Redox reaction of the Pd0 complex bearing the Trost ligand with meso-cycloalkene-1,4-biscarbonates leading to a diamidato PdII complex and 1,3-cycloalkadienes: Enantioselective desymmetrization versus catalyst deactivation

Article abstract of DOI:10.1002/chem.200902739

The Pd⁰ complex 1 that bears the Trost ligand 2 undergoes a facile redox reaction with 1,4-biscarbonates 5b-d and rac-22 under formation of the diamidato-Pd^II complex 7 and the corresponding 1,3-cycloalkadienes 8b-d. The redox deacti

Full text of DOI:10.1002/chem.200902739

DOI: 10.1002/chem.200902739
Redox Reaction of the Pd0 Complex Bearing the Trost Ligand with meso-
Cycloalkene-1,4-biscarbonates Leading to a Diamidato PdII Complex and 1,3-
Cycloalkadienes: Enantioselective Desymmetrization Versus Catalyst
Deactivation
Vasily N. Tsarev, Dennis Wolters, and Hans-Joachim Gais*[a]
Abstract: The Pd0 complex 1 that bears
the Trost ligand 2 undergoes a facile
redox reaction with 1,4-biscarbonates
5b–d and rac-22 under formation of
the diamidato–PdII complex 7 and the
corresponding 1,3-cycloalkadienes 8b–
d. The redox deactivation of complex 1
was the dominating pathway in the re-
12b (96% ee, 92%). Formation of car-
bonate 12b involves two consecutive
inter- and intramolecular substitution
reactions of the p-allyl–PdII complexes
16b and 18b, respectively, with O-nu-
cleophiles and presumably proceeds
through the hydrogen carbonate 17b as
key intermediate. The intermediate for-
mation of 17b is also indicated by the
conversion of alcohol rac-6b to carbon-
tioselective synthesis of diol 13b. The
reaction of the seven-membered biscar-
À
bonate 5c with ent-1 and HCO3 af-
forded carbonate ent-12c (99% ee,
39%). The Pd0 complex 1 is stable in
solution and suffers no intramolecular
redox reaction with formation of com-
plex
7 and dihydrogen as recently
À
action of 5b–d with HCO3 at room
claimed for the similar Pd0 complex 9.
Instead, complex 1 is rapidly oxidized
by dioxygen to give the stable PdII
complex 7. Thus, formation of the PdII
complex 10 from 9 was most likely due
to an oxidation by dioxygen. Oxidative
workup (air) of the reaction mixture
stemming from the desymmetrization
of 5c catalyzed by 1 gave the PdII com-
plex 7 in high yield besides carbonate
12c.
temperature. However, at 08C the six-
membered biscarbonate 5b, catalytic
À
ate 12b upon treatment with HCO3
À
amounts of complex 1, and HCO3
and 1. The Pd0-catalyzed desymmetri-
zation of 5b with formation of 12b and
its hydrolysis allow an efficient enan-
mainly reacted in an allylic alkylation,
which led to a highly selective desym-
metrization of the substrate and gave
alcohol 6b with ꢀ99% ee in 66%
yield. An increase of the catalyst load-
ing in the reaction of 5b with 1 and
Keywords: alkylation · oxidation ·
palladium · redox chemistry · Trost
ligand
À
HCO3 afforded the bicyclic carbonate
Introduction
allylic sulfur derivatives.[10–18] Recently, we have described
an enantioselective synthesis of allylic alcohols through the
The Pd0 complex 1 that bears the Trost ligand 2[1–5]
(Scheme 1) is one of the most versatile catalysts for enantio-
selective allylic alkylation.[6–9] It is applicable to a broad
range of cyclic and acyclic allylic substrates and has found
extensive use in organic synthesis. Recently, a structure-
based rationale had been advanced for the selectivity in the
asymmetric allylic alkylation catalyzed by 1.[5] We had suc-
cessfully applied catalyst 1 in the kinetic resolution of race-
mic allylic carbonates[10–15] and enantioselective synthesis of
[a] Dr. V. N. Tsarev, Dipl.-Chem. D. Wolters, Prof. Dr. H.-J. Gais
Institute of Organic Chemistry, RWTH Aachen University
Landoltweg 1, 52074 Aachen (Germany)
Fax : (+49)241-8094710
Scheme 1. Deracemization of the allylic carbonate rac-3 with the Pd0
complex 1 (dba=dibenzylideneacetone) and HCO3 in CH2Cl2 and H2O.
À
E-mail: gais@rwth-aachen.de
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FULL PAPER
Pd0-catalyzed deracemization of racemic allylic carbonates
with 1 in CH2Cl2/H2O[19,20] as exemplified by the synthesis of
alcohol 4 from carbonate rac-3.[20] The deracemization en-
tails an allylic alkylation of the hydrogen carbonate ion
In the course of this work a report had appeared claiming
that the analogous Pd0 complex 9 is unstable and undergoes
a facile intramolecular redox reaction with formation of the
diamidato–bis(phosphanyl)–PdII complex 10 and dihydrogen
(Scheme 3).[28] Because of the considerable synthetic bearing
it would have, should complex 1 also suffer such a redox re-
action, its stability was studied. We demonstrate that com-
plex 1 is stable under the conditions used and undergoes no
intramolecular redox reaction but instead is rapidly oxidized
by dioxygen to the PdII complex 7.[26,29]
À
(HCO3 ),[19–21] which is formed through the ionization of the
À
substrate from MeCO3 and H2O and may also be supplied
from an external source. Application of this method to the
enantioselective desymmetrization of the meso-configured
biscarbonate 5a afforded the allylic alcohol 6a (Scheme 2),
which served as starting material for the synthesis of a key
prostaglandin building block.[20]
Scheme 2. Pd0-catalyzed desymmetrization of meso-biscarbonate 5a with
À
complex 1 and HCO3
.
The efficient synthesis of 6a from 5a prompted us to
study the potential of this desymmetrization for the enantio-
selective synthesis of the cyclic allylic alcohols 6b–d from
the corresponding meso-configured allylic carbonates 5b–d.
Alcohols 6b–d, which had been previously obtained through
an enzyme-catalyzed desymmetrization, are, like 6a, also
useful chiral building blocks for the synthesis of natural
products.[22–25]
Scheme 3. Alleged intramolecular redox reaction of the Pd0 complex 9.[28]
Despite the many applications 1 has found in enantiose-
lective synthesis, a recycling of the complex has not been de-
scribed. It is shown that an oxidative workup in allylic alky-
lation with dioxygen allows a high yield recovery of Pd and
ligand 2 as complex 7.
Results and Discussion
Synthesis of the biscarbonates: The meso-configured biscar-
bonates 5b–d were synthesized in good yield from the corre-
sponding diols 11b–d upon treatment of the latter with
nBuLi and the subsequent quenching of the corresponding
dilithium salts with methyl chloroformiate (Scheme 4).
Pd0-catalyzed desymmetrization: To our surprise, formation
of alcohol 6b could not be observed upon treatment of bis-
In this paper we describe a new and unexpected redox re-
action between the Pd0 complex 1 and the allylic biscarbon-
ates 5b–d that leads to the diamidato–bis(phosphanyl)–PdII
complex 7[26–29] and the corresponding 1,3-dienes 8b–d. It is
furthermore shown that in the case of the allylic biscarbon-
ates 5b and 5c the redox reaction can be sufficiently sup-
pressed and a highly enantioselective desymmetrization ach-
ieved.
Scheme 4. Synthesis of the cyclic meso-biscarbonates 5b–d.
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H.-J. Gais et al.
Table 1. Pd-catalyzed desymmetrization of biscarbonate 5b with complex
1 at 08C.
carbonate 5b with complex 1 under the standard conditions.
A mixture of 5b, [Pd2A(dba)3]·CHCl3 (2 mol%; dba=diben-
zylideneacetone), ligand (4 mol%), and KHCO3 in
CH2Cl2/H2O (9:1; two phases) was rapidly stirred at room
temperature (RT) for 24 h under argon (Scheme 5), after
which time the biscarbonate was recovered in high yield.
Neither a change of the solvent to THF nor an increase of
the catalyst loading to 4 mol% resulted in the formation of
6b at room temperature.
CTHUNGTRENNUNG
Entry
1
6b
Yield [%]
12b
Yield [%]
2
[mol%]
ee [%]
ee [%]
1
2
2
4
2
2
4
4
2
4
66
21
37
12
38
–
20
30
–
ꢀ99
ꢀ99
ꢀ99
ꢀ99
ꢀ99
–
ꢀ99
ꢀ99
–
–
–
–
–
2[a]
3
40
11
10
92
–
94
94
96
96
–
4[b]
5[c]
6[c]
7[c,d]
8[e]
9[e]
11
88
90
97
[a] Reaction was run for 2 h at 08C and then for 46 h at room tempera-
ture. [b] Biscarbonate 5b was slowly added to a solution of 1 within 7 h.
