Rh(III)-catalyzed directed C-H bond amidation of ferrocenes with isocyanates

Article abstract of DOI:10.3762/bjoc.8.212

[RhCp*(OAc)₂(H₂O)] [Cp* = pentamethylcyclopentadienyl] catalyzed the C-H bond amidation of ferrocenes possessing directing groups with isocyanates in the presence of 2 equiv/Rh of HBF4·OEt2. A variety of disubstituted ferrocenes were prepared in high yields, or excellent diastereoselectivities.

Full text of DOI:10.3762/bjoc.8.212

Rh(III)-catalyzed directed C–H bond amidation of
ferrocenes with isocyanates
Satoshi Takebayashi, Tsubasa Shizuno, Takashi Otani
and Takanori Shibata*
Letter
Open Access
Address:
Beilstein J. Org. Chem. 2012, 8, 1844–1848.
Department of Chemistry and Biochemistry, School of Advanced
Science and Engineering, Waseda University, Okubo, Shinjuku,
Tokyo, 169-8555, Japan
doi:10.3762/bjoc.8.212
Received: 11 July 2012
Accepted: 01 October 2012
Published: 29 October 2012
Email:
Takanori Shibata* - tshibata@waseda.jp
This article is part of the Thematic Series "C–H Functionalization".
Guest Editor: H. M. L. Davies
* Corresponding author
Keywords:
amidation; C–H activation; C–H functionalization; ferrocene; rhodium
© 2012 Takebayashi et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
[RhCp*(OAc)2(H2O)] [Cp* = pentamethylcyclopentadienyl] catalyzed the C–H bond amidation of ferrocenes possessing directing
groups with isocyanates in the presence of 2 equiv/Rh of HBF4·OEt2. A variety of disubstituted ferrocenes were prepared in high
yields, or excellent diastereoselectivities.
Introduction
Ferrocene and its derivatives are among the most useful herbicide (S)-metolachlor on a scale of more than 10000 tons/
organometallic compounds because of their chemical and year [5].
thermal stabilities, structures, and redox activity [1,2]. One of
the most remarkable applications of ferrocene derivatives is as Planar chiral 1,2-disubstituted ferrocene derivatives are usually
chiral ligands [3,4]. A variety of chiral ferrocenyl ligands with synthesized by using diastereoselective ortho-lithiation of
several substitution patterns have been successfully utilized for monosubstituted ferrocenes with an appropriate chiral ortho-
enantioselective catalysis in both academia and industry. In par- directing substituent such as chiral amines, sulfoxides, and oxa-
ticular, planar chiral 1,2-disubstituted ferrocenyl scaffolds have zolines [3]. However, this method suffers from low atom
been extensively studied, and are among a few premier chiral economy, and requires stoichiometric amounts of metal
ligand structures. For instance, a 1,2-disubstituted ferrocenyl reagents. Functionalization of ferrocene derivatives by tran-
ligand, Xyliphos ((R)-1-[(S)-2-(diphenylphosphanyl)ferro- sition-metal-catalyzed enantioselective C–H activation is a
cenyl]ethyl bis(3,5-dimethylphenyl)phosphane) is used for potentially more atom-economical alternative. However, only a
iridium-catalyzed enantioselective hydrogenation to produce the few catalytic C–H activation reactions of ferrocenes have been
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Beilstein J. Org. Chem. 2012, 8, 1844–1848.
reported to date, and there is only one report of enantioselective this manuscript, we report application of this nonoxidative
C–H activation of ferrocenes [6-9]. Schmalz et al. reported the Rh(III) catalysis to synthesize planar chiral 1,2-disubstituted
first catalytic C–H activation of ferrocenes using a Cu-catalyzed ferrocene derivatives.
intramolecular carbene insertion into a Cp–H bond [6]. Further,
they showed that the reaction could be enantioselective if chiral Results and Discussion
bisoxazoline ligands were used. However, the substrate scope of We chose the reaction of ferrocenyl imine 1a and phenyl
this reaction is narrow because of intramolecular reaction. More isocyanate as a model reaction and screened several catalysts
recently, chiral oxazoline-directed diastereoselective arylation (Table 1). The cationic Cp*Ir(III) catalyst, which was used in
of ferrocenes was reported based on a Pd(II)-catalyzed oxida- our previous report [23], did not catalyze the reaction at all
tive coupling reaction [8]. Most of the reactions in this report (Table 1, entry 1). Under copper-salt-free conditions, cationic
are, however, stoichiometric, and the yields of the catalytic Cp*Ir(III) catalyst did not give the product, but cationic
reactions were low. Although there are a number of reports on Cp*Rh(III) selectively afforded monoamidated 1,2-disubsti-
stoichiometric directed C–H activation of ferrocenes by using tuted ferrocene derivative 2a, albeit in low yield (Table 1,
electrophilic metal centers such as Pd(II), Pt(II), and Ru(II) [10- entries 2 and 3). The catalyst with BF4− anion showed the
12], this communication describes a metal-catalyzed directed highest activity (Table 1, entries 3–5). The reaction also
