What is the role of the silanol auxiliary in this hydroxyselenylation? To answer this question, we conducted a series of control experiments (Scheme 6). With (E)-tert-butyl(hex-2-en-1-yloxy)dimethylsilane, regioisomeric selenohydrin products 5 and 6 formed in a combined yield of 35% (Scheme 6A). With trans-2-hexen-1-ol, selenohydrin product 7 formed in >20:1 regioselectivity and in 50% yield (Scheme 6B); this yield is ~20% lower than with the corresponding silanol starting material (Scheme 4, Compound 4a). Thus, a hydroxyl group is essential for regiocontrol in this and related reactions, 28-30 but a free alcohol leads to a drop in yield. Overall, we conclude that the silanol auxiliary serves as a unique protecting group, capable of maintaining >20:1 regioselectivity in the reaction due to presence of the Si-OH.