Planar Chiral Phosphoramidites with a Paracyclophane Scaffold: Synthesis, Gold(I) Complexes, and Enantioselective Cycloisomerization of Dienynes

Article abstract of DOI:10.1002/chem.201504658

The key structural feature of the new phosphoramidites is a paracyclophane scaffold in which two aryl rings are tethered by both a 1,8-biphenylene unit and a O-P-O bridge. Suitable aryl substituents generate planar chirality. The corresponding gold(I) com

Full text of DOI:10.1002/chem.201504658

DOI: 10.1002/chem.201504658
Communication
&
Organic Synthesis
Planar Chiral Phosphoramidites with a Paracyclophane Scaffold:
Synthesis, Gold(I) Complexes, and Enantioselective
Cycloisomerization of Dienynes
Zhiyong Wu, KØvin Isaac, Pascal Retailleau, Jean-FranÅois Betzer,* Arnaud Voituriez,* and
Angela Marinetti[a]
Abstract: The key structural feature of the new phosphor-
amidites is a paracyclophane scaffold in which two aryl
rings are tethered by both a 1,8-biphenylene unit and
a OÀPÀO bridge. Suitable aryl substituents generate
planar chirality. The corresponding gold(I) complexes
promote the cycloisomerization of prochiral nitrogen-teth-
ered dienynes. These reactions afford bicyclo[4.1.0]hep-
tene derivatives displaying three contiguous stereogenic
centers, with very high diastereoselectivity and up to
95% ee.
Scheme 1. a) BINOL (I),[5a,b,e,g,h] SPINOL (II),[5e,g] and TADDOL (III)-derived[5c,d,f,i]
chiral phosphoramidites commonly used in gold catalysis; b) targeted
phosphoramidites (IV).
The gold(I)-catalyzed cycloisomerizations of unsaturated sub-
strates (alkenes, alkynes, allenes) represent powerful tools for
the synthesis of a wide range of carbocyclic, heterocyclic, and
A quick overview of the phosphoramidite-type ligands[7]
polycyclic molecules. Recent studies in this field have led to
the discovery of highly effective catalytic systems, to a better
understanding of reaction mechanisms, as well as to applica-
tions to the synthesis of natural products and bioactive com-
pounds.[1] Moreover, the development of asymmetric variants
has increased significantly the utility and scope of these
reactions.[2] Among others, phosphorus ligands have been de-
signed for building chiral catalysts, by following three main
strategies: the use of bimetallic gold complexes of atropiso-
meric diphosphines,[3] the use of chiral counterions (mainly
chiral phosphoric acid derivatives),[4] and the use of phosphora-
midite ligands with bulky and extended substituents surround-
ing the metallic centre.[5] In addition to these well-established
strategies, our group has recently developed a new approach
based on the use of phosphahelicenes as chiral ligands.[6]
Generally speaking, the specific linear coordination of gold(I)
complexes positions the chiral ligand opposite to the reactive
site with respect to the gold center, rendering the design of
efficient chiral ligands highly challenging.
commonly applied to gold catalysis shows that they display
either central or axial chirality, as typified in Scheme 1a, while,
to the best of our knowledge, planar-chiral phosphoramidites
have not been used so far.[3a,8] Thus, in order to complement
and expand the potential of gold-catalyzed enantioselective
reactions, we have designed new phosphoramidite ligands
with planar chirality of the general formula IV, in which the
phosphorus function is included into a disubstituted paracyclo-
phane scaffold. The targeted paracyclophanes display a 1,1’-bi-
phenylene unit and
a
OÀPÀO chain tethering the
arylidene groups.[9] Planar chirality is generated by adding aryl
substituents (Ar) on both sides of the paracyclophane moiety.
Based on our previous work on phosphoric acids with para-
cyclophane structures,[9] we anticipated that these phosphor-
amidites will display peculiar geometries, giving a singular
orientation of the aryl substituents around the metal center
(see hereafter, Figure 1). In this respect our new ligands will
hopefully complement the known ones, as far as the spatial ar-
rangement of the aryl substituents in complexes such as I or III
(Scheme 1), is known to induce fine-tuning of the stereo-
chemical control in catalytic reactions.
[a] Z. Wu, K. Isaac, Dr. P. Retailleau, Dr. J.-F. Betzer, Dr. A. Voituriez,
Dr. A. Marinetti
As an initial study, we report here on the synthesis of the
representative phosphoramidites 1 (Scheme 2) and preliminary
investigations on the catalytic behavior of their gold
complexes.
