New chiral calixsalen chromium complexes: Recyclable asymmetric catalysts

Article abstract of DOI:10.1002/chem.201001012

A chiral N,N'-bis(salicylidene)ethylenediamine (salen) polymer has been prepared by a condensation reaction between a thio-phenedisalicyladehyde derivative and (S,S)-cyclohexane-1,2-diamine. This polymeric compound was demonstrated to possess a cyclic structure with two to five repetitive units. The addition of chromium(II) salts led to the generation of a chiral catalyst that could be recovered as an insoluble powder. The performance of this new calixsalen-type catalyst was examined in various transformations, particularly in its ability to promote nucleophilic epoxide ring opening under heterogeneous conditions. The target products were obtained in high yields and with improved selectivity compared with those obtained by using analogous linear polymers. The arrangement of the catalytic sites in the cyclic structure is probably more suitable for the necessary cooperative bimetallic pathway of this demanding reaction. The catalyst could be successfully recycled. This approach represents the first use of calixsalen complexes under heterogeneous catalytic conditions.

Full text of DOI:10.1002/chem.201001012

DOI: 10.1002/chem.201001012
New Chiral Calixsalen Chromium Complexes: Recyclable Asymmetric
Catalysts**
Anaꢀs Zulauf, Mohamed Mellah, and Emmanuelle Schulz*[a]
Abstract: A chiral N,N’-bis(salicylide-
ne)ethylenediamine (salen) polymer
has been prepared by a condensation
powder. The performance of this new
calixsalen-type catalyst was examined
in various transformations, particularly
in its ability to promote nucleophilic
epoxide ring opening under heteroge-
neous conditions. The target products
were obtained in high yields and with
improved selectivity compared with
those obtained by using analogous
linear polymers. The arrangement of
the catalytic sites in the cyclic structure
is probably more suitable for the neces-
sary cooperative bimetallic pathway of
this demanding reaction. The catalyst
could be successfully recycled. This ap-
proach represents the first use of calix-
salen complexes under heterogeneous
catalytic conditions.
reaction
between
a
thio-
phenedisalicyladehyde derivative and
(S,S)-cyclohexane-1,2-diamine.
This
polymeric compound was demonstrat-
ed to possess a cyclic structure with
two to five repetitive units. The addi-
tion of chromium(II) salts led to the
generation of a chiral catalyst that
could be recovered as an insoluble
Keywords: asymmetric catalysis
catalyst recycling · heterogeneous
catalysis N,O ligands
ring opening
·
·
·
Introduction
of organic[6] or inorganic supports.[7] Other methodologies
involve noncovalent interactions with different supports
based on, for instance, electrostatic exchanges.[8] Whereas
the first method involves expensive, time-consuming organic
syntheses, the second can suffer from low recyclability due
to significant catalyst leaching. Procedures that combine the
advantages of both homo- and heterogeneous catalyses
often come from modifications of the ligand at some dis-
tance from the metallic coordinating sites, leading to a mo-
nomer that is suitable for polymerization within the main
chain. Some very recent examples have been described deal-
ing with coordination polymers.[9] The most developed pro-
cedure, however, involves polycondensation reactions be-
tween suitably modified diamine and disalicyladehyde deriv-
atives. For instance, Kureshy and co-workers described the
synthesis of salen oligomers by polycondensation, and pre-
pared the corresponding chromium and manganese com-
plexes.[10,11] These catalysts were soluble in the reaction
media and were active for kinetic aminolytic epoxide resolu-
tion[10] and for kinetic oxidative resolution of secondary al-
cohols.[11] The complexes were used as homogeneous cata-
lysts and could be recovered and efficiently reused after pre-
