| Abstract
| - Facile E,Z-isomerization due to the rotation around the SiSi double bond occurs in SiASiBSiSiSiASiB (SiA = t-BuMe2Si and SiB = i-Pr2MeSi). The free energy of activation as determined by dynamic 29Si NMR is ca. 15 kcal/mol at 300 K, which is 10 kcal/mol lower than those of other stable disilenes with aryl and alkyl substituents.
- Stable disilenes having two different trialkylsilyl substituents [SiASiBSiSiSiASiB ((E)-2 and (Z)-2)and SiA2SiSiSiB2 (3), where SiA = t-BuMe2Si and SiB= i-Pr2MeSi] were prepared and characterized. Disilene(E)-2 was found by X-ray crystallography to have thehighest planarity around the SiSi bond among isolatedtetrasilyldisilenes. Temperature-dependent 29Si NMRspectra of a mixture of (E)- and (Z)-2 revealed the facileintramolecular isomerization between (E)-2 and (Z)-2with a free energy of activation less than 15 kcal/molat 300 K; the value is ca. 10 kcal/mol smaller than thoseof E,Z-isomerization of other known stable disilenes witharyl or alkyl substituents. The origin of the low barrierfor the silyl-substituted disilenes is ascribed to theeffective σ−π interaction at the transition state of theisomerization. In addition to the rotational isomerization, the isomerization of 2 to 3 was also observed athigher temperatures.
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