Selectivity manipulation of bicyclization reactions of 1,5-Bis(1,2- allenylketone)s: Pd versus Rh and electronic effect

Article abstract of DOI:10.1002/chem.201003546

Select Rh! The rhodium(I)-catalyzed double cyclization of 1,5-bis(1,2-allenylketone)s gives TNF-α inhibitor (A)-type skeletons at room temperature in acetone or 1,2-dichloroethane (DCE, see scheme; Ts=Tosyl). Compared with [PdCl₂(MeCN)₂

Full text of DOI:10.1002/chem.201003546

DOI: 10.1002/chem.201003546
Selectivity Manipulation of Bicyclization Reactions of 1,5-Bis(1,2-
allenylketone)s: Pd versus Rh and Electronic Effect
Youqian Deng, Chunling Fu, and Shengming Ma*[a]
In memory of S. Maꢀs father, Mr. Xinwei Ma, who passed away last September
Recently, much attention has been paid to the synthesis of
furan-fused bicyclic compounds[1–4] due to their importance
in medicinal chemistry. For example, it has been reported
that the furan-fused cycloheptanes, such as TNF-a inhibitors
(Scheme 1A)[5] and a-adrenergic receptor antagonists
(Scheme 1B),[6] are a class of compounds with important
pharmacological activities. It should be noted that the pres-
ence of the nitrogen atom in the seven-membered ring is
crucial for the biological activity of this type of compounds.
Thus, it is highly desirable to develop efficient methods that
may include different carbon or heteroatom units into the
seven-membered ring for further biological screening.
pared efficiently from a PdII-catalyzed cyclization reaction
of 1,5-bis(1,2-allenylketone)s, providing that the second al-
lenyl ketone unit would serve as the Michael acceptor, in-
stead of the b-heteroatom (LG) elimination in Equa-
tion (1).[10] If this is viable, the carbon or heteroatom unit,
which used as the tether to link the two allenyl ketone units,
would be easily assembled into the seven-membered ring
(Scheme 2). However, based on Hashmiꢀs early observa-
Scheme 1. Furan-fused bicyclic or tricyclic compounds with pharmacolog-
ical activities.
On the other hand, recently, we have developed a palla-
dium(II)-catalyzed tandem bicyclization reaction of symmet-
rical 1,w-bisallenols to form 2,5-dihydrofuran-fused bicyclic
skeletons, in which the second OH moiety in the second
allene unit serves as the leaving group after intramolecular
oxypalladation-carbopalladation to finally form the two exo-
cyclic C=C bonds [Eq. (1)].[7]
Scheme 2. Possible products of the cyclization of bis(allenylketone)s 1, il-
lustrating the selectivity problems (X=carbon or heteroatom).
The transition-metal-catalyzed monocyclization of simple
1,2-allenyl ketones has been well studied by Marshall,
Hashmi, Gevorgyan, and our group.[8,9] With this in mind,
we reasoned that furan-fused cycloheptenes would be pre-
tion,[10] we envisioned that there are three major challenges
of this strategy: 1) the issue of selectivity for the location of
the nonaromatic C=C bond in the products (forming prod-
ucts 2 and/or 3); 2) the control of selectivity for the unsym-
metrical substrates (R1 versus R2 in products 2 and 3); and
in addition, 3) other possible products would be dumb-bell-
type bisfurans 4[8a,9,10] and tricyclic products 5 shown in
Scheme 2. Herein we wish to report a rhodium(I)-catalyzed
double cyclization of 1,5-bis(1,2-allenylketone)s that afford-
ed diversified furan-fused bicyclic products 2 with a very
high selectivity.
[a] Dr. Y. Deng, Prof. Dr. C. Fu, Prof. Dr. S. Ma
Laboratory of Molecular Recognition and Synthesis
Department of Chemistry, Zhejiang University
Hangzhou 310027, Zhejiang (P. R. China)
Fax : (+86)21-64167510
E-mail: masm@sioc.ac.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003546.
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COMMUNICATION
Table 1. Solvent effects on the PdII-catalyzed cyclization reaction of
Table 2. Catalyst effects on the RhI-catalyzed cyclization reaction of
1,5-bis(1,2-allenylketone)s 1a (Ts=tosyl).
1,5-bis(1,2-allenylketone)s 1a.
