Tetra-n-butylammonium borohydride

Photochemical oxidation of electron-rich alkenes with the simultaneous reduction of the initially formed peroxide with tetra-n-butylammonium borohydride to the hydroxy compound has been reported, but the procedure has not been shown to be generally useful [16]. 

Early use of the low-molecular-weight quaternary ammonium borohydrides in hydrocarbon solvents showed little advantage over the use of sodium borohydride in aqueous or alcoholic media. Although the ammonium salts in benzene appealed to be capable of effecting all the normal reductions exhibited by the sodium salt in water, they appeared to be generally less reactive. This is well illustrated by the recrystallization of tetra-n-butylammonium borohydride from acetone, if the operation is performed rapidly [5,6],... 

Kinetic studies established that tetra-n-butylammonium borohydride in dichloromethane was a very effective reducing agent and that, by using stoichiometric amounts of the ammonium salt under homogeneous conditions, the relative case of reduction of various classes of carbonyl compounds was the same as that recorded for the sodium salt in a hydroxylic solvent, i.e. acid chlorides aldehydes > ketones esters. However, the reactivities, ranging from rapid reduction of acid chlorides at -780 C to incomplete reduction of esters at four days at 250 C, indicated the greater selectivity of the ammonium salts, compared with sodium borohydride [9], particularly as, under these conditions, conjugated C=C double bonds are not reduced. 

Reduction of carbonyl groups using tetra-n-butylammonium borohydride... 

A difference in the reactivities and selectivities between tetra-n-butylammonium borohydride and sodium borohydride in the reduction of conjugated ketones is well illustrated with A1-9 2-octalone (Scheme 11.3) [17], Reduction with the sodium salt in tetrahydrofuran is relatively slow and produces the allylic alcohol (1) and the saturated alcohol (2) in a 1.2 1 ratio whereas, in contrast, tetra-n-butylammonium borohydride produces the non-conjugated alcohol (3) (50%) and the saturated alcohol (2) (47%), with minor amounts of the ketone (4), and the allylic alcohol (1) [16]. It has been proposed that (3) results from an initial unprecedented formation of a dienolate anion and its subsequent reduction. 

Much emphasis has been placed on the selectivity of quaternary ammonium borohydrides in their reduction of aldehydes and ketones [18-20]. Predictably, steric factors are important, as are mesomeric electronic effects in the case of 4-substituted benzaldehydes. However, comparison of the relative merits of the use of tetraethyl-ammonium, or tetra-n-butylammonium borohydride in dichloromethane, and of sodium borohydride in isopropanol, has shown that, in the competitive reduction of benzaldehyde and acetophenone, each system preferentially reduces the aldehyde and that the ratio of benzyl alcohol to 1-phenylethanol is invariably ca. 4 1 [18-20], Thus, the only advantage in the use of the ammonium salts would appear to facilitate the use of non-hydroxylic solvents. In all reductions, the use of the more lipophilic tetra-n-butylammonium salt is to be preferred and the only advantage in using the tetraethylammonium salt is its ready removal from the reaction mixture by dissolution in water. 

Trithiocarbonate 5,5-dioxides are reduced to the trithiocarbonates by tetra-n-butylammonium borohydride at ca. -25°C [29]. 

Alkali metal borohydrides are frequently used for the reduction of rc-electron-deficient heteroaromatic systems, but reduction of jt-electron-excessive arenes is generally possible only after protonation of the systems [e.g. 35-37]. The use of tetra-n-butylammonium borohydride under neutral conditions for the conversion of alkylindoles into indolines [38] is therefore somewhat unusual. Reduction of indoles by diborane under strongly alkaline conditions involves the initial interaction of the indolyl anion with the diborane to form an amino-borane which, under the basic conditions, reacts with a second molecule of diborane to produce the indoline [39]. The reaction of tetra-n-butylammonium borohydride with indoles could also proceed via the intermediate formation of diborane. 

Selective reductions. Raber et al.1 have compared the ability of tetra-n-butylammonium borohydride, tetraethylammonium borohydride, and sodium borohydride to effect selective reduction of aldehydes in the presence of ketones, and conclude that no one of these reagents is generally effective for this purpose and that the three reagents are generally similar. 

Reduction of acetanilide with tetra-n-butylammonium borohydride in the presence of dichloromethane followed by treatment with HCl gives 74% N-ethylaniline hydrochloride. ... 

Another convenient and efficient method for the reduction of nitriles to the amines is with tetra-n-butylammonium borohydride in refluxing dichloromethane. The chemospecificity of tetra-n-butylammonium borohydride towards organic cyano and amide compounds... 

Somewhat surprisingly, it has been reported that both tetra-n-butylammonium borohydride and zinc borohydride, in the absence of acid, are capable of reducing indoles to indolines (equations 72 and... 

Although indoles are not normally reduced under neutral conditions with LAH or NBH, tetra-n-butylammonium borohydride has recently been ap-... 

Treatment of the long-chain imidazolium salts 263 with potassium, sodium, and tetra-n-butylammonium borohydrides affords the acyclic diamines 264 and 265. The isomer ratio was not reported. Sodium cyanobo-rohydride failed to reduce the same salt under a variety of conditions. The imidazoline 266 reacts with LAH in THF above — 10°C to give the 1,2-di-aminoethane 267. Sodium borohydride is a less effective reducing agent in this reaction, achieving the same conversion over a longer period of time. Other less reactive hydrides [LiBH4, NaBHjCN, LiAlH(0-t-Bu)j] do not react. 

Tetra-n-butylammonium borohydride, (C4H9)4NBH4. Mol. wt. 257.31 soluble in CH2CI2, insoluble in ether. 

ALDEHYDES Bis(4-methylpiperazinyl)aluminum hydride. Di(r) -cyclopentadienyl)-(chloro)hydridozitconium(IV). Dihalobis(triphenylphosphine)paIIadium(H). 7,8-Dimethyl-l,5-dihydro-2,4-benzodithiepin. Grignard reagents. Lithium bis(ethylene-dioxyboryOmethide. 3-Methyl-I-phenyl-2-phospholene. 2-Methyl-2-thiazoline. Methylthioacetic acid. 3-Methylthio-l,4-diphenyl-s-triazium iodide. Sodium meth-oxide. Methylthiomethyl N,N-dimethyldithiocarbamate. Sodium tetracarbonylferrate(II). Tetra-n-butylammonium borohydride. Triethylallyloxysilane. N,4,4-Trimethyl-2-oxazolinium iodide. 

Full details of the reductive power of 9-borabicyclo[3,3,l]nonane have appeared and the susceptibility to reduction of aldehyde, ketone, and a number of other functional groups has been reviewed. Since tetra-n-butylammonium borohydride in dichloromethane reduces organic compounds with the same selectivity exhibited by sodium borohydride in aqueous or alcoholic media, the former system might prove the reagent of choice in those cases in which the use of protic solvents would lead to undesirable side-reactions. ... 

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