Rh-catalyzed reductive cyclization of enynes using ethanol as a source of hydrogen

Article abstract of DOI:10.1002/chem.201101745

H-eartfelt donation: A Rh-catalyzed reductive cyclization of unactivated 1,6-enynes was developed that uses an alcohol as hydrogen donor under mild reaction conditions (see scheme). The reaction is an environmentally friendly synthetic method and proceeds

Full text of DOI:10.1002/chem.201101745

DOI: 10.1002/chem.201101745
Rh-Catalyzed Reductive Cyclization of Enynes Using Ethanol
as a Source of Hydrogen
Ji Hoon Park, Soo Min Kim, and Young Keun Chung*[a]
The catalytic hydrogenation has been established as a
gen-transfer reaction.[10] However, they are irrelevant to a
À
À
powerful and mechanistically novel strategy for carbon–
carbon bond formation and is in the spotlight of synthetic
chemists. In particular, Krische and co-workers played a
major role in developing hydrogenation as a new method
for catalytic cross-coupling.[1] Hydrogen is the cleanest and
most cost-effective chemical reducing agent. Recent envi-
ronmental concerns about green chemistry have prompted
us to find a substitute for hydrogen. Thus, we envisioned an
in situ generation of hydrogen from easily available and
nontoxic chemicals and finally chose ethanol as the source
of hydrogen in the absence of any oxidants.[2] The oxidant-
free catalytic dehydrogenation of alcohols is well known in
the presence of homogeneous[3] and heterogeneous cata-
lysts.[4] However, the use of the in situ generated hydrogen
C C bond formation. Recently, the C C bond formation by
borrowing hydrogen (dehydrogenative alcohol activation)
has been developed.[11] As far as we are aware, this is the
first practical use of hydrogen generated from ethanol in the
reductive cyclization of enynes. An enantioselective reduc-
tive cyclization in the presence of hydrogen gas and rhodi-
um catalyst had been reported[9b] several years ago and etha-
nol has been used in diene hydro-hydroxyethylation.[12]
Herein we communicate our preliminary results.
According to the previous study,[13] [Rh(CO)Cl
ACTHNUTRGNEG(NU dppp)]2 is
one of the best rhodium compounds in sequential dehydro-
genation of ethanol and decarbonylation of acetaldehyde.
However, it is well known[9a] that cationic rhodium com-
plexes display good activity in reductive cyclization. Thus, to
enable both reactions to proceed smoothly, a combination
À
in the hydrogen-mediated C C bond formation is relatively
rare.[1c,5] In most cases, iPrOH has been used as a hydrogen
source. The generation of hydrogen from alcohols has been
studied[6] and sometimes the generated hydrogen has been
used in hydrocarbonylation reactions.[7] To yield any hydro-
of [Rh(CO)ClACTHUNGTERNNU(G dppp)]2 and AgOTf (2 equiv) was our first
trial as a catalyst (Scheme 1).
À
gen-mediated C C bond formation, a cascade of two con-
ceptually different catalytic reactions should occur, for ex-
ample, a hydrogen generation from ethanol and a hydroge-
native carbon–carbon bond formation, in the presence of
one or two catalysts.
Scheme 1.
Recently, there has been a demonstration of a strategy in-
volving multifunctional catalysts in a one-pot to coopera-
tively catalyze a chemical reaction, which broadens the reac-
tion scope within organic synthesis.[8] In reductive cycliza-
tion, a variety of metal compounds from base metals, such
as iron and nickel to expensive and precious metals includ-
ing rhodium, platinum, and palladium have been used as the
catalyst.[1,9] When a palladium compound is used as a cata-
lyst, a hydride donor such as HSiEt3 has to be added. In the
case of a nickel-catalyzed reaction, excess iPr2Zn has to be
added. Until now, there has been no report on the use of hy-
drogen generated from ethanol in the reductive cyclization.
In some cases, the generated hydrogen is used in the hydro-
We carried out the initial study using 1 as the substrate.