Institut de Chimie des Substances Naturelles
ICSN - CNRS UPR 2301, UniversitØ Paris-Sud, UniversitØ Paris-Saclay
1, av. de la Terrasse, 91198 Gif-sur-Yvette (France)
E-mail: arnaud.voituriez@cnrs.fr
jean-francois.betzer@cnrs.fr
The targeted phosphoramidites 1 display a paracyclophane
ring in which a 1,8-biphenylene unit tethers the arylidene
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504658.
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Scheme 2. Synthesis of the diastereomeric phosphoramidites 1 and the
corresponding gold complexes 3.
bis[(S)-1-phenylethyl]amino group on phosphorus. For clarity
and comparison purposes, the two phenyl groups of the
terphenyl units of (Rp)-3b have been deleted.
Figure 1. A) ORTEP drawing for the gold complex (Rp,S,S)-3b. Selected bond
lengths [Š]: PÀAu: 2.22, AuÀCl: 2.27; nonbonding distances: a=3.83,
b=3.43, AuÀC15=3.49. Bond angle [8]: P-Au-Cl: 178.3. B) Simplified mercury
view of the gold complex (Rp,S,S)-3b (two phenyl groups of each m-
terphenyl unit have been deleted). C) Mercury view of the Monophos-type
phosphoramidite gold complex (Sa,S,S)-Ia.[5b, 12] D) Overlying of the gold
complexes Ia (light grey) and 3b’ (dark grey).
The schematic views of the two phosphoramidite complexes
have been overlaid in Figure 1D, so that their OÀPÀO units
and their gold centers are lined up. From this view, the diver-
gent geometrical features of the two complexes and the differ-
ent orientation of the aromatic substituents around the gold
center appear unambiguously. The planar chiral phosphorami-
dite ligand in 3b lacks the C2-symmetry typically found in
chiral phosphoramidites. As a consequence, two different AuÀ
Cipso nonbonding distances can be measured, at 3.494 and
6.019 Š, respectively. The first one is significantly shorter and
the second one significantly longer than the average AuÀCipso
distance in the C2-symmetric complex Ia (4.650 Š).
The divergent structural features of these two complexes
might generate divergent catalytic behaviors and applications.
However, at first, the potential of the new complexes 3a–c in
terms of chiral induction must be established. This is the
purpose of the preliminary catalytic studies reported hereafter.
The new gold complexes 3a–c have been used as catalysts
for the cycloisomerization of N-tethered enynes into bicy-
clo[4.1.0]heptanes, by starting with the benchmark substrate
4[3h,i,5f,10] (Table 1). Activation of 3 with AgBF4 afforded good
catalysts giving total conversion after 24 h at room tempera-
ture, with enantiomeric excesses of 26, 77, and 64% for 3a,
3b, and 3c, respectively (Table 1, entries 1–3).
groups. The same biphenylene tether had been used in our
previous work to synthesize chiral paracyclophane-based phos-
phoric acids.[9] The synthesis of the desired phosphoramidites
1 has been carried out by reacting the m-terphenyl-substituted
diphenol 2 with ((S)-Ph(Me)CH)2NPCl2, after deprotonation with
NaH (Scheme 2). This macrocyclization reaction occurs in high
yield (85%), since the phenol units in the substrate are
constrained in a parallel arrangement, perfectly suitable for
cyclization.
The reaction affords a mixture of the three expected diaste-
reomers that have not been separated at this step. Instead, the
mixture has been reacted with (Me2S)AuCl to afford the corre-
sponding mixture of gold complexes 3a–c. Compound 3a was
isolated in pure form by column chromatography (d (31P)=
116 ppm in CHCl3; [a]D =À105 (c=1 in CDCl3)). It displays an
achiral paracyclophane unit in which the two m-terphenyl sub-
stituents are located on the same side of the macrocyclic ring.
The epimeric compounds 3b and 3c were separated by
fractional crystallization from a saturated ethyl acetate/n-hep-
tane solution. Compound 3b (d (31P)=114 ppm; [a]D =À132
(c=1 in CHCl3)) was obtained first, as a crystalline solid, and 3c
(d(31P)=111 ppm; [a]D = +78 (c=0.1 in CHCl3)) was isolated
then from the mother liquor. The structural features and
stereochemistry of 3b could be unambiguously determined
from the X-ray structure shown in Figure 1A. Compound 3b
displays a chiral (Rp)-configured paracyclophane scaffold.
Figure 1B–D are intended to compare the structural features
of the gold complex ((Rp)-3b, Figure 1B) with those of the
Complex 3b has been selected then for further experiments.
