Reduction ofalkenes

The ability of sol-gel entrapped compounds to catalyze reactions that do not take place by the non-entrapped materials is even more pronounced when an alumina sol-gel matrix is used. For example, while dihydrogen treated of triosmium- and triruthenium dodecacarbonyls [i.e., H40s4(C0)i2 andH4Ru4(CO)i2] are poor hydrogenation catalysts (see e.g., Ma, 1990) their immobilized versions in alumina sol-gel are highly active catalysts for the reduction ofalkenes and nitro groups at 60°C under 20-25 atm, H2 (Schemes 24-27 and 24-28). The alumina sol-gel entrapped ruthenium carbonyl complex even catalyzes... 

The enantioselection in certain catalytic hydrogenations is very dependent on the pressure of H2 which, in turn, determines the equilibrium concentration, [H2]sat, of H2 in the solution. A thorough recent analysis [2] of solubility and mass-transfer processes has shown, that it is the availability of dissolved H2 in the solution, rather than [ H2]sal, which influences the enantioselectivity, and the former is determined by the relation between the chemical and the mass-transfer rates. The solubility of hydrogen in water is 8 x 10 4 M, which is about 20% of the solubility in MeOH, 3.8 x 10 1 M (both at 20°C, 1 bar total pressure [102]). It follows that working in an aqueous solution is equivalent to using methanolic reaction mixtures under reduced pressure of H2, and that reactions in which the enantioselectivity is known to be pressure-sensitive (such as those described in [92] and in [97]) cannot be strictly compared at the same partial pressure of dihydrogen. The limited solubility of H2 may influence the chemoselectivity, too (reduction vs. isomerization ofalkenes) [103]. 

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