Rhodium(I)-catalyzed cycloisomerization of benzylallene-alkynes through C-H activation

Article abstract of DOI:10.1002/anie.201403990

The efficient Rh^I-catalyzed cycloisomerization of benzylallene-alkynes produced the tricyclo[9.4.0.0^3,8] pentadecapentaene skeleton through a C sp 2-H bond activation in good yields. A plausible reaction mechanism proceeds via oxidative addition of the acetylenic C-H bond to Rh^I, an ene-type cyclization to the vinylidenecarbene- Rh^I intermediate, and an electrophilic aromatic substitution with the vinylidenecarbene species. It was proposed based on deuteration and competition experiments.

Full text of DOI:10.1002/anie.201403990

.
Angewandte
Communications
DOI: 10.1002/anie.201403990
Cyclization
Rhodium(I)-Catalyzed Cycloisomerization of Benzylallene-Alkynes
À
through C H Activation**
Yasuaki Kawaguchi, Shigeo Yasuda, Akira Kaneko, Yuki Oura, and Chisato Mukai*
Abstract: The efficient RhI-catalyzed cycloisomerization of
opened, let alone be used as a C4 building block.[2e–g,6] The
production of 2 (n = 2) could tentatively be rationalized by
the initial formation of the rhodabicyclo[4.3.0] intermediate 3
(n = 2),[3,7] followed by b-C elimination,[2e–g,6] which would
release the cyclobutane ring strain (26.3 kcalmolÀ1),[8] giving
rise to the nine-membered rhodabicycle 4. Reductive elimi-
nation of 4 would then provide the final products. The
successful application of this methodology to the cyclo-
pentane derivative 1 (n = 3) afforded the nine-membered
bicyclic compounds 2 (n = 3).[3c] This novel [7+2] cycloaddi-
tion involves the unprecedented cleavage of the “normal-
sized” cyclopentane ring by releasing its small strain energy of
6.3 kcalmolÀ1[8] presumably via the intermediate 3 (n = 3).[3]
Thus, it might be assumed that the allene-alkyne unit works as
a highly reactive p component toward the RhI catalyst[9]
resulting in the formation of the rhodabicyclic intermediate,
benzylallene-alkynes produced the tricyclo[9.4.0.03,8]penta-
À
decapentaene skeleton through a Csp2 H bond activation in
good yields. A plausible reaction mechanism proceeds via
I
À
oxidative addition of the acetylenic C H bond to Rh , an ene-
type cyclization to the vinylidenecarbene–RhI intermediate,
and an electrophilic aromatic substitution with the vinylidene-
carbene species. It was proposed based on deuteration and
competition experiments.
[1]
À
IÀn the last decade, transition-metal-catalyzed C H and
C C[2] bond-activation reactions have been extensively
studied as powerful step- and atom-economical methods for
the synthesis of complex organic molecules. These activation
reactions often require directing groups in the substrates,
which direct transition metals to the right position close to the
reactive sites. The relief of strain would be an alternative
À
À
which would subsequently activate the C C and/or C H bond
À
À
driving force to facilitate the cleavage of C C bonds on
near the Rh species. Here we describe another type of C H
cycloalkanes. We recently disclosed that the RhI-catalyzed
activation of benzylallene-alkynes which formally involves
À
cycloaddition of allenylcyclopropane-alkynes
1
(n = 1)
cleavage of a Csp2 H bond on the benzene ring to furnish the
afforded the bicyclo[5.4.0]undecatrienes 2 (n = 1)[3a,4] in the
[5+2] ring-closing manner (Scheme 1). The reaction was
tricyclo[9.4.0.03,8]pentadecapentaene derivatives in high
yields (Scheme 2).
Scheme 2. This study: RhI-catalyzed cycloisomerization of benzylallene-
alkynes.
Scheme 1. Previous study: RhI-catalyzed cycloaddition of allenylcycloal-
kane-alkynes (n=1–3).[3]
In our initial experiments we used the benzylallene-
alkyne 5a possessing a phenylsulfonyl group on the allenyl
moiety. Treatment of 5a with 10 mol% of [RhCl(PPh3)3],
which was effective for the ring-opening of the allenylcyclo-
pentane-alkyne 1,[3c] in refluxing toluene for 9 h surprisingly
produced the tricyclo[9.4.0.03,8]pentadecapentaene derivative
6a in 61% yield (Table 1, entry 1). The reaction with
[RhCl(CO)(PPh3)2] was less efficient and the yield was
lower (entry 2). Neither [RhCl(dppp)2], which is a suitable
catalyst for the ring-opening of the allenylcyclobutane,[3b] or
its CO analogue, [{RhCl(CO)(dppp)}2], provided good results
(entries 3 and 4). Several other catalysts, such as
[{RhCl(CO)2}2], [{RhCl(cod)}2], [Rh(cod)2]BF4/PPh3, and
[Rh(cod)2]OTf/PPh3, were examined, but all of them except
for [{RhCl(CO)2}2] furnished poor results. Indeed, the reac-
tion with [{RhCl(CO)2}2] at 808C was complete within 2 h to
afford 6a in 93% yield (entry 5).
