Silylium ion-promoted dehydrogenative cyclization: Synthesis of silicon-containing compounds derived from alkynes

Article abstract of DOI:10.1039/c4cc01648c

Treatment of dialkylbenzylsilane (1) with trityl tetrakis(pentafluorophenyl)borate (TPFPB) in the presence of terminal or internal alkynes (3) and 2,6-di-tert-butyl-4-methylpyridine gave the corresponding 1,2-dihydro-2-silanaphthalene derivatives (4) in 3

Full text of DOI:10.1039/c4cc01648c

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Silylium ion-promoted dehydrogenative
cyclization: synthesis of silicon-containing
compounds derived from alkynes†‡
Hidekazu Arii,a Takashi Kurihara,a Kunio Mochida*a and Takayuki Kawashima*b
Cite this: Chem. Commun., 2014,
50, 6649
Received 5th March 2014,
Accepted 30th April 2014
DOI: 10.1039/c4cc01648c
www.rsc.org/chemcomm
Treatment of dialkylbenzylsilane (1) with trityl tetrakis(pentafluoro- ions to the X–Y p-bonds allow electrophilic substitution on
phenyl)borate (TPFPB) in the presence of terminal or internal alkynes the aromatic ring.
(3) and 2,6-di-tert-butyl-4-methylpyridine gave the corresponding
Kawashima et al. have synthesized dibenzosilole and trisila-
sumanene derivatives through the sila-Friedel–Crafts reaction,
where the silylium ion forms a five-membered p-complex with an
1,2-dihydro-2-silanaphthalene derivatives (4) in 34–82% yields.
Lewis acids are often utilized as reactants or catalysts to activate intramolecular aromatic moiety and consequent deprotonation
inert multiple bond-containing compounds in organic syntheses. of the p-complex with 2,6-lutidine produces the dibenzosilole
A sextet silylium ion1 with a vacant p-orbital has an intrinsically derivatives.11 Herein, we decided to use dialkylbenzylsilanes
high electrophilicity and can interact with p-donor nucleophiles (R2BnSiH, 1), in which case solvent-coordinated silylium ions
such as alkenes, alkynes, and aromatic compounds, as well as [R2BnSiÁarene]+ (2) interact with the appropriate unsaturated
ketones with a coordinative lone pair of electrons.2,3 In contrast compounds to give the b-silyl cations. They undergo intramolecular
to exergonic bond formation with a carbocation, an electronically electrophilic substitution to the benzyl group to afford the corre-
positive silicon center tends to bind to Lewis bases reversibly, sponding 6-membered ring compounds (Scheme 1). In this report,
so it has the potential to play the role of a catalyst4–8 as well as a we describe the reactions of the silylium ions derived from 1 with
high reactive reagent.9 The silylium ion, which is derived from alkynes followed by intramolecular aromatic electrophilic substitu-
treatment of the corresponding hydrosilane with the trityl tion reactions of the resulting ethenyl cations.
cation, exists usually as silane10 or arene adducts2e and permits
A solution of trityl tetrakis(pentafluorophenyl)borate (TPFPB)
the transition metal-free consecutive hydrosilylation of unsatu- in benzene was slowly added to a benzene solution of benzyldi-
rated compounds.4 Oestreich et al. have reported that the methylsilane (1a), a base and trimethylsilylacetylene (3a) to give
unique silylium ion bearing the ferrocenyl group undergoes a the 1,2-dihydro-2-silanaphthalene derivative (4aa) except for the
catalytic [4+2] cycloaddition with dienes or a,b-unsaturated case in entry 3, in which a mixture of 4aa and 1,2,3,4-tetrahydro-
carbonyl compounds.6d
2-silanaphthalene 5aa was obtained (Scheme 1, details of method
Recently, Mu¨ller and coworkers have successfully introduced A are provided in the ESI‡). Among bases that were examined
carbon dioxide activated by the simple trialkylsilylium ion into an (Table 1, entries 1–4), 2,6-di-tert-butyl-4-methylpyridine (DTBMP)
aromatic ring through electrophilic substitution.9a These facts gave the best result, namely, a sila-cycle 4aa was obtained as
prompted us to develop a silylium ion-promoted Friedel–Crafts a colorless oil in 58% isolated yield. The regiochemistry of 4aa
reaction with unsaturated compounds, i.e., it was hypothesized was determined by correlation of the ethenyl proton with the
that the b-silyl cations derived by the addition of silylium ipso carbon on the aromatic ring in an HMBC experiment. The
sterically small bases such as 2,6-lutidine and the proton sponge
relative to DTBMP were less effective for the dehydrogenative
a Department of Chemistry, Gakushuin University, 1-5-1 Mejiro, Toshima-ku,
171-8588 Tokyo, Japan. E-mail: kunio.mochida@gakushuin.ac.jp
b Graduate School of Science and Technology, Gunma University, 1-5-1 Tenjin-cho,
Kiryu, 376-8515 Gunma, Japan. E-mail: kawashima.t@gunma-u.ac.jp
† This work is dedicated to Professor Renji Okazaki on the occasion of his 77th
birthday.
