Ruthenium-Catalyzed Silylation of Ethylene by Disilanes

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Abstract

The bis(dihydrogen) ruthenium complex RuH2(H2)2(PCy3)2 (1) and the ethylene complex RuΗ(C2Η4)[P(η3-C6H8)Cy2](PCy3) (2), obtained by addition of ethylene to 1, catalyze efficiently the silylation of ethylene with HSiMe2SiMe2H (I) and with the series of disilanes HSiMe2(CH2)nSiMe2H (n = 2, II; n = 3, III; n = 4, IV). Reaction of I with ethylene produces the monosilanes (CH2CH)SiMe2(CH2CH3) (I.b) and (CH3CH2)SiMe2(CH2CH3) (I.c) resulting from the cleavage of the Si−Si bond and the functionalization of the Si−H bonds. For II−IV, three processes, hydrosilylation, dehydrogenative silylation, and cyclization, are in competition, leading to the formation of acyclic monofunctionalized intermediates HSiMe2(CH2)nSiMe2(CHCH2) (II.f−IV.f) and HSiMe2(CH2)nSiMe2(CH2CH3) (II.g−IV.g), acyclic difunctionalized compounds (CH2CH)SiMe2(CH2)nSiMe2(CHCH2) (II.a−IV.a), (CH2CH)SiMe2(CH2)nSiMe2(CH2CH3) (II.b−IV.b), and (CH2CH3)SiMe2(CH2)nSiMe2(CH2CH3) (II.c−IV.c), together with cyclic products [(SiMe2)(CH2)3(SiMe2)](CHCH2) (II.d−IV.d) and [(SiMe2)(CH2)3(SiMe2)](CH2CH3) (II.e−IV.e). Kinetic studies reveal a consecutive reaction path for the formation of acyclic difunctionalized compounds via the monofunctionalized intermediates. Saturated and unsaturated cyclic products form concurrently to monofunctionalized disilanes. The rates and the selectivity of the reactions are dramatically influenced by the chain length between the two silicon atoms, the fastest conversion and the highest concentration of vinyl products being observed in the case of a long chain. In contrast, formation of cyclic compounds is favored for n = 2 or 3. Mechanistic studies show that different complexes are obtained, depending on the order of addition of ethylene and disilane. In the presence of the disilane II, 2 converts into a mixture of RuH2{(η2-HSiMe2)2(CH2)2}(PCy3)2 (3a) and RuH2(SiMe2(CH2)2SiMe2H){P(η3-C6H8)Cy2}(PCy3) (6). In contrast, in the presence of ethylene, RuH2{(η2-HSiMe2)2(CH2)n}(PCy3)2 (n = 2, 3a; n = 3, 3b), leads to the formation of a new ethylene complex RuH2(C2H4)2(PCy3)2 (4). Compound 4 is ultimately converted to 2 after total consumption of the liberated disilane. All these complexes are involved in the catalytic processes.

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