Cascade Reaction of Alkynols and 7-Oxabenzonorbornadienes Involving Transient Hemiketal Group Directed C-H Activation and Synergistic RhIII/ScIII Catalysis

Article abstract of DOI:10.1021/acs.orglett.6b02587

As the first cascade C-H activation directed by a transient group, reaction of alkynols and 7-oxabenzonorbornadienes has been achieved via synergistic rhodium and scandium catalysis to afford spirocyclic dihydrobenzo[a]fluorenefurans. This transformation

Full text of DOI:10.1021/acs.orglett.6b02587

Letter
pubs.acs.org/OrgLett
Cascade Reaction of Alkynols and 7‑Oxabenzonorbornadienes
Involving Transient Hemiketal Group Directed C−H Activation and
III
III
Synergistic Rh /Sc Catalysis
†
†
†
†
‡
†
Deng Yuan Li, Liang Liang Jiang, Shuang Chen, Zheng Lu Huang, Li Dang, Xin Yan Wu,
,
†
and Pei Nian Liu*
†Shanghai Key Laboratory of Functional Materials Chemistry and Key Lab for Advanced Materials, School of Chemistry and
Molecular Engineering, East China University of Science and Technology, Meilong Road 130, Shanghai 200237, China
‡Department of Chemistry, South University of Science and Technology of China, Shenzhen 518055, China
*
S Supporting Information
ABSTRACT: As the first cascade C−H activation directed by a
transient group, reaction of alkynols and 7-oxabenzonorborna-
dienes has been achieved via synergistic rhodium and scandium
catalysis to afford spirocyclic dihydrobenzo[a]fluorenefurans. This
transformation proceeds by a transient hemiketal group directed
C−H activation, dehydrative naphthylation, and intramolecular
Prins-type cyclization. Mechanistic studies and density functional
theory calculations indicate that the rate-determining step is C−H
bond cleavage and both the transient hemiketal group and
III
III
synergistic Rh /Sc catalysis play key roles.
4
irect, transition-metal-catalyzed C−H activation has
requirements of green chemistry. Moreover, cascade reactions
D
emerged as a powerful and promising tool for constructing
of alkynols have also been extensively reported and successfully
applied for the synthesis of oxygen-containing heterocyclic
compounds. Most of these reactions proceed via cyclo6-
isomerization of alkynols, together with diverse transformations
such as Prins-type cyclization, Diels−Alder reaction, and
Povarov reaction. Recently, we developed a series of cascade
reactions of alkynols, including a cascade reaction of alkynols
with alkynes involving in situ generated group-directed C−H
1
diverse heterocyclic systems. They usually require that the
5
−10
substrate contains a directing group that accelerates the reaction
and ensures regioselectivity. Preinstallation and removal of these
directing groups sometimes require tedious procedures and
harsh reaction conditions incompatible with other functional
groups. To avoid these problems, transient group-directed C−H
activation has been developed and successfully applied in the
7
8
9
10
2,3
10d
I
II
presence of Rh or Pd catalyst. Using 2-amino-3-picoline as
activation.
I
the transient directing group, Jun reported an Rh -catalyzed
Cascade reactions can be supported by synergistic catalysis, in
which two reactants are simultaneously activated by two different
catalysts in a single chemical reaction. This type of catalysis can
make previously elusive or unattainable reactions possible, and it
can substantially increase the efficiency as well as chemo-, regio-,
hydroacylation of aldehyde with alkene via aldehydic C−H
2
a
I
activation. Subsequently, Dong reported Rh -catalyzed α-
alkylation and -alkenylation of ketones via vinyl C−H activation,
in which an enamine intermediate with a pyridine moiety acts as
11
2b−d
and stereoselectivity of existing reactions. Typical synergistic
the transient directing group.
Using the reversible in situ
1
2
13
catalytic systems are organo−metal, metal−metal, and
transesterification strategy of phosphinite ligand and phenol, in
14a
14b,c
acid−
or organo−photosensitizer.
To our knowledge,
which the phosphinite ligand acts as the transient directing
I
most reports of synergistic catalysis have involved one-step
reactions. Few studies have used two metals as synergistic
group, Lewis reported Rh -catalyzed ortho-alkylation of phe-
2
e
I
nols, while Bedford developed Rh -catalyzed ortho-arylation of
13
2
f
II
catalysts in multistep cascade C−H activation.
Taking advantage of the ring-opening ability of 7-oxabenzo-
phenols. More recently, Yu reported Pd -catalyzed arylation of
aldehydes and ketones using amino acid as the transient directing
3
a
II
norbornadiene for constructing new carbon−carbon and
group, while Dong published the Pd -catalyzed arylation of free
primary alkylamines and anilines using 8-formylquinoline-
15
carbon−heteratom bonds, we report here a novel cascade
3b
reaction of alkynols and 7-oxabenzonorbornadienes by syner-
derived imine as the transient directing group. However, the
more challenging cascade C−H activation directed by a transient
group has not been reported yet.
