Synthesis of [RhCl(CO)(cyclopentadienone)]2 from [RhCl(cod)]2 and a 1,6-diyne under CO: Application to Rh(i)-catalyzed tandem [2+2+1] carbonylative cycloaddition of diynes and Claisen rearrangement

Article abstract of DOI:10.1039/c000747a

Although Rh(i)Cl(CO)(cpd) (cpd = cyclopentadienone) complexes were identified more than 40 years ago, their exact structures have not been determined because of the polymeric nature of these complexes. We determined the structure of [Rh(i)Cl(CO)(cpd)]₂, which was formed by the reaction of [Rh(cod)Cl]₂ with a 1,6-diyne under CO. In addition, based on determination of the structure of the [Rh(i)Cl(CO)(cpd)]₂ complex, we identified a new catalytic tandem reaction - the Rh-catalyzed [2+2+1] carbonylative cycloaddition of phenoxide-substituted diynes and Claisen rearrangement.

Full text of DOI:10.1039/c000747a

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Synthesis of [RhCl(CO)(cyclopentadienone)]2 from [RhCl(cod)]2 and
a 1,6-diyne under CO: application to Rh(I)-catalyzed tandem [2+2+1]
carbonylative cycloaddition of diynes and Claisen rearrangementw
Sang Ick Lee, Yoshiya Fukumoto and Naoto Chatani*
Received (in Cambridge, UK) 12th January 2010, Accepted 16th March 2010
First published as an Advance Article on the web 31st March 2010
DOI: 10.1039/c000747a
Although Rh(I)Cl(CO)(cpd) (cpd
=
cyclopentadienone)
complexes were identified more than 40 years ago, their exact
structures have not been determined because of the polymeric
nature of these complexes. We determined the structure of
[Rh(I)Cl(CO)(cpd)]2, which was formed by the reaction of
[Rh(cod)Cl]2 with a 1,6-diyne under CO. In addition, based on
determination of the structure of the [Rh(I)Cl(CO)(cpd)]2
complex, we identified a new catalytic tandem reaction—the
Rh-catalyzed [2+2+1] carbonylative cycloaddition of phenoxide-
substituted diynes and Claisen rearrangement.
Scheme 1 McVey and Maitlis report.10b
formation of [Rh(I)Cl(CO)(cpd)]n complex (3) from
[RhCl(CO)2]2 (1) and an alkyne10a or tetraphenylcyclopenta-
dienone (2)10b more than four decades ago (Scheme 1). The
exact structure of complex 3 remains unknown because of the
poor solubility of polymeric complexes.
a
Transition-metal-catalyzed carbonylative cycloaddition is
a
powerful method for the enhancement of molecular complexity
and the construction of polycyclic carbonyl compounds.1
A typical example of this type of reaction is the carbonylative
[2+2+1] cycloaddition of enynes. This reaction, which is
referred to as the Pauson–Khand (P–K) reaction, allows for
the direct and efficient synthesis of cyclopentenones.2 Since Jeong
et al. first reported the catalysis of an intramolecular P–K
reaction,3 much attention has been paid to Rh catalysts for
carbonylative cycloaddition.4 Subsequently, Rh-catalyzed carbo-
nylative cycloaddition has been widely studied through the use of
structural analogues of various substrates, such as allenynes,
dienynes, and trienes.5 In contrast to the numerous studies of
these substrates, Rh-catalyzed carbonylative cycloaddition of
diynes has rarely been studied. This is somewhat surprising
Here, we report our structural characterization by X-ray
analysis of [RhCl(CO)(cpd)]2, which was formed by the reaction
of a 1,6-diyne with [Rh(cod)Cl]2 under CO. In addition, based
on the structural characteristics of the [RhCl(CO)(cpd)]2, a
new tandem carbonylative [2+2+1] cycloaddition of diynes
and Claisen rearrangement was discovered.11
Carbonylation of diyne 5 in the presence of a catalytic
amount of [RhCl(cod)]2 gave the orange-yellow Rh complex
6 with concomitant formation of the bicyclo[3.3.0]octadienone
7 in 14% yield (eqn (1)).12
