Cobalt/rhodium heterobimetallic nanoparticle-catalyzed carbonylative [2+2+1] cycloaddition of allenes and bisallenes to Pauson-Khand-type reaction products

Article abstract of DOI:10.1039/b718107h

The first catalytic intra- and intermolecular [2+2+1] cocyclization reactions of allenes and carbon monoxide have been developed. In the Co ₂Rh₂ heterobimetallic nanoparticle-catalyzed carbonylative [2+2+1] cycloaddition of allenes a

Full text of DOI:10.1039/b718107h

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Cobalt/rhodium heterobimetallic nanoparticle-catalyzed carbonylative
[2+2+1] cycloaddition of allenes and bisallenes to Pauson–Khand-type
reaction productswz
Ji Hoon Park, Eunha Kim, Hyeong-Mook Kim, Soo Young Choi and
Young Keun Chung*
Received (in Cambridge, UK) 23rd November 2007, Accepted 11th March 2008
First published as an Advance Article on the web 11th April 2008
DOI: 10.1039/b718107h
The first catalytic intra- and intermolecular [2+2+1] cocycliza-
tion reactions of allenes and carbon monoxide have been devel-
oped. In the Co2Rh2 heterobimetallic nanoparticle-catalyzed
carbonylative [2+2+1] cycloaddition of allenes and carbon
monoxide, the allenes formally serve both as an excellent alkene-
and alkyne-like moiety within a Pauson–Khand-type process.
Scheme 1 Co/Rh nanoparticle catalyzed carbonylation reaction of
phenylallene with 1-phenylpropyne.
Recently, the chemistry of allenes has attracted a lot of
attention, presumably due to their high reactivity.1 In parti-
cular, they are being increasingly used in metal-catalyzed
annulation reactions2 due to their unique reactivity and special
manipulation possibilities. However, if the reactivity of allenes
is not controlled properly, the reactions of such substrates tend
to be complex and useless. In this respect, the allenic
Pauson–Khand reaction is a very useful process.3
When phenylallene was reacted with 2 atm of CO in the
presence of a catalytic amount (5 mol%) of Co2Rh2 at 130 1C for
18 h, two carbonylated products, 1b and 1c, were isolated in 59%
and 3% yield, respectively (Scheme 2). The product was found to
be a cyclic enone bearing two phenyl groups. Thus, the allenes
formally serve both as an excellent alkene and alkyne-type
moiety in Co2Rh2-catalyzed [2+2+1] cycloaddition with carbon
monoxide when compared to the classical Pauson–Khand pro-
cess. About two decades ago, Pasto et al. reported7 the formation
of 1b in the stoichiometric carbonylative reaction of phenylallene
with tris(triphenylphosphine)nickel(0). However, they obtained
1c in 4.8% yield as one of eight major by-products.
In the exploration of the use of Co/Rh nanoparticles as a
catalyst,4 we recently studied a Co/Rh nanoparticle-catalyzed
allenic Pauson–Khand reaction.5 The catalytic system was quite
effective to give a Pauson–Khand reaction product. As an
extension of the previous study, we recently tested an inter-
molecular allenic Pauson–Khand reaction. When an
intermolecular Pauson–Khand reaction of phenylallene with
1-phenylpropyne was examined, two unknown compounds were
isolated instead of the allenic Pauson–Khand reaction product
Using phenylallene as a substrate, we screened the reaction
conditions including the reaction temperature, the reaction
time, and the CO pressure (Table 1). The reaction solvent
(toluene) was adopted from the previous study.5 The reaction
was highly sensitive to and dependent upon the reaction
temperature and the reaction time. The yield was higher at
130 1C as opposed to 100 1C. However, when the reaction time
was lengthened, the yield decreased. Thus, the best yield (80%)
was obtained when the temperature was 130 1C and the
reaction time was 6 h. As shown in Table 1, the reaction of
1a in the presence of [Rh(CO)2Cl]2 at 130 1C for 6 h gave a
mixture of 1b and 1c in 32% and 25% yields, respectively.
1
(Scheme 1). According to the H NMR, the reaction product
did not seem to have incorporated the 1-phenyl propyne moiety.
Instead, carbonylative cycloaddition products were obtained.
An X-ray diffraction analysis (Fig. 1) showed that the products
had been constructed from two allene units and carbon mon-
oxide. Although many transition metal-catalyzed cycloaddition
of allenes have been disclosed,2,6 there has been no report on the
catalytic formation of cyclopentenones via a [2+2+1] cycload-
dition of two allenes and carbon monoxide. Encouraged by this
observation, we examined the reaction more closely. Here we
communicate our preliminary results.
