Regioselectivity of diborane and alkylboranes toward representative alkenes

Percent of Boron Added at Less Substituted Carbon Alkenes 

Hydroborating reagent 1-Hexene 2-Methyl-1-butene 4-Methyl-2-pentene CX. Styrene 

These reagents are prepared by hydroboration of the appropriate alkene using control of stoichiometry to terminate the hydroboration at the desired stage. 

Further changes in reactivity and regioselectivity can be achieved by use of chloroalkylboranes and by use of the dimethyl sulfide complexes of the alkylboranes as is indicated by the data in Table 4.4. 

The hydroboration reaction is also highly specific with regard to the stereochemistry of addition. The addition occurs stereospecifically syn through a four-center transition state with essentially simultaneous bonding to boron and hydrogen. Both the new C-B and C-H bonds are therefore formed from the same side of the multiple bond. In molecular orbital terms, the addition reaction is viewed as taking place by interaction of the olefin tt orbital with the empty p orbital on trivalent boron. Formation of the carbon-boron bond is accompanied by concerted rupture of a B-H bond  

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Table 4.5. Regioselectivity of Diborane and Alkylboranes toward Representative Alkenes...

Regioselectivity of diborane and alkylboranes toward representative alkenes... 

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