Boron preparation

The solubility of transition elements in o-rh boron is low. The purity of a-rh boron prepared by thermal decomposition of BI3 is high, although that of -rh prepared from the same reactants is low. Crystallization of a-rh boron from a Pt melt requires pure chemicals. The solubility of transition and inner transition elements in -tetragonal boron is not known, but that in j8-rh boron is deseribed in 6.7.2.5.2 and 6.7.2.5.3. 

Liquid K, in equilibrium with its vapor, does not react with B 1100°C (p = 10.7 10 N m" ). Above this T, a selective reaction is observed, leading to the formation of the hexaboride, KBs. Whereas Li and Na react equally with both a- and 3-rh boron prepared at 800°C by reducing BBr3 with hydrogen in a quartz tube , K combines only with the a-rh variety. The reaction of Li and Na with B is instantaneous that of K is slower to be complete, it requires about 18 h at 1100°C... 

How is crystalline boron prepared Write a balanced equation for the reaction. 

Cyano compounds liquid crystals, 12, 278 in silver(III) complexes, 2, 241 Cyanocuprates, with copper, 2, 186 Cyano derivatives, a-arylation, 1, 361 Cyanosilanes, applications, 9, 322 Cyclic acetals, and Grignard reagent reactivity, 9, 53 Cyclic alkenes, asymmetric hydrosilylation, 10, 830 Cyclic alkynes, strained, with platinum, 8, 644 Cyclic allyl boronates, preparation, 9, 196 Cyclic allylic esters, alkylation, 11, 91 Cyclic amides, ring-opening polymerization, via lanthanide catalysis, 4, 145... 

Boron prepared in this way contains magnesium and oxides of boron as impurities, and the purity ranges from 80% to 95% when the preceding procedure is used. The boron produced... 

Boron prepared in this way appears as black crystals having a density of 2.34 g cm-3. The structure of the boron unit cell in the crystalline material is a regular icosahedral structure containing 20 equilateral triangles meeting at 12 vertices. Figure 8.1 shows the structure of the B12 icosahedron (4 symmetry). 

A terse paper by Kaga et al. reports the synthesis of dienamides in moderate yield by palladium-catalyzed coupling of P-bromoacrylamides with alkenyl boronates. (Scheme 13) [138]. Employing alkenyl boronates derived from appropriate alkynes, the strategy was applied to the synthesis of piperlongumine, 36(n=0), piperine, 42, trichonine, 9(n=14), pellitorine, 5, and the trienyl amide, 48. An alkyl boronate, prepared from l-(3, 4 -methylenedioxyphenyl)-l,5-hexadiene, was similarly used for the synthesis of the non-conjugated dienamide, pipernonaline, 46(n=4) (Scheme 13) [138]. 

Boron.—Preparation of Organoboranes. The year 1977 was one of consolidation in hydroboration. Detailed investigations of the reactions of 9-borabicy-clo[3,3,l]nonane (9-BBN) with cycloalkenes and non-conjugated dienes were reported in full. The reagent shows a remarkable degree of positional and stereochemical selectivity during hydroboration. Details of the applications of borane-dimethyl sulphide have appeared, and hydroborations using dimethyl... 

Bower. J. G., Elemental Boron, Preparation, Properties and Applications, in Progress in Boron Chemistry, (R. Brotherton and H. Steinberg, eds.), Pergamon Press, Oxford, UK (1969)... 

Cyclic l,3-dienyl-2-boronates, prepared via a ring-dosir metathesis reaction, have been described [86]. Treatment with an excess of dienophile (nitroethylene, acrolein, methyl vinyl ketone) furnished in all cases one major product the minor components being the P-epimer and the endo regioisomer (Scheme 9.40). 

A number of novel l-aza-2-bora-3-oxacyclopentanes such as (19) have been prepared. Compound (20) is the first monomeric pyrazolylborane containing trigonal boron. Preparation of compounds (21) by reaction of RBHa with carbonyldihydrazine has been reported. ... 

---