assumed to proceed through cleavage of the cyclopropane
ring driven by the release of the high strain energy (27.5 kcal
molÀ1).[5] A similar ring construction could be realized using
allenylcyclobutane-alkynes 1 (n = 2) producing the eight-
membered bicyclic compounds 2 (n = 2)[3b] in high yields
([6+2] cycloaddition). The simple cyclobutane ring without
any activating functional group could generally not be
[*] Y. Kawaguchi, Dr. S. Yasuda, A. Kaneko, Y. Oura, Prof. Dr. C. Mukai
Division of Pharmaceutical Sciences
Graduate School of Medicinal Sciences, Kanazawa University
Kakuma-machi, Kanazawa 920-1192 (Japan)
E-mail: mukai@p.kanazawa-u.ac.jp
[**] We are grateful for the support of this research by a Grant-in-Aid for
Scientific Research from the Ministry of Education, Culture, Sports,
Science, and Technology (Japan).
Two sets of complementary and interchangeable condi-
tions (10 mol% [{RhCl(CO)2}2] or [RhCl(PPh3)3] in toluene
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201403990.
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Table 1: Optimization of reaction conditions for RhI-catalyzed cyclo-
isomerization of benzylallene-alkyne 5a.
5j and 5k with dibutyl or cyclohexyl groups instead of
a dimethyl group could be converted into the tricyclic adducts
6j (80%) and 6k (72%) (entries 9 and 10). The phenyl
congener 5l required rather drastic conditions (heating at
1508C in a microwave reactor for 15 h, with [RhCl(PPh3)3])
and gave a low yield (entry 11). As mentioned in Scheme 1,
the allenylcyclobutane derivatives 1 (n = 2, R = alkyl) pro-
duced the ring-expanded products 2 presumably via 3. We
thought it would be interesting to investigate the reaction of
allenylcyclobutane derivative 5m, which has a phenyl group
instead of the alkyl substituent, in the presence of the Rh
catalyst. Thus, the standard conditions ([{RhCl(CO)2}2] in
toluene at reflux) were applied to compound 5m. The starting
material 5m was completely consumed within 0.2 h, but an
intractable mixture was obtained (entry 12).
Entry
RhI complex
t [h]
Yield [%][a]
1
[RhCl(PPh3)3]
9
61
2
3
[RhCl(CO)(PPh3)2]
[RhCl(dppp)2]
24
6
40
complex mixture
complex mixture
93
4
[{RhCl(CO)dppp}2]
[{RhCl(CO)2}2]
3
2
5[b]
[a] Yield of the isolated product. [b] Reaction was performed at 808C.
heated at reflux) were applied to several benzylallene-alkynes
(Table 2). The malonate derivative 5b with the gem-disub-
stituent effect[10] was treated with [{RhCl(CO)2}2] to give the
product 6b in 94% yield (entry 1).[11] The nitrogen and
Two control experiments were performed. Upon exposure
of 7 to the standard conditions, the desired product was not
produced at all, but the triene compound 8 was obtained in
85% yield as a 1:1 mixture of (E) and (Z) isomers (Scheme 3).
Table 2: RhI-catalyzed cycloisomerization of benzylallene-alkyne 5.
Entry
5
R1
R2
X
RhI
cat.
t [h] 6:
Yield [%][a]
Scheme 3. RhI-catalyzed cycloisomerization of 7 and 9.
