Enantio- and regioselective intermolecular benzylic and allylic C-H bond amination

Article abstract of DOI:10.1002/anie.201208906

Smooth salen: Ru(CO)-salen complex 1 is an effective catalyst for asymmetric benzylic and allylic C-H bond amination using 2-(trimethylsilyl) ethanesulfonyl azide (SESN₃) as the nitrene source. The reaction proceeded with high enantioselectivity and excellent regioselectivity. An ethyl group can be selectively aminated, even in the presence of an n-propyl group. No migration or isomerization of the double bond was observed. Copyright

Full text of DOI:10.1002/anie.201208906

Angewandte
Chemie
DOI: 10.1002/anie.201208906
À
C H Amination
À
Enantio- and Regioselective Intermolecular Benzylic and Allylic C H
Bond Amination**
Yota Nishioka, Tatsuya Uchida, and Tsutomu Katsuki*
À
C N bonds appear in the frameworks of various natural
products, and their stereoselective formation is an important
objective in organic synthesis. With the benefit of asymmetric
nitrene-transfer catalysis by transition metal complexes,[1]
remarkable advancements have been made in stereoselective
À
C N bond formation, and many highly enantioselective
methods have been reported.[2–4] However, these reactions
use low-atom-efficient nitrene precursors such as N-tosylimi-
À
nophenyliodinane or in situ prepared equivalents. Thus, C H
amination using atom-efficient azides, especially ones bearing
a readily removable N-protecting group, as nitrene precursors
have attracted a growing interest.[5] Although efficient
À
Scheme 1. Benzylic C H amination of indane with iridium–salen com-
plexes as catalysts. TBDPS=tert-butyldiphenylsilyl.
À
methods for C H amination using azides have been recently
reported, they are not enantioselective,[6–8] and enantiocontrol
À
in C H amination using an azide is still a great challenge.
À
We recently found that intramolecular benzylic C H
a 3,5-dihalo-substituted phenyl group at C2’’, were good
catalysts for asymmetric aziridination with azides as nitrene
precursors.[10] During the study, we observed that the reac-
tions of some conjugated olefins with complex 3 gave allylic
amination could be implemented with high enantioselectivity
using iridium–salen complex 1 as the catalyst,[9] and further
À
explored an intermolecular version of C H amination with
À
complex 1 at room temperature. However, the amination of
indane was slow and the enantioselectivity was poor
(Scheme 1).
Previously, we discovered that Ru(CO)–salen complexes
(3–5, Figure 1), especially durable ones (4 and 5) bearing
C H amination products, albeit with moderate enantioselec-
tivity and low yields.[10c] Taking this observation as a clue, we
À
investigated the asymmetric catalysis of C H amination by
a Ru(CO)–salen complex. Herein, we report highly enantio-
À
and regioselective benzylic and allylic C H amination.
À
We first examined benzylic C H amination of indane
using a series of (Ra,R)-Ru(CO)–salen complexes, which
have cyclohexanediamine as the diamine subunit, as catalysts
in the presence of 2-(trimethylsilyl)ethanesulfonyl azide[11,12]
(SES azide; caution: this compound is potentially explosive
and must be handled in a fume hood.)[13] and 4 ꢀ molecular
sieves (MS) at À108C in 1,1,2,2-tetrachloroethane. Complex 3
À
was found to catalyze C H amination with high enantiose-
lectivity, but low yield (Table 1, entry 1). The reaction with
the robust complex 5, which bears a 3,5-dichloro-4-trimethyl-
silylphenyl group at C2’’[10a] gave better yield, but the
Figure 1. Ru(CO)–salen complexes. TMS=trimethylsilyl.
À
enantioselectivity was moderate (entry 2). The C H amina-
tion product was accompanied by a small or trace amount of
a diamination product (Table 1, footnote [c]). The yield was
improved by using complex 6, which bears a 3,5-dichloro-
phenyl group; however, 6 showed somewhat lower enantio-
selectivity than 3 (entry 3). These results suggested that
a complex bearing a less bulky and poorly oxidizable aryl
[*] Y. Nishioka, Dr. T. Uchida, Prof. T. Katsuki
Department of Chemistry, Faculty of Science, Graduate School, and
International Institute for Carbon-Neutral Energy Research
(WPI-I2CNER), Kyushu University, Hakozaki
Higashi-ku, Fukuoka 812-8581 (Japan)
E-mail: katsuscc@chem.kyushu-univ.jp
[**] Financial support from Nissan Chemical Industries, Ltd., the World
Premier International Research Center Initiative (WPI-I2CNER), the
Global COE Programs, “Science for Future Molecular Systems”, and
Grants-in-Aid for Scientific Research (23245009) from MEXT
(Japan) is gratefully acknowledged. We are also grateful to Hideki
Kobayashi and Kenji Suzuki, of Nissan Chemical Industries, Ltd. for
DSC measurement of SESN3.
