residue heated—without isolation of the N-oxide—with fuming nitric acid and sulfuric acid at 90° for 14 hr. The reaction mixture is then poured on to ice, neutralized, and extracted with ether, yielding over 90%
of 4-nitropyridine-N-oxide.
The nitro group can also be introduced into position 4 in many deriva
tives of pyridine-N-oxide [see, e.g. refs. (200,221, 224-227,237, 270-273) ] . If this position is already occupied, nitration does not occur (194,273).
The nitro group can, however, be introduced at other positions under the influence of substituents already present (226,227,274-277).
If benzoyl nitrate is allowed to stand with pyridine-N-oxide in chloroform for 4 days at room temperature, a very low yield of 3-nitro-pyridine-N-oxide is obtained (278).
Sulfonation—Electrophilic substitution
Surprisingly, the sulfonation of pyridine-N-oxide proceeds quite dif
ferently from the normal nitration. If the amine oxide is treated at 150°
with 20% oleum in the presence of mercuric sulfate, the unchanged starting material is obtained in 60% yield, and no sulfonic acid is formed
(204). It is only by using the conditions required for the sulfonation of pyridine itself, namely heating with 20% oleum and mercuric sulfate for 22 hr at 230°, that a sulfonic acid is formed; it is, however, found to be pyridine-3-sulfonic acid N-oxide ( L X X I ) (204).
Conversely, reaction with mercuric acetate gives the 4-substituted pyridine-mercury compound, which was isolated as 4-(chloromercuri)-pyridine-N-oxide following the addition of saturated salt solution
(LXXII) (279). Treatment of L X X I I with bromine yields 4-bromopyri-dine-N-oxide (279). According to van Ammers and den Hertog (280), on the other hand, substitution occurs in the 2- or 2- and 6-positions, not position 4.
Other electrophilic reactions, e.g. bromination, chlorosulfonation, and Friedel-Crafts reactions have as little effect on pyridine-N-oxide as they have on pyridine itself (204).
INTRODUCTION OF SUBSTITUENTS INTO PYRIDINE RING 89
REACTIONS USING 4-NITROPYRIDINE-N-OXIDES
The nitro group in the 4-position of pyridine-N-oxide is distinguished by an extraordinary reactivity, with the result that numerous 4-substi
tuted pyridine derivatives can be prepared using this nitro compound.
4-Nitropyridine-N-oxide itself is a stable substance; recrystallization from acetone or water yields yellow crystals, m.p. 1 5 9 ° (200). Mere warming with aqueous alkali or with hydrohalic acids causes a re
placement of the nitro group. If 4-nitropyridine-N-oxide is warmed with dilute sodium hydroxide solution, a vigorous reaction ensues but no uni
form product is formed. 4,4/-Azopyridine-N-oxide (281) and 4,4'-azopyri-dine were isolated from the reaction mixture (225). Besides these, 4-hy
droxypyridine is probably also formed (225); it becomes the main product on the addition of hydrogen peroxide to the sodium hydroxide solution (282). If 4-nitropyridine-N-oxide is heated with ammonia, 4,4'-azopyri-dinedi-N-oxide and a small quantity of 4-aminopyridine-N-oxide are produced (283). Prolonged warming with hydrobromic or hydrochloric acid yields mainly 4-bromopyridine-N-oxide (LXXIII) (284) [see also ref. (225)] and 4-chloropyridine-N-oxide, respectively (225).
A particularly good method for synthesizing 4-chloro- or 4-bromo
pyridine-N-oxide is based on the reaction between the nitro derivative and acetyl chloride or bromide; an excess of the acid halide is used and the reaction is carried out at 5 0 ° (LXXIV) (200,285).
The action of phosphorus oxychloride on 4-nitropyridine-N-oxide gives 4-chloropyridine-N-oxide as the major product if a temperature of 70° is maintained; at 100°, 2,4-dichloropyridine is formed (200,285).
The preparation of 4-hydroxypyridine-N-oxide is accomplished by warming 4-nitropyridine-N-oxide with acetic anhydride and
dimethyl-aniline on a water bath ( L X X V ) (200); [see also ref. (237)].
