Synthesis of Chiral Indenylcobalt(I) Complexes and Their Evaluation in Asymmetric [2+2+2] Cycloaddition Reactions

Article abstract of DOI:10.1055/s-0035-1560433

The syntheses of two new indenyl-based cobalt(I) complexes were reported and their properties as catalysts in asymmetric cyclotrimerizations evaluated. While one complex was synthesized from a functionalized BINOL derivative by cross-coupling with 2-indenylboronate, the other complex was derived from a known chiral indenyl cobalt(I) complex by exchange of COD for two phosphite ligands, delivering the first chiral indenyl cobalt(I) complex, which can easily be activated thermally. The prepared complexes were tested in asymmetric cyclization reactions of triynes as well as diynes with nitriles to deliver chiral triaryls and heterobiaryls, respectively. While the BINOL-based precatalysts promoted the cyclizations in good yield without selectivity, the bisphosphite cobalt(I) complex induces chirality with up to 80% ee under photochemical as well as under thermal conditions.

Full text of DOI:10.1055/s-0035-1560433

SYNTHESIS
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© Georg Thieme Verlag Stuttgart · New York
2016, 48, A–J
feature
A
P. Jungk et al.
Feature
Synthesis
Synthesis of Chiral Indenylcobalt(I) Complexes and Their Evaluation
in Asymmetric [2+2+2] Cycloaddition Reactions
generation of novel
indenyl-based precatalysts:
Phillip Jungk
Tobias Täufer
Indre Thiel1
evaluation of properties in asymmetric
cyclotrimerization reactions yielding
heterobiaryls and triaryls:
Marko Hapke*2
OMOM
OMOM
Leibniz Institute for Catalysis at the University of
Rostock (LIKAT Rostock), Albert-Einstein-Straße 29A,
18059 Rostock, Germany
Co
R
O
O
marko.hapke@catalysis.de
N
catalysis
*
OMe
*
*
Dedicated to Prof. Dr. Detlef Heller on the occasion
of his 60th birthday
D & hn
i-Pr
Co
(EtO)3P
P(OEt)3
Received: 08.02.2016
Accepted after revision: 19.02.2016
Published online: 22.03.2016
of symmetrical mannitol-based ligands or those relying on
a BINOL backbone is often lengthy, including critical reac-
tion steps with lower yields, making the overall procedure
rather tedious.13 The group of Heller was the first to de-
velope various chiral (CpR)Co(COD) complexes, for example,
on the basis of tartaric acid14 and menthyl derivatives15 as
well as the first chiral indenyl-derived cobalt complex,
which proved to be highly selective in photochemical
[2+2+2] cycloaddition reactions.16 Taking a closer look at
the asymmetric [2+2+2] cycloaddition reaction the distin-
guished position of the metals of group 9 (cobalt, rhodium,
and iridium) as catalysts in this reaction is revealed.17 Ex-
clusively for rhodium and iridium, in situ catalytic systems
exist up to date, which have found rather broad applica-
tion.18,19 In the case of cobalt, the defined cobalt(I) complex
1 is still the only applied catalyst.
DOI: 10.1055/s-0035-1560433; Art ID: ss-2016-e0098-fa
Abstract The syntheses of two new indenyl-based cobalt(I) complexes
were reported and their properties as catalysts in asymmetric cyclotri-
merizations evaluated. While one complex was synthesized from a
functionalized BINOL derivative by cross-coupling with 2-indenylboro-
nate, the other complex was derived from a known chiral indenyl co-
balt(I) complex by exchange of COD for two phosphite ligands, deliver-
ing the first chiral indenyl cobalt(I) complex, which can easily be
activated thermally. The prepared complexes were tested in asymmet-
ric cyclization reactions of triynes as well as diynes with nitriles to deliv-
er chiral triaryls and heterobiaryls, respectively. While the BINOL-based
precatalysts promoted the cyclizations in good yield without selectivity,
the bisphosphite cobalt(I) complex induces chirality with up to 80% ee
under photochemical as well as under thermal conditions.
Key words cobalt, asymmetric catalysis, [2+2+2] cycloaddition, inde-
Different approaches to obtain or induce chirality in
[2+2+2] cycloaddition reactions are presented in Scheme 1.
The first pathway is the cycloaddition of a chiral substrate
in the presence of an achiral catalyst (a) by conservation of
the chiral information through the reaction step as de-
scribed by Heller et al.20 or (b) by forming a second stereo-
center in the resulting product, thus leading to diastereo-
meric products, which are separable by flash chromatogra-
phy, as developed by our group.21 The second approach is
the use of achiral substrates in the presence of a chiral cata-
lyst to ideally obtain an enantiomerically pure product, as
presented in example (c) using precatalyst 1 under photo-
chemical conditions.16
Having acquired expertise in the syntheses of new types
of cobalt(I) complexes as catalysts for [2+2+2] cycloaddition
reactions,22–24 our group became interested in the synthe-
ses of novel chiral cobalt(I) complexes for asymmetric
[2+2+2] cycloadditions. Only a small library of chiral inde-
nyl complexes exists, which have been used in asymmetric
nyl complexes, phosphites, metal complexation
Chiral cyclopentadiene and indene derivatives and their
corresponding transition metal complexes have been wide-
ly explored, including their relevance in asymmetric cataly-
sis.3 Halterman et al. described in systematic investigations,
the syntheses and applications of a number of different chi-
ral cyclopentadienyl (Cp) ligands and complexes of the met-
als Fe, Ti, Zr, Mo, Ru, Rh, Nb, and Hf.3–8 Although a series of
chiral Cp ligands were synthetically accessible in good
yields, their complexation to cobalt has only been described
in two cases: a cobalt-biscarbonyl5e,9 and a cobalt-bisphos-
phine system.10 Very recently Cramer et al. developed a dif-
ferent kind of chiral Cp ligands possessing C2 symmetry and
used them, for example, for C–H functionalizations with
the corresponding Rh(I) complex.11 With the analogous
Ir(III) complexes they were able to promote enantioselec-
tive cyclizations, furnishing cyclopropanes.12 The synthesis
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2016, 48, A–J

B
P. Jungk et al.
Feature
Synthesis
catalysis,4 mostly derived from menthyl-substituted inde-
nyl moieties.4,15,25b–d,25f Fewer examples are based on inde-
nyl moieties including a chiral axis to naphthol substitu-
ents25a,g,h and only one example reports an alkyloxazoline-
substituted indenyl moiety, which binds to the metal center
via both, the indenyl group and the substituent.25e To the
best of our knowledge no chiral indenyl bearing a binaphth-
yl moiety or a corresponding complex with a noncoordinat-
ing binaphthyl ligand exists. Accordingly, we investigated
the synthesis of new chiral indenyl-based ligands, starting
from enantiomerically pure (S)-BINOL building new chiral
indenyl ligands based on a binaphthyl framework.
