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Heterocylic Diazo Compounds
Heterocyclic Diazo Compounds J. M. TEDDER
Queem’e College, Dundee, Scotland
I. Introduction . 11. Methods of Preparation . 111. Heterocyclic Diazo Compounds . A. Diazopyrroles . B. Diazoindoles . C. 3-Diazocarbazole . D. Diazopyrazoles . E. Diazoindazole . F. Diazoimidazole . G. Diazopurines . H. Diazotriazoles and Diazotetrazole . IV. Applications of Heterocyclic Diazo Compolunds
.
l
.
3
. .
8 8
. . . . . . .
11 13 13 15 16 16 18 18
.
.
I. Introduction Stable five-membered heterocyclic diazo compounds form an interesting group of compounds. The first example, 3-diazoindazole, was reported by Bamberger in 1899.l To reinterpret Bamberger’s suggestion for its structure in modern terms is difficult since the structure proposed for indazole itself would be unacceptable. However Bamberger regarded the diazoindazole as an anhydride of the diazonium hydroxide and he suggested it contained a four-membered ring (which he called a triazolen ring). We can therefore depict his structure as in 1.
(1) ‘I
1
Indazoletriazolen,” Bamberger, 1899
E. Bamberger, Ber. Deut. Chena. Ges. 32, 1773 (18991. 1
2
J. M. TEDDER
[Sec. I.
I n their first paper on diazoindoles some 5 years later Angeli and D’Angelo again regard the diazo compound as the anhydride of the diazonium hydroxide.2 They offered two structures (2 and 3) one of which is very close to today’s picture.
( 31 (2) Structures for diazoindole, Angeli and D’Angelo, 1904
It is interesting that Taylor and Baker in their brilliant rewriting of Sidgwick’s “Organic Chemistry of Nitrogen ” should have written of 3-diazodi- and triphenylpyrroles : “the composition of these so-called diazopyrroles resembles that of the aliphatic diazocompounds but their structure is not fully known.’JSThe real interpretation of the structure and stability of heterocyclic diazo compounds was not forthcoming until 1959. I n 1953 Doering and De Puy reported the synthesis of diazocyclopentadiene (4) apparently unaware that Formula 4b is heterocyclic analogs had been known 50 years bef01-e.~ supposed to represent a combination of the four possible canonical
forms with a negative charge on unsubstituted carbon atoms. The cyclopentadienyl ring then contains six r electrons necessary to make a closed aromatic shell. I n fact, the infrared spectrum of diazocyclopentadiene, and of most heterocyclic diazo compounds, shows absorption at 2100 cm-l characteristic of aliphatic diazo compounds (cf. 2200 cm-l for aromatic and heterocyclic diazonium salts). Nevertheless, the exceptional stability of diazocyclopentadiene and the fact that it will undergo electrophilic substitution suggest that the 2 3 4
A. Angeli and A. D’Angelo, Atti Reale A c c d . Lincei 13, 268 (1904) T. W. J. Taylor and W. Baker, “The Organic Chemistry of Nitrogen,” p. 480. Oxford Univ. Press, London and New York, 1937. W. von E. Doering and C. H. De Puy, J . Am. Chem. SOC.75,5955 (1953).
Sec. 11.1
HETEROCYCLIC DIAZO COMPOUNDS
3
canonical forms represented by formula 4b do contribute to the over-all structure of the molecules. The heterocyclic diazo compounds such as diazopyrroles bear the same relation to diazocyclopentadiene that pyridine does to benzene. Just as pyridine is a base and forms a pyridinium ion, in acid solution, so diazopyrroles (5) form pyrrole diazonium salts ( 6 ) .
(5)
H (6)
We shall first discuss the preparation of heterocyclic diazo compounds as a class and shall then consider separately the individual characteristics of the various types, e.g., diazopyrroles, diazopyrazoles, etc.
11. Methods of Preparation 3-Diazoindazole (11) was first prepared by the diazotization of 3-aminoindazole (9) followed by the treatment of the resulting diazonium salt (10) with base,l, This remains the most important method of synthesis for the whole class of heterocyclic diazo compounds. (See p. 4.) Pyrrole and indole diazonium salts are acidic and lose a proton even in dilute acid to yield the diazo compound. Pyrazole diazonium salts, on the other hand, are only feebly acidic and the diazo compound is liberated only in quite strong alkali. In many cases the difficulty in obtaining the heterocyclic diazo compound has mainly centered round the preparation of the preceding amino compound. Nitration 5
E. Bamberger, Ann. Chem. 305, 289 (1899).
4
[Sec. 11.
J. M. TEDDER
(10)
(9)
(11)
of the heterocyclic nucleus is rarely a satisfactory reaction and for diazopyrroles and indoles nitrosation and reduction of the resulting nitroso compound is a suitable alternative.0 ,N
(14)
yo
(15)
Another route extemively used in the pyrazole series goes via a Curtius rearrangement ( 1 6 4 9 ) .' One of the factors which stimulated recent interest in heterocyclic diazo compounds was the application of the technique of direct 6
7
F. Angelico, Atti Reale Accad. Lincei 14, 167 (1905). H. Reimlinger, A. v. Overstraeten, and H. G. Viche, Chem. Ber. 94, 1036 (1961).
See. 11.1
HETEROCYCLIC DIAZO COMPOUNDS
5
introduction of the diazonium group. It has been shown that a wide variety of aromatic nuclei can be converted into diazonium salts in one experimental step. Phenol ethers and polyalkylbenzenes require nitrosyl sulfuric acid (either crystalline or prepared in situ from sodium nitrite and concentrated sulfuric acid),8 deactivated nuclei such as nitro compounds require in addition mercuric ions as catalyst^,^ but phenols l o and tertiary aromatic amines l1 are converted into diazonium salts in good yield by treatment with excess of nitrous acid. It is customary to emphasize to students the similarity in chemical reactions of phenols and pyrroles. 2,3,5-Triphenylpyrrole (20) was therefore treated with an acetone solution of nitrous acid and the corresponding diazo compound (21) was obtained l 2 (initially the yield was poor but in subsequent work reasonable yields were obtained).
+
HCl NaNOn aqueous acetone
The analogy between phenols and pyrroles is particularly apt in this case because the equilibrium between a phenol diazonium salt (22) and a diazooxide (23) is exactly analogous to that between a pyrrole diazonium salt (24) and a diazopyrrole (25). (See p. 6.) At first it was believed that the reaction could not be applied to pyrroles in which the 2-position was vacant, l 2 but subsequent work
* J. M. Tedder, J. Chem. SOC.p. 4003 (1957).
