Preparation and properties of boranes

The perception by R. Schaeffer that nido-BeHio (stmcture 10, pp. 154, 159) could act as a Lewis base towards reactive (vacant orbital) borance radicals has led to several new conjucto-boranes, e.g. °  

A useful route to B-B bonded conjuncto-boranes involves the photolysis of parent nido-boranes. Thus, ultraviolet irradiation of B5H9 (9) yields the three isomers of conjuncto-Q o f, 

The synthesis of c/oso-borane dianions B H (1-7) relies principally on thermolysis reactions of boranes in the presence of either BH4 or amino-borane adducts. D xhe yields 

BsHi2 Decomp above —35° — BgHn Decomp above —30° — 

Boranes are extremely reactive compounds and several are spontaneously flammable in air. Arachno-horanes tend to be more reactive (and less stable to thermal decomposition) than nWo-boranes and reactivity also diminishes with increasing mol wt. C/oso-borane anions are exceptionally stable and their general chemical behaviour has suggested the term three-dimensional aromaticity . 

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Full details have appeared of the preparation and properties of 9-borabicyclo-[3,3,l]nonane. Treatment of cyclo-octa-1,5-diene with diborane in THF gave a mixture of 1,4- and 1,5-adducts which gave a mixture of 1,4- and 1,5-dihydroxycyclo-octanes after oxidation-hydrolysis. However, if the solution of alkyl boranes is refluxed for 2 h, crystalline 9-borabicyclo[3,3,l]nonane is obtained on cooling and can be oxidized to give cis-1,5-dihydroxycyclo-octane exclusively. Treatment of trialkylboranes with ferric chloride or ferric isocyanate in aqueous THF gave the corresponding alkyl chlorides and alkyl thiocyanates, e.g. cis-cyclo-octene was converted into chlorocyclo-octane. ... 

A study has appeared which describes the preparation and properties of hydrolytically stable cyanohydro(pyrrolyl-l)borates and chiral boron-containing amine-cyano(pyrrolyl-l)borane complexes. Finally, two reports have appeared concerning the preparation, thermal stability, and stereochemistry of silyl-substituted aminoboranes. ... 

The state of the art of reductions with metal hydrides a decade ago was the subject of comprehensive reviews. A detailed survey of reductions of carbonyl compounds with alkali and alkaline earth metal hydrides, borane and derivatives, alane and derivatives, metal borohydrides, metal aluminohydrides, silanes, stannanes and transition metal hydrides was compiled. The properties, preparation and applications of each reagent were discussed together with methods for their determination, handling techniques... 

The stability and equilibrium properties of neutral borane complexes are not well known. Instead, the recent chemistry of the borane complexes is predominantly characterized by preparative and structural investigations. This is even more obvious for the charged borane complexes. 

Since 1964, a large number of borane adducts have been prepared through either the direct reaction of diborane(6) with Lewis bases or through less direct procedures which involve displacement of hydride ion from BH4. Table 3 below lists most of these materials and some of their properties. 

Alfred Stock (131) and his collaborators were the first to characterize a series of boranes. Properties and many reactions reported by them (1912-1936) have required but little revision to this day. For one hundred years prior to the investigations of Stock and his co-woikers, the boron hydrides had been produced intermittently, but they were not identified or analyzed correctly. Stock s work stimulated others, notably Burg and Schlesinger, to enter this field so that from 1930 until 1942, the date of an important review (HI), structural studies were carried out, and the abandon with which the boranes enter into chemical combination with themselves and with other molecules became evident. Development of improved, or even new, experimental methods has necessitated reinterpretation of most of the early structural results. As would be expected, preparative studies carried out prior to about 1942 have required less revision, providing a firm foundation on which a great extension of synthetic work could be built. 

The radical cyclization of enynes by the use of tris(phenylseleno)borane has been utilized for the synthesis of a-kainoids, a class of compounds with neuroexcitant properties. An example is shown in equation 38 where the enyne, prepared by alkylation of 4,4-diethoxycarbonyl-4-trifluoroacetamidobut-2-enoates withprop-2-ynyl bromide, was treated with (PhSe)3B and AIBN to give in 55% the desired product. 

Zinc nitrate (Zn(N03)2) and dimethylamine borane (DMAB) with a reagent grade from Sigma-Aldrich were used to synthesize the thin film of ZnO on the Pt-IPMC electrodes. The chemical deposition method was used because the conventional methods such as metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), and DC or RF sputtering require temperatures from 200 °C to 800 °C, while IPMC could not withstand such high temperatures. ZnO thin films were synthesized on the Pt IPMC in an aqueous solution composed of 0.1 mol/L zinc nitrate hydrous and 0.1 mol/L DMAB maintained at 60 °C. As the electrical and optical properties of ZnO film depend on the DMBA concentration. ZnO film prepared from the 0.1 mol/L DMAB solution showed the best results. The optimum deposition condition for ZnO as reported in [Izaki and Katayama (2000)] was used. 

Both deprotection methods are very stereoselective, with complete retention of configuration at the phosphorus atom. Losses of optical purity have been reported very rarely. The reader must be aware that deboronation is not always required since phosphine boranes can be used directly in transition metal homogeneous catalysis. Juge and co-workers " combined the reducing properties of the borane moiety to prepare, in situ, Cu(I), Pd(0) and Rh(I) metal complexes by mixing Cu(II), Pd(II) and Rh(III) salts and phosphine boranes. 

Plesek, J. Hermanek, S., Chemistry of boranes. fV. Preparation, properties and behavior of magnesium borohydride towards lewis bases. Collect. Czech. Chem. Commun. 1966, 31, 3845-3858. 

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