Tetraaryl-1, 4-bis(dicyano-methylene)cyclohexadienes and their dihydro derivatives

United States Patent Office

3,408,367

Patented Oct. 29, 1968

2

3,408,367

TETRAARYL - 1,4 - BIS(DICYANO - METHYLENE)CYCLOHEXADIENES AND THEIR DIHYDRO DERIVATIVES

Sam Andreades, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware

No Drawing. Filed July 12, 1965, Ser. No. 471,460

Claims. (CI. 260—396)

ABSTRACT OF THE DISCLOSURE

Tetraaryl - 1,4 - bis(dicyanomethylene)cyclohexadienes (I) and their dihydro derivatives (II) are claimed. The electrolytic reductive dimerization of a dicyanomethylenecyclopropene yield II which is converted into I by oxidation. The cyclohexadienes I are useful as polymerization inhibitors and the dihydro derivatives are useful as intermediates to the cyclohexadienes.

Description of the invention

This invention relates to a new class of chemicalcompounds and to their preparation, and more particularly, to tetraphenyl substituted and tetra(substituted phenyl) substituted l,4-bis(dicyanomethylene)cyclohexadienes, and to their dihydro derivatives which are also known as tetraphenyl substituted and tetra (substituted phenyl) substituted l,4-bis(dicyanomethyl)benzenes.

The novel compounds of this invention may be represented by the formulae

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wherein R_1; R₂, R₃ and R₄ may be alike or different, with each having 6-18 carbon atoms and being phenyl or phenyl substituted with 1-4 substituents selected from the group consisting of halogen (fluorine, chlorine, bromine or iodine), nitro, alkyl, alkoxy, each of which has 1-12 carbon atoms, and NR₅R₆ wherein R₅ and R₆ may be alike or different and are hydrogen or alkyl of 1-12 carbon atoms. Further, the substituents on the aforesaid phenyl group may be ortho, meta or para to the valence bond by which the phenyl group is attached to the carbocyclic diene or aromatic ring, with the provisos that iodo and alkyl substituents only may be meta or para, and only one substituent may be ortho. In the preferred embodiments of this invention R_b R₂, R₃ and R₄ are either phenyl or monoalkyl substituted phenyl, with said alkyl substituent having 1-6 carbon atoms.

The dihydro compounds of this invention, i.e., the 1,4bis(dicyanomethyl)benzenes, are prepared by an electrolytic reductive dimerization of a l,2-disubstituted-3-dicyanomethylenecyclopropene, said 1,2-substituents being as defined hereinabove for Rj to R₄. The cathode potential during the electrolysis must be sufficiently negative to reduce the methylenecyclopropene, but not so low as to cause decomposition of the solvent or electrolyte employed. Hence, although cathode potentials more negative than —2.0 volts are operable, most generally cathode potentials of —0.9 to —2.0 volts (S.C.E.) are employed. Preferably, to minimize undesirable side reactions the cathode potential is maintained in the range —1.5 to — 1.7 volts (S.C.E.). The cathodic material may be selected from a wide variety of substances, although platinum or mercury is preferred herein. The reductive dimerization generally is carried out under ambient conditions 5 of temperature and pressure, i.e., at room temperature of about 25° C. and at about atmospheric pressure. Temperatures of —50° to +100° C. are operable, however, and sub atmospheric as well as superatmospheric pressures can be employed. The electrolytic reduction can be car10 Tied out with or without a cell-dividing membrane to separate the cathode and anode compartments. In an undivided cell the anode may be comprised of any material which is compatible with the reaction conditions. Platinum, gold, copper or carbon usually is employed. Pref15 erably, the process is carried out in a divided cell with a mercury cathode, and an unfused “Vycor” salt bridge tube is used. The solvent utilized in the electrolysis should be stable to relatively large negative and positive potentials. Operable solvents include dimethylformamide, dimethyl20 acetamide, nitromethane and acetone. The preferred solvent is acetonitrile since it not only exhibits the requisite stability but it is an excellent solvent for the appropriate electrolyte and reactants employed herein. The electrolyte, likewise, should be chosen from materials which are 25 stable under reaction conditions. Examples of operable electrolytes include tetraethylammonium percholorate, tetrabutylammonium perchlorate and tetrapropylammonium-p-toluenesulfonate. The amount of current passed during the reductive electrolysis should be about one fara30 day per mole of reactant although excess or insufficient current may be employed. Cosolvents, although not necessary, may be utilized, and preferably aprotic solvents. The product of the electrolytic reaction is the dianion of a tetraphenyl- or tetra-(substituted phenyl)-!,4-bis(di35 cyanomethyl)benzene. Protonation of this material is achieved by acidification of the reaction mixture. Finally, the dihydro compounds of this invention are converted to the quinodimethan structure by oxidation, e.g., with chlorine as shown in Example HI, or electrochemically. In 40 the latter type process the oxidation is carried out at a cathode potential in the range —0.2 to +0.8 volt (S.C.E.). Voltages in excess of 0.8 volt, although opperable, provide undesirable side reactions. Moreover, voltages more negative than —0.2 volt, although operable, 45 provide a sufficiently reductive environment that the quinodimethan is reduced to its anion radical (discussed hereinbelow). The optimum and preferable oxidation cathode potential required to minimizze the aforesaid side reactions is in the range +0.25 to +0.35 volt.