J. M. Tedder and G. Theaker, J. Chem. SOC.p. 4008 (1957). J. M. Tedder and G. Theaker, J. Chem. SOC.p. 2573 (1958). 11 H. P. Patel and J. M. Tedder, J. Chem. Soc. p. 4889 (1963). 12 J. M. Tedder and B. Webster, J. Chem. SOC. p. 3270 (1960). 13 H. P. Patel, Ph.D. Thesis, University of Sheffield, 1963. 9
10
6
[Sec. 11.
J. M. TEDDER
6
h'
N-
Ill
It
N+ I
Q+.-
0
OH
+ HX
(25)
(24)
showed that such compounds could be converted into 2-diazopyrr01es.l~This was the first synthesis of this class of compound and in view of the known instability of 2-aminopyrroles it may well prove to be the only route. Certainly the only reported reactions of unstable 2-aminopyrroles with nitrous acid led to decomp~sition.'~The technique of the direct introduction of the diazonium group has also been successfully applied to indoles and pyrazoles16*(see also footnotes 22 and 29). The reactions involve the initial formation of the nitroso compound which reacts further with the nitrous acid present to yield the diazonium nitrate. The reaction has been shown to involve exactly 3 moles of nitrous acid and the mechanism is probably as follows : HNOa
2HNOa
ArH+Ar-Nd+Ar--N=N Ar--N=O
NO __f
'
Ar--N-N=O II
+
NO,
NO __f
n
Ar-N-N=O
__f
Ar-N=N-0--N
+ ,0NO
I1
0
J. M. Tedder and B. Webster, J . Chem. SOC.p. 1638 (1962). H. Fischer, H. Guggemos, and A. Schafer, Ann. Chem. 540,45 (1939). 15* H. P. Patel, J . M. Tedder, and B. Webster, Chem. Ind. (London) p. 1163 14 15
(1961).
16 J.
M. Tedder and G. Theaker, Tetrahedron, 5, 288 (1969).
See. 11.1
HETEROCYCLIC DIAZO COMPOUNDS
7
With molecules as reactive as pyrroles, nitrate ions lead to nitration, and treatment of 2,5-diphenylpyrrole (26) with nitrous acid yielded (28) as well as the expected the 3-diazo-4-nitro-2,5-diphenylpyrrole 3-diazo-2,5-diphenylpyrrole (27).l 2
The nitration was avoided by employing a two-step process. The pyrrole (29) was converted into 3-nitroso-2,5-diphenylpyrroleby treatment with pentylnitrite and sodium ethoxide. The resulting nitroso compound was converted into the diazo compound (31) by treatment with gaseous nitric oxide. Exactly the same problem was encountered in the preparation of 2-diazo-3,5-diphenylpyrrole. l2
Treatment of 0- or p-hydroxybenzoic acids with buffered nitrous acid resulted in the decarboxylation of the acid and the introduction of a
8
J. M . TEDDER
[Sec. 111. A.
nitroso group or a diazonium group in the site originally occupied by the carboxyl gr0up.l’ The same reaction has been applied t o a pyrrole carboxylic acid (35) and although some diazo compound (36) was formed the reaction
was complicated by side reactions and the yield was too small for this to be considered as a practicable method of preparing diazo compounds. An obvious alternative route to heterocyclic diazo compounds would be that employed by Doering and De Puy for the synthesis of diazocyclopentadiene. However, attempts to do this have not been very successful.l8 111. Heterocyclic Diazo Compounds
A. DIAZOPYRROLES Diazopyrroles are stable yellow crystalline solids. They are lightsensitive and decompose on heating but they can be kept indefinitely in the cool and dark. They are weak bases, forming relatively stable diazonium salts with strong acids, which can be isolated as crystalline solids. Soluble in organic solvents such as acetone, ethanol, or chloroform, diazopyrroles are insoluble in water. There is a very marked difference in the stability of the 3-diazopyrroles and the 2-diazopyrroles. The former require no special precautions in handling but the latter, although stable once crystalline, decompose slowly in solution. The yellow diazopyrroles have an absorption maximum in the region 320-400 mp (extinction coefficient varying greatly, depending on the other substituents in the pyrrole ring) in their ultraviolet spectra and all show the characteristic diazo peak (2080-2180 cm-l) in their infrared spectra. I n general the infrared diazo absorption 17
18
J. M. Tedder and G. Theaker, J . Chem. SOC.p. 257 (1959). B. Webster, Ph.D. Thesis, University of Sheffield, 1961.
Sec. 111. A.]
9
HETEROCYCLIC DIAZO COMPOUNDS
occurs a t slightly shorter wavelengths for the 2-diazopyrroles than for the corresponding 3-diazo compounds (see Table I). TABLE I
SPECTRAL CHARACTERISTICSOF SOMETYPICAL DIAZOPYRROLES 12,14
Vmax
(cm-1) 2138 2172 2146
(cm-l)
(mp)
122 145 81 79.5-80
2095 2150 2062 2155
387 339 360 333
Vmx
~________
_ _
35 1
A,,
M.P. ("C)
Although a preliminary study of the effect of light on 3-diazo-2,4,5triphenylpyrrole (21) has been reported, the products of photolysis have not been identified.I8" Diazopyrroles will not, of course, couple with phenols or phenolate anions. Pyrrole diazonium salts are too weakly reactive to couple with phenols and in aqueous alkali necessary for phenolate anions to be present the insoluble diazopyrrole is precipitated. A dilute acid solution of 3-diazopyrrole and phloroglucinol showed no signs of coupling even after a month. However, heating a diazopyrrole with a phenol (either fusing them together, or better refluxing a solution of them in an organic solvent, e.g., chloroform) results in the formation of the corresponding azo dye. Presumably there is some proton transfer from the phenol t o the diazopyrrole, so that the phenolate anion and the pyrrole diazonium salt are formed together and then couple immediately.12 There is a report in the literature of the diazotization of ethyl 4-a~mino-3,5-dimethylpyrrole-2-carboxylate to yield a diazonium salt which couples with alkaline ,$-naphthol. l o , 2o However, the experiF. Angelico, Atti. Re& Accad. Lineei 17, 655 (1908). H. Fischer and A . Stern, Ann. Chern. 446, 240 (1926). 20 H. Fischer and K. Zeile, Ann. Chem. 483, 257 (1930).
laa 19
10
[Sec. 111. A.
J. M. TEDDER
mental section of this paper20 clearly describes the formation of a brown precipitate when the coupling was carried out in sodium bicarbonate solution. When the product was recrystallized from chloroform the solution turned red and the correct azo dye was obtained. Repetition of this work confirmed the experimental observations in every detail. However, the initial brown precipitate was in fact the diazopyrrole and /3-naphthol coprecipitated. Almost all the coupling takes place in the “recrystallization” procedure (36-39).
