A New Class of C2-Symmetric Chiral Cyclopentadienyl Ligand Derived from Ferrocene Scaffold: Design, Synthesis and Application

Article abstract of DOI:10.1002/chem.202001814

A new class of C₂-symmetric, chiral cyclopentadienyl ligand based on planar chiral ferrocene backbone was developed. A series of its corresponding rhodium(I), iridium(I), and ruthenium(II) complexes were prepared as well. In addition, the rhodium(I) complexes were evaluated in the asymmetric catalytic intramolecular amidoarylation of olefin-tethered benzamides via C?H activation.

Full text of DOI:10.1002/chem.202001814

Accepted Article
Accepted Article
Title: A New Class of C2-Symmetric Chiral Cp Ligand Derived from
Ferrocene Scaffold: Design, Synthesis and Application
Authors: Hao Liang, Laxmaiah Vasamsetty, Teng Li, Jijun Jiang,
Xingying Pang, and Jun Wang
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.202001814
Link to VoR: https://doi.org/10.1002/chem.202001814
01/2020

10.1002/chem.202001814
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A New Class of C2-Symmetric Chiral Cp Ligand Derived from
Ferrocene Scaffold: Design, Synthesis and Application
Hao Liang, Laxmaiah Vasamsetty, Teng Li, Jijun Jiang, Xingying Pang, and Jun Wang*
[a]
H. Liang, Dr. L. Vasamsetty, Dr. T. Li, Prof. Dr. J. Jiang, X. Pang, Prof. Dr. J. Wang
Key Laboratory of Bioinorganic and Synthetic Chemistry of Ministry of Education, School of Chemistry, Sun Yat-Sen University
Guangzhou, 510275 (P. R. China)
E-mail: wangjun23@mail.sysu.edu.cn
Supporting information for this article is given via a link at the end of the document.
Abstract: A new class of C2-symmetric, chiral cyclopentadienyl
ligand based on planar chiral ferrocene backbone was developed. A
series of its corresponding rhodium(I), iridium(I), and ruthenium(II)
complexes were prepared as well. In addition, the rhodium(I)
complexes were evaluated in the asymmetric catalytic intramolecular
amidoarylation of olefin-tethered benzamides via C-H activation.
cinnamaldehyde[14], [15] by Cramer et al., the binaphthol derived
Cp ligand II with multisubstituents on the Cp ring[16] as well as
[18]
the chiral spirobiindane Cp ligand IV[17],
by You et al., the
chiral piperidine fused Cp ligand V by Antonchick, Waldmann
and et al.[19], [20] Interestingly, a planar chiral Cp rhodium catalyst
bearing a prochiral Cp ligand VII was also developed by
Perekalin et al.[21] Nevertheless, these limited families of Cp
ligands can hardly meet the ever-growing needs of the rapid
development of asymmetric C-H activation. Therefore, exploring
new chiral Cp ligands must continue being a key and urgent task
for chemists. Herein, we present a new class of C2-symmetric
chiral Cp ligands FcCp containing a planar chiral ferrocene[22]
backbone. It is noteworthy that previously reported chiral Cp
ligands are all based on centrally (I, V, VI) or axially (II, III, IV)
chiral skeletons, whereas examples based on planar chiral
scaffold have never been reported so far. In addition,
preparation and characterization of a series of corresponding
rhodium(I), iridium(I), and ruthenium(II) complexes are reported.
Application of rhodium(I) complexes in the asymmetric catalytic
C-H activation to synthesize various chiral fused tricyclic
compounds is demonstrated as well.
Transition-metal catalyzed asymmetric C-H activation constitutes
one of the most important and challenging research frontiers in
modern organic chemistry.[1] In 2012, Cramer et al.[2] and Rovis,
Ward et al.[3] independently realized an asymmetric C-H
activation by respectively using the mannitol derived chiral
cyclopentadienyl (Cp) rhodium catalyst and the artificial enzyme
bound Cp rhodium catalyst. Remarkably, it is the first time to
show that chiral cyclopentadienylmetal (CpM)[4] catalyst is highly
potential to realize asymmetric C-H activation reactions[5]
characterized by involving inert C-H bond cleavage and
simultaneous C-M bond formation, which is of milestone
significance. The universal applicability of this strategy was
subsequently well demonstrated by numerous follow-up
studies.[6] As three free coordination sites are demanded by the
CpM catalyzed C-H activation reactions, the effective chiral Cp
metal catalysts should be those generated from chiral Cp
ligands with noncoordinating substituents.[6] Whereas the ones
derived from chiral Cp ligands with coordinating substituents[7] or
from the complexation of both achiral Cp ligands and chiral
ligands[8] are inapplicable, though they have been found useful
in other types of asymmetric reactions.
As depicted in Scheme 1, our designed synthetic route for
the new Cp ligand FcCp includes initial installation of two chiral
auxiliaries on each Cp ring of ferrocene, generation of the planar
chiral 1,1',2,2'-tetrasubstituted ferrocene by diastereoselective
di-ortho-functionalization, conversion of chiral auxiliaries to
