Rhodium(I)-catalyzed cyclization of allenynes with a carbonyl group through unusual insertion of a C=O bond into a rhodacycle intermediate

Article abstract of DOI:10.1002/anie.201308824

Rhodium(I)-catalyzed cyclization of allenynes with a tethered carbonyl group was investigated. An unusual insertion of a C=O bond into the C(sp ²)-rhodium bond of a rhodacycle intermediate occurs via a highly strained transition state. Direct reductive elimination from the obtained rhodacyle intermediate proceeds to give a tricyclic product containing an 8-oxabicyclo[3.2.1]octane skeleton, while β-hydride elimination from the same intermediate gives products that contain fused five- and seven-membered rings in high yields. Unusual insertion: During the title reaction, a C=O bond is inserted into the C(sp²)-Rh bond of rhodacycle intermediate I. The insertion occurs via a highly strained transition state. Direct reductive elimination from II gives a tricyclic product containing an 8-oxabicyclo[3.2.1] octane skeleton, whereas β-hydride elimination from II gives products with fused five- and seven-membered rings. Copyright

Full text of DOI:10.1002/anie.201308824

Angewandte
Chemie
DOI: 10.1002/anie.201308824
Cycloadditions
Rhodium(I)-Catalyzed Cyclization of Allenynes with a Carbonyl
=
Group through Unusual Insertion of a C O Bond into a Rhodacycle
Intermediate**
Yoshihiro Oonishi,* Takayuki Yokoe, Akihito Hosotani, and Yoshihiro Sato*
Abstract: Rhodium(I)-catalyzed cyclization of allenynes with
Recently, we have reported a RhI-catalyzed [6+2] cyclo-
addition of 4-allenals with alkynes or alkenes[5] in a tether
[Scheme 2, Eq. (1)].[5a] During ongoing investigation of this
cycloaddition, it was found that reaction of 1a with [Rh-
(dppe)]ClO4 (10 mol%) instead of [Rh(IMes)(cod)]ClO4 did
not produce the expected product 2a, which contains fused
five- and eight-membered rings, but produced the bicyclic
alcohol 3a, which contains fused six- and seven-membered
rings, in 27% yield.[6–8] The formation of 3a could not be
explained by the mechanism of the reported [6+2] cyclo-
addition, but it might be formed via the above-mentioned
unknown intermediate Cꢀ followed by b-hydride elimination
[Scheme 2, Eq. (2)]. This unexpected result prompted us to
investigate RhI-catalyzed cycloaddition of allenynes with
tethered aldehydes giving the product 3a.
a tethered carbonyl group was investigated. An unusual
2
=
insertion of a C O bond into the C(sp )–rhodium bond of
a rhodacycle intermediate occurs via a highly strained
transition state. Direct reductive elimination from the obtained
rhodacyle intermediate proceeds to give a tricyclic product
containing an 8-oxabicyclo[3.2.1]octane skeleton, while b-
hydride elimination from the same intermediate gives products
that contain fused five- and seven-membered rings in high
yields.
T
ransition-metal-catalyzed [2+2+2] cycloadditions of two
À
=
C C multiple bonds with C O bonds, as in aldehydes and
ketones, are useful methodologies for the construction of
oxygen-containing polycyclic compounds.[1–4] Intramolecular
variants are particularly attractive reactions that enable us to
easily access polycyclic compounds from acyclic substrates in
one pot [Scheme 1, Eq. (1)].[2a,g,h,3]
These cycloadditions begin with the formation of the
metalacycle intermediate A through oxidative cycloaddition
À
of two multiple C C bonds to a low-valent transition-metal
complex, and cyclized products are produced through inser-
=
À
tion of a C O bond into the M C bond (a) of the
intermediate A followed by reductive elimination from the
intermediate B. In these reaction processes, if insertion of
=
À
a C O bond into the M C bond (b) of A occurs, the
intermediate C would be produced [Scheme 1, Eq. (2)].
However, most transition-metal-catalyzed cycloadditions
proceed through intermediate B,[2a,g,h,3] and there has been
no report on cyclization through the intermediate C, probably
owing to the highly strained transition state A’’.