cipitation in a suitable solvent. In other examples, however,
a new class of compound was obtained in which the targeted
polymer was not linear, but possessed a macrocyclic struc-
ture, named calixsalen. Jablonski and Li prepared such mac-
rocycles with the help of barium salts as templates and ob-
Chiral N,N’-bis(salicylidene)ethylenediamine (salen) com-
plexes have been intensively studied because they constitute
one of the main catalyst families that can be used to prepare
valuable, highly enantioenriched synthons.[1] Associated with
numerous metals, they promote the catalytic enantioselec-
ACTHNUTRGNEUNG
tive formation of carbon–carbon[2] or carbon–heteroatom[3]
bonds with high efficiencies in terms of both activity and se-
lectivity. Following the principles of green chemistry,[4] one
major goal is now to establish efficient procedures for the
recovery and reuse of such catalysts. However, although
many studies have already been directed towards the fine-
tuning of new methods for catalyst recycling,[5] no satisfacto-
ry global solution exists that affords the targeted enantio-
pure product in high yield, free of metal traces, in an eco-
nomic and environmentally friendly process. Various hetero-
genization procedures have been described that involve the
modification of the salen structure through covalent grafting
[a] Dr. A. Zulauf, Dr. M. Mellah, Dr. E. Schulz
Equipe de Catalyse Molꢀculaire
ICMMO - UMR CNRS 8182 - Universitꢀ Paris Sud XI
Bat 420-91405 Orsay cedex (France)
Fax : (+33)169-15-4680
E-mail: emmanuelle.schulz@u-psud.fr
[**] Salen=N,N’-bis(salicylidene)ethylenediamine.
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FULL PAPER
tained the corresponding manganese complexes, which
proved to be active, soluble catalysts that promoted alkene
epoxidation reactions.[12] Some reports describe the synthesis
of a range of bridged calixsalens,[13] and a number of them
were driven towards the selective formation of macrocycles
with defined size.[14] Jacobsen et al. reported the synthesis of
cobalt–calixsalen complexes, which were obtained as a mix-
ture of different oligomers.[15,16] These catalysts were used
for epoxide kinetic resolution under homogeneous condi-
tions to yield the targeted products with excellent enantiose-
lectivities.[15] This mixture was also recently used to mediate
intramolecular oxetane ring opening.[16] However, to the
best of our knowledge, no complexes involving calixsalen-
type structures have yet been used as asymmetric heteroge-
neous catalysts.
were also tested as promoters for heterogeneous asymmetric
catalysis and the results were compared with those obtained
with their linear homologues.
Results and Discussion
Synthesis of new polysalen complexes by polycondensation:
Aiming to at least match the performance of the best of the
electrogenerated Poly-cat complexes, we prepared the new
disalicylaldehyde compound 3 as a monomer to be intro-
duced into the polycondensation reaction with well-known
(S,S)-cyclohexane-1,2-diamine. The first route was based on
a Suzuki-type coupling reaction between 5-bromo-3-tert-
butyl-2-hydroxybenzaldehyde (1) and commercially avail-
able thiophene-2,5-diboronic acid. Such coupling with a di-
boronic derivative is, however, not a straightforward proce-
dure.[20] Although this strategy was recently reported with
analogous compounds,[21] it did not lead to reproducible re-
sults in our hands. The opposite
We have previously described the synthesis of chiral thio-
phene salen chromium monomers and their efficient poly-
merization by electrochemical anodic oxidation.[17] The mon-
omers (for instance, see Hom-cat in Scheme 1) proved to be
approach, however, afforded
the expected product 3 (see
Scheme 2). The preparation of
boronic ester 2 could be fine-
tuned by combining the proce-
dures described by Nocera and
Yang[22] and DiMauro and Vi-
tullo[23] through the use of bis-
Scheme 1. Electrosynthesis of an insoluble chromium thiophene–salen complex.
