Rhodium-catalyzed [2+2+2] cycloaddition of diynes with carbodiimides and carbon dioxide under ambient conditions

Article abstract of DOI:10.1002/chem.201304623

It has been established that a cationic rhodium(I)/H₈-binap complex is able to catalyze the [2+2+2] cycloaddition of diynes with carbodiimides and carbon dioxide under ambient conditions. Enantio- and/or regioselective variants of these reactions are also disclosed. Turn around: It has been established that a cationic rhodium(I)/H₈-binap complex is able to catalyze the [2+2+2] cycloaddition of diynes with carbodiimides and carbon dioxide under ambient conditions. Enantio- and/or regioselective variants of these reactions are also disclosed. Copyright

Full text of DOI:10.1002/chem.201304623

DOI: 10.1002/chem.201304623
Communication
&
Cycloaddition
Rhodium-Catalyzed [2+2+2] Cycloaddition of Diynes with
Carbodiimides and Carbon Dioxide under Ambient Conditions
Masahiro Ishii,[a] Fumiya Mori,[a] and Ken Tanaka*[a, b]
Abstract: It has been established that a cationic rho-
dium(I)/H8-binap complex is able to catalyze the [2+2+2]
cycloaddition of diynes with carbodiimides and carbon di-
oxide under ambient conditions. Enantio- and/or regio-
selective variants of these reactions are also disclosed.
The transition-metal-catalyzed [2+2+2] cycloadditions involv-
Scheme 1. Transition-metal-catalyzed [2+2+2] cycloadditions of diynes with
carbodiimides and carbon dioxide.
ing heterocumulenes have been extensively explored for the
efficient synthesis of heterocycles because of their atom eco-
nomical and convergent nature.[1] In particular, the [2+2+2] cy-
cloaddition involving isocyanates is an efficient method for the
synthesis of nitrogen heterocycles.[2–10] For example, a number
of the efficient transition-metal-catalyzed [2+2+2] cycloaddi-
tions of two alkynes including diynes with isocyanates to form
substituted 2-pyridones have been achieved under mild reac-
tion conditions by using cobalt,[2] nickel,[3] ruthenium,[4] iridi-
um,[5] and rhodium[6–8] catalysts. On the contrary, the transition-
metal-catalyzed [2+2+2] cycloadditions of two alkynes with
carbodiimides and carbon dioxide[11] require harsh reaction
conditions.[2b,e,10a,12–17] Cobalt(I)-cyclopentadiene complexes are
able to catalyze the [2+2+2] cycloaddition of diynes or two
monoynes with carbodiimides, however, microwave irradiation
and/or high reaction temperature are necessary (Sche-
me 1).[2b,e,12] Very recently, it was reported that [RhCl(PPh3)3] is
able to catalyze the complete intramolecular [2+2+2] cycload-
dition of bis(propargylphenyl)carbodiimides under toluene
reflux, whereas the intermolecular reaction was not includ-
ed.[13a] Cobalt,[2e] nickel[15] and rhodium[16] complexes are able
to catalyze the [2+2+2] cycloaddition of diynes or two mono-
ynes with carbon dioxide at RT–1308C, however, these reac-
tions suffer from low product yields and/or employment of ex-
tremely high pressure carbon dioxide. The use of nickel(0)-N-
heterocyclic carbene complexes allows the employment at-
mospheric pressure carbon dioxide in the [2+2+2] cycloaddi-
tion with internal diynes, whereas elevated reaction tempera-
ture (608C) is still required (Scheme 1).[17] On the other hand,
our research group previously reported that cationic rho-
dium(I)/biaryl bisphosphine complexes are able to catalyze the
[2+2+2] cycloaddition of diynes or two monoynes with isocya-
nates at room temperature.[6b,c,f–h] Herein, we have achieved
the [2+2+2] cycloadditions of diynes with carbodiimides and
atmospheric pressure carbon dioxide at room temperature
using the cationic rhodium(I)/biaryl bisphosphine complexes as
catalysts.
We first examined the reactions of malonate-linked terminal
1,6-diyne 1a with diaryl carbodiimide 2a (1.1 equiv) in the
presence of
a cationic rhodium(I)/bisphosphine catalysts
(10 mol%) at room temperature (Table 1, entries 1–9). In this
catalyst screening, a (CH2Cl)2 solution of 1a and 2a was added
to a (CH2Cl)2 solution of the rhodium catalyst at room tempera-
ture. Pleasingly, all the bisphosphine complexes examined
were able to catalyze the room temperature [2+2+2] cycload-
dition of 1a with 2a to give the expected cycloadduct 3aa.
