Final answer:
The alkyl shift in trialkyl borane oxidation changes stereochemistry by causing inversion at the carbon center due to a change from sp2 to sp3 hybridization, affecting chiral centers.
Step-by-step explanation:
The alkyl shift that occurs in trialkyl borane oxidation is a rearrangement that can alter the stereochemistry of the molecule. In these reactions, the migration of an alkyl group from boron to an adjacent carbon occurs simultaneously with the oxidation of boron, usually by hydrogen peroxide. Since boron in trialkyl boranes like BH3 is sp2 hybridized and trigonal planar, the substituents are originally in the same plane. During oxidation, the alkyl shift is accompanied by a change from planar sp2 to tetrahedral sp3 hybridization at carbon, which can invert the configuration at the carbon center if the migrating group moves to a carbon with established stereochemistry. This inversion can change chiral centers and thus alter the overall stereochemistry of the molecule.