Pincer Complex-Catalyzed Allylation of Aldehyde and Imine Substrates via Nucleophilic η1-Allyl Palladium Intermediates

Versatile Reactivity of Metalloid-Substituted π-Allylpalladium Species

π-Allylpalladium complexes can not only serve as electrophilic allylating agents for a broad range of nucleophiles, but also nucleophilic allylating agents for electrophiles depending on their electronic environments. In contrast to these typical reactivities of π-allylpalladium complexes, silylated and borylated π-allylpalladium species show unique reactivities that can allow versatile transformations in addition to simple allylation. Herein, four different types of transformations that are in principle achieved via the inherently reactive silylated and borylated π-allylpalladium species as common intermediates are described. An appropriate selection of ligands of the silylated and borylated π-allylpalladium species can allow control over the reaction pathways.

Direct Access to Palladium(II) Complexes Based on Anionic C,C,C-Phosphonium Ylide Core Pincer Ligand

The reaction of readily available imidazolium-phosphonium salt [MesIm(CH₂)₃PPh₃](OTf)₂ with PdCl₂ in the presence of an excess of Cs₂CO₃ afforded selectively in one step the cationic Pd(II) complex [(C,C,C)Pd(NCMe)](OTf) exhibiting an LX₂-type NHC-ylide-aryl C,C,C-pincer ligand via formal triple C-H bond activation. The replacement of labile MeCN in the latter by CNtBu and CO fragments allowed to estimate the overall electronic properties of this phosphonium ylide core pincer scaffold incorporating three different carbon-based donor ends by IR spectroscopy, cyclic voltammetry, and molecular orbital analysis, revealing its significantly higher electron-rich character compared to the structurally close NHC core pincer system with two phosphonium ylide extremities. The pincer complex [(C,C,C)Pd(CO)](OTf) represents a rare example of Pd(II) carbonyl species stable at room temperature and characterized by X-ray diffraction analysis. The treatment of isostructural cationic complexes [(C,C,C)Pd(NCMe)](OTf) and [(C,C,C)Pd(CO)](OTf) with (allyl)MgBr and nBuLi led to the formation of zwitterionic phosphonium organopalladates [(C,C,C)PdBr] and [(C,C,C)Pd(COnBu)], respectively.

Electroreductive Cobalt-Catalyzed Carboxylation: Cross-Electrophile Electrocoupling with Atmospheric CO2

The chemical use of CO2 as an inexpensive, nontoxic C1 synthon is of utmost topical interest in the context of carbon capture and utilization (CCU). We present the merger of cobalt catalysis and electrochemical synthesis for mild catalytic carboxylations of allylic chlorides with CO2 . Styrylacetic acid derivatives were obtained with moderate to good yields and good functional group tolerance. The thus-obtained products are useful as versatile synthons of γ-arylbutyrolactones. Cyclic voltammetry and in operando kinetic analysis were performed to provide mechanistic insights into the electrocatalytic carboxylation with CO2 .

Hydroamination of Aromatic Alkynes to Imines Catalyzed by Pd(II)-Anthraphos Complexes

Herein, we report the synthesis, characterization, and catalytic performance of cationic Pd(II)-Anthraphos complexes in the intermolecular hydroamination of aromatic alkynes with aromatic amines. The reaction proceeds with 0.18 mol % of catalyst loading, at 90 °C for 4 h under neat conditions. Good to excellent yields could be obtained for a broad range of amines and alkynes.

Halide exchange studies of novel Pd(ii) NNN-pincer complexes

Palladium(ii) complexes with an NNN type pincer ligand (pip2NNN = 2,6-bis(piperdyl-methyl)pyridine) are synthesized and characterized. Electronic and 1H NMR spectra point to decreasing filled/filled repulsions between the dπ(Pd) orbitals and the halide lone pair orbitals along the Cl < Br < I series. For all complexes, the most downfield α-piperidyl resonance of the pip2NNN ligand is sensitive to changes in the coordinated halide while the meta-pyridyl and benzylic resonances are sensitive to changes in the counter anion. This sensitivity is utilized to study halide association and exchange at the fourth coordination site. Conductivity and 1H NMR spectroscopy confirm the interaction between the exogenous anion (Cl-, Br-, BF4 -) and Pd(pip2NNN)X+ (X = Cl, Br).

Toward ideal carbon dioxide functionalization

This Perspective recapitulates recent developments of carbon dioxide utilization in carbon-carbon bond formation reactions, with an intention of paving a way toward sustainable CO2-functionalization and its tangible applications in synthetic chemistry. CO2 functionalization reactions possess intrinsic drawbacks: the high kinetic inertness and thermodynamic stability of CO2. Numerous procedures for CO2 utilization depend on energy-intensive processes (i.e. high pressure and/or temperature), often solely relying on reactive substrates, hampering its general applications. Recent efforts thus have been dedicated to catalytic CO2-utilization under ambient reaction conditions, however, it is still limited to a few activation modes and the use of reactive substrates. Herein, ideal CO2-functionalization with particular emphasis on sustainability will be discussed based on the following sub-categories; (1) metal-catalyzed 'reductive' carboxylation reaction of halides, olefins and allyl alcohols, (2) photochemical CO2-utilization, (3) redox-neutral CO2-functionalization, and (4) enantioselective catalysis incorporating CO2 to form C-CO2 bonds (excluding strain mediated reactions with epoxide- and aziridine-based substrates). Recent progress in these fields will be discussed with the proposed reaction mechanisms and selected examples, highlighting redox-neutral, umpolung, and asymmetric carboxylation to postulate ideal CO2 functionalization reactions to be developed in the near future.

