Computational Investigations of Transition Metal Catalysis, Organocatalysis and Bonding Scenarios of Main Group Complexes

De, Sriman (2022) Computational Investigations of Transition Metal Catalysis, Organocatalysis and Bonding Scenarios of Main Group Complexes. PhD thesis, Indian Institute of Science Education and Research Kolkata.

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Abstract

The research work presented in this Ph.D. Thesis is conducted in two main fields i.e., computational understanding of mechanisms in transition metal chemistry as well as main group (Group 13 and 14) chemistry. The role of catalyst, its activation and regeneration role of substrates, ligands, solvents and other factors controlling the chemical transformation along with devising new reaction strategies remains my central interest. Furthermore, my interest also includes understanding of the structure-bonding correlation in many newly synthesized, main group (Group 13 and 14) species. Transition metal catalyzed double-bond migration remains a fascinating tool to achieve numerous functionalized olefin systems. Due to potential quantitative atom economy, several studies have been performed towards the development of new catalysts for this reaction. Recently in 2012, Gooßen et. al. have shown that dimeric Pd(I) species [PdI(μ-Br)PtBu₃]₂ which was first reported by Mingos and co-workers acts as a highly efficient catalyst for double-bond migration. Dimeric Pd(I) species [PdI(μ-Br)(PtBu₃]₂ as a highly efficient catalyst for the isomerization reaction is not only applicable for simple alkenes but also for the other unsaturated compounds viz. fatty acids, allylic esters, arenes, amides, ethers and alcohols with moderate E/Z selectivity. This unusual dimeric Pd(I) species converted the fatty acids to an equilibrium mixture of double-bond isomers within less than an hour. Motivated by the experimental results, we have explored the double-bond migration reaction in presence of the same dimeric Pd(I) catalyst [PdI(μ-Br)PtBu3₃]₂. In Chapters 2 and 3, we have endeavoured to address some important issues which are very relevant to the mechanistic aspect. The primary motivation was to pinpoint the actual catalyst responsible for the olefin migration reaction. Is there any direct reactivity of this Pd(I) dimer which was previously suggested by the group of Schoenebeck in case of carbon-carbon cross-coupling reactions? What is the driving force for the generation the active catalyst? Beside the mechanistic study, we have also pointed out the influence of the substrate structure on the efficiency of the reactions and seek the origin of stereoselectivity of the isomerization outcome. To the best of our knowledge, the present computational study for the first time unravel the activation pathways of dinuclear [PdI(μ-Br)PtBu3]2 to generate the catalytic species responsible for the double bond migration. The mechanistic studies presented herein are likely to serve as the basis for the rational design of more efficient catalyst in case of C–C double bond migration reactions. Over the past few decades, interest in 1,2,4-triazoles which were first discovered by Bladin have gained popularity due to their pharmacological benefits such as antibacterial, antifungal, hypoglycaemic, antihypertensive and analgesic properties. In addition to the biological activity, 1,2,4-triazoles shows remarkable potential towards metal binding due to presence of multiple heteroatoms that can easily coordinate to the metal center. However, the metal free 1,2,4-trazole derivatives have also exhibited interesting and useful properties as materials. Due to the biological and chemical importance of the 1,2,4-triazole and its derivatives, various synthetic tools have already been reported. Recently, Maji and co-workers have synthesized 1,2,4-triazole derivatives with the help of N-tosylhydrazones and anilines via tris(pentafluorophenyl) borane B(C₆F₅)₃. In Chapter 4, we narrate the detailed reaction mechanism and give insight into the driving force for the formation of 1,2,4-triazole moiety. Additionally, we will address the specific role of B(C₆F₅)₃ in the reaction. Finally, we will conclude the study by discussing the product distribution for unsymmetrical couplings. Recently, Singh and co-workers have synthesized the boron and silicon complexes in presence of bicyclic(Alkyl)(Amino)Carbene (BICAAC) as a stabilizing species. In Chapter 5, we will describe the electronic structure, bonding scenario and photophysical properties of some BICAAC based B and Si compounds with the help of DFT calculations and other computational techniques like NBO, AIM, EDA analysis etc. Our computational results show a nice correlation with the available experimental findings. Cyanosilylation of carbonyl functional group is the most fundamental carbon–carbon bond forming reaction and the resultant “Cyanohydrins” are important intermediates that can be easily converted into numerous organic and biological compounds like α-hydroxy acids, 1,2-diols, α-amino alcohols, etc. Due to their importance in organic synthesis and also life science research, several contributions have been dedicated in the development of cyanohydrin synthesis. Chapter 6 describes the computational investigation on the cyanosilylation reaction of carbonyl compounds using aluminum organic catalysts. Importantly, the initial attack of TMSCN is favored over carbonyl moiety. The pathway is comprised of four elementary steps, (i) coordination of the Me₃SiCN to Al center, (ii) addition of carbonyl compounds to the Al coordinated Me₃SiCN complex, (iii) activation of C−Si bond by the carbonyls and finally (iv) product formation via elimination. Activation of C−Si bond by the addition of carbonyl compounds is the rate limiting among the overall reaction steps.

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