Facile and systematic access to the least-coordinating WCA [(RO)Al-F-Al(OR)] and its more Lewis-basic brother [F-Al(OR)] (R = C(CF))

By reaction of the Lewis acid Me 3 Si-F-Al(OR F ) 3 with a series of [PF 6 ] − salts, gaseous PF 5 and Me 3 Si-F are liberated and salts of the anion [F-Al(OR F ) 3 ] − ([f-al] − ; R F = C(CF 3 ) 3 ) can be obtained. By addition of another equivalent of Me 3 Si-F-Al(OR F ) 3 to [f-al] − , gaseous Me 3 Si-F is released and salts of the least coordinating anion [(R F O) 3 Al-F-Al(OR F ) 3 ] − ([al-f-al] − ) are formed. Both procedures work for a series of synthetically useful cations including Ag + , [NO] + , [Ph 3 C] + and in very clean reactions with 5 g batch sizes giving excellent yields typically exceeding 90%. In addition, the synthesis of Me 3 Si-F-Al(OR F ) 3 has been optimized and scaled up to 85 g batches in an one-pot procedure. These anions could previously only be obtained by difficult to control decomposition reactions of [Al(OR F ) 4 ] − or by halide abstraction reactions with Me 3 Si-F-Al(OR F ) 3 , generating relatively large countercations that are unsuited for further use as universal starting materials. Especially [al-f-al] − is of interest for the stabilization of reactive cations, since it is even weaker coordinating than [Al(OR F ) 4 ] − and more stable against strong electrophiles. This bridged anion can be seen as an adduct of [f-al] − and Al(OR F ) 3 . Thus, it is similarly Lewis acidic as BF 3 and eventually reacts with nucleophiles (Nu) from the reaction environment to yield Nu-Al(OR F ) 3 and [f-al] − . This prevents working with [al-f-al] − salts in ethereal or other donor solvents. By contrast, the [f-al] − anion is no longer Lewis acidic and may therefore be used for reactions involving stronger nucleophiles than the [al-f-al] − anion can withstand. Subsequently it may be transformed into the [al-f-al] − salt by simple addition of one equivalent of Me 3 Si-F-Al(OR F ) 3 . The least-coordinating anion [(R F O) 3 Al-F-Al(OR F ) 3 ] − and its more Lewis-basic brother [F-Al(OR F ) 3 ] − (R F = C(CF 3 ) 3 ).