Page 210 - Mechanism and Theory in Organic Chemistry
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Bimolecular Nucleophilic Substitution at Sulfur  199

     But Westheimer has pointed  out that inversion would also result if the entering
     and leaving groups occupy radial positions as in 12.67

                                      X.,,  j'
                                      R-S-R




     Some displacements on tricoordinated sulfur in which entering and leaving groups
     are linked proceed with inversion on sulfur. These apparently must have a transi-
     tion state similar to 12. For example, in Reaction 4.35 the entering and leaving
     groups are most likely part of the same six-membered ring in the activated com-
     plex-a   formation that cannot be  accommodated by 11 (180" bond angles be-
     tween X and Y) but can be by 12.68 Cram has suggested that if the entering and












                                          TsN=S=NTs      H,C,
                                                +      +      S--t  NTs   (4.35)
                                                            ./ :
                                            TsN=S=O         .,
                                                              Br
      leaving  groups  are  the  most  electronegative  groups of  the trigonal bipyramid
      and if the entering and leaving groups  are not  tied  together  in  a  ring system,
      then structure 11 is of lower energy. If either of these conditions is not fulfilled,
      then 12 may be more stable.
          Retention of configuration can only occur, in a reaction that has a trigonal
      bipyramidal  transition  state,  if  the  leaving  group occupies  an apical,  and  the
      entering group a radial position  (or vice versa), as in 13. Three cases of retention









       (a) P. C. Haake and F. H. Westheimer, J. Amer. Chem. Soc., 83,  1 102 (1961). (b) The electron pair
      in 12 is shown in the equatorial rather than in the axial position  because the axial positions are pre-
      ferred  by  the more  electronegative groups and  the equatorial  by  the  more  electropositive  groups.
      (c) The structure shown  in  11 arises from attack of  the  nucleophile on the face of  the tetrahedron
      opposite to the apex occupied by  the leaving group. We have already discussed  the simple bonding
      model for 11 in Section 4.2 (p.  175) ; it is readily apparent from this model that 11 could be a transi-
      tion state for nucleophilic  substitution on any sp3 hybridized atom. The structures 12 and 13, on the
      other hand, arise from attack of the nucleophile on an edge of the tetrahedron. If the central atom is
      carbon or some other element that does not  have empty d orbitals, then there is no simple  bonding
      model that corresponds to  12 or  13; if d orbitals are present, oriented toward the edge of  the tetra-
      hedron being attacked, they can accept the electrons of the nucleophile.
        See note 63,  p.  197 and 66, p.  198.
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