Effects of Structural Factors in Silyl Ethers Derived from Terminal Acetylenic Alcohols on 1,4-O→Csp Migration of the Silyl Group

Article abstract of DOI:10.1023/A:1025529428307

Effects of structural factors in silyl ethers derived from terminal acetylenic alcohols on 1,4-O-→C_sp migration of the silyl group in the Iotsitch reagent were studied. The effect of steric factor at the carbon atom neighboring to the reaction center was found to be stronger than that at the silicon atom in the migrating group.

Full text of DOI:10.1023/A:1025529428307

Russian Journal of Organic Chemistry, Vol. 39, No. 3, 2003, pp. 336 339. Translated from Zhurnal Organicheskoi Khimii, Vol. 39, No. 3, 2003,
pp. 368 371.
Original Russian Text Copyright
2003 by Medvedeva, Novokshonov, Mareev, Borisova.
Dedicated to Full Member of the Russian Academy of Sciences
I.P. Beletskaya on Her Jubilee
Effects of Structural Factors in Silyl Ethers Derived
from Terminal Acetylenic Alcohols on 1,4-O C Migration
sp
of the Silyl Group
A. S. Medvedeva, V. V. Novokshonov, A. V. Mareev, and A. I. Borisova
Irkutsk Institute of Chemistry, Siberian Division, Russian Academy of Sciences,
ul. Favorskogo 1, Irkutsk, 664033 Russia
e-mail: traid@irioch.irk.ru
Received July 5, 2002
Abstract Effects of structural factors in silyl ethers derived from terminal acetylenic alcohols on 1,4-O Csp
migration of the silyl group in the Iotsitch reagent were studied. The effect of steric factor at the carbon atom
neighboring to the reaction center was found to be stronger than that at the silicon atom in the migrating group.
In the preceding communications we reported on
,4-O Csp migration of the trimethylsilyl group in
-trimethylsiloxy-1-propyne by the action of ethyl-
purpose, we have synthesized trimethylsilyl ethers of
alicyclic terminal acetylenic alcohols I and II, 2-(1-tri-
methylsiloxycyclopentyl)ethyne (III) and 2-(1-tri-
methylsiloxycyclohexyl)ethyne (IV), as well as
chloromethyl(dimethyl)silyl and tert-butyl(dimethyl)-
silyl 2-propynyl ethers V and VI.
1
3
magnesium bromide to give (after hydrolysis) 85% of
-trimethylsilyl-2-propyn-1-ol [1 3]. An analogous
3
X Csp migration of the R M group occurs by the
3
action of Grignard compounds on propynes of the
We anticipated that steric hindrances to O Csp
migration of the trimethylsilyl group in trimethylsilyl
ethers derived from tertiary acetylenic alcohols of the
alicyclic series would be lower than in the acyclic
analog, trimethylsilyl 1,1-dimethyl-2-propynyl ether.
The silylation of tertiary acetylenic alcohols was
performed following the procedure developed by us
previously; this procedure utilizes hexamethyldisila-
zane as silylating agent and a new catalyst, ethylene-
dinitramine (EDNA) [6]. As might be expected, the
silylation of alcohols I and II was much more difficult
than the silylation of 2-propynyl alcohol (VII). The
latter reaction was complete in 5 min in the presence
of 0.01 mol % of EDNA; the process was accom-
panied by strong heat evolution, and the yield of the
product was quantitative. Trimethylsilyl ethers I and
II were synthesized by the action of hexamethyldi-
silazane in the presence of 0.1 mol % of EDNA at
elevated temperature. A considerable difference in the
reactivities of alcohols I and II should be noted. Ether
III was obtained in 95% yield in 3.5 h at 70 C,
while previously unknown cyclohexyl analog IV was
isolated in 60% yield after heating at 120 130 C for
general formula HC CCH XMR , where M = Si, Ge;
2
3
X = O, S [4, 5]. This reaction demonstrates a radically
new procedure for introduction of Si(Ge) Csp bond
into acetylenic alcohols and thiols. We previously
found that the structure of trimethylsilyl alkynyl
ethers strongly influences the efficiency of their
isomerization into trimethylsilylacetylenic alcohols:
The isomerization is considerably hindered when the
trimethylsiloxy group occupies - rather than -posi-
tion with respect to the triple bond or there are sub-
stituents at the carbon atom neighboring to the reac-
tion center (O Si) in 3-trimethylsiloxy-1-propyne.
For example, trimethylsilyl 1,1-dimethyl-2-propynyl
ether does not undergo isomerization into 4-trimethyl-
silyl-2-methyl-3-butyn-2-ol under the conditions
optimal for isomerization of trimethylsilyl 2-propynyl
ether [2].
While continuing our studies on 1,4-O C migra-
sp
tion of the R Si group in silyl ethers derived from
3
terminal acetylenic alcohols, we examined the effect
of steric factors in the initial acetylenic alcohol and
migrating group on the isomerization process. For this
1
070-4280/03/3903-0336$25.00 2003 MAIK Nauka/Interperiodica

