Sodium aluminum hydride reduction kinetics

With less hindered hydride donors, particularly sodium borohydride and lithium aluminum hydride, cyclohexanones give predominantly the equatorial alcohol. There has been less agreement about the factors that lead to this result. The equatorial alcohols are, of course, the more stable of the two isomers. The stereochemistry of hydride reduction is determined by kinetic control, but it was argued that the relative stability of the equatorial alcohol might be reflected in the transition state and be the dominant factor when no major steric problems intervened. The term product development control was introduced to indicate this explanation of the reaction stereochemistry. A number of objections were raised to this idea, primarily on the basis of the Hammond principle. The common hydride reductions are exothermic reactions with low activation energies. The transition state should resemble starting ketone and reflect little of the structural features that are present in the product, so that it is difficult to see why product stability should determine the product composition. 

Other advanced methods for the manufacture of NaBH4 have been investigated by Toyota Motor Co and academic researchers in Japan. Most notable is the use of magnesium hydride for direct reduction of boron oxide feedstocks, including anhydrous borax, for initial synthesis of NaBlfj (Equation 16.15) and reduction of anhydrous sodium metaborate for regeneration of spent borohy-dride (Equation 16.16) [38], Aluminum metal has also been used instead of magnesium and catalysts employed to improve kinetics. 

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