Regioselectivity and diasteroselectivity in Pt(II)-mediated "green" catalytic epoxidation of terminal alkenes with hydrogen peroxide: Mechanistic insight into a peculiar substrate selectivity

Article abstract of DOI:10.1021/ja071142x

Recently developed electron-poor Pt(II) catalyst 1 with the "green" oxidant 35% hydrogen peroxide displays high activity and complete substrate selectivity in the epoxidation of terminal alkenes because of stringent steric and electronic requirements. In the presence of isolated dienes bearing terminal and internal double bonds, epoxidation is completely regioselective toward the production of terminal epoxides. Insight into the mechanism is gained by means of a reaction progress kinetic analysis approach that underlines the peculiar role of 1 in activating both the alkene and H ₂O₂ in the rate-determining step providing a rare example of nucleophilic oxidation of alkenes by H₂O₂.

Full text of DOI:10.1021/ja071142x

Published on Web 05/27/2007
Regioselectivity and Diasteroselectivity in Pt(II)-Mediated
“Green” Catalytic Epoxidation of Terminal Alkenes with
Hydrogen Peroxide: Mechanistic Insight into a Peculiar
Substrate Selectivity
Marco Colladon,† Alessandro Scarso,† Paolo Sgarbossa,‡ Rino A. Michelin,‡ and
Giorgio Strukul*,†
Contribution from the Dipartimento di Chimica, UniVersita` Ca’ Foscari di Venezia, Dorsoduro
2137, I-30123 Venice, Italy, and Dipartimento di Processi Chimici dell’Ingegneria, UniVersita` di
PadoVa, Via F. Marzolo 9, 35131 PadoVa, Italy
Received February 16, 2007; E-mail: strukul@unive.it
Abstract: Recently developed electron-poor Pt(II) catalyst 1 with the “green” oxidant 35% hydrogen peroxide
displays high activity and complete substrate selectivity in the epoxidation of terminal alkenes because of
stringent steric and electronic requirements. In the presence of isolated dienes bearing terminal and internal
double bonds, epoxidation is completely regioselective toward the production of terminal epoxides. Insight
into the mechanism is gained by means of a reaction progress kinetic analysis approach that underlines
the peculiar role of 1 in activating both the alkene and H2O2 in the rate-determining step providing a rare
example of nucleophilic oxidation of alkenes by H2O2.
advantages, such as high atom-efficiency,7 moderate cost, safe
handling and storage, and production of water as the only
Introduction
Highly selective oxy-functionalization of organic substrates
is a rather challenging field that requires the design of specific
catalysts along with tailoring many variables such as substrate,
oxidant nature, and experimental conditions with the ultimate
goal of achieving high activity, selectivity, and productivity
under mild experimental conditions.1 In particular, the oxidation
of alkenes to the corresponding epoxides is a well-documented
reaction that has been investigated for decades because epoxides
represent important commodities and, at the same time, pivotal
building blocks for organic synthesis, both from the industrial
and academic points of view.2 Although heterogeneous methods
for the epoxidation of alkenes have been developed,3 the highest
selectivities have been observed under homogeneous conditions4
with metal-containing or purely organic catalysts.
byproduct8 making it the most interesting oxidant after molecular
oxygen and stimulating its use in liquid-phase oxidations,
especially for fine-chemicals production.9
The number of transition-metal complexes able to efficiently
activate hydrogen peroxide toward different alkenes is relatively
large.4 Nevertheless, most of them are generally active toward
a limited class of substrates such as, e.g., allylic alcohols where
the presence of the hydroxyl group allows easy coordination to
the metal active site,10 or unfunctionalized alkenes, where good
performance could be observed only for electron-rich CdC
double bonds, or styrene derivatives where peculiar reactivity
is imparted by the presence of the conjugated aromatic ring. In
this framework, a lack of methods is evident for an efficient
and selective epoxidation of terminal, unfunctionalized alkenes
that are intrinsically poorly reactive substrates toward electro-
philic oxidation. In this respect, worthy of mention are
complexes of Ru(III),11 W(VI),12-15 Mn(II),16,17 Re(V),18 and
Several oxidants have been tested over the years for such
reactions; nevertheless, in recent years the interest toward
hydrogen peroxide is overcoming all the other ones because of
stringent environmental concerns. In fact, as recently outlined
by Beller,5 H2O2 is characterized by unique features and
6
(7) Trost, B. M. Angew. Chem., Int. Ed. 1995, 34, 259.
(8) (a) Strukul, G., Ed. Catalytic Oxidations with Hydrogen Peroxide as
Oxidant; Kluwer Academic: Dordrecht, The Netherlands, 1992. (b)
Grigoropoulou, G.; Clark, J. H.; Elings, J. A. Green Chem. 2003, 5, 1.
(9) Noyori, R.; Aoki, M.; Sato, K. Chem. Commun. 2003, 1977.
(10) Adam, W.; Wirth, T. Acc. Chem. Res. 1999, 32, 703.
(11) Klawonn, M.; Tse, M. K.; Bhor, S.; Do¨bler, C.; Beller, M. J. Mol. Cat. A
2004, 218, 13.
† Universita` Ca’ Foscari di Venezia.
‡ Universita` di Padova.
(1) Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice;
Oxford University Press: New York 1998.
(2) van Leeuwen, P. W. N. M. Homogeneous Catalysis; KluwerAcademic
Publishers: Dordrecht, 2004.
(3) For review articles: (a) De Vos, D. E.; Sels, B. F.; Jacobs, P. A. AdV.
Synth. Catal. 2003, 345, 457. (b) Lambert, R. M.; Williams, F. J.; Cropley,
R. L.; Palermo, A. J. Mol. Cat. A 2005, 228, 27.
(12) Sato, K.; Aoki, M.; Ogawa, M.; Hashimoto, T.; Noyori, R. J. Org. Chem.
1996, 61, 8310.
(13) (a) Venturello, C.; Alneri, E.; Ricci, M. J. Org. Chem. 1983, 48, 3831. (b)
Venturello, C.; D’Aloisio, R. J. Org. Chem. 1988, 53, 1553.
(14) (a) Murphy, A.; Pace, A.; Stack, T. D. P. Org. Lett. 2004, 6, 3119. (b)
Murphy, A.; Dubois, G.; Stack, T. D. P. J. Am. Chem. Soc. 2003, 125,
5250. (c) Banfi, S.; Montanari, F.; Quici, S.; Barkanova, S. V.; Lakiya, O.