Pyridyl-4-ether-N-oxides and the corresponding thioether-N-oxides are obtained with equal ease by allowing the nitro compound to react
N O 9 2 % O L X X I V
( C H3C O )20 C , H5N ( C H3)2
1 0 0 % 0 N O H
O L X X V O 77 %
with alkoxides or phenoxides and thiophenoxides, respectively {200, 225, 283,286). A 70% yield of 4-ethoxypyridine-N-oxide is obtained by this route (LXXVI) (225). The reagents mentioned also cause ready ether formation with 4-chloropyridine-N-oxide (200,286).
Catalytic hydrogenation of 4-benzyloxypyridine-N-oxide with pal
ladium-charcoal in methanol gives 4-hydroxypyridine-N-oxide with elimination of toluene (200). As in pyridine itself, the phenoxy and thio-phenoxy groups in the 4-position of pyridine-N-oxide exhibit a high re
activity and can be replaced by heating with primary or secondary amines, thus allowing the introduction of the amino group in this posi
tion (LXXVII) (200,286). The same reaction also occurs with 4-chloro
pyridine-N-oxide (LXXVII) (200,286), though not without the danger of removing the N-oxide function (221).
On prolonged standing with thiourea, 4-chloropyridine-N-oxide af
fords an 80% yield of pyridyl-4-isothiouronium chloride N-oxide and hydrolysis of the latter with cold sodium hydroxide solution results in the formation of 4-mercaptopyridine-N-oxide (287) [see also ref. (200)].
The salt obtained by allowing this mercaptan to react with mercuric acetate possesses high bacteriostatic activity (194)- If the isothiouronium salt is evaporated down with ammonia on a water bath, 4,4'-dipyridyl sulfide N-oxide is obtained (287); this compound is also formed in small quantities by the action of hydrogen sulfide on 4-nitropyridine-N-oxide
L X X V I I
I N T R O D U C T I O N O F S U B S T I T U E N T S I N T O P Y R I D I N E R I N G 91
in the presence of pyridine (194). These reactions are illustrated in reac
tion scheme L X X V I I I (200).
N N - » Q N N N H N H
N a N 02 H , S N a O H N H 4 O H ( 5 0 %) ( 1 5 ° )
L X X V I I
The reduction of 4-nitropyridine-N-oxide with phosphorus trichloride has been referred to above; in this reaction the nitro group remains un
affected. If this process is combined with the action of acetyl chloride, 4-chloropyridine is obtained (175). Most of the reduction methods de
scribed above (pp. 84-85) can be applied directly to 4-substituted pyridine-N-oxides obtained from 4-nitropyridine-N-oxide. Under the action of re
ducing agents, the behavior of 4-nitropyridine-N-oxide resembles that of nitrobenzene inasmuch as azo, azoxy, and hydrazo compounds are formed in addition to 4-aminopyridine-N-oxide; 4-aminopyridine is only produced in acid solution (LXXVIII) (200,288).
4-Aminopyridine-N-oxide, unlike 4-aminopyridine, is readily diazo-tized (200). The diazonium salt can be coupled, yielding azo dyes (200).
Furthermore, the diazo group can be replaced by a halogen, a nitrile, and a thiocyano group (200).
The reactions depicted in this section also apply to many substituted pyridine-N-oxides. Since they offer nothing of a particularly novel char
acter, however, these methods are not discussed.
Appendix
Tertiary Pyridines a n d Pyridinium S a l t s
The sulfonation of 2,6-di-£er£-butylpyridine using S 03 in liquid S 02
does not yield the corresponding 4-pyridinesulfonic acid as previously thought (10), but 2,6-di-£er£-butyl-3-pyridinesulfonic acid; this was re
cently proved with the aid of nuclear magnetic resonance spectra (289).
2-Chloropyridine is obtained in 65-80% yield in the chlorination of pyridine with chlorine and sulfur dioxide at 340-370° (290). No halogen exchange is undergone by either 2-chloropyridine, 2-chloropyridine-N-oxide, 2-chloropyridine hydrochloride, or 2-bromopyridine when heated with potassium fluoride in dimethylformamide; 2-chloro-3-nitro- and 2-chloro-5-nitropyridine, on the other hand, give the corresponding fluoro compounds by this treatment (291).
