Stereoselective intermolecular allylic C-H trifluoroacetoxylation of functionalized alkenes

Article abstract of DOI:10.1021/ja302457p

Pd-catalyzed allylic C-H trifluoroacetoxylation of substituted alkenes was performed using PhI(OCOCF₃)₂ as the oxidant and acyloxy source. Trifluoroacetoxylation of monosubstituted cyclopentenes and cyclohexenes proceeds with excellent regio- and diastereoselectivity. Studies with one of the possible (ν³-allyl)Pd(II) intermediates suggest that the reaction proceeds via stereoselective formation of Pd(IV) intermediates and subsequent stereo- and regioselective reductive elimination of the product.

Full text of DOI:10.1021/ja302457p

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Communication
Stereoselective Intermolecular Allylic C-H
Trifluoroacetoxylation of Functionalized Alkenes
Rauful Alam, Lukasz T. Pilarski, Elias Pershagen, and Kalman J. Szabo
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja302457p • Publication Date (Web): 15 May 2012
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Rauful Alam, Lukasz T. Pilarski, Elias Pershagen and Kálmán J. Szabó*#
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Department of Organic Chemistry, Stockholm University, SE-106 91 Stockholm, Sweden
kalman@organ.su.se
toxylation we envisaged that reductive elimination of the tri-
fluoroacetate from electron deficient Pd(III) or Pd(IV) could be
realized without using BQ or other activator ligands.
ABSTRACT. Palladium-catalyzed allylic C-H trifluoroacetoxyla-
tion of substituted alkenes was performed using PhI(OCOCF3)2
(1a) as oxidant and acyloxy source. Trifluoroacetoxylation of
monosubstituted cyclopentenes and cyclohexenes proceeds with
excellent regio- and diastereoselectivity. Studies with one of the
possible ( 3-allyl)palladium(II) intermediates suggest that the
reaction proceeds via stereoselective formation of Pd(IV) inter-
mediates and subsequent stereo- and regioselective reductive
elimination of the product.
Scheme 1. Basic strategies for selective allylic C-H acyloxyla-
tion of alkenes
Palladium-catalyzed allylic C-H acyloxylation reactions repre-
sent one of the first and most widespread applications in C-H
functionalizations.1-6 This research field was established in the
1990s by the work of Tsuji,7 Åkermark,8-9 Bäckvall,10-11 McMur-
ry12 and Kocovsky.12 In these early studies useful protocols and
mechanistic studies were described for the conversion of simple,
often symmetrical alkenes to allylic acetates using palladium cata-
lysts in the presence benzoquinone (BQ) as a crucial additive.
More recently, efforts have focussed on the development of C-H
acyloxylations for the functionalization of more complex organic
molecules, as exemplified in the work of White,13-18 Stahl,3,5-6,19
Muzart,20 Kaneda,21 Stambuli,22-23 Bercaw24 and ourselves.25-26
This pursuit raises a number of challenges, including two key
aspects of selectivity: i) in the C-H activation step, when several
non-equivalent allylic C-H bonds are available; and ii) in the nu-
cleophilic attack of the acyloxy group, when non-equivalent car-
bons can be functionalized, such as in ( 3-allyl)palladium species.
If a high level of selectivity control cannot be achieved for both of
these two steps, an intractable mixture of product regio- and ste-
reoisomers may be formed. One major strategy is to use ligand
control to govern the selectivity. For example, White and co-
workers15-18 employed serial ligand catalysis (SLC) based on the
application of sulfoxide and benzoquinone (BQ) ligands for high-
ly selective C-H acyloxylation reactions. Many other creative
ideas to replace BQ with alternative ligand/oxidant combinations,
and thereby increase reactivity and selectivity, have been report-
ed.19-20,22,24 Although many basic synthetic problems have been
solved by the recent advances in catalytic C-H acyloxylation,
notorious selectivity and reactivity issues persist. In this commu-
nication we focus on two important questions a) realization of
allylic C-H trifluoroacetoxylation reactions, which have never
been reported, and are probably difficult to achieve through tradi-
tional Pd(0)/Pd(II) manifolds; and b) achievement of highly stere-
oselective intermolecular C-H acyloxylation of functionalized
cyclic alkenes. Herein, we describe meeting these challenges us-
ing an approach based on the application of hypervalent iodines,
which are able to oxidize the palladium catalyst to the Pd(III) or
Pd(IV) oxidation state.27-33 When Pd(II) is the highest possible
oxidation state in the C-H functionalization, activator ligands
(such as BQ) are required to create electron deficiency on palladi-
um, which is necessary to induce reductive elimination (e.g.
