Synthesis of fluorine-containing multisubstituted phenanthridines by rhodium-catalyzed alkyne [2+2+2] cycloaddition and tandem sp2 C-H difluoromethylenation

Article abstract of DOI:10.1002/chem.201300288

A highly efficient method for the synthesis of fluorine-containing multisubstituted phenanthridines through Rh-catalyzed alkyne [2+2+2] cycloaddition reactions has been developed. This method exhibits excellent functional-group compatibility. When a bromodifluoromethyl group, rather than a trifluoromethyl group, was employed in the cycloaddition reaction, more-complicated polycyclic compounds were obtained through tandem Rh-catalyzed cycloaddition/C-H difluoromethylenation. This route provides convenient access to fluorine-containing polycyclic compounds. [2+2+2] Little boys: Fluorine-containing multisubstituted phenanthridines have been synthesized through Rh-catalyzed alkyne [2+2+2] cycloaddition reactions (see scheme; FG=functional group). Polycyclic compounds were also obtained through Rh-catalyzed C-H difluoromethylenation. Copyright

Full text of DOI:10.1002/chem.201300288

DOI: 10.1002/chem.201300288
Synthesis of Fluorine-Containing Multisubstituted Phenanthridines by
2
ꢀ
Rhodium-Catalyzed Alkyne [2+2+2] Cycloaddition and Tandem sp C H
Difluoromethylenation
Yajun Li,[a, b] Jiangtao Zhu,[b] Lisi Zhang,[b] Yongming Wu,*[b] and Yuefa Gong*[a]
Abstract: A highly efficient method for the synthesis of fluorine-containing multi-
substituted phenanthridines through Rh-catalyzed alkyne [2+2+2] cycloaddition
reactions has been developed. This method exhibits excellent functional-group
Keywords: cycloaddition · fluorine ·
compatibility. When a bromodifluoromethyl group, rather than a trifluoromethyl
homogeneous catalysis · polycycles ·
group, was employed in the cycloaddition reaction, more-complicated polycyclic
rhodium
ꢀ
compounds were obtained through tandem Rh-catalyzed cycloaddition/C H di-
fluoromethylenation. This route provides convenient access to fluorine-containing
polycyclic compounds.
Introduction
Given these above-mentioned advantages, [2+2+2] cyclo-
addition reactions are not only useful for constructing
simple cyclic compounds but they are also a good method
for the assembly of more-complicated and challenging com-
pounds. For example, Tanaka et al. have reported the syn-
thesis of enantioenriched [9]helicene-like molecules through
double [2+2+2] cycloaddition reactions,[6] Shibata et al.
have reported the enantioselective synthesis of chiral tripo-
dal cage compounds through the [2+2+2] cycloaddition of
branched triynes,[7] and Witulski and Alayrac have reported
the synthesis of fused carbazoles by alkyne cyclotrimeriza-
tion reactions.[8]
Phenanthridines are important core structures in a large
class of compounds, including many significant natural prod-
ucts, biologically and therapeutically active compounds, and
functional materials.[9] Thus, numerous different strategies
for the construction of phenanthridines have been ex-
plored.[10] Meanwhile, the incorporation of fluorine moieties
into the phenanthridine motif could lead to great changes in
its properties, such as its conformation, solubility, lipophilici-
ty, metabolic stability, hydrogen-bonding ability, and chemi-
cal reactivity.[11] However, methods for introducing fluorine
atom(s) into phenanthridines are still scarce. Herein, we
report a new method for the synthesis of fluorine-containing
phenanthridines by using an alkyne [2+2+2] cycloaddition
strategy.
