Oxidative 1,2-difunctionalization of activated alkenes with benzylic C(sp3)-H bonds and aryl C(sp2)-H bonds

Article abstract of DOI:10.1039/c3cc45861j

DTBP (di-tert-butyl peroxide) is utilized to mediate oxidative 1,2-difunctionalization of activated alkenes with an aryl C(sp²)-H bond and a benzylic C(sp³)-H bond for the synthesis of functionalized oxindoles. This reaction is a new organomediated strategy for alkene difunctionalization facilitated by Lewis acids. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3cc45861j

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Oxidative 1,2-Difunctionalization of Activated Alkenes with Benzylic
C(sp3)-H Bonds and Aryl C(sp2)-H Bonds
Ming-Bo Zhou, Cheng-Yong Wang, Ren-Jie Song, Yu Liu, Wen-Ting Wei, and Jin-Heng Li*
Received (in XXX, XXX) Xth XXXXXXXXX 200X, Accepted Xth XXXXXXXXX 200X
First published on the web Xth XXXXXXXXX 200X
DOI: 10.1039/b000000x
5
activated alkenes with benzylic C(sp3)ꢀH bonds and aryl
DTBP (di-tert-butyl peroxide) is utilized to mediate oxidative 1,2-
C(sp2)ꢀH bonds by C(sp3)ꢀC(sp3)/C(sp2)ꢀC(sp3) bonds
formation cascade (Scheme 1b).4 It is noteworthy that this
55 organomediated approach can be promoted by a catalytic
amount of Lewis acid.
difunctionalization of activated alkenes with an aryl C(sp2)-H
bond and a benzylic C(sp3)-H bond for the synthesis of
10 functionalized oxindole. This reaction is a new organomediated
strategy for alkene difunctionalization facilitated by Lewis acids.
a) The reported organocatalyzed arylalkylation by a C-H functionalization
Difunctionalization of alkenes has become
a
popular
O
R
R3
R3
R3
OH
methodology due to the significance of the products as
building blocks in the synthesis of natural products and
15 pharmaceutical molecules.1ꢀ4 Among the reported elegant
transformations of the alkene difunctionalization, recent
works on transition metalꢀcatalyzed oxidative CꢀH
functionalization/carbocyclization of alkenes by two carbonꢀ
carbon bonds simultaneous formation are really fascinating
20 from a synthetic standpoint[2] because their cyclic products are
common components in biologically active molecules.5 To our
RR'CHOH
TBHP
C
ref. 2i
RCHO
R'
R
O
TBHP
EtOAc, 105 o
ref. 2f
O
N
R1
O
100 o
C
N
R2
N
R2
R1
R1
R4 = aryl, alkyl, OEt, NR2
L (15 mol%)
R
PhI(O2CR)2 (2.5 equiv)
CsO2CtBu (3 equiv)
O
N
R1
R = Me, tBu
N
R1
O
CaCO3 (3 equiv)
N
N
R2
R2
DMA, 110 o
C
L
ref. 3
b) This work: DTBP-meidated arylalkylation via dual C-H functionalization
R4
R4
R5
R6
Ar
R3
R3
R5
Ar
R6
IrCl3 (3 mol%)
DTBP (1.2 equiv)
120 oC, 24 h
knowledge,
however,
the
oxidative
CꢀH
+
H
functionalization/cyclization of alkenes is quite rare. The first
oxidative arylalkylation of an activated alkene with a aryl
O
N
R1
1
O
R2
N
R1
R2
2
25 C(sp2)ꢀH bond and a
α
ꢀC(sp3)ꢀH bond of alkyl nitriles using
3
Scheme 1 Oxidative Alkylarylation of Alkenes
the Pd(OAc)2/PhI(OAc)2 catalytic system has been developed
by Liu and coꢀworkers.2aꢀb Subsequently, they have used the
same Pd(OAc)2/PhI(OAc)2 catalytic system to realize
aryltrifluoromethylation reaction of an activated alkene with
30 an aryl C(sp2)ꢀH bond and TMSCF3.2c Very recently, we
illustrated an oxidative alkylarylation of activated alkenes
with an aryl C(sp2)ꢀH bond and a C(sp3)ꢀH bond adjacent to a
heteroatom using the FeCl3/TBHP catalytic system under
neutral conditions.[2d] However, the transition metalꢀcatalyzed
35 oxidative difunctionalization reactions suffer from the need of
nitrogenꢀcontained ligands2 and/or a stoichiometric amount of
additives (AgF2aꢀb or CsF2c) limiting their diversities and
Our investigation focused on optimizing oxidants, Lewis
60 acids, solvents and reaction temperatures for the reaction
between NꢀmethylꢀNꢀphenylmethacrylamide (1a) and toluene
(2a) (Table 1). Interestingly, amine 1a could react with
toluene (2a) in the presence of DTBP alone, providing an
oxindole product 3aa in 53% yield (entry 1). Encouraged by
65 these,
three
other
oxidants,
TBHP
(tertꢀbutyl
hydrogenperoxide), BPO (benzoyl peroxide) and PhI(OAc)2,
were subsequently test: the oxidants effected the reaction, but
they were less efficient than DTBP (entry 1 vs. entries 2ꢀ4).
