Ring-Opening Iodo- and Bromosilation of Cyclic Ethers by Treatment with Iodo- and Bromotrialkylsilane Equivalents

Full text of DOI:10.1021/jo990722c

8
024
J . Org. Chem. 1999, 64, 8024-8026
Rin g-Op en in g Iod o- a n d Br om osila tion of
Cyclic Eth er s by Tr ea tm en t w ith Iod o- a n d
Br om otr ia lk ylsila n e Equ iva len ts
EtBr, AllylBr). The reactions led to the one-step synthesis
of bifunctional halo(siloxy)alkanes, including a haloge-
nated silyl enol ether. They may be potentially useful in
8
organic synthesis as silyl-protected hydroxyalkyl halides.
J oji Ohshita, Arihiro Iwata, Fujio Kanetani, and
Atsutaka Kunai*
Resu lts a n d Discu ssion
Department of Applied Chemistry, Faculty of Engineering,
Hiroshima University, Higashi-Hiroshima 739-8527, J apan
Table 1 summarizes the ring-opening halosilation of
cyclic ethers with the use of iodosilane or bromosilane
equivalents 1a ,b. When a mixture of tetrahydrofuran
Yasushi Yamamoto and Chinami Matui
(THF) and 0.99 equiv of the reagent composed of Et
2
-
NSiMe
3
and MeI (1a ) was heated at 80-90 °C for 5 h in
Silicone-Electronic Materials Research Center,
Shin-Etsu Chemical Co., Ltd., 1-10 Hitomi, Matsuida,
Gunma 379-0224, J apan
toluene, 1-iodo-4-trimethylsiloxybutane (3a ) was obtained
in 81% isolated yield as shown in Table 1. No other
volatile products were detected by either H NMR
spectrometry and GLC analysis of the reaction mixture.
With 2.4 equiv of 1a , THF was converted into 1,4-
diiodobutane (3c) in 53% yield, although several uniden-
tified products were also found to be formed in low yields
by GC-MS analysis of the reaction mixture. Similarly,
tetrahydropyran (THP) underwent smooth iodosilation
with reagent 1a under the same conditions to afford
1
Received April 29, 1999
In tr od u ction
Iodo- and bromosilanes play a vital role in organic
synthetic chemistry, and their synthetic utilities have
1
been demonstrated. Interaction of cyclic ethers with iodo-
or bromotrimethylsilane affords the respective ring-
opened iodo- or bromosilation products in good yields.2
Iodo- and bromosilanes, however, exhibit a high tendency
to undergo hydrolytic cleavage of the Si-halogen bond
with atmospheric moisture, leading to the formation of
the respective silanols, unless the silicon atom is substi-
tuted by protecting substituent(s) with appropriate steric
bulkiness,3 and therefore, iodo- and bromosilanes are
usually difficult to handle as compared with the other
halosilanes, chloro- and fluorosilanes.
1
-iodo-5-(trimethylsiloxy)pentane (4a ) in 74% yield. Di-
iodopentane (4c) was obtained in 33% yield from the
reaction of THP with 2.8 equiv of 1a .
A reaction of 2-methyltetrahydrofuran (2-MeTHF) with
1
.2 equiv of reagent 1a gave a 82:18 mixture of iodosi-
lation products 5a and 6a in 75% yield. No diiodation
products were detected in the reaction mixture, in
contrast to the fact that the independent reaction of
2
-MeTHF with 1 equiv of Me
3
SiI gave 1,4-diiodopentane
(5c) in 34% yield without any detectable formation of 5a
Recently, we have reported that 1:1 mixtures of di-
and 6a . To obtain better regioselectivity of the iodosila-
tion of 2-MeTHF, we carried out the reactions of 2-MeTHF
ethylaminotrimethylsilane with methyl iodide (Et
2 3
NSiMe /
MeI) and allyl bromide (Et NSiMe /AllylBr) behave as
2
3
using bulkier aminosilanes instead of Et
ever, attempted iodosilation of 2-MeTHF with Et
NSiEtMe /MeI, Et NSiEt /MeI, and Et NSiPhMe /MeI
was unsuccessful, and 2-MeTHF was recovered un-
changed from these reactions. No reactions took place
when i-PrI or OctylI was used instead of MeI in 1a .
