Simple and fast synthesis of new axially chiral bipyridine N, N'-dioxides for highly enantioselective allylation of aldehydes

Article abstract of DOI:10.1002/adsc.200900224

Unsymmetrically 3,3'-substituted axially chiral bis(tetrahydroisoquinoline) N,N'-dioxides can be prepared in just three steps. They exhibit unique catalytic activity (turnover frequency, enantioselectivity, substrate scope) in the asymmetric allylation of

Full text of DOI:10.1002/adsc.200900224

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DOI: 10.1002/adsc.200900224
Simple and Fast Synthesis of New Axially Chiral Bipyridine N,N’-
Dioxides for Highly Enantioselective Allylation of Aldehydes
a
a
b
a,b,
ˇ
Aneta Kadlcꢀkovꢁ, Radim Hrdina, Irena Valterovꢁ, and Martin Kotora *
a
Department of Organic and Nuclear Chemistry, Faculty of Science, Charles University in Prague, Hlavova 8, 12843 Praha
2, Czech Republic
Fax : (+42)-0221-951-326; phone: (+42)-0221-951-334; e-mail: kotora@natur.cuni.cz
Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo n. 2, 16610
b
Praha 6, Czech Republic
Received: April 3, 2009; Published online: June 2, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900224.
Abstract: Unsymmetrically 3,3’-substituted axially
chiral bis(tetrahydroisoquinoline) N,N’-dioxides can
be prepared in just three steps. They exhibit unique
catalytic activity (turnover frequency, enantioselec-
tivity, substrate scope) in the asymmetric allylation
of aromatic aldehydes (up to 96% ee). The product
of the enantioselective allylation of benzaldehyde
served as a building block for the preparation of an
intermediate useful in the enantioselective synthesis
of diospongines.
gines A and B.[14b] Thus the present situation creates
an ideal environment for further activities regarding
the synthesis of new and potentially generally applica-
ble catalysts.
In this regard we have found that axially chiral pyr-
idine and bipyridine N-oxides could be easily accessed
by using the cyclotrimerization of diynes with nitriles
catalyzed by CpCo(CO)2. Microwave irradiation sig-
nificantly improved the course of the reaction result-
ing in higher product yields and shorter reaction
times in comparison with the thermal variant or even
made it to take place in cases where the thermal ini-
tiation totally fails.[15] The prepared axially chiral pyri-
dine N-oxides had aryltetrahydroisoquinoline,[15a] pyr-
idyl-tetrahydroisoquinoline,[15b,c] isoquinolinyltetrahy-
Keywords: asymmetric catalysis; cobalt; cyclotrime-
rization; microwave heating; organic catalysis
droisoquinoline,[15c]
line)[15d] frameworks. All of these compounds were
and
bis(tetrahydroisoquino-
There is no doubt that the development of organoca- tested as chiral catalysts for the asymmetric allylation
talysis in the last decade has provided organic chem- of aldehydes. Although the last group of catalysts[15d]
ists with a number of useful synthetic methods, espe- proved to have interesting features (they possess high
cially in the area of asymmetric reactions.[1] One of catalytic activity and the enantiodiscrimination is pro-
the fast growing areas is catalysis by Lewis bases, foundly dependent on the reaction solvent used), the
among which a prominent role is played by pyridine enantioselectivity stayed below 90% ee.
N-oxides. Their popularity stems from the fact that
Our next effort was focused on the increase of
due to the strong donating ability they activate the enantioselectivity to an acceptable level as well as the
À
C Si bond of halosilanes to such an extent that it simplicity of bipyridine N,N-dioxides resolution into
reacts with various functional groups.[2] The area of enantiomerically pure compounds, which has usually
catalytically active N-oxides for the allylation of alde- been carried out by crystallization or by HPLC. Since
hydes[3] spans from monodentate pyridine N-oxides the previously prepared catalysts A and catalyst B
with additional chelating functional groups such as (Figure 1) showed generally good catalytic activity
OMe,[4–7] nitrogen-containing functionalities,[8–10] bi- and reasonable enantioselectivity, we presumed that
pyridine N,N’-dioxides,[11,12] to terpyridine N,N’,N’’-tri- the synthesis of a compound bearing both functional
oxides.[13] All of these catalysts have pros and cons groups, that is, phenyl and tetrahydrofuranyl, would
with respect to substrate specifity, enantioselectivity, afford a catalyst with better properties (Figure 1).
catalytic activity, preparation, etc. Notwithstanding,
We expected that the presence of an additional
the obtained homoallylic alcohols could serve as im- center of chirality on the tetrahydrofuranyl moiety
portant building blocks for the synthesis of various would result in an easier separation of diasteroisom-
compounds such as fluoxetine[14a] and (À)-diospon-
Adv. Synth. Catal. 2009, 351, 1279 – 1283
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Aneta Kadlcꢀkovꢁ et al.