Foldi (292) has recently described an interesting reaction between 4-alkylpyridines and benzene- or p-toluenesulfonyl chloride. Loss of a proton results in the linking of the arylsulfonyl group to the C atom attached to position 4 of the alkylpyridine.
The investigation by Wibaut and Broekman (293) into the polymeri
zation of 4-chloropyridine revealed that substances possessing the prop
erties of pyridyl-4-chloropyridinium chlorides are formed; rapid hy
drolysis affords 4-hydroxypyridine and N-(4'-pyridyl)-4-pyridone.
Jerchel and Jakob (294) have recently reported an interesting rear
rangement undergone by pyridyl aminophenyl ethers. If the dihydrochlo-rides of the 4-pyridyl aminophenyl ethers containing the N H2 group in position 2, 3, or 4 of the benzene nucleus, are warmed, good yields of the corresponding pyridyl-4-hydroxyphenylamines are obtained.
76 %
I N T R O D U C T I O N O F S U B S T I T U E N T S I N T O P Y R I D I N E R I N G 93 2-Pyridyl aminophenyl ethers also undergo this reaction (294).
Further investigation into the action of nitric acid on 4-mercapto
pyridine has shown that the main product is the dinitrate of di-4-pyridyl disulfide (295,296), not 4-pyridinesulfonic acid (155). The desired sul
fonic acid is obtained from 4-pyridinethioI, nitric acid, and chlorine, or nitric acid, hydrochloric acid, and chlorine (295).
S H
f \ H N O , (HCI) a n d C l2
k J ( 1 ) 0 ° , 2 h r s (2) 7 0 - 8 0 °
Further preparations of 4-pyridinesulfonic acid are based on the oxidation of 4-pyridinethiol with hydrogen peroxide in glacial acetic acid
(296), the oxidation of the barium salt with H202 (297), the treatment of 4-chloropyridine with sodium sulfite (275,295,297) or the action of sodium bisulfite on 4-hydroxypyridine (297). According to Evans and Brown (297), the best method of preparing 4-pyridinesulfonic acid con
sists in the decomposition of 4-pyridylpyridinium chloride hydrochloride with sodium sulfite or bisulfite (298).
HCI
•N
/><3
N a , S 03( N a H S 08)) ^ 3N _So8H n — / \ — 1 4 0 - 1 6 0 ° , 6 - 1 0 h r se — XCI©
4-Pyridinesulfonyl chloride is obtained from 4-mercaptopyridine and chlorine in concentrated hydrochloric acid at 0° (299). It is not isolated, but allowed to react directly with various bases.
P y r i d i n e - N - o x i d e s
Evans and associates (800) have recently proposed the use of a mix
ture of trifluoroacetic acid and H202 (30%) in the preparation of pyri
dine-N-oxides from bases that are not readily oxidized; they succeeded in obtaining 2,6-dibromopyridine-N-oxide in 70% yield by means of this method.
The catalytic reduction of pyridine-N-oxides with Raney nickel and hydrogen does not take place in the presence of glacial acetic acid/acetic anhydride exclusively (219), but also in methanol; even then the addi
tion of glacial acetic acid is advisable (801). According to the investiga
tions of Howard and Olszewski (802), the N-oxide oxygen can be elimi
nated with triphenylphosphine at elevated temperatures. In contrast to the above, aromatic amine oxides in boiling glacial acetic acid were found to be stable to triethyl- or triphenylphosphine (808). Removal of the oxygen from pyridine-N-oxides is also brought about by sulfur and its compounds at temperatures of 115-150° (804).
S O , H
The action of acetic anhydride on pyridine-N-oxides has been re
peatedly investigated recently. 2-Styrylpyridine-N-oxide and 2-propenyl-pyridine-N-oxide are hydroxylated by acetic anhydride both at the ethylenic linkage and in the pyridine nucleus (305).