suitable leaving groups, and finally installation of Cp motif by
twofold nucleophilic substitutions.
Scheme 1. Design of Cp ligands with planar chiral ferrocene scaffolds
Figure 1. Cp ligands studied in asymmetric C-H activation reactions
As shown in Scheme 2, to commence our synthesis, 1,1'-
diformylferrocene 2 was prepared from ferrocene 1 in 88% yield
by a dilithiation-diformylation sequence.[23] It was then allowed to
react with (S)-2-(methoxymethyl)pyrrolidine 3 to form the chiral
ferrocenyl diamine 4 in 87% yield via reductive amination with
Therefore, it is crucial to develop suitable chiral Cp ligands
with noncoordinating substituents. However, in the past eight
years, merely few classes of chiral Cp ligands were developed
for the study of asymmetric C-H activation reactions (Figure 1),
including the chiral Cp ligands I,[9] II,[10] III and VI respectively
derived from mannitol,[2], [11] binaphthol[12], [13], biphenol[13z], and
Ti(Oi-Pr)4
and
NaBH4.[24]
Highly
diastereoselective
difunctionalization of 4 was achieved by di-ortho-lithiation with s-
1
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BuLi and subsequent quenching with various electrophiles (RX)
according to Marinetti's protocol (>95:5 dr),[25] affording diverse
planar chiral 1,1',2,2'-tetrasubstituted ferrocenes 5a-d in 40-86%
yields as pure single diastereomers. As quaternary ammonium
group at the α-position of ferrocene could act as a leaving
group,[26] we first attempted the Cp installation by using
ferrocene quaternary ammonium as intermediate. Delightedly, it
was found the ferrocene ammonium iodide prepared from 5a
and MeI reacted smoothly with CpNa in the presence of NaH, KI
in DMF, affording the desired Cp derivatives as an inseparable
mixture of 6a and 7a (6a:7a = 25:2) in 58% yield over two steps.
Alternatively, inspired by the chemistry that dimethylamino group
in the α-position of the ferrocene could be facilely substituted by
chlorine upon treatment with methylchloroformate via a α-
ferrocenyl carbocation intermediate,[27] we envisioned that the
chiral pyrrolidine moieties in ferrocenyl diamine 5 might be
cleaved and replaced by chlorines too under similar reaction
conditions. Given the potential instability of ferrocenyl
dichloride,[28] it would be better to generate and use it directly
into the following Cp installation step without purification
operations. Along this line, we decided to attempt the low-boiling
acyl chloride (b.p. 51 oC) instead of chloroformate as the
chlorination reagent and dichloromethane as the solvent, which
could all be removed readily and thoroughly after reaction,
simply by rotary evaporation under reduced pressure. In addition,
the acetyl pyrrolidine byproduct produced in the chlorination step
may not affect the Cp installation reaction. Gladly, this protocol
proved feasible. The ferrocenyl dichloride crude product
prepared from 5b-d and AcCl reacted well with CpNa in the
presence of NaH, providing the desired Cp derivatives as
inseparable mixtures of isomers 6b-d and 7b-d in 23-50% yield
over two steps. Subsequently, heating the mixture of isomers 6
and 7 at 190-220 oC exclusively gave the targeted product FcCp
7a-d in 91-99% yield via thermal rearrangement of the spiro
isomers 6a-d to 7a-d.
Besides the above 2,2'-dimethyl and disilyl ferrocenyl Cp
ligands 7a-d, we also successfully synthesized 2,2'-
dialkoxyferrocenyl Cp ligands 7g-h. As illustrated in Scheme 3,
according to our sequential chlorination and Cp installation
protocol, the diiodoferrocene 5e obtained from the highly
diastereoselective iodination[29] of diaminoferrocene 4 could be
conveniently transformed to the ferrocenyl Cp product as an
inseparable mixture of 6e and 7e (6e:7e = 3:1) in 49% yield over
two steps. By treating this mixture with HOAc and cupric oxide,
the di-acetoxylated product 6f was obtained in 64% yield (based
on the amount of 6e).[30] Delightedly, converting acetoxy to
alkoxy could be facilely achieved by a one-pot two-step
procedure involving de-acylation with NaH-HMPA and in-situ
quenching with alkylhalide.[31] For instance, upon quenching with
MeI and i-PrI, the dialkoxy ferrocenes 6g and 6h were
respectively obtained in 59% and 68% yield. Finally, thermal
rearrangement of 6g and 6h at 220 oC led to the desired
dialkoxyferrocenyl Cp ligands 7g and 7h in 93% and 99% yield,
respectively.
Scheme 3. Synthesis of 2,2'- dialkoxyferrocenyl Cp ligands 7g-h
As shown in Scheme 4, with various chiral ferrocenyl Cp
ligands 7 in hand, the corresponding CpM (M = Rh, Ir, Ru) metal
complexes were prepared according to typical procedures,[12, 14]
including the CpRhI complexes 8a-f (53-86% yield), the CpIrI
complex 9 (78% yield) and the CpRuII complexes 10a-b (62-82%
yield). Notably, the structures and absolute configurations of
chiral Cp ligands were unambiguously confirmed by X-ray
