Enantioselective allylation of aldehydes catalyzed by diastereoisomeric bis(tetrahydroisoquinoline) N,N′-dioxides

Article abstract of DOI:10.1002/ejoc.201001219

Enantioselective allylation of aromatic and α,β-unsaturated aldehydes with allyltrichlorosilane catalyzed by two diastereoisomeric (R,R _ax,R)- and (R,S_ax,R)-bis-1,1′-[5,6,7,8-tetrahydro-3- (tetrahydrofuran-2-yl)isoquinoline] N,N′-dio

Full text of DOI:10.1002/ejoc.201001219

FULL PAPER
DOI: 10.1002/ejoc.201001219
Enantioselective Allylation of Aldehydes Catalyzed by Diastereoisomeric
Bis(tetrahydroisoquinoline) N,NЈ-Dioxides
[a]
[a]
[b]
[a,b]
ˇ
Klára Vlasaná, Radim Hrdina, Irena Valterová, and Martin Kotora*
Keywords: Organocatalysis / Allylation / Asymmetric catalysis / Lewis bases
Enantioselective allylation of aromatic and α,β-unsaturated
aldehydes with allyltrichlorosilane catalyzed by two dia-
stereoisomeric (R,Rax,R)- and (R,Sax,R)-bis-1,1Ј-[5,6,7,8-tetra-
hydro-3-(tetrahydrofuran-2-yl)isoquinoline] N,NЈ-dioxides
was studied. The course of the reaction was profoundly influ-
enced by the chosen solvent. The (R,Sax,R) catalyst efficiently
promoted the reaction in THF with enantioselectivity up to
96%. On the other hand, the allylation of aromatic aldehydes
in the presence of the (R,Rax,R) catalyst proceeded only in
MeCN (up to 67%ee), and the level of asymmetric induction
was strongly influenced by the presence of electron-donating
and -accepting groups in the aldehyde. The allylation of α,β-
unsaturated aldehydes proceeded only in dichloromethane
(enantioselectivity up to 68%).
Introduction
corporated into various frameworks, and (iii) aliphatic N-
oxides. The first group comprises 1,1Ј-bis(isoquinolyls) and
2,2Ј-bipyridyls.[15,16] In the second and structurally more di-
verse group, the pyridine ring can be fused with the ter-
pene,[17,18] biaryl,[19] metallocene,[20] and paracyclophane
frameworks,[21] or attached to a chiral binaphthyl unit
through a linker.[22] The third group is represented by chiral
pyrrolidine N-oxides.[23]
Recently, this laboratory developed a procedure for the
synthesis of various axially chiral N,NЈ-dioxides with the
bipyridine scaffold based on CpCo(CO)2-catalyzed
[2+2+2]-cyclotrimerization of alkynes (diynes) with nitriles
under microwave irradiation. These included axially chiral
pyridyl(tetrahydroisoquinoline)s,[22] isoquinolyl(tetrahydro-
sioquinoline)s,[25] symmetrically 3,3Ј-substituted bis(tetra-
hydroisoquinolyl)s,[26] and unsymmetrically 3,3Ј-substituted
bis(tetrahydroisoquinolyl)s.[27,28] Unsymmetrically 3,3Ј-sub-
stituted bis(tetrahydroisoquinolyl)s, in particular, were
proven to catalyze the highly enantioselective allylation of
aromatic and α,β-unsaturated aldehydes with ee values
reaching up to 99%.