Catalyst (5 mol%)
[RhCl(PPh3)3]
[RhCl(CO)(PPh3)2]
[RhCl(COD)]2
[RhCl(CO)2]2
[RhCl(CO)2]2
T
2a/3a
NMR yield of 2a [%]
Solvent
NMR yield of (2a+3a) [%][a]
16 (69/31)[b]
21 (89/19)[b]
53 (83/17)[b]
trace
1
2
3
4
5
N
U
reflux
reflux
reflux
reflux
RT
–
–
95:5
>99:1
>99:1
0[a]
1
2
3
4
5
6
7
DMF
DMA
R
0[b]
52
85
DMSO
CH3NO2
THF
CH3CN
acetone
83 (80)[c]
26 (85/15)[b]
68 (90:10)[b]
54 (83:17)[b,c]
[a] 95% of 1a was recovered. [b] 71% of 1a was recovered. [c] Isolated
yield.
[a] Determined by 1H NMR analysis with 1,3,5-trimethylbenzene as the
internal standard. [b] NMR ratio of 2a/3a. [c] The reaction was conduct-
ed at RT.
reaction temperature showed that the reaction may proceed
smoothly even at RT, which was defined as the standard
conditions (Table 2, entry 5). The double-cycloisomerization
product 4a and triple-cyclization product 5a (Scheme 2)
Initially, we tested the reaction of N-tethered 1,5-bis(1,2-
allenylketone)s 1a catalyzed by 5 mol% of [PdCl2ACTHNUTRGNEN(UG MeCN)2]
in different solvents (Table 1).[7] Although the results in
DMF, dimethylamide (DMA), DMSO, CH3NO2, and THF
are poor (Table 1, entries 1–5), the reaction in CH3CN pro-
were not observed even under the catalysis of [RhClACHTUNGTRENNUNG(PPh3)3]
(Table 2, entry 1).[8a] The structure of 2a was further estab-
lished by X-ray diffraction studies (Figure 1).[11] Here, a
liable ligand is clearly required to allow the cyclization,
which should involve the coordination of the C=C bonds
with the metal. As a comparison, under this set of new con-
ceeded smoothly to afford furoACTHNUTRGNEUGN[3,4-c]azepine derivative 2a
together with the C=C bond regioisomer 3a with a ratio of
90:10 in 68% combined yield (Table 1, entry 6).
To improve the selectivity of 2a/3a, we tried many reac-
tion parameters; however, the results were not improved. It
is even difficult to convert 3a into 2a, by adding a base or
an acid, without a big loss in yield. In addition, the scope of
this Pd-catalyzed reaction is extremely limited, because no
reaction occurred with 1 f and 1g, which had different
carbon tethers (Scheme 3).
ditions with [PdCl2ACHTGUNTERN(NUG MeCN)2] as the catalyst, the reaction
still afforded a mixture (Table 1, entry 7).
Scheme 3. The very limited scope of the PdII-catalyzed protocol.
Thus, we started to check the possible use of RhI catalysts
Figure 1. ORTEP representation of 2a.
to address the issue of regioselectivity (2a versus 3a) and
the scope of the reaction. Disappointingly, [RhCl
and [RhCl(CO)(PPh3)2] failed to afford the expected prod-
uct 2a in acetone (Table 2, entries 1 and 2). But luckily and
as expected, when [RhCl(COD)2]2 was used, 2a was formed
in 52% NMR yield along with 3a in a higher selectivity of
95:5 (2a/3a, Table 2, entry 3). Furthermore, the selectivity
was improved to >99:1 when the reaction was conducted in
acetone under reflux with [RhCl(CO)2]2 as the catalyst to
give a yield of 85% (Table 2, entry 4). Finally, screening the
(PPh3)3][8a]
ACHTUNGTRENNUNG
G
With this set of optimized reaction conditions in hand, the
scope of the cyclization reactions was demonstrated and
some typical results are summarized in Table 3. The reaction
of 1,5-bis(1,2-allenylketone)s 1a, 1b, 1c, and 1d with NTs as
AHCTUNGTRENNUNG
the tether yielded furoACTHNUGRTENUNG[3,4-c]azepines 2a, 2b, 2c, and 2d in
good yields (Table 3, entries 1–4). The sulfone tether may
also be introduced (Table 3, entry 5). Fortunately, the
1,5-bis(1,2-allenylketone)s 1 f and 1g, with a carbon tether,
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S. Ma et al.
Table 3. Substrate scope for the cyclization reaction of 1,5-bis(1,2-allenyl-
ketone)s 1.
X
R
Isolated yield of 2 [%]
1
2
3
4
NTs
NTs
NTs
NTs
SO2
C2H5 (1a)
CH3 (1b)
i-C3H7 (1c)
n-C5H11 (1d)
C2H5 (1e)
n-C6H13 (1 f)
C2H5 (1g)
i-C3H7 (1h)
n-C4H9 (1i)
Ph (1j)
80 (2a)
84 (2b)
77 (2c)
75 (2d)
76 (2e)
77 (2 f)
65 (2g)
72 (2h)
66 (2i)
44 (2j)
66 (2j)
Scheme 4. The reaction of the oxygen-tethered 1k.
