Diborane system

Hydroboration affords an efficient preparation of the 5a-A -system (141, for example) from A" -3-ketones. Reaction with diborane followed by decomposition of the organoboron intermediate with refluxing acetic anhydride gives good yields of olefins. Ketones must be protected, and alcohols are transformed to acetates. A -7-Ketones yield 5oc-A -olefins (for example, 138). 

Iodine azide, on the other hand, forms pure adducts with A -, A - and A -steroids by a mechanism analogous to that proposed for iodine isocyanate additions. Reduction of such adducts can lead to aziridines. However, most reducing agents effect elimination of the elements of iodine azide from the /mwj -diaxial adducts of the A - and A -olefins rather than reduction of the azide function to the iodo amine. Thus, this sequence appears to be of little value for the synthesis of A-, B- or C-ring aziridines. It is worthy to note that based on experience with nonsteroidal systems the application of electrophilic reducing agents such as diborane or lithium aluminum hydride-aluminum chloride may yet prove effective for the desired reduction. Lithium aluminum hydride accomplishes aziridine formation from the A -adducts, Le., 16 -azido-17a-iodoandrostanes (97) in a one-step reaction. The scope of this addition has been considerably enhanced by the recent... 

Diborane in THF reduces epoxides, but the yields are low, and other products are formed by pathways that result from the electrophilic nature of diborane.133 Better yields are obtained when BH4 is included in the reaction system, but the electrophilic nature of diborane is still evident because the dominant product results from addition of the hydride at the more-substituted carbon.134... 

Dieses bindet sich zunachst sicher an noch nicht umgesetzte Al—H-Bin-dungen zu Alkoxy-aluminiumboranaten, die erst in der letzten Phase der Reaktion mit B(OPh)3 zu Diboran und Al(OPh)3 abreagieren. Dieser letzte Schritt scheint im System AlH3/B(OPri)3 nicht mehr moglich zu sein. 

At about the same time, the pyrolysis of diborane was studied by Bragg et al.88 in the temperature range 90-130 °C. These workers again used a static system (reaction vessel volume 212 cm3) and followed the conversion both by measurement of pressure increase and by determination of the amount of hydrogen formed. The system was also examined by mass spectrometric analysis. The empirical rate law was found to be... 

Although the exact mechanism is speculative, it has been found that carboxylic acids, but not esters, are reduced directly to the corresponding alcohols on reaction with benzyltriethylammonium borohydride in the presence of trichlorosilane [21]. Yields are generally in excess of 85%. The failure of the system to reduce esters suggests that diborane is not produced during the reaction [22],... 

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. 

Figure 1 is a sketch of a complete apparatus for the preparation of tetrachloro-diborane(4). Boron chlorides are very corrosive and attack many elastomers and vacuum greases, so only Teflon, Viton, and halocarbon greases should be used for seals and lubrication in the vacuum system. 

Borazine, B3N3H6, was first prepared by thermolysis of the diborane ammonia adduct [(BH2)(NH3)2] [BH4] . More convenient procedures for the laboratory preparation of this important ring system in multigram quantities involve either (a) the decomposition of ammonia- borane [eqn (9.1)] or (b) the reaction between ammonium sulfate and sodium borohydride [eqn (9.2)]. The latter method provides a convenient and economical synthesis of borazine. 

The two metal atoms make use of hybrid orbitals as indicated, and each of these overlaps a hydrogen orbital that is centered midway between the metals but substantially above the metal-metal axis. For a dimeric species, such as diborane, this is repeated with a second set of orbitals and hydrogen atom symmetrically placed on the opposite side of the metal-metal axis. Examination of this model shows that each of the terminal bonds is a normal 2-electron-2-center bond. Only the bonds between the metal centers are nonconventional. An extensive review of these systems has been given by Lipscomb (69). 

The ESR spectra of a series of triarylboron anion radicals have been determined and show selective line-broadening effects (95). The radical anions of diborane, generated in an argon matrix at 77 K by irradiation with a-rays have been studied by ESR (121). The vanadium-aluminum complex obtained on mixing dichlorobis(7r-cyclopentadienyl)vanadium with ethylaluminum dichloride, a system that catalyzes the polymerization of ethylene to give a polymer with good characteristics, has been characterized by ESR and ultraviolet studies (44). 

The three-centered, two-electron hydride bridge, which is prevalent in boron hydride chemistry, has been very well characterized in the ir and Raman spectroscopy of diborane and certain metal borohydrides. A brief review of these data will be given at this point because they afford insight into hydride-bridged systems. 

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