Stereomutation of Pentavalent Compounds: Validating the Berry Pseudorotation, Redressing Ugi’s Turnstile Rotation, and Revealing the Two- and Three-Arm Turnstiles

A general reaction mechanism describes the qualitative change in
chemical topology along the reaction pathway. On the basis of this
principle, we present a method to characterize intramolecular substituent
permutation in pentavalent compounds. A full description of the geometry
around five-coordinate atoms using internal coordinates enables the
analysis of the structural changes along the stereomutational intrinsic
reaction coordinate. The fluxional behavior of experimentally known
pentavalent phosphoranes, silicates, and transition-metal complexes
has been investigated by density functional theory calculations, and
three principal mechanisms have been identified: Berry pseudorotation,
threefold cyclic permutation, and half-twist axial−equatorial
interchange. The frequently cited turnstile rotation is shown to be
equivalent to the Berry pseudorotation. In combination with graph
theory, this approach provides a means to systematically investigate
the stereomutation of pentavalent molecules and potentially identify
hitherto-unknown mechanisms.