Chapter 12, Problem 12.11YT

Interpretation Introduction

Interpretation:

To complete each of the hydroboration steps by adding the curved arrows, when borane add to propene to produce two different products, and to indicate the pertinent steric repulsion that is present in each reaction, as well as the partial charges that develop in the transition state, similar to Equations 12-36 and 12-37, and to determine which reaction is favored.

Concept introduction:

Hydroboration is stereospecific with the H and ${\text{BH}}_{\text{2}}$ groups adding to the alkene in a syn fashion. Hydroboration is also regioselective. The H atom primarily adds to the C with the greater number of alkyl groups, whereas the ${\text{BH}}_{\text{2}}$ group adds to the C with the fewer number of alkyl groups. When ${\text{BH}}_3$ adds to an alkene, steric repulsion directs the larger group, ${\text{BH}}_{\text{2}}$, to the carbon atom with the fewer number of alkyl groups.

In the transition states a partial positive charge appears on the C atom that has a partial bond to H. This is because the step is driven by the formation of a bond between the electron-rich alkene and the electron-poor B atom, so the C-B bond is formed to a greater extent in the transition state than the C-H bond. The transition state is more stable when the ${\text{BH}}_{\text{2}}$ group adds to the C atom with fewer number of alkyl groups because that allows the partial positive charge that is generated to appear on the C atom with the greater number of alkyl groups. The additional alkyl groups are electron donating and stabilize that partial positive charge.