Control of Enantioselectivity in Rhodium(I) Catalysis by Planar Chiral Dibenzo[a,e]cyclooctatetraenes

Article abstract of DOI:10.1002/chem.201704816

Planar chiral 5,11-disubstiuted dibenzo[a,e]cyclo-octatetraenes (dbCOTs) have been developed as the first useful chiral homologs to dbCOT-ligands for asymmetric applications. Methods enabling the preparation of such compounds on a gram-scale in enantiomerically pure form are described. Evaluated as ligands in rhodium(I)-catalyzed 1,4- and 1,2-arylation reactions, tertiary and quarternary stereogenic centers were formed with excellent yields and selectivities of up to >99 % ee. A catalytic asymmetric synthesis of a key cyclization precursor to (?)-penifulvin A highlights the system in an applied context.

Full text of DOI:10.1002/chem.201704816

A Journal of
Accepted Article
Title: Control of enantioselectivity in rhodium(I)-catalysis by planar
chiral dibenzo[a,e]cyclooctatetraenes
Authors: Michaela Christina Melcher, Trpimir Ivšić, Charlotte Olagnon,
Christina Tenten, Arne Lützen, and Daniel Strand
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To be cited as: Chem. Eur. J. 10.1002/chem.201704816
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Control of enantioselectivity in rhodium(I)-catalysis by planar
chiral dibenzo[a,e]cyclooctatetraenes
Michaela-Christina Melcher,[a] Trpimir Ivšić,[a] Charlotte Olagnon, [a] Christina Tenten,[b] Arne Lützen,[b]
and Daniel Strand[a],*
Abstract: Planar chiral 5,11-disubstiuted dibenzo[a,e]cyclo-
octatetraenes (dbCOTs) have been developed as the first useful
chiral homologs to dbCOT-ligands for asymmetric applications.
Methods enabling preparation of such compounds on a gram-scale
in enantiomerically pure form are described. Evaluated as ligands in
rhodium(I) catalyzed 1,4- and 1,2-arylation reactions, tertiary and
quarternary stereogenic centers were formed with excellent yields
and selectivities of up to >99% ee. A catalytic asymmetric synthesis
of a key cyclization precursor to (-)-penifulvin A highlights the system
in an applied context.
stereoisomers and the dual substitution would contribute
configurational integrity to the structure. Despite current interest
in dbCOT derivatives, not only in catalysis and catalyst design,
but also in areas like molecular recognition,11 responsive
materials,12 and light emitting materials,13 there is a lack of
general methods for formation of dbCOTs with this 5,11-pattern
of substitution.14
Herein, we report the development of a practical approach
to the synthesis and chiral resolution of 5,11-dbCOT derivatives
illustrated by Me2dbCOT 3a15 and Ph2dbCOT 3b. Both compounds
proved to be readily resolved by both chromatographic and
crystallization techniques which grants access to gram-
quantities of these compounds. Importantly, the structures were
found to be stereochemically rigid at ambient temperature and
coordinate readily to form stable complexes with rhodium(I). As
catalysts, these perform excellently in asymmetric catalysis, to
readily provide enantioselectivities of up to >99% ee.
Furthermore, application of 3b as a ligand in an asymmetric
synthesis of a key cyclization precursor in Mulzer’s synthesis of
(-)-penifulvin A exemplifies the utility of this approach also in
multistep enantioselective syntheses of natural products.16,17
The
cyclooctatetraenes (dbCOTs), where interconversion of
enantiomers can occur dynamically via tub-to-tub ring-
inversion, has inspired synthetic and mechanistic interest for
over half
century.1 More recently, dbCOT has found
planar
chirality
of
substituted
dibenzo[a,e]
a
a
prominence as a bis-h2-ligand that confers exceptional catalytic
activity and stability to late transition metals. Notable examples
where dbCOT-metal complexes are known to exhibit superior
performance in catalysis spans iridium(I) catalyzed allylic
substitutions,2
rhodium(I)
catalyzed
polymerizations,3
ruthenium(0) catalyzed coupling reactions,4 and rhodium(I)5a,b
and iridium(I)6 catalyzed cycloaddition reactions. Examples of
chiral dbCOTs as ligands to transition metals are, on the other
hand, scarce. Grützmacher7 reported a conceptually intriguing
enantiomerically pure planar chiral rhodium(I) salt of 1, and
Hayashi a planar chiral rhodium(I) complex of cycloocta-1,5-
diene derivative 2 (Figure 1).8 Unfortunately, these complexes
gave only limited efficiency in asymmetric catalysis.9 The utility
of [8]annulene ligands like dbCOT would thus benefit
significantly from development of derivatives also capable of
efficient asymmetric induction. Structures of this type would
moreover fill a particular niche in such catalyst systems where
chirality cannot be readily introduced by other components of the
ligand sphere.
