Baeyer-Villiger oxidation of ketones catalyzed by platinum(II) lewis acid complexes containing coordinated electron-poor fluorinated diphosphines

Article abstract of DOI:10.1021/om049264a

The synthesis and characterization of new hydroxo-bridged platinum(II) complexes of the type [P(μ-OH)(P-P)]₂[BF₄]₂, (1-4), where P-P = (C₆H_5-nF_n) ₂PCH₂CH₂P(C₆H_5-nF _n)2 (n = 2 (2Fdppe) (1), 3 (3Fdppe) (2), 4 (4Fdppe) (3), 5 (dfppe) (4), are reported. These compounds have been used in the Baeyer-Villiger oxidation of 2-methylcyclohexanone using 35% hydrogen peroxide as oxidant. The reactions were performed at 25, 50, and 70°C in a chlorinated solvent/H ₂O two-phase system. Among the fluorinated catalysts, complex 4 was found to be the best one in the oxidation of cyclic ketones; however, it is ineffective toward acyclic ketones. The Lewis acidity of the platinum(II) complexes with coordinated fluorinated diphosphines was investigated through the determination of the wavenumber shift Δν = ν(C≡N) _coord - ν(C≡N)_free of the isocyanide group in complexes of the type [PtCl(CN-2,6-(CH₃)₂C ₆H₃)(P-P)[BF₄] (P-P = 2Fdppe, 3Fdppe, 4Fdppe, dfppe, dppe). This latter parameter, which represents a measure of the electrophilicity of the metal center, was then correlated to the catalytic activity of complexes 1-4 in the Baeyer-Villiger oxidation of ketones.

Full text of DOI:10.1021/om049264a

1012
Organometallics 2005, 24, 1012-1017
Baeyer-Villiger Oxidation of Ketones Catalyzed by
Platinum(II) Lewis Acid Complexes Containing
Coordinated Electron-Poor Fluorinated Diphosphines
Rino A. Michelin,*,† Erika Pizzo,† Alessandro Scarso,‡ Paolo Sgarbossa,†
Giorgio Strukul,*,‡ and Augusto Tassan†
Dipartimento di Processi Chimici dell’Ingegneria, Universita` di Padova,
Via F. Marzolo 9, 35131 Padua, Italy, and Dipartimento di Chimica,
Universita` di Venezia, Dorsoduro 2137, 30123 Venice, Italy
Received September 22, 2004
The synthesis and characterization of new hydroxo-bridged platinum(II) complexes of the
type [P(µ-OH)(P-P)]2[BF4]2 (1-4), where P-P ) (C6H5-nFn)2PCH2CH2P(C6H5-nFn)2 (n ) 2
(2Fdppe) (1), 3 (3Fdppe) (2), 4 (4Fdppe) (3), 5 (dfppe) (4), are reported. These compounds
have been used in the Baeyer-Villiger oxidation of 2-methylcyclohexanone using 35%
hydrogen peroxide as oxidant. The reactions were performed at 25, 50, and 70 °C in a
chlorinated solvent/H2O two-phase system. Among the fluorinated catalysts, complex 4 was
found to be the best one in the oxidation of cyclic ketones; however, it is ineffective toward
acyclic ketones. The Lewis acidity of the platinum(II) complexes with coordinated fluorinated
diphosphines was investigated through the determination of the wavenumber shift ∆νj )
νj(CtN)coord - νj(CtN)free of the isocyanide group in complexes of the type [PtCl(CN-2,6-
(CH3)2C6H3)(P-P)][BF4] (P-P ) 2Fdppe, 3Fdppe, 4Fdppe, dfppe, dppe). This latter param-
eter, which represents a measure of the electrophilicity of the metal center, was then
correlated to the catalytic activity of complexes 1-4 in the Baeyer-Villiger oxidation of
ketones.
Scheme 1. Baeyer-Villiger Oxidation of Acyclic
and Cyclic Ketones with H2O2 Catalyzed by
Binuclear Pt(II) Complexesa
1. Introduction
The oxidation of organic substrates by hydrogen
peroxide is a process attracting great interest from both
the scientific and industrial points of view, because it
is in principle environmentally safe, as the reduction
product of H2O2 is water. Key steps in the oxygen
transfer process are the activation of hydrogen peroxide
and/or the substrate. Most homogeneous catalysts re-
ported so far behave as Lewis acids, usually containing
transition metals in a high oxidation state with free or
easily accessible coordination sites at the metal center.1
In the past two decades some of us have extensively
studied the properties of cationic Pd(II) and Pt(II)
complexes in a variety of oxidation reactions using
hydrogen peroxide as oxidant.2 A typical feature dis-
played by the metal in the latter reactions is the ability
to increase the reactivity of the substrate by coordina-
tion, thereby making it more susceptible to nucleophilic
attack by the peroxidic oxidant. An essential require-
ment to promote this kind of reactivity is the presence
a P-P ) Ph2P(CH2)nPPh2, n ) 1-4.
of a positive charge on the metal, which therefore
possesses a significant Lewis acidity.
A class of catalytically active complexes is represented
by OH-bridged dimeric cationic compounds of the type
{[Pt(µ-OH)(P-P)]2}2+, where P-P is a chelating diphos-
phine which may be also chiral. These complexes are
able to promote the Baeyer-Villiger (BV) oxidation of
cyclic and acyclic ketones in the presence of H2O2
(Scheme 1)3.
