Synthesis and Applications of Novel Chiral NHC Precursors. Synthesis of Urea Derivatives through Decomposition of Cu-NHC under Air. Iron-mediated Synthesis of Dihydroquinoxalinones
|Abstract:||Since its first isolation, N-heterocyclic carbenes (NHC) have been found very useful to coordinate with metals and serve as ligand in catalysis. With the advantages of environmental friendliness, abundance and being less expensive, iron as a metal catalyst has received growing attention in recent decades. Despite that many Fe-NHCs have been synthesized, chiral Fe-NHC for asymmetric catalysis is still in its infancy. In comparison to precious metals, copper as a versatile and less expensive transition metal also has recieved much attention. However, the development of chiral Cu-NHC as efficient catalyst is still challenging. Thus, several types of novel chiral NHC ligand precursors have been synthesized. The synthesis of chiral Fe-NHCs and Cu-NHCs were attempted using those chiral precursors. It was found that the Fe-NHCs and Cu-NHCs would decompose under air. On the other hand, the applications of in situ generated generated chiral Fe-NHCs and Cu-NHCs were carried out for hydrosilylation reactions, Mukaiyama aldol reactions, insertion of metal-carbene into SiH bond and Heck-type reactions. The in situ generated Fe-NHCs were found not applicable in the hydrosilylation of acetophenone. For the Mukaiyama aldol reactions, the conditions using in situ generated Fe-NHCs led to the desired products in up to 88% yield. However, no enantioselectivity was observed for all attempts, probably due to the deactivation of NHC ligand. The insertion reaction of metal-carbene into SiH bond catalyzed by in situ generated Cu-NHCs afforded up to 84% yield and 24% ee of product. Besides, the Heck-type reactions were tested using a chiral Pd-NHC as catalyst. The reactions afforded up to 91% yield, but no enantioselectivity was observed. Furthermore, the decompositions of different types of Cu-NHCs and Ag-NHCs in solutions under humid air were studied. The Cu-NHCs underwent hydrolysis and oxidation to generate imidazoliums and urea derivatives under air. The Ag-NHCs were hydrolyzed to yield formamides or imidazoliums in solution under humid air. Subsequently, a new synthetic method of urea derivative using copper and air as oxidant was developed, which provided moderate to very good yields for sterically unhindered substrates. The mild oxidation conditions are suitable for the synthesis of urea derivatives possessing alkyl, benzyl, aryl, primary hydroxy, acid-sensitive tertbutyloxycarbonyl group, and tertiary amine groups. In the last project, a general and efficient synthesis of enantiopure dihydroquinoxalinones has been developed. The reductive cyclization of N-(o-nitroaryl)amino esters was performed by using iron and zinc metal under mild conditions to afford dihydroquinoxalinones in moderate to high yields and high enantiomeric purity.|
|Document Type:||Thèse de doctorat|
|Open Access Date:||13 November 2019|
|Collection:||Thèses et mémoires|
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