Tunable Cascade Reactions of Alkynols with Alkynes under Combined Sc(OTf)3 and Rhodium Catalysis

Article abstract of DOI:10.1002/anie.201508914

Two tunable cascade reactions of alkynols and alkynes have been developed by combining Sc(OTf)₃ and rhodium catalysis. In the absence of H₂O, an endo-cycloisomerization/C-H activation cascade reaction provided 2,3-dihydronaphtho[1,2-b]furans in good to high yields. In the presence of H₂O, the product of alkynol hydration underwent an addition/C-H activation cascade reaction with an alkyne, which led to the formation of 4,5-dihydro-3H-spiro[furan-2,1′-isochromene] derivatives in good yields under mild reaction conditions. Mechanistic studies of the cascade reactions indicated that the rate-determining step involves C-H bond cleavage and that the hydration of the alkynol plays a key role in switching between the two reaction pathways.

Full text of DOI:10.1002/anie.201508914

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International Edition: DOI: 10.1002/anie.201508914
German Edition: DOI: 10.1002/ange.201508914
Cyclization Reactions
Tunable Cascade Reactions of Alkynols with Alkynes under Combined
Sc(OTf)3 and Rhodium Catalysis
Deng Yuan Li, Hao Jie Chen, and Pei Nian Liu*
Abstract: Two tunable cascade reactions of alkynols and
alkynes have been developed by combining Sc(OTf)3 and
rhodium catalysis. In the absence of H2O, an endo-cyclo-
derivatives.[12a] Nakanowatari and Ackermann developed
a similar transformation catalyzed by a ruthenium com-
plex.[12b]
À
isomerization/C H activation cascade reaction provided 2,3-
dihydronaphtho[1,2-b]furans in good to high yields. In the
presence of H2O, the product of alkynol hydration underwent
As part of our research program on the construction of
heterocycles from alkynols[13] and rhodium-catalyzed C H
À
activation,[14] we report herein the development of two
tunable cascade reactions of alkynols with alkynes by merging
Sc(OTf)3 and rhodium catalysis. The same starting material
was transformed into two different types of products: 2,3-
dihydronaphtho[1,2-b]furans and 4,5-dihydro-3H-spiro-
[furan-2,1’-isochromene] derivatives (Scheme 1).
À
an addition/C H activation cascade reaction with an alkyne,
which led to the formation of 4,5-dihydro-3H-spiro[furan-2,1’-
isochromene] derivatives in good yields under mild reaction
conditions. Mechanistic studies of the cascade reactions
indicated that the rate-determining step involves C H bond
cleavage and that the hydration of the alkynol plays a key role
in switching between the two reaction pathways.
À
The reaction of alkynol 1a with alkyne 2a was initially
performed in 1,2-dichloroethane (1,2-DCE) in the presence
T
ransition-metal-catalyzed
cascade
transformations of alkynols have
attracted extensive interest because
they enable the highly efficient syn-
thesis of oxygen-containing heterocy-
clic compounds, including natural prod-
ucts and drugs.[1–9] Most of these reac-
tions are catalyzed by tungsten,[2]
gold,[3] platinum,[4] palladium,[5] and
copper catalysts[6] and involve intramo-
lecular cycloisomerization of alkynols
together with diverse transformations,
such as
a
Prins-type cyclization,[7]
Diels–Alder reaction,[8] or Povarov
reaction.[9] However, a cascade reaction
Scheme 1. Tunable cascade reactions of alkynols with alkynes. Cp*=1,2,3,4,5-pentamethylcyclo-
pentadienyl, Tf=trifluoromethanesulfonyl.
À
of alkynols that involves C H activa-
tion has not been reported.