crystallographic analysis of the CpRhI complex 8e and the
CpRuII complex 10b. Interestingly, it was found from these
crystal structures that the chiral ferrocenyl moieties fold
somewhat away from the metal centres, which could be
attributed to the high flexibility of the ferrocenyl Cp rings.
However, it is hard to predict to what extent the ferrocenyl
moiety will continue keeping this favoured conformation in
solution. It was envisioned that this new class of chiral CpM
catalysts might perform differently from the existing ones in the
asymmetric C-H activation.
Scheme 2. Synthesis of 2,2'-dimethyl and disilyl ferrocenyl Cp ligands 7a-d
2
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entrya
catalyst
8a
yield (%)b
>99
>99
92
ee (%)c
19
1
2
8b
8c
67
3
49
4
8d
8e
76
66
5
82
18
6
8f
>99
n.r.
>99
50
28
Scheme 4. Synthesis of chiral ferrocenyl CpM (M = Rh, Ir, Ru) complexes
7
9
-
Then, the potential utility of these new CpMI complexes in
asymmetric catalysis was examined with the intramolecular
amidoarylation of the olefin-tethered benzamide 11a as the
model reaction, which was previously reported by Rovis et al.[32]
and Glorius et al.,[33] but in a non-asymmetric fashion. In addition,
during the exploration of asymmetric intramolecular
hydroarylation of alkenes, 12d was once observed as the side
product by Cramer et al.[13c] Among the rhodium precatalysts 8a-
f, it was found that 8b bearing SiEt3 substituents was optimal,
furnishing the tricyclic product 12a in >99% yield with 67% ee
(Table 1, entries 1-6). In contrast, the CpIrI complex 9 was
unable to catalyze this reaction (entry 7). Moreover, for the sake
of comparisons, some widely used chiral CpRhI precatalysts 13-
16 reported by Cramer et al. and You et al. were evaluated in
this reaction. However, the cinnamaldehyde derived chiral CpRhI
13 gave >99%yield whereas with poor enantioselectivity of 14%
ee (entry 8). The atropchiral binaphthyl CpRhI 14 gave
comparable enantioselectivity of 70% ee in contrast to 8b, but in
an inferior yield of 50% (entry 9). Employing chiral spirobiindane
CpRhI 15 only led to trace amount of product with moderate
enantioselectivity of 52% ee (entry 10). Lastly, the mannitol
derived chiral CpRhI 16 provided a good yield of 82% but with
low enantioselectivity of 37% ee (entry 11). These results
explicitly indicated that the ferrocene based Cp catalysts well
complemented the existing chiral Cp catalysts in terms of the
catalytic property.
8
13
14
9
14
70
10
11
15
4
52
16
82
37
a11a (0.05 mmol, 1.0 equiv), catalyst (5 mol%), (BzO)2 (5 mol%),
hexafluoroisopropanol (HFIP, 0.2 mL), rt, 17 h. bDetermined by 1H NMR
analysis using CH2Br2 as internal standard. cDetermined by HPLC.
Further studies revealed that, in the presence of precatalyst 8b
and (BzO)2 at 0 ℃, diverse alkene-tethered benzamides, such
as those bearing 1,1-disubstituted alkenes (11a-e, i),
monosubstituted alkenes with varied chain length (11f-g), and
1,2-disubstituted alkene (11h), could all be smoothly converted
into the corresponding tricyclic lactams in moderate to good
yields and enantioselectivities (Table 2). Notably, the
enantiopure lactam 12e was facilely obtained via
recrystallization from dichloromethane/methanol (6:1), and the
absolute configuration of the major enantiomer of 12e was
determined to be S by X-ray crystallographic analysis.
Table 2. Substrate scopea
Table 1. Evaluation of various Cp complexes in asymmetric C-H activation
3
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A series of corresponding chiral CpRhI, CpIrI, and CpRuII
complexes have been prepared. The potential use of these
metal complexes as catalysts has been demonstrated by a chiral
CpRhIII catalyzed asymmetric intramolecular amidoarylation of
olefin-tethered benzamides via C-H activation. Further
applications of these chiral ferrocenyl Cp metal complexes and
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Acknowledgements
We thank the National Natural Science Foundation of China for
financial support (21971263).
Keywords: chiral cyclopentadiene • asymmetric C-H activation •
planar chiral • ferrocene • rhodium
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10.1002/chem.202001814
Chemistry - A European Journal
COMMUNICATION
COMMUNICATION
Hao Liang, Laxmaiah Vasamsetty, Teng
Li, Jijun Jiang, Xingying Pang, and Jun
Wang*
Page No. – Page No.
A New Class of C2-Symmetric Chiral
Cp Ligand Derived from Ferrocene
Scaffold: Design, Synthesis and
Application
A new class of C2-symmetric, chiral cyclopentadienyl ligand based on planar chiral
ferrocene backbone was developed. A series of its corresponding rhodium(I),
iridium(I), and ruthenium(II) complexes were prepared as well. In addition, the
rhodium(I) complexes were evaluated in the asymmetric catalytic intramolecular
amidoarylation of olefin-tethered benzamides via C-H activation.
6
This article is protected by copyright. All rights reserved.