[c] Addition of 1.4 equiv of KHCO3. [d] Reaction was performed at room
temperature. [e] Reaction was performed under a CO2 atmosphere.
Scheme 5. Attempted Pd0-catalyzed desymmetrization of meso-biscarbon-
ate 5b with 1 and HCO3 at room temperature.
resulted in a 85% conversion of the substrate and gave a
mixture of the bicyclic carbonate 12b and alcohol 6b in a
ratio of 1.1:1 (entry 3). The formation of a bicyclic carbon-
ate in the reaction of a meso-biscarbonate at a higher load-
ing of 1 had been previously observed in the case of the re-
action of 5a, which gave carbonate 12a as byproduct.[20]
When biscarbonate 5b was slowly added to catalyst 1 under
otherwise identical conditions, a mixture of 12b and 6b was
formed in the same ratio but in lower yield (entry 4). Reac-
tion of 5b with 1 in the presence of 1.4 equiv of KHCO3 fur-
nished a mixture of 6b and 12b in a ratio of 3.8:1 (entry 5).
An efficient synthesis of the bicyclic carbonate 12b (92%
yield, 96% ee, 100% conversion of 5b after 15 h) was ac-
complished by running the reaction of 5b with 4 mol% of 1
in the presence of 1.4 equiv of KHCO3 at 08C (entry 6). Per-
forming this reaction under the same conditions but at room
temperature gave alcohol 6b (ꢀ99% ee) in only 20% yield
and none of 12b (entry 7).
À
À
The apparent inertness of 5b towards 1 and HCO3 was
very startling because of the following reasons. The allylic
alkylation of the biscarbonate with C-nucleophiles catalyzed
by 1 has already been described,[30] and various cyclic and
acyclic racemic allylic monomethyl carbonates have been
successfully converted to the corresponding allylic alcohols
by following the standard protocol.[19,20] During an investiga-
tion of the Pd0-catalyzed desymmetrization of the five-mem-
À
bered biscarbonate 5a with 1 and HCO3 under the stan-
dard conditions, we had noticed that both the conversion of
the substrate and yield of alcohol 6a significantly increased
by running the reaction at 08C instead of room tempera-
ture.[20] Surprisingly, under otherwise identical conditions,
À
conducting the reaction of 5b with 1 and HCO3 at 08C in-
stead of room temperature for 24 h gave alcohol 6b with
ꢀ99% ee in 66% yield (Scheme 6). However, the conver-
sion of 5b was still incomplete despite the long reaction
time and the biscarbonate was recovered in 30% yield
(Table 1, entry 1).
Treatment of the seven-membered biscarbonate 5c with
À
catalyst 1 and HCO3 under the standard conditions at
room temperature saw no formation of alcohol 6c and/or
the bicyclic carbonate 12c. At a higher catalyst loading and
08C, carbonate 12c but not alcohol 6c was isolated in 23%
yield with 91% ee (vide infra). However, biscarbonate 5c
was recovered in 70% yield. A conversion of the eight-
membered biscarbonate 5d to 6d and/or 12d upon treat-
À
ment with 1 and HCO3 under the standard conditions
could not be observed.
The absolute configuration of alcohol 6b was assigned in
analogy to that of alcohol 6a.[20] The absolute configuration
of carbonates 12b and 12c was determined by hydrolysis to
the corresponding diols 13b and 13c (Scheme 7) and com-
parison of their optical rotation with those reported for ent-
13b[31–34] and ent-13c,[33–35] respectively. The ee values of al-
cohol 6b and diol 13b were determined through conversion
to the corresponding benzoates 14 and 15 and analysis by
chiral HPLC. The ee value of carbonate 12c was determined
by chiral GC.
Scheme 6. Pd0-catalyzed desymmetrization of biscarbonate 5 with com-
plex 1 and HCO3 at 08C.
À
When the reaction of 5b with 1 was conducted first at
08C for 2 h and then at room temperature for 46 h under
otherwise identical conditions, 6b was only obtained in 21%
yield with ꢀ99% ee (Table 1, entry 2). Decreasing the cata-
lyst loading to 1 mol% saw no formation of alcohol 6b. On
the other hand, increasing the catalyst loading to 4 mol%
The Pd0-catalyzed desymmetrization of biscarbonate 5b
with formation of carbonate 12b allows access to diol 13b,
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Pd0 Complex with the Trost Ligand
FULL PAPER
Scheme 7. Derivatization of the bicyclic carbonates 12b and 12c and al-
cohol 6b (4-DMAP=4-dimethylaminopyridine, Bz=benzoyl).
an efficient enantioselective synthesis of which was not
available up to now.[33–35]
Formation of alcohol 6 and carbonate 12 from 1 and bis-
carbonate 5 is rationalized as shown in Scheme 8.[20] The
enantiotopos-differentiating reaction of the meso-biscarbon-
ate with complex 1 involves, with high selectivity, the me-
thoxycarbonyloxy group at the S-configured stereogenic
center and affords the trans-configured p-allyl–PdII complex
16.[30,36,37] The methyl carbonate ion, which is formed during
À
the ionization of 5, is hydrolyzed, thus giving HCO3 and
MeOH.[19,20] The thus-formed (and eventually externally
À
supplied) HCO3 selectively attacks the p-allyl complex 16
to furnish the cis-configured hydrogen carbonate 17, which
decomposes because of the presence of water and delivers
alcohol 6 and CO2.[19,20] In a competing reaction, the hydro-
gen carbonate 17 or its anion reacts with complex 1 to yield
the p-allyl–PdII complex 18, which undergoes an intramolec-
ular substitution[21] to furnish carbonate 12 and complex 1.
Support for this mechanistic scheme, and in particular for
the intramolecular substitution of 18, comes from the asym-
metric synthesis of cyclic 1,2-carbonates through the Pd-cat-
Scheme 8. Proposed mechanism for the Pd0-catalyzed formation of alco-
hol 6 and carbonates 12 and ent-12 from biscarbonate 5.
À
alyzed deracemization of vinyl epoxides with 1 and HCO3
lytic experiments were performed with 5b, in which 1) the
À
in CH2Cl2/H2O.[21] It was proposed that the racemic vinyl ep-
oxide reacts with 1 under formation of the corresponding
alkoxy-substituted p-allyl–PdII complexes. Their reaction
with CO2 generates the corresponding oxycarbonyoxy-sub-
stituted p-allyl–PdII complexes, the intramolecular substitu-
tion of which yields the cyclic carbonate and 1. Whereas the
reaction of biscarbonate 5 with catalyst 1 involves a matched
ionization, carbonate 17 has to undergo with 1 a mismatched
ionization. It had been shown that monomeric 1 is in equi-
librium with oligomeric species (see below), which react in
mismatched ionization much faster than monomeric 1.[2–5]
Thus, oligomers of 1 are perhaps responsible for the conver-
sion of the hydrogen carbonate 17 to complex 18. To sub-
stantiate the mechanistic proposal of Scheme 8, further cata-
catalyst loading and the concentration of HCO3 were in-
creased, 2) CO2 was added, or 3) all three parameters were
altered (cf. Table 1). These experiments clearly show, in ac-
cordance with Scheme 8, that an increase of the catalyst
loading (entry 1 vs. 3, entry 5 vs. 6, and entry 8 vs. 9) and an
À
increase of the concentration of HCO3 (entry 1 vs. 5, and
entry 2 vs. 8) resulted in an increased formation of carbon-
ate 12b. An inspection of Table 1 (entry 3, without addition
of KHCO3 and entry 6, addition of KHCO3) shows that in
the reaction of 5b with 1 not only an increase of the catalyst
loading but also the pH from approximately 7 to 9 favors
the formation of the bicyclic carbonate 12b. This observa-
tion would also be in accordance with Scheme 8. At pH 9
the concentration of the deprotonated hydrogen carbonate
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H.-J. Gais et al.
17b should be higher, a feature which would facilitate the
formation of complex 18b and thus of 12b. An alternative
route to hydrogen carbonate 17 encompasses the reaction of
termediate) upon treatment of biscarbonate 5 with complex
1 and HCO3 at 08C but not at room temperature, the
À
origin of these apparently contradicting results was probed.
À
À
alcohol 6 with CO2 or HCO3 (vide infra).[38,21] Support for
The reaction of 5 with 1 and HCO3 is slow because of the
this notion comes from an independent but similar experi-
ment with the racemic alcohol rac-6b, which was obtained
from diol 11b and chloroformiate by using only 1 equiv of
nBuLi (cf. Scheme 4). Treatment of rac-6b with 8 mol% of
1 and 1.4 equiv of KHCO3 in CH2Cl2/H2O at 08C gave car-
bonate 12b with 46% ee in 88% yield (Scheme 9).
low nucleophilicity of the anion and the conversion of the
substrates was not complete. Altogether these results and
those listed in Table 1 hinted a competing deactivation of
catalyst 1; they were fast at room temperature and slow at
08C. A control experiment with the monocarbonate rac-20
and 1 was performed at room temperature under identical
conditions to check the activity of the catalyst under the
standard reaction conditions (Scheme 10). Here, alcohol 21
Scheme 9. Pd0-catalyzed synthesis of carbonate 12b from alcohol rac-6b
À
and CO2 or HCO3
.