À
Scheme 1. Intramolecular cycloadditions of C C multiple bonds with
=
C O bonds.
[*] Dr. Y. Oonishi, T. Yokoe, A. Hosotani, Prof. Dr. Y. Sato
Faculty of Pharmaceutical Sciences, Hokkaido University
Nishi 6, Kita 12, Kita-ku, Sapporo 060-0812 (Japan)
E-mail: biyo@pharm.hokudai.ac.jp
Homepage: http://gouka.pharm.hokudai.ac.jp/FSC/jpn/page/
top_page.htm
Prof. Dr. Y. Sato
ACT-C, Japan Science and Technology Agency (JST)
Nishi 6, Kita 12, Kita-ku, Sapporo 060-0812 (Japan)
[**] This work was supported in part by Grants-in-Aid for Scientific
Research (B) (No. 23390001) from the Japan Society for the
Promotion of Science (JSPS) and by a Grant-in-Aid for Scientific
Research on Innovative Areas “Molecular Activation Directed
toward Straightforward Synthesis (No. 25105701)” from MEXT
(Japan). A.H. thanks JSPS for providing a Research Fellowship for
Young Scientists.
Scheme 2. RhI-catalyzed cycloadditions of 1a with [Rh(IMes)(cod)]ClO4
or [Rh(dppe)]ClO4. IMes=1,3-di(2,4,6-trimethylphenyl)imidazolin-2-yli-
dene, cod=cyclooctadiene, dppe=bis(diphenylphosphanyl)ethane.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201308824.
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Table 2: RhI-catalyzed cyclization of various substrates.[a,b]
To improve the yield of 3a, cycloaddition of 1a under
various conditions was reinvestigated. However, the yield of
3a was only improved to 39% when using [Rh(dppe)]ClO4
(10 mol%) in ClCH2CH2Cl at room temperature. Thus, the
substrate was changed from 1a to 4a, which had two carbon
units between allene and alkyne (Table 1). The cyclization of
4a with [Rh(dppe)]ClO4 (10 mol%) in ClCH2CH2Cl at 508C
for 1 h gave the desired bicyclic compound 5a in 80% yield
(Table 1, entry 1).[9] Screening of RhI complexes in the
Entry
Substrate
t [h]
Product
1
4b: R=CH2OTBS
4c: R=TMS
4d: R=Cl
2
1
1
1
5b: 72%
5c: 91%
5d: 75%
5e: 88%
2[c]
3
4
4e: R=CO2Me
Table 1: RhI-catalyzed cycloaddition of 4a.
5
6
7
4 f: R=H
4g: R=OMe
4h: R=CO2Me
1
2
1
5 f: 83%
5g: 82%
5h: 76%
Entry
RhI complex
t [h]
Yield [%]
1[a]
2
[Rh(dppe)]ClO4
[RhCl(PPh3)3]
1
18
24
26
1
80
5[d,e]
3[a]
4[a]
5[a]
6[b]
7[c]
[Rh(dppb)]ClO4
[Rh(DPEphos)]ClO4
[Rh(dppbz)]ClO4
[Rh(dppbz)]ClO4
[Rh(IMes)(cod)]ClO4
–
[e]
[e]
–
91
80
–
2
15
8
4i
1
5i: 71%
[a] Reactions were carried out using 10 mol% [Rh(ligand)]ClO4 at 508C.
[Rh(ligand)]ClO4 was generated in situ from [Rh(ligand)(nbd)]ClO4
under an atmosphere of hydrogen. [b] The reaction was carried out using
2 mol% [Rh(dppbz)]ClO4. [Rh(dppbz)]ClO4 was generated in situ from
[Rh(dppbz)(nbd)]ClO4 under an atmosphere of hydrogen. nbd=nor-
bornadiene, dppb=bis(diphenylphosphanyl)butane, dppbz=1,2-bis-
(diphenylphosphanyl)benzene, DPEphos=bis(2-(diphenylphosphanyl)-
phenyl)ether. [c] The reaction was carried out using 10 mol% [Rh-
(IMes)(cod)]ClO4 generated in situ from [Rh(IMes)(cod)]Cl (10 mol%)
[a] Reactions were carried out using 10 mol% [Rh(dppbz)]ClO4 at 508C.
