DIANANE-CrIII-salen complexes as highly enantioselective catalysts for hetero-Diels-Alder reactions of aldehydes with dienes

Article abstract of DOI:10.1002/ejoc.200600359

A new type of chromium-salen complex bearing DIANANE (endo,endo-2,5- diaminonorbornane) as chiral backbone has been synthesized and applied to the hetero-Diels-Alder reaction of Danishefsky's diene with various aldehydes. The reactions afford the correspo

Full text of DOI:10.1002/ejoc.200600359

FULL PAPER
DOI: 10.1002/ejoc.200600359
DIANANE-CrIII-salen Complexes as Highly Enantioselective Catalysts for
Hetero-Diels–Alder Reactions of Aldehydes with Dienes
Albrecht Berkessel*[a] and Nadine Vogl[a]
Keywords: Asymmetric catalysis / Chromium / Hetero-Diels–Alder reaction / Homogeneous catalysis
A new type of chromium-salen complex bearing DIANANE
(endo,endo-2,5-diaminonorbornane) as chiral backbone has
been synthesized and applied to the hetero-Diels–Alder reac-
tion of Danishefsky’s diene with various aldehydes. The reac-
tions afford the corresponding 2-substituted 2,3-dihydro-4H-
pyran-4-ones in high yields and enantioselectivities (up to
96% ee). The effect of different counteranions of the complex
on reactivity as well as enantioselectivity has been investi-
gated. Besides Danishefsky’s diene, reaction of the less reac-
tive 1-methoxybutadiene with benzaldehyde and ethyl
glyoxylate can be effected by the chromium catalysts as well.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
Introduction
BIII [23] AlIII [6,24,25] CrIII [7,10,11,26] EuIII [27] YbIII [28,29]
, , , ,
,
MnIV,[10,26] and TiIV[15,30,31] in combination with chiral li-
gands containing carboxamidate,[9] BINOL,[31–33] 3,3Ј-di-
substituted BINOL,[6,12,24] salen,[7,10,19,26,34] bisoxa-
zoline,[21,31,33,35] and triflylamide[29] catalyze the HDA cy-
cloaddition. In this context, the catalysts developed by Ding
et al.[12,13,36] and Jacobsen et al.[7,8,11,17] deserve special
mention (Zn and Cr complexes, respectively) as enantio-
selectivities of up to 99% have been obtained for the cyclo-
addition of benzaldehyde to Danishefsky’s diene.
The asymmetric hetero-Diels–Alder (HDA) cycload-
dition of dienes with aldehydes is a powerful and well-inves-
tigated reaction in organic synthesis. In principle, up to
three new stereocenters can be formed in a single step (see
Scheme 1, equation a).
The chiral pyran derivatives generated by this reaction
are versatile building blocks for the synthesis of many biolo-
gically active compounds including, for example, carbo-
hydrates,[1] pheromones,[2] antitumor agents,[3] and sesqui-
terpenoids.[4]
While many catalytic systems give good yields and
Since the discovery by Danishefsky et al.[5a] that Lewis enantioselectivities in the enantioselective version of the
acids catalyze the hetero-Diels–Alder reaction between al- HDA reaction of Danishefsky’s diene with aldehydes, only
dehydes and electron-rich dienes such as 1-methoxy-3-(tri- a few methods for the [4+2] cycloaddition of less-electron-
methylsilyloxy)butadiene
(Danishefsky’s
diene;
see rich dienes with aldehydes have been developed so far. This
Scheme 1, equation b), a variety of chiral Lewis acid cata- reaction requires strong Lewis acid catalysts that can acti-
lysts have been developed.[5b,6–17] It has been shown that vate the aldehyde but which do not promote side reactions,
different metal centers like CoII,[18,19] CuII,[20,21] RhII,[9,22] for example polymerization of sensitive dienes such as 1-
Scheme 1. HDA reaction of dienes with aldehydes (equation a) and of Danishefsky’s diene with aldehydes (equation b).
methoxybuta-1,3-diene. Very good results in the cycload-
dition of 1-methoxybuta-1,3-diene derivatives with glyoxyl-
ates have been obtained by the method developed by Mi-
[a] Institut für Organische Chemie, Universität zu Köln,
Greinstr. 4, 50939 Köln, Germany
Fax: +49-221-4705102
E-mail: berkessel@uni-koeln.de
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A. Berkessel, N. Vogl
FULL PAPER
kami et al.[37] using the BINOL-Ti complex (ee up to 96%). by adding excess CrCl2 to a solution of the deprotonated
However, at least 10 mol-% of the catalyst is necessary and salen ligand 2 in thf under inert atmosphere and stirring for
the method requires substantial technical experience, as re- 12 h at room temperature. The CrIII complex could be iso-
ported by Kalesse et al.[38] Excellent diastereo- and enantio- lated in 78% yield by subsequent oxidation in air for 12 h,
selectivities in the reaction of several less reactive dienes followed by an aqueous work-up. The perchlorate (3b), tos-
(compared to Danishefsky’s diene) with nonactivated alde- ylate (3c), and tetrafluoroborate (3d) salts were obtained
hydes have been reported by Jacobsen et al. employing a from the chloride 3a upon treatment with the different sil-
CrIII complex with a chiral tridentate Schiff-base ligand as ver salts in tert-butyl methyl ether (mtbe). Complex 3e bear-
–
catalyst.[8]
ing BPh4 as counteranion was synthesized from 3b upon
As far as salen ligands are concerned, substantial effort reaction with NaBPh4.
has been invested to varying the steric and electronic prop-
erties of the salicylic aldehyde moiety, whereas the influence
of the diamine has been much less investigated. We decided
to replace the usually employed 1,2-diamines like cyclo-
hexanediamine by a 1,4-diamine with the aim of investiga-
ting how an increase in the N–N distance influences the
reactivity and selectivity of the corresponding metal–salen
complexes (Figure 1). Recently, we reported an effective
enantioselective synthesis of DIANANE (1), which is a new
chiral 1,4-diamine, and its corresponding salen ligand 2.[39]
Furthermore, the latter ligand has become commercially
available. In this paper, we describe the synthesis of chiral
CrIII-salen complexes 3 bearing the DIANANE backbone
and their use as catalysts for the asymmetric HDA reaction
of dienes with aldehydes.
