A Synergistic Bimetallic Ti/Co-Catalyzed Isomerization of Epoxides to Allylic Alcohols Enabled by Two-State Reactivity

Isomerization of epoxides into versatile allylic alcohols is an atom-economical synthetic method to afford vicinal bifunctional groups. Comprehensive density functional theory (DFT) calculations were carried out to elucidate the complex mechanism of a bimetallic Ti/Co-catalyzed selective isomerization of epoxides to allyl alcohols by examining several possible pathways. Our results suggest a possible mechanism involving (1) radical-type epoxide ring opening catalyzed by Cp2Ti(III)Cl leading to a Ti(IV)-bound β-alkyl radical, (2) hydrogen-atom transfer (HAT) catalyzed by the Co(II) catalyst to form the Ti(IV)-enolate and Co(III)–H intermediate, (3) protonation to give the alcohols, and (4) proton abstraction to form the Co(I) species followed by electron transfer to regenerate the active Co(II) and Ti(III) species. Moreover, bimetallic catalysis and two-state reactivity enable the key rate-determining HAT step. Furthermore, a subtle balance between dispersion-driven bimetallic processes and entropy-driven monometallic processes determines the most favorable pathway, among which the monometallic process is energetically more favorable in all steps except the vital hydrogen-atom transfer step. Our study should provide an in-depth mechanistic understanding of bimetallic catalysis

New Insights and Predictions into Complex Homogeneous Reactions Enabled by Computational Chemistry in Synergy with Experiments: Isotopes and Mechanisms

ConspectusHomogeneous catalysis and biocatalysis have been widely applied in synthetic, medicinal, and energy chemistry as well as synthetic biology. Driven by developments of new computational chemistry methods and better computer hardware, computational chemistry has become an essentially indispensable mechanistic “instrument” to help understand structures and decipher reaction mechanisms in catalysis. In addition, synergy between computational and experimental chemistry deepens our mechanistic understanding, which further promotes the rational design of new catalysts. In this Account, we summarize new or deeper mechanistic insights (including isotope, dispersion, and dynamical effects) into several complex homogeneous reactions from our systematic computational studies along with subsequent experimental studies by different groups. Apart from uncovering new mechanisms in some reactions, a few computational predictions (such as excited-state heavy-atom tunneling, steric-controlled enantioswitching, and a new geminal addition mechanism) based on our mechanistic insights were further verified by ensuing experiments.The Zimmerman group developed a photoinduced triplet di-π-methane rearrangement to form cyclopropane derivatives. Recently, our computational study predicted the first excited-state heavy-atom (carbon) quantum tunneling in one triplet di-π-methane rearrangement, in which the reaction rates and 12C/13C kinetic isotope effects (KIEs) can be enhanced by quantum tunneling at low temperatures. This unprecedented excited-state heavy-atom tunneling in a photoinduced reaction has recently been verified by an experimental 12C/13C KIE study by the Singleton group. Such combined computational and experimental studies should open up opportunities to discover more rare excited-state heavy-atom tunneling in other photoinduced reactions. In addition, we found unexpectedly large secondary KIE values in the five-coordinate Fe­(III)-catalyzed hetero-Diels–Alder pathway, even with substantial C–C bond formation, due to the non-negligible equilibrium isotope effect (EIE) derived from altered metal coordination. Therefore, these KIE values cannot reliably reflect transition-state structures for the five-coordinate metal pathway. Furthermore, our density functional theory (DFT) quasi-classical molecular dynamics (MD) simulations demonstrated that the coordination mode and/or spin state of the iron metal as well as an electric field can affect the dynamics of this reaction (e.g., the dynamically stepwise process, the entrance/exit reaction channels).Moreover, we unveiled a new reaction mechanism to account for the uncommon Ru­(II)-catalyzed geminal-addition semihydrogenation and hydroboration of silyl alkynes. Our proposed key gem-Ru­(II)–carbene intermediates derived from double migrations on the same alkyne carbon were verified by crossover experiments. Additionally, our DFT MD simulations suggested that the first hydrogen migration transition-state structures may directly and quickly form the key gem-Ru–carbene structures, thereby “bypassing” the second migration step. Furthermore, our extensive study revealed the origin of the enantioselectivity of the Cu­(I)-catalyzed 1,3-dipolar cycloaddition of azomethine ylides with β-substituted alkenyl bicyclic heteroarenes enabled by dual coordination of both substrates. Such mechanistic insights promoted our computational predictions of the enantioselectivity reversal for the corresponding monocyclic heteroarene substrates and the regiospecific addition to the less reactive internal CC bond of one diene substrate. These predictions were proven by our experimental collaborators. Finally, our mechanistic insights into a few other reactions are also presented. Overall, we hope that these interactive computational and experimental studies enrich our mechanistic understanding and aid in reaction development

