Triflide

Against this background it is important that—quite fitting in this still new millennium— the first catalytic Friedel-Crafts acylations of (still relatively electron-rich) aromatic compounds were reported (Figure 5.35). Trifluoromethane sulfonates ( triflates ) of rare-earth metals, e. g., scandium(III)triflate, accomplish Friedel-Crafts acylations with amounts of as little as 1 mole percent. Something similar is true of the tris(trifluoromethanesulfonyl)-methides ( triflides ) of rare-earth metals. Unlike conventional Lewis acids, the cited rare-earth metal salts can form 1 1 complexes with the ketone produced, but these are so unstable that the Lewis acid can re-enter the reaction. Whether this works analogously for the third catalytic system of Figure 5.35 is unclear. 

A related compound scandium tris(trifluoromethylsulphonyl)methanide, Sc[(F3CS02)3C]3 (scandium triflide) has been prepared and used as a catalyst in the nitration of o-nitrotoluene.209 Scandium triflide has proved to be a better catalyst than scandium triflate for the aromatic nitration of the electron deficient o-nitrotoluene. 

The literature preparation of the triflide anion is non-trivial involving handling of gases and non-commercial materials. Accordingly, we have recently developed a novel and convenient synthesis of the triflide anion from readily available triflic anhydride and trimethylsilylmethyllithium (Scheme 6).33... 

Scheme 6 Preparation oftriflidic acid and ytterbium(IIl) and scandium(III) triflide...

Ytterbium(III) tris(trifluoromethanesulfonyl)methide (ytterbium(III) triflide) was applied in a catalytic amount to catalyse FC processes. Only 1 mol% of ytterbium (III) triflide at 80 C allowed acylation of thiophene and 3-methylthiophene with acid anhydrides [35]. Acylation of thiophene led exclusively to 2-substitution 3-methylthiophene gave a mixture of 2-acylated (8) and 5-acylated (9) products (Scheme 15) [35]. 

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