Scheme 10. Pd0-catalyzed deracemization of the monomethyl carbonate
rac-20.
Another interesting feature of the reaction of biscarbon-
ates 5a–c with complex 1 in CH2Cl2/H2O in the presence of
HCO3 is the variation of the ee value of 12b and 12c de-
was obtained in 92% yield with 97% ee. In a further experi-
ment, biscarbonate 5b was added to a solution of 4 mol%
of catalyst 1 in CH2Cl2/H2O at room temperature, and after
an incubation time of 2 h monocarbonate rac-20 was added
to the mixture, which was then kept for 24 h at room tem-
perature. In this case, formation of alcohol 21 could not be
detected. Carbonates 5b and rac-20 were recovered and al-
cohol 6b was isolated in only 7% yield (ꢀ99% ee).
À
pending on the reaction conditions and the relative low ee
value of 12a (66%).[20] The first step of the reaction of 5
with 1 to 12, the formation of the p-allyl–PdII complex 16, is
generally highly enantioselective.[30,36,37] Since this step deter-
mines, according to the mechanistic proposal of Scheme 8,
the ee value of 12, there ought also to be a process that
leads to the formation of ent-12. A competing intramolecu-
lar substitution of 16 could occur to afford the carbenium
ion 19, the hydrolysis of which would yield the enantiomeric
carbonate ent-12. Indeed, the corresponding p-allyl–PdII
complexes that carry a tosylcarbamoyloxy instead of a me-
thoxycarbonyloxy group exclusively undergo such an intra-
molecular substitution.[39] However, these complexes are
generated from the corresponding meso-bis(tosylcarbamoy-
loxy)cycloalkenes and 1 under nonaqueous conditions in the
absence of an external nucleophile. In addition, the nucleo-
philicity of the tosylcarbamoyloxy group perhaps exceeds
that of the methoxycarbonyloxy group.
These results strongly suggested that a yet unknown reac-
tion of catalyst 1 with biscarbonate 5b but not monocarbon-
ate rac-20 caused a deactivation of the catalyst. However, in
principle a deactivation of catalyst 1 could also have oc-
curred through an oxidation with dioxygen with formation
of the PdII complex 7 (Scheme 11).[26,29] Although all reac-
tions with biscarbonate 5b had been conducted in degassed
solvents under argon, this possibility could not be rigorously
excluded. Finally, a deactivation of 1 through an intramolec-
ular redox reaction with formation of complex 7 and dihy-
drogen had to be considered since such a reaction had been
previously reported for the analogous Pd0 complex 9 that
gave the PdII complex 10.[28] To exclude these two modes of
deactivation and to see whether complex 1 also undergoes
an intramolecular redox reaction, 31P NMR spectroscopic
A recent elegant experimental and theoretical study of a
cationic h3-cyclohexenyl–PdII complex containing 2 as ligand
revealed that the amide groups are placed in close proximity
À
to the Pd atom. It was proposed that the amide N H plays
a crucial role in both directing the attack of the nucleophile
at the C atom of the allyl unit and the ionization of the
faster reacting enantiomer of the cyclohexenyl ester.[5] The
high degree and similar sense of asymmetric induction re-
corded in the reaction of 1 and nucleophiles with cyclohex-
enyl esters and the meso-configured ester 5 suggest that the
selectivities of the reaction of the meso-diesters can also be
rationalized by this model.[5]
Scheme 11. Oxidation of the Pd0 complex 1 to the PdII complex 7 with di-
Stability and oxidation of complex 1 by biscarbonates and
dioxygen: Having observed a formation of alcohol 6 (as in-
oxygen.
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Pd0 Complex with the Trost Ligand
FULL PAPER
experiments with 1 at room temperature were performed.
Solutions of 1 at 9 mm (corresponding to a catalyst loading
of 2 mol%) and 18 mm (corresponding to a catalyst loading
hydrogen carbonate ion. Our results suggest that the report-
ed formation of the analogous PdII complex 10 from the Pd0
complex 9[28] was most likely due to an oxidation with dioxy-
gen and not caused by an intramolecular redox reaction.
Because of the above results, an intramolecular redox de-
activation of complex 1 can be safely dismissed as the cause
for its deactivation in the treatment with biscarbonate 5b
of 4 mol%) were prepared by dissolving [Pd2ACTHUNTRGNE(UGN dba)3]·CHCl3
(1 equiv) and 2 (2 equiv) in degassed CD2Cl2 under argon in
an NMR spectroscopy tube, which was closed with a plastic
cap. The 31P NMR spectra of both solutions taken after
20 min of their preparation showed the two doublets of the
Pd0 complex 1 with the P,P-coordinated ligand 2 at dP =20.9
and 23.9 ppm[28,29] and two broadened signals at dP =22.0
and 27.9 ppm; these stem from oligomers of 1. In addition,
the characteristic singlet of the PdII complex 7 (dP =
26.1 ppm) with the P,P,N,N-tetradentate-coordinated depro-
tonated ligand 2 was present.[27,28] After the solution con-
taining 1 at an initial concentration of 9 mm was kept for
18 h at room temperature in the NMR spectroscopy tube, a
31P NMR spectrum only showed the presence of the PdII
complex 7 and none of 1 and its oligomers. A 31P NMR
spectrum of the solution containing 1 at an initial concentra-
tion of 18 mm still showed after 18 h the presence of 1 and
oligomers besides 7, and only after a further 44 h had
elapsed had complex 1 completely disappeared and only
complex 7 remained. The slower decomposition of 1 at
higher concentrations can perhaps be related to the higher
stability of the oligomeric catalyst. Formation of the PdII
complex 7 from the Pd0 complex 1 under these conditions
could be ascribed either to an intramolecular redox reaction
or oxidation by dioxygen that had entered the NMR spec-
troscopy tube despite its being closed under argon with a
plastic cap or to both reaction pathways. Therefore, the
NMR spectroscopic experiments were repeated by using sol-
utions of 1 in CD2Cl2 and CD2Cl2/H2O (9:1) contained in
NMR spectroscopy tubes, which had been sealed with an
acetylene/dioxygen microburner to rigorously exclude a con-
tamination by dioxygen. The solvents, ligand 2, and [Pd2-
À
and HCO3 at room temperature. However, because of the
facile oxidation of 1, the possibility of a dioxygen contami-
nation as the reason for the failure of this experiment re-
mained. Therefore, two preparative stoichiometric experi-
À
ments with biscarbonate 5b, complex 1, and HCO3 in
CH2Cl2/H2O at room temperature and 08C were run in
sealed flasks under argon by using degassed solvents and re-
agents. After a reaction time of 24 h, the flasks were opened
and the mixtures were analyzed after standard workup, pu-
rification, and derivatization. In the experiment at room
temperature, biscarbonate 5b was recovered in 90% yield.
In contrast, the experiment at 08C gave alcohol 6b in 56%
yield with 97% ee, and carbonate 5b was recovered in 40%
yield. These results unequivocally show that a yet unknown
deactivation of catalyst 1 must have occurred upon reaction
with biscarbonate 5b, which is much faster at room temper-
À
ature than the allylic alkylation of HCO3 . To determine the
À
fate of the reagents, the reaction of 5b with 1 and HCO3
was followed by NMR spectroscopy. The NMR spectroscopy
tube was successively charged with [Pd2ACTHNUGTRNE(UGN dba)3]·CHCl3, 2,
KHCO3, 5b (stoichiometric amounts), and CD2Cl2/H2O
(9:1) at room temperature under the exclusion of dioxygen.
A
31P NMR spectrum, which was taken immediately after
the NMR spectroscopy tube was sealed, showed only the
presence of the PdII complex 7 and none of complex 1. An
1H NMR spectrum revealed in addition to 7 the presence of
diene 8b in stoichiometric amounts (Scheme 12). Thus, a
new and facile stoichiometric redox reaction had occurred
between the Pd0 complex 1 and biscarbonate 5b. To see
whether the redox reaction between 5b and 1 is a general
one for cyclic allylic 1,4-biscarbonates and the Pd0 complex,
the stoichiometric reaction of the seven- and eight-mem-
bered biscarbonates 5c and 5d, respectively, and the race-
mic trans-configured biscarbonate rac-22 with 1 at room
ACHTUNGTRENNUNG(dba)3]·CHCl3 were degassed by several freeze–thaw cycles
and the NMR spectroscopy tubes were sealed at À788C
under vacuum and argon. The 31P NMR spectra of the solu-
tions of 1 only showed the presence of the monomeric com-
plex 1 and its oligomers and none of the PdII complex 7.