Prior art: Grützmacher 7
Hayashi 8
Ph
Ph
Ar
M
M
2
1
As ligand in Hayashi-Miyaura
reactions: up to 62% ee
up to 43% ee
This work:
5
11
R
R
M
Me2dbCOT 3a (R = Me)
Ph2dbCOT 3b (R = Ph)
up to >99% ee
Figure 1. Planar chiral bis-h2 C=C ligands in catalysis
We reasoned that substitution at both the 5 and 11
positions of dbCOT could offer a ligand that retains the unique p-
acidity and resistance to metallation associated with dbCOT,10
and provide improved performance with respect to chiral
At the outset, we envisioned ditriflate 6 as an attractive
building block from which cross-coupling reactions would enable
divergent access to 5,11-disubstituted dbCOTs (Scheme 1). In
practice, a multigram synthesis of this compound was achieved
from abundantly available (>100 g scale) dibromide 4.18 Addition
of AgOTs in the previously reported Kornblum oxidation of 4 to
diketone 519 doubled the yield of this reaction and proved
essential to the overall scalability of the synthesis. Me2dbCOT 3a
and Ph2dbCOT 3b were then targeted for the present study to
cover stereoelectronic diversity in catalysis.
Access to 3a and 3b in enantiomerically pure form is
desirable not only for use in catalysis, but also for investigation
of their respective physical properties. Pleasingly, the
compounds were shown readily separable by chiral HPLC, also
on a semi-preparative scale.
induction compared to prior systems.
A two-fold axis of
symmetry allows fewer reaction pathways leading to undesired
[a]
[b]
M.Sc. M.-C. Melcher, Dr. T. Ivšić, M.Sc. C. Olagnon, Dr. D. Strand
Centre for Analysis and Synthesis
Lund University
Naturvetarvägen 14, 221 00, Sweden
daniel.strand@chem.lu.se
Dr. C. Tenten, Prof. Dr. A. Lützen
Kekulé-Institute of Organic Chemistry and Biochemistry
University of Bonn
Gerhard-Domagk-Str. 1, 53121 Bonn, Germany
Supporting information for this article is given via a link at the end of
the document. CCDCs 1568856-1568861
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10.1002/chem.201704816
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Br
Fe(acac)3, MeMgBr,
NMP, 81% (R = Me, 3a)
O
Tf2O, 2,6-di-tBu-4-
methylpyridine
2,6-lutidine,
AgOTs
[Rh2(µ-Cl)2(C2H4)4
]
or
DMSO, 75 °C,
2h, 65%
DCE, 50°C, 24 h
67%
CH2Cl2, rt, 24-48 h
R = Me rac-7a: 95%,
Ph rac-7b: 99%
PEPPSI-IPr, PhMgBr,
LiBr, 62% (R = Ph, 3b)
R
R
TfO
R
R
OTf
Rh
2
4
5
6
Br
O
Cl
enantiomers
separable by
chromatography
fractional crystallization
(S,S)-Me2dbCOT 8aa
from EtOAc/C6H6 86%
(+)-DABN =
-
SbF6
H2N
H2N
-
SbF6
(S,S)-Ph2dbCOT 8ba
from EtOAc/PhMe 64%
HCl conc.
AgSbF6
R
H2N
R
(S,S)-7a
Rh+
NH2
R
R
Me
Me
MeCN
(S,S)-9
AgSbF6
MeCN/DCE, 48 h
Rh+
Rh
2
MeCN, 2h,
rt, 99%
R = Me: 99% (S,S)-7a
R = Ph: 90% (S,S)-7b
70% (R,R)-7b
Cl
(R,R)-Ph2dbCOT 8ba
from CH2Cl2/C6H6 58%
MeCN
DCE/MeCN, rt, 24h
mixture of
diastereomers
single diastereomers
single enantiomers
Scheme 1. Synthesis and resolution of Me2dbCOT 3a and Ph2dbCOT 3b and their rhodium(I) complexes 7-9 (for clarity, only the (S,S)-stereoisomers of 3a/b, 7a/b,
and 8a/b are shown; a) Stereochemical designation refers to the dbCOT ligand. Yields are given based on the stated diastereomer.