L.; Kopranenkov, V. N.; Luk’yanets, E. A. Tetrahedron Lett. 1995, 36,
2317.
(4) Lane, B. S.; Burgess, K. Chem. ReV. 2003, 103, 2457.
(5) Tse, M. K.; Bhor, S.; Klawonn, M.; Anilkumar, G.; Jiao, H.; Spannenberg,
A.; Do¨bler, C.; Ma¨gerlein, W.; Hugl, H.; Beller, M. Chem.sEur. J. 2006,
16, 1875 and references therein.
(6) Campos-Martin, J. M.; Blanco-Brieva, G.; Fierro, J. L. G. Angew. Chem.,
Int. Ed. 2006, 45, 6962.
9
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Pt-Mediated Nucleophilic Oxidation of Alkenes by H2O2
A R T I C L E S
Fe(III),19,20 although relatively high metal loading, presence of
additives, moderately high temperatures, and in many cases use
of overstoichiometric amounts of hydrogen peroxide are gener-
ally required due to parallel partial decomposition of the oxidant
catalyzed by the complex itself. Very recently, bis-(µ-hydroxo)-
bridged divanadium species implemented in a peroxotungstate
framework showed highly selective and efficient H2O2 epoxi-
dation of alkenes, in particular toward terminal alkenes with
very good regioselectivity in diene epoxidation.21
Scheme 1. Epoxidation of Terminal Olefins with 35% Hydrogen
Peroxide Catalyzed by Electron-Poor Pt(II) Complex 1
Thus far, catalyst design has been aimed mainly at oxidant
activation and has paid little or no attention to the interaction
between the metal center and the alkene. Requirement for a
wider scope, successful epoxidation of simple terminal alkenes
seems to be a new catalyst design activating the substrate instead
of the oxidant, hence changing the role between electrophile
and nucleophile in the system.
Table 1. Catalytic Epoxidation of Various Alkenes with Hydrogen
Peroxide Mediated by 1: Substrate Scope of the Reactiona
Very recently, we reported about the peculiar activity of
second generation,22 electron poor Pt(II) complexes containing
a pentafluorophenyl residue with general formula [(P-P)Pt-
(C6F5)(H2O)]+ (P-P ) diphosphine), and their application as
selective epoxidation catalysts toward terminal unfunctionalized
alkenes, with 2% mol catalyst loading, yields up to 89%,
operating under mild conditions, and with the use of only 1
equiv of hydrogen peroxide.23 The same class of complexes
bearing chiral diphosphine ligands showed ee values up to 98%
in the epoxidation of terminal alkenes.24
Focusing on the most active complex of the achiral series
[(dppe)Pt(C6F5)(H2O)]OTf 1 (dppe ) 1,2-bis(diphenylphosphi-
no)ethane), herein we provide further insight into its extremely
high substrate selectivity and explore in detail aspects like
regioselectivity and diasteroselectivity in the oxidation of a wide
range of substrates (Scheme 1). Further details into the selectiv-
ity of this catalyst are provided by a mechanistic investigation
performed employing the reaction progress kinetic analysis
approach.25
Results and Discussion
Substrate Specificity: Scope of the Epoxidation Reaction.
The substrate scope of the reaction was thoroughly investigated
(15) Mizuno, N.; Yamaguchi, K.; Kamata, K. Coord. Chem. ReV. 2005, 249,
1944.
(16) De Vos, D. E.; Sels, B. F.; Reynaers, M.; Subba Rao, Y. V.; Jacobs, P. A.
Tetrahedron Lett. 1998, 39, 3221.
(17) Murphy, A.; Stack, T. D. J. Mol. Cat. A 2006, 251, 78.
(18) (a) Rudolph, J.; Reddy, K. L.; Chiang, J. P.; Sharpless, K. B. J. Am. Chem.
Soc. 1997, 119, 6189. (b) Cope´ret, C.; Adolfsson, H.; Sharpless, K. B.
Chem. Commun. 1997 1565.
(19) Dubois, G.; Murphy, A.; Stack, T. D. Org. Lett. 2003, 5, 2469.
(20) White, M. C.; Doyle, A. G.; Jacobsen, E. N. J. Am. Chem. Soc. 2001, 123,
7194.
(21) (a) Nakagawa, Y.; Kamata, K.; Kotani, M.; Yamaguchi, K.; Mizuno, N.
Angew. Chem., Int. Ed. 2005, 44, 5136. (b) Mizuno, N.; Nakagawa, Y.;
Yamaguchi, K. J. Mol. Cat. A 2006, 251, 286.
(22) First generation of epoxidation catalyst with general formula [(PP)PtCF3-
(H2O)]+: (a) Baccin, C.; Gusso, A.; Pinna, F.; Strukul, G. Organometallics
1995, 14, 1161. (b) Sinigaglia, R.; Michelin, R. A.; Pinna, F.; Strukul, G.
Organometallics 1987, 6, 728. (c) Zanardo, A.; Michelin, R. A.; Pinna, F.;
Strukul, G. Inorg. Chem. 1989, 28, 1648. (d) Strukul, G.; Michelin, R. A.
J. Am. Chem. Soc. 1985, 107, 7563. (e) Zanardo, A.; Pinna, F.; Michelin,
R. A.; Strukul, G. Inorg. Chem. 1988, 27, 1966.
a Experimental conditions: Substrate ) 0.83 mmol, H2O2 ) 0.83 mmol,
[1] )2% mol, solvent ) 1 mL of dichloroethane (DCE) at RT. bYield
determined by GC analysis. cReaction performed at 0 °C, [1] )3.5% mol;
1
yield determined by H NMR integration.
with catalyst 1 exploring the reactivity toward different sub-
strates bearing alkyl substitution as well as various functional
groups on the alkyl chain (Table 1). In a previous paper dealing
with the same catalyst23 we discovered that disubstituted alkenes
(e.g., cyclohexene, methylene cyclohexane) as well as styrene
are not suitable substrates, but terminal double bonds can be
efficiently epoxidized. As shown in Table 1, 1 shows high
(23) Pizzo, E.; Sgarbossa, P.; Scarso, A.; Michelin, R. A.; Strukul, G.
Organometallics 2006, 25, 3056.
(24) Colladon, M.; Scarso, A.; Sgarbossa, P.; Michelin, R. A.; Strukul, G. J.
Am. Chem. Soc. 2006, 128, 14006.
