The nature of the catalytically active species in olefin dioxygenation with PhI(OAc)2: Metal or proton?

Article abstract of DOI:10.1021/ja110805b

Evidence for the protiocatalytic nature of the diacetoxylation of alkenes using PhI(OAc)₂ as oxidant is presented. Systematic studies into the catalytic activity in the presence of proton-trapping and metal-complexing agents indicate that protons act as catalysts in the reaction. Using triflic acid as catalyst, the selectivity and reaction rate of the conversion is similar or superior to most efficient metal-based catalysts. Metal cations, such as Pd(II) and Cu(II), may interact with the oxidant in the initiation phase of the catalytic transformation; however, 1 equiv of strong acid is produced in the first cycle which then functions as the active catalyst. Based on a kinetic study as well as in situ mass spectrometry, a mechanistic cycle for the proton-catalyzed reaction, which is consistent with all experimental data presented in this work, is proposed.

Full text of DOI:10.1021/ja110805b

ARTICLE
pubs.acs.org/JACS
The Nature of the Catalytically Active Species in Olefin Dioxygenation
with PhI(OAc)2: Metal or Proton?
Yan-Biao Kang† and Lutz H. Gade*,†,‡
†Catalysis Research Laboratory (CaRLa), Im Neuenheimer Feld 584, 69120 Heidelberg, Germany
‡Anorganisch-Chemisches Institut, Universit€at Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany
S Supporting Information
b
ABSTRACT: Evidence for the protiocatalytic nature of the diacetoxylation
of alkenes using PhI(OAc)2 as oxidant is presented. Systematic studies into
the catalytic activity in the presence of proton-trapping and metal-complexing
agents indicate that protons act as catalysts in the reaction. Using triflic acid as
catalyst, the selectivity and reaction rate of the conversion is similar or
superior to most efficient metal-based catalysts. Metal cations, such as Pd(II)
and Cu(II), may interact with the oxidant in the initiation phase of the
catalytic transformation; however, 1 equiv of strong acid is produced in the
first cycle which then functions as the active catalyst. Based on a kinetic study
as well as in situ mass spectrometry, a mechanistic cycle for the proton-
catalyzed reaction, which is consistent with all experimental data presented in
this work, is proposed.
Scheme 1. Approaches for Dioxygenation or Diacetoxylation
of Alkenes
’ INTRODUCTION
There are several classical methods for the dioxygenation of
alkenes that are still widely used in organic synthesis. These
include two-step-reactions, such as the Woodward-Prevost
reaction (pathway a, Scheme 1)1 and the epoxidation followed
by ring-opening (pathway c).2 Both operate under relatively mild
reaction conditions, even though the former is not a catalytic
reaction. The one-step dioxygenation of alkenes catalyzed by
OsO4,3 along with its asymmetric version, the Sharpless
dihydroxylation,4 are among the most widely used methods
but involve an expensive and highly toxic catalyst (pathway b).
The development of new efficient catalytic dioxygenation meth-
ods for alkenes has therefore remained a challenge.
In recent years, there has been considerable interest in the
development of alternative dihydroxylation protocols involving
other metal catalysts. With the emergence of palladium(IV)
chemistry in catalysis,5-13 the dioxygenation of alkenes has been
reinvestigated using Pd- or Cu-catalysts instead of the very toxic
amino acetoxylations.6h It is notable that Chai and co-workers
reported the same type of reaction for a copper(II) catalyst, in
which the triflate counterion also proved to be essential. In this
case, all other copper salts tested, such as Cu(OAc)2 and CuCl2,
were catalytically inert.14b Claims for a mechanism involving
CuIII intermediates were based on in situ high resolution mass
spectrometry. However, closer inspection of the data revealed
significant discrepancies between the expected and observed
data, along with the observation of many other unassigned ion
peaks, which make this interpretation questionable.