Non-chromatographic preparative resolution of unfunctionalized
hydrocarbons of this type is more challenging, but
Grützmacher’s method for resolution of 1 via diastereomeric
rhodium(I) complexes turned out to be surprisingly general and
provides a convenient alternative to chromatography.7a Racemic
rhodium dimers 7a and 7b were thus synthesized by reaction of
The free energy barriers for tub-to-tub inversion
(racemization) of 3a and 3b were investigated using a stop-and-
go heating experiment. Propylene carbonate solutions of either
ligand were heated at three different temperatures and the
deterioration of enantiomeric excess monitored over time by
chiral HPLC. Both complexes exhibited a free energy barrier for
racemization of ~30 kcal mol-1 with small entropic contributions.
Thus, both structures were shown to sustain prolonged
moderate heating without extensive racemization, a prerequisite
for more general use in coordination chemistry and catalysis. In
practice, complexation of enantiomerically pure Ph2dbCOT 3b
with [Rh2(µ-Cl)2(C2H2)4] was shown to give 7b without detectible
racemization.
the
appropriate
dbCOT
ligand
with
[(C2H4)2RhCl]2.
Diastereomeric mixtures of complexes 8a and 8b were then
prepared by reaction of 7a or 7b, respectively, with AgSbF6 and
enantiomerically pure 1,1’-diamino-2,2’-binaphtyl (DABN).
Prudent solvent selection allowed for fractional crystallization
and isolation of either diastereomer of 8b in pure form.20 The
(S,S)-Me2dbCOT diastereomer of 8a could also be readily and
selectively crystallized but we were not able to identify
conditions for selective crystallization of its diastereomer. Each
of the resolved complexes could be reverted back to the
corresponding dimers (S,S)-7a, (S,S)-7b or (R,R)-7b, which are
directly suitable as catalysts or for use as catalyst precursors.
Both 7a and 7b were found to be stable for months at ambient
conditions without signs of decomposition or loss of
stereochemical integrity. The absolute configurations of (R,R)-7a
(obtained via resolution with (–)-DABN) and (S,S)-7b were
determined by scXRD analysis using anomalous dispersion
(Figure 2). To add to the number of catalyst candidates, the
cationic complex (S,S)-9 was also synthesized by treatment of
(S,S)-7a with AgSbF6 in MeCN.
COD
(R,R)-3a (R = Me, 90%)
(R,R)-7a/b
(R,R)-3b (R = Ph, 65%)
CH2Cl2
R
R
single enantiomers
Δ
R = Me: ΔH‡ = 32.0 kcal mol-1
ΔS‡ = 6.2 e.U.
R = Ph: ΔH‡ = 28.4 kcal mol-1
ΔS‡ = -6.2 e.U.
R
R
R
R
(S,S)-3a/b
(R,R)-3a/b
Scheme 2. Decomplexation (top) and investigation of the tub-to-tub inversion
barrier of 3a and 3b (bottom).
Treatment of (R,R)-7a or (R,R)-7b with an excess of
cycloocta-1,5-diene gave a clean decomplexation of the dbCOT
ligands without detectible racemization (Scheme 2).
The ability of 3a and 3b to induce chiral discrimination in
catalysis was first evaluated in rhodium(I)-catalyzed carbon-
carbon-bond forming conjugate additions21 under Hayashi-
Miyaura conditions.22 In
a
model reaction between 2-
cyclopentenone and phenyl boronic acid, catalysts 7a, 7b, and 9
produced cyclopentanone 10a in excellent yields and with
selectivities of 95-99% ee (Table 1).
Table 1. Evaluation of 7a, 7b, and 9 as catalysts in Hayashi-Miyaura
reactions.
O
catalyst (1 mol%), PhB(OH)2 (2 equiv.),
O
base (0.5 equiv.)
PhMe/H2O (10/1 v/V), rt, 1h
10a
Ph
Figure 2. scXRD structures of rhodium complexes (R,R)-7a, (S,S)-7b and rac-
9 (thermal ellipsoids are shown at 30% probability; hydrogen atoms, counter
ions, and solvate molecules are omitted for clarity).
entry
1
base
catalyst
Yield [%]a
94
ee [%]b
97
K3PO4
(S,S)-7a
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by 1H NMR spectroscopy using an internal standard. [c] (R,R)-7a was used as
the catalyst. The major enantiomer is opposite to that shown. [d] A 1,4/1,2
double addition product was formed in 50% with 7a and in 19% with 7b.
