Page 206 - Mechanism and Theory in Organic Chemistry

Bimolecular Nucleophilic Substitution at Sulfur 195
Figure 4.7 Overlap of a d orbital on sulfur with a ,!I orbital on an entering nucleophile.
Dicoordinated Sulfur
In its outer electronic shell divalent sulfur has two s and four p electrons, and it
also has five empty 3d orbitals. A bimolecular nucleophilic displacement reaction
on sulfur might then occur in a single step; or an intermediate such as 7, in which
the sulfur accepts the pair of electrons of the entering Lewis base into one of its
X
+ I
Y: + X-S-R - Y-S-R
empty d orbitals, might be on the reaction path. For example, Figure 4.7 shows
the overlap of an empty 3d orbital with a fullp orbital on an incoming nucleophile.
The available evidence suggests that a one-step displacement is the usual
59 For example,
pathway, but that some reactions may involve an ir~terrnediate.~~,
if 7 does lie on the reaction path, then electron-withdrawing substituents on sulfur
should stabilize it and the transition states for its formation and decomposition:
The reaction should be faster than if there are electron-releasing groups on sulfur.
The data in Table 4.9, however, show that for Reaction 4.27 the rate is acceler-
ated by electron-withdrawing and electron-donating substituents in much the
same way as are rates of direct displacements on para-substituted benzyl chlorides.
An intermediate such as 7 thus seems precluded from the pathway for this
reaction. Substitutions on divalent sulfur normally proceed in a one-step dis-
placement in which both bond making and bond breaking occur at the transition
state. When electron-donating substituents are present in the substrate, bond
breaking is further advanced than bond making; and when electron-with-
drawing groups are there, the opposite is true. In either case the transition
state can be stabilized (see also Section 4.3, p. 183).
J. rimer. Chem. Soc., 90, 4076 (1968); (c) M. A. Sabol and K. K. Andersen, J. Amer. Chem. Soc., 91,
3603 (1 969).
,
,
58 For recent summaries of cases in which an intermediate may be involved in nucleophilic substi-
tution on dicoordinate sulfur, see: (a) E. Ciuffarin and F. Griselli, J. Amer. Chem. Soc., 92, 6015
(1970); (b) L. Senatore, E. Ciuffarin, and A. Fava, J. Amer. Chem. Soc., 92, 3035 (1970); (c) E.
Ciuffarin, J. Org. Chem., 35, 2006 (1970).
59 Pryor has suggested that the addition-elimination mechanism involving intermediate 7 occurs
when the attacking group is highly nucleophilic, the leaving group poor, and the centralsulfur highly
electronegative (see note 54 (c), p. 194). These same criteria for a two step mechanism would also
arise from a consideration of a two-dimensional reaction coordinatc diagram.