osmium.14 Very recently, Dong et al. reported a Pd(OTf)2-
15-18
catalyzed diacetoxylation of alkenes using PhI(OAc)2
as
oxidant.14a In this case, a cationic palladium complex with
electron-rich diphosphines as ligands as well as the use of the
anionic triflato ligand were claimed to be essential for the
reaction because, for instance, Pd(OAc)2 itself was found to be
inert. Although a PdIV intermediate was invoked in the proposed
catalytic cycle for this reaction, no detailed mechanistic investiga-
tions were carried out. Related diacetoxylations were also ob-
served as leading to undesired side products in Pd-catalyzed
Received: December 2, 2010
Published: February 16, 2011
r
2011 American Chemical Society
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Table 1. Catalyst Screening of Metal Salts and Brønsted Acidsa
entry
catalyst
loading (mol %)
% convb
entry
catalyst
Pd(OAc)2
loading (mol %)
% convb
1
Ni(ClO4)2 6H2O
1
100
94
88
10
3.8
1
17d
18d
19d
20g
21g
22d
23
1
6
3
2
Ni(ClO4)2 6H2O
0.1
Cu(OAc)2
CuCl2
1
2
3
3
Ni(ClO4)2 6H2O
0.05
0.01
0.001
0.0001
5
1
7
3
4
Ni(ClO4)2 6H2O
Ca(ClO4)2 4H2O
10
10
10
10
10
1
99
99
2
3
3
5
Ni(ClO4)2 6H2O
Ca(OTf)2
CaCl2
3
6
7c
8d
9e
10f,d
11f,d
12f,d
13f,d
14f,d
15f,d
16f,d
Ni(ClO4)2 6H2O
3
Ni(Cl)2 glyme
6
Mg(OTf)2
100
100
100
23
7
3
Cu(OTf)2
0.5
99
99
99
96
95
95
43
3
24
Mg(ClO4)2 6H2O
3
d g
,
Co(BF4)2
10
25
26
27
Ba(ClO4)2(99.999%)
Ba(ClO4)2(99.999%)
acetic acid (>99.99%)
HClO4(99.999%)
triflic acid (99%)
trifluoroacetic acid
HCl
d g
,
Pd(L)(H2O)2(OTf)2
Pd(L)(H2O)2(OTf)2
Pd(L)(H2O)2(OTf)2
Pd(L)(H2O)2(OTf)2
Pd(L)(H2O)2(OTf)2
Pd(L)(H2O)2(OTf)2
Pd(L)(H2O)2(OTf)2
1
0.1
175
0.1
0.1
0.1
0.1
0.1
0.5
0.25
0.1
28g
29g
30
93
88
2
0.03
0.01
0.001
31
2
2
32
4-CF3-benzoic acid
2
a Reaction conditions: 1 (0.13) mmol; 2a (0.1 mmol); CH2Cl2 (1 mL). 3a forms after a rearrangement of phenyl group in the reaction (see ref 19 for
details). b Determined by 600 MHz 1H NMR. c Refers to NiCl2-CH3OCH2CH2OCH3. d Reaction time: 20 min. e Co(BF4)2 6H2O was used. f (R)-
3
BINAP-Pd(H2O)2(OTf)2 was used. g ICP-OES measurements show the trace amount of metals: Cu <1 ppm and Pd <5 ppm; see Supporting
Information for details.
The nature of the active catalyst in both the palladium- and
copper-catalyzed diacetoxylations thus remains to be resolved.
However, this point is essential for the further development of
the method as a synthetic tool. In view of this uncertainty, we set
out to further explore the mechanism of dioxygenation reaction
of alkenes using PhI(OAc)2 as oxidant and to identify the active
catalyst(s). In this work, we provide new mechanistic insight into
this reaction, both its intramolecular and intermolecular version,
the catalytically active species involved, and the key role that
triflate may play as a weakly coordinating anionic ligand or
counterion.
’ RESULTS AND DISCUSSION
Figure 1. The comparison of the conversion curves for the Pd-, Ca-, and
TfOH-catalyzed diacetoxylation of styrene. Note that no inflection point
was seen for the triflic acid-catalyzed transformation.
