A Unified Approach to Customized Chiral Carbon-Bridged ansa-Metallocenes

Article abstract of DOI:10.1002/chem.201804497

Chiral ansa-metallocenes are privileged catalysts for a range of stereoselective transformations. Their synthesis, however, has remained a tremendous challenge, which has prevented a broad and systematic exploration for applications in synthesis and catalysis. A modular approach to such ansa-metallocenes that enables a facile modification of the ring substitution and the ligand bridge, as well as the introduction of various core metals, is described. The complexes were formed with good rac-selectivity and could be isolated with high purity. The strength of the approach was demonstrated by the synthesis of several new and previously known complexes, including a unique helical chiral ansa-metallocene. Using a chiral ligand, a moderate central-to-planar chirality transfer was observed.

Full text of DOI:10.1002/chem.201804497

A Journal of
Accepted Article
Title: A Unified Approach to Customized Chiral Carbon-Bridged ansa-
Metallocenes
Authors: Stefan Wiesler, Michael Andreas Bau, Sara Laila Younas,
Hieu-Trinh Luu, Daniel Kratzert, and Jan Streuff
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To be cited as: Chem. Eur. J. 10.1002/chem.201804497
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Supported by

10.1002/chem.201804497
Chemistry - A European Journal
COMMUNICATION
A Unified Approach to Customized Chiral Carbon-Bridged ansa-
Metallocenes
Stefan Wiesler,[a] Michael A. Bau,[a] Sara L. Younas,[a] Hieu-Trinh Luu,[a] Daniel Kratzert,[b] and Jan
Streuff*[a]
a)
Abstract: Chiral ansa-metallocenes are privileged catalysts for a
range of stereoselective transformations. Their synthesis, however,
has remained a tremendous challenge, which prevented a broad and
systematic exploration for applications in synthesis and catalysis.
We describe a modular approach to such ansa-metallocenes that
enables a facile modification of the ring substitution and the ligand
bridge, as well as the introduction of various core metals. The
complexes are formed with good rac-selectivity and can be isolated
in high purity. The strength of the approach is demonstrated by the
synthesis of a number of new and previously known complexes,
including a unique helical chiral ansa-metallocene. Using a chiral
ligand, a moderate central-to-planar chirality transfer is observed.
1: [M] = TiCl2
2: [M] = ZrCl2
[M]
[M]
–
–
chiral
meso
free
ebthi-ligand
(achiral)
ebthi-metallocene ebthi-metallocene
(desired)
(undesired)
b)
R
customization of
the bridge
O O
HO2C
CO2H
R
X
X
n
R'
diacid
R'
+
n
R'
Br
R'
[M]
[M]
n
2
n
n
R
R
R
n
vinyl bromide
R
modified
ligand precursors
customization
of the chiral
pocket
Planar chiral ansa-metallocenes belong to the class of so-called
privileged catalysts.[1] In particular, planar chiral ansa-
titanium(IV) and -zirconium(IV) complexes based on the ebthi-
ligand (1, 2), as introduced by Brintzinger in 1982, have
triumphed in a number of different transformations by showing
unprecedented stereoselectivity and efficiency (Scheme 1a,
modular
disconnection
readily-available
materials
Scheme 1. a) The diastereomers of ebthi-metallocenes and the free
deprotonated ebthi-ligand b) Retrosynthetic analysis for the synthesis of chiral
Cp-annulated ansa-metallocenes.
ebthi
=
ethylenebis(4,5,6,7-tetrahydroinden-1-yl)).[2] These
include stereoregular Ziegler-Natta polymerizations, asymmetric
alkene carbometallations, carbonyl and imine reductions, and
reductive umpolung reactions such as the enantioselective
ketonitrile radical cyclization recently developed by us.[3,4]
The stereoinduction in catalyses with ebthi-metallocenes
such as 1 or 2 originates from the steric shielding of two
quadrants by the tetrahydroindenyl motifs of the ligand
(Scheme 1b). Therefore, the synthesis had to enable
a
Despite the wide application of these ansa-metallocene catalysts,
the synthesis of structurally modified derivatives still presents an
extraordinary synthetic challenge.[5] Frequently encountered
issues with ebthi-type catalysts are: 1) only upon coordination of
the achiral ligand to the core metal, a chiral complex is received.