‘CHs
/=
The azo dyes are typical red crystalline compounds; those derived from 2-diazopyrroles (40) form lakes with the transition metals. l 4 Although attempts to make pyrrole diazonium salts couple intermolecularly have been unsuccessful, 3-diazo-2,4,5-triphenylpyrrole (21) on prolonged heating in dilute sulfuric acid undergoes internal coupling (21+41).21 That coupling occurred with the phenyl group in the 4-position rather than the 2-position is shown by the formation of the diketone (42) on oxidation with nitric acid. 21
F. Angelic0 and F. Monforte, Gazz. Chim. ItaE. 53, 795 (1923).
Sec. 111. B.]
HETEROCYCLIC DIAZO COMPOUNDS
8
11
HO
Possible structure of complex formed between transition metal cations and azo dyes derived from 2-diazopyrroles.
B. DIAZOINDOLES Diazoindoles are very similar in their properties to diazopyrroles (see Table 11). 3-Diazoindoles are stable crystalline compounds sensitive to light and forming stable diazonium salts with strong acids. 3Diazoindoles cannot be prepared by the direct reaction of the indole with nitrous acid, presumably because the nitroso compound (43) completely rearranges to the oximoimine form (44).22(See p. 12.) 3-Diazoindoles will couple with phenols under the same conditions as those described for diazopyrroles. The benzindoles were very much 22
H. P. Pate1 and J. M. Tedder, J . Chern. SOC. p. 4593 (1963).
12
[Sec. 111. B.
J. M. TEDDER
TABLE I1 SPECTROSCOPICPROPERTIES OF SOME TYPICAL DIAZOINDOLES 22 M.p. ("C)
(em-1)
(mp)
108 (116) 116
2120 2085
350 ( E = 11,600) 347 (c=9300)
138 167 143
2100 2100 2085
346 (c=4000) 367.9 ( c = 7700) 359 (c=6500)
Diazindazole 3-Diazo-2-phenylindole 3-Diazo-2-( p-met,hoxyphenyl)indole 3-Diazo-S-methoxy-2-( p-methoxypheny1)indole 3-Diazo-2-phenyl-6,7-benzindole 3-Diazo-2-phenyl-4,5-benzindole
vmax
L a x
less reactive and 3-diazo-2-phenyl-4,5-benzindole would not couple with /I-naphthol when the two were fused together at 130" for an hour.22 3-Diazo-2-phenylindole (45) when heated with aqueous acid does not undergo intramolecular coupling like triphenyldiazopyrrole (21) but instead undergoes an intermolecular coupling with the elimination of nitrogen 23
aqueous acid heat
(45) 23
8
HN
, ' X=N Q
H
(46)
V. Castellana and A. d'Angelo, Atti Reale Accad. Lincei 14, 145 (1905).
See. 111. D.]
HETEROCYCLIC DIAZO COMPOUNDS
13
c. 3-DIAZOCARBAZOLE I n 1901 Ruff and Stein obtained carbazole-3-diazonium chloride by the diazotization of 3-amino~arbazole.~~ They reported that treatment of a cold solution of the diazonium chloride (47) with concentrated sodium hydroxide yielded a red compound, which when analyzed gave inexplicable results. Subsequent work by Morgan and Read showed that a very unstable diazo compound (48) could be obtained.25 The diazo compound was far less stable than either the
It~ coupled instantly with diazopyrroles or the d i a z ~ i n d o l e s . ~ 8-naphthol, and treatment of an aqueous solution of the diazonium chloride with alkaline /?-naphthol led to immediate coupling and precipitation of the azo dye. I n this connection, in unreported experiments the present reviewer found that diazotization of 5-aminoindole yielded a diazonium salt but no stable diazo compound when the aqueous solution was treated with alkali. D. DIAZOPYRAZOLES I n 1896 Knorr and Stolz succeeded in diazotizing 4-amino-lphenyl-2,3-dimethylpyrazoleand so obtained the first diazonium salt with a pyrazole nucleus.2s Much later, Morgan and Reilly diazotized 4-amino-3,5-dimethylpyrazoleand studied the reactions of the resultant diazonium salt (49).27328 I n aqueous solution it coupled with 8-naphthol and the crystalline diazonium chloride was quite stable. However, these workers did not isolate the free diazo compound. The diazo compounds can be readily obtained by treating the 24 25
26 27 28
0. Ruff and V. Stein, Ber. Deut. Ch.em. Gea. 34, 1668 (1901). G. T. Morgan and H. N. Read, J . Chem. SOC.p. 2709 (1922). L. Knorr and F. Stolz, Ann. Chem. 293, 68 (1896). G. T. Morgan and J. Reilly, J . Chem. SOC.103, 808 (1913). G. T. Morgan and J. Reilly, J . Chem. SOC.p. 439 (1914).
14
[Sec. 111. D.
J. M. TEDDER
aqueous solutions of the diazonium salts with 29-31 The diazo compounds are in some cases soluble in water and are best obtained by extraction with organic solvents (see Table 111).
- v + fCH3
dilute
L
NMCH3 t
H (49) m.p. 165-168" (decomp.) YmaX 2235 cm-1
(50) m.p. 64" vmax 2185 cm-1
TABLE I11 SPECTROSCOPIC PROPERTIES OF SOME TYPICAL DIAZOPYRAZOLES 7, 29*31
Diazopyrazole 3-Diazopyrazole 100 (decornp.) 3-Diazo-5-benzoyl-4-phenylpyrazole Unstable 3-Diazo-5-benzyl-4-phenylpyrazole 4-Dia~0-3,5-dirnethylpyrazole 64 4-Diazo-3,ELdiphenylpyrazole 224 4-Diazo-5(3)-ethoxy-3(5)-methylpyrazole Oil Infusible below 300 4 -Diazo- 3-benzy 1 5 - phenylpyrazole 4 -Diazo- 3-benzoyl- 5-phenylp yrazole 148
-
2130 2140 2120 2195 2189 2120 2120 2190
270 272
-
354 -
The 3(5)-diazopyrazolesare noticeably less stable than the 4-diazo
compound^.^^ Both are light-sensitive and both couple with phenols
in organic solvents. The dyes so obtained form lakes with transition metal ions (cf. dyes from 2-diazopyrroles above, e.g., 40).13 The coupled product from 3-diazopyrazole and ,!?-naphtholis not in fact the simple azo dye but a condensation product derived from it.7 H. P. Pate1 and J. M. Tedder, J . Chem. Soc. p. 4589 (1963). D. G. Farnurn and P. Pates, Chem. I n d . (London) p. 659 (1960). 31 D. G. Farnum and P. Yates, J. Am. Chena. SOC. 84, 1399 (1962).
29
30
Sec. 111. E.]
