Nucleophilic Fluorination with Aqueous Bifluoride Solution: Effect of the Phase-Transfer Catalyst

Nucleophilic fluorination of sulfonyl chlorides, acyl chlorides, and alkyl sulfonates with saturated aqueous solution of potassium bifluoride (KHF₂) was studied under liquid–liquid two-phase conditions. Original “on-water” procedure, reported by Sharpless et al. (J. Org. Chem. 2016, 81, 11360−11362, DOI: 10.1021/acs.joc.6b01423), was tested on model 1-octanesulfonyl chloride in the presence of phase transfer catalysts, some of which appeared to be beneficial for the reaction rate. Despite the high hydration energy of the fluoride ions, the catalytic system displayed numerous features typical for interfacial transportation of the nucleophilic species, being controlled by amount and structure of the catalyst, lipophilicity of the catalyst’s counterion, and rate of stirring. Besides for synthesis of acyl fluorides presence of 1 mol % of tetrabutylammonium chloride affected the selectivity of the reaction by minimizing formation of carboxylic acids and anhydrides. The presented results suggest that aqueous solutions of bifluorides (or synthetically equivalent systems accessible by acidification of alkali metal fluoride solutions) can be efficient sources of the fluoride ions under two-phase conditions, provided that rate of the intrinsic reaction is sufficiently high. The methodology supplements family of nucleophilic fluorinations, delivering a more reactive form of the solvated anions

A Missing Relative: A Hoveyda–Grubbs Metathesis Catalyst Bearing a Peri-Substituted Naphthalene Framework

Molecular scaffolds of polycyclic aromatic hydrocarbons can serve as unique tools to control the molecular and electronic structure of coordination compounds. Herein, we report the synthesis and properties of a Hoveyda–Grubbs metathesis catalyst bearing a chelating benzylidene ligand assembled on peri-substituted naphthalene. In contrast to other reported naphthalene-based complexes (Barbasiewicz, M.; Grela, K. Chem. Eur. J. 2008, 14, 9330−9337), it exhibits a very fast initiation behavior, attributed to a distorted molecular structure and reduced π-electron delocalization within the chelate ring

Synthesis and Properties of Bimetallic Hoveyda–Grubbs Metathesis Catalysts

The catalytic activity of ruthenium Hoveyda–Grubbs complexes in olefin metathesis is a function of complex steric and electronic effects acting on initiation and propagation steps. In order to study the π-electron factors influencing the initiation process, we attempted syntheses of bimetallic complexes with common organic ligands bearing two chelate rings. While most of the studied ligand exchange reactions of the isomeric bis-chelating benzene derivatives gave mixtures of unstable complexes, a homodinuclear derivative of 1,4-dimethoxy-2,5-divinylbenzene was sparingly soluble and precipitated from the reaction mixture in a pure form. The complex was studied with spectroscopic and X-ray methods, which confirmed the symmetrical bimetallic structure. However, in model metathesis reactions the catalyst displayed activity very comparable to the related monometallic complexes. This suggests that in the bimetallic system two consecutive initiation processes of the metathesis catalyst (first, bimetallic complex + olefin → monometallic complex + propagating species; second, monometallic complex + olefin → styrene + propagating species) proceed at similar rates and, thus, no cooperativity between the two steps is displayed. Properties of the family of bimetallic complexes were probed with NMR studies, and π-electronic effects operating in the systems were discussed

A Missing Relative: A Hoveyda–Grubbs Metathesis Catalyst Bearing a Peri-Substituted Naphthalene Framework

Molecular scaffolds of polycyclic aromatic hydrocarbons can serve as unique tools to control the molecular and electronic structure of coordination compounds. Herein, we report the synthesis and properties of a Hoveyda–Grubbs metathesis catalyst bearing a chelating benzylidene ligand assembled on peri-substituted naphthalene. In contrast to other reported naphthalene-based complexes (Barbasiewicz, M.; Grela, K. Chem. Eur. J. 2008, 14, 9330−9337), it exhibits a very fast initiation behavior, attributed to a distorted molecular structure and reduced π-electron delocalization within the chelate ring

Synthesis and Properties of Bimetallic Hoveyda–Grubbs Metathesis Catalysts

The catalytic activity of ruthenium Hoveyda–Grubbs complexes in olefin metathesis is a function of complex steric and electronic effects acting on initiation and propagation steps. In order to study the π-electron factors influencing the initiation process, we attempted syntheses of bimetallic complexes with common organic ligands bearing two chelate rings. While most of the studied ligand exchange reactions of the isomeric bis-chelating benzene derivatives gave mixtures of unstable complexes, a homodinuclear derivative of 1,4-dimethoxy-2,5-divinylbenzene was sparingly soluble and precipitated from the reaction mixture in a pure form. The complex was studied with spectroscopic and X-ray methods, which confirmed the symmetrical bimetallic structure. However, in model metathesis reactions the catalyst displayed activity very comparable to the related monometallic complexes. This suggests that in the bimetallic system two consecutive initiation processes of the metathesis catalyst (first, bimetallic complex + olefin → monometallic complex + propagating species; second, monometallic complex + olefin → styrene + propagating species) proceed at similar rates and, thus, no cooperativity between the two steps is displayed. Properties of the family of bimetallic complexes were probed with NMR studies, and π-electronic effects operating in the systems were discussed

Olefination with Sulfonyl Halides and Esters: Scope, Limitations, and Mechanistic Studies of the Hawkins Reaction

Carbanions of alkanesulfonyl halides and esters react with nonenolizable carbonyl compounds to give olefins. Mechanistic studies reveal that initial aldol-type addition of the carbanions is followed by cyclization–fragmentation to alkenes, and the leaving group on the sulfonyl moiety (RSO₂X) controls carbanion stability and rate of the olefin formation

Olefination with Sulfonyl Halides and Esters: Scope, Limitations, and Mechanistic Studies of the Hawkins Reaction

Carbanions of alkanesulfonyl halides and esters react with nonenolizable carbonyl compounds to give olefins. Mechanistic studies reveal that initial aldol-type addition of the carbanions is followed by cyclization–fragmentation to alkenes, and the leaving group on the sulfonyl moiety (RSO₂X) controls carbanion stability and rate of the olefin formation

Nitration Under Continuous Flow Conditions: Convenient Synthesis of 2‑Isopropoxy-5-nitrobenzaldehyde, an Important Building Block in the Preparation of Nitro-Substituted Hoveyda–Grubbs Metathesis Catalyst

Herein, we describe the use of continuous flow chemistry for selective, efficient and reproducible nitration of 2-isopropoxybenzaldehyde to produce the desired 2-isopropoxy-5-nitrobenzaldehyde, an important building block in the preparation of a ligand of nitro-substituted Hoveyda–Grubbs metathesis catalyst. Nitration was done with red fuming HNO₃, and this challenging and hazardous process was performed using a flow-through silicon-glass microreactor equipped with a set of temperature sensors, and with a productivity of 13 g/h, providing us with a reproducible chemical process amenable for production of sufficient quantities of 2-isopropoxy-5-nitrobenzaldehyde for ongoing large-scale synthesis of nitro-substituted Hoveyda–Grubbs metathesis catalyst