† Universita` di Padova.
‡ Universita` di Venezia.
(1) (a) Jørgensen, K. A.; Chem. Rev. 1989, 89, 431. (b) Strukul, G.,
Ed. Catalytic Oxidations with Hydrogen Peroxide as Oxidant;
Kluwer: Dordrecht, The Netherlands, 1992. (c) Jones, C. W. Applica-
tions of Hydrogen Peroxide and Derivatives; Royal Society of Chemis-
try: Cambridge. U.K., 1999.
(2) See for example: (a) Strukul, G.; Michelin, R. A. J. Am. Chem.
Soc. 1985, 107, 7563. (b) Marsella, A.; Agapakis, S.; Pinna, F.; Strukul,
G. Organometallics 1992, 11, 3578. (c) Del Todesco Frisone, M.; Pinna,
F.; Strukul, G. Organometallics 1993, 12, 148. (d) Baccin, C.; Gusso,
A.; Pinna, F.; Strukul, G. Organometallics 1995, 14, 1161. (e) Strukul,
G. Angew. Chem., Int. Ed. 1998, 37, 1198.
Recently, we have also observed that the BV oxidation
is very Lewis acid demanding. Thus, the use of highly
acidic [(P-P-O)Pt]2+ type complexes (P-P-O ) triphos
monoxide) has allowed driving the selectivity toward BV
(3) (a) Gusso, A.; Baccin, C.; Pinna, F.; Strukul, G. Organometallics
1994, 13, 3442. (b) Gavagnin, R.; Cataldo, M.; Pinna, F.; Strukul, G.
Organometallics 1998, 17, 661. (c) Paneghetti, C.; Gavagnin, R.; Pinna,
F.; Strukul, G. Organometallics 1999, 18, 5056.
10.1021/om049264a CCC: $30.25 © 2005 American Chemical Society
Publication on Web 01/19/2005

Baeyer-Villiger Oxidation of Ketones
Organometallics, Vol. 24, No. 5, 2005 1013
Chart 1. Fluorinated
Scheme 2a
1,2-Bis(diphenylphosphino)ethane Ligands
a P-P ) 2Fdppe (1), 3Fdppe (2), 4Fdppe (3), dfppe (4).
products vs epoxides in the oxidation of unsaturated
ketones.4 This observation prompted us to explore the
use of other Lewis acidic Pt(II) complexes with the aim
of developing more active and selective catalysts.
In this work we will report the effect of the presence
of electron-withdrawing substituents at the phosphorus
atoms of the P-P ligands on the Lewis acidity of the
Pt(II) metal center in the complexes {[Pt(µ-OH)(P-
P)]2}2+. To this purpose, electron-poor diphosphines
formally derived from progressive substitution of phenyl
hydrogen of dppe (1,2-bis(diphenylphosphino)ethane)
with electronegative fluorine atoms were employed.
Their catalytic activity was tested in the Baeyer-
Villiger oxidation of 2-methylcyclohexanone and, in the
case of the dfppe complex, of other cyclic and acyclic
ketones using hydrogen peroxide as oxidant.
Finally, we will also report preliminary results on the
correlation between the observed catalytic activity and
the Lewis acidity of the synthesized complexes using
2,6-dimethylphenyl isocyanide as a probe molecule,
taking advantage of the sensitivity of the CtN stretch-
ing mode to the Lewis acidity of the metal center in
complexes of the type [PtCl(CN-2,6-(CH3)2-C6H3)(P-P)]-
[BF4].5
Table 1. 31P{1H} NMR Data of [(P-P)Pt(µ-OH)]2
2+
Bridging Hydroxo Complexes
complex
diphosphine
1JP-Pt (Hz)
dppe
3624
3685
3694
3721
3754
1
2
3
4
2Fdppe
3Fdppe
4Fdppe
dfppe
for the Baeyer-Villiger oxidation of cyclic and acyclic
ketones (Scheme 2). The procedure involves the initial
substitution of cyclooctadiene (COD) from [PtCl2(COD)]
by the diphosphine (P-P ) (C6H5-nFn)2PCH2CH2P-
(C6H5-nFn)2, n ) 2-5) to afford the corresponding
complexes [PtCl2(P-P)] in ca. 70-90% yield. Subse-
quent chloride abstraction from [PtCl2(P-P)] by treat-
ment with 2 equiv of AgBF4 in wet MeOH/acetone
solutions affords the bridging hydroxo dimers [Pt(µ-OH)-
(P-P)]2[BF4]2, which were isolated in high yield (>90%)
as white solids upon addition of diethyl ether.
The complexes [PtCl2(4Fdppe)], [Pt(µ-OH)(xFdppe)]2-
[BF4]2 (x ) 2 (1), 3 (2), 4 (3)), and [Pt(µ-OH)(dfppe)]2-
[BF4]2 (4) are new compounds, and they have been
characterized by IR, multinuclear NMR, and elemental
analysis (see Experimental Section).
2. Results and Discussion
The IR spectrum of [PtCl2(4Fdppe)] shows typical Pt-
Cl stretchings at 325 and 335 cm-1, while in the 31P-
{1H} NMR spectrum the P atoms appear as a singlet
resonance at δ 42.7 ppm flanked by 195Pt satellites
(1JPt-P ) 3650 Hz). The IR spectra of 1-4 show a
medium-intensity O-H stretching band in the range
3480-3369 cm-1, typical for this class of complex,8 and
a strong broad band at ca. 1060 cm-1 due to the BF4
group. Their 31P{1H} NMR data along with those known
for [(dppe)Pt(µ-OH)]22+ are reported in the Experimental
Section. As can be seen, there is a steady increase in
the value of the P-Pt coupling constant, which shifts
Synthesis and Characterization of the Catalysts.
