Mechanistic features of the copper-free Sonogashira reaction from ESI-MS

Recent Progress on One-Pot Multisubstrate Screening

Traditionally, new synthetic reactions have been developed using a model substrate to screen reaction conditions before testing the optimized conditions with a range of more complex substrates. In 1998, Gao and Kagan pooled multiple substrates in one pot to study the generality of an enantioselective method. Although such one-pot multisubstrate screenings may be powerful, few applications have appeared in the literature. With the advancement of various chromatography techniques, it may be time to revisit this underutilized platform. This review article discusses the applications of one-pot multisubstrate screenings as a method for developing new synthetic methods.

Elucidating the reaction mechanism of a palladium-palladium dual catalytic process through kinetic studies of proposed elementary steps

In the synergistic dual catalytic process, the kinetics of the catalytic cycles must be balanced for the successful outcome of the reaction. Therefore, the analysis of the kinetics of the independent catalytic cycles is essential for such reactions, as it enables their relational optimization as well as their design. Here we describe an analysis of the mechanism of a catalytic synergistic bimetallic reaction through the experimental study of a palladium-catalysed cross-coupling of aryl halides with terminal alkynes, an example of a monometallic dual catalytic process. The proposed mechanism of the investigated reaction was disassembled into two palladium catalytic cycles and further into elementary reactions, and each step was studied independently. The described mechanistic analysis allowed us to identify the rate-determining step of the catalytic process by comparing the rates of the elementary reactions under similar reaction conditions, balanced kinetics of the palladium catalytic cycles, and also in which step which reagent enters the catalytic cycle and how.

Towards determining molecular structure with ESI-MS backed by computational methods: structures of subnanoclusters of Pd and Cu chlorides, ion dynamics in vacuum, and challenges to the methodology

Determining most stable structures of sub-nanoscale ionic clusters in ESI-MS spectra with quantum chemical modeling.

A mechanistic investigation of the Pd-catalyzed cross-coupling between N-tosylhydrazones and aryl halides

The cross-coupling of N-tosylhydrazones and aryl halides forms carbon-carbon bonds, producing 1,1-disubstituted alkenes. Though it has proven extremely useful in several fields of chemistry, its mechanism remains experimentally unexplored. Combining benchtop NMR and real-time mass spectrometry afforded the ability to monitor the catalytic intermediates as well as the rate of product formation.

Mechanistic Study of Pd/NHC-Catalyzed Sonogashira Reaction: Discovery of NHC-Ethynyl Coupling Process

The product of a revealed transformation-NHC-ethynyl coupling-was observed as a catalyst transformation pathway in the Sonogashira cross-coupling, catalyzed by Pd/NHC complexes. The 2-ethynylated azolium salt was isolated in individual form and fully characterized, including X-ray analysis. A number of possible intermediates of this transformation with common formulae (NHC)_n Pd(C₂ Ph) (n=1,2) were observed and subjected to collision-induced dissociation (CID) and infrared multiphoton dissociation (IRMPD) experiments to elucidate their structure. Measured bond dissociation energies (BDEs) and IRMPD spectra were in an excellent agreement with quantum calculations for coupling product π-complexes with Pd⁰ . Molecular dynamics simulations confirmed the observed multiple CID fragmentation pathways. An unconventional methodology to study catalyst evolution suggests the reported transformation to be considered in the development of new catalytic systems for alkyne functionalization reactions.

In Situ Laser Scattering Electrospray Ionization Mass Spectrometry and Its Application in the Mechanism Study of Photoinduced Direct C-H Arylation of Heteroarenes

An in situ laser scattering electrospray ionization mass spectrometry (LS-ESI-MS) was developed, where the laser scattering was simply achieved through the laser radiation of the "media" modified on the capillary. The laser scattering extended the reaction window and powerfully promoted the reaction yield of the photoinduced organic reaction, which enables the trace intermediates to be efficiently tracked in real time. For instance, the key radical cation in the photoinduced direct C-H arylation of heteroarenes was captured inventively, which provided direct experimental evidence for the verification of the reaction mechanism. Together with the characterization of oxidative photocatalytic Ru(III) intermediate, the integral insight into the process of visible-light-mediated direct C-H arylation of heteroarenes was confirmed. This approach is facile, powerful, and promising in the mechanism study of organic reaction.