15
HETEROCYCLIC DIAZO COMPOUNDS
(51)
(19)
4-Diazo-3-benzyl-5-phenylpyrazole (52) undergoes an intramolecular coupling between the diazo group and the methylene
I
\
(53)
(52)
Photolysis of 4-diazo-3-benzoyl-5-phenylpyrazole(54) and of 3-diazo-5-benzyl-4-phenylpyrazole (55) in benzene solution led to the evolution of nitrogen and the formation in both cases of 3-benzoyl4,5-diphenylpyra~ole.~l Photolysis of the 4-diazo derivative (54) in
C6H5 CsHsZ
Coca&
H N
hv
a
COCsHs
C6H5
benzene -N/ + N
(55)
aqueous acetone gave the 4-hydroxy compound, and photolysis in acetic acid gave the 4-acetoxy compound. However, similar photolysis of the 3(5)-diazo compound (55) in aqueous acetone resulted in the loss of nitrogen to yield 3-benzoyl-4-phenylpyrazole. 31
E. DIAZOINDAZOLE Bamberger's original preparation of diazoindazole has already been referred to and most of the work on this compound is Bamberger's.' The physical properties of the crystalline compound (m.p. 106", v,,,
16
J . M. TEDDER
[Sec. 111. G.
2120 cm-l) are similar to those of the diazopyrazoles. Like them it is soluble in most organic solvents (except petroleum) and is moderately soluble in water. It readily forms stable diazonium salts with strong acids. The diazo compound (as well as the diazonium salt) couples with /3-naphthol and N,N-dimethylaniline. The dye formed from ,%naphthol readily loses water when warmed in amyl alcohol, yielding a compound presumably having a structure analogous to that obtained when 3-diazopyrazole couples with /3-naphthol (see 51 above).
F. DIAZOIMIDAZOLE Diazoimidazoles do not appear to have received very extensive study. The only fully authenticated diazoimidazole appears t o be 5-diazoimidazole-4-carboxamide(57). 3 2 Few reactions have been reported for this compound beyond its cyclization to 2-azahypoxanthine (58).
(57) m.p. 210” (explosive) vmaX 2190 cm-1
G. DIAZOPURINES Diazotization of some 8-aminopurines has been reported to yield stable diazopurines. Gomberg reported that diazotization of 8-aminocaffeine yielded a very unstable “diazocaffeine.” 33 Later, Hans Fischer described crystalline 8-diazoxanthine and 8-diazotheophylline. 3 4 He proposed the same four-membered ring structure for the diazo group that Bamberger originally suggesbed in his structure of diazoindazole (1).These compounds have recently been reinvestigated and it has been suggested that they have the same diazo Y. F. Shealy, R. F. Struck, L. B. Holum, and J. A. Montgomery, J. Org. Chem. 26, 2396 (1961). 33 M. Gomberg, Am. Chem. J. 23, 61 (1901). 34 H. Fisoher, 2.Physiol. Chem. 60, 69 (1909). 32
Sec. 111. G.]
HETEROCYCLIC DIAZO COMPOUNDS
17
structure as the compounds discussed 36 However, their stability could also be due to contributions from a zwitterion structure (59 and 60) as with diazooxides.
Jones and Robins claimed to have excluded this possibility by preparing 8-amino-9-methylxanthine(61).35This compound did not
(61)
yield a stable diazo compound. However, this argument does not rule out the importance of zwitterionic structures ; for example, 8-diazotheophylline can be represented as a hybrid of structures (62 and 63).
There is strong spectroscopic evidence that 62 is the predominant canonical form. The triple-bond infrared absorption for these compounds is in the diazonium salt range (i.e.) diazotheophylline, v,,, = 2225 cm-' ; diazoxanthine, urnax= 2250-2400 cm-I). Their general properties and chemical behavior are much more akin to those 35 36
J. W. Jones and R . K. Robins, J . A m . Chem. SOC.82, 3773 (1960). G. A. Usbeck, J . W. Jones, arid R. K. Robins, J. Am. Chem. SOC.83, 1113 (1961).
18
[Sec. IV.
J. M. TEDDER
of diazooxides. Structure 63 will, of course, make some contribution t o the ground state of these molecules, but it seems better to regard them as diazooxides and they will for this reason receive no further discussion here.
H. DIAZOTRIAZOLES AND DIAZOTETRAZOLE 5-Aminotetrazole on treatment with nitrous acid in dilute aqueous solution yields a diazonium salt which undergoes the usual coupling reactions.37On treatment with alkali the solution becomes yellow and probably diazotetrazole is present. However, if the reaction is carried out in moderately concentrated solution, an explosive mixture is obtained! The reviewer has had dramatic, if harmless, experience of this system. After one originally mild explosion had shattered the reaction vessel, the solution, now scattered about the room and the experimenter, produced small detonations for the next 4 hours! The very formula of the compound explains its instability (64).
(64)
There is one report of a diazotriazole.32 Diazotization of 5-aminotriazole-4-carboxamide (65) with pentylnitrite in acetic acid yielded 5-diazotriazole-4-carboxamide(66). The one reaction reported for this compound was its cyclization to yield 2,8-diazahypoxanthine (67).
Ro
HaNHCoNHn xco ”\/ N-
XN+
/NHz
+
H
(65)
NkN/N
+
(66) m.p. 175” (explosive decomp.)
H
HN\#
(67)
vmaX 2210 cm-1
IV. Applications of Heterocyclic Diazo Compounds The light-sensitive properties of heterocyclic diazo compounds have been utilized for two different photoreproduction processes, The first 37
J. Thiele, Ann. Chew. 270, 46 (1892).
Sec. IV.]
HETEROCYCLIC DIAZO COMPOUNDS
19
of these is a lithographic process. 38 The heterocyclic diazo compound is deposited upoii a suitable metal or plastic surface ; the coated plate is exposed to light. The photolysis products and the original diazo compound have different solubilities and the image is developed by treatment with a suitable solvent. This image, which is described as highly ink- and grease-receptive, is used in offset printing. The second process is a so-called “dye-line” p r o c e ~ s40. ~The ~~ heterocyclic diazo compound and a phenol are coated together on paper together with various additional compounds t o prevent “fogging” of the image. The tracing to be copied is then placed on top and illuminated with ultraviolet light. The lines of the tracing cast a shadow and prevent the diazo compound being decomposed directly underneath them ; elsewhere the diazo compound is destroyed. The coated paper is then removed and heated, under which conditions the diazo compound couples with the phenol, producing an image of the original drawing. This process is intended to be an improvement on the conventional “ dye-line ” photocopying processes in which ordinary stabilized diazonium salts are used and in which coupling is induced by chemical methods (i.e., by rendering the paper alkaline either by washing it with alkali or by exposing it to ammonia vapor). British Patent 816,382; Chem. Abstr. 55, 188 (1961). British Patent 977,326; Chena. Abstr. 62, 6607 (1965). 40 British Patent 988.221 ; Chem. Abstr. 62, 15636 (1965).
