Phosphahelicenes in asymmetric organocatalysis: [3+2] cyclizations of γ-substituted allenes and electron-poor olefins

Article abstract of DOI:10.1002/anie.201500299

The first use of phosphahelicene in enantioselective organocatalysis is reported. New chiral phosphahelicenes have been prepared and enable highly enantioselective [3+2] cyclization reactions between arylidene- or alkylidenemalononitriles and γ-substituted allenoates or cyanoallenes. These reactions afford cyclopentene derivatives in both high yields and diastereoselectivities, with enantiomeric excesses of up to 97 %.

Full text of DOI:10.1002/anie.201500299

Angewandte
Chemie
DOI: 10.1002/anie.201500299
Asymmetric Catalysis
Phosphahelicenes in Asymmetric Organocatalysis: [3+2] Cyclizations
of g-Substituted Allenes and Electron-Poor Olefins**
Maxime Gicquel, Yang Zhang, Paul Aillard, Pascal Retailleau, Arnaud Voituriez,* and
Angela Marinetti*
Abstract: The first use of phosphahelicene in enantioselective
organocatalysis is reported. New chiral phosphahelicenes have
been prepared and enable highly enantioselective [3+2]
cyclization reactions between arylidene- or alkylidenemalono-
nitriles and g-substituted allenoates or cyanoallenes. These
reactions afford cyclopentene derivatives in both high yields
and diastereoselectivities, with enantiomeric excesses of up to
9
7%.
A
lthough phosphine organocatalysis has been long known, it
Figure 1. P-menthyl-phosphahelicenes from previous work and P-Ipc*-
phosphahelicenes from this work.
is only during the last fifteen years or so that it has turned into
[
1]
a versatile, highly useful synthetic method. Especially
endless efforts have been devoted to the development of
[
2]
enantioselective variants of these reactions. Notably, recent
studies have focused on the stereochemical control of the
organocatalytic [3+2] cyclizations of allenes with alkenes,
[Ipc* = (1R,2R,3R,5S)-2,6,6-trimethyl-bicyclo[3.1.1]-heptan-
3-yl], as typified by 3 in Figure 1. The rigid bicyclic structure
of the chiral Ipc* auxiliary was anticipated to possibly induce
high stereochemical control, especially in reactions such as
organocatalytic processes which involve the phosphorus
center itself.
Our strategy for the synthesis of the new P-Ipc*-substi-
tuted phosphahelicene oxide 8 (Scheme 1) relies on the
oxidative photochemical cyclization of the diarylolefin 7 as
[
3]
widely known as the Luꢀs reaction. In this field remarkable
advances have been achieved by taking advantage of either
chiral cyclic phosphines or bifunctional (polyfunctional)
[4]
[5]
acyclic phosphines, which display axial, planar, or central
[
6]
chirality. We disclose here the first examples of highly
enantioselective [3+2] cyclizations achieved by means of
helically chiral phosphorus derivatives, namely the phospha-
[
7]
helicenes 3 (Figure 1).
Our group has recently disclosed a new series of
phosphahelicenes with phosphole units embedded at the
[
8]
end of a helical sequence of aromatic rings. In this series, the
chiral phospha[6]helicene 1 (P-Men*-HelPhos; Figure 1;
Men* =l-menthyl) and its phosphathiahelicene analogue 2
(P-Men*-HelPhos-S), demonstrated good potential as ligands
in gold catalysis, thus giving ee values of up to 96% in 1,6-
[
9]
enyne cycloisomerization reactions. Next, in looking for
more extensive uses of these helical phosphines in catalytic
processes, including organocatalysis, we expanded our inves-
tigations to new compounds in which the l-menthyl group on
phosphorus was replaced by an isopinocampheyl group
[*] M. Gicquel, Y. Zhang, P. Aillard, Dr. P. Retailleau, Dr. A. Voituriez,
Dr. A. Marinetti
Institut de Chimie des Substances Naturelles
CNRS UPR 2301, Universitꢀ Paris-Sud
91198 Gif-sur-Yvette (France)
E-mail: arnaud.voituriez@cnrs.fr
angela.marinetti@cnrs.fr
Scheme 1. Synthesis of the phosphahelicene oxide (P)-8. a) 1. tBuLi,
[
**] Research reported in this paper has been supported by the CNRS.
The authors thank the Institut de Chimie des Substances Naturelles,
the China Scholarship Council, and the University of Paris Sud for
grants to M.G., Y.Z. and P.A.
