Intramolecular Direct C-H Arylation via a Metallocenic Radical Pathway: Stereospecific Approach to Planar-Chiral Ferrocenes

Article abstract of DOI:10.1021/acs.orglett.7b02995

Transition metal-free synthesis of planar-chiral 1,2-fused ferrocenes via intramolecular direct C-H bond arylation was achieved. The C-H arylation reactions promoted by a catalytic amount of 1,10-phenanthroline highlighted a unique planar-chiral metallocenic radical intermediate, generated from iodoferrocenes via a single-electron transfer process.

Full text of DOI:10.1021/acs.orglett.7b02995

Letter
pubs.acs.org/OrgLett
Intramolecular Direct C−H Arylation via a Metallocenic Radical
Pathway: Stereospecific Approach to Planar-Chiral Ferrocenes
Yang Liu,‡ Jiancong Xu,‡ Jinling Zhang, Xiaohua Xu,* and Zhong Jin*
State Key Laboratory and Institute Elementoorganic Chemistry, College of Chemistry, Nankai University, Tianjin 300071, China
S
* Supporting Information
ABSTRACT: Transition metal-free synthesis of planar-chiral
1,2-fused ferrocenes via intramolecular direct C−H bond
arylation was achieved. The C−H arylation reactions promoted
by a catalytic amount of 1,10-phenanthroline highlighted a unique
planar-chiral metallocenic radical intermediate, generated from
iodoferrocenes via a single-electron transfer process.
lanar-chiral ferrocenes are a class of intriguing organo-
tion for synthesis of planar-chiral benzosiloloferrocenes has also
been developed by the groups of Shibata,8 He,9 and Murai and
Takai,10 independently. In addition, gold- or platinum-catalyzed
cycloisomerization of ortho-alkynylaryl ferrocenes was demon-
strated to afford planar-chiral naphthene-fused ferrocenes with
high enantioselectivity.11
P
metallic molecules in the fields of organic synthesis,
materials science, and medicinal chemistry.1 In particular,
application of ligands based on planar-chiral ferrocene scaffolds
in transition metal-catalyzed asymmetric synthesis have
witnessed enormous progress over the past decades.2 Recently,
planar-chiral 1,2-fused ferrocenes have proven also to be highly
efficient chiral ligands and catalysts in asymmetric catalysis
(Scheme 1a).3 However, to date, efficient approaches to these
Despite this tremendous progress, development of alternative
methodologies for asymmetric synthesis of this class of
organometallics still remains necessary given the significant
importance of planar-chiral ferrocenes. Since the first report on
transition metal-free coupling of haloarenes with N-heterocycles
was published by Itami and co-workers in 2008,12 this class of
cross-coupling reaction between aryl halides and unactivated
arenes has received extensive interest in light of its transition
metal-free merit.13 In the presence of an organic strong base,
typically KOtBu, inter- and intramolecular cross-coupling of aryl
halides with arenes could be promoted by various organic
molecules, such as 1,10-phenanthrolines, 1,2-diamines, and
others.14,15 Encouraged by the aforementioned success, we
envisioned that planar-chiral 1,2-fused ferrocenes could be
synthesized via intramolecular direct C−H arylation under
transition metal-free conditions. We herein report a stereo-
specific approach to planar-chiral 1,2-fused ferrocenes via
intramolecular direct C−H bond arylation involving a chiral
metallocenic radical pathway (Scheme 1c).
