metal hydride reductions Massey University（减少金属氢化物梅西大学）.pdf

REDUCTIONS • Invaluable process • Can be used to remove functionality from a molecule • A versatile method for introducing stereocentres • Formally reduction is the gain of electrons but it is more easy to visualise it as the gain of hydrogen (although this far from mechanistically correct) Metal Hydrides • The most common metal hydrides are lithium aluminium hydride (LiAlH ) and sodium 4 borohydride (NaBH ) 4 • There are differences mechanistically • In many cases the lithium cation is vital for reaction • number of repetitions • lithium activates depends on sterics of carbonyl the carbonyl Li H H H Al Li Al 2 H Al + O 3 O O H Al O – 2 – H + H RR H RR H RR R R 4 • addition of a crown compound • each addition is slower can prevent reduction by • alkoxide electron-withdrawing removing lithium group so reduces reactivity of hydride • In NaBH reactions cation is not import