Selektive Oxidationsreaktionen mit molekularem Sauerstoff in komprimiertem Kohlendioxid

The present dissertation is divided into two research projects. In the first one the possibility for a synthesis of organic carbonates starting from alkenes, oxygen, carbon dioxide and aldehydes as coreductants was investigated. The first part of the consecutive reaction sequence-the stainless steel initiated synthesis of epoxides using O2/aldehyde mixtures in supercritical carbon dioxide-was successively performed by Loeker.[1] This result bares the potential for an in situ synthesis of organic carbonates by the addition of suitable catalysts. However, established catalysts of different structure and mode of operation for CO2 insertion proved not to be compatible with the reaction conditions of the epoxidation reaction. The coexistence of Lewis-base and a Brönsted-acid sites was identified as a main difficulty for the development of active and resistant catalysts. In the second project the coupled oxidation of cycloalkanes or alkylarenes with acetaldehyde (2.0 equiv.) and molecular oxygen (2.1 equiv.) was for the first time found to occur effectively in compressed carbon dioxide (16 equiv.) under mild multiphase conditions.[2] No catalyst is required but high pressure ATR-FT-IR online measurements show that the free radical reaction is heterogeneously initiated by the stainless steel of the reactor walls. Since experiments in a titanium autoclave led to comparable conversions, the initiation seems to be relative independent in terms of the reactor material. For secondary carbon atoms a high keto to alcohol ratio is observed (3.5-7.9), most probably due to the fast consecutive oxidation of alcoholic intermediates. Since C-C-scission reactions are detected only to a very small extend, tertiary carbon atoms are transformed to the corresponding alcohols with high selectivity. The obtained product distributions suggest that the strong influence of acetaldehyde is related to its function not only the determining oxygen activator and an efficient H-atom donating agent for peroxo and oxo radicals, but also a crucial reductant for hydroperoxo intermediates. In comparison with compressed N2 or Ar carbon dioxide was proven to increase the yields of alkane oxygenates under identical reaction conditions. Although the experimental results allow at present no full explanation for the enhanced conversion in CO2, a set of exclusive effects in CO2 as a reaction medium were described which are believed to play an important role for the enhanced performance in this medium. [1] F. Loeker, W. Leitner, Chem. Eur. J., 2000, 6, 2011-2015. [2] N. Theyssen, W. Leitner, Chem. Commun., 2002, 410-411.