Steric effects in the Baeyer-Villiger oxidation of ketones catalyzed by platinum(II) lewis acid complexes with coordinated electron-donor alkyl diphosphines

Article abstract of DOI:10.1021/om070077x

The synthesis and characterization of new hydroxo-bridged platinum(II) complexes of the type [Pt(μ-OH)(P-P)]₂[BF₄]₂ (1a-d), where P-P = R₂PCH₂CH₂PR₂ (R = Me, dmpe, 1a; Et, depe,

Full text of DOI:10.1021/om070077x

2714
Organometallics 2007, 26, 2714-2719
Steric Effects in the Baeyer-Villiger Oxidation of Ketones
Catalyzed by Platinum(II) Lewis Acid Complexes with Coordinated
Electron-Donor Alkyl Diphosphines
Paolo Sgarbossa,† Alessandro Scarso,‡ Rino A. Michelin,*,† and Giorgio Strukul*,‡
Dipartimento di Processi Chimici dell’Ingegneria, UniVersita` di PadoVa, Via F. Marzolo 9, 35131 PadoVa,
Italy, and Dipartimento di Chimica, UniVersita` Ca’ Foscari di Venezia, Dorsoduro 2137, 30123,
Venice, Italy
ReceiVed January 26, 2007
The synthesis and characterization of new hydroxo-bridged platinum(II) complexes of the type [Pt-
(µ-OH)(P-P)]2[BF4]2 (1a-d), where P-P ) R2PCH2CH2PR2 (R ) Me, dmpe, 1a; Et, depe, 1b; i-Pr,
dippe, 1c; Cy, dcype, 1d), and [Pt(P-P)(H2O)2][BF4]2) (where R) t-Bu, dtbpe, 1e) are reported. These
complexes were tested in the Baeyer-Villiger oxidation of 2-methylcyclohexanone, 2-methylcyclopen-
tanone, and cyclobutanone with 35% hydrogen peroxide as oxidant. The reactions were performed at 20
and 70 °C in a chlorinated solvent/H2O two-phase system. Within the above-reported series of complexes,
1e gave the highest catalytic activity and productivity in the oxidation of cyclic ketones. The Lewis
acidity of 1a-e was investigated through the determination of the wavenumber shift ∆νj ) νj(CtN)coord
- νj(CtN)free of the isocyanide moiety in complexes of the type [PtCl(CN-2,6-(Me)2C6H3)(P-P)][BF4]
(P-P ) 2a-e). The values of ∆νj showed that compounds 2a-e have comparable Lewis acidity, thus
indicating that the difference in the catalytic activity observed for 1a-e must be largely ascribed to
steric requirements imparted by the alkyl diphosphine ligand.
The synthetic importance of this oxidation reaction together
with unique features such as the use of H2O2 as an environ-
mentally friendly oxidant, the mild experimental conditions, and
the possibility of performing the reaction in an enantioselective
fashion prompted us toward a systematic study of the individual
variables affecting the activity of this class of catalysts. In
particular, in recent years, our group thoroughly investigated
the effect of the bite angle of the diphosphine4b and more
recently the effect of the Lewis acid character of the complexes.
The latter factor was systematically investigated with two
different approaches: (i) performing the synthesis of the Pt(II)
complex bearing 1,2-bis(dimethoxyphosphino)ethane (Pom-
Pom) as chelating ligand5 and (ii) preparing a series of
complexes containing differently fluorinated tetra-aryl diphos-
phines.6 In the latter case, a clear correlation between the number
of fluorine atoms and the catalytic activity was observed, while
in the former case, enhancement of both activity and productivity
due to the less electron donating character of the ligands was
observed comparing isosteric ligands. Both results are consistent
with the common observation that high Lewis acidity parallels
high catalytic activity in oxidation reactions7 due to better
electrophilic activation of the substrate. One major problem
concerning these catalysts is their limited lifetime under
(strongly oxidizing) reaction conditions. In fact, during catalytic
experiments the diphosphine ligand starts being oxidized in a
parallel side reaction until the catalyst is completely destroyed.
1. Introduction
The Baeyer-Villiger (BV) oxidation of ketones using organic
peroxy acids is an important reaction that finds wide application
in organic synthesis.1 However, its catalytic version using other
oxidants such as hydrogen peroxide or sodium hypochlorite
is still rather elusive, and factors leading to high catalytic activity
have not yet been completely understood. In fact, in the
past 20 years the number of active homogeneous and hetero-
geneous catalysts discovered for such oxidation reaction is
still very limited and has not allowed a thorough investiga-
tion of the factors (electronic and steric) that control the
reaction.