Among them, H8-binap (entry 1) and dppe (entry 8) were
found to be the most effective ligands (Figure 1). In order to
improve the yield of 3aa, increasing the amount of 2a
(2 equiv) was tested. Although the identical yield of 3aa was
observed by using dppe as a ligand (entry 10), the significantly
improved yield of 3aa was observed by using H8-binap as
ligand (entry 11). Further improved yield of 3aa was achieved
when a (CH2Cl)2 solution of 1a was added dropwise over 5 min
to a (CH2Cl)2 solution of 2a and the rhodium catalyst
(entry 12), whereas prolonged addition time over 30 min de-
creased the yield of 3aa. This operation effectively suppressed
the undesired homo-[2+2+2] cycloaddition of 1a. Unfortu-
nately, decreasing the catalyst loading to 5 mol% lowered the
yield of 3aa (entry 13).
[a] M. Ishii, F. Mori, Prof. Dr. K. Tanaka
Department of Applied Chemistry, Graduate School of Engineering
Tokyo University of Agriculture and Technology
Koganei, Tokyo184-8588 (Japan)
E-mail: tanaka-k@cc.tuat.ac.jp
Homepage: http://www.tuat.ac.jp/~tanaka-k/
[b] Prof. Dr. K. Tanaka
JST, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012 (Japan)
Thus, we explored the scope of this process under the
above-optimized reaction conditions (Scheme 2). With respect
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304623.
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Table 1. Optimization of reaction conditions for rhodium-catalyzed
[2+2+2] cycloaddition of 1a with 2a.[a]
Entry
Ligand
Catalyst [mol%]
2a [equiv]
Yield [%][b]
1
2
3
4
5
6
H8-binap
binap
segphos
biphep
dppf
dppb
dppp
dppe
dppm
10
10
10
10
10
10
10
10
10
10
10
10
5
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
1.1
2.0
2.0
2.0
2.0
43
42
36
35
27
28
28
54
34
54
69
77
59
7[c]
8[c]
9[c]
10
11
12[d]
13[d,e]
dppe
H8-binap
H8-binap
H8-binap
[a] [Rh(cod)2]BF4 (0.010 mmol), ligand (0.010 mmol), 1a (0.10 mmol), 2a
(0.11–0.20 mmol), and (CH2Cl)2 (2.0 mL) were used. A (CH2Cl)2 solution of
1a and 2a was added to a (CH2Cl)2 solution of the Rh catalyst; [b] isolat-
ed yield; [c] [Rh(nbd)2]BF4 was used; [d] 1a was added dropwise to
a
(CH2Cl)2 solution of 2a and the Rh catalyst over 5 min; [e] 1a
(0.20 mmol), 2a (0.40 mmol), and (CH2Cl)2 (4.0 mL) were used.
Scheme 2. Rhodium-catalyzed [2+2+2] cycloadditions of diynes 1 with car-
bodiimides 2. [Rh(cod)2]BF4 (0.010 mmol), H8-binap (0.010 mmol),
1 (0.10 mmol), 2 (0.11–0.20 mmol), and (CH2Cl)2 (2.0 mL) were used. A
(CH2Cl)2 solution of 1 was added dropwise to a (CH2Cl)2 solution of 2 and
the Rh catalyst over 5 min. The cited yields are of the isolated products.
[a] A (CH2Cl)2 solution of 1 and 2 was added to a (CH2Cl)2 solution of the Rh
catalyst.
Figure 1. Structures of bisphosphine ligands.
to diynes, malonate- and dimethoxypropane-linked terminal
1,6-diynes 1a–c treated with 2a gave the corresponding cyclo-
adducts 3aa–ca in high yields. However, the use of tosyl-
amide-linked terminal 1,6-diyne 1d lowered the cycloadduct
yield (3da) due to the rapid homo-[2+2+2] cycloaddition of
1d. Importantly, methylene-linked terminal 1,6-diyne 1e
smoothly reacted with 2a to give cycloadduct 3ea in high
yield despite the absence of the Thorpe–Ingold effect.[18] Not
only 1,6-diynes, but also 1,7-diynes 1 f,g could be employed. In
the reactions of internal 1,6-diynes 1h–l, the use of 1.1 equiv
of 2a was optimum and the slow addition of substrates were
not required due to low reactivity of internal 1,6-diynes to-
wards the homo-[2+2+2] cycloaddition. With respect to carbo-
diimides, not only diaryl carbodiimide 2a but also dialkyl car-
bodiimide 2b could be employed.[19] Next, the regioselective
[2+2+2] cycloadditions using unsymmetrical 1,6-diynes were
examined. The reaction of unsymmetrical 1,6-diyne 1m, pos-
sessing hydrogen and a methyl group at each alkyne terminus,
with 2a furnished 3ma with high regioselectivity along with
minor regioisomer 3ma’. On the other hand, lower regioselec-
tivity was observed in the reaction of methyl- and phenyl-sub-
stituted unsymmetrical 1,6-diyne 1n with 2a.