As shown in Scheme 1, treatment of 1 in the presence of a
catalytic amount of [Rh(CO)ClACHTNUGTRENUNG(dppp)]2/AgOTf (4 mol%) in
toluene/EtOH (2 mL/0.5 mL) at 808C for 18 h gave a reduc-
tive cyclization product 1a in 54% yield with a 40% recov-
ery of the reactant. Thus we confirmed that ethanol could
be used as a hydrogen source in the reductive cyclization of
1. Most known dehydrogenation reactions of ethanol are in
the presence of heterogeneous catalysts.[14] However, in our
reaction, the dehydrogenation of ethanol occurred in the
presence of the homogeneous catalyst under mild reaction
conditions. Furthermore, no cycloisomerized product was
observed in the reaction of 1.
Encouraged by the above result, enyne 1 was chosen as a
model substrate, and [RhACTHNUTRGNE(NUG cod)Cl]2/AgOTf was used as a cat-
[a] Dr. J. H. Park, Dr. S. M. Kim, Prof. Dr. Y. K. Chung
Intelligent Textile System Research Center and
Department of Chemistry, College of Natural Sciences
Seoul National University Institution, Seoul 151-747 (Korea)
Fax : (+82)2-889-0310
alyst for the optimization of reaction conditions. Initial in-
vestigations (Table 1, entries 1, 2, and 9–11) were focused on
finding optimal solvents, and we found that solvent effect
played a crucial role in obtaining good activity. The best
yield (86%) was obtained in EtOH/H2O (2.0 mL/0.2 mL)
E-mail: ykchung@snu.ac.kr
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101745.
10852
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Chem. Eur. J. 2011, 17, 10852 – 10856

COMMUNICATION
Table 1. Reaction of 1 under various reaction conditions.[a]
Table 2. Rh-catalyzed reductive cyclization of various enynes using etha-
nol.[a]
Entry
Additive
EtOH
[mL]
Toluene
[mL]
t
1a
[h]
[%][b]
Entry
Reactant
Conditions[b]
Product
Yield
[%][c]
1
2
3
4
5
6
7
8
dppp
dppp
dppe
dppf
BIPHEP
bipyridyl
dcyhpp
Xantphos
dppp/H2O (0.1 mL)
dppp/H2O (0.2 mL)
dppp/H2O (0.5 mL)
0.5
2
2
0.5
2
2
2
2
2
2
0
18
18
24
24
12
12
12
12
12
12
12
54 (40)[c]
61[d]
0.5
0.5
0.5
0.5
0.5
0.5
2
n.r.
n.r.
52[d]
1
2
3
4
R=4-C6H4OCH3
4-C6H4Cl
naphthyl
2
3
4
5
A
A
A
A
2a 84
n.r
n.r.
n.r.
3a 76
4a 88
5a 48
CH3
9
10
11
40 (50)[c]
86[d]
2
2
0
0
10 (80)[c]
[a] 1 (0.15 mmol), Rh dimer complex (4 mol%), AgOTf (8 mol%), and
phosphine additive (8 mol%) were reacted in EtOH/toluene solvent.
[b] Isolated product yield. [c] Recovered starting materials. [d] A mixture
of dimers was obtained. dppe=1,3-bis(diphenylphosphino)ethane;
dppp=1,3-bis(diphenylphosphino)propane; dppf=1,1’-bis(diphenylphos-
5
6
R=phenyl
3,5-C6H3ACHTUNTRGNEUNG(CH3)2
6
7
A
A
6a 92
7a 78
7
8
B
8a 49[d]
phino)ferrocene;
BIPHEP=2,2’-bis(diphenylphosphino)-1,1’-biphenyl;
dcyhpp=1,3-bis(dicyclphexylphosphino)propane; Xantphos=4,5-bis(di-
phenylphosphino)-9,9-dimethylxanthene.
8
9
B
9a 72
solution. Various additives (Table 1, entries 1 and 3–8) were
subsequently evaluated for catalyst activation. Apart from
BIPHEP, other additives were detrimental and no reaction
was observed. A catalytic system, [RhACTHNUTRGNEN(UG cod)Cl]2/BIPHEP/
AgOTf, gave 1a and 1b in 52 and 45% yields, respectively.
9
10
11
12
B
B
B
10a 64
10
11[e]
11a 76
A brief survey of other metal complexes such as [Ir-
A
ACHTUNGTRNE(NUGN cod)Cl]2/dppp/AgOTf
12a 36[d,f]
AHCTUNGTRENNUNG
choice. One important point to note about this is that the
yield of the reaction is highly sensitive to the order of addi-
tion of the rhodium complex, the substrate, and the solvent.
