Page 209 - Mechanism and Theory in Organic Chemistry
P. 209

occur in resolvable enantioneric pairs, and those in which sulfur is part of a sub-
                stituted  ring system form  isolable geometric  (cis-trans)  isomers. In most  cases,
                the  product  of a  nucleophilic  substitution  on  tricoordinated  sulfur  is  found  to
                have the opposite configuration from the starting material.
                     For  example, by optical rotary dispersion  studies,  Mislow has shown  that
                Reaction 4.33 goes with  100 percent  inversion of c~nfiguration.~~












                 Similarly, Johnson  has carried out the following two-step isomeri~ation:~~















                 The product was obtained completely inverted in 93 percent yield from the sul-
                fonium  ion.  Since  inversion  on  sulfur  could  not  have  taken  place  in  the  0-
                 alkylation  step,  which  involves only  the  oxygen  atom,  it  must  have  occurred
                 during the displacement of -OCH,CH,  by  -OH.
                     What is  the structure of the transition state or intermediate that leads to
                 inversion ? All stable tetracoordinated sulfur compounds closely resemble trigonal
                 bipyramid~.~~ Thus it seems most likely that the tetracoordinated species leading
                to  substitution  on  tricoordinated  sulfur  will  also be  a  trigonal bipryamid.  We
                might immediately assume that the position the entering and leaving groups take
                up in  this  structure is  the same as their  position  in the transition  state for S,2
                 substitution at carbon-that   is,  that  the transition  state or intermediate would
                 be 11 in which the entering and leaving groups occupy the apical positions.










                 84  K.  Mislow, M.  M. Greene, P. Laur, J. T. Mellilo, T. Simmons, and A. L. Ternay, Jr.,  J. Amer.
                 Chem. SOC., 87, 1958 (1965).
                 O5  C. R. Johnson, J. Amer. Chem. SOC., 85, 1020 (1963).
                  D. J. Cram, J. Day, D.  R.  Rayner,  D.  M. v.  Schriltz, D. J. Duchamp,  and  D.  C.  Garwood, J.
                 Amer.  Chem. SOC., 92, 7369 (1970).
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