À
group at C2’’ might be an appropriate catalyst for C H
amination. Thus, we examined reactions with complexes 7–10,
which bear a robust and less bulky group at C2’’ (entries 4–
7).[14] A high enantioselectivity of 95% ee and better yield
were obtained when 8 was used as a catalyst (entry 5).
Complex 9 was poorly active as a catalyst (entry 6) and
complex 10, the diastereomer of 8, did not show catalytic
activity (entry 7). We next examined the reaction with 8 at
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201208906.
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À
À
Table 1: Benzylic C H amination of indane using Ru(CO)–salen com-
Table 2: Benzylic and allylic C H amination using Ru(CO)–salen com-
plexes 3–10 as catalysts.[a]
plex 8 as a catalyst.[a]
Entry
1[d]
Substrate
t [h]
4
Yield [%][b]
77
ee [%][c]
96
2[d]
2
97(90)[e]
94(93)[e]
Entry
Cat.
Solvent
T [8C]
Yield [%][b,c]
ee [%][d]
1
2
3
4
5
6
7
8
3
5
6
7
8
9
10
8
8
8
(CHCl2)2
(CHCl2)2
(CHCl2)2
(CHCl2)2
(CHCl2)2
(CHCl2)2
(CHCl2)2
(CHCl2)2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
À10
À10
À10
À10
À10
À10
À10
25
25
25
0
À10
À25
À10
À10
4
33
44
42
51
trace
n.r.
24
47
5
85
56
83
92
95
n.d.
–
91
89
65
93
94
94
94
94
3[d]
4[d]
4
6
82
75
94
97
5[d]
3
2
73[f]
99
87
93
96
6[d]
9
10[e]
11
12
13
14[f]
15[f,g]
7[g,h]
24
79
8
8
8
8
64
79
56
8[d]
9[d]
24
3
n.r.
77
–
CH2Cl2
CH2Cl2
83
90[h]
96
8
10[g]
11[g]
24
24
61
69
94
91
[a] Reactions were run with an indane/azide molar ratio of 1:1 at À108C
with Ru catalyst (2 mol%), 4 ꢀ MS (20 mg), and a 1.0m substrate
concentration on a 0.1 mmol scale under nitrogen, unless otherwise
mentioned. [b] Yield of isolated product. [c] A trace amount of diami-
nation product (monoamine/diamine >20:1) was detected by 1H NMR
spectroscopy in all cases except for entries 1, 6, 7, and 10. Reaction with
5 gave the diamination product in 14% yield together with the major
monoamination product. [d] Determined by HPLC analysis on a chiral
stationary phase (DAICEL AD-3). [e] Run in the absence of 4 ꢀ MS.
[f] Run on a 0.3 mmol scale at 3.0m substrate concentration. [g] Run
with an indane/azide molar ratio of 1.3:1; the azide was completely
consumed. [h] Yield is based on the amount of azide used. n.d.=not
determined, n.r.=no reaction.
12[i]
24
60
93
13[d]
14[g]
15[d]
10
24
4
81
91
93
93
99 (93)[j]
97
16[g]
17[g]
24
24
34[k]
30[l]
–
88
18[g]
24
51
46
258C, but the yield and the enantioselectivity were both
reduced, to 24% and 91% ee, respectively (entry 8). Under
these conditions, we examined solvent effects for this
reaction[15] and found that dichloromethane gave a better
yield (47%), albeit with slightly lower enantioselectivity
(89% ee) (entry 9). Reaction in the absence of 4 ꢀ MS gave
the product in only 5% yield (entry 10). Thus, the reaction
was examined at lower temperatures in dichloromethane in
the presence of 4 ꢀ MS (entries 11–13), and high enantiose-
lectivity along with improved yield were obtained at À108C
(entry 12). The reaction at a high substrate concentration
(3.0m) further improved the yield (entry 14), and the highest
yield was obtained with a slight excess of indane (1.3 equiv)
(entry 15). Under these optimized conditions, we reexamined
the amination of indane with iridium–salen complexes 1 and
2. However, the reaction with with these catalysts gave less
than 1% yield (Scheme 1).
