Lanthanides cyanates

Table 2. Molar conductance (A) for the hexachloride and the hexaisothio-cyanate complexes of the lanthanides in nitrobenzene 35, 36)...

Complexes with cyanate were prepared [143] by reaction of anhydrous lanthanide chlorides with an excess of tetraethylammonium cyanate in absolute ethanol. The complex has the formula [(C2H5)4N]3Ln(NCO)6, where Ln = Eu, Gd, Dy, Ho, Er, and Yb and they are indicated to be six-coordinate and cyanate is N-bonded as evidenced by IR spectra [147]. Mixed complexes [(C2H5)4N]3[Ln(NCO)3X3], where Ln = Dy, Er and Yb and X = C1, NCS have been synthesized and they appear to be of six-coordinate [144]. Mixed complexes of the type [(C2H5)4N]3[Ln(NC0)3(N03)3], where Ln = La, Pr, Nd, Sm, Eu, Gd, Er and Yb have been synthesized and the IR spectra shows N-bonded cyanate, bidentate nitrate and the overall coordination number amounting to nine [145]. 

IV. Chalcogenocyanate Complexes of the Transition Elements A. Scandium, Yttrium, Lanthanum, and the Lanthanides i. Cyanates... 

In catalytic processes with enzymes such as D-oxynitrilase and (R) xynitrilase (mandelonitrilase) or synthetic peptides such as cyclo[(5)-phenylalanyl-(5)-histidyl], or in reaction with TMS-CN pro-mot by chiral titanium(IV) reagents or with lanthanide trichlorides, hydrogen cyanide adds to numerous aldehydes to form optically active cyanohydrins. The optically active Lewis acids (8) can also be used as a catalyst. Cyanation of chiral cyclic acetals with TMS-CN in the presence of titanium(IV) chloride gives cyanohydrin ethers, which on hydrolysis lead to optically active cyanohydrins. An optically active cyanohyrMn can also be prepared from racemic RR C(OH)CN by complexation with bru-... 

A silylene-bridged lanthanide complex was introduced by Luo et al. as a new entry of Lewis acid catalyst to cyanation of ketones (31). The complex was prepared from 9-fluorenylmethylphenylsilylene and LnClj by the action of butyUithiirm. In cyanosilylation of acetophenones, Pr complex (6) was found out to be one of the most active catalysts to afford TMS ether up to >99% yield (Scheme 13.13). PrCh itself failed to catalyze the reaction. The reactivity was dependent on the substituent on the aromatic ring. 

Kobayashi et al. disclosed reversed chemoselectivity of lanthanide Lewis acid over conventional Lewis acid in cyanation reaction. The comparitive reaction was performed with equimolar mixture of aldehydes, imines, and TMSCN, affording Strecker-type product exclusively with 20mol% of Yb(OTf)3 [10]. In sharp contrast, SnCU required more than a stoichiometric amount of reagent and gave cyanohydrin preferentially (Table 13.15). 

---