Selenonium salts metallation

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

Synthesis of a-selenovinyl metals (1 -seleno-1-alkenyl metals) by metallation of vinyl selenides 2.6.2.23 Synthesis of a-metalloalkyl selenoxides, selenones and selenonium salts... 

Among these methods, the metallation reaction is the method of choice for selenides bearing electron-withdrawing groups able to stabilize the carbanionic center. This also applies to selenoxides, selenones and selenonium salts. 

Alkyl metals are not suitable for the metallation of tiiese compounds. For example, BuaU reacts at -78 C on the seletuum atom of selenonium salts and produces a novel organolithium in which the carb-aruonic center is more stabilized (Scheme 50, a and b), whereas alkylmagnesium bronudes and chlmides unexpectedly lead to decyl bromide and chloride respectively on reaction with decyl phenyl selenrme (Scheme 50, c). 

Allyldimethyl- and allylmethylphenyl-selenonium salts react under similar conditions with potassium r-butoxide (THF, 20 "C, 20 h) and lead to homoallyl selenides in very high yields, including the particularly strained alkylidenecyclopropane derivatives (Scheme 65, b). These result from the metallation of... 

---