Screening of Metal Salts for Catalytic Activity in the
Intramolecular Diacetoxylation of Alkenes. As indicated,
both palladium and copper salts have been reported as catalysts
palladium in these salts, if present, were well below the 1 and 5
ppm level, respectively (Supporting Information). Since at these
concentrations none of the transition metal salts was found to be
active, “hidden” catalysis by these metals may be ruled out.
A similar behavior was observed for the intermolecular diacetox-
ylation of styrene with PhI(OAc)2 (eq 2). For comparison the
conversion curves for the Pd-, Ca-, and TfOH-catalyzed reaction are
displayed in Figure 1, using 5 mol % of HOTf and [(R)-BINAP-
Pd(H2O)2(OTf)2] as well as 11 mol % of Ca(OTf)2. First of all, we
note that the Ca(OTf)2 catalysis occurs with much lower activity
and that [(R)-BINAP-Pd(H2O)2(OTf)2] and HOTf display very
similar activities at the same catalyst loading. However, the metal-
catalyzed reactions are characterized by sigmoidal conversion curves
which may indicate the (reversible) generation of the catalytically
active species in the initial stages of the reaction and its gradual
disappearance near the end.
for the diacetoxylation of alkenes. It was thus essential to
establish to which degree these two metals were necessary, or
in fact, whether a metal-based catalyst in general was an indis-
pensible prerequisite. The results of the screening of metal salts
and complexes for the intramolecular diacetoxylation repre-
sented in eq 1 are summarized in Table 1. Besides Cu(OTf)2
and Pd(OTf)2, other transition metal Lewis acids such as
Ni(ClO4)2 and Co(BF4)2 were also found to be efficient
catalysts, while the corresponding salts with more basic or
strongly coordinating counterions, such as Pd(OAc)2, Cu-
(OAc)2, CuCl2, and NiCl2, proved to be inactive.
Surprisingly, we found that alkali earth metal salts also
displayed catalytic activity (entries 20-26). For instance, using
0.1 mol % of extremely pure Ba(ClO4)2, a full conversion of 2a
was obtained within 20 min (entry 26). ICP-OES analysis of the
calcium and barium salts established that traces of copper and
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Table 2. Proton-Trapping Study on the Intramolecular
Reactiona
entry
catalyst
additive
% convb
1
HOTf
-
99
<4
33
11
1
2
HOTf
HOTf
HOTf
HOTf
EDTA
AcOH
5% H-sponge
20% Bu4NOAc
100% Bu4NOAc
200% Bu4NOAc
-
3
4
5
6
0
7
-
0
8
9c
Cu(OTf)2
-
99
99
0
Cu(OTf)2
1% EDTA
Figure 2. The comparison of the catalytic conversion curves for the
diacetoxylation of styrene with PhI(OAc)2 as oxidant, obtained for
different amounts of [(R)-BINAP-Pd(H2O)2(OTf)2] and compared to
5% of TfOH as catalyst.
10c
11
12
13
14
15c
16c
17
18
19
20d
21c,d
22c,d
23d
24d
Cu(OTf)2
1% EDTAþ5% H-sponge
5% H-sponge
20% Bu4NOAc
100% Bu4NOAc
-
Cu(OTf)2
0
Cu(OTf)2
7
Cu(OTf)2
0
It is clear from these results that high valent PdIV or CuIII are
not necessarily involved in the catalytic diacetoxylation with
PhI(OAc)2. Remarkably, several Brønsted acids proved to cata-
lyze the reaction, albeit under very specific conditions. We note
that in a previous study of a noncatalytic version of this reaction19
no conversion had been observed in the presence of a dilute
acidic solution. In our investigations of the reaction, very low
activity and conversion was found in the presence of AcOH (175
mol %, entry 27), while 0.1 mol % of HClO4 or HOTf gave rise to
complete conversion (entries 28 and 29). Again, ICP-OES analyses
of the acids established levels of Cu and Pd of below 1 and 5 ppm,
thus again ruling out hidden metal catalysis. However, under the
same reaction conditions, other strong acids only induced very
low conversions (entries 30-32), thus raising the question as to
the specificity of the observed acid catalysis.