(pinacolato)diboron and diphe-
nylphosphinoferrocene (dppf)
palladium dichloride as a cata-
suitable catalysts for the promotion of diverse, well-known
chromium-driven reactions, and the corresponding insoluble
complexes (Poly-cat) were efficiently used for the same
transformations as heterogeneous and reusable chiral cata-
lysts.[18] Comparable results in terms of activity and selectivi-
ty could be obtained from the homo- and heterogeneous
catalyses of hetero-Diels–Alder and Henry reactions.[18a–c]
When applied to epoxide ring opening, however, Poly-cat
led to a dramatic decrease in the enantioselectivity values
relative to its homogeneous counterpart, Hom-cat.[18d] An
explanation for these results may possibly be found in the
mechanistic pathway proposed to describe the ring opening
of epoxides.[3b,19] Indeed, a bimolecular mechanism has now
been confirmed that involves two catalytic species acting in
a cooperative way to activate both the epoxide ring and the
nucleophile. The arrangement of the catalytic sites in the
electrogenerated Poly-cat complex in a ꢂpearl necklaceꢃ
manner is probably not favorable for this specific optimal
positioning of the catalytic sites. Taking these results into ac-
count, we report herein our efforts to gain deeper insights
into the course of the epoxide ring-opening reaction in the
presence of the electrogenerated heterogeneous salen com-
plexes. Furthermore, we have prepared structurally distinct
polysalen chromium derivatives through a chemical polycon-
densation reaction that has led to the generation of new
chiral calixsalen–thiophen compounds. These derivatives
lyst under microwave activation. Compound 2 was isolated
in a low but constant yield of 33%.
Starting from boronic ester 2 and 2,5-dibromothiophene,
a double Suzuki coupling was successfully performed with
potassium carbonate as base and palladium tetrakisdiphe-
nylphosphane as catalyst, which was used in a very low
ratio.[24] Dimer 3 was isolated in nearly quantitative yield
after purification by silica gel chromatography. Subsequent
condensation with (S,S)-cyclohexane-1,2-diamine was per-
formed in THF, in which both compounds were soluble. At
the end of the transformation, anhydrous methanol was
added, leading to the precipitation of a yellow powder,
1
which was recovered by filtration. The H NMR spectrum in
CDCl3 revealed a number of similar signals in very close
proximity, which was clearly indicative of a mixture of dif-
ferent compounds possessing similar structures. This mixture
was further analyzed by MALDI-TOF spectroscopy, and the
results indeed indicated the presence of different macrocy-
cles possessing two to five salen units, with each repetitive
unit (with the loss of a water molecule) having a molecular
mass of 514 gmolÀ1, thus confirming the presence of cyclic
structures 4.
Attempts were made to separate the macrocycles to
obtain well-defined molecules. However, neither silica gel
chromatography nor separation by steric exclusion on type
LH-20 material led to satisfactory results; NMR spectrosco-
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11109

E. Schulz et al.
508C afforded the products 6 or
8 with shorter reaction times
(Table 1, entries 2 and 7). The
enantioselectivity values for
both compounds nevertheless
remained less impressive than
the results reported by Jacobsen
et al.,[26] clearly indicating that
replacement of the sterically
hindered donating tert-butyl
group by a thiophene moiety is
accompanied by a distinct re-
duction of catalyst selectivity.
These values are, however, sig-
nificant enough to be compared
to those arising from the use of
their heterogeneous counter-
parts, Poly-cat and Calix-cat.
Poly-cat was then used as an
insoluble complex in MTBE
and introduced into the trans-
formation of 5; the reaction
Scheme 2. Synthesis of a new thiophene–salen chromium Calix-cat.
py analysis always indicated mixtures of compounds. This
mixture was therefore dissolved in THF and stirred with
chromium chloride salts, first under an argon atmosphere
and then in the presence of air, following a procedure de-
scribed by Jacobsen et al. for the preparation of salen–chro-
mium complexes.[25] Calix-cat complexes could thus be iso-
lated in high yield as a brown powder, for which the elemen-
tal analysis revealed a slight deficit in the chromium content
compared with the total number of possible anchoring sites.
These new chiral complexes were then tested, as a mixture,
for their ability to promote a number of asymmetric catalyt-
ic reactions and, particularly, the asymmetric ring opening
of epoxides.
Table 1. Homogeneous and heterogeneous thiophene–salen chromium
complexes in the epoxide ring-opening reactions.
