Rhodium(III)-Catalyzed C-H Activation/Alkyne Annulation by Weak Coordination of Peresters with O-O Bond as an Internal Oxidant

Article abstract of DOI:10.1021/acs.orglett.5b02291

A redox-economic strategy has been developed, involved in an efficient Rh(III)-catalyzed oxidative C-H activation and alkyne annulation with perester as the oxidizing directing group. In this process, the cleavage of an oxidizing O-O bond as an internal oxidant is described for the first time. This reaction could be carried out under mild conditions and exhibits excellent regioselectivity and wide functional groups tolerance.

Full text of DOI:10.1021/acs.orglett.5b02291

Letter
pubs.acs.org/OrgLett
Rhodium(III)-Catalyzed C−H Activation/Alkyne Annulation by Weak
Coordination of Peresters with O−O Bond as an Internal Oxidant
Jiayu Mo, Lianhui Wang, and Xiuling Cui*
Engineering Research Center of Molecular Medicine, Ministry of Education, Key Laboratory of Xiamen Marine and Gene Drugs,
Institute of Molecular Medicine and School of Biomedical Sciences, Huaqiao University, Xiamen 361021, P. R. China
S
* Supporting Information
ABSTRACT: A redox-economic strategy has been developed,
involved in an efficient Rh(III)-catalyzed oxidative C−H
activation and alkyne annulation with perester as the oxidizing
directing group. In this process, the cleavage of an oxidizing
O−O bond as an internal oxidant is described for the first time. This reaction could be carried out under mild conditions and
exhibits excellent regioselectivity and wide functional groups tolerance.
ransition-metal-catalyzed functionalization of inert C−H
bonds by oxidative couplings of substrates bearing C−C
Scheme 1. Strategies for the Redox-Neutral C−H Activation
T
multiple bonds has attracted significant interest since it avoids
the multistep preparation of preactivated starting materials and
thus allows an overall streamlining of organic synthesis.1
Concerning the catalytic mechanism, it generally involves a
high-oxidation-state metal species as a promoter and generates
the desired products as well as low-oxidation-state metal species
produced by reductive elimination. Therefore, stoichiometric or
excessive amounts of oxidants are generally required to
regenerate the active catalyst, which would provide a
stoichiometric amount of waste and reduce the overall
“greenness” of the process. To overcome this drawback, a
novel redox-neutral strategy via the use of an oxidizing directing
group (internal oxidant)2 has been recently pioneered by
Hartwig,3 Yu,4 Glorius,5 Guimond and Fagnou,6 Ackermann7
and our group.8 So far, internal oxidants are limited to the
oxidizing potential of N−O, N−N, and O−N bonds in oximes,9
N-pivaloxybenzamide,6b,10 hydrazines,11 phenoxyamides,12 N-
oxides,8,13 and others.14 Exploitations of other covalent bonds
containing heteroatoms in the redox-neutral C−H activation are
extremely rare.15
bond formation involved in an internal oxidant is rarely
explored,12b,d,14d partly owing to the large energy gap between
M−O HOMO and M−C LUMO frontier orbitals.20 Thus, there
would be some formidable challenges in this redox-economic
process: (1) The coordination of the oxygen atom in aryl
peroxides is weak.1d,h To the best of our knowledge, aryl
peroxides have not been successfully used as directing groups in
the directed C−H activations. (2) The O−O bond of aryl
peroxides is thermally labile and would undergo decomposition
to generate free radicals under various factors, such as
temperature or ultraviolet radiation. (3) The initial C−H
activation via metal catalysis and the subsequent O−O bond
cleavage need to be compatible, and the leaving moiety at the late
stage should not inhibit the active metal species. As a
consequence, we hence became intrigued by developing
Peresters are easily available and widely applied as oxidants or
radical initiators in organic synthesis16 due to their weak O−O
bond and the strong dependence of the decomposition time in
their structures.17 We deduced that the O−O bond from the
easily available aryl peresters could serve as an internal oxidant
for the transition-metal-catalyzed direct C−H activation. In this
view, using a perester as an oxidizing directing group will be of
great value compared to the ones using O−N or N−N bonds as
an internal oxidant (Scheme 1a, b), with the leaving O-containing
moiety resulting in a more environmentally friendly alcohol as
the sole byproduct (Scheme 1c). In addition, isocoumarins are
important structural motifs of various bioactive natural
products.18
Transition-metal-catalyzed oxidative annulations of arylcar-
boxylic acids and alkynes have recently been developed for the
isocoumarins construction, which requires stoichiometric
external oxidants and high temperatures.19 Moreover, C−O
Received: August 14, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b02291
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
ab
Rh(III)-catalyzed direct coupling of tert-butyl peroxybenzoate
(TBPB) with alkynes through C−H/O−O bonds functionaliza-
tion with a perester as an efficient oxidizing directing group
(Scheme 1c).