The partially fluorinated diphosphines (Chart 1) 2Fdppe
and 3Fdppe and also 4Fdppe, which was not previously
reported, were all prepared according to a published
procedure6 involving the reaction of 1,2-bis(dichloro-
phosphino)ethane, Cl2PCH2CH2PCl2, with the corre-
sponding LiArF species (ArF ) C6H3F2, C6H2F3, C6HF4).
The perfluorinated diphosphine dfppe is a commercially
available product and was used as received.
The spectroscopic characterization of the new 4Fdppe
ligand is reported in the Experimental Section. In
particular, its 31P{1H} NMR spectrum exhibits an
absorption at -32.8 ppm, which is upfield from the
resonance reported for the unsubstitued dppe (-12.5
ppm),7 as also observed for 2Fdppe, 3Fdppe, and dfppe
(-31.9, -30.2, and -44.5 ppm, respectively).6
2+
from 3624 Hz for [(dppe)Pt(µ-OH)]2 to 3754 Hz for 4
(Table 1). This behavior can be associated with an
increasing polarization of the Pt-O bond trans to the
phosphine, as was suggested by Appleton and Bennett
many years ago,9 and seems to reflect the increasing
electron-withdrawing character of the diphosphine ligand.
The catalysts were prepared according to the general
method reported in the literature8 and previously
employed in the preparation of those reported in ref 3
1
The OH resonance could be detected in the H NMR
spectra only at low temperature because of overlapping
with CH2 multiplets at room temperature.
(4) (a) Brunetta, A.; Strukul, G. Eur. J. Inorg. Chem. 2004, 5, 1030.
(b) Brunetta, A.; Sgarbossa, P.; Strukul, G. Catal. Today, in press.
(5) (a) Michelin, R. A.; da Silva, M. F. C. G.; Pombeiro, A. J. L. Coord.
Chem. Rev. 2001, 218, 75. (b) Belluco, U.; Michelin, R. A.; Uguagliati,
P.; Crociani, B. J. Organomet. Chem. 1983, 250, 565.
(6) Wursche, R.; Debaerdemaeker, T.; Klinga, M.; Rieger, B. Eur.
J. Inorg. Chem. 2000, 2063.
(8) (a) Li, J. J.; Li, W.; Sharp, P. R. Inorg. Chem. 1996, 25, 604. (b)
Bandini, A. L.; Banditelli, G.; Demartin, F.; Manassero, M.; Minghetti,
G. Gazz. Chim. Ital. 1993, 123, 417. (c) Scarcia, V.; Furlani, A.; Longato,
B.; Corain, B.; Pilloni, G. Inorg. Chim. Acta 1988, 153, 67. (d) Bushnell,
G. W.; Dixon, K. R.; Hunter, R. G.; McFarland, J. J. Can. J. Chem.
1972, 50, 3694.
(7) Garrou, P. E. Inorg. Chem. 1975, 14, 1435.
(9) Appleton, T. G.; Bennett, M. A. Inorg. Chem. 1978, 17, 738

1014 Organometallics, Vol. 24, No. 5, 2005
Michelin et al.
Scheme 3a
a P-P ) 2Fdppe (5), 3Fdppe (6), 4Fdppe (7), dfppe (8), dppe (9).
Table 2. Selected IR Dataa of the Isocyanide
Complexes [Pt(P-P)(CN-2,6-(CH3)2C6H3)Cl]+
complex
ligand
νjCtN,coord (cm-1
)
∆νj (cm-1
)
9
5
6
7
8
dppe
2207
2211
2213
2215
2221
85
89
91
93
99
2Fdppe
3Fdppe
4Fdppe
dfppe
a Values found in CH2Cl2 solution. νjCN(CN-2,6-(CH3)2C6H3 free)
2122 cm-1
.
Determination of the Lewis Acidity of the Com-
plexes. We tried to correlate the catalytic activity of
complexes 1-4 with the Lewis acidity of the metal
center arising from the withdrawing ability of the
coordinated electron-poor diphosphine. It is known5 that
the value of the CtN stretching (or the wavenumber
shift ∆νj ) νj(CtN)coord - νj(CtN)free) of a transition-
metal-coordinated isocyanide gives information about
the electrophilicity of the isocyanide carbon atom, which
in turn is related to the electron density and, hence, to
the Lewis acidity of the metal center. Thus, it is
expected that a larger electron-withdrawing effect by
the metal will give rise to a larger wavenumber shift.
On this basis, using 2,6-dimethylphenyl isocyanide as
a probe molecule, we have synthesized a homologous
series of isocyanide complexes of the general formula
[PtCl(CN-2,6-(CH3)2C6H3)(P-P)]+ (P-P ) 2Fdppe (5),
3Fdppe (6), 4Fdppe (7), dfppe (8), dppe (9)), which differ
only in the diphosphine ligand. They have been obtained
from the corresponding [PtCl2(P-P)] derivatives by
initial halide abstraction with an equivalent amount of
AgBF4 in CH2Cl2 and subsequent treatment with a
stoichiometric amount of the isocyanide ligand (8 and
9; Scheme 3, route a) or by halide abstraction with the
isocyanide ligand and NaBF4 (5-7; Scheme 3, route b).