because Muller et al.6 reported that diynes react efficiently with
¨
a stoichiometric amount of RhCl(PPh3)3 under CO to give CPDs
(CPD = cyclopentadienone). CPD-containing compounds have
received considerable attention due to their high reactivity in
decarbonylative cycloaddition with alkynes, in which CPDs
act as a dienophile.7 Isolated CPDs have also been used as
ligands in metal complexes, which are efficient catalysts for
transfer hydrogenation reactions.8 CPD–metal complexes
have also been synthesized by the reaction of diynes (or two
alkynes) with transition metal carbonyl compounds, such as
Fe(CO)5, CpCo(CO)2, and Os3(CO)12. The structures of
the CPD metal complexes have been characterized by X-ray
crystallography.9 Rh(I)Cl(CO)(cpd) complexes also have been
known for a long time.10 Maitlis and McVey reported the
ð1Þ
Because the Rh-complex 6 showed poor solubility in common
organic solvents, spectral data for complex 6 were unavailable,
except for the results of IR spectroscopy. The IR spectrum of 6
in KBr exhibited two sharp carbonyl-stretching frequencies at
2015 cmꢀ1 and 1664 cmꢀ1, which corresponded to the terminal
metal carbonyl and the organic CQO, respectively.
Interestingly, when the amount of [RhCl(cod)]2 loaded was
increased to 50 mol% at 25 1C in 1,2-dichloroethane (DCE)/
Et2O, 5 was completely consumed and 6 was isolated as the
sole product without formation of 7 (eqn (2)).
Department of Applied Chemistry, Faculty of Engineering,
Osaka University, Suita, Osaka 565-0871, Japan.
E-mail: chatani@chem.eng.osaka-u.ac.jp
w Electronic supplementary information (ESI) available: Spectral data
for all new compounds and crystallographic data of 6 and 8. CCDC
752604–752606. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c000747a
ð2Þ
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Scheme 3 A working hypothesis.
trans-RhCl(CO)(PPh3)2 (9), as confirmed by 1H and 31P NMR
and by IR spectroscopy.
Fig. 1 X-Ray crystallography of the complex 6.
Next, we focused our attention on substrate modification to
achieve tandem catalytic reactions via an Rh–CPD intermediate.14
We postulated that structural changes in CPD would liberate Rh
metal from coordination, thereby making the reaction catalytic.
We assumed that a metal coordinated to CPD would show
properties similar to those of a metal coordinated to alkenes
or alkynes. In transition metal-catalyzed [3,3]-sigmatropic
rearrangement, the reaction becomes favorable by enhancing
electrophilicity through metal coordination (Scheme 3). However,
Rh-catalyzed Claisen rearrangement reactions are much less
studied than Pd-catalyzed rearrangement.15
Fortunately, we were able to obtain a single crystal of 6 by
slow diffusion of 5 into [RhCl(cod)]2 (1) (see ESIw). X-Ray
crystallography confirmed the structure of 6, which was a
chloro-bridged dimer of Rh(CO)(cpd) (Fig. 1). It is important
to note that, as far as we know, this is the first structural
characterization of a chloro-bridged Rh(I)–CPD carbonyl
complex.13 It is assumed that the poor reactivity of diynes
toward Rh-catalyzed carbonylative cycloaddition might be
due to the formation of a stable RhCl(CO)(cpd) complex,
which deactivates the Rh-catalyst and prevents additional
catalytic cycles. In fact, when 5 was reacted with a catalytic
amount of [Rh(cod)Cl]2 under CO even at 120 1C, 70% of 5
was recovered from the reaction shown by eqn (1).
Based on these assumptions, we attempted Rh-catalyzed
carbonylative cycloaddition of the phenoxide-substituted
diyne 10a under CO. We successfully isolated the bicycle-
[3.3.0]octadiene derivative with an ortho substituted phenol
11a, which was formed by tandem carbonylation and Claisen
rearrangement (eqn (3)).