Figure 2. DIANANE-CrIII-salen complexes 3a–e bearing different
counteranions.
Catalytic Asymmetric HDA Reaction of Danishefsky’s
Diene with Benzaldehyde
We first employed the chiral CrIII-salen 3a to optimize
the reaction conditions for the HDA between Danishefsky’s
diene (4) and benzaldehyde (5a) [Equation (1)].
(1)
First, the influence of water and air on the course of the
reaction was studied. It was found that conversion and yield
were higher when the HDA cycloaddition was conducted
under an inert atmosphere in the presence of molecular
sieves (∆ up to 13%). Of the various solvents employed,
mtbe turned out to be the best one with respect to catalytic
activity and enantioselectivity. The effect of catalyst loading
is summarized in Table 1.
Figure 1. Salen ligands derived from trans-1,2-diaminocyclohexane
and DIANANE, and the corresponding N–N distances. a) N–N
distance in the ligand derived from 3,5-di-tert-butylsalicylic alde-
hyde[40]. b) N–N distances in various differently substituted salen
ligands[39,41]
Table 1. Effect of catalyst loading on the reaction of diene 4 with
benzaldehyde (5a).[a]
Entry
Cat. loading Conv.[b,c]
Yield[c]
[%]
ee[d]
[%]
[mol-%]
[%]
1
2
3
4
5
8
4
2
1
91
90
61
32
32
87
80
42
19
13
92
92
91
85
76
Results and Discussion
0.5
Preparation of the Chromium-Salen Complexes
[a] All reactions were performed in mtbe in the presence of dry 3-
Å molecular sieves at –21 °C; reaction time: 28 h. [b] Based on the
aldehyde. [c] Determined by capillary GC using Ph2O as internal
standard. [d] Determined by capillary GC.
The salen ligand 2 (Figure 1) was prepared according to
our previous publication.[39] The corresponding CrIII-salen
complex 3a (Figure 2) was synthesized following the pro-
cedure of Jacobsen et al.[7] with the exception that triethyl-
Optimal conditions were found at a catalyst loading of
amine had to be added in order to effect quantitative com- 4 mol-%, where a high yield of product was obtained
plex formation. First, the CrII-salen complex was formed (entry 2, conv. 90%, yield: 80%) after 28 h. The (S)-(+)-
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DIANANE-CrIII-salen Complexes as Highly Enantioselective Catalysts
FULL PAPER
enantiomer of the HDA product 6a was formed with an poor aldehyde 5f (67% ee, entry 6), which could be isolated
enantiomeric excess of 92%. A lower catalyst loading or in a moderate yield of 45%. Unfortunately, pivalaldehyde,
further lowering of the temperature retards the cycload- pyrrol-2-carbaldehyde, and pyridine-2-carbaldehyde, which
dition. At higher temperatures, faster conversion but lower were tested during these studies as well, did not show any
enantiomeric excess was observed.
reactivity towards 4. For pyridine-2-carbaldehyde, this ob-
This initial success prompted us to apply the optimized servation may be explained by its coordinating ability
reaction conditions of Table 1 (entry 2) to the asymmetric towards transition metal catalysts in general, which results
HDA of 4 with a number of other aldehydes 5. The results in deactivation of the chromium complex 3a.
are summarized in Table 2.
In order to investigate the effect of the counteranion on
the activity and selectivity of the catalyst, we synthesized
the chromium complexes 3b–e from complex 3a by anion
exchange. The performance of these complexes in the asym-
metric HDA cycloaddition of 4 with 5a is summarized in
Table 3.
Table 2. Asymmetric HDA of different aldehydes 5a–f with diene 4
employing catalyst 3a.[a]
Table 3. Effect of the catalyst counterion on the asymmetric HDA
of benzaldehyde with diene 4.[a]
Entry
5
T
[°C]
t
[h]
Conv.[b] Yield[c] ee[d]
[%]
[%]
[%]
1
2
5a
5b
–21 to –24
28
68
24
24
43
20
91
92
96
58
85
97
98
87
100
80
84
85
46
32
93
72
54
45
92 (92)
95 (95)
90 (90)
93 (93)
96 (96)
90 (98)
84 (98)
79 (79)
61 (67)
Entry
Catalyst
Conv.[b,c]
[%]
Yield[c]
[%]
ee[d]
[%]
–12
–16
–12
–12
–16
5 to 10
–35 to –26
3
4
5c
5d
1
2
3
4
5
3a
3b
3c
3d
3e
62
55
14
70
39
61
13
3
5
19
95
94
90
50
87
23
5
6
5e
5f
68[e]
69[f]
[a] All reactions were performed in mtbe in the presence of dry 3-
Å molecular sieves using 4 mol-% of catalyst. [b] Determined by
capillary GC using Ph2O as internal standard, based on the alde-
hyde. [c] Yield of isolated products. [d] Determined by capillary
GC, values in brackets are the ee values of the isolated products
after column chromatography; for 6d (entry 4), the value in brack-
ets resulted from additional crystallization. [e] 22 h at 5 °C and 46 h
at 10 °C. [f] 18 h at –35 °C and 51 h at –26 °C.