2) The formation of the undesired achiral meso-metallocene
diastereomer is often preferred. 3) The difficult purification and
final resolution to give the desired enantiopure ansa-metallocene
result in a high operational complexity and reduced yields. While
some of these issues could be addressed for the synthesis of
single example complexes,[5,6] a systematic exploration of this
unique catalyst class has remained unachievable. Although
derivatives containing modified tetrahydroindenyl motifs have
been prepared,[5c,6h] no general method for the straightforward
introduction of such modifications has been described. To this
end, we herein report a broadly applicable modular synthesis of
structurally modified chiral ansa-metallocenes.
straightforward customization of these ligand parts. Moreover, it
was our goal to achieve a full customization of the complex,
including changes in the bridge length and structure, as well as
installing different core metals. To avoid the manipulation of
metallocene intermediates, the complex formation had to be
carried out as the final step. We further reasoned that using the
free bis-tetrahydroindenyl ligand for the complex formation
would avoid the formation of the undesired meso diastereomer
due to the lack of π-stacking interactions. The free ligand was
then further disconnected to a central diacid building block and
two equivalents of a functionalized alkenylmetal reagent that
was readily accessible from
precursor.
a corresponding halogenated
The synthesis was developed on the basis of complex 1,
which involved a careful optimization of each step (Scheme 2).
The key elements of the synthesis of the free ligand were the
olefination of
a
bis-enone, followed by
a
double
cyclopropanation and
a
double carbene rearrangement
(Skattebøl rearrangement).[7] Such a sequence had previously
been applied in the synthesis of a myrtanyl-derived ansa-
titanocene and the carbene-rearrangement itself had been
introduced for the preparation of certain Cp-based ansa-
complexes.[6d,h,7] However, this sequence had not been
advanced to a general protocol for the preparation of ansa-
metallocenes. Furthermore, we sought to avoid the use of toxic
reagents such as HMPA or organomercury compounds and
aimed for a practical and reliable protocol for the purification and
isolation of the racemic complexes.
[a]
[b]
S. Wiesler, M. A. Bau, S. L. Younas, Dr. H.-T. Luu, Dr. J. Streuff
Institut für Organische Chemie
Albert-Ludwigs-Universität Freiburg
Albertstr. 21, 79104 Freiburg, Germany
E-mail: jan.streuff@ocbc.uni-freiburg.de
Dr. Daniel Kratzert
Institut für Anorganische und Analytische Chemie
Albert-Ludwigs-Universität Freiburg
Albertstr. 21, 79104 Freiburg, Germany
Supporting information for this article is given via a link at the end of
the document.
1
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10.1002/chem.201804497
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COMMUNICATION
MgBr•LiCl
b) [Ph3PCH3]+ Br–
c)
CHBr3, KOH
BnMe3NCl (10 mol%)
a)
O O
Me
O O
Me
4
KOt-Bu
N
N
OMe
70–96%
79–88%
96%
MeO
3
5
6
(7a:7b:7c = 3.8:5.4:1)
Br
Br
Br
Br
Br
Br
Br
Br
Br
e)
n-BuLi
TiCl3•3THF
d) MeLi
Cl
Ti
+
7a
7b
Br
Cl
carbene
then 6N HCl, air
Br
Br
rearrangement
8
60% from 7a–c
+ double bond isomers
rac-1
7c
32–49% overall
Scheme 2. Synthesis of rac-(ebthi)titanium(IV) dichloride (rac-1).
