Synthesis and Characterization of Imine-Palladacycles Containing Imidate “Pseudohalide” Ligands: Efficient Suzuki–Miyaura Cross-Coupling Precatalysts and Their Activation To Give Pd0Ln Species (L = Phosphine)

Rapid plugged flow synthesis of nucleoside analogues via Suzuki-Miyaura coupling and heck Alkenylation of 5-Iodo-2'-deoxyuridine (or cytidine)

Nucleosides modification via conventional cross-coupling has been performed using different catalytic systems and found to take place via long reaction times. However, since the pandemic, nucleoside-based antivirals and vaccines have received widespread attention and the requirement for rapid modification and synthesis of these moieties has become a major objective for researchers. To address this challenge, we describe the development of a rapid flow-based cross-coupling synthesis protocol for a variety of C5-pyrimidine substituted nucleosides. The protocol allows for facile access to multiple nucleoside analogues in very good yields in a few minutes compared to conventional batch chemistry. To highlight the utility of our approach, the synthesis of an anti-HSV drug, BVDU was also achieved in an efficient manner using our new protocol.

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Supplementary Information

The online version contains supplementary material available at 10.1007/s41981-023-00265-1.

Water-Soluble Pd-Imidate Complexes as Versatile Catalysts for the Modification of Unprotected Halonucleosides

Modification of unprotected nucleosides has been attracting continuous interest, since these building blocks themselves and their phosphate-upgraded corresponding nucleotides have shown a plethora of uses in fields like biochemistry or pharmacy. Pd-catalyzed cross-coupling reactions, conducted in water or its mixtures with polar organic solvents, have frequently been the researchers' choice for the functionalization of the purine/pyrimidine base of the unprotected nucleosides. In this scenario, the availability of hydrophilic ligands and its water-soluble palladium complexes has markedly set the pace of the advances. The approach of our group to the synthesis of such complexes, Pd-imidates specifically, has faced critical stages, namely the jump to synthesize water soluble complexes from our experience working in conventional solvents, the preparation of phosphine free complexes and the overall goal of getting catalytic systems able to work close to room temperature. The continuous feedback with Kapdi's group, experienced in the chemistry of nucleosides, has produced over the last decade the interesting results in both fields presented here.

Imine Palladacycles: Synthesis, Structural Analysis and Application in Suzuki–Miyaura Cross Coupling in Semi-Aqueous Media

Treatment of the imines a–c with palladium(II) acetate in acetic acid yielded the μ-acetate dinuclear complexes 1a–c, which readily reacted with sodium chloride or bromide to provide μ-halide analogues. The reaction of the latter with nitrogen, phosphorus and oxygen donor nucleophiles yielded new imine palladacycles following the cleavage of the Pd₂X₂ unit. The complexes were fully characterized by microanalysis, ¹H, ¹³C and ³¹P NMR spectroscopies, as appropriate. The compounds were applied as catalysts in the Suzuki–Miyaura coupling reaction in aqueous and semi-aqueous media.

On the energetic stability of halogen bonds involving metals: implications in crystal engineering

This work reports a combined computational and experimental analysis of the ability of square planar d8 transition metal complexes to establish unconventional halogen bonding interactions with chloro-, bromo- and iodopentafluorobenzene...

Transmetalation of boronic acids and their derivatives: mechanistic elucidation and relevance to catalysis

The Suzuki-Miyaura reaction (the cross-coupling reaction of boronic acids with organic halides catalysed by Pd complexes) has been recognised as a useful synthetic organic reaction that forms a C(sp²)-C(sp²) bond. The catalytic cycle of the reaction involves the transmetalation of aryl- and alkenylboronic acids with Pd(II) complexes. It migrates the aryl and alkenyl groups of boronic acid to Pd and produces a Pd-C bond. Many studies have investigated the mechanism of transmetalation. They elucidated the mechanism of the organometallic reaction and its role as a fundamental step in catalytic reactions. This perspective reviews studies on the transmetalation of aryl- and alkenylboronic acids with Pd(II) complexes. Emphasis was laid on the structures and chemical properties of the intermediate Pd complexes and the effects of OH^- on the pathways of the catalytic Suzuki-Miyaura reaction. The reactions of arylboronic acids with Rh(I)-OH complexes were investigated, which are relevant to the mechanism of Rh-catalysed addition of aryl boronic acids to enones and aldehydes. Recent studies on the transmetalation of boronic acids with other late transition metals such as Fe(II), Co(I), Pt(II), Au(III), and Au(I) are presented with the related catalytic reactions and their utilisation in the synthesis of aromatic molecules and π-conjugated materials.