Complexes 5-9 display the νj(CtN) absorption in the
range 2207-2221 cm-1 and ∆νj values in the range 99-
85 cm-1 in CH2Cl2 (Table 2). Table 2 shows that upon
increasing the number of fluorine atoms in the phenyl
groups of the diphosphine, ∆νj increases and hence the
acidity of the metal center increases. Complex 8, having
coordinated dfppe, shows the highest wavenumber shift
(∆νj ) 99 cm-1), and consequently it should possess the
highest Lewis acidity among the five complexes. This
is also in agreement with its highest catalytic activity
in the oxidation of cyclic ketones (see below).
Figure 1. Oxidation of 2-methylcyclohexanone catalyzed
2+
2+
by [Pt(µ-OH)(xFdppe)]2 (1-4) and [Pt(µ-OH)(dppe)]2
.
Reaction conditions: Pt, 0.017 mmol; ketone, 1.7 mmol;
H2O2, 1.7 mmol; DCM as solvent; T, 25 °C.
typical reaction profiles (percent conversion vs time) are
reported in Figure 1.
As can be observed, the reactions generally begin with
relatively fast rates but tend to level off rather quickly.
After about 300 min catalytic conversion ceased in all
reactions. At 25 °C the selectivity to lactone is >98%.
Increasing the reaction temperature obviously results
not only in a significant increase in both the initial rate
and the maximum conversion but also in a decrease in
selectivity since, by the end of the reaction, the forma-
tion of 5-methyl-6-hydroxyhexanoic acid arising from
hydrolysis of the lactone main product becomes evident.
It has to be pointed out that, while the reaction
conditions reported above and in Figure 2 allow rela-
tively high conversions and reaction rates, they do not
allow a complete solubilization of all the complexes in
the reaction medium. It was observed that the solubility
of the catalyst decreases as the degree of fluorine
substitution increases. Therefore, an analysis of the
effects of the diphosphine on the catalytic properties of
the catalysts was not possible.
To solve this problem, a new set of conditions was
chosen (Pt, 4.6 µmol; ketone, 1.5 mmol; H2O2, 1.8 mmol;
DCE, 3 mL; T, 50 °C). Results are shown in Figure 2
and Table 3. In the latter, the selectivity reported refers
to heptalactone, while the only byproduct is 5-methyl-
6-hydroxyhexanoic acid. Typical turnover numbers are
in the 20-120 range. As can be seen, the trends
observed for both the initial rate and the conversion as
a function of the degree of fluorine substitution parallel
those reported in Table 2 for the ∆ν value of the
coordinated isocyanide and in Table 1 for the P-Pt
coupling constants. In other words, it can be concluded
that the Lewis acidity of the catalyst increases both the
Catalytic Studies. Catalytic studies were performed
in the oxidation of 2-methylcyclohexanone with com-
mercial 35% hydrogen peroxide using 1-4 and [Pt(µ-
OH)(dppe)]2[BF4]2 as catalysts (Pt, 0.017 mmol; ketone,
1.7 mmol; H2O2, 1.7 mmol). The reactions were carried
out in two-phase solvent systems (CH2Cl2/H2O or DCE/
H2O) at two different temperatures (25 and 70 °C). Some

Baeyer-Villiger Oxidation of Ketones
Organometallics, Vol. 24, No. 5, 2005 1015
Table 4. Summary of the Catalytic Data
for the Oxidation of Different Ketones with
Hydrogen Peroxide Catalyzed by
[Pt(µ-OH)(dfppe)]2[BF4]2 (4)a
105(init
temp time rate) conversn selectivity
entry
ketone
(°C) (h)
(M/s)
(%)
(%)
1
2
3
2,6-dimethylcyclo- 25
24
1
0.33
6.2
13
20
100
95
hexanone
70
2-methylcyclo-
hexanone
25
70
24
1
2.8
28b
60
85
98
97
4
5
2-methylcyclo-
pentanone
25
70
24
1
1.7
27b
33
65
100
100
6
7
cyclobutanone
25
0
24
1
6.7
0.33
85
56
100
100
8
9
methyl tert-butyl
ketone
25
70
24
1
0
0
0
3
0
100
10
Figure 2. Oxidation of 2-methylcyclohexanone catalyzed
a Experimental conditions: Pt, 0.017 mmol; ketone, 1.7 mmol;
H2O2, 1.7 mmol; DCE, 3.0 mL. b Diffusion controlled.
2+
2+
by [Pt(µ-OH)(xFdppe)]2 (1-4) and [Pt(µ-OH)(dppe)]2
.
Reaction conditions: Pt, 4.6 µmol; ketone, 1.5 mmol; H2O2,
1.8 mmol; DCE as solvent; T, 50 °C.
Figure 3 shows that the reactions generally begin
with relatively fast rates but tend to level off rather
quickly. Reactions are generally finished in about 500
min. Methyl tert-butyl ketone (the only acyclic ketone)
is not oxidized. Figure 3 shows that the leveling off in
conversion is practically complete after about 400 min
at 25 °C. Increasing the reaction temperature obviously
results in a significant increase in both the initial rates
and the maximum conversions (the leveling off occurs
at about 200 min). At 70 °C reactions may be controlled
by the diffusion of H2O2 from the aqueous to the organic
phase,10 as was found, for example, in the case of
2-methylcyclohexanone and 2-methylcyclopentanone
(Table 4, entries 4 and 6). Cyclobutanone always yields
very high conversions and rates, but this is not surpris-
ing, as ring strain makes it by far the most reactive
substrate.
