Oxidant-Triggered C1-Benzylation of Isoquinoline by Iodine-Catalyzed Cross-Dehydrogenative-Coupling with Methylarenes

Article abstract of DOI:10.1055/s-0036-1588331

A practical iodine-catalyzed oxidative functionalization of isoquinolines with methylarenes is developed, which can be triggered by the selected oxidants to produce C₁- or N-benzyl-substituted products selectively. This method utilizes readily available isoquinolines and methylarenes as starting materials and proceeds under metal-free conditions with broad substrate scope with respect to methylarenes, avoiding the usage of expensive metal catalysts and generation of halide and metal wastes.

Full text of DOI:10.1055/s-0036-1588331

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© Georg Thieme Verlag Stuttgart · New York
2016, 27, A–E
letter
A
X. Shi et al.
Letter
Synlett
Oxidant-Triggered C1-Benzylation of Isoquinoline by Iodine-
Catalyzed Cross-Dehydrogenative-Coupling with Methylarenes
Xin Shia
I
2 (5 mol%)
I
2 (5 mol%)
DTBP (3 equiv)
Feng Zhangb
Wen-Kun Luoa
Luo Yang*a
TBHP (3 equiv)
H
N
+
N
N
130 °C
120 °C
R
R
R
R = alkyl, halo, cyano etc.
O
H
a Key Laboratory for Environmentally Friendly Chemistry
and Application of the Ministry of Education, College of
Chemistry, Xiangtan University, Xiangtan, Hunan
411105, P. R. of China
Advantages:
• Oxidant-triggered C1- or N-benzylation
• Metal- and additive-free conditions
• 20 examples and up to 72% yield
yangluo@xtu.edu.cn
zhangf@iccas.ac.cn
b College of Science, Hunan Agricultural University,
Hunan 410128, P. R. of China
Received: 06.08.2016
Accepted after revision: 19.09.2016
Published online: 07.11.2016
bonds as radical sources, which, however, were confined to
simple symmetric alkanes or ethers to suppress regioiso-
mers.5
DOI: 10.1055/s-0036-1588331; Art ID: st-2016-w0515-l
O
O
Abstract A practical iodine-catalyzed oxidative functionalization of
isoquinolines with methylarenes is developed, which can be triggered
by the selected oxidants to produce C1- or N-benzyl-substituted prod-
ucts selectively. This method utilizes readily available isoquinolines and
methylarenes as starting materials and proceeds under metal-free con-
ditions with broad substrate scope with respect to methylarenes, avoid-
ing the usage of expensive metal catalysts and generation of halide and
metal wastes.
N
N
O
HO
O
O
O
OR
O
R = H, palaudine
R = Me, papaverine
sevanine
NH
HO
Key words benzylation, Minisci reaction, iodine, isoquinoline, iso-
quinolinone
OH
coclaurine
Figure 1 Examples of C1-benzyl-substituted (tetrahydro)isoquinolines-
fused natural products
C1-Substituted isoquinolines and tetrahydroisoquino-
lines are abundant in pharmaceutics and widely used as li-
gands in organic synthesis.1 For example, natural products
palaudine, papaverine, sevanine and coclaurine all contain
C1-benzyl-substituted (tetrahydro)isoquinoline motif (Fig-
ure 1), which feature narcotic, spasmolytic, dopaminergic,
ion-channel modulating and cytotoxic properties.2 Great ef-
forts have been developed to construct these electron-defi-
cient heterocycles, but the studies on the direct functional-
ization of the heterocyclic C(sp2)–H bonds, especially under
metal-free conditions, have been far less. Different from the
easy alkylation of electron-rich arenes via Friedel–Crafts re-
action, the similar alkylation of these heterocycles is un-
likely due to their innate electron-deficient reactivity. In
contrast, the addition of nucleophilic alkyl radicals to het-
erocycles, the Minisci reaction,3,4 is well developed and
widely used in organic synthesis. However, the disadvan-
tages of the classic Minisci reaction include limited func-
tionalized precursors (carboxylic acids or alkyl halides) and
the addition of metal catalysts (Ag, for example) up to stoi-
chiometric amounts. The recent progresses for the Minisci
reaction have included the direct use of certain C(sp3)–H
Recently, a wonderful Y(OTf)3-catalyzed C1-benzylation
of isoquinolines with methylarenes was discovered by Liu
et al. (Scheme 1, path a).6a However, considering the costs of
rare earth metal catalyst and difficulties to remove the
trace transition metal impurities from the final products,
especially for the late-stage functionalization of biologically
active pharmaceuticals,4c,7 a general method for the alkyla-
tion of heterocycles under metal-free conditions would be
highly desired. In this context, an efficient alkylation of iso-
quinoline via metal-free oxidative decarbonylative coupling
with aliphatic aldehydes had previously been developed by
our group.8a This reaction was further developed by Guin
group using dioxygen to replace peroxides as the radical
initiator.8b
On the other hand, the use of molecular iodine or iodide
anion as a catalyst is a promising strategy and has attracted
increasing attention.9 As our ongoing interests in the ben-
zylic C(sp3)–H bonds transformation for the functionaliza-
tion of heterocycles,10a–e we recently reported the iodine-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