Starting from the bis-N,O-dimethylhydroxylamide (bis-
Weinreb amide) of succinic acid (3), a double Grignard addition
of cyclohexenylmagnesium bromide lithium chloride adduct (4)
was carried out. A combination of magnesium powder and
lithium chloride together with dibromoethane and TMSCl as
initiating additives was found to give reproducible concentrations
of the organometallic reagent and high yields for the double
addition reaction (70–96%). Among several olefination methods,
a double Wittig methylenation was identified as the most suitable
procedure for the preparation of the tetraene 6. The reaction
gave typically high yields (79–88%), and 6 could be easily
purified by flash chromatography despite its unpolar nature.
Tetraene 6 was then bis-dibromocyclopropanated in almost
quantitative yield by portion-wise addition of bromoform and
potassium hydroxide in the presence of a phase transfer catalyst.
A selective double cyclopropanation was achieved (once on
each half of the molecule) without formation of the tri- or
tetracyclopropanated products, resulting in an inseparable
diastereomeric mixture of the double-addition regioisomers 7a–c
(the regioisomers could be assigned by NMR). Importantly, all
isomers underwent the rearrangement to the free bis-
cylopentadiene ligand 8 upon treatment with methyllithium. The
formation of 8 together with several double bond isomers could
To demonstrate the modularity of this route, we applied it to
the synthesis of modified ansa-titanocenes such as complexes
with annulated seven- and eight-membered rings that could not
be accessed previously, as well as
a
C3-bridged ansa-
and
metallocene (Scheme 3).[9] Starting from either
3
a
cycloheptenyl or cyclooctenyl Grignard reagent (9, 10), or 4 in
combination with the bis-Weinreb amide of glutaric acid (11),
good to excellent yields were observed for the steps leading to
the bis-cyclopropanated ligand precursors [steps a)–c)]. The
MgBr•LiCl
MgBr•LiCl
O
O
O
O
+
3
+
3
+ 4
N
N
3
11
9
10
a) 81%
b) 69%
c) 81%
a) 61%
b) 70%
c) >99%
a) >99%
b) 61%
c) 92%
d),e) (>15:1 d.r.)
d)*,e) (5.1:1 d.r.)
d),e) (1.4:1 d.r.)
Cl
Ti
Cl
Ti
Cl
Ti
Cl
Cl
Cl
rac-12, 58%
26% overall
rac-13, 57%
24% overall
rac-14, 27%
15% overall
1
be confirmed by H NMR analysis of the crude mixture, but due
to the unstable nature of 8, it was found to be the most
convenient to directly submit the crude material to the
deprotonation
with
n-butyllithium
and
reaction
with
trichlorotitanium(III) tris-tetrahydrofuran complex (TiCl3•3THF) in
a single operational sequence. Treatment with aqueous HCl and
exposure to air then gave the desired ansa-metallocene in a
high rac:meso ratio of 5.0–6.7:1 (crude 1H NMR) and column
chromatography provided pure rac-1 in an excellent 60% yield.
Alternative protocols for the complex formation involving the
isolation of the double-deprotonated ligand or using
tetrakis(dimethylamine)metal precursors were inferior.[6h,8,9]
Overall, the five-step (four operational sequences) procedure
resulted in a very good 32–49% overall yield of (rac)-1. At this
point, it should be noted that a direct comparison of our results
for the synthesis of known metallocenes with previous reports
was not possible in most cases. Furthermore, it was our goal to
highlight the generality of our method rather than discussing
potential improvements.[10]
X-ray of rac-12
X-ray of rac-13
Scheme 3. Synthesis of ansa-titanocenes 12–14. Conditions for steps a)–e)
are identical to Scheme 2. The yield of steps d)+e) is reported for the purified
rac-isomer. X-ray structures of complexes rac-12 and rac-13. Only one
enantiomer is shown. Solvent molecules have been omitted for clarity, thermal
ellipsoids are shown at the 50% probability level. * Step d) was carried out with
i-PrMgCl•LiCl instead of MeLi.
