Green C1-Chemistry: Reactions with carbon dioxide and carbon dioxide derivatives
This dissertation deals in part I with the synthesis of cyclic organic carbonates from the most sustainable C1 building block: CO2. Part II deals with the synthesis of amidines from carbodiimides, the diimino derivatives of CO2, or from thioureas as more accessible substrates, respectively. Part...
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|Summary:||This dissertation deals in part I with the synthesis of cyclic organic carbonates from the most sustainable C1 building block: CO2. Part II deals with the synthesis of amidines from carbodiimides, the diimino derivatives of CO2, or from thioureas as more accessible substrates, respectively.
Aqueous two phase synthesis of cyclic carbonates from CO2 and epoxides
The focus of this chapter was on the development of a multiple phase catalysis for the preparation of organic carbonates with water as sustainable solvent. The aim was to utilise the miscibility gap between water and most cyclic organic carbonates for an energy efficient catalyst recycling. Additionally, water acts as co-catalyst via hydrogen bonding to the epoxide. The developed water-soluble organocatalysts 9a-Br demonstrated a high productivity for the carboxylation of propylene oxide (94% yield, 95% selectivity) and butylene oxide (95% yield, 97% selectivity). Through modification of the catalyst into surfactant shighly hydrophobic epoxide could be used as substrates as well.
The catalyst could be recycled multiple times through simple phase separation, though a slow decline in productivity was observed. However, it was demonstrated that the catalyst 9a Br can be regenerated in situ by addition of small quantities of aqueous HBr after each run. This is explained by replacing the bromide lost through the formation of bromohydrin as trace product.
Synthesis of cyclic carbonates from olefines and H2O2
1,2 hexane carbonate was obtained in 22% yield from olefines, CO2 and H2O2 under solvent free conditions. Towards oxidation by H2O2 stable, but less nucleophilic anions NO3- and Cl- were used as nucleophilic carboxylation catalysts instead of oxidation sensitive bromide and iodide. Unfortunately, decomposition of the quaternary ammonium cation of the ionic catalysts inhibited the implementation of an efficient orthogonal tandem catalysis as one pot synthesis. Additionally, both reaction steps favour different temperatures.
Organic superbases as catalysts for the coupling of CO2 and epoxides
In this chapter carbodiphosphorane and phosphazenyl phosphine superbases have been used as organocatalysts for the carboxylation of epoxides for the first time. The C- and P-bases, provided by IGOR MARTIN and SEBASTIAN WAGNER, were highly active organocatalysts. Epichlorohydrin conversion >80% with yields up to 99% at full selectivity were achieved with just 0.01-0.1 mol% and TONs up to 8600 .
The catalysts were also robust and remained active after 8 h reaction time and in presence of moisture. The free C-and P-base as well as the corresponding CO2- or water-adduct can be used as precursor. Additionally, it was shown that a too strong nucleophilic character of the superbase catalysts leads to the undesired polymerisation of the epoxide. Investigation of the reaction mechanism revealed that, regardless of if the free base or the CO2-adduct is employed, the onium bicarbonate of the superbase is present at the end of the reaction. This suggests that the reaction proceeds via an onium bicarbonate formed in situ from the superbase, CO2 and traces of moisture as catalytically active species.
Preparation of carbodiimides from thioureas
Diisopropyl carbodiimide was prepared from diisopropyl thiourea via dehydrodesulfurisation with strong bases and thiophilic metal cations as sulfur acceptors. The thiophilicity of the metal and the strength of the base impact the yield of the reaction. However, the concept could not be transferred to a wider selection of thioureas or ureas. The isolation of unstable primary alkyl substituted carbodiimides was not feasible.
One pot synthesis of amidines from thioureas
In 2 position alkyl, aryl or alkinyl substituted amidines were prepared from dialkyl and diaryl thioureas with MeLi, nBuLi, PhC≡CLi or PhLi. The desulfurisation does not require an additional sulfur acceptor next to the lithium cation itself. Through simple modification of the synthetic protocol, the method was extended to the synthesis of an unsymmetrical guanidine and a selection of 2-bis(amino)methylene nitriles. The required nucleophiles were prepared in situ by the same lithium base used for the deprotonation of the thiourea. This enables one pot conditions for these reactions. The isolation of carbodiimide intermediates is not necessary. This method allows the synthesis of several valuable compound classes from a single cheap, stable and accessible thiourea as starting material without the need for toxic metals.|