ꢀ
788C, Et O; 2. H O , CH Cl ; 3. TBAF, CH Cl , 76%; b) Cs CO ,
2 2 2 2 2 2 2 2 3
PhNTf , DMF, RT, 4h, 79%, 1:1 epimers ratio; c) [Pd(SPhos) Cl ],
2
2
2
Cs CO , THF/H O (10:1), 808C, 5h, 88%; d) hn, I , propylene oxide,
2
3
2
2
cyclohexane/THF (70:1), 1h, 80%. DMF=N,N-dimethylformamide,
TBAF=tetra-n-butylammonium fluoride, TBS=tert-butyldimethylsilyl,
Tf =trifluoromethanesulfonyl, THF=tetrahydrofuran.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201500299.
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
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.
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Communications
the key step. It is based on the use of the P-Ipc*-substituted
phosphindole oxide 5 as the key building block. The
phosphindole oxide 5 is available as a mixture of two epimers,
the P-Ipc*-substituted phosphahelicenes (P)-3 and (M)-3,
and the previously known P-menthyl-substituted HelPhos
[
13]
(P)-1 were engaged as catalysts in the [3+2] cyclization
between benzylidenemalononitrile (9a) and ethyl 6-phenyl-
[
10]
starting from (isopinocampheyl)dichlorophosphine and the
[
14,15]
dibromoolefin 4, by the dilithiation/cyclization sequence
hexa-2,3-dienoate (10a).
We were pleased to see that,
[
9a]
shown in Scheme 1. The two epimers of 5 were separated
by column chromatography and the subsequent reactions
have been carried out then on single epimers. Thus, the Suzuki
despite the stereochemical lability of the phosphorus center,
these phosphines display high regio-, diastereo-, and enantio-
selectivity in this organocatalytic reaction (Table 1). Both
2
D
5
31
coupling of (ꢀ)-5 [½aꢁ ¼ꢀ92 (c = 1.6, CHCl ), P NMR: d
3
[
8a]
=
55 ppm] with the olefinic boronate 6 led to the enantio-
2
5
merically pure tetrasubstituted olefin (+)-7 [½aꢁ ¼ + 112 (c =
D
Table 1: Screening of the HelPhos catalysts in an organocatalytic [3+2]
cyclization reaction.
3
1
1
.5, CHCl ), P NMR: d = 57 ppm].
3
The final photocyclization step in Scheme 1 proved to be
remarkably efficient and diastereoselective: starting from
+)-7 it gave a single epimer of the desired phosphahelicene
oxide 8 in 80% yield. The positive [a] value of 8, [a] =
(
D
D
+
2375 (c = 0.5, CHCl ), indicates that its helical scaffold has
3
Entry
PR
3
*
d.r. [%]
Yield [%]
ee [%]
a P-configuration. The analogous photocyclization of (ꢀ)-7
1
2
3
P-Men*-HelPhos (P)-1
P-Ipc*-HelPhos (P)-3
P-Ipc*-HelPhos (P)-3
P-Ipc*-HelPhos (M)-3
>95:5
>95:5
>95:5
85:15
30
37
91
83
35
89 (+)
95 (+)
96 (+)
68 (ꢀ)
8 (+)
gives the phosphahelicene (M)-8 in 70% yield upon isolation
[
[a] = ꢀ2430 (c = 1, CHCl ); see Supporting Information for
D
3
[a][b]
details]. In both cases, the photochemical cyclization step
takes place with much higher yield than the analogous
4
5
[c]
(ꢀ)-5’
90:10
[
8a,9]
photocyclizations of P-l-menthyl-substituted olefins.
