Science of Synthesis

Cox, L. R., Science of Synthesis, (2008) 36, 101.

Group 1 and 2 metal hydrides have only rarely been used as reducing agents, principally because these reagents exhibit poor reducing activity as a result of their high lattice energies. The use of such inexpensive, readily available, low-molecular-weight metal hydrides, however, is attractive, and various approaches have been developed to increase their activity. Varying the method of metal hydride preparation offers one approach. “Active” lithium hydride has been prepared by the action of molecular hydrogen on the butyllithium–N,N,N′,N′-tetramethylethylenediamine complex or tert-butyllithium in hexane.[‌441‌] Lithium hydride prepared in this fashion is an excellent reducing agent for aldehydes (and ketones) possessing nonenolizable protons to give alkanols ‌53‌ (‌Table 15‌, entries 1, 2). “Active” sodium hydride and potassium hydride have also been accessed from hydrogen and the corresponding metal tert-butoxide.[‌441‌] Ashby obtained “active” magnesium hydride by treating diethylmagnesium with lithium aluminum hydride (entry 3).[‌442‌] Reaction of the magnesium hydride generated in this way with bis(2,6-diisopropoxyphenoxy)magnesium in tetrahydrofuran affords the more stable aryloxymagnesium hydride, which is also an effective reducing agent (entry 4).

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References

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• 8.Meeeee-Meee, (8888) 8/8e, 888.