EFFECTS OF STRUCTURAL FACTORS IN SILYL ETHERS
337
1
5 h. Presumably, the reason is the different steric
addition to alcohol I (ratio 1:1). These data indicate
that ether III is relatively stable (as compared to
trimethylsilyl ether VIII) under conditions of Si O
bond heterolysis by the action of both Iotsitch
(Grignard) reagent and 5% hydrochloric acid.
structures of the substrates containing 5- and 6-mem-
bered rings. Due to planar structure of the five-
membered ring in alcohol I, steric hindrances at
the reaction center to attack by silylating agent are
considerably lower than in cyclohexyl analog II.
3
-Chloromethyl(dimethyl)siloxy-1-propyne (V) was
obtained from 2-propynyl alcohol and bis(chloro-
methyl)tetramethyldisilazane in the presence of
0.01 mol % of EDNA (70 75 C, 2 h; yield 82%).
V, IX, R = CH Cl; VI, X, R = t-Bu.
2
These results supplement our previous data on the
high sensitivity of the rearrangement under study to
steric effects at the carbon atom attached to oxygen
in silyl ethers derived from acetylenic alcohols.
Using 3-tert-butyl(dimethyl)siloxy-1-propyne (VI)
and 3-chloromethyl(dimethyl)siloxy-1-propyne (V) as
examples, we examined the effect of the migrating
I, CR = cyclo-C H ; II, CR = cyclo-C H ; III, R = Me,
2
5
11
2
6
13
CR = cyclo-C H ; IV, R = Me, CR = cyclo-C H ; V, R =
2
5
11
2
6
13
H, R = CH Cl; VII, R = H; VIII, R = H, R = Me.
2
Our attempt to synthesize hitherto unknown 3-tert-
group (R Si) structure. The isomerization of ether VI
3
butyl(dimethyl)siloxy-1-propyne (VI) by the action
of tert-butyl(chloro)dimethylsilane on 2-propynyl
alcohol (VII) in the presence of pyridine was unsuc-
cessful. The reaction was accompanied by strong
tarring, and the yield of silyl ether VI was poor.
Therefore, we have developed a procedure for chemo-
selective metalation of the hydroxy group in VII by
ethylmagnesium bromide to obtain 2-propynyloxy-
magnesium bromide. The subsequent treatment with
chlorotrimethylsilane gave ether VIII which contained
no impurity of isomeric 3-trimethylsilyl-2-propyn-1-ol
under the above conditions in the presence of EtMgBr
gave the expected product, previously unknown 3-tert-
butyl(dimethyl)silyl-2-propyn-1-ol (X) in 27% yield.
When one methyl group in the Me Si substituent of
3
ether VIII was replaced by chloromethyl group, the
yield of the rearrangement product, 3-chloromethyl-
(
dimethyl)silyl-2-propyn-1-ol (IX) under the optimal
conditions was considerably lower (48%). These data
may be explained by electron-acceptor properties of
the chloromethyl group, which facilitate heterolysis of
the Si O bond by the action of Grignard reagent.
(product of the reaction of the Iotsitch reagent with
chlorotrimethylsilane). Following the same procedure,
ether VI was obtained in 58% yield.
Thus the results of our study show much stronger
effect of steric environment of the carbon atom nearest
to the reaction center on O C migration of the silyl
sp
group in silyl alkynyl ethers, as compared to the
corresponding steric effect of the silicon-containing
moiety. In keeping with our concept implying inter-
molecular mechanism of the process [2], the rate-
determining stage is heterolytic dissociation of the
Si O bond. It might be expected a priori that steric
hindrances at the reaction center, created by both
acetylenic and silicon-containing fragments, would
hamper formation of transition complex and hence the
isomerization process. However, the greater contribu-
tion of steric hindrances at the -carbon atom rather
than at the silyl group to nucleophilic attack on the
silicon atom was not obvious.
VI, R = t-Bu; VIII, R = Me.
Ethers III and IV were subjected to isomerization
under the conditions optimal for O Csp migration
of the trimethylsilyl group in ether VIII (EtMgBr,
THF, reflux, 9 h) [2 4], as well as by the action of
butyllithium. However, after hydrolysis with 5%
hydrochloric acid, we isolated only the corresponding
terminal acetylenic alcohols I and II instead of the
expected C -silylated acetylenic alcohols. In the reac-
tion with EtMgBr, initial silyl ether III was isolated in
sp
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 3 2003