[a] Reaction temperature=808C. [b] As an additional experiment, reac-
3
1
Reduction of the phosphine oxide (P)-8 ( P NMR: d =
9 ppm) was carried out at 1008C with phenylsilane and
tion in entry 3 has been carried out at a 5 mol% catalyst loading: total
conversion was attained after 48 h at 808C, thus giving 11a in 96% ee.
5
catalytic
amounts
of
bis(4-nitrophenyl)phosphate
[
c] The phosphindole oxide (ꢀ)-5 was reduced to (ꢀ)-5’ with PhSiH , (4-
3
[
11]
(
Scheme 2). Under these reaction conditions, the trivalent
NO C H O) P(O)OH and used then as the catalyst.
2 6 4 2
phosphahelicenes (P)-1 and (P)-3 afforded the cyclopentene
1a as the unique [3+2] cyclization product, which results
from the a-addition of the allenoate to the olefin (Michael-
1
[16]
type addition of the allenoate through its a-carbon atom).
The syn-isomer was formed preferentially with greater than
5:5 diastereomeric ratio. The use of (P)-1 as the catalyst
provided 11a in a moderate 30% yield with a high ee value
entry 1). Gratifyingly, the same product 11a could be
9
(
Scheme 2. Reduction of the phosphahelicene oxide (P)-8. a) PhSiH3,
4-NO C H O) P(O)OH, toluene, 1008C, 4h, 2:3 epimers ratio;
obtained in much higher enantiomeric excess (95% ee) and
37% yield, by using the newly synthesized (P)-3 as the
catalyst (entry 2). The yield could be further increased to
(
2
6
4
2
b) HSiCl , toluene, ꢀ208C, 1h, >10:1 epimers ratio at ꢀ208C.
3
9
1%, while retaining the same 96% enantiomeric excess, by
carrying out the reaction at 808C (entry 3). In analogous
experiments, the opposite epimer, (M)-3, afforded the
expected product 11a in 68% ee only (entry 4). This result
demonstrates that the relative configurations of the isopino-
campheyl group and the helical scaffolds are suitably matched
to attain good enantioselectivity levels. For comparison
purposes, (ꢀ)-5 (Scheme 1) was reduced into the correspond-
ing trivalent phosphine (ꢀ)-5ꢀ and tested as a catalyst for the
same reaction (entry 5). It provided a very low ee value, thus
showing that helical chirality plays a major role in the
stereochemical control of these cyclizations.
P-Ipc*-HelPhos [(P)-3] was obtained as a mixture of two
epimers in a 2:3 ratio ( P NMR: d = 11 and 6 ppm). Alter-
natively, reduction of (P)-8 was carried out at low temper-
3
1
ature with HSiCl in toluene. The reaction mixture was
3
3
1
monitored by P NMR spectroscopy and showed that the
reduction occurs at ꢀ208C, thus giving the two epimers in
3
1
a greater than 10:1 ratio (major epimer: P NMR d =
1
0
1 ppm). Epimerization at phosphorus took place slowly at
8C (1:1 ratio after 1 h), and the thermodynamic ratio of 2:3
was attained after 0.5 hours of heating at 608C. The use of
a HSiCl /NEt mixture as the reducing agent gave the same
The scope of these enantioselective cyclizations was then
investigated by using (P)-3 as the catalyst. A wide range of
substrates proved to be suitable for these reactions (Table 2).
At first, we reacted ethyl 6-phenylhexa-2,3-dienoate with
various arylidenemalononitrile derivatives (entries 1–10).
3
3
results. These experiments show that the stereogenic center of
these benzofused phospholes is configurationally unstable
even at 08C, and thus fully supports previous literature
[12]
studies.