Table 3. Summary of the Catalytic Data for the
Oxidation of 2-Methylcyclohexanone with
Hydrogen Peroxide Catalyzed by
[Pt(µ-OH)(xFdppe)]2[BF4]2 (1-4) and
[Pt(µ-OH)(dppe)]2[BF4]2 at 50 °Ca
105(init rate) conversn selectivity
complex diphosphine
dppe
(M/s)
(%) (3 h)
(%) (3 h)
1.4
4.1
5.0
5.2
14
11
22
21
21
37
86
99
98
98
95
1
2
3
4
2Fdppe
3Fdppe
4Fdppe
dfppe
a Experimental conditions: Pt, 4.6 µmol; ketone, 1.5 mmol;
H2O2, 1.8 mmol; DCE, 3 mL; T, 50 °C.
Some randomly performed analyses of the residual
hydrogen peroxide in the final reaction mixture indicate
that the consumed oxidant corresponds to the maximum
conversion observed. In other words, there are no side
reactions leading to consumption of the oxidant. These
observations imply that reactions stop because of cata-
lyst deactivation and not depletion of the oxidant.
To investigate this point, the catalysts were checked
in randomly selected reactions carried out with 1-4 at
25 and 70 °C. 31P NMR spectra showed the disappear-
ance of the signals characteristic of 1-4 and the
presence of resonances typical for the oxides of the
ligands (δ 25.7, 23.6, 22.8, and 34.9 ppm, respectively),
thus demonstrating the decomposition of the catalysts.
Chemical shifts for authentic samples of the different
diphosphine dioxides were obtained by direct oxidation
of the free diphosphines with hydrogen peroxide.
Figure 3. Oxidation of ketones catalyzed by [Pt(µ-OH)-
(dfppe)]22+ (4). Reaction conditions: Pt, 0.017 mmol; ketone,
1.7 mmol; H2O2, 1.7 mmol; DCM as solvent; T, 25 °C.
3. Conclusions
activity and the conversion in the BV oxidation of
2-methylcyclohexanone.
In this work we have systematically changed the
electrophilicity of a homologous series of cationic Pt(II)
centers (and hence their Lewis acidity) by varying the
degree of fluorine substitution of a series of tetraaryl
diphosphine ligands that were employed for the syn-
thesis of µ-hydroxo complexes. The systematic change
in Lewis acidity of the different complexes is shown by
The complex [Pt(µ-OH)(dfppe)]2[BF4]2 (4) is the most
active, and we tested its catalytic activity in the oxida-
tion of a series of cyclic and acyclic ketones with
hydrogen peroxide. The reactions were carried out in
two-phase solvent systems (CH2Cl2/H2O and DCE/H2O)
at two different temperatures (25 and 70 °C). Some
typical reaction profiles (percent conversion vs time) are
reported in Figure 3, while initial rates and conversions
are collected in Table 4.
(10) Zanardo, A.; Pinna, F.; Michelin, R. A.; Strukul, G. Inorg. Chem.
1988, 27, 1966.

1016 Organometallics, Vol. 24, No. 5, 2005
Michelin et al.
vacuo to yield a white product. Recrystallization from ethanol/
acetone afforded 4Fddpe as a white crystalline solid. Yield:
1.80 g, 65.9%. Anal. Calcd for C26H8F16P2: C, 45.50; H, 1.17.
Found: C, 45.70; H, 1.18. 1H NMR (δ, (CD3)2CO): 7.27 (m,
Ar), 2.49 (m, PCH2). 19F NMR (δ, (CD3)2CO): -132.27 (m, o-F),
-140.41 (m, p-F), -155.38 (m, m-F), -157.20 (m, m-F).;
31P{1H} NMR (δ, (CD3)2CO): -32.75 (m, P).
spectroscopic evidence, the most convincing of which is
the increase in frequency of the CtN stretching mode
of coordinated 2,6-dimethylphenyl isocyanide used as
a probe molecule.
In agreement with previous observations,4a we have
found a systematic increase in activity in the catalytic
Baeyer-Villiger oxidation of ketones with hydrogen
peroxide when the Lewis acidity of the metal center
increases, which represents a key issue for the develop-
ment of “green” catalysts with high activity for this
synthetically useful reaction.13
(b) Synthesis of [PtCl2(4Fdppe)]. To a solution of [PtCl2-
(COD)] (0.405 g, 1.082 mmol) in dichloromethane (40 mL) at
room temperature was added 0.802 g (1.168 mmol) of 1,2-bis-
(bis(2,3,4,5-tetrafluorophenyl)phosphino)ethane. The reaction
mixture was stirred under nitrogen overnight, and then, after
concentration, the suspension was treated with Et2O to give
a white solid, which was filtered off and dried under vacuum.
