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

Abstract

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.

The modern interpretation of the Wittig reaction mechanism

“…90 Indeed, it 65 is only recently that definitive computational and experimental results have appeared that shed light on the elusive details of this part of the mechanism (see the section on the modern mechanstic interpretation for the Wittig reaction). 40,91 In any case, it is the origin of the stereoselectivity induced in the OPA intermediate 70 that is of greatest interest since this stereochemistry is retained exactly in the alkene product after decomposition, and this was the primary focus in the work of Vedejs and co-workers.…”

“…90 50 Recently, Lamertsma & co-workers have shown that permutation of ligands in trigonal bipyramidal species may occur in a single step by processes that are comprised of multiple consecutive Berry pseudorotations. 91,101,102 González, López-Ortiz & co-workers have shown that one or more of these single 55 step mechanisms is highly likely to operate in the interconversion (stereomutation) of OPAs very similar to those produced in typical Wittig reactions. 40 Although there exists at least one example of a Wittig reaction of a chiral ylide in which the chirality of the phosphorus centre is retained in the phosphine 60 oxide product, 79 the implication of the results of González, López-Ortiz & co-workers is that retention of configuration at phosphorus is unlikely in any Wittig reaction in which OPA stereomutation is faster than OPA decomposition.…”

Chiral Hypervalent, Pentacoordinated Phosphoranes

“…It should be noted here that the very recent DFT calculations of the fluxional behavior of experimentally known pentavalent molecules have suggested the equivalence of the turnstile rotation with the Berry pseudorotation. This suggestion is based on identification of three principal mechanisms by which the substituents interchange can be achieved (Berry pseudorotation, threefold cyclic permutation, and half-twist axial-equatorial interchange) [27]. ( Figure 3).…”

Synthesis of 2,3-Dihydro-1-phenylbenzo[b]phosphole (1-Phenylphosphindane) and Its Use as a Mechanistic Test in the Asymmetric Appel Reaction: Decisive Evidence against Involvement of Pseudorotation in the Stereoselecting Step

“…3 Such intermediates can undergo a fast intramolecular ligand exchange process, termed Berry pseudorotation,4,5 that may have a significant effect on the outcome of the reaction. 6,7 However, in the case of the Appel reaction conditions, 8 while the involvement of chloro-and alkoxy-phosphonium species is well established, 9,10,11 the extent of participation of pentaco-ordinate phosphorane intermediates is less clear.…”