Ruthenium(II) and Manganese(I) catalyzed organic transformations via hydrogen transfer reactions

• Ruthenium(II) und Mangan(I) katalysierte Organische Umsetzungen via Wasserstoff-Transfer-Reaktionen

Kaithal, Akash; Leitner, Walter (Thesis advisor); Quadrelli, Alessandra (Thesis advisor)

Aachen (2020)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2020. - Dissertation, Université de Lyon, 2020

Abstract

The present dissertation focuses on new organic transformations enabled by hydrogen transfer reactions using Mn(I) and Ru(II) pincer complexes. The primary focus deals with the study of Mn(I) and Ru(II) complexes and their similar reactivity in hydrogen transfer reactions which includes hydrogen borrowing and reduction reactions. Chapter 1 is a general introduction of hydrogen-transfer reactions reported for Mn(I) complexes and their similar reactivity with the Ru(II) complexes that are well-established for the hydrogen-transfer reactions. Chapter 2 focuses on the utilization of MeOH as a C1 source for the synthesis of fine chemicals, pharmaceuticals and alternative fuels. The selective -methylation of alcohols was achieved using methanol as a C1 source. Various ruthenium complexes were investigated for this transformation and a Ru-MACHO-BH pincer complex revealed the best catalytic results. Mechanistic studies and DFT calculations confirmed that the reaction proceeds via “Hydrogen borrowing pathways” and involved metal-ligand cooperation on the ruthenium metal center. Chapter 3 also deals with the selective -methylation of alcohols using methanol as a C1 source. However, in this chapter, earth-abundant and air-stable manganese pincer complexes were investigated. The reactivity of Mn(I) pincer complexes with Ru(II) pincer complexes was compared. Numerous manganese pincer complexes were synthesized and checked for this process where the Mn-MACHO-iPr complex demonstrates the optimum results with high selectivity and high yield to the corresponding desired product. Chapter 4 demonstrates the formation of substituted cycloalkanes using secondary alcohols or ketones and diols as initial substrates employing Mn-MACHO-iPr complex as pre-catalyst. The reaction studies showed that the Mn-MACHO-iPr complex revealed better reactivity in comparison to the Ru-MACHO-BH complex. Various substituted cycloalkane rings such as substituted cyclopentane, cyclohexane, and cycloheptane rings were synthesized employing the Mn-MACHO-iPr complex. Mechanistic studies revealed that the reaction proceeds via “hydrogen borrowing pathways”. Chapter 5 addresses the selective deuteration of primary and aliphatic alcohols using D2O as a deuterium source. The already established Mn-MACHO-iPr complex was investigated for this transformation which showed selective deuterations of benzylic alcohols at the alpha positions and alpha and beta deuteration for aliphatic alcohols. Chapter 6 deals with the selective hydrogenation of cyclic carbonates to the analogous methanol and diols. Several manganese pincer complexes were synthesized to confirm the activity towards this transformation. Air-stable Mn-MACHO-iPr pincer complex showed the best catalytic activity with high turnover numbers and selective preparation to the corresponding methanol and diols. Chapter 7 discusses the preparation of methoxy-borane and boronate-diols via selective reduction of cyclic and linear carbonates and CO2 using pinacolborane as a reducing agent. A newly synthesized manganese pincer complex was explored for this process which revealed the high efficiency and selectivity towards this transformation.