Introduction <P>Ytterbium trifluoromethansulfonate [Yb(OTf)
3 ] has
been widely used in organic syntheses in the last few years.
[
¹ ]
Yb(OTf)
3 is
a strong Lewis acid
[
² ]
due
to the hard character of Yb
³+ ion and
the presence of electron-deficient triflate in its coordination
sphere. In contrast to traditional Lewis acids, such as AlCl
3 ,
BF
3 , TiCl
4 , and SnCl
4 , which are
often used in stoichiometric amounts, only catalytic amounts of
Yb(OTf)
3 are necessary. Moreover it can be easily recovered
and reused without loss of activity. Interestingly, Yb(OTf)
3 remains
catalytically active in the presence of many Lewis bases containing
nitrogen, oxygen, phosphorus or sulfur atoms. The resulting water-compatibility
of Yb(OTf)
3
[
³ ]
is
one of its well-known advantages, with respect to traditional Lewis
acids that are very sensitive and easily decomposed or deactivated
in the presence of small amounts of water. The most interesting point
from a synthetic point of view is that Yb(OTf)
3 -catalyzed
reactions are clean, while Yb(OTf)
3 is regarded as environmentally
friendly catalyst. Ytterbium triflate is prepared by heating ytterbium(III)
oxide or chloride in an aqueous trifluoromethansulfonic acid solution
(Scheme 1).
[
4 ]
[
5 ]
</P>
Scheme 1
<P>This reagent has been used in numerous organic transformations,
[
¹ ]
e.g. in aldol reactions,
[
6 ]
Kharasch-type additions,
[
7 ]
glycosylations,
[
8 ]
Friedel-Crafts acylations,
[
9 ]
dealkoxyacetylations,
[
¹0 ]
syntheses of β-enaminones,
[
¹¹ ]
etc. This article
describes some major applications in organic synthesis in the recent
years.</P>
