Oxygen transfer from sulfoxides: Oxidation of alkylarenes catalyzed by a polyoxomolybdate, [PMo12O40]3-

Article abstract of DOI:10.1021/ja0178721

The polyoxomolydate of the Keggin structure, PMo₁₂O₄₀^3-, catalyzes, under anaerobic conditions, oxygen transfer from sulfoxides to alkylarenes such as xanthene and diphenylmethane to yield xanthen-9-one and benzophenone, respectively. With use of ¹⁷O and ¹⁸O labeled phenylmethylsulfoxide it was shown that the sulfoxide is complexed by the polyoxometalate and the oxygen is transferred from the sulfoxide to the alkylarene. There is a good correlation between the reaction rate and the heterolytic benzylic C-H bond energy indicating a hydride transfer reaction from the alkylarene to the polyoxometalate-sulfoxide complex. In the case of triphenylmethane the resulting carbocation reacts to yield 9-phenylfluorene as the major product. The reaction kinetics supports such a reaction pathway. Copyright

Full text of DOI:10.1021/ja0178721

Published on Web 03/27/2002
Oxygen Transfer from Sulfoxides: Oxidation of Alkylarenes Catalyzed by a
3-
40
Polyoxomolybdate, [PMo12O ]
Alexander M. Khenkin and Ronny Neumann*
Department of Organic Chemistry, Weizmann Institute of Science, RehoVot 76100, Israel
Received December 23, 2001
Oxygen transfer reactions are of basic importance in both
Table 1. Oxidation of Alkylarenes with PMSO Catalyzed by
4
Q3[PMo12O40]a
organic-synthetic and biological settings. In this context, the use
of sulfoxides as oxidants is also of significant interest. Notably, in
synthetic organic chemistry, the Swern reaction and its numerous
variants use sulfoxides, primarily DMSO, for the oxidation of
1
alcohols to aldehydes and ketones. The oxidation reaction is not
catalytic and requires a stoichiometric amount of an electrophilic
reagent. In addition, deoxygenation of sulfoxides to sulfides
catalyzed by metal complexes with oxygen transfer to the metal
complex or to reduced species such as hydrohalic acids, phosphines,
2
carbenes, and carbon monoxide is also well established. Despite
these well-known cases describing oxygen-transfer reactions with
sulfoxides, to the best of our knowledge, there have been no reports
of catalytic oxygen transfer from a sulfoxide to a hydrocarbon.
In the context of ongoing interest in the catalytic activity of
polyoxometalates in oxidation reactions with various oxygen
3
donors, we have now found that polyoxometalates can also activate
sulfoxides and lead to oxidation of alkylarenes. Specifically, the
4
VI
4
oxidized Keggin-type polyoxomolybdate, Q
3
[PMo 12
O
40] ( Q )
+
n-C H
4 9
N ), catalyzes the oxidation of alkyl aromatics to ketones,
eq 1, or alternatively catalyzes their oxydehydrogenation, eq 2.
In Table 1, the results on the oxidation of alkylarenes with
4
phenylmethylsulfoxide (PMSO) catalyzed by Q
3
[PMo12
O40] are
4
presented. For secondary alkylarene substrates without hydrogen
R to the benzylic position, there was oxygenation at the benzylic
position and selective formation of the diaryl ketone. The oxygen-
ation of triphenylmethane was not selective (see below). For other
substrates, 9,10-dihydroanthracene, 9,10-dihydrophenanthrene, and
a
Reaction conditions: 1 mmol of PMSO, 1 mmol of substrate, 3.3 µmol
of Q3PMo12O40 in 1 mL of DCB, 170 °C, 15 h, Ar. Conversion is mol %
of PMSO reacted, TON is mol of products from alkylarene/mol of
Q3PMo12O40.
1
,2-diphenylethane, the presence of the R-hydrogen instead led
also apparent; i.e., DPSO is less reactive than PMSO. Other
4
4
mostly to oxydehydrogenation. The reaction stoichiometry as
indicated in eqs 1 and 2 was verified within (10% by gas
chromatography. The catalyst was stable under reaction conditions
as surveyed by following the 31P NMR spectrum.
Use of the oxidation of xanthene to xanthen-9-one as a model
reaction also enabled the observation of the effect of the sulfoxide
structure and the catalyst on the reactivity. With use of the following
molybdates, such as Q
less active catalysts per Mo atom compared to Q
2
MoO
4
and Q
6
Mo
7
O
24, were significantly
4
3
[PMo12O40].
MoO
2 2
(acac) was initially reactive but quickly yielded an un-
identified black precipitate. The analogous polyoxotungstate,
4
3
Q [PW12O40], was inactive. Some insight into the reaction mech-
anism is most warranted. Thus, the rates of oxygenation of xanthene,
9,10-dihydroanthracene, diphenylmethane, fluorene, and triphenyl-
5
reaction conditionss0.2 M xanthene, 1 M sulfoxide, 3.3 mM
methane were plotted as a function of the oxidation potential, the
4
6
Q
3
[PMo12
O40] in 1,2-dichlorobenzene (DCB) at 170 °C, Ar, 2 hs
homolytic benzylic C-H bond energy, and the heterolytic benzylic
7
the relative reactivity for various sulfoxides related to PMSO was
the following: dimethyl sulfoxide (DMSO), 0.4 < diphenylsul-
foxide (DPSO), 0.8 < phenylmethylsulfoxide (PMSO), 1.0 <
C-H bond energy, Figure 1.
Clearly, the best correlation (r ) 0.99) was for the rate as a
2
function of the heterolytic benzylic C-H bond energy with an
obvious deviation for triphenylmethane. This correlation suggests
that hydride abstraction and formation of a benzylic carbocation is
involved in the reaction pathway and tends to discount alkylarene
4-nitrophenylmethylsulfoxide (NPMSO), 1.2. On one hand, the
results indicate that electron-withdrawing groups increase reactivity,
NPMSO > PMSO > DMSO. On the other hand, a steric effect is
4198
9
J. AM. CHEM. SOC. 2002, 124, 4198-4199
10.1021/ja0178721 CCC: $22.00 © 2002 American Chemical Society

