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 Mechanistic Origin of Ligand Effects on Exhaustive Functionalization During Pd-Catalyzed Cross-Coupling of Dihaloarenes(American Chemical Society, 2024-04) Larson, Nathaniel G.; Norman, Jacob P.; Neufeldt, SharonWe describe a detailed investigation into why bulky ligands─those that enable catalysis at “12e–” Pd0─tend to promote overfunctionalization during Pd-catalyzed cross-couplings of dihalogenated substrates. After one cross-coupling event takes place, PdL initially remains coordinated to the π system of the nascent product. Selectivity for mono- vs difunctionalization arises from the relative rates of π-decomplexation versus a second oxidative addition. Under the Suzuki coupling conditions in this work, direct dissociation of 12e– PdL from the π-complex cannot outcompete oxidative addition. Instead, Pd must be displaced from the π-complex as 14e– PdL(L’) by a second incoming ligand L’. The incoming ligand is another molecule of dichloroarene if the reaction conditions do not include π-coordinating solvents or additives. More overfunctionalization tends to result when increased ligand or substrate sterics raises the energy of the bimolecular transition state for separating 14e– PdL(L’) from the monocross-coupled product. This work has practical implications for optimizing the selectivity in cross-couplings involving multiple halogens. For example, we demonstrate that small coordinating additives like DMSO can largely suppress overfunctionalization and that the precatalyst structure can also impact selectivity.Item Solvent coordination to palladium can invert the selectivity of oxidative addition(Royal Society of Chemistry, 2022-01) Elias, Emily K; Rehbein, Steven M; Neufeldt, Sharon R.In the presence of the bulky monophosphine PtBu3, palladium usually prefers to react with Ar–Cl over Ar–OTf bonds. However, strongly coordinating solvents can bind to palladium, inducing a reversal of selectivity.