Rhodium-catalyzed benzylic fluorination of trichloroacetimidates

Recent Advancements in the Synthesis of α-fluoroalkylated Azine-derived Heterocycles through Direct Fluorination

The review highlights recent advancements in the synthesis of α-fluoro and α,α- difluoroalkylated azines, focusing on two main approaches. The first approach involves nucleophilic deoxofluorination, wherein α-hydroxy- or α-oxoalkylated azines are treated with diethylaminosulfur trifluoride or other S-F reagents to introduce fluorine atoms. The second approach employs direct electrophilic benzylic fluorination, whereby alkylazines undergo fluorination using N-F reagents. Both methods provide flexibility in designing and synthesizing fluoroalkylated heterocycles.

Photoactivated Pyridine Directed Fluorination through Hydrogen Atom Transfer

We have established hydrogen atom transfer (HAT) as the key player in a directed, photopromoted fluorination of pyridylic groups. The Lewis basic pyridyl nitrogen directs amine radical dication propagated HAT and Selectfluor fluorination of various ortho substituents in a highly regioselective manner with little to no side product formation. A variety of pyridines and quinolines were employed to showcase the directing capability of the nitrogen atom. Additionally, both experimental and computational data are provided that illuminate how this mechanism differs from and complements prior work in the area.

Acid Catalyzed N-Alkylation of Pyrazoles with Trichloroacetimidates

N-Alkyl pyrazoles are important heterocycles in organic and medicinal chemistry, demonstrating a wide range of biological activity. A new method for the N-alkylation of pyrazoles has been developed using trichloroacetimidate electrophiles and a Brønsted acid catalyst. These reactions provide ready access to N-alkyl pyrazoles which are present in a variety of medicinally relevant lead structures. Benzylic, phenethyl and benzhydryl trichloroacetimidates provide good yields of the N-alkyl pyrazole products. Unsymmetrical pyrazoles provide a mixture of the two possible regioisomers, with the major product being controlled by sterics. This methodology provides an alternative to other alkylation methods that require strong base or high temperature.

Nucleophilic Fluorination Catalyzed by a Cyclometallated Rhodium Complex

Quantitative catalytic nucleophilic fluorination of a range of acyl chlorides to acyl fluorides was promoted by a cyclometallated rhodium complex [(η5,κ2C-C5Me4CH2C6F5CH2NC3H2NMe)- RhCl] (1). 1 can be prepared in high yields from commercially available starting materials using a one-pot method. The catalyst could be separated, regenerated, and reused. Rapid quantitative fluorination generated the fluoride analogue of the active pharmaceutical ingredient probenecid. Infrared in situ monitoring verified the clean conversion of the substrates to products. VTNA graphical kinetic analysis and DFT calculations lead to a postulated reaction mechanism involving a nucleophilic Rh-F bond.

The Attractiveness of the Ternary Rh-Pd-Pt Alloys for CO Oxidation Process

Ternary alloys of platinum group metals attract a growing interest due to their unique catalytic properties. The present research is aimed to synthesize a series of Rh-Pd-Pt alloys with varied ratios of metals using a single-source precursor approach. Rhodium and palladium are partly miscible metals, while each of these metals is unlimitedly miscible with platinum. Thermolysis of complex salts used as a precursor results in the formation of metastable systems. The 3D nanostructure alloys are being formed after the complete decomposition of the single-source precursor. High-resolution transmission electron microscopic studies have shown that the nanoalloys are composed of interconnected polycrystalline ligaments with a mean diameter of 50 nm. The single-phase composition is confirmed by an X-ray diffraction analysis. The ratio of metals plays an important role in determining the catalytic activity of alumina-supported alloys and their thermal stability. According to UV-vis spectroscopy data, the higher palladium portion corresponds to worse dispersion of initially prepared, fresh catalysts. Treatment of the catalysts under prompt thermal aging conditions (up to 800 °C) causes redispersion of palladium-rich alloyed nanoparticles, thus leading to improved catalytic activity and thermal stability.

Stereospecific Isomerization of Allylic Halides via Ion Pairs with Induced Noncovalent Chirality

A regioselective protocol for the synthesis of substituted allylic chlorides, bromides, and fluorides has been established. Remarkably, the method can be applied to the enantioselective synthesis of challenging chiral allylic chlorides. When the allylic halides are treated with the base triazabicyclodecene as the catalyst, a [1,3]-proton shift takes place, giving the corresponding vinyl halides in excellent yields with excellent Z:E ratios. Furthermore, the [1,3]-proton shift takes place with an outstanding level of chirality transfer from chiral allylic alcohols (≤98%) to give chiral trifluoromethylated vinyl chlorides.

Cobalt-Catalyzed Asymmetric Cross-Coupling Reaction of Fluorinated Secondary Benzyl Bromides with Lithium Aryl Boronates/ZnBr2

A cobalt-catalyzed asymmetric cross-coupling of α-bromo-α-fluorotoluene derivatives with a variety of aryl zincates derived from lithium aryl n-butyl pinacol boronates and ZnBr₂ under mild reaction conditions was described. In addition to mild reaction conditions, another advantage includes the compatibility of various common functional groups such as fluoride, chloride, bromide, cyano, or ester groups. Furthermore, this protocol was successfully applied to the enantioselective synthesis of three fluorinated derivatives of biologically active compounds or drug molecules.

Synthesis of 1,1'-Diarylethanes and Related Systems by Displacement of Trichloroacetimidates with Trimethylaluminum

Benzylic trichloroacetimidates are readily displaced by trimethylaluminum under Lewis acid promoted conditions to provide the corresponding methyl substitution product. This method is a convenient way to access 1,1'-diarylethanes and related systems, which play a significant role in medicinal chemistry, with a number of systems owing their biological activity to this functionality. Most benzylic substrates undergo ready displacement, with electron deficient systems being the exception. The use of an enantiopure imidate showed significant racemization, implicating the formation of a cationic intermediate.