N-heterocyclic carbene (NHC)-rhodium-catalyzed carbonylative C-C bond formation of allenols with arylboronic acids under carbon monoxide

Article abstract of DOI:10.1002/adsc.201100309

(3-Butyl-1-methylimidazole)(1,5-cyclooctadiene)rhodium chloride [Rh(Ibm)(cod)Cl] was found to be an efficient catalyst in the carbonylative arylation of allenols with arylboronic acids in the presence of carbon monoxide. Efficient, simple, and versatile m

Full text of DOI:10.1002/adsc.201100309

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DOI: 10.1002/adsc.201100309
N-Heterocyclic Carbene (NHC)-Rhodium-Catalyzed
À
Carbonylative C C Bond Formation of Allenols with
Arylboronic Acids under Carbon Monoxide
Soo Young Choia and Young Keun Chunga,*
a
Intelligent Textile System Research Center, and Department of Chemistry, College of Natural Sciences, Seoul National
University, Seoul 151-747, Korea
Fax : (+82)-2-889-0310; phone: (+82)-2-880-6662; e-mail: ykchung@snu.ac.kr
Received: April 22, 2011; Revised: June 11, 2011; Published online: October 10, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adcs.201100309.
Abstract: (3-Butyl-1-methylimidazole)(1,5-cyclooc-
tadiene)rhodium chloride [Rh(Ibm)(cod)Cl] was
tion. Recently, we studied the rhodium NHC-cata-
lyzed C C bond formation of allenols in the presence
À
G
E
found to be an efficient catalyst in the carbonylative
arylation of allenols with arylboronic acids in the
presence of carbon monoxide. Efficient, simple, and
versatile methods for the synthesis of 2-methylene-
1-arylbut-3-en-1-ones have thus been developed.
of arylboronic acids and observed that Rh-catalyzed
carbonylative arylation took place under mild reac-
tion conditions (at 508C under 1 atm of CO). For the
carbonylation of allenols, a palladium-catalyzed reac-
tion (1–20 atm of CO) had already been published.[15]
Moreover, the Rh-catalyzed carbonylative arylation
of alkynes or unsaturated ketones (at 808C under 5~
20 atm of CO) was recently documented.[16] However,
as far as we are aware, a carbonylative arylation of al-
lenols has not been reported. In the rhodium-cata-
lyzed carbonylative arylation of allenols, we expected
aryl 1,3-butadien-2-yl ketones as reaction products
which could be used as important intermediates to
Keywords: allenic alcohols; carbonylative arylation;
À
C C bond formation; N-heterocyclic carbenes; rho-
dium
Allenes are quite important compounds in organic construct complex molecules. Herein, we describe the
synthesis and synthetic organic chemistry.[1] Recently, rhodium NHC-catalyzed carbonylative arylation reac-
allenes and their chemistry have received much atten- tions under mild reaction conditions.
tion.[2] They are much more reactive than other al-
Our initial study used allenic alcohol 1a as a model
kenes. Many synthetic strategies make use of their substrate and Rh(Ibm)(cod)Cl (Ibm=3-butyl-1-meth-
A
ACHTUNGTRENNUNG
high reactivity in reactions. While we were studying ylimidazole; cod=1,5-cyclooctadiene) as a catalyst
the carbonylation of unsaturated hydrocarbons,[3] we (Table 1). The reaction of 1a with 1.5 equivalents of
became interested in the chemistry of allenols and phenylboronic acid (2A) in the presence of Rh
ACHTUNGTRENNUNG(Ibm)-
synthesized allenols to make use of their high reactivi- (cod)Cl (5 mol%) in toluene/water (v/v, 10:1) at 808C
AHCTUNGTRENNUNG
ty in our reactions. Allenols have already been used for 2 h gave the carbonylative arylated product (3aA)
in various reactions including cyclopropanation,[4] in 44% yield together with the arylated product
coupling reactions,[5] and cyclocarbonylation.[6] More- (4aA) in 14% yield (entry 1). When the reaction tem-
over, they are versatile building blocks for the synthe- perature was lowered to 508C (entry 2), the yield of
sis of 2,5-dihydrofurans,[7] vinylic epoxides,[8] a,b-unsa- 3aA increased to 84% and the formation of 4aA was
turated ketones,[9] and numerous other valuable com- not observed. However, further lowering of the reac-
pounds.
tion temperature to room temperature afforded 3aA
Since the stable N-heterocyclic carbenes (NHC) in 56% yield over 18 h of a reaction time (entry 3).
have been isolated,[10] much attention has been given Strangely, either increasing the amount of 2A
to the chemistry of NHC catalysts in organic synthe- (entry 4) or raising the CO pressure to 3 atm (entry 6)
sis.[11–13] We also have been involved in studying the was not helpful for the formation of 3aA. Either de-
use of transition metal NHC complexes in catalytic creasing the amount of 2A (entry 5) or catalyst
organic reactions.[14] Thus, we decided to use rhodium (entry 7) led to a slightly lower yield (62% and 78%,
À
NHC compounds as a catalyst in C C bond forma- respectively). When the reaction medium was
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Table 1. Optimization of the carbonylative arylation of allenol 1a.[a]
Entry
Catalyst
PhB(OH)2
CO [atm]
Solvent (v/v)
Temp. [8C]
Time [h]
Yields [%][b]
3aA
4aA
1
2
3
4
5
6
7
8
5 mol%
5 mol%
5 mol%
5 mol%
5 mol%
5 mol%
2.5 mol%
5 mol%
5 mol%
5 mol%
5 mol%
5 mol%
5 mol%
1.5 equiv.