Yield: 0.956 g, 92.8%. Anal. Calcd for C26H8Cl2F16P2Pt: C,
32.79; H, 0.85. Found: C, 32.44; H, 0.84. IR (ν˜, PE): 335, 325
(s, Pt-Cl). 1H NMR (δ, (CD3)2CO): 7.86 (m, Ar), 3.11 (m,
PCH2). 19F NMR (δ, (CD3)2CO): -126.72 (m, o-F), -138.64 (m,
p-F), -149.05 (m, m-F), -154.85 (m, m-F). 31P{1H} NMR (δ,
Unfortunately, while the activity of the catalysts can
be very high (reactions are even diffusion controlled
under certain conditions), their lifetime (and hence their
productivity) can be severely limited. Studies are un-
derway to systematically investigate the role of the
electron-withdrawing substituents on the stability of the
complexes under oxidizing conditions and possibly find
catalysts able to conjugate activity and stability under
suitably mild conditions.
1
(CD3)2CO): 42.67 (s, JPt-P ) 3650 Hz).
(c) Synthesis of [Pt(µ-OH)(2Fdppe)]2[BF4]2 (1). To a
solution of [PtCl2(2Fdppe)] (0.81 g, 1.00 mmol) in acetone (50
mL) and methanol (25 mL) at room temperature was added
2.10 mL (2.10 mmol) of a 1.0 M acetone solution of AgBF4.
The reaction mixture was stirred under nitrogen for 2 h, and
then the solid AgCl that formed was filtered off. After
concentration, the solution was treated with Et2O to give a
white solid, which was filtered off and dried under vacuum.
Yield: 0.84 g, 99.6%. Anal. Calcd for C52H34B2F24O2P4Pt2: C,
37.12; H, 2.04. Found: C, 37.28; H, 2.04. IR (ν˜, Nujol): 3480
(s, OH). 1H NMR (δ, CD3CN): 7.75 (m, Ar), 7.15 (m, Ar), 2.64
(m, PCH2). 1H NMR (δ, CD3CN, 270 K): 2.82 (s, OH). 19F NMR
(δ, CD3CN): -96.85 (m, o-F), -102.34 (m, p-F), -152.27 (s,
BF4-). 31P{1H} NMR (δ, CD3CN): 28.75 (s, 1JPt-P ) 3685 Hz).
(d) Synthesis of [Pt(µ-OH)(3Fdppe)]2[BF4]2 (2). This
compound was prepared by using a procedure similar to that
described above for complex 1, starting from [PtCl2(3Fdppe)]
(0.21 g, 0.24 mmol). Yield 0.20 g, 90.7%. Anal. Calcd for
C52H26B2F32O2P4Pt2: C, 34.20; H, 1.43. Found: C, 34.31; H,
1.35. IR (ν˜, Nujol): 3398 (s, OH). 1H NMR (δ, (CD3)2CO): 7.80
(m, Ar), 7.46 (m, Ar), 2.79 (m, PCH2). 1H NMR (δ, CD3CN, 260
K): 2.80 (s, OH). 19F NMR (δ, CD3CN): -101.53 (m, o-F),
-125.33 (m, p-F), -142.35 (m, m-F), -152.43 (s, BF4-). 31P-
4. Experimental Section
General Procedures and Materials. All work was carried
out with the exclusion of atmospheric oxygen under a dini-
trogen atmosphere using standard Schlenk techniques. Sol-
vents were dried and purified according to standard methods.
Substrates were purified by passing through neutral alumina
and stored in the dark at low temperature. Hydrogen peroxide
(35% Fluka), dfppe (Aldrich), and 54% HBF4 in ether (Aldrich)
were commercial products and were used without purification.
[PtCl2(COD)],11 [PtCl2(dfppe)],12 [Pt(µ-OH)(dppe)]2[BF4]2,8c and
1,2-bis(bis(x-fluorophenyl)phosphino)ethane (x ) 2,4 and 2,4,5)
and their platinum dichloro complexes6 were synthesized by
following procedures reported in the literature. IR spectra were
taken on a Nicolet Instrument Corp. AVATAR 320 FT-IR
spectrophotometer in CH2Cl2 solution using CaF2 windows; the
wavenumbers are given in cm-1 1H NMR, 19F NMR, and
.
31P{1H} NMR spectra were run at 298 K, unless otherwise
stated, on a Bruker AC200 spectrometer operating at 200.13,
81.015, and 188.25 MHz, respectively; δ values in ppm are
relative to SiMe4, 85% H3PO4, and CFCl3. GLC measurements
were taken on a Hewlett-Packard 5890A gas chromatograph
equipped with an FID detector (gas carrier He). Identification
of products was made with GLC by comparison with authentic
samples. The elemental analyses were performed by the
Department of Analytical, Inorganic, and Organometallic
Chemistry of the University of Padua.
1
{1H} NMR (δ, CD3CN): 29.78 (s, JPt-P ) 3694 Hz).