Intelligent Textile System Research Center, Department of Chemistry,
College of Natural Sciences, Seoul National University, Seoul 151-
747, Korea. E-mail: ykchung@snu.ac.kr; Fax: +82-2-889-0310;
Tel: +82-2-880-6662
w Electronic supplementary information (ESI) available: Experimen-
tal. See DOI: 10.1039/b718107h
Fig. 1 X-Ray structures of 1b and 1c with 30% probability thermal
z CCDC 650888 & 650889. For crystallographic data in CIF or other
electronic format see DOI: 10.1039/b718107h
ellipsoids.
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Table 2 Co2Rh2-Catalyzed [2+2+1] cycloaddition of allenesa
Scheme 2 Carbonylative cycloaddition of allene 1a.
Encouraged by the above results, we screened other allenes
under the optimized reaction conditions (Table 2).y In all cases
except entry 5, compounds b and c are easily separated by
Entry
R
Product
Yield (%) (b/c)b
1
2
3
4
5
6
7
a
4-Tolyl
2a
3a
4a
5a
6a
7a
8a
2b + 2c
3b + 3c
4b + 4c
5b
6b + 6c
7b + 7c
8b + 8c
84(70/14)
84(71/13)
92(72/20)
61(61/0)
84(64/20)c
76(68/8)
59(52/7)
3,5-Dimethylphenyl
1-Naphthyl
4-Anisyl
4-Acetylphenyl
4-Chlorophenyl
n-Hexyl
1
chromatography and show characteristic H NMR patterns.
1
Thus we can easily differentiate them by H NMR study: for
example, the benzylic proton in b is shifted up-field with
respect to the corresponding proton in c. When 4-tolylallene
(2a) was used as a substrate, 2b was obtained in 70% yield,
with concomitant formation of 2c in 14%. In the case of 1,3-
dimethyl-5-(1,2-propadienyl)benzene (3a), the yields of 3b and
3c were 71% and 13%, respectively. Treatment of (naphtha-
len-1-yl)allene (4a) under the same reaction conditions led to
the isolation of 4b isomer in 72% yield, with isomer 4c in 20%
yield. Interestingly, a reaction of p-anisylallene (5a) under the
same reaction conditions gave 5b in 61% yield as the sole
product. When 40-acetylphenylallene (6a) was used as a sub-
strate, 6b and 6c were obtained in 64% and 20% yields,
respectively. In the same way, reaction of p-chlorophenylallene
(7a) gave 7b and 7c in 68% and 8% yields, respectively, and
reaction of 1,2-nonadiene (8a) gave 8b and 8c in 52% and 7%
yields, respectively. The yield of the minor isomer (c) was
somewhat dependent upon the substituent on the allene. The
overall yields of carbonylative cycloaddition reactions, apart
from entries 4 and 7, were quite high (76–92%).
Reaction condition: 1.0 mmol allene, 5 mol% Co2Rh2 catalyst, 5 mL
b
c
toluene, 130 1C, CO (2 atm), and 6 h. Isolated yield. Yields of 6b
and 6c were determined by H NMR.
1
result from these annulation processes. However, only one
regioisomer was isolated. The isolated regioisomer was easily
1
characterized by H NMR peak of the vinyl proton (see ESIw).
Introduction of a methyl to the position between the allene and
N-Ts did not alter the yield of the reaction (10b, entry 1).
However, introduction of a phenyl group onto the allene (11b,
entry 2) slightly enhanced the yield to 74%. In contrast, intro-
duction of a methyl group onto the allene at the same position
(12b, entry 3) slightly decreased the yield to 53%. Reaction of a
bisallene (13a) with an oxygen tether gave 13b in 45% yield.
Interestingly, introduction of substitutents on the terminal
positions of an allene (14a) led to the isolation of a monocyclic
seven-membered triene, 14b, as the sole product (Scheme 3). In the
absence of carbon monoxide, no reaction was observed. Thus, the
presence of carbon monoxide was indispensable. Very recently,
Lu and Ma have reported9 the [RhClL2]2 (L = CO, COD)-
catalyzed conversion of 1,5-bisallenes to seven-membered trienes.
Considering the results obtained above, we postulated a
mechanism for this newly developed intramolecular [2+2+1]
cocyclization of allenes and CO, as shown in Scheme 4.
Coordination of carbon monoxide to the nanoparticles gen-
erates I as an active metal species. Then, the next step involves
We next investigated the use of bisallenes as substrates in a
Co2Rh2-catalyzed carbonylative cycloaddition reaction. Com-
pared to the use of allenes as substrates, the use of bisallenes is
rather uncommon, as seen from the fact that only a handful of
reactions have been reported.8 We screened the reaction
conditions including the amount of the catalyst, the CO
pressure, the reaction temperature, and the reaction time
(Table 3). Treatment of 9a led to the isolation of bicyclic
enone 9b. We have provisionally established a set of optimum
reaction conditions to be as follows: 5 mol% Co2Rh2, toluene,
2 atm CO, 100 1C, and 4 h.
