Bismuth-catalyzed cyclization of amino-1,6-enynes

Article abstract of DOI:10.1002/hc.20490

Bi(OTf)₃ effectively catalyzed the cyclization of amino-1,6-enynes, leading to bicyclic amine and cyclopentadienyl amino compounds. The selectivity of the products depends on the substitution pattern of the olefinic moiety. ¹⁹F NMR trace experiment of the reaction and other results indicate a novel alkynophilicity of the bismuth species.

Full text of DOI:10.1002/hc.20490

Heteroatom Chemistry
Volume 19, Number 6, 2008
Bismuth-Catalyzed Cyclization of
Amino-1,6-enynes
Kimihiro Komeyama, Motoyoshi Miyagi, and Ken Takaki
Department of Chemistry and Chemical Engineering, Graduate School of Engineering,
Hiroshima University, 1-4-1 Kagamiyama, Higahi-Hiroshima, 739-8527, Japan
Received 12 November 2007; revised 6 February 2008, 5 April 2008
carboxylic acids to provide the corresponding hete-
ABSTRACT: Bi(OTf)3 effectively catalyzed the cy-
clization of amino-1,6-enynes, leading to bicyclic
amine and cyclopentadienyl amino compounds. The
selectivity of the products depends on the sub-
stitution pattern of the olefinic moiety. 19F NMR
trace experiment of the reaction and other results
indicate a novel alkynophilicity of the bismuth
species. 2008 Wiley Periodicals, Inc. Heteroatom Chem
19:644–648, 2008; Published online in Wiley InterScience
(www.interscience.wiley.com). DOI 10.1002/hc.20490
rocyclic compounds, respectively, in excellent yields
[15–17]. In these reports, the iron could activate
both donors and acceptors via dual mode. However,
the catalyst showed lower activity for intramolecu-
lar nucleophilic addition of amino group to alkynes
than that to olefins. Thus, we researched other cata-
lysts that can be used for alkyne activation free from
environmental problem. Gratifyingly, Bi(OTf)3 was
found to reveal a high alkynophilicity. That is, the
catalyst promoted cyclization of amino-1,6-enynes
to yield aminobicyclic compounds, wherein the re-
action mode drastically depended on a stability of
the cationic intermediate. In this communication,
we would like to report these unprecedented results.
ꢀ
C
INTRODUCTION
Heterofunctionalization of carbon–carbon multiple
bonds is one of the most desirable and elegant tool
for the construction of carbon-heteroatom tethers
from simple substrate, and the methodology has
been widely recognized as a key step in organic
chemistry [1–3]. This transformation was rapidly de-
veloped by a high alkynophilicity of the noble metal
catalysts such as gold, silver, platinum, and mer-
cury, which accomplished a diverse set of nucle-
ophilic addition of N- and C-nucleophiles to olefins
[4,5], alkynes [6–9], allenes [10,11], and enynes [12–
14]. Despite excellent results in this field, these com-
plexes are expensive, partially toxic, and not suit-
able for industrial use. Therefore, the development
of more practical catalyst system is highly desired.
On the other hand, we have recently reported iron
salts-catalyzed intramolecular heterofunctionaliza-
tion of unactivated olefins with amines, alcohols, and
RESULTS AND DISCUSSION
When amino-1,6-enyne 1a was treated with 5 mol%
of Bi(OTf)3 at room temperature for 70 min in ace-
tonitrile, bicyclic materials 2a and 2aꢁ were obtained
in 93% total yield with a ratio of 86/14 (Table 1,
entry 1). Optimization studies revealed that the re-
action is sensitive to the solvent. Thus, MeCN, DCE,
and toluene were suitable for the transformation,
whereas the lower conversion and product selectiv-
ity were observed with MeNO2 and 1,4-dioxane.
Bi(OTf)3 was effective catalyst in the transforma-
tion to comparison with other representative transi-
tion metals as illustrated in Table 2. In the absence
of the catalyst, the reaction did not proceed at all
under the similar conditions. Hard Lewis acids such
as AlCl3 and Sc(OTf)3 were ineffective for the cy-
clization to give a slight yield of bicyclic product 2
with high 2a-selectivity (Table 1, entries 1 and 2).
