Silica chloride in the presence of NaI is a useful system for the efficient and selective conversion of TMS, TBDMS and THP ethers into their corresponding iodides

Article abstract of DOI:10.1016/S0040-4039(02)01706-9

Direct and highly selective conversion of benzylic, allylic and propargylic TMS, TBDMS and THP ethers into their corresponding iodides with the SiO₂-Cl/NaI system is described. Reactions were conducted in CH₃CN at room temperature. Aliphatic silyl and tetrahydropyranyl ethers remained almost intact under similar reaction conditions.

Full text of DOI:10.1016/S0040-4039(02)01706-9

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 7139–7141
Silica chloride in the presence of NaI is a useful system for the
efficient and selective conversion of TMS, TBDMS and THP
ethers into their corresponding iodides
Habib Firouzabadi,* Nasser Iranpoor* and Hassan Hazarkhani
Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran
Received 2 July 2002; revised 3 August 2002; accepted 15 August 2002
Abstract—Direct and highly selective conversion of benzylic, allylic and propargylic TMS, TBDMS and THP ethers into their
corresponding iodides with the SiO –Cl/NaI system is described. Reactions were conducted in CH CN at room temperature.
2
3
Aliphatic silyl and tetrahydropyranyl ethers remained almost intact under similar reaction conditions. © 2002 Published by
Elsevier Science Ltd.
7
b
The utility of the trialkylsilyl group and dihydropyran
as protecting groups for hydroxy functions during
multi step synthesis has been well documented. Direct
carbonyl compounds, transformation of sulfoxides to
7c
thioethers and the selective tetrahydropyranylation of
symmetrical diols. We have used our high capacity
silica chloride for transthioacetalization of acetals and
transformation of acylals into 1,3-dithiolanes, 1,3-dithi-
1
7d
7
e
transformation of silyl and tetrahydropyranyl ethers
2a–d
into the corresponding halides is an attractive
7f
method for the generation of iodides which are impor-
tant precursors for the preparation of organometallics
and also free radical reactions as well as nucleophilic
anes and 1,3-oxathiolanes, dehydration of tertiary
7g
benzylic alcohols, deprotection of thioacetals and ring
7h
expansion of cyclic thioacetals. We now report that
solid silica chloride in the presence of NaI can be
efficiently employed for the preparation of iodides
under heterogeneous conditions. This system has been
successfully used for direct and highly selective conver-
sion of benzylic, allylic and propargylic TMS, TBDMS
and THP-ethers into their corresponding iodides in
high yields (Scheme 1, Table 1).
2
d–i
substitutions.
The effective use of TMSI for the
conversion of alcohols to the corresponding iodides has
3
been well recognized. Moreover, it has been shown
that chlorosilanes in combination with iodide anion
4
convert alcohols to the iodides via in situ generation of
iodosilanes. In addition, solid supports have found wide
5
applications in organic reactions. They mostly facili-
tate the work-up of the reaction and usually high
selectivity accompanied with high yields of the products
are observed. Silica gel is one of the extensively used6
supports for different purposes in organic chemistry.
Modified silica supports for functional group transfor-
mation are also of interest. Silica chloride has been
reported to be an efficient reagent for the preparation
As shown in Table 1, primary and secondary benzylic
TMS, TBDMS and THP-ethers were easily converted
into their benzylic iodides in good to excellent yields.
Allylic and propargylic TMS, TBDMS and THP-ethers
were smoothly converted to their iodides in good yields.
However, aliphatic TMS, TBDMS, THP-ethers sur-
vived under similar reaction conditions.
7
a
of sulfobenzylsilica, the selective thioacetalization of
In order to show the high selectivity of the method, we
have carried out the successful conversion of a benzylic
TMS-ether in the presence of a secondary aliphatic one
in a single molecule (Scheme 2). The structure of the
product was determined by NMR spectroscopy and
chemical reactions.
Scheme 1.
In summary, this system is efficient, cheap, and easily
available and reacts under mild reaction conditions for
*
0
Corresponding authors.
040-4039/02/$ - see front matter © 2002 Published by Elsevier Science Ltd.
PII: S0040-4039(02)01706-9

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H. Firouzabadi et al. / Tetrahedron Letters 43 (2002) 7139–7141
Table 1. Selective conversion of TMS, TBDMS and THP
ethers into their corresponding iodides by the silica chlo-
ride/NaI system in CH CN
3
Scheme 2.
trimethylsilyl iodide (TMSI) with hazardous vapors.
This system can also be used at elevated temperatures
where required very easily. Work-up of the reaction
mixtures is easy and the reactions are not time-
consuming.
Typical procedure for the conversion of TMS, TBDMS
and THP-ethers into their corresponding iodides: To a
solution of sodium iodide (2.25 g, 10 mmol) in CH CN
3
7e
(
30 mL), silica chloride (1–1.5 g) and the ether (2
mmol) were added and the resulting mixture was stirred
at room temperature. After completion of the reaction
(
Table 1, TLC, n-hexane/EtOAc, 5/1), powdered
Na S O (0.5 g) was added to the mixture (to destroy
2
2
3
the liberated iodine in the reaction mixture) and the
solvent was evaporated under reduced pressure. The
resulting solid mixture was added to a silica gel pad (5
cm thick) and was washed with petroleum ether (bp
6
0–80°C, 150 ml). The resulting solution was evapo-
rated to dryness to afford the almost pure iodide in
good to excellent yields.
Acknowledgements
The authors are thankful to the Shiraz University
Research Council for the support of this work.
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