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Figure 1. Design of bipyridine N,N’-dioxide 1.
ers in comparison to the tedious HPLC resolution of
enantiomers.[16]
It was initially envisioned that a stepwise synthesis
of the pyridine ring could provide a suitable method
for its preparation. Once again, as in our previous
work, tetrayne 2 was chosen as a starting material.
Attempts to selectively monocyclotrimerize 2 with
benzonitrile using Okamoto conditions (CoCl2·6H2O,
Zn, dppe, NMP or DMF)[17] were met only with par-
tial success. Disappointingly, the reaction proceeded
with low selectivity giving the expected 1-diynyltetra-
hydroisoquinoline 3 only in 10% isolated yield.[15d]
Varying the reaction conditions or molar ratios of
starting compounds did not change the situation. Also
Scheme 1. Synthesis of (R,R)-1 and (S,R)-1.
the second step, the catalytic cyclotrimerization reactions were carried out in the presence of 1 mol%
[CpCo(CO)2] of 3 with (R)-tetrahydrofurannitrile of 1 at À408C (CH2Cl2 and PhCl) or at À788C
under microwave irradiation yielded the desired bi- (THF). Initially, the reactions catalyzed by the more
pyridine 4 in just 28% isolated yield. Again, the abundant bipyridine N,N’-dioxide (R,R)-1 were exam-
change of the reaction conditions or the catalytic ined. The allylation of benzaldehyde 6a to homoallylic
system did not lead to any improvement. An attempt alcohol 7a proceeded with full conversion in all sol-
to use (R)-tetrahydrofurannitrile for the first cyclotri- vents within 1 hour. The lowest asymmetric induction,
merization failed: no product was isolated.
53% ee (entry 1), was observed in MeCN. On the
Therefore we chose the one-pot cyclotrimerization other hand, in PhCl and THF the ees were as high as
of tetrayne 2 with benzonitrile and (R)-tetrahydrofur- 93% (entries 2 and 3).
annitrile, which could provide a straightforward route
As in our previous reports, the same solvent effect
to the unsymmetrically substituted bipyridine 4. was observed: the catalyst with R-axial chirality gave
Against all odds, the one-pot cyclotrimerization of all the S-product in MeCN, but the R-product was ob-
three components in the presence of CpCo(CO)2 tained in PhCl and THF (and vice versa). Comparison
(20 mol%) under microwave irradiation yielded the of allylations of 4-methoxybenzaldehyde 6b and 4-tri-
desired bipyridine 4 in a reasonable 28% isolated fluoromethylbenzaldehyde 6f to the corresponding
yield along with the Ph-substituted symmetrical bipyr- homoallylic alcohols in PhCl (7b, 55% ee, entry 4; 7f,
idine 5 (20%) (Scheme 1).[18,19] (The combined yield 67% ee, entry 9) and THF (7b, 86% ee, entry 5; 7f,
of the cyclotrimerization products was 48%, which is 95% ee, entry 10) revealed THF as the superior sol-
almost the same as in the previous synthesis of sym- vent to achieve high enantioselectivity. The reactions
metrically substituted bis(tetrahydroisoquinolines).
with 4-methyl-, 4-fluoro, 4-chloro-, and 4-cyanoben-
Oxidation of 4 by MCPBA (3 equiv.) in dichloro- zaldehydes 6c–6e and 6g proceeded with good enan-
methane proceeded uneventfully to give a 3/1 mixture tioselectivities: 7c (85% ee), 7d (84% ee), 7e (89%
of diastereoisomeric bipyridine N,N-dioxides (R,R)-1 ee), and 7g (83% ee) (entries 6–8, and 11). Good
and (S,R)-1 in 50% isolated yield (Scheme 1). Both enantioselectivity (7h, 87% ee) was also observed in
diastereoisomers were readily separated by a column the allylation of 3-chlorobenzaldehyde 6h (entry 12).
chromatography on silica gel.
However, a sharp decrease in asymmetric induction
Having thus secured the preparation of (R,R)-1 and to 46% ee was observed for 2-chlorobenzaldehyde
(S,R)-1 in three steps (including the preparation of 2), (entry 13). Next, a set of experiments was performed
we set out to explore their catalytic activity and enan- with (S,R)-1 under identical conditions. The allylation
tioselectivity in the allylation of variously substituted of benzaldehyde 6a in MeCN proceeded again with a
benzaldehydes with allyltrichlorosilane (Table 1). The low enantioselectivity of 44% ee (entry 1), but in
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Simple and Fast Synthesis of New Axially Chiral Bipyridine N,N’-Dioxides
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Table 1. Allylations of substituted benzaldehydes 6 to homoallylic alcohols 7 catalyzed by 1.