Even after the solutions of 1 were kept in the sealed NMR
spectroscopy tube for three months at room temperature,
formation of complex 7 could not be detected. Interestingly,
when the sealed NMR spectroscopy tubes that contained
the solution of 1 were opened under argon and closed with
a plastic cap, a 31P NMR spectrum, which was recorded im-
mediately afterwards, already showed the formation of com-
1
temperature was also followed by 31P and H NMR spectros-
copy. The redox reaction of 1 with 5c, 5d, and rac-22 was
fast in all cases and gave complex 7 together with stoichio-
metric amounts of the corresponding dienes 8c, 8d, and 8b.
Oxidation of complex 1 is confined to allylic 1,4-biscarbon-
ates. An NMR spectroscopic investigation of the deracemi-
zation of the monomethyl carbonate rac-20 with catalytic
amounts of 1 under standard conditions by using a sealed
NMR spectroscopy tube only showed the consumption of
the substrate and formation of the allylic alcohol 20. Neither
the formation of diene 8b nor that of complex 7 could be
detected. Complex 1 was still present, the oxidation of
which to 7 only started upon opening the NMR spectrosco-
py tube. The PdII complex 7 showed no activity in allylic al-
kylation. Treatment of biscarbonate 5b with 7 under the
standard conditions did not lead to a noticeable reaction.
1
plex 7. The 31P and H NMR spectra revealed after approxi-
mately 1 d only the signals of complex 7. These results un-
equivocally demonstrate that 1) the Pd0 complex 1 does,
under the conditions applied, not undergo an intramolecular
redox reaction with formation of the PdII complex 7 but in-
stead, 2) is very sensitive towards dioxygen and rapidly oxi-
dized to complex 7. Oxidation of 1, which is typically used
in only catalytic amounts in allylic alkylation, with dioxygen
can become a severe side reaction in the case of nucleo-
philes that have a low nucleophilicity, as, for example, the
Chem. Eur. J. 2010, 16, 2904 – 2915
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2909

H.-J. Gais et al.
Scheme 12. Redox reaction of complex 1 with biscarbonates 5 and rac-22
with formation of complex 7 and diene 8.
Having observed a facile redox reaction of complex 1
with biscarbonates 5c,d and rac-22, it was of interest to see
whether Pd0 complexes that contain a phosphane and are
devoid of an amide group are also amendable to a redox re-
action with the biscarbonates. NMR spectroscopy experi-
ments were performed with stoichiometric amounts of bis-
carbonate 5b and [{Pd
ACHTUNGTERN(GUN allyl)Cl}2], [Pd2ACHTUNTGREN(NUNG dba)3]·CHCl3, or [Pd-
ACHTUNGTRENNUNG
standard conditions in CD2Cl2 in sealed NMR spectroscopy
tubes. In no case could the formation of diene 8b be ob-
served. This is in contrast to the reported conversion of acy-
clic and bicyclic allylic 1,4-biscarbonates to the correspond-
ing 1,3-dienes upon treatment with catalytic amounts of
Scheme 13. Proposed mechanism for the redox reaction of the Pd0 com-
plex 1 with biscarbonate 5.
[Pd
phosphanes including PPh3, or even better, phosphites in-
cluding P
(OiPr)3.[40] The mechanism of this synthetically
2ACHTUNGTRENNUNG(dba)3]·CHCl3 and nearly stoichiometric amounts of aryl
T
plex that carries a h3-cyclohexenyl group.[5] A nucleophilic
attack of the second amide N atom of the p-allyl–PdII com-
plex 23 at the Pd atom instead of at the C atom of the p-
useful conversion of allylic 1,4-biscarbonates to the corre-
sponding 1,3-dienes is not known.[40]
allyl–PdII unit generates the second Pd N bond and leads
À
The formation of the 1,3-diene 8 and complex 7 in the re-
action of the allylic biscarbonate 5 with complex 1 in
CH2Cl2 or CH2Cl2/H2O is rationalized as shown in
Scheme 13. Reaction of complex 1 with 5 furnishes the p-
allyl–PdII complex 16 with methyl carbonate (hydrogen car-
bonate) as counterion. Intramolecular attack of the amide N
atom at the Pd atom delivers the p-allyl–PdII complex 23·H+
under elimination of the Pd atom and the methyl carbonate
ion to the formation of diene 8 and the protonated tetra-
coordinate PdII complex 7·H+ (not shown in Scheme 13),
which is deprotonated by the methyl carbonate (hydrogen
carbonate) ion with formation of 7. Attack of the N atom of
23 at the Pd atom may be preceded by a dissociation of a P
atom. The facile redox reaction of the trans-configured bis-
carbonate rac-22 with 1, which should take a similar mecha-
nistic path, shows that the trans configuration of complex 16
is not a prerequisite for the reactions depicted in Scheme 13
to occur. The decisive steps according to Scheme 13 are the
À
(not shown in Scheme 13) with generation of a Pd N bond,
which is deprotonated by the methyl carbonate (hydrogen
carbonate) ion to give 23. Attack of the N atom of complex
16 at the Pd atom is perhaps facilitated by a close proximity
of the amide group as observed in the corresponding com-
2910
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Chem. Eur. J. 2010, 16, 2904 – 2915

Pd0 Complex with the Trost Ligand
FULL PAPER
intramolecular attack of the N atom of the p-allyl–PdII com-
plexes 16 and 23 at the Pd atom with formation of the neu-
tral p-allyl–PdII complex 23 and complex 7, respectively. The
first step is apparently faster at room temperature than the
intermolecular substitution of 16 by HCO3 to give carbon-
ate 17 (cf. Scheme 8).
and higher catalyst loading followed by an oxidation of the
mixture with air (Scheme 15). Biscarbonate 5c was treated
with catalyst 1 (19 mol%) and KHCO3 in degassed CH2Cl2/
H2O (9:1) at 08C under argon for 24 h. Then the mixture
was stirred for 12 h under air at room temperature. Addition
of diethyl ether to the organic phase and centrifugation of
the suspension afforded the slightly impure complex 7 in
90% yield. In addition, carbonate 12c (68% ee) was isolated
in 44% yield and biscarbonate 5c was recovered in 16%
yield.
À
The stoichiometric redox reaction between the Pd0 com-
plex 1 and biscarbonate 5 with formation of diene 8 and the
PdII complex 7 is mechanistically different from the Pd0-cat-
alyzed elimination of allylic monocarbonates, which also
yields 1,3-dienes.[41,42] It is interesting to note that the re-
verse transformation to the one depicted in Scheme 12, the
generation of 1,4-bisacyloxycycloalk-2-enes and a Pd0 com-
plex from the corresponding 1,3-cycloalkadienes and a PdII
complex, is well established.[43]
According to NMR spectroscopy, the reaction of equimo-
lar amounts of 1 and biscarbonates 5b–d gave stoichiometric
amounts of the corresponding dienes 8b–d and complex 7.
Quantitative isolation of dienes 8b–d proved to be difficult
because of their volatility. Therefore, diene 8b, for example,
formed upon reaction of 1 with 5b, was trapped with dieno-
philes 24a and 24b as the corresponding cycloadducts 25a
and 25b (Scheme 14). Complex 1 was treated with biscar-
Scheme 15. Isolation of complex 7 in the desymmetrization of biscarbon-
ate 5c with 1 through oxidative workup.
Conclusion
Scheme 14. Trapping of diene 8b as cycloadducts 25a and 25b.
Catalyst 1 and biscarbonates 5 and rac-22 undergo without
the requirement of dioxygen a facile redox reaction whereby
the former is oxidized to the PdII complex 7 and the latter
reduced to the 1,3-diene 8. The redox deactivation of 1 with
allylic 1,4-biscarbonates, which involves two intramolecular
substitutions at the Pd atom, dominates over the allylic alky-
lation at room temperature because of the poor nucleophi-
licity of the hydrogen carbonate ion. The p-allyl–PdII com-
plex 16 is most likely the key intermediate in both compet-
ing reaction paths, the allylic alkylation by means of inter-
molecular nucleophilic substitution and the redox reaction
by means of intramolecular nucleophilic addition/deprotona-
tion. At lower temperatures, allylic alkylation is preferred,
which proceeds in the case of the six-membered biscarbon-
ate with high enantioselectivity. The alcohol formed in this
bonate 5b (1 equiv) in CH2Cl2 at room temperature for
15 min. Addition of diethyl ether and centrifugation of the
thus-formed suspension of 7 afforded the PdII complex in
82% yield. The solution of the diene 8b in CH2Cl2/Et2O
also containing dba was treated with dienophiles 24a and
24b following the careful removal of diethyl ether at normal
pressure, which furnished cycloadducts 25a[44] and 25b[45] in
75 and 80% yield, respectively. Reaction of 8b with the di-
enophiles in the presence of diethyl ether was very slow.