Metal Centers as Nucleophiles: Oxymoron of Halogen Bond-Involving Crystal Engineering

This review highlights recent studies discovering unconventional halogen bonding (HaB) that involves positively charged metal centers. These centers provide their filled d‐orbitals for HaB, and thus behave as nucleophilic components toward the noncovalent interaction. This role of some electron‐rich transition metal centers can be considered an oxymoron in the sense that the metal is, in most cases, formally cationic; consequently, its electron donor function is unexpected. The importance of Ha⋅⋅⋅d‐[M] (Ha=halogen; M is Group 9 (Rh, Ir), 10 (Ni, Pd, Pt), or 11 (Cu, Au)) interactions in crystal engineering is emphasized by showing remarkable examples (reported and uncovered by our processing of the Cambridge Structural Database), where this Ha⋅⋅⋅d‐[M] directional interaction guides the formation of solid supramolecular assemblies of different dimensionalities.

Reductive activation of PdII-precatalysts via decarboxylation of pivalate in direct C-H arylation reactions

The mechanism of the reductive activation of Pd^II pre-catalysts has been extensively studied, but remains poorly understood. Herein, a combined computational and experimental approach is employed to clearly identify a Pd^II reduction process that has not been considered thus far. Pivalate, assumed to be a general base, was found to decarboxylate and act as a reductant, suggesting an alternative explanation for the superior performance of pivalic acid as an additive in Pd-catalyzed direct C-H arylation reactions.

Palladacycle-Catalyzed Triple Suzuki Coupling Strategy for the Synthesis of Anthracene-Based OLED Emitters

The development of the site-selective Suzuki-Miyaura cross-coupling of dibromoanthracene as an efficient strategy toward organic light emitting diodes (OLEDs) is disclosed in this article. An unprecedented step-economic palladacycle-promoted triple Suzuki coupling protocol allowed the synthesis of three new OLED emitters and could prove to be a useful general strategy for researchers working in this field. Characterization of the synthesized molecules by UV-vis spectroscopy and thermogravimetric analysis-differential scanning calorimetry followed by density functional theory studies of the different properties strongly confirms the derivatives possess more significant hole mobility character than electron transfer capability.

Understanding the Unusual Reduction Mechanism of Pd(II) to Pd(I): Uncovering Hidden Species and Implications in Catalytic Cross-Coupling Reactions

The reduction of Pd(II) intermediates to Pd(0) is a key elementary step in a vast number of Pd-catalyzed processes, ranging from cross-coupling, C-H activation, to Wacker chemistry. For one of the most powerful new generation phosphine ligands, PtBu₃, oxidation state Pd(I), and not Pd(0), is generated upon reduction from Pd(II). The mechanism of the reduction of Pd(II) to Pd(I) has been investigated by means of experimental and computational studies for the formation of the highly active precatalyst {Pd(μ-Br)(PtBu₃)}₂. The formation of dinuclear Pd(I), as opposed to the Pd(0) complex, (tBu₃P)₂Pd was shown to depend on the stoichiometry of Pd to phosphine ligand, the order of addition of the reagents, and, most importantly, the nature of the palladium precursor and the choice of the phosphine ligand utilized. In addition, through experiments on gram scale in palladium, mechanistically important additional Pd- and phosphine-containing species were detected. An ionic Pd(II)Br₃ dimer side product was isolated, characterized, and identified as the crucial driving force in the mechanism of formation of the Pd(I) bromide dimer. The potential impact of the presence of these side species for in situ formed Pd complexes in catalysis was investigated in Buchwald-Hartwig, α-arylation, and Suzuki-Miyaura reactions. The use of preformed and isolated Pd(I) bromide dimer as a precatalyst provided superior results, in terms of catalytic activity, in comparison to catalysts generated in situ.

Non-covalent stacking interactions directing the structural and photophysical features of mono- and dinuclear cyclometalated palladium(ii) complexes

Cyclometallated Pd(ii) complexes with different stackings show emission in solid state in relationship to them.