À
Rhodium-catalyzed C H activation
has emerged as a powerful and promising tool for the
construction of diverse heterocyclic systems in organic syn-
thesis.[10] Activation usually requires the presence of a direct-
ing group to accelerate the reaction and ensure regioselec-
tivity. Weakly coordinating groups, such as aliphatic hydroxy
of [{Cp*RhCl2}2] (2.5 mol%) and Cu(OAc)2·H2O (2.0 equiv)
at 808C. When AgSbF6 (10 mol%) was added, the product 3a
was obtained in 33% yield along with an unexpected product
4a in 10% yield (Table 1, entries 1 and 2). Single-crystal X-
ray diffraction analysis of product 4a[15] confirmed the
structure. We then focused on optimizing the formation of
product 3a. Lower yields were observed with other rhodium
catalysts, such as [Cp*Rh(OAc)2], [Cp*Rh(CH3CN)3](SbF6)2,
and [Cp*Rh(CH3CN)3](BF4)2. With commonly used oxidants,
such as AgOAc, Na2S2O8, and PhI(OAc)2, low yields were
observed (Table 1, entries 3–5). The screening of various
silver salts showed that AgOTf gave a better yield than
AgOAc or AgPF6 (Table 1, entries 6–8). The presence of
AgOTf might promote the formation of an active cationic
rhodium catalyst with trifluoromethanesulfonate anion as the
counter ion. Of the Lewis acids tested, Sc(OTf)3 gave the best
result (Table 1, entries 9–12). The addition of AcOH
(1 equiv) and the use anhydrous copper acetate increased
À
groups, have been used as directing groups in C H activa-
tion.[11] Miura, Satoh, and co-workers reported a rhodium-
À
catalyzed, hydroxy-group-directed C H bond activation of
benzyl and allyl alcohols for the preparation of isochromene
[*] Dr. D. Y. Li, H. J. Chen, Prof. Dr. P. N. Liu
Shanghai Key Laboratory of Functional Materials Chemistry
Key Lab for Advanced Materials and Institute of Fine Chemicals
East China University of Science and Technology
Meilong Road 130, Shanghai, 200237 (China)
E-mail: liupn@ecust.edu.cn
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201508914.
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Table 1: Optimization of the cascade reactions of alkynol 1a with alkyne
2a.[a]
After optimizing the reaction conditions for the endo-
À
cycloisomerization/C H activation cascade reaction, we
investigated the scope of the reaction by varying the
substrates (Scheme 2). Alkynols with various substituents,
Entry
Lewis acid
Oxidant
Additive
Yield [%][b]
3a
4a
1
2
3
–
Cu(OAc)2·H2O
Cu(OAc)2·H2O
AgOAc
–
–
–
ND
33
16
ND
10
–
AgSbF6
AgSbF6
4
5
6
7
8
9
AgSbF6
AgSbF6
AgOAc
AgBF4
Na2S2O8
PhI(OAc)2
–
–
–
–
–
–
–
–
trace
ND
ND
44
64
71
63
76
78
80
81
ND
71
55
39
–
–
–
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2
trace
trace
trace
trace
trace
trace
trace
–
ND
trace
trace
trace
48
72
77
75
36
56
59
55
AgOTf
Cu(OTf)2
Fe(OTf)3
Ce(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
Sc(OTf)3
10
11
12
13
14
15[c]
16
17
18
19
20[d]
21[e]
22[f]
23[e,g]
24[e,h]
25[e,i]
26[e]
–
AcOH
AcOH
AcOH
H2O
PivOH
CF3SO3H
HFIP
HFIP+H2O
HFIP+H2O
HFIP+H2O
HFIP+H2O
HFIP+H2O
HFIP+H2O
H2O
Cu(OAc)2
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
Cu(OAc)2·H2O
30
trace
trace
trace
24
16
11
18
[a] Reaction conditions: 1a (0.45 mmol), 2a (0.30 mmol), [{Cp*RhCl2}2]
(0.0075 mmol), Lewis acid (0.03 mmol), oxidant (0.60 mmol), additive
(0.30 mmol), 1,2-DCE (1.5 mL), 808C, 24 h, nitrogen atmosphere.
[b] Yield of the isolated product. [c] The reaction was carried out without
[{Cp*RhCl2}2]. [d] H2O (1.8 mmol). [e] H2O (2.7 mmol), 30 h. [f] H2O
(3.6 mmol), 48 h. [g] HFIP (0.60 mmol). [h] Sc(OTf)3 (0.06 mmol).
[i] Cu(OAc)2·H2O (0.36 mmol). ND=not detected, Piv=pivaloyl.
Scheme 2. Scope of the cascade reaction of alkynols 1 with alkynes 2
to give 2,3-dihydronaphtho[1,2-b]furans 3. Reaction conditions:
1 (0.45 mmol), 2 (0.30 mmol), [{Cp*RhCl2}2] (0.0075 mmol), Sc(OTf)3
(0.03 mmol), HOAc (0.30 mmol), Cu(OAc)2 (0.60 mmol), 1,2-DCE
(1.5 mL), 808C, nitrogen atmosphere. Yields are for the isolated
product. [a] The product contained the non-isolable isomer 3t’
(R2 =Ph, R3 =Me; 3% yield).
the yield of product 3a to 81% (Table 1, entries 13 and 14).
