Cationic platinum-catalyzed etherification by intra- and intermolecular dehydration of alcohols

Article abstract of DOI:10.1055/s-2004-835664

Catalytic etherification of diols proceeds to give various cyclic ethers by use of cationic platinum salt, which is in situ prepared from PtCl₂ and AgSbF₆. Etherification of benzylic alcohols is also possible by intermolecular dehydration. Both of intra- and intermolecular etherifications smoothly proceed even under an atmosphere of air.

Full text of DOI:10.1055/s-2004-835664

152
LETTER
Cationic Platinum-Catalyzed Etherification by Intra- and Intermolecular
Dehydration of Alcohols
Cationic
P
latinu
a
m-Catalyz
k
edEtherifica
a
tion nori Shibata,* Ryo Fujiwara, Yasunori Ueno
Department of Chemistry, School of Science and Engineering, Waseda University, Shinjuku, Tokyo, 169-8555, Japan
E-mail: tshibata@waseda.jp
Received 13 September 2004
Table 1 Catalytic Etherification of 1,5-Diol Using Platinum Salts
Abstract: Catalytic etherification of diols proceeds to give various
cyclic ethers by use of cationic platinum salt, which is in situ pre-
pared from PtCl2 and AgSbF6. Etherification of benzylic alcohols is
also possible by intermolecular dehydration. Both of intra- and
intermolecular etherifications smoothly proceed even under an
atmosphere of air.
PtCl2 (2 mol%)
OH
OH
Ph
O
Ph
Ag salt (5 mol%)
Ph
Ph
DCE, 60 °C, 1–2 h
Entry
Ag salt
Yield (%)
NRc
NRc
78
Key words: platinum, ethers, dehydration, diols, alcohols
1
None
2
AgBF4
AgOTf
AgSbF6
AgSbF6
AgSbF6
Williamson ether synthesis is an established protocol for
the synthesis of ethers from alcohols and halides, how-
ever, it is generally examined under the strongly basic
conditions and a stoichiometric amount of salts is
formed.1 On the other hand, dehydration of alcohols is
more direct synthesis of ethers, and protic acid catalysts2
like amberlyst3 and nafion-H4 were reported for the ether-
ification of diols. Lewis acid is another choice of a pro-
moter for the dehydrative ether synthesis2 and a
stoichiometric amount of zinc chloride was used for the
synthesis of cyclic ethers.5 In these years, catalytic dehy-
drative etherifications of allylic and benzylic alcohols
were also reported by use of various metal catalysts.6,7
3
4
94
5a
6b
94
NRc
a Under an atmosphere of air.
b Without PtCl2.
c NR = no reaction.
Table 2 shows the results of cationic platinum-catalyzed
etherification of various diols under an atmosphere of ar-
gon or air.9,10 A 1,4-diol gave 2,5-disubstituted tetrahy-
drofuran under the same reaction conditions (entry 1). At
higher reaction temperature, even 0.1 mol% of platinum
catalyst could work efficiently to give the cyclic ether in
excellent yield (entry 3). Unsymmetrical diols also react-
ed and di- and mono-substituted tetrahydrofurans were
obtained (entries 4–6). Other aryl groups could be also tol-
erated as substituents on 1,5-diols and the corresponding
tetrahydropyran derivatives were provided (entries 7, 8).
Under the dilute reaction conditions, a 1,6-diol also cy-
clized to a 7-membered cyclic ether (entry 9). Not only
benzylic alcohols, but alkyl alcohols were also substrates
(entries 10–14). From a diol, having tertiary and primary
alcohols, a 2,2-dialkyl-substituted tetrahydrofuran was
obtained (entries 10, 11).11 1,11-Diphenylundecane-4,8-
diol gave a symmetrical 2,6-dialkyl-substituted tetrahy-
dropyran in good yield (entry 12). 2-Alkyl- and 2-alkynyl-
substituted tetrahydropyrans were also provided from the
corresponding diols (entries 13–15). It is noteworthy that,
even under an atmosphere of air, comparable yields were
achieved (entries 5, 11, 14).
We here disclose a catalytic ether synthesis using a cation-
ic platinum salt. Intra- and intermolecular dehydrations of
alcohols were examined under an atmosphere of argon
and air.
We chose Pt(II) salt because of its high Lewis acidity.8
Dehydration of 1,5-diphenylpentane-1,5-diol was exam-
ined by use of a catalytic amount of platinum dichloride in
1,2-dichloroethane (DCE) under an atmosphere of argon,
however, no reaction proceeded at 60 °C (Table 1, entry
1). In order to increase Lewis acidity, silver salts were
added for the generation of cationic platinum (entries 2–
4). As a result, the choice of counter anion was important
and 2,6-disubstituted tetrahydropyran was obtained in an
excellent yield9 by the combination of PtCl2 and AgSbF6
(entry 4). Argon atmosphere was not needed and no
decrease of catalytic activity was observed even under an
atmosphere of air (entry 5). Silver salt did not operate as a
catalyst by itself (entry 6), therefore, the cationic platinum
salt was an efficient catalyst for dehydrative etherifica-
tion.
Intermolecular dehydration of benzylic and allyl alcohols
also proceeded by use of the cationic platinum catalyst
(Table 3).12 The reaction of 1-phenylethanol and allyl al-
cohol gave the corresponding ether in high yield (entry 1),
moreover, the present etherification proceeded even under
SYNLETT 2005, No. 1, pp 0152–0154
Advanced online publication: 29.11.2004
DOI: 10.1055/s-2004-835664; Art ID: U26004ST
© Georg Thieme Verlag Stuttgart · New York
0
5.
0
1.
2
0
0
5