Since the seminal study by Reppe et al., transition-metal-
catalyzed alkyne [2+2+2] cycloaddition reactions have
become a powerful tool for the construction of substituted
aromatic and nonaromatic cyclic compounds.[1] Compared
with other remarkable reactions that are catalyzed by transi-
tion metals, such as cross-coupling reactions, these ring-
forming reactions could be able to accomplish the required
chemistry in a single step, rather than through tedious and
low-yielding multistep strategies.[2]
A large number of such cycloaddition reactions with a va-
riety of transition metals, such as Co, Ni, Pd, Rh, Ru, Ir, Fe,
Zr, Cr, Ti, etc., have been well-documented.[3] Under opti-
mized conditions, a broad range of substituted cyclic mole-
cules, such as benzenes, pyridines, pyridones, 1,3-cyclohexa-
dienes, pyrones, and pyrans, can be obtained from various
unsaturated substrates, such as alkynes, nitriles, isocyanates,
alkenes, carbon dioxide, aldehydes, and ketones.[4] More-
over, typically, no extra base or acid is required to facilitate
these reactions. Thus, a wide range of functional groups,
such as esters, ketones, amides, nitriles, alcohols, amines, hal-
ogens, and even sulfides, can be well-tolerated.[5]
[a] Y. Li, Prof. Dr. Y. Gong
School of Chemistry and Chemical Engineering
Huazhong University of Science and Technology
1037 Luoyu Road, Wuhan, Hubei 430074 (P.R. China)
E-mail: gongyf@mail.hust.edu.cn
Results and Discussion
[b] Y. Li, Dr. J. Zhu, L. Zhang, Prof. Dr. Y. Wu
Key Laboratory of Organofluorine Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Science
Based on our recent work on the synthesis of fluorine-con-
taining alkynylimines,[12] we envisioned a strategy that in-
volved [2+2+2] cycloaddition reactions between diynes 3 or
5 and various alkynes for the synthesis of fluorine-contain-
ing phenanthridines (Scheme 1). Diynes 3 and 5 were pre-
pared in three steps starting from commercially available 2-
345 Lingling Road, Shanghai 200032 (P.R. China)
E-mail: ymwu@sioc.ac.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300288.
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FULL PAPER
was rather slow and required 12 h to obtain high conversion
(Table 1, entry 2).[14] The catalyst loading seemed to have
minimal influence on the reaction yield (Table 1, entries 1,
3, 6, and 7) and the reaction could go completion within 8 h,
even in the presence of 1 mol% catalyst (Table 1, entry 7).
Polar solvents, such as DMF and THF, were also appropriate
solvents for this transformation (Table 1, entries 4 and 5). Fi-
nally, the optimized conditions were ascertained to be
Scheme 1. Strategy for the A!ABC ring formation.
5 mol% [RhCl
atmosphere.
ACHTUNGTERN(NUNG PPh3)3] in toluene at 908C under a nitrogen
iodoaniline (Scheme 2). After Sonogashira reactions be-
tween terminal alkynes and 2-iodoaniline, intermediates 1
were treated with trifluoroacetic acid or bromodifluoroace-
tic acid under certain conditions to form intermediates 2.[13]
Then, intermediates 2 underwent Sonogashira reactions to
give diynes 3 and 5 in good overall yields.[12b]
To investigate the scope and limitations of this method,
we treated various diynes with 3-hexyne as the electrophile
partner. When R1 was a phenyl ring and R4 represented an-
other aromatic moiety, both electron-donating groups and
electron-withdrawing groups on the aromatic ring had a neg-
ligible effect on the transformations, thus giving their corre-
sponding products in excellent yields (Table 2). These data
Table 2. Substrate scope of the [2+2+2] cycloaddition reaction.[a]
R1
R4
R2 and R3
Product
Yield
[%][b]
1
2
3
4
5
6
7
8
9
10
11
12[c]
13
14
15
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Bu
Bu
H
Ph
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Et
Ph
CH2OH
CO2Me
4a
4b
4c
4d
4e
4 f
4g
4h
4i
4j
4k
4l
4m
4n
4o
96
88
85
90
92
85
96
89
92
88
72
48
95
98
37
Scheme 2. Synthesis of cycloaddition precursors 3 and 5.
p-OMeC6H4
p-ClC6H4
p-FC6H4
o-ClC6H4
p-NO2C6H4
2-thienyl
tBu
Bu
Ph
Bu
Ph
Initial studies were focused on the Rh-catalyzed [2+2+2]
cycloaddition reaction between diyne 3a and 3-hexyne. The
reaction was optimized with respect to the solvent, tempera-
ture, time, and catalyst loading. As shown in Table 1, in the
presence of 10 mol% [RhClACHTNUGTRNEUNG(PPh3)3], this reaction was
smoothly completed in toluene at 908C within 1 h (Table 1,
entry 1). However, when performed at 608C, the reaction
Ph
Ph
Ph
Ph
Ph
Ph
Table 1. Optimization of the [2+2+2] cycloaddition reaction catalyzed
by Wilkinsonꢁs catalyst.[a]
[a] General conditions: Diyne 3, alkyne (2 equiv), [RhClACHTUNGTRENNUNG(PPh3)3]
(5 mol%), toluene, 908C. [b] Yield of isolated product. [c] The R1 group
in starting material 3l was a trimethylsilyl group.