19[g]
24
24
n.r.
n.r.
–
–
20[g,m]
21[d,h]
22[g]
2
99
98
79
99
24
[a] The reactions were run at À108C with a substrate/azide molar ratio of
1.3:1, along with catalyst 8 (4 mol%) and 4 ꢀ (or 5 ꢀ) MS on a 0.3 mmol
scale, unless otherwise mentioned. [b] Yield of isolated product based on
the azide. [c] Determined by HPLC analysis on a chiral stationary phase
(see the Supporting Information for details). [d] Run with 5 ꢀ MS
(20 mg) under solvent-free conditions. [e] The data in parentheses are
the yield of isolated product and the enantioselectivity obtained from
a 2 mmol scale reaction. [f] Yield of the para-isomer (Meta-isomer: 92%
ee, 20% yield). [g] Run with CH2Cl2 (100 mL) and 4 ꢀ MS (20 mg). [h] The
reaction was run at 08C. [i] Run with CH2Cl2 (50 mL) and 4 ꢀ MS (20 mg).
[j] The number in the parentheses is the yield of the reaction with a 1:1
substrate/azide molar ratio. [k] C1 amination product. [l] Yield of the 4-
aminated product; the yields of 1-aminated and 1,4-diaminated products
were 21% and 3%, respectively. Yields were determined by 1H NMR
analysis of the crude mixture using phenanthrene as an internal
standard. [m] Run in the presence of (Z)-3-heptene (1.3 equiv). n.r. =no
reaction, TBDPS=tert-butyldiphenylsilyl.
À
Accordingly, C H amination of various alkylarenes and
alkenes was examined with 8 as the catalyst under the
optimized conditions with 1.3 equivalents of alkylarene or
alkene (Table 2). The reaction of ethylbenzene 11 in dichloro-
methane was highly enantioselective, but the yield was
modest. However, an acceptable yield without deterioration
of the enantioselectivity was obtained using 8 (4 mol%) in the
presence of 5 ꢀ MS[16] under solvent-free conditions (entry 1).
The reactions of other alkylarenes were also carried out under
solvent-free conditions. The reactions of 4-substituted ethyl-
benzenes 12, 13, and 14 proceeded with high enantioselectiv-
2
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ity and good to high yield (entries 2–4). The reaction was also
effective when carried out on a 2 mmol scale (data in
parentheses, entry 2). The reaction of 5-methoxy-2,3-dihy-
dro-1H-indene 15 occurred preferentially at the benzylic
position para to the methoxy group with a high enantiose-
lectivity (para/meta = 1.0:0.27; entry 5).[17] The amination of
2,3-dihydrobenzofuran 16 and N-acetylindoline 17 occurred
at the benzylic position with high enantioselectivity (entries 6
À
and 7). However, n-propylbenzene 18 did not undergo C H
Scheme 2. Reactions of a radical clock and (Z)-1-phenyl-1-butene.
amination (entry 8), whereas the amination of 1-ethyl-4-
propylbenzene 19 was extremely regioselective and occurred
only at the benzylic carbon of the ethyl group with the same
enantioselectivity as that observed in the amination of
ethylbenzene (entry 9). Complex 8 did not dissolve in
alkene, and so the amination of (E)-3-hexene 20 was carried
out in dichloromethane in the presence of 4 ꢀ MS (20 mg) to
give solely (E)-2-(N-SES-amino)-3-hexene with high enan-
tioselectivity (entry 10). We next examined the reactions of
(E)-3-alkenes 21, 22, and 23, which have two different
methylene carbons. The reactions were also extremely
regioselective and occurred only at the ethyl group to give
the corresponding (E)-2-(N-SES-amino)-3-alkenes as a single
product with high enantioselectivity (entries 11–13). The (E)-
disubstituted conjugated alkenes 24 and 25 also underwent
À
a short-lived radical, we examined the C H amination of cis-
2-ethyl-1-phenylcyclopropane and (Z)-1-phenyl-1-butene
under the present conditions. cis-2-Ethyl-1-phenylcyclopro-
pane did not react, but (Z)-1-phenyl-1-butene gave (Z)-3-(N-
SES-amino)-1-phenyl-1-butene (18% yield) together with an
aziridine product (12%; Scheme 2b). HPLC analysis of the
amination product did not detect any (E)-3-(N-SES-amino)-
1-phenyl-1-butene.[22] This result supports a concerted mech-
À
anism for the present C H amination; however, the inter-
mediacy of a short-lived radical species cannot be completely
ruled out, if the radical rebound is an extremely rapid step.
