Titanocene borohydrides

The different reactivities of aromatic and aliphatic ketones can be exploited in the same way by carrying out the selective reduction of the latter. Reduction with Zn(BH4)2-1.5 DMF in MeCN or ZnlCNBH lj in EtjO in the presence of trace amounts of water is a good choice [HJl, KK5], as is NaBH4 in /-PrOH or UBH4 in diglyme [PSl] (Figure 3.9). Titanocene borohydride also reduces aliphatic ketones faster than aromatic ones [BS6]. 

Reduction. The reagent is prepared from Cp TiClj and NaBH in DME. The reagent prepared in situ works equally well, and the reduction profile is not changed, even as borane is cogenerated, owing to the superior reactivity of titanocene borohydride. 

Per-acylated pyranoid and furanoid glycosyl bromides were easily reduced in high yield to the corresponding anhydroalditols by catalytic quantities of titanocene borohydride (prepared in situ from Cp2TiCl2 and NaBH4). ... 

The XeChlor process is a reductive dechlorination process for the treatment of a wide array of organic halides, including conversion of polychlorinated biphenyls (PCBs) into biphenyl or dioxins into simple dibenzofuran. The dechlorination catalyst is prepared in situ from titanocene dichloride sodium borohydride is the reducing agent. 

Only a few chiral catalysts based on metals other than rhodium and ruthenium have been reported. The titanocene complexes used by Buchwald et al. [109] for the highly enantioselective hydrogenation of enamines have aheady been mentioned in Section 3.4 (cf. Fig. 32). Cobalt semicorrin complexes have proven to be efficient catalysts for the enantioselective reduction of a,P-unsaturated carboxylic esters and amides using sodium borohydride as the reducing agent [ 156, 157]. Other chiral cobalt complexes have also been studied but with less success... 

Perchloric acid Phosphomolybdic acid Phosphorus oxychloride Phosphorus pentachloride Phosphorus trichloride y-Picoline Polyphosphoric acid Potassium silicate Rhodium Selenium Selenium dioxide Silica gel Silver oxide (ous) Sodium borohydride Sodium silicate Strontium carbonate Sulfur dioxide Tantalum Tellurium Tetraisopropyl di (dioctylphosphito) titanate Titanocene dichloride Trichloromethylphosphonic acid Tristriphenylphosphine rhodium carbonyl hydride Tungsten carbide Vermiculite Ytterbium oxide Zinc chloride Zinc dust Zinc 2-ethylhexanoate Zirconium potassium hexafluoride... 

Sulphurated borohydrides can reduce oximes to amines the intermediate hydroxylamines are isolable. Sodium borohydride reduces nitriles to amines in high yield in the presence of Raney nickel as catalyst. The titanocene-promoted fixation-reduction of molecular nitrogen has been utilized in the conversion of ketones into amides and acid chlorides into nitriles in an overall reductive deoxygenation, as exemplified in Scheme 109. 

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