The essential role of the rhodium catalyst was confirmed by
a reaction in its absence, in which no detectable product 3a
was formed (Table 1, entry 15).
such as Ph, MeO, Me, tBu, or Cl, at the para position of the
benzene ring gave the corresponding products 3b–f in 50–
86% yield. The alkynol substituted with a m-Me group gave
the desired product 3g selectively as a single isomer in 67%
yield, but shifting the Me substituent to the ortho position led
to no detectable product. This result demonstrates that steric
hindrance at the benzene ring inhibited the reaction. Alkynols
substituted with dioxol or secondary hydroxy groups gave the
corresponding products 3h (single isomer) and 3i in 76 and
64% yield, respectively, thus indicating that the reaction is
slightly influenced by steric hindrance near the hydroxy
group.
Next, various aryl-substituted alkynes, featuring either
electron-donating or electron-withdrawing groups, reacted
efficiently with alkynol 1a to form the desired products 3j–q
in 49–73% yield. Single-crystal X-ray diffraction analysis of
product 3l[15] confirmed the structure of the products. A
cascade reaction of dialkyl-substituted alkynes with alkynol
We then optimized the reaction conditions for the
formation of product 4a with Cu(OAc)2·H2O as the oxidant.
The addition of 3,3-hexafluoro-2-propanol (HFIP) rather
than H2O, PivOH, or CF3SO3H increased the yield of product
4a to 48% (Table 1, entries 16–19). The addition of more H2O
showed that 9.0 equivalents gave the best yield of product 4a
(Table 1, entries 20–22). An increase in the HFIP or Sc(OTf)3
loading decreased the yield of product 4a, as did a decrease in
the Cu(OAc)2·H2O loading (Table 1, entries 23–25). A reac-
tion without HFIP gave product 4a in just 55% yield, thus
illustrating the positive effect of HFIP, which might provide
suitable acidic conditions for the reaction to form product 4a
(Table 1, entry 26).
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1a produced the desired products 3r and 3s in moderate
hydroxy group also underwent the transformation to give the
yields. To investigate the regioselectivity of this cascade
reaction, we treated an unsymmetrical alkyne bearing one
phenyl group and one methyl group with alkynol 1a, and
obtained product 3t in 76% yield with very high regioselec-
tivity (22:1), although the non-isolable isomer 3t’ with the
positions of the R2 and R3 substituents reversed was also
detected in 3% yield.
We tested various alkynols 1 in the reaction under the
optimized conditions for the formation of product 4a
(Scheme 3). When R1 on the phenyl group of the alkynol
desired product 4i in 70% yield, thus showing that the
reaction is insensitive to steric hindrance near the hydroxy
group.
Various alkynes 2 were also tested for their ability to react
with alkynol 1a (Scheme 3). Derivatives of 1,2-diphenyl-
ethyne with electron-donating or electron-withdrawing
groups on the phenyl ring all reacted well to deliver products
4j–q in good yields. The unsymmetrical alkyne prop-1-yn-1-
ylbenzene was transformed into the single isomer 4r in 49%
yield.
To clarify the mechanism of these two cascade reactions,
we determined the kinetic isotope effect (KIE) of the cascade
reaction to afford product 3a. A KIE value of 3.0 was
determined from two parallel reactions and a KIE value of 2.4
was determined from a competition reaction (see the
Supporting Information). The corresponding KIE values
determined for the cascade reaction to afford product 4a
were 3.9 and 4.3 (see the Supporting Information). These
À
results suggest that C H bond cleavage is the rate-determin-
ing step in both cascade reactions.
To gain a deeper understanding of the mechanism of the
cascade reactions, we explored some control experiments. In
the reaction of 4-([1,1’-biphenyl]-4-yl)but-3-yn-1-ol (1b) with
[{Cp*RhCl2}2] (2.5 mol%) and Sc(OTf)3 (10 mol%) in 1,2-
DCE, the endo-cycloisomerization product 5-phenyl-2,3-
dihydrofuran could be detected by GC–MS. In a similar
reaction of 1b with Sc(OTf)3 (10 mol%) but without
[{Cp*RhCl2}2], 5-phenyl-2,3-dihydrofuran was not detected.
Scheme 3. Scope of the cascade reaction of alkynols 1 with alkynes 2
to give 4,5-dihydro-3H-spiro[furan-2,1’-isochromene] derivatives 4.
Reaction conditions: 1 (0.45 mmol), 2 (0.30 mmol), [{Cp*RhCl2}2]
(0.0075 mmol), Sc(OTf)3 (0.03 mmol), HFIP (0.30 mmol), H2O
(2.7 mmol), Cu(OAc)2·H2O (0.60 mmol), 1,2-DCE (1.5 mL), 808C,
nitrogen atmosphere. Yields are for the isolated product. [a] The
diastereomeric ratio was determined by 1H NMR spectroscopy.
was an electron-donating or electron-withdrawing substitu-
ent, the alkynol readily underwent the cascade reaction with
alkyne 2a to give products 4a–g in moderate to good yields.