Some years ago, we reported the successful use of a class of
[(P-P)Pt(µ-OH)]22+-soluble complexes (P-P ) different diphos-
phines, including chiral ones) that proved to be the most active
catalysts known to date for the BV reaction.2 These complexes
operate under very mild conditions (room temperature, atmo-
spheric pressure), using a green oxidant such as hydrogen
peroxide.3 They also allow performing the enantioselective
version of this reaction with ee’s up to 80% observed in the
desymmetrization of meso-substituted cyclohexanones.4
* Corresponding authors. E-mail: strukul@unive.it; rino.michelin@unipd.it.
† Universita` di Padova.
‡ Universita` Ca’ Foscari di Venezia.
(1) (a) Strukul, G. Angew. Chem., Int. Ed. Engl. 1998, 37, 1198-1209.
(b) ten Brink, G.-J.; Arends, I. W. C. E.; Sheldon, R. A. Chem. ReV. 2004,
104, 4105-4124. (c) Renz, M.; Meunier, B. Eur. J. Org. Chem. 1999, 737-
750.
(2) (a) Gusso, A.; Baccin, C.; Pinna, F.; Strukul, G. Organometallics
1994, 13, 3442-3451. (b) Gavagnin, R.; Cataldo, M.; Pinna, F.; Strukul,
G. Organometallics 1998, 17, 661-667.
(3) (a) Lane, B. S.; Burgess, K. Chem. ReV. 2003, 103, 2457-2473. (b)
Grigoropoulou, G.; Clark, J. H.; Elings, J. A. Green Chem. 2003, 5, 1-7.
(c) Noyori R.; Aoki M.; Sato K. Chem. Commun. 2003, 1977-1986. (d)
Strukul, G., Ed. Catalytic Oxidations with Hydrogen Peroxide as Oxidant;
Kluwer Academic: Dordrecht, The Netherlands, 1992.
(4) (a) Gusso, A.; Baccin, C.; Pinna, F.; Strukul, G. Organometallics
1994, 13, 3442-3451. (b) Paneghetti, C.; Gavagnin, R.; Pinna, F.; Strukul,
G. Organometallics 1999, 18, 5057-5065.
(5) Sgarbossa, P.; Scarso, A.; Pizzo, E.; Mazzega Sbovata, S.; Tassan,
A.; Michelin, R. A.; Strukul, G. J. Mol. Catal. A 2006, 261, 202-206.
(6) Michelin, R. A.; Pizzo, E.; Scarso, A.; Sgarbossa, P.; Strukul, G.;
Tassan, A. Organometallics 2005, 24, 1012-1017.
(7) (a) Corma, A.; Garcia, H. Chem. ReV. 2003, 103, 4307-4366. (b)
Corma, A.; Garcia, H. Chem. ReV. 2002, 102, 3837-3892.
10.1021/om070077x CCC: $37.00 © 2007 American Chemical Society
Publication on Web 04/10/2007

Oxidation of Ketones Catalyzed by Pt(II) Lewis Acid Complexes
Organometallics, Vol. 26, No. 10, 2007 2715
Scheme 1. Baeyer-Villiger Oxidation of Acyclic and Cyclic
Chart 1. Pt(II) Dimeric µ-OH Complexes 1a-d and
Monomeric 1e with Coordinated Alkyl Diphosphines
Ketones with H2O2 Catalyzed by Pt(II) Complexes
Scheme 2. Equilibrium between Dimeric and Active
Monomeric Species in the BV Oxidation Mediated by
Dimeric Pt(II) Complexes
of dimeric complexes more favorable (Scheme 2). On the other
hand, the employment of ligands with a more pronounced
donating character will also decrease the Lewis acidity at the
metal center, and this, in turn, could result in a decrease of
catalytic activity. An investigation on which one of the two
effects will predominate is crucial for the further development
of more active environmentally friendly Pt(II) complexes in the
BV oxidation with hydrogen peroxide.
Nevertheless, moderate turnover numbers (TON) up to 65 in
the BV oxidation of 2-methylcyclohexanone can be attained.6
A second issue that deserves attention is related to the
necessity to improve catalytic activity through the increase of
the concentration of the active monomeric species according
to the equilibrium depicted in Scheme 2, which, for complexes
bearing aryl diphosphines, is a little shifted to the right. In fact,
kinetic studies performed with this type of dimeric complexes
have suggested that the most likely active species is a mono-
meric hydroperoxo complex derived from the bridge splitting
of the µ-hydroxo dimeric complex by substrate and oxidant
coordination, and consequently the Lewis acidity of the metal
center is crucial for the activation of the ketone toward
nucleophilic attack by the oxidant.8
2. Experimental Section
2.1. General Procedures and Materials. All work was carried
out with the exclusion of atmospheric oxygen under a dinitrogen
atmosphere using standard Schlenk techniques. Solvents 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), dmpe and depe
(Aldrich), and AgBF4 (Aldrich) were commercial products and used
without purification; [PtMe2(COD)],10 [PtCl2(dcype)],11[PtCl2-
(dtbpe)],12 and 1,2-bis(diisopropylphosphino)ethane)13 were syn-
thesized according to literature methods. The complexes [PtCl2-
(dmpe)], [PtCl2(depe)], and [PtCl2(dippe)] were prepared following
a modified procedure with respect to that previously reported.14,15
IR spectra were taken on a FT-IR AVATAR 320 spectropho-
tometer (Nicolet Instrument Corporation) in CH2Cl2 solution using
CaF2 windows; the wavenumbers are given in cm-1. 1H NMR and
31P{1H} NMR spectra were run at 298 K, unless otherwise stated,
on a Bruker AC200 spectrometer operating at 200.13 and 81.015
MHz, respectively; δ values in ppm are relative to SiMe4 and 85%
H3PO4. GLC measurements were taken on a Hewlett-Packard
5890A gas chromatograph equipped with a FID detector (carrier
gas He). Identification of products was made with GLC by
The above points are addressed in the present paper where
we report on recent studies in the BV oxidation of cyclic ketones
with hydrogen peroxide using a series of dimeric complexes of
2+
the type [(P-P)Pt(µ-OH)]2 (1a-d) and a monomeric [(P-
P)Pt(H2O)2]2+ (1e) with 1,2-bis(dialkyphosphino)ethane ligands,
where the alkyl substituents at phosphorus have been systemati-
cally changed (Chart 1).