2
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COMMUNICATION
MgBr
rearrangement-complexation
sequence
then
gave
the
1. [Ph3PCH3]+ Br–
KOt-Bu
complexes 12 and 13 in very good ratios favoring the rac
complex (>15:1 in the case of 12). For the formation of 13, the
carbene rearrangement was initiated with i-PrMgCl•LiCl instead
of MeLi, which prevented an undesired overmethylation of the
ligand.[11] The pure racemic complexes 12 and 13 were isolated
either by crystallization from the crude mixture or by column
chromatography using silanized silica gel in 58% and 68% yield,
respectively.[12] The preparation of the literature-known C3-
bridged complex 14 resulted in a reduced rac:meso ratio of
1.4:1,[9] which was most likely caused by the non-symmetric
position of the (CH2)3-bridge that led to steric interactions with
the tetrahydroindenyl groups. Nevertheless, both diastereomers
could be conveniently separated via column chromatography,
affording 27% of rac-14 and 12% of the meso-diastereomer. The
total yield of 15% for the whole sequence was significantly
improved over the original preparation of rac-14 (5%). X-ray
analyses of rac-12 and rac-13 unambiguously confirmed the
structures and revealed that the seven- and eight-membered
rings were twisted away from the titanium center, which likely
also occurred in solution (Scheme 3).[13] Hence, steric shielding
of the titanium center was only weakly enhanced in comparison
to 1.
>99%
O O
15
3
84%
2.
CHBr3, KOH
BnMe3NCl (10 mol%)
86%
16
1. i-PrMgCl•LiCl
2.
n-BuLi
TiCl3•3 THF
Br
Br
Br
Br
Cl
Ti
Cl
then: 6N HCl, air
(8.3:1 d.r.)
17a
(+ regioisomers 17b,c)
rac-18, 43%
31% overall
X-ray of rac-18
α
α = –31.6º
We, therefore, designed a complex that would contain a less
flexible ring structure and an increased occupation of two
opposing quadrants (Scheme 4). Starting from 3 and the alkenyl
Grignard reagent 15, diketone 16 was prepared. This diketone
was then advanced to the bis-cyclopropanated precursors
17a–c, which smoothly underwent the rearrangement and
complex formation with TiCl3•3THF to give titanocene complex
rac-18 and its meso diastereomer in a very good ratio of 8.3:1.
Again, i-PrMgCl•LiCl was used for the rearrangement and
purification by chromatography afforded rac-18 in 43% yield
(31% overall yield) together with traces of the meso-
diastereomer (ratio ca. 21:1). An X-ray analysis then confirmed
that the shielding of the metal center of this complex was
significantly enhanced in comparison to previous complexes.[13]
Furthermore, titanocene 18 represented a new class of a
helicene-like structure, with an ansa-metallocene embedded into
its framework that induced the chirality of the helix
(Scheme 4).[14] The terminal arene units of the ligand were bent
towards the center, resulting in a warped structure with a
negative twist angle between the phenyl ring plane and the Cl-
Ti-Cl plane of α = –31.6º to –48.2º. This was in sharp contrast to
the structures of previously prepared metallocene helicenes and
appeared to be a result of favorable H-Cl hydrogen bonds
(d = 3.0–3.2 Å) and the conformational preference of the seven-
membered rings.[15]
To show the feasibility of the new route for the introduction of
different core metals, the known racemic zirconocene 2, and
hafnocene 19 were prepared from the isomeric mixture of the
common cyclopropanated intermediate (7a–c, Scheme 5).[2b,16]
The sequence of carbene rearrangement and complex formation
again proceeded with good rac:meso selectivity (2.5:1 and 3.9:1,
respectively) and the pure racemic complexes were obtained
after washing with pentane and crystallization from toluene in
yields of 51% (2) and 20% (19). Since no NMR data had been
reported for hafnocene 19, the structure was confirmed by X-ray
analysis and comparison with reported structural data.[16b]
Scheme 4. Synthesis of rac-18, a new type of metallocene helicene and X-ray
analysis of rac-18. A negative twist angle (α) of the phenyl rings is observed.