1
The reduction procedure (Scheme 2) was applied also to
When the R substituent of the malononitrile derivative was
the synthesis of the epimeric phosphahelicene (M)-3. Both
either phenyl (entries 1 and 2) or a mono- or disubstituted
2
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Angew. Chem. Int. Ed. 2015, 54, 1 – 5
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Chemie
[
a]
Table 2: Scope and limitations of the reaction.
Finally, to illustrate the versatility of the new catalyst (P)-
, we employed it in [3+2] cyclizations between 2-arylidene-
3
malononitriles and g-substituted buta-2,3-dienenitriles. To the
best of our knowledge, cyanoallenes have been used only
once in enantioselective phosphine-catalyzed cyclizations.
These reactions involved imines as the cyclization partners
and led to 2,5-dihydro-1H-pyrroles in up to 60% ee. In our
work, the enantioselective [3+2] cyclizations between 9 and
the buta-2,3-dienenitriles 12 have been carried out in the
presence of 10 mol% of (P)-3 at room temperature
1
2
Entry
1
R
R
d.r. [%]
Yield [%]
ee [%]
[
17]
Ph
Ph
4-ClC H4
4-BrC H4
4-OMeC H4
4-MeC H4
3-BrC H4
2-BrC H4
3,4-Cl C H
1-naphthyl
2-furyl
N-Me-2-indolyl (CH ) Ph
2-thienyl
cyclohexyl
Ph
Ph
Ph
4-BrC H4
Ph
Ph
(CH ) Ph
(CH ) Ph
2 2
(CH ) Ph
2 2
(CH ) Ph
(CH ) Ph
(CH ) Ph
(CH ) Ph
2 2
(CH ) Ph
>95:5
>95:5
95:5
90:10 83 (11d)
>95:5
>95:5
95:5
>95:5
90:10 80 (11i)
95:5
91 (11a)
92 (11b)
84 (11c)
96
96
94
94
94
95
92
2
2
[
b]
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
6
6
2 2
(Scheme 3). The corresponding cyclopentenes 13a–c were
84 (11e)
91 (11 f)
78 (11g)
6
2 2
6
2 2
6
94 (11h) 87
6
2 2
(CH ) Ph
(CH ) Ph
(CH ) Ph
88
85
95
89
2
6
3
2 2
0
1
2
3
4
5
6
7
8
9
0
79 (11j)
2
2
90:10 90 (11k)
95:5 56 (11l)
2
2
[
c]
2
2
(CH ) Ph
90:10 80 (11m) 95
>95:5 60 (11n) 82
>95:5 86 (11o) 95
(CH ) CO Me >95:5 89 (11p) 96
2
2
(CH
2
)
2
Ph
CH -C H
9
2
5
Scheme 3. [3+2] cyclizations on g-substituted buta-2,3-dienenitriles.
obtained in high yields, with total regioselectivity (a-adducts),
2
2
2
(CH ) Cl
>95:5
>95:5
95:5
70 (11q)
71 (11r)
73 (11s)
74 (11t)
93
89
94
97
2
3
CH3
CH3
Ph
6
>95:5
[
18]
and total diastereoselectivity (syn isomers). The enantio-
meric excesses were in the range of 83–88%. Thus, reactions
in Scheme 3 represent the first examples of highly enantio-
selective phosphine-catalyzed cyclizations on cyanoallenes.
In summary, we have developed stereoselective access to
a new series of chiral phosphahelicenes displaying an
isopinocampheyl group on phosphorus. We have demon-
strated the high potential of these phosphahelicenes in
enantioselective nucleophilic organocatalysis by the develop-
ment of [3+2] cyclization reactions between activated olefins
and g-substituted allenes giving ee values of up to 97%. Our
results afford the first evidence for efficient stereochemical
control of organocatalytic processes induced by helically
chiral phosphines. From a synthetic point of view, these
catalytic reactions provide an unprecedented and versatile
approach to a series of enantioenriched, highly-substituted
cyclopentenes derivatives.