Scheme 1a).3,10-11,34-35 Based on our previous work25 and recent
studies by Hou and co-workers36 on oxidative C-H trifluoroace-
As mentioned above, White and co-workers have presented a
number of spectacular applications of stereoselective cyclization
via allylic C-H acyloxylation reactions.15,17 There are, however,
very few studies demonstrating diastereoselective intermolecular
allylic C-H acyloxylations.8 We predicted that using highly oxi-
dized species should enable allylic C-H functionalization at reac-
tion temperatures low enough to permit high diastereoselectivity.
We,25 and subsequently the Stambuli group,23 have shown that
PhI(OAc)2 (1b) can be used as an oxidant in C-H acyloxylation
reactions instead of BQ or other activator ligands (Scheme 1b).
We have now found that even stronger oxidants, such as
PhI(OCOCF3)2 (1a), can also be used, further extending the syn-
thetic scope of the palladium-catalyzed C-H acyloxylation to the
synthesis of allylic trifluoroacetates from alkenes. Moreover, in
line with our prediction, this makes possible the diastereoselective
C-H acyloxylation of functionalized cyclic alkenes (Schemes 1c
and 2). Thus, alkene substrates (2) were reacted (Scheme 2) with
PhI(OCOCF3)2 1a (also called PIFA) in the presence of catalytic
amounts (5 mol%) of Pd(OAc)2 in dimethyl carbonate (DMC) as
solvent, affording allylic trifluoroacetates in good to excellent
yields (Table 1). Our choice of DMC as solvent is based on its
high polarity and stability to 1a (which can oxidize possible alter-
natives, like THF and DMSO). Allylic trifluoroacetates are more
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reactive substrates in Pd(0) catalyzed allylic substitution than
in the literature. For example, Åkermark and co-workers8 reported
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allylic acetates.1-2,37-39 As Pd(0) does not form under the strongly
oxidative conditions, product degradation via oxidative addition
can be avoided. Both terminal (2a, 2d-e) and internal (2b-c) al-
kenes react with high regioselectivity using 1a as oxidant and
source of the nucleophile, comparably with previously published
reactions using 1b.23,25 In the case of the acyclic substrates 2a-e,
the reaction proceeds faster in the presence of LiOCOCF3. The
reactions are conducted typically at 40 °C in the presence of car-
boxylate and sulfonate substituents (entries 1-3 and 5), while
phenyl ketone 2d is more reactive and a full conversion could be
achieved at 25 °C (entry 4). Even benzylic trifluoroacetoxylation
could be performed with 2f, however the reaction required much
harsher conditions than for its allylic counterparts. It was shown28
that 2f can smoothly acetoxylated by 1b.
the palladium-catalyzed acetoxylation of 2k using BQ (Scheme 1a
type reaction) at 50°C, resulting in five regio- and stereoisomers
with the main regioisomers being formed in nearly a 1:1 ratio.
Our new system permits considerably lower operating
temperatures, granting access to much improved selectivity.
Table 1. Palladium-catalyzed C-H trifluoroacetoxylation by
PhI(OCOCF3) 1aa
Scheme 2. Scope and reaction conditions
The most interesting results were observed for C-H
trifluoroacetoxylation of cyclic substrates 2g-m. The cyclic
alkenes reacted much faster than the acyclic analogs. Thus, the
reaction temperature for monosubstituted substrates 2g, 2i-m
could be lowered to 0 °C. In fact, it is very unusual that C-H
functionalization reactions can be performed under such mild
conditions. As the melting point of dimethylcarbonate is above 0
°C, we conducted these reactions in a mixture of DMC/CH2Cl2
(10:1). Disubstitution at the same carbon (2h) leads drop in
reactivity (c.f. reaction temperatures in entries 7 and 8) probably
due to steric effects on the C-H activation step. To our delight, the
triflouroacetoxylation reactions proceeded with a very high regio
and diastereoselectivity. Considering the fact that in substrates 2g
and 2i-m four non-equivalent C-H bonds can be functionalized
and there is a possiblity for allylic rearrangement, at least six
different regio- and stereoisomers could be expected. Hence the
reaction proceeds with a high stereochemistry and the major
product is a 1,4-regioisomer (type I, Scheme 2), while the 1,2-
regioisomer (type II) is formed as a minor product. An exception
is the trifluoroacetoxylation of 2g which gives only a single regio-
and stereoisomer 3g (entry 7). The regioselectivity (see the I/II
ratio in Table 1) depends on the ring size and on the ring
substituents. The allylic C-H trifluoroacetoxylation is more
selective for cyclopentene than for cyclohexene derivatives (c.f
entries 7 with 11-12). In the presence of the ester functionality
significantly higher regioselectivity is obtained than with the
amide functionality (c.f. entries 7 and 10). A drop of the
regioselectivity of C-H allylation reactions in the presence of
amide functionality was also reported by Stambuli and co-
workers.22 Interestingly, we could also observe some difference in
regioselectivity between the methyl (2k) and benzyl (2l) ester
functionalities (c.f. entries 11 and 12). The high regio- and
stereoselectivity for the intermolecular C-H functionalization of
monosubstituted cyclopentenes and cyclohexenes is a very
important synthetic advantage of the above method (Scheme 2).
