Tetrahedron Volume 44 issue 21 1988 [doi 10.1016_s0040-4020(01)90106-1] Branko Juršić -- Synthetic...
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Transcript of Tetrahedron Volume 44 issue 21 1988 [doi 10.1016_s0040-4020(01)90106-1] Branko Juršić -- Synthetic...
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8/10/2019 Tetrahedron Volume 44 issue 21 1988 [doi 10.1016_s0040-4020(01)90106-1] Branko Juri -- Synthetic application
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SYNTHETICAPPLICATIONOF MICELLARCATALYSIS.
WILLIAMSONSSYNTHESISOF ETHERS
BRANKO URSIC
Laboratory of Organic Chemistry and Biochemistry, Faculty of Science,
University of Zagreb, Strossmayerov trg 14, 41000 Zagreb, Yugoslavia
Received
in UK 16
August1988)
Abstract
- A simple, rapid and efficient procedure for the preparation of
di-alkyl, simple phenyl-alkyl and hindered phenyl-alkyl ethers has been de-
veloped. Based on the principle of micellar catalysis the method involves
alkylation of the alkoxide or the phenoxide ion with an alkyl chloride at
80C in the presence of cationfc mieelles. For the preparation of phenyl-
-alkyl ethera normal micelles ware used, while for the di-alkyl ethers rc-
verse micelles were more effective. By increasing the ionic strength of the
solution the rate of formation of phenyl-alkyl ethers could be increased.
Preparation of ethers is an important synthetic reaction for which a wide variety of procedures
has been developed during the last hundred years.
The Williamson reaction, discovered in 1850,
is still the best general method for the preparation of unsymmetrical as well as symmetrical
ethers.
1
However, this method is not satisfactory for ethers containing a tertiary group (because
of eliminations while with secondary groups low yields are obtained.
2
By the use of phase-transfer
catalysis an improvement of the Williamson reaction was achieved.
3
A number of very useful synthe-
tic applications of cellar catalysis has been reported,
4
but to our knowledge, no detailed stu-
dies on the application of this technique to the synthesis of ethers.
There are few available procedures for the conversion of phenols or alcohols into ethers which
do not comprise the initial formation of the corresponding phenoxide or alkoxide ions. Among these
the most widely used is the direct alkylation with diazomethane or diazoalkane5
which is mostly li-
mited to methyl ethers and seldom used because of the obnoxious nature of the reagent. Alkylation
has also been accomplished with orthocarbonate esters,
6
dialkyl oxalate esters,?
onium salts,
and
by treating phenols with alcohols in the presence of dicyclohexylcarbodiimide.
9
However, none of
these methods can be considered as a general method for the preparation of ethers.
In our work we used aqueous micelles to achieve perturbations which are synthetically favourable
and succeeded in controlling the aromatic brominativn
10
and enone reduction.
11
In this paper a new
synthesis of ethers performed under mild conditions in the presence of normal and reverse micellea
is reported.
RESULTSAND DISCUSSION
As already mentioned cationie micelles fCTAB)* are more effective than anionic vnes (SDS)** in the
synthesis of phenyl ethers (see Table 1).
So the alkylation rate of phenol in SDS is lower by a factor
o
100 1000
than in CTAB micelle. Cetyltrimethyl ammonium phenvxide.
formed by the reaction of CTAB
cetyltrimethylammonium bromide
**sodium dodecyl sulfate
6677
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78
8.
JURSIC
Table 1. Yields f alkyl phenyl thers hOR prepared rom phenol nd alkyl
halide n the presence f CTAB as micelle
RX
CH31
CH3Cti2Br
CH3CH2CH2Br
t+,HQCl
n-C5H,,Cl
n-C10H2,Cl
n-C,2H25Cl
n-C,6H33Br
PhOR
Yield, RX PhOR
Yield, '
PhOCH3 97 CH2=CH-CH2Cl PhOCH2CWH2 96
PhOCH2CH3
93
PhCH2Cl PhOCH2Ph
95
PhOCH2CH2CH3
95
PhCH2CH2Cl PhOCH2CH2Ph
87
PhOCIiHg
87
Cl(CH212Cl PhOfCH212OPh
89
PhOC5H1,
91
Cl(CH2$Cl PhO(CH&OPh
86
PbC10H21
89
Cl(CH2)ACl PhO(CH~)~OP~
91
PhCV25
86
Cl(CH215Cl
PhOlCH2)50Ph
93
PhoC16H33
63
Er(CH2)6Br PhO(CH2160Ph
90
'Refers to isolated aterial.
