Nuclear magnetic resonance applied to the study of chemical reactions

Author(s)

Uenifer Rodrigues Couto

• Document type: Doctoral Thesis
• Press: Campinas, SP.
• Institution: Universidade Estadual de Campinas (UNICAMP). Instituto de Química
• Defense date: 2022-12-07

Examining board members

Cláudio Francisco Tormena; Marco Antonio Barbosa Ferreira; Luciano Morais Lião; Júlio Cezar Pastre; Caio Costa Oliveira

• Advisor: Cláudio Francisco Tormena

Abstract

Nuclear Magnetic Resonance (NMR) is a spectroscopic technique that deals with magnetic active nuclei, being able to differentiate them according to the chemical environment and interactions with neighboring magnetic active nuclei. The technique’s robustness and its ability to identify and characterize chemical compounds made it the tool of choice for spectroscopic characterization, and it has become one of the main tools of modern organic chemistry. In this work we intend to show that in addition to spectroscopic characterization, NMR is a powerful technique to explore chemical reactions. For this, we explored the use of 3JCH for the assignment of trisubstituted double bonds. It was possible to detect a dependence of 3JCH with the orientation of substituents that could compromise the application of this parameter in the assignment of the double bond configuration. The formyl substituent orientation was able to cause an inversion of the expected pattern for the 3JCH couplings, which indicates that this parameter should be used with caution. We also explored the origin of the selectivity (E or Z) observed in the Knoevenagel condensation. The selectivity, was previously assigned by us to a spontaneous isomerization, but by exploring the phenomena, we were able to discover that this is not always the case. The isomerization seems to be dependent of the substituent pattern, solvent and presence of acidic or basic substances. Our results suggest that isomerization process is present in the reaction conditions, and it can establish a thermodynamic equilibrium between the E and Z isomers that will overcome any preference due to the reaction mechanism. As a last application of NMR, we demonstrate how the use of simple NMR techniques associated with a careful experimental design can provide useful information about the Gewald reaction. Furthermore, we presented two different approaches to the acquisition of the kinetic profile by NMR for this heterogeneous reaction. The first exploits a competitive reaction to stall the reaction progress of aliquots taken from the reaction vessel, which was later analyzed by NMR. The second approach involves developing of a low-cost system for the real-time monitoring of the reaction directly from the reaction vessel. This system has potential application for reactions other than the Gewald reaction, as well as the application to hyphenation techniques (AU)

• FAPESP's process: 16/24331-4 - Nuclear Magnetic Resonance spectroscopy and theoretical calculation for real-time reaction monitoring and mechanistic investigation
• Grantee: Uenifer Rodrigues Couto
• Support Opportunities: Scholarships in Brazil - Doctorate