Complex borohydrides

Considering the decomposition of alkali metal borohydrides to form metal hydrides, the reaction can be described as follows (Eq. (5.2))  

For alkaline earth borohydrides the following decomposition reactions are possible 

Thermal desorption studies revealed for Li-, Na, and K-borohydrides the formation of alkali hydride as decomposition products above 430 °C. While Mg-, Sc, and Zr-borohydrides decompose through the formation of intermediate phases and/or borides, Zn(BH4)2 decomposes directly to elemental Zn [5]. 

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XRD Characterization The powder x-ray diffraction of the mechano-chemically milled complex borohydride has been carried out by the Philips X pert diffractometer with Cu-Koi radiation of X= 5.4060 A. The incident and diffraction slit width used for the measurements are 1° and 2° respectively. The sample holder was covered with Polyethylene tape (foil) with an O-ring seal in an N2 filled glove box in order to avoid or at least minimize the 02/moisture pickup during the XRD measurements. The diffraction from the tape was calibrated without the actual sample and found to be occurring at 29 angles of 22° and 24°, respectively. The XRD phase identification and particle size calculation has been carried out using PANalytical X pert Highscore software, version l.Of. 

Treatment of aldehydes or ketones with ammonia, primary or secondary amines in reducing media is called reductive alkylation (of ammonia or amines) or reductive amination (of aldehydes or ketones). Reducing agents are most frequently hydrogen in the presence of catalysts such as platinum, nickel or Raney nickel [955], complex borohydrides [705, 954, 955], formaldehyde or formic acid [522]. 

Consequently, by choosing proper conditions, especially the ratios of the carbonyl compound to the amino compound, very good yields of the desired amines can be obtained [322, 953]. In catalytic hydrogenations alkylation of amines was also achieved by alcohols under the conditions when they may be dehydrogenated to the carbonyl compounds [803]. The reaction of aldehydes and ketones with ammonia and amines in the presence of hydrogen is carried out on catalysts platinum oxide [957], nickel [803, 958] or Raney nickel [956, 959,960]. Yields range from low (23-35%) to very high (93%). An alternative route is the use of complex borohydrides sodium borohydride [954], lithium cyanoborohydride [955] and sodium cyanoborohydride [103] in aqueous-alcoholic solutions of pH 5-8. 

Other reagents used for the preparation of lactones from acid anhydrides are lithium borohydride [1019], lithium triethylborohydride (Superhydride ) [1019] and lithium tris sec-butyl)borohydride (L-Selectride ) [1019]. Of the three complex borohydrides the last one is most stereoselective in the reduction of 3-methylphthalic anhydride, 3-methoxyphthalic anhydride, and 1-methoxynaphthalene-2,3-dicarboxylic anhydride. It reduces the less sterically hindered carbonyl group with 85-90% stereoselectivity and is 83-91% yield [1019]. 

Other reagents used for reduction are boranes and complex borohydrides. Lithium borohydride whose reducing power lies between that of lithium aluminum hydride and that of sodium borohydride reacts with esters sluggishly and requires refluxing for several hours in ether or tetrahydrofuran (in which it is more soluble) [750]. The reduction of esters with lithium borohydride is strongly catalyzed by boranes such as B-methoxy-9-bora-bicyclo[3.3.1]nonane and some other complex lithium borohydrides such as lithium triethylborohydride and lithium 9-borabicyclo[3.3.1]nonane. Addition of 10mol% of such hydrides shortens the time necessary for complete reduction of esters in ether or tetrahydrofuran from 8 hours to 0.5-1 hour [1060],... 

Stereoselective reductions based on complexed borohydrides have also proved of value in many instances in particular they have been of use in the synthesis of epimeric cyclic alcohols. For example, the reduction of 4-t-butylcyclo-hexanone to the cis-alcohol [99.5%, arising from equatorial hydride ion attack (i)] is effected by L-Selectride (lithium tri-s-butylborohydride, cf. Section 4.2.49, p. 448), or LS-Selectride53 (lithium trisiamylborohydride, cf. Section 4.2.49, p. 448) but to the trans-alcohol [98%, arising from axial hydride ion attack (ii)] with lithium butylborohydride.54 The experimental details of these reductions are given in Expt 5.34. 

Selective reductions This complex borohydride is particularly useful for selective 1,2-reduction of acyclic a,/ -cnones and of conjugated cyclohexenones to allylic alcohols. However, the 1,2-selectivity is less marked with conjugated cyclopentenones. The reagent reduces unhindered cyclic ketones to the more stable (equatorial) alcohols with stereoselectivity greater than that of sodium borohydride. 

Borohydride regeneration is complex. Borohydrides form multi-hydrates. 

In 1939 Schlesinger et al. prepared A1(BH4)3, the first example of a complex metal borohydride [2]. Starting from aluminum trimethyl and diborane, they tried to synthesize AIH3, but the complex borohydride A1(BH4)3 was produced as in Eq. (5.1). 

By the action of complex borohydrides (Ph3P)2CuBH4 [DF1,W4] or (Ph3P)3CuCNBH3 [HM2]. These reductions take place at room temperature in acetone, and only acid chlorides are sensitive under these conditions. 

Preparation. This complex borohydride is prepared conveniently by the reaction of sodium borohydride and zinc chloride in ether solution. 

Barkhordarian, G., Klassen, T., Domheim, M. and Bormann, R. (2007) Unexpected kinetic effect of MgB2 in reactive hydride composites containing complex borohydrides. Journal of Alloys and Compounds, 440, L18-L21. 

A complex ion, tetrahydridoborate(III) (borohydride, BH4"), can be formed it has very strong reducing properties. Other, more complex, borohydride ions exist. 

This complex borohydride is prepared by addition of r-butyllithium in n-pentane to trisiamylborane in THF at —78°. 

Complex borohydrides are not suitable for this reduction, since they preferentially reduce the ester carbonyl. 

The reagent reduces camphor to the exo-alcohol, but only slowly (25°, 72 hours), probably because of the large steric requirements of this complex borohydride. 

KoUonitscli, J. Euchs, O. Gabor, V., New and known complex borohydrides and some of their apphca-tions in organic syntheses. Nature 1954,173, 125—126. 

This stable complex borohydride, which could be named Na(PEGBH2), is soluble in PEG itself and can exhibit an enhanced reactivity, if compared with NaBH in hydroxylie solvents. Further, Na(PEGBH2) can be formed... 

In order to gain more informations on the nature of the complex borohydride Na(PEGBH2), the reduction of esters was examined using NaBH in PEG 200, 300 and monomethoxy PEG. From these results it appears that methyl benzoate is completely reduced in PEG 200, whereas the same reduction in PEG 300 is complicated by a partial hydrolysis of the ester to benzoic acid. Finally, if one of the two hydroxy groups of PEG is blocked as ether no reduction at all takes place (Scheme 7). 

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