Similar low activity and no conversion of 1a were
Correspondence to: Kimihiro Komeyama; e-mail: kkome@
hiroshima-u.ac.jp.
c
ꢀ
2008 Wiley Periodicals, Inc.
644

Bismuth-Catalyzed Cyclization of Amino-1,6-enynes 645
TABLE 1 Solvent Effect for Bi(OTf)3-Catalyzed Cyclization of 1a
Ph
Ph
Ph
NHTs
5 mol% Bi(OTf)3
solvent, rt, 70 min
+
TsN
TsN
1a
2a
2a´
Entry
Solvent
Conversion (%)a
Total Yield (%)a,b
1
2
3
4
5
MeCN
DCE
Toluene
MeNO2
97
100
83
93
22
93
82
75
53
15
(86/14)
(76/24)
(74/26)
(79/21)
(68/32)
1,4-Dioxane
aDetermined by 1H NMR.
bParenthesis value indicates isomeric ratio of 2a and 2aꢁ.
TABLE 2 Catalyst Screening for Cyclization of 1a
Ts
N
Ph
Ph
NHTs
5 mol% catalyst
MeCN, rt, 70 min
Ph
+
TsN
2a or 2a´
1a
3
Product and Yield (%)a,b
Entry
Catalyst
Conversion (%)a
2
3
1
AlCl3
Sc(OTf)3
Fe(OTf)3
Fe(OTf)2
Bi(OTf)3
Ni(OTf)2
Zn(OTf)2
PdCl2
4
12
44
4 (100:0)
7 (99 > :1)
10 (91: 9)
No reaction
93 (86:14)
No reaction
0
2 (99 > :1)
0
9 (99> 1)
0
4
9
2
3c
4
5
6
7
8
97
0
4
60
100
42
2
22
29
0
9
PtCl2
AgOTf
10d
aDetermined by 1H NMR.
bParenthesis value indicates isomeric ratio of 2a and 2aꢀ.
cReaction was carried out at 80◦C for 2 h.
dReaction was carried out for 14 h.
observed in the case of borderline acid such as Fe(II),
Ni(II), and Zn(II) triflates (entries 4 with 6 and 7),
but Fe(OTf)3 catalyst yielded a moderate conver-
sion of the substrate, albeit with low product con-
tribution, wherein any unassailable oligomers were
formed (entry 3). Interestingly, although π-acid cat-
alyst, PdCl2 and PtCl2, afforded moderate to high
consumption of 1a, monocyclic amine 3 and the
oligomer were mainly produced (entries 8 and 9).
In contrast, exposure of 1a with AgOTf catalyst gave
a negligible yield of 2 (9%) by NMR measurement of
the complexed reaction mixture. These findings in-
dicate that Bi(OTf)3 plays an individual role in this
cyclization.
reaction smoothly proceeded to yield a mixture of
isomers, 2a-d and 2aꢁ-d, wherein deuterium was in-
corporated at an allylic methyl group (2a-d: 87% D,
2aꢁ-d: 82% D). This result provided the definite ev-
idence for the origin of the product structure: The
skeleton was formed by connection of olefin with
both alkyne and amine moieties.
Ph
Ph
Ph
NHTs
5 mol% Bi(OTf)3
(1)
+
MeCN, rt, 70 min
93% [84:16]
TsN
TsN
D
D
2a
-
d
2a´-d
(82% D)
D
(87% D)
1a
-d
(95% D)
5 mol% Bi(OTf)3
or
10 mol% TfOH
To get information about the construction of
carbon-skeleton of the products 2a and 2aꢁ, the re-
action of deuterated aminoenyne 1a-d (95% D) was
carried out under similar conditions (Eq. (1)). The
Ts
N
Ph
Ph
NHTs
+
(2)
MeCN-d6, rt
TsN
2a or 2a´
Ph
3
1a
Heteroatom Chemistry DOI 10.1002/hc

646 Komeyama, Miyagi, and Takaki
FIGURE 1 19F NMR monitoring of the reaction of 1a in acetonitrile-d3 with Bi(OTf)3 (5 mol%); (b) after 15 min; (c) 60 min, and
with TfOH (15 mol%); (e) after 30 min; (f) after 60 min. The spectra (a) and (d) indicate the resonance of Bi(OTf)3 and TfOH in
acetonitrile-d3, respectively.
