
involvement of carbon radicals. Our recent work has shown
that selenide anion is not involved. Another factor to consider
7
II
5
2 2
H O , which we have used in our work with PhSeSePh and
8
II
IV
Bu
3
P already cited. We modify our concept of the Fe –Fe
9
7
21 21
IV
is the relative rates of reaction (3 3 10 and 2.3 3 10 m
s
)
manifold as shown in Scheme 3. Normally, the Fe –CHR
species can fragment into Fe and a carbon radical. This,
however, does not happen when PhSe is one of the ligands.
2
for radicals with PhSeH and PhSeSePh, respectively.6
b
III
2
In the present study, PhSeSePh was added to a system where
S and O were being passed through a mixture of cyclohex-
H
2
2
In our recent work on the phenylselenation reaction we
ane and picolinic acid (Table 1). This afforded ketone, alcohol
and a very high yield of phenylselenocyclohexane with respect
showed by 77Se NMR spectroscopy that the reaction of
PhSeSePh and Bu
excellent method for quantitative synthesis of PhSeH in situ.
The presence of the excess Bu P guaranteed that there was no
3
P (50% excess) with a drop of water was an
to the amount of PhSeSePh added (entries 1 and 2). Unlike in
5
the earlier study using H
2
O
2
, when H
2
S was used the picolinic
3
Table 1 Effect of selenide on Gif oxidationa
Ligand
Entry (mmol)
Selenide
(mmol)
FeCl
(mmol)
2
·4H
2
O/
Cyclohexane/
(mmol)
Ketone/
(mmol)
Alcohol/
(mmol)
PhSeC
(mmol)
6
H
11
/
Conversionb
(%)
Yieldc
(%)
1
2
3
4
5
6
7
Picolinic acid (3)
Picolinic acid (3)
PhSeSePh (1)
PhSeH (1.5)
PhSeH (2.5)
PhSeSePh (1.5)
PhSeSePh (1)
PhSeSePh (1)
PhSeSePh (1)
1
1
1
1
1
1
1
20
20
20
20
20
5
0.75
0.15
0.74
1.28
1.00
0.19
0.10
2.31
0.51
1.32
2.50
2.19
0.40
0.15
1.81
1.46
2.51
3.03
1.98
0.95
0.75
24.35
10.6
22.85
30.85
25.85
30.8
90.5
97.3
100
100
99
—
—
—
—
—
47.5
37.5
2
50
a
Ligand (0–3 mmol), FeCl
2
·4H
2
O (0.3–1 mmol), cyclohexane (2–20 mmol), selenide (1–2.5 mmol), 4-tert-butylpyridine (2 ml), MeCN (31 ml). O
2
(g) and
H
2
S (g) were passed at atmospheric pressure through the reaction mixture at room temperature for 3–4 h. The products were analyzed by GC, naphthalene
b
c
was used as internal standard. Conversion based on cyclohexane. Yield based on selenide.
Chem. Commun., 1998
821

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Table 2 Phenyselenation in the presence of Bu
3
Pa
LiCl/
PhSeSePh/
mmol
Bu
mmol
3
P/
Chloride/
mmol
Ketone/
mmol
Alcohol/
mmol
PhSeC
mmol
6 11
H /
Entry
mmol
Conditions
b
1
2
3
4
5
6
7
8
1
2
2
2
4
4
4
4
1
1
8
60
3
3
6
6
6
6
2
—
—
—
—
—
—
—
—
20
20
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
S–O
S–O
S–H
S–H
S–H
S–H
S–H
S–H
2
2
2
2
2
2
2
2
—
—
—
—
—
—
—
—
0.73
0.39
—
—
—
—
—
—
—
1.98
1.07
—
—
—
—
—
—
—
1.85
0.83
0.74
1.30
0.85
1.54
1.92
2.55
0.69
1.05
b
c
O
O
O
O
O
O
2
2
2
2
2
2
(1)
(2)
(1)
(2)
(3)
(4)
c
c
c
c
c
d
9
O
O
2
(1)
(2)
0.23
0.34
d
1
0
2
2
—
—
a
Picoline acid (3 mmol), FeCl
2 2
·4H O (1 mmol), cyclohexane (20 mmol), PhSeH (0.5–4 mmol), 4-tert-butylpyridine (2 ml), MeCN (31 ml). The products
b
were analyzed by GC, naphthalene was used as internal standard.
3
O
2
(g) and H
2
O (g) were passed through the reaction mixture at room temperature for
(1–2 mmol) was added at 0 °C.
c
(1–4 mmol) was added. d
–4 h.
H
2
S (g) was passed through the reaction mixture at 0 °C when H H O
2
O
2
2 2
picolinic acid) and the correct amount of a suitable pyridine
base (here 4-tert-butylpyridine). If the formation of the chloride
can only take place via radical formation then the presence of
PhSeH would remove the radical and no chloride would be
formed. In fact entries 9 and 10 show that chloride formation is
in competition with the phenylselenation reaction.
Ligand coupling
FeIV
FeII
+
PhSe
SePh
Scheme 1
FeII + •O2H
Finally we examined an oxidant, tert-butyl hydroperoxide
FeIII OOH
or
II
(
TBHP), that always reacts with Fe to make tert-butoxy
FeIII + H2O2
OOCHR2
12–14
radicals.
