化學題目求詳解

it's about steric repulsion between the alkyl groups.

a dative covalent bond is formed when the electron lone pair on N approaches the electron-deficient B.
for 2,6-dimethylpyridine, the two methyl groups on ortho-positions has steric repulsion against the methyls on boron. this raises the energy of the molecule, leading to instability of the adduct. thus 2,6-dimethylpyridine is less reactive than solely pyridine toward trimethylboron.