[a] All reactions were performed at –24 °C in mtbe in the presence
of dry 3-Å molecular sieves using 4 mol-% of catalyst 2a; reaction
time: 24 h. [b] Based on the aldehyde. [c] Determined by capillary
GC using Ph2O as internal standard. [d] Determined by capillary
GC.
From Jacobsen’s work it is known that CrIII-salen com-
–
Aromatic as well as aliphatic aldehydes could be acti- plexes bearing less coordinating counterions such as BF4
vated by the chiral Lewis acid 3a such that they underwent and B(Ar)4 are less reactive and less enantioselective than
–
cycloaddition with diene 4 in an asymmetric way. The ex- complexes with strongly coordinating anions.[4] Of our
pected dihydropyranones 6a–f could be obtained in moder- CrIII-salen complexes 3a–e tested in the HDA cycload-
ate to good yields and, in most cases, excellent enantio- dition, only 3a shows satisfactory results in terms of yield
selectivities (Ͼ90% ee). In particular, besides benzaldehyde and enantioselectivity. With complexes 3b–e, either the con-
5a (entry 1), the aliphatic aldehydes heptanal (5b) and cy- version was very low (Table 3, entries 3 and 5) or the selec-
clohexylcarbaldehyde (5c) as well as furfural (5d) also re- tivity for HDA cyclization was poor due to side reactions
acted with Danishefsky’s diene to afford the corresponding (Table 3, entries 2 and 4). Catalysts 3d and 3e, which bear
dihydropyranones 6a–d with high ee (93%, 96%, and 90% less coordinating counterions, proved to be less enantiose-
ee, respectively; see entries 2–4) after acidic workup. In the lective than the catalysts bearing more strongly coordinat-
case of 5d, the HDA product crystallized readily after puri- ing anions (3a–c). This is in accordance with the findings
fication by column chromatography. This single crystalli- by Jacobsen and co-workers.[7]
zation resulted in a significantly enhanced enantiomeric ex-
Next, the suitability of catalyst 3a for the HDA of 1-
cess (98% ee, entry 4). trans-Cinnamic aldehyde (5e), which methoxybuta-1,3-diene (7), a diene of lower electron den-
is an α,β-unsaturated aldehyde, underwent cycloaddition sity, with benzaldehyde (5a) and ethyl glyoxylate (5f) was
relatively sluggishly. Therefore, the reaction temperature investigated; the results are summarized in Table 4.
had to be raised to 10 °C (entry 5), whereupon it provides
With 5a and 7 as substrates, a slow reaction could be
the dihydropyranone 6e in moderate yield and enantio- effected by catalyst 3a. After 64 h reaction time, an alde-
selectivity (54% yield, 79% ee, entry 5). A somewhat lower hyde conversion of 35% was obtained and the product was
enantioselectivity was obtained with the electron- isolated in 11% yield as a mixture of diastereomers. The cis
Eur. J. Org. Chem. 2006, 5029–5035
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A. Berkessel, N. Vogl
FULL PAPER
Table 4. HDA of 1-methoxybuta-1,3-diene (7) with 5a and 5f em- 5f, producing the cis diastereomers 8b and 9b in high excess
ploying catalyst 3a.[a]
(up to 95:5 dr) and moderate to high enantioselectivity (up
to 90% ee).
Experimental Section
General: All reactions were performed in oven-dried glassware un-
der argon in a glove box, or using standard Schlenk techniques.
Molecular sieves were activated prior to use by heating in vacuo.
5
T
[°C]
t
[h]
Conv.[b] Yield dr
ee[d]
ee[d]
Solvents were dried and distilled using standard techniques. The
aldehydes were freshly distilled prior to use. 1-Methoxy-3-(trimeth-
ylsilyloxy)-1,3-butadiene (Danishefsky’s diene, 4) was purchased
from Fluka (Ն95%) or Aldrich (90%, distilled prior to use). 1-
Methoxybuta-1,3-diene was purchased from Fluka (50% solution
in toluene) and used without further purification. The ligand 2 is
commercially available from TCI (Tokyo Chemical Industries).
NMR spectra were recorded with a Bruker AC 300 instrument.
The IR measurements were performed with a Perkin–Elmer Para-
gon 1000 spectrometer. ESI mass spectra were recorded with a Fin-
nigan MAT 900 spectrometer, and GC mass spectra with a Hew-
lett–Packard HP 6890 gas chromatograph with an HP 5973 mass
selective detector. Capillary GC data were obtained with a Hew-
lett–Packard HP 6890 gas chromatograph with a Chirasil-Dex col-
umn (25 m). The signs of optical rotation were determined with a
Perkin–Elmer polarimeter 343plus.
[%]
8–9[c] trans:cis 8a/9a 8b/9b
[%]
[%]
[%]
5a
5f
55 to 65
64[e] 35
11
45
5:95
25:75
14
17
75
90
–21 to –16 69[f] 64
[a] All reactions were performed in mtbe in the presence of dry 3-
Å molecular sieves. With 5a 6 mol-% of catalyst was employed,
whereas with 5f 4 mol-% was employed. [b] Determined by capil-
lary GC using Ph2O as internal standard, based on the aldehyde.