Structure, Anion, and Solvent Effects on Cation Response in ESI-MS

The abundance of an ion in an electrospray ionization mass spectrum is dependent on many factors beyond just solution concentration. Even in cases where the analytes of interest are permanently charged (under study here are ammonium and phosphonium ions) and do not rely on protonation or other chemical processes to acquire the necessary charge, factors such as cation structure, molecular weight, solvent, and the identity of the anion can affect results. Screening of a variety of combinations of cations, anions, and solvents provided insight into some of the more important factors. Rigid cations and anions that conferred high conductivity tended to provide the highest responses. The solvent that most closely reflected actual solution composition was acetonitrile, while methanol, acetonitrile/water, and dichloromethane produced a higher degree of discrimination between different ions. Functional groups that had affinity for the solvent tended to depress response. These observations will provide predictive power when accounting for analytes that for reasons of high reactivity can not be isolated.

Mechanism of copper-free Sonogashira reaction operates through palladium-palladium transmetallation

The seminal contributions by Sonogashira, Cassar and Heck in mid 1970s on Pd/Cu- and Pd-catalysed (copper-free) coupling of acetylenes with aryl or vinyl halides have evolved in myriad applications. Despite the enormous success both in academia and in industry, however, critical mechanistic questions of this cross-coupling process remain unresolved. In this study, experimental evidence and computational support is provided for the mechanism of copper-free Sonogashira cross-coupling reaction. In contrast to the consensus monometallic mechanism, the revealed pathway proceeds through a tandem Pd/Pd cycle linked via a multistep transmetallation process. This cycle is virtually identical to the Pd/Cu tandem mechanism of copper co-catalysed Sonogashira cross-couplings, but the role of Cu^I is played by a set of Pd^II species. Phosphine dissociation from the square-planar reactants to form transient three-coordinate Pd species initiates transmetallation and represents the rate-determining step of the process.

Organic Reactions in Microdroplets: Reaction Acceleration Revealed by Mass Spectrometry

The striking finding that reaction acceleration occurs in confined-volume solutions sets up an apparent conundrum: Microdroplets formed by spray ionization can be used to monitor the course of bulk-phase reactions and also to accelerate reactions between the reagents in such a reaction. This Minireview introduces droplet and thin-film acceleration phenomena and summarizes recent methods applied to study accelerated reactions in confined-volume, high-surface-area solutions. Conditions that dictate either simple monitoring or acceleration are reconciled in the occurrence of discontinuous and complete desolvation as the endpoint of droplet evolution. The contrasting features of microdroplet and bulk-solution reactions are described together with possible mechanisms that drive reaction acceleration in microdroplets. Current applications of droplet microreactors are noted as is reaction acceleration in confined volumes and possible future scale-up.

Fluoride-mediated rearrangement of phenylfluorosilanes

Combining Ph3SiF and fluoride ion under conditions used for the Hiyama coupling causes rapid formation of the expected [Ph3SiF2]−; however, real-time electrospray mass spectrometric analysis reveals that phenyl-fluoride exchange occurs concomitantly, also producing substantial quantities of [PhnSiF5–n]− (n = 0–2). The exchange process is verified using 19F NMR spectroscopy. This observation may have implications for Hiyama reaction protocols, which use transmetallation from triaryldifluorosilicates as a key step in cross-coupling.

Anionic Palladium(0) and Palladium(II) Ate Complexes

Palladium ate complexes are frequently invoked as important intermediates in Heck and cross-coupling reactions, but so far have largely eluded characterization at the molecular level. Here, we use electrospray-ionization mass spectrometry, electrical conductivity measurements, and NMR spectroscopy to show that the electron-poor catalyst [L₃ Pd] (L=tris[3,5-bis(trifluoromethyl)phenyl]phosphine) readily reacts with Br^- ions to afford the anionic, zero-valent ate complex [L₃ PdBr]^- . In contrast, more-electron-rich Pd catalysts display lower tendencies toward the formation of ate complexes. Combining [L₃ Pd] with LiI and an aryl iodide substrate (ArI) results in the observation of the Pd^II ate complex [L₂ Pd(Ar)I₂ ]^- .

Trinuclear complexes of palladium(ii) with chalcogenated N-heterocyclic carbenes: catalysis of selective nitrile–primary amide interconversion and Sonogashira coupling

The interconversion of nitriles to amides and Sonogashira coupling were catalyzed with Pd(ii) complexes (0.5–2 mol%).