Ytterbium Trifluoromethansulfonate

Introduction

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 AlCl₃, BF₃, TiCl₄, and SnCl₄, 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]

Scheme 1

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.

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