[Rh(dppbz)]ClO4 was generated in situ from [Rh(dppbz)(nbd)]ClO4
under an atmosphere of hydrogen. TBS=tert-butyldimethylsilyl,
TMS=trimethylsilyl. [b] E=CO2Me. [c] In the presence of MS4A.
the alkyne part, proceeded smoothly, giving the desired
products in high yields (Table 2, entries 5–7). When 4i, the
reaction of which was expected to give a heterobicyclic
compound, was treated with [Rh(dppbz)]ClO4 (10 mol%) in
ClCH2CH2Cl at 508C for 1 h, the desired compound 5i was
obtained in 71% yield (Table 2, entry 8).[10]
1
and AgClO4 (10 mol%). [d] Yield was determined by H NMR spectros-
copy using 1,3,5-trimethoxybenzene as an internal standard. [e] The
starting material was recovered in 66% (entry 2), 75% (entry 3), and
56% (entry 4) yields.
A possible reaction mechanism for the formation of 5
from 4 is depicted in Scheme 3. The rhodacycle I would be
reaction of 4a was carried out, and it was found that the use
of [RhCl(PPh3)3], [Rh(dppb)]ClO4, or [Rh(DPEphos)]ClO4
was not effective for this cycloaddition (Table 1, entries 2–4).
Surprisingly, the reaction using [Rh(dppbz)]ClO4 afforded the
cyclic compound 5a in 91% yield (Table 1, entry 5). Further-
more, the catalyst loading could be reduced to 2 mol% under
similar conditions, thereby giving 5a in 80% yield (Table 1,
entry 6). The use of [Rh(IMes)(cod)]ClO4, which was effec-
tive for the above-mentioned [6+2] cycloaddition,[5a] gave
a complex mixture in the cyclization of 4a (Table 1, entry 7).
Next, the cyclization of various substrates using [Rh-
(dppbz)]ClO4 was examined (Table 2). The cyclization of 4b,
having a silyloxy group in the tether, gave the cyclic
compound 5b in 72% yield (Table 2, entry 1). The use of
substrate 4c, which has a TMS group at the alkyne part,
afforded 5c in 91% yield (Table 2, entry 2). In the reaction of
4d and 4e, which have an electron-withdrawing group such as
a chlorine atom and ester on the alkyne moiety, the
corresponding products 5d and 5e were obtained in 75%
and 88% yields, respectively (Table 2, entries 3 and 4). The
cyclization of 4 f–4h, which have various aromatic moieties on
formed by oxidative cycloaddition of the alkyne part and
I
=
external C C bond of the allene moiety of 4 to the Rh
complex.[3,11–13] Insertion of an aldehyde moiety of 4 into the
C(sp2)–rhodium bond of I would occur to give the rhodacycle
II, from which b-hydride elimination followed by reductive
Scheme 3. Possible reaction mechanism.
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Table 3: Cyclization through reductive elimination of rhodacycle II.[a,b]
elimination from III would occur to give the bicyclic
compound 5.
Entry
Substrate
t [h]
Product
The most critical step in the mechanism should be the one
from I to II (i.e., from A to C via A’’ in Scheme 1) since
2
=
insertion of a C O bond into the C(sp )–rhodium bond would
yield a highly strained transition state. Thus, to obtain insights
into the reaction course, we prepared the substrate 4a in an
enantiomerically enriched form (81% ee) and subjected it to
the optimal conditions. As a result, the substrate (R)-4a was
converted to (S)-5a in 86% yield, 76% ee, which is explain-
able according to the mechanism shown in Scheme 3. Thus,
the stereospecific formation of a chiral rhodacycle intermedi-
ate I’ from the chiral substrate 4a occurs, and then the
1
2
4k: R=Me
4l: R=Et
1
24
6k: 87%
6l: 90%
=
intermediate II’ is produced by insertion of a C O into the
C(sp2)–rhodium bond of I’ (Scheme 4).[14]
3
4
5
4m: R=H
4n: R=OMe
4o: R=CO2Me
24
24
24
6m: 42%
6n: –[c]
6o: 85%
6
4p
1
6p: 81%
[a] Reactions were carried out using 10 mol% [Rh(dppbz)]ClO4 in the
presence of MS4A at reflux. [Rh(dppbz)]ClO4 was generated in situ from
[Rh(dppbz)(nbd)]ClO4 under an atmosphere of hydrogen.