Furthermore, we showed that the appropriate choice of
the reaction medium,[26,27a,28] that is, the solvent, was cru-
cial for the course of the reaction and for asymmetric induc-
tion. In summary, although both reaction mechanisms pro-
ceed through six-membered transition states, electrophilic
solvents (CH2Cl2, MeCN, etc.) promote the initial forma-
tion of pentacoordinate cationic silicon species, whereas
nonelectrophilic solvents (toluene, THF, etc.) promote the
formation of hexacoordinate neutral silicon species.[29]
These observations and results thus provided the necessary
background to understand the high level of asymmetric in-
duction observed for allylations carried out in THF.[27a,28]
Taking into account the easy and fast preparation of
(R,Rax,R)- and (R,Sax,R)-bis-1,1Ј-[5,6,7,8-tetrahydro-3-
Enantioselective organocatalytic reactions have been in a
spotlight of chemistry for more than a decade. This special
attention is driven by several factors: (i) a catalyst can often
be prepared easily from readily available enantiomerically
pure starting material, (ii) catalysis is usually carried out
under mild reaction conditions, (iii) catalysts do not contain
metal atoms.[1] Although there are considerable potential
benefits tied to the use of Lewis acid and/or base catalysts,
the latter has been studied to a considerably lesser extent
than the former.[2] Nonetheless, among Lewis base catalysts
a special area is occupied by N-oxides. A considerable nega-
tive charge located on the oxygen atoms makes them highly
basic molecules capable of interacting (activating) Lewis ac-
ids such as organosilanes.[3,4] Of numerous reactions, special
attention is devoted to the enantioselective allylation of al-
dehydes, because it gives rise to chiral homoallyl alcohols
that can serve as convenient building blocks for the synthe-
sis of more complex compounds.[5] Typical examples in-
clude the syntheses of fluoxetine,[6] diospongine,[7] epi-
calyxin F,[8] rhoiptelol,[9] goniothalamin,[10] hyptolide,[11]
fostriecine,[12] symbiodinolide,[13] and papulacandin D.[14]
This resulted in the development of a series of organocata-
lysts having a chiral scaffold. These compounds could be
roughly divided into three groups: (i) N,NЈ-dioxides with
two pyridine moieties (most often bipyridines) possessing a
stereogenic axis, (ii) N-oxides having the pyridine ring in-
[a] Department of Organic and Nuclear Chemistry, Faculty of
Science, Charles University in Prague,
Hlavova 8, 12843 Praha 2, Czech Republic
Fax: +420-221-951-326
E-mail: kotora@natur.cuni.cz
[b] Institute of Organic Chemistry and Biochemistry, Academy of
Sciences of the Czech Republic,
Flemingovo nám. 2, 16610 Praha 6, Czech Republic
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201001219.
7040
© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2010, 7040–7044

Enantioselective Allylation of Aldehydes
(tetrahydrofuran-2-yl)isoquinoline] N,NЈ-dioxides (1;[26] reasonable and was within the range 73–88%ee for 3a, 3d,
Figure 1) and the above-mentioned solvent effects, we de- and 3f–i. The only exception was the allylation of 2-chloro-
cided to screen the enantioselective allylation of aromatic benzaldehyde (2e), where homoallyl alcohol 3e was ob-
aldehydes in detail and also to check its scope in the case tained in with only 40%ee (Table 1, Entry 5).
of α,β-unsaturated benzaldehydes.
To increase the yields of the homoallyl alcohols, the
amount of (R,Sax,R)-1 was increased to 2 mol-% and the
reaction temperature was raised to –40 °C. Over 11 vari-
ously substituted benzaldehydes bearing electron-donating
or -accepting groups were subjected to the allylation with
allyltrichlorosilane. In general, the reactions proceeded in
good yields with high levels of asymmetric induction, re-
gardless of the electronic properties of the substituents. It
is also worth noting that an increase in the reaction tem-
perature from –78 to –40 °C did not have any detrimental
effect on the level of asymmetric induction. Thus, allylation
of benzaldehyde (2a) and para-substituted benzaldehydes 2d
and 2g–k yielded the corresponding products with ee values
in the range 75–92% (Table 1, Entries 1, 4, 7–10). In a sim-
ilar manner, the allylation of meta-substituted benzalde-
hydes 2c and 2f proceeded to give products 3c and 3f with
92 and 84%ee, respectively (Table 1, Entries 3 and 6). Only
in the case of ortho-substituted benzaldehydes 2b and 2e
was a lower level of asymmetric induction observed: 68%
for 3b and 25% for 3e (Table 1, Entries 2 and 5).
Figure 1. Diastereoisomeric bis(tetrahydroisoquinolyl) N,NЈ-diox-
ides (R,Rax,R)-1 and (R,Sax,R)-1.
Results and Discussion
Initially, we decided to check the allylation of benzalde-
hydes 2 with allyltrichlorosilane under the previously used
conditions with catalyst 1 (1 mol-%) at –78 °C (Scheme 1).
The inability of (R,Rax,R)-1 to catalyze the allylation of
benzaldehydes in THF prompted us to switch the solvent
to MeCN (Table 2). Gratifyingly, the altered reaction me-
dium had a positive effect on the course of the reaction.
The allylation of benzaldehydes 2a and 2d–i proceeded with
full conversion, giving the corresponding homoallyl
alcohols 3a and 3d–i quantitatively (Table 2, Entries 1–7).
On the other hand, the level of asymmetric induction was
highly dependent on the substituent attached to the aro-
matic ring. In the case of benzaldehyde (2a), the level of
Scheme 1. Allylation of benzaldehydes 2 to homoallyl alcohols 3.