and sodium henolate n water is a surface ctive ompound hich dissolves lkyl halides. he phe-
noxy ion, as evidenced y
1
H NMR spectroscopy,
2
resides n the tern layer of the normal icelle 0
the nucleophilic isplacement f phenoxy on on primary lkyl chloride akes place on the inter-
f ce
of micelle
fFigure f.
Figure 1
It was of interest o examine he
Table 2. Etherificationf hindered henols ith
butyl chloride n the presence f CTAB as micelle
ArOH
Yield,
2,6-Dimethylphenol
95
2,6-Dimethyl-4-t-butylphenol 82
2,&Dimethoxyphenol
91
Z,d-Di-t-butylphenol 80
aRefer to isolated aterial.
applicability f the presented ethod to the esterification
11
of highly indered henols,
here generation f the phenoxide onswas reported o be difficult.*
A number f highly hindered henol8 ere therefore reated ith butyl chloride nder miceilar on-
ditions ith 20% sodium ydroxide. xcellent ields f the corresponding ryl butyl ethers ere
obtained Table ). As expected, here was no reaction n the absence f CTAB, which was confirmed
(by gas chromatography) or the etherification f 2,6-dimethylphenolith butyl chloride.
This synthesis f aryl ethers
s very
convenient ecause he work-up
s
simple nd involves
a
turation f the reaction ixture ith sodium hloride,
iltration, xtraction nd purification
either by distillation r crystallization.
he products btained re 98% pure as determined y
gas
chromatography nd 'H NMR analysis.
Preparation f mixed n-alkyl ther8 from the corresponding lcohols nd n-alkyl hlorides n
20% or 50% aqueous odium r potassium ydroxide n the presence f normal icelles s not 80 ef-
fective, ather low yields ere obtained nd a certain mount f symmetric ther was formed (40%).
This yield was raised to 40% when a large axce88 f alkyl chloride 83 used. The reaction f ben-
zyl alcohol ith a two-fold xcess f I-pentyl hloride n 50% potassium ydroxide n the pre-
sence of CTAB resulted n a very poor yield (30%) f the expected enzyl ther: he major product
was diphenyl ther. owever,
he desired enzyl entyl ther was obtained n an excellent ield
when the reaction as performed nder the liquid-solid
4
reverse cellar conditions. he experi-
mental conditions or the solid-liquid everse icellar water-pool )rocess re exceedingly
simple. therification s usually carried out in tbe presence f the organic hase talky1 hloride
and alcohol1 nd with a two-fold xoess f sodium r potassium ydroxide s the solid hase. n cases
where solid products ere formed tetrahydrofuranolution f alkyl chloride nd alcohol as used
as a liquid hase. he results re presented n Table 3. The amount f water plays an important
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8/10/2019 Tetrahedron Volume 44 issue 21 1988 [doi 10.1016_s0040-4020(01)90106-1] Branko Juri -- Synthetic application
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yntheticapplicationfmicetlaratalysis
6679
role in the solid-liquid everse lcellar atalysis:
n the absence f water no nucleophilic is-
placement eaction etherification)akes place.
his finding s similar o that valid for phase
transfer ystems.