C O MMU N I C A T I O N S
Figure 1. The rate of alkylarene oxygenation as a function of heterolytic
and homolytic C-H bond strength, and oxidation potential. Reaction
conditions: 1 mmol PMSO, 1 mmol substrate, 3.3 µmol Q3PMo12O40, 170
°
0
C, Ar. The initial rates were computed as -d[substrate]/dt and were 0.26,
Figure 2. The rate of xanthene oxygenation as a function of initial PMSO
concentration. Reaction conditions: 0.6 M xanthene, 1.65-6.6 M PMSO,
10 mM Q PMo O in DCB at 170 °C. The insert is the Lineweaver-
.15, 0.021, 0.014, and 0.006 mmol/h for xanthene (X), 9,10-dihydroan-
thracene (DHA), diphenylmethane (DPM), fluorene (F), and triphenyl-
methane (TPM), respectively.
3
12 40
Burk plot (r2 ) 0.97); maximum rate, 0.05 M/h; Michaelis constant, K
m
)
1
M.
Scheme 1. Formation of 9-Phenylfluorene from Triphenylmethane
yielding a carbocation intermediate (Figure 1, formation of 9-phe-
nylfluorene from triphenylmethane and the positive effect of
electron-withdrawing groups in the sulfoxide) and a reduced
polyoxometalate. Then the carbocation is oxygenated by the
activation by hydrogen abstraction or electron-transfer pathways.
The formation of the carbocation is corroborated by the observation
of 9-phenylfluorene as the major product in the reaction of
18
sulfoxide ( O-labeling experiments) to give the oxygenated product
and a proton is released. Water may exchange with an intermediate
formed at this stage. The reduced polyoxometalate is reoxidized
8
triphenylmethane, Scheme 1. This reaction along with the differ-
11
quickly by an additional equivalent of sulfoxide to yield the
ence in the oxygen transfer step and reaction stoichiometry, eq 1,
probably account for the outlying result observed for triphenyl-
oxidized Q
3
PMo12
O
40, sulfide, and water. The quantitative formation
18
of the latter (GC-MS) was verified by using O-labeled PMSO.
H D
methane. A kinetic isotope effect (k /k ) of 3.4 for the oxygenation
Acknowledgment. This research was supported by the Basic
Research Foundation administered by the Israeli Academy of
Science and Humanities and the Israel Ministry of Science.
of xanthene/xanthene-d was also measured in support of C-H bond
2
cleavage in the rate-determining step.
Isotope-labeling experiments also gave significant insight. First,
the 17O NMR of a solution of 1.4 M PMSO- O (∼0 ppm, ∼8%
17
Supporting Information Available: The reaction kinetics of
xanthene oxygenation as a function of xanthene and polyoxometalate
concentration, and reaction temperature (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
9
4
3
enriched) and 2.8 mM Q [PMo12O40] in DCB showed a new peak
at 181 ppm, indicating a coordination interaction between PMSO
and the polyoxomolybdate. This chemical shift is distinctly different
compared to chemical shifts for bridging and terminal oxygens,
1
0
∼
550 and ∼930 ppm, and rules out oxygen exchange between
References
4
the sulfoxide and Q
3
[PMo12
O
40]. Second, a reaction between 1 M
(
1) Mancuso, A. J.; Swern, D. Synthesis 1981, 165-185. Tidwell, T. T. Org.
React. 1990, 39, 297-572. Tidwell, T. T. Synthesis 1990, 857-870.
2) Kukushkin, V. Y. Coord. Chem. ReV. 1995, 139, 375-407.
3) Mizuno, N.; Misono, M. Chem. ReV. 1998, 98, 171-199. Hill, C. L.;
Prosser-McCartha, C. M. Coord. Chem. ReV. 1995, 143, 407-455.