2 3
NSiMe . How-
synthetic equivalents of iodo- and bromotrimethylsilane,
respectively, and the reactions with epoxides afford the
2
-
4
2
2
3
2
2
corresponding ring-opened 1,2-halosilation products. The
2 3
reagent of Et NSiMe /MeI reacts also with alkyl esters
to give high yields of trimethylsilyl esters with liberation
5
of iodoalkanes. More recently, we have demonstrated
2 3
However, with Et NSiMe /EtI (1a ′) the reaction pro-
that silyl enol ethers are obtained from the reactions of
ceeded with higher regioselectivity to give a 89:11
mixture of 5a and 6a in 73% yield. Similar to THF and
THP, 2-MeTHF was transformed into diiodide 5c in 51%
yield by treating it with 2.2 equiv of 1a . In contrast to
the reaction of 2-MeTHF with 1a , tetrahydrofurfuryl
alcohol reacted with 2.0 equiv of 1a to produce product
2 3
NSiMe
/MeI.6
the respective aliphatic ketones with Et
We also found that treatment of 1:1 mixtures of hydrosi-
lanes and alkyl iodides with a catalytic amount of PdCl
SiH/R′I(PdCl ), produces high yields of iodosilanes by
hydride-iodide exchange.
2
,
R
3
2
7
To explore further the scope of synthetic utilities of
these reaction systems, we studied ring-opening halosi-
lation of five- and six-membered cyclic ethers, which are
7
a exclusively, by the cleavage of the C4-O bond. No
products arising from C1-O cleavage were obtained. In
this reaction, silation of the hydroxyl group of tetrahy-
drofurfuryl alcohol would occur prior to the C-O bond
cleavage, which gives rise to a bulkier trimethylsiloxy
group relative to the methyl group to lead to better
regioselectivity.
2
less strained than epoxides, with the reagents of Et -
1
1
1
3 3 2
NSiMe /R X and Et SiH/R X(PdCl ) (R X ) MeI, EtI,
(
1) Larson, G. L. In The Chemistry of Organic Silicon Compounds;
Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; Part 1, Chapter
1.
2) Kr u¨ erke, U. Chem. Ber. 1962, 95, 174.
3) Walsh, R. In The Chemistry of Organic Silicon Compounds;
Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989; Part 1, Chapter
1
Heating 2,5-dihydrofuran and 1a at 80-90 °C for 7 h
gave a mixture of stereoisomers, 9a and 10a , in a ratio
of 96:4 in 67% yield. Monitoring the reaction progress
(
(
5
.
1
by H NMR spectrometry indicated that 9a was produced
(
4) Yamamoto, Y.; Shimizu, H.; Matui, C.; Chinda, M. Main Group
Chem. 1996, 1, 409.
5) Yamamoto, Y.; Shimizu, H.; Hamada, Y. J . Organomet. Chem.
996, 509, 119.
6) Yamamoto, Y.; Matui, C. Organometallics 1997, 16, 2204.
7) Kunai, A.; Sakurai, T.; Toyoda, E.; Ishikawa, M. Organometallics
994, 13, 3233.
as the initial product and was then gradually converted
into an equilibrium mixture of 9a and 10a (Table 2). The
similar reaction of 2,3-dihydrofuran at 50-60 °C led to
(
1
1
(
(
(8) Lalonde, M.; Chan, T. H. Synthesis 1985, 815.
1
0.1021/jo990722c CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/24/1999

Notes
J . Org. Chem., Vol. 64, No. 21, 1999 8025
Ta ble 1. Rin g-Op en in g Ha losila tion of Cyclic Eth er s w ith Rea gen t 1 a t 80-90 °Ca
a
Reactions were carried out in toluene, using 1a (Et2NSiMe3/2MeI), 1a ′ (Et2NSiMe3/2EtI), 1b (Et2NSiMe3/2AllylBr), 1b′ (Et2NSiMe3/
b
c
d
2
EtBr). At 50-60 °C. At 110 °C. Trimethylsilyl tetrahydrofurfuryl ether was obtained in 11% yield. e Trimethylsilyl tetrahydrofurfuryl
ether was obtained in 7% yield.