(25) For a review article on the potentiality of this kinetic approach on the
investigation of reaction mechanism: Blackmond, D. G. Angew. Chem.,
Int. Ed. 2005, 44, 4302. For an insightful example applied to a coupling
reaction: Mathew, J. S.; Klussmann, M.; Iwamura, H.; Valera, F.; Futran,
A.; Emanuelsson, E. A. C.; Blackmond, D. G. J. Org. Chem. 2006, 71,
4711.
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A R T I C L E S
Colladon et al.
Scheme 2. Regioselective Epoxidation of cis-1,4-Hexadiene Bearing a Terminal and a cis Disubstituted Double Bond with 35% Hydrogen
Peroxide Catalyzed by 1 Compared to the Stoichiometric Epoxidation Performed with m-CPBA
activity toward monofunctionalized linear terminal alkenes with
a slight decrease in productivity with an increase in the length
of the alkyl chain (entries 2-5). The smaller substrate of the
series, propene (entry 1), was efficiently epoxidized in a few
hours with 78% yield at low temperature with a catalyst loading
as low as 3.5%. Comparing entries 3 with 7 and 8 makes it
clear that a methyl substituent in the γ-position of the alkyl
chain of the substrate influenced the epoxidation reaction only
slightly, while methyl substitution in the â-position decreased
the yield in epoxide. Further substitution in the same position
hampered the reaction almost completely (entry 9) as did
substitution in the R-position (entry 6). From these observations
it clearly appears that the present catalytic system is specific
for terminal alkenes and it is very sensitive to the steric
properties of the substrate; in fact, the closer the substituent to
the double bond, the lower the yield in epoxide.
Another class of suitable alkenes is based on allyl benzene
derivatives, which can be considered as â-substituted terminal
double bonds (entries 10-12). Substitution with methoxy
residues does not influence directly the electron density on the
double bond but decreased the yield in epoxide probably because
of increased steric bulkiness or competition of the oxygen donor
with the CdC double bond for coordination at Pt. At the same
time, the system did not withstand the presence of coordinating
heteroatoms in the side chain; in fact 3-butenol, 3-cyano-
propene, allyl chloride, 5-hexenoic acid, and allyl imidazole are
all nonreactive substrates. Even more surprisingly substitution
of a methylene residue in the alkyl chain of 1-hexene with an
oxygen atom as in the case of allyl ethyl ether (entries 3 and
14) caused a severe drop in reactivity, while for allyl phenyl
ether the yield reached a maximum of 5% (entry 15). Analo-
gously allyl acetate (entry 13), albeit comparable in steric
hindrance with other substrates, turned out to be almost
unreactive, because of the presence of heteroatoms in the side
chain. These observations clearly speak for the dramatic effect
of heteroatoms acting as potential ligands toward the catalytic
site of the Pt(II) complex 1, even though water present in large
excess from H2O2 did not hamper the reaction. In this respect,
it has to be pointed out that even if water may be a good
competitor for coordination to the metal, its concentration in
DCE is about 2 orders of magnitude lower than that of the
substrate under standard reaction conditions. To have further
confirmation about the detrimental effect of heteroatoms, we
performed the epoxidation of 1-octene with catalyst 1 and H2O2
in dichloroethane as the solvent with an increasing amount of
tetrahydrofurane (THF) or methanol. In a 2:1 DCE/THF solution
the epoxide yield after 24 h decreased to 54%, and a reaction
performed in pure THF led only to a 12% yield. Pure methanol
as the solvent showed an even stronger effect yielding only 6%
terminal 1,2-epoxyoctane.
Experiments with a decreasing amount of catalyst in the case
of 1-hexene as the substrate were performed. A gradual decrease
of yield and concomitant increase in the TON was observed
(cat 2%, yield 89%, TON 45; cat 1%, yield 85%, TON 85; cat
0.4%, yield 63%, TON 158) which clearly speaks for the
intrinsic robustness of the present catalyst under the strongly
oxidizing experimental conditions. On the contrary, an experi-
ment carried out in neat 1-hexene did not show product
formation because of catalyst insolubility in pure alkene.
Regioselective Dienes Epoxidation. Regioselective mono-
oxidation of dienes is a challenging oxidative step which leads
to highly valuable synthons for organic synthesis, in particular
good results were reported for conjugated dienes with a
preference for the oxidation of more electron rich cis26 and
trans27 double bonds.
The high regioselectivity of catalyst 1 toward terminal
unfunctionalized alkenes is emphasized by the experiments
reported in Scheme 2. As a typical example, cis-1,4-hexadiene
bears both a terminal and a cis CdC bond and, in the presence
of a stoichiometric amount of m-chloroperbenzoic acid (m-
CPBA), leads mainly to cis-4,5-epoxy-1-hexene due to the
electrophilic epoxidation of the more electron-rich internal
double bond. On the contrary, when the epoxidation is per-
formed with catalyst 1 and 1 equiv of H2O2, the regioselectivity
of the reaction is completely inverted, favoring the product with
the terminal oxirane ring (Scheme 2). In Table 2 are reported a
few examples of such an inversion of regioselectivity in the
epoxidation with m-CPBA and with H2O2 catalyzed by 1. The
complete regioselectivity toward unsubstituted terminal olefins
observed with the former system is, to the best of our
knowledge, for some substrates comparable and for others better
than that observed with the best catalyst reported so far, i.e.,
V(III) containing polyoxometalates.21
(26) (a) Chang, S.; Heid, R. M.; Jacobsen, E. N. Tetrahedron Lett. 1994, 35,
669. (b) Burke, C. P.; Shi, Y. Angew. Chem., Int. Ed. 2006, 45, 4475.
(27) (a) Zhang, W.; Lee, N. H.; Jacobsen, E. N. J. Am. Chem. Soc. 1994, 116,
425. (b) Frohn, M.; Dalkiewicz, M.; Tu, Y.; Wang, Z.-X.; Shi, Y. J. Org.
Chem. 1998, 63, 2948.
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Pt-Mediated Nucleophilic Oxidation of Alkenes by H2O2
A R T I C L E S
Table 2. Regioselectivity Comparison in the Epoxidation of Various Dienes with Hydrogen Peroxide Catalyzed by 1 and with a Standard
Electrophilic Oxidant Like m-CPBAa
a Experimental conditions: Substrate ) 0.83 mmol, H2O2 ) 0.83 mmol, [1] ) 2% mol, solvent ) 1 mL of dichloroethane (DCE) at RT. bSum of
c
1
d
e
epoxides. Determined by GC and H NMR analysis. [Terminal monosubstituted epoxide]/[terminal disubstituted epoxide]. 7% of side products.