Extensive screening revealed that a series of Lewis acids, such
70 as FeCl3, Fe(acac)2, CuCl2, ZnCl2, InBr3, CoCl2, PdCl2, RhCl3,
RhCl3 and IrCl3, could facilitate the reaction, and IrCl3
displayed the highest activity (entry 5 and Table S1 in
Supplementary Information).2b,7 Among the reaction
applications. Thus, the development of
a novel and
mechanistically distinct strategy for expanding the oxidative
40 alkene 1,2ꢀdifunctionalization method is an urgent need.
Recently, Liu and coꢀworkers established
a
new
o
organocatalyzed strategy for arylalkylation of an activated
alkene with an aryl C(sp2)ꢀH bond and an alkyl radical from
the in-situ decomposition of PhI(O2CR)2,3 albeit the
temperature examined, it turned out that the reaction at 120 C
75 gave the best results (entries 5ꢀ7). Gratifyingly, the reaction at
3 mol% IrCl3 gave the identical results to those of 10 mol%
IrCl3 after prolonging the reaction time to 24 h (entry 5 vs.
entry 8). We were happy to observe that DCP (dicumyl
peroxide) has the same reactivity with DTBP (entry 9). The
80 results indicated that the reaction could successfully proceed
in anhydrous DMSO medium (entry 10), but use of NMP
solvent resulted in trace detectable oxindole 3aa (entry 11).
However, the yield was lowered slightly at 1.2 equiv DTBP
45 requirement of both nitrogenꢀcontained ligands and
a
stoichiometric amount of mixed bases (CsO2CtBu/Ca2CO3)
(Scheme 1a). This3 and recent advance in radicalꢀinvolved
benzylic C−H functionalization6 encouraged us to exploit the
feasibility of the organomediated oxidative strategy in alkene
50 difunctionalization with a benzylic C−H bond. Herein we
report a new DTBPꢀmediated oxidative 1,2ꢀalkylarylation of
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DOI: 10.1039/C3CC45861J
(entry 12). It is noteworthy that 10 mmol scale of acrylamide 40 be tolerated, thereby facilitating additional modifications at
1a still furnishes product 3aa in good yield (entry 13).
