Cadmium borohydride complexes

Zinc-modified cyanoborohydride, prepared from anhydrous zinc chloride and sodium cyanoborohy-dride in the ratio 1 2 in ether, selectively reduced aldehydes and ketones but not acids, anhydrides, esters and tertiary amides. In methanol the reactivity paralleled the unmodified reagent. Zinc and cadmium borohydrides form solid complexes with DMF, which may prove to be convenient sources of the reducing agents.Aromatic and a,p-unsaturated ketones were reduced much more slowly than saturated ketones, so chemoselective reduction should be possible. 

These two complex hydrides have been used also in inorganic syntheses. Lithium aluminum hydride may be used to prepare unstable hydrides in ether at low temperatures from the appropriate halides cadmium hydride and mercury hydride (Chap. 2) have been so prepared. A number of additional borohydrides, aluminum hydrides, and even a gallohydride (LiGaH4) have been reported. 

The easily prepared, stable solid reagent diphenylamine-borane (Ph2NH BH8) has been shown to be more reactive than aliphatic amine-boranes and almost as reactive as borane-THF for the reduction of ketones acids are also reduced to alcohols. Polyethylene glycols (PEG) catalyse the reduction of ketones by sodium borohydride under phase-transfer (PT) conditions, for example in solid-liquid PT with PEG as solvent. The solid zinc borohydride-dimethylformamide complex reduces aldehydes and ketones to alcohols, but only one hydrogen atom from each tetrahydridoborate unit is utilized. The different rates of reduction of various classes of ketone (saturated aliphatic faster than aromatic, and a -unsaturated very slow) suggest a possible selectivity between ketones. The corresponding cadmium complex, prepared in situ, reacts similarly. Lithium methylborohydride, LiMeBHj, prepared as shown in equation (1), where... 

Other Preparations.—Carboxylic acids have been converted into aldehydes through di-isobutylaluminium hydride reduction of 3-acylthiazolidine or 2-thiazoline-2-thiol ester intermediates. Bis(triphenylphosphine)copper(l) tetrahydroborate, (Ph3P)2CuBH4, shows promise as a new reagent for the reduction of acid chlorides to aldehydes. The same conversion can be accomplished using sodium borohydride in a mixture of acetonitrile and hexamethyl-phosphoramide containing a cadmium(il) chloride-dimethylformamide complex. ... 

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