Helicenes with embedded phosphole units in enantioselective gold catalysis

Article abstract of DOI:10.1002/anie.201308377

This paper discloses the first uses of phosphahelicenes as chiral ligands in transition-metal catalysis. Unlike all known helical phosphines used so far in catalysis, the phosphorus function of phosphahelicenes is embedded in the helical structure itself. This crucial structural feature originates unprecedented catalytic behaviors and efficiency. An appropriate design and fine tuning allowed both high catalytic activity and good enantiomeric excesses to be attained in the gold promoted cycloisomerizations of N-tethered 1,6-enynes and dien-ynes. A twist of gold: Phosphahelicenes were for the first time used as chiral ligands in transition-metal catalysis. Unlike all helical phosphines used so far in catalysis, the phosphorus atom of phosphahelicenes is embedded within the helical structure (see scheme). Structural design allowed both high catalytic activity and good ee values to be attained in gold-promoted cycloisomerizations of N-tethered 1,6-enynes and dienynes. Copyright

Full text of DOI:10.1002/anie.201308377

Angewandte
Chemie
DOI: 10.1002/anie.201308377
Asymmetric Gold Catalysis
Helicenes with Embedded Phosphole Units in Enantioselective Gold
Catalysis**
Keihann Yavari, Paul Aillard, Yang Zhang, Frꢀdꢀrick Nuter, Pascal Retailleau,
Arnaud Voituriez,* and Angela Marinetti*
Abstract: This paper discloses the first uses of phosphaheli-
cenes as chiral ligands in transition-metal catalysis. Unlike all
known helical phosphines used so far in catalysis, the
phosphorus function of phosphahelicenes is embedded in the
helical structure itself. This crucial structural feature originates
unprecedented catalytic behaviors and efficiency. An appro-
priate design and fine tuning allowed both high catalytic
activity and good enantiomeric excesses to be attained in the
gold promoted cycloisomerizations of N-tethered 1,6-enynes
and dien-ynes.
Figure 1. Design of phosphorus-based chiral catalysts for gold-pro-
moted enantioselective reactions.
T
his work demonstrates the first uses of phosphole-contain-
ing helicenes in organometallic catalysis and affords unpre-
cedented evidence for the high potential of helically chiral
trivalent phosphines in enantioselective gold catalysis.[1]
It is well recognized that gold-based catalysts play
a unique role in homogeneous catalytic processes, due to
their high catalytic activity, which is usually associated with
good product selectivity.[2] Most particularly, in the field of
cycloisomerizations, pioneering work from Echavarren and
co-workers on cationic gold(I) catalysts[3] opened the way to
a number of highly powerful synthetic methods and applica-
tions.[4] The development of enantioselective variants of
cycloisomerizations and other gold-promoted processes[5]
was hampered initially by the intrinsic structural features of
gold(I) complexes, which make ligand design especially
challenging. The major drawback of dicoordinated gold(I)
complexes relates to their linear geometry, which brings the
active reaction site opposite to the chiral ligand and requires
the building of an extended chiral pocket.
and extended substituents embracing the gold center and its
reactive site (Figure 1b).[8,9]
We propose here an alternative ligand design, based on
a class of trivalent monodentate phosphines that display an
extended helical structure, typified by the ligand in I in
Figure 1c. Indeed, we have recently disclosed a flexible access
to helical structures with embedded phosphole units.[10,11]
From these ligands, a stable gold complex of the general
formula I (Ar: fused phenyl ring) could be prepared, in
racemic form, with the rigid polyaromatic moiety of the
helicene folded toward the gold center and, thus, screening
[12]
À
the distant Au Cl bond.
We have postulated that this
unique arrangement, combined with appropriate substitution
patterns, might be a favorable feature for building chiral gold
complexes for enantioselective catalysis. The preliminary
investigations reported hereafter validate this new design.
Our work first involved the synthesis of several new gold
complexes of helical phosphines in which the terminal units of
the helical sequences are either benzophospholes (13) or a-
aryl-substituted phospholes (9–12). In order to obtain enan-
tiomerically pure phosphines easily through diastereoselec-
tive procedures, we employed a chiral P-bonded menthyl
group as a chiral auxiliary.