Entry
Catalyst
T [8C]
t [h]
Yield [%][a]
ee [%][b]
1
2
3
Hom-cat[c]
Poly-cat[d]
20
50
50
50
50
72
24
24
24
24
98
70
85
90
97
49
59
5
5
6
4[e]
5[f]
6
7
8
Hom-cat[c]
Poly-cat[d]
20
50
50
72
24
24
40
63
95
88
70
24
Catalytic tests under homogeneous and heterogeneous con-
ditions: In 1995 Jacobsen and co-workers discovered the
ability of chromium–salen complexes to very efficiently pro-
mote the ring opening of epoxides with trimethylsilylazide
as a nucleophile.[26] They reported high activity and enantio-
selectivity values of up to 98%. Because mechanistic studies
revealed a cooperative bimetallic pathway involving the si-
multaneous activation of both species,[19] some articles have
reported the preparation of chromium dimers to favor this
cooperativity.[8a,10a,27] We thus aimed to test our catalysts
under both homo- and heterogeneous conditions in epoxide
ring-opening reactions using commercially available 7-oxa-
[a] Isolated yield. [b] Enantiomeric excess determined by chiral GC anal-
yses. [c] 2 mol%. [d] 4 mol%. [e] 1st reuse of Poly-cat. [f] 2nd reuse of
Poly-cat.
was performed at 508C to favor mass transfer under the het-
erogeneous conditions. The expected product 6 could be sat-
isfactorily isolated in a very good 85% yield, however, in
nearly racemic form (Table 1, entry 3). The complex Poly-
cat could be recovered as an insoluble powder in the follow-
ing way: After complete conversion of the substrate, the so-
lution was removed from the Schlenk tube through a filtra-
tion syringe and the remaining powdered catalyst was
washed thoroughly with MTBE and then dried. New sub-
strates were then added to the same catalyst to evaluate its
recyclability. This complex showed high stability for two fur-
ther reuses (Table 1, entries 4 and 5) and allowed the prepa-
bicycloACHTUNGTRENNUNG[4.1.0]heptane (5) and 3,6-dioxa-bicycloACHTUTGNREN[NUGN 3.1.0]hexane
(7; prepared according to a reported procedure from dihy-
drofuran)[28] as substrates. The reactions proceeded in
methyl tert-butyl ether (MTBE) in the presence of Hom-cat,
and led to the expected products in satisfactory to high
yields at room temperature (Table 1, entries 1 and 6). The
reactions were quite slow, but increasing the temperature to
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Recyclable Chromium Catalysts
FULL PAPER
ration of compound 6 with continued high yields. Similarly,
Poly-cat catalyzed the ring opening of epoxides 7 at a much
lower level of enantioselectivity than its homogeneous coun-
terpart (Table 1, entry 8). We assume that this significant de-
crease in the selectivity for the transformations that are run
under heterogeneous conditions is due to the structure of
the polymeric catalyst itself, with the chromium coordinating
sites arranged in a pearl necklace manner. The position of
the catalytic sites, which is critical for the cooperative bimet-
allic pathway, is probably not optimal in this case.
The results could be further improved when the ring-
opening reaction of epoxide 7 was performed with trime-
thylsilylazide at room temperature (see Table 3). The first
use of Calix-cat in this transformation allowed the prepara-
tion of the target product 8 in 73% isolated yield with
62% ee, which is a value approaching that obtained with the
corresponding homogeneous catalyst (compare Table 3,
Table 3. Use of Calix-cat in the ring-opening reaction of 7.
To gain more insight into this phenomenon, we then
tested the new Calix-cat complexes. Analogously to Poly-
cat, these new complexes also lack the important tert-butyl
groups in the 5- and 5’-positions of the phenol rings, howev-
er, they possess a cyclic structure that possibly allows a
better arrangement of the catalytic sites for this demanding
epoxide ring-opening reaction.