Isolation of complex 7 in allylic alkylation: Despite the
many applications the Pd0 complex 1 has found in asymmet-
ric allylic alkylation, a recycling of the catalyst, which would
À
be desirable because of the price of [Pd
N
manner is activated upon reaction with HCO3 to yield the
ligand 2, has not been described. The main obstacle to a re-
covery of 1 in allylic alkylation is the facile oxidation of the
complex to the PdII complex 7 by dioxygen. A recycling of 1
may perhaps be accomplished by a two-step procedure that
includes its oxidation to 7 during workup and a reduction of
the latter. Therefore, it was of interest to see whether an ox-
idative workup in allylic alkylation with 1[26] would allow the
isolation of 7 in high yield. Thus, the desymmetrization of
biscarbonate 5c with 1 was run at a somewhat larger scale
corresponding hydrogen carbonate, which suffers a mis-
matched ionization by the catalyst, followed by an intramo-
lecular substitution to give the bicyclic carbonate. The Pd0
complex 1 that bears the Trost ligand undergoes no intramo-
lecular redox reaction with formation of the PdII complex 7
and dihydrogen under the standard conditions of allylic al-
kylation. In contrast to what is stated in the literature, mon-
omeric and oligomeric Pd0 complexes 1 are stable in solu-
tion and do not generate the PdII complex 7 with formation
Chem. Eur. J. 2010, 16, 2904 – 2915
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2911

H.-J. Gais et al.
was diluted with EtOAc (40 mL). The organic layer was washed twice
with water and brine, dried (MgSO4), and concentrated in vacuo. Purifi-
cation by column chromatography on silica gel (cyclohexane/EtOAc, 7:2)
gave the corresponding biscarbonate.
of dihydrogen. Our results strongly suggest that the reported
formation of the analogous PdII complex 10 from the Pd0
complex 9 was due to an oxidation with dioxygen and not
caused by an intramolecular redox reaction. The facile oxi-
dation of the Pd0 complex 1 to the PdII complex 7 by dioxy-
gen allows a recovery of Pd and ligand 2 in allylic alkylation
through an oxidative workup. However, a complete recy-
cling of catalyst 1 from complex 7 has to await the develop-
ment of a procedure for the reduction of the PdII complex.
Biscarbonate 5b: Biscarbonate 5b (3.83 g, 95%) was obtained from diol
1
11b according to GP1 as a colorless oil. H NMR (300 MHz, CDCl3): d=
1.96 (m, 4H), 3.79 (s, 6H), 5.09 (m, 2H), 5.98 ppm (d, J=1.5 Hz, 2H);
13C NMR (75 MHz, CDCl3): d=24.7 (u), 54.8 (d), 70.9 (d), 130.1 (d),
155.4 ppm (u); IR (capillary): n˜ =3472 (w), 2959 (vs), 2681 (w), 2374 (w),
2202 (m), 1746 (vs), 1586 (m), 1399 (m), 1343 (s), 1259 (s), 1153 (m),
1110 (s), 1079 (m), 1052 (w), 1009 (s), 940 (s), 867 (m), 845 (m), 792 cmÀ1
(s); MS (EI, 70 eV): m/z (%): 154 (55), 110 (69), 95 (52), 79 (100), 67
(30), 59 (51), 54 (11), 45 (17); elemental analysis calcd (%) for C10H14O6
(230.21): C 52.17, H 6.13; found: C 52.01, H 6.11.
Experimental Section
Biscarbonate 5c: Biscarbonate 5c (1.30 g, 70%) was obtained from diol
11c according to GP1 as a colorless solid besides the corresponding mon-
ocarbonate (340 mg, 24%). 1H NMR (300 MHz, CDCl3): d=1.58–2.13
(m, 6H), 3.79 (s, 6H), 5.23 (m, 2H), 5.75 ppm (s, 2H); 13C NMR
(75 MHz, CDCl3): d=22.8 (u), 32.3 (u), 54.8 (d), 77.5 (d), 132.1 (d),
155.1 ppm (u); IR (KBr): n˜ =3472 (w), 2940 (m), 2864 (w), 2192 (w),
2015 (w), 1733 (vs), 1652 (w), 1583 (w), 1448 (s) 1336 (s), 1258 (vs), 1160
(m), 1109 (m), 1062 (m), 966 (vs), 940 (s), 885 (m), 856 (m), 825 (m),
782 cmÀ1 (s); MS (EI, 70 eV): m/z (%): 244 (2) [M+], 168 (85), 109 (100),
96 (23), 93 (68), 81 (31), 77 (23), 67 (16), 59 (55); elemental analysis
calcd (%) for C11H16O6 (244.24): C 54.09, H 6.60; found: C 54.29, H 7.02.
General: All reactions were carried out under an argon atmosphere in
dry solvents with syringe and Schlenk techniques in oven-dried glassware.
Diols 11b and 11d were prepared according to the literature from the
corresponding dienes 8b and 8d by means of the corresponding endo-
peroxides,[46,47] which were reduced without isolation with activated Zn in
CH2Cl2/acetic acid.[48,49] Diol 11c and (Æ)-trans-cyclohex-2-ene-1,4-diol
were prepared according to the literature from the corresponding dienes
8c and 8b through
a
Pd-catalyzed 1,4-diacyloxylation.[50, 51] [Pd2-
ACHTUNGTRENNUNG
prepared according to the literature. Carbonate rac-20 was prepared
from cylohex-2-en-1-ol by standard procedures. Cyclohexadiene 8b, cy-
clooctadiene 8d, and dieneophiles 24a and 24b were obtained from com-
mercial sources. THF and Et2O were distilled under argon from lead/
sodium in the presence of benzophenone. CH2Cl2 was distilled from
CaH2. Bulk solvents for column chromatography and extractions were
distilled prior to use. Reagents were obtained from commercial sources
and used directly without further purification unless otherwise specified.
nBuLi was obtained from commercial sources and standardized by titra-
tion with diphenylacetic acid. TLC was performed on E. Merck pre-
coated plates (silica gel 60 F254, layer thickness 0.2 mm), and chromatog-
raphy was performed with E. Merck silica gel (0.040–0.063 mm) in the
flash mode with a positive nitrogen pressure. HPLC was carried out with
a Dynamax SD-1 pump by using Varian 320 UV/Vis and Knauer RI de-
tectors on a Chromasil Si-100 column. 1H, 13C, and 31P NMR spectra
were recorded using Varian Mercury 300 and Varian Inova 400 instru-
ments. 1H and 13C chemical shifts are reported relative to TMS (d=
0.00 ppm) as internal standard and 31P NMR chemical shifts are reported
relative to H3PO4 in D2O (d=0.00 ppm) as external standard. The fol-
lowing abbreviations are used to designate the multiplicity of the peaks
in the 1H NMR spectra: s=singlet, d=doublet, t=triplet, q=quartet,
qt=quintet, sex=sextet, sep=septet, o=octet, m=multiplet, br=broad,
and combinations thereof. Peaks in the 13C NMR spectra are denoted as
“u” for carbons with zero or two protons attached or as “d” for carbons
with one or three attached protons, as determined from the attached
proton test (APT) pulse sequence. Assignments in the 1H NMR spectra
were made by gradient multiple quantum (GMQ) COSY and heteronu-
clear correlation (HETCOR) experiments, and those in the 13C NMR
spectra were made by distortionless enhancement by polarization transfer
(DEPT) experiments. IR spectra were recorded using a Perkin–Elmer
PE 1759 FT instrument, and the abbreviations used to designate the in-
tensity of the peaks are vs=very strong, s=strong, m=medium, and w=
weak. Optical rotations were measured using a Perkin–Elmer 241 polar-
imeter at approximately 228C. Specific rotation is given in degreesꢁ(mL
per dm)ꢁg, and c is in grams per 100 mL. Enantiomer analyses were car-
ried out by HPLC and GC using commercial columns with chiral station-
ary phases as stated in the experimental procedures.
Biscarbonate 5d: Biscarbonate 5d (4.07 g (90%) was obtained from diol
11d according to GP1 as a colorless oil, which solidified on standing.
M.p. 34–358C; 1H NMR (400 MHz, CDCl3): d=1.61 (m, 6H), 2.03 (m,
2H), 3.78 (s, 6H), 5.45 (m, 2H), 5.63 ppm (dd, J=4.4, 1.1 Hz, 2H);
13C NMR (100 MHz, CDCl3): d=22.9 (u), 34.7 (u), 54.7 (d), 75.4 (d),
129.5 (d), 154.8 ppm (u); IR (Universal ATR): n˜ =3006 (w), 2935 (m),
2866 (w), 1744 (vs), 1587 (v), 1444 (s), 1316 (s), 1252 (vs), 1144 (w), 1106
(w), 1077 (w), 996 (s), 947 (m), 925 (s), 864 (m), 791 cmÀ1 (s); MS (EI,
70 eV): m/z (%): 258 (1) [M+], 182 (54), 141 (7), 123 (24), 109 (26), 106
(100), 97 (16), 95 (29), 91 (44), 78 (42), 71 (18), 67 (27), 59 (62), 55 (26);
elemental analysis calcd (%) for C12H18O6 (258.11): C 55.81, H 7.02;
found: C 56.13, H 6.97.
