Chemistry & Biochemistry
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The Department of Chemistry and Biochemistry offers research-oriented programs culminating in the Doctor of Philosophy degree. The faculty in the department have expertise over a broad range of specialty areas including synthesis, structure, spectroscopy, and mechanism. In each of these fields, the strength of the department has been recognized at the international level.
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Item Oxidative Addition of (Hetero)aryl (Pseudo)halides at Palladium(0): Origin and Significance of Divergent Mechanisms(American Chemical Society, 2024-07) Kania, Matthew J.; Reyes, Albert; Neufeldt, Sharon R.Two limiting mechanisms are possible for oxidative addition of (hetero)aryl (pseudo)halides at Pd(0): a 3-centered concerted and a nucleophilic displacement mechanism. Until now, there has been little understanding about when each mechanism is relevant. Prior investigations to distinguish between these pathways were limited to a few specific combinations of the substrate and ligand. Here, we computationally evaluated over 180 transition structures for oxidative addition in order to determine mechanistic trends based on substrate, ligand(s), and coordination number. Natural abundance 13C kinetic isotope effects provide experimental results consistent with computational predictions. Key findings include that (1) differences in highest occupied molecular orbital (HOMO) symmetries dictate that, although 12e– PdL is strongly biased toward a 3-centered concerted mechanism, 14e– PdL2 often prefers a nucleophilic displacement mechanism; (2) ligand electronics and sterics, including ligand bite angle, influence the preferred mechanism of the reaction at PdL2; (3) phenyl triflate always reacts through a displacement mechanism regardless of the catalyst structure due to the stability of a triflate anion and the inability of oxygen to effectively donate electron density to Pd; and (4) the high reactivity of C–X bonds adjacent to nitrogen in pyridine substrates relates to stereoelectronic stabilization of a nucleophilic displacement transition state. This work has implications for controlling rate and selectivity in catalytic couplings, and we demonstrate application of the mechanistic insight toward chemodivergent cross-couplings of bromochloroheteroarenes.Item Experimental and Computational Evaluation of Tantalocene Hydrides for C–H Activation of Arenes(American Chemical Society, 2021-08) Rehbein, Steven M.; Kania, Matthew J.; Neufeldt, Sharon R.Half a century ago, tantalocene hydrides (especially Cp2TaH3, where Cp = η5-C5H5) were reported to catalyze H/D exchange with arenes. However, there has been very little follow-up to the seminal reports, and numerous questions about this chemistry remain unanswered. In an effort to better evaluate the potential of tantalocene hydrides for processes involving C–H activation, we have conducted a series of experimental and computational studies on these complexes. Density functional theory (DFT) calculations support a mechanism for arene C–H activation involving oxidative addition at transient TaIII, rather than a σ-bond metathesis mechanism at TaV. Comparisons were made between thermal and photochemical conditions for the reaction of Cp2TaH3 with benzene-d6, and H/D exchange was found to be moderately faster under thermal conditions. In a reaction with toluene, Cp2TaH3 activates the aromatic C(sp2)–H bonds but not the benzylic bonds. DFT calculations suggest that benzylic C–H activation at TaIII has a barrier similar to aromatic C–H activation, but that formation of a π-complex with Cp2TaH directs preferential aromatic C–H activation. Analogous complexes containing the less labile permethylated ligand Cp* (Cp* = η5-C5Me5) were also evaluated for their ability to catalyze H/D exchange with benzene-d6, but these complexes are less active than Cp2TaH3. DFT calculations indicate that the methyl groups of Cp* disfavor π-coordination of an arene to the TaIII intermediate.