LETTER
Cationic Platinum-Catalyzed Etherification
153
Table 2 Etherification of Various Diols by Cationic Platinum Catalyst
PtCl2 (2 mol%)
OH
R1
R2
R3
AgSbF6 (5 mol%)
O
n
R3
R1
DCE, 60–80 °C
under argon or air
R2
n
OH
n
1
1
1
1
1
1
2
2
3
1
1
2
2
2
2
Entry
1
R1
R2
H
H
H
H
H
H
H
H
H
R3
Temp (°C)
Time (h)
Yield (%)
97
Ph
Ph
60
60
80
60
80
60
60
60
60
60
60
70
80
80
60
0.5
1
2a
Ph
Ph
98
3b
Ph
Ph
0.5
1
95
4
Ph
Me
97
5c
Ph
Me
2
95
6
Ph
H
1
93
7
4-ClC6H4
4-MeOC6H4
Ph
4-ClC6H4
0.5
0.5
1
93
8
4-MeOC6H4
93
9d
Ph
75
10
11c
12d
13
14c
15e
3-Phenylpropyl
3-Phenylpropyl
3-Phenylpropyl
Hexyl
Me
Me
H
H
2
91
H
4
93
3-Phenylpropyl
8
77
H
H
H
H
5
78f
76f
86
Hexyl
H
4
1-Hexynyl
H
5
a PtCl2 (0.5 mol%), AgSbF6 (1.5 mol%).
b PtCl2 (0.1 mol%), AgSbF6 (0.5 mol%).
c Under an atmosphere of air. The other entries were examined under an atmosphere of argon.
d The concentration of diol in DCE is 0.020 M.
e The concentration of diol in DCE is 0.025 M.
f A five-membered cyclic ether (ca. 8%) was included as a side product.
an atmosphere of air without solvent (entry 2). Etherifica- In alcoholic solvents, methyl and ethyl ether syntheses
tion of propargylic and tertiary alcohols occurred at room from benzylic alcohols were possible using 0.5 mol% cat-
temperature (entries 3, 4). Generation of benzylic cation is ionic platinum catalyst (Table 4). Symmetrical ethers
probably important for the following nucleophilic addi- were obtained by dimerization of alcohols, which are
tion of allyl alcohol.13
limited to benzylic ones (Equation 1).
Table 3 Intermolecular Dehydration of Benzylic and Allyl Alco-
Table 4 Intermolecular Etherification in Alcoholic Solvents
hols
PtCl2 (0.5 mol%)
OR3
R1
OH
R2
PtCl2 (4 mol%)
AgSbF6 (2.5 mol%)
OH
R2
O
AgSbF6 (10 mol%)
R1
Ph
R3OH, reflux
OH
(2 equiv)
Ph
R1
R2
+
R1
Ph
Ph
DCE
under argon or air
R2
Entry
R1
R2
R3
Time (h) Yield (%)
Entry
R1
R2
H
Temp (°C) Time (h) Yield (%)
1
2
3
1-Propynyl
1-Propynyl
Me
H
Me
Et
25
82
67
63
1
Me
50
60
r.t.
r.t.
1.5
1
91
94
75
87
H
4
3
2a
3
Me
H
Me
Me
1-Propynyl
Me
H
1
4
Me
2
a Under an atmosphere of air without solvent.
Synlett 2005, No. 1, 152–154 © Thieme Stuttgart · New York