Ta ble 2. Iod osila tion of 1,4-Dih yd r ofu r a n w ith Rea gen t
of a monosilation product, 2-tetrahydrofurfuryl trimeth-
ylsilyl ether, in contrast to the iodosilation of 2-tetrahy-
drofurfuryl alcohol with 1a , which gave 7a as the single
isomer. Carrying out the reaction of tetrahydrofurfuryl
alcohol at higher temperature (110 °C) led to better
regioselectivity (81:19) and a slightly higher yield (57%).
Bromosilation of 2-MeTHF with 1b proceeded with less
regioselectivity, as compared with that with 1a , to give
a 60:40 mixture of products 5b and 6b. Reagent 1b did
not react with 2,5- and 2,3-dihydrofuran and THP.
1
a
temp/°C
time/h
conversion/%
yield/% (9a /10a )
2
8
8
5
1
1
5
3
20
84
3 (0/100)
20 (18/82)
67 (90/10)
0-90
0-90
the stereoselective formation of an iodosilation product,
1a . The reactions of 2,5- and 2,3-dihydrofuran with an
1
excess of reagent 1a resulted in the formation of complex
mixtures and no diiodination products were separated
from the reaction mixtures.
Similar halosilation of cyclic ethers was performed with
the use of reagent Et
AllylBr(PdCl ) (2b). The results are summarized in Table
3. Thus, when THF was treated with 1 equiv of Et SiH
and MeI in the presence of a catalytic amount of PdCl
product 12a was obtained in 70% yield. Other hydrosi-
lanes, PhMe SiH (reagent 2a ′) and Ph MeSiH (reagent
3 2 3
SiH/MeI(PdCl ) (2a ) and Et SiH/
When AllylBr was used (reagent 1b) instead of MeI in
1
a , bromosilation of THF occurred to give product 3b in
2
7
6% yield (Table 1). Using EtBr as the halogen source
3
(1b′) also gave the bromosilation product, although the
2
,
yield was a little lower (53%). In the reaction of 2-tet-
rahydrofurfuryl alcohol with 1b, a mixture of regioiso-
mers of bromobis(trimethylsiloxy)pentanes (7b and 8b)
was obtained in 50% yield, together with the 11% yield
2
2
2a ′′), can also be used as the silyl source to give 12a ′ and
12a ′′, respectively, although the reactions proceeded

8
026 J . Org. Chem., Vol. 64, No. 21, 1999
Ta ble 3. Rin g-Op en in g Ha losila tion of Cyclic Eth er s w ith Rea gen t 2 a t 80-90 °Ca
Notes
a
Reactions were carried out without solvent, using 2a (Et2SiH/MeI(PdCl2)), 2a ′ (PhMe2SiH/MeI(PdCl2)), 2a ′′ (Ph2MeSiH/MeI(PdCl2)),
d
2
b (Et3SiH/AllylBr(PdCl2)), 2c (Et3SiH/MeI(PdCl2(PPh3)2)), 2d (Et3SiH/MeI(Pd(PPh3)4)). b At room temperature. c At 100 °C. Yield based
on the silane used.
7
much slower than that with 2a . Iodosilation of THP,
3
free Et SiI, resulting in a nearly 1:1 mixture of 14a and
2
-MeTHF, and 2-tetrahydrofurfuryl alcohol, with 2a
15a . Fine black precipitates probably due to metallic Pd
were observed in the reaction mixture in this case. On
the other hand, a much higher selectivity for 14a attained
proceeded smoothly to give the expected ring-opened
products (13a -16a ). The reaction of 2-MeTHF proceeded
less selectively relative to that with 1a , and a 45:55
mixture of 14a and 15a was obtained. In contrast to the
reactions with 1a , those of 2,5- and 2,3-dihydrofuran with
reagent 2a gave complex mixtures and no products
analogous to 9a -11a were separated from the mixtures.
with reagent 2c (PdCl
(PPh catalyst) is explained by the formation of homo-
geneous complexes such as Et Si-Pd(L )-I as the active
2 3 2
(PPh ) catalyst) and 2d (Pd-
3 4
)
3
2
species, since clear solutions resulted from these reagent
systems.