Our target molecules based on the binaphthyl frame-
work are molecules A and B (Figure 1), consisting of easily
accessible functionalized BINOL and 2-substituted 1H-in-
dene building blocks, which should be possible to link con-
Biographical Sketches
Phillip Jungk was born in Ber-
lin. He obtained his B.Sc. with
Prof. C. B. W. Stark at the Free
University of Berlin (Germany)
in 2009 and his M.Sc. with Prof.
H.-U. Reißig at the same Univer-
sity in 2011. He then joined the
research group of Prof. M. Hapke
at LIKAT Rostock in 2012 and
finished his Ph.D. in 2016. His
thesis focuses on the develop-
ment of new chiral ligands and
their application as chiral com-
plexes on the basis of cobalt and
iron in asymmetric [2+2+2] cy-
cloaddition reactions.
Tobias Täufer was born in
Crivitz (Germany). He obtained
his B.Sc. with Prof. A. Schulz at
the University of Rostock in
2013 and his M.Sc. degree at
the same University in 2015 in
the group of Prof. P. Langer.
Since May 2015, he conducts
research as a Ph.D. student at
the LIKAT Rostock under the su-
pervision of Prof. A. Schulz and
Prof. M. Hapke in cooperation
with the Science Campus Phos-
phorus Research Rostock and is
focusing on cycloadditions with
unusual PN-precursor mole-
cules.
Indre Thiel studied biochemis-
try at the Heinrich-Heine Uni-
versity Düsseldorf, Germany,
and did her Bachelor thesis at
the University of Cincinnati in
Prof. J. Mack’s group in 2008.
She obtained her M.Sc. degree
in 2010 in the field of bioorgan-
ic chemistry in the group of
Prof. W. Kläui in Düsseldorf. She
then joined Prof. M. Hapke’s re-
search group at the LIKAT in
Rostock and obtained her Ph.D.
in 2013 on the topic of ‘Synthe-
sis of novel CpCo(I) complexes
and their application in [2+2+2]
cycloaddition reactions’, sup-
ported by a Kekulé scholarship
of the Fonds der Chemischen In-
dustrie. Since 2014 she is a
postdoctoral fellow at the ETH
Zürich in the research group of
Prof. C. Copéret.
Marko Hapke studied chemis-
try at the Carl von Ossietzky
University in Oldenburg, Ger-
many and received his diploma
degree in 1999. He obtained his
doctorate under the supervision
of Prof. P. Köll and Prof. A.
Lützen in the field of supramo-
lecular metal complex chemis-
try at the same university. After
his postdoctoral studies in the
area of C–H functionalization
chemistry with Prof. J. F.
Hartwig at Yale University, he
joined the group of Prof. U.
Rosenthal at LIKAT in 2006. The
focus of his independent work-
ing group centers on the devel-
opment of novel first row
transition metal catalysts for cy-
cloaddition reactions and the
understanding of factors deter-
mining the reactivity and selec-
tivity.
He
finished
his
habilitation at the University of
Rostock in 2015 and holds a
professorship at the Johannes
Kepler University in Linz
(Austria) since November of the
same year, with an affiliated
working group at the LIKAT in
Rostock (Germany).
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2016, 48, A–J

C
P. Jungk et al.
Feature
Synthesis
(a) [2+2+2] Cycloaddition with conservation of existing chirality:
N
O
O
[CpCo(COD)]
hexane, 5 h
O
O
N
N
4
+
N
82%
(b) [2+2+2] Cycloaddition with conservation of existing chirality
and additional generation of axial chirality:
R
N
N
CpCo(H2C=CHSiMe3)2
R
N
(5 mol%)
+
OMe
THF or toluene
N
0–40 °C
OMe
44–69%, dr: 1.13–1.75:1
R = Me, Ph, 2-FC6H4
separable diastereomeric atropisomers
(c) Asymmetric [2+2+2] cycloaddition using chiral cobalt complexes:
catalyst:
i-Pr
R
R
N
N
1 (1–2 mol%)
+
Co
OMe
OMe
THF, hν, –20 °C
Me
R = Me, t-Bu, Ph
66-86%, 90–93% ee
1
Scheme 1 Commonly used cobalt(I) catalysts for [2+2+2] cycloadditions
veniently by a cross-coupling reaction. The binaphthyl
framework is a promising structural feature to provide chi-
ral induction by protecting one side of the resulting com-
plex with its axial chirality. Furthermore, 2-substituted 1H-
indene compounds are often commercially available or eas-
ily synthesized by literature known procedures in compari-
son to 1-substituted 1H-indenes, which are less well docu-
mented and would lead to molecules such as C (Figure 1).
The selected ligands should also exemplify the role of possi-
ble rotation around the connecting bond between the
binaphthyl and the metal-coordinated indene moiety,
which should certainly differ for molecules A and B. Easily
cleavable protecting groups of the free hydroxyl groups of
the BINOL should also allow further derivatization.
The synthesis sequence was initiated by the protection
of the BINOL hydroxyl moieties with methoxymethyl
(MOM) groups following a known literature procedure.26
Attempts to borylate the binaphthyl backbone in the 3- or
3′-position27 to obtain a suitable substrate for a Suzuki–Mi-
yaura coupling were troublesome due to irreproducibility
of the isolated yields. Bromination of the BINOL at the 3-po-
sition and subsequent coupling of derivative 2 with the bor-
ylated indene 3 led to the desired product 4 in excellent
yield and purity (Scheme 2) with an overall yield of 71% for
3 steps after optimization of the reaction conditions, which
included the change of the base from sodium carbonate to
potassium phosphate and a decrease of the reaction time
from 22 to 2 hours.