154
T. Shibata et al.
LETTER
(7) Non-dehydrative ether syntheses of propargyl alcohols
PtCl2 (4 mol%)
OH
Ph
R
Ph
R
catalyzed by transition metal complexes: (a) Nishibayashi,
Y.; Wakiji, I.; Hidai, M. J. Am. Chem. Soc. 2000, 122,
11019. (b) Sherry, B. D.; Radosevich, A. T.; Toste, F. D. J.
Am. Chem. Soc. 2003, 125, 6076.
AgSbF6 (10 mol%)
O
Ph
R
DCE, r.t. to 40 °C
R= Me: 83% (dl/meso = 2.3/1)
(8) Recent examples using platinum salt as Lewis acid
catalysts: (a) Hartman, J. W.; Sperry, L. Tetrahedron Lett.
2004, 45, 3787. (b) Marion, F.; Coulomb, J.; Courillon, C.;
Fensterbank, L.; Malacria, M. Org. Lett. 2004, 6, 1509.
(c) Shimada, T.; Nakamura, I.; Yamamoto, Y. J. Am. Chem.
Soc. 2004, 126, 10546. (d) Nevado, C.; Ferrer, C.;
Echavarren, A. M. Org. Lett. 2004, 6, 3191. (e) Qian, H.;
Han, X.; Widenhoefer, R. A. J. Am. Chem. Soc. 2004, 126,
9536.
R= 1-propynyl: 85% (dl/meso = 1/1))
Equation 1
In summary, we developed a cationic platinum-catalyzed
ether synthesis from alcohols. Catalytic intra- and inter-
molecular dehydration gave various cyclic and acyclic
ethers. Moreover, the platinum catalyst is moisture-toler-
ant and the etherifications efficiently proceeded even
under an atmosphere of air.
(9) All of the 2,n-disubstituted cyclic ethers were obtained as a
mixture of dl and meso isomers (ca. 1:1).
(10) Typical Experimental Procedure for Cyclic Ethers from
Diols (Table 2): PtCl2 (1.1 mg, 0.004 mmol) was placed in
a flask and a 1,2-dichloroethane solution (2.0 mL) of a diol
(0.20 mmol, 0.10 M) was added. To the resulting mixture
was added AgSbF6 (3.6 mg, 0.010 mmol) and the mixture
was stirred at the temperature cited in Table 2 for 0.5–8 h.
After excluding the solvent to the volume of ca. 0.5 mL
under reduced pressure, the obtained mixture was purified
by column chromatography (hexane–EtOAc) using silica gel
to give a pure cyclic ether.
Acknowledgment
This research was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Science
and Technology, Japan.
References
(11) 2-Methyl-2-(3-phenylpropyl)tetrahydrofuran: IR (neat):
1496, 1454, 1057, 748, 700 cm–1. 1H NMR (400 MHz,
CDCl3): d = 1.16 (s, 3 H), 1.51–1.74 (m, 6 H), 1.82–1.95 (m,
2 H), 2.59–2.66 (m, 2 H), 3.74–3.85 (m, 2 H), 7.17–7.29 (m,
5 H). 13C NMR (100 MHz, CDCl3): d = 25.8, 26.1, 26.7,
36.5, 36.7, 40.9, 67.1, 82.5, 125.6, 128.1, 128.3, 142.5.
HRMS (FAB): m/z calcd for C14H20O [M+]: 204.1514.
Found: 204.1508.
(12) Typical Experimental Procedure for Acyclic Ethers
(Table 3): PtCl2 (3.2 mg, 0.012 mmol) was placed in a flask
and a 1,2-dichloroethane solution (4.5 mL) of an alcohol
(0.30 mmol) and allyl alcohol (0.60 mmol) was added. To
the resulting mixture was added AgSbF6 (10.3 mg, 0.030
mmol) and the mixture was stirred at the temperature cited
in Table 3 for 1–2 h. The following procedure is the same as
in ref. 10.
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(13) Complete racemization of a chiral alcohol indicates that the
present etherification proceeded via SN1 pathway
(Scheme 1).
Scheme 1
Synlett 2005, No. 1, 152–154 © Thieme Stuttgart · New York