Boro hydrides

Hydrogenolysis of aryl and alkenyl halides and triflates proceeds by the treatment with various hydride sources. The reaction can be explained by the transmetallation with hydride to form palladium hydride, which undergoes reductive elimination. Several boro hydrides are used for this purpose[680], Deuteration of aromatic rings is possible by the reaction of aryl chlorides with NaBD4681]. 

This group of reagents is commercially available in large quantities some of its members - notably lithium aluminium hydride (LiAlH4), calcium hydride (CaH2), sodium borohydride (NaBH4) and potassium boro-hydride (KBH4) - have found widespread use in the purification of chemicals. 

Although the nature of the general polar effect suggested by Kamernitzsky and Akhrem " to account for axial attack in unhindered ketones is not clear, several groups have reported electrostatic interactions affect the course of borohydride reductions. Thus the keto acid (5a) is not reduced by boro-hydride but its ester (5b) is reduced rapidly further, the reduction of the ester (6b) takes place much more rapidly than that of the acid (6a). Spectroscopic data eliminate the possibility that in (5a) there is an interaction between the acid and ketone groups (e.g. formation of a lactol). The results have been attributed to a direct repulsion by the carboxylate ion as the borohydride ion approaches. " By contrast, House and co-workers observed no electrostatic effect on the stereochemistry of reduction of the keto acid (7). However, in this compound the acid group may occupy conformations in which it does not shield the ketone. Henbest reported that substituting chlorine... 

Semicarbazones are used as protecting groups as a consequence of their stability to reducing agents such as potassium borohydride, sodium boro-hydride and lithium borohydride. Semicarbazones are cleaved by strong acids and by heating in acetic anhydride-pyridine. " ... 

Both the 4- (38) and 6-(31) nitrimines are reduced with sodium boro-hydride to the corresponding y5-nitramine derivatives, e.g., (41)], which are methylated with methyl iodide and potassium carbonate. The A -methyl derivative is reduced in modest yield to the 5a-fluoride (43) with lithium aluminum hydride. 

A 500-ml three-necked flask is fitted with a condenser, a pressure-equalizing dropping funnel, a magnetic stirrer, and a thermometer. The flask is charged with a mixture of 33.6 g (0.48 mole) of 2-methyl-2-butene and 180 ml of a 1 M solution of sodium boro-hydride in diglyme. The flask is cooled in an ice bath and stirring begun. Boron trifluoride etherate (0.24 mole) is added dropwise to the mixture and the solution is stirred at 0° for 2 hours. 

Dihydrostreptomycin sulfate may be prepared from streptomycin sulfate by catalytic hydrogenation (Merck, Pfizer, Cyanamid), electrolytic reduction (Schenley, Olin Mathieson), or by sodium boro hydride reduction (Bristol), or by isolation from a fermentation process (Takeda). 

The synthesis of key intermediate 12, in optically active form, commences with the resolution of racemic trans-2,3-epoxybutyric acid (27), a substance readily obtained by epoxidation of crotonic acid (26) (see Scheme 5). Treatment of racemic 27 with enantio-merically pure (S)-(-)-1 -a-napthylethylamine affords a 1 1 mixture of diastereomeric ammonium salts which can be resolved by recrystallization from absolute ethanol. Acidification of the resolved diastereomeric ammonium salts with methanesulfonic acid and extraction furnishes both epoxy acid enantiomers in eantiomerically pure form. Because the optical rotation and absolute configuration of one of the antipodes was known, the identity of enantiomerically pure epoxy acid, (+)-27, with the absolute configuration required for a synthesis of erythronolide B, could be confirmed. Sequential treatment of (+)-27 with ethyl chloroformate, excess sodium boro-hydride, and 2-methoxypropene with a trace of phosphorous oxychloride affords protected intermediate 28 in an overall yield of 76%. The action of ethyl chloroformate on carboxylic acid (+)-27 affords a mixed carbonic anhydride which is subsequently reduced by sodium borohydride to a primary alcohol. Protection of the primary hydroxyl group in the form of a mixed ketal is achieved easily with 2-methoxypropene and a catalytic amount of phosphorous oxychloride. 

Its solubility characteristics in aqueous systems are such that retention of toxicity to insects by dissolved crystal protein is always suspect, and loss of activity on dissolution owing to denaturation is often observed. The protein is soluble only in relatively strong aqueous alkali. Thus, it has been variously reported to be soluble in 0.01N- to 0.05N sodium hydroxide (1) and alkali at pH 10.5 in the presence of thioglycollate (35) we have also observed its solubility in alkali at pH 9.5 in the presence of urea and potassium boro hydride. One difference between the characteristic proteins produced by various strains of crystalliferous bacilli is observed in the degree of alka-... 

Just prior to this time aluminum borohydride (4) and beryllium boro-hydride (5) had been prepared in our laboratories. These are the most... 