Ba(ClO4)2
99
99
0
Ba(ClO4)2
1% EDTA
Ba(ClO4)2
1% EDTAþ5% H-sponge
5% H-sponge
20% Bu4NOAc
50% Bu4NOAc
-
Ba(ClO4)2
0
Ba(ClO4)2
23
<1
99
99
<1
<1
6
Ba(ClO4)2
Pd(L)(H2O)2(OTf)2
Pd(L)(H2O)2(OTf)2
Pd(L)(H2O)2(OTf)2
Pd(L)(H2O)2(OTf)2
Pd(L)(H2O)2(OTf)2
1% EDTA
1% EDTAþ5% H-sponge
5% H-sponge
20% Bu4NOAc
a Reaction conditions: 1 (0.13 mmol); 2a (0.1 mmol); catalyst (1 mol %);
CH2Cl2 (1 mL); 20 min. Proton sponge refers to N,N,N0,N0-tetra-
b
methyl-1,8-naphthalenediamine. Determined by 600 MHz 1H NMR.
c Added as neutral tetracarboxylic acid. d (R)-BINAP-Pd(H2O)2(OTf)2
Both metal salts as well as triflic and perchloric acid thus
appear to catalyze the diacetoxylation. The nature of the actual
catalytically active species in these reactions therefore remains to
be resolved and, in particular, whether there might be a common
underlying principle. In Figure 2 the conversion curves of the
[(R)-BINAP-Pd(H2O)2(OTf)2]-catalyzed diacetoxylation of
styrene with PhI(OAc)2 as oxidant (eq 2) are compared for
different Pd catalyst loadings (and with HOTf). At all catalyst
loadings, the sigmoidal characteristics of the conversion curves,
already noticed in Figure 1, are found. At the point of inflection,
the activity of the Pd catalyst is essentially the same as that of
HOTf at an equal catalyst loading (in this case: 5 mol %). It
appears that for the Pd catalyst, the active species, which we
propose to be the “excess” proton, is being formed initially, an
aspect that will be further discussed below.
Proton-Trapping Study. To investigate the possible catalytic
role of protons in this dioxygenation reaction, proton-trapping
agents were added to the reaction in eq 1 for various catalysts. As
shown in Table 2, the intramolecular reference reaction catalyzed
by HOTf was effectively inhibited by either 5 mol % of proton
sponge (N,N,N0,N0-tetramethyl-1,8-naphthalenediamine) or
100 mol % of Bu4NOAc (entries 2 and 4).
was used.
other hand, both proton-sponge and Bu4NOAc were efficient
inhibitors for the Cu(OTf)2-, Pd(OTf)2-, and Ba(ClO4)2-cata-
lyzed reaction (entries 10-13, 16-19, 22-24) in the presence
or absence of EDTA.
Analogous proton-trapping experiments were carried out for
the intermolecular dioxygenation of styrene with iodobenzene
diacetate (eq 2). This reaction proceeded smoothly in the
presence of 5 mol % of HOTf to give 97% conversion at room
temperature within 6 h (Table 3, entry 1). When proton sponge
or Bu4NOAc were added at the outset, no conversion was observed
even after 24 h. In the cases of Cu(OTf)2 and Pd(OTf)2, the proton
traps also efficiently suppressed the reaction, and Cu(OAc)2 as well
as Pd(OAc)2, containing the basic acetate as counteranion, were
found to be inactive as noted in previous studies.14
In Figure 3 the conversion curves of the [(R)-BINAP-Pd-
(H2O)2(OTf)2]-catalyzed diacetoxylation of styrene with or
without an equivalent amount of Bu4N(OAc) are displayed.
We note that 1 mol of added base essentially quenches 1 mol of
Pd catalyst. This would be consistent with a mechanistic scenario,
in which the catalytically active proton is generated in a first
reaction step of the metal complex with the oxygenation agent
PhI(OAc)2 (vide infra) and is trapped if additional acetate is
present.
