Atropisomeric chiral dienes in asymmetric catalysis: C2- symmetric (Z,Z)-2,3-Bis[1-(diphenylphosphinyl)ethylidene]tetralin as a highly active lewis base organocatalyst

Article abstract of DOI:10.1002/anie.201308112

Diene catalysts with a twist: The title C₂-symmetric tetralin-fused 1,3-butadiene derivative is atropisomeric and can be resolved into the two helical enantiomers. The optically pure compound showed excellent enantioselectivity as well as unusu

Full text of DOI:10.1002/anie.201308112

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DOI: 10.1002/anie.201308112
Asymmetric Organocatalysis
Atropisomeric Chiral Dienes in Asymmetric Catalysis: C2-Symmetric
(Z,Z)-2,3-Bis[1-(diphenylphosphinyl)ethylidene]tetralin as a Highly
Active Lewis Base Organocatalyst**
Masamichi Ogasawara,* Shunsuke Kotani, Hikaru Nakajima, Haruka Furusho,
Mitsuru Miyasaka, Yasushi Shimoda, Wei-Yi Wu, Masaharu Sugiura, Tamotsu Takahashi,* and
Makoto Nakajima*
“Atropisomeric chirality” is chirality resulting from restricted
rotation about a single bond.[1] The most important atropiso-
meric compounds are appropriately substituted biaryl com-
pounds,[2] which have been utilized in various asymmetric
reactions as useful chiral scaffolds. Other atropisomeric
systems, which are often referred as “non-biaryl atropiso-
mers”,[3] include aryl amides[4] and diaryl ethers,[5] as well as
other compounds,[6] and have undergone rapid development
during the last decade. Analogous atropisomerism in con-
jugated dienes is also feasible. The possibility of diene
atropisomerism was pointed out in the 1950s;[7] however,
such enantiomerically resolvable dienes are still rare.[8] The
ꢀ
energy barrier to rotation about the C C single bond in
a conjugated diene is generally low, and thus the majority of
atropisomeric dienes reported so far (e.g., A in Scheme 1)[7b]
show rapid racemization at ambient temperature.[7,9,10] For
Scheme 1. Examples of atropisomeric compounds.
this reason, the application of atropisomeric dienes in
asymmetric reactions has scarcely been examined. Appa-
rently, the development of diene systems in which atropiso-
merization is completely frozen is indispensable for the
application of these molecules as chiral catalysts or reagents.
derivatives, such as B.[10] Detailed analysis of these helically
chiral molecules revealed that the dienes were fluxional in
solution. Analogous diene structures could be assembled by
the zirconium-mediated reductive cyclization of diynes.[11a]
Doherty, Knight, and co-workers prepared a series of
bisphosphines by this method.[11] Their dienyl bisphosphines
(e.g., C) are also fluxional as free ligands, but their helical-
sense conformation can be frozen upon coordination to
a transition metal.[11] Recently, Doherty and co-workers
reported the preparation of a resolvable atropisomeric
diene phosphine by extensive substitution of the diene
skeleton.[12]
Herein, we report a novel C2-symmetric conformationally
rigid atropisomeric chiral diene 1, which possesses opera-
tional Lewis basic functionality. The compound, which can be
prepared in enantiomerically pure form on a multigram scale,
is an extremely effective Lewis base organocatalyst[13] in the
enantioselective allylation of aldehydes with allylsilanes.[14–16]
Due to the high activity of 1, the allylation reaction can be
conducted at a lower temperature with a lower catalyst
loading. Furthermore, 1 can be used for reactions with (b-
hydrocarbylallyl)trichlorosilanes, the Lewis base catalyzed
asymmetric addition of which to aldehydes is rather difficul-
t.[15a,16c,17]
ꢀ
The incorporation of a fused cyclic structure at the C2 C3
bond of a 1,3-diene increases the conformational rigidity of
the molecule, and bulky substituents on the “inner” sides of
the 1- and 4-positions of the diene force the diene to adopt
a nonplanar helical conformation. In 2000, RajanBabu and
co-workers reported that the palladium-catalyzed cyclized
silylstannylation of 1,6-diynes gave carbocycle-fused diene
[*] Prof. M. Ogasawara, H. Nakajima, Dr. W.-Y. Wu, Prof. T. Takahashi
Catalysis Research Center and Graduate School of Life Science
Hokkaido University
Kita-ku, Sapporo 001-0021 (Japan)
E-mail: ogasawar@cat.hokudai.ac.jp
Prof. S. Kotani
Priority Organization for Innovation and Excellence
Kumamoto University
Kumamoto 862-0973 (Japan)
H. Furusho, Y. Shimoda, Prof. M. Sugiura, Prof. M. Nakajima
Graduate School of Pharmaceutical Sciences, Kumamoto University
Kumamoto 862-0973 (Japan)
E-mail: nakajima@gpo.kumamoto-u.ac.jp
[**] This research was partially supported by a Grant-in-Aid for Scientific
Research on Innovative Areas (“Advanced Molecular Transforma-
tions by Organocatalysts”) from MEXT (Japan; to M.O. and M.N.).