Colorimetric Calcium Probe with Comparison to an Ion-Selective Optode

Design strategies for small molecular probes lay the foundation of numerous synthetic chemosensors. A water-soluble colorimetric calcium molecular probe inspired by the ionophore-based ion-selective optode is presented here with a tunable detection range (around micromolar at pH 7). The binding of Ca2+ resulted in the deprotonation of the probe and thus a significant spectral change, mimicking the ion-exchange process in ion-selective optodes. The 1:1 exchange between Ca2+ and H+ was confirmed with Job’s plot. Computational studies revealed possible monomer and dimer forms of the probe–Ca2+ complexes

A Mechanistic Study of the Cobalt(I)-Catalyzed Amination of Aryl Halides: Effects of Metal and Ligand

Transition-metal-catalyzed amination of aryl halides is a useful approach for the synthesis of medicinal compounds, organic functional materials, and agrochemical compounds. A systematic DFT study has been performed to investigate the mechanism of the Co­(I)-catalyzed amination of aryl halides by LiN­(SiMe3)2 using (PPh3)3CoCl as the precatalyst. Our computational results suggest that the most favorable dissociative concerted C–I activation pathway in a triplet state consists of (a) dissociation of one PPh3 ligand, (b) concerted oxidative addition (OA) of the C–I bond, (c) transmetalation, (d) (optional) dissociation of the second PPh3 ligand, (e) C–N bond-forming reductive elimination (RE), and (f) ligand exchange to regenerate the active species. Comparatively, the associative concerted OA, radical, SH2/SN2, single electron transfer (SET), and σ-bond metathesis pathways should be less favorable due to their higher barriers or unfavorable reaction free energies. The effects of different metals (Rh and Ir) as centers in the catalyst were further examined and found to require higher reaction barriers, due to unfavorable dissociation of their stronger M–PPh3 bonds. These results highlight an advantage of the earth-abundant Co catalysts for the dissociative pathway(s). Overall, our study offers deeper mechanistic insights for the transition-metal-catalyzed amination and guides the design for efficient Co-based catalysts

New Tricks for an Old Dog: Grubbs Catalysts Enable Efficient Hydrogen Production from Aqueous-Phase Methanol Reforming

Herein, we report a new application of the prize-winning Grubbs catalysts, which have been widely applied for olefin metathesis, for hydrogen production from aqueous-phase methanol reforming under easily achievable conditions (1 atm, <100 °C) with negligible CO formation. Out of the catalysts tested, the best turnover frequency (158 h–1) and turnover number (11424, 72 h) were both achieved with a third-generation Grubbs catalyst (G-III). The best TOF was achieved with G-III and is competitive when compared with some of the best results reported (Chem. Rev. 2018, 118, 372−433). Also, G-III is found to be a versatile catalyst for the dehydrogenation of ethanol and formic acid. Mechanistic studies and DFT calculations shed light on the reaction mechanism, which involves an unusual substrate (solvent)-assisted six-membered-ring (σ-bond) metathesis pathway. This work should open up new opportunities in catalyst design in connection with the hydrogen economy and, more generally, with the development of clean and renewable energies

Design and Application of Hybrid Phosphorus Ligands for Enantioselective Rh-Catalyzed Anti-Markovnikov Hydroformylation of Unfunctionalized 1,1-Disubstituted Alkenes

A series of novel hybrid phosphorus ligands were designed and applied to the Rh-catalyzed enantioselective anti-Markovnikov hydroformylation of unfunctionalized 1,1-disubstituted alkenes. By employing the new catalyst, linear aldehydes with β-chirality can be prepared with high yields and enantioselectivities under mild conditions. Furthermore, catalyst loading as low as 0.05 mol % furnished the desired product in good yield and undiminished selectivity, demonstrating the efficiency of this transformation in large-scale synthesis