To develop enantioselective catalysts based on phospho-
rus auxiliaries, three main strategies have been successfully
implemented so far: the use of bimetallic gold complexes of
atropisomeric diphosphines,[6] the use of tightly associated
chiral counterions, mainly chiral phosphoric acid derivatives
(Figure 1a),[7] and the use of phosphoramidites with bulky
a-Aryl-substituted phosphole units were targeted as the
terminal units of the helical sequences, because variation of
the a-aryl groups is expected to enable easy modulation of the
steric environment of the phosphorus atom. The synthesis of
the corresponding helicenes is based on the photochemical
oxidative cyclization of diaryl olefins illustrated in Scheme 1.
According to our general strategy,[10] the synthetic procedure
started with the coupling of a diastereomerically pure P-
menthyl-substituted phosphindole triflate, 1 (Ar: Ph) or 2
(Ar: 3,5-Me2C6H3),[13] with the olefinic boronate 3, under
palladium catalysis. When Pd(SPhos)2Cl2 was used as the
catalyst, the desired tetrasubstituted olefins (RP)-4, (SP)-4,
and (RP)-5 were isolated in good yields (76–92%). These
olefins were then subjected to photochemical oxidative
[*] K. Yavari, P. Aillard, Y. Zhang, Dr. F. Nuter, Dr. P. Retailleau,
Dr. A. Voituriez, Dr. A. Marinetti
Institut de Chimie des Substances Naturelles
CNRS UPR 2301, Centre de Recherche de Gif
91198 Gif-sur-Yvette (France)
E-mail: arnaud.voituriez@cnrs.fr
angela.marinetti@cnrs.fr
[**] The authors thank the Institut de Chimie des Substances Naturelles,
the China Scholarship Council, and the University of Paris Sud for
grants to K.Y., Y.Z., and P.A.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201308377.
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Scheme 2. Synthesis of the chiral gold complexes 9a’. The “exo”/
“endo” labels indicate the position of the gold atom with respect to
the helical scaffold. The a and a’ suffixes indicate compounds with
helical M and P configurations, respectively. a) (EtO)2MeSiH, bis(4-
nitrophenyl) phosphate, toluene; b) NaAuCl4·2H2O, 2,2’-thiodiethanol,
CHCl3/H2O (1:5), 08C, 40 min.
the synthesis of 9a’ (for other examples, see the Supporting
Information). Reduction of the oxide (RP,P)-6a’ with
(EtO)2MeSiH[15] and the subsequent reaction, in situ, of the
corresponding trivalent phosphine with NaAuCl4.2H2O and
2,2-thiodiethanol afforded a mixture of the two diastereo-
Scheme 1. Synthesis of enantiopure helical phosphine oxides. OTf:
trifluoromethanesulfonate; Men: menthyl; SPhos: 2-dicyclohexylphos-
phino-2’,6’-dimethoxybiphenyl.
À
meric complexes in which the P Au bond is oriented either
toward the helical scaffold ((SP,P)-9a’-endo) or in the
opposite direction ((RP,P)-9a’-exo). In this case, formation
of the epimeric pair of gold complexes is likely due to the
configurational lability of the phosphorus center in the
intermediate trivalent phosphole. Nevertheless, the gold
complexes themselves retain enantiomeric integrity, as far
as the helical chirality persists in this process. The stereogenic
phosphorus centers of 4 and 5 play the role of stereochemical
relays (Scheme 1) and epimerize afterwards.
cyclization to be converted into the desired helical com-
pounds. In principle, the photochemical reaction can afford
both [6]helicenes, displaying meta-fused phosphole rings (6 or
7), and [7]helicenes (8), displaying ortho-fused phosphole
rings, because either of the two carbon atoms a and a’ in the
substrates can be involved in the cyclization. We have found
that the outcome of the reaction markedly depends on the
nature and stereochemistry of the substrates.
As shown in Scheme 1, the two epimers of the olefinic
substrate 4 (Ar: Ph) give different product distributions: the
SP-configured substrate gives a 1:1 mixture of the two helical
derivatives (SP,M)-6a and (SP,P)-8a’, whereas the RP-config-
ured substrate affords exclusively the [6]helicene (RP,P)-6a’.
This means that the phosphorus configuration steers the
regioselectivity of the photocyclization reactions. Moreover,
it also controls the configuration of the newly created helical
unit: for instance, the photocyclization of the RP-configured
olefin 4 affords the single diastereomer (RP,P)-6a’, which
displays a P-configured helical scaffold. Also, photocycliza-
tion of (RP)-5 (Ar: 3,5-Me2C6H3) affords the analogous meta-
fused helicene (RP,P)-7a’ as the major diastereomer (9:1
isomer ratio). Thus, the photochemical cyclization proves to
be a well-suited approach to phosphahelicenes, especially due
to its high diastereoselectivity. Moreover, it should be possible
to overcome the scale-up limitations usually associated with
photochemical processes, by using the newly developed
continuous flow strategy.[14]
The structure and stereochemistry of complex (SP,P)-9a’-
endo was ascertained by an X-ray diffraction study. An
ORTEP view of this complex is shown in Figure 2.
