Vanadium complexes silyl

Up to the end of 1972, more than 350 compounds containing silicon-transition-metal bonds had been synthesized, but their distribution among the d-block elements is by no means uniform. It can be inferred that the stability of such complexes decreases from right to left in a transition series and with increasing oxidation state of the transition metal. Nevertheless, it is remarkable that no silyl derivatives of the vanadium group have been prepared and that there are few nickel or palladium derivatives, as can be seen in the complete list of compounds given in Section 6. 

Silylation of Na[V(CO)2(Me2PCH2CH2PMe2)2] affords the vanadium carbyne complex see Carbyne Complexes), V(COSiPh3)(CO)(Me2PCH2CH2PMe2)2, which is supposed to be a precursor complex for the formation of a ] -acetylene... 

The 1,2-addition of a cyanide ion to a carbonyl compound to form a cyanohydrin is a fundamental carbon-carbon bond-forming reaction in organic chemistry, and has frequently been at the forefront of advances in chemical transformations. In 2000, Belokon and North developed a catalytic system based on a vanadium-salen complex (Scheme 9.1). The synthesis of vanadium(iv) complex 1 was accomplished by refluxing a mixture of the corresponding Schiff base with vanadium(iv) sulfate and pyridine in ethanol under an argon atmosphere. A very low catalyst loading of 0.1 mol% was employed to convert aromatic and aliphatic aldehydes to cyanohydrin silyl ethers 3 with enantioselectivities of 68-95% after 24 h. Further investigations... 

Niobium lies directly below vanadium in Group 5 and thus is expected to have high Lewis acidity however, there are few reports on chiral niobium catalysts. This is in contrast to the various asymmetric transformations catalysed by neighbouring Group 4 metal complexes, i.e., titanium and zirconium. In 2005, Kobayashi first reported highly enantioselective niobium Lewis-acid catalysts for the Mannich-type reaction of aldimines 57 with silyl enol ethers 58. To prepare an efficient chiral pentavalent niobium(v) catalyst for the activation of aldimines 57, Kobayashi designed tridentate ligand 56 (Scheme 9.24). 

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