[
a] Reactions were performed under Ar on a 0.10 mmol scale, in degased
toluene (1.0 mL), at 808C; 9/10 ratio=1:2. Regio- and diastereoselec-
tivities were evaluated by H NMR spectroscopy on the crude reaction
1
mixtures. The ee values were determined by HPLC using a chiral
stationary phase. Racemic samples of 11a–t were obtained with 5-
phenyl-dibenzophosphole as the catalyst. [b] Benzyl 6-phenylhexa-2,3-
dienoate was used instead of the corresponding ethyl ester. [c] In
dichloroethane at 808C.
aryl ring (entries 3–9), the expected adducts 11 could be
isolated in high yields, regio- and diastereoselectivities and
uniformly high enantiomeric excesses (87–96% ee). The
1
olefins 9 in which the aryl substituent R displays chloro,
bromo, methyl, and methoxy substituents in various positions,
including the sterically relevant ortho-position (entry 8),
could be used without any decrease in terms of activity or
selectivity. The reaction tolerates 2-heteroarylidenemalono-
nitrile derivatives (entries 11–13). Noteworthy is that the
reaction also proceeds with challenging substrates such as
alkyl-substituted dicyanoolefins, albeit with some decrease in
Received: January 13, 2015
Published online: && &&, &&&&
1
Keywords: allenoates · cyclizations · enantioselectivity ·
enantioselectivity (82% ee for R = cyclohexyl, entry 14).
.
organocatalysis · phosphahelicene
Several g-substituted allenes could be used, thus giving
excellent yields and enantioselectivities, with up to 97% ee
obtained with the phenyl-substituted allene (entries 15–20).
It is worth noting that the only reported examples of
enantioselective [3+2] cyclizations between dicyanoolefins
and g-substituted allenoates relate to substrates in entries 18
and 19 of Table 2. These reactions were performed by using
a chiral aminophosphine as the catalyst and gave the
corresponding cyclopentenes in good yields but in moderate
enantioselectivity. Moreover, they produced the anti-isomers
as the major compounds, instead of the syn derivatives. Thus,
our catalyst largely improves the previously known processes
and complements the known catalyst.
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Retailleau, A. Voituriez, A. Marinetti, Chem. Eur. J. 2013, 19,
939 – 9947; c) P. Aillard, P. Retailleau, A. Voituriez, A. Marine-
9
[17] S. S. Kinderman, J. H. van Maarseveen, H. Hiemstra, Synlett
2011, 1693 – 1696.
[18] Cyclopentenes 13c, as well as 11b and 11l, have been
characterized by X-ray diffraction studies.
tti, Chem. Commun. 2014, 50, 2199 – 2201. For the synthesis of
other phosphahelicenes, see: d) N. Fukawa, T. Osaka, K.
Noguchi, K. Tanaka, Org. Lett. 2010, 12, 1324 – 1327; e) Y.
Sawada, S. Furumi, A. Takai, M. Takeuchi, K. Noguchi, K.
4
www.angewandte.org
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Asymmetric Catalysis
M. Gicquel, Y. Zhang, P. Aillard,
P. Retailleau, A. Voituriez,*
A. Marinetti*
&&&& — &&&&
Phosphahelicenes in Asymmetric
Organocatalysis: [3+2] Cyclizations of g-
Substituted Allenes and Electron-Poor
Olefins
Helical P: Specially designed phospha-
helicenes demonstrate high efficiency
and enantioselectivity in organocatalytic
cyclizations. These new helically chiral
phosphines complement and outperform
previous nucleophilic catalysts used in
this field. Ipc*=(1R,2R,3R,5S)-2,6,6-tri-
methyl-bicyclo[3.1.1]-heptan-3-yl.
Angew. Chem. Int. Ed. 2015, 54, 1 – 5
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!