Iron-Catalyzed Carbenoid Insertion into C(sp 3)-H Bonds

Article abstract of DOI:10.1055/s-0036-1588743

An iron-catalyzed carbenoid insertion into C-H bonds of alkanes was developed with high activity (turnover numbers up to 690 in a gram-scale experiment) and chemoselectivity. This non-heme iron-catalyzed C(sp ³)-H insertion reaction provides an efficient strategy for C-H functionalization.

Full text of DOI:10.1055/s-0036-1588743

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© Georg Thieme Verlag Stuttgart · New York
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letter
A
Q.-Q. Cheng et al.
Letter
Synlett
Iron-Catalyzed Carbenoid Insertion into C(sp3)–H Bonds
Qing-Qing Chenga
1 mol% Fe(ClO4)2⋅xH2O
Ji-Min Yanga
Huan Xua
Shou-Fei Zhu*a,b
1.2 mol% BPMEN
R3
R2
R3
R4
R2
R1
R4
N2
1.2 mol% NaBArF⋅3H2O
C
H
+
R1
CO2Me
CHCl3, reflux
CO2Me
Me
Me
a State Key Laboratory and Institute of Elemento-Organic
Chemistry, College of Chemistry, Nankai University, Tianjin
300071, P. R. of China
b Collaborative Innovation Center of Chemical Science and En-
gineering (Tianjin), Tianjin 300071, P. R. of China
sfzhu@nankai.edu.cn
R
R
1 = aryl
2–4 = alkyl, H
18 examples
up to 84% yield
TON up to 690
N
N
N
N
BPMEN
Received: 15.12.2016
Accepted after revision: 19.02.2017
Published online: 08.03.2017
(turnover numbers, TON up to 690 in a gram-scale experi-
ment) than the iron porphyrin catalysts7a in C(sp3)–H inser-
tion reaction.
DOI: 10.1055/s-0036-1588743; Art ID: st-2016-w0846-l
The initial experiment was performed with methyl α-di-
azophenylacetate (1a) and cyclohexane (2a) in chloroform
at 70 °C, in the presence of 10 mol% iron catalyst generated
in situ from Fe(ClO4)2·xH2O, TMEDA (N,N,N′,N′-tetramethy-
lethylenediamine, 4), and NaBArF (sodium tetrakis[3,5-
bis(trifluoromethyl)phenyl]borate; Table 1, entry 1). The
desired C–H functionalization product 3aa was isolated
with 36% yield, although carbene dimerization was ob-
served as a competing reaction. Increasing the amount of
TMEDA to 24 mol% led to a slight increase in yield. We then
studied different types of bidentate and tridentate ligands
5–10, but no significant improvement in yield was observed
(entries 2–7). Several tetradentate ligands 11–13 were then
examined (entries 8–10). To our delight, when ligand
BPMEN (12) was used, the yield of the product 3aa in-
creased markedly to 69% (entry 9). In the absence of any li-
gand, only a trace amount of 3aa was generated, with car-
bene dimers9 being the predominant products (entry 11).
To further improve the reactivity and selectivity of this
C–H functionalization process, the reaction conditions were
optimized by using ligand 12 (Table 2). First, several iron
precursors were evaluated, with Fe(ClO4)2 giving the best
yield (entries 1–8). The amount of additive was then stud-
ied. NaBArF with 12 mol% loading proved to be most suit-
able (entry 1 vs. entries 9 and 10). The absence of chloro-
form was not favorable in terms of reaction rate and yield
(entry 11). Gratifyingly, increasing the bath temperature
from 70 to 95 °C significantly improved both the reaction
rate and the yield of the desired product 3aa (80%, entry
12). Reducing the catalyst loading to 1 mol% gave the same
level of yield, but a longer reaction time was required (entry
13). Even 0.1 mol% iron catalyst was sufficient to achieve a
Abstract An iron-catalyzed carbenoid insertion into C–H bonds of al-
kanes was developed with high activity (turnover numbers up to 690 in
a gram-scale experiment) and chemoselectivity. This non-heme iron-
catalyzed C(sp3)–H insertion reaction provides an efficient strategy for
C–H functionalization.
