Scandium complexes tris

The Lewis acid-promoted reaction of aldehydes with a-substituted allylsilanes forms 3-silyltetrahydrofurans in good to high yields.169-172 The use of homochiral allylsilanes is very valuable for highly diastereo- and enantio-selective syntheses of tri- and tetrasubstituted tetrahydrofurans (Equation (43)). Catalytic asymmetric [3-1-21-cycloaddition of a-substituted allenylsilanes to aldehydes can be achieved by a chiral scandium complex.173... 

SCANDIUM TRICHLORIDE TRIS(TETRAHYDROFURAN) AND 0-DIKETIMINATE-SUPPORTED SCANDIUM CHLORIDE COMPLEXES... 

The first example of a group 3 NHC complex was reported by Arduengo et al. in 1994 with yttrium (Scheme 14.1) [12]. The seven-coordinate yttrium carbene complex 1 was obtained from the direct reaction of the free carbene 1,3-dimethyl-4,5-dimethylimidazol-2-ylidene with tris(2,2,6,6-tetramethylheptane-3,5-dionato)yttrium, Y(thd)3 and features a NMR doublet resonance for the carbene atom (d 199.4 ppm) with a coupling constant of 33 Hz to the nucleus. The first scandium NHC complexes were only recently described. Cui and coworkers prepared indenyl-functionaUzed NHC carbene scandium complex 2 in 2007 [13,14]. Complex 2 could be structurally characterized by X-ray... 

Analogous methyl compoimds are iU-characterized and appear to be polymeric [129). The air-sensitive phenyl derivatives when first obtained from THF are soluble in benzene, but when dried completely are no longer soluble — apparently due to pol5unerization. The homoaryl complexes of the smaller scandium, and yttrium ions form only the tris complexes Sc(CeH5)3 and Y(C6H5)3 [129) It is apparent that the structure and stabihty of the homoaryls are dependent on the metal ionic radii and the steric bulkiness of the phenyl group. 

Structural analysis of several octahedral Sc(III) complexes have recently been reported. The structure 44) of anhydrous scandium-tris-acetylacetonate, Sc(acac)3, consists of discrete Sc(CH3COCHCOCH3)3 entity, with slight deviation from Dz symmetry. The coordination polyhedron around Sc (III) is a distorted... 

A series of trivalent lanthanoid complexes, scandium, and ytterbium tris-(R)-(-)-1,1 -binaphthyl-2,2 -diyl phosphonate, have been introduced as new chiral and stable Lewis acids for the asymmetric hetero Diels-Alder reaction of Danishefsky s diene and aldehydes. 2,6-Lutidine was found to be an effective additive to improve the enantioselectivity up to 89% ee [66] (Eq. 8A.42). 

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. 

Tredget, C.S., Lawrence, S.C., Ward, B.D. et al. (2005) A family of scandium and yttrium tris((trimethylsilyl)methyl) complexes with neutral N3 donor Ugands. Organometallics, 24, 3136. 

Many of the early reports of complexes with bidentate oxygen ligands involved acac complexes and the /3-diketonate and related ligands were discussed in CCC (1987).210 The /3-diketonate complexes of scandium and yttrium and the lanthanides were also discussed in CCC (1987).1 Many reported systems contained adducts formed by the metal /3-diketonate complexes. In solution some of the complexes with bidentate ligands can expand their coordination number to seven by coordination to a suitable donor ligand. A solvent extraction method has been used to show that Sc(acac)3 does not form an adduct with trioctylphosphine oxide but tris(tropolonato)scandium(III) and tris-(ethylmatolato)scandium(III) do coordinate with trioctylphosphine oxide.211... 

SgO, cycto-Octasulfur monoxide, 21 172 ScCl303Ci2H24, Scandium(IU), trichloro-tris(tetrahydrofuran)-, 21 139 SeC, Carbon selenide, chromium complex, 21 1, 2... 

ScCI3, Scandium chloride, 22 39 ScCl3Cs, Cesium scandium chloride, 22 23 ScC1303C 2Hm, Scandium(lll), trichloro-tris(tetrahydrofuran)-, 21 139 Sc2CI,Cs3, Cesium scandium chloride, 22 25 SeC, Carbon selenide, chromium complex, 21 1, 2 Se2C, Carbon diselenide, 21 6, 7 Se2NC3H,i, Diselenocaibamic acid, N,N-di-ethyl-,... 

HEAT CAPACITY, ENTROPY. ENTHALPY. FREE ENERGY. AND HEAT OF FUSION OF THE TRIS/ACETYLACETONATO/ COMPLEXES OF SCANDIUM/III/. VANADIUM/III/. MANGANESE/III/. 

Finally some homoleptic alkyne complexes of scandium and some rare earths are known. Tris(phenylethynyl) scandium is formed as a dark brown product after treatment of phenylethyne with butyl lithium at — 78°C in hexane, followed by the addition of scandium trichloride at room temperature. The compound is inflammable in air (Hart and Saran, 1968 Hart et al., 1970). The reaction of the anionic... 

---