Ruthenium-Catalyzed Silylation of Ethylene by Disilanes

The bis(dihydrogen) ruthenium complex RuH<INF>2</INF>(H<INF>2</INF>)<INF>2</INF>(PCy<INF>3</INF>)<INF>2</INF> (<BO>1</BO>) and the ethylene complex RuΗ(C<INF>2</INF>Η<INF>4</INF>)[P(η³-C<INF>6</INF>H<INF>8</INF>)Cy<INF>2</INF>](PCy<INF>3</INF>) (<BO>2</BO>), obtained by addition of ethylene to <BO>1</BO>, catalyze efficiently the silylation of ethylene with HSiMe<INF>2</INF>SiMe<INF>2</INF>H (<BO>I</BO>) and with the series of disilanes HSiMe<INF>2</INF>(CH<INF>2</INF>)<INF>n</INF><INF></INF>SiMe<INF>2</INF>H (n = 2, <BO>II</BO>; n = 3, <BO>III</BO>; n = 4, <BO>IV</BO>). Reaction of <BO>I</BO> with ethylene produces the monosilanes (CH<INF>2</INF>&dbd;CH)SiMe<INF>2</INF>(CH<INF>2</INF>CH<INF>3</INF>) (<BO>I.b</BO>) and (CH<INF>3</INF>CH<INF>2</INF>)SiMe<INF>2</INF>(CH<INF>2</INF>CH<INF>3</INF>) (<BO>I.c</BO>) resulting from the cleavage of the Si−Si bond and the functionalization of the Si−H bonds. For <BO>II</BO><BO>−</BO><BO>IV</BO>, three processes, hydrosilylation, dehydrogenative silylation, and cyclization, are in competition, leading to the formation of acyclic monofunctionalized intermediates HSiMe<INF>2</INF>(CH<INF>2</INF>)<INF>n</INF><INF></INF>SiMe<INF>2</INF>(CH&dbd;CH<INF>2</INF>) (<BO>II.f</BO><BO>−</BO><BO>IV.f</BO>) and HSiMe<INF>2</INF>(CH<INF>2</INF>)<INF>n</INF><INF></INF>SiMe<INF>2</INF>(CH<INF>2</INF>CH<INF>3</INF>) (<BO>II.g</BO><BO>−</BO><BO>IV.g</BO>), acyclic difunctionalized compounds (CH<INF>2</INF>&dbd;CH)SiMe<INF>2</INF>(CH<INF>2</INF>)<INF>n</INF><INF></INF>SiMe<INF>2</INF>(CH&dbd;CH<INF>2</INF>) (<BO>II.a</BO><BO>−</BO><BO>IV.a</BO>), (CH<INF>2</INF>&dbd;CH)SiMe<INF>2</INF>(CH<INF>2</INF>)<INF>n</INF><INF></INF>SiMe<INF>2</INF>(CH<INF>2</INF>CH<INF>3</INF>) (<BO>II.b</BO><BO>−</BO><BO>IV.b</BO>), and (CH<INF>2</INF>CH<INF>3</INF>)SiMe<INF>2</INF>(CH<INF>2</INF>)<INF>n</INF><INF></INF>SiMe<INF>2</INF>(CH<INF>2</INF>CH<INF>3</INF>) (<BO>II.c</BO><BO>−</BO><BO>IV.c</BO>), together with cyclic products [(SiMe<INF>2</INF>)(CH<INF>2</INF>)<INF>3</INF>(SiMe<INF>2</INF>)](CH&dbd;CH<INF>2</INF>) (<BO>II.d</BO><BO>−</BO><BO>IV.d</BO>) and [(SiMe<INF>2</INF>)(CH<INF>2</INF>)<INF>3</INF>(SiMe<INF>2</INF>)](CH<INF>2</INF>CH<INF>3</INF>) (<BO>II.e</BO><BO>−</BO><BO>IV.e</BO>). 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 <BO>II</BO>, <BO>2</BO> converts into a mixture of RuH<INF>2</INF>{(η²-HSiMe<INF>2</INF>)<INF>2</INF>(CH<INF>2</INF>)<INF>2</INF>}(PCy<INF>3</INF>)<INF>2</INF> (<BO>3a</BO>) and RuH<INF>2</INF>(SiMe<INF>2</INF>(CH<INF>2</INF>)<INF>2</INF>SiMe<INF>2</INF>H){P(η³-C<INF>6</INF>H<INF>8</INF>)Cy<INF>2</INF>}(PCy<INF>3</INF>) (<BO>6</BO>). In contrast, in the presence of ethylene, RuH<INF>2</INF>{(η²-HSiMe<INF>2</INF>)<INF>2</INF>(CH<INF>2</INF>)<INF>n</INF><INF></INF>}(PCy<INF>3</INF>)<INF>2</INF> (n = 2,<BO> 3a</BO>; n = 3,<BO> 3b</BO>), leads to the formation of a new ethylene complex RuH<INF>2</INF>(C<INF>2</INF>H<INF>4</INF>)<INF>2</INF>(PCy<INF>3</INF>)<INF>2</INF> (<BO>4</BO>). Compound <BO>4 </BO>is ultimately converted to <BO>2</BO> after total consumption of the liberated disilane. All these complexes are involved in the catalytic processes.