Calix-cat, as a complex oligomeric mixture, was initially
used in the transformation of compound 5. The complex
was insoluble in MTBE, the reaction solvent, which allowed
the reaction to be run under heterogeneous conditions. To
our delight, this new chiral catalyst provided the expected
product after 15 h reaction time at 508C with a high isolated
yield (Table 2, entry 1) and 42% ee. This value is not as high
as that obtained with Hom-cat under homogeneous condi-
Run
Yield [%][a]
ee [%][b]
1
2
3
4
5
73
47
66
62
64
62
64
64
67
66
[a] Isolated yield. [b] Enantiomeric excess determined by chiral GC anal-
yses.
entry 1, with Table 1, entry 6). With respect to selectivity,
Calix-cat is thus confirmed to be a superior catalyst to its
equivalent linear analogue, Poly-cat, probably because of its
cyclic structure, which facilitates bimolecular catalysis. The
enantioselectivity value obtained with this heterogeneous
catalyst did not reach the maximal value obtained with the
homogeneous species (i.e., 88%), probably because Calix-
cat is a mixture of differently sized macrocycles, which leads
to an average, not optimized, value. Work is in progress to
separate these macrocycles so that the optimal ring size for
this challenging transformation can be found.
Calix-cat could also be recovered after the first run of the
reaction and new substrates added for its reuse in four new
consecutive runs. To our delight, under these optimized re-
action conditions, compound 8 could be isolated with a re-
markably stable enantioselectivity and in satisfying yields.
Calix-cat derived from chromium thiophene–salen com-
plexes are thus good candidates for the efficient promotion
of epoxide ring-opening reactions, as new chiral heterogene-
ous recyclable catalysts.
Table 2. Use of Calix-cat in the ring-opening reaction of 5.
Run
Yield [%][a]
ee [%][b]
1
2
3
4
5
6
90
87
91
84
94
87
42
36
41
36
22
27
[a] Isolated yield. [b] Enantiomeric excess determined by chiral GC anal-
yses.
tions, but it is much improved compared with the same cata-
lytic reaction run with Poly-cat. We assume that this im-
provement is due to the ability of the cyclic calix structure
to arrange the position of the catalytic sites in a favorable
manner.
Calix-cat, as an insoluble powder, could also be recovered
and reused in successive catalytic runs as reported in
Table 2. This catalytic species showed a high stability in re-
spect to its activity, and product 6 could be isolated in con-
sistently high yield for six cycles. This recycling was, howev-
er, accompanied by a significant decrease in the selectivity
value achieved after each progressive run. This example
nevertheless represents, as far as we know, the first use of
calixsalen complexes in asymmetric catalytic heterogeneous
transformations.
We have previously reported[18] the efficient use of the
electrogenerated polymer Poly-cat to catalyze the hetero
Diels–Alder reaction,[25,29] the Henry reaction,[30] and the ad-
dition of dimethylzinc to aldehydes.[31] Corresponding Calix-
cat salen was also applied to these transformations to test its
suitability as an asymmetric catalyst. The first reaction in-
volved the hetero-Diels–Alder reaction between heptanal 9
and Danishefskyꢃs diene at room temperature in MTBE. 2-
Hexyl-2,3-dihydropyran-4-one (10) was isolated with an
enantioselectivity value similar to that obtained by using
Poly-cat and in a similar yield (Table 4, entries 1–3). It is
known that, for this transformation, no bimolecular mecha-
nism is required for efficient catalysis and, although they
possess clearly different macrostructures, both heterogene-
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E. Schulz et al.
ous catalysts led to the same activity, probably due to a simi-
lar arrangement of catalytic sites. We then tested a different
transformation: the nitroaldol reaction between 2-methoxy-
benzaldehyde (11) and nitromethane, to afford 1-(2-methoxy-
alysts in different transformations and, particularly, in enan-
tioselective epoxide ring opening. They compared well with
the homogeneous monomer counterparts in terms of activi-
ty, and allowed the preparation of target products with satis-
factory levels of enantioselectivity. Furthermore, they
proved to be much more selective than analogous catalysts
possessing a linear polymeric structure, thus confirming that
their cyclic structure was more suitable for transformations
involving a bimolecular activation mechanism. Interestingly,
these macrocyclic chiral chromium complexes were insolu-
ble in the reaction mixture, thus, they act as heterogeneous
catalysts. They could be recovered by filtration and efficient-
ly reused to promote the transformation of a new substrate
batch. As far as we know, this report represents the first use
and recycling of calixsalen complexes under heterogeneous
catalytic conditions. Work is underway to evaluate the abili-
ty of these new structures to promote other asymmetric cat-
alytic transformations, either as a mixture or as isolated spe-
cies.