Scheme 3. Diastereoselective Epoxidation of a Chiral Terminal Alkene with 35% Hydrogen Peroxide Catalyzed by 1 (2% mol)
Analogously to what was reported for V(III) polyoxometal-
ates,21 the exceptional regioselectivity observed with 1 supports
the existence of stringent steric requirements although the
extreme ability in substrate recognition suggests that electronic
effects should also be carefully analyzed.
degree of diastereoselectivity with product distribution that is
dependent on catalyst/oxidant combination.29
In isolated terminal dienes the epoxidation of the first CdC
double bond creates a stereocenter (racemic monoepoxide), and
the subsequent epoxidation of the remaining alkene moiety
occurs in a diastereoselective fashion. In Scheme 4 we report
the results of the diastereoselective epoxidation of 1,4-pentadiene
and 1,5-hexadiene with catalyst 1 and an excess of hydrogen
peroxide.
At 0 °C the reaction with 1,4-pentadiene led to a lower yield
in diepoxides but with a higher diastereoselectivity when com-
pared to the reaction performed with the longer substrate for
which higher yields but a lower diastereoselectivity were
observed. Both effects can be probably ascribed to the strong
steric sensitivity characteristic of catalyst 1. In 1,5-hexadiene
the two double bonds are remote from each other, and they react
almost independently behaving similarly to isolated double bonds
with a high yield in diepoxide and low reciprocal sensing as con-
firmed by the low de. On the contrary with the smaller 1,4-
pentadiene the two double bonds are closer, and this decreases
the yield in diepoxide while increasing the diastereoselectivity.
Diastereoselectivity in Terminal Alkenes Epoxidation.
Spurred by the high selectivity showed by catalyst 1, we investi-
gated also the diastereoselectivity of this complex toward the
epoxidation of a racemic chiral terminal alkene such as 4-methyl-
hexene. This substrate differs from 4-methylpentene only for
the longer alkyl chain (Table 1 entry 7); however, the former
reacts more slowly (25% yield after 24 h at RT) compared to
the shorter analogue, with 25% diastereoselectivity in favor of
the epoxide product with the oxirane ring anti to the methyl
group in the â position (Scheme 3). The low diastereoselectivity
observed at room temperature increases up to 32% with a 21%
yield when the reaction was performed at 0 °C.
The de observed is low, but this is no surprise because of
the small steric difference between the methyl and ethyl group
in the chiral alkene substrate,28 as well as the absence of
attractive interactions that often are responsible for the peculiar
substrate orientation. A well-known example of such behavior
is the oxidation of chiral allylic alcohols that often show a high
(29) For examples of diastereoselective epoxidation of chiral allylic alcohols:
(a) Adam, W.; Degen, H.-G.; Saha-Mo¨ller, C. R. J. Org. Chem. 1999, 64,
1274. (b) Adam, W.; Stegmann, V. R.; Saha-Mo¨ller, C. R. J. Am. Chem.
Soc. 1999, 121, 1879. (c) Adam, W.; Mitchell, C. M.; Saha-Mo¨ller, C. R.
J. Org. Chem. 1999, 64, 3699. Della Sala, G.; Giordano, L.; Lattanzi, A.;
Proto, A.; Scettri, A. Tetrahedron 2000, 56, 3567. (d) Adam, W.; Alsters,
P. L.; Neumann, R.; Saha-Mo¨ller, C. R.; Sloboda-Rozner, D.; Zhang, R. J.
Org. Chem. 2003, 68, 1721. (e) Dusi, M.; Mallat, T.; Baiker, A. Catal.
ReV.sSci. Eng. 2000, 42, 213. (f) Adam, W.; Malisch, W.; Roschmann, K.
J.; Saha-Mo¨ller, C. R.; Schenk, W. A. J. Organomet. Chem. 2002, 661, 3.
(28) For examples of diastereoselective epoxidation of chiral alkenes: (a) Yang,
D.; Jiao, G.-S.; Yip, Y.-C.; Wong, M.-K. J. Org. Chem. 1999, 64, 1635.
(b) Chan, W.-K.; Liu, P.; Yu, W.-Y.; Wong, M.-K.; Che, C.-M. Org. Lett.
2004, 6, 1597. (c) Chan, W.-K.; Wong, M.-K.; Che, C.-M. J. Org. Chem.
2005, 70, 4226. (d) Jensen, A. J.; Luthman, K. Tetrahedron Lett. 1998,
39, 3213.
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A R T I C L E S
Colladon et al.
Scheme 4. Diastereoselective Oxidation of Isolated Terminal Dienesa
a (A) 1,4-Pentadiene and (B) 1,5-hexadiene with excess of hydrogen peroxide catalyzed by 1 (2% mol). Experimental conditions like those in Table 1,
2.1 equiv of H2O2.
Kinetic Investigation by Means of Reaction Progress
Kinetic Analysis. Insight into the mechanism of epoxidation
catalyzed by 1 was obtained performing a series of kinetic
experiments following the approach elegantly described by
Blackmond in a recent review article.25 Starting from a small
number of kinetic experiments following the reaction progress
from the beginning to completion, such a kinetic method allows
us to ascertain catalyst and substrate order, as well as possible
inactivation phenomena like catalyst decomposition or product
inhibition. According to the protocol described by Blackmond,25
hydrogen peroxide was the reactant used in “excess”. Being
the reaction medium biphasic (dichloromethane/water), the
concentration of H2O2 in dichloromethane is determined by the
partition coefficient.30 Clearly the “excess” refers rather to the
Figure 1. Rate vs 1-octene concentration, standard conditions. (b) [1]0 )
15.3 mM; [1-octene]0 ) 0.82 M; [H2O2]0 ) 1.05 M; [excess] ) 0.23 M;
solvent, 0.5 mL of dichloromethane (DCM); T ) 15 °C.
amount present in the whole system than to the actual
concentration of H2O2 present in the organic phase. As will be
clear below from the order observed in H2O2 concentration, this
does not affect the meaning of the results. As long as mass
transfer of the reagents back and forth between the phases is
not the rate-determining step (rds), the results observed by
reaction progress kinetic analysis in the case of biphasic
reactions are correct.
