Metal hydrides diborane

Boron forms a whole series of hydrides. The simplest of these is diborane, BjH. It may be prepared by the reduction of boron trichloride in ether by lithium aluminium hydride. This is a general method for the preparation of non-metallic hydrides. 

The reaction of metal hydrides and BF depends on the stoichiometry as well as the nature of the metal hydride. For example, LiH and BF may form diborane (6) orUthiumborohydride (31,32) ... 

Boron trifluoride is used for the preparation of boranes (see Boron compounds). Diborane is obtained from reaction with alkafl metal hydrides organoboranes are obtained with a suitable Grignard reagent. 

Addition of diborane (eq. 52) under the latter conditions renders the production of MBH essentially continuous until consumption of the metal hydride is complete because trimethyl borate is regenerated. 

War research forced us to explore new synthetic routes and we discovered the alkali metal hydride route to diborane. This solved the synthetic problem. At the same time we discovered sodium borohydride and developed simple synthetic methods for its preparation and manufacture. 

Alkali metals, finely divided aluminum and magnesium particles, hydrazine, diborane, metal hydrides, and hydrogen are strong reducing agents [35]. An example of a significant problem is the possible explosive reaction between light metals and carbon tetrachloride which is itself a stable compound [57]. 

Alkali metal hydrides react with diborane to form metal borohydrides B2He+2NaH 2NaBH4... 

Atkah metal hydrides too abstract protons from boranes. While water is produced with basic hydroxides, hydrogen is liberated with hydrides. Except diborane, all other boron hydrides undergo similar reactions, liberating hydrogen ... 

Alkali metal hydrides reduce boron trichloride to diborane at ordinary temperatures ... 

Mdssbauer spectra of bonding and structure in, 15 184-187 reactions with diborane, 16 213 stabilization of, 5 17, 18-19 cyanates, 17 297, 298 cyanide complexes of, 8 143-144 cyclometallated bipyridine complex, 30 76 diazene complexes, 27 231-232 dinitrogen complexes, 27 215, 217 diphosphine complexes of, 14 208-219 dithiocarbamates, 23 253-254 -1,2-dithiolene complexes, 22 323-327 hydrogen bonding, 22 327 halide complexes with phosphine, etc., 6 25 hexaflouride, structure, 27 104 hydride complexes, 20 235, 248-281, see also Transition metal-hydride complexes... 

Figure D shows some olefin insertion reactions. Hydride additions to olefins have been known for a long while. Among these many examples, manganese hydrocarbonyl, and cobalt hydrocarbonyl, magnesium hydride, diborane, alkylalu-minum hydrides, germanium and tin hydrides all add quite readily to olefins. These last two cases are questionable because the mechanism is not clear. Some of these additions occur without a catalyst some are speeded up by ultraviolet light some are catalyzed by Group VIII metals. So it is not clear whether all these reactions are the same or whether there are several different mechanisms.

The main methods of reducing ketones to alcohols are (a) use of complex metal hydrides (b) use of alkali metals in alcohols or liquid ammonia or amines 221 (c) catalytic hydrogenation 14,217 (d) Meerwein-Ponndorf reduction.169,249 The reduction of organic compounds by complex metal hydrides, first reported in 1947,174 is a widely used technique. This chapter reviews first the main metal hydride reagents, their reactivities towards various functional groups and the conditions under which they are used to reduce ketones. The reduction of ketones by hydrides is then discussed under the headings of mechanism and stereochemistry, reduction of unsaturated ketones, and stereochemistry and selectivity of reduction of steroidal ketones. Finally reductions with the mixed hydride reagent of lithium aluminum hydride and aluminum chloride, with diborane and with iridium complexes, are briefly described. 

Diborane, B2H6, also reduces many carbonyl groups. In contrast to the metal hydride reagents, diborane is a relatively electrophilic reagent, as witnessed by its ability to add to carbon-carbon double bonds. 

The heteroboranes are derived ultimately from boranes, which, in turn, can be built up from lower molecular weight boron hydrides, the simplest of which is diborane, B2H6. There are a number of industrial processes described for the synthesis of diborane, most involving the reduction of boron trihalides with metal hydrides, as shown in equation (1). Other methods involve the reduction of trihaloboranes with group 1... 

Rednction of boron trihaUdes to elemental boron can be accomplished by heating with alkali metals, alkaline earth metals, or hydrogen. Under the proper conditions, rednctions of this type can also yield diborane and, under selected conditions, boron subhalides (see below). Metal hydrides also react with boron trihalides to give diborane. Boron nitride and boron carbide have been prepared by the high-temperature reductions of boron trihalides with ammonia and methane, respectively, and deposited on metal substrates by CVD. 

Similarly, the parent hydride, diborane(4), has been prepared only in the form of complexes with Lewis bases 30, 43, 72). Attempted reduction of B2CI4 with various metallic hydrides leads to the formation of diborane(6) and other boron hydrides 104, 109). 

Diborane can be prepared by a variety of methods, the most common being the reduction of boron trihalides with active metal hydrides - and the reaction of hydroborate salts with boron trifluoride, tin(II) chloride, sulfuric acid, methane-sulfonic acid, orthophosphoric acid, or polyphosphoric acid. Although diborane is commercially available in bulk quantities, we have found the reaction of potassium hydroborate, KBH4, with 85% orthophosphoric acid to be convenient for the rapid preparation in a vacuum line of small quantities of this material. ... 

Opposite regioselectivity to that of regular metal hydride reductions has been observed in the NaBH4 reductions of oxiranes in the presence of triethylamine under photochemical conditions (hydride attacks the more highly substituted carbon) <92CC1133>. Diborane (B2H6) likewise tends to reduce oxiranes at the sterically more hindered oxirane carbon the mechanism and stereochemistry of the diborane reduction in connection with aliphatic oxiranes has been studied <82H(l8)28l>. 

The first report of pure alkali metal borohydride appeared in 1940 by Schlesinger and Brown ° who synthesized LiBH4 by the reaction of ethyllithium with B2H6. The direct reaction of the corresponding metal/metal hydrides with diborane in etheral solvents under suitable conditions produces high yields of the borohydrides " " ... 

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