The enantiomerically pure phosphole-terminated helical
phosphine oxides 6–8 were converted into the corresponding
gold complexes 9–13 in two steps, as typified in Scheme 2 by
Figure 2. X-ray crystal structure of (SP ,P)-9a’-endo.
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Table 1: Gold-promoted enantioselective enyne cycloisomerizations.[a]
The same procedure as that used for the synthesis of the
gold complexes 9 was then applied to the phosphine oxides
6a, 7a’, and 8a’ (Scheme 1) to afford complexes (SP,M)-9a-
exo and (RP,M)-9a-endo in a 2:1 ratio, (SP,P)-11a’-endo as the
only observed isomer, and the [7]helicene derivatives (SP,P)-
10a’-exo and (RP,P)-10a’-endo in a 1:3 ratio. Individual
diastereomers of the epimeric gold complexes were separated
by column chromatography. The structures of these com-
plexes are represented in Scheme 3.
Entry
Catalyst
ee
Conv.
[%]
Config.[c]
[%][b]
1
2
(SP ,M)-13a-endo
(RP ,P)-10a’-endo
7
70
43
1S,6R
1S,6R
35
3
4
(RP ,M)-9a-endo
(SP ,M)-9a-exo
42
n.d.
90
<10
1S,6R
–
5
(RP ,P)-9a’-exo
n.d.
<5
–
6
7
8
(SP ,P)-9a’-endo
(SP ,P)-11a’-endo
(SP ,P)-12a’-endo
81
84
82
>95
>95
>95
1R,6S
1R,6S
1R,6S
[a] Ts: toluene-4-sulfonyl. [b] n.d.: not determined. [c] The configuration
of the bicyclic derivative 15 was assigned by comparison with known
samples.[16j] The (1R,6S)-configured bicycles display positive optical
rotation values (c=1, CH2Cl2).
be efficient and enantioselective catalysts, provided that some
precise structural requirements are fulfilled, according to the
observed trends hereafter. Firstly, exo complexes, in which the
gold atom occupies the external face of the helical structure,
are almost inactive (Table 1, entries 4 and 5), whereas endo
complexes display good catalytic activity. With exo com-
plexes, dark gold deposits formed shortly after removal of the
chloride ligand with AgBF4 and addition of the substrate.
Such deactivation of the exo complexes cannot be of steric
origin, as far as the gold atom is expected to be more exposed
in the exo complexes than in the corresponding endo isomers.
We can postulate that the low-coordination, cationic gold
intermediates obtained from endo complexes are stabilized
either by the steric hindrance of the folded ligand or by p-
arene–gold interactions,[17] although the X-ray crystal struc-
ture of 9a’ displays long nonbonding distances (ꢀ 3.6 ꢀ)
between the Au atom and the terminal aromatic ring of the
helicene. Secondly, the relative configurations of the three
stereogenic elements of the ligands, that is, the menthyl group,
the phosphorus center and the helical scaffold, should be
suitably matched to attain good enantioselectivity levels:
entries 3 and 6 in Table 1 show that the l-menthyl unit
combines more favorably with an S-configured phosphorus
center that is with a P-configured helical scaffold.
Scheme 3. Gold complexes of helical phosphines.
In further experiments, analogous synthetic strategies
were applied to the synthesis of the complex (SP,P)-12a’-endo,
also shown in Scheme 3, which displays a methoxy substituent
on the terminal arene ring, opposite to the phosphole unit.
Moreover, the gold complexes (SP,M)-13a-endo and (RP,M)-
13a-exo, which contain benzophosphole-terminated [6]heli-
cenes as the ligands,[10b] were prepared by the same procedure
as that represented in Scheme 2.
After it had been demonstrated that a whole series of
phosphahelicene–gold complexes, with various structural
features, is easily available by the method described above,
the next step of this work was the screening of these
complexes in selected catalytic reactions. To this end, we
investigated the cycloisomerization of the N-tethered 1,6-
enyne 14 into the bicyclo[4.1.0]heptene 15.[8d,16] Selected
results are displayed in Table 1.
