Theoretical Explanation of Reaction Site Selectivity in the Addition of a Phenoxy Group to Perfluoropyridine

Halogenated Phenylpyridines Possessing Chemo-Selectivity for Diverse Molecular Architectures

Pentafluoropyridine was used as a molecular building block for the installation of aryl bromides, affording a series of multisubstituted halogenated arenes. This operationally simplistic methodology offers precise regioselectivity, ease of scalability, and high purity. 19F Nuclear magnetic resonance (NMR) served as a key diagnostic tool for structural characterization, given the sensitivity with various aryl bromine substitutions on the fluorinated pyridine ring. Furthermore, molecular modeling simulations offered insight into this new class of halogenated phenylpyridines and their unique electronic and reactive properties. This study also demonstrates examples of efficient chemo-selectivity upon either metal-catalyzed aryl-aryl coupling or nucleophilic aromatic substitution of the aryl bromide or fluorinated pyridine scaffold, respectively. A diverse pool of polyarylene structures with high degree of complexity, functionalized linear polymers, and controlled network architectures were achieved from this simple methodology.

An evolved pyrrolysyl-tRNA synthetase with polysubstrate specificity expands the toolbox for engineering enzymes with incorporation of noncanonical amino acids

Aminoacyl-tRNA synthetase (aaRS) is a core component for genetic code expansion (GCE), a powerful technique that enables the incorporation of noncanonical amino acids (ncAAs) into a protein. The aaRS with polyspecificity can be exploited in incorporating additional ncAAs into a protein without the evolution of new, orthogonal aaRS/tRNA pair, which hence provides a useful tool for probing the enzyme mechanism or expanding protein function. A variant (N346A/C348A) of pyrrolysyl-tRNA synthetase from Methanosarcina mazei (MmPylRS) exhibited a wide substrate scope of accepting over 40 phenylalanine derivatives. However, for most of the substrates, the incorporation efficiency was low. Here, a MbPylRS (N311A/C313A) variant was constructed that showed higher ncAA incorporation efficiency than its homologous MmPylRS (N346A/C348A). Next, N-terminal of MbPylRS (N311A/C313A) was engineered by a greedy combination of single variants identified previously, resulting in an IPE (N311A/C313A/V31I/T56P/A100E) variant with significantly improved activity against various ncAAs. Activity of IPE was then tested toward 43 novel ncAAs, and 16 of them were identified to be accepted by the variant. The variant hence could incorporate nearly 60 ncAAs in total into proteins. With the utility of this variant, eight various ncAAs were then incorporated into a lanthanide-dependent alcohol dehydrogenase PedH. Incorporation of phenyllactic acid improved the catalytic efficiency of PedH toward methanol by 1.8-fold, indicating the role of modifying protein main chain in enzyme engineering. Incorporation of O-tert-Butyl-L-tyrosine modified the enantioselectivity of PedH by influencing the interactions between substrate and protein. Enzymatic characterization and molecular dynamics simulations revealed the mechanism of ncAAs affecting PedH catalysis. This study provides a PylRS variant with high activity and substrate promiscuity, which increases the utility of GCE in enzyme mechanism illustration and engineering.

Perfluoropyridine: Discovery, Chemistry, and Applications in Polymers and Material Science

Perfluoropyridine (PFPy) is an organofluorine compound that has been employed for a variety of applications, from straightforward chemical synthesis to more advanced functions, such as fluorinated networks and polymers. This can be directly attributed to the highly reactive nature of PFPy, especially towards nucleophilic aromatic substitution (SNAr). The aim of this review is to highlight the discovery and synthesis of PFPy, discuss its reactive nature towards SNAr, and to summarize known reports of the utilization and thermal analysis of PFPy containing fluoropolymers and fluorinated network materials.