Scheme 1. Planar-Chiral Ferrocenes (a) and Approach to
Planar-Chiral Ferrocenes via Intramolecular C−H Arylation
(b and c)
At the outset, chiral ferrocenocarboxamide (Sp)-2a (96:4 er),
easily prepared from ferrocenocarboxylic acid according to the
previous procedure,16 was employed as a model reactant and
various reaction conditions were optimized (Table 1) (see the
Supporting Information for the details). Among the alkali metal
tert-butoxides, KOtBu was shown to be the optimal base (entries
1−3, Table 1). The efficacy of 1,10-phenanthrolines was greatly
associated with their substitution patterns in the scaffold. A
coupling reaction promoted by 1,10-phenanthroline (C1) gave
the arylation product 3a in 92% yield, while with 2,9-dimethyl
phenanthroline (C2) as an additive only a trace amount of
planar-chiral 1,2-fused ferrocenes are rather rare. In 1997,
Schmalz and co-workers accomplished a copper-catalyzed
enantioselective insertion of carbenoids into a Cp−H bond of
ferrocene giving the optically active ferrocene-fused cyclic
ketones.4 Very recently, intensive efforts on transition metal-
catalyzed direct C−H functionalization for synthesis of planar-
chiral ferrocene-fused derivatives have been made (Scheme 1b).
The research groups of You,5 Gu,6 and Liu and Zhao7 have
independently reported a palladium-catalyzed enantioselective
intramolecular direct C−H arylation with halobenzenes leading
to planar-chiral indenone- and quinolinone-fused ferrocenes. A
rhodium-catalyzed enantioselective intramolecular C−H silyla-
Received: September 24, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02995
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Letter
anion was likely to be disassociated with the CpFe+ ion. When
the resulting cyclopentadienyl radical anion turn upside down
and recoordinated to the CpFe+ ion, racemization occurred and
an enantiomer of the original chiral ferrocenyl radical was
obtained. The polar solvent, for example, THF and DME, were
reasoned to be beneficial to the solvolytic dissociation of the
CpFe+ cation and preferred to cause racemization.
Table 1. Optimization of Reaction Conditions
With the optimal conditions in hand, the generality of the
protocol with a wide array of anilines was then examined. As
shown in Scheme 2, in all cases planar-chiral ferrocene-fused
Scheme 2. Intramolecular Direct C−H Arylation with Planar-
a
Chiral Iodoferrocenes
a
b
entry
base (equiv)
additive
solvent
THF
yield (%)
er
nd
1
LiOtBu (3.0)
NaOtBu (3.0)
KOtBu (3.0)
KOtBu (4.0)
KOtBu (4.0)
KOtBu (4.0)
KOtBu (4.0)
KOtBu (4.0)
KOtBu (4.0)
KOtBu (4.0)
KOtBu (4.0)
KOtBu (4.0)
KOtBu (4.0)
KOtBu (4.0)
KOtBu (4.0)
KOtBu (4.0)
KOtBu (4.0)
KOtBu (4.0)
KOtBu (4.0)
C1
C1
C1
C1
C2
C3
C4
C5
C6
C7
C8
C9
C10
C1
C1
C1
C1
C1
C1
C1
<5
39
83
92
<5
80
76
35
0
2
THF
nd
nd
3
THF
4
THF
92:8
nd
5
THF
6
THF
nd
7
THF
nd
8
THF
nd
9
THF
nd
10
11
12
13
14
15
16
17
18
19
20
THF
45
40
38
27
81
64
82
79
75
73
65
nd
THF
nd
THF
nd
THF
nd
a
Reaction conditions: (Sp)-2 (0.2 mmol), Phen (20 mol %), KOtBu (4
DME
86:14
90:10
97:3
93:7
95:5
96:4
95:5
equiv), benzene (2 mL), 80 °C, 10 h, under Ar. Isolated yields and er
dioxane
benzene
toluene
m-xylene
p-xylene
mesitylene
c
values are determined by HPLC. The er values of (Sp)-2 are reported
in parentheses.
products were obtained in 74−90% yields with complete
retention of configuration. Anilines with electron-neutral alkyl
groups, such as methyl, ethyl, and tert-butyl groups (3b−d), and
electron-rich alkoxyl group (3e) were well tolerated in the
reactions. Fluoro- and chloro-substituted anilines (3f and 3g)
were also compatible substrates, affording the arylation products
in excellent yields. As we know, intramolecular direct C−H
arylation of iodobenzenes with the substituted arenes normally
provided a mixture of regioisomers, which resulted from 5-exo-
trig and 6-exo/endo-trig cyclization pathways (vide infra).15b,c,e
Gratifyingly, intramolecular arylation reaction of iodoferrocenes
with substituted anilines proceeded with specific regioselectivity.
