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5B.10 PHOSPHAZENES  181


                  REVIEW PROBLEM 5B.16
                  Suggest a mechanism for the general phosphazene synthesis mentioned above
                  (5B.49).



                  At present, phosphazenes are probably best known as neutral organic superbases, an
               application pioneered by Reinhard Schwesinger in the 1980s. They are a big step up from
               classic amine, amidine, and guanidine bases in terms of basicity. Two commonly used phos-
               phazene bases, BEMP and t-Bu-P4 (“Schwesinger base”) are depicted in Figure 5B.3; the
               pK ’s of their conjugate acids are also compared with those of selected traditional nitrogen
                  a
               bases:

                                                         R 2               R 5
                         N                           N                  N

                                       N                    R 3  R 1           R 3
                                                  R 1    N           N      N
                                                         R 4         R 2   R 4
                         N
                       DMAP        Hünig’s base     Amidine          Guanidine
                    pK a(BH+)  ~9.2  pK a(BH+)  ~11.4  pK a(BH+)  ~12  pK a(BH+)  ~11.5–13.0

                                                               t-Bu
                                                             −
                                                       NMe 2  N       NMe 2
                             N     NEt 2             +          +       +
                               +  P    t-Bu     Me 2 N  P  N  P   N   P   NMe 2
                                    N                      −      −
                                N   −                  NMe 2  −  N    NMe 2
                                                                +
                                                       Me 2 N  P  NMe 2
                                                              NMe 2
                               BEMP                          t-Bu-P4
                            pK a(BH+)  ~27.5               pK a(BH+)  ~42.4
                          Selected uncharged nitrogen bases and pK ’s of their protonated forms (the higher the
                                                         a
               Figure 5B.3
               value, the stronger the base).

                  The highly basic phosphazenes thus provide an alternative to organolithium bases such
               as lithium dialkylamides and alkyllithiums. Milder reaction conditions and better solubility
               are two key advantages associated with phosphazenes. Another important consideration is
               the lack of a coordinating cation: naked enolates and other anions obtained with phosp-
               hazenes often exhibit enhanced reactivity, relative to their lithium salts.
                  Phosphazene synthesis is conceptually rather simple, as illustrated below for a “P1 base.”
               The P–N skeleton is first put together via a series of nucleophilic displacements (presum-
               ably of the S 2-Si type):
                          N
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