Alternative Metal Borohydrides Li, Zn, Ce

In previous sections, the metal counterion for hydride reducing agents has been largely restricted to sodium, lithium, and potassium. When different metal hydrides are used, there can be a significant effect on 

This reagent was used to reduce 112 to the hydroxymethyl derivative (113) in 91%, without racemization of the stereogenic center at C5, taken from Smith s synthesis of pyrrolizidine alkaloids.Hojo showed that silica gel is an effective catalyst for several other reactions. 1  

It is possible to reduce epoxides at the more substituted position, with high selectivity, by using zinc borohydride supported on silica gel.l When 1-nonene oxide (2-heptyloxirane) was treated with this reagent, an 

88% yield of 1-nonanol was obtained. In general, however, epoxides can be difficult to reduce with Zn(BH4)2 When 1-nonene oxide was treated with Zn(BH4)2 in THF, only a small amount of reduction occurred and a mixture of regioisomeric products was obtained.Reductive cleavage of a nitrile group has also been observed when a mixture of zinc borohydride and silver tetrafluoroborate (AgBF4) was used, as in the conversion of 118 to 119 in Husson s synthesis of dihydropinidine.l  

---