(e) Synthesis of [Pt(µ-OH)(4Fdppe)]2[BF4]2 (3). This
compound was prepared by using a procedure similar to that
described above for complex 1, starting from [PtCl2(4Fdppe)]
(0.46 g, 0.49 mmol). Yield: 0.36 g, 75.6%. Anal. Calcd for
C52H18B2F40O2P4Pt2: C, 31.70; H, 0.92. Found: C, 31.91; H,
1
0.95. IR (ν˜, Nujol): 3400 (s, OH). H NMR (δ, CD3CN): 7.55
(m, Ar), 2.78 (m, PCH2). 1H NMR (δ, CD3CN, 250 K): 2.89 (s,
OH). 19F NMR (δ, CD3CN): -125.73 (m, o-F), -138.18 (m, p-F),
-147.11 (m, m-F), -153.90 (m, m-F), -152.52 (s, BF4-). 31P-
Synthesis. (a) Synthesis of 1,2-Bis(bis(2,3,4,5-tetrafluo-
rophenyl)phosphino)ethane (4Fdppe). In a 500 mL Schlenk
flask equipped with a dropping funnel, 1-bromo-2,3,4,5-tet-
rafluorobenzene (2.7 mL, 21.8 mmol) was dissolved in dry
diethyl ether (40 mL) and the solution was cooled to -78 °C
by employing a liquid N2/2-propanol bath. Over a period of 20
min, a solution of n-buthyllitium in n-hexane (13.5 mL, 1.6
M) was added by means of the same dropping funnel. Follow-
ing this addition, stirring was continued for 30 min. The second
funnel was charged with a solution of P,P′-(1,2-ethanediyl)-
bis(dichlorophosphine) (0.92 g, 3.97 mmol) in diethyl ether (15
mL), which was added dropwise at low temperature. The
solution was then warmed to room temperature. After hy-
drolysis with degassed water, the ethereal phase was sepa-
rated, washed with water and brine, and then dried with
anhydrous magnesium sulfate. The solvent was evaporated in
1
{1H} NMR (δ, CD3CN): 31.72 (s, JPt-P ) 3721 Hz).
(f) Synthesis of [Pt(µ-OH)(dfppe)]2[BF4]2 (4). To a sus-
pension of [PtCl2(dfppe)] (0.50 g, 0.49 mmol) in methanol
(35 mL) at room temperature was added 1.15 mL (0.98 mmol)
of an acetone solution of AgBF4. The reaction mixture was
stirred under nitrogen for 2 h, and then the solid AgCl that
formed was filtered off. After concentration, the solution was
treated with Et2O to give a white solid, which was filtered and
dried under vacuum. Yield: 0.48 g, 93.0%. Anal. Calcd for
C52H10B2F48O2P4Pt2: C, 29.54; H, 0.48. Found: C, 30.05; H,
1
0.32. IR (ν˜, Nujol): 3369 (s, OH), 1061 (s, BF4). H NMR (δ,
2
(CD3)2CO, 270 K): 4.22 (s, OH, JPt-H ) 3.00 Hz), 3.44 (m,
CH2). 31P{1H} NMR (δ, (CD3)2CO): 5.40 (s, 1JPt-P ) 3754 Hz).
19F NMR (δ, (CD3)2CO, room temperature): -128.8 (d, o-C6F5,
3
3JF-F ) 14.3 Hz), -145.9 (t, p-C6F5, JF-F ) 19.0 Hz), -160.6
(11) Clark, H. C.; Manzer, L. E. J. Organomet. Chem. 1973, 59, 411.
(12) Merwin, R. K.; Schanabel, R. C.; Koola, J. D.; Roddick, D. M.
Organometallics 1992, 11, 2972.
(13) ten Brink, G.-J.; Arends, I. W. C. E.; Sheldon, R. A. Chem. Rev.
2004, 104, 4105.
(pseudo t, m-C6F5, JF-F ) 18.3 Hz), -153.5 (s, BF4-).
3
(g) Synthesis of [PtCl(CN-2,6-(CH3)2C6H3)(2Fdppe)]-
[BF4] (5). To a solution of [PtCl2(2Fdppe)] (0.14 g, 0.17 mmol)

Baeyer-Villiger Oxidation of Ketones
Organometallics, Vol. 24, No. 5, 2005 1017
3
3
in acetone (35 mL) was added, with stirring, 0.10 g (0.91 mmol)
of NaBF4. The solution was then treated dropwise with a
solution of 0.023 g (0.18 mmol) of 1,6-dimethylphenyl isocya-
nide in 12 mL of acetone for 20 min. The reaction mixture was
stirred under nitrogen for 1 h, and then the solid NaCl that
formed was filtered off. After concentration, the solution was
treated with Et2O to give a yellowish solid, which was filtered
off, washed with Et2O, and dried under vacuum. Yield: 0.12
(d, o-C6F5, JF-F ) 20.9 Hz), -128.6 (d, o-C6F5, JF-F ) 10.4
3
Hz), -144.5 (t, p-C6F5, JF-F ) 19.8 Hz), -145.2 (t, p-C6F5,
3JF-F ) 19.8 Hz), -160.0 (pseudo t, m-C6F5, 3JF-F ) 19.6 Hz),
-160.6 (pseudo t, m-C6F5, 3JF-F ) 19.6 Hz), -153.5 (s, BF4-).
(k) Synthesis of [PtCl(CN-2,6-(CH3)2C6H3)(dppe)][BF4]
(9). This compound was prepared using a procedure similar
to that described above for complex 8, starting from [PtCl2-
(dppe)] (0.40 g, 0.60 mmol). Yield: 0.48 g, 94.0%. IR (ν˜, CH2-
1
1
g, 73.7%. IR (ν˜, CH2Cl2): 2211 (s, CtN). H NMR (δ, CDCl3):
Cl2): 2208 (s, CtN). IR (ν˜, PE): 314 (s, Pt-Cl). H NMR (δ,
7.79 (m, Ph), 7.08 (m, Ph), 3.03 (m, PCH2), 2.22 (s, CH3). 31P-
CDCl3): 7.0-8.0 (m, Ph), 2.5-3.3 (m, CH2), 2.05 (s, CH3). 31P-
{1H} NMR (δ, CDCl3): 44.0 (d, PCl-trans, 1JPt-P ) 3166 Hz, 2JP-P
) 6.6 Hz), 45.9 (d, PC-trans, 1JPt-P ) 2844 Hz, 2JP-P ) 6.6 Hz).