The design of complexes 1a-e is based on the assumption
that the use of more electron-donating alkyldiphosphines would
stabilize the P-Pt bond and make it more resistant toward
oxidation, as it is known that the oxidation of a Pt(II)-
coordinated phosphine is a process that occurs at the P donor
atoms when they dissociate from the metal.9
In principle, this formulation should (i) stabilize to some
extent the complexes against phosphine oxidation due to a less
labile P-Pt bond; (ii) allow complexes of similar Lewis acidity,
with a stronger P trans labilizing effect compared to aryl-
diphosphines; and (iii) check the effect of a larger concentration
of monomeric species due to the influence of the steric hindrance
on the position of the equilibrium that makes the dissociation
(10) Clark, H. C.; Manzer, L. E. J. Organomet. Chem. 1973, 59, 411-
428.
(11) Hackett, M.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 1449-
1462.
(12) Yoshida, Y.; Yamagata, T.; Tulip, T. H.; Ibers, J. A.; Otsuka, S. J.
Am. Chem. Soc. 1978, 100, 2063-2073.
(13) Fryzuk, M. D.; Jones, T.; Einstein, F. W. B. Organometallics 1984,
3, 185-191.
(14) (a) Anderson, G. K.; Lumetta, G. J. Inorg. Chem. 1987, 26, 1518-
1524. (b) Harnisch, J. A.; Angelici, R. J. Inorg. Chim. Acta 2000, 300-
302, 273-279.
(8) Gavagnin, R.; Cataldo, M.; Pinna, F.; Strukul, G. Organometallics
1998, 17, 661-667.
(9) Halpern, J. Phosphorus, Sulfur Silicon Relat. Elem. 1983, 18, 307-
(15) Edelbach, B. L.; Vicic, D. A.; Lachicotte, R. J.; Jones, W. D.
Organometallics 1998, 17, 4784-4794.
310.

2716 Organometallics, Vol. 26, No. 10, 2007
Sgarbossa et al.
Scheme 3. Attempted Synthesis of [PtCl2(P-P)] Complexes
with P-P ) dmpe, depe, and dippe
M AgBF4 solution in acetone. The reaction mixture was stirred for
2 h. Then the solid AgCl was filtered off. Upon concentration, the
solution was treated with diethyl ether to give the product as a
white solid. Yield: 0.84 g, 99.6%. Anal. Calcd for C12H34B2F8O2P4-
Pt2: C, 16.05; H, 3.82. Found: C, 16.14; H, 3.91. IR (ν˜, Nujol):
3566 (s, OH). 1H NMR (δ, CD3CN): 1.69-2.13 (m, PCH2 + CH3).
1
31P{1H} NMR (δ, CD3CN): 24.51 (s, JPt-P ) 3426 Hz).
2.2.5. Synthesis of [Pt(µ-OH)(depe)]2[BF4]2 (1b). This com-
pound was prepared as described for 1a starting from [PtCl2(depe)]
(0.21 g, 0.24 mmol). Yield: 0.20 g, 90.7%. Anal. Calcd for
C20H50B2F8O2P4Pt2: C, 23.78; H, 4.99. Found: C, 23.74; H, 5.12.
IR (ν˜, Nujol): 3532 (s, OH). 1H NMR (δ, (CD3)2CO): 2.02-2.17
(m, CH2), 1.21-1.38 (m, CH3). 31P{1H} NMR (δ, (CD3)2CO):
Scheme 4. Synthesis of the Complexes [PtCl2(P-P)] with
P-P ) dmpe, depe, and dippe
1
51.55 (s, JPt-P ) 3469 Hz).