38
39
Queem’e College, Dundee, Scotland
I. Introduction . 11. Methods of Preparation . 111. Heterocyclic Diazo Compounds . A. Diazopyrroles . B. Diazoindoles . C. 3-Diazocarbazole . D. Diazopyrazoles . E. Diazoindazole . F. Diazoimidazole . G. Diazopurines . H. Diazotriazoles and Diazotetrazole . IV. Applications of Heterocyclic Diazo Compolunds
.
l
.
3
. .
8 8
. . . . . . .
11 13 13 15 16 16 18 18
.
.
I. Introduction Stable five-membered heterocyclic diazo compounds form an interesting group of compounds. The first example, 3-diazoindazole, was reported by Bamberger in 1899.l To reinterpret Bamberger’s suggestion for its structure in modern terms is difficult since the structure proposed for indazole itself would be unacceptable. However Bamberger regarded the diazoindazole as an anhydride of the diazonium hydroxide and he suggested it contained a four-membered ring (which he called a triazolen ring). We can therefore depict his structure as in 1.
(1) ‘I
1
Indazoletriazolen,” Bamberger, 1899
E. Bamberger, Ber. Deut. Chena. Ges. 32, 1773 (18991. 1
2
J. M. TEDDER
[Sec. I.
I n their first paper on diazoindoles some 5 years later Angeli and D’Angelo again regard the diazo compound as the anhydride of the diazonium hydroxide.2 They offered two structures (2 and 3) one of which is very close to today’s picture.
( 31 (2) Structures for diazoindole, Angeli and D’Angelo, 1904
It is interesting that Taylor and Baker in their brilliant rewriting of Sidgwick’s “Organic Chemistry of Nitrogen ” should have written of 3-diazodi- and triphenylpyrroles : “the composition of these so-called diazopyrroles resembles that of the aliphatic diazocompounds but their structure is not fully known.’JSThe real interpretation of the structure and stability of heterocyclic diazo compounds was not forthcoming until 1959. I n 1953 Doering and De Puy reported the synthesis of diazocyclopentadiene (4) apparently unaware that Formula 4b is heterocyclic analogs had been known 50 years bef01-e.~ supposed to represent a combination of the four possible canonical
forms with a negative charge on unsubstituted carbon atoms. The cyclopentadienyl ring then contains six r electrons necessary to make a closed aromatic shell. I n fact, the infrared spectrum of diazocyclopentadiene, and of most heterocyclic diazo compounds, shows absorption at 2100 cm-l characteristic of aliphatic diazo compounds (cf. 2200 cm-l for aromatic and heterocyclic diazonium salts). Nevertheless, the exceptional stability of diazocyclopentadiene and the fact that it will undergo electrophilic substitution suggest that the 2 3 4
A. Angeli and A. D’Angelo, Atti Reale A c c d . Lincei 13, 268 (1904) T. W. J. Taylor and W. Baker, “The Organic Chemistry of Nitrogen,” p. 480. Oxford Univ. Press, London and New York, 1937. W. von E. Doering and C. H. De Puy, J . Am. Chem. SOC.75,5955 (1953).
Sec. 11.1
HETEROCYCLIC DIAZO COMPOUNDS
3
canonical forms represented by formula 4b do contribute to the over-all structure of the molecules. The heterocyclic diazo compounds such as diazopyrroles bear the same relation to diazocyclopentadiene that pyridine does to benzene. Just as pyridine is a base and forms a pyridinium ion, in acid solution, so diazopyrroles (5) form pyrrole diazonium salts ( 6 ) .
(5)
H (6)
We shall first discuss the preparation of heterocyclic diazo compounds as a class and shall then consider separately the individual characteristics of the various types, e.g., diazopyrroles, diazopyrazoles, etc.
11. Methods of Preparation 3-Diazoindazole (11) was first prepared by the diazotization of 3-aminoindazole (9) followed by the treatment of the resulting diazonium salt (10) with base,l, This remains the most important method of synthesis for the whole class of heterocyclic diazo compounds. (See p. 4.) Pyrrole and indole diazonium salts are acidic and lose a proton even in dilute acid to yield the diazo compound. Pyrazole diazonium salts, on the other hand, are only feebly acidic and the diazo compound is liberated only in quite strong alkali. In many cases the difficulty in obtaining the heterocyclic diazo compound has mainly centered round the preparation of the preceding amino compound. Nitration 5
E. Bamberger, Ann. Chem. 305, 289 (1899).
4
[Sec. 11.
J. M. TEDDER
(10)
(9)
(11)
of the heterocyclic nucleus is rarely a satisfactory reaction and for diazopyrroles and indoles nitrosation and reduction of the resulting nitroso compound is a suitable alternative.0 ,N
(14)
yo
(15)
Another route extemively used in the pyrazole series goes via a Curtius rearrangement ( 1 6 4 9 ) .' One of the factors which stimulated recent interest in heterocyclic diazo compounds was the application of the technique of direct 6
7
F. Angelico, Atti Reale Accad. Lincei 14, 167 (1905). H. Reimlinger, A. v. Overstraeten, and H. G. Viche, Chem. Ber. 94, 1036 (1961).
See. 11.1
HETEROCYCLIC DIAZO COMPOUNDS
5
introduction of the diazonium group. It has been shown that a wide variety of aromatic nuclei can be converted into diazonium salts in one experimental step. Phenol ethers and polyalkylbenzenes require nitrosyl sulfuric acid (either crystalline or prepared in situ from sodium nitrite and concentrated sulfuric acid),8 deactivated nuclei such as nitro compounds require in addition mercuric ions as catalyst^,^ but phenols l o and tertiary aromatic amines l1 are converted into diazonium salts in good yield by treatment with excess of nitrous acid. It is customary to emphasize to students the similarity in chemical reactions of phenols and pyrroles. 2,3,5-Triphenylpyrrole (20) was therefore treated with an acetone solution of nitrous acid and the corresponding diazo compound (21) was obtained l 2 (initially the yield was poor but in subsequent work reasonable yields were obtained).
+
HCl NaNOn aqueous acetone
The analogy between phenols and pyrroles is particularly apt in this case because the equilibrium between a phenol diazonium salt (22) and a diazooxide (23) is exactly analogous to that between a pyrrole diazonium salt (24) and a diazopyrrole (25). (See p. 6.) At first it was believed that the reaction could not be applied to pyrroles in which the 2-position was vacant, l 2 but subsequent work
* J. M. Tedder, J. Chem. SOC.p. 4003 (1957).