B
X. Shi et al.
Letter
Synlett
target product, so we deduced that the molecular iodine
played a crucial role in this transformation. Either decreas-
ing the catalyst loading to 2.5 mol% or increasing to 10
mol% afforded lower yields, and 5 mol% was proved to be
the optimal value. The yield also decreased rapidly when
the reaction was run at lower temperatures of 120 °C and
110 °C, and no reaction took place at 100 °C, which might
support our theory that the hemolytic cleavage of DTBP is
the initial step for this reaction. No reaction occurred in the
absence of iodine catalyst. It is worth noting that this reac-
tion can also be conducted in other solvents such as chloro-
benzene, with excess p-xylene (10 equiv) added as reactant,
but leading to decreased yields (Table 1, entry 16).
N
R
Y(OTf)3 (5 mol% )
DTBP (3 equiv)
120 °C
path (a)
Liu et. al.
I2 (5 mol % )
TBHP (3.5 equiv)
H
+
N
N
120 °C
R
R
path (b)
O
H
our previous work
I2 (cat.)
DTBP
heat
path (c)
this work
Table 1 Optimization of the CDC of Isoquinoline with p-Xylenea
N
N
R
1a
[I] (10 mol%)
[O], 12 h
N
+
+
N
H
Scheme 1 The cross-dehydrogenative-coupling (CDC) of isoquinoline
with methylarenes
O
3a
4a
2a
catalyzed oxidative functionalization of isoquinoline with
benzylic C–H bonds via N-alkylation and amidation cascade
to provide isoquinolin-1(2H)-ones, with TBHP (tert-butyl
hydroperoxide) as the oxidant (Scheme 1, path b).10a Unex-
pectedly, when using DTBP (di-tert-butyl peroxide) as the
oxidant instead, the reaction switched to afford C1-ben-
zylated isoquinoline product predominantly.
Entry
[I] (mol%)
2 (5)
[O] (equiv)
Temp (°C)
Yield (%)b 3a (4a)
I
1
2
DTBP (3)
TBHP (3)
H2O2 (3)
K2S2O8 (3)
BPO (3)
130
130
130
130
130
130
130
130
130
130
130
130
120
110
100
130
71 (<2)
0 (72)
0
I2 (5)
3
I2 (5)
4
I2 (5)
0
5
I2 (5)
0
With our previous studies on iodine-catalyzed oxidative
functionalization of isoquinoline with benzylic C–H bonds
to provide isoquinolin-1(2H)-ones in mind,10a we first tried
the reaction of isoquinoline (1a) and p-xylene (2a) with
DTBP as the oxidant in the presence of 5 mol% molecular io-
dine as catalyst, which produced the C1-benzyl isoquinoline
3a in 71% yield, and the yield of N-benzyl isoquinolinone 4a
was less than 2% as determined by GC (Table 1, entry 1). In
contrast, the control reaction using TBHP as oxidant pro-
duced the N-benzyl isoquinolinone 4a in 72% yield, and the
C1-benzyl isoquinoline 3a could not be detected by GC (Ta-
ble 1, entry 2). It seems that by simply switching the oxi-
dants (and at the same time, the radical initiator), the reac-
tion selectively generated the C1 or N-benzyl-substituted
products. So other oxidants such as aqueous hydroperox-
ide, potassium persulfate and benzoyl peroxide were tried
but failed in this reaction (Table 1, entries 3–5). Next, other
anionic iodine source (CuI, KI and TBAI) and cationic iodine
(NIS, N-iodosuccinimide) as catalysts were examined.
