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
−Me2Si(CH2)3SiMe2−
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
−Me2Si(CH2)3SiMe2−
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
(η5-cyclopentadienyl)(η3-allyl)palladium(II)
catalyst to give
poly(Me2Si(CH2)3SiMe2)
compounds with high molecular weights.
In CH2C12 solution the phosphinate anion BrCH2CH2(CH2),CH2(Ph)P(0)Ocyclises only 4.3 times faster when n = 0 (five-membered ring product) than when n = 1; for the thiophosphinate anion CICH2CH2(CH2),CH2(Ph)P(S)Ocyclisation (via sulfur) is still only 30 times faster when n = 0 than when n = 1.
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