Biscarbonate rac-22: Biscarbonate rac-22 (3.91 g, 97%) was obtained
from (Æ)-trans-cyclohex-2-ene-1,4-diol according to GP1 as a colorless
solid. M.p. 86–878C; 1H NMR (400 MHz, CDCl3): d=1.79 (m, 2H), 2.19
(m, 2H), 3.79 (s, 6H), 5.18 (m, 2H), 5.99 ppm (d, J=1.4 Hz, 2H);
13C NMR (100 MHz, CDCl3): d=25.5 (u), 54.8 (d), 71.0 (d), 130.0 (d),
155.1 ppm (u); IR (Universal ATR): n˜ =3463 (w), 2959 (w), 2877 (w),
2195 (w), 2049 (w), 1731 (vs), 1585 (w), 1446 (s), 1400 (w), 1361 (w), 1322
(m), 1246 (vs), 1102 (m), 1003 (s), 930 (s), 897 (m), 790 (s), 758 cmÀ1 (m);
MS (EI, 70 eV): m/z (%): 154 (75), 110 (72), 95 (51), 79 (100), 67 (28), 59
(47), 45 (28); elemental analysis calcd (%) for C10H14O6 (230.21): C
52.17, H 6.13; found: C 52.28, H 5.87.
Alcohol 6b:
A Schlenk flask was successively charged with [Pd2-
AHCTUNTGREGUN(NN dba)3]·CHCl3 (41 mg, 0.04 mmol), ligand 2 (55 mg, 0.08 mmol), and de-
gassed CH2Cl2 (9 mL). After the mixture was stirred at room tempera-
ture for 15 min, it was cooled to 08C. Then degassed water (1 mL) and
carbonate 5b (460 mg, 2 mmol) were added. After the mixture was rapid-
ly stirred at 08C for 24 h, it was poured into cyclohexane/EtOAc (1:1;
200 mL) and the mixture was filtered through a plug of silica gel (0.0040–
0.063 mm; 3ꢁ2 cm). The organic phase was dried (MgSO4) and concen-
trated in vacuo. Purification by chromatography on silica gel (cyclohex-
ane/EtOAc, 7:2) afforded alcohol 6b (227 mg 66%) of ꢀ99% ee (deter-
mined by HPLC after benzoylation to 14) as a colorless oil. [a]D =+
106.0 (c=1.0 in CH2Cl2); 1H NMR (300 MHz, CDCl3): d=1.91 (m, 4H),
2.18 (brs, 1H), 3.78 (s, 3H), 4.17 (m, 1H), 5.06 (m, 1H), 5.85 (m, 1H),
6.00 ppm (m, 1H; 13C NMR (75 MHz, CDCl3): d=25.1 (u), 27.9 (u), 54.7
(d), 65.4 (d), 71.0 (d), 126.8 (d), 135.7 (d), 155.4 ppm (u); IR (capillary):
n˜ =3403 (s), 3028 (s), 2956 (vs), 2857 (m), 2675 (w), 2460 (w), 2200 (w),
1745 (vs), 1584 (w), 1541 (w), 1444 (s), 1397 (w), 1341 (s), 1264 (vs), 1224
(s), 1149 (m), 1112 (w), 1067 (s), 1010 (s), 970 (s), 943 (s), 913 (s), 867
(w), 831 (w), 792 (s), 757 cmÀ1 (vs); MS (CI, isobutane): m/z (%): 173
(10) [M++1], 155 (100), 135 (8), 101 (6), 97 (66), 79 (4), 77 (10); elemen-
General procedure for synthesis of the cyclic biscarbonates 5b–d and rac-
22 (GP1): nBuLi (24.2 mL of 1.6m in hexanes, 38.7 mmol) was added
dropwise to a solution of the diol (11b, 11d, and (Æ)-trans-cyclohex-2-
ene-1,4-diol; 17.5 mmol) in THF (60 mL). The slurry was stirred first at
08C for 10 min and then at room temperature for 20 min. Then the mix-
ture was cooled to 08C and methyl chloroformiate (5.4 mL, 70 mmol)
was added. After the mixture was stirred at room temperature for 2 h, it
2912
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Pd0 Complex with the Trost Ligand
FULL PAPER
tal analysis calcd (%) for C8H12O4 (172.07): C 55.81, H 7.02; found: C
55.61, H 7.18.
and the combined organic phases were washed with 1m HCl, saturated
aqueous NaHCO3, and brine, dried (Na2SO4), and concentrated in vacuo.
Purification by column chromatography on silica gel (n-pentane/Et2O,
5:1) gave benzoate 14 (81 mg, 97%) as a colorless oil. HPLC (Chiralpak
IA, n-heptane/iPrOH, 99.9:0.1, flow rate 0.75 mLminÀ1, l=254 nm): tR
(ent-14)=18.43 min, tR (14)=20.70 min, ꢀ99% ee; [a]D =À49.5 (c=1.0
in CH2Cl2); 1H NMR (400 MHz, CDCl3): d=2.04 (m, 4H), 3.81 (s, 3H),
5.15 (m, 1H), 5.50 (m, 1H), 6.04 (qd, J=11.0, 3.0 Hz, 2H), 7,44 (t, J=
8.0 Hz, 2H), 7.56 (t, J=7.4 Hz, 1H), 8.05 ppm (d, J=6.9 Hz, 2H);
13C NMR (100 MHz, CDCl3): d=24.9 (u), 25.1 (u), 54.8 (d), 67.7 (d), 71.1
(d), 128.3 (d), 129.5 (d), 129.6 (d), 130.2 (u), 130.9 (d), 133.0 (d), 155.3
(u), 165.9 ppm (u); IR (capillary): n˜ =3421 (w), 2963 (m), 1746 (vs), 1717
(vs), 1601 (v), 1584 (w), 1541 (w), 1445 (m), 1398 (w), 1343 (w), 1315 (w),
1264 (vs), 1212 (w), 1176 (w), 1110 (s), 1071 (w), 1052 (w), 1012 (s), 941
(m), 924 (m), 792 (m), 755 cmÀ1 (w); MS (CI, CH4): m/z (%): 276 (1)
[M+], 201 (100), 155 (76), 133 (3), 123 (41), 111 (6), 105 (74), 91 (2), 79
(35), 77 (38); elemental analysis calcd (%) for C15H16O5 (276.10): C
65.21, H 5.84; found: C 65.02, H 5.79.
Carbonate 12b: A Schlenk flask was successively charged with [Pd2-
ACHTUNGTRENNUNG(dba)3]·CHCl3 (83 mg, 0.08 mmol), ligand 2 (110 mg, 0.16 mmol), KHCO3
(280 mg, 2.8 mmol), and degassed CH2Cl2 (9 mL). Then the mixture was
stirred at room temperature for 15 min. After the mixture was cooled to
08C, it was treated with degassed water (1 mL) and carbonate 5b
(460 mg, 2 mmol) and the mixture was rapidly stirred at 08C for 24 h.
Then the mixture was poured into cyclohexane/EtOAc (1:1; 200 mL) and
filtered through a plug of silica gel (0.0040–0.063 mm; 3ꢁ2 cm). The or-
ganic phase was dried (MgSO4) and concentrated in vacuo. Purification
by chromatography on silica gel (cyclohexane/EtOAc, 7:2) afforded car-
bonate 12b (258 mg, 92%) of 96% ee (determined by HPLC after hy-
drolysis to 13b and benzoylation to 15) as a colorless oil. [a]D =+5.0 (c=
11.0 in CH2Cl2); 1H NMR (300 MHz, CDCl3): d=1.94 (m, 1H), 2.10 (m,
2H), 2.30 (m, 1H), 4.93 (m, 1H), 5.03 (m, 1H), 5.79 (dqtd, J=10.1, 1.7,
0.5 Hz, 1H), 6.24 ppm (dtd, J=10.1, 4.0, 0.8 Hz, 1H); 13C NMR
(75 MHz, CDCl3): d=19.5 (u), 21.2 (u), 72.2 (d), 74.9 (d), 121.3 (d), 135.3
(d), 155.0 ppm (u); IR (capillary): n˜ =3040 (w), 2957 (vs), 2930 (vs), 2859
(s), 1928 (w), 1801 (vs), 1726 (vs), 1652 (w), 1600 (w), 1550 (w), 1459
(m), 1377 (m), 1354 (m), 1280 (s), 1203 (m), 1160 (vs), 1077 (s), 1045
(vs), 1022 (vs), 978 (w), 961 (w), 928 (w), 875 (w), 852 (w), 773 cmÀ1 (m);
MS (CI, CH4): m/z (%): 141 (49) [M++1], 97 (19), 79 (100), 68 (3), 63
(6); elemental analysis calcd (%) for C7H8O3 (140.05): C 59.99, H 5.75;
found: C 59.74, H 5.50.