indicate that this reaction has good functional-group toler-
ance (Table 2, entries 1–7). In addition, when R4 was an ali-
phatic group, the reaction could also proceed smoothly, thus
offering the corresponding phenanthridines in about 90%
yield (Table 2, entries 8 and 9). When both R1 and R4 were
aliphatic chains, the yields of the corresponding products de-
creased slightly (Table 2, entries 10 and 11). When R1 was a
trimethylsilyl group, the corresponding cycloaddition prod-
uct could also be obtained, albeit as the desilylated product
in a relatively low yield (Table 2, entry 12). Finally, we fo-
cused our attention on the electrophile partner. Whereas
electron-neutral alkynes, such as diphenylacetylene (Table 2,
entry 13), and electron-rich alkynes, such as 2-butyne-1,4-
Catalyst
[mol%]
Solvent
T
[8C]
t
Yield[b]
[%]
N
[h]
1
2
3
4
5
6
7
10
10
5
5
5
toluene
toluene
toluene
DMF
THF
toluene
toluene
90
60
90
90
70
90
90
1
12
2
92
93
96
90
89
92
93
2
2
5
8
2.5
1
[a] General conditions: Diyne 3a, 3-hexyne (2 equiv), [RhClACTHUNTRGNE(UNG PPh3)3], sol-
vent. [b] Yield of isolated product.
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Y. Wu, Y. Gong et al.
diol (Table 2, entry 14), efficiently underwent the cycloaddi-
tion reaction, electron-deficient alkynes, such as dimethyl
acetylenedicarboxylate (DMAD), were not good electro-
philes for this transformation (Table 2, entry 15).[15]
Regioselectivity is an important issue in the reactions be-
tween diynes and asymmetric alkynes.[16] To test the regiose-
lectivity of this catalytic system, the reaction between com-
pound 3j and 3-3-dimethyl-1-butyne was carried out. In
agreement with previous reports, excellent regioselectivity
(>95:5) was observed when bulky groups were involved
[Eq. (1)].[16]
thridines (7) from diynes (5) as the starting substrates. The
reaction conditions were then optimized. First, we mixed
substrate 5a with 3-hexyne, 5 mol% [RhClACTHNUTRGNE(UNG PPh3)3], and
1 equivalent of Ag2CO3 in toluene or 1,4-dioxane at 908C or
1208C (Table 3). However, these reactions gave compound
6a as the major product (Table 3, entry 1–3). Because the
alkyne [2+2+2] cycloaddition reaction didnꢁt require the as-
sistance of Ag2CO3, Ag2CO3 was added as an additive after
the completion of the alkyne [2+2+2] cycloaddition reac-
tion. Unfortunately, a similar result was obtained (Table 3,
entry 4). These results imply that the catalyst might lose its
activity during the reaction.
Thus, another portion of the
catalyst was added to the reac-
tion system along with the
silver reagent during the
second stage. Gratifyingly, the
reaction exclusively afforded
the desired product (7a) in
Table 3. Optimization of the cascade reaction catalyzed by Wilkinsonꢁs
catalyst.
To our surprise, when the trifluoromethyl group on the
diynes was replaced by a bromodifluoromethyl group, the
reaction of diyne 5a with 3-hexyne not only gave the desired
product (6a), but also afforded compound 7a [Eq. (2)], as
confirmed by X-ray crystallography (Figure 1).[17] This unex-
pected result implies a facile pathway for the construction of
more-complicated polycyclic compounds.
Conditions
Yield 6a Yield 7a
[%]
[%]
1
2
3
4
U
U
80
76
73
66
16
18
19
22
AHCTUNGTRENNUNG
5
6
7
1) [RhCl
2) [RhCl
1) [RhCl
2) [RhCl
1) [RhCl
2) [RhCl
N
0
0
0
75
79
90 (85)[a]
[a] Yield of the isolated product is given in parentheses.
Figure 1. Single-crystal X-ray structure ORTEP diagram of compound
7a. Hydrogen atoms have been omitted for clarity and the thermal ellip-
soids are set at the 30% probability level.
75% yield (Table 3, entry 5). Finally, we identified the opti-
mal reaction conditions after several attempts. During the
first stage, the diynes reacted with 3-hexyne in toluene at
Recently, we have reported a Rh-catalyzed intramolecular
coupling reaction that involved
a
bromodifluoromethyl
908C in the presence of 5 mol% [RhCl
termediates 6. During the second stage, 1 equivalent of
Ag2CO3, a second portion of [RhCl(PPh3)3] (5 mol%), and
ACHTUNGTRENN(NUG PPh3)3] to form in-
group.[18] Encouraged by these above-mentioned results, we
became interested in developing a tandem cycloaddition/
AHCTUNGTRENNUNG
ꢀ
C H functionalization strategy to directly construct phenan-
1,4-dioxane were added to the reaction mixture. After 4 h at
1158C, the reaction gave prod-
uct 7a in 90% yield (Table 3,
entry 7).
