Titanium complexes with cyanides

Vrachnou, E. Graetzel, M. McEvoy, A. J. Efficient visible light photoresponse following surface complexation of titanium dioxide with transition metal cyanides, J. Electroanal. Chem. Interfacial Electrochem. 1989, 258, 193. 

With regard to PK-type reactions, Buchwald studied titanium species as efficient catalysts in the PKR and in PK-like reactions with cyanides. Following preliminary results with [Cp2Ti(PMe3)2] and [Cp2TiCl2] [79-81], they reported a more practical procedure which improved the TON using commercial titanocene dicarbonyl (33) [82,83]. This complex is able to catalyze the... 

Narasaka has reported that TADDOL-Ti dichloride catalyzes the asymmetric addition of trimethylsilylcyanide to aromatic and aliphatic aldehydes (Sch. 63) [148]. The reactions proceed only in the presence of MS 4A. In reactions with aliphatic aldehydes a chiral cyanotitanium species obtained by mixing of the TADDOL-Ti dichloride and trimethylsilylcyanide before addition of the aldehydes acts as a better chiral cyanating agent and affords higher enantiomeric excesses. Chiral titanium complexes obtained from an alcohol ligand and salicylaldehyde-type Schiff bases and a salen ligand have been reported to catalyze the asymmetric addition of hydrogen cyanide or... 

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-... 

In contrast to the low yield when hydrogen chloride is employed, an 88% yield of 2,4,6-triphenyl- 1,3,5-triazine (7) is obtained when chlorosulfonic acid is used as catalyst in a molecular ratio of 3 1 (CiSOjH/PhCN) at 0-5 C C and a reaction time of 12 to 24 hours.174 Trifluo-romethanesulfonic acid as a catalyst and solvent trimerizes benzonitrile at 91 °C in a yield of 66%.175 Lewis acids alone, such as aluminum, zinc, iron or titanium chlorides, phosphorus pentachloride, and boron trifluoride, have a considerably lower catalytic activity than the corresponding mixtures of Lewis acid with various promotors, such as mineral acids, organic acids and water. These differences are attributed to a change in the structure of the active complexes with the aryl cyanides. 

Two probable systems TiCl and alkyl cyanides form complexes of the type (R-C=N)2TiCl4 [6] and titanium amides (R2N) Ti also complex with the -C=N group [5] as below ... 

The addition of cyanide to imine (Strecker-type reaction) can also be catalyzed by chiral Ti complexes. In a pioneering work reported by Snapper and Hoveyda, a series of titanium complexes of tripeptide-based Schiffbase ligands have been discovered by combinatorial chemistry approach for the addition of TMSCN to imine derivatives with >93% conversion of substrates and excellent enantioselectivities (85-97% ee) [227]. This methodology has provided a practical procedure for the... 

The complications which result from the hydrolysis of alkali metal cyanides in aqueous media may be avoided by the use of non-aqueous solvents. The one most often employed is liquid ammonia, in which derivatives of some of the lanthanides and of titanium(III) may be obtained from the metal halides and cyanide.13 By addition of potassium as reductant, complexes of cobalt(O), nickel(O), titanium(II) and titanium(III) may be prepared and a complex of zirconium(0) has been obtained in a remarkable disproportion of zirconium(III) into zirconium(IV) and zirconium(0).14 Other solvents which have been shown to be suitable for halide-cyanide exchange reactions include ethanol, methanol, tetrahydrofuran, dimethyl sulfoxide and dimethylformamide. With their aid, species of different stoichiometry from those isolated from aqueous media can sometimes be made [Hg(CN)3], for example, is obtained as its cesium salt form CsF, KCN and Hg(CN)2 in ethanol.15... 

An alternative TS for asymmetric addition of cyanide to aldehydes catalysed by titanium-salen complexes has been proposed, based on a comparison with a related iron-salen complex for which a crystal stmcture is reported.252... 

Another way to prepare a reusable catalyst was proposed by Venkataraman. The ligand 25, bearing a linear polyethyleneglycol (PEG) moiety (MW 5 kDa) was used in the titanium-catalysed addition of tri-methylsilyl cyanide to benzaldehyde (Figure 7.1). The silylated cyanohydrin was obtained in more than 95% yield and 86% enantiomeric excess after 24 h at room temperature, with only 0.1 mol% of catalyst. The titanium-salen complex was separated from the reaction mixture by dialysis by means of a Soxhlet apparatus. 

Roll cell. These are sandwich constructions consisting of a packed stainless steel or titanium mesh cathode, separator and a screen anode rolled up like a swiss roll. These cells can operate with fluid velocities of 1-10 cm s and with apparent current densities of 10-200 mA cm at the separator. This type of cell is a concentrator device for metal ions, the metal is recovered from the cell by leaching or by anodic dissolution. An economic analysis [23] showed that waste water treatment with this cell is highly competitive with ion-exchange technology. Typical applications for metal ion removal are recovery of copper and Hg from waste stream recovery of Ag from a used fixer solution down to a silver concentration of 0.1 ppm and the treatment of zinc cyanide plating bath rinse waters which contain the Zn(CN)5 complex ion. 

Other works by the same group on the cyanation of a-ketoesters with acetyl cyanide revealed that, when using a ketone as substrate, a chiral base such as cinchonidine became responsible for enantioselection instead of the chiral titanium(iv) salen complex (Scheme 7.18), leading to the corresponding tertiary cyanohydrins in good enantioselectivities of up to 82% ee. ... 

Reactions with Oxiranes, Oxetanes, and Aziridines. Lewis acids, lanthanide salts, and titanium tetraisopropoxide or aluminum isopropoxide catalyze the reactions of cyanotri-methylsilane with oxiranes, oxetanes, and aziridines, yielding ring-opened products. The nature of the products and the regio-selectivity of the reaction are primarily dependent on the nature of the Lewis acid, the substitution pattern in the substrate, and the reaction conditions. Monosubstituted oxiranes undergo regiospe-cific cleavage to form 3-(trimethylsiloxy)nitriles when refluxed with a slight excess of cyanotrimethylsilane in the presence of a catalytic amount of potassium cyanide-18-crown-6 complex (eqs 8-10). The addition of cyanide occurs exclusively at the least-substituted carbon. 

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