In Silico Study of Reaction Mechanisms Towards Main-group and Metal Mediated Bond Activations

Mondal, Totan (2018) In Silico Study of Reaction Mechanisms Towards Main-group and Metal Mediated Bond Activations. PhD thesis, Indian Institute of Science Education and Research Kolkata.

PDF (PhD thesis of Totan Mondal (12RS033)) 12RS033.pdf - Submitted Version Restricted to Repository staff only Download (12MB)

Abstract

This thesis work has been devoted to the computational understanding of transition metal as well as main group chemistry. The role of ligands, additives, solvents, counter-ions and other factors in controlling the reaction and devising new reaction strategies remains my central interest. Apart from mechanistic investigations, my interest also includes an understanding of the structure-bonding correlation in many newly synthesized, unique main group compounds. In 2012, Mandal et al. has reported the activation of less reactive C–Cl bond under ambient reaction conditions by dimeric Pd(II)-aNHC (aNHC = abnormal N-heterocyclic carbene) complex. That was the first report of utilizing aNHC-Pd complex in homogeneous catalysis. Despite these fascinating outcomes, they were unable to trap or even characterize any intermediate as well as active species responsible for this reaction. In Chapters 2 and 3, we endeavored to address some important questions highly relevant to the mechanistic aspect. The primary motivation was to pinpoint the actual catalytic species that undergoes the oxidative addition reaction. Importantly, the presence of the excess base plays a critical role in the generation of active species. The formation of Pd(0) catalyst from Pd(II) species was highly energy demanding. Curiously, a surprising Pd(II)/Pd(IV) pathway has been observed to be more positive than as often as possible watched, conventional Pd(0)/Pd(II) pathway. The role of base and the effect of counter-cations in the critical transmetalation and reductive elimination events was investigated. Additionally, the negative role played by the counter cations in the transmetalation step was successfully addressed. Chapter 4 narrates the detailed mechanistic discussion of less explored intermolecular Pd-catalyzed carbonylative coupling reaction of aryl bromides relying on C(sp²)‒H activation process. We will discuss the formation of active catalyst responsible for the reaction as well as explore diverse pathways for the carbonylative coupling reaction. Moreover, our investigation will also address the chemoselective activation of C−Br bond and role of other controlling factors in obtaining the desired product. Finally, we will conclude the study by discussing the effect of different substituents on reaction rates. Chapter 5 describes the computational investigation of the regioselective C−F bond activation of pentafluoropyridine (PFP) by group 14 dialkylamino metalylenes. The reason behind the group 14 central atom (M = Si(II), Ge(II), and Sn(II) and substituents (−NMe₂, −NiPr₂,−Cl, −NH₂, and −PH₂) dependent switching of oxidative addition to the metathesis reaction route is uncovered using state-of-the-art theoretical methods to provide a systematic classification of the individual mode of reactions. Finally, Activation Strain Model (ASM) is utilized to get brief insight into the barrier originating via different mode reactions, namely oxidative addition and metathesis. In Chapter 6, comprehensive DFT calculations have been performed to explore and understand the mechanistic details for the formation of unprecedented phosphaboraheterocycle compound formed by the reaction of [(PhC)₄BPh] and NaOCP. Importantly, the nucleophilic O-attack of phosphaethynolate⁻O−C≡P is preferred over the nucleophilic P-attack of phosphaketenide O=C=P⁻ toward the B center of the pentaphenylborole unit. The intramolecular nucleophilic attack of the C center to the electrophilic C center of ⁻O−C≡P fragment is found to be the RDS step among the overall reaction steps. Recently, the synthesis of C₂ in zero oxidation state has been for the first time obtained by Roesky and co-workers in presence of cyclic alkyl(amino) carbenes (cAACs) as a stabilizing species. The major thrust of my last thesis Chapter is to understand the effect of different carbene ligand systems in stabilizing the low valent E2 species (E = C, Si, Ge, Sn, and Pb). The structure and bonding relationship for this class of compounds are nicely illustrated using DFT calculations and other methods including NBO, AIM analysis and other quantum mechanical techniques. Calculated results are found to have good correlation with the available experimental observations.

Actions (login required)

View Item