3
38
MEDVEDEVA et al.
0.5 h, and the mixture was stirred for 50 min. A finely
EXPERIMENTAL
The IR spectra were recorded on a Specord 75IR
crystalline solid precipitated. After appropriate treat-
ment and removal of the solvent, the residue (9.5 g)
was distilled under reduced pressure. Yield of ether
VI 5.3 g (54%), bp 102 105 C (16 mm). IR spectrum
1
13
spectrometer. The H and C NMR spectra were
obtained on a Bruker DPX-400 instrument using
CDCl as solvent and HMDS as internal reference.
Analysis by GLC was performed using an LKhM-80
chromatograph equipped with a thermal conductivity
detector; carrier gas helium; 1500 3.0-mm column;
stationary phase 10% of polymethylsiloxane on
Chromaton.
1
(film), , cm : 1250 (Si C), 2119 (C C), 3300
3
1
( C H). H NMR spectrum, , ppm: 0.08 s [6H,
(CH ) Si], 0.91 s (9H, t-BuSi), 2.40 t (1H, HC , J =
3
2
1
3
2.5 Hz), 4.31 d (2H, CH O, J = 2.5 Hz). C NMR
2
spectrum, , ppm: 5.04 [(CH ) Si], 18.40 [(CH ) C],
C
3 2
3 3
25.93 [(CH ) C], 51.55 (CH O), 73.02 (HC C),
3
3
2
8
2.50 (HC C).
1-Ethynylcyclopentanol (I) [7] and 1-ethynylcyclo-
hexanol (II) [8] were synthesized by the Favorsky
3-Trimethylsiloxy-1-propyne (VIII). a. A solu-
reaction.
tion of 5.6 g (0.1 mol) of 2-propynyl alcohol in 10 ml
of THF was quickly added at 10 C (cooling with ice
water) to the Grignard compound prepared from 2.4 g
(
1-Trimethylsiloxycyclopentyl)ethyne (III).
A mixture of 3.56 g (0.032 mol) of alcohol I, 2.6 g
0.016 mol) of hexamethyldisilazane, and 0.1 mol %
(
0.1 mol) of magnesium and 10.9 g (0.1 mol) of ethyl
bromide in 40 ml of THF. The mixture was stirred for
5 min at 18 20 C, and 10.9 g (0.1 mol) of chloro-
(
of EDNA was stirred for 3.5 h at 70 C. Vacuum dis-
2
tillation gave 4.9 g (95%) of ether III, bp 64 65 C
16 mm), n2 1.4410 [9].
0
trimethylsilane in 10 ml of THF was added over
a period of 5 min. The mixture was stirred for 50 min
and diluted with 50 ml of dry hexane. An abundant
fine precipitate separated, the solvent was removed
by decanting, the precipitate was washed with hexane,
and the filtrate was concentrated under reduced pres-
sure. Yield of ether VIII 43% (GLC).
b. A mixture of 5.6 g (0.1 mol) of 2-propynyl
alcohol, 8.1 g (0.05 mol) of hexamethyldisilazane,
and 0.01 mol % of EDNA was stirred for 5 min at
(
D
(
1-Trimethylsiloxycyclohexyl)ethyne (IV).
A mixture of 6.2 g (0.05 mol) of alcohol II, 4.04 g
0.025 mol) of hexamethyldisilazane, and 0.1 mol %
(
of EDNA was stirred for 15 h at 120 130 C. Vacuum
distillation gave 6.1 g (60%) of ether IV, bp 69.5
2
0
7
2 C (14 mm), n 1.4550. IR spectrum (film),
,
D
1
cm : 1250 (Si C), 2102 (C C), 3310 ( C H).
1
H NMR spectrum, , ppm: 0.19 s [9H, (CH ) Si];
3
3
1
.27 m (2H), 1.56 m (4H), and 1.83 m (4H, cyclo-
18 20 C. According to the GLC data, the yield of
1
3
C H ), 2.44 s (1H, HC ). C NMR spectrum, C,
6
10
ether VIII was 96%.
4
3
ppm: 2.31 [(CH ) Si]; 23.12 (C ), 25.45 (C ), 39.97
3
3
2
1
Attempted isomerization of (1-trimethylsiloxy-
cyclopentyl)ethyne (III) by the action of EtMgBr.
A solution of 1.82 g (0.01 mol) of ether III in 10 ml
of THF was added to the Grignard compound pre-
pared from 0.24 g (0.01 mol) of magnesium and 1.1 g
(
C ), 72.16 (C ) (cyclo-C H ); 73.38 (HC ); 88.23
6 10
(
HC C).
-Chloromethyl(dimethyl)siloxy-1-propyne (V).
A mixture of 5.6 g (0.1 mol) of 2-propynyl alcohol,
1.5 g (0.05 mol) of bis(chloromethyl)tetramethyldi-
3
1
(
0.01 mol) of ethyl bromide in 20 ml of THF. The
silazane, and 0.01 mol % of EDNA was stirred for 2 h
at 70 75 C. According to the GLC data, the yield of
silyl ether V was 82%. Vacuum distillation gave
mixture was heated under reflux for 16 h with stirring.
It was then treated with a saturated solution of NH Cl
4
and extracted with chloroform, and the organic phase
1
n
3
2.7 g (78%) of ether V, bp 64.5 65 C (25 mm),
was dried over MgSO . The solvent was removed
4
2
0
1
1.4494. IR spectrum (film), , cm : 2105 (C C),
D
under reduced pressure. According to the GLC data,
the residue, 1.8 g, was a mixture of 1-ethynylcyclo-
pentanol (I) and initial ether III at a ratio of 1:1.
280 ( C H). Published data [10]: bp 78 79 C
25 mm), n2 1.4488.
0
D
(
3
-tert-Butyl(dimethyl)siloxy-1-propyne (VI).
Attempted isomerization of (1-trimethylsiloxy-
cyclohexyl)ethyne (IV) by the action of EtMgBr.
A solution of 1.96 g (0.01 mol) of ether IV in 10 ml
of THF was added to the Grignard compound pre-
pared from 0.24 g (0.01 mol) of magnesium and 1.1 g
(0.01 mol) of ethyl bromide in 20 ml of THF. The
mixture was heated under reflux for 7 h with stirring.
After appropriate treatment, 1.6 g of 1-ethynylcyclo-
hexanol (II) was isolated.
A solution of 2.8 g (0.05 mol) of 2-propynyl alcohol
in 10 ml of THF was added over a period of 5 min at
0 C to the Grignard compound prepared from 1.22 g
1
(
0.05 mol) of magnesium and 5.45 g (0.05 mol) of
ethyl bromide in 20 ml of THF. The mixture was
stirred for 40 min on cooling with ice water, a solu-
tion of 7.5 g (0.05 mol) of tert-butyl(chloro)dimethyl-
silane in 10 ml of THF was added over a period of
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 3 2003

