Discussion
In the following reaction
the major products X and Y, respectively, are:
- A.
- B.
- C.
- D.
Answer: C
X and Y are 2-methyl-2-(2-phenylsulfinylethyl)-1,3-dithiane and methyl vinyl ketone.
n-BuLi removes the C2 proton of 2-methyl-1,3-dithiane, giving an acyl-anion equivalent. It adds in conjugate fashion to phenyl vinyl sulfoxide, a Michael acceptor, so X carries a CH2CH2S(O)Ph chain on C2.
Refluxing in xylene triggers a syn (Ei) sulfoxide elimination through a five-membered cyclic transition state, losing PhSOH and leaving a vinyl group. HgCl2 with CdCO3 in water then hydrolyses the dithiane to the ketone, giving CH3COCH=CH2, methyl vinyl ketone. The phenyl vinyl sulfoxide has served as a vinyl cation equivalent.
(A) shows the vinyl compound as X, but the elimination needs the heating of the second step. (B) keeps a phenyl in Y, though phenyl leaves with the sulfur. (D) has the anion attacking sulfur rather than the alkene.
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