The effect of the counterion has been investigated as shown
in Table 1, entries 4–7 by combining 3b with various silver
salts. These experiments have highlighted 3b/AgSbF6 as the
best catalyst, which allows total conversion even at 08C. A
good, 79% enantiomeric excess has been attained under these
conditions (entry 8).[11]
After these encouraging experiments, we have considered
a variant of this cycloisomerization reaction in which the pro-
chiral dienynes 6a–i are used as the substrates (Table 2). In this
case, the chiral catalyst is expected to differentiate the enantio-
gold complex of
a Monophos-type phosphoramidite Ia
(Figure 1C).[5b] Both phosphoramidites display the same
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catalytic activity and enantioselectivity. They demonstrate that
the paracyclophane scaffold based on a biphenylene unit is
chemically and configurationally stable under the conditions of
gold catalysis. From here, it will be possible to modulate
extensively the substitution pattern of this paracyclophane
scaffold,[9] by changing both the aryl and the nitrogen
substituents, in order to expand and optimize their uses in a
variety of gold-catalyzed processes.
Table 1. Screening of the chiral gold catalysts 3 in the cycloisomerization
of the N-tethered 1,6-enyne 4.
Entry
[Au*]
X
T [8C]
Yield [%][a]
ee [%][b]
1
2
3
4
5
6
7
8
3a
3b
3c
3b
3b
3b
3b
3b
BF4
BF4
BF4
BF4
OTf
NTf2
SbF6
SbF6
RT
RT
RT
10
RT
RT
RT
0
89
91
95
25
72
65
95
94
26 (1R)
77 (1S)
64 (1R)
78 (1S)
55 (1S)
66 (1S)
74 (1S)
79 (1S)
In conclusion, we have synthesized and characterized new
chiral phosphoramidites displaying an unprecedented paracy-
clophane structure. We have demonstrated, for the first time,
the ability of planar-chiral gold(I) complexes to attain high
levels of enantioselectivities in enyne-cycloisomerization reac-
tions. In particular, starting from N-tethered prochiral dienynes,
the corresponding bicyclo[4.1.0]heptane derivatives with three
contiguous stereocenters, were obtained in good yields, with
excellent diastereoselectivity and up to 95% ee. The new
ligands reported here expand the well-established series of
cyclic phosphoramidites as chiral auxiliaries in asymmetric
catalysis. Further studies on their catalytic applications are
ongoing.
[a] Isolated yield. [b] Determined by HPLC on a chiral stationary phase.
Table 2. Enantioselective cycloisomerization of N-tethered 1,6-dienynes.
Experimental Section
General procedure for AuI-catalyzed cycloisomerizations of
1,6-enynes
Entry
Ar
Prod.
Yield [%][a]
d.r.[b]
95:5
95:5
95:5
>95:5
>95:5
95:5
ee [%][c]
1
2
3
4
5
6
7
8
9
Ph
7a
7b
7c
7d
7e
7 f
7g
7h
7i
92
71
33
95
96
95
93
90
94
90
84
67
88
86
84
83
87
95
3,4-Cl-C6H3
4-NO2-C6H4
3,5-Me-C6H3
2-MeO-C6H4
3-MeO -C6H4
4-MeO-C6H4
2,4-MeO-C6H3
2,6-MeO-C6H3
To a solution of the gold(I) catalyst (2.5 mg, 0.002 mmol, 4 mol%)
and the enyne or dienyne substrates 4 or 6 (0.05 mmol, 1 equiv) in
toluene (1.5 mL) at 08C, AgSbF6 (1.4 mg, 0.004 mmol, 8 mol%) was
added. The mixture was stirred for 20–72 h. The reaction was
monitored by 1H NMR. Volatils were removed under reduced
pressure and the final product was purified by column chromatog-
raphy (heptane/ethyl acetate=90:10 to 80:20). Enantiomeric
excesses have been measured by chiral HPLC analysis. Samples
of racemic compounds have been obtained by (acetonitrile)[(2-
biphenyl)di-tert-butylphosphine]gold(I) hexafluoroantimonate or
PtCl2-promoted cycloisomerizations.
95:5
>95:5
>95:5
[a] Isolated yield. [b] Determined by 1H NMR of the crude product.
[c] Determined by HPLC analysis on a chiral stationary phase.
6-Phenyl-3-tosyl-2-vinyl-3-azabicyclo[4.1.0]hept-4-ene[10] (7a)
topic vinyl groups of the substrate, that is, to control the ste-
reochemistry of both the cyclopropane ring and the additional
stereogenic centre in the final product. Reactions of this class
have been carried out so far by means of chiral platinum cata-
lysts that gave good enantiomeric excesses (80–95% ee, 4 ex-
amples), but only moderate catalytic activity (28–66% yield).[10]
The gold complex 3b, activated by AgSbF6, proved to be
a very efficient catalyst in terms of both catalytic activity and
stereoselectivity. The expected bicyclic compound 7a (Ar=Ph)
was obtained in 92% yield, 95/5 d.r., and 90% ee (Table 2,
entry 1). Enantiomeric excesses of 83–95% were obtained also
for the dichloro-substituted substrate 6b (entry 2) and the
electron-rich substrates 6d–i (6 examples, entries 4–9). Notably,
the 4-nitrophenyl-substituted dienyne 7c, which is known to
be inert under platinum catalysis,[10] was converted into the
desired bicyclo[4.1.0]heptane in 33% yield and 67% ee when
using 3b as the catalyst (entry 3).