III
III
gistic Rh /Sc catalysis. This reaction proceeds through a
transient hemiketal group directed C−H activation, followed by
Alkynols have attracted substantial interest as important
synthons because they can undergo cycloisomerization to yield
exo- or endo-heterocycles with 100% atom efficiency, fulfilling the
Received: August 29, 2016
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02587
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Organic Letters
Letter
dehydrative naphthylation and intramolecular Prins-type cycliza-
tion. The process yields spirocyclic dihydrobenzo[a]-
fluorenefurans with excellent regioselectivity and good yield
and scandium played pivotal roles in the reaction (entry 2).
While 2.5 mol % of [Cp*RhCl ] and 5 mol % of Sc(OTf)
2
2
3
proved to be optimal, the yield dropped only marginally with 5
(Scheme 1). Mechanistic studies and density functional theory
mol % equivalents of [Cp*RhCl ] (entries 3 and 4). Other
2
2
common catalysts such as Cp*CoI (CO) or [RuCl (p-
2
2
Scheme 1. Cascade C−H Activation Directed by a Transient
Hemiketal Group
Cymene)] were less effective, as were other scandium catalysts
2
such as ScCl or Sc(OAc) (entries 5 and 6). Testing various
3
3
additives showed that AgOAc (50 mol %) facilitates cascade C−
H activation (entries 7 and 8). Omitting PivOH led to low yields
of product 3a (entries 9−11), suggesting that the additive creates
suitably acidic conditions for the reaction. Omitting H O led to
2
very low yields of 3a, confirming its essential role (entry 12).
Increasing or decreasing H O loading did not improve the yield
2
(
entries 13 and 14). Interestingly, enhancing 1,2-DCE from 1.5
to 3 mL increased the yield to 71% (entry 15), suggesting that the
amount of solvent provides a suitable concentration of reactants
to promote the formation of desired product.
With the optimized reaction conditions in hand, we explored
the scope of this cascade reaction (Scheme 2). Alkynols with
a
Scheme 2. Substrate Scope
(
DFT) calculations confirm that both the transient H O-derived
2
III
III
hemiketal group and synergistic Rh /Sc catalysis play key roles
in the casacde reaction. To the best of our knowledge, this
protocol is the first example of cascade C−H activation directed
by a transient group.
We began investigations of the cascade C−H activation using
a and 2a as model substrates. After screening various reaction
1
conditions, we isolated the desired pirocyclic dihydrobenzo[a]-
fluorenefuran 3a in 63% yield using the catalysts [Cp*RhCl2]2
(
(
2.5 mol %) and Sc(OTf) (5 mol %) and the additives AgOAc
3
0.5 equiv), PivOH (1.5 equiv), and H O (6 equiv) (Table 1,
2
entry 1). Then we clarified the role of each reactant by
performing various control experiments. As expected, rhodium
Table 1. Optimized Reaction Conditions
b
a
yield
Reaction conditions: 1 (0.45 mmol), 2 (0.30 mmol), [Cp*RhCl2]
a
2
entry
variations from reaction conditions
without [Cp*RhCl ] or Sc(OTf)
5 mol % instead of 2.5 mol % [Cp*RhCl2]2
10 mol % instead of 5 mol % Sc(OTf)3
(%)
(0.0075 mmol), Sc(OTf)3 (0.015 mmol), AgOAc (0.15 mmol),
1
2
3
4
5
63
nd
62
18
nd
PivOH (0.45 mmol), H O (1.8 mmol), 1,2-DCE (3 mL), 80 °C, 18 h,
2
b
under N . Yields of isolated products are given. The diastereomeric
2
2
3
2
1
ratio was determined using H NMR spectroscopy.
Cp*CoI (CO) or [RuCl (p-Cymene)] instead of
2
2
2
functional groups such as Me, n-hexyl, t-Bu, MeO, BnO, PhO,
TsO, Cl, or I at the para position of the aryl ring were tolerated,
giving the desired products 3b−j in good yields. Single-crystal X-
ray diffraction analysis of 3e confirmed the structure of products
[
Cp*RhCl ]
2
2
6
7
8
9
ScCl or Sc(OAc) used instead of Sc(OTf)
3
nd
nd
60
22
nd
18
7
3
3
CsOAc, NaOAc, or NaOPiv instead of AgOAc
100 mol % instead of 50 mol % AgOAc
without PivOH
(
see the Supporting Information). However, alkynols bearing
strongly electron-withdrawing groups (CF or CN) did not react.
3
10
11
12
13
14
15
HOTf instead of PivOH
Alkynols substituted with phenyl, alkenyl, or ethynyl at the para
position of the aryl ring were also tolerated, giving the products
HFIP instead of PivOH
without H O
2
3
k−n in good yields.
3 equiv instead of 6 equiv of H O
61
60
71
2
Alkynols substituted with m-Me or -MeO reacted well with 2a,
12 equiv instead of 6 equiv of H O
2
selectively producing 3o and 3p as single isomers in 70% and
3% yields, respectively. Alkynols substituted with o-MeO also
1,2-DCE (3 mL) used
7
a
gave the desired product 3q in slightly lower yield of 52%.