20 mmol) in MeCN (31 ml) and 4-tert-butylpyridine (2 ml)
containing FeCl ·4H O (1 mmol) and picolinic acid (3 mmol)
with passage of H S, only traces of oxidation (0.05 mmol) were
When TBHP (3 mmol) was added to cyclohexane
(
FeIII
CH2R2
H2O2–O2
2
2
2
I –
seen. From workup, 3 mmol of tert-butyl alcohol were
recovered. This result is in keeping with the reduction of tert-
butoxy radicals by H S. We conclude that carbon and oxygen
2
FeV CHR2
ICHR2
H2S
FeIV CHR2
radicals do not play a role in the synergistic oxidation of
saturated hydrocarbons.
Scheme 2
We thank Unilever, for support of this work. We thank Dr
J. A. Smith for the 31P NMR experiment.
CH2R2
FeII + H2O2
FeII OOH
Scheme 3
FeIV CHR2
fast
very fast
Notes and References
†
E-mail:dhrbarton@tamu.edu
1
D. H. R. Barton, T. Li and J. MacKinnon, Chem. Commun., 1997,
557.
adventitious oxidation of PhSeH back to PhSeSePh. We studied
this reaction by P NMR spectroscopy and showed that the
reduction was complete in 2 min (experiment by Dr J. A.
31
2 Handbook of Chemistry and Physics, ed. R. C. Weast, 67th edn., CRC
Press, Boca Raton, Florida, 1986, pp. F-178–184.
3
4
D. H. R. Barton and D. Doller, Acc. Chem. Res., 1992, 25, 504.
D. H. R. Barton, D. Crich and W. B. Motherwell, J. Chem. Soc., Chem.
Commun., 1984, 242.
2 2
Smith). Before adding H O we waited for 10–20 min to make
sure that the reduction was complete.
Table 2 shows further experiments in which Bu
the presence of PhSeSePh. Entries 1 and 2 used the H
3
P is used in
S–O
5
6
D. H. R. Barton, G. Lalic and J. A. Smith, Tetrahedron, in the press.
D. H. R. Barton, J. Boivin and P. Le Coupanec, J. Chem. Soc., Chem.
Commun., 1987, 1379; G. Balavoine, D. H. R. Barton, J. Boivin, P. Le
Coupanec and P. Lelandais, New J. Chem., 1989, 13, 691; D. H. R.
Barton and B. Chabot, Tetrahedron, 1996, 52, 10 287; (b) M. Newcomb
and M. B. Marek, J. Am. Chem. Soc., 1990, 112, 9662; M. Newcomb,
Tetrahedron, 1993, 49, 1151.
2
2
system as studied in Table 1. For entry 1 the yield of
phenylselenocyclohexane was 92%, whilst the total activation
including ketone and alcohol was 4.56 mmol. This is a good
1
conversion as judged by past experiments. All the selenium
was present as PhSeH prior to reaction with the hydrocarbon yet
the ketone and alcohol were formed in significant amounts. So
7
D. H. R. Barton, S. D. B e´ vi e` re, W. Chavasiri, E. Csuhai, D. Doller and
W.-G. Liu, J. Am. Chem. Soc., 1992, 114, 2147.
not only are no radicals present but also the Bu
with the iron species prior to activation by the hydrocarbon.
Using H as a minor component in the presence of an
3
P does not react
8
9
M. J. Perkins, Chem. Soc. Rev., 1996, 25, 229.
D. H. R. Barton, M. Costas Salgueiro and J. MacKinnon, Tetrahedron,
2 2
O
1
997, 53, 7417.
excess of PhSeH, the yield of the phenylselenocyclohexane (the
only product) was 74 and 85% for entries 3 and 5, respectively
10 F. A. Cotton and A. G. Stanislowski, J. Am. Chem. Soc., 1974, 96,
5074.
11 D. H. R. Barton, B. Hu, D. K. Taylor and R. U. Rojas Wahl, J. Chem.
Soc., Perkin Trans. 2, 1996, 1031.
2 F. Minisci and F. Fontana, Tetrahedron Lett., 1994, 35, 1427; F.
Minisci, F. Fontana, S. Araneo and F. Recupero, J. Chem. Soc., Chem.
Commun., 1994, 1823; Tetrahedron Lett., 1994, 35, 3759.
3 I. W. C. E. Arends, K. U. Ingold and D. D. M. Wayner, J. Am. Chem.
Soc., 1995, 117, 4710; P. A. MacFaul, I. W. C. E. Arends, K. U. Ingold
and D. D. M. Wayner, J. Chem. Soc., Perkin, Trans 2, 1997, 135.
(
for 1 mmol of H
respectively (for 2 mmol of H
entries 5 and 6 were due to an increase in the available PhSeH.
A further increase in the amount of H (entries 7 and 8)
reduced the yield with respect to H to 64%.
The formation of cyclohexyl chloride in the Fe –Fe
2
O
2
), and 65 and 77% for entries 4 and 6,
2 2
O
). The increased yields in
1
2 2
O
2 2
O
1
II
IV
11
manifold is usually accepted to imply the reaction of a carbon
radical with an Fe –Cl bond. The reaction can also be
considered as a ligand-coupling reaction with an Fe –C bond
III
14 D. H. R. Barton, V. N. Le Gloahec, H. Patin and F. Launay, New J.
Chem., in the press; D. H. R. Barton, V. N. Le Gloahec and H. Patin,
ibid., in the press.
IV
as in formation of the selenide (Scheme 1). The formation of
cyclohexyl chloride using H
reaction, requires the presence of the right carboxylic acid (here
2
O
2
, like the phenylselenation
Received in Corvallis, OR, USA, 14 November 1997; revised manuscript
received, 15th January 1998; 8/00552D
822
Chem. Commun., 1998