4- Fluoropyridines

Fluoropyridine is readily hydroly2ed to 2-pyridone in 60% yield by reflux in 6 Ai hydrochloric acid (383). It is quite reactive with nucleophiles. For example, the halogen mobiUty ratio from the comparative methoxydehalogenation of 2-fluoropyridine and 2-chloropyridine was 85.5/1 at 99.5°C (384). This labihty of fluorine has been utili2ed to prepare fluorine-free 0-2-pyridyl oximes of 3-oxo steroids from 2-fluoropyridine for possible use as antifertihty agents (385). 

Metahation of 2-fluoropyridine with lithium diisopropylamide (LDA) gives 2-fluoro-3-hthiopyridine, thereby providing entry to 3-substituted pyridines (388). This technique has been used to make fluorine analogues of the antitumor eUipticines (389). 

Derivatives such as 3-fluoro-4-nitropyridine [13505-01 -6] (396) or the 1-oxide [769-54-0] (397) have been used to characteri2e amino acids and peptides. 5-Eluoro-3-pyridinemethanol [22620-32-2] has been patented as an antihpolytic agent (398). A promising antidepressant, l-(3-fluoro-2-pyridyl)pipera2ine hydrochloride [85386-84-1] is based on 2-chloro-3-fluoropyridine [17282-04-1] (399). 

Fluoropyridine. This isomer can be prepared in 54—81% yield by dia2oti2ation of 4-aminopyridine in anhydrous hydrogen fluoride (370,371,400). Eree 4-fluoropyridine readily undergoes self-quaterni2ation to give pyridyl pyridinium salts (401) stabili2ation can be effected as the hydrochloride salt (371,400). Numerous 4-fluoropyridinium salts, eg, 4-fluoro-l-methylpyridinium iodide, have been converted to novel penicillins (387,402). 

Amines or ammonia replace activated halogens on the ting, but competing pyridyne [7129-66-0] (46) formation is observed for attack at 3- and 4-halo substituents, eg, in 3-bromopyridine [626-55-1] (39). The most acidic hydrogen in 3-halopyridines (except 3-fluoropyridine) has been shown to be the one in the 4-position. Hence, the 3,4-pyridyne is usually postulated to be an intermediate instead of a 2,3-pyridyne. Product distribution (40% (33) and 20% (34)) tends to support the 3,4-pyridyne also. 

Selective fluonnation in polar solvents has proved commercially successful in the synthesis of 5 fluorouracil and its pyrimidine relatives, an extensive subject that will be discussed in another section Selective fluonnation of enolates [47], enols [48], and silyl enol ethers [49] resulted in preparation of a/phn-fluoro ketones, fieto-diketones, heta-ketoesters, and aldehydes The reactions of fluorine with these functionalities is most probably an addition to the ene followed by elimination of fluonde ion or hydrogen fluoride rather than a simple substitution In a similar vein, selective fluonnation of pyridmes to give 2-fluoropyridines was shown to proceed through pyridine difluondes [50]... 

The A -fluoropyridinium salts produce 2-fluoropyridines on treatment with base. A carbene mechanism has been proposed [78] (equation 36). 

None of the 3-halogenopyridines yield 2-piperidinopyridine. This substance was obtained as the only product from the reaction of 2-fluoropyridine (24, X = F) with lithium piperidide under the same conditions in 97% yield. Finally, it was found that 4-chloropyridine (32, X = Cl) was converted in 95% total yield into a mixture of 0.4% of 3-piperidino- (29, Y = NC5H10) and 99.6% of 4-piperidino-pyridine (34, Y = NCsHio)- Thus, in contrast to the amination with potassium amide, 4-chloropyridine reacts with lithium piperidide almost exclusively via the addition product 33 (X = Cl, Y = NC5H10). 

In an extensive investigation of the behavior of halogeno-pyridines and -quinolines towards potassium amide in liquid ammonia, tendencies to react according to the hetaryne type and other mechanisms were compared. The following results may be mentioned. 3-Fluoropyridine does not react via 3,4-pyridyne, but yields fluoro derivatives of 4,4 - and 2,4 -bipyridine. 3,6-Dibromopyridine is converted first into 6-bromo-3,4-pyridyne and not into 6-amino-3-bromopyridine. 2-Bromoquinoline yields 2-methylquinazoline together with 2-aminoquinohne. ... 

Fluorination of pyridine (90TL775), uracil (90T3093), and octaethyl-porphyrin [88JCS(P1)1735] has been described using cesium fluorooxysulfate. The outcome of the former was strongly dependent on the solvent. For example, with pyridine in methanol no fluorination was observed and 2-methoxypyridine was obtained. 2-Fluoropyridine was isolated when cyclohexane was the solvent (90TL775). 

An unusual difluoroboryl imidate (10) was isolated during decomposition of (9) and its stability arose from a strong intramolecular bond between the pyridine N and boron. Formation of the desired 2-amino-5-fluoropyridine followed by the use of aqueous sodium hydroxide (89S905). 

There are several reports on the decomposition of diazonium salts, in particular from pyridines, in anhydrous HF [81JFC( 18)497 88JFC(38)435]. Mildness of conditions and improved yields are benefits. The elusive 4-fluoropyridine can be obtained using this method [81JFC( 18)497]. 

Nitrosonium tetrafluoroborate has been proposed as an alternative to NaN02/HBF4. 2-Fluoropyridine is obtained in 69% yield on warming a CH2C12 mixture of NOBF4 with 2-NHrpyridine (90EUP430434). 

A novel route to 2-fluoropyridines involved the base-induced decomposition of substituted N-fluoropyridinium salts. Abstraction of the 2-H produces a singlet carbene (11) that removes F from a counterion. This is in contrast to the reaction with C nucleophiles, which are fluorinated, and is attributed to the high stability of C—F compared to O—F and N—F (89JOC1726). 

The ease of dehalogenation of C H X by Ni(ll)/ IMes HCl 1/NaO Pr decreased in the order 1 > Br > Cl F. Subsequent work showed that a 1 1 combination of Ni and NHC in the presence of NaOCHEt resulted in enhanced reactivity towards aryl fluorides [6], Again, the A-mesityl substituted ligand IMes HCl 1 imparted the highest level of catalytic activity. Table 8.2 illustrates that hydrodefluorination is sensitive to both the nature of the substituents on the aromatic ring and the specific regioisomer. Thus, 2- or 4-fluorotoluene (Table 8.2, entry 2) proceeded to only 30% conversion after 15 h, whereas quantitative conversion of 2-fluoroanisole (Table 8.2, entry 3) and high conversion of 3-fluoropyridine (Table 8.2, entry 5) was achieved in only 2-3.5 h. The reactivity of 2-fluoropyridine was compromised by more efficient nucleophilic aromatic substitution. 

Carbon and Proton NMR Data. 13C and 11NMR chemical shift and coupling constant data for 2-fluoropyridine, 2-fluoroquino-lines, and 2-fluoroquinoxoline are provided in Scheme 3.62. 

The solid produced by action of bromine trifluoride on pyridine in carbon tetrachloride ignites when dry. 2-Fluoropyridine reacts similarly. 

A similar reaction of 1,2,4-triazole with 2-cyano-3-fluoropyridine 33 gave 3-[l,2,4]-triazol-l-yl-pyridine-2-carboni-trile 34 in a yield of 92% after purification (Equation 7) <2004JME2995>. 

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