Moreover, the reaction tolerated multi-substitutions at both the
meta and para positions, affording products 3r−u as single
isomers in 46−67% yields. The substrate 4-(naphthalen-2-
Reaction conditions: 1a (0.45 mmol), 2a (0.30 mmol), [Cp*RhCl2]2
0.0075 mmol), Sc(OTf)3 (0.015 mmol), AgOAc (0.15 mmol),
(
PivOH (0.45 mmol), H O (1.8 mmol), 1,2-DCE (1.5 mL), 80 °C, 18
h, under N . Yields of isolated products are given. nd = not detected.
2
b
2
B
DOI: 10.1021/acs.orglett.6b02587
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Letter
yl)but-3-yn-1-ol reacted with 2a, giving a single isomer 3v in 65%
yield. Heteroaryl alkynols, in contrast, did not react with 2a to
form the desired products under the current conditions.
1a and 5 identify the hydration product 5 as the key intermediate
in the cascade reaction.
Morever, the reaction was carried out with 1a and D O in the
2
Alkynols substituted with secondary hydroxyl groups gave
products 3w and 3x in respective yields of 60% and 78%, but
those substituted with tertiary hydroxyl groups did not
participate in the reaction. These results indicate that hindrance
near the hydroxyl group inhibits the reaction. Not only the
bicyclic olefin 2a but also the substituted 7-oxabenzonorborna-
dienes reacted efficiently with 1a, giving the products 4a, 4b, and
absence of 2a, affording product [D ]5 (60% deuteration ratio)
in 71% yield (eq 3). When the reaction was repeated in the
2
presence of 2a, [D ]3a (60% deuteration ratio in tetrahydrofuran
2
ring) was isolated in a slightly lower yield of 61%, and H/D
exchange was observed from the different aryl ring of [D ]3a
2
with deuteration ratios of 31% and 14% (eq 4), respectively.
These results further demonstrate that C−H bond cleavage is a
reversible step in the cascade reaction.
4c in moderate yields. Note that the other alkyl- or arylalkenes
such as tert-butyl acrylate, 1-hexene, norbornene, and styrene did
not react with 1a to give the corresponding spirocyclic products.
To shed light on the mechanism of cascade reaction, kinetic
isotope effects (KIEs) were measured (Scheme 3). Two parallel
Next, we performed an intramolecular reaction under standard
conditions using alkynol 1b as reactant (eq 5). Product 3a was
isolated in 73% yield, and a similar yield was obtained when the
reaction was repeated in the absence of [Cp*RhCl ] , Sc(OTf) ,
2
2
3
or H O. When 1a was allowed to react with 2a within 6 h under
2
Scheme 3. Studies of the Kinetic Isotope Effects
standard conditions, the naphthylation intermediate E was
+
detectable by EI-HRMS (calcd for C H O [M] 290.1307,
20
18
2
found 290.1310, Scheme 5). These results suggest that the
reaction involves a Prins-type cyclization step.
Scheme 5. Proposed Mechanism
reactions determined a KIE value of 7.3. A competition reaction
gave a KIE value of 5.7. These results suggest that C−H bond
cleavage is the rate-determining step in the cascade reaction.
To explore the mechanism of the cascade reaction in greater
detail, we performed some control experiments (Scheme 4).
Under standard conditions but in the absence of 2a, 1a reacted
with H O to form hydration product 5 in 70% isolated yield (eq
2
1
). When 5 reacted with 2a under the same conditions, the
product 3a was isolated in 73% yield (eq 2). Similar yields from
Scheme 4. Control Experiments
To clarify the role of Sc(OTf) in the cascade reaction, we
3
performed DFT calculations on the direct C−H activation step
involving 1a and 2a (see the Supporting Information for details)
III
III
and established a synergistic Rh /Sc catalysis pathway to be
the lowest energy (Scheme 5). In the pathway, hydration/
addition of alkynol 1a and H O forms the hemiketal A in the
2
III
presence of Sc(OTf) and active rhodium catalyst [Cp*Rh ]
generated from the [Cp*RhCl ] /AgOAc. Then, A undergoes
3
2
2
hydroxyl-directed C−H activation assisted by a pivalate or
acetate anion via concerted metalation−deprotonation for which
16,17
the calculated activation free energy is 31.8 kcal/mol.
This
step reversibly generates arene rhodium intermediate B, which
undergoes alkene insertion with [Sc]2a derived from 2a and
Sc(OTf) with an activation barrier as low as 25.9 kcal/mol,
3
generating bimetallic intermediate C. Subsequent β-oxygen
elimination with a lower activation free energy of 5.3 kcal/mol
generates the intermediate D. However, in the absence of
Sc(OTf) , intermediate C would have to overcome a higher
3
activation free energy of 46.6 kcal/mol to afford the β-oxygen
C
DOI: 10.1021/acs.orglett.6b02587
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Organic Letters
Letter
elimination intermediate D. This calculated activation free
energy is much higher than the free energy of C−H activation
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III
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III
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
Corresponding Author
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ACKNOWLEDGMENTS
This research was supported by the National Natural Science
Foundation of China (Project Nos. 21421004, 21372072,
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DOI: 10.1021/acs.orglett.6b02587
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