A
(S,R)-2[a]
ACHTUNGTRENNUNG
Entry
R
Solvent
T [8C]
Yield [%][b]
ee [%][c]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
6a, H
6a, H
6a, H
MeCN
PhCl
THF
PhCl
THF
THF
THF
THF
PhCl
THF
THF
THF
THF
À40
À40
À78
À40
À78
À78
À78
À78
À40
À78
À78
À78
À78
7a, 100
7a, 100
7a, 100
7b, 93
7b, 100
7c, 99
7d, 81
7e, 89
7f, 76
53 (S)
93 (R)
93 (R)
55 (R)
86 (R)
85 (R)
84 (R)
89 (R)
67 (R)
95 (R)
83 (R)
87 (R)
46 (R)
100
100
98
–
90
99
99
91
–
91
93
78
42
44 (R)
94 (S)
96 (S)
–
91 (S)
94 (S)
95 (S)
93 (S)
–
87 (S)
88 (S)
92 (S)
14 (S)
6b, 4-MeO
6b, 4-MeO
6c, 4-Me
6d, 4-F
6e, 4-Cl
6f, 4-CF3
6f, 4-CF3
6g, 4-CN
6h, 3-Cl
6i, 2-Cl
9
10
11
12
13
7f, 76
7g, 55
7h, 75
7i, 45
[a]
1 mol%, unless otherwise noted.
GC yields.
Determined by GC.
[b]
[c]
PhCl the corresponding homoallylic alcohol was ob-
In an attempt to assess the scope of the catalytic re-
tained with 94% ee (entry 2) and it rose to 96% action with respect to other aldehydes, the allylations
(entry 3) when the solvent was switched to THF. The of n-octanal 9a and cyclohexylcarbaldehyde 9b were
allylations of 4-methoxy-, 4-methyl-, 4-fluoro-, 4- carried out in CHCl3 (it was shown that chloroform is
chloro-, and 4-trifluoromethylbenzaldehydes 6b–6f the solvent of choice for the allylation of aliphatic al-
proceeded mostly in very good enantioselectivities: dehydes[21]). Thus, the allylations of 9a and 9b cata-
7b (91% ee), 7c (94% ee), 7d (95% ee), 7e (93% ee), lyzed by (R,R)-1 provided products 10a and 10b with
7f (87% ee), and 7g (88% ee) (entries 3, 5–8, and 11). higher enantioselectivity (Table 2, entries 1 and 2) in
It is also worth of mentioning that unusually high ee comparison with the catalysis by (S,R)-1 (Table 2, en-
(92%) was obtained for the meta-substituted substrate tries 3 and 4).
(entry 12). A surprising drop in asymmetric induction
(14% ee) was again observed for 2-chlorobenzalde- catalyst for highly enantioselective allylation of ben-
hyde. zaldehydes bearing electron-withdrawing as well as
From a general point of view, (S,R)-1 is the better
The allylation of benzaldehyde 6a with E-crotyltri- electron-donating groups. On the other hand in some
chlorosilane, prepared as a 6:1 E/Z mixture by the specific cases, the superior enantioselection was ach-
CuCl-catalyzed reaction of crotyl chloride with ieved with (R,R)-1, for example, the allylation of 4-tri-
HSiCl3,[20] was briefly explored to assess the scope of fluoromethylbenzaldehyde 6f gave 7f with 95% ee
the reaction (Scheme 2). The reaction carried out in (entry 10).
PhCl afforded a 2.4/1 mixture of anti/syn diastereoiso-
Regarding the high asymmetric induction, the cru-
mers with good enantioselectivity (91 and 87% ee) in cial role is played by THF, which enables the reaction
82% yield.
mechanism to proceed through the sterically more
crowded neutral six-coordinate silicon species leading
to higher selectivities.[22] The comparison of catalysts
1 with the 3-tetrahydrofuranyl-3’-unsubstituted deri-
vative[15d] (allylation proceeded with 49% ee) clearly
indicates that the simultaneous substitution in posi-
tions 3 and 3’ is essential for high enantioselectivity.
In addition, the comparison with symmetrically substi-
tuted 3,3’-diphenyl-bis(tetrahydroisoquinoline) N,N’-
dioxide[15d] seems to confirm that an unsymmetrical
environment may favour non-equivalent coordination
Scheme 2. Crotylation of benzaldehyde 6a catalyzed by
(R,R)-1.
À
of the N O moieties to the silicon atom, as indicated
Adv. Synth. Catal. 2009, 351, 1279 – 1283
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Aneta Kadlcꢀkovꢁ et al.