In the case at hand, a plot of the rate vs 1-octene concentration
under standard conditions is reported in Figure 1 and shows
that the reaction is first order in alkene. The plot is reproducible
even changing the stirring rate (1500 and 1000 rpm), thus
indicating the absence of mass transfer limitations. Actually,
the plot shows a small positive intercept on the abscissa
indicating that when the rate goes to zero there is still some
alkene left. This may be an indication that catalyst concentration
during the reaction progress is not constant,31 and especially
that partial decomposition of the catalyst may occur in the highly
oxidizing conditions in which the present reaction is carried
out. Nevertheless, such a detrimental side reaction does not seem
to hamper the epoxidation considering the high yields in epoxide
observed. Support for this view was found by carrying out an
experiment with different alkene concentrations but the same
[excess] showing that the new plot does not overlay to the
original one (Figure 2). Indeed, deviation from overlapping is
small (as is the intercept on the abscissa) indicating that catalyst
deactivation might be rather limited. An alternative view to
justify the positive intercept could be a possible inhibition by
product. This was checked by adding some free 1,2-epoxyoctane
to the reaction system. Perfect overlapping of the new set of
data to the original one rules out this possibility (Figure 3).
A new experiment was carried out using a different [ex-
cess], keeping [alkene]0 constant and increasing [H2O2]0. The
corresponding plot of rate vs [alkene] overlays the original one
(Figure 4) indicating that the reaction is zero order in [H2O2].
A further increase in [H2O2]0 up to 1.8 M provided a rate vs
[alkene] profile completely overlapping the previous ones.
Reaction order with respect to catalyst concentration was
determined by carrying out experiments at different [catalyst]0.
Plots of turnover frequency (TOF) vs [alkene] clearly overlapped
and prove the first-order kinetic dependence in [1] (Figure 5),
(30) The partition coefficient of H2O2 in dichloroethane/water biphasic system
DCE/H2O ) 1/367 was determined following the procedure reported in
Wolfe, W. C. Anal. Chem. 1962, 34, 1328. Due to the higher polarity of
DCM compared to DCE, it is likely that the partition coefficient for the
former chlorinated solvent should be lower then 1/300.
(31) (a) Rosner, T.; Pfaltz, A.; Blackmond, D. G. J. Am. Chem. Soc. 2001, 123,
4621. (b) Singh, U. K.; Strieter, E. R.; Blackmond, D. G.; Buchwald, S. L.
J. Am. Chem. Soc. 2002, 124, 14104. (c) Shekhar, S.; Ryberg, P.; Hartwig,
J. F.; Mathew, J. S.; Blackmond, D. G.; Strieter, E. R.; Buchwald, S. L. J.
Am. Chem. Soc. 2006, 128, 3584.
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Pt-Mediated Nucleophilic Oxidation of Alkenes by H2O2
A R T I C L E S
Figure 5. Turnover frequency vs 1-octene concentration with different [1]0.
(b) [1]0 ) 15.3 mM; [1-octene]0 ) 0.82 M; [H2O2]0 ) 1.05 M; [excess] )
0.23 M; (0) [1]0 ) 23.1 mM; [1-octene]0 ) 0.82 M; [H2O2]0 ) 1.05 M;
[excess] ) 0.23 M; solvent, 0.5 mL of DCM; T ) 15 °C.
Figure 2. Rate vs 1-octene concentration under same “[excess]” conditions.
(b) [1]0 ) 15.3 mM; [1-octene]0 ) 0.82 M; [H2O2]0 ) 1.05 M; [excess] )
0.23 M; (4) [1]0 ) 15.3 mM; [1-octene]0 ) 1.09 M; [H2O2]0 ) 1.32 mM;
[excess] ) 0.23 M; solvent, 0.5 mL of DCM; T ) 15 °C.
Scheme 5. Possible Reaction Pathways for the Catalytic
Epoxidation of 1-Octene with Hydrogen Peroxide Catalyzed by
Complex 1
reported in Scheme 5, where in path A [Pt]0 is equivalent to
the concentration of the starting complex [1] ) (Pt+), whereas
according to path B, [Pt]0 is equivalent to the concentration of
a newly formed [(dppe)Pt(C6F5)(OOH)] species (Pt-OOH). The
latter mechanism is suggested on the basis of previous observa-
tions with [(PP)Pt(CF3)(solv)]+ interaction with hydrogen
peroxide leading to the formation of Pt-hydroperoxo species.22
In order to gain more insight into the species involved in the
reaction a series of 31P NMR experiments were performed under
the experimental conditions of Figure 1. The spectrum of the
starting complex 1 in CDCl3 (Figure 6, spectrum A) shows two
signals at δ 29.0 ppm 1JP-Pt 4353 Hz and 44.5 ppm 1JP-Pt 2309
Hz, the latter with evidence for P-F coupling. Addition of an
excess of water to this complex (spectrum B) leads to the
formation of a slightly different species (P1-H2Otrans 29.9 ppm
1JP-Pt 3474 Hz; P2-C6F5trans 44.9 ppm 1JP-Pt 2320 Hz), indicating
that the latter is indeed the aquo complex whereas the initial
species observed in spectrum A is most likely a solvento
(CD2Cl2) complex. Addition of 50 equiv of H2O2 to a solution
of 1 in CD2Cl2 leads to a spectrum identical to spectrum B. This
observation seems to rule out pathway B, because the formation
of a hydroperoxy platinum species would require a significant
Figure 3. Rate vs 1-octene concentration for the experiment with product
addition. (b) [1]0 ) 15.3 mM; [1-octene]0 ) 0.82 M; [H2O2]0 ) 1.05 M;
[excess] ) 0.23 M; (]) [1]0 ) 15.3 mM; [1-octene]0 ) 0.82 M; [H2O2]0
) 1.05 M; [1,2-epoxyoctane]0 ) 0.163 M; [excess] ) 0.23 M; solvent, 0.5
mL of DCE; T ) 15 °C.
Figure 4. Rate vs 1-octene concentration under different “[excess]”
conditions. (b) [1]0 ) 15.3 mM; [1-octene]0 ) 0.82 M; [H2O2]0 ) 1.05
M; [excess] ) 0.23 M; (O) [1]0 ) 15.3 mM; [1-octene]0 ) 0.82 M; [H2O2]0
) 1.18 M; [excess] ) 0.36 M; solvent, 0.5 mL of DCM; T ) 15 °C.