It remains unclear whether the present cycliza-
tion would be caused by Brønsted acid, TfOH, or not,
which may be generated from the bismuth catalyst
in situ. Because Brønsted acids are known to activate
the siloxy alkynes, they may exhibit a similar effect
for terminal alkyne moiety of 1a [18]. To prove this
point, the reactions of 1a with Bi(OTf)3 and TfOH
catalysts were compared by 19F NMR monitoring
(Eq. (2)). If TfOH was generated in the reaction,
signal of the acid or its amine salt should be ob-
served in the spectra. As illustrated in Fig. 1, when
aminoenyne 1a was treated with catalytic amount of
Bi(OTf)3 (δ–78.06 ppm, a), its signal completely dis-
appeared at once, and then a new signal appeared
at from −77.98 to −77.95 ppm in the period of 15–
60 min without any other signals (b and c). In the
meantime, most of 1a has been converted to only
2a and 2aꢁ. In contrast, the reaction with TfOH
(δ–77.69 ppm, d) exhibited a broad signal at from
−78.17 to −78.15 ppm in the period of 30–60 min
(e and f), giving rise to the products 2a and 2aꢁ in
75% total yield together with pyrrolidine 3a (8%)
as a minor product. However, the conversion of 1a
with TfOH was much slower than that with Bi(OTf)3,
even it took 5 h to reach to 95% conversion. There-
fore, it is likely that the present reaction is catalyzed
by Bi(OTf)3, not by TfOH.
cating a strong alkynophilicity of Bi(OTf)3 (Eq. (3)).
Increase of catalyst lording (10 mol%) and reaction
temperature (80◦C) changed the product selectivity
to provide aminobicyclic alkene 2b as a single iso-
mer in 40% yield along with 4 (39% yield) (Eq. (4)).
In contrast, 1-phenyl substituted aminoenyne 1c
showed different reaction manner. Thus, the enyne
cyclization took place to yield cyclopentadiene 5
without participation of amino moiety (Eq. (5)). In-
terestingly, treatment of enyne 6 with the bismuth
catalyst furnished similar cyclopentadiene deriva-
tive 7, even so in different conditions (Eq. (6)).
Ts
N
5 mol% Bi(OTf)3
(3)
MeCN
rt, 3 h
Ph
Ph
NHTs
4
15%
Ph
1b
10 mol% Bi(OTf)3
+
4
(4)
TsN
MeCN
39%
80 C, 17 h
2b
40%
Ph
Ph
NHTs
NHTs
5 mol% Bi(OTf)3
(5)
(6)
MeCN, rt, 17 h
34%
Ph
Ph
1c
5
7
To test the generality of the reaction, we tried out
the reaction of differently substituted aminoenynes.
The reaction of 1b gave dihydropyrrole 4 in low yield
under standard conditions. It is, however, notewor-
thy that monocyclic amine 4 was derived from only
alkynyl unit of 1b, not from olefinic moiety, indi-
Ph
CN
Ph
CN
5 mol% Bi(OTf)3
toluene
100 C, 1 day
80%
Ph
Ph
6
Heteroatom Chemistry DOI 10.1002/hc

Bismuth-Catalyzed Cyclization of Amino-1,6-enynes 647
Ph
dle LUMO energy between hard and soft acid ones
[19,20]. Studies on the product selectivity and the
reaction scope are currently in progress.
TsHN
Ph
Ph
TsN
R1
5
2
Ph
[Bi]
EXPERIMENTAL
TsHN
R1
[Bi]
Ts
N
Substrate1
Representative Procedure for Tandem
Hydroamination of 1a
Ph
4
Ph
Ph
A solution of aminoenyne 1a (51.5 mg, 0.14 mol) and
Bi(OTf)3 (4.6 mg, 7.0 μmol) in DCE (1.4 mL) was
stirred at room temperature for 70 min under N2 at-
mosphere. The reaction mixture was passed through
a short silica-gel column with ether eluent. Purifica-
tion of the crude product by column chromatogra-
phy (hexane/AcOEt = 10:1, SiO2) gave the product
2a and 2aꢁ in 70% and 5% yields, respectively.