For para-substituted anilines (3b−g), only 6-exo/endo-trig
cyclization products were produced as a single isomer. While
for substrate derived from m-methyl aniline, an intramolecular
coupling reaction occurred regioselectively at the 2-position to
give product 3h as a single isomer, which probably attributed to a
kinetically favored 5-exo-trig ipso attack on the aniline ring to
form a spirocyclohexadienyl radical intermediate. Disappoint-
ingly, instead of amide 2a with 2-bromo congener no desired
coupling product was observed.
KOtBu (4.0)
a
b
Isolated yields. Determined by HPLC analysis. nd: not detected.
product was obtained (entries 4 and 5). 1,10-Phenanthroline
(C6) possessing a nitro group at the 6-position inhibited the
transformation completely (entry 9). 1,1′-Bipyridines and 1,2-
diamines were also efficient catalysts for this reaction, albeit in
lower yields (entries 10−13). Although excellent yield was
achieved with tetrahydrofuran (THF) as a solvent, the partial
racemization in product 3a was observed (entry 4). Therefore, a
variety of organic solvents were systematically examined for
improving enantioselectivity. Encouragingly, when coupling
reactions were carried out in nonpolar solvents such as benzene,
xylene, and mesitylene, product (Rp)-3a was obtained with
complete retention of configuration (entries 16 and 18−20).
Among them, benzene was shown to be the most efficient one.
Notably, intermolecular coupling products between iodoferro-
cene and solvents, for example, benzene, were not detected
under the reaction conditions.
In regard to partial racemization in the arylation products, a
proper interpretation involving the decomplexation of the
planar-chiral ferrocenyl radical was suggested. It was reported
that planar-chiral compounds were prone to racemization in the
presence of Brønsted’s or Lewis’s acids, or in the polar solvents,
such as nitromethane.17 After a single electron transfer (SET)
initiation from the planar-chiral iodoferrocenes and dissociation
of I− anion, a planar-chiral ferrocenyl radical was thus
generated,18 in which the substituted cyclopentadienyl radical
Next, to further understand the arylation process we
investigated intramolecular coupling of o-haloaniline-derived
amides 4 (Scheme 3). As expected, coupling reactions of
substrates 4 proceeded smoothly in tetrahydrofuran (THF) to
afford the racemic product 3 in 80−92% yields. Unlike coupling
of iodoferrocenes 2, o-bromoaniline-derived substrates were also
compatible with the present protocol affording the target
products in good yields (3a, 3h, and 3m). Their structures
B
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Letter
with a methyl group in the other Cp ring was well tolerated in the
reaction (6k). Again, the structures of this class of
isoquinolinone-fused ferrocenes were completely validated by
single crystal X-ray diffraction analysis of compound 6d.19
A series of radical-trapping experiments were conducted for
better understanding of the mechanism of this C−H arylation
reaction. For amide 2a, the addition of 1 equiv of radical
scavenger (2,2,6,6-tetramethylpiperidine-1-oxyl, TEMPO)
caused the reaction to halt completely. Relatively, under the
same conditions intramolecular C−H arylation reactions of
substrates 4a and 5a still afforded the corresponding coupling
products 3a and 6a in a comparable yield (82% and 69%),
respectively.19 As a result, it could be inferred that intramolecular
direct C−H arylation of substrates 2 (Scheme 2) must involve a
radical pathway, while for reactions of substrates 4 and 5
(Scheme 3 and 5) a benzyne mechanism15a cannot be ruled out
strictly. The deuteration experiment on aromatic C−H arylation
of 2b was performed simultaneously.19 The observed kinetic
isotope effect (KIE) (kH/kD ≈ 1.25) also indicated that the
cleavage of aromatic C−H bonds did not involve the rate-
determining step.