Key words iron catalysis, carbenoid insertion, C–H functionalization,
chemoselectivity, diazo compounds, alkanes
The direct functionalization of inert C(sp3)–H bonds is a
state-of-the-art and challenging topic in modern organic
chemistry.1 Transition-metal-catalyzed carbenoid insertion
into C(sp3)–H bonds is a classic and efficient C–H function-
alization strategy.2 However, this transformation has tradi-
tionally involved dirhodium(II) complexes as catalysts.3 Re-
cently, several iridium complexes were also developed as
catalysts in this reaction.4 The limited availability as well as
high price and toxicity of precious metals has driven aca-
demia and industry to develop base metals, especially iron,
as alternative catalysts for this important transformation.5,6
Few iron-mediated carbenoid insertions into C(sp3)–H have
been developed using iron porphyrins as catalysts or stoi-
chiometric reagents.7 As iron porphyrin has a special planar
coordinative structure, it is difficult to tune its reactivity
and selectivity through ligand modifications. Thus, the de-
velopment of non-heme iron catalysts for C(sp3)–H inser-
tion reaction is highly desired. As part of our continuing ef-
forts to develop iron-catalyzed reactions,8 we herein report
a non-heme iron-catalyzed C(sp3)–H insertion reaction us-
ing the iron complex of tetradentate nitrogen ligand BPMEN
(N,N′-dimethyl-N,N′-bis(pyridin-2-ylmethyl)ethane-1,2-di-
amine, 12) as a catalyst, which exhibited higher activity
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–D

B
Q.-Q. Cheng et al.
Letter
Synlett
Table 1 Iron-Catalyzed C–H Functionalization of Cyclohexane: Ligand
Table 2 Iron-Catalyzed C–H Functionalization of Cyclohexane: Optimi-
Evaluationa
zation of Reaction Conditionsa
10 mol% [Fe]
12 mol% 12
12 mol% NaBArF⋅3H2O
10 mol% Fe(ClO4)2⋅xH2O
N2
12 mol% ligand
N2
12 mol% NaBArF⋅3H2O
OMe
OMe
+
+
Ph
OMe
Ph
OMe
CHCl3, 70 °C
CHCl3
Ph
Ph
O
O
O
O
1a
2a
5
3aa
1a
2a
3aa
Entry [Fe]
Temp (°C)b Time (h) 3aa (%)c
1a (%)c
Ph2P
NMe2
PPh2
Me2N
Ph2P
Me
N
N
N
1
2
Fe(ClO4)2·xH2O
Fe(OTf)2
70
70
70
70
70
70
70
70
70
70
70
95
95
95
48
48
48
48
48
48
48
48
48
36
48
10
36
60
69
54
62
56
24
51
45
51
58
47
60
80
82
73
–
4
6
7
11
9
3
Fe(OAc)2
Ph2P
PPh2
N
N
4
FeSO4·7H2O
Fe(BF4)2·6H2O
FeCl2
12
–
N
N
N
PPh2
PtBu2
Me
PtBu2
Me
5
8
9
10
6
16
–
7
Fe(ClO4)3·6H2O
FeCl3
Me
N
Me
N
N
N
N
N
8
24
10
–
N
N
N
9d
10e
11f
12
13g
14h
Fe(ClO4)2·xH2O
Fe(ClO4)2·xH2O
Fe(ClO4)2·xH2O
Fe(ClO4)2·xH2O
Fe(ClO4)2·xH2O
Fe(ClO4)2·xH2O
N
Me
Me
N
11
12
13
8
Entry
Ligand
Time (h)
3aa (%)b
1a (%)b
–
–
1
2
4
5
48
20
48
40
48
48
48
48
48
48
10
36 (43)c
11
–
–
–
a Reaction conditions: [Fe] (0.03 mmol), 12 (0.036 mmol), NaBArF
(0.036 mmol), 1a (0.3 mmol), 2a/CHCl3 (3:1 mL).
b Bath temperature.
3
6
37
–
4
7
44
–
c Isolated yield or recovery.
5
8
32
6
d In the absence of NaBArF.
e Using 24 mol% NaBArF.
6
9
38
16
–
f In the absence of CHCl3.
g Using 1 mol% catalyst.
7
10
11
12
13
none
30
h Using 0.1 mol% catalyst.