However, alkynols bearing more strongly electron withdraw-
ing groups, such as CF3 or CN, did not undergo the cascade
reaction. When the benzene ring of the alkynol was changed
to a thiophene ring, the reaction with alkyne 2a afforded
product 4h in moderate yield. An alkynol with a secondary
Scheme 4. Mechanistic studies of the cascade reactions of alkynols
with alkynes. [a] The same yield was observed for the reactions with
and without Sc(OTf)3.
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These results demonstrate that
the rhodium complex is the real
catalyst for the endo-cycloiso-
merization step of the alkynol,
whereas Sc(OTf)3 might just sta-
bilize and reactivate the cationic
rhodium complexes in the endo-
cycloisomerization process.[16]
Under the standard condi-
tions to form product 4a, but in
the absence of alkyne 2a, alky-
nol 1a reacted with H2O to form
product 5, which was isolated in
71% yield with or without Sc-
(OTf)3 [Scheme 4, Eq. (1)].
When the hydration product 5
reacted with 2a under the same
conditions, the corresponding
product 4a was obtained in
78% yield [Scheme 4, Eq. (2)].
These results indicated that the
hydration product 5 was the key
intermediate in the process to
Scheme 5. Proposed mechanism for the cascade reactions of alkynols with alkynes.
form product 4a.
To further evaluate the role
of H2O in the formation of
In summary, we have developed tunable cascade reactions
of alkynols and alkynes through the combination of Sc(OTf)3
and rhodium catalysis. The reactions provide two different
kinds of products in good yields. Mechanistic studies demon-
product 4a, we carried out the reaction of alkynol 1a in the
absence or presence of alkyne 2a with D2O (11.0 equiv). The
products [D]5 (77% deuteration ratio) and [D]4a (75%
deuteration ratio) were obtained from the two reactions, both
in 71% yield [Scheme 4, Eq. (3) and (4)]. When we repeated
the reaction of 1a and 2a in the presence of H218O (95% 18O),
we isolated the corresponding product [18O]4a in 73%
isolated yield, with 85% 18O incorporation at the position
indicated [Scheme 4, Eq. (5)]. The above results demonstrate
that the formation of product 4a involves the addition of the
hydroxy group to the carbonyl group of hydration intermedi-
ate 5.
À
strate that C H bond cleavage is the rate-determining step
ꢀ
and that the hydration of the C C bond of alkynols plays an
important role in switching between the two reaction path-
ways. This approach appears to be the first cascade reaction of
À
alkynols involving cycloisomerization and C H activation,
and may provide a concise synthetic strategy for the
construction of drugs and natural products.
On the basis of our mechanistic studies and previous
relevant reports,[11d,12a,17] we propose the following mechanism
for the cascade reactions of alkynols with alkynes through
combined Sc(OTf)3 and rhodium catalysis (Scheme 5). In the
absence of H2O, endo-cycloisomerization of alkynol 1a occurs
to generate intermediate A in the presence of the active
rhodium catalyst [Cp*RhIII] generated from [{Cp*RhCl2}2]/
Acknowledgements
We greatly appreciate the insightful comments from
a reviewer in helping us to clarify the mechanism. This
research was supported by the NSFC/China (Project Nos.
21421004, 21190033, 21172069, and 21372072), the NCET
(NCET-13-0798), the Basic Research Program of the Shang-
hai Committee of Science and Technology (Project No.
13M1400802), the Eastern Scholar Distinguished Professor
Program, and the Fundamental Research Funds for the
Central Universities.
À
Sc(OTf)3 (path I). Intermediate A may undergo C H activa-
tion to first generate the arene–rhodium intermediate B or B’,
which undergoes alkyne insertion with alkyne 2a and
À
a second C H cleavage step to form the seven-membered
rhodacycle C. Upon subsequent reductive elimination, the
product 3a is generated, and the resulting [Cp*RhI] species
may be oxidized by a copper(II) salt to regenerate the active
rhodium catalyst [Cp*RhIII]. In the presence of H2O, the
addition of H2O to alkynol 1a gives intermediate 5, which
undergoes intramolecular addition of the hydroxy group to
the carbonyl group to form the hemiketal D (path II). After
Keywords: alkynes · alkynols · cascade reactions ·
À
C H activation · homogeneous catalysis
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Angew. Chem. 2016, 128, 381–385
À
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Received: September 23, 2015
Published online: November 4, 2015
Angew. Chem. Int. Ed. 2016, 55, 373 –377
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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