On the other hand, EDTA as neutral tetracarboxylic acid,
which was added as a metal complexation agent, had no inhibitive
effect on the Cu(OTf)2, Ba(ClO4)2, and (R)-BINAP-Pd(H2O)2-
(OTf)2-catalyzed dioxygenation (entries 9, 15, and 21), while the
ligand EDTA itself (in its neutral, protonated form) was inactive
(entry 6). This indicated that the scavenging of the metal by a
complexone did not impede the activity of the system. On the
Substrate Scope of the Metal Catalysts and the Proton.
The obervations made in the catalyst screening suggested the
possibility of the proton alone acting as a catalytically active
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Table 3. Proton-Trapping Study on the Intermolecular Reaction of Styrene with Iodobenzene Diacetate
entry
catalyst
HOTf (5 mol %)
additive (mol %)
t, h
% conv
1
-
6
97
0
2
HOTf (5 mol %)
6 mol % H-sponge
100 mol % Bu4NOAc
6 mol % Bu4NOAc
-
24
24
24
4
3
HOTf (5 mol %)
0
4
HOTf (5 mol %)
0
5
Cu(OTf)2 (5 mol %)
44
32
0
6
Cu(OTf)2 (5 mol %)
6 mol % Bu4NOAc
12 mol % Bu4NOAc
6 mol % H-sponge
12 mol % H-sponge
-
4
7
Cu(OTf)2 (5 mol %)
24
4
8
Cu(OTf)2 (5 mol %)
36
0
9
Cu(OTf)2(5 mol %)
24
4
10
11
12
13
14
15
(R)BINAP-Pd(H2O)2(OTf)2 (5 mol %)
(R)BINAP-Pd(H2O)2(OTf)2 (5 mol %)
(R)BINAP-Pd(H2O)2(OTf)2 (5 mol %)
Pd(OAc)2 (5 mol %)
66
0
6 mol % H-sponge
6 mol % Bu4NOAc
-
24
24
4
0
0
CuCl2 (5 mol %)
-
4
0
Cu(OAc)2 (5 mol %)
-
4
0
a Reaction conditions: 1 (0.5 mmol); 2 (0.25 mmol); CH2Cl2 (0.5 mL); AcOH (12.5 μL). b Determined by 600 MHz 1 H NMR.
of water as a side reaction. Notably, the latter occurred exclusively
in the diacetoxylation of 1-allylcyclohexanol 2f in the presence of
water (entry 9).
As far as the intermolecular version of this reaction is con-
cerned, triflic acid was efficient for a variety of alkenes. In a
comparative study, HOTf gave the same or better yields and
diastereoselectivities than the Pd(OTf)2 or Cu(OTf)2 catalysts
(entries 1-4, Table 5). The reactions were carried out with or
without water in glacial acetic acid, giving the same yields but
quite different diastereoselectivities (entries 3 and 4). As dis-
cussed below, a possible explanation is the reaction of a non-
metalated cyclic cationic intermediate with water, generating a
nonacetylated hydroxyl group, which in turn is transformed to
OAc after treatment with acetic anhydride in the workup (vide
infra).14a Both cis- and trans-stilbene gave the syn-diastereomer
as the major product (entries 1-6), which may be due to an
intramolecular rotation within the cationic intermediate. Cyclic
and terminal aliphatic alkenes also gave good yields and diaster-
eoselectivities (entries 7 and 8 and entries 14 and 15) as did
styrene itself and its derivates (entries 9 and 10). The reaction in
wet acetic acid gave the 2-hydroxyl derivative (entries 11 and 13),
underscoring the interpretation of the role of water given above.
In the presence of water in acetic acid, very similar diastereos-
electivites were obtained as reported in ref 14a. However, in dry
acetic acid, diastereoselectivities decreased, probably due to the
change in the attacking nucleophile. For example, cis-stilbene
afforded the diacetoxylation products with the ratio of syn/anti of
10/1 and 6/1 in our case (entry 4, Table 5) and in ref14a,
respectively. When the reactions were carried out in dry acetic
acid, in contrast, very low diastereoselectivities were observed
both with the palladium catalyst or triflic acid (entries 2 and 3).
Finally, the dioxygenation of R,β-unsaturated ester 4k also gave
the corresponding reaction product in moderate yield (entry 16).
