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4.11 GLYCOL CLEAVAGE BY LEAD TETRAACETATE  125
               trivalent thallium, many tetravalent lead compounds are oxidizing. The functioning of the
               now-discontinued antiknock agent tetraethyllead depends on its facile dissociation to lead
               atoms and ethyl radicals, which results in smooth, controlled combustion of gasoline. In
               organic chemistry, LTA in acetic acid is widely used as an oxidizing agent, particularly for
               cleaving glycols (1,2-diols):



                   HO        OH     AcO                    R 1               R 4
                                                − Pb(OAc) 2
                       C   C    +      Pb   OAc              C   O +   O   C      (4.63)
                   R 1       3  R 4  AcO          − HOAc   R 2               R 3
                     R 2    R        AcO


                  The mechanism involves the formation of a cyclic tetravalent “plumbate ester” interme-
               diate:

                         AcO                        OAc                   OAc
                                                AcO                   AcO
                            Pb  OAc
                                                    Pb  OAc               Pb
                       AcO                                                    OAc
                                                               − HOAc
                          AcO
                  H   O      O   H            H   O +    O   H          O      O   H
                                          −  OAc
                        C  C                        C   C                 C   C
                     R 1      R 4                R 1  2   3  R 4       R 1       R 4
                       R 2  R 3                    R     R               R 2  R 3
                                        OAc
                                   AcO
                                                              AcO   OAc
                                        Pb  OAc
                                                                 Pb
                                      O      O   H
                                                               O     O
                                       C   C
                                                                C  C
                                     R 1      R 4           R 1        4
                                      R 2   R 3                2      R
                                                              R     R 3
                                                                                  (4.64)
               The five-membered ring then disintegrates via a pericyclic process to generate two carbonyl
               fragments and lead diacetate.

                               AcO    OAc

                                   Pb
                                O     O               R 1              R 4
                                           − Pb(OAc) 2                            (4.65)
                                 C   C                   C   O +  O  C
                             R 1         R 4
                                R 2   R 3             R 2              R 3


                  Interestingly, a cyclic mechanism is not an absolute requirement for explaining the for-
               mation of the observed products. An E2-like elimination from a plumbate monoester can
               also account for the observed products.
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