Rhodium(III)-catalyzed directed peri -C-H alkenylation of anthracene derivatives

Article abstract of DOI:10.1021/ol501926b

Rhodium(III)-catalyzed oxidative coupling reactions of anthracene-9- carboxylic acid derivatives with electron-deficient olefins are reported. A cationic rhodium(III) catalyst, in combination with a copper(II) oxidant, promotes selective monoalkenylation

Full text of DOI:10.1021/ol501926b

Letter
pubs.acs.org/OrgLett
Rhodium(III)-Catalyzed Directed peri-C−H Alkenylation of
Anthracene Derivatives
Jaganathan Karthikeyan and Naohiko Yoshikai*
Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University,
Singapore 637371, Singapore
S
* Supporting Information
ABSTRACT: Rhodium(III)-catalyzed oxidative coupling re-
actions of anthracene-9-carboxylic acid derivatives with
electron-deficient olefins are reported. A cationic rhodium(III)
catalyst, in combination with a copper(II) oxidant, promotes
selective monoalkenylation of anthracene-9-carboxamide,
affording 1-alkenylanthracene-9-carboxamide in moderate to
good yields. A similar catalytic system also promotes the
reaction of anthracene-9-carboxylic acid and an electron-deficient olefin, which affords a lactone derivative through C−H
alkenylation followed by intramolecular conjugate addition.
he past few decades have witnessed significant develop-
ment in the transition-metal-catalyzed, directing-group-
Scheme 1. Rhodium(III)-Catalyzed Directed Oxidative C−H
Alkenylation with Olefins
T
assisted functionalization of aromatic compounds.1 With the
extensive development of transition metal catalysts, heteroatom
directing groups, and carbon- and heteroatom-based coupling
partners, this strategy now offers a myriad of methods for the
conversion of aromatic C−H bonds into C−C and C−
heteroatom bonds, some of which have found applications in
pharmaceuticals and other functional molecules.2 Given this
background as well as the importance of polycyclic aromatic
hydrocarbons (PAHs) in organic functional materials,3 it was
thought somewhat strange that such polyaromatic compounds
have not been frequently employed as platforms in directed C−
H functionalization.4
In the past several years, there has been explosive
development in rhodium(III)-catalyzed directed C−H func-
tionalization reactions of aromatic compounds under oxidative
and nonoxidative conditions.5 With respect to oxidative ortho-
alkenylation with olefins, rhodium(III) catalysis has allowed the
use of benzoic acids,6 acetanilides,7 acetophenones,8 benza-
mides,8,9 benzoic esters,10 aryl oxime ethers,11 phenol
carbamate,12 and many others13 as substrates, which form
five- or six-membered rhodacycles as the key intermediates
(Scheme 1, eq 1). On the other hand, polyaromatic substrates
for directed peri-functionalization have been mostly limited to
1-naphthol and 1-aminonaphthalene derivatives that can form a
five-membered rhodacycle upon peri-C−H activation,14 and
other 1-functionalized naphthalene derivatives typically under-
go C−H activation at the ortho-position rather than the peri-
position. To our knowledge, functionalized anthracenes and
higher acenes, including readily available anthracene-9-carbox-
ylic acid and its derivatives, have been rarely used as substrates
for the rhodium(III) catalysis.15 With our recent development
of a synthetic route to anthracene derivatives that features
cobalt-catalyzed branch-selective addition of aromatic imines to
styrenes,16,17 we became interested in the possibility of directed
and nondirected C−H functionalization of such PAH
derivatives. Here, we report on a rhodium(III)-catalyzed
oxidative coupling reaction of an anthracene-9-carboxamide
and an electron-deficient olefin, which results in selective
monoalkenylation of the proximal peri-position of the
anthracene core (Scheme 1, eq 2).
Our effort was initially focused on the alkenylation of
anthracen-9-yl(pyrrolidin-1-yl)methanone 1a with ethyl acryl-
ate 2a (Table 1). Upon screening various reaction conditions,
we managed to promote the desired reaction using [Cp*Rh-
(MeCN)3](SbF6)2 (5 mol %), Cu(OAc)2·H2O (2 equiv), and
Received: July 3, 2014
Published: July 28, 2014
© 2014 American Chemical Society
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dx.doi.org/10.1021/ol501926b | Org. Lett. 2014, 16, 4224−4227