The regio- and enantioselective [2+2+2] cycloaddition of
unsymmetrical 1,6-diynes, possessing hydrogen and an ortho-
substituted phenyl group at each alkyne terminus, with 2a
was also examined (Scheme 3).[20,21] Pleasingly, methylene-
linked 1,6-diyne 1o, possessing the 2-chloro-substituted
phenyl group, reacted with 2a in the presence of the cationic
rhodium(I)/(S)-H8-binap catalyst at room temperature to give
axially chiral heterobiaryl 3oa[22] as a single regioisomer with
good yield and ee values. The use of (R)-DTBM-segphos as
a ligand significantly increased the ee value, although the
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Table 2. Optimization of reaction conditions for rhodium-catalyzed
[2+2+2] cycloadditions of 1h with CO2.[a]
Entry Ligand
Catalyst Solvent Prestirring Addition
[mol%]
Yield
time [min] time [min] [%][b]
1
2
3
4
5
6
7
8
H8-binap 20
H8-binap 20
H8-binap 20
H8-binap 20
H8-binap 20
(CH2Cl)2 30
10
10
10
10
10
10
10
10
10
10
10
10
10
10
30
48
19
35
<5
<5
41
19
9
25
0
0
0
0
18
59
78
77
94
90
42
79
CH2Cl2
C6H5Cl
30
30
Scheme 3. Rhodium-catalyzed regio- and enantioselective [2+2+2] cycload-
dition of unsymmetrical diynes 1o–q with carbodiimide 2a.
toluene 30
THF 30
binap
20
(CH2Cl)2 30
(CH2Cl)2 30
(CH2Cl)2 30
(CH2Cl)2 30
(CH2Cl)2 30
(CH2Cl)2 30
(CH2Cl)2 30
(CH2Cl)2 30
(CH2Cl)2 30
(CH2Cl)2 30
(CH2Cl)2 30
(CH2Cl)2 60
segphos 20
product yield decreased. Malonate-linked 1,6-diyne 1p also re-
acted with 2a to give heterobiaryl 3pa with high ee value. Al-
though the product yield decreased, 1,6-diyne 1q, possessing
the methoxycarbonyl-substituted phenyl group, also reacted
with 2a to give heterobiaryl 3qa with high ee value.
biphep
dppf
dppb
dppp
dppe
20
20
20
20
20
20
5
5
5
5
5
9
10
11[c]
12[c]
13[c]
14
15
16
17
18
19[d]
dppm
The successful cationic rhodium(I)/H8-binap complex-cata-
lyzed [2+2+2] cycloaddition of diynes with carbodiimides
prompted our investigation into the use of carbon dioxide in
place of the carbodiimide. Pleasingly, after prestirring
a (CH2Cl)2 solution of the cationic rhodium(I)/H8-binap complex
(20 mol%) under atmospheric pressure carbon dioxide,
a (CH2Cl)2 solution of malonate-linked internal 1,6-diyne 1h
was added dropwise over 10 min at room temperature and
the resulting solution was stirred for 16 h to give the corre-
sponding bicyclic 2-pyrone 4h in moderate yield (Table 2,
entry 1). Effect of solvents (entries 1–5) and ligands (entries 6–
13) were then examined, which revealed that the use of
(CH2Cl)2 as a solvent and H8-binap as a ligand is the best com-
bination (entry 1). Decreasing the catalyst loading to 5 mol%
significantly lowered the yield of 4h due to the formation of
the homo-[2+2+2] cycloaddition product of 1h (entry 14). In
order to suppress the undesired homo-[2+2+2] cycloaddition,
the slow addition of 1h was examined. Gratifyingly, the addi-
tion of 1h over 30 min significantly improved the yield of 4h
(entry 15). Prolonged addition time (120 min) further improved
the yield of 4h (entry 16). The effect of prestirring time of the
catalyst solution under carbon dioxide was also examined (5–
60 min, entries 16–18). Interestingly, short stirring time (5 min)
significantly increased the yield of 4h as a result of the sup-
pression of the homo-[2+2+2] cycloaddition of 1h
(entry 18).[23] This reaction was found to be very rapid and was
completed after only 1 h (entry 19). The reaction under re-
duced carbon dioxide pressure (0.5 atm) significantly de-
creased the yield of 4h (entry 20). Finally, the use of inexpen-
sive binap instead of H8-binap was tested under the best con-
ditions shown in entry 19, but the yield of 3aa decreased
(entry 21).