8
62
7
9
69
–
10
11
37
42
–
thoxy substituents on their phenyl rings, respectively, gave
slightly lower yields (entries 6 and 8). The steric bulk of the
ortho substituents of α-aryl-α-diazoacetates 1i–k did not
affect the yield of reaction (entries 9–11). α-Diazo-α-(2-
naphthyl)acetate 1m was also a suitable substrate for the
reaction (entry 13). Other cycloalkanes including cyclopen-
tane (2b), cycloheptane (2c), and cyclooctane (2d), also re-
acted smoothly to furnish the desired products 3ab–ad in
good yields (entries 14–16). The iron catalyst Fe/BPMEN ex-
hibited high sensitivity to the diazo compounds, and the re-
action only afforded complex reaction mixture when ben-
zyl α-diazoacetate or benzyl α-diazopropionate was used.
The Fe/BPMEN-catalyzed C–H functionalization of al-
kanes is easy to scale-up. A gram-scale reaction of 1a and
2a was carried out with 0.1 mol% catalyst, and 1.0 g (69%
yield) of the desired product 3aa was isolated (Scheme 1). It
should be noted that the diazo compound can be added in
one potion in all the above reactions, which indicates that
the reaction has excellent chemoselectivity. In contrast,
trace
a Reaction conditions: Fe(ClO4)2 (0.03 mmol), ligand (0.036 mmol), NaBArF
(0.036 mmol), 1a (0.3 mmol), 2a/CHCl3 (3:1 mL), bath temperature =
70 °C.
b Isolated yield (3aa) or recovery (1a).
c The yield in parentheses was obtained using 24 mol% TMEDA.
satisfactory result (entry 14). Notably, this process dis-
played a high chemoselectivity, with only trace amounts of
carbene dimers observed. Moreover, despite the crystalliza-
tion water in iron precursors and NaBArF, no O–H insertion
product was detected in the reaction.8a
The generality and substrate scope of the reaction were
investigated under the optimized reaction conditions. A va-
riety of α-diazo esters 1a–m smoothly underwent the reac-
tion with cyclohexane (2a) to afford the corresponding
products 3aa–ma in good yields (Table 3, entries 1–13). Di-
azo compounds 1f and 1h, which have 3-chloro and 3-me-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–D

C
Q.-Q. Cheng et al.
Letter
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Table 3 Iron-Catalyzed C–H Functionalization of Cycloalkanes: Sub-
product 16a and the ylide rearrangement product 16b in
approximately 1:1.6 ratio (Scheme 2 c). This chemoselectiv-
ity is reversed in comparison with the previously reported
iron porphyrin-catalyzed reaction (16a/16b = 3.4:1).7a The
major diastereomer of 16a was methyl (S*)-2-phenyl-2-
((R*)-tetrahydrofuran-2-yl)acetate, which is consistent
with the reaction catalyzed by Rh2(S-DOSP)4.3b
strate Scopea
(CH2)n
1 mol% Fe(ClO4)2⋅xH2O
1.2 mol% 12
N2
1.2 mol% NaBArF⋅3H2O
OMe
R1
OMe
+
(CH2)n
R1
CHCl3, reflux, 36 h
O
O
1
2
3
N2
1 mol% Fe(ClO4)2⋅xH2O
1.2 mol% 12
1.2 mol% NaBArF⋅3H2O
Entry
R1
Ph
1
n
2
3
Yield (%)b
OMe
Ph
1
2
1a
1b
1c
1d
1e
1f
2
2
2
2
2
2
2
2
2
2
2
2
2
1
3
4
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b
2c
2d
3aa
3ba
3ca
3da
3ea
3fa
3ga
3ha
3ia
82
73
77
84
71
56
76
61
76
73
77
72
66
72
84
78
OMe
OMe
O
1a
(a)
Ph
Ph
+
+
4-ClC6H4
4-PhC6H4
4-MeC6H4
4-MeOC6H4
3-ClC6H4
3-MeC6H4
3-MeOC6H4
2-FC6H4
CHCl3, reflux, 48 h
O
O
3
14a, 20% yield
14b, 19% yield
4
N2
1 mol% Fe(ClO4)2⋅xH2O
1.2 mol% 12
1.2 mol% NaBArF⋅3H2O
5
OMe
Ph
6
OMe
OMe
O
1a
Ph
Ph
(b)
+
7
1g
1h
1i
+
CHCl3, reflux, 48 h
O
O
8
15a, 43% yield
15b, 19% yield
dr = 1.4:1
9
N2
10
11
12
13
14c
15
16
2-ClC6H4
2-MeC6H4
1j
3ja
OMe
1 mol% Fe(ClO4)2⋅xH2O
1.2 mol% 12
1a 1.2 mol% NaBArF⋅3H2O
O
Ph
1k
1l
3ka
3la
O
O
3,4-O2CH2C6H3
OMe
Ph
OMe
(c)
+
+
Ph
CHCl3, reflux, 36 h
2-naphthyl
1m
1a
1a
1a
3ma
3ab
3ac
O
O
O
Ph
Ph
Ph
16a, 26% yield
16b, 41% yield
dr = 1.2:1
3ad
O
OMe
Ph
a Reaction conditions: Fe(ClO4)2·xH2O (0.003 mmol), 12 (0.0036 mmol),
NaBArF (0.0036 mmol), 1 (0.3 mmol), 2/CHCl3 (3:1 mL).