Ph
TsHN
TsHN
Ph
TsHN
R2
1 = H
R2 = H or Me
R1 = Ph
R2 = H
[Bi]
R2
[Bi]
R
[Bi]
H
R1
B
A
C
SCHEME 1 Plausible reaction mechanisms.
Based on these results, the reaction process
could be explained as depicted in Scheme 1. Thus,
preferential coordination of the bismuth species to
the alkyne moiety of 1 would yield a key interme-
diate A. Then, in the case of 1,1-disubstituted olefin
1a (R1 = H; R2 = Me), A would exclusively changed
to the tertiary carbocation species B, which was fol-
lowed by nucleophilic addition of the amino group
to give the product 2 after protonation and isomer-
ization. In contrast, because the intermediate B de-
rived from 1b (R1 = R2 = H) was less stable sec-
ondary cation than that from 1a, direct amination
of alkyne moiety of the corresponding intermediate
A could compete with the cyclizations via B, leading
to both 4 and 2b, respectively. In addition, substrate
1c (R1 = Ph; R2 = H) would choice the intermediate
C due to high stability of benzylic cation. β-Proton
elimination of C followed by isomerization, instead
of intramolecular addition of amino group would
yield the cyclopentadienyl derivative 5.
3,5-Dimethyl-1-phenyl-6-tosyl-6-azabicyclo[3.2.1]-
oct-2-ene (2a). Isolated as a colorless oil; H NMR
1
(CDCl3, 270.05 MHz) δ: 1.46 (3H, s), 1.66, (3H, s),
2.09 (2H, s), 2.12 (1H, d, J = 17.8 Hz), 2.40 (3H,
s), 2.65 (1H, d, J = 17.8 Hz), 3.51 (1H, d, J = 8.1
Hz), 4.00 (1H, d, J = 8.1 Hz), 5.53 (1H, s), 7.19–7.35
(7H, m), 7.74 (2H, d, J = 8.1 Hz); 13C NMR (CDCl3,
67.8 MHz) δ: 21.3, 21.7, 25.4, 44.5, 45.9, 49.0, 62.2,
66.0, 125.8, 126.7, 126.8, 128.5, 129.0, 129.2, 129.6,
133.4, 139.1, 142.5, 143.2; LRMS m/z: 367 (M+, 2),
184 (36), 169 (48), 91 (100), 65 (42); HRMS calcd
for C22H25NO2S (M+) 367.1606, found 367.1615.
3,5-Dimethyl-1-phenyl-6-tosyl-6-azabicyclo[3.2.1]-
1
oct-3-ene (2aꢀ). Isolated as a colorless oil; H NMR
(CDCl3, 395.75 MHz) δ: 1.31 (3H, s), 1.68 (3H, s),
2.03 (1H, d, J = 10.5 Hz), 2.12 (1H, d, J = 17.7 Hz),
2.22 (1H, d, J = 17.7 Hz), 2.29 (1H, d, J = 10.5
Hz), 2.42 (3H, s), 3.70–3.71 (2H, m), 5.36 (1H, s),
7.17–7.34 (7H, m), 7.72 (2H, d, J = 8.2 Hz); 13C NMR
(CDCl3, 67.8 MHz) δ: 14.1, 21.5, 44.1, 48.1, 48.6,
61.5, 63.6, 125.2, 126.6, 127.4, 127.8, 128.3, 128.6,
129.0, 129.1, 134.8, 137.4, 142.7, 145.8; LRMS m/z:
367 (M+, 3.7), 183 (25), 169 (33), 91 (100), 65 (41);
HRMS calcd for C22H25NO2S (M+) 367.1606, found:
367.1603.
CONCLUSION
In conclusion, we have demonstrated a novel
alkynophilicity of Bi(OTf)3 catalyst in the cyclization
of amino-1,6-enynes. The reaction mode depended
on the substitution of the olefinic moiety because
of the stability of the cationic intermediate at all.