[c] Yield of isolated products. [d] Determined by capillary GC.
[e] 41 h at 55 °C and 23 h at 65 °C. [f] 45 h at –21 °C and 24 h at
–16 °C.
diastereomer 8b was formed in large excess with respect to
the trans diastereomer 8a (dr 95:5). The former was ob-
tained with an enantiomeric excess of 75%, while the minor
trans diastereomer was obtained with 14% ee. Moderate
conversion and yield were obtained after 69 h in the cyclo-
addition of 5f and 7 in the presence of catalyst 3a. In this
reaction, a mixture of diastereomers 9a (trans) and 9b (cis)
was formed in a ratio of 25:75. The minor trans product
was obtained with an ee of 17% and the predominant cis
product with an ee of 90%. The observation that the cis
product 9b is formed in excess is in accordance with the
Synthesis of the Catalyst 3a: Triethylamine (345 µL, 2.48 mmol)
was added to a solution of the salen ligand 2 (448 mg, 0.8 mmol)
in absolute and degassed thf (20 mL). After stirring for half an
hour at room temperature, CrCl2 (108 mg, 0.88 mmol) was added.
After several minutes, the reaction mixture turned from yellow to
brown. Stirring was continued at room temperature for 12 h with
exclusion of air. The flask was then opened to air and allowed to
stir for an additional 12 h. The solution was diluted with mtbe
(90 mL) and washed with saturated NH4Cl solution (3×50 mL)
and saturated NaCl solution (3×50 mL). The organic suspension
was dried with Na2SO4 and then decanted from the drying agent.
Complex 3a was then isolated by filtration as a brown solid. After
drying in vacuo, the chromium complex 3a was obtained in 78%
findings of Jurczak et al.,[42] who employed several CrIII
-
salen ligands developed by Jacobsen for the HDA cycload-
dition of 7 with n-butyl glyoxylate. The maximum enantio-
meric excess obtained in the latter study for the cis dia-
stereomer was 80%, with C2-symmetrical salen ligands.
With our catalytic system, we obtained somewhat higher
enantioselectivity in a comparable reaction.
yield (400 mg). IR (KBr): ν = 3610, 3420, 2953, 2869, 1613, 1548,
˜
1534, 1415, 1359, 1323, 1258, 1202, 1189, 1172, 1036, 841, 784,
747, 520, 487 cm–1. UV/Vis: λmax = 403.0 nm. MS (ESI, CH2Cl2/
MeOH): m/z (%) 642.40 [M+(54Cr) – Cl + MeOH], 641.40
[M+(53Cr) – Cl + MeOH], 640.40 (100) [M+(52Cr) – Cl + MeOH],
638.36 [M+(50Cr) – Cl + MeOH], 610.38 [M+(54Cr) – Cl], 609.37
[M+(53Cr) – Cl], 608.36 [M+(52Cr) – Cl], 606.36 [M+(50Cr) – Cl];
exact mass (ESI) calculated for C37H52CrN2O2 [M – Cl]+: 608.343;
found 608.342.
The catalytic results using our CrIII-salen complexes are
complementary to those using chiral CrIII-porphyrins,
which were studied in our laboratory.[43] While the CrIII
-
salen complexes bearing DIANANE as backbone (reported
in this paper) produce the cis-diastereomer 8b in excess, the
CrIII-porphyrins give predominant access to the trans dia-
stereomer 8a.
Representative Procedure for the Synthesis of Complexes 3b,c,d: The
corresponding silver salt (1 equiv.) was added to a solution of the
chromium complex 3a in absolute mtbe and the heterogeneous mix-
ture was stirred for one day at room temperature in the dark. The
reaction mixture was then filtered through celite and the solvent
was removed in vacuo.
Conclusions
Synthesis of Complex 3b: Complex 3b was obtained in a yield of
77% from 78.0 µmol of complex 3a and 1 equiv. of AgClO4 in mtbe
(12 mL). MS (ESIpos, CH2Cl2/MeOH): m/z (%) 643.33, 642.35
[M+(54Cr) – ClO4 + MeOH], 641.36 [M+(53Cr) – ClO4 + MeOH],
640.35 (100) [M+(52Cr) – ClO4 + MeOH], 638.36 [M+(50Cr) – ClO4
+ MeOH], 610.34 [M+(54Cr) – ClO4], 609.32 [M+(53Cr) – ClO4],
We have shown that the readily available CrIII-salen cata-
lyst 3a performs excellently in a number of HDA cycload-
ditions of Danishefsky’s diene 4 with various aldehydes, af-
fording enantiomeric excesses of greater than 90%. Besides
4, less reactive 1-methoxybuta-1,3-diene (7) also shows re-
activity towards the electronically diverse aldehydes 5a and 608.33 [M+(52Cr) – ClO4]. MS (ESIneg, CH2Cl2/MeOH): m/z (%)
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DIANANE-CrIII-salen Complexes as Highly Enantioselective Catalysts
101.15 [ClO4, (37Cl)], 99.17 (100) [ClO4, (35Cl)]; exact mass (ESI)
FULL PAPER
= 14.4 Hz, 1 H), 2.64 (ddd, J1 = 16.8, J2 = 3.5, J3 = 1.3 Hz, 1 H)
calculated for C37H52CrN2O2 [M – ClO4]+: 608.343; found 608.344. ppm. 13C NMR (CDCl3, 75 MHz): δ = 192.4 (s), 163.4 (d), 137.6
(s), 128.8 (d), 128.7 (d), 126.0 (d), 107.2 (d), 81.0 (d), 43.1 ppm (t).
