Kumar Vanka

A Combined Density Functional Theory and Molecular Mechanics (QM/MM) Study of Single-Site Ethylene Polymerization Catalyzed by [(C~6H~5N=CH)-C~4H~3N]~2-RM^+ {M = Ti, Zr} in the Presence of the Counterion CH~3B(C~6F~5)~3^-

Calculations have been carried out to investigate the insertion of the ethylene monomer into the M−methyl bond for the systems [(C6H5NCH)C4H3N]2-CH3M-μ-CH3-B(C6F5)3 (M = Ti and Zr), using density functional theory. Second insertion studies have also been conducted for the [(C6H5NCH)C4H3N]2-C3H7Ti-μ-CH3-B(C6F5)3 system. A validated QM/MM model was used to represent the counterion. The C6H5 groups in the ligands were modeled with QM/MM, with hydrogens being used as the capping atoms. With R = Me (the initiation step), approach of the ethylene cis to the μ-Me bridge (cis approach) and from between the pyrrolide imine ligands (trans approach) were considered, along a path leading first to uptake of ethylene to form a π-complex followed by insertion of ethylene into the M−R bond. For the titanium-based system, the uptake was found to be rate determining for the cis approach, the total barrier being 12.9 kcal/mol, and the insertion barrier was found to be rate determining for the trans approach, the total barri...

Effect of Donors on the Activation Mechanism in Ziegler–Natta Catalysis: A Computational Study

Full quantum chemical calculations, using density functional theory (DFT), have been conducted to explain the effect of donors on the “activation mechanism” in the Ziegler–Natta (Z–N) catalyst system. In the activation mechanism, the inactive TiIVCl4 catalyst converts into the active TiIIICl2Et catalyst with the help of the AlEt3 present in the system. The donors that have been considered in this study are: ethyl benzoate (eb), two representative diether cases, a phthalate donor, and a silyl ester donor. The results indicate that eb and the diether donor cases donor have a negative effect on the barriers for the activation mechanism. However, the eb donor can be displaced from the MgCl2 surface by AlEt3, which matches experimental observations. For the phthalate, silyl ester and TiCl3–OC4H8Cl cases, the results indicate that a significant induction period would be present in Z–N systems employing such donors or having such a catalytic center, before catalysis could commence.

A Density Functional Study of Ethylene Insertion into the M‐methyl (M = Ti, Zr) Bond for Different Catalysts, with a QM/MM Model for the Counterion, B(C6F5)3CH3−

Single site homogeneous catalysts have been studied extensively in recent years as alternatives to traditional heterogeneous catalysts. The current theoretical study uses density functional theory to study the insertion process of the ethylene monomer into the titanium-carbon chain for contact ion-pair systems of the type [L 1 L 2 TiCH 3 -μ-CH 3 -B(C 6 F 5 ) 3 ]. where L 1 , L 2 , are Cp, NPH 3 , and other ligands. Different modes of approach cis and trans to the μ-CH 3 bridge were considered. The counterion, B(C 6 F 5 ) 3 CH 3 , was modeled by QM/MM methods. The value of ΔH t o l -the total barrier to insertion-was found to be positive (in the range of 4-15 kcal/mol). The ability of the ancillary ligands. L 1 and L 2 , to stabilize the ion-pair was found to be an important factor in determining the value of ΔH t o l . On replacing the titanium metal center with zirconium, the ΔH values were found to he lowered (in the range of 2 -9 kcal/mol), indicating that they would be better catalysts than their titanium analogues. The size of the ligands L 1 and L. was increased by replacing hydrogens in the ligands with tertiary butyl groups. The value of ΔH s o l was found to increase (in the range of 10-28 kcal/mol) in contrast to the simple systems, for both the cis and trans cases of approach, with the cis mode of approach giving lower values of ΔH t o t . Solvent effects were incorporated with cyclohexane (e = 2.023) as the solvent, and were found to have a minor influence. ′(0.5-1.5) kcal/mol) on the insertion barrier for all the cases studied.

Secondary Interactions Arrest the Hemiaminal Intermediate To Invert the Modus Operandi of Schiff Base Reaction: A Route to Benzoxazinones

Discovered by Hugo Schiff, condensation between amine and aldehyde represents one of the most ubiquitous reactions in chemistry. This classical reaction is widely used to manufacture pharmaceuticals and fine chemicals. However, the rapid and reversible formation of Schiff base prohibits formation of alternative products, of which benzoxazinones are an important class. Therefore, manipulating the reactivity of two partners to invert the course of this reaction is an elusive target. Presented here is a synthetic strategy that regulates the sequence of Schiff base reaction via weak secondary interactions. Guided by the computational models, reaction between 2,3,4,5,6-pentafluoro-benzaldehyde with 2-amino-6-methylbenzoic acid revealed quantitative (99%) formation of 5-methyl-2-(perfluorophenyl)-1,2-dihydro-4H-benzo[d][1,3]oxazin-4-one (15). Electron donating and electron withdrawing ortho-substituents on 2-aminobenzoic acid resulted in the production of benzoxazinones 9-36. The mode of action was tracked using low temperature NMR, UV-vis spectroscopy, and isotopic (18O) labeling experiments. These spectroscopic mechanistic investigations revealed that the hemiaminal intermediate is arrested by the hydrogen-bonding motif to yield benzoxazinone. Thus, the mechanistic investigations and DFT calculations categorically rule out the possibility of in situ imine formation followed by ring-closing, but support instead hydrogen-bond assisted ring-closing to prodrugs. This unprecedented reaction represents an interesting and competitive alternative to metal catalyzed and classical methods of preparing benzoxazinone.

Less Frustration, More Activity ‐ Interesting Theoretical Insights into Frustrated Lewis Pairs for Hydrogenation Catalysis

The field of frustrated Lewis pair (FLP) chemistry has seen rapid development in only a few years. FLPs have performed most spectacularly in hydrogenation catalysis: a wide variety of FLP‐based systems can catalyze the hydrogenation of a range of different substrates, including imines, enamines, ketones, alkynes, and alkenes. However, FLP‐based hydrogenation catalysts are yet to match the efficiency of their transition‐metal counterparts. The current investigation reveals an important aspect of FLPs that can be exploited to improve their efficiency, that is, the more sterically hindered the FLP catalyst is, the lower is its turnover frequency. Full quantum chemical calculations with DFT for a family of different, experimentally known hydrogenation FLP catalysts shows that superior FLP catalysts can be designed by reducing the frustration (by reducing the steric demand and acid/base strength) in the FLP. However, as lowering the steric demand without reduction in the frustration can result in unwanted side reactions, the design of the most efficient FLP catalysts depends on tuning the system so that both the steric demand and the frustration are reduced appropriately.

Exploring the activation of olefin polymerisation catalysts with density functional theory

Density Functional Theory has been used to study the activation of different olefin polymerisation catalysts by different activators. The results show that biscyclopentadienyl catalyst systems would act as the best catalysts and the activators of the type [CPh 3 + ][A - ] would be the best at activating such systems. The competition between different species present in solution for the vacant active site in the catalyst was studied for the [(1,2Me 2 Cp) 2 ZrMe + ][B(C 6 F 5 ) 3 CH 3 - ] system and the pre-catalyst and AlMe3 were found to be the compounds most likely to form dormant products in solution.