Table 4. Monomeric and polymeric chromium thiophen–salen complexes
as chiral catalysts.
Entry
Catalyst
Yield [%][a]
ee [%][b]
1
2
3
Hom-cat[c]
Poly-cat[d]
Calix-cat[d]
53
63
58
72
63
61
4
5
6
Hom-cat[c]
Poly-cat[d]
Calix-cat[d]
88
92
77
74
54
65
Experimental Section
General methods: All reactions were carried out under an argon atmos-
phere in oven-dried glassware with magnetic stirring. Solvents were dis-
tilled before use: THF from sodium metal/benzophenone, and MTBE,
DME, 1,4-dioxane, and CH2Cl2 from calcium hydride. 7-Oxa-bicyclo-
7
8
9
Hom-cat[c]
Poly-cat[d]
Calix-cat[d]
79
5
30
93
20
60
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
[a] Isolated yield. [b] Enantiomeric excess determined by chiral GC or
HPLC analyses. [c] 2 mol%. [d] 4 mol%.
dihydrofuran.[28] The epoxides were distilled before use and trimethylsily-
lazide was used as received from commercial sources. The synthesis of
Hom-cat and Poly-cat were performed as previously described, as were
the hetero Diels–Alder, the Henry reaction, and the addition of dime-
thylzinc to benzaldehyde.[18] 1H and 13C NMR spectra were recorded with
either Bruker AM 360 (360 MHz), AM 300 (300 MHz), or AM 250
(250 MHz) instruments with samples dissolved in CDCl3; data are report-
phenyl)-2-nitroethanol (12; Table 4, entries 4–6). In this
case, again, Calix-cat was found to be an efficient catalyst,
affording the expected product in satisfactory yield and
enantioselectivity. Under these heterogeneous conditions,
the transfer of chirality occurred smoothly from both insolu-
ble polymers. Finally, the addition of dimethylzinc to alde-
hydes, recently described by Cozzi and Kotrusz,[31] was also
attempted in the presence of Calix-cat. The monomeric
chromium thiophene–salen species Hom-cat showed high
activity and enantioselectivity for this transformation, pro-
viding compound 14 from benzaldehyde with 93% ee
(Table 4, entry 7). Surprisingly, we found that the electro-
generated complex Poly-cat was almost inactive for this
transformation (Table 4, entry 8), whereas Calix-cat afforded
1-phenylethanol with 60% ee, albeit in low yield (Table 4,
entry 9). The mechanism of this reaction is not well under-
stood, but the increase in enantioselectivity obtained
through the use of cyclic structures may indicate a possible
bimetallic intermediate.
1
ed in ppm with the solvent signal as reference (d=7.27 ppm for H NMR
and d=77 ppm for 13C NMR). Optical rotations were measured at the
sodium D-line for solutions in 10 cm cells with a Perkin–Elmer 241 polar-
imeter. IR spectra were recorded as KBr disks with a Perkin–Elmer spec-
trometer. Mass spectra were recorded with a Finnigan MAT 95 S spec-
trometer. UV/Vis experiments were performed with a Uvikon polarime-
ter from Bio-tek instruments. GC analyses were carried out with a
GC430 Varian instrument equipped with a FID detector and a split/split-
less injector.
3-tert-Butyl-2-hydroxy-5-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)ben-
zaldehyde (2): 3-tert-Butyl-5-bromo-2-hydroxybenzaldehyde (500 mg,
1.94 mmol), bis(pinacolato)diborane (543 mg, 2.14 mmol), potassium car-
bonate (538 mg, 3.89 mmol), and [PdCl2ACTHNUGTRNEUNG(dppf)] (159 mg, 0.19 mmol) were
introduced into a 10 mL microwave reaction vessel, and 1,4-dioxane
(2.5 mL) was added. The vessel was sealed and the mixture was heated in
the microwave for 45 min at 1408C (100 W) then allowed to cool to RT.
