Synthesis of Organosilicon Macrocycles. Palladium-Catalyzed Ring-Enlargement Oligomerization of Cyclic Disilanes via Si−Si σ-Bond Metathesis

Bis( tert -alkyl isocyanide)palladium(0) complexes catalyzed a ring-enlargement oligomerization of 1,1,2,2-tetramethyl-1,2-disilacyclopentane through Si−Si σ-bond metathesis to give cyclic oligomers up to the 40-membered octamer. The cyclic structure of the tetramer was established by the single-crystal X-ray diffraction method, which showed that the four Si−Si bonds of the tetramer were in a close to parallel arrangement. Reactivities of the dimer, trimer, and tetramer in the presence of bis( tert -butyl isocyanide)palladium(0) revealed that the present Si−Si σ-bond metathesis proceeded reversibly, in which the −Me 2 Si(CH 2 ) 3 SiMe 2 − unit of the disilacyclopentane was successively inserted into the Si−Si bond of the oligomer to give higher oligomers. The intermediary 6-membered bis(organosilyl)bis( tert -alkyl isocyanide)palladium complex, which arose from oxidative addition of the disilacyclopentane onto bis( tert -alkyl isocyanide)palladium(0), was isolated and characterized by the single-crystal X-ray method. The insertion of the −Me 2 Si(CH 2 ) 3 SiMe 2 − unit also occurred with linear disilanes and digermane to give linear oligomers. The cyclic trimer and tetramer reacted with 2,6-diisopropylphenyl isocyanide in the presence of Pd(OAc) 2 to afford an 18-membered-ring triimine and 24-membered-ring tetraimine, respectively, in which the isocyanides were inserted into all the Si−Si linkages. Oxidation of the cyclic oligomers with trimethylamine oxide gave the corresponding cyclic oligo(disiloxane) species in quantitative yields. Unexpected polymerization of the disilacyclopentane occurred in the presence of the (η 5 -cyclopentadienyl)(η 3 -allyl)palladium(II) catalyst to give poly(Me 2 Si(CH 2 ) 3 SiMe 2 ) compounds with high molecular weights.