Dialkylcarbene and quinoid carbene complexes of group VIII and IX transition metal porphyrins : bonding and catalysis

Abstract

Transition metal-catalyzed carbene transfer reactions are a powerful tool for constructing carbon-carbon and carbon-heteroatom bonds and have therefore enjoyed wide application in modern organic chemistry. The divalent carbene carbon can introduce two new functional groups (the two carbene substituents) into the product molecule in a single transformation, making this methodology extremely useful and versatile in the synthesis of complex structures. Currently, metal-carbene species bearing electron-withdrawing groups, especially those derived from alpha-diazocarbonyl compounds, are predominantly used for method development and synthetic applications; while some new types of carbene species, such as dialkylcarbene and quinoid carbene which have already shown some promise in building up carbocyclic chains and functional aromatic moieties, are poorly understood in terms of their interactions with the metal center and their potential reactivities. In this thesis, dialkylcarbene and quinoid carbene are thoroughly investigated via both isolable metal-carbene complexes and catalytic carbene transfer reactions. Chapters 3 and 4 focus on metal dialkylcarbene chemistry. In Chapter 3, a synthetic protocol for cobalt porphyrin-catalyzed Buchner reaction/arene cyclopropanation of dialkylcarbene is described. Prior to this work, dialkylcarbene was only known to undergo sp3 C-H insertion and cyclopropanation reactions; the methodology presented here further expands the scope of dialkylcarbene catalysis, giving rise to some interesting heterocycles fused with carbocyclic rings in high yields and with high selectivity. Chapter 4 describes the isolation and characterization of a series of group 8 metal dialkylcarbene complexes. This project began as a mechanistic investigation of the dialkylcarbene transfer catalysis and the middle/late transition metal dialkylcarbene intermediates involved in the catalytic cycle. These features have been far less explored than their early transition metal counterparts (Schrock alkylidenes). 2-Adamantylidene was chosen as a rigid model of dialkylcarbene leading to remarkably stable group 8 metal complexes. The uniqueness of dialkylcarbene is further elaborated by comparison with other commonly used carbene ligands, and its role as a supporting ligand in mediating some challenging carbene transfer reactions was also investigated. Metal quinoid carbene chemistry is explored in the following two chapters. In Chapter 5, the isolation and characterization of a series of ruthenium porphyrin quinoid carbene complexes, including the first crystal structures of metal-quinoid carbene complexes are presented. In addition to their carbene transfer reactivity with nitrosoarenes, the hydrogen atom transfer (HAT) reactivity of these complexes with weak X-H (X = C, N, S) bonds was also observed, thus presenting a unique, dual reactivity feature. Furthermore, both reactivities can be developed into catalytic versions and are promising in synthetic applications. An sp3 C-H arylation strategy based on iridium porphyrin-catalyzed quinoid carbene transfer reactions is described in Chapter 6. This catalytic transformation takes place by harnessing the HAT reactivity of metal quinoid carbene compounds as described in Chapter 5, and switching the metal center from ruthenium to iridium to enable the carbene radical and alkyl radical to undergo further radical rebound to afford the final product. This arylation process, the mechanism of which is substantiated by experimental evidence, presents a rare example of a radical mechanism in carbene C-H insertion catalysis.