Yield: 92%; 95:5 d.r.; 1H NMR (500 MHz, CDCl3): d=7.70 (d, J=
7.5 Hz, 2H), 7.33 (d, J=8.0 Hz, 2H), 7.27 (d, J=7.0 Hz, 2H), 7.22–
7.15 (m, 3H), 6.30 (d, J=8.0 Hz, 1H), 5.87 (ddd, J=16.5, 10.5,
6.0 Hz, 1H), 5.59 (d, J=8.0 Hz, 1H), 5.28 (d, J=16.5 Hz, 1H), 5.15
(d, J=11.0 Hz, 1H), 4.80 (d, J=6.0 Hz, 1H), 2.44 (s, 3H), 1.77 (dd,
J=8.0, 7.0 Hz, 1H), 1.28 (dd, J=8.0, 5.0 Hz, 1H), 0.40 ppm (dd, J=
7.0, 5.0 Hz, 1H); 13C NMR (75 MHz, CDCl3): 143.8 (C), 143.6 (C),
136.5 (C), 136.2 (CH), 129.9 (CH), 128.6 (CH), 127.4 (CH), 127.1 (CH),
126.5 (CH), 118.7 (CH), 117.9 (CH), 116.5 (CH2), 52.9 (CH), 36.0 (CH),
22.6 (C), 21.7 (CH3), 21.2 ppm (CH2); HPLC analysis: 90% ee (CHIR-
ALPAK IC, 258C, 1% EtOH/n-heptane, 1 mLminÀ1
, 220 nm,
retention times: 29.7 min (major) and 33.4 min (minor)).
6-(2,6-Dimethoxyphenyl)-3-tosyl-2-vinyl-3-azabicyclo[4.1.0]-
hept-4-ene (7i)
Yield: 94%; >95:5 d.r.; melting point: 217–2188C; 1H NMR
(300 MHz, CDCl3): d=7.70 (d, J=8.1 Hz, 2H), 7.30 (d, J=8.1 Hz,
2H), 7.15 (t, J=8.4 Hz, 1H), 6.48 (d, J=8.4 Hz, 2H), 6.19 (d, J=
Overall, the experiments above demonstrate the efficiency
of the new planar-chiral phosphoramidite 3b, both in terms of 8.1 Hz, 1H), 6.09 (ddd, J=17.0, 10.2, 7.2 Hz, 1H), 5.29 (d, J=8.4 Hz,
Chem. Eur. J. 2016, 22, 3278 – 3281
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3280 ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
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1H), 5.28 (d, J=17.0 Hz, 1H), 5.12 (d, J=10.2 Hz, 1H), 4.79 (d, J=
7.2 Hz, 1H), 3.74 (s, 6H), 2.43 (s, 3H), 1.79 (t, J=7.8 Hz, 1H), 0.99
(dd, J=8.7, 4.8 Hz, 1H), 0.63 ppm (t, J=5.7 Hz, 1H); 13C NMR
(75 MHz, CDCl3): 159.8 (C), 143.4 (C), 137.6 (CH), 136.8 (C), 129.7
(CH), 128.4 (CH), 127.3 (CH), 119.24 (C), 119.17 (CH), 117.6 (CH),
114.1 (CH2), 103.9 (CH), 55.5 (CH3), 54.7 (CH), 35.1 (CH), 23.5 (CH2),
21.7 (CH3), 14.3 ppm (C); IR: n˜max =2936, 1639, 1590, 1471, 1351,
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1249, 1166, 1109, 989, 912, 880, 873, 812, 783, 729, 708, 668 cmÀ1
;
HRMS (ESI): calcd for C23H26NO4S [M+H]+: 412.1583; found:
412.1610; HPLC analysis: 95% ee (CHIRALPAKꢁ IC, 258C, 20%
iPrOH/n-heptane, 0.95 mLminÀ1, 224 nm, retention times: 16.5 min
(major) and 21.5 min (minor)).
Acknowledgements
This work was supported by the I.C.S.N. and the Agence
Nationale de la Recherche (ANR Blanc SIMI7 2011, “Chiracid”).
We acknowledge the CSC (Chinese Scholarship Council) for the
PhD grant to Z.W. and the ANR for the PhD grant to K.I.
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Keywords: cycloisomerization
catalysis · phosphoramidite · planar chirality
·
enantioselectivity
·
gold
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Received: November 19, 2015
Published online on February 2, 2016
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