Only one enantiomer is shown. Solvent molecules have been omitted for
clarity and thermal ellipsoids are shown at the 50% probability level.
1.
MeLi
1.
MeLi
2. n-BuLi, ZrCl4
2. n-BuLi, HfCl4
Cl
Cl
Cl
Cl
Hf
Zr
7a–c
51%
20%
rac-19
rac-2
11–16% overall
27–41% overall
Scheme 5. Synthesis of ansa-zirconocene 2 and ansa-hafnocene 19.
Finally, we probed whether a chiral ligand would lead to a
central-to-planar chirality transfer during the complex formation.
Previous studies using certain conformationally fixed ligands had
been successful in this regard, but employing linear, chiral
bridges led to poor stereoselectivity and the undesired pseudo-
meso metallocene as major product.[5b,17] Therefore, the
synthesis of chiracene-based titanocene 20 was carried out
(Scheme 6).[6b] The enantiopure bis-Weinreb amide (R,R)-21
prepared from readily available (R,R)-2,3-dimethylsuccinic
acid[18] was converted into the corresponding bis-enone 22 by
the addition of cyclohexenylmagnesium bromide lithium chloride.
No erosion of the enantiopurity took place in this step
(>99.9% ee). The methylenation of 22 to 23 was then
successfully achieved by applying
a modified Lombardo
olefination procedure developed by Takai and Utimoto (48–52%
yield).[19] Other common methylenation protocols gave inferior
results. The bis-dibromocyclopropanation was then carried out
as before and the ligand precursor 24 was obtained in good
3
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10.1002/chem.201804497
Chemistry - A European Journal
COMMUNICATION
[1]
[2]
T. P. Yoon, E. N. Jacobsen, Science 2003, 299, 1691–1693.
yield as a mixture of regio- and diastereomers. After the
rearrangement to the free chiral C2-symmetric ligand, the
conversion into the desired complex was achieved by
deprotonation with NaHMDS and reaction with TiCl3•3THF. A
comparison with the literature NMR data revealed that the
desired stereoisomer (S,S,R,R)-20 was indeed formed as major
product together with the pseudo-enantiomeric complex
(R,R,R,R)-25 and the pseudo-meso complex (R,S,R,R)-26
(20:25:26 = 3.6:1:2.8). Chromatography using silanized silica gel
allowed the separation of 26 (5% yield) from 20 and 25, which
were isolated as a 3.7:1 mixture in 28% yield. A further
separation of 20 from 25 was not undertaken, since it had
previously been described.[6b] Although the formation of 25 was
not completely suppressed, this synthesis of 20 served as a
proof of principle for two reasons: 1) The desired stereoisomer
was formed as major product in the complex formation, and 2) a
moderate central-to-planar chirality transfer was observed.
a) F. R. W. P. Wild, L. Zsolnai, G. Huttner, H. H. Brintzinger, J.
Organomet. Chem. 1982, 232, 233–247; b) F. R. W. P. Wild, M.
Wasiucionek, G. Huttner, H. H. Brintzinger, J. Organomet. Chem. 1985,
288, 63–67; c) S. Collins, B. A. Kuntz, N. J. Taylor, D. G. Ward, J.
Organomet. Chem. 1988, 342, 21–29.
[3]
[4]
a) H. H. Brintzinger, D. Fischer, R. Mülhaupt, B. Rieger, R. M.
Waymouth, Angew. Chem. Int. Ed. Engl. 1995, 34, 1143–1170; Angew.
Chem. 1995, 107, 1255–1283; b) A. H. Hoveyda, J. P. Morken, Angew.
