Biological Applications and Mechanistic Insights of Ligand-Accelerated Palladium-Catalyzed O-Deallylation

This thesis discusses the biological activity and mechanistic aspects of the ligand-accelerated palladium-catalyzed O-deallylation. Previously, Pohorilets (Koide Group) reported a tri(2-furyl)phosphine (TFP) analog, iPhos, which successfully uncaged resorufin allyl ether in cell culture media containing serum. The primary objective of the current work is to evaluate the kinetics of Pd(OAc)2-iPhos, investigate its activity in living cells, and explore its potential in bioorthogonal applications. Pd(OAc)2-iPhos outperformed a benchmark Ru complex in complete culture media, yielding a second-order rate constant of 36.2 M-1s-1. The high bioorthogonal activity was also demonstrated in intracellular uncaging of allyl Pittsburgh Green ether. The uncaging can not only be performed in living cells maintained in HBSS, but also in living cells maintained in complete culture media. Complete culture media contains proteins and vitamins abundantly to sustain the cell growth, and such biological matrix is detrimental for transition metals. Therefore, the compatibility of Pd(OAc)2-iPhos with complete culture media highlights the improvement of Pd catalyst for using under a bioorthogonal context. The high activity of Pd(OAc)2-iPhos was corroborated by cryogenic 31P NMR. The addition of Pd(OAc)2 to the solution of iPhos broadened the peak corresponding to iPhos, indicating the rapid equilibrium between the Pd-bound and unbound iPhos. This broadened peak became sharper as the temperature decreased. Importantly, this observation was absent with Pd(OAc)2-TFP. The cryogenic 31P NMR studies suggested that a labile Pd-P interaction facilitates the catalytic activity likely by enhancing the π-philicity of the Pd center. To explore other applications for Pd(OAc)2-iPhos, uncaging of caged nuclear localization signal (NLS), delivering membrane-impermeable aspartate and small molecules were tested. Pd(OAc)2-iPhos was unable to achieve the desired outcome. However, these failed attempts offered new insights into the future applications of Pd(OAc)2-iPhos. Collectively, current work reported Pd(OAc)2-iPhos, a highly bioorthogonally active Pd-phosphine complex which remained active in living cells within an unmodified cell culture environment. The mechanistic insights provided by cryogenic 31P NMR suggest that the π-philic nature of Pd complex was crucial for the bioorthogonal activity. The present study on Pd complex added valuable insights for the future development of bioorthogonal Pd catalysts.