electrophilic C–H activation of an electron-rich Cp ring of proceeded in the presence of an isolated cationic Cp*Rh cata-
ferrocene.
lyst, which simplifies the reaction setup and is required to
prevent the redox reaction between AgBF4 and 1a [24].
Pentamethylcyclopentadienyl (Cp*)Rh(III) is known to catalyze However, the use of [RhCp*(MeCN)3](BF4)2 [17] resulted in
electrophilic activation of aryl C–H bonds, typically in the pres- lower yield likely due to the coordination of MeCN (Table 1,
ence of an acetate ligand, and is used for oxidative C–C-bond- entry 6). The yield significantly increased when the combina-
formation reactions [13]. Recently, several reports of cationic tion of [RhCp*(OAc)2(H2O)] [25] and 2 equiv/Rh of
Cp*Rh(III)-catalyzed nonoxidative C–C-bond-formation reac- HBF4·OEt2 was used to form dicationic Cp*Rh species in situ
tions have been disclosed [14-22]. For example, Ellman et al. (Table 1, entry 7) [26].
and Shi et al. reported that Cp*Rh(III) complexes catalyzed the
reaction of aryl C–H bonds to imines, isocyanates, and alde- We examined a variety of isocyanates under the same reaction
hydes by directed electrophilic activation of aryl C–H bonds at conditions given in entry 7 in Table 1 except with a lower cata-
relatively low temperature and under oxidant-free conditions lyst loading of [RhCp*(OAc)2(H2O)]. The reaction proceeded
[15-21]. We also reported that cationic Cp*Ir(III) complexes, smoothly with 5 mol % of [RhCp*(OAc)2(H2O)] along with a
combined with 1 equiv/Ir of Cu(OAc)2, catalyzed the directed slight decrease of yield (Table 2, entry 1). Both electron-rich
C–H activation of aryl C–H bonds at room temperature [23]. In and -poor aryl isocyantes showed similar reactivity in the
Table 1: Screening of catalysts.
entry
catalyst (mol %)
additive (mol %)
yield (%)
1a
2
[IrCp*Cl2]2 (5)
AgSbF6 (20), Cu(OAc)·H2O (20)
AgSbF6 (10)
0
[IrCp*Cl2]2 (2.5)
0
3b
4b
5b
6
[RhCp*Cl2]2 (2.5)
AgSbF6 (10)
30
20
42
29
86
[RhCp*Cl2]2 (2.5)
AgOTf (10)
[RhCp*Cl2]2 (2.5)
AgBF4 (10)
[RhCp*(MeCN)3](BF4)2 (10)
[RhCp*(OAc)2(H2O)] (10)
none
7
HBF4·OEt2 (20)
aThe reaction was examined at 100 °C for 1 h in 1,2-dichloroethane using 4-methoxyphenyl isocyanate. bThe reaction time was 24 h.
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Beilstein J. Org. Chem. 2012, 8, 1844–1848.
Table 2: Scope of isocyanates.
entry
R
time (h)
product
yield (%)
1
2
3
4
5
6
Ph
3
3
2a
2b
2c
2d
2e
2f
74
85
87
84
41
19
4-MeOC6H4
4-ClC6H4
benzyl
3
3
n-butyl
24
24
cyclohexyl
present reaction (Table 2, entries 2 and 3). The use of benzyl The absolute configuration of planar chirality in 3c was deter-
isocyanate also formed the monoamidated product 2d (Table 2, mined to be S by X-ray crystallography (Figure 1). The absolute
entry 4). It required a longer reaction time, but alkyl isocyanates configuration is consistent with the previous report of diastereo-
were also available.
selective ortho-lithiation of 1b [27].
We next examined a diastereoselective reaction using a Conclusion
commercially available chiral oxazolyl ferrocene 1b, and the In conclusion, a Cp*Rh(III)-catalyzed reaction between ferro-
reaction was conducted under the same reaction conditions used cenyl C–H bonds and isocyanates was developed to synthesize
in Table 2. Several isocyanates were submitted to the reaction a variety of 1,2-disubstituted ferrocenes. The use of the
with 1b, and planar chiral 1,2-disubstituted ferrocenes 3a–f commercially available chiral oxazolyl ferrocene enabled us to
were obtained with high diastereoselectivity, but the yields in synthesize planar chiral 1,2-disubstituted ferrocenes with excel-
all cases were moderate because of a low conversion ratio lent diastereoselectivity. The present reaction is a rare example
(Table 3). Lower coordination ability of the oxazolyl group of catalytic methods to construct planar chiral ferrocenes. We
compared to the imino one probably decreased the reactivity. are currently investigating an enantioselective reaction.
Table 3: Diastereoselective reaction by using chiral oxazolyl ferrocene 1b.
entry
R
time (h)
product
yield (%)
1
2
3
4
5
6
Ph
2
2
3a
3b
3c
3d
3e
3f
43
24
38
32
69
38
4-MeOC6H4
4-ClC6H4
benzyl
2
2
n-butyl
24
24
cyclohexyl
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Supporting Information
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Experimental procedures and physical properties of new
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CIF of complex 3c.
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Acknowledgements
This work was supported by the Japan Society for the Promo-
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Special Research Projects (No. 2011A-896). We greatly appre-
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