Mechanistic Investigation of Reactions Involving Organometallic Catalysis and Main Group Compound Mediated Transformations: A Computational Study

Maity, Bholanath (2017) Mechanistic Investigation of Reactions Involving Organometallic Catalysis and Main Group Compound Mediated Transformations: A Computational Study. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

The present thesis titled “Mechanistic Investigation of Reactions Involving Organometallic Catalysis and Main Group Compound Mediated Transformations: A Computational Study” has been divided into two independent parts: Part-I and Part-II. The main objective of Part-I is to describe the detailed mechanism of rutheniumcatalyzed hydroamidation/hydrocarboxylation of terminal alkynes with the aim of understanding the key factors controlling the regio- and stereoselectivity of product formation. These types of reactions provide excellent methodologies for the synthesis of enamides and enol-esters. The main challenge of this method arises due to formation of product mixture, Markovnikov and anti-Markovnikov E- or Z- products (Scheme 1). Control of regio- and stereoselectivity by modulating the electronic and steric environment in the catalytic systems has gained a serious attention in chemical community since last few decades. There are several approaches developed to control the selectivity in catalytic hydroamidation/hydrocarboxylation by the effective use of external ligands, additives and solvent mediums. We have performed DFT calculations at M06L-D3(SMD)/LANL2TZ(f)/6- 311+G(d,p)//M06L/LANL2DZ/6-31G(d) level to explore a complete catalytic mechanism for ruthenium-catalyzed hydrocarboxylation of terminal alkynes with special focus on the role of solvent in controlling the regio- and stereoselectivity of the addition process. The calculated results not only demonstrate the plausible reaction routes of the reaction but also elucidate the preference of a particular isomeric product with respect to other. In DCM medium, the calculations predict that Markovnikov addition is energetically more facile than vinylidene complex formation (Scheme 4). On the contrary, a strong coordinating THF solvent facilitates the vinylidene formation via an alternative route, which in turn allows the favorable formation of Z-selective anti- Markovnikov product generation. This is in excellent correlation with the experimental findings. This detailed understanding of the reaction mechanism will serve as the basis for the rational design of more efficient catalysts for enol-ester synthesis with a new level of activity in terms of regio- and stereoselective product formation. This part of the thesis deals with the metal free asymmetric transformations and remarkable chemistry of some main group radical species in terms of their electronic structure, bonding and specific reactivities. We have performed computational analysis to predict electronic structure and bonding scenario of some silicon containing radical and biradical systems, stabilized by cAAC and explain their reactivity using DFT calculations, ab initio studies and other bonding analysis like natural bonding orbital (NBO) and atoms in molecules (AIM). Based on calculated results we have suggested that in all the cAAC stabilized radical systems the unpaired electron is mostly localized on the CcAAC and slightly delocalized to neighbouring nitrogen atom. These results are further substantiated by electron density and Laplacian values obtained from AIM calculations. Furthermore, our computed EPR parameters for doublet systems, show strong correlation with experimental values.

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