Diol 13b: K2CO3 (181 mg, 1.28 mmol) was added portionwise to a solu-
tion of carbonate 12b (30 mg, 0.21 mmol) in MeOH (5 mL) and the mix-
ture was stirred at room temperature for 1 h. The mixture was concen-
trated in vacuo, the residue was dissolved in water (5 mL), and the solu-
tion was extracted with EtOAc (5ꢁ10 mL). The combined organic
phases were dried (MgSO4) and concentrated in vacuo, which gave diol
13b (24.2 mg, 99%) as a slightly yellow oil. The thus-obtained diol was
pure enough for benzoylation. Purification by column chromatography
(cyclohexane/EtOAc, 1:9) gave diol 13b (24 mg, 98%) of 96% ee (deter-
mined by HPLC after benzoylation to 15) as a colorless oil. [a]D =À172.0
Carbonate 12c: A Schlenk flask was successively charged with [Pd2-
ACHTUNGTRENNUNG(dba)3]·CHCl3 (70 mg, 0.067 mmol), ligand 2 (97 mg, 0.14 mmol), KHCO3
(127 mg, 1.26 mmol), and degassed CH2Cl2 (18 mL). After the mixture
was stirred at room temperature for 15 min, it was cooled to 08C and suc-
cessively treated with degassed water (2 mL) and carbonate 5c (200 mg,
0.82 mmol) in degassed CH2Cl2 (18 mL). After the mixture was rapidly
stirred at 08C for 24 h, it was poured into cyclohexane/EtOAc (1:1)
(200 mL) and the mixture was filtered through a plug of silica gel
(0.0040–0.063 mm; 3ꢁ2 cm). The organic phase was dried (MgSO4) and
concentrated in vacuo. Purification by chromatography on silica gel (cy-
clohexane/EtOAc, 7:2) afforded carbonate 12c (20 mg, 16%) of 91% ee
(Lipodex-gamma, H2, tR (12c)=94.1 min, tR (ent-12c)=94.7 min) as a
colorless oil and biscarbonate 5c (164 mg, 82%). [a]D =À30.0 (c=0.5 in
CH2Cl2); 1H NMR (400 MHz, CDCl3): d=1.53–2.32 (m, 6H), 4.77–4.85
(m, 1H), 5.40 (m, 1H), 5.61–5.66 (m, 1H), 5.81–5.89 ppm (m, 1H);
13C NMR (100 MHz, CDCl3): d=18.4 (u), 27.5 (u), 27.6 (u), 77.7 (d), 78.3
(d), 123.5 (d), 131.3 (d), 154.1 ppm (u); IR (capillary): n˜ =3569 (w), 2943
(m), 2870 (w), 1786 (vs), 1658 (w), 1545 (w), 1457 (m) 1362 (s), 1263 (w),
1166 (vs), 1055 (vs), 881 (w), 841 (m), 770 cmÀ1 (m); MS (EI, 70 eV): m/z
(%): 155 (10), 111 (1), 109 (4), 92 (45), 81 (28), 68 (86), 67 (100), 54 (51);
elemental analysis calcd (%) for C8H10O3 (154.16): C 62.33, H 6.54;
found: C 62.54, H 6.41.
1
(c=1 in CH2Cl2); H NMR (400 MHz, CDCl3): d=1.65 (m, 1H), 1.72 (m,
1H), 1.96 (m, 1H), 2.14 (m, 1H), 3.56 (brs, 2H), 3.75 (dt, J=9.3, 3.8 Hz,
1H), 4.05 (brs, 1H), 5.64 (dqt, J=10.2, 1.9 Hz, 1H), 5.78 ppm (dtd, J=
10.2, 3.7, 0.8 Hz, 1H); 13C NMR (100 MHz, CDCl3): d=23.7 (u), 25.8 (u),
66.5 (d), 68.9 (d), 126.9 (d), 131.0 ppm (d); IR (capillary): n˜ =3773 (vs),
3027 (m), 2922 (s), 2655 (w), 1650 (m), 1434 (s), 1404 (s), 1323 (w), 1252
(m), 1226 (m), 1193 (w), 1151 (m), 1080 (vs), 995 (s), 972 (m), 914 (m),
878 (w), 862 (w), 824 (m), 754 cmÀ1 (s); MS (EI, 70 eV): m/z (%): 114 (1)
[M+], 97 (2), 79 (1), 70 (100), 67 (4), 57 (2), 55 (9), 53 (4), 51 (4), 45 (2);
elemental analysis calcd (%) for C6H10O2 (114.07): C 63.14, H 8.83;
found: C 63.08, H 8.74.
Diol 13c: A mixture of carbonate 12c (50 mg, 0.32 mmol) of 68% ee and
K2CO3 (200 mg, 1.45 mmol) in MeOH (5 mL) was stirred at room tem-
perature for 16 h. Then the mixture was concentrated in vacuo, the resi-
due was dissolved in water (5 mL), and the solution was extracted with
EtOAc (4ꢁ10 mL). The combined organic phases were dried (MgSO4)
and concentrated in vacuo. Purification by column chromatography (cy-
clohexane/EtOAc, 1:9) gave diol 13c (20 mg, 45%). [a]D =À65.0 (c=
0.95 in CHCl3); 1H NMR (400 MHz, CDCl3): d=1.45–1.70 (m, 2H), 1.80
(m, 1H), 1.90–2.15 (m, 2H), 2.23 (m, 1H), 2.55 (brs, 2H), 3.95 (m, 1H),
4.45 (dd, J=1.7, 1.9 Hz, 1H), 5.55 (dqt, J=11.5, 1.9 Hz, 1H), 5.91 ppm
(m, 1H); 13C NMR (100 MHz, CDCl3): d=20.8 (u), 28.5 (u), 33.9 (u),
72.5 (d), 73.9 (d), 131.8 (d), 132.6 ppm (d); IR (capillary): n˜ =3379 (vs),
3027 (m), 2923 (vs), 2677 (w), 1655 (m), 1442 (s), 1385 (m), 1351 (w),
1273 (s), 1171 (w), 1034 (vs), 994 (vs), 942 (m), 905 (m), 872 (m), 833
(m), 771 (m), 729 (w), 674 cmÀ1 (m); MS (ESI, 70 eV): m/z (%): 111 (1)
[M+ÀOH], 92 (100), 67 (6).
Carbonate ent-12c: A Schlenk flask was successively charged with [Pd2-
ACHTUNGTRENNUNG(dba)3]·CHCl3 (156 mg, 0.151 mmol), ligand ent-2 (225 mg, 0.33 mmol),
KHCO3 (255 mg, 2.55 mmol), and degassed CH2Cl2 (15 mL). After the
mixture was stirred at room temperature for 5 min, it was cooled to 08C
and successively treated with degassed water (2.5 mL) and carbonate 5c
(415 mg, 1.70 mmol) in degassed CH2Cl2 (8 mL). After the mixture was
rapidly stirred at 08C for 24 h, it was warmed to room temperature and
stirred for 12 h under air. The organic phase was separated from the
aqueous phase with a syringe and treated with diethyl ether (360 mL).
The suspension was centrifuged and the solid was washed with diethyl
ether. The combined organic phases were dried (MgSO4) and concentrat-
ed in vacuo. Purification by chromatography on silica gel (cyclohexane/
EtOAc, 7:2) gave carbonate ent-12c (102 mg, 39%) of 99% ee as a color-
less oil. [a]D =+35.0 (c=1.2 in CH2Cl2).
Dibenzoate 15: The synthesis of dibenzoate 15 (115 mg, 95%) from diol
13b was performed according to the procedure used for the synthesis of
monobenzoate 14 with the exception that 0.1 equiv of 4-DMAP, 3 equiv
of NEt3, and 2.1 equiv of PhCOCl were used. Colorless oil; HPLC (Chir-
alpak IA, n-heptane/iPrOH=99.2:0.8, flow rate 0.75 mLminÀ1
, l=
254 nm): tR (ent-15)=31.17 min, tR (15)=32.31 min, 96% ee, [a]D =
À118.0 (c=1.0 in CH2Cl2); 1H NMR (300 MHz, CDCl3): d=1.93 (m,
1H), 2.06–2.35 (m, 3H), 5.36 (dt, J=10.1, 3.5 Hz, 1H), 5.71 (m, 1H), 5.77
(m, 1H), 5.98 (dt, J=9.6, 3.2 Hz, 1H), 7.24 (t, J=7.9 Hz, 2H), 7.30 (t, J=
8.2 Hz, 1H), 7.40 (m, 2H), 7.85 (d, J=8.4 Hz, 2H), 7.93 ppm (d, J=
8.4 Hz, 2H); 13C NMR (75 MHz, CDCl3): d=23.6 (u), 24.0 (u), 67.4 (d),
70.3 (d), 123.5 (d), 128.3 (d), 128.4 (d), 129.6 (d), 129.7 (d), 130.3 (u),
130.4 (u), 132.9 (d), 133.0 (d), 133.1 (d), 165.8 (u), 166.0 ppm (u); IR (ca-
Benzoate 14: Alcohol 6b (52 mg, 0.3 mmol) and 4-DMAP (4-DMAP=4-
dimethylaminopyridine; 1.8 mg, 0.015 mmol) were dissolved in CH2Cl2
(2 mL) and the solution was cooled to 08C. The mixture was successively
treated with NEt3 (0.06 mL, 0.45 mmol) and PhCOCl (0.037 mL,
0.315 mmol). Then the cooling bath was removed, and after the mixture
was stirred for 2 h, it was poured onto a mixture of ice and saturated
aqueous NaHCO3. The mixture was extracted with CH2Cl2 (3ꢁ10 mL)
Chem. Eur. J. 2010, 16, 2904 – 2915
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2913

H.-J. Gais et al.