Computational Study of Enantioselectivity in the Asymmetric Allylation of Aldehydes with Chiral Pt(II) Phosphinite Complexes

Computational investigations on the enantioselectivity observed in the allylation of cinnamaldehyde have been carried out using four closely related chiral platinum catalysts with ascorbic acid-based bisphosphinite ligands. Enantioselectivity depends on the substitution of hydrogen on the hydroxyl group by benzyl group(s) in the ascorbic acid framework. The key intermediate is assumed to be one, where both the aldehyde (via oxygen) and the η¹ allyl group are coordinated to the metal center, and C-C bond formation between the terminal allylic carbon and carbonyl carbon is the rate-determining step. A screening process has been adopted to select potentially relevant reactant adducts from a total of several hundred theoretically possible conformers by molecular mechanics (MM) calculations. Only 50 transition state structures (TSs) lead to the product based on a two-layer QM/MM study employing B3LYP and UFF methods. Out of these 50 TSs, around 10 structures were selected for the final step QM calculation at B3LYP/LANL2TZ(f)(Pt),6-311G(d,p)//B3LYP/LANL2DZ(Pt),6-31G(d,p) level of theory. This computational approach predicts chirality induction correctly in most of the seven catalysts examined. The successful prediction of enantioselectivity carried out using the MM, QM/MM, and QM approaches suggests that the described simplified procedure could be useful in other catalyst systems also.

Lithium Diisopropylamide Catalyzed Allylic C-H Bond Alkylation with Styrenes

Allylic substitution reactions, a well-established approach for new bond construction, often need transition-metal catalysts and stoichiometric amounts of organometallic reagents, strong bases, or oxidants. Lithium diisopropylamide (LDA), a widely used and commercially available Brønsted base, is herein reported to catalyze the allylic C-H bond addition of 1,3-diarylpropenes to styrenes. Preliminary mechanism studies have provided a solid structure of the π-allyllithium intermediate and revealed the unique catalytic roles of LDA and its conjugate acid diisopropylamine.

Catalytic Nucleophilic Allylation Driven by the Water-Gas Shift Reaction

The ruthenium-catalyzed allylation of aldehydes with allylic pro-nucleophiles has been demonstrated to be an efficient means to form carbon-carbon bonds under mild conditions. The evolution of this reaction from the initial serendipitous discovery to its general synthetic scope is detailed, highlighting the roles of water, CO, and amine in the generation of a more complete catalytic cycle. The use of unsymmetrical allylic pro-nucleophiles was shown to give preferential product formation through the modulation of reaction conditions. Both (E)-cinnamyl acetate and vinyl oxirane were efficiently used to form the anti-branched products (up to >20:1 anti/syn) and E-linear products (up to >20:1 E/Z) in high selectivity with aromatic, α,β-unsaturated, and aliphatic aldehydes, respectively. Attempts to render the reaction enantioselective are highlighted and include enantioenrichment of up to 75:25 for benzaldehyde.

Cobalt-Catalyzed Allylic C(sp3)-H Carboxylation with CO2

Catalytic carboxylation of the allylic C(sp³)-H bond of terminal alkenes with CO₂ was developed with the aid of a Co/Xantphos complex. A wide range of allylarenes and 1,4-dienes were successfully transformed into the linear styrylacetic acid and hexa-3,5-dienoic acid derivatives in moderate to high yields, with excellent regioselectivity. The carboxylation showed remarkable functional group tolerability, so that selective addition to CO₂ occurred in the presence of other carbonyl groups such as amide, ester, and ketone. Since styrylacetic acid derivatives can be readily converted into optically active γ-butyrolactones through Sharpless asymmetric dihydroxylation, this allylic C(sp³)-H carboxylation showcases a facile synthesis of γ-butyrolactones from simple allylarenes via short steps.

Palladacycles having normal and spiro chelate rings designed from bi- and tridentate ligands with an indole core: structure, synthesis and applications as catalysts

Palladacycles with normal and spiro rings catalyze Suzuki–Miyaura coupling of ArBr/ArCl/allylation of aldehydes at 0.001/1 mol% loading.

Development of (quinolinyl)amido-based pincer palladium complexes: a robust and phosphine-free catalyst system for C–H arylation of benzothiazoles

Well-defined (quinolinyl)amido-pincer palladium complexes are developed and employed for the catalytic C–H bond arylation of benzothiazoles with aryl iodides, which can be recycled and reused for several cycles.

Waste management in zinc promoted allylation of aldehyde

The waste zinc material in Zn(0) promoted Grignard–Barbier type allylation of aldehydes has been successfully utilized as a reusable material for the adsorption of various dyes and also converted into the corresponding hexagonal wurtzite phase of ZnO.

Palladacycles of unsymmetrical (N,C−,E) (E = S/Se) pincers based on indole: their synthesis, structure and application in the catalysis of Heck coupling and allylation of aldehydes

Palladacycles of Schiff bases of 1-(2-phenylsulfanyl/selenylethyl)-1H-indole-3-carbaldehyde with benzyl amine catalyze Heck coupling and allylation at 0.1–0.3 and 1.0 mol% respectively.

Asymmetric palladium-catalyzed umpolung cyclization of allylic acetate-aldehyde using formate as a reductant

Asymmetic palladium/chiral diphosphine-catalyzed umpolung cyclization of allylic acetate-aldehyde using formate as a reductant affordscis-disubstituted pyrrolidine, tetrahydrofuran, and spiro carbocycle.