The absolute configuration of the major enantiomer was assigned
Synthesis of Complex 3c: Complex 3c was obtained in a yield of
92% from 77.6 µmol of complex 3a and 1 equiv. of AgTos in mtbe
as (+)-S based on comparison of the measured optical rotation
(12 mL). MS (ESIpos, CH2Cl2/MeOH): m/z (%) 643.32, 642.31
with the literature value.[7]
[M+(54Cr) – Tos + MeOH], 641.31 [M+(53Cr) – Tos + MeOH],
640.31 (100) [M+(52Cr) – Tos + MeOH], 638.31 [M+(50Cr) – Tos +
(R)-(+)-2-Hexyl-2,3-dihydro-4H-pyran-4-one
(6b):
Column
MeOH], 611.31, 610.31 [M+(54Cr) – Tos], 609.31 [M+(53Cr) – Tos],
chromatography (n-hexane/Et2O, 3:1) afforded 6b as a clear oil. The
chromatographed material had 90% ee (reaction at –12 °C) and
93% ee (reaction at –16 °C). Chiral GC analysis [WCOT-FS CP-
Chirasil-Dex CB, 90 °C (7 min), 15 °Cmin–1, 150 °C (16 min),
608.31 [M+(52Cr) – Tos], 606.31 [M+(50Cr) – Tos]. MS (ESIneg
,
CH2Cl2/MeOH): m/z (%) 173.22 [Tos (34S)], 171.16 (100) [Tos
(32S)], 121.24 [Tos (32S) – C4H2]; exact mass (ESI) calculated for
C37H52CrN2O2 [M – Tos]+: 608.343; found 608.344.
15 °Cmin–1, 180 °C (2 min)]: tR(major)
= 23.9, tR(minor) =
24.5 min. GC-MS [HP5-MS, 100 °C (5 min), 20 °Cmin–1, 200 °C
(15 min), 20 °Cmin–1, 280 °C (10 min)]: tR = 9.87 min; m/z 182
[M+], 167, 153, 139, 124, 111, 97, 84, 71 (100%), 55. 1H NMR
(CDCl3, 300 MHz): δ = 7.34 (d, J = 6.0 Hz, 1 H), 5.39 (d, J =
6.0 Hz, 1 H), 4.43–4.33 (m, 1 H), 2.56–2.43 (m, 2 H), 1.86–1.74 (m,
1 H), 1.67–1.57 (m, 1 H), 1.46–1.23 (m, 8 H), 0.87 (t, J = 6.4 Hz,
3 H) ppm. 13C NMR (CDCl3, 75 MHz): δ = 192.8 (s), 163.6 (d),
106.9 (d), 79.6 (d), 41.8 (t), 34.4 (t), 31.6 (t), 29.0 (t), 24.7 (t), 22.5
(t), 14.0 ppm (q). The absolute configuration of the major enanti-
omer was assigned as (+)-R by analogy to compounds 6a and 6c–e.
Synthesis of Complex 3d: Complex 3d was obtained in a yield of
76% from 383 µmol of complex 3a and 1 equiv. of AgBF4 in mtbe
(50 mL). MS (ESIpos, CH2Cl2/MeOH): m/z (%) 643.33, 642.35
[M+(54Cr) – BF4 + MeOH], 641.36 [M+(53Cr) – BF4 + MeOH],
640.35 [M+(52Cr) – BF4 + MeOH], 638.36 [M+(50Cr) – BF4
+
MeOH], 610.34 [M+(54Cr) – BF4], 609.32 [M+(53Cr) – BF4], 608.33
[M+(52Cr) – BF4], 606.31 [M+(50Cr) – BF4]; exact mass (ESI) calcu-
lated for C37H52CrN2O2 [M – BF4]+: 608.343; found 608.344.
Synthesis of Complex 3e: NaBPh4 (5 equiv.) was added to a solution
of complex 3b (78.0 µmol) in mtbe (12 mL) and the heterogeneous
mixture was stirred at room temperature for one day. Further mtbe
(60 mL) was then added and the reaction mixture was washed with
water (3×40 mL). The organic phase was dried with Na2SO4, and
the solvent was removed in vacuo. Complex 3e was isolated as a
brown solid in 98% yield. MS (ESIpos, CH2Cl2/MeOH): m/z (%)
643.49, 642.47 [M+(54Cr) – BPh4 + MeOH], 641.47 [M+(53Cr) –
BPh4 + MeOH], 640.46 [M+(52Cr) – BPh4 + MeOH], 638.47
[M+(50Cr) – BPh4 + MeOH], 611.46, 610.46 [M+(54Cr) – BPh4],
609.45 [M+(53Cr) – BPh4], 608.45 (100) [M+(52Cr) – BPh4], 606.45
(S)-(+)-2-Cyclohexyl-2,3-dihydro-4H-pyran-4-one (6c): Column
chromatography (n-hexane/Et2O, 3:1) afforded 6c as a clear oil. The
chromatographed material had 96% ee. Chiral GC analysis
[WCOT-FS CP-Chirasil-Dex CB, 90 °C (7 min), 15 °Cmin–1,
160 °C (39 min), 15 °Cmin–1, 180 °C (2 min)]: tR(major) = 25.3,
tR(minor)
= 26.3 min. GC-MS [HP5-MS, 100 °C (5 min),
20 °Cmin–1, 200 °C (15 min), 20 °Cmin–1, 280 °C (10 min)]: tR
=
10.47 min; m/z 180 [M+], 162, 151, 136, 122, 110, 95, 81 (100%),
67, 55. 1H NMR (CDCl3, 300 MHz): δ = 7.36 (d, J = 5.9 Hz, 1 H),
[M+(50Cr) – BPh4], 367.35, 366.35, 365.35, 364.35. MS (ESIneg
,
5.38 (d, J = 5.9 Hz, 1 H), 4.15 (ddd, J1 = 14.3, J2 = 5.6, J3
=
CH2Cl2/MeOH): m/z 321.18, 320.21 [BPh4, (11B, 13C)], 319.18
[BPh4, (11B)], 318.19 [BPh4, (10B)]; exact mass (ESI) calculated for
C37H52CrN2O2 [M – BPh4]+: 608.343; found 608.344.