Catalytic Studies. These were carried out in a 10 mL
round-bottomed flask equipped with a stopcock for vacuum/
N2 operations and a sidearm fitted with a screw-capped silicone
septum to allow sampling. Stirring was performed by a Teflon-
coated bar driven externally by a magnetic stirrer. Constant
temperature (20, 50, or 70 °C) was maintained by water
circulation through an external jacket connected with a
thermostat. The possible presence of diffusional problems was
determined by the dependence of conversion vs time plots on
the stirring rate. The concentration of the commercial 35%
H2O2 solution was checked iodometrically prior to use.
The following general procedure was followed: the required
amount of catalyst was placed in solid form in the reactor,
which was evacuated and filled with N2. Purified, N2-saturated
substrate was added under N2 flow, followed by the required
amount of 1,2-dichloromethane. After thermostating at the
required temperature for a few minutes, the H2O2 solution in
the appropriate amount was injected through the septum, and
the time was started.
{1H} NMR (δ, CDCl3): 22.43 (s, PCl-trans 1JPt-P ) 3270 Hz),
,
41.13 (s, PC-trans,
1JPt-P ) 2917 Hz). 19F NMR (δ, (CD3)2CO,
room temperature): -95.41, -96.08 (m, o-F), -98.94, -99.84
(m, p-F), -154.49 (s, BF4-).
(h) Synthesis of [PtCl(CN-2,6-(CH3)2C6H3)(3Fdppe)]-
[BF4] (6). This compound was prepared using a procedure
similar to that described above for complex 5, starting from
[PtCl2(3Fdppe)] (0.10 g, 0.11 mmol). Yield: 0.08 g, 58.4%. IR
1
(ν˜, CH2Cl2): 2213 (s, CtN). H NMR (δ, (CD3)2CO): 7.90 (m,
Ph), 7.35 (m, Ph), 3.43 (m, PCH2), 2.26 (s, CH3). 31P{1H} NMR
1
(δ, (CD3)2CO): 30.05 (s, PCl-trans, JPt-P ) 3286 Hz), 44.64 (s,
1
P
C-trans, JPt-P ) 2912 Hz). 19F NMR (δ, (CD3)2CO): -100.24
(m, o-F), -124.88, -125.35 (m, p-F), -141.27, -142.98 (m,
m-F), -151.98 (s, BF4-).
(i) Synthesis of [PtCl(CN-2,6-(CH3)2C6H3)(4Fdppe)]-
[BF4] (7). This compound was prepared using a procedure
similar to that described above for complex 5, starting from
[PtCl2(4Fdppe)] (0.13 g, 0.14 mmol). Yield: 0.11 g, 70.4%. IR
(ν˜, CH2Cl2): 2215 (s, CtN). 1H NMR (δ, CDCl3): 7.33 (m, Ph),
3.13 (m, PCH2), 2.35 (s, CH3). 31P{1H} NMR (δ, CDCl3): 26.28
(s, PCl-trans, 1JPt-P ) 3304 Hz), 45.10 (s, PC-trans, 1JPt-P ) 2925
Hz). 19F NMR (δ, CDCl3): -125.54, -126.63 (m, o-F), -133.52,
-134.85 (m, p-F), -143.60, -143.71 (m, m-F), -151.08 (m,
m-F), -152.59 (s, BF4-).
All reactions were monitored with GLC by direct injection
of samples taken periodically from the reaction mixtures with
a microsyringe. Prior quenching of the samples by adding an
excess of LiCl was found to be unnecessary. Initial rate data
were determined from conversion vs time plots. Separation of
the products was performed on a 25 m HP-5 capillary column
using a flame ionization detector.
(j) Synthesis of [PtCl(CN-2,6-(CH3)2C6H3)(dfppe)][BF4]
(8). To a suspension of [PtCl2(dfppe)] (0.25 g, 0.24 mmol) in
acetone (5 mL) and dichloromethane (50 mL) was added, with
stirring, 0.28 mL (0.24 mmol) of a 1.0 M acetone solution of
AgBF4. The reaction mixture was stirred under nitrogen for
20 min, and the solid AgCl that formed was filtered off. To
the solution was added 2,6-dimethylphenyl isocyanide (0.03
g, 0.24 mmol), and this mixture was stirred at room temper-
ature for 2 h. It was then concentrated under vacuum and
treated with Et2O. The white solid that precipitated was
filtered off, washed with Et2O, and dried under vacuum.
Yield: 0.11 g, 37.0%. IR (ν˜, CH2Cl2): 2221 (s, CtN). 31P{1H}
Acknowledgment. R.A.M. and G.S. thank the
MIUR, the University of Padua, and the University of
Venice for financial support (PRIN and COFIN 2003).
R.A.M. thanks the CIRCC (Consorzio Interuniversitario
Reattivita` Chimica e Catalisi) for a fellowship to E.P.
G.S. wishes to thank Johnson-Matthey for the loan of
platinum.
1
NMR (δ, (CD3)2CO): 8.3 (s, PCl-trans, JPt-P ) 3368 Hz), 26.3
(s, PC-trans, 1JPt-P ) 2929 Hz). 19F NMR (δ, (CD3)2CO): -127.0
OM049264A