An Enantioselective Oxidative C-H/C-H Cross-Coupling Reaction: Highly Efficient Method to Prepare Planar Chiral Ferrocenes

Article abstract of DOI:10.1021/jacs.6b00127

A Pd-catalyzed, asymmetric oxidative cross-coupling reaction between ferrocenes and heteroarenes is described. The process, which takes place via a twofold C-H bond activation pathway, proceeds with modest to high efficiencies (36-86%) and high levels of regio- and enantioselectivity (95-99% ee). In the reaction, air oxygen serves as a green oxidant and excess amounts of the coupling partners are not required. The process is the first example of a catalytic asymmetric biaryl coupling reaction that occurs via double C-H bond activation. Finally, the generated coupling products can be readily transformed into chiral ligands and catalysts.

Full text of DOI:10.1021/jacs.6b00127

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Communication
An Enantioselective Oxidative C–H/C–H Cross-Coupling Reaction:
Highly Efficient Method to Prepare Planar Chiral Ferrocenes
De-Wei Gao, Qing Gu, and Shu-Li You
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.6b00127 • Publication Date (Web): 18 Feb 2016
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An Enantioselective Oxidative C–H/C–H Cross-Coupling Re-
action: Highly Efficient Method to Prepare Planar Chiral
Ferrocenes
De-Wei Gao, Qing Gu, and Shu-Li You*
9
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, 345 Lingling Lu, Shanghai 200032, China
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Supporting Information Placeholder
A major breakthrough in the area of catalytic asymmetric
C–H bond activation was made recently by Yu and his
coworkers. This group demonstrated that mono-protected
amino acids can be utilized as chiral ligands in these pro-
cesses.9 Recently, a few remarkable Pd(II)-catalyzed asym-
metric C–H functionalization reactions of ferrocene deriva-
tives have been described, as exemplified by processes de-
veloped in our recent studies of asymmetric C–H arylations
with aryl boronic acids.8a Removal of the need for pre-
activation of arene substrates, would simplify this process.
For this purpose, we envisioned that planar chiral ferro-
cenes could be synthesized directly from ferrocene and un-
functionalized arenes via two-fold C–H activation process
(Figure 1c).
ABSTRACT: A Pd-catalyzed, asymmetric oxidative cross-
coupling reaction between ferrocenes and heteroarenes is
described. The process, which takes place via a two-fold C–
H bond activation pathway, proceeds with modest to high
efficiencies (36-86%) and high levels of regioselectivity and
enantioselectivity (95-99% ee). In the reaction, air oxygen
serves as a green oxidant and excess amounts of the
coupling partners are not required. The process is the first
example of a catalytic asymmetric biaryl coupling reaction
that occurs via double C–H bond activation. Finally, the
generated coupling products can be readily transformed
into chiral ligands and catalysts.
Asymmetric C–H bond functionalization has attracted the
attention of chemists1 owing to its unprecedented applica-
tions in organic synthesis.2 In the past few years, a number
of coupling reactions between unfunctionalized arenes (Ar–
H) and aryl halides or aryl organometallics have been un-
covered. Although the processes produce a variety of chiral
compounds, pre-functionalization of at least one of the cou-
pling partners is required (Figure 1a).1 From the viewpoint
of atom and step economy, an enantioselective cross-
coupling reaction of two unfunctionalized arenes (Ar–H) via
a double C–H bond activation pathway would undoubtedly
be an extremely attractive process for construction of enan-
tio-enriched compounds. A strategy of this type would have
numerous advantages such as low cost, and minimized ma-
nipulations and waste. To the best of our knowledge, only
racemic versions of these types of coupling reactions have
been described owing to the fact that chiral recognition of
inert C–H bonds of arene has not been well addressed (Fig-
ure 1b).3 Additionally, the processes are typically limited by
poor regioselectivities and by the required use of large ex-
cesses of coupling partners and stoichiometric amounts of
metal oxidants. Therefore, the development of a method for
carrying out asymmetric, two-fold oxidative C–H cross-
coupling reactions, although extremely challenging, is high-
ly desirable.4
Figure 1. Asymmetric cross-coupling for C–C bond for-
mation. a, Asymmetric cross-coupling reaction involving
one C–H bond activation. b, Two-fold oxidative C–H cross-
coupling reaction in a racemic manner. c, Asymmetric oxi-
dative C–H/ C–H cross-coupling reaction. FG = functional
group. * = symbol of chirality. Ar = arene. Fc = ferrocene.
