J. Appl. Cryst.の発刊に際して

Dialkylbiaryl phosphines are a valuable class of ligand for Pd-catalyzed amination reactions and have been applied in a range of contexts. This perspective attempts to aid the reader in the selection of the best choice of reaction conditions and ligand of this class for the most commonly encountered and practically important substrate combinations.

Dialkylbiaryl phosphines in Pd-catalyzed amination: a user's guide

Current methods for functional group interconversion have, for the most part, relied on relatively strong driving forces which often require highly reactive reagents to generate irreversibly a desired product in high yield and selectivity. These approaches generally prevent the use of the same catalytic strategy to perform the reverse reaction. Here we describe a catalytic functional group metathesis approach to interconvert, under CO-free conditions, two synthetically important classes of electrophiles that are often employed in the preparation of pharmaceuticals and agrochemicals-aroyl chlorides (ArCOCl) and aryl iodides (ArI). Our reaction design relies on the implementation of a key reversible ligand C-P bond cleavage event, which enables a non-innocent, metathesis-active phosphine ligand to mediate a rapid aryl group transfer between the two different electrophiles. Beyond enabling a practical and safer approach to the interconversion of ArCOCl and ArI, this type of ligand non-innocence provides a blueprint for the development of a broad range of functional group metathesis reactions employing synthetically relevant aryl electrophiles.

Metathesis-active ligands enable a catalytic functional group metathesis between aroyl chlorides and aryl iodides

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 CuI is played by a set of PdII 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.

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

We describe the development of a new method to use palladium catalysis to form functionalized aromatics: via the metathesis of covalent σ-bonds between Ar–X fragments. This transformation demonstrates the dynamic nature of palladium-based oxidative addition/reductive elimination and offers a straightforward approach to incorporate reactive functional groups into aryl halides through exchange reactions. The reaction has been exploited to assemble acid chlorides without the use of high energy halogenating or toxic reagents and, instead, via the metathesis of aryl iodides with other acid chlorides.

Functional Group Transposition: A Palladium-Catalyzed Metathesis of Ar-X σ-Bonds and Acid Chloride Synthesis.

We describe herein computational studies on the unusual ability of Pd(PtBu3 )2 to catalyze formation of highly reactive acid chlorides from aryl halides and carbon monoxide. These show a synergistic role of carbon monoxide in concert with the large cone angle PtBu3 that dramatically lowers the barrier to reductive elimination. The tertiary structure of the phosphine is found to be critical in allowing CO association and the generation of a high energy, four coordinate (CO)(PR3 )Pd(COAr)Cl intermediate. The stability of this complex, and the barrier to elimination, is highly dependent upon phosphine structure, with the tertiary steric bulk of PtBu3 favoring product formation over other ligands. These data suggest that even difficult reductive eliminations can be rapid with CO association and ligand manipulation. This study also represents the first detailed exploration of all the steps involved in palladium-catalyzed carbonylation reactions with simple phosphine ligands, including the key rate-determining steps and palladium(0) catalyst resting state in carbonylations.

Computational Study of the Palladium‐Catalyzed Carbonylative Synthesis of Aromatic Acid Chlorides: The Synergistic Effect of PtBu3 and CO on Reductive Elimination

The title compound, [Co(C10H15)(C2H4)2], was prepared by Na/Hg reduction of [Co2(C10H15)2(μ-Cl)2] in THF under an ethyl­ene atmosphere and crystallized from pentane at 193 K. The Co—C(olefin) bonds have an average length of 2.022 (2) A, while the Co—C(penta­dien­yl) bonds average 2.103 (19) A. The olefin C=C bonds are 1.410 (1) A. The dihedral angle between the planes defined by the cyclo­penta­dienyl ligand and the two olefin ligands is 0.25 (12)°. In the crystal, mol­ecules are linked into chains by C—H⋯π inter­actions.

https://journals.iucr.org/e/issues/2016/09/00/is5456/is5456.pdf

Crystal structure of bis­(η2-ethyl­ene)(η5-penta­methyl­cyclo­penta­dien­yl)cobalt

The title hydrated mol­ecular salt, C4H12N+·C4H5O6−·H2O, was prepared by deprotonation of enanti­opure l-tartaric acid with racemic sec-butyl­amine in water. Only one enanti­omer was observed crystallographically, resulting from the combination of (S)-sec-butyl­amine with l-tartaric acid. The sec-butyl­ammonium moiety is disordered over two conformations related by rotation around the CH–CH2 bond; the refined occupancy ratio is 0.68 (1):0.32 (1). In the crystal, mol­ecules are linked through a network of O—H⋯O and N—H⋯O hydrogen-bonding inter­actions, between the ammonium H atoms, the tartrate hy­droxy H atoms, and the inter­stitial water, forming a three-dimensional supra­molecular structure.

https://journals.iucr.org/e/issues/2017/05/00/su5364/su5364.pdf

Kai E. O. Ylijoki

Papers

Computational Study of the Palladium‐Catalyzed Carbonylative Synthesis of Aromatic Acid Chlorides: The Synergistic Effect of PtBu3 and CO on Reductive Elimination

Crystal structure of bis(η2-ethylene)(η5-pentamethylcyclopentadienyl)cobalt

Crystal structure of (S)-sec-butylammonium l-tartrate monohydrate

Kai E. O. Ylijoki

Papers

Computational Study of the Palladium‐Catalyzed Carbonylative Synthesis of Aromatic Acid Chlorides: The Synergistic Effect of PtBu3 and CO on Reductive Elimination

Crystal structure of bis­(η2-ethyl­ene)(η5-penta­methyl­cyclo­penta­dien­yl)cobalt

Crystal structure of (S)-sec-butyl­ammonium l-tartrate monohydrate

Crystal structure of bis(η2-ethylene)(η5-pentamethylcyclopentadienyl)cobalt

Crystal structure of (S)-sec-butylammonium l-tartrate monohydrate