3-Chloro-, 3-bromo-, and 3-fluoropyridine-N-oxide react with acetic anhydride to give the corresponding 2-acetoxy-3-halopyridines (306).
The acetic anhydride method was also invoked in the synthesis of 3- and 6-alkylpicolinic acid, 5-methylpicolinic acid (307) and pyridinealdehyde-N-oxides (308,309).
Papers by both Furukawa (310) and Traynelis and Martello (311) deal with the question of the mechanism of this reaction.
The replacement of acetic anhydride by trifluoroacetic anhydride is recommended, as the latter reacts even at 40° and is more readily saponified (312). The reaction between 2-picoline-N-oxide and ketene in the presence of catalytic quantities of H2S 04 results in the formation of 2-acetoxypicoline, which is obtained from the same N-oxide by the action of acetic anhydride (313).
A recently discovered reaction between pyridine-N-oxides and potas
sium cyanide is not without interest. The action of alkyl halides or di
methyl sulfate on pyridine-N-oxide yields quaternary compounds which can be converted by potassium cyanide into 2- and 4-cyanopyridine at room temperature (314,315).
Picoline- and lutidine-N-oxides also undergo this reaction (314,315).
In a detailed study of the sulfonation of pyridine-N-oxide, M. van
S O , H N
4 0 - 4 5 % 0 . 5 - 1 . 0 % 2 - 2 . 5 %
I N T R O D U C T I O N O F S U B S T I T U E N T S I N T O P Y R I D I N E R I N G 95
Ammers and H. J. den Hertog (316) detected small quantities of isomers in addition to the main product, 3-pyridinesulfonic acid N-oxide; 45%
of the pyridine-N-oxide subjected to the reaction remained unchanged.
Experimental
N- (2,6-Dichlorobenzyl) -4-phenacyl-l ,4-dihydropyridine (114, 115).
N-(2,6-Dichlorobenzyl) pyridinium bromide (114) (6.38 gm) in meth
anol (25 ml) is treated with acetophenone (5 ml) and p-nitrosodimethyl-aniline (1.8 gm) in a closed round-bottomed flask fitted with in- and outlet tubes. Nitrogen is passed into the solution—which is maintained at 20°—until the air in the flask is expelled, and sodium hydroxide solu
tion (2iV, 20 ml) added. A red coloration develops after a few minutes and the solution becomes slightly warm. After cooling, crystallization is induced by scratching and completed by the addition of water (25 ml).
After 3 hr the crystals are filtered and washed with water and a little alcohol, yielding the crude product (5.4 gm, 76%).
4-Phenacylpyridine (114,115). The compound described above (3.56 gm) is heated with hydrobromic acid (66%, 3 ml) in a tube for 1 hr at 180°. After cooling, the 4-phenacylpyridine hydrobromide formed may be freed from 2,6-dichlorobenzyl bromide by filtering and washing with ether followed by acetone. The aqueous fraction of the filtrate is evap
orated under reduced pressure on a wrater bath; crystallization, initiated by scratching in the presence of a little methanol, is completed by the addition of ether. The total yield of hydrobromide amounts to 2.6 gm (93%). 4-Phenacylpyridine, liberated from the salt by alkali, is re-crystallized from petroleum ether or acetone and has a m.p. of 115°
(115).
N-Pyridyl-4-pyridinium chloride hydrochloride (120,161). Dry pyri
dine (300 ml) is vigorously stirred with thionyl chloride (technical grade, 900 gm) in a 1 liter three-necked flask fitted with a dropping fun
nel, stirrer, and reflux condenser protected by a calcium chloride tube.
An internal temperature of 20° is easily maintained by external cooling with flowing water and a controlled dropping rate. After the addition of SOCl2 is completed the mixture is allowed to stand for 3 days at room temperature. The excess of thionyl chloride is removed by vacuum dis
tillation, the temperature of the water bath being gradually raised to 100°; this temperature is maintained for 2 hr following the complete removal of thionyl chloride. The solid residue left in the flask is con
verted into a homogeneous crystalline mass by boiling with absolute methanol (200 ml), cooled to 0°, and filtered. The crude product, washed with a little alcohol and dried at 110°, has a m.p. of 145-148°
and the yield is 260 gm (60%). Further purification is effected by
dis-solving the crude product in a little hot 2N hydrochloric acid, filtering, and treating the filtrate repeatedly with animal charcoal. Evaporation under vacuum and addition of alcohol cause the separation of off-white crystals; after cooling these are filtered, washed with alcohol, and dried.