It is instructive to compare the results obtained in this study for
the intra- and intermolecular diacetoxylations/dioxygenations
summarized in Tables 4 and 5 using triflic acid as catalyst to those
obtained previously as well as in our own study as a result of
Figure 3. The comparison of the [(R)-BINAP-Pd(H2O)2(OTf)2]-
catalyzed (3 mol %) diacetoxylation of styrene with or without an
equivalent amount of Bu4NOAc: 1 mol of added base essentially
quenches 1 mol of Pd catalyst.
species in the diacetoxylation of alkenes with iodobenzene
diacetate. Therefore, the generality of the catalytic capability of
protons in this type of reaction had to be established and
compared to previous observations made in the apparent metal
catalysis.14 As shown in Table 4, both copper triflate and triflic
acid itself were efficient, the latter possessing only trace amounts
of transition metals (<1 ppm of Co, Cu, and Ni, <5 ppm of Pd).
The less reactive substrates required acetic acid as solvent and
elevated reaction temperatures (entries 3-9). For the substrates
in entries 4 and 5 possessing longer chains between the carboxyl
and acetyloxy groups, both cyclic and acyclic products were
observed by 1H NMR. To avoid this problem, water was added to
the reaction mixture, which was subsequently worked up by
treatment with acetic anhydride to obtain clean diacetoxylations
of the terminal alkenes. By tuning the reaction conditions with or
without addition of water, both the cyclic ethers and the acyclic
diacetoxylation products were obtained, respectively (entries 6
and 7). The cyclizations involving carboxyl groups, giving
lactones, proceeded more cleanly compared to those of hydroxyl
groups, yielding cyclic ethers that may be due to the elimination
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Table 4. Intramolecular Diacetoxylations of Alkenes with Iodobenzene Diacetatea
a Reaction conditions: 1 (0.65 mmol); 2 (0.50 mmol); solvent (1 mL). b Isolated yield. c Reaction product 3a is formed by a rearrangement step following
the initial diacetoxylation; see ref 19. d With 3 equiv of H2O; after reaction, if treated with acetic anhydride, OH was transformed to OAc. e Using 1.0
mmol of 2 and 0.2 mL of AcOH.
catalysis by a palladium triflato complex14a as well as copper
triflate.14b The intramolecular dioxygenation of 2e with triflic acid
(entry 7, Table 4) gave the reaction product in 72% yield
compared to 78% and 80% yield reported in the literature using
phosphine palladium triflate and copper triflate as catalysts,
respectively.14a,b Furthermore, the dioxygenation of 2f catalyzed
by triflic acid led to almost the same yield of the reaction product
as the Pd(dppp)(H2O)2(OTf)2 catalyst reported by Dong et al.14a
For the intermolecular diacetoxylation reaction, very similar
results were obtained under the same reaction conditions. For
example, the diacetoxylation of styrene catalyzed by Pd(dppp)-
(H2O)2(OTf)2, copper triflate, and triflic acid (entry 9, table 5)
at room temperature gave the corresponding reaction products
in 90%, 80% (at 40 °C), and 91% yields, respectively. The
diacetoxylations of 4b, 4i, and 4j catalyzed by triflic acid
(Table 5) gave the same or higher yields and diastereoselectiv-
ities (same major diastereomers) as those catalyzed by the
palladium and copper triflate catalysts.14a,b In conclusion, in both
the intra- and intermolecular diacetoxylations, triflic acid always
gave comparable yields and diastereoselectivities to those ob-
tained with the metal triflato complexes. Assuming a general
catalytic principal for this transformation, this further supports
the aforementioned hypothesis that the diacetoxylation of
alkenes with iodobenzene diacetate is a proton-catalyzed
reaction.
Kinetic Study of the HOTf-Catalyzed Diacetoxylation of
Styrene with Iodobenzene Diacetate. The diacetoxylation of
styrene with iodobenzene diacetate may be conveniently fol-
lowed by 1H NMR which was employed in a kinetic study using
added 2-bromo-1,3-xylene as internal standard. This established
a first-order dependence of the reaction rate on both the
concentration of HOTf (Figure 4) and the oxidant PhI(OAc)2
(Figure5).