Among them, CuI was found to be totally inactive, KI gave
lower yield, and KI and TBAI resulted in similar results as
those obtained using molecular iodine (Table 1, entries 6–
9). Later, the loading of molecular iodine and the influence
of temperature were carefully evaluated (Table 1, entries
10–15). The blank control experiment failed to produce the
6
CuI (10)
KI (10)
TBAI (10)
NIS(10)
I2 (0)
DTBP (3)
DTBP (3)
DTBP (3)
DTBP (3)
DTBP (3)
DTBP (3)
DTBP (3)
DTBP (3)
DTBP (3)
DTBP (3)
DTBP (3)
0
7
50
8
71
9
70
10
11
12
13
14
15
16c
trace
66
I2 (2.5)
I2 (10)
I2 (5)
62
50
I2 (5)
45
I2 (5)
<2
I2 (5)
54
a Conditions: 1a (0.4 mmol), catalyst (mol%), oxidant (equiv), in p-xylene
(2a, 1.0 mL), reacted for 12 h under air atmosphere unless otherwise noted.
b Isolated yields of 3a, the yield of 4a is given in parentheses.
c A mixture of solvents p-xylene (10 equiv) and chlorobenzene (1.0 mL) was
used.
The generality of this iodine-catalyzed cross-dehydro-
genative-coupling of isoquinoline with benzylic C–H bonds
was subsequently investigated. The substrate scope for the
methylarene moiety is listed in Scheme 2. Methylarenes
bearing electron-donating or electron-withdrawing substit-
uents were successfully transformed into the desired C1-
benzylated isoquinolines in high yields, such as methyl (2c–
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

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X. Shi et al.
Letter
Synlett
e), methoxy (2f), halo (2g–j and 2m), methoxycarbonyl (3k)
and cyano group (2l). Among them, 1-methoxy-4-methyl-
benzene (2f) gave a much lower yield of 40%, which might
be caused by the loss of molecular iodine during the elec-
trophilic iodination of election-rich arene. To our delight,
both the m-xylene (2d) and o-xylene (2e) reacted with iso-
quinoline readily and good yields were realized, which indi-
cated that there was no obvious steric hindrance effect for
this CDC reaction. Gratifyingly, the reaction could be satis-
factorily performed on a 1-gram scale of isoquinoline, pro-
viding 3a in a slightly lower yield of 64%. For all of these
substrate tested, the N-benzyl isoquinolinone product 4
was not detected by GC–MS, so the reaction shows high
chemoselectivity.
Encouraged by the exciting results obtained through the
CDC of isoquinoline (1a) with different methylarenes, ap-
plication of this iodine-catalyzed CDC to other substituted
isoquinolines was investigated next (Scheme 3). Isoquino-
lines 1b–g substituted at different positions turned out to
be suitable substrates for this transformation. Among them,
4-bromoisoquinoline resulted in a much lower yield, due to
the generation of debromination by-product. It is a pity that
the CDC of quinoline and p-xylene provided 2-(4-methyl-
benzyl)quinoline in a much lower yield of 18%, which might
be due to its decreased reactivity compared with isoquino-
line.
R
R
I2 (5 mol%)
N
H
+
N
DTBP (3 equiv)
I2 (5 mol%)
N
130 °C, 12 h
H
+
1b–g
2a
3n–s
N
DTBP (3 equiv)
130 °C, 12 h
R
R
1a
2a–m
3a–m
tBu
Ph
N
N
N
N
N
N
3n, 60%
3p, 70%
Br
3o, 65%
Br
3a, 71%
3b, 66%
3c, 67%
Br
N
N
N
N
N
N
3q, 63%
3r, 50%
3s, 40%
OMe
3d, 68%
3e, 72%
3f, 40%
Scheme 3 The influence of isoquinolines on the iodine-catalyzed CDC.
Reagents and conditions: azaarene 1b–g (0.4 mmol), I2 (5 mol %), DTBP
(1.2 mmol, 3 equiv), in p-xylene (2a, 1.0 mL) was reacted for 12 h under
air atmosphere unless otherwise noted; isolated yields.