Recrystallization from methanol affords colorless needles, m.p. 151°.
4-Pyridyl 4-nitrophenyl ether (118). A mixture of N-pyridyl-4-py-ridinium chloride hydrochloride (10 gm) and p-nitrophenol (6.6 gm) is heated at 160-165° in an oil bath, forming a clear melt within a few minutes. On cooling, this is treated with 10% sodium carbonate solu
tion, filtered, and the filtration residue washed with sodium hydroxide solution and water. Recrystallization from aqueous alcohol gives needles
(4.5 gm, 47%), m.p. 128°.
4-Chloropyridine (118,161). N-Pyridyl-4-pyridinium chloride hydro
chloride (200 gm, crude product) is thoroughly ground to a powder, in
timately mixed with phosphorus pentachloride (180 gm) and heated to 160° in an oil bath, in a 500-ml flask equipped with an air condenser.
As the internal temperature rises to 165-170°, a dark, mobile melt is formed. The reaction mixture is maintained at a bath temperature of 180° for a further 6 hr. Ice water is carefully added through the con
denser, accompanied by efficient cooling, until a clear solution is ob
tained; the solution is transferred to a larger flask and basified with sodium hydroxide solution, the operation being carried out with vigorous stirring and careful cooling. 4-Chloropyridine and pyridine come over to
gether when the mixture is steam-distilled, and are separated by the ad
dition of potassium carbonate. The aqueous phase is extracted with ether; the ethereal extracts are used to dissolve the previously separated bases. After drying with KOH, the product is distilled in the apparatus shown in Fig. 1. The apparatus must first be thoroughly rinsed with alcoholic potassium hydroxide and dried. After removal of the solvent at normal pressure, pyridine comes over first at 57-59°/100 mm, followed by 4-chloropyridine at 63-64°/50 mm. The yield of 4-chloropyridine amounts to 72 gm (73%).
4-Phenylaminopyridine (192). N-Pyridyl-4-pyridinium chloride hy
drochloride (2.3 gm) and aniline hydrochloride (2.6 gm) are heated at 180° for 90 min, dissolved in water on cooling and warmed with animal charcoal. A large excess of sodium hydroxide is added and the mixture brought to the boil; crystals separated on cooling which are filtered and recrystallized from methanol/water to give 4-phenylaminopyridine, m.p.
172°, in quantitative yield.
4-n-Butylaminopyridine (192). 4-Pyridyl phenyl ether (120) (2.6 gm) and n-butylamine hydrochloride (4.9 gm) are heated at 180° for 2 hr, and at 200° for the last 5 min. The mixture is freed from unreacted starting material by dissolving the cooled melt in water, rendering the
I N T R O D U C T I O N O F S U B S T I T U E N T S I N T O P Y R I D I N E R I N G 97 solution alkaline with sodium hydroxide solution and steam-distilling.
On cooling, the amine produced crystallizes out from the oily residue.
The crude product obtained by filtration is dried and recrystallized from petroleum ether (b.p. 40-70°) yielding colorless needles of 4-n-butyl-aminopyridine (1.6 gm, 70%), m.p. 65°.
F i g . 1. A p p a r a t u s for t h e d i s t i l l a t i o n of 4 - c h l o r o p y r i d i n e
4-Pyridyl n-cetyl thioether (118). Hydrogen sulfide is passed for 2 hr into a suspension of N-pyridyl-4-pyridinium chloride hydrochloride (15 gm) in pyridine (15 ml) maintained at approximately 70°, forming a clear solution. n-Cetyl chloride (20 gm) is added, the mixture heated in a tube for 12 hr at 145° and the product dissolved in warm water (150 ml). After the addition of HCI (2Ar, 20 ml) and cooling, the scarcely soluble hydrochloride of the thioether can be largely freed from water and pyridine by filtration. The moist product is suspended in water, treated with sodium hydroxide solution (2N) and vigorously stirred at 50° for 15 min. A semisolid, reddish mass forms on cooling which can be brought into solution with hot methanol/ethanol; treatment with animal charcoal yields white flakes (11 gm, 52%), m.p. 52°.