The dicyanomethylenecyclopropenes utilized as reactants in the electrolytic reductive dimerization of the present invention are claimed in co-pending co-assigned application, Ser. No. 410,747, filed Nov. 12, 1964, now U.S. Patent 3,366,664.

The following examples are intended to illustrate but not limit the products and process of this invention.

EXAMPLE I l,4-bis(dicyanomethyl)-2,3,5,6-tetraphenylbenzene

NC H ON

<img file="US3408367A_D0002.tif" />

NC H CN

7Q A solution of 100 mg. of l,2-diphenyl-3-dicyanomethylenecyclopropene and 2.2 g. of tetraethylammonium perchlorate in 100 ml. of acetonitrile is electrolytically re3,408,367

4 duced using a stirred mercury pool cathode at a potential of —1.70 volts (S.C.E.). The cell is divided by using an anode contained in an unfused “Vycor” chamber. A drop in current flow is noted when the number of fardays passed is equivalent to the amount of starting ma- 5 terial (assuming a one electron per molecule reduction), indicating that the reduction proceeds with 100% current efficiency. The reaction is discontinued and the reaction mixture is evaporated to dryness, taken up in 100 ml. of distilled water and filtered to give 56 mg. of an intractable ₁₀solid. The filtrate, which is alkaline, is acidified with one drop of concentrated hydrochloric acid. The yellow precipitated solid which is removed by filtration and dried weighs 55.1 mg. and is identified as l,4-bis(dicyanomethyl)-2,3,5,6-tetraphenylbenzene, also known as tetra- 15 phenyldihydrotetracyanoquinodimethan or tetraphenyldihydro-TCNQ. The product contains some tetraphenyltetracyanoquinodimethan as shown by the nitrile bands appearing at 4.4.μ and 4.5/z in the infrared spectrum (KBr disc). An analytical sample is obtained by recrystalliza- 20 tion from acetonitrile.

Analysis.—Calcd. for C₃₆H₂₂N₄: C, 84.68; H, 4.34; N, 10.98; mol. wt. 510. Found: C, 83.90; H, 4.48; N, 10.98; mol. wt. 510, 10.74.

The molecular weight is determined by direct intro- 25 duction of the sample into the isatron of a mass spectrometer.

The 56 mg. of intractable solid recovered above is a mixture of tetraphenyltetracyanoquinodimethan and an unidentified material. Oxidation of the solid with chlorine 30 yields unstable orange crystals believed to be 1,4-bis(chlorodicyanomethyl)-2,3,5,6-tetraphenylbenzene which decomposes on attempted recrystallization.

EXAMPLE If

Example I is repeated using a platinum cathode. From this example is recovered 31 mg. of tetraphenyldihydrotetracyanoquinodimethan. Recrystallization of the product from 1.0 ml. of acetonitrile gives 7 mg. of white crystals, M.P. 329-332° C. (dec., sealed capillary). The infrared spectrum is identical to that of the product of Example I and exhibits both aromatic and aliphatic C-H stretching, based upon absorption bands at 3.23/x and 3.41/x, and weak CN, based upon absorption at 4.4μ; additional bands are noted at 6.25μ and 6.32μ.

EXAMPLE III

2,3,5,6-tetraphenyl-7,7,8,8-tetracyanoquinodimethan

<img file="US3408367A_D0003.tif" />

A solution of 25 mg. of tetraphenyldihydrotetracyanoquinodimethan in 10 ml. of acetonitrile containing one drop of triethylamine is oxidized by passing an excess of chlorine gas through the solution at room temperature. The immediate formation of a yellow color is observed. gj After 5-10 minutes the introduction of the chlorine gas is discontinued and the solution is evaporated to dryness to give 24.2 mg. of orange crystals of 2,3,5,6-tetraphenyl-7,7,8,8-tetracyanoquinodimethan. The infrared spectrum shows nitrile absorption at 4.5μ and a strong γθ absorption band at 6.73μ. Recrystallization of the product from 0.8 ml. of acetonitrile gives an analytical sample of 11 mg. of orange-red crystals, M.P. 318-320° C. (dec., sealed capillary). The infrared spectrum for this material (KBr pellet) shows neither aliphatic nor aro- 75 matic C-H stretching absorption; bands are noted at 6.3 μ,

6.7 μ, and 6.9μ.

Analysis.—Calcd. for C₃₆H₂₀N₄: C, 85.02; H, 3.96; N,

11.02; mol. wt. 508.54. Found: C, 84.74; H, 4.45; N,

11.26; mol. wt. 508 (mass spec., direct introduction).