Palladium complexes, PdCl
agents 2c and 2d ), can also catalyze the iodosilation of
-MeTHF with Et SiH/MeI, leading to better regioselec-
2 3 4 3 4
(PPh ) and Pd(PPh ) (re-
Exp er im en ta l Section
2
3
tivities than 2a . Bromosilation of THF proceeded with
the use of 2b to give 12b. However, no reactions of 2,3-
and 2,5-dihydrofuran, and THP, took place with reagent
Reactions were carried out under an atmosphere of dry argon.
Illustrative procedures for halosilation reactions of cyclic ethers
with reagents 1 and 2 are as follows.
Rea ction of THF w ith 1a . A mixture of diethylaminotrim-
ethylsilane (7.78 g, 53.7 mmol), THF (3.89 g, 54.0 mmol), methyl
iodide (20.3 g, 143 mmol), and toluene (20 mL) was stirred at
2
b.
In conclusion, we have demonstrated that reagents of
1
1
Et
R
PhMe
2
3
NSiMe /R X (1, R X ) MeI, EtI, EtBr, AllylBr) and
1 1 2
8
0-90 °C for 5 h. After the resulting ammonium salts were
2
3
SiH/R X(PdCl
2
) (2, R X ) MeI, AllylBr; R
3
) Et
3
,
filtered, the solvent was evaporated and the residue was distilled
under reduced pressure to give 1-iodo-4-trimethylsiloxybutane
2
, Ph Me) can be used as synthetic equivalents of
2
(3a , 11.9 g; 81%).
iodo- and bromosilanes. These reagents reacted with
cyclic ethers, leading to the clean formation of halosila-
tion products.
Rea ction of THF w ith 2a . A mixture of triethylsilane (3.87
g, 33.4 mmol), THF (2.36 g, 32.7 mmol), methyl iodide (6.74 g,
7.5 mmol), and palladium dichloride (70 mg, 0.40 mmol) was
4
Remarkable changes in regioselectivity observed for
iodosilation of 2-MeTHF may provide information on the
reactive species. From dominant formation of 5a with
stirred at room temperature for 5 h. After evaporation of the
excess methyl iodide, the residue was distilled under reduced
pressure to give 1-iodo-4-triethylsiloxybutane (12a , 7.22 g; 70%).
1
1
aminosilane reagents 1a (R X ) MeI) and 1a ′ (R X )
EtI), actual species in 1a and 1a ′ are postulated to be
Ack n ow led gm en t. This work was supported in part
by Grant-in-Aid Scientific Research, No. 09450331 from
the Ministry of Education, Science, Sports, and Culture.
We thank Sankyo Kasei Co. Ltd. and Sumitomo Electric
Industry for financial support. We also thank Shin-Etsu
Chemical Co. Ltd. for the gift of chlorosilanes.
3
bulky ammonium salts that are in equilibrium with Me -
SiI, as presented below. This hypothesis is also supported
by the fact that the reaction did not occur when bulkier
Et
Et
2
NSiEtMe
NSiMe
2
2 3
and Et NSiEt were employed in place of
2
3
.
1
1
+
Et N-SiMe + R I f [Et R N -
2
3
2
Su p p or tin g In for m a tion Ava ila ble: Detailed experi-
mental procedures and spectral and analytical data for the
products. This material is available free of charge via the
Internet at http://pubs.acs.org.
-
1
SiMe ]I a Et R N + Me SiI
3
2
3
In contrast, the active species in the reaction with
hydrosilane reagent 2a (PdCl catalyst) is undoubtedly
2
J O990722C