Page 266 - Arrow Pushing in Inorganic Chemistry A Logical Approach to the Chemistry of the Main Group Elements
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GROUP 16 ELEMENTS: THE CHALCOGENS
                246
                shown below:

                      O                    O
                                                 SeAr
                             ArSeX
                             via enol
                            or enolate
                                                    O      Ar                   O
                                                             +
                                                          Se
                                         [O]                   −  − ArSeOH
                                                              O


                                                                                  (6.133)
                  In a typical procedure, the carbonyl compound is deprotonated to yield the enolate,
                which is then selenylated by any of a number of selenylating agents ArSeX, where X = Cl,
                Br, CN, SeAr, and so on. The   -selenyl carbonyl compound is oxidized to the selenoxide
                by H O , a peroxyacid such as mCPBA, or ozone. The selenoxide is decomposed in situ,
                      2
                    2
                typically by warming, to yield the desired   ,  -unsaturated carbonyl compound.
                  We won’t discuss the mechanisms of the first two steps—selenylation and oxidation;
                you are welcome to work them out! For the selenoxide elimination step, stereochemical
                evidence suggests a five-membered cyclic transition state, as shown below:

                                     O      Ar                    O
                                              +
                                           Se   −
                                               O   − ArSeOH
                                                                                  (6.134)

                                            H
                                           H

                The following is a good example of the overall transformation:

                                   O                           O

                                                      S
                                             1. 2 LDA, Ph 2 2                     (6.135)
                                      N                           N
                                             2. mCPBA
                                             3. PhMe, 110 °C




                  REVIEW PROBLEM 6.23*
                  The Grieco–Sharpless elimination provides a mild and powerful method for dehy-
                  drating primary alcohols to terminal alkenes, as exemplified by the following reaction
                  sequence:
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