Surprisingly, in the presence of (R,Rax,R)-1 the reactions asymmetric induction was mediocre (48%ee for 3a; Table 2,
did not proceed at all. Then, allylation of selected aldehydes Entry 1). The presence of the electron-donating MeO sub-
2a and 2d–i with (R,Sax,R)-1 was attempted. In this case, stituent in the para position of 2d resulted in an increased
the allylation took place; however, yields of the correspond- enantioselectivity of the allylation: 67%ee for 3d (Table 2,
ing homoallyl alcohols 3a and 3d–i after 24 h reaction time Entry 2). However, the presence of electron-accepting
were rather marginal within the range of 2–6% (Table 1). groups led to a low or marginal level of asymmetric induc-
On the other hand, the level of asymmetric induction was tion. Thus, allylation of 2f–h furnished 3f–h with ee values
Table 1. Allylation of aromatic aldehydes 2 to homoallyl alcohols 3 catalyzed by (R,Sax,R)-1 in THF (24 h).
Entry
Aldehyde 2
R
Product 3
Reaction performed at –78 °C[a]
Reaction performed at –40 °C[b]
Yield [%][c]
3
ee [%][c]
80 (S)
Yield [%][c,d]
ee [%][c]
1
2
3
4
5
6
7
8
9
2a
2b
2c
2d
2e
2f
2g
2h
2i
H
3a
3b
3c
3d
3e
3f
3g
3h
3i
96
88
87
78
51
80
87
90
75
86
86
81 (S)
68 (S)
92 (S)
83 (S)
25 (S)
84 (S)
89 (S)
92 (S)
75 (S)
92 (S)
88 (S)
2-MeO
3-MeO
4-MeO
2-Cl
3-Cl
4-Cl
2
2
5
6
5
5
76 (S)
40 (S)
73 (S)
87 (S)
88 (S)
80 (S)
4-F
4-CF3
4-CN
4-Me
10
11
2j
2k
3j
3k
[a] Reactions were catalyzed by 1 mol-% of (R,Sax,R)-1. [b] Reactions were catalyzed by 2 mol-% of (R,Sax,R)-1. [c] Determined by GC.
[d] Isolated yields were on average lower by 10–15%.
Eur. J. Org. Chem. 2010, 7040–7044
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K. Vlasˇaná, R. Hrdina, I. Valterová, M. Kotora
Table 2. Allylation of benzaldehydes 2 to homoallyl alcohols 3 catalyzed by (R,Rax,R)-1 and (R,Sax,R)-1 in MeCN (–40 °C, 24 h).
FULL PAPER
Entry
Aldehyde 2
R
Product 3
(R,Rax,R)-1[a]
(R,Sax,R)-1[a,b]
Yield [%][c,d]
ee [%][c]
Yield [%][c,d]
ee [%][c]
1
2
3
4
5
6
7
2a
2d
2e
2f
2g
2h
2i
H
4-MeO
2-Cl
3-Cl
4-Cl
4-F
4-CF3
3a
3d
3e
3f
3g
3h
3i
Ͼ98
Ͼ98
Ͼ98
Ͼ98
Ͼ98
Ͼ98
Ͼ98
48 (S)
67 (S)
2 (S)
26 (S)
32 (S)
50 (S)
9 (S)
Ͼ98
Ͼ98
46 (R)
0
Ͼ98
16 (R)
[a] Reactions were catalyzed by 1 mol-% of 1. [b] The data were taken from ref.[26a] [c] Determined by GC. [d] Isolated yields were on
average lower by 10–15%.
in the range 26–50% (Table 2, Entries 4–6) and that of 2e bearing electron-accepting groups (Cl, F). This trend has
and 2i yielded 3e and 3i with 2 and 9%ee, respectively already been observed by others.[15a,16b]
(Table 2, Entries 3 and 7). The previously reported al-
We recently showed that allylation of α,β-unsaturated al-
lylations catalyzed with (R,Sax,R)-1 in MeCN are added for dehydes catalyzed by unsymmetrically 3,3Ј-substituted bis-
comparison.[26a] Although the yields of homoallyl alcohols (tetrahydroisoquinolyl)s could proceed with enantio-
were quantitative, the level of asymmetric induction was selectivity up to 99%ee.[28] In this respect, the comparison
low: 3a was obtained with 46%ee (Table 2, Entry 1) and 3i of catalytic activity and the level of asymmetric induction
in 16%ee (Table 2, Entry 7). Surprisingly, the allylation of achieved in the presence of 1 was interesting (Table 3).
2b did not result in any asymmetric induction at all
(Table 2, Entry 2).
