3,3-Dimethylbut-1-ene is treated with $\mathrm{Hg}(\mathrm{OAc}){}_2$ in water/THF, followed by $\mathrm{NaBH}{}_4$. The major product is

Oxymercuration-demercuration gives Markovnikov hydration through a mercurinium ion, so no carbocation rearrangement takes place. OH goes to C-2 and the product is $(\mathrm{CH}{}_3){}_3\mathrm{C}-\mathrm{CH}(\mathrm{OH})-\mathrm{CH}{}_3$. The rearranged 2,3-dimethylbutan-2-ol would form only in acid-catalysed hydration.