Kinetics of nucleophilic substitution reactions of polyfluoroaromatic compounds

Dehalogenation of aromatics by nucleophilic aromatic substitution

Nucleophilic aromatic substitution has been implicated as a mechanism for both the biotic and abiotic hydrodehalogenation of aromatics. Two mechanisms for the aqueous dehalogenation of aromatics involving nucleophilic aromatic substitution with hydride as a nucleophile are investigated using a validated density functional and continuum solvation protocol. For chlorinated and brominated aromatics, nucleophilic addition ortho to carbon-halogen bonds via an anionic intermediate is predicted to be the preferred mechanism in the majority of cases, while concerted substitution is predicted to be preferred for most fluorinated aromatics. Nucleophilic aromatic substitution reactions with the hydroxide and hydrosulfide anions as nucleophiles are also investigated and compared.

Exploring and exploiting polar-π interactions with fluorinated aromatic amino acids

Fluorination has become an increasingly attractive strategy in protein engineering for both basic research and biomedical applications. Thus researchers would like to understand the consequences of fluorination to the structure, stability, and function of target proteins. Although a substantial amount of work has focused on understanding the properties of fluorinated aliphatic amino acids, much less is known about fluorinated aromatic residues. In addition, polar-π interactions, often referred to as aromatic interactions, may play a significant role in protein folding and protein-protein interactions. Fluorination of aromatic residues presents an ideal strategy for probing polar-π interactions in proteins. This Account summarizes the recent studies of the incorporation of fluorinated aromatic amino acids into proteins. Herein we discuss the effects of fluorinating aromatic residues and rationalize them in the context of polar-π interactions. The results strongly support the proposal that polar-π interactions are energetically significant to protein folding and function. For example, an edge-face interaction of a pair of phenylalanines contributes as much as -1 kcal/mol to protein stability, while cation-π interactions can be much stronger. Furthermore, this new knowledge provides guidelines for protein engineering with fluorination. Importantly, incorporating perfluorinated aromatic residues into proteins enables novel mechanisms of molecular recognition that do not exist in native proteins, such as arene-perfluoroarene stacking. Such novel mechanisms can be used for programming protein folding specificity and engineering peptide-based materials.

Trimethylsilyl- and trimethylstannyldimethylphosphane--convenient and versatile reagents for the synthesis of polyfluoroaryldimethylphosphanes

Trimethylsilyldimethylphosphane (Me3SiPMe2) and the corresponding tin compound (Me3SnPMe2) were used as reagents for the substitution of fluorine by the Me2P group in polyfluoroarenes C6F5X (X = F, H, Cl, CF3) and C5NF5. The reactions occur even under mild conditions (T = 0-20 C), either in benzene or without solvent, to give as a rule 4-X-1-(dimethylphosphano)tetrafluorobenzenes (XC6F4PMe2, 1-4) and 4-(dimethylphosphano)tetrafluoropyridine (C5NF4PMe2, 5), respectively, in yields between 75 and 95%. In the case of C6F6, double substitution is also observed, which affords 1,4-bis(dimethylphosphano)tetrafluorobenzene (6). A very efficient route to the compounds XC6F4PMe2 (X = F, H, Cl, CF3) and C5NF4PMe2 was developed as a one-pot reaction of the corresponding fluoroarenes with tetramethyldiphosphane (P2Me4) and trimethyltin hydride (Me3SnH) at moderate temperatures. This process was tested for C6F6 and perfluorobiphenyl which gave C6F5PMe2 (1) and 4,4'-bis(dimethylphosphano)octafluorobiphenyl (7), respectively. The results, which included kinetic measurements that used the intensities of the 31P signals, revealed the influence of the substrate type on the rate of reaction in the sequence: C5NF5>C6F5CF3> C6F5Cl, C6F5PMe2>C6F5H>C6F6>> C6H5F. Ab initio calculations were carried out on the model reactions of pentafluoropyridine with silylphosphane, phosphane or phosphide to discriminate between possible reaction mechanisms. The novel phosphanes were characterised by spectroscopic investigations (NMR, MS), by preparation of the related thiophosphanes ArFP(=S)Me2 (8-14), their spectroscopic and analytic data and single crystal X-ray diffraction studies on five of these derivatives.