Next, we investigated the thermal stability of 6 and its
reactivity toward other ligands that might possibly be used
as additives to catalyze the liberation of CPD. We envisioned
that Rh(I)-catalyzed incorporation of CO into a diyne could be
accomplished by liberation of CPD from Rh(I) metal. But,
similar to other MLn–CPD complexes, CPD was strongly
coordinated to the rhodium center to fulfill 18e configuration.
To our surprise, 6 exhibited high stability when exposed to
high temperatures o200 1C. By contrast to the results
reported by McVey and Maitlis,10b when 6 was heated at
130 1C for 20 h, the decarbonylation of complex 6 was not
observed. Likewise, the liberation of 7 was not detected by GC
analysis.
ð3Þ
It should be noted that thermal Claisen rearrangement
usually requires a high reaction temperature (4200 1C). Thus,
formation of the RhCl(CO)(cpd) complex makes the Claisen
rearrangement a favorable reaction. Encouraged by this
positive result, we examined the scope of substrates for the
Rh-catalyzed tandem [2+2+1] carbonylative/Claisen
rearrangement (Table 1). As expected, substrate reactivity
was strongly influenced by the substituent (R1) that was
located in a sterically hindered environment (entries 1–4).
The cyclopropyl-substituted substrate 10b showed comparable
reactivity (entry 2). But the substrate with the Pr group gave
only a trace amount of the expected compound (entry 3). In
the case of R1 = H, a complicated mixture was obtained
(entry 4). Next, we observed the effect of arene substituents.
We expected that Claisen rearrangement would be accelerated
by electron-donating substituents. Surprisingly, use of an
o-methyl-substituted phenyl ring, as in 10e, gave a mixture
of ortho (45%)- and para (12%)-substituted phenol products,
11e and 12e (entry 5). However, use of R1 = c-Pr selectively
gave the ortho-substituted phenol product (entry 6).
Para-substitution of the phenyl ring exclusively gave ortho-
substituted phenol products (entries 7–9). We propose that the
In the presence of excess pyridine (as a solvent), 6 was
transformed into the RhCl(cpd)(py)2 complex (8) in quantitative
yield (498%) (Scheme 2). This result is similar to the pyridine
coordination of a RhCl(CO)(cpd) complex described by
McVey and Maitlis.10b The structure of complex 8 was also
confirmed by X-ray analysis (see ESIw). When 6 was reacted
with excess PPh3, CPD was liberated from Rh, and 7
was isolated in high yield with the concomitant formation of
Scheme 2
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Table
1 Rh(I)-catalyzed tandem carbonylative [2+2+1] cyclo-
Notes and references
addition and Claisen rearrangementa
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R. J. Donovan, Chem. Rev., 1996, 96, 635; N. Chatani, Chem.
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2 For recent reviews on the Pauson–Khand reaction, see:
(a) Y. K. Chung, Coord. Chem. Rev., 1999, 188, 297;
(b) K. Brummond and J. L. Kent, Tetrahedron, 2000, 56, 3263;
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4 Y. Koga, T. Kobayashi and K. Narasaka, Chem. Lett., 1998, 249;
N. Jeong, S. Lee and B. K. Sung, Organometallics, 1998, 17, 3642.
5 For allenynes see: (a) C. Mukai, I. Nomura, K. Yamanishi and
M. Hanaoka, Org. Lett., 2002, 4, 1755; (b) K. M. Brummond,
H. Chen, K. D. Fisher, A. D. Kerekes, B. Rickards, P. C. Sill and
S. J. Geib, Org. Lett., 2002, 4, 1931. For dienynes: ; (c) P. A. Wender,
N. M. Deschamps and G. G. Gamber, Angew. Chem., Int. Ed., 2003,
42, 5948. For trienes: ; (d) P. A. Wender, M. P. Croatt and
N. M. Deschamps, J. Am. Chem. Soc., 2004, 126, 5948.
6 (a) E. Muller, Synthesis, 1974, 761; (b) E. A. Kelly, G. A. Wright
¨
and P. M. Maitlis, J. Chem. Soc., Dalton Trans., 1979, 178.
