Synthesis and structure of a four-coordinate aluminum alkyl cation/HB(C6F5)3 salt:: Implication in a B(C6F5)3-catalyzed hydroalumination reaction of benzophenone or benzaldehyde

The reaction of the bulky aminophenol 2-(CH2NMe2)-4-Me-6-CPh3-C6H3OH (1) with Al(Bu-i)(3) yields via isobutane elimination the formation of the diisobutyl Al complex {6-(CH2NMe2)-2-CPh3-4-Me-C6H2O}Al(Bu-i)(2) (2) in high yield. Reaction of 2 with B(C6F5)(3) in the presence of NMe2Ph affords, along with isobutene, the salt compound [{6-(CH2NMe2)-2CPh(3)-4-Me-C6H2O}Al(Bu-i)(NMe2Ph)][HB(C6F5)(3)] (3), consisting of a four-coordinate Lewis acidic Al cation associated with the borohydride HB(C6F5)(3)(-), whose solid-state structure was determined by X-ray crystallography. When complex 2 is reacted with B(C6F5)(3) (1 and 0.05 equiv) in the presence of benzophenone and benzaldehyde, the quantitative conversion to the hydroalumination products 4 and 5, respectively, is observed. This B(C6F5)(3)-catalyzed hydroalumination reaction, via a B(C6F5)(3)-mediated hydride abstraction/transfer reaction, represents a new type of reactivity for Al alkyl cations and illustrates the higher reactivity of Al cation[HB(C6F5)(3)(-) salts versus the classical Al cation/MeB(C6F5)(3)(-) and B(C6F5)(4)(-) salts.