H8-binap
H8-binap
H8-binap
H8-binap
H8-binap
H8-binap
120
120
120
120
120
120
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
5
5
5
5
5
20[d,e] H8-binap
21[d]
binap
5
5
[a] [Rh(cod)2]BF4 (0.010–0.040 mmol), ligand (0.010–0.040 mmol), 1h
(0.20 mmol), CO2 (1 atm), and solvent (5.0 mL) were used. A solution of
1h was added to a solution of the Rh catalyst at RT over 10–120 min;
[b] isolated yield; [c] [Rh(nbd)2]BF4 was used; [d] reaction time: 1 h;
[e] CO2 (0.5 atm) and N2 (0.5 atm).
sessing free hydroxyl groups, also reacted with carbon dioxide
to give the corresponding diol 4v in moderate yield. Hetero-
atom-linked internal 1,6-diynes 1j,k could be employed, but
the product yields were low. Sterically demanding diethyl-sub-
stituted internal 1,6-diyne 1w smoothly reacted with carbon
dioxide to give the corresponding bicyclic 2-pyrone 4w in
high yield. However, malonate-linked terminal 1,6-diyne 1a
and methylene-linked internal 1,6-diyne 1l failed to react with
atmospheric pressure carbon dioxide due to the rapid homo-
[2+2+2] cycloaddition of diynes. Next, the regioselective
[2+2+2] cycloadditions using unsymmetrical 1,6-diynes were
examined. Although unsymmetrical 1,6-diynes 1m,n employed
in Table 2 failed to react with atmospheric pressure carbon di-
oxide, the reaction of ethyl- and methyl-substituted unsym-
metrical 1,6-diyne 1x proceeded to give 4xa (major) and 4xa’
(minor) with moderate regioselectivity. The reaction of iso-
propyl- and methyl-substituted unsymmetrical 1,6-diyne 1y
with carbon dioxide also proceeded to give 4ya (major) and
4ya’ (minor) with high regioselectivity.
Thus, we explored the scope of this process under the
above-optimized reaction conditions (Scheme 4). With respect
to diynes, malonate- (1h,i), 1,3-diketone- (1r,s), and 1,3-dialkox-
ypropane-linked internal 1,6-diynes (1t,u) reacted with atmos-
pheric pressure carbon dioxide to give the corresponding bicy-
clic 2-pyrones 4 in good to excellent yields. 1,6-Diyne 1v, pos-
As the reactions of diynes with atmospheric pressure carbon
dioxide proceeded more smoothly than those with carbodi-
imides, a large-scale reaction was attempted at low-catalyst
loading. Pleasingly, the reaction of 1h (236 mg) with atmos-
pheric pressure carbon dioxide proceeded at room tempera-
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Scheme 6. Possible reaction mechanism (R1 =large substituent, R2 =small
substituent).
catalyst. Alternatively, oxidative coupling of the sterically less
demanding alkyne moiety of diyne 1 and carbodiimide 2 or
carbon dioxide furnishes heterorhodacyclopentene C. Insertion
of another alkyne moiety of 1 would also furnish the same rho-
dacycle B.
In order to determine the reaction pathway, the enantiose-
lective desymmetrization reactions of phenylacetate-derived
1,6-diynes 1a and 1b were examined. If the reaction of pro-
ceeds via rhodacycle A an almost racemic cycloadduct would
be generated due to the absence of the interaction between
a quaternary chiral center and a chiral ligand. On the contrary,
if the reaction proceeds via rhodacycle C, an enantioenriched
cycloadduct would be generated due to the presence of such
interaction. In our previous report, the rhodium(I)/(R)-binap
complex-catalyzed [2+2+2] cycloaddition of 1a with phenyl
isothiocyanate proceeded to give enantioenriched cycloadduct
with 61% ee presumably via the corresponding azarhodacyclo-
pentene intermediate.[24] The cationic rhodium(I)/(S)-H8-binap
complex-catalyzed [2+2+2] cycloadditions between 1a,b and
2a proceeded to give cycloadducts 3aa and 3ba with low ee
values (Scheme 7). On the other hand, the corresponding cy-
Scheme 4. Rhodium-catalyzed [2+2+2] cycloaddition of diynes 1 with CO2.
