Transformation of carbon dioxide

Biocatalytic carboxylation

Dwindling petroleum feedstocks and increased CO(2)-concentrations in the atmosphere currently open the concept of using CO(2) as raw material for the synthesis of well-defined organic compounds. In parallel to recent advances in the chemical CO(2)-fixation, enzymatic (biocatalytic) carboxylation is currently being investigated at an increased pace. On the one hand, this critical review provides a concise overview on highly specific biosynthetic pathways for CO(2)-fixation and, on the other hand, a summary of biodegradation (detoxification) processes involving enzymes which possess relaxed substrate specificities, which allow their application for the regioselective carboxylation of organic substrates to furnish the corresponding carboxylic acids (145 references).

Mechanism of cyclic carbonate synthesis from epoxides and CO2

Three interconnected catalytic cycles account for the title reaction catalyzed by a bimetallic aluminum(salen) complex and Bu(4)NBr. In the first, Bu(4)NBr acts as a nucleophile to activate the epoxide. In the second, Bu(3)N generated in situ serves to activate CO(2). In the third, the aluminum(salen) complex brings the two activated species together so that the key bonds can be formed intramolecularly.

Binding of 4-(N,N-dimethylamino)pyridine to Salen- and Salan-Cr(III) cations: a mechanistic understanding on the difference in their catalytic activity for CO2/epoxide copolymerization

The coordination chemistry of 4-(N,N-dimethylamino)pyridine (DMAP) with Salen- or Salan- (where the tetradentate N,N'-disubstituted bisaminophenoxide is designated as Salan, a saturated version of the Schiff-base Salen ligand) chromium complexes was studied by electrospray ionization mass spectrometry (ESI-MS). The relative stabilities of mono DMAP adducts of these chromium complexes were characterized by collision-induced dissociation (CID) and further discussed with regard to the activity in catalyzing CO(2)/epoxide copolymerization. [SalenCr](+) cations preferably bind two DMAP molecules to form six-coordinated complex ions, while [SalanCr](+) cations usually bind one molecule of DMAP to form five-coordinated complex ions, which were found to be relatively unstable. The remarkable difference in the coordination of DMAP to these two chromium complexes resulted in a significant difference in catalytic activity for the alternating copolymerization of CO(2) and propylene oxide. In the presence of 1 equiv of DMAP, the activity of the chromium-Salan complex 2a was up to 86 h(-1) of TOF at ambient temperature, which was about 30 times that of the corresponding chromium-Salen complex 1a. In sharp contrast to a long induction period up to 2 h with the use of 1a in conjunction with DMAP as catalyst, no initiation time or a very short one was observed in the binary 2a/DMAP catalyst systems. The initiator role of DMAP was confirmed by continuous determination of the propagating polymer species at various intervals using ESI-MS, which in combination with a kinetic study by means of infrared spectroscopy resulted in a mechanistic understanding on the difference in activity of the two catalyst systems for CO(2)/epoxide copolymerization.