Borohydride, zinc

Other Borohydrides. Potassium borohydride was formerly used in color reversal development of photographic film and was preferred over sodium borohydride because of its much lower hygroscopicity. Because other borohydrides are made from sodium borohydride, they are correspondingly more expensive. Generally their reducing properties are not sufficiently different to warrant the added cost. Zinc borohydride [17611-70-0] Zn(BH 2> however, has found many appHcations in stereoselective reductions. It is less basic than NaBH, but is not commercially available owing to poor thermal stabihty. It is usually prepared on site in an ether solvent. Zinc borohydride was initially appHed to stereoselective ketone reductions, especially in prostaglandin syntheses (36), and later to aldehydes, acid haHdes, and esters (37). 

In the early work on the synthesis of prostaglandins, zinc borohydride was used for the reduction of the 15-ketone function and a 1 1 mixture of epimeric 15(S)- and 15(/ )-alcohols was generally obtained. Subsequent studies led to reaction conditions for highly selective reduction to the desired 15(S)-alcohol. Some of the results are summarized in the following table. The most practical method is E which utilizes borane as the stoichiometric reductant and a chiral, enzyme-like catalyst which is shown. 

Pr)4, " borohydride-exchange resin,and formic acid. When the last is used, the process is called the Wallach reaction. Conjugated aldehydes are converted to alkenyl-amines with the amine/silica gel followed by reduction with zinc borohydride.In the particular case where primary or secondary amines are reductively methylated with formaldehyde and formic acid, the method is called the Esch-weiler-Clarke procedure. It is possible to use ammonium (or amine) salts of formic acid, " or formamides, as a substitute for the Wallach conditions. This method is called the Leuckart reaction,and in this case the products obtained are often the N-formyl derivatives of the amines instead of the free amines. Primary and secondary amines can be iV-ethylated (e.g., ArNHR ArNREt) by treatment with NaBH4 in acetic acid. Aldehydes react with aniline in the presence of Mont-morillonite KIO clay and microwaves to give the amine. Formaldehyde with formic acid converts secondary amines to the N-methyl derivative with microwave irradiation. 

Zinc borohydride, which is also a useful reagent,66 is prepared by reaction of ZnCl2 with NaBH4 in THF. Owing to the stronger Lewis acid character of Zn2+, Zn(BH4)2 is more reactive than NaBH4 toward esters and amides and reduces them to alcohols and amines, respectively.67 Zn(BH4)2 reduces carboxylic acids to primary alcohols.68 The reagent also smoothly reduces a-aminoacids to (3-aminoalcohols.69... 

Zinc borohydride has been found to effect very efficient reductive amination in the presence of silica. The amine and carbonyl compound are mixed with silica and the powder is then treated with a solution of Zn(BH4)2. Excellent yields are also obtained for unsaturated aldehydes and ketones.96... 

Many borohydrides are highly unstable and have to be used as freshly prepared ethereal solutions. However there are instances where the polymer-supported versions are more stable e. g. an Amberlyst anion exchange resin supported borohydride and cyanoborohydride [61], polyvinylpyridine supported zinc borohydride [62] and the corresponding zirconium borohydride [63]. Such compounds, in their labeled forms, should turn out to be very useful. 

TABLE 3-3. Asymmetric Acylation Using (2A,5A)-24 and Subsequent Stereoselective Reduction with Zinc Borohydride in THF at —78°C... 

If the perhydrooxazolo[3,2- ]pyridine now possesses a nitrile at C-5, the aminonitrile can be reduced with sodium borohydride as exemplified in Scheme 89 <1998T8783>. Another option is to generate an intermediate iminium ion and to further reduce it with zinc borohydride <1998T8783>. 

The high-yield reductive methylation of numerous alkyl and arylamines and of dialkyl-and alkyl-arylamines with paraformaldehyde in the presence of zinc chloride and zinc borohydride has been reported (equation 52)151. 

E. Jeon, Y. Whan Cho, Mechanochemical synthesis and thermal decomposition of zinc borohydride, J. Alloys Compd. 422 (2006) 273-275. 

The first asymmetric synthesis of (-)-monomorine I, an enantiomer of the natural alkaloid, by Husson and co-workers starts with the chiral 2-cyano-6-oxazolopiperidine synthon (385) prepared from (-)-phenylglycinol (384), glu-taraldehyde (383), and KCN (443). Alkylation of 385 with an iodo ketal led to the formation of a single product (386). The cyano acetal (386) was treated with silver tetrafluoroborate and then zinc borohydride to afford a 3 2 mixture of C-6 epimeric oxazolidine (387) having the (2S) configuration. Reaction of 387 with... 

The analogue in which carbon replaces oxygen in the enol ring should of course avoid the stability problem. The synthesis of this compound initially follows a scheme similar to that pioneered by the Corey group. Thus, acylation of the ester (7-2) with the anion from trimethyl phosphonate yields the activated phosphonate (7-3). Reaction of the yhde from that intermediate with the lactone (7-4) leads to a compound (7-5) that incorporates the lower side chain of natural prostaglandins. This is then taken on to lactone (7-6) by sequential reduction by means of zinc borohydride, removal of the biphenyl ester by saponification, and protection of the hydroxyl groups as tetrahydropyranyl ethers. 

Sodium triacetoxyborohydride, 283 Titanium(III) chloride, 302 Tributyltin hydride, 316 Zinc borohydride, 167 of alkenes to alkanes (R)-(-F)- and (S)-(-)-2,2 -Bis(di-phenylphosphine)-l,1 -binaphthyl, 36 [1,4-Bis(diphenylphosphine)-butanej(cycloheptatriene)-rhodium(I) tetrafluoroborate, 89 [ 1,4-Bis(diphenylphosphine)butane]-(norbornadiene)rhodium(I) tetrafluoroborate, 37... 

Sodium triacetoxyborohydride, 283 Tetrabutylammonium iodide-Boron trifluoride etherate, 287 Tributylborane, 325 Tributyl borate, 326 Triethylborane, 33, 292 Trimethyl borate, 218 Zinc borohydride, 167 Chromium Compounds Arene(tricarbonyl)chromium complexes, 19... 

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