3.3 Hz, 1 H), 2.54 (dd, J1 = 16.7, J2 = 14.3 Hz, 1 H), 2.38 (dd, J1
= 16.7, J2 = 3.3 Hz, 1 H), 1.95–1.55 (m, 6 H), 1.35–0.94 (m, 5 H)
ppm. 13C NMR (CDCl3, 75 MHz): δ = 193.8 (s), 164.0 (d), 106.7
(d), 83.6 (d), 41.4 (d), 39.0 (t), 28.2 (t), 28.0 (t), 26.2 (t), 25.9 (t),
25.8 ppm (t). The absolute configuration of the excess enantiomer
was assigned as (+)-S based on comparison of the measured optical
rotation with the literature value.[7]
Representative Procedure for the HDA Reaction of Danishefsky’s
Diene (4) with Aldehydes 5 Catalyzed by Complexes 3: Aldehyde 5
(0.25 mmol) and diphenyl ether (19.8 µL, 0.125 mmol, GC stan-
dard) were dissolved in absolute mtbe (0.3 mL) and cooled to the
temperature specified in the tables. Molecular sieves (3 Å) and a
catalytic amount of the chromium complex (4 mol-%, unless other-
wise stated) were then added and the reaction was started by add-
ing Danishefsky’s diene (4; 0.3 mmol). For quenching the reaction,
5 drops of triflic acid were added and the mixture was stirred for
half an hour. Samples were taken before the reaction was started
and after the reaction was stopped, and analyzed by chiral GC.
The HDA products were purified by column chromatography of
the reaction mixture after washing with water. The enantiomeric
excess of the chromatographically pure products was measured
again by capillary GC.
(S)-(+)-2-(2-Furyl)-2,3-dihydro-4H-pyran-4-one
(6d):
Column
chromatography (n-hexane/Et2O, 3:1) gave 6d as a colorless solid.
The enantiomeric excess before work-up ranged from 84 to 90%
(depending on the reaction temperature). The chromatographed
material had 98% ee after additional recrystallization. M.p. 70 °C;
chiral GC analysis [WCOT-FS CP-Chirasil-Dex CB, 90 °C (7 min),
15 °Cmin–1, 150 °C (16 min), 15 °Cmin–1, 180 °C (2 min)]:
tR(major) = 21.3, tR(minor) = 22.0 min. GC-MS [HP5-MS, 100 °C
(5 min), 20 °Cmin–1, 200 °C (15 min), 20 °Cmin–1, 280 °C
(10 min)]: tR = 8.81 min; m/z 164 [M+], 146, 134, 122, 108, 94
1
(100%), 81, 66, 55. H NMR (CDCl3, 300 MHz): δ = 7.45 (dd, J1
= 1.5, J2 = 0.9 Hz, 1 H), 7.35 (d, J = 6.2 Hz, 1 H), 6.43 (dd, J1 =
3.3, J2 = 0.5 Hz, 1 H), 6.38 (dd, J1 = 3.3, J2 = 1.8 Hz, 1 H), 5.50–
5.42 (m, 2 H), 3.07 (dd, J1 = 16.9, J2 = 12.8 Hz, 1 H), 2.71 (dd, J1
= 16.9, J2 = 4.1 Hz, 1 H) ppm. 13C NMR (CDCl3, 75 MHz): δ =
191.4 (s), 162.5 (d), 149.9 (s), 143.6 (d), 110.5 (d), 109.7 (d), 107.3
(d), 73.5 (d), 39.8 ppm (t). The absolute configuration of the excess
enantiomer was assigned as (+)-S based on comparison of the mea-
sured optical rotation with the literature value.[7]
(S)-(+)-2-Phenyl-2,3-dihydro-4H-pyran-4-one
(6a):
Column
chromatography (n-hexane/Et2O, 3:1) afforded 6a as a clear oil. The
chromatographed material had 92% ee (reaction at –21 °C) and
95% ee (reaction at –24 °C). Chiral GC analysis [WCOT-FS CP-
Chirasil-Dex CB, 170 °C, isothermal]: tR(major) = 11.1, tR(minor)
= 11.7 min. GC-MS [HP5-MS, 100 °C (5 min), 20 °Cmin–1, 200 °C
(15 min), 20 °Cmin–1, 280 °C (10 min)]: tR = 9.77 min; m/z 174
[M+], 156, 128, 104 (100%), 78, 51. IR (film): ν = 3058, 3027, 2925,
˜
1719, 1664, 1598, 1576, 1494, 1449, 1330, 1176, 977, 758, 698,
(S)-(+)-2-[(E)-Styryl]-2,3-dihydro-4H-pyran-4-one (6e): Column
chromatography (n-hexane/Et2O, 3:1) afforded 6e as a clear oil. The
chromatographed material had 79% ee. Chiral GC analysis
[WCOT-FS CP-Chirasil-Dex CB, 90 °C (7 min), 15 °Cmin–1, 160
1
553 cm–1. H NMR (CDCl3, 300 MHz): δ = 7.48 (dd, J1 = 6.1, J2
= 0.7 Hz, 1 H), 7.41–7.35 (m, 5 H), 5.51 (dd, J1 = 6.1, J2 = 1.3 Hz,
1 H), 5.41 (dd, J1 = 14.4, J2 = 3.5 Hz, 1 H), 2.89 (dd, J1 = 16.8, J2
Eur. J. Org. Chem. 2006, 5029–5035
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
5033

A. Berkessel, N. Vogl
FULL PAPER
°C (65 min), 15 °Cmin–1, 180 °C (2 min)]: tR(major) = 69.4, (m, 1 H), 4.72 (dd, J1 = 9.9, J2 = 4.0 Hz, 1 H), 3.47 (s, 3 H), 2.36–
tR(minor)
=
71.9 min. GC-MS [HP5-MS, 100 °C (5 min),
2.12 (m, 2 H) ppm. 13C NMR (CDCl3, 75 MHz): δ = 137.5 (s),
129.6 (d), 128.4 (d), 128.3 (d), 127.5 (d), 125.7 (d), 99.2 (d), 74.0
(d), 55.2 (q), 32.9 ppm (t).