Chem. Int. Ed. 1996, 35, 1262–1284; Angew. Chem. 1996, 108, 1378–
1401.
a) J. Streuff, M. Feurer, P. Bichovski, G. Frey, U. Gellrich, Angew.
Chem. Int. Ed. 2012, 51, 8661–8664; Angew. Chem. 2012, 124, 8789–
8792. See also: b) J. Streuff, M. Feurer, G. Frey, A. Steffani, S.
Kacprzak, J. Weweler, L. H. Leijendekker, D. Kratzert, D. A. Plattner, J.
Am. Chem. Soc. 2015, 137, 14396–14405; c) J. Streuff, Synthesis 2013,
45, 281–307.
[5]
[6]
a) R. L. Halterman, Chem. Rev. 1992, 92, 965–994; b) P. J. Shapiro,
Coord. Chem. Rev. 2002, 231, 67–81; c) Y. Qian, J. Huang, M. D. Bala,
B. Lian, H. Zhang, H. Zhang, Chem. Rev. 2003, 103, 2633–2690.
a) J. A. Bandy, M. L. H. Green, I. M. Gardiner, K. Prout, J. Chem. Soc.,
Dalton Trans. 1991, 2207–2216; b) A. L. Rheingold, N. P. Robinson, J.
Whelan, B. Bosnich, Organometallics 1992, 11, 1869–1876; c) M. E.
Huttenloch, J. Diebold, U. Rief, H. H. Brintzinger, A. M. Gilbert, T. J.
Katz, Organometallics 1992, 11, 3600–3607; d) S. C. Sutton, M. H.
Nantz, S. R. Parkin, Organometallics 1993, 12, 2248–2257; e) R. L.
Halterman, T. M. Ramsey, N. A. Pailes, M. A. Khan, J. Organomet.
Chem. 1995, 497, 43–53; f) B. Chin, S. L. Buchwald, J. Org. Chem.
1996, 61, 5650–5651; g) G. M. Diamond, R. F. Jordan, J. L. Petersen, J.
Am. Chem. Soc. 1996, 118, 8024–8033; h) D. F. Taber, B. Balijepalli,
K.-K. Liu, S. Kong, A. L. Rheingold, F. R. Askham, J. Org. Chem. 1999,
64, 4525–4527; i) M. Ringwald, R. Stürmer, H.-H. Brintzinger, J. Am.
Chem. Soc. 1999, 121, 1524–1527; j) H.-R. H. Damrau, E. Royo, S.
Obert, F. Schaper, A. Weeber, H. H. Brintzinger, Organometallics 2001,
20, 5258–5265; k) M. D. LoCoco, X. Zhang, R. F. Jordan, J. Am. Chem.
Soc. 2004, 126, 15231–15244; l) L. Xu, K. Mikami, Tetrahedron Lett.
2004, 45, 9215–9217.
MgBr•LiCl
O
Me
O
Me Me
N
4
MeO
N
OMe
Et2O, 23 ºC
60%
Me
O
Me Me
21
O
22, >99.9% ee
Br
Br
CHBr3, KOH
BnMe3NCl
(10 mol%)
CH2I2
Zn, TiCl4
cat. PbCl2
Br
Br
Me
48–52%
76%
Me
23
Me
Me
24 (+ isomers)
1.
2.
MeLi
NaHMDS
TiCl3•3THF
Me
Me
Me
Me
Me
Cl
Cl
Cl
Cl
Cl
Cl
Ti
+
+
Ti
Ti
–40 ºC to reflux
then 12N HCl, air
Me
crude:
3.6:1:2.8 d.r.
(S,S,R,R)-20
(R,R,R,R)-25
(R,S,R,R)-26
[7]
a) L. Skattebøl, Tetrahedron 1967, 23, 1107–1117. See also: b) B.
Dorer, M.-H. Prosenc, U. Rief, H. H. Brintzinger, Organometallics 1994,
13, 3868–3872; c) R. B. Grossman, J.-C. Tsai, W. M. Davis, A.