2
3
4
5
6
K3PO4
K3PO4
tBuOK
KOH
(S,S)-7b
99
95
99
92
94
>99
95
(S,S)-9
(S,S)-7a
(S,S)-7a
(S,S)-7a
97
In general, the diphenyl substituted 7b was found both more
efficient and selective than the methyl substituted 7a. Even so,
7a gave over 99% ee with certain substrates, which is
noteworthy given the limited steric demands of the methyl
steering groups. The moderate to fair yields obtained for certain
substrate combinations were attributed to the lability of the
boronic acids under the basic reaction conditions used (entries
5-9,15). The facial selectivity of addition for both ligands follows
models for sterically related ligands where the ene-one is
oriented to minimize steric interactions with the ligand
substituents.23b,d
The successful application in 1,4-additions prompted us to
also evaluate 7b also in a more challenging transformation: The
formation of tertiary glycolates by 1,2-addition of arylboronic
acids to α-ketomethylesters (Table 3).24 Diene ligated catalysts
have previously met with very limited success for such
substrates carrying sp2-hybridized substituents on the keto-
group (<10% ee).25 Following a brief optimization, rhodium dimer
7b was shown to give 1,2-addition product 11a in a quantitative
yield and with a useful enanantioselectivity of 91% ee.26
Variation of the boronic acid component, utilizing both aryl and
heteroaryl substituents, gave the desired 1,2-addition products
in excellent yields and with selectivities of up to 94% ee.
Minimization of steric clashes between the substrate aryl group
and the phenyl substituents of the ligand likely accounts for the
facial selectivity in this reaction. Accordingly, increased steric
bulk of the ester moiety, which is in proximity to the ligand
phenyl group, resulted in a significantly reduced selectivity (see
SI for details).
97
(iPr)2NH
96
[a] Determined by 1H NMR spectroscopy using an internal standard. [b]
Determined by HPLC on a chiral stationary phase. See SI for details.
The choice of base and solvent had only minor impact on
reaction performance. Catalysts 7a and 7b were then evaluated
with various combinations of structurally diverse 1,4-acceptors
and boronic acid derivatives (Table 2). Cyclic and linear ene-
ones as well as a range of boronic acids including aryl,
heteroaryl, and cinnamyl derivatives were screened and high
enantioselectivities were maintained across the substrate scope.
In the reaction between phenylboronic acid and cyclohexenone,
1,2-addition to the product ketone was observed as a competing
side reaction (Entry 8). This was particularly pronounced with 7a
(50%), and with this catalyst, traces of the corresponding double
addition product were also seen in entry 1. The observation is
interesting as double addition of this type is rarely found under
ambient conditions with traditional diene ligands.22
Table 2. Scope of 7a and 7b catalyzed asymmetric Hayashi-Miyaura
reactionsa.
catalyst (2 mol%),
R'B(OH)2 (2 equiv.),
K3PO4 (0.5 equiv.)
O
O
O
R
proposed
stereochemical
pathway
X
X
Rh
R'
R'
PhMe/H2O, rt
O
R
O
1.
2.
3.
O
Table 3. Scope of 7b catalyzed asymmetric 1,2-arylation reactionsa
Me
10b
10c
10d
Ph
(S,S)-7b (2 mol%),
RB(OH)2 (2.0 equiv.)
K3PO4 (0.5 equiv.)
O
O
MeO
HO
R
F
(S,S)-7a: 93% ee (66%)
(S,S)-7b: >99% ee (quant.)b
OMe
95% ee (83%)
>99% ee (quant.)b
OMe
OMe
O
>99% ee (76%)
99% ee (quant.)b
O
Rh
O
O
proposed
rt, aq. PhMe
R
O
stereochemical
pathway
Ph
4.
5.
6.
O
1.
2.
3.
NMe2
Cl
OMe
Cl
Cl
OH
O
OH
OH
F
10f
10g
S
10e
N/a
11a
11b
11c
NMe2
OMe
F
O
OMe
O
OMe
O
96% ee (48%)b
>99% ee (38%)
N/a
>99% ee (47%)
89% ee (99%)
90% ee (99%)
94% ee (80%)
>99% ee (30%)
O
4.
5.
6.
8.
9.
7.