2.2.6. Synthesis of [Pt(µ-OH)(dippe)]2[BF4]2 (1c). This com-
pound was prepared as reported for 1a starting from [PtCl2(dippe)]
(0.46 g, 0.49 mmol). Yield: 0.36 g, 75.6%. Anal. Calcd for
C28H66B2F8O2P4Pt2: C, 29.96; H, 5.93. Found: C, 29.88; H, 5.90.
IR (ν˜, Nujol): 3512 (s, OH). 1H NMR (δ, (CD3)2CO): 2.07-2.12
(m, Ar), 1.26-1.49 (m, PCH2). 31P{1H} NMR (δ, (CD3)2CO):
Scheme 5. Synthesis of the Complexes 1a-e
1
68.60 (s, JPt-P ) 3488 Hz).
2.2.7. Synthesis of [Pt(µ-OH)(dcype)]2[BF4]2 (1d). This com-
pound was prepared as reported for 1a starting from [PtCl2(dcype)]
(0.20 g, 0.29 mmol). Yield: 0.18 g, 87.1%. Anal. Calcd for
C52H98B2F8O2P4Pt2: C, 43.28; H, 6.85. Found: C, 43.24; H, 6.72.
1
IR (ν˜, Nujol): 3540 (s, OH). H NMR (δ, DMSO-d6): 3.46 (s,
OH), 2.52 (m, PCH2), 1.31-2.11 (m, Cy). 31P{1H} NMR (δ,
1
DMSO-d6): 59.31 (s, JPt-P ) 3470 Hz).
comparison with authentic samples. The elemental analyses were
performed by the Department of Analytical, Inorganic and Orga-
nometallic Chemistry of the Universita` di Padova.
2.2.8. Synthesis of [Pt(dtbpe)(H2O)2][BF4]2 (1e). To a suspen-
sion of [PtCl2(dtbpe)] (0.12 g, 0.20 mmol) in acetone (20 mL) and
methanol (10 mL) at RT was added 0.41 mL (0.41 mmol) of a 1.0
M AgBF4 solution in acetone. The reaction mixture was stirred for
2 h. Then the solid AgCl was filtered off. Upon concentration, the
solution was treated with diethyl ether to precipitate the product as
awhitesolid.Yield: 0.048g,38.1%.Anal.CalcdforC18H44B2F8O2P2-
2.2. Synthesis. 2.2.1. Synthesis of [PtCl2(dmpe)]. To a solution
of [PtMe2(COD)] (0.352 g, 1.057 mmol) in dichloromethane (20
mL) at RT was added 0.180 mL (1.079 mmol) of 1,2-bis-
(dimethylphosphino)ethane (dmpe). To the mixture was then added
20 mL of methanol and 0.220 mL (3.094 mmol) of acetyl chloride
(evolution of CH4). After 2 h the solution was reduced to a small
volume under reduced pressure and treated with methanol to
precipitate a white solid. The solid was filtered, washed with
ethanol, and dried under vacuum. Yield: 0.316 g, 71.7%. Anal.
Calcd for C6H16Cl2P2Pt: C, 17.32; H, 3.88. Found: C, 17.54; H,
1
Pt: C, 29.90; H, 6.13. Found: C, 29.54; H, 6.12. H NMR (δ,
CDCl3): 3.48 (s, OH), 2.19-2.55 (m, PCH2), 1.55 (s, t-Bu). 31P-
1
{1H} NMR (δ, CDCl3): 78.17 (s, JPt-P ) 3926 Hz).
2.2.9. Synthesis of [PtCl(CN-2,6-(CH3)2C6H3)(dmpe)][BF4]
(2a). To a solution of [PtCl2(dmpe)] (0.084 g, 0.201 mmol) in
acetone (20 mL) was added 0.12 g (1.10 mmol) of NaBF4 under
stirring. A solution of 1,6-dimethylphenyl isocyanide (0.027 g, 0.205
mmol) in 12 mL of acetone was added by dropping in 20 min.
Then the mixture was stirred for 1 h and the solid NaCl was filtered
off. Upon concentration, the solution was treated with diethyl ether
to give a white solid, which was filtered, washed with Et2O, and
dried under vacuum. Yield: 0.10 g, 86.9%. Anal. Calcd for C15H25-
BClF4NP2Pt: C, 30.10; H, 4.21; N, 2.34. Found: C, 30.24; H, 4.13;
1
4.12. IR (ν˜, PE): 230, 280 (s, Pt-Cl). H NMR (δ, (CD3)2SO):
2.94 (m, CH2), 1.73 (m, CH3). 31P{1H} NMR (δ, (CD3)2SO): 34.26
1
(s, JPt-P ) 3521 Hz).
2.2.2. Synthesis of [PtCl2(depe)]. The procedure is similar to
that reported above for the synthesis of [PtCl2(dmpe)], starting from
0.403 g (1.21 mmol) of [PtMe2(COD)] and 0.290 mL (1.24 mmol)
of 1,2-bis(diethylphosphino)ethane (depe). Upon addition of 20 mL
of methanol and 0.25 mL (3.5 mmol) of acetyl chloride, a white
solid was isolated. Yield: 0.424 g, 74.1%. Anal. Calcd for C10H24-
Cl2P2Pt: C, 25.43; H, 5.12. Found: C, 25.33; H, 5.30. IR (ν˜, PE):
1
N, 2.29. IR (ν˜, CH2Cl2): 2202 (s, CtN). H NMR (δ, CDCl3):
7.35-7.17 (m, Ph); 2.32 (m, CH2); 1.98 (m, CH3). 31P{1H} NMR
(δ, CDCl3): 30.74 (s, PCl-trans, 1JPt-P ) 3051 Hz); 41.09 (s, PC-trans
1JPt-P ) 2729 Hz).
,
1
303, 279 (s, Pt-Cl). H NMR (δ, CDCl3): 1.71-2.36 (m, CH2-
CH3), 1.83 (m, CH2), 1.24 (dt, CH3). 31P{1H} NMR (δ, CDCl3):
2.2.10. Synthesis of [PtCl(CN-2,6-(CH3)2C6H3)(depe)][BF4]
(2b). This compound was prepared according to a procedure similar
to that described above for 2a starting from [PtCl2(depe)] (0.15 g,
0.32 mmol). Yield: 0.19 g, 89.9%. Anal. Calcd for C19H33BClF4-
NP2Pt: C, 34.85; H, 5.08; N, 2.14. Found: C, 34.64; H, 4.82; N,
1
57.85 (s, JPt-P ) 3545 Hz).
2.2.3. Synthesis of [PtCl2(dippe)]. The procedure is similar to
that reported above for the synthesis of [PtCl2(dmpe)], starting from
0.224 g (0.627 mmol) of [PtMe2(COD)] and 0.185 g (0.694 mmol)
of 1,2-bis(diisopropylphosphino)ethane (dippe). Upon addition of
20 mL of methanol and 0.10 mL (1.4 mol) of acetyl chloride, a
white solid was isolated. Yield: 0.237 g, 66.6%. Anal. Calcd for
1
2.19. IR (ν˜, CH2Cl2): 2199 (s, CtN). H NMR (δ, (CD3)2CO):
7.17-7.36 (m, Ph); 2.11-2.46 (m, PCH2); 1.19-1.38 (s, CH3).
31P{1H} NMR (δ, (CD3)2CO): 60.10 (s, PCl-trans, JPt-P ) 3061
1
1
1
Hz); 63.24 (s, PC-trans, JPt-P ) 2751 Hz).
C14H32Cl2P2Pt: C, 31.83; H, 6.10. Found: C, 31.46; H, 6.15. H
NMR (δ, CDCl3): 2.54-2.73 (m, CH), 1.72-1.78 (m, PCH2),
2.2.11. Synthesis of [PtCl(CN-2,6-(CH3)2C6H3)(dippe)][BF4]
(2c). This compound was prepared according to a procedure similar
to that reported for 2a starting from [PtCl2(dippe)] (0.11 g, 0.20
mmol). Yield: 0.13 g, 91.4%. Anal. Calcd for C23H41BClF4NP2Pt:
C, 38.86; H, 5.81; N, 1.97. Found: C, 38.74; H, 5.92; N, 1.99%.
IR (ν˜, CH2Cl2): 2198 (s, CtN). 1H NMR (δ, CDCl3): 7.18-7.40
1
1.16-1.46 (m, CH3). 31P {1H} NMR (δ, CDCl3): 72.35 (s, JPt-P
) 3571 Hz).
2.2.4. Synthesis of [Pt(µ-OH)(dmpe)]2[BF4]2 (1a). To a suspen-
sion of [PtCl2(dmpe)] (0.81 g, 1.00 mmol) in acetone (50 mL) and
methanol (25 mL) at RT was added 2.10 mL (2.10 mmol) of a 1.0

Oxidation of Ketones Catalyzed by Pt(II) Lewis Acid Complexes
Organometallics, Vol. 26, No. 10, 2007 2717
1
Table 1. 31P{1H} NMR Data for JP-Pt (Hz) of the
Scheme 6. Synthesis of the Complexes
[PtCl(CN-2,6-(CH3)2C6H3)(P-P)](BF4), 2a-e
2+
[(P-P)Pt(µ-OH)]2 Hydroxo-Bridging Complexes (1a-d)
and of [(dppe)Pt(µ-OH)]2 for Comparison Purposes
2+
complex
diphosphine
1JP-Pt (Hz)
dppe
dmpe
depe
dippe
dcype
3624
3426
3469
3488
3470
1a
1b
1c
1d
(m, Ph); 2.11-2.82 (m, PCH2 + PCH); 1.26-1.47 (s, CH3). 31P-
{1H} NMR (δ,CDCl3): 81.47 (s, PCl-trans, 1JPt-P ) 3098 Hz); 76.37
in dichloromethane with 1 equiv of dmpe, depe, and dippe lead
always to the precipitation of a white solid, as also reported by
Andersen,14 which was only soluble in polar solvents such as
acetone or methanol. The 31P{1H} NMR spectra show in each
1
(s, PC-trans, JPt-P ) 2807 Hz).
