Yttrium complexes addition

The addition of trimethylsilyl (TMS) cyanide to aldehydes produces TMS-protected cyanohydrins. In a recent investigation a titanium salen-type catalyst has been employed to catalyse trimethylsilylcyanide addition to benzaldehyde at ambient temperature1118]. Several other protocols have been published which also lead to optically active products. One of the more successful has been described by Abiko et al. employing a yttrium complex derived from the chiral 1,3-diketone (41)[119] as the catalyst, while Shibasaki has used BINOL, modified so as to incorporate Lewis base units adjacent to the phenol moieties, as the chiral complexing agent11201. 

Since the Ms hydrides may have interesting chemical and physical properties in addition to their remarkable structure, it was desirable to have a high-yield synthesis of these species. Investigation of the reactivity of the yttrium complexes [(CsH5)2Y(/i-H)(thf)]2 with a variety of lithium reagents has led to a synthesis of [(CsHs)2YH]3H Li(thf)4 in 75% yield according to Eq. (19) (56). 

The concept may be extended by additional coordination of weakly binding peripheral donor functions to the metal centre, as exemplified by the yttrium complex displayed in Fig. 5. The 2-fluorophenyl groups provide an "active" ligand periphery which stabilizes the highly acidic Y-center [12],... 

Reaction of pyridine with the corresponding fluorenyl(hydrido)yttrium complex [ (3,6-But2Flu)SiR2NBut Y(/r-H)(THF)]2 selectively gave the 1,4-addition product, which was characterized by single crystal X-ray diffraction (Scheme 179).676... 

In addition to this wide range of uses for yttrium, new applications of yttrium complexes are under continual development. Examples include hydrate with halide and other simple ligands, with oxygen donor ligands with lanthanides, or mixed (e.g., metal diyttrium-barium and yttrium-copper), with nitrogen donor ligands, with... 

W.-X. Zhang, M. Nishiura, Z. Hou, Catalytic addition of secondary amines to carbodiimides by a halfsandwich yttrium complex an efficient route to N,N, N",N"-tetrasubstituted guanidines. Syrdett, 2006 1213. 

The reaction reportedly takes place without the addition of base. The corresponding yttrium complexes in which Q = CH3, C2H5, n-CsHy, and i-CjH were prepared by the same workers at a later date but it was found that the ligand binds in the neutral form (Kuma and Yamada, 1975). From the analyses and conductivity measurements it was concluded that the compounds should be formulated as [Y(LH)3Cl2]Cl. The only apparent synthetic differences were that the reaction with the lanthanides was carried out at temperatures less than 50°C in the absence of a solvent whereas the reaction with yttrium took place in refluxing ethanol. 

Recently, the intermolecular Markovnikov addition of simple aliphatic primary amines to unactivated 1-alkenes was achieved using the binaphtholate yttrium complex 63a-Y (Fig. 15) [68]. The reaction requires high temperatures and a 9-15-fold excess of the alkene was employed (Table 20). Secondary amines and internal alkenes are imreactive moreover, sterically hindered terminal olefins also displayed significantly diminished reactivity (Table 20, entry 5). Enantioselectivities of up to 61% ee were achieved despite the harsh reaction conditions. 

In addition to the systems just mentioned, recent kinetic and mechanistic studies have included those involving copper(II) (409,410) and zinc(II) (411) species, various binuclear metal(II) complexes of first row transition elements (412-414), especially iron (407), cobalt (415), copper (305,416), and zinc (417,418), yttrium (419,420) and lanthanide (421,422) species, and thorium(IV) (423). 

A catalytic asymmetric epoxidation reaction of a,/l-unsaturated esters via conjugate addition of an oxidant using chiral yttrium-biphenyldiol-Ph3As=0 complexes has been developed. Yields up to 90% and ees up to 99% have been achieved.43... 

Use of less basic triphenylsiloxide ligands also allowed the isolation of mononuclear complexes (Table 1) [46,47]. In addition to the silylamide route, anhydrous nitrates and isopropoxides were employed as synthetic precursors (Eqs. 1,2). The siloxide bridges in the solvent-free dimeric systems Ln2(OSiPh3)6 are readily disrupted by donor solvents like THF, OP Bu3 or DME. The m s-THF adducts of lanthanum, cerium (Fig. 5) and yttrium adopt an approximately /ac-octahedral geometry. 

A mixed metal siloxide complex YCu(OSiPh3)4(PMe2Ph) was obtained by a simple addition reaction (Eq. 3) [48], The geometry around the yttrium atom is distorted tetrahedral. The molecule is cleaved in solution by Lewis basic solvents like THF. 

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