Gutiérrez, S. L. Buchwald, Organometallics 1994, 13, 3892–3896.
G. M. Diamond, S. Rodewald, R. F. Jordan, Organometallics 1995, 14,
5–7.
pseudo-enantiomers
28% (20+25)
pseudo-meso
5%
(3.7:1 d.r.)
[8]
[9]
Scheme 6. Synthesis of titanocene 20 from an enantiopure ansa-ligand.
W. Röll, L. Zsolnai, G. Huttner, H. H. Brintzinger, J. Organomet. Chem.
1987, 322, 65–70.
In summary,
a broadly applicable synthesis of chiral
[10] A compilation of the previous results for the synthesis of 1, 2, 14, 19,
and 20 can be found in the Supporting Information.
annulated ansa-metallocenes has been developed. It allows the
rapid assembly of customized complexes with good rac-
selectivity from alkenyl Grignard reagents and bis-Weinreb
amides. This has been demonstrated by installing structural
modifications at the ebthi-framework and the bridge, as well as
the introduction of Ti, Zr, and Hf as core metals. Furthermore, a
unique helical ansa-metallocene has been prepared that may
find application in future stereoselective catalyses.
[11] For precedence of an initiation by Br-Mg exchange, see: M. S. Baird, A.
V. Nizovtsev, I. G. Bolesov, Tetrahedron 2002, 58, 1581–1593.
[12] For the preparation of silanized silica gel, see the Supporting
Information.
[13] CCDC entries 1578500 (12), 1578518 (13) and 1848143 (18) contain
the supplementary crystallographic data for this paper. These data can
be obtained from The Cambridge Crystallographic Data Centre.
[14] N. Saleh, C. Shen, J. Crassous, Chem. Sci. 2014, 5, 3680–3694.
[15] The two enantiomers of the unit cell showed
a slightly different
conformation, probably due to crystal packing effects.
[16] a) J. A. Ewen, L. Haspeslagh, J. L. Atwood, H. Zhang, J. Am. Chem.
Soc. 1987, 109, 6544–6545; b) CCDC number 1160511.
[17] For an example featuring a bulky, conformationally fixed ligand, see: Z.
Chen, R. L. Halterman, J. Am. Chem. Soc. 1992, 114, 2276–2277.
[18] Both enantiomers are commercially available. We prepared (R,R)-21
following a literature procedure: C.-D. Lu, A. Zakarian, Org. Synth.
2008, 85, 158–171.
Acknowledgements
This work was funded by the Deutsche Forschungsgemeinschaft
(DFG) – projects 333382543 and 408295365.
Keywords: helical structures • ligand design • metallocenes •
[19] K. Takai, T. Kakiuchi, Y. Kataoka, K. Utimoto, J. Org. Chem. 1994, 59,
2668–2670.
rearrangement • titanium
4
This article is protected by copyright. All rights reserved.

10.1002/chem.201804497
Chemistry - A European Journal
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
A modular synthesis of annulated
chiral ansa-metallocenes is presented
that enables the facile construction of
customized complexes for asymmetric
catalysis. It allows the straightforward
modification of the chiral pocket and the
introduction of various bridge units. The
approach is demonstrated on eight
examples, covering Ti-, Zr-, and Hf-
based metallocenes, a chiral ligand,
and a novel metallocene helicene.
Stefan Wiesler, Michael A. Bau, Sara L.
Younas, Dr. Hieu-Trinh Luu, Dr. Daniel
Kratzert, Dr. Jan Streuff*
MeO
O
O
OMe
N
N
R
R'
n
R'
[M]
+
Page No. – Page No.
modular
synthesis
MgX
n
A Unified Approach to Customized
Chiral Carbon-Bridged ansa-
Metallocenes
R
n
R
5
This article is protected by copyright. All rights reserved.