At the outset, allylation of cinnamaldehydes 4a–g cata-
lyzed by (R,Sax,R)-1 in THF was carried out at –40 °C. As
Comparison of the allylations of aromatic aldehydes car- expected, the absolute configuration of the corresponding
ried out in THF and MeCN clearly supports previous ob- homoallyl alcohols 5 was S. Rather mediocre enantio-
servations and the conclusion that the solvent crucially af- selectivity was observed in the allylations of cinnamal-
fects the course of the reaction as well as the level of asym- dehyde (4a, 66%ee) and 4-methoxycinnamaldehyde (4b,
metric induction. The reactions carried out with catalysts 63%ee), which bear electron-donating group (Table 3, En-
possessing an (R) stereogenic axis gave rise to products with tries 1 and 2). The presence of electron-accepting groups in
(R) configuration, whereas those carried out with an (S) 4c–e led to an increase in asymmetric induction: 94%ee for
stereogenic axis proceeded to give products with (S) config- 5c, 79%ee for 5d, and 72% ee for 5e (Table 3, Entries 3–5).
uration. In MeCN, the opposite was true: the catalyst with Gratifyingly, the presence of substituents in the α-position
an (R) stereogenic axis yielded products with (S) configura- (chloro and methyl groups) to the carbonyl group in 4f and
tion and vice versa. For the reactions carried out in THF, 4g led to products 5f and 5g with high enantioselectivity
the electronic properties of the substituents attached to the (96%ee; Table 3, Entries 6 and 7), which was higher than
aromatic ring did not have any substantial effect on the those obtained previously.[26] The allylation of aliphatic α,β-
level of asymmetric induction, whereas in MeCN, benzalde- unsaturated aldehydes 4h and 4i bearing a methyl group at
hydes bearing electron-donating substituents (MeO) were the α-position led to products 5h and 5i with ee values of
allylated with a higher level of enantioselectivity than those 70 and 80%ee, respectively.
Table 3. Allylation of α,β-unsaturated aldehydes with (R,Rax,R)-1 and (R,Sax,R)-1 in THF and dichloromethane (–40 °C, 3–6 h).
Entry
Aldehyde 4
R1
R2
Product 5
(R,Sax,R)-1[a] in THF
(R,Rax,R)-1[a] in CH2Cl2
Yield [%][b]
ee [%][c]
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
4a
4b
4c
4d
4e
4f
4g
4h
4i
C6H5
4-MeOC6H4
4-BrC6H4
4-FC6H4
4-NO2C6H4
C6H5
H
H
H
H
H
Me
Cl
Me
Me
5a
5b
5c
5d
5e
5f
5g
5h
5i
70
53
75
58
73
82
60
52
60
66 (S)
63 (S)
94 (S)
79 (S)
72 (S)[d]
96 (S)
96 (S)
70 (R)
80 (R)
55
40
97
82
93
95
60
50
50
23 (S)
28 (S)
15 (S)
17 (S)
4 (S)[d]
68 (S)
62 (S)
61 (R)
62 (R)
C6H5
Me
Et
[a] Reactions were catalyzed by 1 mol-% of 1. [b] Determined by NMR spectroscopy (isolated yields were on average lower by 10–15%).
[c] Determined by GC. [d] Determined by HPLC.
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Eur. J. Org. Chem. 2010, 7040–7044

Enantioselective Allylation of Aldehydes
The (R,Rax,R)-1-catalyzed allylation reaction of cinnam- groups (Table 3, Entry 3) and α-substituted cinnamal-
aldehydes 4 in THF did not proceed as it did for the benzal- dehydes with up to 96%ee (higher asymmetric induction in
dehyde substrates. Interestingly, the reaction also did not comparison with other related catalysts;[28] Table 1, En-
take place in MeCN. Fortunately, switching the reaction tries 6 and 7). Secondly, solvent effects play an important
medium to CH2Cl2 resulted in a smooth reaction. On the role, which, in spite of the current knowledge, is still diffi-
other hand, an increase in the catalytic activity had a detri- cult to fully comprehend. Thirdly, a fast and easy synthetic
mental effect on the level of enantioselectivity. Thus, the route to catalysts 1 on the preparative scale from commer-
observed level of asymmetric induction for the allylation of cially available compounds could make them useful syn-
cinnamaldehydes 4a–d was rather low (15–28%ee; Table 3, thetic tools in asymmetric synthesis.
