Bis(allyl)aluminum Cation, Tris(allyl)aluminum, and Tetrakis(allyl)aluminate: Synthesis, Characterization, and Reactivity

Cationic, neutral, and anionic aluminum allyl compounds were synthesized, and their reactivity toward electrophiles was studied. The THF adduct of the previously elusive tris(allyl)aluminum, [Al(eta(1)-C3H5)(3)(THF)] (1), was isolated as an oil, Protonolysis of one allyl ligand in 1 using [NEt3H]- [BPh4] gave the cationic bis(allyl)aluminum, a fragment of the crystalline [Al(eta(1)-C3H5)(2)(THF)(3-n)](+)- [BPh4](-)center dot(n+1)THF(n = 0, 1) (2). Single-crystal X-ray diffraction of [Al(eta(1)-C3H5)(2)(THF)(2)](+)[BPh4](-) (2a) revealed a tetrahedral aluminum center, while [Al(eta(1)-C3H5)(2)(THF)(3)](+)[BPh4](-) (2b) contains a trigonal-bipyramidal aluminum center with both allyl ligands in the equatorial plane. The tetrakis(allyl)aluminate K+[Al(eta(1)-C3H5)(4)](-) (3) was also synthesized from the reaction of 1 with K(C3H5). Reactions of the allyl compounds 1-3 with (i) benzophenone, (ii) allyl halides C3H5X (X = Cl, Br, I), and (iii) halogen X-2 (X = Br, I) showed considerable difference with respect to the ionic charge of the aluminum allyl.