b Isolated yield.
O
c Reaction time: 60 h.
Scheme 2 Iron-catalyzed C–H functionalization of other hydrocarbon
substrates
slow addition of diazo compound was generally required in
the C–H insertion reactions catalyzed by other metal cata-
lysts to avoid carbene dimerization.2–4
To gain an insight into the reaction mechanism, we con-
ducted kinetic isotope effect (KIE) experiments (Scheme 3).
A primary KIE (kH/kD = 2.00) was observed, which is consis-
tent with that reported in the rhodium-catalyzed C(sp3)–H
insertion and implies that the C–H bond cleavage might be
involved in the rate-limiting step.11 A detailed mechanism
study is under way in our laboratory.
In addition to cycloalkanes, linear and branched alkanes
are also suitable substrates for the reaction. The C–H inser-
tion of n-hexane occurred at the secondary C–H bonds un-
der the standard conditions (Scheme 2 a). When 2-methyl-
pentane was subjected to the reaction, the tertiary C–H in-
sertion product 15a was obtained as the major product
(Scheme 2 b).10 Notably, no product of carbene insertion
into the primary C–H bonds was observed in these reac-
tions. In addition, the reaction of tetrahydrofuran under
standard conditions led to two products, the C–H insertion
OMe
1 mol% Fe(ClO4)2⋅xH2O
Ph
1.2 mol% 12
1.2 mol% NaBArF⋅3H2O
O
N2
2a
3aa
OMe
+
Ph
CHCl3, reflux, 36 h
O
D12
0.1 mol% Fe(ClO4)2⋅xH2O
0.12 mol% 12
0.12 mol% NaBArF⋅3H2O
D11
1a
N2
kH/kD = 2.00
D
2a-d12
OMe
+
OMe
Ph
OMe
Ph
CHCl3, reflux, 60 h
Ph
(2a/2a-d12 = 1:1)
O
O
O
3aa-d12
1a, 1.1 g
2a
3aa, 1.0 g, 69% yield
Scheme 1 Gram-scale synthesis
Scheme 3 Kinetic isotope effect experiments
© Georg Thieme Verlag Stuttgart · New York — Synlett 2017, 28, A–D

D
Q.-Q. Cheng et al.
Letter
Synlett
In summary, the non-heme iron-catalyzed carbene in-
sertion into C(sp3)–H bonds of alkanes has been realized.12
The readily available iron complex of BPMEN was found to
be a powerful and highly chemoselective catalyst for the re-
action. The easy modification of BPMEN leaves rooms for
the development of iron catalysts with tunable reactivity
and selectivity. This iron-catalyzed C–H insertion reaction
provides an efficient strategy for C–H functionalization of
alkanes.
(5) For reviews on iron catalysis, see: (a) Iron Catalysis in Organic
Chemistry: Reactions and Applications; Plietker, B., Ed.; Wiley-
VCH: Weinheim, 2008. (b) Iron Catalysis: Fundamentals and
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Plietker, B., Ed.; Springer-Verlag: Berlin/Heidelberg, 2011.