5
Table 3. Yields of dfalkyl thers repared rom alkyl halides nd
alcohols
in
the
presence of
CTAB as reverse fcelle
R'OH
RX R'OR
Yield, '
CH30H
C2H5OH
n-C3H70H
n-C5H,,0H
(CH~)~CHOH
n-C4H90H
n-C5H,,OH
CH3CH2CH(CH3)OH
(CH3)3COH
1CH313COH
(CH~)~COH
(CH3j3C0H
(CH~)~COH
PhCH?OH
n-C4H9OH
n-C5H,,0H
n-C,2H250H
c>a
H
OH
n-C5H,,Cl
n-C5HIIC1
n-C5H,,Cl
n-C3H.,Br
n-C5H11C1
n-C5H,,Cl
n-C4H9Cl
n-C5H,,C1
CH31
CH3CH2Br
n-C3H7Br
n-C4H9Cl
n-C5H,,C1
CH2sCH-CH2Cl
CH2=CH-CH2C1
CH2=CH-CH2C1
CH2= CH-CH2Cl
CH31
n-C4H9Cl
cH30C5Hll
C2H50C5H11
c3H70c5Hll
C3H70c5Hll
(CH3)$HOC5H,,
C4H90C5Hll
C4H90c5Hll
~H3CH~CH(CH3)OC~H,,
KH313COCH3
KH313COCH2CH3
(CH3)3COC3H7
CH313COC4H9
fCH313COC5H1,
PhCH20CH2CHzCH2
C4H90CH2CH.CH2
C5H,,0CH2CH=CH2
C,2H250CH2CH=CH2
5
OCH
3
0
C4H9
a
a
86
81
91
a7
a5
a2
a5
a3
87
86
87
91
95
96
90
81
87
aRefers to isolated aterial.
With an excess f water two non-miscible iquid-phases re formed nd the yield of the reaction s
decreased. he schematic resentation f the proposed atalysis s given in Figure . The reverse
micelle issolves odium ydroxide nd removes t from the solid state to the organic ulk; the
hydroxide on is a very strong ase and forms alkoxide on thus enabling he displacement eaction
with alkyl chloride n the interface f the reverse icelle.
surfactant
A
water)
Figure . Schematic resentation f
water-pool reverse icelle) atalysis n
the esterification f alcohols ith alkyl chlorides
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6680
B JURW
EXPERIMENTAL
Melting points were determined on a Kofler hot-stage microscope apparatus and are uncorrected.
IR spectra were recorded on a Perkin-Elmer Model 257 grating infrared spectrophotometer using a
standard liquid film and Nujol mull techniques.
Nuclear magnetic resonance spectra were recorded on
a JEOL 90 FXQ spectrometer, as solutions in deuteriochloroform, using tetramethylsilane as a in-
ternal standard. Where possible, the identity of products was confirmed by comparing their IR and
NMR spectra with those of commercial samples.
Analytical gas-liquid chromatography was done on a
Vsrian odel 920 gas
chromatograph with standard
Carbowax 0M columns, ommercially vailable
(Fluka) sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB) were used without
further purification.
Starting phenols,
alcohols, and alkyl chloride were purchased commercially and, where necessary
purified before use. Analytical and spectral data for the product ether8 were consistent,with the
assigned structures. The products were 98% pure as determined by gas chromatography and H NMR
analysis.
General rocedure or the preparation f phenolic thers
To a 20 aqueous solution of sodium hydroxide (1000 mL1 the corresponding phenol (0.1 molf and
cetyltrimethylammonium bromide (3.2 g; 0.1 mol) were added.
The mixture was heated to 60C for 0.5
hr and then allowed to cool to ambient temperature, after which the corresponding alkyl halide (0.12
mol) wa8 added with stirring. After re-heating to 8O*C for 5 hrs,
the reaction mixture was saturated
with sodium chloride and the aqueous layer separated by filtration from the solid sodium chloride.
The aqueous solution was extracted with diethyl ether (x31. The ethereal extract was washed twice
with 20% sodium hydroxide to remove unreacted phenol,
and then with saturated sodium chloride so-
lution. After drying with sodium sulfate, the solvent was evaporated and the residual ether puri-
fied by distillation or crystallization.
When methyl iodide was used as an alkylation agent, due to hydrolysis a notable excess of
methyl iodide (x5) was required.
General
procedure or the preparation f
unsymmetrical alkyl ethers
A mixture of alkyl chloride (0.2 mol),
alcohol (0.2 mol), water (1.0 mL), cetyltrimethylammo-
nium bromide (3.64 g; 0.01 mol), sodium hydroxide 516 g; 0.4 mol), and, in the case of solid
products, tetrahydrofuran (50 mL) was heated to 70 C and stirred vigorously overnight. The reac-
tion suspension was then allowed to cool and the solid residue was separated by filtration. The
residue was washed several times with ether or tetrahydrofuran, the organic solutions were com-
bined and dried over anhydrous sodium sulfate.
Ether or tetrahydrofuran was carefully evaporated
and the oily residue purified by distillation or crystallization.
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