Real-Time Monitoring of Solid-Liquid Slurries: Optimized Synthesis of Tetrabenazine

Halide Salts Alleviate TMSOK Inhibition in Suzuki-Miyaura Cross-Couplings

The Suzuki-Miyaura cross-coupling (SMC) remains one of the most widely used transformations available to chemists. Recently, robust new conditions achieving rapid reactivity under homogeneous aprotic conditions enabled by the use of potassium trimethylsilanolate (TMSOK) as a base were reported. However, the strong inhibitory effect of TMSOK restricts the generality of such conditions. Moreover, the basic nature of TMSOK impedes the use of protic heterocycles as substrates, as these latter anionic species are even more potent catalyst inhibitors. Herein, we report a thorough mechanistic study of these novel SMC conditions. Halide salt additives were found to provide a dramatic rate acceleration and mitigate the inhibitory effect of TMSOK. NMR experiments revealed that this is largely achieved by impacting the unexpected formation of inactive [LnPd(Ar)(μ-OH)]2, favoring the formation of active LnPd(Ar)(X) instead. These findings enabled an impressive substrate scope even at low catalyst loadings (0.1 mol %). Finally, halide additives were observed to enable the use of protic heterocyclic substrates, which could otherwise completely inhibit reactivity.

Investigating the Origin of Epimerization Attenuation during Pd-Catalyzed Cross-Coupling Reactions

Palladium-catalyzed cross-couplings remain among the most robust methodologies to form carbon-carbon and carbon-heteroatom bonds. In particular, carbon-nitrogen (C-N) couplings (Buchwald-Hartwig aminations) find widespread use in fine chemicals industries. The use of base in these reactions is critical for catalyst activation and proton sequestration. Base selection also plays an important role in process design, as strongly basic conditions can impact sensitive stereocenters and result in erosion of stereochemical purity. Herein we investigate the role of a Pd catalyst in suppressing base-mediated epimerization of a sultam stereocenter during a C-N cross-coupling reaction to access the RORγ inhibitor GDC-0022. Online high-performance liquid chromatography-mass spectrometry (HPLC-MS) was employed to acquire reaction time course profiles and to delineate epimerization behavior, identify decomposition pathways, and monitor Pd-containing species. Our ability to monitor organopalladium complexes in real time by HPLC-MS provided strong evidence that the degree of epimerization was correlated to the Pd speciation in solution. Specifically, Pd(II) complexes were associated with mitigating epimerization of six-membered sultams. Additional studies showed that the suppression of epimerization in the presence of Pd(II) can impact Pd-catalyzed reactions of other substrates such as enolizable ketones, thus providing practical insight on the execution and optimization of such processes.

Simultaneous reaction- and analytical model building using dynamic flow experiments to accelerate process development

In modern pharmaceutical research, the demand for expeditious development of synthetic routes to active pharmaceutical ingredients (APIs) has led to a paradigm shift towards data-rich process development. Conventional methodologies encompass prolonged timelines for the development of both a reaction model and analytical models. The development of both methods are often separated into different departments and can require an iterative optimization process. Addressing this issue, we introduce an innovative dual modeling approach, combining the development of a Process Analytical Technology (PAT) strategy with reaction optimization. This integrated approach is exemplified in diverse amidation reactions and the synthesis of the API benznidazole. The platform, characterized by a high degree of automation and minimal operator involvement, achieves PAT calibration through a "standard addition" approach. Dynamic experiments are executed to screen a broad process space and gather data for fitting kinetic parameters. Employing an open-source software program facilitates rapid kinetic parameter fitting and additional in silico optimization within minutes. This highly automated workflow not only expedites the understanding and optimization of chemical processes, but also holds significant promise for time and resource savings within the pharmaceutical industry.

Phase transfer catalysts shift the pathway to transmetalation in biphasic Suzuki-Miyaura cross-couplings

The Suzuki-Miyaura coupling is a widely used C-C bond forming reaction. Numerous mechanistic studies have enabled the use of low catalyst loadings and broad functional group tolerance. However, the dominant mode of transmetalation remains controversial and likely depends on the conditions employed. Herein we detail a mechanistic study of the palladium-catalyzed Suzuki-Miyaura coupling under biphasic conditions. The use of phase transfer catalysts results in a remarkable 12-fold rate enhancement in the targeted system. A shift from an oxo-palladium based transmetalation to a boronate-based pathway lies at the root of this activity. Furthermore, a study of the impact of different water loadings reveals reducing the proportion of the aqueous phase increases the reaction rate, contrary to reaction conditions typically employed in the literature. The importance of these findings is highlighted by achieving an exceptionally broad substrate scope with benzylic electrophiles using a 10-fold reduction in catalyst loading relative to literature precedent.

Insights into the synthesis of NHC-stabilized Au nanoclusters through real-time reaction monitoring

Atomically precise gold nanoclusters (AuNCs) are interesting nanomaterials with potential applications in catalysis, bioimaging and optoelectronics. Their compositions and properties are commonly evaluated by various analytical techniques, including UV-vis spectroscopy, NMR spectroscopy, ESI mass spectrometry, and single-crystal X-ray diffraction. While these techniques have provided detailed insights into the structure and properties of nanoclusters, synthetic methods still suffer from a lack of in situ and real-time reaction monitoring methodologies. This limits insight into the mechanism of formation of AuNCs and hinders attempts at optimization. We have demonstrated the utility of HPLC-MS as a monitoring methodology in the synthesis of two NHC-protected gold nanoclusters: [Au13(NHC)9Cl3]2+ and [Au24(NHC)14Cl2H3]3+. Herein we show that HPLC coupled with mass spectrometry and 13C NMR spectroscopy of labelled derivatives enables new insight into critical reaction dynamics of AuNCs synthesis and rapid reaction optimization.

Continuous addition kinetic elucidation: catalyst and reactant order, rate constant, and poisoning from a single experiment

Kinetic analysis of catalytic reactions is a powerful tool for mechanistic elucidation but is often challenging to perform, limiting understanding and therefore development of these reactions. Establishing order in a catalyst is usually achieved by running several reactions at different loadings, which is both time-consuming and complicated by the challenge of maintaining consistent run-to-run experimental conditions. Continuous addition kinetic elucidation (CAKE) was developed to circumvent these issues by continuously injecting a catalyst into a reaction, while monitoring reaction progress over time. For reactions that are mth order in a single yield-limiting reactant and nth order in catalyst, a plot of reactant concentration against time has a shape dependent only on the orders m and n. Therefore, fitting experimental CAKE data (using open access code or a convenient web tool) allows the reactant and catalyst orders, rate constant, and the amount of complete catalyst inhibition to be determined from a single experiment. Kinetic information obtained from CAKE experiments showed good agreement with the literature.

Enantioselective synthesis of (−)-tetrabenazine via continuous crystallization-induced diastereomer transformation

A multi-well continuous CIDT approach with inline racemization of the solution phase is presented. Using two in-house built PATs and a flow reactor, we were able to successfully crystallize an enantiopure salt of TBZ, the active metabolite of the tardive dyskinesia drug valbenazine. Despite discovering an undesired racemic solid phase, inline racemization combined with careful control of crystallization conditions allowed for multigram quantities of enantiopure material to be harvested using our setup. Critically, this control was made possible by the use of PATs to observe and quantify the composition of both the solid and solution phases.

A novel enantioselective route to tetrabenazine has been developed using continuous CIDT in a multiwell crystallization/racemization device outfitted with real-time HPLC to visualize and control the dynamic process.