As summarized in Scheme 3,
a
range of phenanthridines
that contained electron-donat-
ing or electron-withdrawing
groups were formed in moder-
ate-to-high yields. These reac-
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Synthesis of Fluorine-Containing Phenanthridines
FULL PAPER
Scheme 3. One-pot synthesis of polycyclic compounds 7.
tions are compatible with a number of functional groups, in-
cluding halogen (7d and 7i), ester (7g), ketone (7h), and
even cyano groups (7e). It is well-known that the cyano
group could participate in [2+2+2] cycloaddition reactions,
which would disturb the outcome of the product.[3,5] Howev-
er, we didnꢁt identify any other product apart from the de-
sired product, which was obtained in moderate yield. Diynes
that were connected with an aliphatic chain or a hydrogen
atom could also give the corresponding products (7j and
7k) in moderate yields. Moreover, other alkynes, such as di-
phenylacetylene, could also be used as the reaction partner
(7l).
Based on our preliminary data, we propose a plausible re-
action mechanism, as shown in Scheme 4. Cycle I shows the
Scheme 4. Proposed mechanism for the formation of phenanthridines 4 and 7.
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Y. Wu, Y. Gong et al.
mechanism of the alkyne [2+2+2] cycloaddition reaction
[1] a) W. Reppe, O. Schichting, K. Klager, T. Toepel, Justus Liebigs
Ann. Chem. 1948, 560, 1–92; b) W. Reppe, W. J. Schweckendiek,
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ꢀ
and cycle II shows the mechanism of the C H difluoro-
A
with RhI to form intermediate B, which then coordinates
with the alkyne to give intermediate C. Then, addition or in-
sertion of the alkyne to metallacycle C occurs to give inter-
mediates D, E, or F. The phenanthridine product is formed
upon reductive elimination of the metal. When the bromodi-
fluoromethyl group, rather than the trifluoromethyl group,
is involved in the reaction, according to our previous re-
sults,[18] intermediate G (cycle II) undergoes oxidative addi-
tion to form intermediate H. This latter species undergoes
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In conclusion, we have developed a highly efficient method
for the construction of fluorine-containing multisubstituted
phenanthridines through Rh-catalyzed diyne [2+2+2] cyclo-
addition reactions. This new method offers the mild and
direct synthesis of multisubstituted phenanthridines with
good functional-group tolerance. Moreover, excellent regio-
selectivity in this reaction was obtained when bulky groups
were employed. With substrates that contained bromodi-
fluoromethyl groups, more-complicated polycyclic com-
pounds were obtained through a tandem Rh-catalyzed cy-
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ꢀ
cloaddition/C H difluoromethylenation process, which
might be useful in constructing polycyclic fluorine-contain-
ing compounds.
Experimental Section
General procedure for the synthesis of compound 4: Diyne 3 (0.2 mmol),
the alkyne (2 equiv), and Wilkinsonꢁs catalyst were stirred together in tol-
uene (2 mL) at 908C under a nitrogen atmosphere. After the reaction
was complete (by TLC), the volatile compounds were removed in vacuo
and the crude residue was purified by column chromatography on silica
gel (EtOAc/petroleum ether) to give the product.
General procedure for synthesis of compound 7: Diyne 5 (0.2 mmol), 3-
hexyne (2 equiv), and Wilkinsonꢁs catalyst were stirred together in tol-
uene (2 mL) at 908C under a nitrogen atmosphere. After the reaction
was complete (by TLC), Ag2CO3 (1 equiv) and a second portion of
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gen atmosphere, followed by 1,4-dioxane. Then, the mixture was stirred
at 1158C for about 4 h. After the reaction was complete (by TLC), the
volatile compounds were removed in vacuo and the crude residue was
purified by column chromatography on silica gel (EtOAc/petroleum
ether) to give the product.
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[2+2+2] cycloaddition reactions involving dimethyl acetylenedicar-
Acknowledgements
We thank the National Science Foundation of China (21172239) for
financial support.
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Synthesis of Fluorine-Containing Phenanthridines
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Received: January 25, 2013
Published online: April 24, 2013
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8299