Supercritical carbon dioxide (scCO2) is an environmentally benign reaction medium for highly efficient rhodium-catalyzed hydroformylation reactions. Olefinic substrates can be hydroformylated in scCO2 at 40−65 °C to give the corresponding aldehydes in practically quantitative yields. The reaction course of the hydroformylation of 1-octene in scCO2 was analyzed in detail by online-GC monitoring. The influence of reaction parameters such as temperature, synthesis gas pressure, and [P]/Rh ratio on reaction rates and selectivities is grossly similar to the effects observed in conventional solvents. Maximum turnover frequencies of 1375, 500, and 115 h-1 were determined as lower limits for the catalytic activities under the present conditions for the unmodfied, phosphine-modified, and phosphite-modified systems, respectively. With unmodified catalysts, the hydroformylation rates are considerably higher in scCO2 than in organic solvents or liquid CO2 under otherwise identical conditions. Modified catalytic systems formed with perfluoralkyl-substituted triarylphosphine and triaralyphosphite ligands lead to higher regioselectivities than those found in conventional solvents. A constant overall n/iso ratio of 5−6 was achieved with “CO2-philic” triarylphosphines, whereas it increased in an apparent linear fashion during the reaction from approximately 6 to over 9 with the phosphite ligand. Olefin isomerization, which is a typical side reaction for phosphite-modified systems in conventional solvents, was effectively suppressed in scCO2.

Liquid or supercritical carbon dioxide (scCO(2)) is a versatile reaction medium for ring-opening metathesis polymerization (ROMP) and ring-closing olefin metathesis (RCM) reactions using well-defined metal catalysts. The molybdenum alkylidene complex 1 and ruthenium carbenes 2 and 3 bearing PCy(3) or N-heterocyclic carbene ligands, respectively, can be used and are found to exhibit efficiency similar to that in chlorinated organic solvents. While compound 1 is readily soluble in scCO(2), complexes 2 and 3 behave like heterogeneous catalysts in this reaction medium. Importantly, however, the unique properties of scCO(2) provide significant advantages beyond simple solvent replacement. This pertains to highly convenient workup procedures both for polymeric and low molecular weight products, to catalyst immobilization, to reaction tuning by density control (RCM versus acyclic diene metathesis polymerization), and to applications of scCO(2) as a protective medium for basic amine functions. The latter phenomenon is explained by the reversible formation of the corresponding carbamic acid as evidenced by (1)H NMR data obtained in compressed CO(2). Together with its environmentally and toxicologically benign character, these unique physicochemical features sum up to a very attractive solvent profile of carbon dioxide for sustainable synthesis and production.

COMMUNICATIONS [13] Owing to its d' configuration the manganese(i1) ion is isotropic and not very sensitive to geometrlcal factors. Therefore it is quite unlikely that such factors can reduce the magnetic exchange coupling mediated by a hydroxo bridge. In this respect, it is worth noting that Jwas also evaluated to be -9 cm-' in an (p-hydroxo)bis(p-carboxy1ato)Mnt1Mn"' complex [14].perfluorinated alkyl alcohols to the reaction mixture for neutral complexes with Binap-type arylphosphane~.[~~] Both methods lead only to a moderate increase in solubility. The solubilizer is not directly bound to the reaction center, which results only in a weak, nonpermanent interaction. The use of the BAr, anion is further restricted to ionic compounds, and application of additives is of course not desirable for practical reasons.We have now developed a generally applicable methodology to increase the solubility of arylphosphanes and their metal complexes in scC0, without changing their catalytic properties. The concept is outlined in Scheme 1 for complexes containing a

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The oxidation state of titanium in titanium dioxide is commonly assumed to be +4. This assumption is used ubiquitously to rationalize phenomena observed with TiO 2 . We present a comprehensive electronic structure investigation of Ti ions, TiO 2 molecules and TiO 2 bulk crystals, using different density functional theory and wave function-based approaches, which suggests a lower oxidation state (+3). Specifically, there is evidence of a significant remaining contribution from valence s and d electrons of Ti, including the presence of a nuclear cusp around the Ti core. The charge corresponding to valence s and d states of Ti amounts to 1 e. The commonly assumed picture may therefore have to be revised.