In summary, we have achieved a highly enantio- and
À
À
highly enantioselective C H amination (entries 14 and 15).
regioselective intermolecular benzylic and allylic C H bond
À
Metal–nitrenoid species generally insert into methylene C H
amination using the new Ru(CO)–salen complex 8, which
bears a durable and less bulky 2,6-difluorophenyl group, as
the catalyst and SES azide as the nitrene precursor. Under the
present conditions, only ethyl, methyl, and cyclic methylene
groups in the allylic or benzylic position can be aminated. It is
noteworthy that an ethyl group can be selectively aminated in
a highly enantioselective manner, even in the presence of an
n-propyl group (Table 2, entries 9 and 11).
bonds in preference to methyl C H bonds.[3a,7f,9] However, the
amination of (E)-2-octene 26 occurred at C1, albeit slowly
À
À
(entry 16). No C H amination at C4 was observed. The
reaction of (E)-2-pentene 27 gave a 10:7:1 mixture of 4-
aminated/1-aminated/1,4-diaminated products in 54% yield
(entry 17). On the other hand, (Z)-alkenes are poor substrates
for this amination. The reaction of cyclohexene 28 was slow
and only modestly enantioselective (entry 18), whereas the
reaction of (Z)-3-heptene 29 did not occur at all (entry 19).
(Z)-3-Heptene 29 was more strongly coordinated to complex
8 than (E)-3-heptene 21, and this coordination blocked the
decomposition of SESN3.[18] Thus, the amination of 20 did not
proceed in the presence of 29 (entry 20). However, the
reactions of tri-substituted olefins proceeded with good to
high enantioselectivity. 1-Phenylcyclohexene 30 underwent
Experimental Section
Method A (Table 2, Entries 7, 10–12, 14, 16–20, and 22; reactions in
CH2Cl2): The reaction was carried out in a Schlenk tube (5 mL) under
N2. Substrate (0.39 mmol), SES azide (0.3 mmol, 67.0 mg, 58.0 mL)
and 4 ꢀ MS (20 mg) were added in dry CH2Cl2 (the amount is given in
the Table 2 footnotes) and stirred at À108C or 08C for 30 min.
Subsequently, 8 (12.0 mmol, 12.3 mg, 4 mol%) was added, and the
whole mixture was stirred for 24 h at À108C or 08C. The reaction
mixture was filtered through a pad of Celite and concentrated. The
residue was purified by silica gel chromatography (n-hexane/
AcOEt = 20:1 to 4:1) to obtain an amination product.
À
C H amination to give 1-phenyl-3-(N-SES-amino)cyclohex-
ene with 79% ee (entry 21), and the reaction of 2-ethylindene
31 proceeded with high enantioselectivity (entry 22). No
amination at C1 was observed.
À
Each reaction was repeated at least three times. One of the
experiments was carried out in the presence of phenanthrene. An
aliquot was taken after the reaction was complete and analyzed by
1H NMR spectroscopy.
There is controversy about whether C H amination
through a putative metal–nitrenoid intermediate proceeds in
a concerted manner[3a,19] or in a stepwise manner.[20] To get
insight into the mechanism, we examined the amination of
trans-(2-ethylcyclopropyl)benzene, which is a radical clock
(Scheme 2a).[21] The reaction gave only two diastereomeric
amination products, albeit in low yields. Neither the ring-
opened product nor any other product was detected by
1H NMR analysis.[22] Recently, Du Bois et al.[23a] and White
et al.[23b] independently proposed the intermediacy of a short-
Method B (Table 2, Entries 1–6, 8, 9, 13, 15, and 21; under solvent
free condition): The reaction was carried out in a Schlenk tube (5 mL)
under N2. Substrate (0.39 mmol), SES azide (0.3 mmol, 67.0 mg,
58.0 mL), and 5 ꢀ MS (20 mg) were mixed and stirred at À108C or
08C for 30 min. Subsequently, 8 (12.0 mmol, 12.3 mg, 4 mol%) was
added and the whole mixture was stirred at À108C or 08C. The
mixture gradually became pasty and the stirrer stopped. After 1 h, the
mixture was diluted with dichloromethane (100 mL). (For reaction
time, see Table 2.) The mixture was filtered through a pad of Celite
and concentrated on a rotatory evaporator. The residue was purified
by silica gel chromatography (n-hexane/AcOEt = 20:1 to 4:1) to
obtain an amination product. The product from Table 2, entry 5 was
À
lived radical species in C H amination through a putative
metal–nitrenoid species, based on the isomerization of a cis-
disubstituted cyclopropane[23a] or a Z-substituted olefin sub-
strate.[23] Thus, to ascertain if this amination proceeds through
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further purified by silica gel chromatography (n-hexane/AcOEt/
EtOH = 20:1:1).