To best of our knowledge, very few, if any, examples are reported
aSubstrate 2 (0.3 mmol) was dissolved in dimethyl carbonate (DMC),
PIFA 1a (0.45 mmol) and Pd(OAc)2 (0.015 mmol, 5 mol %) were then
added and the mixture stirred using the temperatures and times indicated.
For reactions conducted at 0 °C a DMC/CH2Cl2 10:1 mixture was used as
b
solvent. Reaction conditions: temperature / time = °C / h. c Ratio of the
obtained regioisomers (see Scheme 2). d Isolated yield (%). e LiOCOCF3
(0.6 mmol) was used as an additive. f (CF3CO)2O (3.0 mmol) was used as
an additive. g NMR yield.
2
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Surprisingly, on replacing 1a with 1b in reactions with cyclic
substrates (e.g. 2g or 2l) no reaction occurred at 0 °C or elevated
Figure 1. Proposed catalytic cycle for the allylic C-H trifluoro-
acetoxylation reaction
1
temperatures (up to 35 °C). Thus, it seems likely that the highly
selective acyloxylation of monosubstituted cyclic alkenes can only
be performed with trifluoroacetate reagent 1a. Moreover, we
found that Pd(OAc)2 and Pd(OCOCF3)2 showed similar activity as
catalysts, but that only trifluoroacetoxylated products were ob-
served with Pd(OAc)2. No reaction occurred in the absence of Pd
catalyst.
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Scheme 3. Synthesis and reactivity of the presumed (
allyl)palladium(II) intermediate of the reaction
-
Based on the above observation (Scheme 3) and results of the
catalytic reactions described herein, we proposed a catalytic cycle,
as exemplified by the 2m → 3m transformation (entry 13) in Fig-
ure 1. In the first instance, Pd(OAc)2 is directly oxidized by 1a to
give complex 5. The observed acceleration (entries 1-6) of the
reaction by addition of LiOCOCF3 may be explained by formation
of anionic Pd species, which is easy to oxidize. Muzart and co-
workers20 reported that Pd(OAc)2 may form such complexes in
the presence of lithium salts. We also suppose that coordination
of DMC (e.g. L = DMC) may stabilize the formed Pd(IV) species
5. Coordination of 2m is followed by stereoselective C-H activa-
tion of Ha (red colored) in complex 6. A possible mechanism is
The stereochemistry of the reactions (entries 7-13) and our previ-
ous studies25 with 1b (Scheme 1b) suggest that the reaction prob-
ably proceeds via ( 3-allyl)palladium species. However, an inter-
esting mechanistic question is whether the reaction starts with the
formation of an ( 3-allyl)palladium(II) species, which is subse-
quently oxidized to ( 3-allyl)palladium(IV), or whether oxidation
3
of the Pd(II) catalyst precedes formation of an
(
-
allyl)palladium(IV) complex (i.e. without prior formation of the
corresponding ( 3-allyl)palladium(II) species). It is well estab-
lished that ( 3-allyl)palladium(II) complexes can be formed from
alkenes and Pd(II) precursors,40-41 and therefore we studied the
possible formation of such complexes under catalytic conditions.
Complex 4a was prepared from 3m and Pd2(dba)3 in benzene
followed by treatment of the resulting complex with LiCl (Scheme
3). Kurosawa and co-workers have shown that allyl chlorides
undergo stereoselective cis oxidative addition with Pd2(dba)3
when non-coordinating solvents, such as benzene, are used.42-43
We also obtained a single stereoisomeric complex 4a from prod-
uct 3m with Pd2(dba)3 in benzene at room temperature. Chloro
complex 4a was stable enough for purification by silica-gel chro-
matography, and it was obtained in 63% yield. After purification,
ligand exchange was performed by halogen abstraction using
AgOCOCF3 to give complex 4b, which is a conceivable reaction
intermediate in the catalytic C-H trifluoroacetoxylation of 2m
(entry 13). Notably, complex 4b was very stable at ambient condi-
tions and we did not observe reductive elimination to 3m. We
hypothesized that 4b could be oxidized and subsequently undergo
reductive elimination to form 3m on addition of PhI(OCOCF3)2
1a. Therefore, 4b was reacted with 1a for 1h at 0 °C in DMC. In
this process 4b was completely consumed resulting in a complex
mixture, in which only traces of 3m was observed. However, the
catalytic reaction proceeds very cleanly at 0°C, therefore we con-
clude that 4b is not a true reaction intermediate in the catalytic C-
H trifluoroacetoxylation of 2m. As mentioned in the introduction
BQ (or other activator ligand) is necessary to induce reductive
deprotonation44-45 by an acetate (X = OAc) or trifluoroacetate
3
ligand. The driving force of the formation of
(
-
allyl)palladium(IV) complex 7 is the relief of the electron defi-
ciency at the Pd(IV) center by -donation from the allyl moiety.