‡ Electronic supplementary information (ESI) available: Experimental section,
the spectroscopic data and crystallographic data. CCDC 989789 for 4ab and
Scheme 1 Strategy for silylium ion-promoted annulation using 1 and
989790 for 4ae. For ESI and crystallographic data in CIF or other electronic format
unsaturated compounds.
see DOI: 10.1039/c4cc01648c
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Table 1 Optimization of dehydrogenative cyclization using hydrosilane 1a
and trimethylsilylacetylene (3a)
Table 2 Dehydrogenative cyclization using hydrosilanes 1a,b with alkynes
3a–ga
Product yieldb (%)
Silane 1
Alkyne 3
R1
Product
R2
yieldb (%)
Entry
Method
x eq.
Basea
4aa
5aa
Entry
R
c
1
2
3
4
5
A
A
A
A
B
1.5
1.5
1.5
1.5
3
2,6-Lut
Proton sponge
Cs2CO3
DTBMP
DTBMP
3
6
—
—
1
2
3
4
5
6
7
8
Me (1a)
Me (1a)
Me (1a)
Me (1a)
Me (1a)
Me (1a)
Me (1a)
iPr (1b)
iPr (1b)
iPr (1b)
iPr (1b)
iPr (1b)
SiMe3
H
H
H
Ph
H
(3a)
(3b)
(3c)
(3d)
(3e)
(3f)
(3g)
(3a)
(3b)
(3c)
(3e)
(3f)
73 (4aa)
34 (4ab)
38 (4ac)
c
Ph
nBu
tBu
Ph
Et
Ph
H
Ph
nBu
Ph
Et
12
58
73
12
c
—
c
—
c
—
77 (4ae)
a
Et
34d (4af)
2,6-Lut
=
2,6-lutidine, proton sponge
= 1,8-bis(dimethylamino)-
b
e
SiMe3
SiMe3
H
H
Ph
—
naphthalene, DTBMP
yields based on 1a. Not detected.
=
2,6-di-tert-butyl-4-methylpyridine. Isolated
c
71 (4ba)
67 (4bb)
66 (4bc)
82 (4be)
39d (4bf)
9
10
11
12
cyclization because they may coordinate to the resulting silylium ion
2a to interrupt the addition of alkyne.12 In entry 3, use of Cs2CO3
resulted in the formation of a mixture of 4aa and 5aa. As a result
of optimizing the reaction conditions, the isolated yield of 4aa
was increased up to 73% upon addition of 1a to the benzene
solution of TPFPB, DTBMP, and 3a (method B) (entry 5).
Et
a
b
c
Using method B. Isolated yields based on 1. Complex mixture.
d
e
Using method A. Benzyldimethyl(2-phenylethynyl)silane (6ag) was
obtained in 54% yield.
(the b-effect of a silyl group). However, the cationic oligomeriza-
tion of the alkyne with the ethenyl cation A was not completely
prohibited under these conditions, which caused a decrease in
the yields of the sila-cycle 4aa.