N
N
N
Based on our previous study on metal-free oxidative de-
carbonylative coupling of aliphatic aldehydes with
azaarenes8a and literature reports,3,6 we speculated that this
reaction was also realized through a Minisci-type mecha-
nism. To verify this speculation, several mechanistic experi-
ments were designed. First, when 1,1-diphenylethene (2.0
equiv) as radical scavenger was added to prove the genera-
tion of benzyl radical, the reaction of isoquinoline and tolu-
ene failed to provide the C1-benzylation product 3b. Indeed,
the corresponding oxidative benzyl radical addition prod-
uct 5 was isolated in 12% yield (Scheme 4 a). Then, the pre-
liminary kinetic isotope competition experiment of iso-
quinoline with toluene (C7H8) and deuterated toluene
(C7D8) revealed that the KIE value was 2.8 (Scheme 4 b, see
SI for details), which implied that the cleavage of the ben-
zylic C–H bond to produce the benzyl radical was the rate-
determining step. When TBHP was used as oxidant to pro-
duce N-benzyl isoquinolinone selectively, benzyl iodide
could be detected by GC–MS (Scheme 4 c), thus a ‘quaterna-
F
Cl
Br
3g, 63%
3h, 72%
3i, 71%
N
N
N
CO2Me
I
CN
3j, 72%
3k, 55%
3l, 52%
N
N
Br
3a, 64%, 10 mmol scale, 1.5 g
3m, 60%
Scheme 2 The influence of methylarenes on iodine-catalyzed CDC.
Reagents and conditions: 1a (0.4 mmol), I2 (5 mol %), DTBP (1.2 mmol,
3 equiv), in methylarene 2a–m (1.0 mL) was reacted for 12 h under air
atmosphere unless otherwise noted; isolated yields.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

D
X. Shi et al.
Letter
Synlett
ry ammonium salt’ intermediate was proposed. However,
when the oxidant was switched to DTBP, no benzyl iodide
can be detected. Thus, we speculated that the molecular io-
dine as catalyst might play a role of Lewis acid 9a to activate
the isoquinoline and accelerate the subsequent aromatiza-
tion step. Since at the beginning of this reaction, only I2 was
added but no HI was generated yet; at the same time, in the
presence of excess oxidant (DTBP), HI would be readily oxi-
dized to I2. So we proposed that the molecular iodine was
more likely to act as the Lewis acid to activate the isoquino-
line than HI.
In conclusion, we have developed an efficient and prac-
tical iodine-catalyzed cross-dehydrogenative-coupling of
isoquinoline with benzylic C–H bonds via the Minisci-type
mechanism. This reaction can selectively realize C1- or N-
benyzlation of isoquinoline by choosing DTBP or TBHP as
the oxidant, respectively. This method utilizes unfunction-
alized isoquinoline and methylarenes as starting materials
and proceeds under metal-free conditions with good yields,
avoiding the usage of expensive metal catalysts and genera-
tion of halide and metal wastes, and thus is a practical
pathway for the C1-benzylation of isoquinolinone. Further
application of this strategy to other functionalization of
azaarenes is ongoing in our laboratory.
Ph
Ph
(2 equiv)
I2 (5 mol%)
Bn
Ph
H
+
(a)
N
+
Acknowledgment
N
DTBP (3 equiv)
Ph
Bn
130 °C, 12 h
This work was supported by the Opening Fund of Beijing National
Laboratory for Molecular Sciences, Xiangtan University ‘Academic
Leader Program’ (11QDZ20), Hunan Provincial Natural Science Foun-
dation (2016JJ2122). The authors declare no competing financial in-
terest.
1a
2b
3b, N.D.
5, 12%
I2 (5 mol%)
DTBP
C7H8
C7D8
N
N
(b)
+
+
N
130 °C, 6 h
Bn
Bn-d7
C7H8/C7D8 (v/v) = 1:1
KIE = 2.8:1
Supporting Information
I
[O] = TBHP
I2 (5 mol%)
Supporting information for this article is available online at
[O] (3 equiv)
can be detected
I
H
(c)
http://dx.doi.org/10.1055/s-0036-1588331.
S
u
p
p
ortiInfogrmoaitn
S
u
p
p
o
nrtogI
f
rmoaitn
+
N
130 °C, 12 h
[O] = DTBP
(dried by Na)
References and Notes
can't be detected
(1) (a) Topics in Heterocyclic Chemistry, Bioactive Heterocycles V;
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Scheme 4 Mechanistic investigation on the iodine-catalyzed CDC
According to the above analysis, a plausible mechanism
is proposed in Scheme 5, with the reaction of isoquinoline
(1a) and toluene (2b) as an example. First, the homolytic
cleavage of DTBP forms tert-butoxy radical, which abstracts
the benzylic hydrogen atom of toluene to provide the ben-
zyl radical. At the same time, molecular iodine as a Lewis
acid coordinates with isoquinoline to increase its electro-
philicity.9a Then, the benzyl radical adds to the activated
electron-deficient isoquinoline (A) to afford the intermedi-
ate (B), which would eliminate the hydroiodide to afford
the benzylated product 3b. The molecular iodine can be re-
generated by the oxidation of iodide anions by the peroxide
or tert-butyl hypoiodite generated in situ.9d
(2) Hagel, J. M.; Facchini, P. J. Plant Cell Physiol. 2013, 54, 647.
