Development of N-Heterocyclic Carbenes as Organic Catalysts and Efficient Ligands in Palladium Mediated Transformations

• Main Author: Singh, Rohit
• Format: Others
• Published: ScholarWorks@UNO 2007
• Subjects: Palladium; Carbenes; Cross-Coupling; Organic-Catalysis
• Online Access: http://scholarworks.uno.edu/td/576; http://scholarworks.uno.edu/cgi/viewcontent.cgi?article=1576&context=td

N-Heterocyclic carbenes (NHCs) have emerged as appropriate replacements for phosphines in transition metal catalyzed cross-coupling chemistry. The advantages of NHCs over phosphines include ease of handling, minimal toxicity, stability and powerful electron donating properties. Improvement of catalytic processes has become increasingly relevant in light of prospective applications of organic transformations in industry as well as in synthetic laboratories. To that end, NHCs represent an important class of catalysts and catalyst modifiers which mandate continued research efforts. Prospective applications of processes catalyzed by NHCs and NHC-metal catalysts provide a strong impetus to develop them and related methodologies. This dissertation focuses on the development of NHCs and NHC ligated metal complexes in various catalytic transformations. NHC ligated palladium catalysts were synthesized in simplified protocols amenable to large-scale industrial applications. The catalysts were utilized in developing different valuable coupling methodologies. Significant advances were achieved in Suzuki-Miyaura, á-arylation of ketones and Kumada-Tamao-Corriu cross coupling reactions. The focus of the work was to make the synthesized catalysts and their activity in these methodologies acceptable to wider range of applications. The strongly nucleophilic nature and easily tunable steric and electronic properties of NHCs have been exploited to mediate organic transformations by utilizing NHCs as catalysts. The metal-free catalysis has an added advantage of being more environmentally friendly. NHCs have proven to be excellent transesterification catalysts for reactions of alcohol and esters. An efficient catalytic system, widening the scope of N-heterocyclic carbenes catalyzed transesterification/acylation reaction of alcohols is described. The methodology has been expanded to include secondary alcohols as well as phosphorus based esters.