1
5b SO2Ph
5c SO2Ph
5d SO2Ph
5e P(O)(OEt)2 Me
5 f Me
5g nBu
5h tBu
5i Ph
5j SO2Ph
5k SO2Ph
Me
Me
Me
C(CO2Me)2
NTs
O
C(CO2Me)2
C(CO2Me)2
C(CO2Me)2
C(CO2Me)2
C(CO2Me)2
C(CO2Me)2
A
A
A
A
B
B
A
B
A
B
B
A
0.2 6b: 94
0.2 6c: 71
0.2 6d: 62
0.5 6e: 84
0.2 6 f: 95
0.2 6g: 86
2[b]
3[b]
4
A similar treatment of the monomethyl derivative 9 afforded
10 (E/Z = 10:1) in 92% yield. The formation of 8 and 10 could
be well rationalized in terms of the intermediacy of the
rhodabicyclic species A, which undergoes b-hydride elimina-
tion.[9a,12] These two experiments strongly indicated that the
geminal disubstituent moiety at the benzyl position of 5 is
essential for the efficient transformation to the desired
tricyclic compound 6.
5
6
7
8
9
10
Me
Me
Me
Me
nBu
1
6h: 67
0.2 6i: 78
0.2 6j: 80
-(CH2)5- C(CO2Me)2
Ph C(CO2Me)2
-(CH2)3- C(CO2Me)2
1
6k: 72
11[c] 5l SO2Ph
12
15 6l: 31
[d]
5m SO2Ph
0.2
–
The effect of the substituent on the benzene ring was
examined next (Table 3). Substrates 5n,o with a methyl group
at the p- or o-position gave the cyclized products 6n and 6o in
a quantitative and 78% yield, respectively (entries 1 and 2).
In the case of 5p, which has an electron-donating methoxy
[a] Yield of the isolated product. [b] An 0.025m solution was used.
[c] Reaction was performed in a microwave reactor at 1508C. [d] An
intractable mixture was obtained.
oxygen congeners 5c,d produced the corresponding aza
compound 6c (71% yield) and oxa compound 6d (62%
yield), respectively (entries 2 and 3). It was shown that the
phenylsulfonyl substituent on the allenyl moiety was not
essential for this transformation. Thus, the cycloisomerization
of the phosphonate derivative 5e easily occurred to give 6e in
84% yield (entry 4). Furthermore, upon exposure to the
standard conditions with [RhCl(PPh3)3], the benzylallenes
having methyl (5 f), butyl (5g), and phenyl groups (5i) on the
allenyl moiety underwent cycloisomerization to furnish the
tricyclic products 6 f (95% yield), 6g (86% yield), and 6i
(78% yield), respectively (entries 5, 6, and 8). The reaction of
the bulky tert-butyl derivative 5h in the presence of
[{RhCl(CO)2}2] proceeded without any trouble to provide
the desired product 6h in 67% yield (entry 7). The substrates
Table 3: Examination of the effect of the substituent on the benzene ring
of 5.
Entry
5
R1
R2
t [h]
6:
Yield [%][a]
1
2
3
4
5
5n
5o
5p
5q
5r
H
Me
H
H
H
Me
H
OMe
Cl
0.2
0.2
0.2
0.5
2
6n: 99
6o: 78
6p: 71
6q: 71
6r: 82
NO2
[a] Yield of the isolated product.
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group, the reaction was complete within 0.2 h, but the yield of
6p (71%) slightly decreased because of the formation of
unidentified by-products. On the other hand, compounds 5q
and 5r with electron-withdrawing chloro and nitro groups
needed a prolonged reaction time (0.5 h and 2 h; 6q: 71%
and 6r: 82% yield; entries 4 and 5). Thus, it was concluded
that the ring-closing reaction consistently occurred irrespec-
tive of the electronic properties of the substituent on the
benzene ring of 5, although electron-withdrawing groups tend
to slow down the reaction.
Treatment of the one-carbon-shortened substrate 11 with
the RhI catalyst effected the construction of the tricyclic
framework to produce 12 in 51% yield [Eq. (1)].
Scheme 5. Proposed reaction mechanism.
An ene-type cyclization of intermediate B then forms the
vinylidenecarbene–RhI intermediate C.[13,14] Activation of
À
a Csp2 H bond on the benzene ring could happen at this
stage by nucleophilic addition of the pendant benzene ring to
the a-carbon of the vinylidenecarbene species C to give D. A
subsequent proton transfer from the benzene ring to the Rh
center would then produce the intermediate E,[15] which
undergoes reductive elimination to form the product 6.
A competition experiment using an equimolar amount of
5b and [D5]5b revealed a kinetic isotope effect (KIE) value of
To obtain information on the mechanism of the novel
transformation of 5 into 6, we performed three experiments
with the deuterated substrates, [D5]5b, [D1]5b, and [D1]5b’
(Scheme 4). Treatment of the pentadeuterated substrate
1.0,[16] indicating that the cleavage of the C H bond of the
À
benzene ring is not the rate-determining step. Furthermore,
the similarly low KIE value (1.2) observed in the experiment
using 5b and [D1]5b[16] suggested that oxidative addition of
I
À
the acetylenic C H bond to Rh is also not the rate-
determining step. These results indirectly support the pro-
posed reaction mechanism, which involves the conversion of
the intermediate C to E through a stepwise Friedel–Crafts-
type substitution reaction on the benzene ring.