The mixture was filtered through Celite and the solvents were removed
under reduced pressure. The residue was purified by flash chromatogra-
phy on silica gel (heptane/diethyl ether 90:10) to afford 2 as a yellow
solid (175 mg, 33%). M.p. 86–888C; 1H NMR (250 MHz, CDCl3): d=
12.00 (s, 1H), 9.91 (s, 1H), 7.93–7.89 (m, 2H), 1.44 (s, 9H), 1.36 ppm (s,
12H); 13C NMR (90 MHz, CDCl3): d=197.5, 163.6, 140.0, 139.9, 137.4,
120.4, 83.9, 34.8, 29.2, 24.8 ppm; MS (EI): m/z (%): 304 (20), 290 (17),
289 (100), 288 (26).
Conclusion
2,5-Di[5-(3-tert-butyl-2-hydroxybenzaldehyde)]thiophene (3): A Schlenk
tube was charged with 2 (175 mg, 0.58 mmol), 2,5-dibromothiophene
An efficient synthetic procedure for the preparation of new
chiral calixsalen-type chromium complexes has been de-
scribed. These compounds were tested as heterogeneous cat-
(26 mL, 0.23 mmol), [Pd
ACHTNUGRTEN(UNGN PPh3)4] (8 mg, 0.007 mmol), and K2CO3 (48 mg,
0.35 mmol) and the mixture was placed under argon by successive
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Chem. Eur. J. 2010, 16, 11108 – 11114

Recyclable Chromium Catalysts
FULL PAPER
vacuum–argon cycles (3 h). Thoroughly degassed DME (460 mL) and de-
gassed water (154 mL) were introduced into the Schlenk tube by using a
cannula. The mixture was heated at 1008C for 24 h, then water (10 mL)
was added and the aqueous layer was extracted with CH2Cl2. The organic
layer was dried over MgSO4 and the solvents were removed under re-
duced pressure. The residue was purified by flash chromatography on
silica gel (cyclohexane/ethyl acetate, 90:10) to afford 3 as a yellow solid
(112 mg, 96%). M.p. 1358C; 1H NMR (360 MHz, CDCl3): d=11.86 (s,
2H), 9.95 (s, 2H), 7.81 (d, J=2.2 Hz, 2H), 7.64 (d, J=2.2 Hz, 2H), 7.23
(s, 2H), 1.52 ppm (s, 18H); 13C NMR (90 MHz, CDCl3): d=197.0, 160.7,
142.1, 139.1, 131.6, 128.5, 125.8, 123.4, 120.6, 35.0, 29.3 ppm. HRMS
(ESI): m/z: calcd for C26H27O4S [M – H]: 435.1630; found: 435.1637.
1.71–1.57 (m, 2H), 1.41–1.02 (m, 4H), 0.14 ppm (s, 9H); 13C NMR
(90 MHz, CDCl3): d=75.1, 66.6, 34.5, 30.4, 24.0, 23.8, À0.1 ppm. MS
(IE): m/z (%): 170 (32), 143 (21), 142 (69), 129 (26), 118 (29), 75 (86), 73
(100), 45 (15).