1.5 equiv.
1.5 equiv.
3 equiv.
1.1 equiv.
1.5 equiv.
1.5 equiv.
1.5 equiv.
1.5 equiv.
1.5 equiv.
1.5 equiv.
1.5 equiv.
1.5 equiv.
1
1
1
1
1
3
1
1
1
1
1
1
1
toluene/H2O (10/1)
toluene/H2O (10/1)
toluene/H2O (10/1)
toluene/H2O (10/1)
toluene/H2O (10/1)
toluene/H2O (10/1)
toluene/H2O (10/1)
toluene/H2O (1/1)
H2O
toluene
dioxane
DCE/H2O (10/1)
cyclohexane/H2O (10/1)
80
50
r.t.
50
50
50
50
50
50
50
50
50
50
2
2
18
3
3.5
2
4
2
1
44
84
56
74
62
76
78
69
21
44
33
69
81
14
–
–
–
–
–
–
–
14
–
9
10
11
12
13
15
18
2
–
–
–
4
[a]
0.5 mmol 1a and ca. 2 mL solvent were used under a balloon of CO. For entries 1–7, toluene/H2O (2 mL/0.2 mL).
Isolated yields.
[b]
Table 2. Optimization of a rhodium source.[a]
changed to toluene/water (v/v 1:1), water, toluene, di-
oxane or DCE/water (v/v, 10:1) (entries 8–12), the
yields of 3aA were 69%, 21%, 44%, 33% and 69%
yields, respectively. Moreover, formation of 3aA or
4aA was not observed in a mixture solvent of water
with CH3NO2, dioxane, THF, CH3CN, or 2-propanol.
In contrast, when a mixture of cyclohexane/water (v/v,
10:1) was used as a reaction medium (entry 13), the
yield of 3aA was still 81% in 4 h of a reaction time.
Thus, the reaction was highly sensitive to the reaction
medium.
Entry
Rhodium source
Rh(Ibm)(cod)Cl
Time [h]
Yields [%][b]
1
G
N
2
84
80
31
21
76 (6)
78 (11)
81
11
11
2
3
Rh(Ibu)(cod)Cl
Rh(IMes)(cod)Cl
Rh(IPr)(cod)Cl
(cod)Cl]2
(cod)OH]2
N
R
3
U
18
24
2
4
U
5[c]
6
N
E
2
7
8
9
10
Rh
Rh(acac) (CO)2
Rh(dppp)2Cl
Rh(CO)Cl(PPh3)2
G
2
3
18
18
Neutral rhodium complexes such as, for example,
53
Rh
[Rh
Rh(acac)(CO)2, Rh
were screened as catalysts (Table 2 and Figure 1). In
the cases of Rh(IMes)(cod)Cl, Rh(IPr)(cod)Cl,
Rh(dppp)2Cl, and Rh(CO)Cl(PPh3)2, poor yields of
3aA were observed even with longer reaction times.
When [Rh(cod)Cl]2 and [Rh(cod)OH]2 were used as
A
E
E
G
N
ACHTUNGTRENNUNG
[a]
Reaction conditions: 0.5 mmol 1a, 0.75 mmol 2A, Rh cat-
alyst (5 mol% Rh), 2.2 mL toluene/H2O (10/1), 508C.
Isolated yield and the yield of 4aA in parenthesis.
0.3 equivalents of KOH used.
A
E
N
ACHTUNGTRENNUNG
A
R
ACHTUNGTRENNUNG
[b]
[c]
A
E
N
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
A
ACHTUNGTRENNUNG
catalysts, high yields of 3aA (76% and 78%, respec-
tively) were observed with a concomitant formation
of 4aA (6% and 11%, respectively). In the cases of
Rh
(Ibu)
E
N
ACHTUNGTRENNUNG
comparable to that of Rh
N
ACHTUNGTRENNUNG
that the steric effect of the NHC group plays a more
important role than the electronic effect of the NHC
group. With the results described above, our opti-
mized reaction conditions were established as follows:
5 mol%
Rh
G
(cod)Cl,
1.5 equivalents
of
PhB(OH)2, toluene/H2O (2 mL/0.2 mL), 1 atm CO, Figure 1. NHC-Rh catalysts used in this study.