Bisphosphine 2 was prepared from 1,2-bis(2-butynyl)ben-
zene by a simple one-pot operation based on a reported
method.[11a] Subsequent oxidation of 2 with H2O2 afforded
phosphine oxide 1 in quantitative yield (Scheme 2). In the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201308112.
13798
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Scheme 2. Synthesis and chiral resolution of the atropisomeric chiral
dienyl bisphosphine dioxide 1.
Figure 1. Ball-and-stick drawing of the single-crystal X-ray structure of
1H NMR spectrum of 2 in C6D6, the methylene hydrogen
atoms at the 1- and 4-positions of the tetralin skeleton were
detected at d = 3.60 and 3.95 ppm (J = 16.9 Hz) as a pair of
well-resolved AB doublets. This observation indicates that
the two bulky diphenylphosphanyl substituents in 2 interact
with each other, which prevents the molecule from adopting
a coplanar conformation. The diene-based helical chirality
thus induced in 2 makes the two hydrogen atoms in each CH2
moiety diastereotopic with each other. Indeed, 2 could be
resolved into the two helical enantiomers in practically
enantiomerically pure form by HPLC on a chiral stationary
phase. However, to our disappointment, 2 showed slow
interconversion between the two enantiomers (i.e., racemi-
zation). Although the resolved sample of 2 had an ee value of
> 99.5% immediately after resolution, the enantiomeric
purity dropped to 49.5% ee over 12 h when stirred as
a solution in n-octane at 408C. The activation energy of the
racemization at this temperature was calculated to be DG°-
(313 K) = 25.3 ꢁ 0.3 kcalmolꢀ1. On the other hand, phosphine
oxide 1 displayed remarkable conformational rigidity with
respect to the diene helicity. Optical resolution of racemic
1 with commercial (+)- or (ꢀ)-dibenzoyl tartaric acid (DBT)
as the resolving reagent gave (+)- and (ꢀ)-1 in isomerically
pure form. The resolved sample of (+)-1 was dissolved in
xylenes and heated at 1358C for 24 h. After this time, the
HPLC analysis of (+)-1 on a chiral stationary phase showed
no signs of racemization. On the basis of an assumed
detection limit of 0.2% ee for this HPLC analysis, we
estimated the lower limit of the activation energy of the
racemization of 1 at 1358C to be DG°(408 K) ꢂ 38.4 kcal
molꢀ1. Both the preparation and the resolution of rac-1 are
simple operations that can be scaled up readily: a 20-gram
scale synthesis was conducted without any difficulties.
[(M)-(+)-1/d-(+)-DBT]·EtOH with selected atom numbering. The
(+)-DBT and EtOH moieties are shown as wireframe drawings for
clarity.
uration of the dextrorotatory enantiomer of 1 was unambig-
uously defined as M. As shown in Figure 1, the array of the
=
=
four conjugated P O and C C double bonds [O(1)–P(1)–
C(1)–C(2)–C(3)–C(4)–P(2)–O(2)] forms a characteristic hel-
ical motif in 1. The two phosphinyl oxygen atoms, O(1) and
O(2), interact considerably with each other, which might be
a major factor preventing 1 from racemization.
