Scandium crown ether complexes

Although some crown ether complexes of scandium were made at the time that the lanthanide complexes were first made, it is only recently that they have been investigated systematically and definitive structural information has become available, mainly due to Willey and co-workers... 

In contrast, the 1.8 A diameter of the cavity in the tetradentate 12-crown-4 ring is too small for a [ScCy ion to fit, so an extra-cavity (half-sandwich) structure (12) has been suggested for [ScCl2(12-crown-4)(MeCN)] SbCl6, though no diffraction data are available. Unlike the intracavity crown ether complexes already mentioned, a MeCN is believed to coordinate, again reflecting scandium s preference for seven-coordination. ... 

Using this strategy the first scandium trimethyl complex was successfully isolated by the reaction of ScCb with three equivalents of MeLi in the presence of aza-crown ether (Figure 8.9) [33]. 

In recent years, many chiral catalysts for the enantioselective synthesis of optical active 1,5-dicarbonyl compounds have been developed, such as chiral crown ethers with potassium salt bases and chiral palladium complexes, including bimetallic systems. Nakajima and coworkers reported on enantioselective Michael reactions of S-keto esters to a,/3-unsaturated carbonyl compounds in the presence of a chiral biquinoline N,N dioxide-scandium complex, which catalyzed the additions in high yields and with enan-tioselectivities up to 84% ee . Kobayashi and coworkers found that the combination of Sc(OTf)3 with the chiral bipyridine ligand 149 (equation 41) was also effective as a chiral catalyst for asymmetric Michael additions of 1,3-dicarbonyl compounds 147 to a,/3-unsaturated ketones 148. The corresponding Michael adducts 150 were obtained in good to high yields with excellent enantiomeric excesses in most cases (Table 10). 

Crown ethers and the lanthanide complexes were extensively covered in CCC (1987)218 but complexes of scandium and yttrium were not mentioned with the exception of [Y(NCS)3(18-crown-6].1... 

A novel scandium complex formed from scandium triflate in the presence of 18-crown-6 and p-sulfonatocalix[4]arene has been investigated.224 The crown ether resides in cavities created by two calixarenes from adjacent polymeric sheets. There are two types of scandium ions in the complex, one type is bound to the phenolic oxygen of the calixarene and the second exists as the hydroxide bound dimer ion [Sc(OH)2(H2O)10]4+. A similar complex may also be prepared without the crown ether. 

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