[b] E=CO2Me. [c] A complex mixture including the desired compound
was obtained.
Scheme 4. Chiral transfer reaction.
Additionally, we prepared the substrate 4j having no
hydrogen atom at the b-position of the aldehyde moiety to
prevent b-hydride elimination from an oxa-rhodacycle inter-
mediate such as II’ in Scheme 4. When 4j was treated with
[Rh(dppbz)]ClO4 (10 mol%) at reflux for 3 h, we obtained 6j,
which has an 8-oxabicyclo[3.2.1]octane structure, in 84%
yield;[15,16] 6j was surely formed through direct reductive
elimination from the oxa-rhodacycle intermediate II’’
(Scheme 5). Furthermore, the cyclization of 4j under a CO
atmosphere afforded the tricyclic lactone 7j in 62% yield
along with the cyclic compound 6j in 24% yield, the structure
of which was unambiguously determined by X-ray analysis.[17]
The product 7j should be produced through the insertion of
CO into the oxa-rhodacycle intermediate II’’, and all of the
results in Schemes 4 and 5 strongly support the mechanism in
Scheme 3.
Next, we turned our attention to investigating the scope of
carbonyl groups in the cyclization giving a product having an
8-oxabicyclo[3.2.1] skeleton (Table 3). The cyclization of 4k
and 4l, which have a dialkyl ketone moiety (R = Me or Et)
instead of an aldehyde, proceeded to give 8-oxabicyclo-
[3.2.1]octane derivatives in 87% and 90% yields, respectively,
when using MS4A as an additive (Table 3, entries 1 and 2).
When aryl ketones 4m–o were employed in this cyclization,
4o, having an electron-withdrawing group at the aromatic
ring, afforded the corresponding cyclic compound 6o in high
yield, while a complex mixture was obtained in the case of 4n,
bearing an electron-donating group at the aromatic ring
(Table 3, entries 3–5). Gratifyingly, this cyclization was appli-
cable for sterically hindered silyl ketone 4p, and 6p was
obtained in 81% yield, the structure of which was also
unambiguously determined by X-ray analysis (Table 3,
entry 6).[17]
In conclusion, we succeeded in developing novel RhI-
catalyzed cyclizations of allenynes with a tethered carbonyl
=
group, wherein an unusual insertion of a C O bond into the
C(sp2)–rhodium bond of rhodacycle intermediate I occurs
nevertheless via a highly strained transition state, and to our
knowledge, a metalacycle intermediate such as II has been
unknown in the literature. Direct reductive elimination from
II proceeds to give a tricyclic product containing an 8-
oxabicyclo[3.2.1]octane skeleton, while b-hydride elimination
from II gives products that contain fused five- and seven-
membered rings via intermediate III in high yields. It is
Scheme 5. Cyclizations of 4j.
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[7] Selected examples of RhI-catalyzed cyclization using aldehyde
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Lett. 2002, 43, 7031; e) Z. Shen, H. A. Khan, V. M. Dong, J. Am.
Chem. Soc. 2008, 130, 2916; f) Z. Shen, P. K. Dornan, H. A.
Khan, T. K. Woo, V. M. Dong, J. Am. Chem. Soc. 2009, 131, 1077;
g) M. M. Coulter, P. K. Dornan, V. M. Dong, J. Am. Chem. Soc.
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Lee, J. Am. Chem. Soc. 2000, 122, 2385; o) B. Bennacer, M.
Fujiwara, I. Ojima, Org. Lett. 2004, 6, 3589; p) P. A. Wender,
G. G. Gamber, R. D. Hubbard, L. Zhang, J. Am. Chem. Soc.
2002, 124, 2876; q) H. A. Wegner, A. de Meijere, P. A. Wender,
J. Am. Chem. Soc. 2005, 127, 6530; r) Y. Wang, J. Wang, J. Su, F.
Huang, L. Jiao, Y. Liang, D. Yang, S. Zhang, P. A. Wender, Z.-X.