We next investigated the potential of the novel helically
chiral phosphine oxide 1 as a chiral organocatalyst.[13]
Although the Lewis base catalyzed asymmetric allylation of
aldehydes has been studied extensively, most allylsilane
pronucleophiles used in previous studies had an unsubstituted
or g-substituted allyl group.[14–16] Reaction with (b-hydro-
carbylallyl)trichlorosilanes are relatively unexplored, and
most reported examples showed unsatisfactory enantioselec-
tivity.[15a,16c,17] The results of our allylation studies with b-
substituted allylsilanes are summarized in Table 1. As a pro-
totypical reaction, the addition of methallyltrichlorosilane
(4x) to benzaldehyde (3a) was examined under our previ-
ously described reaction conditions with slight modifica-
tions.[15a] In the presence of (M)-1 (1.0 mol%) in propionitrile,
the reaction of 3a with 4x proceeded smoothly at ꢀ788C to
give the homoallylic alcohol 5ax in 87% yield with 87% ee
(Table 1, entry 1). Phosphine oxide 1 showed unusually high
catalytic activity, in contrast with the much lower activity of
other chiral phosphine oxide catalysts used so far in enantio-
selective allylation reactions, in which a catalyst loading of
more than 10 mol% is typically required for the completion
of the reaction.[14–17] For example, under identical conditions,
the reaction catalyzed by (S)-binapo (binap dioxide), which is
one of the best catalysts described to date for this type of
reaction,[15a] gave 5ax in only 22% yield with 79% ee (Table 1,
entry 2). We took advantage of the high catalytic activity of
(M)-1 to decrease the catalyst loading to as low as 0.1 mol%
without appreciable loss of enantioselectivity, although the
reaction took longer to reach completion (Table 1, entry 3).
30
A 1:1 complex of (+)-1, ½aꢃD ¼ + 0.17 (c = 1.02 in CHCl3),
and d-(+)-DBT was recrystallized from ethanol to yield
colorless crystals suitable for X-ray crystal-structure analysis
(Figure 1).[18] The molecular complex cocrystallized with one
molecule of ethanol, and hydrogen bonds were detected
between O(1) and H(35) as well as between O(2) and H(49).
By internal comparison with (+)-DBT, the absolute config-
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Table 1: Enantioselective addition of (b-hydrocarbylallyl)trichlorosilanes to aldehydes with chiral
phosphine oxide catalysts.
excellent (Table 1, entry 11). The
reaction with 2-naphthaldehyde
(3e) afforded 5ez with 93% ee
(Table 1, entry 12). The present
reaction could be applied to an
aliphatic conjugated enal: the reac-
tion of 4z with cinnamaldehyde
(3 f) proceeded smoothly to provide
5 fz with 83% ee (Table 1, entry 13).
The excellent performance of (M)-
1 was further demonstrated in reac-
tions of 4x with 3d and 3e in the
presence of 1 mol% of the catalyst
at ꢀ788C to give 5dx and 5ex with
90 and 89% ee, respectively
(Table 1, entries 14 and 15).
Phosphine oxide 1 is robust
under the reaction conditions de-
scribed in Table 1 and could be
recovered readily from the reaction
mixture by standard silica-gel
column chromatography. HPLC
analysis of recovered (M)-1 on
a chiral stationary phase showed
no racemization with respect to the
atropisomeric chirality.
Entry
3
4
Catalyst
Catalyst
loading
[mol%]
T
t
[h]
Yield
[%][b]
ee
[8C][a]
[%][c]
1
2
3
4
5
6
7
8
9
10
11
12
13
14[e]
15[e]
3a
3a
3a
3a
3a
3a
3a
3a
3b
3c
3d
3e
3 f
4x
4x
4x
4x
4y
4y
4z
4z
4z
4z
4z
4z
4z
4x
4x
(M)-1
(S)-binapo
(M)-1
(M)-1
(M)-1
(S)-binapo
(M)-1
(S)-binapo
(M)-1
(M)-1
(M)-1
1.0
1.0
0.1
1.0
1.0
1.0
10
10
10
10
10
ꢀ78
ꢀ78
ꢀ78
ꢀ90
ꢀ78
ꢀ78
ꢀ78
ꢀ78
ꢀ78
ꢀ78
ꢀ78
ꢀ78
ꢀ78
ꢀ78
ꢀ78
3
3
24
9
87 (5ax)
87 (S)
79 (R)
86 (S)
90 (S)
85 (S)
85 (R)
92 (S)
22 (5ax)
87 (5ax)
88 (5ax)
70 (5ay)
22 (5ay)
89 (5az)
<5 (5az)
92 (5bz)
90 (5cz)
55 (5dz)
77 (5ez)
91 (5 fz)
85 (5dx)
87 (5ex)
24
24
48
48
48
48
48
48
48
24
24
[d]
–
93
93
90
93
83
90 (S)
89
(M)-1
(M)-1
(M)-1
(M)-1
10
10
1.0
1.0
The key intermediate in the
allylation reaction, a cationic five-
coordinate silicon species D, is gen-
erated by the coordination of the
phosphine oxide to the allylsilane 4
(Scheme 3).[13,15a] Intermediate D is
3d
3e
[a] Bath temperature. [b] Yield of the isolated product after silica-gel chromatography. [c] The ee value
was determined by HPLC analysis on a chiral stationary phase (see the Supporting Information for
details). [d] The ee value was not determined. [e] The reaction was carried out without nBu4N·I.