Compound 5aa is considered to be formed by hydride abstrac-
tion of the protonated 4aa from the Si–H bond of 1a. Silylium
ion-promoted catalytic hydrosilylation of various unsaturated
compounds such as alkenes, alkynes, ketones, and imines
has been reported,4 and it is thought that the unsaturated
compound activated by addition of the silylium ion abstracts
the hydride from the hydrosilane to afford the hydrosilylation
product and regenerates the silylium ion. Therefore, the above
formation mechanism for 5aa seems to be reasonable. In the
present stoichiometric reaction using 1a and 3a, the cation
center of the transient ethenyl cation A could approach the
intramolecular benzyl group, and the subsequent Friedel–
Crafts reaction via cyclohexadienyl cation B resulted in the
formation of 4aa (Scheme 2). Interestingly, when 3a was used as an
alkyne, the isomer with the trimethylsilyl group at the 3-position
was predominant because two silyl groups can stabilize the b-cation
Using the optimized method B, the scope of hydrosilanes 1
and alkynes 3 was examined and the results are summarized
in Table 2.13 The cyclization using hydrosilane 1a and phenyl-
acetylene (3b) or 1-hexyne (3c) except for trimethylsilylacetylene
(3a) as a terminal alkyne (entries 1–4) gave the corresponding
4-substituted 1,2-dihydro-2-silanaphthalene 4ab or 4ac, indicating
that the secondary ethenyl cation in A is more stable than the
primary one. X-ray crystallographic analysis of 4ab showed that
the C(3)–C(4) bond is a double bond (1.347(2) Å) and that the
phenyl group is located on the C(4) atom (Fig. 1). The presence of
the tert-butyl group prohibited the formation of 4ad (entry 4), as
expected, because of its steric hindrance for intramolecular
electrophilic substitution of the benzyl group. Mu¨ller et al. have
reported that ethenyl cations bearing tert-butyl or aromatic
groups at the a-position and two silyl groups at the b-position
can be isolated and are stable even in benzene.14 The remarkable
stability of the isolated ethenyl cations seems to be attributable
to the b-effect by two silyl groups and steric hindrance of the
a-substituent. In our system, most of the resulting ethenyl cations
maintain the moderate electrophilic reactivity so that aromatic
substitution can occur, in contrast to the Mu¨ller’s system. Internal
alkynes such as diphenylacetylene (3e) and 3-hexyne (3f) also
yielded the corresponding cyclization products in 77% and 34%
isolated yields for 4ae and 4af, respectively (entries 5 and 6). The
crystal structure of 4ae was revealed to be similar to that of 4ab,
except for the phenyl group at the 3-position in the 1,2-dihydro-2-
silanaphthalene backbone (Fig. S1, ESI‡). The good yield for the
annulation of sterically bulky diphenylacetylene is attributed
to a prohibition of alkyne oligomerization by the resulting
Scheme 2 Plausible mechanism for the formation of 4 from hydrosilane
1 and alkynes 3.
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Scheme 4 Catalytic cyclization using hydrosilane 1 and alkyne 3c.
in entry 3 of Table 1, the catalytic cycle as shown in Scheme 3 can
be designed. That is, silylium ion 2 formed from 1 is reacted with
alkyne 3 to give cyclohexadienyl cation B. Then, carbocation C
formed by the H-migration and aromatization from cyclohexa-
dienyl cation B16 abstracts the hydride from the Si–H bond of 1
to produce 5 with regeneration of silylium ion 2, which adds to
the alkyne to construct the catalytic cycle (Scheme 3). Hence, the
reaction using 1 and 1-hexyne (3c) was examined in a catalytic
amount of TPFPB in benzene. Upon using 3c, the cyclization
products 5ac and 5bc were obtained in 33% and 50% isolated
yields, respectively (Scheme 4). This catalytic reaction is very
interesting from the viewpoint of a transition metal-free reaction
and a multistep catalytic reaction involving two types of carbo-
cations with different reactivities.
In conclusion, we developed a dehydrogenative cyclization
procedure using hydrosilanes and alkynes that is promoted by a
transient silylium ion to give 1,2-dihydro-2-silanaphthalene
derivatives. The regioselectivity of alkynes for the cyclization
depended on the thermodynamic stability of the corresponding
cations by resonance with the benzene ring, hyperconjugation
of the alkyl group or the b-effect of another silyl group. 1,2,3,4-
Tetrahydro-2-silanaphthalene derivatives were also successfully
synthesized in the transition metal-free catalytic system containing
cyclization and reduction steps.
Fig. 1 Crystal structure of one of the two independent molecules in 4ab,
showing 50% probability of thermal ellipsoids.
ethenyl cation. Use of phenyl(trimethylsilyl)acetylene (3g) gave
the phenylethynylation product (6ag) in 54% isolated yield with
no observation of 4ag (entry 7) since an electropositive silicon
atom in the trimethylsilyl group allows the nucleophilic attack of
DTBMP, resulting in the desilylation reaction to give 6ag.
To elucidate the steric effect of the substituent on the silicon
center, we performed the reactions of 1b possessing two isopropyl
groups on the silicon atom with alkynes (entries 8–12). The dehydro-
genative cyclization with alkynes occurred in moderate-to-good
yields except for 3-hexyne (3f), although the cyclization using 1b
required a longer reaction time (30 min) than that required
using 1a (15 min). The product yield for the reaction of 1b with
3b or 3c improved compared with that of 1a, which can be
understood by the Thorpe–Ingold effect.15 The regioselectivity
for the cyclization was maintained despite the sterically bulky
isopropyl group on the silicon atom.
Application of the stoichiometric system to a catalytic one is
important from the viewpoint of reduction of waste. Consider-
ing the aforementioned formation mechanism of 5aa, shown
This work was financially supported by a Grant-in-Aid for
Young Scientists (No. 25810026, H.A.) from the Ministry of Educa-
tion, Culture, Sports, Science and Technology, Japan.
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6652 | Chem. Commun., 2014, 50, 6649--6652
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