2.2.12. Synthesis of [PtCl(CN-2,6-(CH3)2C6H3)(dcype)]2[BF4]2
(2d). This compound was prepared according to a procedure similar
to that reported for 2a starting from [PtCl2(dcype)] (0.099 g, 0.144
mmol). Yield: 0.106 g, 84.4%. Anal. Calcd for C27H49BClF4NP2-
Pt: C, 42.28; H, 6.44; N, 1.83. Found: C, 42.22; H, 6.32; N, 1.92.
IR (ν˜, CH2Cl2): 2196 (s, CtN). 1H NMR (δ, CDCl3): 7.19-7.38
(m, Ph); 1.32-2.45 (m, PCH2 + Cy). 31P{1H} NMR (δ, CDCl3):
1
case a singlet resonance flanked by 195Pt satellites with JP-Pt
values in the range 2200-2300 Hz, considerably lower than
those expected for [Pt(µ-OH)(P-P)]2[BF4]2 complexes and in
agreement with similar Pt(II) bis-chelated complexes.16 On the
other hand, the NMR and far-IR analysis of the complexes are
in accordance with the formation of bis-chelate diphosphine
complexes probably due to the high nucleophilicity of the
ligands employed (Scheme 3).
In order to avoid undesired secondary reactions, dimethyl Pt-
(II) complex [PtMe2(COD)] was employed in order to prevent
bis-chelation by the diphosphine ligands, and it was reacted with
the appropriate P-P ligands to form the dimethyl diphosphine
complexes [PtMe2(P-P)] by displacement of COD (Scheme 4).
These latter complexes were then converted in good yields to
the final [PtCl2(P-P)] derivatives upon treatment in situ with a
solution of acetyl chloride in methanol as in situ source of HCl
(Scheme 4).
1
1
73.18 (s, PCl-trans, JPt-P ) 3102 Hz); 68.77 (s, PC-trans, JPt-P
)
2798 Hz).
2.2.13. Synthesis of [PtCl(CN-2,6-(CH3)2C6H3)(dtbpe)]2[BF4]2
(2e). This compound was prepared using a procedure similar to
that described above for 2a starting from [PtCl2(dtbpe)] (0.030 g,
0.052 mmol). Yield: 0.039 g, 98.4%. Anal. Calcd for C35H57BClF4-
NP2Pt: C, 48.26; H, 6.59; N, 1.61. Found: C, 48.34; H, 6.52; N,
1.59. IR (ν˜, CH2Cl2): 2198 (s, CtN). 1H NMR (δ, CDCl3): 7.17-
7.36 (m, Ph); 2.21-2.57 (m, PCH2); 1.44 (s, CH3); 1.73 (s, CH3).
1
31P{1H} NMR (δ, CDCl3): 93.33 (s, PCl-trans, JPt-P ) 3164 Hz);
1
81.44 (s, PC-trans, JPt-P ) 2884 Hz).
2.3. 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 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 conver-
sion versus 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 the reactor. Subsequently the
substrate was added followed by the required amount of 1,2-
dichloromethane. Then the vial was thermostatted at the required
temperature for 10 min, and subsequently the appropriate amount
of 35% H2O2 solution was injected through the septum and time
was started.
The final µ-hydroxo-bridged dimeric Pt(II) catalysts were then
synthesized according to a general procedure2b (Scheme 5)
starting from the dichloro complexes by abstraction of the
chlorides with AgBF4 in a wet solvent mixture.
The IR spectra in Nujol mulls of 1a-d show a medium-
intensity O-H stretching band in the range 3566-3512 cm-1
,
typical for this class of complex, and a strong broad band at ca.
1060 cm-1 due to the BF4 group. Selected 31P{1H} NMR data
are reported in Table 1. It is observed that there is a steady
increase in the value of the P-Pt coupling constant from 3426
Hz for 1a to 3488 Hz for 1c. This behavior can be associated
with an increasing weakening of the Pt-O bond trans to the P
atom probably ascribed to the increasing steric hindrance of
the diphosphine ligand. Because of overlap with other reso-
1
nances, H NMR spectra the OH signal cannot be detected
except for 1d.
All reactions were monitored with GLC by quick direct injection
of samples taken periodically from the reaction mixtures with a
microsyringe employing n-decane as internal standard. Prior
quenching of the samples by adding an excess of LiCl was found
unnecessary. Initial rate data were determined from conversion
versus time plots. Separation of the products was performed on a
25 m HP-5 capillary column using a flame ionization detector.
