Advices For Studying Organic Chemistry

3) Both electronic and steric factors accounts for the anti-Markovnikov orientation

of the addition.

A Mechanism for the Reaction

Hydroboration

B B B

H

H H

H

H H

CC

H 3 C H

HH

H 3 C CCH

HH

H 3 C CCH

HH

H H

H

+

π complex

δ−

δ+

Four-center transition state

++

Addition takes place through the initial formation of a π complex, which changes

into a cyclic four-center transition state with the boron atom adding to the less

hindered carbon atom. The dashed bonds in the transition state represent bonds

that are partially formed or partially broken.

CC

H B

H 3 C

H H

H

HH

The transition state passes over to become an alkylborane. The other B−H bonds

of the alkylborane can undergo similar additions, leading finally to a trialkylborane.

(^)

11.6B THE STEREOCHEMISTRY OF HYDROBORATION

1. The transition state for the hydroboration requires that the boron atom and the
   hydrogen atom add to the same face of the double bond ⇒ a syn addition.

CC syn addition CC

H B