Organic Letters
Letter
a
a
Table 1. Screening of Reaction Conditions
Table 2. Scope of peri-C−H Alkenylation
b
entry
deviation from standard conditions
none
yield (%)
b
entry
R1
R2
R3
3, yield (%)
c
1
2
3
4
5
6
7
8
72
c
d
1
pyrrolidinyl
pyrrolidinyl
pyrrolidinyl
pyrrolidinyl
pyrrolidinyl
pyrrolidinyl
pyrrolidinyl
pyrrolidinyl
piperidinyl
morpholinyl
NMe2
H
H
H
H
H
H
H
H
H
H
H
H
H
Ph
CO2Et
CO2Me
CO2Bu
CO2t-Bu
CO2Bn
SO2Ph
CN
3aa, 72
3ab, 80
3ac, 74
3ad, 64
3ae, 61
3af, 36
0
[Cp*RhCl2]2/AgSbF6 as the catalyst
PhCl instead of DCE
49
65
36
24
5
2
3
t-AmylOH instead of DCE
1,4-Dioxane instead of DCE
DMF instead of DCE
4
5
6
AgOAc instead of Cu(OAc)2·H2O
Ag2CO3 instead of Cu(OAc)2·H2O
49
35
7
8
Ph
0
a
b
The reaction was performed on a 0.2 mmol scale. Determined by 1H
c
9
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
CO2Me
3bb, 66
3cb, 61
3db, 48
3eb, 42
3fb, 48
3gb, 70
3hb, 71
3ib, 64
3jb, 64
NMR using mesitylene as an internal standard. Isolated yield.
d
10
11
12
13
14
15
16
17
18
[Cp*RhCl2]2 (2.5 mol %) and AgSbF6 (10 mol %) were used.
NEt2
1,2-dichloroethane (DCE) as the catalyst, oxidant, and solvent,
respectively, which afforded the peri-monoalkenylated product
3aa in 72% yield (Table 1, entry 1). The use of a combination
of Cp*RhCl2 dimer and AgSbF6 instead of the cationic
rhodium catalyst resulted in a lower yield (Table 1, entry 2).
While a comparable result was obtained using PhCl instead of
DCE (Table 1, entry 3), more polar and coordinating solvents
such as t-AmylOH, 1,4-dioxane, and DMF were detrimental to
the reaction (Table 1, entries 4−6). Replacement of the Cu(II)
oxidant to a Ag(I) oxidant such as AgOAc and Ag2CO3 led to
lower yields (Table 1, entries 7 and 8).
NHiPr
pyrrolidinyl
pyrrolidinyl
pyrrolidinyl
pyrrolidinyl
pyrrolidinyl
m-Tol
PhCC
morpholinyl
Br
CO2Me
3kb, 56
a
b
c
The reaction was performed on a 0.2 mmol scale. Isolated yield. 2.5
equiv of ethyl acrylate were used.
As briefly mentioned previously, the combination of cobalt-
catalyzed branch-selective addition of aryl aldimine to styrene
and subsequent indium-catalyzed dehydrative cyclization allows
for the facile synthesis of methylated anthracene derivatives,
such as 7-methyltetraphene 6 (Scheme 2).16a The methyl group
Under the conditions in entry 1, the unreacted starting
material 1a was mostly recovered and no identifiable byproduct
was detected. Further modifications of the reaction parameters
(e.g., catalyst loading, amount of 2a, temperature) have not
allowed us to achieve significant improvement of the
conversion of 1a and the yield of 3aa. It should be noted
that, when using 10 equiv of 2a, we observed the formation of a
small amount of a peri-dialkenylation product (<5% yield
a
Scheme 2. Alkenylation of Tetraphene-7-carboxamide
1
according to H NMR analysis).
With the optimized reaction conditions in hand, we next
explored the scope of anthracene amides and olefins for the
present peri-alkenylation reaction (Table 2). The reactions of
1a with acrylates proceeded smoothly regardless of the alkyl
group, affording the corresponding alkenylation products 3aa−
3ae in yields of 61%−80% (Table 2, entries 1−5), while phenyl
vinyl sulfone reacted more sluggishly (Table 2, entry 6).
Acrylonitrile and styrene failed to participate in the reaction
(Table 2, entries 7 and 8). In the coupling with methyl acrylate
2b, pyrrolidine-derived 1a afforded the highest yield among the
tertiary amides examined (Table 2, entry 1), followed by those
derived from other cyclic amines (Table 2, entries 9 and 10)
and then by N,N-dimethyl- and N,N-diethylamides (Table 2,
entries 11 and 12). N-Isopropylamide also underwent the peri-
alkenylation reaction with 2b to afford the desired product 3fb
in a moderate yield of 48% (Table 2, entry 13). Note that, in
this case, intramolecular cyclization of the amide group and the
acrylate moiety was not observed. Anthracene-9-carboxamides
bearing extra substituents such as aryl, alkynyl, amino, and
bromo groups on the 10-position all participated in the reaction
with 2b, affording the corresponding alkenylation products
3gb−3kb in reasonable yields (Table 2, entries 14−18).
a
See the Supporting Information for the conversion of 6 to 1l.
of 6 was then converted to a tertiary amide group in a few steps,
thus affording unsymmetrical tetraphene-7-carboxamide 1l.
Because both of the peri-positions of 1l are sterically
unhindered, not unexpectedly, the rhodium(III)-catalyzed
reaction of 1l and 2b afforded a mixture of regioisomers 3lb
and 3lb′ in an approximate ratio of 1:1.
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Organic Letters
Letter
Oxidative peri-functionalization of parent anthracene-9-
carboxylic acid 7 was also achieved (Scheme 3). Thus, the
Table 3. Photophysical Properties of Selected Alkenylated
Anthracenes
a
b
c
entry 1 or 3
λmax/nm
log ε
λem/nm
ΦF
Scheme 3. peri-C−H Alkenylation/Cyclization of
a
1
2
3
4
5
6
1a
347, 366, 385 4.18, 4.36, 4.33 389, 412, 436
0.18
0.42
0.44
0.44
0.55
0.028
Anthracene-9-carboxylic Acid
3ab
3af
3gb
3ib
3jb
391
4.24
473
485
484
487
467
394
4.08
401
4.15
417, 438
409
4.52, 4.51
4.13
a
b
In CH2Cl2. Excited at the longest absorption maxima wavelength
c
except for 1a (366 nm) and 3ib (417 nm). Determined with
anthracene as a standard.
a
The reaction was performed on a 0.2 mmol scale.
its simple derivatives (Table 3, entry 1). Such fine structures
disappeared in most of the peri-alkenylated derivatives (Table 3,
entries 2−6), except for the 10-alkynyl derivative 3ib which
showed two absorption maxima (Table 3, entry 5). Expectedly,
the peri-alkenyl group of 3ab and 3af caused a substantial red
shift of λmax and λem compared to 1a (Table 3, entries 2 and 3),
while further influence of the 10-substituent (3gb, 3ib, 3jb) was
relatively small (Table 3, entries 4−6). The peri-alkenylated
derivatives showed light blue-green fluorescence (Table 3,
entries 2−5), with the exception of 10-morpholino derivative
3jb which was almost nonfluorescent presumably due to excited
state intramolecular electron transfer (Table 3, entry 6).
In summary, we have demonstrated that a rhodium(III)
catalyst is capable of catalyzing peri-C−H alkenylation of
anthracene-9-carboxamides and anthracene-9-carboxylic acid
with electron-deficient alkenes under oxidative conditions.
Further improvement of the catalytic activity and exploration of
other carbon- and heteroatom-based coupling partners may
lead to versatile methods for the functionalization of anthracene
and higher acenes, producing functionalized polyaromatic
compounds that are not readily accessible by conventional
methods.
reactions of 7 and electron-deficient olefins proceeded under
modified reaction conditions employing CsOAc as an additive
and tert-amyl alcohol instead of DCE as a solvent, thus
affording lactone derivatives 8b, 8g, and 8i in good yields,
presumably through C−H alkenylation followed by intra-
molecular conjugate addition.6 Note that the reaction of 7 and
2b in the absence of CsOAc afforded 8b in a lower yield of
70%. We speculate that CsOAc acts as a base to facilitate the
formation of rhodium(III) anthracene-9-carboxylate, which
would be an intermediate prior to C−H activation.18
While our attempts to characterize a reaction intermediate
have not been successful, we assume that the present peri-
alkenylation reaction involves a catalytic cycle that is essentially
the same as those proposed for related ortho-alkenylation
reactions (Scheme 4).5 Thus, the rhodium(III) catalyst
Scheme 4. Plausible Catalytic Cycle
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details and characterization of new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: nyoshikai@ntu.edu.sg.
Notes
undergoes peri-C−H activation of the anthracene substrate 1
to form a rhodacycle A with concomitant deprotonation.
Insertion of olefin 2 into the C−Rh bond of A gives an
alkylrhodium intermediate B, which may or may not retain the
chelation of the amide oxygen atom. Subsequent β-hydride
elimination results in the formation of the product 3 and
reduction of rhodium(III) to rhodium(I). Reoxidation of the
rhodium(I) species with the copper(II) oxidant regenerates the
active rhodium(III) catalyst.
Table 3 summarizes the UV−visible absorption and
fluorescence properties of 1a and selected peri-alkenylated
anthracene derivatives. The starting material 1a showed
multiple absorption and emission peaks reflecting the vibra-
tional fine structure, which is characteristic to anthracene and
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Singapore National Research
Foundation (NRF-RF2009-05) and Nanyang Technological
University.
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