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Table 2. Allylations of aliphatic aldehydes (RCHO) 9 to ho-
moallylic alcohols 10 catalyzed by 1.
oxides – that exhibit unique catalytic activity (turn-
over frequency, enantioselectivity, substrate scope) in
the enantioselective allylation of aromatic aldehydes
(up to 96% ee). The efficiency (just three steps from
commercially available material) and potentially mod-
ular nature of the synthesis should allow for further
tuning of the catalyst properties. Further studies
aimed at diversifying the structure of the catalysts
and their applications in other reactions are under
way.
Entry
R
Catalyst
Yield [%][a]
ee [%][b]
1
2
3
4
9a, n-C7H15
9b, c-C6H11
9a, n-C7H15
9b, c-C6H11
N
10a, 90
10b, 95
10a, 88
10b, 94
60 (S)
67 (R)
43 (S)
38 (R)
[a]
GC yields.
Determined by GC.
[b]
Experimental procedures, analytical and spectral
data, and copies of the NMR spectra for the key com-
by previous calculations.[22] This may thus enhance the pounds are available in the Supporting Information.
preferential dissociation of a more weakly bound
À
N O moiety to allow coordination of the incoming
carbonyl group while keeping the rigid transtion state Experimental Section
necessary for the high asymmetric induction.
Although the enantiodiscriminating properties of General Procedure for Allylation of Aromatic
both catalysts are comparable with other ones such as Aldehydes 6
[4]
ˇ
´
those prepared by Kocovsky et al. (long reaction
times and high catalyst loadingd), Hayashi[12] (low ees
in the allylation of substrates with electron-withdraw-
ing groups), Andrus[10] (long reaction times), their use
has several considerable advantages: i) it is easier to
prepare them from a simple starting material (just
three steps, no special resolution techniques are re-
quired, just simple column chromatography), ii) short
reaction times (the reaction is usually finished with
1 h even at À788C, iii) low catalyst loads (usually
1 mol%).
To a solution of bipyridine-N,N’-dioxides (R,R)-1 or (S,R)-1
(0.004 mmol, 2 mg) in THF, PhCl or MeCN (1 mL), alde-
hyde (0.4 mmol), diisopropylethylamine (0.6 mmol, 104 mL,
77 mg) and allyltrichlorosilane (0.6 mmol, 85 mL, 51 mg)
were added at À78 or À408C and the reaction mixture was
stirred for 1 hour. The reaction was quenched with brine
(4 mL), organic layer was separated, dried over MgSO4 and
filtrated on silica gel. Yields and ees were determinated by
GC (HP-Chiral ß column, 30 mꢃ0.25 mm, oven: 808C for
1 min, then 18CminÀ1 to 1608C, 5 min).
In order to verify the synthetic application of the
prepared homoallylic alcohols, we decided to prepare
lactone 12, which is a known intermediate used in the
synthesis of diospongines.[14b] Esterification of (R)-7a,
prepared by the allylation of benzaldehyde 6a cata-
lyzed by (R,R)-1 (Table 1, entry 3), with acryloyl chlo-
ride afforded ester 11 in good 80% isolated yield
(Scheme 3). Then 11 was subjected to a ring-closing
metathesis reaction catalyzed by Grubbs 2nd genera-
Acknowledgements
Authors gratefully acknowledge financial support from the
Czech Science Foundation (Grant No. 203/08/0350) and Min-
istry of Education of the Czech Republic to the Center for
Structural and Synthetic Application of Transition Metal
Complexes (Project Nos. LC06070, MSM0021620857, and
Z40550506).
tion
catalysts,[23]
(H2IMes)ACHTNGUTER(UNNG PCy3)Cl2Ru=CHPh,
(5 mol%), affording the known intermediate 12. The
RCM carried out in toluene yielded 12 in 93% yield
(~90% ee).[24] Interestingly, the use of CH2Cl2 as the
solvent resulted in a lower yield (75%) and in a loss
of enantiopurity (75% ee).[25]
In summary, we have presented a new class of ax-
ially chiral bipyridine N,N’-dioxides – 3,3’-unsymmet-
rically substituted bis(tetrahydroisoquinoline) N,N’-di-
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ˇ
ˇ
Hrdina, A. Kadlcꢀkovꢁ, I. Valterovꢁ, J. Hodacovꢁ, M.
[25] The origin of this observation is not currently clear and
will be subject of further investigation.
Kotora, Tetrahedron: Asymmetry 2006, 17, 3185–3191;
ˇ
c) R. Hrdina, I. Valterovꢁ, J. Hodacovꢁ, M. Kotora,
Adv. Synth. Catal. 2009, 351, 1279 – 1283
ꢂ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
1283