1
shift of the signal relative to P trans to O and a JP-Pt in the
3200-3500 Hz range.32 On the contrary, addition of 50 equiv
of 1-octene to a solution of 1 in CD2Cl2 caused the formation
(spectrum C) of a new species characterized by signals at lower
field (P1-C)Ctrans 46.7 ppm 1JP-Pt 3476 Hz; P2-C6F5trans 45.7 ppm
1JP-Pt 2146 Hz) in particular for the P atom trans to the neutral
coordinating ligand, while P trans to C6F5 changes only slightly.
The relative intensities between the original and the new species
even though a negligible difference between the two profiles is
present, probably due to the moderate catalyst deactivation as
observed in Figure 5. The kinetic analysis reported above leads
to the following rate expression:
Rate ) d[epox]/dt ) k[Pt]0[alkene]
Reaction Mechanism. The above-described kinetic analysis
is consistent with two possible alternative reaction schemes
(32) Appleton, T. G.; Bennett, M. A. Inorg. Chem. 1978, 17, 738.
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A R T I C L E S
Colladon et al.
Figure 6. 31P{1H} NMR of (A) catalyst 1; (B) 1 with H2O; (C) 1 with addition of 50 equiv of 1-hexene; (D) 1 with addition of 50 equiv of 1-hexene and
50 equiv of H2O2.
Table 3. Reversible Coordination of Alkenes to 1a
This point was further investigated by NMR analysis of metal
alkene formation with substrates that proved catalytically
unreactive according to Table 1. For a more hindered methylene-
cyclohexane and vinyl-cyclohexane, as well as styrene or cis-
2-heptene, clearly no or negligible metal-alkene species were
observed. These experiments are in agreement with the general
coordination preference of Pt(II) complexes with alkenes where
the presence of electron-withdrawing groups on the alkene
encourages back-donation and enhances coordination, while on
the contrary electron-donating groups make the alkene bind more
weakly.33 For the reaction at hand, clearly no reaction is possible
in the absence of a reversible metal-alkene pre-equilibrium
interaction. Such observations rules out pathway A in Scheme
5.
a Experimental conditions: Substrate ) 0.83 mmol, [1] ) 2% mol,
solvent ) 0.5 mL of CDCl3 at RT. bRelative ratio of [1‚alkene]/[1]
Addition to 1 of 50 equiv of 1-octene and excess H2O
(independently of the addition order) showed the presence of
four different species (spectrum D, Figure 6). These are the
solvento complex, the aquo complex (all together approximately
90% of all the Pt(II) species), the olefin complex, and a new
species with signals at δ 28.7 ppm and 41.9-42.5 ppm with
no evidence for P-F coupling. The latter could probably be
ascribed to both olefin and water containing penta-coordinate
species where one P donor is in equatorial position, the other P
is in axial position, and the C6F5 ligand is in equatorial position.
Five-coordinate Pt(II) complexes with alkene neutral ligands
are known in the literature,34 and they usually assume trigonal
bipyramidal geometry. Addition of 50 equiv of H2O2 instead
of water leads to a spectrum identical to spectrum D. Differences
with respect to D after 24 h are the slow formation of traces of
1,2-bis(diphenylphosphino)ethane dioxide (39 ppm), clearly
supporting the partial degradation of the catalyst, and the
depletion after several hours of the olefin-containing species
when free olefin concentration is decreased because of the
determined by integration of the 31P NMR spectra.
are 1 to 1.6. The decrease of the 1JP-Pt coupling constant for P
trans to the neutral ligand is in agreement for alkene coordina-
tion in square planar complexes.32
Alkene coordination is the most obvious explanation to this
1
NMR experiment and is supported also by H NMR of 1 in a
solution saturated with propene where new signals at higher
field compared to the free alkene are observed. 31P NMR
experiments performed at different temperatures showed dif-
ferent amounts of 1 and the low field species with coordinated
propene proving the reversibility of the substrate coordination
step (see Supporting Information). Moreover, if alkene coor-
dination was the rds, the largely prevailing species in solution
should be complex 1, and as a consequence, signals of the 1‚
alkene, which is a species just after the rds, should disappear
when H2O2 is present and the epoxidation reaction occurs.
It is clear that alkene coordination is therefore a reversible
pre-equilibrium that takes place rapidly and whose position is
due to the steric and electronic features of the alkene. In fact,
increasing steric hindrance of the substrate in closer proximity
of the double CdC bond decreases alkene pre-equilibrium
coordination (Table 3) and hampers the subsequent epoxidation
step (see Supporting Information).
(33) (a) Crabtree, R. H. The Organometallic Chemistry of the Transition Metals;
John Wiley & Sons: 1988. (b) Collman, J. P.; Hegedus, L. S.; Norton, J.
T.; Finke, R. G. Principles and Applications of Organotransition Metal
Chemistry; University Science Books: Mill Wallet, CA, 1987.
(34) (a) Fanizzi, F. P.; Margotta, N.; Lanfranchi, M.; Tiripicchio, A.; Pacchioni,
G.; Natile, G. Eur. J. Inorg. Chem. 2004, 1705. (b) Albano, V. G.; Natile,
G.; Panunzi, A. Coord. Chem. ReV. 1994, 133, 67.
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Pt-Mediated Nucleophilic Oxidation of Alkenes by H2O2
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Figure 7. 31P{1H} NMR at different times taken during the reaction of 1-octene with H2O2 catalyzed by 1. Experimental conditions as those in Figure 1.
catalytic reaction, again indicating that they are in equilibrium
with the original catalyst 1 (Figure 7).
perfluoro-aryl ligand present in catalyst 1 on the epoxidation
reaction, in particular its interaction (if any) with H2O2. It is
well-known that H2O2 is both a better hydrogen bond donor
and a better acceptor than H2O;40 therefore, in order to
investigate plausible hydrogen bonding between the oxidant and
the fluorine atoms of the C6F5 residue,39d we first performed
19F NMR experiments on 1 in the presence of H2O2, 1-octene
and both reagents under catalytic conditions. Such measurements
did not allow any unequivocal assignment of the resonances of
the C6F5 moiety because addition of either 1-octene or H2O2
caused changes in the chemical shifts due to both direct
interaction with the fluorine atoms and direct coordination to
the metal center. As observed by 31P NMR investigation, several
species are therefore present in solution all with different 19F
spectra which are not easily assignable. In order to exclude the
latter effect and investigate only the interaction of the reagents
with the fluorine atoms, the complex [(dppe)Pt(C6F5)Cl] was
employed as a nonreactive analogue of catalyst 1. Addition of
H2O2 to [(dppe)Pt(C6F5)Cl] caused a small but substantial
deshielding of meta and para fluorine atoms of about 0.1 ppm
that could well account for hydrogen bonding between such
fluorine atoms and H2O2 dissolved in CDCl3.41 At the same
time, no signals for the original complex are observed. It is
therefore likely that H2O2 efficiently binds to the meta and para
fluorine atoms of C6F5 by means of hydrogen bonding to all
the different Pt(II) species present in solution as is observed by
31P NMR analysis (Figure 7). Such an interaction can be
described as an equilibrium strongly shifted toward the forma-
The apparent discrepancy between kinetic and NMR studies
is not to be ascribed to different stirring or diffusional conditions
because rate data calculated from 1H NMR spectra overlay with
those obtained by GC analysis (Supporting Information), at least
for a significant part of the experiment. To overcome this
problem a different explanation is therefore necessary.