Entries 1–4). In the case of 4-nitrocinnamaldehyde, the
asymmetric induction was marginal (4%ee; Table 3, En-
Experimental Section
try 5). Only in the case of cinnamaldehydes 4f and 4g did
the enantioselectivity reach 68 and 62%ee, respectively
(Table 3, Entries 6 and 7). A similar enantioselectivity was
General Procedure for the Enantioselective Allylation of Benzalde-
hydes 2 with Allyltrichlorosilane Catalyzed by 1: To a solution of 1
observed also in the allylation of α,β-unsaturated aldehydes (0.005 or 0.0025 mmol), a benzaldehyde (0.25 mmol), diisopropyle-
thylamine (54 μL, 0.31 mmol) in a solvent (0.5 mL) at –40 °C (or
–78 °C) was added allyltrichlorosilane (46 μL, 0.31 mmol), and the
reaction mixture was stirred for 24 h. The reaction was quenched
with saturated aqueous NaHCO3 (1 mL), and the organic layer was
separated and dried with MgSO4. Yields and ee values were deter-
mined by GC (HP-Chiral β, 30 mϫ0.25 mm, oven: 80 °C for
1 min, then 1 °C/min to 160 °C, 5 min at that temperature, flow:
1.5 mL/min). Spectral characteristics of the prepared compounds
were in agreement with the previously reported data.[24a,25]
4h and 4i, which gave rise to homoallylic alcohols 5h
(61%ee) and 5i (62%ee).
The dramatic difference in the level of asymmetric induc-
tion in the allylations in THF and in MeCN or dichloro-
methane can be reasonably explained by different reaction
mechanisms. In the former solvent, the existence of a neu-
tral hexacoordinate silicon intermediate is presumed to be
responsible for a more conformationally rigid transition
state, whereas in the case of the latter solvent, the course of
the reaction proceeds through a cationic pentacoordinate
species that gives rise to conformationally more flexible in-
termediates.[29]
On the other hand, despite the current level of knowl-
edge regarding the role of the structure of the catalyst used,
as well as that of a solvent, on the course of the reaction
and the level of asymmetric induction on experimental as
well as theoretical levels,[19b,28,29] practical experiments still
confirm that there is a number of subtle effects that may
crucially control the overall outcome. A typical example is
the lack of catalytic activity of (R,Rax,R)-1 in the allylation
of benzaldehydes in THF and that of cinnamaldehydes in
THF and MeCN. Although there is no evidence, participa-
tion of the oxygen atom belonging to the chiral tetra-
hydrofuran moiety in the coordination to allyltrichlorosil-
ane to form a coordinatively saturated silicon species that
is not able to react with the aldehyde cannot be excluded.
On the other hand, a pleasant surprise was the high level
of asymmetric induction observed in the case of the al-
lylation of α-substituted cinnamaldehydes 4f and 4g, which
could be explained by the existence of a more conforma-
tionally rigid transition state resulting in more selective chi-
rality transfer.
General Procedure for the Enantioselective Allylation of Benzalde-
hydes 2 with Allyltrichlorosilane Catalyzed by 1: To a solution of 1
(0.004 mmol), an α,β-unsaturated aldehyde (0.4 mmol), diisopropy-
lethylamine (104 μL, 0.6 mmol) in a solvent (2 mL) at –40 °C was
added allyltrichlorosilane (85 μL, 0.6 mmol), and the reaction mix-
ture was stirred for 3–6 h. The reaction was quenched with satu-
rated aqueous NaHCO3 (1 mL), and the organic layer was sepa-
rated and dried with MgSO4. Yields and ee values were determined
by GC (HP-Chiral β, 30 mϫ0.25 mm, oven: 80 °C for 1 min, then
1 °C/min to 160 °C, 5 min at that temperature, flow: 1.5 mL/min).
Spectral characteristics of the prepared compounds were in agree-
ment with the previously reported data.[24a,25]
Supporting Information (see footnote on the first page of this arti-
cle): Experimental details.
Acknowledgments
The authors acknowledge financial support from the Czech Science
Foundation (grant No. 203/08/0350), Ministry of Education, Youth
and Sports of the Czech Republic (grants Nos. LC06070,
MSM0021620857, and Z40550506), and Charles University in
Prague (grant No. SVV 261205/2010).
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The results presented clearly emphasize several points.
Firstly, 3,3Ј-substituted bis(tetrahydroisoquinolyl)s could be
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hydes bearing electron-donating or -accepting groups with
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© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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K. Vlasˇaná, R. Hrdina, I. Valterová, M. Kotora
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Received: August 28, 2010
Published Online: November 12, 2010
7044
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Eur. J. Org. Chem. 2010, 7040–7044