The synthesis performed starting from dtbpe led to the
formation of complex 1e, whose spectroscopic properties are
markedly different from those of the series of dimeric complexes
1a-d. In fact, 1e shows a 31P NMR spectrum characterized by
a singlet at 78.17 ppm flanked by 195Pt satellites with 1JP-Pt of
3926 Hz. The value of the coupling constant is much higher
than that expected for a classical µ-OH dimeric structure, being
more typical for a neutral coordinating ligand like a molecule
of an organic solvent or a molecule of water.17 This suggests
that the diphosphine dtbpe is too bulky to allow the formation
of the dimeric µ-OH species, and simple monomeric bis-aquo
complex [(dtbpe)Pt(H2O)2]2+ is preferentially obtained. The
latter species was confirmed, observing via NMR that exactly
the same spectra were obtained by addition of other silver salts
(e.g., AgPF6, AgOTf) to dtbpePtCl2.
3. Results and Discussion
3.1. Synthesis and Characterization of the Complexes. The
synthesis of the new hydroxo-bridged Pt(II) complexes [Pt(µ-
OH)(P-P)]2[BF4]2 (1a-d) involved the initial preparation of
the dichloro derivatives [PtCl2(P-P)]. While compounds with
P-P ) dcype and dtbpe were prepared according to general
routes based on metathesis of L2 ligand(s) in [PtCl2L2]
complexes (L2 ) COD, 2 PhCN)11,12 by the alkyl diphosphines,
those with P-P ) dmpe, depe, and dippe required an alternative
synthetic procedure. In fact, the metathesis reactions carried out
(16) Berning, D. E.; Noll, B. C.; DuBois, D. L. J. Am. Chem. Soc. 1999,
121, 11432-11447.
(17) Appleton, T. G.; Bennett, M. A. Inorg. Chem. 1978, 17, 738-747.

2718 Organometallics, Vol. 26, No. 10, 2007
Sgarbossa et al.
1
Table 2. Selected IR Dataa and 31P{1H} NMR JPt-P(trans-C)
Table 3. Catalytic Baeyer-Villiger Oxidation of Cyclic
Ketones with Hydrogen Peroxide Catalyzed by Dimeric
Pt(II) Catalysts 1a-ea
for the Isocyanide Complexes 2a-e and
[PtCl(CN-2,6-(CH3)2C6H3) (dppe)]+ as Reference
complex ligand νjC≡N coord (cm-1
)
∆νj (cm-1
)
1JPt-P(trans-C) (Hz)
dppeb
dmpe
depe
dippe
dcype
dtbpe
2207
2202
2199
2198
2196
2198
85
80
77
76
74
76
2844
2729
2751
2807
2798
2884
2a
2b
2c
2d
2e
a Values taken in CH2Cl2 solution. νjCN for free CN-2,6-(Me2)C6H3 2122
cm-1. b See ref 6.
Figure 1. Oxidation of 2-methylcyclohexanone catalyzed by
catalysts 1a (0), 1b (9),1c (2),1d ([), and 1e (b). Reaction
conditions: Pt, 0.017 mmol; ketone, 1.7 mmol; H2O2, 1.7 mmol;
dichloroethane (DCE) 3 mL as solvent; T, 20 °C.
3.2. Lewis Acidity Measurements. It is known that the value
of the CtN stretching (or the wavenumber shift ∆νj ) νj(Ct
N)coord - νj(CtN)free) of a transition metal coordinated isocya-
nide provides 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.18
This approach was previously tested successfully and provided
a qualitative correlation between activity and Lewis acid
character when applied to a class of homologous complexes
containing differently fluorinated diphosphines.6
a Reaction conditions: Pt, 0.017 mmol; ketone, 1.7 mmol; H2O2, 1.7
mmol; dichloroethane (DCE) 3 mL as solvent; T, 20 °C. Experiment carried
b
Following the same approach, we prepared a homologous
series of model isocyanide complexes 2 of the general formula
[PtCl(CN-2,6-(Me)2C6H3)(P-P)]+ (P-P ) dmpe (2a), depe
(2b), dippe (2c), dcype (2d), and dtbpe (2e)). They were
obtained as reported in Scheme 6 starting from the correspond-
ing [PtCl2(P-P)] derivatives by initial halide abstraction and
subsequent reaction with 2,6-dimethylphenyl isocyanide in the
presence of NaBF4 in acetone.
out at 70 °C.
moieties, thus indicating that complexes 1a-e are characterized
by similar Lewis acid character. As a consequence of the IR
study described above, it appears reasonable to ascribe the
differences in the catalytic activity shown by the series of µ-OH
complexes 1a-e in the Baeyer-Villiger oxidation (see later)
mainly to the different steric properties of the diphosphines. It
is likely that an increase of the steric bulkiness of the ligand
determines a shift to the right of the equilibrium depicted in
Scheme 2, thus increasing the concentration of the monomeric
active species.
