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A Simple and Efficient Iron-Catalyzed Intramolecular Hydroamination of Unactivated Olefins.

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Communications
Cyclizations
sulfonyl)pyrrolidine (2 a) was isolated in quantitative yield
without any by-products [Eq. (1)].
DOI: 10.1002/anie.200503789
A Simple and Efficient Iron-Catalyzed
Intramolecular Hydroamination of Unactivated
Olefins
Kimihiro Komeyama,* Takayuki Morimoto, and
Ken Takaki*
Nitrogen heterocycles have attracted significant interest
among synthetic and medicinal chemists over the years,
because they form the core structures and are key synthetic
intermediates of natural products.[1] The importance of intramolecular hydroamination in the synthesis of these heterocycles has been widely recognized, and much effort has been
made towards exploiting this technique.[2] The most promising
hydroamination ever reported includes two distinctive metalcatalyzed processes: amine activation and olefin activation. In
the former process, the transformation takes place with early
transition metals through amido and imido species.[3]
Although the catalysts reveal good reactivity towards unactivated olefins, high compatibility of the substrates would not
be expected because of the extreme sensitivity of the metals
to heteroatom functional groups.[4] In the latter process, latetransition-metal complexes are the catalysts of choice, but
they have been used only for allenes[5] and alkynes,[6] not
alkenes. Moreover, the reaction is often accompanied by
olefin isomerization[7] and oxidative amination.[8] Recently, a
platinum catalyst with a high capacity for intramolecular
hydroamination of N-alkylaminoolefins was reported by
Widenhoefer et al.[9] However, this procedure requires
proper loading of the expensive catalyst and high temperature
(120 8C). Therefore, the development of a more practical
catalyst system that can be used for hydroamination with
fewer limitations under mild conditions is desired.
Although the high affinity of iron for olefins was proved
by many reactions with the cyclopentadienyliron dicarbonyl
(Fp) species,[10, 11] as well as by recent examples with iron
salts,[12] it has, surprisingly, not hitherto been applied in
catalytic hydroamination. Herein we report our investigations
into FeCl3-catalyzed intramolecular hydroamination, in which
we found that the iron activity was superior to that of
conventional transition-metal catalysts.
When
2,2-dimethyl-1-(4-toluenesulfonylamino)pent-4ene (1 a) was treated with FeCl3 (10 mol %) in 1,2-dichloroethane (DCE) at 80 8C for 2 h, 2,4,4-trimethyl-1-(4-toluene-
[*] K. Komeyama, T. Morimoto, Dr. K. Takaki
Department of Chemistry and Chemical Engineering
Graduate School of Engineering, Hiroshima University
Kagamiyama, Higashi-Hiroshima 739-8527 (Japan)
Fax: (+ 81) 82-424-5494
E-mail: kkome@hiroshima-u.ac.jp
ktakaki@hiroshima-u.ac.jp
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
2938
Notably, the presence of water did not prevent the
reaction, that is, the same results were obtained with
FeCl3�H2O under air. Although FeCl2�H2O gave a moderate yield of 2 a (73 %), no reaction occurred in the presence of
other iron salts such as Fe(NO3)3, Fe2(SO4)3, and Fe(acac)3
(acac = acetylacetonate). For comparison with these results,
the reaction was carried out with many transition-metal
chlorides and triflates (OTf) under similar conditions but
under a nitrogen atmosphere.[13] Of the catalysts tested,
Cu(OTf)2 and AgOTf gave 2 a in quantitative and 78 % yields,
respectively, but longer reaction times ( 16 h) were necessary for the reaction to be complete.
The reaction yield depended on the solvent used
(Table 1). DCE and hexane were the most suitable for the
transformation (Table 1, entries 1 and 2), whereas little or no
2 a was obtained with coordinative solvents such as benzene,
1,4-dioxane, tetrahydrofuran, 2-propanol, dimethyl sulfoxide,
and N,N-dimethylformamide (Table 1, entries 3?8).
