Forskning ved Københavns Universitet - Københavns Universitet


Activation of CO2 for Umpolung Carboxylation

Publikation: Bog/antologi/afhandling/rapportPh.d.-afhandlingForskning

  • Martin Juhl
Chapter 1 will first describe the use of CO2 as a C1 source as well as some general trends in CO2 functionalization. An introduction to carboxylic acid formation with and without CO2 is then presented. The main focus will be on the scarce literature involving nitrogen base mediated carboxylations. The literature is compared in order to elucidate how DBU might activate CO2 thereby making it more electrophilic. Chapter 2 will describe the development of a novel carboxylation reaction of aryl aldehydes in the formation of aryl α-keto acids. An umpolung strategy using KCN was applied in order to access an acyl anion synthon. Good to excellent yields of α-keto acids were achieved when DBU and Ti(OiPr)4 was added to promote to promote product formation at atmospheric pressure. It is postulated that DBU increases the electrophilicity of CO2 while Ti(OiPr)4 stabilizes the product by impeding decarboxylation. Mechanistic investigations yielded a good understanding of not just the mechanism towards formation of product but also all the by-products. Furthermore, a reductive amination procedure was developed to access α-amino acids from the α-keto acids. Chapter 3 will describe the development of a chiral DBU analogue for performing catalytic asymmetric carboxylation. α-Amino silanes were carboxylated in the formation of enantiomeric enriched α-amino acids. This is the first case of an organocatalytic enantioselective C-C bond forming carboxylation. The procedure was further applied in the development of a highly proficient carboxylation procedure of secondary benzylic silanes. Mechanistic considerations are discussed suggesting a possible asymmetric counteranion directed catalysis (ACDC) type mechanism. Chapter 4 describes the development of a novel cyanation procedure of carbonyl compounds mediated by CO2 or CS2. It was found that quantitative formation of cyanohydrin can be accomplished when KCN and CO2 was used in combination. From mechanistic investigations it was observed that only 1 equivalence of CO2 was needed. In order to demonstrate that a cyanohydrin carbonate was generated in situ a procedure for generating cyanohydrin methyl xanthate esters was developed. The combination of the two cyanation procedures were revealed as strategies for the elongation of aldehydes. Preliminary investigations into developing an enantioselective cyanation reaction was also demonstrated yielding the desired enantio enriched cyanohydrin in up to 25% ee. Chapter 5 will describe the development of a palladium catalyzed homocoupling reaction of boronic ester pyrazole units. The procedure allows for the formation of bipyrazoles which can be applied in the formation the synthesis of metal-organic framworks (MOF). The MOF was then tested as an additive to achieve a higher local concentration of CO2 for a carboxylation reaction. Hence, performing carboxylation reaction at atmospheric pressure which are frequently conducted at a high CO2 pressure.
ForlagDepartment of Chemistry, Faculty of Science, University of Copenhagen
StatusUdgivet - 2019

ID: 229736380