Complexes 2a-e were characterized by IR and multinuclear
NMR spectroscopies. Selected IR and 31P{1H} NMR data are
reported in Table 2.
Complexes 2a-e display the νj(CtN) absorption in the range
2202-2196 cm-1 and the ∆νj values in the range 80-74 cm-1
in CH2Cl2 solution (Table 2). The average ∆νj value displayed
by the alkyl-diphosphine complexes is ca. 8 cm-1 lower than
that observed for the tetra-aryl dppe derivative (Table 2) and
even lower than that reported for the corresponding fluorinated
aryl diphosphine compounds, which displayed ∆νj values in the
range 99-85 cm-1.6 This feature supports that dialkyl diphos-
phine complexes here reported display a lower Lewis acidity
that decreases with increasing steric bulkiness of the alkyl
3.3. Baeyer-Villiger Oxidation of Cyclic Ketones. Catalytic
tests performed with 1a-e were carried out on cyclobutanone,
2-methylcyclopentanone, and 2-methylcyclohexanone either at
20 or 70 °C using 1 mol % [Pt] catalyst in all cases. Some
typical reaction profiles for the room-temperature oxidation of
2-methylcyclohexanone with the different catalysts are collected
in Figure 1. As shown, the reactions are virtually finished after
about 4-5 h, and conversions and initial rates follow similar
trends for each individual catalyst. As a consequence, the
conversion after a certain time can be reasonably assumed as
an indication of the overall activity as well as the lifetime of
the catalysts (TON).
(18) (a) Michelin, R. A.; da Silva, M. F. C. G.; Pombeiro, A. J. L. Coord.
Chem. ReV. 2001, 218, 75-112. (b) Belluco, U.; Michelin, R. A.; Uguagliati,
P.; Crociani, B. J. Organomet. Chem. 1983, 250, 565-587.

Oxidation of Ketones Catalyzed by Pt(II) Lewis Acid Complexes
Organometallics, Vol. 26, No. 10, 2007 2719
A summary of the conversions obtained with the different
substrates and the different combinations of substrate and
catalyst is reported in Table 3. As can be seen, high activities
for all complexes are observed only in the case of cyclobu-
tanone. This is no surprise, as the four-membered cyclobutanone
ring is intrinsically reactive, while the other substrates better
show the differences in activity of the catalysts. As a general
observation, with methylcyclopentanone and methylcyclohex-
anone all the catalysts 1a-e led to the regioselective formation
of the normal lactone due to migration of the more substituted
carbon atom.
high activity and high productivity in the BV oxidation of cyclic
ketones. Although catalytic activity and TON’s are in general
relatively moderate, no better catalysts have been reported so
far. We systematically changed the steric bulkiness of a
homologous series of cationic Pt(II) catalysts by acting on the
size and branching degree of alkyl diphosphine ligands that were
employed for the synthesis of µ-hydroxo complexes. Similar
Lewis acidity was proved by means of spectroscopic evidence
by analyzing the CtN stretching mode of coordinated 2,6-
dimethylphenyl isocyanide used as a probe molecule.
A positive trend in catalytic activity and TON was observed
with increasing steric bulkiness in the series of complexes
employed, with a maximum for the monomeric 1e. This
behavior associated with increasing size of the diphosphine alkyl
moiety is evident also in the case of cyclobutanone, albeit more
moderate. Performing the reaction at 70 °C had a substantial
effect only on the rate of the reaction but not on the productivity
and robustness of the catalysts, and yields comparable to the
reactions performed at 20 °C were observed. Complex 1c was
more active also than the corresponding complex [Pt(µ-OH)-
(dppe)]2(BF4)2 bearing aryl diphosphines.8 This observation
clearly speaks for the prevailing positive effect of alkyldiphos-
phines in enhancing catalyst lifetime as well as catalytic activity,
and the detrimental effect on activity due to decreased Lewis
acidity can be efficiently overcome by increasing steric bulki-
ness.
We observed a systematic increase in activity and productivity
with increasing steric bulkiness of the ligand in complexes 1a-d
likely due to a more favorable dissociation of the dimeric into
monomeric catalyst structure, the latter being the active catalytic
species, as confirmed by the highest activity of 1e. These results
represent key information for the development of new catalysts
for the “green” BV oxidation of ketones with hydrogen peroxide
and prompt us toward the development of Pt(II) complexes
implementing bulky ligands in order to attain high activity. In
particular, new strategies to favor the formation of monomeric
species in solution are currently underway and will be shortly
reported.
Acknowledgment. R.A.M. and G.S. thank MIUR, the
Universita´ di Padova, and the Universita´ Ca’ Foscari di Venezia
for financial support (PRIN and COFIN 2003). G.S. thanks
Johnson-Matthey for the loan of platinum.
4. Conclusion
This work represents a further step in the investigation and
optimization of the properties of Pt(II) complexes that favor
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