AHCTUNGTERG(NNUN 3R,4S)-3-Azido-4-trimethylsiloxytetrahydrofurane (8): Colorless oil;
[a]2D0 =À20.8 (c 1.02, CHCl3) for 36% ee, Lit.[26]: [a]2D3 =+64.0 (c 2.35,
CHCl3) for 98% ee material. The absolute stereochemistry was assigned
as (3R,4S) based on comparison of the measured rotation with the litera-
ture value.[26] The ee was determined by GC analysis using a chiraldex
column (50 m ꢄ 0.25 mm ꢄ 0.25 mm, 1208C), which resolved both enantio-
(minor)=7.86 min]. 1H NMR (360 MHz,
mers [tRACHTNUGTRNENUG(major)=7.73 min, tRCAHTUNGTRENNNUG
CDCl3): d=4.25–4.21 (m, 1H), 3.99 (ddd, J=4.6, 9.4, 19.7 Hz, 2H), 3.83–
3.74 (m, 2H), 3.6 (dd, J=2.7, 9.4 Hz, 1H), 0.15 ppm (s, 9H); 13C NMR
(90 MHz, CDCl3): d=76.2, 74.1, 70.2, 67.6, À0.3 ppm; MS (EI): m/z (%):
144 (14), 117 (26), 102 (51), 101 (73), 75 (30), 73 (100), 59 (24), 45 (20).
Calixsalen ligand 4: (S,S)-Cyclohexane-1,2-diamine (145 mg, 1.27 mmol)
was added to a solution of dialdehyde 3 (553 mg, 1.27 mmol) in THF
(18 mL) with continuous stirring, and the mixture was heated at 708C for
24 h. The reaction was cooled to RT, THF was partially evaporated and
methanol was added. The solid was filtered and washed with cold metha-
nol to afford 4 as a yellow powder (538 mg, 83%). M.p. 173–1778C;
1H NMR (360 MHz, CDCl3): d=14.40–13.72 (m, 2H), 8.65–8.28 (m, 2H),
7.68–6.72 (m, 6H), 3.48–3.05 (m, 2H), 2.18–1.62 (m, 8H), 1.48 ppm (s,
18H); 13C NMR (90 MHz, CDCl3): d=213.8, 165.4, 160,2, 160.1, 160.0,
142.5, 142.4, 137.8, 137.7, 127.4, 127.2, 126.6, 126.4, 124.6, 124.5, 122.5,
122.4, 118.6, 118.5, 118.4, 34.9, 29.5 ppm. IR (KBr): n˜ =2936, 2862, 1629,
1439 cmÀ1; [a]D20 =À33 (c 1.0, CHCl3); UV/Vis (CH3CN): lmax (e)=221,
250, 347 nm.
Acknowledgements
The CNRS, the Ministꢅre de lꢃEnseignement Supꢀrieur et de la Recher-
che and the program “Chimie et Dꢀveloppement Durables” from CNRS
are acknowledged for financial support.
Calix-cat: Chiral ligand 4 (150 mg, 0.29 mmol) in anhydrous, degassed
THF (5 mL) was added to a solution of anhydrous CrCl2 (39.4 mg,
0.32 mmol) in anhydrous, degassed THF (5 mL). The resulting brown so-
lution was stirred for 2 h under argon and then exposed to air. Stirring
was continued overnight to give a dark-brown solution, which was diluted
with CH2Cl2 and washed with sat. NH4Cl and brine. The organic layer
was dried over MgSO4 and solvents were removed under reduced pres-
sure to afford the expected complex (130 mg, 74%). IR (KBr): n˜ =2939,
2862, 1622 cmÀ1; elemental analysis calcd (%) for C32H36ClCrN2O2S2
(632): C 64.04, H 6.05, N 4.67, S 5.34, Cl 5.91, Cr 8.66; found: C 59.70, H
6.82, N 3.76, S 3.99, Cl 4.63, Cr 4.95; [a]2D0 =+371 (c 0.18, CHCl3); UV/
Vis (CH3CN) : lmax (e)=226, 359 nm.
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A
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ACHTUNGTRENNUNG =
(1R,2R)-1-Azido-2-trimethylsiloxycyclohexane (6): Colorless oil; [a]D20
+18.5 (c 0.99, CHCl3) for 58% ee, Lit.[26]: [a]D23 =À22.4 (c 2.87, CHCl3)
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column (50 m ꢄ 0.25 mm ꢄ 0.25 mm, 1208C), which resolved both enantio-
mers [t
G
N
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Chem. Eur. J. 2010, 16, 11108 – 11114
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Received: April 19, 2010
Published online: August 4, 2010
11114
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Chem. Eur. J. 2010, 16, 11108 – 11114