The benzophosphole-terminated complex (SP,M)-13a-
endo and complex (RP,P)-10a’-endo, which contains a helical
phosphine ligand with an ortho-fused phosphole unit, dis-
played significant catalytic activity (70 and 43% conversion
rates, respectively, after 24 h at room temperature). However,
they gave low levels of enantiocontrol (7 and 35% ee,
respectively; entries 1 and 2 in Table 1).
In the end, among the four epimeric complexes 9 (Table 1,
entries 3–6), (SP,P)-9a’-endo proved to be the best catalyst by
giving the highest enantioselectivity (ee = 81%) and display-
ing an excellent catalytic activity.
Gratifyingly, however, the gold complexes of helicenes
with meta-fused phosphole units (called “HelPHOS” ligands)
and a-aryl substituents relative to the phosphorus atom (9, 11,
and 12) gave much more convincing results. They proved to
The last two entries in Table 1 show that variations of both
the a-aryl substituents with regard to the phosphorus center
and the terminal aryl unit[18] of the helical structure cause
slight modulations of the enantioselectivity levels. The highest
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ee value (84%) is attained with the 3,5-dimethylphenyl-
substituted complex (SP,P)-11a’ (Table 1, entry 7).
complexes (SP,P)-9a’-endo and (SP,P)-12a’-endo, under even
milder conditions than with platinum catalysis. If R is H, the
reaction affords the [4.1.0]bicycloheptene 17, whereas the
reaction affords the rearranged product 18 if R is a phenyl
group. The vinylcyclopropane–cyclopentene rearrangement
leading to 18 takes place at room temperature, in a reaction
that is likely to be promoted by gold itself.[20] Both 17 and 18
have been obtained in ꢀ 85% ee. For 18, the highest ee value
(85%) was attained with complex (SP,P)-12a’, which contains
the MeO-substituted helical ligand.
In conclusion, we have demonstrated that 1) gold com-
plexes are available from helicenes with terminal benzo-
phosphole or a-aryl-substituted phosphole units, 2) these
complexes afford efficient precatalysts for 1,6-enyne cyclo-
isomerizations if their geometric features force the gold
center to be oriented toward and shielded by the helical
moiety, and 3) the specifically designed helicenes displaying
a-aryl-substituted phosphole units (HelPHOS ligands) create
a suitable, three-dimensional environment for gold centers
and induce efficient chiral discrimination in these enyne
cycloisomerizations. HelPHOS ligands are easily available in
enantioenriched form from P-menthyl-substituted phosp-
holes. A wide structural diversity can be generated by
changing either the aryl substituent of the phosphole ring or
the terminal aromatic unit of the helical sequence. The highly
modular synthetic approach will hopefully facilitate optimi-
zation of these ligands for further applications in gold
catalysis.
From the results in Table 1, it can also be noted that, in the
[6]helicene series, the sense of chiral induction systematically
correlates to the configuration of the helical scaffold. In other
words, the effect of helical chirality overcomes the effect of
the carbon-centered chirality of the menthyl unit.
A tentative rational about the high chiral induction
attained with the HelPHOS–Au complexes 9a’, 11a’, and
12a’ in these challenging reactions can be found from the
schematic view of the complex (SP,P)-9a’-endo shown in
Figure 3 (derived from the X-ray data in Figure 2). It can be
Figure 3. View of (SP ,P)-9a’-endo and the corresponding cycloisomeri-
zation product (1R,6S)-15.
seen that three quadrants around the gold atom are highly
hindered: the helical sequence of aromatic rings hides the
bottom-right space, the menthyl group hides the upper-right
space, and the phenyl substituent hides the upper-left space.
Only the bottom-left quadrant is available to accommodate
the substrates and their most hindered groups during the
stereodetermining steps. This constraint is likely to play a key
role in the observed stereoselection, as already postulated for
Pt[16j] and Rh[16f] complexes in analogous processes.
Received: September 25, 2013
Published online: December 6, 2013
Keywords: asymmetric catalysis · cycloisomerization · enynes ·
.
gold · helicenes · phosphorus
The potential of the HelPHOS–gold complexes in catal-
ysis has been established further by studies on the cyclo-
isomerization of the N-tethered dienynes 16 (Scheme 4).
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[12] Other examples of gold complexes displaying racemic helical
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Angew. Chem. Int. Ed. 2014, 53, 861 –865
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