Pyridine-N-oxide {200). A mixture of pyridine (40 gm), glacial acetic acid (100 ml), and hydrogen peroxide (50 ml) is heated at 70-80° for 8 hr; after 3 hr, more perhydrol (35 ml) is added. The solution is then evaporated under vacuum to half its volume, diluted with water (100 ml), and evaporated under vacuum as far as possible. Small portions of anhydrous sodium carbonate are added until a solid crystalline mass is obtained, when the amine oxide can be extracted by shaking with chloro
form (ca. 300 ml). After standing for several hours at room temperature the undissolved salt is filtered off, the filtrate dried with Na2S04 and the solvent evaporated. Pyridine-N-oxide (40-45 gm, 85-95%) distils at 138-140°/15 mm.
Purification of 4-methylpyridine via 4~methylpyridine-N-oxide {219). (a) 4-Methylpyridine (23 gm), glacial acetic acid (120 ml), and perhydrol (40 ml) are treated as described above. The residue left after removal of the chloroform can in this case be recrystallized from a large quantity of acetone, yielding white needles of 4-methylpyridine-N-oxide
(21.6 gm. 80%), m.p. 186-186.5°.
(b) The amine oxide (20 gm) is dissolved in glacial acetic acid (60 ml) and acetic anhydride (10 m l ) ; Raney nickel (1 gm) is added and the mixture hydrogenated with vigorous stirring at room temperature.
When the uptake of hydrogen has stopped the major portion of the catalyst is removed by filtration; the filtrate, to which cone. HCI (20 ml) has been added, is evaporated at 80° at the water pump to a sirupy consistency. Addition of dilute sodium hydroxide solution causes precipi
tation of nickel hydroxide. The free base is steam-distilled, separated with NaOH, and dried for several days with solid NaOH. The last traces of the base may be extracted from the aqueous layer with ether. Dis
tillation of the combined portions of the base yields pure 4-methyl
pyridine (39.2 gm, 84%), b.p. 144-145°/760 mm.
4-Nitropyridine-N-oxide (200). Pyridine-N-oxide (10 gm) is dissolved in a mixture of nitric acid (12 gm, d = 1.48) and cone, sulfuric acid
(30 ml) and warmed at 128-130° for 3 to 4 hr in an oil bath. On cooling, the reaction mixture is poured on to ice and neutralized by the addition of small portions of solid sodium carbonate. As a crystalline precipitate separates out, the mixture is filtered and the solid washed with ice water.
The filtrate is basifled with cooling and shaken with chloroform, yielding a further crop of the nitro compound which is isolated by evaporation of the solvent. Recrystallization of the combined products from acetone or water gives yellow crystals of 4-nitropyridine-N-oxide (10.6 gm, 72%), m.p. 159°.
4-Chloropyridine-N-oxide (200). 4-Nitropyridine-N-oxide (8 gm) is added portion wise to acetyl chloride (40 ml) placed in a flask equipped
INTRODUCTION OF SUBSTITUENTS INTO PYRIDINE RING 99 with a reflux condenser. After gentle initial warming, a vigorous reac
tion ensues. The mixture is heated at 50° for half an hour, during which time a crystalline mass is formed. This is carefully dissolved in water, basified with sodium hydroxide solution and extracted with chloroform.
The solution of the base is dried with sodium carbonate and the solvent evaporated; recrystallization of the residue from acetone yields white needles of 4-chloropyridine-N-oxide (4.0 gm, 55%) m.p. 169.5° with decomposition. Higher yields are reported in smaller-scale reactions
(200).
ACKNOWLEDGMENT
We thank the Deutsche Forschungsgemeinschaft for financial assistance, and Dr. J. Heider for his valuable help in editing the manuscript.
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