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5B.11 THE COREY–WINTER OLEFINATION  185
               5B.11*  THE COREY–WINTER OLEFINATION


               This two-step sequence for converting vicinal diols to olefins is of considerable mechanistic
               interest. The reagents are thiophosgene (CSCl ) for the first step and a trialkylphosphite,
                                                     2
               P(OR) , for the second.
                     3
                                                    S

                                HO       OH         C       R 1      R 4
                                              1.  Cl    Cl                       (5B.63)
                                   C   C                       C   C
                                                 2. P(OR) 3
                                R 1       R 4                R 2     R 3
                                  R 2   R 3
               The first step is relatively unremarkable, analogous to standard carbonyl chemistry:
                                                  H
                              Cl                     +   Cl             1
                     H                       R 1   O      Cl          R
                                 C   S     R 2                      R 2      O
                        O                       C      C                C
                   R 1  C     Cl                C      S −                     C   S
                  R 2                      R 3                      R 3  C
                         C   O                4    O                         O
                                             R                         4
                                                                      R
                      R 3  R 4  H                 H
                                                                                 (5B.64)
               The second step, involving the reaction of the cyclic thiocarbonate intermediate formed
               above with trialkylphosphite, is of great interest. The phosphite P attacks the S of the thio-
               carbonyl group, generating a cyclic dialkoxycarbene, as shown below:


                     R 1                            R 1         RO  +
                  R 2      O            OR       R 2      O        P   OR
                       C                              C       −       OR
                              C   S    P                     C   S
                       C                  OR          C
                   R 3     O           OR        R 3      O
                     R 4                            R 4     R 1
                                                         R 2      O             OR
                                                              C           −   +
                                                                     C  +  S   P
                                                              C                   OR
                                                         R 3      O             OR
                                                            R 4
                                                                                 (5B.65)
               Carbene formation appears to be facilitated by at least two significant driving forces: forma-
                                    +
                                       –
               tion of the rather stable P –S (BDE 335 kJ/mol) linkage, and the electronic stabilization
               of the carbene by the alkoxy groups.
                  A second molecule of trialkylphosphite now reacts with the dialkoxycarbene intermedi-
               ate to generate the olefin product.
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