(c) Bolm, C.; Legros, J.; Le Paih, J.; Zani, L. Chem. Rev. 2004, 104,
6217. (d) Enthaler, S.; Junge, K.; Beller, M. Angew. Chem. Int. Ed.
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75, 6061. (f) Gopalaiah, K. Chem. Rev. 2013, 113, 3248. (g) Zhu,
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H.-J. Chem. Rev. 2015, 115, 3170. (i) Guo, N.; Zhu, S.-F. Chin. J.
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(6) For a review on iron-catalyzed C–H functionalization, see:
(a) Sun, C.-L.; Li, B.-J.; Shi, Z.-J. Chem. Rev. 2011, 111, 1293. For
selected examples, see: (b) Li, Z.; Cao, L.; Li, C.-J. Angew. Chem.
Int. Ed. 2007, 46, 6505. (c) Li, Y.-Z.; Li, B.-J.; Lu, X.-Y.; Lin, S.; Shi,
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Q.; Fan, C.-A.; Zhang, F.-M.; Shi, L. Angew. Chem. Int. Ed. 2009, 48,
8761. (e) Yoshikai, N.; Mieczkowski, A.; Matsumoto, A.; Ilies, L.;
Nakamura, E. J. Am. Chem. Soc. 2010, 132, 5568. (f) Shang, R.;
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135, 6030. (g) Ratnikov, M. O.; Xu, X.; Doyle, M. P. J. Am. Chem.
Soc. 2013, 135, 9475.
(7) For an iron porphyrin-catalyzed C–H insertion, see: (a) Mbuvi,
H. M.; Woo, L. K. Organometallics 2008, 27, 637. For stoichio-
metric C–H insertion of iron porphyrin carbenes, see: (b) Li, Y.;
Huang, J.-S.; Zhou, Z.-Y.; Che, C.-M.; You, X.-Z. J. Am. Chem. Soc.
2002, 124, 13185.
Acknowledgment
We thank the National Natural Science Foundation of China
(21625204; 21421062; 21290182), the National Basic Research Pro-
gram of China (2012CB821600), the ‘111’ project (B06005) of the
Ministry of Education of China, and the National Program for Support
of Top-notch Young Professionals for financial support.
Supporting Information
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0036-1588743.
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ortioInfgrmoaitn
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References and Notes
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(9) Dimethyl 2,3-diphenylmaleate and dimethyl 2,3-diphenylfuma-
rate.
(10) The yields of 14a, 14b, 15a, and 15b were determined by 1H
NMR spectroscopic analysis of the crude reaction mixture using
pyrazine as the internal standard.
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(12) Methyl 2-cyclohexyl-2-phenylacetate (3aa); Typical Proce-
dure: Powered Fe(ClO4)2·xH2O (1.0 mg, 0.003 mmol, 1 mol%),
ligand 12 (1.0 mg, 0.0036 mmol, 1.2 mol%), and NaBArF (3.4 mg,
0.0036 mmol, 1.2 mol%) were introduced into an oven-dried
Schlenk tube in an argon-filled glovebox. After CHCl3 (1 mL) was
injected into the Schlenk tube, the solution was stirred at 25 °C
under argon atmosphere for 4 h. Cyclohexane (2a; 3 mL) and
methyl α-diazophenylacetate (1a; 52.9 mg, 0.3 mmol) were
then successively introduced into the system. The resulting
mixture was stirred at a bath temperature of 95 °C for 36 h.
After concentration in vacuo, the residue was purified by flash
chromatography on silica gel (petroleum ether/ethyl acetate,
30:1 v/v) to give methyl 2-cyclohexyl-2-phenylacetate (3aa) as
a colorless oil. Yield: 57.2 mg (0.246 mmol, 82%). 1H NMR (400
MHz, CDCl3): δ = 7.39–7.20 (m, 5 H), 3.64 (s, 3 H), 3.22 (d, J =
10.7 Hz, 1 H), 2.08–1.94 (m, 1 H), 1.83–1.57 (m, 4 H), 1.33–0.99
(m, 5 H), 0.79–0.68 (m, 1 H). 13C NMR (101 MHz, CDCl3): δ =
174.4, 137.8, 128.5, 128.4, 127.1, 58.8, 51.7, 41.0, 32.0, 30.4,
26.3, 26.0, 25.9.
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