Addition of metal hydrides to alkenes and alkynes

The most important examples concern B, A1 and Si. (Additions of non-metal hydrides N—H, P—H, 0—H, S—H and H—Hal are not included here.) These reactions are not as simple as the equation suggests and several different mechanisms can operate, even in the reactions of hydrides of one element such as silicon. 

The addition of boron hydrides to alkenes and alkynes is known as hydrobor-ation. It provides a simple laboratory route to boron alkyls and thence to a wide range of organic compounds (p. 66). It takes place under mild conditions at room temperature in an ether solvent. 

If propene and Pr Al are heated at 140-200°C under pressure, an insertion of the alkene into A1—C bonds occurs. This is followed by elimination of the dimer of propene, 2-methylpentene. 

With ethene, further chain growth occurs, leading to oligomers (p. 80). 

Gallium hydrides also add to alkenes. For example, Et GaH adds 1-decene giving Et2GaC3oH2i, and the dimer (HGaCl2)2 adds to terminal olefins. 

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