The synthesis of molecule B was achieved via the syn-
thesis of 8 following a multi-step procedure already report-
ed for the (R)-BINOL (Scheme 3).28 The BINOL-monophos-
phate ester 7, obtained in excellent yield, was crucial for the
synthesis of bromide 8 in the following step. Reductive
metalation of the 2-position of the binaphthyl derivative 7
in the presence of in situ-generated lithium naphthalide
was followed by bromination applying 1,2-dibromo-
1,1,2,2-tetrafluoroethane, yielding naphthyl bromide 8 in
55% yield, slightly contaminated with the defunctionalized
congener 9, which was inseparable by chromatography
OPG
OPG
OPG
OPG
OPG
A
B
C
Figure 1 Potential chiral indenyl ligands as target molecules
(PG = Protecting Group or other functionality)
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2016, 48, A–J

D
P. Jungk et al.
Feature
Synthesis
Pd(PPh3)4 (3 mol%)
base
O
O
OMOM
OMOM
OMOM
OMOM
+
B
solvent, 100 °C, t
Br
2
3
4
base
Na2CO3
K3PO4
solvent
toluene 22 h
DMF 2 h
t
yield
52%
>95%
A
B
Scheme 2 Cross-coupling key step in the synthesis of ligand 4
(Scheme 3). However, this mixture can be applied directly
in the next step since 9 does not react under these condi-
tions.
in an acceptable yield of 33% (Scheme 5). The removal of
free triphenylphosphine resulting from the exchange for
the COD ligand turned out to be troublesome. Using n-Buli
for deprotonation complex 11 could only be purified after
column chromatography under inert conditions followed by
treatment with a polymer-based triphenylphosphine scav-
enger as reported by Lipshutz et al.29
DIPEA
MOMCl
NaH
ClP(O)(OEt)2
OMOM
OH
OH
OH
CH2Cl2
0 °C, 2.5 h
THF/DMF (3:1)
25 °C
1) KH
THF, 25 °C, 30 min
2) CoCl(PPh3)3
5
6 (64%)
OMOM
OMOM
OMOM
OMOM
25 °C, 2 h
3) COD, toluene
110 °C, 2 h
i) Li
naphthalene
THF, –78 °C
OMOM
OMOM
Br/H
Co
O
O
4
ii) BrCF2CF2Br
25 °C
P
OEt
EtO
11 (33%)
7 (>95%)
mixture of 8 (55%)
and 9 (12%)
Scheme 5 Synthesis of the chiral cobalt(I) complex 11
Scheme 3 Synthesis of compound 8 via a multi-step procedure start-
ing from (S)-BINOL (5)
We also investigated the complexation of ligand 10,
however, while the reaction seemed to proceed successful-
ly, the isolation and unambiguous characterization of the
respective complex was not possible.
We were able to obtain crystals of 11 from an n-hexane
solution, which were analyzed by X-ray diffraction. The
molecular structure showed the indene ring being coplanar
to the substituted naphthyl ring. The distance of the Cp-
centroid to the cobalt [1.737(2) Å] is identical to that of
complex 1 determined by Heller et al. [1.732(3) Å] (Figure
2).15
The cross-coupling reaction of the mixture of 8/9 with
the indenylboronate 3 was conducted under the optimized
reactions conditions used before and furnished ligand 10 in
excellent yield (Scheme 4).
The ligand synthesis was followed by complexation of
the ligands to the cobalt center, which often requires opti-
mization. We found that the use of potassium hydride as
deprotonating agent for ligand 4 and CoCl(PPh3)3 in the
presence of 1,5-cyclooctadiene (COD) furnished complex 11
O
B
O
OMOM
OMOM
OMOM
3
+
Br/H
Pd(PPh3)4 (5 mol%)
K3PO4
DMF, 100 °C, 2.5 h
mixture of 8 (82%)
and 9 (18%)
10
(>95% in relation to 8)
9
(unchanged)
Scheme 4 Final assembly of ligand 10
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2016, 48, A–J

E
P. Jungk et al.
Feature
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The evaluation of complex 11 in a completely intramo-
lecular [2+2+2] cycloaddition of the ether-bridged triyne 15
gave the resulting product 16 only in 18% yield as a racemic
mixture (Scheme 6).
O
O
O
complex 11 (5 mol%)
THF, hν, 25 °C, 24 h
*
*
O
18% racemic mixture
d/l:meso = 1:1.4
15
16
Figure 2 ORTEP presentation of complex 11 (30% probability, H atoms
omitted for clarity)
Scheme 6 Evaluation of complex 11 in the intramolecular cycloaddi-
tion of triyne 15
The obtained structure in the solid state suggests that
the chiral information of the biaryl axis in 11 might be too
far away from the cobalt center to influence the (stereo-
chemical) reaction outcome. Nevertheless, having complex
11 in hand we evaluated its catalytic activity as well as its
potential chiral induction in the partial intramolecular
[2+2+2] cycloaddition reaction of diyne 12 and benzonitrile
(13), yielding pyridine 14. We applied photochemical reac-
tion conditions identical to those used for complex 1. In-
creasing the temperature from –10 to +20 °C led to a small
increase in yield, but indeed we unfortunately did not ob-
serve any selectivity in the formed product (Table 1, entries
1–3). In comparison, complex 1 showed a high selectivity of
93% ee at –20 °C. Increasing the temperature led to a more
active but slightly less selective catalyst (entries 4–6).
Presumably none of the sides of the formed cobaltacycle
intermediate resulting after oxidative cyclization of diyne
12 with complex 11 is shielded enough by the second naph-
thyl ring of the chiral binaphthyl framework, leading to an
unselective subsequent reaction with the nitrile. This situa-
tion could be expected to be different for a catalyst derived
from ligand structure B (see Figure 1), where the connec-
tion in the 2-position of the BINOL naphthyl ring puts the
indenyl group closer to the biaryls axis and also the second
naphthyl ring. We investigated different metalation meth-
ods for ligand 10 to coordinate it to the cobalt center, how-
ever, unfortunately, the resulting cobalt(I) complex was
synthetically not accessible in pure form via any route. The
broader context for these investigations also encompasses
the question, if the chiral information from the indenyl-
attached BINOL moiety is sufficient to induce chirality in
the cyclization products or if the presence of two elements
of chirality (planar chirality and axial chirality) like in com-
plex 1 is a requirement for successful induction of chirality
during the cycloaddition process. In the latter case, the at-
tachment of a chiral substituent in the 2-position, avoiding
the formation of planar chirality at the indenyl ring, would
generally lead to only little or no chiral induction at all. On
the other hand, if the planar chirality prevails in the induc-
tion of product chirality, the chirality of the attached sub-
stituent in the 1-indenyl position will possess only inferior
importance.