Remarkably, zeroth order dependence on the reaction rate was
established for the substrate styrene (Figure 6), indicating a
mechanism in which the rate-determining step precedes the
interaction of the oxidant with the olefin.
In Situ Mass Spectra and Discussion of a Potential Reac-
tion Mechanism. In situ high resolution ESI mass spectra
recorded during the diacetoxylation of styrene and R-methyl-
styrene are depicted in Figure 7A and 7B, respectively.
Samples were taken 1-5 min after mixing of the reagents
and diluted with CH3CN prior to the injection into the mass
spectrometer.
First of all, we note the presence of peaks at m/z = 262.95635,
344.95942, and 260.93336, which are assigned to the ions
[PhI(OAc)]þ, [PhI(OAc)2 - Na]þ, and [PhI(OAc)2 - K]þ,
respectively. All three species are derived from the oxidizing
agent alone and were also found previously in ESI mass spectra of
PhI(OAc)2.20 Notably, an additional mass peak in Figure 7A at
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Table 5. Intermolecular Diacetoxylations of Alkenes with Iodobenzene Diacetate a
a Reaction conditions: 1 (0.65 mmol); 4 (0.50 mmol); AcOH (1 mL). b Isolated yield. c (R)-BINAP-Pd(H2O)2(OTf)2 was used. d With 3 equiv of H2O;
after reaction, if treated with acetic anhydride, OH was transformed to OAc.
m/z = 306.99782 and the corresponding peak in Figure 7B at
m/z = 321.01347 are assigned to [PhCCH2(IPh)]þ and [Ph-
(Me)CCH2(IPh)]þ, respectively, which are species generated by
the interaction of the oxidant with styrene and R-methylstyrene.
They are thought to be derived from one of the key intermediates
in the catalytic cycle for the proton-catalyzed diacetoxylation of
alkenes.
The proposed mechanism of the catalytic cycle for the
diacetoxylation of styrene (as example of alkene substrates in
general) is represented in Scheme 2. Upon protonation of
iodobenzene diacetate, one molecule of AcOH is eliminated,
giving rise to the cationic intermediate A. The possibility of the
protolytic formation of A has been previously demonstrated and
is supported by a series of X-ray crystal structure determinations
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Figure 4. Dependence of the initial rate on TfOH: (A) Plots of [styrene] vs time using TfOH; (B) initial rate vs TfOH (mol %). Initial rates were
calculated as the slopes of time zero using Originlab software. Reaction conditions (initial): [styrene] = 0.50 M (0.25 mmol), [PhI(OAc)2] = 0.7 M,
TfOH (1-7 mol %), AcOH (5.0 M), CD2Cl2, RT, recorded on 600 MHz 1H NMR.
Figure 5. Dependence of initial rate on initial [PhI(OAc)2]: (A) Plots of [styrene] vs time and (B) initial rate vs initial [PhI(OAc)2]. Initial rates were
calculated as the slopes of time zero using Originlab software. Reaction conditions: [styrene] = 0.50 M, [PhI(OAc)2] = 0.6-1.0 M, [TfOH] = 25 mM,
AcOH (5.0 M), CD2Cl2, RT, recorded on 600 MHz 1H NMR.
Figure 6. Dependence of initial rate on initial [styrene]: (A) Plots of [styrene] vs time, (B) initial rate vs initial [styrene]. Initial rates were calculated as
the slopes of time zero using Originlab software. Reaction conditions: [styrene] = 0.30-0.70 M, [PhI(OAc)2] = 0.7 M, [TfOH] = 25 mM, AcOH (5.0
M), CD2Cl2, RT, recorded on 600 MHz 1H NMR.