Kozhevnikov, I. V. Chem. ReV. 1998, 98, 171-198. Neumann, R. Prog.
Inorg. Chem. 1998, 47, 317-370.
1
8
9
PMSO- O (85% enriched), 0.2 M xanthene, and 3.3 mM
(
(
Q
3
PMo12
O
40 in DCB at 170 °C yielded xanthene-9-one with 62%
18
16
O enrichment. Third, a reaction between 1 M PMSO- O, 0.2 M
xanthene, and 3.3 mM Q
3
PMo12
O
40 in DCB at 170 °C in the
O (95% enrichment) gave xanthen-9-one
with 36% 1 O; however, the PMSO remained unlabeled. Indepen-
18
presence of 1 mmol H
2
(4) Reactions were carried out in 20 mL glass Schlenk flasks. The flasks
were loaded with the appropriate components and degassed by 3 successive
8
“
freeze-thaw-pump” cycles and filled with dry Ar. The reactions were
18
dently, PMSO was not labeled with H
absence of Q
The kinetics of the oxygenation reaction was also studied. In
40
the oxygenation of xanthene with PMSO catalyzed by Q PMo12O ,
2
O in the presence or
carried out in a thermostated oil bath and analyses were carried out by
GC and GC-MS.
3 40
PMo12O .
(
(
(
5) The oxidation potential is assumed to be proportional to the ionization
potential of the hydrocarbon. The data are from NIST at http://
webbook.nist.gov/chemistry.
3
6) The energy of the homolytic bond disassociation is assumed to be
proportional to the gas-phase IR absorption of the benzylic C-H bond.
The data are from NIST at http://webbook.nist.gov/chemistry.
7) -∆Ghydride is the free energy of R-H heterolytic bond disassociation:
the reaction displayed Michaelis-Menten-type kinetic behavior as
a function of PMSO concentration, Figure 2. A weak binding
constant, K
m
) 1 M, between PMSO and the polyoxometalate was
+
-
R-H f R + H . Data from: Cheng, J. P.; Handoo, K. L.; Parker, V.
D. J. Am. Chem. Soc. 1993, 115, 2655-2660. Handoo, K. L.; Cheng, J.
P.; Parker, V. D. J. Am. Chem. Soc. 1993, 115, 5067-5072.
computed. Additionally, van’t Hoff plots (Supporting Information)
showed the reaction to be of simple first order in xanthene and
(
8) In a control experiment, reaction of the authentic carbocation, trityl
tetrafluorborate, at reaction conditions as given in Table 1, yielded
triphenylmethanol/9-phenylfluorene in a 26/74 ratio, very similar to the
also approximately first order in Q
3
PMo12O40. An Arrhenius plot
(
Supporting Information) yielded the following activation param-
3
0/70 ratio observed in the oxidation reaction
q
q
17
18
17O (10.2% 17O) and
eters: E
a
) 12.85 kcal/mol; ∆H 25°C ) 12.26 kcal/mol; ∆S 25°C
)
(9) O- and O-labeled PMSO were prepared with H
2
18 18
2
H O (95% O), using the procedure described in: Okruszek, A. J. Label.
-
22.0 cal/(mol K).
From the experimental evidence a conceivable reaction path-
way for oxygen transfer may be postulated as follows. First, a
Compd. Radiopharm. 1983, 20, 741-743.
(10) Filowitz, M.; Ho, R. K. C.; Klemperer, W. G.; Shum, W. Inorg. Chem.
1979, 18, 93-103.
(
11) Reduced H PMo12O40 (prepared by reduction of Q PMo12O40 with Zn/
2
Q
3
3
17
3
Q PMo12
O
40-sulfoxide complex is formed ( O NMR, Michaelis
HCl) is oxidized by PMSO at rates that are at least 2 orders of magnitude
faster than the oxidation of xanthene.
kinetics). No reaction occurs in the absence of either polyoxometa-
late or sulfoxide. This complex catalyzes hydride abstraction,
JA0178721
J. AM. CHEM. SOC.
9
VOL. 124, NO. 16, 2002 4199