J. M. Tedder and G. Theaker, J. Chem. SOC.p. 4008 (1957). J. M. Tedder and G. Theaker, J. Chem. SOC.p. 2573 (1958). 11 H. P. Patel and J. M. Tedder, J. Chem. Soc. p. 4889 (1963). 12 J. M. Tedder and B. Webster, J. Chem. SOC. p. 3270 (1960). 13 H. P. Patel, Ph.D. Thesis, University of Sheffield, 1963. 9
10
6
[Sec. 11.
J. M. TEDDER
6
h'
N-
Ill
It
N+ I
Q+.-
0
OH
+ HX
(25)
(24)
showed that such compounds could be converted into 2-diazopyrr01es.l~This was the first synthesis of this class of compound and in view of the known instability of 2-aminopyrroles it may well prove to be the only route. Certainly the only reported reactions of unstable 2-aminopyrroles with nitrous acid led to decomp~sition.'~The technique of the direct introduction of the diazonium group has also been successfully applied to indoles and pyrazoles16*(see also footnotes 22 and 29). The reactions involve the initial formation of the nitroso compound which reacts further with the nitrous acid present to yield the diazonium nitrate. The reaction has been shown to involve exactly 3 moles of nitrous acid and the mechanism is probably as follows : HNOa
2HNOa
ArH+Ar-Nd+Ar--N=N Ar--N=O
NO __f
'
Ar--N-N=O II
+
NO,
NO __f
n
Ar-N-N=O
__f
Ar-N=N-0--N
+ ,0NO
I1
0
J. M. Tedder and B. Webster, J . Chem. SOC.p. 1638 (1962). H. Fischer, H. Guggemos, and A. Schafer, Ann. Chem. 540,45 (1939). 15* H. P. Patel, J . M. Tedder, and B. Webster, Chem. Ind. (London) p. 1163 14 15
(1961).
16 J.
M. Tedder and G. Theaker, Tetrahedron, 5, 288 (1969).
See. 11.1
HETEROCYCLIC DIAZO COMPOUNDS
7
With molecules as reactive as pyrroles, nitrate ions lead to nitration, and treatment of 2,5-diphenylpyrrole (26) with nitrous acid yielded (28) as well as the expected the 3-diazo-4-nitro-2,5-diphenylpyrrole 3-diazo-2,5-diphenylpyrrole (27).l 2
The nitration was avoided by employing a two-step process. The pyrrole (29) was converted into 3-nitroso-2,5-diphenylpyrroleby treatment with pentylnitrite and sodium ethoxide. The resulting nitroso compound was converted into the diazo compound (31) by treatment with gaseous nitric oxide. Exactly the same problem was encountered in the preparation of 2-diazo-3,5-diphenylpyrrole. l2
Treatment of 0- or p-hydroxybenzoic acids with buffered nitrous acid resulted in the decarboxylation of the acid and the introduction of a
8
J. M . TEDDER
[Sec. 111. A.
nitroso group or a diazonium group in the site originally occupied by the carboxyl gr0up.l’ The same reaction has been applied t o a pyrrole carboxylic acid (35) and although some diazo compound (36) was formed the reaction
was complicated by side reactions and the yield was too small for this to be considered as a practicable method of preparing diazo compounds. An obvious alternative route to heterocyclic diazo compounds would be that employed by Doering and De Puy for the synthesis of diazocyclopentadiene. However, attempts to do this have not been very successful.l8 111. Heterocyclic Diazo Compounds
A. DIAZOPYRROLES Diazopyrroles are stable yellow crystalline solids. They are lightsensitive and decompose on heating but they can be kept indefinitely in the cool and dark. They are weak bases, forming relatively stable diazonium salts with strong acids, which can be isolated as crystalline solids. Soluble in organic solvents such as acetone, ethanol, or chloroform, diazopyrroles are insoluble in water. There is a very marked difference in the stability of the 3-diazopyrroles and the 2-diazopyrroles. The former require no special precautions in handling but the latter, although stable once crystalline, decompose slowly in solution. The yellow diazopyrroles have an absorption maximum in the region 320-400 mp (extinction coefficient varying greatly, depending on the other substituents in the pyrrole ring) in their ultraviolet spectra and all show the characteristic diazo peak (2080-2180 cm-l) in their infrared spectra. I n general the infrared diazo absorption 17
18
J. M. Tedder and G. Theaker, J . Chem. SOC.p. 257 (1959). B. Webster, Ph.D. Thesis, University of Sheffield, 1961.
Sec. 111. A.]
9
HETEROCYCLIC DIAZO COMPOUNDS
occurs a t slightly shorter wavelengths for the 2-diazopyrroles than for the corresponding 3-diazo compounds (see Table I). TABLE I
SPECTRAL CHARACTERISTICSOF SOMETYPICAL DIAZOPYRROLES 12,14
Vmax
(cm-1) 2138 2172 2146
(cm-l)
(mp)
122 145 81 79.5-80
2095 2150 2062 2155
387 339 360 333
Vmx
~________
_ _
35 1
A,,
M.P. ("C)
Although a preliminary study of the effect of light on 3-diazo-2,4,5triphenylpyrrole (21) has been reported, the products of photolysis have not been identified.I8" Diazopyrroles will not, of course, couple with phenols or phenolate anions. Pyrrole diazonium salts are too weakly reactive to couple with phenols and in aqueous alkali necessary for phenolate anions to be present the insoluble diazopyrrole is precipitated. A dilute acid solution of 3-diazopyrrole and phloroglucinol showed no signs of coupling even after a month. However, heating a diazopyrrole with a phenol (either fusing them together, or better refluxing a solution of them in an organic solvent, e.g., chloroform) results in the formation of the corresponding azo dye. Presumably there is some proton transfer from the phenol t o the diazopyrrole, so that the phenolate anion and the pyrrole diazonium salt are formed together and then couple immediately.12 There is a report in the literature of the diazotization of ethyl 4-a~mino-3,5-dimethylpyrrole-2-carboxylate to yield a diazonium salt which couples with alkaline ,$-naphthol. l o , 2o However, the experiF. Angelico, Atti. Re& Accad. Lineei 17, 655 (1908). H. Fischer and A . Stern, Ann. Chern. 446, 240 (1926). 20 H. Fischer and K. Zeile, Ann. Chem. 483, 257 (1930).
laa 19
10
[Sec. 111. A.
J. M. TEDDER
mental section of this paper20 clearly describes the formation of a brown precipitate when the coupling was carried out in sodium bicarbonate solution. When the product was recrystallized from chloroform the solution turned red and the correct azo dye was obtained. Repetition of this work confirmed the experimental observations in every detail. However, the initial brown precipitate was in fact the diazopyrrole and /3-naphthol coprecipitated. Almost all the coupling takes place in the “recrystallization” procedure (36-39).
