Redox-Switchable Cross Metathesis and Acyclic Diene Metathesis Polymerization

Department of ChemistryAcyclic diene metathesis (ADMET) polymerization grant the opportunity to synthesize precisely defined polymers. One of the limitations of this method is the lack of control on the molecular weight of the final products. We reasoned that the use of redox switchable catalysis can be used to achieve enhanced levels of control. Herein, a Ru(II)-based redox-switchable catalyst, containing a quinone-annulated N-heterocyclic carbine is used in order to modulate the reactivity of ADMET polymerization by using redox agents (CoCp2 and DDQ). The oxidation state of the ligand switched by reducing and oxidizing agents allow us to control directly the kinetic of the reaction. Whereas the neutral form of the complex catalyzes the reaction (in the same rate as commercially-available catalyst), the reduced complex inhibits the reaction by a decrease in the rate constant by around one order of magnitude, compared to the neutral form. Furthermore, the molecular weight of polymers by ADMET polymerization with this redox-switchable catalysis was related to the same chemistry as mentioned before. In this thesis we investigate the cross metathesis (CM) reaction because it shares the fundamental mechanism with ADMET polymerization and we will show that it is possible to modulate the molecular weights of the final products.clos

Potentiostatically Controlled Olefin Metathesis

A Ru(II) complex supported by an N-heterocyclic carbene annulated to a redox-active naphthoquinone (NQ) was interrogated using a range of potentiodynamic and potentiostatic electrochemical techniques. The complex exhibited two redox processes, one of which was attributed to the Ru(II)IRu(III) couple (E-1/2 = +1.10 V vs a saturated calomel electrode) and the other to the NQJNIQ(-) couple (E-1/2 = -0.62 V). Using potentiostatic coulometry or bulk electrolysis, the application of a fixed negative potential (-0.9S V) to electrodes placed in a dichloromethane solution containing the complex resulted in a reduction reaction. The complex was quantitatively reduced within minutes, as determined by coulometry, and subsequently oxidized to its initial, neutral form through the application of a relatively positive potential (+0.34 V) over similar periods of time. The interconversion process was found to be reversible and used to modulate a series of ring-closing metathesis and ring-opening metathesis polymerization reactions. While relatively high activities were observed when the neutral form of the catalyst was employed, the reaction rates were attenuated upon in situ, potentiostatic reduction. Toggling between relatively negative or positive potentials enabled the aforementioned olefin metathesis reactions to be switched between fast and slow states

Redox- and light-switchable N-heterocyclic carbenes: a "soup-to-nuts'' course on contemporary structure-activity relationships

Switchable catalysts respond to various types of stimuli in a manner that results in distinct structural or electronic changes. When each state exhibits a different activity, selectivity, or solubility, the corresponding catalyst may be used to control chemical reactions in a temporally-or spatially-resolved fashion. N-Heterocyclic carbenes (NHCs) are versatile scaffolds for building switchable catalysts and many examples that respond to changes in electrochemical potential or light have been introduced. Such types of switchable NHCs will be described in this Feature Article. The accompanying discussions include design considerations, characterization methodology, quantification of the underlying switching phenomena, and utility in catalytic applications. Challenges for the field as well as perspectives on potential opportunities for future development are also provided

Redox- and light-switchable N-heterocyclic carbenes: A "soup-to-nuts" course on contemporary structure-activity relationships

Switchable catalysts respond to various types of stimuli in a manner that results in distinct structural or electronic changes. When each state exhibits a different activity, selectivity, or solubility, the corresponding catalyst may be used to control chemical reactions in a temporally- or spatially-resolved fashion. N-Heterocyclic carbenes (NHCs) are versatile scaffolds for building switchable catalysts and many examples that respond to changes in electrochemical potential or light have been introduced. Such types of switchable NHCs will be described in this Feature Article. The accompanying discussions include design considerations, characterization methodology, quantification of the underlying switching phenomena, and utility in catalytic applications. Challenges for the field as well as perspectives on potential opportunities for future development are also provided. © 2019 The Royal Society of Chemistry

Redox-switchable olefin cross metathesis (CM) reactions and acyclic diene metathesis (ADMET) polymerizations

We show that redox-switchable catalysis may be used to control acyclic diene metathesis (ADMET) polymerizations and related reactions. A Ru(ii) complex was found to display catalytic activities that were dependent on the oxidation state of a quinone-containing ligand. While the neutral form of the complex was found to catalyze ADMET polymerizations at rates that were commensurate with a commercially-available catalyst, significantly lower activities were observed when the complex was reduced. Using the rate differential, a series of ADMET polymerizations were modulated by alternately reducing and oxidizing the catalyst over time. A similar approach was also used to regulate the molecular weights of the polymers produced. Cross metathesis reactions and computational studies were performed in parallel to gain a deeper understanding of the underlying redox-switchable chemistry