(3) For reviews on Minisci reaction, see: (a) McCallum, T.; Jouanno,
L.-A.; Cannillo, A.; Barriault, L. Synlett 2016, 27, 1282.
(b) Tauber, J.; Imbr, D.; Opatz, T. Molecules 2014, 19, 16190.
(c) Duncton, M. A. J. Med. Chem. Commun. 2011, 2, 1135.
(d) Punta, C.; Minisci, F. Trends Heterocycl. Chem. 2008, 13, 1.
(4) For recent examples on Minisci-type alkylation of heterocycles
with functionalized starting materials as radical sources, see:
(a) Daniel, G. M. S.; Kimberly, A. W.; Joseph, P. L.; Anthony, A. E.
Tetrahedron Lett. 2015, 56, 4063. (b) DiRocco, D. A.; Dykstra, K.;
Krska, S.; Vachal, P.; Conway, D. V.; Tudge, M. Angew. Chem. Int.
Ed. 2014, 53, 4802. (c) Bohman, B.; Berntsson, B.; Dixon, R. C.
M.; Stewart, C. D.; Barrow, R. A. Org. Lett. 2014, 16, 2787. (d) Xia,
R.; Xie, M.-S.; Niu, H.-Y.; Qu, G.-R.; Guo, H.-M. Org. Lett. 2014,
16, 444. (e) Leverrier, A.; Bero, J.; Frederich, M.; Quetin-Leclercq,
J.; Palermo, J. Eur. J. Med. Chem. 2013, 66, 355. (f) Duncton, M. A.
J.; Estiarte, M. A.; Johnson, R. J.; Cox, M.; O’Mahony, D. J. R.;
Edwards, W. T.; Kelly, M. G. J. Org. Chem. 2009, 74, 6354.
tBuOOBut
tBuO•
H2C
H
Ph
N
Ph
tBuO•
tBuOH
2b
N
N
H
I
tBuOI
(5) For Minisci-type alkylation of heterocycles with C(sp3)–H bonds
as radical sources, see: (a) Devariab, S.; Shah, B. A. Chem.
Commun. 2016, 52, 1490. (b) Jin, J.; MacMillan, D. W. C. Angew.
Chem. Int. Ed. 2015, 54, 1565. (c) Antonchick, A. P.; Burgmann, L.
Angew. Chem. Int. Ed. 2013, 52, 3267. (d) Wu, Z.; Pi, C.; Cui, X.;
I2
+
HI
Bn
3b
δ–
Bn
δ+
N
I
B
I
A
tBuOI
HI
tBuOH
+ I2
Scheme 5 Proposed mechanism for the iodine-catalyzed CDC
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E

E
X. Shi et al.
Letter
Synlett
Bai, J.; Wu, Y. Adv. Synth. Catal. 2013, 355, 1971. (e) Li, X.; Wang,
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(11) A general experimental procedure for this CDC is described as
following: an oven-dried reaction vessel was charged with iso-
quinoline (1a, 0.4 mmol, 1 equiv), I2 (0.02 mmol, 5.2 mg, 5
mol%), DTBP (di-tert-butyl peroxide, 1.2 mmol, 3 equiv) in p-
xylene (1 mL) under air. The vessel was sealed and heated at 130
°C for 12 h, then cooled to r.t. The resulting mixture was trans-
ferred to a silica gel column and eluted with hexanes and EtOAc
(12:1) to give 1-(4-methylbenzyl)isoquinoline (3a) in 71% yield
as a yellow oil. 1H NMR (400 MHz, CDCl3): δ = 8.49 (d, J = 5.6 Hz,
1 H), 8.15 (d, J = 8.0 Hz, 1 H), 7.80 (d, J = 8.0 Hz, 1 H), 7.62 (t, J =
7.4 Hz, 1 H), 7.50–7.55 (m, 2 H), 7.17 (d, J = 7.3 Hz, 2 H), 7.05 (d,
J = 7.4 Hz, 2 H), 4.63 (s, 2 H), 2.27 (s, 3 H). 13C NMR (100 MHz,
CDCl3): δ = 160.48, 142.11, 136.67, 136.49, 135.81, 129.88,
129.30, 128.59, 127.41, 127.28, 127.25, 125.94, 119.82, 41.76,
21.07.
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, A–E