Tungsten(0)- and rhenium(I)-catalyzed tandem cyclization of acetylenic dienol silyl ethers based on geminal carbo-functionalization of alkynes

Article abstract of DOI:10.1002/chem.201003019

Tungsten(0)- and rhenium(I)-catalyzed reactions of acetylenic dienol silyl ethers based on the concept of geminal carbo-functionalization of alkynes are reported. Treatment of 3-siloxy-1,3-diene-7-ynes with catalytic amounts of [W(CO)₆] or [ReCl(CO)₅] under photoirradiation conditions gives synthetically useful bicyclo[3.3.0]octane derivatives in good yields. Extremely high catalytic activity is noted for the rhenium(I) complex. The reaction has been extended to substrates containing a nitrogen atom in their tethers. In this case, two kinds of synthetically useful heterocyclic compounds-the 2-azabicyclo[3.3.0]octane derivatives 9 and the monocyclic dihydropyrroles 10, with allenyl substituents-are obtained, and selective preparation of either product can be achieved through the use of an appropriate combination of the nitrogen substituent and the type of the rhenium(I) catalyst. The 2-azabicyclo[3.3.0]octane derivatives 9 are obtained selectively by carrying out treatment of N-Ns derivatives in the presence of [ReCl(CO) ₄(PPh₃)], whereas the dihydropyrrole derivatives 10 are obtained by treatment of N-Mbs derivatives with [ReCl(CO)₅]/ AgSbF₆. Finally, we have applied this geminal carbo-functionalization to one-carbon-elongated substrates containing N-Ts moieties in their tethers. Selective 5-exo cyclization is achieved in the presence of gold(I) or rhenium(I) catalysts, whereas 6-endo cyclization is observed on use of [W(CO) ₆]. Geminal carbo-functionalization of 3-siloxy-1,3-dien-7-ynes leading to bicyclo[3.3.0]octane derivatives is achieved through electrophilic activation of alkynes by tungsten(0) and rhenium(I) catalysts (see graphic). Extremely high activity is noted for rhenium(I) catalysts. Furthermore, selective preparation of two different classes of heterocyclic compounds from 5-aza-3-siloxy-1,3-dien-7-ynes is also achieved by appropriate choice of the rhenium(I) catalyst and the protecting group on the nitrogen. Copyright

Full text of DOI:10.1002/chem.201003019

FULL PAPER
DOI: 10.1002/chem.201003019
Tungsten(0)- and Rhenium(I)-Catalyzed Tandem Cyclization of Acetylenic
Dienol Silyl Ethers Based on Geminal Carbo-Functionalization of Alkynes
Hiroyuki Kusama, Yusuke Karibe, Rie Imai, Yuji Onizawa, Hokuto Yamabe, and
Nobuharu Iwasawa*[a]
Abstract: Tungsten(0)- and rhenium(I)-
catalyzed reactions of acetylenic dienol
silyl ethers based on the concept of
geminal carbo-functionalization of al-
kynes are reported. Treatment of 3-
siloxy-1,3-diene-7-ynes with catalytic
amounts of [W(CO)6] or [ReCl(CO)5]
under photoirradiation conditions gives
heterocyclic
compounds—the
2-
9
are obtained selectively by carrying out
À
azabicyclo[3.3.0]octane derivatives
G
treatment of N Ns derivatives in the
and the monocyclic dihydropyrroles 10,
with allenyl substituents—are obtained,
and selective preparation of either
product can be achieved through the
use of an appropriate combination of
the nitrogen substituent and the type
of the rhenium(I) catalyst. The 2-
presence
of
[ReCl(CO)4ACHTUNGTRENNUNG(PPh3)],
whereas the dihydropyrrole derivatives
À
10 are obtained by treatment of N
Mbs derivatives with [ReCl(CO)5]/
AgSbF6. Finally, we have applied this
geminal carbo-functionalization to one-
carbon-elongated substrates containing
synthetically useful bicycloACHTNUTRGNE[NUG 3.3.0]octane
À
derivatives in good yields. Extremely
high catalytic activity is noted for the
rhenium(I) complex. The reaction has
been extended to substrates containing
a nitrogen atom in their tethers. In this
case, two kinds of synthetically useful
azabicyclo
[3.3.0]octane derivatives
9
N Ts moieties in their tethers. Selec-
tive 5-exo cyclization is achieved in the
presence of gold(I) or rhenium(I) cata-
lysts, whereas 6-endo cyclization is ob-
served on use of [W(CO)6].
Keywords: alkynes · cyclization ·
nitrogen heterocycles · rhenium ·
tungsten
Introduction
tuted products (Scheme 1b)—has remained unexplored in
spite of its high potential as a useful synthetic method.[2]
Recently, transition-metal-catalyzed electrophilic activa-
tion of alkynes has been extensively studied for the con-
struction of carbo- and heterocyclic compounds. In particu-
lar, gold(I) and platinum(II) complexes have emerged as
powerful catalysts for reactions of this type, and a very rich
enyne chemistry has made it possible to prepare a variety of
synthetically useful cyclic compounds atom-economically
(Scheme 2).[2,3] With these alkynophilic transition-metal cat-
1,2-Vicinal carbo-functionalization of alkynes can easily be
carried out by their carbometalation in the presence of vari-
ous kinds of organometallic reagents such as Cu, Al, Zr, and
so forth, followed by treatment of the resulting alkenyl met-
allic species with carbon electrophiles (Scheme 1a).[1] This
reaction provides a powerful method for the synthesis of
polysubstituted olefins in a one-pot operation.[1] On the
other hand, geminal carbo-functionalization of alkynes—ad-
dition of carbon nucleophiles and carbon electrophiles se-
quentially to the same alkyne carbon to afford 1,1-disubsti-
Scheme 2. Geminal carbo-functionalization: generation of cyclopropyl
carbenes from enynes.
Scheme 1. Vicinal (a) or geminal (b) functionalization of alkynes.
alysts a kind of geminal carbo-functionalization reaction—
cyclopropanation to generate cyclopropyl carbene complex
intermediates—was achieved,[3,4] and almost all the reactions
so far reported belong to this reaction type.
We have previously reported that [W(CO)5(L)] can acti-
vate terminal alkynes effectively and catalytically for the in-
tramolecular nucleophilic attack of enol silyl ethers to give
various kinds of carbocycles.[5] In these reactions, the pro-
duced alkenyltungsten moieties were protonated by the
[a] Dr. H. Kusama, Y. Karibe, R. Imai, Dr. Y. Onizawa, Dr. H. Yamabe,
Prof. Dr. N. Iwasawa
Department of Chemistry, Tokyo Institute of Technology
2-12-1, O-okayama, Meguro-ku, Tokyo 1528551 (Japan)
Fax : (+81)3-5734-2931
E-mail: niwasawa@chem.titech.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003019.
Chem. Eur. J. 2011, 17, 4839 – 4848
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4839

In this article we report a full account of this work, in-
cluding reaction scope, synthetic application, and further ap-
plication to the synthesis of heterocyclic compounds.[9]
Results and Discussion
Development of tungsten(0)- and rhenium(I)-catalyzed
geminal carbo-functionalization: We first examined the reac-
tion behavior of the w-acetylenic dienol silyl ether 1a
(Table 1) in toluene in the presence of an equimolar amount
Scheme 3. [W(CO)5(L)]-catalyzed 5-endo-dig cyclization of 6-siloxy-5-en-
1-ynes.
of preformed [W(CO)5ACTHNUTRGNEUNG
(toluene)][10] and molecular sieves
Table 1. Reaction behavior of the dienol silyl ether 1a.
added water to give cyclic ketones with regeneration of the
catalysts (Scheme 3).
During these studies, we thought of utilizing the alkenyl-
tungsten intermediates for further carbon–carbon bond for-
mation, because these intermediates could be regarded as a
anions of tungsten–carbene complexes.[6] We expected that
synthetically useful reactions based on the geminal carbo-
functionalization strategy might be achieved through appro-
priate design of the starting materials. We then considered
the reaction behavior of the acetylenic dienol silyl ethers 1
(Scheme 4) as substrates. Treatment of the acetylenic dienol
Entry Conditions (mol%)
T, t
Yield
[%]
2a (a/b)
3a
1
[W(CO)
G
87
(90:10)
86
(90:10)
59
(57:43)
65
13
14
41
35
–
2
[W(CO)6], hn (10)
PtCl2 (10)
3
4
AuBr3 (10)
(89:11)
–
5[a]
6
A
R
E
84
(78:22)
86
16
14
7
A
Scheme 4. Tandem cyclization of dienol silyl ethers.
(88:12)
91 (93:7)
84
8
9
U
RT, 3 h
RT, 23 h
98
68
9
16
(71:29)
–
86
silyl ethers 1 with appropriate electrophilic transition-metal
complexes should give zwitterionic intermediates of type B
through 5-endo nucleophilic cyclization of the enol silyl
ether systems to the electrophilically activated h2-alkyne
complexes A, and the resulting alkenyl metallic moieties
should then attack the a,b-unsaturated silyloxonium moiet-
ies intramolecularly at the position b to the metal to gener-
ate bicyclic unstabilized carbene complexes of type C. Final-
ly, the unstabilized carbene complexes should undergo a typ-
ical carbene reaction, such as [1,2] hydrogen migration,[7] to
10
11
[W(CO)6], hn (0.5)
[ReCl(CO)5], hn (0.5)
RT, 16 h
RT, 16 h
<5
92
–
14
G
(81:19)
[a] CH2Cl2 was used as a solvent.
(4 ꢁ). The reaction proceeded as expected and the desired
bicyclo[3.3.0]octane derivative 2a was obtained in moderate
AHCTUNGTRENNUNG
yield as a mixture of diastereomers (mainly a-Ph product)
accompanied by a small amount of the tricyclic compound
3a (Table 1, entry 1). The formation of the tricyclic com-
pound 3a explicitly implicates the presence of the carbene
complex intermediate C, in which the carbene moiety inserts
give the bicycloACHTUNGTRENNUNG[3.3.0]octane derivatives 2 with regeneration
of the reactive catalytic species. These reactions would
afford an efficient method for the synthesis of synthetically
À
useful bicyclo[3.3.0]octane derivatives, which constitute
into the neighboring benzylic C H bond. When carried out
under photoirradiation conditions, the reaction proceeded to
completion even in the presence of only 10 mol% of
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Tungsten(0)- and Rhenium(I)-Catalyzed Tandem Cyclization
FULL PAPER
Table 2. Rhenium- and tungsten-catalyzed reaction behavior of dienol
silyl ethers.
[W(CO)6] (Table 1, entry 2). Because the expected geminal
carbo-functionalization had been found to proceed catalyti-
cally in the presence of [W(CO)5(L)], we further examined
the possibility of carrying out the reaction more efficiently
through the use of other transition-metal complexes.
[2,3]
[2,3]
PtCl2
(67% yield, Table 1, entry 3) and AuBr3
(79%
yield, entry 4) also showed high catalytic activity for this
transformation, and the proportion of the tricyclic com-
pound 3a formed was larger in these cases than in the tung-
sten-catalyzed reaction. Surprisingly, a cationic gold(I) com-
plex, often used as a powerful catalyst for the electrophilic
activation of alkynes,[2,3] was not suitable for this reaction,
probably due to the poor nucleophilicity of the alkenyl-
Entry R1, R2, R3
G
[W(CO)6] Yield
A
[%] (a/b)
1
2
3
4
5
6
7
Me, H, Me (1b) 0.5
Et, H, Me (1c) 0.5
5
72
(73:27)
79
ACHTUNGTRENNUNG
10
20
5
iPr, H, Me (1d)
Ph, H, Me (1e)
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
81
(41:59)
67
(20:80)
79
(27:73)
77
ined.[11] [ReBr(CO)5] showed moderate activity under heat-
ing conditions, but under photoirradiation conditions a very
high catalytic activity was achieved (Table 1, entries 6 and
7). Furthermore, [ReCl(CO)5] was found to have an ex-
tremely high activity: the reaction even took place in the
presence of only 0.5 mol% [ReCl(CO)5] to give the cyclized
products 2a and 3a in 92% yield and in a ratio of 86:14
upon carrying out the reaction in toluene under photoirra-
diation conditions (Table 1, entry 11).
When the [W(CO)5(L)]-promoted reaction was carried
out in the presence of a protonating agent (MeOH), the
monocyclic cyclopentene derivative 4 (Scheme 5), the pro-
tonated product of the zwitterionic intermediate B, was
mainly obtained. This bicyclization reaction is thus likely to
proceed in a stepwise manner.
Me, Me, Me
(1 f)
Ph, Me, Me
(1g)
20
10
5
Ph, Me, H (1h)
(75:25)
sponding trisubstituted diene (Table 2, entry 1 vs. 5). Fur-
thermore, a substrate possessing a terminal alkyne moiety
(1h) could also be employed for this reaction (Table 2,
entry 7). The [ReCl(CO)5]-catalyzed reaction gave results
superior in most cases to those of the [W(CO)5(L)]-cata-
lyzed reaction: that is, the reactions of 1a–1g gave the bicy-
clic enol silyl ethers in good yields (67–81%) in the pres-
ence of 5–20 mol% [W(CO)6] under photoirradiation condi-
tions in toluene, whereas the [ReCl(CO)4(L)]-catalyzed re-
action required much smaller amounts of [ReCl(CO)5] (only
0.5–3 mol%).
In particular, the rhenium-catalyzed reactions of the
dienol silyl ethers 1i and 1j (Table 3), with no substituents
at their b positions (1-positions), afforded the desired bicy-
Scheme 5. Cyclization in the presence of a proton source.
Table 3. Rhenium- and tungsten-catalyzed reaction behavior of 1-unsub-
stituted dienol silyl ethers.
Having established that [W(CO)6] and [ReCl(CO)5] cata-
lyzed the geminal carbo-functionalization reaction very effi-
ciently, the generality of the reaction was examined, and the
results are summarized in Table 2.[12] The trisubstituted
dienes 1b–1d, with a methyl, primary, or secondary alkyl
group at their b positions (R1), were cyclized to afford the
corresponding bicyclic enol silyl ethers (R1-a product major)
in good yields (Table 2, entries 1–3). The Ph-substituted
diene (1e) also gave a good result, but no stereoselectivity
with regard to the Ph substituent was observed (Table 2,
entry 4).
It should be noted that no tricyclic enol silyl ether of type
3 was obtained when a substrate without a gem-diester
moiety was employed. Even the tetrasubstituted dienes (1 f,
1g) reacted to afford the corresponding substituted bicyclic
enol silyl ethers in good yields. In these cases, the stereose-
lectivity of the products was different from that of the corre-
R
N
Yield
[%]
[W(CO)6]
[mol%]
Yield
[%]
ACHTUNGTRENNUNG
Me (1i)
H (1j)
81
51
100
100
17
0
clic enol silyl ethers in moderate to good yields in the pres-
ence only of 1 mol% [ReCl(CO)5], whereas the correspond-
ing tungsten-catalyzed reactions proceeded in very low
yields even with stoichiometric amounts of [W(CO)5(L)].
Next, to gain an insight into the higher reactivity of the
rhenium carbonyl complex, the charge distributions in the
rhenium[13]– and tungsten–alkyne p complexes were evaluat-
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4841

N. Iwasawa et al.
58% yields, respectively, after acid treatment.[16] Conjugate
addition of dimethyl propargylmalonate sodium salt to 5 in
the presence of TIPS triflate gave the w-acetylenic enol silyl
ethers 6-a and 6-b in yields of 18 and 36%, respectively.[5b]
Furthermore, treatment of the two isomers 6-a and 6-b
again with catalytic amounts of [W(CO)6] and with water
(3 molequiv) under photoirradiation conditions in THF
gave the tricyclic ketones 7-a and 7-b, each containing a tri-
quinane skeleton, in yields of 52 and 78%, respectively
(Scheme 6a and b). The basic carbon skeleton of triquinanes
can thus easily be constructed with the aid of Re- and W-
catalyzed cyclizations.
Table 4. Charge distributions in the p-alkyne complexes D and E.
D
E
C1
C2
C3
C4
Mulliken
NBO
0.120
0.026
0.120
0.026
0.180
0.070
0.119
0.018
ed by DFT calculations[14] with but-2-yne as a model sub-
strate. As shown in Table 4, one of the alkyne carbon atoms
in the rhenium complex E has more positive charge than
the alkyne carbon atoms in the tungsten complex D, which
suggests that the rhenium–alkyne p complex is more elec-
trophilically activated than the corresponding tungsten com-
plex. This calculation is in good agreement with the higher
reactivity of the rhenium-catalyzed reaction.
Rhenium-catalyzed selective preparation of nitrogen-con-
taining cyclic compounds: We next applied these reactions
to the synthesis of azabicycloACHTNUTRGNE[UNG 3.3.0]octane derivatives. When
À
the dienol silyl ether 8a (Scheme 7), containing an N Ts
component in the tether, was treated with [W(CO)6]
Synthetic application of tandem cyclization—synthesis of tri-
quinane skeletons: This tandem cyclization protocol pro-
vides a facile method for the preparation of bicyclo-
ACHTUNGTRENNUNG[3.3.0]octane derivatives, which constitute the basic carbon
skeletons of many natural products,[8] in a single-step fashion
from easily available acyclic substrates. This protocol was
therefore applied to a concise synthesis of the basic carbon
skeleton of triquinanes.[15] Treatment of the dienol silyl ether
1k (Scheme 6), containing a methoxy group at the diene ter-
minus, either with [W(CO)6] (5 mol%) in diethyl ether or
with [ReCl(CO)5] (0.5 mol%) in toluene under photoirra-
diation conditions gave the bicyclic dienone 2k in 95 or
Scheme 7. Reaction behavior of the dienol silyl ether 8a, containing an
À
N Ts component in the tether.
(10 mol%) under photoirradiation conditions, the expected
bicyclic silyl enol ether 9a was obtained in 68% yield as a
mixture of diastereoisomers. However, when the same reac-
tion was carried out with [ReCl(CO)5] (10 mol%) under
heating conditions in toluene in the presence of molecular
sieves (4 ꢁ),[17] the unexpected monocyclic dihydropyrrole
10a, containing an allenyl substituent, was obtained in 43%
yield, along with 46% yield of the bicyclic silyl enol ether
9a.[2c,18] At that point, it was thought that the expected 2-
azabicycloACTHNUTRGNEUG[N 3.3.0]octane derivative 9a was obtained in a simi-
lar geminal carbo-functionalization, whereas the dihydropyr-
role derivative 10a was obtained through a ring-opening re-
action of the zwitterionic intermediate F. The ring-open re-
action was thought to be caused by electron donation from
the alkenylmetallic moiety to the sulfonamide group to give
the acyclic intermediate G, containing an allenyl substituent,
followed by 1,4-addition of the anion of the sulfonamide to
the a,b-unsaturated silyloxonium moiety (Scheme 7, path b).
Because two kinds of synthetically useful heterocyclic com-
pounds were found to be obtained on treatment with
Scheme 6. Synthesis of triquinane skeletons.
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Tungsten(0)- and Rhenium(I)-Catalyzed Tandem Cyclization
FULL PAPER
Table 7. Catalyst screening for the selective preparation of the dihydro-
pyrrole 10d.
[ReCl(CO)5], we decided to examine this reaction in more
detail to allow the selective preparation of these two types
of products.
Firstly, the reaction behavior of substrates with a variety
of sulfonyl groups on the nitrogen atom was examined with
the goal of achieving selective preparation of either 9 or 10,
in the expectation that arylsulfonamide units containing
electron-withdrawing groups would favor the formation of
dihydropyrroles 10, because of their better ability as elimi-
nating groups, whereas those with electron-donating groups
Entry
Catalyst
X
t [h]
Yield [%]
9d (a/b)
10d (cis/trans)
1
2
3
A
0
0
10
10
10
0.5
4
0.5
0.5
5
28 (92:8)
55 (100:0)
33 (70:30)
51 (100:0)
58 (100:0)
75 (70:30)
N
N
47 (85:15)
31 (70:30)
18 (90:10)
11 (90:10)
U
would favor the formation of the 2-azabicycloACTHNURTGNEU[GN 3.3.0]octane
derivatives 9 (Table 5). However, it was a surprise to find
4[a]
5[a,b]
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
[a] 1,2-Dichloroethane was used as solvent. [b] 1 mol% of [ReCl(CO)5]
was used.
Table 5. Examination of the effects of a variety of sulfonyl groups on the
nitrogen atom.
[ReCl(CO)
azabicyclo[3.3.0]octane derivatives 9, whereas the cationic
(PPh3)][19] to favor the formation of the 2-
4ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
rhenium catalyst prepared in situ from [ReCl(CO)5] and
[20]
AgSbF6 favored the formation of the dihydropyrrole de-
rivatives 10. The best conditions for the selective prepara-
Entry
Ar
Yield [%]
tion of the 2-azabicyclo
dihydropyrrole derivatives 10 are as follows. A 2-azabicyclo-
AHCTUNGTREG[NNUN 3.3.0]octane derivative 9 is obtained selectively by carrying
ACHTUNGTRENN[UNG 3.3.0]octane derivatives 9 and of the
9 (a/b)
10
43
34
17
55
1
2
3
4
46 (90:10)
49 (90:10)
76 (90:10)
28 (92:8)
À
out the treatment of the N Ns derivative with [ReCl(CO)4-
(PPh3)] (5 mol%) in toluene at 808C, whereas a dihydropyr-
AHCTUNGTRENNUNG
role derivative 10 is obtained with good selectivity by treat-
À
ment of the N Mbs derivative with [ReCl(CO)5] (1 mol%)
and AgSbF6 (10 mol%) in dichloroethane at 808C.[21]
Rhenium(I)-catalyzed selective preparation of two types
of nitrogen-containing cyclic compounds from 5-aza-3-
siloxy-1,3-diene-7-ynes had been achieved, so the generality
of these reactions was examined with several substrates.[22]
Firstly, the generality of the preparation of the bicyclic com-
that the 2-azabicycloACTHNUGRTENUNG[3.3.0]octane derivative 9c was ob-
tained as a major product when the substrate containing a
p-nitrobenezenesulfonyl (Ns) group was used (Table 5,
entry 3), whereas the presence of a p-methoxybenezenesul-
fonyl (Mbs) group increased the formation of the dihydro-
pyrrole 10d (Table 5, entry 4).
À
pounds 9 was examined with use of N Ns derivatives and
[ReCl(CO)
U
The use of other rhenium catalysts was also examined,
both to achieve selective preparation of the two types of
products and to provide further information on the mecha-
nisms of these reactions. As summarized in Table 6 and
ed dienes (R=Ph, 1-naphthyl) were cyclized to afford the
corresponding 2-azabicyclo[3.3.0]octanes 9c and 9e as a
mixture of diastereomers in good yields and with high selec-
AHCTUNGTRENNUNG
À
Table 7, examination of several rhenium catalysts with N
Table 8. Generality of 2-azabicycloACHTUNTRGNEUNG[3.3.0]octane synthesis.
À
Ns and N Mbs substrates revealed the tendency for neutral
Table 6. Catalyst screening for the selective preparation of the bicyclic
compound 9c.
Entry
R1, R2, R3
X
t [h]
Yield [%]
9 (a/b)
10
1
2
Ph, H, H (8c)
1-naphthyl, H, H (8e)
iPr, H, H (8 f)
5
12.5
0.5
1
4.5
4.5
1.5
0.5
81 (60:40)
92 (85:15)
37 (16:84)
87 (20:80)
91 (30:70)
78 (60:40)
33 (50:50)
4
10
10
5
10
5
trace
trace
0
Entry Catalyst (mol%)/additive T [8C], t [h]
Yield [%]
9c (a/b) 10c
3[a,b]
4
Ph, H, Me (8g)
1
G
80, 0.5
76 (90:10) 17
61 (85:15) 21
5
1-naphthyl, H, Me (8h)
Ph, Me, Me (8i)
iPr, H, Me (8j)
0
2[a]
3
6[c]
7[d]
trace
80, 1
79 (70:30)
5
10
17[e]
reflux, 0.5
80, 12.5
[a] Concentration was 0.01m. [b] Molecular sieves (5 ꢁ) were used in-
stead of molecular sieves (4 ꢁ). [c] TBS enol ether was employed.
[d] Concentration was 0.05m. [e] cis/trans=14:86.
4
G
CHTUNGTRENNUNG
81 (60:40)
4
[a] 1,2-Dichloroethane was used as solvent.
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N. Iwasawa et al.
tivity (Table 8, entries 1 and 2). The alkyl-substituted diene
could also be employed to give the corresponding 2-
azabicycloACHTUNGTRENNUNG[3.3.0]octane 9 f selectively, albeit in low yield
(Table 8, entry 3). Next, the reaction behavior of internal al-
kynes with a methyl group at the alkyne terminus was exam-
ined. The phenyl- and 1-naphthyl-substituted dienes afford-
ed the corresponding 2-azabicycloACHTUNTRGNEUNG[3.3.0]octanes 9g and 9h
with methyl substituents at their C5 positions in good yields
without the formation of the dihydropyrrole derivatives 10g
or 10h (Table 8, entries 4 and 5). In these cases, the R1-b
isomers of the 2-azabicycloACTHNUTRGENUGN[3.3.0]octanes 9g and 9h were
each obtained as the major diastereomer. The reaction of a
tetrasubstituted diene also proceeded to give the corre-
sponding bicyclic enol silyl ether 9i selectively and in good
yield (Table 8, entry 6). However, the reaction of the alkyl-
substituted diene 8j proceeded only in moderate yield and
with moderate selectivity (Table 8, entry 7).[23]
Next, the generality of selective preparation of dihydro-
pyrrole derivatives of type 10 was examined under cationic
rhenium(I) conditions (Table 9). The aryl-substituted dienes
could be successfully converted into the corresponding dihy-
Scheme 8. A possible mechanism for the formation of allenes of type 10.
occurs first in the zwitterionic intermediate F to give the
azabicyclic ammonium intermediate H (Scheme 8),[24] which
then undergoes elimination of the dihydropyrrole derivative
10 with an allenic substituent along with the rhenium cata-
lyst.
À
The observations that the N Ns derivatives mainly gave
À
the 2-azabicyclo
[3.3.0]octane derivatives 9, whereas the N
Mbs derivatives preferentially gave the dihydropyrrole de-
rivatives 10 could be explained on the basis of this mecha-
nism because the nucleophilicity of the nitrogen atom in the
Table 9. Generality of dihydropyrrole synthesis.
À
À
N Mbs system is higher than that in the N Ns system
(Scheme 9). Thus, in the zwitterionic intermediate of type F,
Entry R1, R2, R3
X
t [h] Yield [%]
9 (a/b)
10 (cis/trans)
1
2
3
Ph, H, H (8d)
1-naphthyl, H, H (8k)
Ph, Me, H (8l)
1
5
4
2
0.5
11 (90:10) 75 (70:30)
14 (76:24) 70 (90:10)
9 (90:10) 86 (30:70)
11 (60:40) 69 (40:60)
52 (50:50) 31 (94:6)
41 (60:40) 58 (20:80)
14 (60:40) 72 (20:80)
1
1
5
4[a]
5
iPr, H, H (8m)
Ph, H, Me (8n)
Ph, Me, Me (8o)
iPr, H, Me (8p)
10 0.1
10 0.5
5
6
7[a]
0.5
[a] Concentration was 0.01m.
Scheme 9.
dropyrroles 10, each with an allenic substituent, with high
selectivity as mixtures of diastereomers even with 1 mol%
of the cationic rhenium catalyst (Table 9, entries 1–3). The
alkyl-substituted dienes also gave good results for the selec-
tive preparation of dihydropyrroles 10m and 10p irrespec-
tive of the presence or absence of alkyne substituents
(Table 9, entries 4 and 7), whereas the aryl-substituted
dienes 8n and 8o, with internal alkyne moieties, gave two
types of products with concomitant formation of the dihy-
dropyrroles 10n and 10o (Table 9, entries 5 and 6).
Here we would like to discuss the mechanism of the reac-
tion, in particular with regard to the formation of the dihy-
dropyrroles with allenic substituents. As already mentioned,
the effect of the substituent on the arylsulfonyl group was
not compatible with the initially proposed elimination mech-
anism (see Scheme 7). From the data obtained so far, we be-
lieve at present that nucleophilic addition of the sulfona-
mide nitrogen to the a,b-unsaturated silyloxonium moiety
instead of the alkenylrhenium moiety, the nitrogen atoms of
À
the N Mbs derivatives preferentially underwent addition to
the a,b-unsaturated silyloxonium moieties to give the bicy-
clic ammonium salts H, which gave the dihydropyrroles with
regeneration of the catalyst as shown in Scheme 8. Use of
the cationic rhenium catalyst should also retard the nucleo-
philic attack of the alkenylrhenium moieties in the zwitter-
ionic intermediates F, resulting in the favorable nitrogen
attack onto the a,b-unsaturated silyloxonium moieties.
These reactions were thus able to provide two synthetically
useful nitrogen-containing cyclic compounds selectively
through variation of the substituent on the nitrogen and the
kind of rhenium catalyst.
Tungsten-catalyzed tandem cyclization through 6-endo-dig
cyclization: Finally, we applied this geminal carbo-function-
À
alization to one-carbon-elongated substrates containing N
4844
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Chem. Eur. J. 2011, 17, 4839 – 4848

Tungsten(0)- and Rhenium(I)-Catalyzed Tandem Cyclization
FULL PAPER
Ts components in their tethers. We had already reported
that on use of the cationic gold(I) catalyst, the dienol silyl
ethers 11a diastereoselectively gave the bicycloACTHNUTRGNE[NUG 4.3.0]nonane
H bonds in the carbene complex intermediate generated by
5-exo cyclization.[25] No other cyclized products were ob-
tained. This high insertion ability of the carbene complex in-
termediate is a remarkable characteristic of the rhenium(I)
catalyst. On the other hand, when 11a was treated with an
equimolar amount of [W(CO)6] under photoirradiation con-
ditions, the desired geminal carbo-functionalization through
6-endo-dig cyclization occurred to give the 3-azabicyclo-
derivatives 12a (Scheme 10), with different stereochemistry
AHCTUNGTRENNUNG
[4.3.0]nonane derivative 15a as a single diastereoisomer[28]
in 40% yield. In this case, no 5-exo-dig products 12a, 13a,
or 14a were observed.[29] Further examination to improve
the yield of the product and to reduce the amount of the
catalyst did not give satisfactory results.
We next examined the generality of this tungsten-cata-
lyzed geminal carbo-functionalization through 6-endo-dig
cyclization (Table 11). With the three substrates 11a–11c,
use of the gold catalyst selectively gave the 8-azabicyclo-
Scheme 10. Gold-catalyzed geminal carbo-functionalization of 11a
through 5-exo-dig cyclization.
Table 11. Generality of tungsten-catalyzed geminal carbo-functionaliza-
tion through 6-endo-dig cyclization.
from the thermal Diels–Alder products.[25] In these reac-
tions, geminal carbo-functionalization through initial 5-exo
cyclization gave a bicyclic carbene complex such as I, which
underwent 1,2-alkyl migration of the benzylic carbon to give
the product with regeneration of the catalyst.[26]
Entry
Conditions[a]
(R1, R2, X)
Yield [%]
12
15
During these studies, we thought of the possibility—based
on previous experimental results[27]—of geminal carbo-func-
tionalization through initial 6-endo cyclization in the pres-
ence of tungsten catalyst. Surprisingly, treatment of 11a
with [ReCl(CO)5] (10 mol%) gave a 78% yield of the tricy-
clic compounds 13a and 14a (Table 10), produced by inser-
1
2
3
4
A
B
A
B
A
B
Ph, Me, Ts (11a)
Me, Me, Ts (11b)
Ph, Ph, Ms (11c)
77
–
68
–
72
–
–
40
–
37
–
52
5
6[b]
[a] Conditions A: [AuClACTHNUTRGNEUNG(PPh3)]/AgSbF6 (10 mol%), molecular sieves
À
tion of the carbene complex moiety into the neighboring C
(4 ꢁ), 1,2-dichloroethane, 808C, 1 h. Conditions B: [W(CO)6]
(100 mol%), molecular sieves (4 ꢁ), toluene, hn, 5–10 min. [b] An 18%
yield of the tricyclic compounds 13c and 14c was obtained.
Table 10. Catalyst screening for geminal carbo-functionalization through
6-endo-dig cyclization.
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
derivatives 15, the 6-endo cyclized products, diastereoselec-
tively and in moderate yields. We have thus succeeded in
complementary construction of two types of azabicyclo-
AHCTUNGTREG[NNUN 4.3.0]nonane derivatives, core skeletons of several biologi-
cally active natural products,[30] through appropriate choice
of transition-metal complexes.
Conclusion
Entry Catalyst (mol%)
T, t
Yield [%]
12a 13a+14a
(13a/14a)
15a
We have developed tungsten(0)- and rhenium(I)-catalyzed
geminal carbo-functionalization reactions of 3-siloxy-1,3-
diene-7-ynes to give synthetically useful bicycloACTHNUTRGNE[NUG 3.3.0]octane
1[a]
2
N
E
808C, 1 h 77
0
0
derivatives. Exceptionally high activity of [ReCl(CO)5] for
these reactions is noted. The reactions were applied to the
construction of the basic carbon skeletons of triquinanes. Se-
lective preparation of two kinds of nitrogen-containing
cyclic compounds from 5-aza-3-siloxy-1,3-diene-7-ynes was
0
78 (50:50)
0
0
20 min
RT,
10 min
3
0
40
[a] 1,2-Dichloroethane was used as solvent.
Chem. Eur. J. 2011, 17, 4839 – 4848
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www.chemeurj.org
4845

N. Iwasawa et al.
The reaction mixture was then filtered through a short pad of Celite, and
the filtrate was concentrated under reduced pressure. Water was then
poured onto the residue and organic materials were extracted twice with
ethyl acetate. The combined organic layer was washed with brine and
dried over anhydrous MgSO4. The filtrate was concentrated and the re-
sulting crude product was purified by PTLC (toluene/EtOAc 9:1) to give
a mixture of 9c (40.3 mg, 76%) as a mixture of diastereomers (a/b 90:10)
and 10c (9.2 mg, 17%) as a single isomer.
also achieved through appropriate choice of the catalyst and
the protecting group on nitrogen. Furthermore, control of
the 5-exo and 6-endo cyclizations of one-carbon-elongated
substrates was achieved by use of gold and tungsten cata-
lysts, respectively. These reactions demonstrate the high syn-
thetic utility of geminal carbo-functionalization of alkynes
and also show new potential for the use of tungsten and rhe-
nium complexes in organic synthesis.
Preparation of the bicyclic compounds 9: A typical procedure for the
cyclizations of 8c and 8e–8j is described for the reaction with 8g as sub-
strate. Compound 8g (55.4 mg, 0.10 mmol) in degassed toluene (1 mL)
was added to a mixture of [ReCl(CO)4ACHTNUGTRENUNG(PPh3)] (3.0 mg, 0.005 mmol) and
Experimental Section
activated molecular sieves (4 ꢁ, 100 mg), and the resulting mixture was
stirred at 808C until TLC showed the complete disappearance of 8g. The
reaction mixture was then filtered through a short pad of Celite, and the
filtrate was concentrated under reduced pressure. Water was then poured
onto the residue, and organic materials were extracted twice with ethyl
acetate. The combined organic layer was washed with brine and dried
over anhydrous MgSO4. The filtrate was concentrated and the resulting
crude product was purified by PTLC (hexane/EtOAc 8:2) to give 9g
(48.2 mg, 87%) as a mixture of diastereomers (a/b 20:80).
Cyclizations of the dienol silyl ethers 1: A typical procedure for the cycli-
zations of 1a–1j is described for the reaction with 1a as substrate. Com-
pound 1a (292 mg, 0.60 mmol) in degassed toluene (1.0 mL) was added
to a mixture of [ReCl(CO)5] (1.1 mg, 0.0030 mmol, 0.5 mol%) and acti-
vated molecular sieves (4 ꢁ, 100 mg), and the resulting mixture was
stirred under photoirradiation conditions (250 W super high-pressure Hg
lamp) until TLC showed the complete disappearance of 1a. The reaction
mixture was then filtered through a short pad of silica gel and the filtrate
was concentrated under reduced pressure. The residue was purified by
PTLC (hexane/EtOAc 9:1) to give a mixture of 2a and 3a (2a-a/2a-b/3a
69:17:14, 269 mg, 0.55 mmol) in 92% yield.
Preparation of the dihydropyrroles 10: A typical procedure for the cycli-
zations of 8d and 8k–8p is described for the reaction with 8l as substrate.
Compound 8l (106.7 mg, 0.20 mmol) in degassed dichloroethane (2 mL)
was added to a mixture of [ReCl(CO)5] (0.7 mg, 0.002 mmol) and activat-
ed molecular sieves (4 ꢁ, 100 mg), and the resulting mixture was stirred
at 808C until TLC showed the complete disappearance of 8l. The reac-
tion mixture was then filtered through a short pad of Celite, and the fil-
trate was concentrated under reduced pressure. Water was then poured
onto the residue and organic materials were extracted twice with ethyl
acetate. The combined organic layer was washed with brine and dried
over anhydrous MgSO4. The filtrate was concentrated and the resulting
crude product was purified by PTLC (toluene/EtOAc 96:4) to give a mix-
ture of 9l (9.3 mg, 9%) as a mixture of diastereomers (a/b 90:10) and
10l (92.0 mg, 86%) as a mixture of diastereomers (cis/trans 30:70).
Cyclization and hydrolysis of the dienol silyl ether 1k: Compound 1k
(215 mg, 0.67 mmol) in degassed diethyl ether (1.0 mL) was added to a
mixture of [W(CO)6] (13 mg, 0.037 mmol, 0.05 equiv) and activated mo-
lecular sieves (4 ꢁ). After the mixture had been photoirradiated (250 W
super high-pressure Hg lamp) for 7.5 h, hydrochloric acid (1m, ca. 2 mL)
was added and the mixture was further stirred for 30 min. The organic
layer was extracted with ether, washed with sat. NaHCO3 solution and
brine, and dried over MgSO4. The bicyclic ketone 5 was so volatile that
the solvent was not removed completely. The crude ketone was purified
by PTLC (diethyl ether/hexane 20%), and the eluent (diethyl ether) was
then removed under atmospheric pressure. The ketone was used for the
next reaction without complete removal of the eluent (GC yield: 95%).