Yu, J. Am. Chem. Soc. 2007, 129, 10060; s) F. Huang, Z.-K. Yao,
Y. Wang, Y. Wang, J. Zhang, Z.-X. Yu, Chem. Asian J. 2010, 5,
1555; t) Z.-K. Yao, J. Li, Z.-X. Yu, Org. Lett. 2011, 13, 134, and
also see Ref. [2a].
known that polycyclic compounds con-
taining an 8-oxabicyclo[3.2.1]skeleton
such as englerins[18] have an interesting
biological activity, and the present
cyclization is a unique methodology
for construction of such a skeleton.
Further studies along this line includ-
ing applications to the synthesis of natural products are in
progress.
Received: October 10, 2013
Revised: October 29, 2013
Published online: December 4, 2013
Keywords: alkynes · allenes · carbonyl compounds ·
cycloadditions · rhodium
.
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I
=
[2] Examples of Rh -catalyzed [2+2+2] cycloaddition involving C
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Am. Chem. Soc. 2005, 127, 17756; b) J. R. Kong, M. J. Krische, J.
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[9] The structure of 5a was determined by 2D NMR spectroscopy
(see the Supporting Information).
[10] The reaction of substrate 4 having a terminal alkyne under
similar conditions did not proceed, but unreacted starting
material was recovered in 26% yield.
[11] Recent reviews on transition metal-catalyzed cycloaddition of
allenes and unsaturated bonds: a) F. Lꢄpez, J. L. MascareÇas,
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[4] Examples of transition-metal-mediated or -catalyzed [2+2+2]
=
=
cycloadditions involving C O bonds: a) D. F. Harvey, B. M.
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[6] A similar tendency, that is, the formation of a rhodacycle
À
through oxidative cycloaddition of C C multiple bonds occurs in
À
preference to oxidative addition of a C H bond of an aldehyde
to a RhI complex when using dppe as a ligand, has been also
observed for a RhI-catalyzed cycloisomerization reported by us,
although the reason is unclear at this stage, see: a) Y. Sato, Y.
Oonishi, M. Mori, Organometallics 2003, 22, 30; b) Y. Oonishi,
A. Saito, M. Mori, Y. Sato, Synthesis 2009, 969.
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3
3
À
tion of allenyne through C(sp ) C(sp ) bond activation: i) F.
Inagaki, K. Sugikubo, Y. Oura, C. Mukai, Chem. Eur. J. 2011, 17,
9062; j) C. Mukai, Y. Ohta, Y. Oura, Y. Kawaguchi, F. Inagaki, J.
Am. Chem. Soc. 2012, 134, 19580, and also see Ref. [8c]. An
example of RhI-catalyzed [2+2+2] cycloaddition of allenynes
with alkynes: k) X. Jiang, S. Ma, J. Am. Chem. Soc. 2007, 129,
11600.
[15] The structure of 6j was determined by INADEQUATE and
HMBC (see the Supporting Information).
[16] Wender (Ref. [8a,b]), Mukai (Ref. [8c]), and Tang (Ref. [8d,e])
have also reported on the access to the similar bicyclic
compounds including 7-membered rings through RhI-catalyzed
cycloaddition. According to these previous methods, however, 8-
oxabicyclo[3.2.1]octane skeleton having an oxygen-bridged
structure cannot be constructed. Also, see: K. E. O. Ylijoki,
J. M. Stryker, Chem. Rev. 2013, 113, 2244.
[17] CCDC 963500 (7j) and 963499 (6p) contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
[13] Our recent study of RhI-catalyzed cyclization of allenyne
À
through C H bond activation: Y. Oonishi, Y. Kitano, Y. Sato,
Angew. Chem. 2012, 124, 7158; Angew. Chem. Int. Ed. 2012, 51,
7052, and also see Ref. [12j].
[14] The absolute configuration of (S)-5a was determined by
a modified Mosherꢅs method after reaction with MTPACl (see
the Supporting Information).
[18] A recent review of the synthesis of englerins: R. H. Pouwer, J.-A.
Richard, C.-C. Tseng, D. Y.-K. Chen, Chem. Asian J. 2012, 7, 22.
Angew. Chem. Int. Ed. 2014, 53, 1135 –1139
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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