[Rh(cod)2]BF4 (0.010 mmol), H8-binap (0.010 mmol), 1 (0.20 mmol), CO2
(1 atm), and (CH2Cl)2 (5.0 mL) were used. After stirring a (CH2Cl)2 solution of
the Rh catalyst at RT under CO2 (1 atm) for 5 min, a (CH2Cl)2 solution of
1 was added to this solution over 120 min. The cited yields are of the isolat-
ed products.
ture for 1 h by using 1 mol% of the Rh catalyst, although bicy-
clic 2-pyrones 4h was obtained in somewhat lower yield due
to the formation of the homo-[2+2+2] cycloaddition product
of 1h (Scheme 5).
Scheme 7. Rhodium-catalyzed enantioselective desymmetrization of 1,6-
diynes 1a,b with carbodiimide 2a.
cloadditions using (R)-DTBM-segphos as ligand gave 3aa and
3ba with 33 and 23% ee, respectively. These results would
suggest that in addition to rhodacycle A, rhodacycle C contrib-
utes, at least in part, to the formation of 3aa and 3ba.
The cationic rhodium(I)/(S)-H8-binap complex-catalyzed
[2+2+2] cycloadditions between 1b and carbon dioxide also
proceeded to give cycloadduct 4b with 20% ee, although no
reaction was observed when using (R)-DTBM-segphos as
a ligand (Scheme 8). This result would also suggest the contri-
bution of rhodacycle C, at least in part, to the formation of bi-
cyclic 2-pyrone 4b.
Scheme 5. [2+2+2] Cycloaddition of 1,6-diyne 1h with CO2 using 1 mol%
of the rhodium catalyst.
Possible mechanisms for the regioselective [2+2+2] cycload-
ditions are shown in Scheme 6. Oxidative coupling of two
alkyne moieties of diyne 1 furnishes rhodacyclopentadiene A.
Regioselective insertion of carbodiimide 2 or carbon dioxide
between the sterically less demanding RhÀC bond furnishes
rhodacycle B. Reductive elimination affords cycloadduct 3 or 4,
respectively, as a major product and regenerates the rhodium
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In conclusion, we have established that a cationic rho-
dium(I)/H8-binap complex is able to catalyze the [2+2+2] cy-
cloaddition of diynes with carbodiimides and carbon dioxide
under ambient conditions. Enantio- and/or regioselective var-
iants of these reactions were also disclosed. Mechanistic stud-
ies implied that in addition to a rhodacyclopentadiene inter-
mediate, a heterorhodacyclopentene intermediate contributes,
at least in part, to the [2+2+2] cycloaddition reactions of
diynes with carbodiimides and carbon dioxide. Future work
will focus on further utilization of the cationic rhodium(I)/biaryl
bisphosphine complexes for the [2+2+2] cycloaddition involv-
ing heterocummulenes.
[7] The rhodium-catalyzed [2+2+2] cycloadditions of tethered alkene-iso-
cyanates with alkynes have been reported. See: a) R. T. Yu, T. Rovis, J.
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719.
Acknowledgements
This work was supported partly by a Grant-in-Aid for Scientific
Research (No. 20675002) from the Ministry of Education, Cul-
ture, Sports, Science and Technology (MEXT, Japan) and ACT-C
from Japan Science and Technology Agency (JST). We are
grateful to Umicore for generous support in supplying the rho-
dium complexes, and Takasago International Corporation for
the gift of H8-binap, segphos, and DTBM-segphos.
[8] For reviews of the rhodium-catalyzed [2+2+2] cycloaddition, see:
refs. [1a] (chapter 4), [1b], [1c], [1r], and [1v].
[9] For the transition-metal-catalyzed [2+2+2] cycloaddition of alkynes
with two isocyanates to form substituted pyrimidine-2,4-diones, see:
H. A. Duong, J. Louie, Tetrahedron 2006, 62, 7547. See also ref. [7d].
[10] For examples using stoichiometric transition-metal complexes, see: a) T.
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Keywords: cycloaddition · carbodiimides · carbon dioxide ·
diynes · rhodium
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tion at 808C (from 90 to 6% ee for 6 h).
[23] A CDCl3 solution of the rhodium(I)/H8-binap catalyst was stirred under
carbon dioxide (1 atm) at RT for 1 h and the resulting solution was ana-
lyzed by NMR spectroscopy. No change was observed in both 1H and
31P NMR spectra before and after treatment with carbon dioxide. There-
fore, the reason for the yield increase in short stirring time (5 min) of
the catalyst solution under carbon dioxide is not clear at the present
stage.
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could not be isolated in a pure form by a silica gel chromatography
due to its very high polarity.
Received: November 26, 2013
Revised: December 3, 2013
Published online on January 29, 2014
Chem. Eur. J. 2014, 20, 2169 – 2174
www.chemeurj.org
2174
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim