Rhenium-Catalyzed Cyclization via 1,2-Iodine and 1,5-Hydrogen Migration for the Synthesis of 2-Iodo-1 H-indenes

Article abstract of DOI:10.1021/acs.orglett.9b02380

A rhenium complex catalyzed the formation of 2-iodo-1H-indene derivatives through iodine and hydrogen migration of 3-iodopropargyl ethers. The reaction proceeded via generation of 1-iodoalkenylrhenium carbene species by sequential 1,2-iodine and 1,5-hydro

Full text of DOI:10.1021/acs.orglett.9b02380

Letter
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Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Rhenium-Catalyzed Cyclization via 1,2-Iodine and 1,5-Hydrogen
Migration for the Synthesis of 2‑Iodo‑1H‑indenes
Masahito Murai*,†,‡ and Kazuhiko Takai*,†
†Division of Applied Chemistry, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushimanaka,
Kita-ku, Okayama 700-8530, Japan
‡Department of Chemistry, Graduate School of Science, Nagoya University, Furo, Chikusa, Nagoya 464-8602, Japan
S
* Supporting Information
ABSTRACT: A rhenium complex catalyzed the formation of 2-iodo-
1H-indene derivatives through iodine and hydrogen migration of 3-
iodopropargyl ethers. The reaction proceeded via generation of 1-
iodoalkenylrhenium carbene species by sequential 1,2-iodine and 1,5-
hydrogen shifts with readily available precursors under neutral
conditions. The reaction mechanism and the reactivity of the generated
alkenylcarbene species were also investigated.
ransition-metal-catalyzed 1,2-H shift of terminal acety-
lenes is a powerful and operationally simple method to
alkynes, which involves the formation of rhenium vinylidene
species generated via a 1,2-H shift of the terminal proton of
acetylene.6,7 We envisioned that rhenium complexes may also
serve as promoters for the generation of halogen-substituted
vinylidene species via 1,2-halogen shift of haloalkynes.8 To
investigate the catalytic generation of iodovinylidene species
via 1,2-I shift of 1-iodo-2-alkylalkynes derived from aliphatic
acetylenes, a propargyl ether 1 was selected as a precursor in
this work. Since the resulting vinylidene species possesses an
electrophilic carbene center attached with electron-with-
drawing iodo substituents, further conversion into 1-
iodoalkenyl carbene species A was hypothesized to occur via
sequential 1,5-H shift followed by elimination of an aldehyde
(eq 1).9 Although an alkenyl carbene species is widely used for
T
provide synthetically useful vinylidene species atom-effi-
ciently.1 Reactivity of the resulting vinylidene species is
influenced by substituents adjacent to the generated carbene
carbon centers. Therefore, vinylidene formation from alkynes
with functional groups, such as SiR3, GeR3, SnR3, SR, SeR, or a
halogen group, at the terminus via their 1,2-shift is an efficient
and straightforward approach to reactive and functionalized
vinylidene species.2,3 Among the functional groups, the iodo
group is an important coupling partner, and the precursors,
iodoalkynes, are an attractive class of alkynes having unique
reactivity.4 Since the seminal report by Iwasawa describing
tungsten-catalyzed cycloisomerization leading to 1-iodonaph-
thalenes via generation of iodovinylidene species,3a,5 1,2-I shifts
have been used in several unique transformations by Furstner
̈
and Hashimi et al. (Scheme 1).3b−g However, most of the
iodoalkynes used in the previous studies were derived from
arylacetylenes,3a−d,f,g and the activity of catalysts other than
tungsten, ruthenium, and gold complexes is unknown.
Recently, we have reported rhenium-catalyzed anti-Markov-
nikov addition reaction of carbon nucleophiles to terminal
the construction of various heterocycles by cycloadditions and
annulations,9d,10 those that possess an iodo group at the
alkenyl or carbene carbon are rare.
Scheme 1. Generation of Iodovinylidene Species via 1,2-I
Shift of Iodoalkyne
The present study began with treatment of benzyl propargyl
ether 1a with a catalytic amount of a transition-metal complex
in 1,4-dioxane (0.10 M) at 80 °C. 2-Iodo-1H-indene 2a was
formed with the use of several rhenium catalysts, such as
ReBr(CO)5, ReI(CO)5, and [ReBr(CO)3(thf)]2 (Table 1,
entries 1−3). In contrast, other metal complexes, including
W(CO)5(thf), AuCl, and IPrAuNTh2, which were effective for
the generation of iodovinylidene species via 1,2-I shift in a
previous study,3 did not provide 2a even at higher temper-
atures due to competitive decomposition of 1a.11 Although a
Received: July 10, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02380
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Table 1. Optimization of Reaction Conditions
entry
cat. Re
solvent
R
substrate
yield (%)
1
2
3
4
5
6
7
8
9
ReBr(CO)5
ReI(CO)5
[ReBr(CO)3(thf)]2
Re2(CO)10
[ReBr(CO)3(thf)]2
[ReBr(CO)3(thf)]2
[ReBr(CO)3(thf)]2
[ReBr(CO)3(thf)]2
[ReBr(CO)3(thf)]2
[ReBr(CO)3(thf)]2
1,4-dioxane
dioxane
dioxane
dioxane
THF
ClCH2CH2Cl
dioxane
dioxane
dioxane
dioxane
Ph
Ph
Ph
Ph
Ph
Ph
OMe
Et
1a
1a
1a
1a
1a
1a
1aa
1ab
1ac
1ad
71
70
78 (77)
22
32
7
63
68
71
a
4-MeOC6H4
4-BrC6H4
10
74
a
120 °C for 18 h.
higher temperature (120 °C) is required, 2a was also obtained,
albeit in low yield using Re2(CO)10 as a catalyst (entry 4). The
proper choice of solvents was crucial, and formation of 2a was
The reaction could be applicable to benzyl propargyl ether 1f
derived from dibenzosuberone, and indene-fused tetracycle 2f
was formed in moderate yield. Precursor 1g, which contained a
m-methoxyphenyl group, reacted preferentially at the less
sterically hindered position to afford 2g along with its
regioisomer 2g′ as a minor product (eq 2).
n
observed in ethereal solvents, such as THF, CPME, Bu2O,
and DME (entries 5 and 6).12 Substituents on the oxygen atom
slightly affected reaction efficiency, and precursors 1aa, 1ab,
1ac, and 1ad containing methoxymethoxy, propoxy, or
benzyloxy groups with electron-donating and -withdrawing
functional groups were converted to 2a in lower yields
compared to those with 1a (entries 7−10).13,14
Under the optimized reaction conditions listed in entry 3 of
Table 1, the scope and functional group tolerance were
investigated briefly with several benzyl propargyl ethers 1
(Figure 1). Functional groups such as methoxy, fluoro, and
A proposed mechanism for the formation of 2-iodo-1H-
indenes 2 in the current reaction is shown in Scheme 2. First,
Scheme 2. Proposed Reaction Mechanism for Formation of
2-Iodo-1H-indenes 2
alkenyl carbene species A′ was generated via sequential 1,2-I
and 1,5-H shifts, followed by elimination of benzaldehyde, as
shown in eq 1. A deuterium-labeling experiment using 1a-d
confirmed that a 1,5-shift of the deuterium atom from the
benzylic position of 1a onto the alkenyl carbon of 2a (Scheme
3a). Furthermore, a parallel reaction of 1a and 1a-d in a
separate flask provided a KIE value of 1.08, which suggested
that the hydrogen shift of 1 was not involved in the rate-
determining step (Scheme 3b). Due to the relatively fast 1,5-H
shift, direct insertion of a vinylidene species into the benzylic
C−H bond3g leading to 3-iodo-2,5-dihydrofuran derivatives
did not occur. A crossover experiment using 1a-d and 1c in the
same flask provided 2a-d and 2c, and no exchange of the
deuterium label was observed (Scheme 3c). Thus, the current
1,5-H shift occurred through an intramolecular process.
Because cyclization of 1e, which contained electron-with-
drawing trifluoromethyl groups, required higher temperatures,
the carbenoid carbon of A′ possessed electrophilic character.
Thus, nucleophilic attack of the aryl group on the carbene
Figure 1. Rhenium-catalyzed synthesis of 2-iodo-1H-indene deriva-
tives 2. (a) At 100 °C. Mixture with double-bond isomer 2e′ in a ratio
of 69:31 (see the SI for the structure of 2e′). (b) At 65 °C.
chloro groups were tolerated to provide the corresponding 2-
iodo-1H-indene derivatives 2b, 2c, and 2d, respectively, in
good yields. Higher reaction temperatures were necessary for
complete conversion of trifluoromethyl group substituted
precursor 1e leading to 2e. In this case, transposition of the
double bond to yield 2e′ occurred competitively due to the
presence of an acidic benzyl proton in the structure of 2e.15
B
DOI: 10.1021/acs.orglett.9b02380
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occurred very quickly, and products derived from vinylidene
formation via a 1,2-I shift were not observed. The
corresponding iodoallene 3j was also obtained in the reaction
of sec-propargyl ether 1j. These results indicate that selectivity
of these 1,n-shifts was influenced by the steric environment of
the substituents at the propargyl positions. Although the
expected vinylidene species was not generated, these trans-
formations are also useful as a novel approach to iodoallenes,
which are useful synthetic building blocks to introduce two
consecutive allenyl double bonds into target molecules.
Scheme 3. Control Experiments Using Deuterated Substrate
1a-d (Ar = 4-FC6H4)
The impact of an iodo group at the alkyne terminal on the
efficiency of the 1,2-shift was estimated by comparison of
reactivity with the corresponding terminal alkyne (Table 2).
Table 2. Effect of Terminal Substituents of Alkynes on 1,2-
Shifts
carbon, followed by subsequent elimination of rhenium species
and aromatization, is most plausible for formation of the
indene skeletons (Scheme 2).16 As an alternate pathway,
however, direct insertion of the carbenoid carbon of A′ into a
nearby aromatic C−H bond followed by transposition of
double bond would also afford 2.
The corresponding 2-iodo-1H-indene 2h was also obtained
from benzyl propargyl ether 1h containing phenyl and methyl
groups at the propargyl position. In this system, the
alkenylcarbene intermediate B, having a carbene center
situated anti to the phenyl group, was thought to be
predominantly B′ due to steric repulsion.17 However,
formation of 2h in moderate yield suggested that B could be
interconverted to B′ via zwitterionic intermediate C (Scheme
4).18
a
yield (%)
entry
X
product
1 h
3 h
15 h
78
65 (62)
1
2
I
H
2a
4
71
28
78 (77)
41
a
1
Determined by H NMR. Values in parentheses are isolated yields.
Under the current optimized reaction conditions, iodoalkyne
1a afforded the cyclization product 2a more rapidly (entry 1)
than the corresponding terminal alkyne (entry 2). A previous
study on rhenium-catalyzed anti-Markovnikov addition to
terminal alkynes revealed that vinylidene formation via 1,2-H
shift was relatively slow and involved in the rate-determining
step.6 These results suggest that 1,2-I shift leading to
iodovinylidene species occurred much faster and did not
involve the rate-determining step. A crossover experiment
using 1a-d and a terminal alkyne having two 4-fluorophenyl
groups at the propargyl position also revealed that this 1,2-I
shift occurred through an intramolecular process (see eq S1,
SI).
Scheme 4. Interconversion of Alkenylcarbene Intermediate
B
The optimized rhenium catalyst, [ReBr(CO)3(thf)]2, was
also effective for vinylidene formation from iodoalkynes
derived from arylacetylenes. For example, formation of 3-
iodo-1H-indene derivative 6 via insertion of C(sp3)−H bond
into the rhenium vinylidene species proceeded efficiently
under identical conditions (Scheme 6).3g
Scheme 6. Rhenium-Catalyzed Cycloisomerization of
Iodoalkyne 5 Derived from Arylacetylene
In contrast, iodoallene 3i was obtained in 88% yield when
benzyl propargyl ether 1i, which did not possess aryl groups at
the propargyl position, was used as a precursor (Scheme 5).19
The 1,5-H shift followed by elimination of benzaldehyde
Scheme 5. Formation of Iodoallenes 3 without 1,2-I Shift
In conclusion, 3-iodopropargyl ethers were found to be
efficient 1-iodoalkenyl carbene generators when using rhenium
catalysis under neutral conditions without additional phos-
phine- or nitrogen-based ligands. Iodoalkynes derived from
alkylalkynes were applicable as precursors, and the carbene
intermediates generated were trapped efficiently through
C
DOI: 10.1021/acs.orglett.9b02380
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
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(11) 2a was not obtained with the following catalysts (5 mol % M)
in dioxane at 100 °C: TaCl5, Cr(CO)5(thf), Mo(CO)6,
W(CO)5(thf), WCl6, Mn2(CO)10, MnBr(CO)5, ReCl5, Fe2(CO)9,
[RuCl2(CO)3]2, Ru3(CO)12, [Rh(OAc)2]2, [RhCl(cod)]2, Ir4(CO)12,
PtCl2, AuCl, AuBr3, IPrAuNTh2, Au(PPh3)SbF6, and GaCl3.
(12) Effect of other solvents with 2.5 mol % of [ReBr(CO)3(thf)]2 at
80 °C for 3 h: yield of 2a was 12% in toluene, 9% in cyclohexane, 41%
in Bu2O, 63% in CPME, 43% in DME, 17% in AcOEt, 0% in MeCN
or DMF. Effect of temperature with 2.5 mol % of [ReBr(CO)3(thf)]2
in 1,4-dioxane for 3 h: 68% at 70 °C, 58% at 90 °C. Additives,
including amines, phosphines, and MS4A failed to improve the yield
of 2a.
(PDF)
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: masahito.murai@chem.nagoya-u.ac.jp.
*E-mail: ktakai@cc.okayama-u.ac.jp.
ORCID
Masahito Murai: 0000-0002-9694-123X
Kazuhiko Takai: 0000-0002-2572-0851
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by a Grant-in-Aid for
Scientific Research (No. 18H03911 and 19H02719) from
MEXT, Japan. The authors gratefully thank Jun Namikoshi and
Honami Ueda (Okayama University) for work provided on the
preliminary study.
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