5
6[a]
7
CACHTUNGTRENNUNG(CO2Me)2
Table 4. Reactions of aromatic–aliphatic-mixed 1,5-bis(1,2-allenyl ke-
tone)s 1.
CH2
CH2
CH2
CH2
CH2
8[b]
9
10[c]
11[c]
Ph (1j)
[a] The reaction was conducted under reflux. [b] The reaction time is 2 h.
[c] The reaction was conducted in 1,2-dichloroethane (DCE) at 708C for
1 h.
Aryl
Alkyl
t
Yield of
Yield of
[min]
2 [%][a]
2’ [%][a]
1
2
3
4
Ph
nBu (1l)
Bn[c] (1m)
nBu (1n)
iPr (1o)
20
60
20
20
74 (2l)
55 (2m)
77 (2n)
60 (2o)
8 (2l’)
fails to proceed under the catalysis of [PdCl2ACTHNUTRGNEN(UG MeCN)2]
p-O2NC6H4
p-O2NC6H4
p-O2NC6H4
3[b] (2m’)
0 (2n’)
0 (2o’)
(Scheme 2), but react smoothly under the catalysis of
[RhCl(CO)2]2 (Table 3, entries 6 and 7), although a higher
reaction temperature was required when 1,5-bis(1,2-allenyl-
ketone) 1 f, with a malonate tether, was used (Table 3,
entry 6). The seven-membered carbon ring seems to be pre-
ferred, since 1,5-bis(1,2-allenylketone)s 1h and 1i with a
CH2 tether under the standard conditions provide the fused
[5.3.0]bicyclic products 2h and 2i in good yields (Table 3,
entries 8 and 9). By applying this protocol, the reaction of
phenyl-substituted 1,5-bis(1,2-allenylketone) 1j afforded
[a] Isolated yield. [b] NMR yield. [c] Bn=benzyl.
1
product 2j in 44% yield (Table 3, entry 10). H NMR spec-
tra of the crude products indicated the formation of only
one product.
However, the reaction of 1,5-bis(1,2-allenylketone)s 1k,
with the oxygen tether, failed to afford the expected product
2k effectively and selectively under the standard conditions.
Further screening led to the observation that the fused
[5.3.0]bicyclic product 2k was formed smoothly in 58% iso-
lated yield when the reaction was conducted in DCE at
708C (Scheme 4). With this new set of reaction conditions,
the yield of 2j was also improved (Table 3, entry 11).
Furthermore, unsymmetrical diketones selectively afford
products 2l and 2m with n-butyl or benzyl allenyl ketone
cyclization and the phenyl or p-nitrophenyl allenyl ketone
serving as the Michael acceptor (Table 4). A combination of
nBu or iPr with p-nitrophenyl
Figure 2. ORTEP representation of 2o.
the electron-donating aryl allenyl ketone cyclizes, whereas
the electron-withdrawing aryl allenyl ketone accepts the
conjugate addition from the in situ formed furanyl–rhodium
intermediate to selectively form of products 2p and 2p’ in
55% and 7% isolated yields, respectively (Scheme 5).
To further unveil the nature of the selectivity observed
with the rhodium catalyst, a mixture of 2a and 3a in a ratio
even led to an exclusive forma-
tion of 2n and 2o. The selectivi-
ty observed here has been es-
tablished by X-ray diffraction
studies of 2o (Figure 2).[12]
Surprisingly, even for 1p with
a p-nitro and pMeO group on
each phenyl ring, respectively, Scheme 5. Reactions of the unsymmetrical 1,5-bis(1,2-allenylketone)s 1p.
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Rhodium-Catalyzed Bicyclization Reactions
COMMUNICATION
of 87:13 was treated with 5 mol% [RhCl(CO)2]2 for 1 h
under the standard reaction conditions or even 12 h with an
unchanged 2a/3a ratio (Scheme 6). This clearly indicates
bound rhodium dienolate intermediate M2b must have
formed exclusively; this yields 2 as the only product.