Table 1 Catalytic Evaluation of Complex 11 and Comparison with
Complex 1
Ph
N
[complex]
PhCN
+
*
OMe
OMe
THF, T, t
12
13
14
Entry
Complex
Temp/hν
Time (h) Yield (%) Selectivity
Since CpCo(I)-bisphosphite complexes were recognized
as catalysts possessing a very interesting reactivity profile
for cyclization reactions at moderate temperatures23 we fol-
lowed this approach accordingly for the synthesis of new
chiral cobalt(I) complexes. For this purpose, complex 1 was
synthesized in moderate yield by an optimized literature
procedure (Scheme 7).15 The problematic separation of un-
reacted starting material 17 from the desired complex 1 re-
quired the separation of unconsumed ligand from the prod-
1
11 (5 mol%)
11 (5 mol%)
11 (5 mol%)
1 (1 mol%)
1 (1 mol%)
1 (1 mol%)
–10 °C, hν
5 °C, hν
24
24
24
72
24
24
66
61
71
86
86
79
–
2
–
3
20 °C, hν
–20 °C, hν
3 °C, hν
–
416
516
616
93% ee
89% ee
82% ee
20 °C, hν
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2016, 48, A–J

F
P. Jungk et al.
Feature
Synthesis
i-Pr
1) n-BuLi
i-Pr
i-Pr
THF, 25 °C, 2 h
2) CoCl(PPh3)3
25 °C, 18 h
Me
Co
Co
+
Me
3) COD
25 °C, 2 h
Me
Me
i-Pr
observed byproduct
17
1 (29%)
Scheme 7 Synthesis of chiral cobalt(I) complex 1
uct complex by bulb-to-bulb distillation. As by-product a
bisindenyl Co(II) complex was isolated and characterized by
X-ray structure analysis.
Following our previously reported procedure to ex-
change COD for phosphite ligands under photochemical
conditions23 we were able to generate the chiral bisphos-
phite complex 18 from 1 via a simple substitution reaction
at room temperature in very good yield (Scheme 8).
reaction times, unfortunately the enantioselectivity was
significantly lower (entry 2). The lower ee value for the re-
action carried out at 80 °C can most likely be attributed to
the partial racemization of the biaryl product 14. Earlier
stereochemical studies of biaryls including 14 showed that
significant racemization can occur at temperatures above
90–100 °C.18 Under irradiation conditions at –20 °C basical-
ly no reaction was observed after 24 hours (entry 3). By in-
creasing the temperature to 0 °C, 14 was isolated in good
yield and good enantioselectivity of 64% ee (entry 4).
The optimized thermal cyclization conditions (Table 2,
entry 1) were tested with three other nitriles as substrates
(Table 3).
i-Pr
i-Pr
P(OEt)3 (2 equiv)
Co
Co
THF, hν, 25 °C
(EtO)3P
P(OEt)3
1
18 (96%)
Table 3 Screening Conditions for Complex 18 for the Reaction be-
tween Diyne 12 and Nitriles 19, 21, and 23
Scheme 8 Synthesis of complex 18 by exchange of the neutral ligands
R
Based on earlier studies complex 18 was supposed to be
activated already at comparably mild temperatures, since
[CpCo{P(OEt)3}2] proved to be very active in the co-cyclotri-
merization reaction of diynes and nitriles already at 50 °C.23
complex 18
N
(10 mol%)
RCN
+
*
OMe
OMe
THF, 50 °C, 120 h
Table 2 Screening Conditions for Complex 18 for the Reaction be-
tween Diyne 12 and Benzonitrile (13)
12
20, 22, 24
Entry
1
R
Conver-
sion (%)
Biaryl product yield Selectivity
(conversion yield)
Ph
complex 18
(5 mol%)
Me (19)
24
20 22% (95%)
80% ee (R)
N
PhCN
+
F
*
OMe
OMe
T, t
2
3
3
22 traces
–
(21)
12
13
14
OMe
MeO
MeO
MeO
Entry Solvent
Temp
Time (h) Yield (%) Selectivity
28
24 20% (71%)
70% ee (R)
1
2
3
4
THF
50 °C
70
24
24
24
20
54
2
66% ee (R)
27% ee (R)
–
toluene
THF
80 °C
(23)
–20 °C, hν
0 °C, hν
THF
86
64% ee (R)
The reaction time was increased to 120 hours to maxi-
mize the conversion. Furthermore, the starting material
was reisolated to calculate the conversion and conversion
yield, respectively. The results for acetonitrile (19) (biaryl
20, Table 3, entry 1) are comparable to benzonitrile with a
nearly identical yield but higher selectivity (80% ee).
Switching to the bis-ortho-substituted benzonitrile 21 only
The reaction of 12 and 13 in the presence of 5 mol% 18
proceeded very slowly at 50 °C. After reaction times as long
as 70 hours, only 20% of 14, however, with an enantiomeric
excess of 66% ee, was obtained (Table 2, entry 1). Raising the
temperature to 80 °C resulted in higher yields and shorter
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2016, 48, A–J

G
P. Jungk et al.
Feature
Synthesis
very little conversion was observed (entry 2). Investigation
of 3,4,5-trimethoxy benzonitrile (23) as cyclization sub-
strate led to the corresponding biaryl 24 with yield and se-
lectivity comparable to 20 (entry 3). Although the yields are
mediocre, the presented results are the first examples of
thermal enantioselective indenyl cobalt(I)-catalyzed co-cy-
clotrimerization of diynes and nitriles.
Further investigations of complex 18 were carried out
regarding the completely intramolecular [2+2+2] carbocy-
clization with the ether-bridged triyne 15.30 Cyclization of
15 gave a yield of 77% cyclization product 16 with a very
low enantiomeric excess of 7% ee (Scheme 9). Use of the
structurally identical malonate-bridged congener of 15
proceeded with similar yield but negligible enantioselectiv-
ity.
ett Packard HP 1100 with DAD, chiralyzer and RI detector and chiral
columns. HRMS (ESI-TOF) was performed on an Agilent 6210 Time-
of-Flight LC/MS and HRMS (EI) and MS (EI/CI) on a Thermo Electron
Finnigan MAT 95-XP or an Agilent 6890 N/5973. Elemental analysis
was performed with a Leco Microanalysator-TruSpec CHNS (C, H) and
a PerkinElmer UV/VIS-spectrometer Lambda 2 (P). Optical rotation
was measured on an Anton Paar polarimeter MCP 200 and melting
points with a Mettler Toledo MP70 melting point system (not correct-
ed). X-ray analysis was performed on a Bruker Kappa APEX II Duo dif-
fractometer. Flash chromatography was performed on silica gel with
solvent mixtures of n-hexane, EtOAc, cyclohexane, CH2Cl2, toluene,
and Et2O. Column chromatography was performed under argon with
degassed silica and solvent mixtures of anhydrous solvents (see
above).
