Cyanate, metal carbonyl complexes

Transition metal organometallic complexes like dicarbonyl cyclopentadienyl iron [128], tricarbonyl cyclopentadienyl manganese [129] and iron-arene complexes [130,131] have also been reported as photoinitiators for photochemical crosslinking of cyanate esters. Photosubstitution of carbonyl groups by -OCN during irradiation initiates the reaction in the former case whereas photochemical dissociation of arene triggers it in the latter system. 

Optically active cyanohydrins can be easily transformed to P-hydroxy amines, a-hydroxy and a-amino carboxylic acids, which represent versatile intermediates for the synthesis of biologically important compounds, including insecticides and medicines [189, 190]. Asymmetric cyanation of carbonyl compounds catalyzed by chiral metal complexes, particularly titanium compounds, has provided one of the most convenient protocols to the access of these type of compounds. The first example of catalytic asymmetric cyanation of aliphatic aldehydes was realized in Reetz s group using BINOL-Ti complex as the catalyst to give the cyanohydrins in up to 82% ee [104] (Scheme 14.85). 

Since the addition of HCN to carbonyl compounds proceeds in the presence of a catalytic base, asynunetric cyanation should also be possible by using metal alkoxides such as boron, aluminum, titanium, zirconium, and lanthanoids through modification with an optically active substituent or ligand. Mori and Inoue reported that the titanium complex of an acyclic dipeptide composed of... 

The currently accepted view of the oxidation state of the metal centers in the di-iron subsite in the enzyme resting state is that of distal iron(I) and proximal iron(II). The simple dinuclear complexes that best model the structure of the di-iron subsite are di-iron carbonyls bridged by a tripodal thiolate ligand. These complexes may undergo two successive cyanation steps to yield a complex that structurally resembles closely the CO-inhibited form of the enzyme (Fig. 4a). 

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