PENTAMETHYLDISILANYL-SUBSTITUTED CYCLOPEN TADIENES - SYNTHESIS, STRUCTURE AND DYNAMIC BEHAVIOR

The pentamethyldisilanyl-substituted cyclopentadienes Me(n)C(5)H(6-n-m )(Si(2)Me(5))(m) (for n = 0: 1 (m = 1), 2 (m = 2), 3 (m = 3), 4 (m = 4 ); for n = 1: 5 (m = 1), 7 (m = 2), 9 (m = 3); for n = 3: 13 (m = 1), 14 (m = 2); for n = 4: 15 (m = I)) are accessible in good yields by tr eatment of the corresponding cyclopentadienyllithium compounds with Me (5)Si(2)Cl. The mono-Me(5)Si(2)-substituted species 1 and 5 are presen t only to a small extend in form of vinylic isomers and to a greater e xtend as isomers with the Me(5)Si(2)-group in allylic position; the la tter possess a dynamic structure due to sigmatropic rearrangements. In the twice-Me(5)Si(2)-substituted cyclopentadienes 2 and 7, the 5,5 an d 2,5 isomers are observed, which can be interconverted by silatropic shifts; in addition, the presence of two vinylic isomers can be proved in the case of 2. In the cyclopentadiene species 3 and 9 with three M e(5)Si(2) groups, only the 2,5,5 isomers can be detected by NMR spectr oscopy. Compound 3 possesses a fluxional structure and can thus be dep rotonated. On the other hand, 9 does not show a fluxional behaviour an d thus cannot be deprotonated. The cyclopentadiene 4 with four Me(5)Si (2) substituents possesses a static structure and cannot be deprotonat ed. The 2,3,5,5 position of the substituents is proved by an X-ray cry stal structure analysis. Only two Me(5)Si(2) groups can be incorporate d in the carbon skeleton of 1,2,4-trimethylcyclopentadiene, whereby co mpounds of the type 1,2,4-Me(3)C(5)H(3-n)(Si(2)Me(5))(n) (13: n = 1; 1 4: n = 2) are formed. Surprisingly, 14 cannot be deprotonated with (n) BuLi and KH, respectively. The reaction of Me(4)C(5)HLi with Me(5)Si(2 )Cl leads to the cyclopentadiene Me(4)C(5)HSi(2)Me(5) (15). Though com pound 15 can be deprotonated, further reaction of the resulting anion with Me(5)Si(2)Cl does not lead to the expected cyclopentadiene Me(4)C (5)(Si(2)Me(5))(2) (16). On the other hand, 16 can be prepared by meta llation of 14 with C8K and further reaction with CH3I. In contrast to 14, compound 4 cannot be deprotonated with C8K; the reaction of 4 with C8K and CH3I leads to 9 via Si-C bond splitting. The pentamethyldisil anyl-substituted pentamethylcyclopentadiene Me(5)C(5)Si(2)Me(5) (17) i s obtained by reaction of Me(5)C(5)K with Me(5)Si(2)Cl; compound 17 sh ows dynamic behaviour; the migration of the Me(5)Si(2) group is slower than that of the Me(3)Si group in Me(5)C(5)SiMe(3). Three ElMe(3) gro ups can be introduced stepwise into the 1,2,4-Me(3)C(5)H(3) molecule, as demonstrated by the exemplary synthesis of the cyclopentadienes 1,2 ,4-Me(3)C(5)H(3-n)(SiMe(3))(n) (10: n = 1; 11: n = 2) and 1,2,4-Me(3)C (5)(SiMe(3))(2)SnMe(3) (12).