When the substrate was added to the alcohol solution of the
rhodium catalyst, a relatively high yield was observed. How-
ever, when the alcohol was added to the mixture of the rho-
dium catalyst and the enyne substrate, almost no reductive
cyclization was observed. We envisioned that a catalytically
active rhodium species would be generated from a reaction
between the rhodium catalyst and alcohol. However, a reac-
tion of the rhodium catalyst with the enyne substrate gener-
ated a stable rhodium species, resulting in no catalytic reac-
tion.
12
13
B
13a 59
[a] Compound 1 (0.15 mmol), [RhACHTNUTRGENNG(U cod)Cl]2 (4 mol%), AgOTf (8 mol%),
and dppp (8 mol%) were reacted at 808C for 12 h. [b] Condition A: The
reaction was carried out in a mixture of ethanol (2 mL) and water
(0.2 mL); B: The reaction was carried out in a mixture of ethanol
(0.5 mL) and toluene (2.0 mL). [c] Isolated product yield. [d] Reactant re-
covered. [e] Reaction time: 36 h. [f] The product was obtained with small
amount of an unknown byproduct.
structural modifications can be accommodated, as can be
À
With the optimal reaction conditions in hand, we thus ex-
amined the scope of the multicatalytic dehydrogenation of
ethanol and reductive cyclization of a variety of 1,5-enynes
(Table 2). One of two reaction conditions was used depend-
ing upon the solubility of a substrate in ethanol. When the
substrate showed a good solubility in ethanol, the reaction
was carried out in a mixture of ethanol and water (2.0 mL/
0.2 mL; conditions A). When the substrate was hardly solu-
ble in ethanol, the reaction was carried out in a mixture of
ethanol and toluene (0.5 mL/2.0 mL; conditions B). Amino,
oxygenated, and C-based substrates are readily cyclized to
the corresponding hetero- and carbocycles with Rh (4
mol%) at 808C. The desired cyclized product was generated
with high selectivity for the Z-configured alkene. Substantial
seen in entries 8, 11, and 12 (Table 2). For N Ts tethered
1,6-enynes, various substrates with arylalkynyl having an
electron-donating substituent (Table 2, entry 1) or having an
electron-withdrawing substituent (Table 2, entry 2) at the
para position and with sterically bulky arylalkynyl (Table 2,
entry 3) had proven to be reliable substrates for the cycliza-
tion, leading to the synthesis of (Z)-3-benzylidene-4-methyl-
1-tosylpyrrolidines in good yields (84–88%). However, the
substrate with an alkylalkynyl substituent gave a moderate
yield (48%, Table 2, entry 4). For O-tethered 1,6-enynes, a
substituent of the alkene seems to have much more influ-
ence on the yield of the reaction than a substituent of the
alkyne. An introduction of a methyl to the inner position of
an alkene (Table 2, entry 7) led to the isolation of 8a in
Chem. Eur. J. 2011, 17, 10852 – 10856
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www.chemeurj.org
10853

Y. K. Chung et al.
49% yield with a recovery of the reactant (57%). However,
introduction of two methyl groups at the meta positions of
arylalkynyl (Table 2, entry 6) or an introduction of cyclohex-
yl (entry 8) still gave high yields (78 and 72% yields, respec-
tively). For carbon-tethered 1,6-enynes, malonate-tethered
substrates were first tried under the optimized reaction con-
ditions, but all the reactants were recovered. This observa-
tion is quite different from that of Krische and Jang.[6a] They
observed formation of a hydrogen-mediated reductive cycli-
zation product in high yield (80–91%) in the presence of
Scheme 4.
A reaction of 1 with CH3CH2OD was also performed
(Scheme 4). The rhodium-catalyzed reductive cyclization of
1 gave [D]H1a in 62% yield with an isotopic label exclu-
sively in the vinyl position of the exo alkenyl substituent.
