Catalysis scandium

Finally, Inanaga reported a rare example of scandium catalysis with a preformed complex bearing a fluorinated axial chiral phosphonate. This fluorination of 3-keto esters employed N-fluoro pyridinum triflate as reagent, a rare case in which NFSl did not represent the superior reagent [46]. 

In the very recent past, metal complex catalysis has been used with advantage for the stereo- and enantio selective syntheses based on the Henry and Michael reactions with SENAs (454-458). The characteristic features of these transformations can be exemplified by catalysis of the reactions of SENAs (327) with functionalized imides (328) by ligated trivalent scandium complexes or mono-and divalent copper complexes (454) (Scheme 3.192). Apparently, the catalyst initially forms a complex with imide (328), which reacts with nitronate (327) to give the key intermediate A. Evidently, diastereo- and enantioselectivity of the process are associated with preferable transformations of this intermediate. 

The enantioselective addition of ally organometallics to carbonyls has become one of the workhorses of organic synthesis. Dennis Hall of the University of Alberta reports (J. Am. Chem. Soc. 125 10160, 2003) the scandium triflate catalysis chiral allylboronic acids become more effective tools. The best of these, the Hoffmann camphor derivative 2, adds to aldehydes under Sc(OTf), catalysis with excellent enantiomeric excess. The reaction works equally well for methallyl, and for the E and Z crotyl boronic acids. The crotyl derivatives react with the expected high diastereocontrol. A limitation to the boronate additions is that branched chain aldehydes give low yields. 

Giuseppone, N., Schmitt, J.-L., Schwartz, E., Lehn, J.-M. Scandium(III) catalysis of transim-ination reactions. Independent and constitutionally coupled reversible processes. J. Am. Chem. Soc. 2005,127, 5528-5539. 

Giuseppone N, Schmitt J-L, Schwartz E, Lehn J-M (2005) Transaminations under scandium triflate catalysis. Independent and constitutionally coupled reactions. J Am Chem Soc 127 5528-5539... 

As already described in Sect. 2.2, the use of scandium(III) ions is also an efficient way to catalyze transimination reactions. The most effective catalysis was observed with the exchange reaction between an oxime of cyclohexanone and benzylhydroxylamine. Interestingly, crossover experiments mixing hydrazones and oximes also proved successful [24]. 

The surfactant-aided Lewis acid catalysis was first demonstrated in the model reaction shown in Table 13.1 [22]. While the reaction proceeded sluggishly in the presence of 10 mol% scandimn triflate (ScfOTOs) in water, a remarkable enhancement of the reactivity was observed when the reaction was carried out in the presence of 10 mol% Sc(OTf)3 in an aqueous solution of sodium dodecyl sulfate (SDS, 20 mol%, 35 mM), and the corresponding aldol adduct was obtained in high yield. It was found that the type of surfactant influenced the yield, and that Triton X-100, a non-ionic surfactant, was also effective in the aldol reaction (but required longer reaction time), while only a trace amount of the adduct was detected when using a representative cationic surfactant, cetyltrimethylammonium bromide (CTAB). The effectiveness of the anionic surfactant is attributed to high local concentration of scandium cation on the surfaces of dispersed organic phases, which are surroimded by the surfactant molecules. 

Ward, B.D., Bellemin-Laponnaz, S., and Gade, L.H. (2005) C3 chiraUty in polymerization catalysis a highly active dicationic scandium(III) catalyst for the isoselective polymerization of 1-hexene. Angewandte Chemie International Edition, 44, 1668. 

The enantioselective total synthesis of (-)-hemiasterlin, a marine tripeptide with cytotoxic and antimitotic activity, was achieved by E. Vedejs and co-workers. The asymmetric Strecker reaction was used to construct the key tetramethyltryptophan subunit. The aldehyde substrate was first converted to the corresponding chiral imine with (R)-2-phenylglycinol under scandium triflate catalysis. The addition of tributyltin cyanide resulted in the formation of a-amino nitriles as an 8 1 mixture of diastereomers. Subsequently the cyano group was converted to a primary amide, and the chiral auxiliary was removed under catalytic hydrogenation conditions. 

The trityl borate [Ph3C][B(C6Fs)4] did not cause CeFs back transfer. However, if the coordination site on Sc had sufficient open, the solvent molecules would coordinate to the metal center to form solvent-separated ion pairs (SSIPs), which were inactive in catalysis of polymerizations. A series of scandium methyl SSIPs 124-128 stabilized by the relatively less steric effect P-diketiminato L24 were prepared by reacting 120 with [Ph3C][B(C6Fs)4] in an arene solvent. The synthetic route is shown in Scheme 41 and the complexes were studied for the relevance to propagation processes in the polymerization of alkene. Competition... 

On the basis of biphasic catalysis concepts, Kobayashi and coworkers recently developed a heterogeneous catalyst consisting of a silica-supported scandium loaded with a hydrophobic IL, for organic reactions in water/ ... 

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