Ligand Syntheses
(S)-3-Bromo-2,2′-bis(methoxymethoxy)-1,1′-binaphthyl (2)
[CAS Reg. No. 1394020-41-7]
O
O
O
In an adaptation of the literature procedure by Harada et al.,31 t-BuLi
(14.5 mL, 1.45 M, 21.1 mmol) was added to a solution of (S)-2,2′-
bis(methoxymethoxy)-1,1′-binaphthyl (3.59 g, 9.58 mmol) in THF
(200 mL) at –78 °C and the reaction mixture was stirred for 1 h at this
temperature. To the resulting solution was added 1,2-dibromo-
1,1,2,2-tetrafluoroethane (4.0 mL, 33.6 mmol) dropwise and the solu-
tion was allowed to warm to r.t. and stirred for an additional hour.
The reaction was quenched with sat. aq NH4Cl and extracted with
CH2Cl2 (3 ×). The combined organic phases were washed with H2O
and brine, and dried (Na2SO4). After flash chromatography with n-
hexane/EtOAc (10:1, v/v) as eluent (S)-3,3′-dibromo-2,2′-bis(me-
thoxymethoxy)-1,1′-binaphthyl (0.79 g, 16%, yellow oil) and the main
product 2 (3.12 g, 72%, colorless solid) were isolated. The analytical
data were in accordance with the literature data.32
complex 18 (10 mol%)
*
*
THF, 65 °C, 72 h
O
77% (7% ee)
d/l:meso = 1:1.1 a
15
16
Scheme 9 Evaluation of complex 18 in the intramolecular cycloaddi-
tion of triyne 15. a Ratio was determined from chiral HPLC analysis.
The presented investigation describes the syntheses and
application of novel chiral indenyl cobalt(I) complexes and
their catalytic evaluation in [2+2+2] cycloaddition reactions
of naphthyldiynes and nitriles. The convenient generation
of novel BINOL-indenyl ligands and their challenging com-
plexation to a cobalt(I) center led to active complexes in
[2+2+2] cycloadditions, however, the chiral BINOL moiety
was too far away from the reaction center to exert any ef-
fect on the selectivity. In an alternative study, we were able
to convert the known chiral indenyl cobalt(I)-COD precata-
lyst 1, active only under photochemical conditions, into a
precatalyst also active under rather mild thermal condi-
tions, using a simple ligand substitution reaction to intro-
duce phosphite ligands. To the best of our knowledge this is
the first example of a thermally activated chiral indenyl co-
balt(I) catalyst with selectivities of up to 80% ee in the syn-
thesis of chiral isoquinolines. Photochemical activation is
also still possible, improving the yields compared to the
thermal reaction.
(S)-3-(1H-Inden-2-yl)-2,2′-bis(methoxymethoxy)-1,1′-binaphthyl
(4)
With K3PO4 as base: In a flame-dried Schlenk flask, compound 2
(450 mg, 1.00 mmol), 2-(1H-inden-2-yl)-4,4,5,5-tetramethyl-1,3,2-
dioxaborolane (240 mg, 1.00 mmol), Pd(PPh3)4 (40 mg, 0.03 mmol,
3 mol%), and K3PO4 (640 mg, 3.00 mmol) were dissolved in DMF
(10 mL) and heated to 100 °C for 2 h. The reaction was stopped by
adding toluene at r.t. and washing of the organic phase with H2O (2 ×).
The organic phase was then dried (Na2SO4) and purified by flash
chromatography with n-hexane/EtOAc (20:1, v/v) to give the product
4 (0.49 g, 99%) as a yellow solid.
With Na2CO3 as base: In a flame-dried Schlenk flask, compound 2
(0.83 g, 1.80 mmol), 2-(1H-inden-2-yl)-4,4,5,5-tetramethyl-1,3,2-di-
oxaborolane (0.43 g, 1.80 mmol), and Pd(PPh3)4 (40 mg, 0.04 mmol,
2 mol%) were dissolved in toluene (20 mL) and a solution of Na2CO3
(1.27 g, 12.0 mmol, 6.0 equiv) in degassed H2O (12 mL) was added.
The resulting mixture was heated to 110 °C for 22 h and quenched
with H2O. After extraction with EtOAc, the combined organic phases
were washed with H2O, and dried (Na2SO4). After flash chromatogra-
phy with n-hexane/EtOAc (10:1, v/v) product 4 (0.45 g, 52%) was iso-
23.7
lated as
a
light yellow solid; mp 112–113 °C; [α]D
–26.79
All experiments were carried out under an inert gas atmosphere (ar-
gon) in flame-dried Schlenk glassware. The anhydrous solvents (THF,
toluene, CH2Cl2, Et2O, and n-hexane) were dried in a solvent purifica-
tion system MD-5 from Inert (former Innovative Technology). All
NMR spectra were recorded on either a Bruker AV 300, AV 400 or Fou-
rier 300 NMR spectrometer. HPLC analysis was performed on a Hewl-
(c = 1.0078 g/100 mL, CHCl3); Rf = 0.17 (4.7% EtOAc/cyclohexane).
1H NMR (CDCl3, 300 MHz): δ = 8.13 (s, 1 H, ArH), 7.99 (d, J = 8.78 Hz,
1 H, ArH), 7.91 (t, J = 8.18 Hz, 2 H, ArH), 7.63 (d, J = 9.09 Hz, 2 H, ArH),
7.54 (d, J = 7.27 Hz, 1 H, ArH), 7.45 (d, J = 7.42 Hz, 1 H, ArH), 7.43–7.37
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2016, 48, A–J

H
P. Jungk et al.
Feature
Synthesis
(m, 2 H, ArH), 7.33–7.29 (m, 3 H, ArH), 7.26–7.19 (m, 3 H, ArH), 5.13
(dd, J = 31.75, 7.01 Hz, 2 H, OCH2), 4.65 (s, 2 H, OCH2), 4.08 (d,
J = 11.5 Hz, 2 H, CH2), 3.26 (s, 3 H, OCH3), 2.53 (s, 3 H, OCH3).
ArH), 7.25–7.12 (m, 7 H, ArH), 7.03 (tt, J = 7.1, 1.7 Hz, 1 H, ArH), 6.64
(br s, 1 H, CH), 5.03 (dd, J = 1.8, 0.8 Hz, 2 H, OCH2), 3.21 (dd, J = 35.6,
22.5 Hz, 2 H, CH2), 3.05 (s, 3 H, OCH3).
13C NMR (CDCl3, 75 MHz): δ = 152.8, 150.9, 145.6, 143.4, 143.0, 134.0,
133.1, 131.4 (CarylH), 130.9, 130.1, 129.6 (CarylH), 129.5, 128.5 (CarylH),
127.9 (CarylH), 127.7 (CarylH), 126.5 (CarylH), 126.4 (CarylH), 126.2
(CarylH), 125.7 (CarylH), 125.6 (CarylH), 125.2 (CarylH), 124.8 (CarylH),
124.0 (CarylH), 123.4 (CarylH), 121.2 (CarylH), 120.9, 116.5 (CH), 98.4
(OCH2), 94.9 (OCH2), 56.3 (OCH3), 55.9 (OCH3), 40.9 (CH2).
