- Cobalt-Catalyzed C(sp2)-H Borylation: Mechanistic Insights Inspire Catalyst Design
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A comprehensive study into the mechanism of bis(phosphino)pyridine (PNP) cobalt-catalyzed C-H borylation of 2,6-lutidine using B2Pin2 (Pin = pinacolate) has been conducted. The experimentally observed rate law, deuterium kinetic isotope effects, and identification of the catalyst resting state support turnover limiting C-H activation from a fully characterized cobalt(I) boryl intermediate. Monitoring the catalytic reaction as a function of time revealed that borylation of the 4-position of the pincer in the cobalt catalyst was faster than arene borylation. Cyclic voltammetry established the electron withdrawing influence of 4-BPin, which slows the rate of C-H oxidative addition and hence overall catalytic turnover. This mechanistic insight inspired the next generation of 4-substituted PNP cobalt catalysts with electron donating and sterically blocking methyl and pyrrolidinyl substituents that exhibited increased activity for the C-H borylation of unactivated arenes. The rationally designed catalysts promote effective turnover with stoichiometric quantities of arene substrate and B2Pin2. Kinetic studies on the improved catalyst, 4-(H)2BPin, established a change in turnover limiting step from C-H oxidative addition to C-B reductive elimination. The iridium congener of the optimized cobalt catalyst, 6-(H)2BPin, was prepared and crystallographically characterized and proved inactive for C-H borylation, a result of the high kinetic barrier for reductive elimination from octahedral Ir(III) complexes.
- Obligacion, Jennifer V.,Semproni, Scott P.,Pappas, Iraklis,Chirik, Paul J.
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supporting information
p. 10645 - 10653
(2016/09/04)
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- Synthesis and characterization of new polysubstituted pyridinium-based ionic liquids: Application as solvents on desulfurization of fuel oils
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The production of transportation fuels which have a very low content of sulfur has became one of the priority challenges for the oil industry worldwide, due to by strict new regulatory requirements. Ionic liquids (ILs) have been proposed as suitable and promising solvents for this purpose due to their excellent qualities as solvents. In this work a series of ten new ILs derived from pyridinium cation substituted with different alkyl chains have been synthesized from 2-alkyl-3,5-dimethylpyridines. The starting materials were prepared by selective metalation of 2,3,5-trimethylpyridine, which allowed the introduction of different alkyl groups in pyridine position 2 with high yields. To test the ILs sulfur-removal capacity, liquid-liquid equilibrium (LLE) data for ternary systems (heptane + thiophene + IL) were determined at T = 298.15 K and atmospheric pressure. Selectivity and solute distribution ratio, calculated from tie-lines, were used to evaluate whether these new ILs could be used as solvents for the extraction of thiophene from heptane. Finally, the experimental LLE data were correlated with the NRLT thermodynamic model.
- Verdia, Pedro,Gonzalez, Emilio J.,Rodriguez-Cabo, Borja,Tojo, Emilia
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experimental part
p. 2768 - 2776
(2011/12/05)
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- Process for producing 2,3,5-collidine and/or 2,3,5,6-tetramethylpyridine
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Process for producing 2,3,5-collidine and/or 2,3,5,6-tetramethylpyridine which is characterized by reacting 3,5-lutidine as the starting material with an aliphatic alcohol having 1 to 4 carbon atoms, in the presence of a catalyst for hydrogenation at a temperature of 200° C. or higher. The above-mentioned 2,3,5-collidine and/or 2,3,5,6-tetramethylpyridine are important compounds as intermediates for synthesizing various pharmaceutical chemicals.
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