Table 3 Reaction of 9a under various reaction conditionsa
We investigated the carbonylative cycloaddition of other
bisallenes under the optimized reaction conditions (Table 4).
For entries 1–3, it is possible for two regioisomeric products to
Table 1 Reaction of 1a under various reaction conditionsa
CO/
Yield
Entry Catalyst
atm Temp/1C Time/h (1b/1c)b
Entry
CO/atm
Temp/1C
Time/h
Yield (%)b
1
2
3
4
5
6
7
8
a
Co2Rh2 5 mol%
1
2
3
2
2
2
2
2
130
130
130
130
130
100
130
100
18
18
18
12
6
12
6
6
53(45/8)
62(59/3)
60(55/5)
72(62/10)
80(68/12)
51(45/6)
57(32/25)
39(39/0)
1
2
3
4
5
6
a
1
2
2
2
3
2
100
100
80
110
100
100
12
12
12
12
4
33
66
n.r.c
55
70
68
b
Co2Rh2 5 mol%
Co2Rh2 5 mol%
Co2Rh2 5 mol%
Co2Rh2 5 mol%
Co2Rh2 5 mol%
[Rh(Co)2Cl]2 5 mol%
[Rh(Co)2Cl]2 5 mol%
4
0.3 mmol of 9a and
5
mol% Co2Rh2 were used. Isolated
c
yield. The reactant was recovered.
b
1.0 mmol of 1a and 5 mL toluene were used. Isolated yield.
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Table 4 Co2Rh2-Catalyzed [2+2+1] cycloaddtion of bisallenesa
H. Nonetheless, it is believed that the 1,3-hydrogen shift is
catalyzed by some Lewis acidic or basic species present within
the reaction mixture after carbonylation. The drive for
isomerization of IV to create the product is the relief of strain
in the twisted diene chromophore.
Entry Bisallene
Product
Yield (%)b
In conclusion, we have developed the first catalytic inter-
and intramolecular [2+2+1] cocyclization of allenes and CO.
This process provides a rapid and atom-economical method
for the synthesis of a variety of cyclopentenones with an
exocyclic double bond, or [5.3]bicyclic enones in one step.
The process developed in this study will enrich the chemistry
of Pauson–Khand-type transformations and related reactions.
This work was supported by grant No. (R01-2005-000-10548-
0) from the Basic Research Program of the Korea Science &
Engineering Foundation, the Korean Government (MOEHRD)
(KRF-2005-070-C00072 and R02-2004-000-10005-0), and the
SRC/ERC program of MOST/KOSEF (R11-2005-065). JHP,
EK, and HMK thank the Brain Korea 21 fellowships.
1
2
3
R1 = Me, R2 = H 10a 10b
R1 = H, R2 = Ph 11a 11b
R1 = H, R2 = Me 12a 12b
67
74
53
4
45
a
Reaction condition: 0.3 mmol allene, 5 mol% Co2Rh2 catalyst, 3 mL
b
toluene, 100 1C, and 4 h. Isolated yield.
Notes and references
y Phenylallene 1a (1.0 mmol, 116 mg), 5 mol% Co2Rh2 (45 mg of the
immobilized Co2Rh2), and toluene (5 mL) were placed in a 100 mL
stainless steel autoclave equipped with a stirring bar. The reactor was
charged with 2 atm of CO and heated at 130 1C for 6 h. After the
reactor was cooled to room temperature, the solution was filtered and
concentrated, and the product isolated by chromatography on a silica
gel column eluting with hexane and ethyl acetate (v/v, 4 : 1) to give 1b
(88 mg, 68% yield) and 1c (13 mg, 12% yield).
Scheme 3 Co2Rh2 catalyzed cycloisomerization of 14a.
1 For reviews, see: (a) K. M. Brummond and J. E. DeForrest,
Synthesis, 2007, 795–818; (b) S. Ma, Chem. Rev., 2005, 105,
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Scheme 4 Proposed mechanism.
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formation of a bis-p-complex II in which p-bond formation
occurs around the unsubstituted double bonds of the allenes.
Coupling in II occurs to produce apparently only III. Carbo-
nylation of III produces IV which undergoes a 1,3-hydrogen
shift to produce the product, b. The hydrogen shift occurs to
the carbon with less substituents, since a substituent on the
carbon is believed to restrict the shift of a hydrogen. Having
stated this, it is acknowledged that this rationalisation does
not explain the regioselectivity observed in the formation of
10b from substrate 10a, where the hydrogen shift in both
possible directions would be to a carbon with only an
9 P. Lu and S. Ma, Org. Lett., 2007, 9, 2095–2097.
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2390 | Chem. Commun., 2008, 2388–2390