13C NMR (CDCl3, 75 MHz): δ = 152.5, 147.2, 144.9, 143.7, 134.4, 134.3,
133.6, 132.9, 131.4, 130.4 (CarylH), 129.8 (CarylH), 129.7, 128.0 (2 ×
C
arylH), 127.8 (CarylH), 127.0 (CarylH), 126.8 (CarylH), 126.7 (CarylH),
126.3 (CarylH), 126.2 (CarylH), 125.8 (CarylH), 125.4 (CarylH), 124.4
(CarylH), 124.2 (CarylH), 123.9, 123.2 (CarylH), 120.9 (CarylH), 116.5 (CH),
94.6 (OCH2), 55.8 (OCH3), 40.9 (CH2).
HRMS (ESI-TOF): m/z [M + H]+ calcd for C33H28O4: 489.2060; found:
HRMS (ESI-TOF): m/z [M + H]+ calcd for C31H24O2: 429.1849; found:
489.2045.
429.1828.
Anal. Calcd for C33H28O4: C, 81.12; H, 5.78. Found: C, 81.38; H, 5.98.
Anal. Calcd for C31H24O2: C, 86.89; H, 5.65. Found: C, 85.78; H, 5.56.
Diethyl (S)-{2′-(methoxymethoxy)-[1,1′-binaphthalen]-2-yl}phos-
Syntheses of Complexes
phonate (7)
(S)-2′-(Methoxymethoxy)-[1,1′-binaphthalen]-2-ol (6; 1.93 g, 5.83
mmol), prepared according to the reported procedure,28 was dis-
solved in a mixture of THF (18 mL) and DMF (5 mL) under argon at-
mosphere at r.t. NaH (0.25 g, 6.12 mmol, 60% in oil) was added to the
solution and stirred for 1 h. Diethyl chlorophosphate (0.89 mL, 6.12
mmol) was added and the reaction mixture was stirred for 1 h. The
reaction was quenched with aq NH4Cl, and the aqueous layer was ex-
tracted with EtOAc. The combined organic layers were washed with
brine, dried (Na2SO4) and evaporated under reduced pressure. The
crude product was purified by column chromatography on silica gel
(n-hexane/EtOAc = 2:1, v/v) to give 7 as a colorless solid (2.65 g, 98%).
The spectral data were in accordance with the literature values of the
R-enantiomer;28c [α]D22 –33.61 (c = 1.1010 g/100 mL, CHCl3); Rf = 0.07
(20% EtOAc/cyclohexane).
(–)-(pS)-(η4-Cycloocta-1,5-diene)(η5-1-neomenthylindenyl)cobalt
([Ind*Co(cod)]) (1)
[CAS Reg. No. 673452-64-7]
To a solution of (+)-3-neomenthylindene (17; 1.27 g, 5.00 mmol) in
THF (15 mL) was added n-BuLi (3.75 mL, 6.0 mmol, 1.6 M solution in
hexane) in one portion via syringe at 25 °C and the mixture was
stirred for 2 h. To the resulting solution was added CoCl(PPh3)3 (5.29
g, 6.00 mmol) as a solid via a glass Schlenk tube and the resulting
mixture was stirred for additional 18 h. To the dark red mixture
COD (0.86 mL, 7.00 mmol) was added and the stirring continued for
2 h. The resulting solution was filtered through a thin pad of degassed
silica gel and eluted with THF. The solvent was removed in vacuo and
the oily residue dissolved in pentane. This solution was filtered again
through a thin pad of degassed silica gel and eluted with pentane. Af-
ter removal of the solvent in vacuo, the residue was distilled via bulb-
to-bulb distillation to remove unreacted ligand. Finally, the resulting
brown residue was purified by column chromatography on degassed
silica gel. Elution with pentane allowed the isolation of the product as
the first red fraction and removal of the eluate caused a dark red pre-
cipitation of 1 (0.61 g, 29%). The bis(η5-1-neomenthylindenyl)cobalt
was obtained as dark red crystals from the slower moving fraction.
The analytical data were in accordance to the data reported in the lit-
erature.15
(S)-2-Bromo-2′-(methoxymethoxy)-1,1′-binaphthyl (8)
In a flame-dried Schlenk flask, Li (8 mg, 1.01 mmol) and naphthalene
(140 mg, 1.01 mmol) were reacted in THF (3 mL) by stirring at r.t. for
3 h. The resulting green solution was cooled to –78 °C and a solution
of 7 (230 mg, 0.50 mmol) in THF (3 mL) was added and stirred for 30
min at this temperature. Then, 1,2-dibromo-1,1,2,2-tetrafluo-
roethane (0.22 mL, 1.81 mmol) was added, the cooling bath removed,
and the solution stirred for 2 h at r.t. The reaction was quenched with
sat. aq NH4Cl and extracted with EtOAc (3 ×). The combined organic
phases were washed with brine and dried (Na2SO4). After flash chro-
matography with n-hexane/EtOAc (20:1, v/v), a mixture of 8 and 9
(0.126 g, 67%) was isolated with a molar ratio of 82:18 as determined
by NMR spectroscopy. The spectral data were in accordance with the
literature values of the R-enantiomer.28c
Complex 11
To a suspension of KH (18 mg, 0.50 mmol) in THF (5 mL) was added
compound 4 (220 mg, 0.50 mmol) and stirred at r.t. for 30 min.
