Reactivity of silyl-substituted allyl compounds with group 4, 5, 9, and 10 metals: Routes to eta(3)-allyls, alkylidenes, and sec-alkyl carbocations

Whereas the reaction of alkali-metal salts of silyl-allyls E+[C3H3(SiMe3)(2)-1,3](-) (E = Li, K) with group 4 and group 5 metal halides gave intractable reduction products, Co(acac)(3) and Ni(acac)(2) reacted with K[C3H3(SiMe3)(2)-1,3] to give Co{eta(3)-C3H3(SiMe3)(2)-1,3}(2) (1) and Ni-{eta(3)-C3H3(SiMe3)(2)-1,3}(2) (2), respectively. The reaction of K[C3H3(SiMe3)(2)-1,3] with Me3SnCl afforded Me3SiCH=CHCH(SiMe3)(SnMe3) (3), which reacted cleanly with TaCl5 to give {eta(3)-C3H3(SiMe3)(2)-1,3}TaCl4 (4). Treatment of this complex with tetramethylethylenediamine led to HCl abstraction, and the allyl complex was transformed into the vinyl-alkylidene compound Me3SiCH=CHC(SiMe3)=TaCl3(TMEDA) (5). Whereas in the case of TaCl5 dehalostannylation was facile, the reaction of 3 with ZrCl4 and HFCl4 took a different course, leading instead to the addition of Me3Sn+ to 3 to give [HC{CH(SiMe3)(SnMe3)}(2)](+)[M2Cl9](-) (6, M = Zr; 7, M = Hf), the first examples of isolable sec-alkyl carbocations. These salts are surprisingly thermally stable and melt > 100 degrees C; this stability is largely due to delocalization of the positive charge over the two tin atoms. The crystal structures of 1, 2, and 5-7 are reported.