20 °Cmin–1, 200 °C (15 min), 20 °Cmin–1, 280 °C (10 min)]: tR
=
13.43 min; m/z 200 [M+], 185, 171, 153, 129 (100%), 115, 91, 77,
51. 1H NMR (CDCl3, 300 MHz): δ = 7.47–7.24 (m, 6 H), 6.71 (dd,
J1 = 16.0, J2 = 0.9 Hz, 1 H), 6.28 (dd, J1 = 16.0, J2 = 6.6 Hz, 1
H), 5.45 (dd, J1 = 6.1, J2 = 1.0 Hz, 1 H), 5.10–5.02 (m, 1 H), 2.72
(dd, J1 = 16.8, J2 = 12.6 Hz, 1 H), 2.60 (ddd, J1 = 16.8, J2 = 4.2,
J3 = 1.0 Hz, 1 H) ppm. 13C NMR (CDCl3, 75 MHz): δ = 190.7 (s),
162.9 (d), 135.6 (s), 133.8 (d), 128.7 (d), 128.5 (d), 126.8 (d), 125.0
(d), 107.3 (d), 79.7 (d), 42.0 ppm (t). The absolute configuration of
the excess enantiomer was assigned as (+)-S based on comparison
of the measured optical rotation with the literature value.[7]
Procedure for the HDA Reaction of 1-Methoxybuta-1,3-diene (7)
with Ethyl Glyoxylate (5f) Using Catalyst 3a: A 50% solution of
5f in toluene (49.56 µL, 0.25 mmol) and diphenyl ether (19.8 µL,
0.125 mmol, GC standard) was added to absolute mtbe (0.25 mL)
and cooled to –21 °C. Molecular sieves (3 Å) and chromium com-
plex 3a (6.5 mg, 10.0 µmol, 4 mol-%) were added and the reaction
was started by adding 1-methoxybuta-1,3-diene (7; 30.2 µL,
0.3 mmol). Samples were taken before the reaction was started and
when the reaction was stopped and analyzed by chiral GC. The
HDA product (diastereomeric mixture) was purified by column
chromatography (silica gel, n-hexane/Et2O, 2:1) to afford a colorless
oil. The enantiomeric excess of both diastereomers was measured
Ethyl (S)-3,4-Dihydro-4-oxo-2H-pyran-2-carboxylate (6f): Column
chromatography (n-hexane/Et2O, 2:1) afforded 6f as a slightly yel-
low oil. The chromatographed material had 67% ee. Chiral GC
analysis [WCOT-FS CP-Chirasil-Dex CB, 90 °C (7 min), by capillary GC.
15 °Cmin–1, 160 (24 min), 15 °Cmin–1, 180 °C (2 min)]: tR(major)
Ethyl 2-Methoxy-5,6-dihydro-2H-pyran-6-carboxylate (9): The puri-
= 15.6, tR(minor) = 15.8 min. GC-MS [HP5-MS, 100 °C (5 min),
20 °Cmin–1, 200 °C (15 min), 20 °Cmin–1, 280 °C (10 min)]: tR
fied material consisted of a 25:75 mixture of the diastereomers 9a
(trans) and 9b (cis). The minor, trans diastereomer 9a had 17% ee
and the major, cis diastereomer 9b had 90% ee. Chiral GC analysis
[WCOT-FS CP-Chirasil-Dex CB, 90 °C (7 min), 15 °Cmin–1,
160 °C (24 min), 15 °Cmin–1, 180 °C (2 min)]: tR(9a, minor) = 12.8,
tR (9a, major) = 13.0 min; tR(9b, major) = 13.5, tR (9b, minor) =
13.7 min. GC-MS [HP5-MS, 100 °C (5 min), 20 °Cmin–1, 200 °C
(15 min), 20 °Cmin–1, 280 °C (10 min)]: tR = 8.0 min (trans, minor),
tR(cis, major) = 8.18 min; m/z 185 [M+ – 1], 171, 155, 140, 127,
113, 99, 81 (100%), 71, 53.
