Kinetics of Palladium(0)‐Allyl Interactions in the Tsuji‐Trost Reaction, derived from Single‐Molecule Fluorescence Microscopy

Abstract

Single-molecule (SM) chemistry is devoted to unravel reaction steps which are hidden in cuvette experiments. Controversies about the substrate activation during the Tsuji-Trost deallylation motivated us to study, on the single-molecule level, the kinetics of the catalyst precursor Pd(PPh 3 ) 4 with our recently designed two-color fluorescent probes. Photochemical, metal-free bypass reactions were found and taken into account by the combina-tion of spectrally separated single-molecule TIRF-microscopy and state-of-the art analysis procedures. Unselective π-complex formation (K D � 10 3 M À 1 ) precedes the insertion of the active catalyst into the CÀ OR bond (RO À = leaving group), indicated by the lacking immediate change of fluorescence color. The formed intermediate then decomposes on a time scale of � 2 -3 s to the deallylated product.

Steady-State Spectroscopy to Single Out the Contact Ion Pair in Excited-State Proton Transfer

“…In previous work, we have established pyrene-based superphotoacids with estimated p K a * values down to −4 in the excited state, high photostability, and high fluorescence quantum yields . They proved to be versatile probes for single-molecule microscopy , and time-resolved spectroscopy. , The focus is now placed on the elementary proton transfer step, that is, the formation of the HBIP: How does the surrounding solvent influence the PT process?…”

Exploring chemistry with single-molecule and -particle fluorescence microscopy

“…Although subensemble fluorescence microscopy techniques have been employed to explore mechanisms in aqueous biological systems since the 1980s [1], the adaptation to study chemical reactions was not straightforward and many challenges had to be overcome [2]. For example, chemists had to translate microscopy techniques to achieve compatibility with the organic solvents [3] and air-free techniques that are often critical for studying organic and organometallic reactions [4,5]. Equally important to these technical barriers, an initial lack of realization of shared scientific interests between single-molecule/-particle microscopy experts and synthetic/mechanistic chemists contributed to the relatively late blooming of the field.…”

“…Furthermore, two-color single-molecule experiments investigated the selectivity of single molecular Grubbs catalysts for chain-elongation or chain-termination reactions; these experiments concluded that the selectivity of catalysts for these competing reactions varies with time [43]. In palladium catalysis, Goldsmith investigated the initiation-step kinetics of an immobilized pyridine-palladium-NHC ('PEPPSI' type) catalyst [44], while Jung studied the palladium-catalyzed Tsuji-Trost deallylation mechanism and discovered that the mechanism involves the formation of a π complex and that it proceeds through a long-lived Pd(II) intermediate [5,45].…”

“…In organic solvents [3,44] Under air-free conditions (glovebox setup and adding solid reagents during imaging) [4,5] Imaging in flow cells [9,32,52] Higher temperature imaging (diffusion at 135°C, chemical reaction at 45°C) [24,52] Types of reactions and processes imaged Hydrolysis [6] Proton-transfer [53] Epoxidation [26,33] Solubilization [4,36] Deallylation [5] Redox (nanoparticle and photocatalysis) [10,11,17,[19][20][21]54] Cross-coupling [46] Oligomerization [14] Polymerization [40][41][42]55] Hydrogenation [56] Cycloaddition [47][48][49][50] Condensation [34,57] Corrosion [15,16] Molecular metal-ligand exchange/dissociation [37][38][39]44] Oxidative addition [4,35,46] Mechanistic questions answered Identification of previously unknown organic and organometallic intermediates …”

Growth Kinetics of Single Polymer Particles in Solution via Active-Feedback 3D Tracking

“…These traditional measurement techniques, however, are incapable of measuring polymer behavior at the single-molecule and -particle level. In contrast, at the other end of the analytical regime, single-molecule/particle techniques can provide information on tethered or otherwise restricted polymer behaviors in real time with high spatial resolution, − but to date cannot measure behavior in freely diffusing solution at time scales beyond millisecond-scale snapshots or provide a way to see changes in the same species over time, leaving the solution behavior of growing polymers at the single-particle level unknown. Therefore, an analytical trade-off has existed up to this point: measure in freely diffusing solution and only acquire ensemble data or measure restricted-diffusion species and reveal single-particle/molecule data.…”