Table 1: Effect of solvent on the intramolecular hydroamination of 1 a
with FeCl3�H2O.[a]
Entry
Solvent
2 a [%][b]
1
2
3
4
5
6
7
8
DCE[c]
hexane
benzene
1,4-dioxane
THF[d]
2-propanol
DMSO[e]
DMF[f ]
56
52
19
9
<1
<1
0
0
[a] Reaction conditions: 1 a (0.5 mmol), FeCl3�H2O (10 mol %), solvent
(5 mL), 80 8C, 1 h under air. [b] NMR spectroscopic yield based on
internal standard. [c] DCE = 1,2-dichloroethane. [d] THF = tetrahydrofuran. [e] DMSO = dimethyl sulfoxide. [f] DMF = N,N-dimethylformamide.
Next, the compatibility of FeCl3 with various types of
aminoolefins 1 a?1 i was investigated under the optimized
conditions in air (Table 2). As in the case of 1 a (Table 2,
entry 1), the 2,2-disubstituted aminoolefin 1 b was smoothly
transformed into the corresponding pyrrolidine 2 b in quantitative yield (Table 2, entry 2). Although 1,2-disubstituted
aminoolefins are known to be the least reactive substrates
toward intramolecular hydroamination,[14] 1 c and 1 d were
completely converted by FeCl3�H2O (10 mol %) into 2 c and
2 d, respectively, within 3 h (Table 2, entries 3 and 4). The high
reactivity did not require dialkyl substituents b to the amino
group, that is, 2 e was produced from 1 e in excellent yield,
albeit with low diastereoselectivity (3.8:1) (Table 2, entry 5).
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 2938 ?2941
Angewandte
Chemie
Table 2: Intramolecular hydroamination of various aminoolefins.[a]
Entry
Substrate
Product
t
[h]
Yield [%][b]
(d.r.)[c]
1
2
97
2
2
> 99
3
3
95
4
3
95
5
2
97 (3.8:1)
9
21
4
96 (1.1:1)
81 (1.3:1)
93 (2.1:1)
38
82 (2:2:1:1)
6
7
8
1 f R = OMe
1 g R = OTs
1h R=I
2f
2g
2h
9
[a] Reaction conditions: aminoolefin 1 (1 mmol), FeCl3�H2O
(10 mol %), 1,2-dichloroethane (10 mL), 80 8C under air. [b] Yield of
isolated product. [c] Determined by 1H or 13C NMR spectroscopy. Ts = ptoluenesulfonyl.
of tetrahydrofurylsulfonamide 4 and pyrrolidinoalcohol 5
( 4:1) in 96 % total yield (Scheme 1, Equation (2)), whereas
a longer reaction time provided the 2-aza-7-oxospirobicyclic
compound 6 in excellent yield with a diastereomer ratio of
4:3:2:1 (Scheme 1, Equation (3)).
Finally, we attempted to extend our methodology to the
synthesis of piperidine and azepane derivatives. Surprisingly,
we found that the construction of five-membered rings was
more favorable than that of six- and seven-membered rings
under the present conditions (Scheme 2).[15] For example,
Scheme 2. Intramolecular hydroamination of aminoolefins with various
carbon-chain lengths.
treatment of 2,2-dimethyl-1-(4-toluenesulfonylamino)hex-5ene (7) with FeCl3�H2O gave 2 c in 72 % yield together with
2-methylpiperidine 8 (24 %) (Scheme 2, Equation (4)). Similarly, exposure of 2,2-dimethyl-1-(4-toluenesulfonylamino)hept-6-ene (9) to iron(iii) chloride did not provide a sevenFunctional groups such as ether, tosylate, and iodide were also
membered ring; instead, 2 d and 2-ethylpiperidine 10 were
permitted in the reaction (Table 2, entries 6?8). In particular,
formed in 63 % and 32 % yield, respectively (Scheme 2,
the iodide-substituted aminoolefin 1 h gave the corresponding
Equation (5)). Because the 2 c/8 and 2 d/10 ratios did not
pyrrolidine 2 h in high yield (93 %) without the elimination of
change further upon separate treatment with the catalyst
iodide (Table 2, entry 8), which is uncommon in late-transiunder the reaction conditions, the above results would be
tion-metal chemistry. Furthermore, the system was also
caused by an isomerization process, probably of the starting
applicable to the synthesis of the 2,7-diazaspirobicyclic
substrates, during the reaction. On the other hand, 2,2compound 2 i (Table 2, entry 9).