O
O
Me
OH
OH
OH
Nap
O
S
10h
10ie
10j
Ph
Ph
Ph
97% ee (51%)b
99% ee (42%)
OMe
O
OMe
O
OMe
11d
121e
11f
97% ee (87%)
95% ee (92%)b
98% ee (50%)
98% ee (80%)
90% ee (81%)
81% ee (94%)
72% ee (98%)
O
10.
11.
O
O
12.
7.
p-Cl-Ph
8.
9.
O
Ph
Ph
CF3
F
OMe
OMe
OMe
i-PrO
OH
OH
10k
97% ee (quant.)b,c
91% ee (99%)
10l
10m
Ph
>99% ee (95%)
>99% ee (quant.)b
OH
98% ee (82%)
99% ee (96%)b
F3C
O
OMe
11g
11h
11i
O
OMe
O
OMe
82% ee (99%)
88% ee (99%)
93% ee (99%)
O
O
13.
i-PrO
a See SI for experimental details and assignment of absolute configurations.
Substructures from boronic acids are highlighted in blue. Isolated yields.
10n
N/a
>95% ee (28%b; 20%)
[a] See SI for experimental details and assignment of absolute configurations.
Substructures from boronic acids are highlighted in blue. [b] Yield determined
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10.1002/chem.201704816
Chemistry - A European Journal
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cyclooctatetraenes, see: d) L. A. Paquette, Acc. Chem. Res. 1993, 26,
57–62 and references therein.
Finally, 7b was evaluated by a short catalytic asymmetric
synthesis of the key cyclization precursor 15 in Mulzer’s
synthesis of the dioxafenestrene natural product (–)-penifulvin A
(16) (Scheme 3). The prior asymmetric approach to this
compound has relied on auxiliary control.16 A 1,4-addition of o-
tolyl boronic acid to ester 12 afforded 13 in high yield and as a
single detected enantiomer. The reaction proceeded without
competing lactonization or interference from the free hydroxyl
group of the substrate. When conducting this reaction on a gram
scale, the catalyst could be recovered in 76% yield by
chromatography.27
[2]
[3]
For selected examples, see: a) S. Spiess, C. Welter, G. Franck, J.-P.
Taquet, G. Helmchen, Angew. Chem. 2008, 120, 7764–7767; Angew.
Chem., Int. Ed. 2008, 47, 7652–7655; b) W. Chen, J. F. Hartwig, J. Am.
Chem. Soc. 2012, 134, 15249–15252; c) Q. Cheng, Y. Wang, S.-L. You,
Angew. Chem. 2016, 128, 3557–3560; Angew. Chem., Int. Ed. 2016,
55, 3496–3499.
P. Zhang, H. Wang, X. Shi, X. Yan, X. Wu, S. Zhang, B. Yao, X. Feng,
J. Zhi, X. Li, B. Tong, J. Shi, L. Wang, Y. Dong, J. Polym. Sci., Part A:
Polym. Chem. 2017, 55, 716–725.
[4]
[5]
H. Yuki, N. Komine, S. Komiya, M. Hirano, Organometallics 2014, 33,
6604–6613.
For dinapthto[a,e]cyclooctatetraene-rhodium(I) in catalysis, see: a) P. A.
Wender, A. B. Lesser, L. E. Sirois, Angew, Chem. 2012, 124, 2790–
2794; Angew, Chem., Int. Ed. 2012, 51, 2736–2740; see also: b) X. Xu,
P. Liu, A. Lesser, L. E. Sirois, P. A. Wender, K. N. Houk, J. Am Chem.
Soc. 2012, 134, 11012–11025.
M.-C. Melcher, H. von Wachenfeldt, A. Sundin, D. Strand, Chem. Eur. J.
2015, 21, 531–535.
a) F. Läng, F. Breher, D. Stein, H. Grützmacher, Organometallics 2005,
24, 2997–3007; see also: ref. 5a.
(o-tol)B(OH)2 (2.0 equiv)
1. TBSOTf, 2,6-lut., DMF
2. LiBH4, MeOH, THF
3. TsCl, DMAP, CH2Cl2
(S,S)-7b (5 mol%),
K3PO4 (1.0 equiv.)
O
O
Me
OH
OH
EtO
[6]
[7]
toluene/H2O (10:1)
89%, >99% ee
EtO
68% over three steps
12
13
O
H
O
O
O
HO
MgBr,
[8]
[9]
A. Kina, K. Ueyama, T. Hayashi, Org. Lett. 2005, 7, 5889–5892.
Ligand 2 gives efficient facial discrimination in rhodium(I) catalysis, but
the selectivity was reported to rapidly erode over time due to
racemization of the catalyst.