Based on the above results, a plausible reaction mecha-
nism is shown in Scheme 5. The dinuclear complex is pro-
posed to be defragmented into a monometallic rhodium
complex I. Subsequently, an alcohol reacts with the catalyst
I liberating DOTf and giving a coordinatively unsaturated
rhodium alkoxide species II. Then, b-hydrogen elimination
from the alkoxide ligand and the coordination of the enyne
give rise to a rhodium enyne hydride species III. In the next
step, two pathways, alkyne hydrometallation and oxidative
cyclization were proposed when hydrogen was used. Howev-
er, they could not be differentiated by experiments. To the
contrary, our experimental results showed that the oxidative
cyclization was much more favorable. Thus, an enyne ligand
coordinates with releasing aldehyde from the rhodium af-
fording IV. During coordination, the alkyne moiety of the
enyne can coordinate to the metal center and the hydride is
transferred to the terminal of the alkene moiety, giving spe-
cies IV. Finally, an oxidative addition of alcohol and a simul-
taneous release of the reductive cyclization product a gener-
ate a free coordination site to restore the coordinately unsa-
turated rhodium alkoxide species II and complete the cata-
lytic cycle.
[RhACHTUNGTRENNUNG(cod)2]X/BIPHEP (X=OTf, BF4). However, when 1,6-
enyne acetal (Table 2, entry 9) was used as a substrate, the
expected reductive cyclization product was obtained in 64%
yield. When an 1,7-enyne was used as a substrate, it under-
went a reductive cyclization to give the expected product in
36% yield with a recovery of the reactant (64%). In the
case of an allylic 2-alkynoate (Table 2, entry 12), the expect-
ed reductive cyclization product was isolated in a reasonable
yield (59%).
On the basis of the results in Table 2, we examined chiral
diphosphine ligand for the development of its asymmetric
variant.[9b] Our preliminary results showed that the reaction
of 1 smoothly proceeded in the presence of (R)-BINAP to
give 1a with 47% and 81% ee, respectively (Scheme 2).[15] It
is expected that optimization will give to a higher yield and
higher ee values.
Scheme 2. An asymmetric reductive cyclization reaction using ethanol as
a hydrogen source.
To gain mechanistic insights, we attempted to detect an
in-situ-generated hydrogen or metal-hydride species from a
reaction of ethanol in the presence of rhodium catalyst in
[D8]toluene at 808C. However, we failed to observe the for-
mation of hydrogen or a metal-hydride species in the reac-
tion. Thus, it seems that the presence of an enyne substrate
is a prerequisite to generate hydrogen or a metal-hydride
species.
A reaction of tosylated amino enyne 1 with [D6]ethanol
was examined to discover whether or not all the hydrogen
atoms used in the reductive cyclization come from ethanol
(Scheme 3). The reaction of 1 with [D6]ethanol in toluene at
1
808C for 12 h gave the product in 43% yield. H NMR spec-
tra show that all the hydrogen atoms used in the reductive
cyclization come from ethanol.
Scheme 3.
Scheme 5. A plausible reaction mechanism.
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Rh-Catalyzed Reductive Cyclization of Enynes
COMMUNICATION
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Experimental Section
Reaction conditions A: [RhACHTNUTRGNE(NUG cod)Cl]2 (3 mg, 4 mol%), AgSbF6 (3 mg,
8 mol%), dppp (4.2 mg, 8 mol%) and ethanol (2.0 mL) were added
under argon gas flow to a flame-dried 50 mL Schlenk tube flask capped
with a rubber septum. After the solution was stirred for 20 min, a sub-
strate (0.15 mmol) and distilled water (0.2 mL) were put into the flask.
The reaction vessel was stirred for 12 h at 808C. The reaction mixture
was purified by flash chromatography on a silica gel column eluting with
n-hexane/EtOAc (v/v=10:1) to afford product.
Reaction conditions B: [RhACHTNUTRGNE(NUG cod)Cl]2 (3 mg, 4 mol%), AgSbF6 (3 mg,
8 mol%), dppp (4.2 mg, 8 mol%) and toluene (2.0 mL) were added
under argon gas flow to a flame-dried 50 mL Schlenk tube flask capped
with a rubber septum. After the solution was stirred for 20 min, ethanol
(0.5 mL) was added. After a moment, a substrate (0.15 mmol) was put
into the flask. The reaction vessel was stirred for 12 h at 808C. The reac-
tion mixture was purified by flash chromatography on a silica gel column
eluting with n-hexane/EtOAc (v/v=10:1) to afford product.
Acknowledgements
This work was supported by the National Research Foundation 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).
Keywords: catalysis
hydrogen · rhodium
· cyclization · enynes · ethanol ·
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[15] The absolute configuration of the enentioenriched exo-methylene
substituted pyrrolidine was not determined.
Received: June 8, 2011
Published online: August 25, 2011
10856
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Chem. Eur. J. 2011, 17, 10852 – 10856