Each reaction was repeated at least three times. One of the
experiments was carried out in the presence of phenanthrene. One
hour after the stirrer stopped, the mixture was diluted with CDCl3 and
analyzed by 1H NMR spectroscopy.
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Received: November 7, 2012
Published online: && &&, &&&&
Keywords: asymmetric catalysis · azides · C-H amination ·
.
nitrenes · salen complexes
À
[1] For reviews on metal-catalyzed C H amination, see: a) H. Lu,
X. P. Zhang, Chem. Soc. Rev. 2011, 40, 1899 – 1909; b) F. Collet,
C. Lescot, P. Dauban, Chem. Soc. Rev. 2011, 40, 1926 – 1936; c) J.
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T. Katsuki, Angew. Chem. 2011, 123, 10058 – 10061; Angew.
Chem. Int. Ed. 2011, 50, 9884 – 9887.
À
[2] For selected papers of enantioselective C H amination, see:
[10] a) H. Kawabata, K. Omura, T. Uchida, T. Katsuki, Chem. Asian
J. 2007, 2, 248 – 256; b) K. Omura, T. Uchida, R. Irie, T. Katsuki,
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[11] For the SES protection of amino groups, see: S. M. Weinreb,
D. M. Demko, T. A. Lessen, J. P. Demers, Tetrahedron Lett. 1986,
27, 2099 – 2102; for the synthesis of SESN3, see the Supporting
Information.
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[12] For the selection of the azide, see the Supporting Information,
Table S1.
[13] For measurement of differential scanning calorimetry, see the
Supporting Information.
À
[3] For asymmetric C H amination using optically active sulfon-
[14] For the synthesis of complex 8, see the Supporting Information.
[15] For the solvent effect on yield and enantioselectivity, see the
Supporting Information, Table S2.
imidamides as the nitrene precursors, see: a) C. Liang, F. Collet,
F. Robert-Peillard, P. Mꢂller, R. H. Dodd, P. Dauban, J. Am.
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c) C. Lescot, B. Darses, F. Collet, P. Retailleau, P. Dauban, J. Org.
Chem. 2012, 77, 7232 – 7240.
[16] Under solvent-free conditions, 5 ꢀ MS was a better additive
than 4 ꢀ MS. Under solvent-free conditions, the reaction
mixture became pasty and the stirrer stopped when the reaction
came to an end. For the work-up procedure, see the Supporting
Information.
[4] Shi and co-workers have reported Pd-catalyzed 3,4-diamination
of terminal olefins using di-tert-butyldiaziridinone as a nitrogen
source, which proceeds through a 1,3-diene intermediate; see:
a) H. Du, W. Yuan, B. Zhao, Y. Shi, J. Am. Chem. Soc. 2007, 129,
7496 – 7497; for an asymmetric version, see: b) H. Du, B. Zhao,
Y. Shi, J. Am. Chem. Soc. 2008, 130, 8590 – 8591; for another
[17] Determined by 1H NMR analysis.
[18] For details, see the Supporting Information.
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À
example of Pd-catalyzed C H amination, see: c) H. Bao, U. K.
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[22] See the Supporting Information for details.
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4
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ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
À
C H Amination
Y. Nishioka, T. Uchida,
T. Katsuki*
&&& — &&&
Enantio- and Regioselective
À
Intermolecular Benzylic and Allylic C H
Bond Amination
Smooth salen: Ru(CO)–salen complex
1 is an effective catalyst for asymmetric
selectivity and excellent regioselectivity.
An ethyl group can be selectively ami-
nated, even in the presence of an n-propyl
group. No migration or isomerization of
the double bond was observed.
À
benzylic and allylic C H bond amination
using 2-(trimethylsilyl)ethanesulfonyl
azide (SESN3) as the nitrene source. The
reaction proceeded with high enantio-
Angew. Chem. Int. Ed. 2013, 52, 1 – 5
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!