pillary): n˜ =3033 (w), 2957 (m), 2854 (w), 1746 (vs), 1716 (vs), 1601 (w),
1584 (w), 1491 (w), 1444 (s), 1398 (w), 1342 (w), 1316 (w), 1263 (vs), 1176
(m), 1109 (s), 1071 (m), 1052 (w), 1012 (s), 980 (w), 939 (m), 924 (m), 857
(w), 792 (m), 7.56 cmÀ1 (m); MS (CI, CH4): m/z (%): 323 (2) [M++1],
305 (1), 235 (1), 201 (100), 139 (2), 123 (5), 105 (34), 79 (2), 77 (2); ele-
mental analysis calcd (%) for C20H18O4 (322.12): C 74.52, H 5.63; found:
C 74.15, H 5.63.
General procedure for the trapping of 8b as adducts 24a and 24b (GP5):
Diethyl ether was carefully distilled off the combined organic phases,
which were obtained according to GP4 from 5b, at normal pressure by
using a column. Then CH2Cl2 (10 mL) was added and the remaining di-
ethyl ether was again carefully distilled off (this procedure was repeated
twice). Then the mixture was treated with CH2Cl2 (10 mL) and the dieno-
phile (24a or 24b). After the mixture was stirred at room temperature
for 5 (for 24a) and 2 h (for 24b), the solvent was removed in vacuo. Pu-
rification by chromatography on silica gel (cyclohexane/EtOAc, 3:1) gave
adducts 24a and 24b.
Reaction of alcohol rac-6b with 1 and KHCO3: A Schlenk flask was suc-
cessively charged with [Pd2ACHTNUTRGNE(UGN dba)3]·CHCl3 (42 mg, 0.04 mmol), ligand 2
(55 mg, 0.08 mmol), degassed CH2Cl2 (9 mL), and KHCO3 (141 mg,
1.4 mmol). After the mixture was stirred at room temperature for 15 min,
it was cooled to 08C and then degassed water (1 mL) and alcohol rac-6b
(172 mg, 1 mmol) were added. After the mixture was rapidly stirred at
08C for 2 h, it was poured into cyclohexane/EtOAc (1:1; 200 mL) and the
mixture was filtered through a plug of silica gel (0.0040–0.063 mm; 3ꢁ
2 cm). The organic phase was dried (MgSO4) and concentrated in vacuo.
Purification by chromatography on silica gel (cyclohexane/EtOAc, 7:2)
afforded carbonate 12b (123 mg, 88%) of 46% ee (determined by HPLC
after hydrolysis to 13b and benzoylation to 15) as a colorless oil.
Adduct 25a: Adduct 24a was synthesized according to GP5 by reaction
of diene 8b obtained according to GP4 with maleic anhydride (24a)
(55 mg, 0.56 mmol). Adduct 25a (74 mg, 74%) was obtained as a slightly
yellow solid. All spectral data of 25a were in complete agreement with
those of a sample independently prepared from diene 8b and 24a accord-
ing to the literature.[44]
Adduct 25b: Adduct 25b was synthesized according to GP5 by the reac-
tion of diene 8b obtained according to GP4 with dienophile 24b (98 mg,
0.56 mmol). Adduct 25b (115 mg, 80%) was obtained as a pale yellow
solid. All spectral data of 25b were in complete agreement with those of
a sample independently prepared from diene 8b and 24b according to
the literature.[45]
General procedure for in situ NMR spectroscopic investigation of the
Pd0 complex 1 in a sealed NMR spectroscopy tube (GP2): A Schlenk-
type NMR spectroscopy tube was successively charged with [Pd2-
A
Desymmetrization of biscarbonate 5c and isolation of 7 after oxidative
degassed CD2Cl2 (0.7 mL) under argon. After the mixture was cooled to
À788C, the tube was evacuated and sealed under vacuum at À788C with
an acetylene/oxygen microburner.
workup:
A
A
Schlenk flask was successively charged with [Pd2-
(330 mg, 0.48 mmol),
KHCO3 (374 mg, 3.72 mmol), and degassed CH2Cl2 (36 mL). Then the
mixture was immediately cooled to 08C and stirred for 5 min under
argon. After the mixture was treated with degassed water (4 mL) and bis-
carbonate 5c (608 mg, 2.5 mmol), it was rapidly stirred at 08C for 24 h
under argon. Then the mixture was warmed to room temperature and
stirred for 12 h under air. The organic phase was separated from the
aqueous phase with a syringe and treated with diethyl ether (360 mL).
Centrifugation of the suspension and washing of the solid with diethyl
ether gave after drying in vacuo the slightly impure complex 7 (316 mg,
90%). The combined organic phases were dried (MgSO4) and concen-
trated in vacuo. Purification by chromatography on silica gel (cyclohex-
ane/EtOAc, 7:2) gave biscarbonate 5c (100 mg, 16%) and carbonate 12c
(171 mg, 44%) of 68% ee as colorless oils. [a]D =À42.0 (c=1.8 in
CH2Cl2).
General procedure for the in situ NMR spectroscopic investigation of the
reaction of the Pd0 complex 1 with biscarbonates 5b–d and rac-22 in a
sealed NMR spectroscopy tube (GP3): A Schlenk-type NMR spectrosco-
py tube equipped with a ground joint was successively charged with [Pd2-
ACHTUNGTRENNUNG(dba)3]·CHCl3 (3.2mg, 0.0031mmol), ligand 2 (4.3mg, 0.0062mmol), bis-
carbonate (5b, 5c, 5d, and rac-22; 0.0062mmol) and degassed CD2Cl2
(0.7mL), and the mixture was cooled to À788C. Then the NMR spectros-
copy tube was evacuated and sealed under vacuo at À788C by using an
acetylene/oxygen microburner. Subsequently the mixture was warmed to
room temperature. The 1H NMR spectra showed in all cases the signals
of the PdII complex 7, the corresponding dienes 8b, 8c, and 8d, and dba,
whereas the 31P NMR spectra showed in all cases the signal of 7. Diene 8
and complex 7 were present in a ratio of approximately 1:1. PdII complex
7: Contained in all mixtures obtained from biscarbonates 5c,d, rac-22,
and 1 according to GP3. 1H NMR (400 MHz, CD2Cl2): d=1.25 (m, 2H),
1.47 (m, 2H), 1.59 (m, 2H), 2.50 (brd, J=12.9 Hz, 2H), 3.68 (m, 2H),
6.72 (m, 2H), 7.09 (m, 4H), 7.14–7.23 (m, 14H), 7.37 (t, J=7.1 Hz, 2H),
7.41 (t, J=7.1 Hz, 2H), 7.50 (t, J=7.4 Hz, 2H), 8.51 ppm (dd, J=8.0,
3.0 Hz, 2H); 31P NMR (162 MHz, CD2Cl2): d=26.0 ppm. The NMR spec-
troscopic data were in complete agreement with those reported in the lit-
erature.[27,28] 1,3-Cyclohexadiene (8b): Contained in the reaction mixture
obtained from biscarbonates 5b and rac-22 and 1 according to GP3.
1H NMR (400 MHz, CD2Cl2): d=2.12 (m, 4H), 5.78 (m, 2H), 5.87 ppm
(m, 2H). 1,3-Cycloheptadiene (8c): Contained in reaction mixture ob-
Acknowledgements
V.N.T. thanks the Alexander von Humboldt Foundation for a postdoctor-
al fellowship. This research was financially supported by the Deutsche
Forschungsgemeinschaft (SFB 380 and GK 440) and the Alexander von
Humboldt Foundation. We thank the referees for valuable suggestions
and comments.
tained from biscarbonate 5c and
1
according to GP3. 1H NMR
(400 MHz, CD2Cl2): d=1.73 (m, 2H), 2.24 (m, 4H), 5.67 ppm (m, 4H).
1,3-Cyclooctadiene (8d): Contained in the reaction mixture obtained
from biscarbonate 5d and 1 according to GP3. 1H NMR (400 MHz,
CD2Cl2): d=1.50 (m, 4H), 2.17 (m, 4H), 5.61 (m, 2H), 5.79 ppm (m,
2H).
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2914
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Chem. Eur. J. 2010, 16, 2904 – 2915

Pd0 Complex with the Trost Ligand
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Received: October 5, 2009
Published online: January 26, 2010
Chem. Eur. J. 2010, 16, 2904 – 2915
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
2915