Brown, H. C., The Reactions of Alkali Metal Hydrides and Boro-hydrides with Lewis Acids of Boron and Aluminum, Congr. Lect., 17th Int. Congr. Pure Appl. Chem. p. 167. Butterworths, London, 1960. 

Acetophenone was reduced in methanol to 1-phenylethanol at 20 °C by boro-hydride moieties coupled to a porous polymer resin [3], In principle, four hydrogen atoms can be released from the borohydride the reactivity, however, decreases with each hydrogen atom lost Experimentally it was shown that the first two atoms mainly contribute and to the reduction the other two remain on the polymer site. 

Another photochemical method was reported by Kessar et al. (157). l-o-Toluyl-3,4-dihydroisoquinoline 299a, derived from dihydroberberine metho salt (298a), was irradiated and then reduced with sodium boro-hydride to provide the ring D-inverted 11,12-oxygenated protoberberine 482 (Scheme 100). 

Price, W. C. The Infrared Absorption Spectra of Some Metal Boro-hydrides. J. Chem. Phys. 17, 1044 (1949). 

Aldehydes attached to the tricyclic ring system can be readily reduced on standard treatment with sodium boro-hydride as illustrated in (Equation 18) for the conversion of aldehyde 88 to alcohol 89 <1997JME2196>. 

The key intermediate 124 was prepared starting with tryptophyl bromide alkylation of 3-acetylpyridine, to give 128 in 95% yield (Fig. 37) [87]. Reduction of 128 with sodium dithionite under buffered (sodium bicarbonate) conditions lead to dihydropyridine 129, which could be cyclized to 130 upon treatment with methanolic HC1. Alternatively, 128 could be converted directly to 130 by sodium dithionite if the sodium bicarbonate was omitted. Oxidation with palladium on carbon produced pyridinium salt 131, which could then be reduced to 124 (as a mixture of isomers) upon reaction with sodium boro-hydride. Alternatively, direct reduction of 128 with sodium borohydride gave a mixture of compounds, from which cyclized derivative 132 could be isolated in 30% yield after column chromatography [88]. Reduction of 132 with lithium tri-f-butoxyaluminum hydride then gave 124 (once again as a mixture of isomers) in 90% yield. 

Tetramethylammonium triacetoxy boro hydride, (CH3)4NBH(OAc)3 (1). Preparation ... 

TBA-BH4 (Table 11.8) in CH2C12 (400 ml) is azeotropically dried by evaporation of ca. 250 ml of the CH2C12 under reduced pressure. The organic substrate (0.1 mmol) is added under argon and the mixture cooled to 0°C. The haloalkane (0.2 mol) (Table 11.8) is added dropwise and the mixture stirred for 30 min at room temperature. The excess boro-hydride is destroyed by the addition of EtOH (25 ml) and the solution is neutralized with HC1 (20%). The aqueous phase is separated and extracted with CH2C12 (2 x 20 ml). The combined organic solutions are washed with H20 (10 ml), dried (MgS04), and fractionally distilled to yield the reduced product. 

A convenient method to affect the oxidation p- to nitrogen in piperidines is based on the anodic oxidation of N-carboalkoxy piperidines (Scheme 35). The electrochemical oxidation of piperidine (152) in the presence of acetic acid and potassium acetate, for example, afforded a mixture of isomeric 2-hydroxy-3-acetoxypiperidines (153) in a combined yield of 93%, following an aqueous workup [61]. Reduction with sodium boro-hydride severed the C-OH bond. Treatment with acid and then base completed a synthesis of pseudoconhydrine (154). 

To circumvent this problem, several methods have been developed (157). Probably, the most prominent method is treatment with sodium boro-hydride (0.1% in PBS, 30 minutes prior to staining). NaBILi is known to neutralize Schiff s bases through reduction of amine-aldehyde compounds into nonfluorescent salts. 

Photolytic. PCB-1254 in a 90% acetonitrile/water solution containing 0.2-0.3 M sodium boro-hydride and irradiated with UV light (X = 254 nm) reacted to yield dechlorinated biphenyls. After 16 h, no chlorinated biphenyls were detected. Without sodium borohydride, only 25% of PCB-1254 were destroyed after 16 h (Epling et al, 1988). In a similar experiment, PCB-1254 (1,000 mg/L) in an alkaline 2-propanol solution was exposed to UV light (X = 254 nm). After 30 min, all of the PCB-1254 isomers were converted to biphenyl. When the radiation source was sunlight, only 25% was degraded after a 20-h exposure. But when the sensitizer phenothiazine (5 mM) was added to the solution, photodechlorination of PCB-1254 was complete after 1 h at 350 nm. In addition, when PCB-1254 contaminated soil was heated at about 80 °C in the presence of di-ferf-butyl peroxide, complete dechlorination to biphenyl was observed (Hawari et al., 1992). 

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