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metallics, 1985, 4, 1459; (b) Y. Shvo, D. Czarie and
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R. A. Dubbert, J. Chem. Soc., Chem. Commun., 1991, 202;
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D. Reshef and Y. Shvo, Organometallics, 1995, 14, 3996;
(d) H. K. Gupta, N. Rampersad, M. Stradiotto and
M. J. McGlinchey, Organometallics, 2000, 19, 184.
10 (a) P. M. Maitlis and S. McVey, J. Organomet. Chem., 1965, 4, 254;
(b) S. McVey and P. M. Maitlis, Can. J. Chem., 1966, 44, 2429;
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11 A. M. Castro, Chem. Rev., 2004, 104, 2939.
12 T. Shibata, K. Yamashita, H. Ishida and K. Takagi, Org. Lett.,
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a
Reactant (0.5 mmol) and [RhCl(cod)]2 (0.025 mmol) in TCE
(2.5 mL, TCE
= 1,1,2,2-tetrachloroethane) at 120–130 1C for
c
b
20 h. Isolated yield. c-Pr = cyclopropyl.
13 Recently, Severin reported fascinating structures of Rh(I)Cl(cpd)
complexes by X-ray analysis. They reported the CPD-concentration-
dependent formation of different types of complexes by
mixing [RhCl(CO)2]2 with 2. However, because the complexation
reaction proceeds via [2+2+1] carbonylative cycloaddition
and [3.3]-sigmatropic rearrangement, as shown in Scheme 3.16
In conclusion, we isolated [RhCl(CO)(cpd)]2 (6) from a
1,6-diyne and [Rh(cod)Cl]2 at room temperature under CO.
The complex showed high stability under thermal conditions.
With the addition of ligands, such as pyridine or triphenyl-
phosphine, [RhCl(CO)(cpd)]2 was converted into either
pyridine-coordinated complex 8 or bicyclo[3.3.0]octadienone
3. The use of a tandem Claisen rearrangement to activate
the Rh-catalyst in the Rh–CPD complex was studied. The
structural modification of reactants successfully activated a
new tandem catalytic carbonylative cycloaddition and Claisen
rearrangement. Additional studies of both the scope
and mechanism of the tandem reaction (ortho- and para-
substitution) are in progress.
was carried out without
a CO atmosphere, they isolated
non-carbonyl-coordinated Rh(I)Cl(cpd) complexes. D. Rechavi,
R. Scopelliti and K. Severin, Organometallics, 2008, 27, 5978.
14 For a paper on a Co-catalyzed tandem carbonylative cycloaddition
of 1,6-diyne via cyclopentadienone intermediates, see: S. U. Son,
Y. K. Chung and S.-G. Lee, J. Org. Chem., 2000, 65, 6142. For a
paper on a Co-catalyzed carbonylative cycloaddition to give CPDs
from 1,6-diynes, see: T. Sugihara, A. Wakabayashi, H. Takao,
H. Imagawa and M. Nishizawa, Chem. Commun., 2001, 2456.
15 For a paper on Rh(I)-catalyzed Claisen rearrangement, see:
P. Eilbracht, A. Gersmeier, D. Lennartz and T. Huber, Synthesis,
1995, 330.
16 The reaction mechanism, particularly that of the formation of the
para-substituted product 12, is not clear at the present time. A
possible mechanism for the formation of the para-substituted
product is the ion-pair mechanism. Although, in most cases,
phenoxide nucleophiles undergo O-alkylation instead of C-alkylation,
C-alkylation at the para position reportedly takes place when
phenol is used as the nucleophile. Nevertheless, at the present
time, we cannot comment on whether the Claisen rearrangement
proceeds via a concerted or an ion-pair mechanism.
S. I. L. wishes to acknowledge financial support from KRF
(2007-357-C00057) (Korea Research Foundation) and JSPS
(Japan Society for the Promotion of Science) for a post
doctoral fellowship.
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Chem. Commun., 2010, 46, 3345–3347 | 3347