Page 212 - Mechanism and Theory in Organic Chemistry

Bimolecular Nucleophilic Substitution at Sulfur 201

starting material. Thus 15 must either be formed in a slow, nonreversible first
step, or 15 is a transition state, not an intermediate.71
Kice, however, has suggested that this 180 exchange experiment is incon-
If
clusi~e.~~ hydroxide ion attacks 14, the most likely first intermediate would be
17, in which the five-membered ring spans an apical and an equatorial position
and in which 0 - and the electron pair (which are less electronegative than OH 73)
occupy equatorial positions. This intermediate is not isoenergetic with 18, the

result of a simple proton transfer, because in 18 the most electronegative ligand,
OH, occupies an equatorial position.74 Kice suggests that 17 might be formed in a
rapid preequilibrium, and its presence might not be detected in the experiment
described above.

Tetracoordinated Sulfur
The mechanism of nucleophilic displacements on tetracoordinated sulfur has
not yet been much studied, but the formation of an intermediate seems possible.
For example, when sulfonate esters are hydrolyzed with 180-enriched
hydroxide (Reaction 4.38) the product sulfonic acids contain 180 but the starting
material recovered after 50 percent reaction does not.75 By the arguments out-
lined on p. 200 this might be evidence for either a one-step displacement or a two-
step mechanism with the first step rate-determining. The latter is made more

0 0
II I1
H180- + R-S-OR + HlsO-S-R + -OR (4.38)
II II
0 0
plausible by the work of Ciuffarin, who has studied the reactions of several
nucleophiles with 19 and has found that the rates of displacement of X are very
similar for X = C1, Br, and I. If departure of the leaving group were involved in

In other reactions on tricoordinate sulfur, not yet well understood, intermediates analogous to 15
areformed. See, for example, B. M. Trost, R. LaRochelle, and R. C. Atkins, J. Amer. Chem. Soc., 91,
2175 (1969) and references in this paper and in E. N. Givens and H. Kwart, J. Amer. Ch. Soc., 90,
378, 386 (1968). See also N. E. Hester, Znt. J. Sulfur Chem., 8, 119 (1973).
J. L. Kice and C. A. Walters, J. Amer. Chem. SOC., 94, 590 (1972).
73 See note 67 (b), p. 199.
74 See note 67 (b), p. 199.
75 D. R. Christman and S. Oae, Chem. Znd. (London), 1251 (1959).