To test the validity of this proposal, an initial investiga-
tion was carried out exploring the enantioselective cross-
coupling reaction of dimethylaminomethylferrocene (1a)
and 1.5 equivalents of benzofuran (2a). In this case, dime-
thylamino group acts as directing group for the first pallada-
tion, and the electron-rich benzofuran facilitates subsequent
electrophilic regioselective palladation. It should be noted
that 1a was used earlier in a stoichiometric asymmetric cy-
clopalladation reaction developed by Sokolov.10 The cross-
coupling process between 1a and 2a takes place in the pres-
ence of 10 mol % of Pd(OAc)2 and 20 mol % of Boc- L-Val-
OH at 70 oC with air as an oxidant to generate product 3a in
Since the time of the serendipitous discovery and later
structural determination of ferrocene, related “sandwich”
compounds have been subjected to extensive studies.5 Fer-
rocenes, possessing planar chirality, have been used in the
synthesis of industrial, pharmaceutical and agrochemical
commodities.6 As a result, considerable effort has been de-
voted to the development of procedures to prepare planar
chiral members of the ferrocene family.7 There is little doubt
that methods employing catalytic asymmetric C–H func-
tionalization would be ideal for introducing substituents on
the ferrocene backbone.8
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a highly C2-regioselective manner and a 44% yield and with
99% ee (Table 1, entry 1).
tion is that asymmetric C–H functionalization takes place
exclusively at the position ortho to the N-directing group (1i
to 3i, 69% yield, 99% ee) even when a phosphoryl-directing
group is present on the ferrocene ring. Finally, the pathway
involving regioselective formation of 2-ferrocenyl-
benzofurans is followed in all cases.
Encouraged by this result, the conditions for this reaction
were optimized. The efficiency of the reaction was found to
be only slightly enhanced by raising the temperature to
80 °C (entry 2). The results of screening commercially
available chiral amino acid derivatives revealed that the use
of Boc- L-Ile-OH leads to an improved yield of 3a without
lowering the level of enantioselectivity (entry 3). Notably,
racemic 3a is generated when Boc- L-Pro-OH is employed
as a chiral ligand, a likely result of the fact that this ligand
does not have a NH moiety that is required for formation of
a bidentate complex with Pd (entry 6). Interestingly, an in-
crease in the loading of benzofuran causes an increase in the
yield of the process (entry 7). Moreover, the use of 10 mol %
of benzoquinone and 4 equivalents of water leads to im-
provement of the efficiency of this process (71% yield, 99%
ee; entry 10).
Table 2. Regio- and enantio-selective cross-
coupling reactions of ferrocene derivativesa,b,c
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Table 1. Enantioselective C–H oxidative cross-
couplinga
entry 2a (equiv.) ligand
yield (%)b ee (%)c
1d
2
1.5
1.5
1.5
1.5
1.5
1.5
2.0
2.0
2.0
2.0
Boc-L-Val-OH
Boc-L-Val-OH
Boc-L-Ile-OH
44
48
53
99
98
98
94
94
0
a
Reaction conditions: 1 (0.3 mmol), 2a (0.6 mmol)
Pd(OAc)2 (10 mol %), Boc-L-Ile-OH (20 mol %), K2CO3
(0.45 mmol), benzoquinone (10 mol %) and water (4.0
equiv.) in DMA under air at 80 °C unless noted otherwise. b
Isolated yield. c Determined by HPLC analysis.
3
4
Boc-L-Phe-OH 45
Boc-L-Nva-OH 48
Boc-L-Pro-OH 15
5
The generality of the direct coupling reaction was ex-
plored using ferrocene (1a) and a variety of heteroarenes
(Table 3). The results show that benzofurans bearing elec-
tronically different substituents (5-Me, 5-OMe, 5-iPr, 5-Ph,
5-F, 5-Cl, 5,7-Me2, 7-tBu, 7-Cl) react with 1a to produce the
corresponding products (3j-r) in satisfactory yields (49-77%)
and with excellent levels of enantioselectivity (97-99% ee).
It is worth noting that reactions of furans bearing various
substituents, regardless of their electronic nature and posi-
tion, take place with exclusive C2 regioselectivity, and in
high yields and levels of enantioselectivity (3s-y). Both C2
and C3 substituted thiophenes also serve as substrates for
the oxidative cross-coupling reaction that form products
(3z-zh) in moderate to high yields and with near perfect
levels of enantiocontrol. Furthermore, reactions occur
smoothly using ketone, ester, aldehyde and acrylate func-
tionalized pyrroles to form the corresponding products
(3zi-zl). In addition, 1-methylindole bearing an electron-
withdrawing group (5-Cl) is a substrate for this process,
which forms the C2-indole product 3zm regioselectively in
6
7e
8f
9g
10h
Boc-L-Ile-OH
Boc-L-Ile-OH
Boc-L-Ile-OH
Boc-L-Ile-OH
60
65
69
71
99
99
99
99
a
Reaction conditions: 1a (0.3 mmol), 2a (0.45 mmol),
Pd(OAc)2 (10 mol %), ligand (20 mol %), K2CO3 (0.3 mmol)
b
in DMA under air at 80 °C unless noted otherwise. Isolat-
c
d
e
ed yield. Determined by HPLC analysis. At 70 °C . With
f
K2CO3 (1.5 equiv.). With K2CO3 (1.5 equiv.) and benzoqui-
g
none (10 mol %). With K2CO3 (1.5 equiv.), benzoquinone
(10 mol %) and water (1.0 equiv.). h With K2CO3 (1.5 equiv.),
benzoquinone (10 mol %) and water (4.0 equiv.).