In conclusion, we have developed an efficient and highly
selective synthesis for 3,4-fused bicyclic furans through a
rhodium(I)-catalyzed cycloisomerization of readily available
1,5-bis(1,2-allenylketone)s[15] under mild conditions. Com-
pared with [PdCl2ACTHNUGTRNE(UGN MeCN)2], [RhCl(CO)2]2 shows an excel-
lent selectivity at RT; this may be explained by the different
bonding nature of palladium and rhodium to the carbon or
oxygen in the organometallic intermediates. Because reports
on such observations are very limited,[13,14] this will also be
very informative for the future design of the reactions in-
volving Pd and Rh enolates or dienolates. In addition, the
selectivity of unsymmetric substrates has been nicely con-
trolled by the electronic effect of nonallenyl groups connect-
ed to the carbonyl functionalities. Due to the potentials of
the products and the excellent selectivities observed, this
method will also be of high interest to organic and medicinal
chemists. Further studies concerning the mechanism and
synthetic applications in this area are being pursued in our
laboratory.
Scheme 6. Possible isomerization between 2a and 3a under the RhI catal-
ysis.
that the excellent selectivity observed with rhodium was not
caused by isomerization between a,b-unsaturated enones 2
and b,g-unsaturated enones 3; it is probably owed to the
direct protonolysis of the oxygen-bound rhodium dienolate
intermediate shown in Scheme 7.
Experimental Section
AHCTUNGTERG[NNUN RhCl(CO)2]2-catalyzed cyclization of 1: Acetone (2 mL) was added to a
mixture of [RhCl(CO)2]2 (2.2 mg, 0.0057 mmol) and 1a (38.0 mg,
0.10 mmol) at RT. The reaction was stirred at RT for 1 h. After the reac-
tion had run to completion, as monitored by TLC, evaporation and
column chromatography on silica gel (petroleum ether/ethyl acetate 5:1)
afforded 2a (30.3 mg, 80%) as a solid, m.p.: 110–1118C (ethyl acetate/
hexane); 1H NMR (300 MHz, CDCl3): d=7.62 (d, J=8.1 Hz, 2H), 7.27
(s, 1H), 7.22 (d, J=8.1 Hz, 2H), 4.31 (s, 2H), 4.04 (s, 2H), 2.68 (q, J=
7.2 Hz, 2H), 2.48 (q, J=7.2 Hz, 2H), 2.39 (s, 3H), 2.03 (s, 3H), 1.11 ppm
(t, J=7.2 Hz, 6H); 13C NMR (75 MHz, CDCl3): d=205.6, 153.4, 143.3,
139.5, 137.3, 135.8, 131.2, 129.4, 127.6, 127.2, 112.5, 47.2, 42.7, 35.4, 21.4,
19.7, 19.4, 12.4, 8.0 ppm; IR (neat): u˜ =2976, 2937, 2877, 1681, 1597, 1493,
1456, 1377, 1337, 1203, 1185, 1159, 1093, 1057, 1004 cmÀ1; MS (EI): m/z:
387 [M]+ (8.01), 330 [MÀC2H5CO]+ (7.23), 232 [MÀTs]+ (100); elemen-
tal analysis calcd (%) for C21H25NO4S: C 65.09, H 6.50, N, 3.61; found: C
64.78, H 6.74, N 3.81.
Scheme 7. Possible mechanism for the formation of 2 and 3.
The formation of the 3,4-fused bicyclic furan skeletons
may be rationalized by the mechanism shown in Scheme 7.
Firstly, the cyclic oxymetallation of the 1,2-allenyl ketone
moiety in 1 with PdII or RhI would form intermediate M1
(M=Pd or Rh). The electron-donating R1 group increases
the nucleophilicity of the carbonyl oxygen atom; this ex-
plains the selectivity for unsymmetrical substrates with RhI
catalysis. Then intramolecular carbometallation of the re-
maining allene unit in M1 would form the p-allylic palladi-
um intermediate M2a due to low oxophilicity of palladi-
um.[13] Due to the presence of the carbonyl group, subse-
quent protonolysis of M2a with H+ may occur. However,
such a reaction at the a or g-position would afford an iso-
meric mixture of 2 and 3, with 2 as the major product due to
the steric effect, and regenerate the catalytically active PdII
species. However, it has been reported that rhodium prefers
oxygen bonding in the enolate-type intermediates.[14] Thus,
we reasoned that by using the rhodium catalyst oxygen-
Acknowledgements
Financial support from the National Basic Research Program of China
(2011CB808700) and the National Natural Science Foundation of China
(20732005) is greatly appreciated. Shengming Ma is a Qiu Shi Adjunct
Professor at Zhejiang University. We thank Xiaojing Ma in this group for
reproducing the results presented in entries 3 and 11 of Table 3 and
Scheme 4.
Keywords: allenes
rhodium
· cyclization · furans · palladium ·
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Received: December 8, 2010
Published online: March 23, 2011
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Chem. Eur. J. 2011, 17, 4976 – 4980