Fluorine atoms can impart peculiar properties to molecules;
for instance there are many examples of hydrogen bonding of
fluorinated drugs35 within the hydrophobic portions of proteins
that enhances pharmacological activity compared to nonfluori-
nated ones. In catalysis, fluorinated solvents, in particular
alcohols, have attracted the attention of many research groups
for several years because of their intrinsic ability in the activation
of H2O2 for oxidation reactions. In most cases such an effect
allows us to perform oxidation reactions in the complete absence
of any sort of metal catalysts, just exploiting the solvating and
hydrogen bonding donor and acceptor ability of fluorinated
moieties that act as organo-catalysts. Sound literature on
fluorinated solvent effects in electrophilic activation of hydrogen
peroxide in epoxidation reactions36 and nucleophilic activation
in a Baeyer-Villiger reaction36b,37 is available. Moreover, many
research groups analyzed in detail the effect of fluorine on the
mechanisms of activation by means of experimental measure-
ments38 and theoretical calculations.39 Spurred by this vast
literature, we decided to investigate the possible effect of the
(35) (a) Carosati, E.; Sciabola, S.; Cruciani, G. J. Med. Chem. 2004, 47, 5114.
(b) Razgulin, A. V.; Mecozzi, S. J. Med. Chem. 2006, 49, 7902.
(36) (a) Berkessel, A.; Adrio, J. A.; Hu¨ttenhain, D.; Neudo¨rfl, J. M. J. Am. Chem.
Soc. 2006, 128, 8421. (b) Neimann, K.; Neumann, R. Org. Lett. 2000, 2,
2861. (c) van Vliet, M. C. A.; Arends, I. W. C. E.; Sheldon, R. A. Chem.
Commun. 1999, 263. (d) van Vliet, M. C. A.; Arends, I. W. C. E.; Sheldon,
R. A. Chem. Commun. 1999, 821. (e) van Vliet, M. C. A.; Arends, I. W.
C. E.; Sheldon, R. A. Synlett 2001, 248.
(39) (a) de Visser, S. P.; Kaneti, J.; Neumann, R.; Shaik, S. J. Org. Chem. 2003,
68, 2903. (b) Berkessel, A.; Adrio, J. A. J. Am. Chem. Soc. 2006, 128,
13412. (c) Schaal, H.; Ha¨ber, T.; Suhm, M. A. J. Phys. Chem. A 2000,
104, 265. (d) Howard, J. A. K.; Hoy, V. J.; O’Hagan, D.; Smith, G. T.
Tetrahedron 1996, 52, 12613. (e) Alkorta, I.; Rozas, I.; Elguero, J. J.
Fluorine Chem. 2000, 101, 233.
(37) (a) Berkessel, A.; Andreae, M. R. M.; Schmickler, H.; Lex, J. Angew. Chem.,
Int. Ed. 2002, 41, 4481. (b) Berkessel, A.; Andreae, M. R. M. Tetrahedron
Lett. 2001, 42, 2293.
(38) (a) Ravikumar, K. S.; Zhang, Y. M.; Be´gue´, J. P.; Bonnet-Delpon, D. Eur.
J. Org. Chem. 1998, 2937. (b) Legros, J.; Crousse, B.; Bourdon, J.; Bonnet-
Delpon, D.; Be´gue´, J. P. Tetrahedron Lett. 2001, 42, 4463. (c) ten Brink,
G.-J.; Vis, J. M.; Arends, I. W. C. E.; Sheldon, R. A. Tetrahedron 2002,
58, 3977.
(40) (a) Goebel, J. R.; Ault, B. S.; Del Bene, J. E. J. Phys. Chem. A 2001, 105,
11365. (b) Rankin, K. N.; Boyd, R. J. J. Comput. Chem. 2001, 22, 1590.
(c) Goebel, J. R.; Ault, B. S.; Del Bene, J. E. J. Phys. Chem. A 2000, 104,
2033. (d) Mo´, O.; Ya´n˜ez, M.; Rozas, I.; Elguero, J. J. Chem. Phys. 1994,
100, 2871. (e) Mo´, O.; Ya´n˜ez, M.; Rozas, I.; Elguero, J. Chem. Phys. Lett.
1994, 219, 45.
(41) A typical example of an H-F hydrogen bond in organometallic com-
plexes: Anil Kumar, P. G.; Pregosin, P. S. Organometallics 2004, 23, 5410.
9
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A R T I C L E S
Colladon et al.
Scheme 6. Mechanistic Hypothesis for Terminal Alkene Epoxidation with Hydrogen Peroxide Mediated by 1 on the Bases of the Results of
Reaction Progress Kinetic Analysis and 31P and 19F NMR Investigation
Conclusion
tion of the C6F5-H2O2 adduct. In Scheme 6 is reported what
we believe is the most likely mechanism of the reaction.