Table 1 Screening Optimal Conditionsa
the halogenated position (Products 3ig and 3jg). Notably,
Table 2 Scope of Substrates (2)a
H
R5
Ar
Ph
H
H
R5
Lewis acid, [O]
R6
IrCl3, DTBP
+
H
Ph
O
+
H
Ar
R6
Ar, 12 h
O
120 oC, Ar, 24 h
N
O
H
N
O
N
N
2a
2
1a
1a
3aa
3
5
R
Entry Lewis acid (mol%)
[O]
Solvent Isolated Yield (%)
DTBP
TBHP
BPO
neat
neat
53
47
22
42
85
80
8
83
81
79
trace
75
86
1
2




N
N
R
O
neat
3
O
N
O
N
O
O
N
PhI(OAc)2
DTBP
DTBP
DTBP
DTBP
DCP
DTBP
DTBP
DTBP
DTBP
neat
4
N
N
R = MeO, 3ad, 73%
R = Br, 3ae, 71%
R = I, 3af, 73%
R = Me, 3ab, 83%
R = MeO, 3ac, 71%
5
IrCl3 (10)
IrCl3 (10)
IrCl3 (10)
IrCl3 (3)
IrCl3 (3)
IrCl3 (3)
IrCl3 (3)
IrCl3 (3)
IrCl3 (3)
neat
neat
neat
neat
3ag, 86%
3ai, 73%
3ah, 68%
6b
7c
N
S
N
8d
Ph
Ph
9d
neat
O
N
O
10d,e
11d,e
12d,f
13d,g
DMSO
NMP
neat
O
N
O
O
N
N
N
3am, 77%
3al, 81%
3an, 67%
3ak, 79%
3aj, 77%
a
neat
45 Reaction conditions: 1a (0.5 mmol), 2 (7.5 mmol), IrCl3 (3 mol%) and
DTBP (1.0 mmol) at 120 oC under argon atmosphere for 24 h.
a Reaction conditions: 1a (0.5 mmol), 2a (7.5 mmol), Lewis acid and [O]
(1.0 mmol) at 120 o C under argon atmosphere for 12 h. b At 130 C. c At
o
metaꢀmethylꢀsubstituted substrate 1k gave a mixture of two
regioselective isomers 3kg and 3kg’ in good total yield with
2:1 ratio. For substrates 1l-1n having a substituent, phenyl, oꢀ
50 tolyl or CH2OAc, at the 2ꢀposition of the acrylamide moiety,
the corresponding oxindoles 3lg-3ng were obtained in high
yields. It was noted that NꢀmethylꢀN-phenylacrylamide (1o), a
terminal alkene, was a suitable substrate, albeit with a slightly
decreased reactivity (Product 3og). Interestingly, substrate 1p
55 with a phenyl group at the terminal alkene offered oxindole
3pg in 70% yield.
90 C. d For 24 h. 2a (5 mmol) in solvent (anhydrous, 0.5 mL). DTBP
o
e
f
(0.6 mmol). g 1a (10 mmol) and 2a (15 equiv) for 48 h.
10
With the optimized conditions in hand, the generality of
this oxidative 1,2ꢀdifunctionalization reaction was probed
(Tables 2 and 3). As shown in Table 2, this reaction can be
applied to
arylmethanes
a
wide range of benzylic CꢀH bonds in
2b-2g, heteroarylmethanes 2h-2j and
15 phenylethanes 2k-2l. Treatment of arylmethanes 2b-2g,
bearing Me, MeO, Br or I groups on the aryl ring, with amide
1a, IrCl3 and DTBP afforded the corresponding oxindoles
3ab-3ag in good yields. Using mesitylene (2g), a monoꢀ
benzylic CꢀH functionalization product 3ag was isolated
20 exclusively in 86% yield. Heteroarylmethanes 2h-2l were
compatible with the optimized conditions, making this
methodology more useful for the preparation of
pharmaceuticals and natural products (Products 3ah-3al). It
was noted that phenylethane (2m) was viable for this
25 oxidative 1,2ꢀdifunctionalization reaction. Interestingly,
cumene (2n) with a tertiary benzyl C(sp3)ꢀH bond was
consistent with the optimized conditions, albeit with slightly
decreased yield (Product 3an).
Table 3 Scope of NꢀArylacrylamides (1)a
R3
N
R4
R4
R3
IrCl3, DTBP
+
120 oC, Ar, 24 h
O
R2
O
R1
N
R1
2g
R2
1
3
R2
R2 = Me, 3eg, 85%
R2 = OMe, 3fg, 81%
R2 = CF3, 3gg, 77%
R1 = Bn, 3bg, 88%
R1 = Ac, 3cg, trace
R1 = H, 3dg, trace
O
O
O
N
N
Next screening disclosed that different Nꢀarylacrylamides
30 were subjected to the optimized conditions (Table 3). The
amide in which the Nꢀmethyl substituent in 1a is replaced by a
benzyl group was also viable for the reaction (Product 3bg);
however, replacement of the methyl substituent by a hydrogen
atom or an Ac group has no reactivity (Products 3cg and 3dg).