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N-Heterocyclic Carbene (NHC)-Rhodium-Catalyzed Carbonylative C C Bond Formation
Table 3. Synthesis of 3a using RhACHTNUGTREN(NUNG Ibm)ACHTUNGTRENNNUG
(cod)Cl as catalyst.[a,b]
Entry
Product
Entry
Product
Entry
3
Product
Entry
4
Product
1
2
2 h, 3aA, 84%
2 h, 3aE, 93%
4 h, 3aB, 90%
6 h, 3aF, 78%
6 h, 3aC, 64%
1 h, 3aG, 93%
6 h, 3aD, 71%
6 h, 3aH, 63%
5
6
7
8[c]
9
10
14
11
15
12
2 h, 3aI, 80%
7 h, 3aJ, 35%
2 h, 3bA, 87%
1.5 h, 3aK, 65%
24 h, 3cA, trace
1 h, 3aL, 41%
13[c]
16[d]
18 h, 3aM, 39%
24 h, 3dA, trace
17[d]
18
19
20
24 h, 3eA, 47%
4 h, 3fA, 70%
3 h, 3gA, 71%
3 h, 3hA, 76%
[a]
Reaction conditions: 0.5 mmol allenic alcohol, 0.75 mmol boronic acid, 5 mol% Rh
0.2 mL water, 508C under a balloon of CO.
Isolated yields.
1.5 mmol boronic acid used.
Stereochemistry was not determined.
ACHUTNGTRENNUG(Ibm)ACHTUNGTERN(NUGN cod)Cl, 2 mL toluene and
[b]
[c]
[d]
508C, and 2 h. When molybdenum hexacarbonyl was 16) as in the cases of NHC ligands bearing sterically
used as carbon monoxide source instead of gaseous hindered backbones, IPr and IMes. In the cases of en-
CO under optimized reaction conditions, the expected tries 15 and 16, the reactants 1c and 1d were recov-
product 3aA was isolated in 8% yield and the arylated ered with 95% and 86% yields, respectively. The ste-
product 4aA in 60% yield.
reochemistry of 3 in entries 18–20 was determined by
Using Rh(Ibm)ACTHNUGRTENUNG(cod)Cl as a catalyst, the carbonyla- nOe experiments. Moreover, the formation of 3 was
G
tive arylation was studied.[17] The results are summar- confirmed by an X-ray diffraction study of 3aB
ized in Table 3. For the reaction between 1a and a va- (Figure 2).[17]
riety of arylboronic acids 2 (entries 1–12), products 3
Based on the above results and previous studies,[18]
were obtained in moderate to high yields (35–93%). a plausible reaction mechanism is shown in Scheme 1.
An electronic and/or a steric effect of a substituent on The reaction is slow to initiate in the absence of
À
the phenyl group of 2 did not appear clearly. Interest- water, suggesting the generation of an LnRh OH (A)
ingly, a reaction with trans-3-phenyl-1-propen-1-ylbor- species. Reaction of A with the boron reagent yields a
À
onic acid 2M produced 39% of 3aM with 10% of 4aM Rh Ar (B) species, which undergoes carbonylation to
À
(entry 13). Reactions of PhB(OH)2 with other allenols generate a Rh C(O)Ar (C) species. Following alkene
À
such as 1b, 1f, 1g, and 1h (entries 14, 18–20) gave cor- coordination and insertion into the Rh C(O)Ar bond
responding carbonylative arylated products 3 in high results in a p-allyl rhodium species (D). b-Oxygen
yields. However, sterically bulky substituent(s) on the elimination and reaction with 1 equivalent of water
allenol was detrimental to the yield (entries 15 and generates the product and regenerates A.
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Soo Young Choi and Young Keun Chung
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Experimental Section
Typical Procedure for the NHC-Rhodium-Catalyzed
Carbonylative Arylation of Allenols with Arylboronic
Acids under Carbon Monoxide
To the solution of RhACTHNUTRGENN(UG Ibm)ACHTUNGTERN(NUGN cod)Cl (0.025 mmol) in a
mixted solvent of toluene (1.0 mL) and water (0.2 mL), phe-
nylboronic acid 2A (0.75 mmol) was added. After the solu-
tion had been stirred for 5 min, 1a (0.5 mmol) in toluene
(1.0 mL) was added to the mixture. The reaction mixture
was charged with carbon monoxide three times. The result-
ing mixture was heated at 508C with stirring for 2 h under a
balloon filled with CO and the reaction was monitored by
TLC. After the reaction mixture had cooled to room tem-
perature, purification by chromatography on silica gel
(eluent: n-hexane/ethyl acetate 20:1 v/v) afforded the prod-
uct 3aA; yield: 84%.
Acknowledgements
Figure 2. X-ray structure of 3aB. Displacement ellipsoids are
depicted at the 30% probability level.
This work was supported by the National Research Founda-
tion of Korea (NRF) (2010-0029663) and the Basic Science
Research Program through the NRF funded by the Ministry
of Education, Science and Technology (R11-2005-065). S.Y.
Choi thanks the BK21 fellowship and Seoul Science Fellow-
ship.
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