Schubert establishes the tonic not with a solid chord, but through a flowing accompaniment that hints at the minor mode immediately.
How do we get from E-flat major to B minor? The distance is a (E-flat to A) followed by a diminished fifth to B. In classical theory, this is a brutal, Neapolitan-like leap. But Schubert bridges it with a single, magical chord: the E-flat diminished seventh at the end of bar 54, which resolves enharmonically to the dominant of B minor (F-sharp major) in bar 55. schubert impromptu op 90 no 2 harmonic analysis