‘CHs
/=
The azo dyes are typical red crystalline compounds; those derived from 2-diazopyrroles (40) form lakes with the transition metals. l 4 Although attempts to make pyrrole diazonium salts couple intermolecularly have been unsuccessful, 3-diazo-2,4,5-triphenylpyrrole (21) on prolonged heating in dilute sulfuric acid undergoes internal coupling (21+41).21 That coupling occurred with the phenyl group in the 4-position rather than the 2-position is shown by the formation of the diketone (42) on oxidation with nitric acid. 21
F. Angelic0 and F. Monforte, Gazz. Chim. ItaE. 53, 795 (1923).
Sec. 111. B.]
HETEROCYCLIC DIAZO COMPOUNDS
8
11
HO
Possible structure of complex formed between transition metal cations and azo dyes derived from 2-diazopyrroles.
B. DIAZOINDOLES Diazoindoles are very similar in their properties to diazopyrroles (see Table 11). 3-Diazoindoles are stable crystalline compounds sensitive to light and forming stable diazonium salts with strong acids. 3Diazoindoles cannot be prepared by the direct reaction of the indole with nitrous acid, presumably because the nitroso compound (43) completely rearranges to the oximoimine form (44).22(See p. 12.) 3-Diazoindoles will couple with phenols under the same conditions as those described for diazopyrroles. The benzindoles were very much 22
H. P. Pate1 and J. M. Tedder, J . Chern. SOC. p. 4593 (1963).
12
[Sec. 111. B.
J. M. TEDDER
TABLE I1 SPECTROSCOPICPROPERTIES OF SOME TYPICAL DIAZOINDOLES 22 M.p. ("C)
(em-1)
(mp)
108 (116) 116
2120 2085
350 ( E = 11,600) 347 (c=9300)
138 167 143
2100 2100 2085
346 (c=4000) 367.9 ( c = 7700) 359 (c=6500)
Diazindazole 3-Diazo-2-phenylindole 3-Diazo-2-( p-met,hoxyphenyl)indole 3-Diazo-S-methoxy-2-( p-methoxypheny1)indole 3-Diazo-2-phenyl-6,7-benzindole 3-Diazo-2-phenyl-4,5-benzindole
vmax
L a x
less reactive and 3-diazo-2-phenyl-4,5-benzindole would not couple with /I-naphthol when the two were fused together at 130" for an hour.22 3-Diazo-2-phenylindole (45) when heated with aqueous acid does not undergo intramolecular coupling like triphenyldiazopyrrole (21) but instead undergoes an intermolecular coupling with the elimination of nitrogen 23
aqueous acid heat
(45) 23
8
HN
, ' X=N Q
H
(46)
V. Castellana and A. d'Angelo, Atti Reale Accad. Lincei 14, 145 (1905).
See. 111. D.]
HETEROCYCLIC DIAZO COMPOUNDS
13
c. 3-DIAZOCARBAZOLE I n 1901 Ruff and Stein obtained carbazole-3-diazonium chloride by the diazotization of 3-amino~arbazole.~~ They reported that treatment of a cold solution of the diazonium chloride (47) with concentrated sodium hydroxide yielded a red compound, which when analyzed gave inexplicable results. Subsequent work by Morgan and Read showed that a very unstable diazo compound (48) could be obtained.25 The diazo compound was far less stable than either the
It~ coupled instantly with diazopyrroles or the d i a z ~ i n d o l e s . ~ 8-naphthol, and treatment of an aqueous solution of the diazonium chloride with alkaline /?-naphthol led to immediate coupling and precipitation of the azo dye. I n this connection, in unreported experiments the present reviewer found that diazotization of 5-aminoindole yielded a diazonium salt but no stable diazo compound when the aqueous solution was treated with alkali. D. DIAZOPYRAZOLES I n 1896 Knorr and Stolz succeeded in diazotizing 4-amino-lphenyl-2,3-dimethylpyrazoleand so obtained the first diazonium salt with a pyrazole nucleus.2s Much later, Morgan and Reilly diazotized 4-amino-3,5-dimethylpyrazoleand studied the reactions of the resultant diazonium salt (49).27328 I n aqueous solution it coupled with 8-naphthol and the crystalline diazonium chloride was quite stable. However, these workers did not isolate the free diazo compound. The diazo compounds can be readily obtained by treating the 24 25
26 27 28
0. Ruff and V. Stein, Ber. Deut. Ch.em. Gea. 34, 1668 (1901). G. T. Morgan and H. N. Read, J . Chem. SOC.p. 2709 (1922). L. Knorr and F. Stolz, Ann. Chem. 293, 68 (1896). G. T. Morgan and J. Reilly, J . Chem. SOC.103, 808 (1913). G. T. Morgan and J. Reilly, J . Chem. SOC.p. 439 (1914).
14
[Sec. 111. D.
J. M. TEDDER
aqueous solutions of the diazonium salts with 29-31 The diazo compounds are in some cases soluble in water and are best obtained by extraction with organic solvents (see Table 111).
- v + fCH3
dilute
L
NMCH3 t
H (49) m.p. 165-168" (decomp.) YmaX 2235 cm-1
(50) m.p. 64" vmax 2185 cm-1
TABLE I11 SPECTROSCOPIC PROPERTIES OF SOME TYPICAL DIAZOPYRAZOLES 7, 29*31
Diazopyrazole 3-Diazopyrazole 100 (decornp.) 3-Diazo-5-benzoyl-4-phenylpyrazole Unstable 3-Diazo-5-benzyl-4-phenylpyrazole 4-Dia~0-3,5-dirnethylpyrazole 64 4-Diazo-3,ELdiphenylpyrazole 224 4-Diazo-5(3)-ethoxy-3(5)-methylpyrazole Oil Infusible below 300 4 -Diazo- 3-benzy 1 5 - phenylpyrazole 4 -Diazo- 3-benzoyl- 5-phenylp yrazole 148
-
2130 2140 2120 2195 2189 2120 2120 2190
270 272
-
354 -
The 3(5)-diazopyrazolesare noticeably less stable than the 4-diazo
compound^.^^ Both are light-sensitive and both couple with phenols
in organic solvents. The dyes so obtained form lakes with transition metal ions (cf. dyes from 2-diazopyrroles above, e.g., 40).13 The coupled product from 3-diazopyrazole and ,!?-naphtholis not in fact the simple azo dye but a condensation product derived from it.7 H. P. Pate1 and J. M. Tedder, J . Chem. Soc. p. 4589 (1963). D. G. Farnurn and P. Pates, Chem. I n d . (London) p. 659 (1960). 31 D. G. Farnum and P. Yates, J. Am. Chena. SOC. 84, 1399 (1962).
29
30
Sec. 111. E.]