Through a glass Schlenk tube solid CoCl(PPh3)3 (420 mg, 0.50 mmol)
was added and the mixture was stirred for an additional 2 h. The sol-
vent was removed under reduced pressure, the residue was dissolved
in toluene (7 mL) and COD (0.08 mL, 0.70 mmol) was added. The mix-
ture was heated to 110 °C for 2 h and cooled down to r.t. The resulting
mixture was filtered through a short pad of degassed silica gel and di-
luted with THF. The solvent was removed under reduced pressure and
the crude product was purified by column chromatography on de-
(S)-2-(1H-Inden-2-yl)-2′-(methoxymethoxy)-1,1′-binaphthyl (10)
In a flame-dried Schlenk flask, a mixture of 8/9 (108 mg, 0.27 mmol),
2-(1H-inden-2-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (72 mg,
0.30 mmol), K3PO4 (185 mg, 0.87 mmol), and Pd(PPh3)4 (17 mg,
0.015 mmol, 5 mol%) were dissolved in DMF (5 mL) and heated to
110 °C for 2.5 h. The reaction mixture was diluted with toluene,
washed with distilled H2O, and dried (Na2SO4). After flash chromatog-
raphy with n-hexane/EtOAc (20:1, v/v), the unfunctionalized product
9 (20 mg, 12% over 2 steps) and the product 10 (116 mg, 99% yield
gassed silica gel with toluene/Et2O (5:1, v/v). Complex 11 was isolated
26.8
as a red solid (100 g, 33%); mp 127.5–128.5 °C; [α]D
(c = 0.0104 g/100 mL, toluene).
–288.46
1H NMR (C6D6, 300 MHz): δ = 8.84 (s, 1 H, ArH), 8.03–7.97 (m, 1 H,
ArH), 7.79–7.74 (m, 1 H, ArH), 7.73–7.62 (m, 3 H, ArH), 7.47 (dd,
J = 8.5, 1.0 Hz, 1 H, ArH), 7.33 (ddd, J = 8.2, 6.8, 1.2 Hz, 1 H, ArH), 7.25–
7.17 (m, 6 H, ArH), 7.08 (ddd, J = 8.2, 6.8, 1.2 Hz, 1 H, ArH), 4.89 (d,
J = 1.7 Hz, 1 H, CH), 4.88–4.80 (m, 2 H, OCH2), 4.77 (d, J = 1.8 Hz, 1 H,
with reference to bromide 8, light yellow solid) were isolated; mp
31.8
102–103 °C; [α]D
–58.88 (c = 1.0020 g/100 mL, CHCl3); Rf = 0.27
(4.7% EtOAc/cyclohexane).
1H NMR (CDCl3, 300 MHz): δ = 8.00 (dd, J = 8.8, 4.2 Hz, 2 H, ArH), 7.91
(dd, J = 8.1, 6.9 Hz, 3 H, ArH), 7.56 (dd, J = 9.1, 1.4 Hz, 1 H, ArH), 7.45
(ddd, J = 8.1, 6.1, 2.0 Hz, 1 H, ArH), 7.35 (ddd, J = 8.1, 6.6, 1.3 Hz 1 H,
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2016, 48, A–J

I
P. Jungk et al.
Feature
Synthesis
CH), 4.72–4.62 (m, 2 H, OCH2), 3.76–3.63 (m, 4 H, COD), 3.02 (s, 3 H,
OCH3), 2.46 (s, 3 H, OCH3), 2.08 (ddd, J = 13.1, 8.5, 5.5 Hz, 4 H, COD),
1.49–1.34 (m, 4 H, COD).
13C NMR (C6D6, 75 MHz): δ = 153.4, 151.7, 134.5, 133.2, 131.6, 131.2,
129.8, 129.6 (CarylH), 128.2 (CarylH), 128.0 (CarylH), 127.8 (CarylH), 127.5,
126.8, 126.6 (CarylH), 126.2 (CarylH), 126.0 (CarylH), 125.8 (CarylH), 125.3
(CarylH), 124.3 (CarylH), 124.2 (CarylH), 124.0 (CarylH), 122.7 (CarylH),
122.6 (CarylH), 121.1, 116.3 (CarylH), 105.9, 105.7, 98.6 (OCH2), 94.5
(OCH2), 75.9 (CH), 75.6 (CH), 69.5 (CH), 69.3 (CH), 55.9 (OCH3), 55.3
(OCH3), 31.0 (CH2), 30.8 (CH2).
Cyclization of Diynes and Nitriles with Chiral Complex 18; General
Procedure
The respective diyne (0.25 mmol) and nitrile (5 equiv with regard to
the diyne) were dissolved in THF (3 mL). A solution of 18 (5 mol%
with regard to the diyne) in THF was added to the reaction mixture
and the solution was stirred at the respective temperature. At the end
of the reaction, the solvent was removed under reduced pressure and
the residue purified by column chromatography with cyclohex-
ane/EtOAc (4:1, v/v) to yield the biaryls. The ee values were deter-
mined by chiral HPLC analysis (Tables 2 and 3).
MS (EI): m/z = 654 (85%, [M]+).
1-(2-Methoxynaphthalen-1-yl)-3-methyl-5,6,7,8-tetrahydroiso-
quinoline (20)
HRMS (EI): m/z [M] calcd for C41H39CoO4: 654.2175; found: 654.2177.
Compound 12 (65.6 mg, 0.25 mmol) and acetonitrile (19; 51.3 mg,
1.25 mmol) were placed in a Schlenk tube and dissolved in THF
(3 mL). Afterwards, a solution of 18 (0.025 M in THF, 1 mL) was added
to the reaction mixture and the solution was stirred at 50 °C for 120
h. At the end of the reaction, the solvent was removed under reduced
pressure and the residue purified by column chromatography with
cyclohexane/EtOAc (4:1, v/v) to yield the product. Compound 20 was
isolated as a white solid (17 mg, 22%, 80% ee). The starting material 12
could be reisolated in pure form (50 mg, 76%). The ee value was deter-
mined by chiral HPLC analysis (Reprosil, heptane/EtOH, 99:1, v/v; 2.0
mL/min). The product was identified by its known analytical data.16
(–)-(pS)-Bis(triethylphosphite)(η5-1-neomenthylindenyl)cobalt
([Ind*Co{P(OEt)3}2]) (18)
A solution of [Ind*Co(cod)] (1; 226 mg, 0.50 mmol) and P(OEt)3 (172
μL, 1.00 mmol) in THF (6 mL) was irradiated for 24 h. The solution was
filtered over a small amount of neutral Al2O3 (Brockman Type I),
which was washed with THF (10 mL). After removal of the solvent un-
der reduced pressure the resulting deep red oil was dried under vacu-
um; yield: 258–341 mg (74–96%).
1H NMR (C6D6, 300 MHz): δ = 7.45 (dd, J = 14.5, 14.1 Hz, 2 H), 7.00 (t,
J = 7.2 Hz, 1 H), 6.85 (t, J = 7.3 Hz, 1 H), 5.61 (s, 1 H), 4.47 (s, 1 H), 3.79
(m, 12 H), 3.57 (s, 2 H), 2.71 (d, J = 12.4 Hz, 1 H), 2.16 (m, 1 H), 1.97 (d,
J = 11.3 Hz, 1 H), 1.64 (d, J = 7.2 Hz, 4 H), 1.15 (t, J = 7.2 Hz, 18 H), 1.09
(d, J = 5.4 Hz, 3 H), 0.95 (d, J = 5.4 Hz, 3 H), 0.46 (d, J = 5.4 Hz, 3 H).
