Direct Conversion of TMS-ethers to THP-ethers Catalyzed by {[K.18-Crown-6]Br3}n

Full text of DOI:10.1080/00304948.2019.1586430

Organic Preparations and Procedures International
The New Journal for Organic Synthesis
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Direct Conversion of TMS-ethers to THP-ethers
Catalyzed by {[K.18-Crown-6]Br3}n
Gholamabbas Chehardoli, Mohammad Ali Zolfigol & Fateme
Derakhshanpanah
To cite this article: Gholamabbas Chehardoli, Mohammad Ali Zolfigol & Fateme
Derakhshanpanah (2019) Direct Conversion of TMS-ethers to THP-ethers Catalyzed by
{[K.18-Crown-6]Br3}n, Organic Preparations and Procedures International, 51:2, 192-197, DOI:
10.1080/00304948.2019.1586430
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Organic Preparations and Procedures International, 51:192–197, 2019
# 2019 Taylor & Francis Group, LLC
ISSN: 0030-4948 print / 1945-5453 online
DOI: 10.1080/00304948.2019.1586430
OPPI BRIEF
Direct Conversion of TMS-ethers to THP-ethers
Catalyzed by {[K.18-Crown-6]Br3}n
Gholamabbas Chehardoli,1
Fateme Derakhshanpanah2
Mohammad Ali Zolfigol,2 and
1Department of Medicinal Chemistry, School of Pharmacy, Hamadan
University of Medical Sciences, Hamadan 6517838695, Iran
2Faculty of Chemistry, Bu-Ali Sina University, Hamadan 6517838683, Iran
As macrocyclic polyethers, crown ethers have attracted remarkable attention. Crown
ether moieties are well-known host compounds in host–guest chemistry. They have
shown a significant ability to form strong complexes with organic and inorganic anions
orþcations, selectively. Among the crown ethers, the cavity size of the 18-crown-6 and
K
diameter are well matched. Thus, 18-crown-6 forms a very stable complex with
Kþ.1,2 Designing catalysts based on crown ethers, which work well under mild condi-
tions, is an attractive and desirable area of research.
As is well known, elemental bromine is very toxic and hazardous. Therefore, using
organic tribromide reagents (OTBs) as safe bromine sources are preferable to molecular
bromine.3 Tribromides have been reported previously. Some examples of these are:
benzimidazolium tribromide,4 benzyltrimethylammonium tribromide,5 tetrabutylammo-
nium tribromide,6 quinolinium tribromide,7 pyridinium tribromide,8 hexamethylenetetr-
amine-bromine,9 and DABCO-bromine.10 Given the efficiency of these compounds, the
development of further synthetic protocols for their synthesis and utility is of interest.
In 2007, we reported {[K.18-crown-6]Br3}n as a particular tribromide type with a
columnar nanotube-like structure (I, Figure 1). We applied this catalyst for the bromi-
nation of aromatic rings and conversion of thiols to disulfides11 and for protection of
amines and oxidation of alcohols as well as protected alcohols.12
Due to its easy installation and removal and stability towards a variety of reaction
conditions, the 3,4-dihydro-2H-pyran (DHP) group is one of the most important and
frequently used protecting groups, especially for alcohol derivatives. The resistance of
THP-ethers towards base-catalyzed attack makes them favorable orthogonal partners for
the protection of alcohols during the synthesis of multifunctional target molecules.
Several reagents, catalysts and methods have been developed for the conversion of alco-
hols to their corresponding THP-ethers.13
Received August 19, 2017; in final form October 19, 2018.
Address correspondence to Gholamabbas Chehardoli, Department of Medicinal Chemistry,
School of Pharmacy, Hamadan University of Medical Sciences, Hamadan 6517838695, Iran.
E-mail: chehardoli@umsha.ac.ir; cheh1002@gmail.com
Color versions of one or more of the figures in the article can be found online at www.tandfon-
line.com/uopp.
192