The high catalytic activity of (M)-1 enabled us to conduct the
reaction at ꢀ908C, at which temperature the enantioselec-
tivity could be further improved to 90% ee (Table 1, entry 4).
The enantioselectivity and reactivity of the asymmetric
allylation reaction are both greatly influenced by the R’
substituent in the allylsilane reagent. The reactions of 4y
(R’ = Ph) were somewhat slower. Although both 1 and binapo
gave the b-phenylallylated product 5ay in 85% ee, 5ay was
formed in higher yield with 1 than with binapo (Table 1,
entries 5 and 6). The introduction of a tert-butyl substituent in
4z improved the enantioselectivity of the reaction, and 5az
was obtained with 92% ee and in 89% yield with (M)-
1 (Table 1, entry 7). The allylsilane 4z was much less reactive
than 4x and 4y, probably because of its steric bulkiness, and
a catalyst loading of 10 mol% and a longer reaction time were
required for a reasonable yield of 5az. The binapo catalyst
was completely ineffective in the reaction with 4z: a trace
amount of 5az was formed under otherwise identical con-
ditions (Table 1, entry 8).
Lewis acidic and captures aldehyde 3 to form a six-coordinate
intermediate E, in which the stereoselective formation of
ꢀ
a C C bond takes place. Subsequent decomplexation of E
releases the silyl homoallyl ether 6 and the phosphine oxide
catalyst for further turnover.
The high catalytic activity of 1 may be attributed to the
distorted structure in D upon the coordination of 1. The X-ray
crystal structures of 1 and binapo are described schematically
in Scheme 4. Whereas the naphthyl–naphthyl dihedral angle
Aldehydes 3 were screened as substrates for this reaction
with pronucleophile 4z and catalyst (M)-1 (Table 1, entries 9–
13). Aromatic aldehydes 3b and 3c bearing electron-donating
groups afforded the corresponding alcohols 5bz and 5cz with
93% ee in good yields (Table 1, entries 9 and 10). On the
other hand, 3d, which contains an electron-withdrawing p-
chloro substituent, was less reactive and gave the addition
product 5dz in 55% yield, but the enantioselectivity was still
Scheme 3. Catalytic cycle of the phosphine oxide catalyzed allylation of
aldehydes.
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ꢀ =
in binapo is 94.28, the dihedral angle between the two P C C
planes in 1 is much smaller at 89.28. The smaller dihedral
ꢀ ꢀ
angle in 1 leads to a narrower O Si O angle in D coordinated
with 1. This distortion around the silicon atom may enhance
the Lewis acidity and nucleophilicity of D. The increased
Lewis acidity of skewed four-coordinate silicon compounds
has been well documented as “strain-release Lewis acid-
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reactivity of the five-coordinate silicon species. Alternatively,
the high catalytic activity of 1 could be explained in terms of
a wider pocket provided by the smaller O Si O angle at the
silicon center in D; this wider pocket would accommodate the
incoming aldehyde more readily.
ꢀ ꢀ
The potential usefulness of 1 as a chiral organocatalyst
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1 (10 mol%) to give the corresponding 1,2-chlorohydrin in
93% yield with 84% ee. Additional applications of 1 are now
under investigation by our research group and will be
reported in due course.
In summary, we have found that C2-symmetric (Z,Z)-2,3-
bis[1-(diphenylphosphinyl)ethylidene]tetralin (1) is atropiso-
meric and conformationally static. After optical resolution
into the two atropisomeric enantiomers, the optically pure
compound was applied as a chiral Lewis base organocatalyst
and showed excellent enantioselectivity as well as unusually
high catalytic activity in reactions of various aldehydes with b-
substituted allyltrichlorosilanes.
ˇ
´
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Received: September 16, 2013
Published online: October 25, 2013
Keywords: allylation · asymmetric synthesis · atropisomerism ·
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