Scandium , ligand exchange

The thenoyltrifluoroacetate (tta) complex, [Sc tta)3 OP(OBu)3)], has been prepared. Scandium also afforded, with l,2-dihydroxybenzene-3,5-disodium sulphonate (Tiron), a monoprotonated chelate below pH 2.5, but above pH 6.0 hydrolysis afforded a monohydroxo-Sc-Tiron complex. Ligand-exchange equilibria for the piperidium salts of [Y(ffac)4] and [Y(tfac)4] have been studied by n.m.r. spectroscopy. The fully fluorinated ligands appear to exchange faster than the partially fluorinated species, and the mixed compounds, [Y(Hffac) (tfac)4 ] , n = 0 — 4, were obtained. 

Scheme 12.10 Synthesis of scandium methyl surface species by consecutive ligand exchange (cf route C in Scheme 12.3).

Polymeric catalysts. A very useful modification of the catalyst is obtained by ligand exchange with a (polyallyl)triflamide, forming polymer-bound scandium triflamide bistriflate. The catalyst has been used in the combinatorial synthesis of a tetrahydroquinoline library from anilines, aldehydes, and alkenes. A related catalyst prepared from ScClj-bHjO and Nafion is effective in several useful synthetic reactions, including allylation, Diels-Alder reaction, and Friedel-Crafts acylation. ... 

PisanieUo DL, Lincoln SF. A proton magnetic resonance study of ligand exchange on the hexakis(l, 1,3,3-tetramethylurea)scandium(III) ion and its N,N-dimethylacetamide analogue. J Chem Soc Dalton Trans. 1980 699—704. 

Anwander and cowotkers (Gerstberger et al., 1999) immobilized rare-earth complexes on the mesopororrs silicate MCM-41 by first grafting [R N(SiHMe2)2 3(thf) c] onto MCM-41, followed by surface-confined ligand exchange with Hfod. This procedure yielded [MCM-41]R(fod) c(thf)3, (R = Sc, Y, La). The performance of this catalyst was tested in the hetero Diels-Alder reaction between trans-l-methoxy-3-trimethylsilyloxy-l,3-butadiene and ben-zaldehyde. A comparable activity was found for the yttiium(III) and scandium(III) com-... 

In this reaction, a C—H bond in methane is made, and a C—H bond of benzene is cleaved, but the oxidation state of the scandium center remains +3. This class of reaction, which is not hmited to early transition metals, is called sigma-bond metathesis. In this mechanism, the metal is first postulated to coordinate the bond to be activated in an rf- fashion, followed by formation of a four-centered transition state that leads to an exchange of ligands at the metal (Figure 14.8). 

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