Having identified optimized conditions (entry 10, Table 1),
an exploration of asymmetric C–H oxidative cross-coupling
reactions of ferrocene derivatives, containing a variety of
electronically different substituents, was carried out. The
results of this effort (Table 2) show that regardless of the
nature of the N-directing group (e.g., pyrrolidinyl, piperidi-
nyl, or methylpropylamino group) the ferrocenes (1b-d)
participate in this process to generate products in high
yields and nearly enantiopure forms. The reaction displays
an excellent functional group tolerance that is reflected by
the fact that sensitive functionality, such as an aldehyde (1e),
unprotected 3o alcohol (1f-g) and silyl group (1h), can be
present on the Cp ring. A particularly interesting observa-
a
51% yield and with 98% ee. Interestingly, 5-
methylbenzothiophene undergoes efficient coupling reac-
tion with 1a to generate 3zn2 and 3zn3 in a 9.4:1 ratio and a
74% combined yield. However, pentafluorobenzene, imid-
azole and oxazole are not suitable substrates for the cou-
pling reaction. Finally, the configurations of the stereogenic
centers in the products of these reactions is assumed to be
Rp based on X-ray crystallographic analysis of single crystals
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of representative enantiopure products 3zh and 3zk (see
the Supporting Information for details).
by using existing methods. As illustrated in Fig. 2A, 3a was
converted through ortho-lithiation and subsequent reac-
tions with electrophiles to an array of planar chiral biden-
tate type, ferrocene ligands (4a-d). Planar chiral ferrocenes
bearing similar privileged scaffolds have been demonstrated
to serve as efficient asymmetric catalysts.6 For example, 4a
is an efficient catalyst in asymmetric diethylzinc addition of
benzaldehyde and 1-naphthaldehyde (90% yield, 86% ee
and 86% yield, 87% ee, Fig. 2B). In spite of the fact that the
enantioselectivity of this process is not as high as other
state-of-art reactions described in the literature,11 the new
method serves as a useful platform for the design of new
ligands and catalysts for asymmetric transformations.
Table 3. Regio- and enantio-selective cross-
coupling reactions of heteroarenesa,b,c
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Figure 2. Transformations of 3a and an asymmetric dieth-
ylzinc addition reaction. A, Transformations of 3a. B, The
application of 4a in asymmetric diethylzinc addition reac-
tion. See the Supporting Information for details.
In the investigation described above, we developed a new
asymmetric two-fold C–H oxidative cross-coupling reaction
of ferrocenes with heteroarenes. These processes take place
with excellent levels of regio- and enantio-selectivity, and
high efficiencies. Consequently, a strategy now exists for
carrying out enantioselective C–H bond activation reactions
without the need for pre-activation of either coupling sub-
strate. Additionally, the employment of air oxygen as a
green oxidant and the absence of the need to use significant
excesses of either coupling partner serve as further advanta-
geous features of the new methodology. Finally, the new
protocol can be incorporated into routes for the synthesis of
a variety of new enantiopure ligands or catalysts for asym-
metric transformations.
ASSOCIATED CONTENT
Supporting Information
Experimental procedures and analystical data for all new
compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
a
Reaction conditions: 1a (0.3 mmol), 2 (0.6 mmol)
Pd(OAc)2 (10 mol %), Boc-L-Ile-OH (20 mol %), K2CO3
(0.45 mmol), benzoquinone (10 mol %) and water (4.0
equiv.) in DMA under air at 80 °C unless noted otherwise. b
AUTHOR INFORMATION
c
d
Isolated yield. Determined by HPLC analysis. With het-
Corresponding Author
e
eroarenes (4 equiv.).
mined.
Enantioselectivity was not deter-
slyou@sioc.ac.cn
To further demonstrate the utility of the new methodolo-
gy developed in this effort, various straightforward trans-
formations of the coupling product 3a were carried out to
produce planar chiral ligands, which are difficult to prepare
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
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