CuI then
TBAF
Mulzer and Gaich
5 steps
H
Me
OTBS
Me
TsO
reference 16
54%
H
14
15
(–)-penifulvin A (16)
[10] For pioneering studies of dbCOT as a metalation-resistant ligand: a) A.
R. Douglas, R. H. Crabtree, Organometallics 1983, 2, 621; dbCOT as a
catalyst poison: b) D. R. Anton, R. H. Crabtree, Organometallics 1983,
2, 855–859.
Scheme 3. Formal catalytic asymmetric synthesis of (–)-penifulvin A.
[11] a) A. Huber, A. May, K. Müllen, Chem. Ber. 1981, 114, 1318–1336; b)
W. Heinz, H. Rader, K. Müllen, Tetrahedron Lett. 1989, 30, 159–162; c)
T. Nishiuchi, Y. Kuwatani, T. Nishinaga, M. Iyoda, Chem. Eur. J. 2009,
15, 6838–6847.
No loss in catalytic activity or selectivity was seen over the
course of three recovery cycles, emphasizing the robust nature
of this complex. The synthesis of 15, and hence, the formal
synthesis of (–)-penifulvin A, was then completed in four
additional steps concluding with a Cu-mediated sp2-sp3 coupling
reaction. With small variations, this strategy should be applicable
to also to related natural products and their structural analogs.
In summary, this work introduces 5,11-disubstituted
dbCOTs as configurationally stable and readily accessible
planar chiral homologs to the parent dbCOT ligand. In particular,
5,11-di-methyl and di-phenyl substituted dbCOTs were shown to
form stable complexes with rhodium(I). When applied in 1,4- and
1,2-arylation reactions, high yields and ee’s up to >99% were
achieved over a wide range of substrates, and in a synthesis of
a key intermediate in Mulzer’s synthesis of (–)-penifulvin A. More
broadly, we envision that 5,11-disubstituted dbCOTs in
enantiopure form will find utility across many catalyst systems
and types of catalytic reactions given the unique inertness and
ligand-properties of these structures. The building blocks
developed during the ligand syntheses should be of value also in
other contexts. Such efforts are in progress and will be reported
in due course.
[12] M. J. Marsella, R. J. Reid, Macromolecules 1999, 32, 5982–5984.
[13] T. Nishida, S. Ohta, F. Xu, K. Shinohara, T. Kamada, H. Akashi, M.
Takezaki, K. Wakamatsu, A. Orita, Org. Lett. 2016, 16, 3988–3991.
[14] a) For synthesis of 5,11-diaryl dbCOTs, see reference 1c. For synthesis
of 5,11-disulfonyl dbCOTs, see: b) A. Orita, D. Hasegawa, T. Nakano, J.
Otera, Chem. Eur. J. 2002, 8, 2000–2004; c) F. Xu, L. Peng, K.
Shinohara, T. Morita, S. Yoshida, T. Hosoya, A. Orita, J. Otera, J. Org.
Chem. 2014, 79, 11592–11608. For an efficient and selective synthesis
of cyclooctatetraenes with this substitution pattern, see: (d) P. A.
Wender, J. P. Christy, A. B. Lesser, M. T. Gieseler, Angew. Chem.
2009, 121, 7823–7826; Angew. Chem., Int. Ed. 2009, 48, 7687–7690.
[15] For an early attempt to synthesize 3a, see: S. Wawzonek, J. Am. Chem.
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Acknowledgements
We thank the Swedish Research Council and the Royal
Physiographical Society in Lund for funding and L. Sirois and T.
J. Paxton (Wender Group) for discussions on penifulvin A.
Keywords: cyclooctatetraenes • planar chirality • asymmetric
catalysis • rhodium(I) complexes • fenestrenes
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Chemistry - A European Journal
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[26] Catalyst 7a gave 12a in 42% ee with an opposite facial selectivity to 7b.
See SI for details.
[27] It is unclear whether dimeric 7b is reformed from a catalytically active
monomeric species or recovered unreacted.
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10.1002/chem.201704816
Chemistry - A European Journal
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Michaela-Christina Melcher,[a] Trpimir
Ivšić, [a] Charlotte Olagnon, [a] Christina
Tenten,[b] Arne Lützen,[b] and Daniel
Strand[a],
*
Page No. – Page No.
Control of enantioselectivity in
rhodium(I)-catalysis by planar chiral
dibenzo[a,e]cyclooctatetraenes
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