35 Gratifyingly, several substituents, such as Me, MeO, CF3, Cl
or I, on the paraꢀ or orthoꢀposition of the Nꢀaryl moiety
wereconsistent with the optimized conditions, and the
corresponding oxindoles 3eg-3jg were obtained in good yields.
Importantly, either Cl or I groups at the orthoꢀposition could
R1
R2 = Me, 3hg, 84%
R2 = Cl, 3ig, 78%
R2 = I, 3jg, 79%
+
O
O
N
N
N
R2
R3
3kg + 3kg' = 84% (2:1)
Ph
AcOH2C
O
N
O
O
O
N
N
N
3og, 68%
R3 =H, 3lg, 87%
R3 = Me, 3mg, 81%
3ng, 87%
3pg, 70%
2
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DOI: 10.1039/C3CC45861J
a
Reaction conditions: 1 (0.5 mmol), 2g (7.5 mmol), IrCl3 (3 mol%) and
DTBP (1.0 mmol) at 120 oC under argon atmosphere for 24 h.
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Consequently, a possible mechanism outlined in Scheme 2
is proposed on the basis of the present results.1ꢀ4,6ꢀ8 Initially,
DTBP (or DCP) is split into ketone, an alkyloxy radical A and
a methyl radical B.8 In the presence of radicals A and/or B, a
methyl hydrogen in substrate 2a is readily abstracted and
transferred into radical intermediate C. Addition of radical
intermediate C to the carbonꢀcarbon double bond of amide 1a
10 furnishes radical intermediate D, followed by intramolecular
cyclization of radical intermediate D with an aryl ring affords
radical intermediate E. Finally, hydrogen abstraction of
radical intermediate E by radicals A and/or B offeres oxindole
3aa. Lewis acids are used to stabilize the radical
15 intermediates.2b,7
50
55
5
2
60
65
O
R7
O
O
R7
+
+
CH3
R7
O
R7
R7 = Me, Ph
B
A
CH3
R7
O
70
Ph CH3
2a
Ph CH2
C
3
4
CH4
R7
OH
N
O
75
1a
Ph
Ph
Ph
CH3
CH4
H
R7
O
5
N
O
N
O
N
O
R7
OH
80
3aa
D
E
Scheme 2. Possible Mechanism.
In summary, we have established a new Lewis acidꢀ
facilitated organomediated strategy for oxidative 1,2ꢀ
20 difunctionalization of activated alkenes with benzylic C(sp3)ꢀ
H bonds and aryl C(sp2)ꢀH bonds. This current method allows
two C(sp3)ꢀC(sp3) and C(sp2)ꢀC(sp3) bonds formation
simultaneous cascade to access functionalized oxindoles. The
expansion of this method to the synthesis of other bioactive
25 molecules is currently underway in our laboratory.
This research was supported by the Natural Science
Foundation of China (No. 21172060), Specialized Research
Fund for the Doctoral Program of Higher Education (No.
20120161110041), Hunan Provincial Natural Science
30 Foundation of China (No. 13JJ2018), and Fundamental
Research Funds for the Central Universities (Hunan
University, No. 2011ꢀ015).
85
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Some control experiments, including the deuterated experiments, the
experiments in the presence of two radical inhibitors and the data of
the reaction between 1a and 2a inꢀsitu determined by GCꢀMS
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95
100
105
110
Notes and references
7
8
a State Key Laboratory of Chemo/Biosensing and Chemometrics, College
35 of Chemistry and Chemical Engineering, Hunan University, Changsha
410082, China. Fax: +86 731 8871 3642; Tel: +86 731 8882 2286;
E-mail: jhli@hnu.edu.cn
† Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
40 DOI: 10.1039/b000000x/
‡ Footnotes should appear here. These might include comments relevant
to but not central to the matter under discussion, limited experimental and
spectral data, and crystallographic data.
1
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45
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