Scheme 2. Scope of Aryl Peresters 1
We commenced our study on the condensation of TBPB (1a)
with diphenylacetylene (2a) (Table 1). Varieties of metal salts as
Table 1. Optimizing Various Parameters for the
a
Condensation of TBPB with Diphenylacetylene
[Rh]2
entry (mol %)
additive 1
(equiv)
additive 2
(equiv)
temp
(°C)
3aa
(%)
solvent
1
2
3
2.5
2.5
2.5
AgOAc (0.3)
CsOAc (0.3)
TFE
TFE
TFE
100
100
100
63
55
65
NaOAc
(0.3)
a
b
[RhCp*Cl2]2 (2.5 mol %). Ratio of regioselectivity was determined
1
by H NMR.
4
5
2.5
2.5
KOAc (0.3)
TFE
TFE
100
100
59
72
CsOPiv
(0.3)
[RhCp*Cl2]2. The withdrawing groups resulted in lower yields.
For instance, TBPB substituted by 4-CO2Me, 4-NO2, 4-COMe,
and 4-CN gave the corresponding products in 40%, 38%, 10%,
and less than 5% yields, respectively. No target products were
found when 4-OH- and NH2-substituted TBPB as well as tert-
butylpyridine-2-carboperoxoate were used as the substrates.
Moreover, good yields were obtained for ortho-substituted
TBPB. For instance, -o-methyl and fluoro-tert-butyl benzoper-
oxoate (1g and 1h) gave the corresponding products 3ga and
3ha in 70% and 78% yields, respectively. It is worth noting that
only a single isomer was obtained, although two chemically
inequivalent ortho C−H bonds exist in meta-substituted TBPB.
The redox-economic C−H bond annulation occurred at the less
hindered C−H bond (3ia, Scheme 2). In contrast, tert-butyl 3-
fluorobenzoperoxoate (1j) gave product 3ja as the sole product,
perhaps because of a secondary directing group effect.21
Naphthyl peresters 1k and 1l afforded the fused target products
3ka and 3la in 87% and 78% yields, respectively. In addition, high
regioselectivity was achieved by using tert-butyl naphthalene-2-
carboperoxoate (1k) as the coupling substrate. It is worth noting
that tert-butyl thiophene-2-carboperoxoate also reacted
smoothly under the standard reaction conditions, delivering
the target product 3ma in 79% yield.
We next examined the scope of the internal alkynes in this
redox-economic C−H bond annulation. As shown in Scheme 3,
various internal alkynes reacted smoothly with TBPB and
afforded the desired 3,4-disubstituted isocoumarins in moderate
to good yields. Both electron-rich, such as bis(4-
methoxylphenyl)acetylene (2b) and di(4-methylphenyl)-
acetylene (2c), and electron-deficient tolanes, such as bis(4-
trifluoromethylphenyl)acetylene (2f), were compatible and gave
the corresponding products (3ab, 3ac, and 3af) in 68%, 82% and
55% yields, respectively. Halogens were tolerated well. Yields of
80% and 66% were obtained for bis(4-chlorophenyl)acetylene
(2d) and bis(4-fluorophenyl)acetylene (2e). Similarly, when
fluorine was substituted at the 3-position of the aryl group, the
yield decreased slightly (3ag), while only trace amounts of the
desired products were detected for 4-COMe-, NO2-, 4-CO2Me-,
CN-, OH-, and NH2-substituted 1,2-diphenylethyne as well as
1,2-di(pyridin-2-yl)ethyne as substrates. Importantly, this
catalytic system was not restricted to aryl-substituted alkynes
6
7
8
9
2.5
1
CsOPiv
(0.3)
TFE
TFE
80
80
80
80
80
60
80
80
68
80
93
50
22
0
CsOPiv
(0.3)
1
CsOPiv
(0.3)
PivOH (1.0) TFE
PivOH (1.0) TFE
PivOH (1.0) TFE
PivOH (1.0) TFE
PivOH (1.0) TFE
b
1
CsOPiv
(0.3)
b
b
b
10
11
12
0.5
1
CsOPiv
(0.3)
CsOPiv
(0.3)
0
CsOPiv
(0.3)
a
Reaction conditions: 1a (0.5 mmol), 2a (0.6 mmol), solvent (2.0
mL), 18 h under N2. 1a (0.5 mmol), 2a (0.75 mmol). TFE: 2,2,2-
trifluoroethanol.
b
additives were screened (entries 1−5). CsOPiv was found to
promote the transformations most efficiently, delivering
isocoumarin 3aa in 72% yield (entry 5). Further investigation
showed that the yield increased to 80% when the temperature
decreased to 80 °C (entry 6), which might be attributed to the
decomposition of peresters at the higher temperature.