9a: 1H NMR (CDCl3, 300 MHz): δ = 6.04–5.99 (m, 1 H), 5.74
(ddd, J1 = 10.1, J2 = 4.8, J3 = 2.6 Hz, 1 H), 4.99–4.96 (m, 1 H),
4.47 (dd, J1 = 8.7, J2 = 6.8 Hz, 1 H), 4.28–4.17 (m, 2 H, superim-
posed by signals of the cis-product), 3.44 (s, 3 H), 2.52–2.42 (m, 2
H), 1.41–1.23 (m, 3 H, superimposed by signals of the cis-product)
ppm. 13C NMR (CDCl3, 75 MHz): δ = 127.6 (d), 125.4 (d), 95.9
(d), 65.8 (d), 61.2 (t), 55.6 (q), 26.0 (t), 14.2 ppm (q).
9b: 1H NMR (CDCl3, 300 MHz): δ = 6.04–5.99 (m, 1 H), 5.67
(ddd, J1 = 10.3, J2 = 4.0, J3 = 2.0 Hz, 1 H), 5.02–5.00 (m, 1 H),
4.35 (dd, J1 = 6.6, J2 = 5.4 Hz, 1 H), 4.28–4.17 (m, 2 H, superim-
posed by signals of the trans-product), 3.48 (s, 3 H), 2.36–2.26 (m,
2 H), 1.41–1.23 (m, 3 H, superimposed by signals of the trans-
product) ppm. 13C NMR (CDCl3, 75 MHz): δ = 127.7 (d), 126.1
(d), 97.2 (d), 69.6 (d), 61.1 (t), 55.6 (q), 27.5 (t), 14.1 ppm (q).
=
8.52; m/z 170 [M+], 154, 142, 125, 113, 101, 97 (100%), 85, 73, 55.
1H NMR (CDCl3, 300 MHz): δ = 7.37 (d, J = 6.1 Hz, 1 H), 5.45
(d, J = 6.1 Hz, 1 H), 4.98 (t, J = 7.8 Hz, 1 H), 4.25 (q, J = 7.1 Hz,
2 H), 2.83 (d, J = 7.8 Hz, 2 H), 1.28 (t, J = 7.1 Hz, 3 H) ppm. 13C
NMR (CDCl3, 75 MHz): δ = 189.4 (s), 187.0 (s), 161.7 (d), 107.9
(d), 76.1 (d), 62.2 (t), 38.3 (t), 14.0 ppm (q). The absolute configura-
tion of the excess enantiomer was assigned as (+)-S based on com-
parison of the measured optical rotation with the literature
value.[16]
Procedure for the HDA Reaction of 1-Methoxybuta-1,3-diene (7)
with Benzaldehyde (5a) Using Catalyst 3a: Benzaldehyde (25.4 µL,
0.25 mmol) and diphenyl ether (19.8 µL, 0.125 mmol, GC stan-
dard) were dissolved in 0.25 mL of absolute mtbe and warmed to
55 °C. Molecular sieves (3 Å) and the chromium complex 3a
(9.75 mg, 15.0 µmol, 6 mol-%) were added and the reaction was
started by adding 1-methoxybuta-1,3-diene (7; 30.2 µL, 0.3 mmol).
After 41 h, the temperature was raised to 65 °C and the reaction
was stirred for another 23 h. Samples were taken before the reac-
tion was started and just before the reaction mixture was purified
and analyzed by chiral GC. The HDA product was purified by
column chromatography (silica gel, n-hexane/Et2O, 3:1) to afford a
clear oil. The enantiomeric excess of both diastereomers was mea-
sured by capillary GC.
2-Methoxy-6-phenyl-5,6-dihydro-2H-pyran (8): The purified mate-
rial consisted of a 5:95 mixture of the diastereomers 8a (trans) and
8b (cis). The minor, trans diastereomer 8a had 14% ee and the
major, cis diastereomer 8b had 75% ee. Chiral GC analysis [WCOT-
FS CP-Chirasil-Dex CB, 90 °C (7 min), 15 °Cmin–1, 170 °C
(16 min), 15 °Cmin–1, 180 °C (2 min)]: tR(8a, minor) = 15.4, tR(8a,
major) = 15.5 min; tR(8b, major) = 16.1, tR(8b, minor) = 16.3 min.
GC-MS [HP5-MS, 100 °C (5 min), 20 °Cmin–1, 200 °C (15 min),
20 °Cmin–1, 280 °C (10 min)]: tR = 9.67 min (trans, minor); m/z
159, 145, 131, 117 (100%), 105, 91, 84, 77, 69, 59, 51; tR = 9.89 min
(cis, major); m/z 159, 144, 129, 104, 84 (100%), 69, 51.
8a: 1H NMR (CDCl3, 300 MHz): δ = 7.40–7.17 (m, 5 H), 6.08–
5.98 (m, 1 H), 5.77 (dtd, J1 = 10.5, J2 = 2.8, J3 = 1.5 Hz, 1 H),
4.96–4.94 (m, 1 H), 4.84 (dd, J1 = 11.0, J2 = 4.2 Hz, 1 H), 3.38 (s,
3 H), 2.36–2.12 (m, 2 H) ppm. 13C NMR (CDCl3, 75 MHz): δ =
142.3 (s), 129.1 (d), 128.5 (d), 127.6 (d), 126.2 (d), 125.5 (d),
96.4 (d), 68.4 (d), 55.3 (q), 32.2 ppm (t).
Acknowledgments
This work was supported by the Fonds der Chemischen Industrie.
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Received: April 25, 2006
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5035