In comparison with other transition metal-catalyzed
cyclizations, the bismuth catalyst is helpful for or-
ganic transformation due to its easy handling and
commercial availability. In the demonstrated trans-
formation, we found that the bismuth employs an
individual effect to activate amino enynes. The ac-
tual role of the bismuth is not yet clear, but the au-
thors believe that the catalyst acts as a dual activator
of both amino group and carbon–carbon multiple
bonds, because the borderline acid has kept a mid-
3-Methyl-1-phenyl-6-tosyl-6-azabicyclo[3.2.1]oct-
2-ene (2b). Isolated as a colorless oil; 1H NMR
(CDCl3, 270.05 Hz) δ: 1.61 (3H, s), 1.63–1.68 (1H,
m), 2.00 (1H, dd, J = 10.5, 1.4 Hz), 2.25–2.42 (2H,
m), 2.40 (3H, s), 3.23 (1H, d, J = 8.4 Hz), 3.91 (1H,
dd, J = 8.4 Hz, 1.4 Hz), 4.34–4.38 (1H, m), 5.58
(1H, m), 7.16 (2H, d, J = 8.1 Hz), 7.23–7.34 (5H,
m), 7.72 (2H, d, J = 8.1 Hz); 13C NMR (CDCl3, 67.8
MHz) δ: 21.5, 22.3, 39.0, 40.1, 47.7, 57.4, 61.1, 125.8,
126.8, 127.0, 128.6, 129.7, 130.7, 132.7, 135.7, 143.1,
Heteroatom Chemistry DOI 10.1002/hc

648 Komeyama, Miyagi, and Takaki
143.4; LRMS m/z: 353 (M+, 6), 198 (75), 169 (70),
91 (100), 65 (50); HRMS calcd for C21H23NO2S (M+)
353.1449, found 353.1445.
7.20–7.43 (10H, m); 13C NMR (CDCl3, 67.8 MHz) δ:
19.3, 38.4, 43.7, 112.5, 112.8, 118.3, 125.7, 126.1,
127.3, 127.9, 128.0, 128.6, 129.0, 137.8, 138.9, 139.0;
HRMS calcd for C20H17N (M+) 271.1361, found
271.1367.
2,2-Dimethyl-4-phenyl-4-(prop-2-ynyl)-1-tosylpy-
1
rrolidine (3). Isolated as a colorless oil; H NMR
(CDCl3, 270.05 MHz) δ: 1.32 (3H, s), 1.63 (3H, s),
1.91 (1H, t, J = 2.7 Hz), 2.18 (1H, d, J = 13.0 Hz),
2.42 (3H, s), 2.47 (1H, dd, J = 16.7, 2.7 Hz), 2.48
(1H, d, J = 13.0 Hz), 2.62 (1H, dd, J = 16.7, 2.7 Hz),
3.61 (1H, d, J = 9.9 Hz), 3.75 (1H, d, J = 9.9 Hz),
7.14 (2H, d, J = 8.2 Hz), 7.23–7.30 (5H, m), 7.77
(2H, d, J = 8.2 Hz); 13C NMR (CDCl3, 67.8 MHz)
δ: 21.5, 29.4, 29.8, 30.9, 46.4, 52.2, 56.9, 64.9, 71.3,
80.6, 126.3, 126.8, 127.4, 129.1, 129.4, 137.9, 142.9,
144.0; LRMS m/z: 367 (M+, 0.3), 352 (27), 155 (24),
91 (100), 65 (45); HRMS calcd for C22H25NO2S (M+)
367.1606, found 367.1608.
ACKNOWLEDGMENTS
This work has been supported financially by the
Grant-in Aid for Scientific Research on Priority
Areas No. 18850015 from the Ministry of Edu-
cation, Science, Sports, Culture, and Technology,
Japan. We are gratefully acknowledged to profes-
sors Yohsuke Yamamoto and Atsushi Kawachi for
19F NMR measurement.
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8.7, 3.5 Hz), 5.95 (1H, s), 6.27 (1H, t, J = 2.0 Hz),
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1
Heteroatom Chemistry DOI 10.1002/hc