Scandium trifluoromethanesulfonate as a recyclable catalyst for efficient methoxymethylation of alcohols

Article abstract of DOI:10.1016/S0040-4039(03)01481-3

Scandium triflate [Sc(OTf)₃] has been found to be a recyclable catalyst for mild highly efficient methoxymethylation of a variety of alcohols using formaldehyde dimethyl acetal (FDMA).

Full text of DOI:10.1016/S0040-4039(03)01481-3

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 6051–6053
Scandium trifluoromethanesulfonate as a recyclable catalyst for
efficient methoxymethylation of alcohols
Babak Karimi* and Leyla Ma’mani
Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), PO Box 45195-159, Gava Zang,
Zanjan, Iran
Received 31 January 2003; revised 2 June 2003; accepted 13 June 2003
Abstract—Scandium triflate [Sc(OTf)3] has been found to be a recyclable catalyst for mild highly efficient methoxymethylation of
a variety of alcohols using formaldehyde dimethyl acetal (FDMA).
© 2003 Published by Elsevier Ltd.
Although tetrahydropyranylation is a commonly used
method for the protection of hydroxy functions,1
methoxymethylation has the advantage over tetra-
hydropyranylation in many instances because the latter
involves the formation of a new asymmetric center,
thus forming diastereomers when it is used for chiral
alcohols.2 Methoxymethyl (MOM) derivatives are fre-
quently prepared by alkylation of alcohols with an
excess of chloromethyl methyl ether in alkaline solu-
tion.1,3 Because of the potent carcinogenic properties of
chloromethyl methyl ether,4 efforts were undertaken to
find an alternative safe method for the preparation of
MOM ethers. A few reports based on formaldehyde
dimethyl acetal (FDMA), an inexpensive reagent, have
appeared in the literature utilizing various catalysts
such as P2O5,5 p-toluenesulfonic acid,6 Nafion-H,7
TMSI,8 molybdenum(VI) acetyl acetonate,9 BF3,10
posed or deactivated in the presence of water or protic
solvents, Sc(OTf)3 is stable and works as a Lewis acid
even in aqueous solutions.17 Thus, unlike other tradi-
tional Lewis acids, Sc(OTf)3 does not decompose under
aqueous work-up conditions and its recycling is often
possible. This makes Sc(OTf)3 relatively environmen-
tally acceptable as a catalyst in functional group trans-
formations. This catalyst has been successfully used for
a wide range of synthetic organic reactions,16 especially
as a Lewis acid in aldol condensation reactions,18
Diels–Alder reactions,19 Friedel–Crafts acylation,20
Michael reactions,21 and Fries rearrangements.22 There
is no report of the application of Sc(OTf)3 as a catalyst
for direct methoxymethylation of alcohols. In this
paper, we wish to disclose an efficient method for
preparing MOM ethers from alcohols by employing
formaldehyde dimethyl acetal (FDMA) using a cata-
lytic amount of Sc(OTf)3.23 Table 1 summarizes the
results and shows the efficiency of the present protocol
for both activated and deactivated benzyl alcohols
including sterically hindered ones such as 2,6-
dichlorobenzyl alcohol (entries 1–6).
Envirocat,11 sulfated zirconia,12 expansive graphite,13
14
,
and FeCl3 dispersed on molecular sieves 3 A. Very
recently, it has also been shown that methoxymethyl-2-
pyridyl sulfide (MOM-ON) is an efficient and neutral
methoxymethylating reagent when used in conjunction
with AgOTf, NaOAc and THF.15 However, although
these methods have shown an improvement in many
instances, many of them suffer from drawbacks such as
using stoichiometric amounts of expensive reagents,15
strong protic acid conditions or destruction of the
catalyst during the work-up procedure. Recently, scan-
dium triflate has been introduced as a promising mild
and selective reagent for a variety of functional group
transformations.16 While most Lewis acids are decom-
Primary, as well as secondary alcohols effectively
reacted with an excess of FMDA and 5 mol% of
Sc(OTf)3 in refluxing CHCl3 to afford the correspond-
ing MOM ethers in good to excellent yields (Table 1,
entries 7–13). It is worth mentioning that by employing
this protocol even hindered tertiary alcohols such as
1-adamantanol and 1-methyl cyclohexanol also fur-
nished the corresponding MOM-ethers in satisfactory
yields (Table 1, entries 14, 15). Neither double bond
migration nor elimination was observed in the case of
either allylic or tertiary alcohols (Table 1, entries 12–
* Corresponding author. Tel.: +98-241-4248013; fax: +98-241-
4249023; e-mail: karimi@iasbs.ac.ir
0040-4039/$ - see front matter © 2003 Published by Elsevier Ltd.
doi:10.1016/S0040-4039(03)01481-3

6052
B. Karimi, L. Ma’mani / Tetrahedron Letters 44 (2003) 6051–6053
Table 1. Methoxymethylation of alcohols using FDMA in
the presence of Sc(OTf)3
In summary, we have demonstrated the use of Sc(OTf)3
for efficient methoxymethylation of a variety of alco-
hols under mild reaction conditions where the catalyst
could be readily recovered and reused thus making the
procedure environmentally acceptable. Further applica-
tions of Sc(OTf)3 especially based upon designing a new
auto-recycling catalytic system are currently underway
in our laboratory.
Acknowledgements
The authors acknowledge the Institute for Advanced
Studies in Basic Sciences (IASBS) Research Council for
support of this work.
References
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14). However, our preliminary examination showed
that the method is not applicable for protection of
hydroxy groups in phenols, because mixtures of uniden-
tified products were formed even at room temperature.
In this protocol, we have also found that Sc(OTf)3 can
be effectively recovered from the reaction mixture dur-
ing the work-up procedure by simple evaporation of the
aqueous phase and drying the solid residue at 90°C
overnight. The percentage recovery in most cases was
more than 91%. Moreover, efficient methoxymethyla-
tion of 4-nitrobenzyl alcohol using recovered Sc(OTf)3
clearly shows that the reactivity of the recovered cata-
lyst does not change considerably after the work-up
(Table 1, entry 4). This undoubtedly renders the above
protocol environmentally acceptable.
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B. Karimi, L. Ma’mani / Tetrahedron Letters 44 (2003) 6051–6053
6053
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ml). The aqueous layer was separated and evaporated
under reduced pressure to afford the recycled catalyst.
The organic layer was washed successively with a 10%
aqueous solution of NaOH (25 mL) and water (15 ml)
and dried over anhydrous Na2SO4. Evaporation of the
solvent under reduced pressure gave the almost pure
MOM ether. Further purification of products could be
achieved by vacuum distillation or recrystallization to
afford the corresponding pure MOM ethers (Table 1).
23. General procedure for the preparation of MOM ethers:
To a solution of alcohol (2 mmol) and FDMA (100
mmol) in CHCl3, Sc(OTf)3 (0.1 mmol) was added and the
mixture was stirred at reflux until complete disappearance
of starting alcohol (as monitored by TLC). After comple-
tion, the reaction was quenched with distilled water (20
mL), and the mixture was extracted with CHCl3 (2×30