Synthesis of aminopyridines from 2-fluoropyridine and lithium amides

C-F bond activation under transition-metal-free conditions

The unique properties of fluorine-containing organic compounds make fluorine substitution attractive for the development of pharmaceuticals and various specialty materials, which have inspired the evolution of diverse C-F bond activation techniques. Although many advances have been made in functionalizations of activated C-F bonds utilizing transition metal complexes, there are fewer approaches available for nonactivated C-F bonds due to the difficulty in oxidative addition of transition metals to the inert C-F bonds. In this regard, using Lewis acid to abstract the fluoride and light/radical initiator to generate the radical intermediate have emerged as powerful tools for activating those inert C-F bonds. Meanwhile, these transition-metal-free processes are greener, economical, and for the pharmaceutical industry, without heavy metal residues. This review provides an overview of recent C-F bond activations and functionalizations under transition-metal-free conditions. The key mechanisms involved are demonstrated and discussed in detail. Finally, a brief discussion on the existing limitations of this field and our perspective are presented.

Amination of Phosphorodiamidate-Substituted Pyridines and Related N-Heterocycles with Magnesium Amides

The amination of various phosphorodiamidate-substituted pyridines, quinolines, and quinoxaline with magnesium amides R₂NMgCl·LiCl proceeds at room temperature within 8 h. Several pharmaceutically active amines were suitable substrates for this amination procedure, and also the antihistaminic tripelennamine was prepared. Additionally, several heterocyclic phosphorodiamidates underwent directed ortho-metalation (D oM) using TMPMgCl·LiCl (TMP = 2,2,6,6-tetramethylpiperidyl) or TMP₂Mg·2LiCl, followed by electrophilic functionalization prior to the amination step, which led to ortho-functionalized aminated N-heterocycles.

Transition-metal-free access to 2-aminopyridine derivatives from 2-fluoropyridine and acetamidine hydrochloride

Under catalyst-free conditions, an efficient method for the synthesis of 2-aminopyridine derivatives through the nucleophilic substitution and hydrolysis of 2-fluoropyridine and acetamidine hydrochloride has been developed. This amination uses inexpensive acetamidine hydrochloride as the ammonia source and has the advantages of a high yield, high chemoselectivity and wide substrate adaptability. The results suggest that other N-heterocycles containing fluorine substituents can also complete the reaction via these reaction conditions and yield the target products.

Nucleophilic amination of methoxypyridines by a sodium hydride-iodide composite

A new protocol for nucleophilic amination of methoxypyridines and their derivatives was developed using sodium hydride (NaH) in the presence of lithium iodide (LiI). The method offers a concise access to various aminopyridines which are potentially of medicinal interest.

Effective, transition metal free and selective C–F activation under mild conditions

A simple and effective aromatic nucleophilic monosubstitution reaction for the synthesis of aromatic amines via selective C–F bond cleavage of various fluoroarenes with primary and secondary aromatic amines is demonstrated.

Mechanism of lithium diisopropylamide-mediated substitution of 2,6-difluoropyridine

Treatment of 2,6-difluoropyridine with lithium diisopropylamide in THF solution at -78 degrees C effects ortholithiation quantitatively. Warming the solution to 0 degrees C converts the aryllithium to 2-fluoro-6-(diisopropylamino)pyridine. Rate studies reveal evidence of a reversal of the ortholithiation and a subsequent 1,2-addition via two monomer-based pathways of stoichiometries [ArH*i-Pr(2)NLi(THF)](double dagger) and [ArH*i-Pr(2)NLi(THF)(3)](double dagger). Computational studies fill in the structural details and provide evidence of a direct substitution without the intermediacy of a Meisenheimer complex.

Probing (1)J(C-F) and (n)J(F-F) spin-spin coupling constants for fluoroazines: an ab initio theoretical investigation

Ab initio equation-of-motion coupled cluster singles and doubles calculations have been carried out to evaluate one-bond C-F coupling constants (1)J(C-F) and three-, four-, and five-bond F-F coupling constants (n)J(F-F) for a series of mono-, di-, and trifluoroazines. The computed (1)J(C-F) and (n)J(F-F) values for these are in good agreement with available experimental coupling constants. The values of (1)J(C-F) vary as the number and positions of N atoms and the number and relative positions of C-F bonds change, but it is difficult to discern general patterns for these changes due to opposing effects of the Fermi contact and paramagnetic spin-orbit terms. The majority of (1)J(C-F) values lie in a range that includes the three monosubstituted pyridines. For trifluoroazines, (1)J(C-F) for a C-F bond that is ortho to two other C-F bonds is greater than (1)J(C-F) for the other two bonds. F-F coupling constants arise in these molecules when the two C-F bonds are ortho, meta, or para. Values of (3)J(F-F) are relatively large and negative, whereas values of (5)J(F-F) are relatively large and positive. (4)J(F-F) may be positive or negative and large or small. The value of this coupling constant depends on the nature of the atom that links the two C-F bonds and the number and positions of N atoms in the ring. The calculations carried out in this study at a reliable level of theory give values for one-bond C-F and n-bond F-F spin-spin coupling constants for the fluoroazines that are not available experimentally. In addition, the patterns that describe the changes that occur in these molecules provide a basis for predicting their values in larger, related systems in the absence of experimental data and direct calculations.

A mild, catalyst-free synthesis of 2-aminopyridines

Alkylation of 2-mercaptopyridine with 1,2-dibromoethane affords a cyclic dihydrothiazolopyridinium salt that can serve as a precursor of 2-aminopyridines. Its reaction with primary or secondary amines, either neat or in DMSO, under mild conditions gives the title compounds.