Theoretical Prediction and Explanation of Reaction Site Selectivity in the Addition of a Phenoxy Group to Perfluoropyrimidine, Perfluoropyridazine, and Perfluoropyrazine

Perfluoroaromatics, such as perfluoropyridine and perfluorobenzene, are privileged synthetic scaffolds in organofluorine methodology, undergoing a series of regioselective substitution reactions with a variety of nucleophiles. This unique chemical behavior allows for the synthesis of many perfluoroaromatic derived molecules with unique and diverse architectures. Recently, it has been demonstrated that perfluoropyridine and perfluorobenzene can be utilized as precursors for a variety of materials, ranging from high performance polyaryl ethers to promising drug scaffolds. In this work, using density functional theory, we investigate the possibility of perfluoropyrimidine, perfluoropyridazine, and perfluoropyrazine participating in similar substitution reactions. We have found that the first nucleophilic addition of a phenoxide group substitution on perfluoropyrimidine and on perfluoropyridazine would happen at a site para to one of the nitrogen atoms. While previous literature points to mesomeric effects as the primary cause of this phenomenon, our work demonstrates that this effect is enhanced by the fact that the transition states for these reactions result in bond angles that allow the phenoxide to π-complex with the electron-deficient diazine ring. The second substitution on perfluoropyrimidine and on perfluoropyridazine is most likely to happen at the site para to the other nitrogen. The second substitution on perfluoropyrazine is most likely to happen at the site para to the first substitution. The activation energies for these reactions are in line with those reported for perfluoropyridine and suggest that these platforms may also be worth investigation in the lab as possible monomers for high performance polymers.

Fluoromaticity: The Molecular Orbital Contributions of Fluorine Substituents to the π-Systems of Aromatic Rings

The addition of fluorine atoms to an aromatic ring brings about an additional set of π-bonding and antibonding orbitals culminating after the addition of the sixth fluorine with a new set of π-aromatic-like orbitals that affect the molecule in a way that we will refer to hereafter as "fluoromaticity". Depending on the number and position of the fluorine atoms, the contributed π-orbitals can even further stabilize the ring leading to smaller bond lengths within the ring and higher resistance to addition reactions. This added ring stability partially explains the high thermostability and chemical resistance found in polymers containing fluorinated aromatics in their architecture. A similar molecular orbital effect is seen with the addition of other halogen atoms to aromatic rings, though to a much smaller degree and not resulting in the additional ring stability.

Carboxylic Acid Deoxyfluorination and One-Pot Amide Bond Formation Using Pentafluoropyridine (PFP)

This work describes the application of pentafluoropyridine (PFP), a cheap commercially available reagent, in the deoxyfluorination of carboxylic acids to acyl fluorides. The acyl fluorides can be formed from a range of acids under mild conditions. We also demonstrate that PFP can be utilized in a one-pot amide bond formation via in situ generation of acyl fluorides. This one-pot deoxyfluorination amide bond-forming reaction gives ready access to amides in yields of ≤94%.

Synthesis of complex unnatural fluorine-containing amino acids

The area of fluorinated amino acid synthesis has seen rapid growth over the past decade. As reports of singly fluorinated natural amino acid derivatives have grown, researchers have turned their attention to develop methodology to access complex proteinogenic examples. A variety of reaction conditions have been employed in this area, exploiting new advances in the wider synthetic community such as photocatalysis and palladium cross-coupling. In addition, novel fluorinated functional groups have also been incorporated into amino acids, with SFX and perfluoro moieties now appearing with more frequency in the literature. This review focuses on synthetic methodology for accessing complex non-proteinogenic amino acids, along with amino acids containing multiple fluorine atoms such as CF3, SF5 and perfluoroaromatic groups.

Protecting Group-Controlled Remote Regioselective Electrophilic Aromatic Halogenation Reactions

Being able to utilize a protecting group to influence remote regiocontrol offers a simple alternative approach to direct late-stage functionalization of complex organic molecules. However, protecting groups that have the ability to influence reaction regioselectivity remote to their local chemical environment are not widely reported in the literature. Herein, we report the development of remote regioselective electrophilic aromatic substitution (S_EAr) reactions that are enabled via the application of the tetrafluoropyridyl (TFP) phenol-protecting group. We demonstrate that through sequential reactions and protection/deprotection of the TFP group, substitution patterns that do not conform to classical S_EAr regioselectivity rules can be readily accessed.