Furthermore, we were pleased to find that reducing the loading
of [RhCp*Cl2]2 from 2.5 to 1 mol % preserved a similar catalytic
activity in the presence of PivOH (entries 7 and 8). Moreover,
the isolated yield was improved to 93% by simply switching the
ratio of 1a and 2a to 1.0:1.5 (Table 1, entry 9). Attempts to lower
the catalyst loading further as well as to decrease the reaction
temperature resulted in reduced yields (entries 10 and 11). No
target product was observed in the absence of rhodium catalyst
(entry 12).
With the optimized reaction conditions in hand, the scope of
aryl peresters for the synthesis of isocoumarins was explored
(Scheme 2). A variety of para-substituted TBPB were tested first.
The desired products were obtained in good to excellent yields
for donating groups (3ba−fa). It is known that sulfur is easily
coordinated with transition metals, making them inactive.
However, tert-butyl 4-(methylthio)benzoperoxoate (1d) was a
suitable substrate in this catalytic system and delivered the
desired product in 84% yield in the presence of 2.5 mol % of
B
DOI: 10.1021/acs.orglett.5b02291
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 3. Scope of Internal Alkynes 2
Scheme 6. Plausible Reaction Mechanism
a
1
Ratio of regioselectivity was determined by H NMR.
but also allowed the oxidative annulation of dialkylalkynes. For
instance, 4-octyne (2h) and 5-decyne (2i) smoothly reacted with
TBPB (1a) under the standard reaction conditions, furnishing
the desired products in 58% and 62% yields, respectively (3ah,
3ai). When unsymmetrical alkynes were employed, excellent
regioselectivity was achieved. Isocoumarins 3aj, 3al, and 3am
were formed in 61%, 73%, and 78% yields as the exclusive
regioisomers, and 3ak was produced in 58% yield as two isomers
in 20:1 ratio.
step was the generation of the active [RhIII] species. Then an
irreversible C−H activation of aryl perester 1a23 was catalyzed by
the active rhodium species through cyclorhodation14d,19 to give
intermediate A, which was coordinated and inserted with
diarylalkyne 2a to form intermediate C. C−O bond formation
by elimination of intermediate C gave intermediate D.9h,11f,14c,d
Product 3aa was released through the redox process of
intermediate D and regenerated the active [RhIII] species (path
I). Another possible pathway (path II) is also possible which
involves a pivalic acid promoted intramolecular nucleophilic
substitution leading to a C−O bond formation and O−O bond
cleavage to afford product 3aa, tert-butyl alcohol, and a [RhIII]
species.11f,14d Since the protonation of O,O-tert-butyl group of
intermediates E by pivalic acid makes it a better leaving group in
path II, while the migration of the [RhI] to the adjacent oxygen
would require a high energy 3-ring transition state in path I, path
II seems to be more favorable in this transformation.
To probe the reaction mechanism, isotope experiments were
carried out (Schemes 4 and 5). First, [D5]-TBPB ([D5]-1a) was
Scheme 4. C−H Functionalization with Labeled Compound
[D5]-1a
In summary, we have developed a mild and efficient Rh(III)-
catalyzed redox-neutral C−H activation of peresters with alkynes
to synthesize various isocoumarins. This is the first example
employing peresters as the oxidizing directing groups in directed
C−H activation reactions. Using this rare oxidizing perester
directing group will broaden the scope of metal-catalyzed C−H
bond functionalizations and may find further applications in the
efficient synthesis of useful complex molecules.
Scheme 5. Kinetic Isotope Effect Studies with Labeled
Compound [D5]-1a
ASSOCIATED CONTENT
* Supporting Information
■
S
subjected to the standard reaction conditions (Scheme 4). No
significant loss of deuterium was observed for the product [D4]-
3aa or recycled substrate [D5]-1a, indicating that the step of
cyclorhodation was irreversible.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b02291.
Experiemental procedures and NMR spectra (PDF)
Then, we conducted parallel experiments using substrates 1a
or [D5]-1a with diphenylacetylene (2a) under the standard
reaction conditions. A significant kinetic isotope effect (kH/kD ≈
2.2) was found, suggesting the irreversible C−H cleavage to be
the rate-determining step (Scheme 5).22
On the basis of the experimental results obtained above and
the literature,2−14,19 a proposed catalytic cycle for this novel
oxidative annulation reaction is depicted in Scheme 6. The first
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: cuixl@hqu.edu.cn. Fax: (+)86-592-6162996.
Present Address
Engineering Research Center of Molecular Medicine, Ministry of
Education, Key Laboratory of Xiamen Marine and Gene Drugs,
C
DOI: 10.1021/acs.orglett.5b02291
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the NSF of China (21572072),
Xiamen Southern Oceanographic Center (13GYY003NF16),
and Huaqiao University.
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DOI: 10.1021/acs.orglett.5b02291
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