Direct Conversion of TMS-ethers to THP-ethers
193
Figure 1. The structure of {[K.18-crown-6]Br3}n .
Scheme 1. Optimized reaction conditions for the direct conversion of TMS-ether to THP-ether.
Herein, we wish to report {[K.18-crown-6]Br3}n as a unique catalyst for the direct
conversion of TMS-ethers to THP-ethers under mild reaction conditions.14–19
We studied the conversion of benzyloxytrimethylsilane (1 mmol) to its correspond-
ing THP-ether derivative using different amounts of {[K.18-crown-6]Br3}n and a diver-
sity of solvents in the presence of DHP at room temperature. The optimized reaction
conditions are 0.001 mmol of {[K.18-crown-6]Br3}n, 1.1 mmol DHP and CH3CN as the
best solvent.
We then converted a range of TMS-ethers to the corresponding THP-ethers under
the optimized reaction conditions (Scheme 1 and Table 1).
As seen in the table, a range of TMS-ethers will undergo this procedure to give the
corresponding THP-ethers as products in good to excellent yields. Spectral data for the
obtained products are consistent with their proposed structures.20,21
According to the earlier literature15 and our newly-obtained results, we propose a
mechanism for the direct conversion of TMS-ethers to the THP-ethers given in Scheme 2.
To investigate the recyclability of the catalyst, the residue after work-up was
washed with ethanol and Br2 was added. The obtained yellow solid was recrystallized
in acetonitrile (67%). {[K.18-Crown-6]Br3}n was generated again and catalyzed the
reaction without any reduction in activity through two cycles.
In conclusion, we report for the first time the direct conversion of TMS-ethers to
THP-ethers catalyzed by {[K.18-crown-6]Br3}n as a unique tribromide type catalyst at
room temperature. The reaction is carried out under mild conditions and yields of prod-
ucts are good to excellent.
Experimental Section
Chemicals were purchased from Merck. THP-ethers products were characterized by com-
parison of their spectra (IR, 1H-NMR and 13C-NMR) with those of known samples.19,20
Nuclear magnetic resonance (NMR) spectra were recorded on a JEOL FX 90 Q
and tetramethylsilane (TMS) was used as internal standard. Infrared spectra were

194
Chehardoli et al.
Table 1
Direct Conversion of TMS-ethers to the THP-ethers Catalyzed by {[K.18-Crown-
6]Br3}n at Room Temperature
Entry
1
TMS-ether (a)
Time (min)
20
Isolated yields (%)
85
2
3
20
10
72
80
4
5
35
12
70
75
6
60
70
7
8
55
35
72
72
9
10
63
80
60
10
11
12
72
20
90
71
13
15
85
(Continued)

Direct Conversion of TMS-ethers to THP-ethers
195
Table 1
(Continued)
41
14
70
15
16
110
48
73
74
17
25
70
18
19
15
46
89
72
Scheme 2. Suggested mechanism for direct conversion of TMS-ether to THP-ether in the pres-
ence of a catalytic amount of {[K.18-crown-6]Br3}n.
recorded on a Perkin Elmer GX FT IR spectrometer. {[K.18-crown-6]Br3}n was pre-
pared according to our previously reported procedure.10 Data from the optimization
study are available from the corresponding author upon request.

196
Chehardoli et al.
General Procedure for the Direct Conversion of TMS-ethers to the THP-ethers
To the TMS-ether (1 mmol) and DHP (1.1 mmol) in CH3CN (4 mL) was added {[K.18-
crown-6]Br3}n (0.00054 g, 0.001 mmol) and 1 drop H2O. The solution was stirred at
room temperature for the proper time as determined by TLC (silica gel/n-hexane/
EtOAc) (see Table 1). After completion of the reaction, CH3CN was removed by evap-
oration on the steam bath. Column chromatography on a short pad of silica gel with
CH3CN afforded pure products in good to excellent yields. The catalyst could be regen-
erated via addition of Br2 to the residue of the reaction, followed by recrystallization
from acetonitrile.
Preparation of 2-Phenethoxytetrahydro-2H-pyran (1b)
To a solution of trimethyl(phenethoxy)silane (1a, 0.194 g, 1 mmol) and DHP (0.092 g,
1.1 mmol) in CH3CN (4 mL) was added to {[K.18-crown-6]Br3}n (0.00054 g,
0.001 mmol) and 1 drop H2O. The solution was stirred at room temperature for
20 minutes. CH3CN was removed by evaporation on the steam bath. Column chroma-
tography on a short pad of silica gel with CH3CN afforded 1b, 0.175 g, 85%, colorless
1
oily liquid. IR (KBr): 1021, 1119, 1455 cmꢀ1; HNMR (90 MHz, CDCl3): d¼ 1.62 (br
s, 6H), 2.92 (t, 2H), 3.67(m, 4H), 4.6(s, 1H), 7.25(s, 5H); 13CNMR (22.5 MHz,
CDCl3): d ¼ 14.01, 20.1, 25.27, 31.02, 56.54, 62.81, 93.23, 120.73, 123.59.
Acknowledgments
The authors acknowledge partial support for this work from the Research Affairs
Office of Hamadan University of Medical Sciences, Hamadan, I.R. Iran, and also the
Center of Excellence in the Development of Chemical Methods, Hamedan, I.R. Iran.
ORCID
Gholamabbas Chehardoli
http://orcid.org/0000-0002-8760-3837
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