Ruthenium benzenes, stereoselectivity

By switching the catalyst from platinum to ruthenium, the stereoselectivity of the i yw-hydrosilylation can be reversed to afford the a h-hydrosilylation product (26) in the presence of catalytic [RuCl2(benzene)]2 (eq 8). ... 

It has also been shown that dimethylsilyl enolates can be activated by diisopropylamine and water and exhibit a high reactivity toward iV-tosyl imines to give Mannich-type reaction products in the absence of a Fewis acid or a Bronsted acid.51 For example, the reaction of [(1-cyclohexen-l-yl)oxy]dimethylsilane with 4-methyl-A -(phenylmethylene)benzene sulfonamide gave re/-4-methyl-N- (f )-[(15)-(2-oxocyclohexyl)phenyl-methyl] benzenesulfonamide (anti-isomer) in 91% yield stereoselectively (99 1 anti syn) (Eq. 11.30). On the other hand, Fi and co-workers reported a ruthenium-catalyzed tandem olefin migration/aldol and Mannich-type reactions by reacting allyl alcohol and imine in protic solvents.52... 

In general, hydrogenation of di- and polysubstituted benzenes leads to the formation of a mixture of stereoisomers the composition of which is closely related to the catalyst used. Nickel is the least stereoselective of the common catalysts, giving product mixtures in which the thermodynamically favored product dominates5. Ruthenium, on the other hand, gives primarily rf.s-substituted cyclohexanes on hydrogenation of 1,2- and 1,3-disubstituted benzenes, while rhodium affords more of the rw-isomers from 1,4-disubstituted compounds6 7. 

The diastereoselective hydrogenation of (5) in equation (11) is catalyzed by ruthenium and rhodium complexes in methanol, while dichloromethane and benzene gave poor results. Cationic complexes are likely intermediates. [Ru(02CCF3)2(PPh3)2] and [Rh(COD)(dppe)]C104 were used as precursors. The selectivity for the anti isomer was up to 99%. The stereoselective hydrogenation... 

These transformations under carbon dioxide pressure have been reinvestigated recently in supercritical CO2 without any other solvent [32]. In particular, the selection of appropriate experimental conditions and the evaluation of other ruthenium precursors such as /rani-RuCl2(P(OEt3))4 [33], RuCl2(pyridine)4 or RuCl2(benzene)(PMe3) [34] have led to improvements in terms of yield and stereoselectivity. 

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