Gold-catalyzed reactions of the dienol silyl ethers 11: A typical proce-
dure for the cyclizations of 11a–11c is described for the reaction with
11a as substrate. Compound 11a (53.9 mg, 0.10 mmol) in degassed di-
Conjugate addition of dimethyl propargylmalonate sodium salt to 5: A
diethyl ether solution (3.0 mL) of dimethyl (prop-2-ynyl)malonate
(161 mg, 0.95 mmol) was added dropwise at 08C over 2 min to NaH
(46 mg, 1.92 mmol). After the mixture had been stirred for 50 min, the
solvent was removed under reduced pressure to afford a yellowish solid,
and then CH2Cl2 (2.0 mL) was added to the mixture. A CH2Cl2 solution
(3.0 mL) of the above ketone 5 was added at 08C to the dimethyl propar-
gylmalonate Na salt, and TIPSOTf (0.22 mL, 0.82 mmol) was then added
successively at the same temperature. After the mixture had been stirred
overnight, the reaction was quenched with phosphate buffer (pH 7). The
mixture was extracted three times with ethyl acetate, and the combined
organic layer was washed with brine and dried over MgSO4. Evaporation
under reduced pressure gave a crude product, which was purified by
silica gel column chromatography (ethyl acetate/hexane 5%) to give the
silyl enol ether 6 (158 mg, 54%) as a mixture of diastereomers (6-a/6-b=
1:2). The enol silyl ethers 6-a and 6-b were separable by PTLC (ethyl
acetate/hexane 10%).
chloroethane (1 mL) was added to a mixture of [AuClACTHUNRGTNEUNG(PPh3)] (4.9 mg,
0.0099 mmol, 10 mol%), AgSbF6 (3.4 mg, 0.0099 mmol, 10 mol%), and
activated molecular sieves (4 ꢁ, 100 mg), and the resulting mixture was
stirred at room temperature until TLC showed the complete disappear-
ance of 11a. The reaction mixture was then filtered through a short pad
of silica gel and the filtrate was concentrated under reduced pressure.
The residue was purified by PTLC (hexane/EtOAc 9:1) to give 12a
(41.5 mg, 0.077 mmol) in 77% yield as a colorless oil.
Rhenium-catalyzed reaction of the dienol silyl ether 11a: Compound 11a
(53.9 mg, 0.10 mmol) in degassed toluene (1 mL) was added to a mixture
of [ReCl(CO)5] (3.6 mg, 0.01 mmol) and activated molecular sieves (4 ꢁ,
100 mg), and the resulting mixture was stirred at 808C until TLC showed
the complete disappearance of 11a. The reaction mixture was then fil-
tered through a short pad of silica gel and the filtrate was concentrated
under reduced pressure. The residue was purified by PTLC (hexane/
EtOAc 9:1) to give a mixture of 13a and 14a (1:1, 42.0 mg, 0.078 mmol)
in 78% yield as a colorless oil.
Synthesis of a triquinane skeleton: A typical procedure for the prepara-
tion of the tricyclic ketones (7-a, 7-b) is described for the reaction with
6-b as substrate. A THF solution (1.0 mL) of 6-b (41.0 mg, 0.089 mmol)
was added to a mixture of [W(CO)6] (3.0 mg, 0.0085 mmol, 0.10 equiv)
and H2O (5.0 mL, 0.28 mmol). After the mixture had been photoirradiat-
ed (200 W high-pressure Hg lamp) for 6 h, the solvent was removed
under reduced pressure to give the crude product, which was purified by
PTLC (ethyl acetate in hexane 10%) to give 7-b (21.0 mg, 78%).
Tungsten-catalyzed reactions of the dienol silyl ethers 11: A typical pro-
cedure for the cyclizations of 11a–11c is described for the reaction with
11a as substrate. Compound 11a (53.9 mg, 0.10 mmol) in degassed tolu-
ene (1 mL) was added to a mixture of [W(CO)6] (35.2 mg, 0.10 mmol)
and activated molecular sieves (4 ꢁ, 100 mg), and the resulting mixture
was stirred under photoirradiation conditions until TLC showed the com-
plete disappearance of 11a. The reaction mixture was then filtered
through a short pad of Celite and the filtrate was concentrated under re-
duced pressure. The residue was purified by PTLC (hexane/EtOAc 9:1)
to give 15a (21.6 mg, 0.040 mmol) in 40% yield as a pale yellow oil.
ACHTUNGTRENNUNG[ReCl(CO)5]-catalyzed reactions of the dienol silyl ethers 8a–8d: A typi-
cal procedure for the cyclizations of 8a–8d is described for the reaction
with 8c as substrate. Compound 8c (52.9 mg, 0.10 mmol) in degassed tol-
uene (1 mL) was added to a mixture of [ReCl(CO)5] (3.6 mg, 0.01 mmol)
and activated molecular sieves (4 ꢁ, 100 mg), and the resulting mixture
was stirred at 808C until TLC showed the complete disappearance of 8c.
4846
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Chem. Eur. J. 2011, 17, 4839 – 4848

Tungsten(0)- and Rhenium(I)-Catalyzed Tandem Cyclization
FULL PAPER
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À
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product 5 during evaporation of solvent.
Chem. Eur. J. 2011, 17, 4839 – 4848
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
4847

N. Iwasawa et al.
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AHCTUNGTRENNUNG
[4.3.0]nonane derivatives as obtained in the AuI-catalyzed reactions;
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there was no difference in reactivity between (Z)-8d and (E)-8d.
For details, see the Supporting Information (7.3).
Received: October 19, 2010
Published online: March 21, 2011
4848
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Chem. Eur. J. 2011, 17, 4839 – 4848