15
HETEROCYCLIC DIAZO COMPOUNDS
(51)
(19)
4-Diazo-3-benzyl-5-phenylpyrazole (52) undergoes an intramolecular coupling between the diazo group and the methylene
I
\
(53)
(52)
Photolysis of 4-diazo-3-benzoyl-5-phenylpyrazole(54) and of 3-diazo-5-benzyl-4-phenylpyrazole (55) in benzene solution led to the evolution of nitrogen and the formation in both cases of 3-benzoyl4,5-diphenylpyra~ole.~l Photolysis of the 4-diazo derivative (54) in
C6H5 CsHsZ
Coca&
H N
hv
a
COCsHs
C6H5
benzene -N/ + N
(55)
aqueous acetone gave the 4-hydroxy compound, and photolysis in acetic acid gave the 4-acetoxy compound. However, similar photolysis of the 3(5)-diazo compound (55) in aqueous acetone resulted in the loss of nitrogen to yield 3-benzoyl-4-phenylpyrazole. 31
E. DIAZOINDAZOLE Bamberger's original preparation of diazoindazole has already been referred to and most of the work on this compound is Bamberger's.' The physical properties of the crystalline compound (m.p. 106", v,,,
16
J . M. TEDDER
[Sec. 111. G.
2120 cm-l) are similar to those of the diazopyrazoles. Like them it is soluble in most organic solvents (except petroleum) and is moderately soluble in water. It readily forms stable diazonium salts with strong acids. The diazo compound (as well as the diazonium salt) couples with /3-naphthol and N,N-dimethylaniline. The dye formed from ,%naphthol readily loses water when warmed in amyl alcohol, yielding a compound presumably having a structure analogous to that obtained when 3-diazopyrazole couples with /3-naphthol (see 51 above).
F. DIAZOIMIDAZOLE Diazoimidazoles do not appear to have received very extensive study. The only fully authenticated diazoimidazole appears t o be 5-diazoimidazole-4-carboxamide(57). 3 2 Few reactions have been reported for this compound beyond its cyclization to 2-azahypoxanthine (58).
(57) m.p. 210” (explosive) vmaX 2190 cm-1
G. DIAZOPURINES Diazotization of some 8-aminopurines has been reported to yield stable diazopurines. Gomberg reported that diazotization of 8-aminocaffeine yielded a very unstable “diazocaffeine.” 33 Later, Hans Fischer described crystalline 8-diazoxanthine and 8-diazotheophylline. 3 4 He proposed the same four-membered ring structure for the diazo group that Bamberger originally suggesbed in his structure of diazoindazole (1).These compounds have recently been reinvestigated and it has been suggested that they have the same diazo Y. F. Shealy, R. F. Struck, L. B. Holum, and J. A. Montgomery, J. Org. Chem. 26, 2396 (1961). 33 M. Gomberg, Am. Chem. J. 23, 61 (1901). 34 H. Fisoher, 2.Physiol. Chem. 60, 69 (1909). 32
Sec. 111. G.]
HETEROCYCLIC DIAZO COMPOUNDS
17
structure as the compounds discussed 36 However, their stability could also be due to contributions from a zwitterion structure (59 and 60) as with diazooxides.
Jones and Robins claimed to have excluded this possibility by preparing 8-amino-9-methylxanthine(61).35This compound did not
(61)
yield a stable diazo compound. However, this argument does not rule out the importance of zwitterionic structures ; for example, 8-diazotheophylline can be represented as a hybrid of structures (62 and 63).
There is strong spectroscopic evidence that 62 is the predominant canonical form. The triple-bond infrared absorption for these compounds is in the diazonium salt range (i.e.) diazotheophylline, v,,, = 2225 cm-' ; diazoxanthine, urnax= 2250-2400 cm-I). Their general properties and chemical behavior are much more akin to those 35 36
J. W. Jones and R . K. Robins, J . A m . Chem. SOC.82, 3773 (1960). G. A. Usbeck, J . W. Jones, arid R. K. Robins, J. Am. Chem. SOC.83, 1113 (1961).
18
[Sec. IV.
J. M. TEDDER
of diazooxides. Structure 63 will, of course, make some contribution t o the ground state of these molecules, but it seems better to regard them as diazooxides and they will for this reason receive no further discussion here.
H. DIAZOTRIAZOLES AND DIAZOTETRAZOLE 5-Aminotetrazole on treatment with nitrous acid in dilute aqueous solution yields a diazonium salt which undergoes the usual coupling reactions.37On treatment with alkali the solution becomes yellow and probably diazotetrazole is present. However, if the reaction is carried out in moderately concentrated solution, an explosive mixture is obtained! The reviewer has had dramatic, if harmless, experience of this system. After one originally mild explosion had shattered the reaction vessel, the solution, now scattered about the room and the experimenter, produced small detonations for the next 4 hours! The very formula of the compound explains its instability (64).
(64)
There is one report of a diazotriazole.32 Diazotization of 5-aminotriazole-4-carboxamide (65) with pentylnitrite in acetic acid yielded 5-diazotriazole-4-carboxamide(66). The one reaction reported for this compound was its cyclization to yield 2,8-diazahypoxanthine (67).
Ro
HaNHCoNHn xco ”\/ N-
XN+
/NHz
+
H
(65)
NkN/N
+
(66) m.p. 175” (explosive decomp.)
H
HN\#
(67)
vmaX 2210 cm-1
IV. Applications of Heterocyclic Diazo Compounds The light-sensitive properties of heterocyclic diazo compounds have been utilized for two different photoreproduction processes, The first 37
J. Thiele, Ann. Chew. 270, 46 (1892).
Sec. IV.]
HETEROCYCLIC DIAZO COMPOUNDS
19
of these is a lithographic process. 38 The heterocyclic diazo compound is deposited upoii a suitable metal or plastic surface ; the coated plate is exposed to light. The photolysis products and the original diazo compound have different solubilities and the image is developed by treatment with a suitable solvent. This image, which is described as highly ink- and grease-receptive, is used in offset printing. The second process is a so-called “dye-line” p r o c e ~ s40. ~The ~~ heterocyclic diazo compound and a phenol are coated together on paper together with various additional compounds t o prevent “fogging” of the image. The tracing to be copied is then placed on top and illuminated with ultraviolet light. The lines of the tracing cast a shadow and prevent the diazo compound being decomposed directly underneath them ; elsewhere the diazo compound is destroyed. The coated paper is then removed and heated, under which conditions the diazo compound couples with the phenol, producing an image of the original drawing. This process is intended to be an improvement on the conventional “ dye-line ” photocopying processes in which ordinary stabilized diazonium salts are used and in which coupling is induced by chemical methods (i.e., by rendering the paper alkaline either by washing it with alkali or by exposing it to ammonia vapor). British Patent 816,382; Chem. Abstr. 55, 188 (1961). British Patent 977,326; Chena. Abstr. 62, 6607 (1965). 40 British Patent 988.221 ; Chem. Abstr. 62, 15636 (1965).
38
39