控制原因Stereoelectronic Effects- The attacking electrophile must obey the principle of maximum overlap of the participating orbitals by perpendicular approach to the plane of atoms which constitute the enolate (enol) function.
C. Stereoselectivity
Also applies to protonation in reprotonation reaction:
Nucleophilic addition to carbonyl compound takes place not at 90°(perpendicular)    but at an angle of 105 ± 5°
Dunitz Tetrahedron 1974, 30, 1563.