Scheme 3. Intramolecular Direct Cp−H Arylation with o-
a
Haloanilines
a
Reaction conditions: 4 (0.2 mmol), Phen (20 mol %), KOtBu (4
c
equiv), THF (2 mL), 80 °C, 10 h, under Ar. Isolated yields. o-
Bromoanilines are used.
were assigned unambiguously by single crystal X-ray diffraction
analysis of compound 3j.19 Furthermore, a scale-up reaction for
substrate 4a was carried out on gram scale and product 3a was
obtained successfully in 73% yield, which demonstrates the
synthetic utility of this transformation (Scheme 4).
Moreover, when replacing the N-methyl group in the amide
3m by an allyl group, C−H arylation afforded an unexpected O-
arylation 2-ferrocenyl benzoxazole 8 in 91% yield (Scheme 6).
Scheme 6. Intramolecular O-Arylation Reaction
Scheme 4. Scale-up Reaction
To the best of our knowledge, the N-allyl group on the amides
could be reductively cleaved via photoactivated radical electron
transfer from an organic electron donor.20 The production of 8
implied strongly the presence of an organic electron donor in the
present C−H arylation process, which initiated an aromatic
radical from aryl halide via a SET process.21
Considering the electron-deficient property of the Cp rings in
the ferrocenocarboxamides 2 and 4, we also investigated
intramolecular direct C−H arylation of electron-rich amino-
ferrocene substrates 5 (Scheme 5). To our delight, under the
To compare the efficacy and determine the regioselectivity of
the C−H arylation product in substituted ferrocene substrates,
the parallel experiments of transition metal-free and Pd-catalyzed
intramolecular C−H arylation reactions for amide substrate 9,
derived from 2-methyl ferrocenocarboxylic acid, were further
carried out (Scheme 7). In both cases, compound 10 was isolated
as the single product.
Scheme 5. Intramolecular Direct Cp−H Arylation with
a
Aminoferrocenes
Scheme 7. Parallel Experiments of Transition Metal-Free and
Pd-Catalyzed Intramolecular C−H Arylation Reactions
a
Reaction conditions: 5 (0.2 mmol), Phen (20 mol %), KOtBu (4
On the basis of the above-mentioned experiments and
previous reports,15 transition metal-free intramolecular aromatic
C−H arylation reactions are considered to proceed via a
homolytic aromatic substitution (HAS)13a mechanism involving
ferrocenyl radicals (see Scheme S1 of SI file). Although radical-
trapping experiments on C−H arylation of substrates 4a and 5a
demonstrated that a benzyne mechanism may also be involved,
observation of product 3h in Scheme 3 approves undoubtedly
the radical pathway.
equiv), THF (2 mL), 80 °C, 10 h, under Ar. Isolated yields.
optimal conditions all o-iodobenzamides coupled smoothly with
aminoferrocenes to give the isoquinolinone-fused ferrocene
derivatives 6 in 62−82% yields. Remarkably, compared to 3- and
4-methoxy benzamides (6d and 6h), intramolecular coupling of
3,4-dimethoxy benzamide with aminoferrocene achieved the
desired product 6j in obviously decreased yield. The substrate
C
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Organic Letters
Letter
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In summary, we have developed a transition metal-free,
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Further study on the reaction mechanism of the metallocenic
radical C−H arylation is underway in our laboratory.
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ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b02995.
Experimental procedures and compound characterization
data (PDF)
Data for compound 3j (CIF)
Data for compound 6d (CIF)
Data for compound 8 (CIF)
AUTHOR INFORMATION
Corresponding Authors
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■
*E-mail: zjin@nankai.edu.cn
*E-mail: xiaohuaxu@nankai.edu.cn
ORCID
Zhong Jin: 0000-0001-5722-7175
Author Contributions
‡Y.L. and J.X. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The financial support from the National Natural Science
Foundation of China (NNSFC) (Nos. 21672116 and
21172111) is acknowledged.
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