Borides preparation

X indicates existence of the borides, x(P) borides prepared under high pressure, o borides having another structure. 

The reduction of a metal oxide or other metal compound by C, B or boron carbide requires higher T than in the other methods available for boride preparation. 

Because of its stability and unusual properties, the crystal growth of TiB2 is the most studied process of boride preparation by CVD. It is interesting from both a scientific and an engineering viewpoint, and because of the availability, low cost and... 

Three new methods for the conversion of alkynes to (Z)-alkenes were reported, although Lindlar semi-hydrogenation still remains as the most convenient method. Copper (I) hydride reagent could reduce alkynes to (Z)-alkenes as shown in Scheme 3 [12]. Yoon employed nickel boride prepared on borohy-dride exchange resin for selective hydrogenation of alkynes to (Z)-alkenes (Scheme 4) [13]. 

Nickel boride (prepared by adding NaBH4 to nickel chloride in methanol - THF) is an efficient, non-pyrophoric reagent for the desulfurization of benzo[ Jthiophene and dibenzothiophene (90CC819, 92JOC1986). The reaction proceeds under very mild conditions and is probably mediated by nickel hydride. 

Nickel boride prepared from Nil2 and two equivalents of LiBH4 [42] was utilized as an oxazaborolidine catalyst support (Scheme 4) [43]. Reaction of nickel boride with 0.1 equivalents of chiral amino alcohol in THF at room temperature gave the anchored catalyst 6, which produced chiral alcohols in optical yields of up to 95%, and which furthermore showed higher activity as regards the reduction of acetophenone derivatives than that of the corresponding homogene-... 

From the discussion in Section II,C it should be clear that there is a relationship between the structures of borides and boranes as far as the boron networks are concerned. A comparison of the literature shows that the known structural diversity of the boranes is much greater than that of the borides. This may well be due to the rigorous conditions required for boride preparation that is, only the most stable boron networks are formed. TTie low-temperature routes illustrated in the foregoing work suggest an approach to presently unknown compounds with novel boride networks. Hence, the multitude of known metalla-boranes constitute a stockpile of potential precursors to metal borides with new and perhaps useful properties. 

Raney nickel is a stronger desulfurizing agent than nickel boride. On the other hand nickel boride, prepared from nickel(II) chloride and sodium borohydride, is more selective since it does not desulfurize aromatic sulfones. An example of such selectivity is desulfurization of p-phenylsulfonylphenyl p-tolyl sulfide (33), which affords diphenyl sulfone in 91% yield and toluene in 84% yield. In contrast. Raney nickel desulfurizes both the sulfide and the sulfone giving a mixture of benzene and toluene (equation 82). 5... 

While various borohydride nickel ratios have been used in these nickel boride preparations maximum P-2 catalytic activity was observed with a 2 1 BH4 Ni ratio.However, when the reduction was run under a hydrogen atmosphere using a 4 1 BH4 Ni ratio, a hydrogenated nickel boride catalyst, the P-3 nickel boride, was obtained.27 This catalyst was somewhat more active than the P-2 catalyst for alkene hydrogenation but it induced signifieantly more double bond isomerization during the reaction. 

Imidazolines can exist as the 2-, 3-, or 4-isomers ((5)- 7)), and whereas (5) can exist as a pair of tautomers, any proton shift in (6) will give (7) by rearrangement. Hydrolysis of A(-unsubstituted 3-imidazolines (6) to a-aminoketones presumably occurs via (7). Nickel boride , prepared in situ, transforms 3-imidazolines into 2-imidazolines by double-bond migration <86H(24)287l>. 

This reaction was improved by sonication in the case of lipidic substrates. The reductive elimination of i /c-dinitro compounds to olefins, usually effected with tributyltin hydride, was achieved successfully by nickel boride (prepared from nickel chloride and sodium borohydride by sonication in methanol for 30 min) giving olefins in excellent yields (Eq. 37). The reagent is prepared by sonication of nickel chloride and sodium borohydride and gives the so-called nickel boride. 

Procedures were proposed for the reduction of aromatic nitro groups to amines. Sodium dithionitei78 and a combination of sulfur and hydrazine on activated carboni79 are reported to provide anilines in good to excellent yields. From aliphatic nitro compounds (Eq. 66), primary amines could be obtained with "nickel boride", prepared from nickel chloride and sodium borohydride.i ... 

Reductions - For unsaturated compounds containing oxygen (allylic alcohols and ethers, ynols, epoxides, a,6-unsaturated ketones, aldehydes, acids and epoxides) nidkel boride (prepared from N1 acetate and NaBH ) selectively and quantitatively hydrogenates the C-C double bonds without rearrangements, hydrogenolysis or carbonyl reductions. 

Adiponitrile hydrogenated with 10% cobalt boride (prepared from NaBH4 and GoGlg in water) at 80° in the presence of NHg in a rotating autoclave hexa-methylenediamine. Y 95.7%. B. D. Polkovnikov, L. K. Freidlin, and A. A. Balandin. Izvest. 1959, 1488 G. A. 54, 1264h. 

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