Finally, the reaction of the substrate 13, which has an
internal alkyne, was examined not only to compare it with the
reactions of the terminal alkyne derivatives, but also to obtain
additional information about the mechanism of this cyclo-
isomerization. Upon exposure to the standard conditions, the
internal alkyne derivative 13 was converted into the tricyclic
compound 14, which was produced as a single diastereomer in
rather lower yield (54%) [Eq. (2)]. The structure of 14 is
Scheme 4. RhI-catalyzed cycloisomerization of [D5]5b, [D1]5b, and
[D1]5b’.
[D5]5b with [{RhCl(CO)2}2] under the standard conditions
produced the deuterated product [D5]6b in 95% yield. It
became apparent that one deuterium atom was exclusively
incorporated at the olefinic position on the seven-membered
ring and the other four deuterium atoms remained on the
benzene ring. In the case of the monodeuterated substrate
[D1]5b, the deuterium atom at the triple bond terminus was
incorporated completely at the allylic position of the seven-
membered ring of [D1]6b. Another monodeuterated sub-
strate [D1]5b’ afforded the same product [D1]6b.
These deuteration experiments provided an informative
insight into the mechanism of the cycloisomerization of 5
(Scheme 5). On the basis of these experiments in combination
with the fact that substrates 5q and 5r with electron-
withdrawing groups required longer reaction times
(Table 3), the following plausible reaction mechanism is
similar to that of 6,[17] but differs in the position of the double
bonds.[18] This result indicates that the reaction pathway from
13 to 14 is different from that for the formation of 6.
In summary, we have developed the novel and efficient
RhI-catalyzed cycloisomerization of benzylallene-alkyne
derivatives leading to the formation of tricyclo[9.4.0.03,8]-
pentadecapentaene derivatives. Preliminary investigations
À
proposed. The oxidative addition of the acetylenic C H
bond to RhI initially occurs to form the RhIII intermediate B.
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using deuterated substrates provided the basis for the
113 – 115.
proposed reaction mechanism which involves the following
I
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[7] In the case of the allenylcyclopropane derivatives 1 (R = H, n =
1), we assumed the initial formation of the six-membered
rhodacycle intermediate because cyclopropane has a fairly high
ring strain energy.[3a] A similar rhodacycle intermediate could be
considered for the reaction of the allenylcyclopropane-alkenes,
see K. Sugikubo, F. Omachi, Y. Miyanaga, F. Inagaki, C.
Matsumoto, C. Mukai, Angew. Chem. 2013, 125, 11579 – 11582;
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À
steps: 1) oxidative addition of acetylenic C H bond to Rh ,
2) ene-type cyclization to form the vinylidenecarbene-RhI
intermediate, and 3) electrophilic aromatic substitution fol-
À
lowed by proton transfer to the rhodium center (Csp2 H bond
activation). The results presented here provide new insight
À
into the chemistry of the C H bond activation and transition-
metal–vinylidene intermediates in catalysis. We are currently
examining the scope and limitations of this method and
conducting further mechanistic studies.
Received: April 4, 2014
Revised: May 1, 2014
Published online: May 30, 2014
Keywords: alkynes · allenes · C–H activation ·
.
cycloisomerization · rhodium
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2006, 128, 6302 – 6303; i) R. Shintani, H. Nakatsu, K. Takatsu, T.
Hayashi, Chem. Eur. J. 2009, 15, 8692 – 8694.
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www.angewandte.org
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Angewandte
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[17] An X-ray analysis of 14 unambiguously established its tri-
cyclo[9.4.0.03,8]pentadecapentaene structure (see the Supporting
Information for details).
[18] The isomerization of the double bonds of both 14 and 6 could not
be observed. We do not have a suitable explanation of the
mechanism for the formation of 14 from 13.
[15] For cyclization reactions involving nucleophilic substitution at
the a-carbon atom of the vinylidenecarbene–rhodium species
and subsequent proton transfer, see Ref. [13e,g].
[16] See the Supporting Information for details of the KIE experi-
ments.
7612
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Angew. Chem. Int. Ed. 2014, 53, 7608 –7612