1-(2-Methoxynaphthalen-1-yl)-3-(3,4,5-trimethoxyphenyl)-
5,6,7,8-tetrahydroisoquinoline (24)
13C NMR (C6D6, 75 MHz): δ = 122.0, 121.9, 119.1, 103.5, 103.2, 96.3,
88.4, 66.4, 59.3, 48.2, 44.3, 35.9, 34.0, 29.7, 29.2, 28.4, 24.1, 23.5, 23.3,
19.4, 16.6 (d, J = 6.6 Hz).
31P NMR (C6D6, 121 MHz): δ = 173.9 (br s), 169.4 (br s).
MS (CI): m/z = 644 [M]+.
Compound 12 (65.6 mg, 0.25 mmol) and nitrile 23 (241.5 mg,
1.25 mmol) were placed in a Schlenk flask and dissolved in THF
(3 mL). The catalyst 18 was added as a solution in THF (1 mL, 0.025 M
in THF) via syringe to the reaction mixture and the solution was
stirred at 50 °C for 120 h. The reaction was stopped and the solvent
was removed under reduced pressure and the residue purified by col-
umn chromatography with cyclohexane/EtOAc (4:1, v/v) to yield the
product 24 (20 mg, 20%, 70% ee). Furthermore, unreacted starting ma-
terial 12 was reisolated (47 mg, 72%). The ee value was determined by
chiral HPLC analysis (Cellulose 4, heptane/EtOH, 90:10, v/v; 0.5
mL/min). The product was identified by its known analytical data.16b
Catalytic Screening
Cyclization of Diyne 12 and Benzonitrile (13) with the Chiral Com-
plex 11; General Procedure
The chiral complex 11 (17 mg, 5 mol%) and compound 12 (130 mg,
0.50 mmol) were placed in a glass photo-reactor under Schlenk con-
ditions and tempered for 1 h. Afterwards, THF (8 mL) was added fol-
lowed by the addition of benzonitrile (13; 1.00 mmol) and the result-
ing solution was irradiated at the stated temperature (Table 1) for 24
h. The reaction was stopped by turning off the lamps and exposing
the reaction vessel to air. After purifying by column chromatography,
product 14 was isolated. The ee values were determined by chiral
HPLC analysis (Reprosil 100, heptane/EtOH, 99:1, v/v; 0.5 mL/min).
The analytical data were in accordance with the literature data.16
Cyclization of Triyne 15 with Chiral Complex 18
Compound 15 (0.20 mL, 0.25 mmol, 1.25 M in THF) was dissolved in
THF (3 mL) and a solution of complex 18 (1 mL, 0.025 M in THF) was
added. The solution was stirred at 65 °C for 3 days and stopped by re-
moving the solvent under reduced pressure. The residue was purified
by column chromatography with cyclohexane/EtOAc (4:1, v/v) to
yield compound 16 (80 mg, 77%, 7% ee). The ee value was determined
by chiral HPLC analysis (Cellulose 2, heptane/i-PrOH, 95:5, v/v; 0.5
mL/min). The analytical data were in accordance with the reported
literature data.33
Cyclization of Triyne 15 with Chiral Complex 11
In a secured glass photo-reactor containing a solution of chiral com-
plex 11 (8 mg, 5 mol%) in THF (4 mL) was added compound 1533
(0.20 mL, 0.25 mmol, 1.25 M in THF) and the mixture irradiated for 24
h at 25 °C. The reaction was stopped by turning off the lamps and ex-
posing the reaction vessel to air. Purification by column chromatogra-
phy with cyclohexane/EtOAc (4:1, v/v) afforded product 16 (19 mg,
18%). The ee value was determined by chiral HPLC analysis (Cellulose
2, heptane/i-PrOH, 95:5, v/v; 0.5 mL/min) and the analytical data
were in accordance to the literature data.33
Acknowledgment
We thank Prof. Dr. Uwe Rosenthal for helpful discussions and com-
ments and would also like to thank the DFG (HA3511/3-1) and the
Leibniz-Gemeinschaft for financial support. I.T. is grateful for a Kekulé
scholarship provided by the Fonds der Chemischen Industrie. T.T.
gratefully acknowledges financial support by the Science Campus
Phosphorus Research Rostock. The authors are indebted to the excel-
lent support provided by our analytical department. We thank Dr.
Anke Spannenberg for the X-ray crystal structure analysis of complex
© Georg Thieme Verlag Stuttgart · New York — Synthesis 2016, 48, A–J

J
P. Jungk et al.
Feature
Synthesis
11. Fabian Fischer is acknowledged for excellent technical support.
The authors thank one of the reviewers for helpful comments and
suggestions.
(16) (a) Gutnov, A.; Heller, B.; Fischer, C.; Drexler, H.-J.;
Spannenberg, A.; Sundermann, B.; Sundermann, C. Angew.
Chem. Int. Ed. 2004, 43, 3795; Angew. Chem. 2004, 116, 3883.
(b) Hapke, M.; Kral, K.; Fischer, C.; Spannenberg, A.; Gutnov, A.;
Redkin, D.; Heller, B. J. Org. Chem. 2010, 75, 3993.
Supporting Information
(17) Weding, N.; Hapke, M. Chem Soc. Rev. 2011, 40, 4525.
(18) (a) Shibata, T.; Fujimoto, T.; Yokota, K.; Takagi, K. J. Am. Chem.
Soc. 2004, 126, 8382. (b) Tanaka, K.; Nishida, G.; Wada, A.;
Noguchi, K. Angew. Chem. Int. Ed. 2004, 43, 6510; Angew. Chem.
2004, 116, 6672.
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0035-1560433.
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
ortioInfgrmoaitn
(19) (a) Tanaka, K. In Transition-Metal-Mediated Aromatic Ring Con-
struction; Tanaka, K., Ed.; Wiley: Hoboken, 2013, 127.
(b) Takeuchi, R. In Transition-Metal-Mediated Aromatic Ring
Construction; Tanaka, K., Ed.; Wiley: Hoboken, 2013, 161.
(20) Heller, B.; Sundermann, B.; Fischer, C.; You, J.; Chen, W.;
Drexler, H.-J.; Knochel, P.; Bonrath, W.; Gutnov, A. J. Org. Chem.
2003, 68, 9221.
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(2) Additional address since November 1, 2015: Johannes Kepler
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