dimethyl-5-phenyl-1-(4-toluenesulfonylamino)pent-4-ene
We also found that a suitably positioned hydroxy group
(11) produced 2-phenylpiperidine 12 in 94 % yield through 6resulted in competition with the sulfonamide functional group
endo-trig cyclization, and no 2-benzylpyrrolidine was
(Scheme 1). Thus, reaction of 3 at 80 8C for 2 h gave a mixture
detected (Scheme 2, Equation (6)).
In conclusion, we have demonstrated a
simple iron-salt-catalyzed intramolecular
hydroamination of unactivated olefins.
The reaction proceeds under mild conditions, and it is not necessary to exclude air
and moisture. Of special importance is the
tolerance of aminoolefins containing halide
moieties, which is rarely observed with
Group 9 and 10 metals. Moreover, 1,2disubstituted aminoolefins readily react in
the present system. With regard to the
FeCl3-catalyzed reaction suggested herein,
we must clarify whether the reaction proScheme 1. Intramolecular hydroamination of 3 with different reaction times.
Angew. Chem. Int. Ed. 2006, 45, 2938 ?2941
2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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2939
Communications
ceeds through the action of an acid catalyst, particularly HCl,
which may be generated from the iron salt under the reaction
conditions. Treatment of 1 e with FeCl3 (10 mol %) in the
presence of 2,6-di-tert-butylpyridine (10 mol %) at 80 8C for
2 h did not provide 2 e.[16] However, no reaction took place
with only HCl (30 mol %) under similar conditions.[17] Based
on these experiments, we concluded that the simple acidcatalyzed pathway does not contribute to the present
reaction. In further studies, we found that the activity of the
catalyst could be much improved by a silver-salt additive,
which allows the reaction of 1 a to be completed within 1 h.
Studies on other mechanistic aspects and the scope and
limitation of the iron-catalyzed reaction are in progress.
Experimental Section
Hydroamination of 1 a: A mixture of 1 a (267 mg, 1 mmol) and
FeCl3�H2O (27 mg, 0.1 mmol) in DCE (10 mL) was heated at 80 8C
for 2 h while being monitored by silica-gel TLC. The reaction was
allowed to cool and quenched with water (10 mL). The aqueous phase
was extracted with diethyl ether (20 mL). The combined organic layer
was washed with brine (10 mL), dried over MgSO4, filtered, and
evaporated. The crude product was purified by column chromatography on silica gel (60?230 mesh) with a hexane/ethyl acetate (3:1)
eluent to provide 2 a (259 mg, 0.97 mmol, 97 %) as a white solid. M.p.:
82?83 8C; 1H NMR (CDCl3): d = 0.53 (3 H, s), 1.02 (3 H, s), 1.34?1.42
(1 H, m), 1.41 (3 H, d, J = 6.3 Hz), 1.72 (1 H, dd, J = 12.6, 7.3 Hz), 2.41
(3 H, s), 3.05 (1 H, d, J = 10.6 Hz), 3.15 (1 H, d, J = 10.6 Hz), 3.56?3.69
(1 H, m), 7.30 (2 H, J = 7.9 Hz), 7.72 ppm (2 H, J = 8.3 Hz); 13C NMR
(CDCl3): d = 21.5, 22.7, 25.8, 26.5, 37.1, 48.8, 55.9, 61.4, 127.4, 129.4,
135.2, 143.0 ppm; MS: m/z (%): 267 ([M+] 5), 252 (100), 155 (51), 91
(86), 56 (98); elemental analysis: calcd (%) for C14H21NO2S: C 62.89,
H 7.92, N 5.24; found: C 63.04, H 7.85, N 5.22.
[4]
[5]
[6]
Received: October 26, 2005
Revised: January 11, 2006
Published online: March 23, 2006
.
Keywords: alkenes � homogeneous catalysis � hydroamination �
iron � nitrogen heterocycles
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