of such species reported by Ochiai and co-workers.21 The cation
A was also observed in the HRMS-ESI study discussed above and
was previously found in the HRMS-ESI mass spectra of PhI-
(OAc)2.20 Our kinetic study established first-order dependence
of the reaction rate on the concentration of added triflic acid as
well as oxidant but zeroth order dependence on the concentra-
tion of the alkene. This suggests that the formation of A is the
rate-determining step in the catalytic cycle and that the cationic
iodoso reagent is rapidly trapped by the alkene (in this case
styrene) to form B. As indicated in the previous section, the
formation of B is supported by the observation of the peak
m/z = 306.99782 and the corresponding peak at m/z =
321.01347 in the in situ recorded HR mass spectra from the
diacetoxylation of styrene and R-methylstyrene. These mass
peaks are assigned to [PhCCH2(IPh)]þ and [Ph(Me)CCH2-
(IPh)]þ, respectively, which are probably formed by elimination
of AcOH from B under the conditions of the HRMS experi-
ments.
In the following step, intermediate B could be attacked by
acetate either from acetic acid attacking externally (pathway b)
or, in an intramolecular rearrangement, from OAc in B (pathway a),
and both intermediates could form the cyclic intermediate E,
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Figure 7. HRMS-ESI experiments for reaction solutions. Reaction conditions: styrene or R-methylstyrene (0.125 mmol), PhI(OAc)2 (0.25 mmol),
TfOH (1 M in AcOH, 50 μL), AcOH (0.45 mL), RT, 1-5 min, diluted with CH3CN before HRMS-ESI tests.
absence of any catalytic activity in the presence of proton-
capturing agents. Finally, the presence of chlorido or other
potentially coordinating ligands, in turn, will suppress the
formation of A due to recombination with the cation and thus
deactivation of the actual oxidizing agent.
Scheme 2. Proposed Mechanism for the TfOH-Catalyzed
Intermolecular Diacetoxylation of Styrene Based on the
Evidence Presented in This Work
’ CONCLUSION
In this work, we have provided evidence for the protiocatalytic
nature of the diacetoxylation of alkenes using PhI(OAc)2 as
oxidant. Metal cations, such as Pd(II) and Cu(II), may interact
with the oxidant in the initiation phase of the catalytic transfor-
mation; however, 1 equiv of strong acid is produced in the first
cycle which then functions as the active catalyst. A mechanistic
cycle for the protiocatalytic reaction, which is consistent with all
experimental data presented in this work, has been proposed.
Based on this discovery of proton catalysis for the reaction at
hand, we developed the intra- and intermolecular triflic acid-
catalyzed dioxygenation for a range of alkene substrates.
On the basis of our observations, particular care has to be
taken in the interpretation of metal catalysis involving Ph(OAc)2
as a substrate. It is clear that Pd-catalyzed reactions6 performed
under basic conditions are not explicable by a simple protolytic
scheme as put forward in this work.
which had also been proposed by Dong et al. in the published
catalytic cycle based on a PdII/PdIV catalytic system.14a Finally,
the cyclic cation is attacked by AcOH to afford the diacetoxyla-
tion product along with with the liberation of a proton which
then acts as catalyst in the next cycle.
What remains to be addressed is the role played by the metal
dications/metal complexes which catalyze the diacetoxylation
of alkenes. The sigmoidal behavior of the conversion curves and
the observation of a point of inflection provide an indication
that the catalytically active species is being generated during an
initiation step. This may be the formation of the cationic key
intermediate A, the generation of which is initially catalyzed by
the Lewis acidic metal dications. In the following reaction steps,
one excess proton is generated provided that the metal salt or
metal complex does not contain a basic counterion such as
acetate. The excess proton(s) then act as (more active) catalyst
in the subsequent reaction cycle, thus making the proton the
effective catalyst also in these cases as demonstrated by the
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details and char-
b
acterization data. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
lutz.gade@uni-hd.de
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’ ACKNOWLEDGMENT
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S.; Guimond, N.; Lecavallier, M.; Fagnou, K. J. Am. Chem. Soc. 2009
131, 3291.
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We are grateful to Dr. J€urgen H. Gross (Univ. of Heidelberg)
for help concerning the in situ recorded mass spectra. Y.-B. Kang
works at CaRLa of the University of Heidelberg, being cofi-
nanced by the University of Heidelberg, the state of Baden-
W€urttemberg, and BASF SE. Support from these institutions is
greatly acknowledged.
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