Such a scheme takes into account all the spectroscopic
evidence on the reaction reported above, including the formation
of an off-cycle metal-olefin complex, and is in agreement with
the kinetic data under the assumption (confirmed by NMR data)
that [Pt0] is essentially given by the solvento and aquo
complexes carrying H2O2 hydrogen-bound to the C6F5 ligand,
all the other species being negligible. Scheme 6 would also
account for the steric effects observed in Table 1 as metal-
alkene formation precedes the rate-determining step, as well as
for the nucleophilic character of the oxygen transfer step as
demonstrated by the unusual regioselectivity observed in dienes
epoxidation (Table 2). This is, to the best of our knowledge, a
rare example of epoxidation of unfunctionalized alkenes by
means of electrophilic alkene and nucleophilic oxidant activation
provided by metal catalysts. In fact, this kind of oxidation
characterized by a concomitant substrate and oxidant activation
is much more common for electron-poor substrates, as demon-
strated recently for the asymmetric nucleophilic epoxidation of
chalcone with hydrogen peroxide catalyzed by polyleucine
catalysts.42
The use of Pt(II) complex 1 demonstrated a high activity and
unusual selectivity compared to other metal-catalyzed epoxi-
dation methods for terminal monofunctionalized alkenes with
hydrogen peroxide as oxidant. In addition to (i) high activity
under mild experimental conditions, (ii) catalyst loading as low
as 2%, and (iii) no need of over-stoichiometric amounts of
oxidant, key features of catalyst 1 are (iv) the extreme substrate
selectivity and (v) the exceptional regioselectivity21 which, in
particular with some dienes, is the highest so far reported for
metal mediated epoxidation. Such properties, along with the
straightforward synthesis of the catalyst, allowed the develop-
ment of an asymmetric and highly regioselective version of the
reaction24 whose results are well interpreted on the basis of the
mechanism observed. Further development on the described
class of Pt(II) catalysts is currently underway with the aim of
disclosing a practical application for highly selective terminal
alkene epoxidation.
Experimental Section
Reagents and Materials. The complex [(dppe)Pt(C6F5)(H2O)]OTf
1 was prepared following the procedure reported in the literature.23
Hydrogen peroxide (35% Aldrich) as well as all the alkene substrates
listed above and 1,2-epoxyoctane are commercial products (Aldrich)
and were used without further purification.
Differences with respect to the mechanism proposed for the
homologous -CF3 derivatives and analyzed by initial rate
kinetic experiments are likely due to the different nature of the
fluorinated ligands. The trifluoromethyl residue is less sterically
demanding with respect to pentafluorophenyl but more electron-
withdrawing.43 It is likely that the higher acidity of CF3 favors
also direct H2O2 coordination, and the reaction proceeds through
a second-order kinetic dependence in catalyst concentration.22e
1
General. H, 31P{1H}, and 19F{1H} NMR spectra were recorded at
298 K, unless otherwise stated, on a Bruker AVANCE 300 spectrometer
(43) Acidity of alcohols: (a) Ballinger, P.; Long, F. A. J. Am. Chem. Soc. 1959,
81, 1050. (b) Korenaga, T.; Kadowaki, K.; Ema, T.; Sakai, T. J. Org. Chem.
2004, 69, 7340. Acidity of carboxylic acids: (c) Takahashi, S.; Cohen, L.
A.; Miller, H. K.; Peake, E. G. J. Org. Chem. 1971, 36, 1205. (d) Strong,
L. E.; Brummel, C. L.; Lindower, P. J. Solution Chem. 1987, 16, 105.
Taft’s σ* values: ref 43b.
(42) Mathew, S. P.; Gunathilagan, S.; Roberts, S. M.; Blackmond, D. G. Org.
Lett. 2005, 7, 4847.
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Pt-Mediated Nucleophilic Oxidation of Alkenes by H2O2
A R T I C L E S
operating at 300.15, 121.50, and 282.38 MHz, respectively. δ values
in ppm are relative to SiMe4, 85% H3PO4, and CFCl3. GLC measure-
ment were taken on a Hewlett-Packard 5890A gas chromatograph
equipped with an FID detector (gas carrier He). All reactions were
monitored on a 25 m HP-5 capillary column. The concentration of
hydrogen peroxide was checked iodometrically prior to use.
peroxide (1 equiv compared to propene) were added and time was
started. Reaction was quenched at the desired time by LiCl addition,
and the yield was determined by H NMR integration.
1
Reaction Progress Kinetic Analysis. The experimental methodology
employed for data acquisition was GC analysis of the progress of the
reaction, monitoring the reactant [1-octene] and product [1,2-epoxy-
octane] over time with n-decane as the internal standard. The concentra-
tion of H2O2 (the reagent in “excess”) was calculated indirectly from
the concentration of alkene, as a sufficient number of direct determina-
tions during reaction progress is experimentally impossible. The mass
balance for H2O2 was checked at the end of the reaction and found
satisfactory. The experimental concentration vs time profiles for epoxide
product was interpolated with a sixth grade polynomial function, which
was subsequently derived to obtain a rate vs time profile.
Oxidation Reactions. These were carried out in a 2 mL vial
equipped with a screw-capped silicone septum to allow sampling.
Stirring was performed by a Teflon-coated bar driven externally by a
magnetic stirrer (700 rpm). Constant temperature was maintained by
water or alcohol circulation through an external jacket connected with
a thermostat. The concentration of the commercial 35% H2O2 solution
was checked iodometrically prior to use. Typically, the proper amount
of catalyst 1 (0.016 mmol, 2% mol) was placed in the vial, followed
by the solvent (1 mL) and internal standard, and then the vial was
thermostatted. After the mixture stirred for 10 min, the substrate (0.83
mmol) was added and the mixture stirred for another 10 min. To this
35% hydrogen peroxide was added in one portion, (0.83 mmol) and
time was started. All reactions (except when the substrate was propene)
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. Separation of the products was performed on a 25 m
HP-5 capillary column with He as carrier using a flame ionization
detector.
Acknowledgment. RAM and GS thank MUR, the Universita`
di Padova, and the Universita` Ca’ Foscari di Venezia for
financial support (PRIN 2003). GS thanks Johnson-Matthey for
the loan of platinum. L. Sperni is gratefully acknowledged for
GC-MS analysis. Special thanks are expressed to Prof. D. G.
Blackmond (Imperial College, London) for fruitful discussions
on reaction progress kinetic analysis method.
Supporting Information Available: 31P NMR spectra for the
coordination of different alkenes to 1 and temperature effect
on pre-equilibrium position in the coordination of propene to
1. Comparison of catalyst’s activity by means of NMR and GC
determination. This material is available free of charge via the
Internet at http://pubs.acs.org.
Propene Oxidation. 5 mL of DCE were placed in a vial thermo-
statted at the desired temperature, and to this propene was bubbled for
30 min until saturation of the solution. 1 mL of such solution was put
in a vial, and catalyst 1 was added (0.016 mmol). The substrate/catalyst
ratio was determined by 1H NMR. The solution was then thermostatted,
and the proper amount of internal standard followed by 35% hydrogen
JA071142X
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