Formation of allylpalladium complexes from alkenes with a cis
double bond, such as in cyclic substrates 2g-m, might be easier
than the same process from an acyclic alkenes 2a-e. This would
explain the above finding that the cyclic substrates are more reac-
tive than the acyclic ones. The palladium atom in 7 is strongly
electron deficient, which allows a rapid reductive elimination
involving the trifluoroacetate ligand. Thus, an activator ligand
(such as BQ, Scheme 1a) is not required to effect reductive elimi-
nation, as the high oxidation state of palladium itself is suffi-
cient.The regioselectivity is probably governed by the hypercon-
jugative interaction between the polar C-C(OPh) -bond and the
-allyl system.46-49 Thus the C-H activation (6 → 7) is stereose-
lective and the reductive elimination (7 → 3m) is both stereo- and
regioselective to impart an exceptionally high selectivity to the
entire allylic C-H functionalization process. The cis counterpart of
3m might have formed by external attack of the trifluoroacetate
on 7.34 However, the lack of cis products shows that the trifluoro-
acetate coordinated to palladium attacks the allyl moiety and not
an external one. An alternative explanation of the trans selectivity
is that the (blue colored) C-He bond of 6 is activated by palladium
to form the cis isomer of 7, which subsequently undergoes exter-
nal attack by a trifluoroacetate ion affording 3m. Of course, the
formation of bimetallic Pd(III)-Pd(III) instead of Pd(IV) species is
also plausible, as with aryl-palladium analogs of 7.30,50 Since the
metal centers of Pd(III) complexes are also electron deficient,
similar reductive elimination processes may operate, as for the 7
elimination (Scheme 1a) in the classical Pd(0)/Pd(II) based pro-
3
cess to generate the allylic product.3,10-11,34-35 As expected for (
-
allyl)palladium(II) complexes,34 4b in the presence of BQ under-
goes a smooth reductive elimination affording 3m.
3
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→ 3m process described above. A further possibility is that the
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reaction proceeds via stereoselective formation of Pd(II) complex
4b followed by oxidation to 7. However, our findings do not sup-
port this route, and it is known that ( 3-allyl)palladium(II) triflu-
roacetate complexes under C-H activation conditions are usually
unstable.41,51 A further mechanistic alternative might be that
Pd(II) serves as a Lewis-acid to activate 1a and also coordinate to
the carbonyl or ester functionality of the substrates. However, this
possibility seems unlikely. For example, the reaction of various
Lewis acids (such as AlCl3,TiCl4 and BF3) with 1a and 2g did not
give even traces of 3g, which was formed cleanly under the same
reaction conditions (entry 7) using Pd(OAc)2.
In summary, we have demonstrated the first catalytic allylic C-H
trifluoroacetoxylation reaction, which is achieved using palladium
catalysis in the presence of PhI(OCOCF3)2. The reaction proceeds
with remarkably high regio- and stereoselectivity and is syntheti-
cally useful for the generation of stereodefined cycloalkenes from
monosubstituted precursors. As allylic trifluoroacetates are more
reactive substrates in allylic substitutions than allylic acetates, we
envisage our method will prove a selective route to the generation
of valuable synthons. Development of the asymmetric version of
the reaction (with substrates 2b-c, 2g-j) and study of the possibili-
ties for chirality transfer (with optically pure form of 2k-m) are
ongoing projects in our laboratory.
Acknowledgement. The authors thank the financial support of the
Swedish Research Council (VR) and the Knut och Alice
Wallenbergs Foundation via the Center of Molecular Catalysis at
Stockholm University (CMCSU). A postdoctoral stipend by the
Carl Tryggers Stiftelse for L. T. P. is gratefully acknowledged.
The authors also thanks Robert Pendrill for help with some NMR
experiments.
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Supporting Information Available: Detailed experimental
procedures and compound characterization data is given. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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Table of contents:
Stereoselective Intermolecular Allylic C-H Trifluoroace-
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toxylation of Functionalized Alkenes
Rauful Alam, Lukasz T. Pilarski, Elias Pershagen and
Kálmán J. Szabó*
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