N-heterocyclic carbene-palladium(II)-1-methylimidazole complex-catalyzed Suzuki-Miyaura coupling of benzyl sulfonates with arylboronic acids

The first example of palladium-catalyzed Suzuki-Miyaura coupling between benzyl sulfonates and arylboronic acids was reported in this paper. In the presence of a well-defined, air-stable and easily available NHC-Pd(II)-Im complex, all reactions worked well to give the desired products in good to almost quantitative yields under the optimal conditions. Electron-rich, -neutral, -poor and sterically-hindered substituents on both substrates are tolerated in such transformation, providing a convenient, efficient and alternative method for the synthesis of diarylmethanes.

σ-Alkenyl endo-palladacycle formation via regiospecific functionalisation of an unreactive NHC-tethered C(sp2)–H bond

A highly unusual cyclometallation reaction in which an aliphatic vinylic C(sp²)–H bond is activated to form a σ-vinyl palladacyclic–NHC complex is reported.

Mechanistic examination of AuIII-mediated 1,5-enyne cycloisomerization by AuBr2(N-imidate)(NHC)/AgX precatalysts – is the active catalyst AuIII or AuI?

The role of Au^I and Au^III species in 1,5-enyne cycloisomerization reactions has been examined in this paper.

[Pd(C∧N)(X)(PPh3)] palladacycles react with 2,4,6-trifluorophenyl boronic acid to give stable transmetallation products of the type [Pd(C∧N)(2,4,6-F3C6H2)(PPh3)]

Evidence for a direct transmetallation between a palladacycle (containing acetate and imidate pseudohalides) with 2,4,6-trifluorophenyl boronic acid is presented in this paper.

New water soluble Pd-imidate complexes as highly efficient catalysts for the synthesis of C5-arylated pyrimidine nucleosides

Recyclable water-soluble Pd complexes were revealed as excellent catalysts for Suzuki–Miyaura cross-coupling of challenging substrates like the antiviral nucleoside analogue 5-iodo-20-deoxyuridine.

Mechanistic origin of antagonist effects of usual anionic bases (OH-, CO3(2-)) as modulated by their countercations (Na+, Cs+, K+) in palladium-catalyzed Suzuki-Miyaura reactions

The mechanism of the reaction of trans-ArPdBrL(2) (Ar=p-Z-C(6)H(4), Z=CN, H; L=PPh(3)) with Ar'B(OH)(2) (Ar'=p-Z'-C(6)H(4), Z'=H, CN, MeO), which is a key step in the Suzuki-Miyaura process, has been established in N,N-dimethylformamide (DMF) with two bases, acetate (nBu(4)NOAc) or carbonate (Cs(2)CO(3)) and compared with that of hydroxide (nBu(4)NOH), reported in our previous work. As anionic bases are inevitably introduced with a countercation M(+) (e.g., M(+)OH(-)), the role of cations in the transmetalation/reductive elimination has been first investigated. Cations M(+) (Na(+), Cs(+), K(+)) are not innocent since they induce an unexpected decelerating effect in the transmetalation via their complexation to the OH ligand in the reactive ArPd(OH)L(2), partly inhibiting its transmetalation with Ar'B(OH)(2). A decreasing reactivity order is observed when M(+) is associated with OH(-): nBu(4)N(+) > K(+) > Cs(+) > Na(+). Acetates lead to the formation of trans-ArPd(OAc)L(2), which does not undergo transmetalation with Ar'B(OH)(2). This explains why acetates are not used as bases in Suzuki-Miyaura reactions that involve Ar'B(OH)(2). Carbonates (Cs(2)CO(3)) give rise to slower reactions than those performed from nBu(4)NOH at the same concentration, even if the reactions are accelerated in the presence of water due to the generation of OH(-). The mechanism of the reaction with carbonates is then similar to that established for nBu(4)NOH, involving ArPd(OH)L(2) in the transmetalation with Ar'B(OH)(2). Due to the low concentration of OH(-) generated from CO(3)(2-) in water, both transmetalation and reductive elimination result slower than those performed from nBu(4)NOH at equal concentrations as Cs(2)CO(3). Therefore, the overall reactivity is finely tuned by the concentration of the common base OH(-) and the ratio [OH(-)]/[Ar'B(OH)(2)]. Hence, the anionic base (pure OH(-) or OH(-) generated from CO(3)(2-)) associated with its countercation (Na(+), Cs(+), K(+)) plays four antagonist kinetic roles: acceleration of the transmetalation by formation of the reactive ArPd(OH)L(2), acceleration of the reductive elimination, deceleration of the transmetalation by formation of unreactive Ar'B(OH)(3)(-) and by complexation of ArPd(OH)L(2) by M(+).