EFFECTS OF STRUCTURAL FACTORS IN SILYL ETHERS
339
2
0
Attempted isomerization of (1-trimethylsiloxy-
cyclopentyl)ethyne (III) by the action of BuLi.
A solution of 1.56 g (0.01 mol) of silyl ether III in
silyl-2-propyn-1-ol (X), bp 91 92 C (10 mm), n
D
1
1.4600. IR spectrum (film), 1 , cm : 1240 (Si C),
2175 (C C), 3350 (O H). H NMR spectrum, ,
3
0 ml of THF was cooled to 36 C, 6 ml of a 1.6 M
ppm: 4.24 s (2H, CH O), 1.99 br.s (1H, OH), 0.91 s
2
1
3
solution of butyllithium (0.01 mol) was added, and
the mixture was stirred for 1 h in an inert atmosphere.
The mixture was treated with 10 ml of a saturated
(9H, t-BuSi), 0.08 s [6H, (CH ) Si]. C NMR spec-
3 2
trum, C, ppm: 104.54 (SiC C), 88.81 (SiC C),
51.56 (CH O), 26.00 [(CH ) C], 16.39 [(CH ) C],
2
3 3
3 3
solution of NH Cl and 10 ml of 5% hydrochloric acid
4.70 (CH Si).
4
3
and extracted with chloroform. The extract was dried
This study was financially supported by the Rus-
sian Foundation for Basic Research (project no. 98-
over MgSO , and the solvent was removed under
4
reduced pressure. According to the GLC data, the
residue, 1.3 g, was initial ether III.
0
3-32931a) and by the Presidium of the Siberian
Division, Russian Academy of Sciences (Resolution
no. 83, March 10, 2000, Siberian Division, Russian
Academy of Sciences).
Attempted isomerization of (1-trimethylsiloxy-
cyclohexyl)ethyne (IV) by the action of BuLi. A so-
lution of 1.96 g (0.01 mol) of silyl ether IV in 30 ml
of THF was cooled to 36 C, 7 ml of a 1.6 M solu-
tion of butyllithium (0.01 mol) was added, and the
mixture was stirred for 1 h in an inert atmosphere.
Appropriate treatment gave 1.5 g of a residue which,
according to the GLC data, was initial ether IV.
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1
2
174 (C C), 3350 (O H). H NMR spectrum,
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2
1
3
(
2H, CH Cl), 0.19 s [6H, (CH ) Si]. C NMR spec-
2 3 2
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2
2
3
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1
956, p. 960.
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and 1.2 g (0.01 mol) of ethyl bromide in 20 ml of
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RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 39 No. 3 2003