The ultraviolet spectrum of this product in acetonitrile shows a N_max at 450 ιημ (¢=25,000), with a broad absorption at 600 ιημ. (¢=14,800). A shoulder running into end absorption is noted at 310 ιημ. For comparison, the ultraviolet spectrum of tetracyanoquinodimethan exhibits a X_max at 395 ιημ (¢=63,000). The ulartviolet spectrum illustrates the utility of this class of compounds as ultraviolet absorbers. Further, the ultraviolet spectrum exhibits a band at 890 ιημ, indicating the presence of a small quantity of tetraethylammonium tetraphenyltetracyanoquinodimethanide, an anion radical salt formed by the partial chemical oxidation of the dihydro compound. The electrolytic preparation of the anion radicals is described below.

EXAMPLE IV

Example I is repeated using in place of the 1,2-diphenyl-3-dicyanomethylenecyclopropene one of the substituted cyclopropenes shown below. In each instance the recovered dihydro product is oxidized to the tetracyanoquinodimethan according to the procedure of Example III. In each of the four experiments R₄ and R₂ are the same and appear in both the reactant and the products.

(a) (b) (c) (d)

REACTANT

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PRODUCTS

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Br

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As indicated above, the preceding examples are illustrative only. Shown below in Table I are additional substituted cyclopropenes which are operable in this invention. It is obvious that mixtures of substituted cyclopropenes likewise may be electrolytically reductively dimerized to give mixtures of products. In such a mixture of reactants either substituted cyclopropene may couple with itself or with another substituted cyclopropene present in the mixture. Moreover, even where a single disubstituted cyclopropene is employed in the dimerization reaction mixtures of products also will be obtained if the substituents on each aryl group are dissimilar. It is to be understood, therefore, that any of the R groups depicted in Table I, i.e., Rj, R₂, R₃ or R₄, may be alike or different. In most cases, because of preferable preparative feasibility Rj, R₂, R₃ and R₄ will be the same. Although only the dihydro products are shown in Table I, in each case the tetracyanoquinodimethan may be obtained by oxidation of the dihydro compound as illustrated in Example III.

3,408,367

TABLE I

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Ri, R2, R3 or R4 in Reactants and Products

CH₃

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<img file="US3408367A_D0009.tif" />

<img file="US3408367A_D0010.tif" />

The tetraphenyl- and tetra(substituted phenyl) tetracyanoquinodimethans of this invention may be converted to anion radical salts of the type reported by L. R. Melby et al. in the J. Am. Chem. Soc., 84, 3374-87 (1962). The anion radical salts of the instant invention are metal or quaternary ammonium salts of the anion radical.

<img file="US3408367A_D0011.tif" />

in wherein each of R_1; R₂, R₃ and R₄ is as defined hereinabove. The instant discovery provides the first known evidence of anion radicals which are derived from tetraphenyl- and tetra(substituted phenyl) tetracyanoquinodimethans. Previously, only unsubstituted 7,7,8,8-tetracyanoquinodimethan, or alkyl substituted derivatives thereof, were known to be convertible to anion radical salts. In addition to the aforesaid reference this prior art disclosure is made in U.S. Patent 3,115,506, issued Dec. 24, 1963 to Acker et al. Further, as to the prior art, Weitz and Schmidt, Berichte, 75, 1921 (1942), have reported the conversion of tetraphenylquinodimethan to a cation radi- 60 cal. Finally, it is worthy of note with regard to the prior art on quinodimethans generally that their syntheses differ markedly from the process of the instant invention. Thus, although di-n-propyldicyanomethylenecyclopropene is known [Kende et al., J. Am. Chem. Soc., 86, 3588 (1965)], it is believed never to have been employed in the preparation of prior art alkyl substituted quinodimethans. Moreover, the unsubstituted dicyanomethylenecyclopropene which would be required for the synthesis of unsubstituted quinodimethans, employing the present process, is believed to be unknown in the prior art, a fact which probably is related to its inherent instability and/ or difficulty of isolation.

The formation of the aforesaid anion radicals from the quinodimethans of the instant invention may be effected by a variety of means. As may be seen from Example III the chlorine oxidation of the dihydro compound provides some anion radical as an intermediate in the quinodi35 methan synthesis. Preferably the dihydro compound is oxidized completely to the quinodimethan and the latter is partially reduced to the anion radical, either by chemical or electrolytic means. Thus, treatment of the quinodimethan with an approximately stoichiometric quantity of 40 a metal iodide, such as potassium iodide, in an appropriate solvent, such as acetonitrile, yields the anion radical. Alternatively, the quinodimethan may be electrolytically reduced under the same environmental conditions previously described except that the cathode potential is maintained 43 within the range —0.6 to -f-0.4 volt (S.C.E.). Preferably the reduction is effected at a cathode potential in the range of —0.05 to—0.15 volt. The current is passed during the reduction until it diminishes to 1% of its initial value. As an example of the electrochemical generation 50 of the anion radical, Example I is repeated carrying out the reductive coupling reaction on 100 mg. of 1,2-diphenyl3-dicyanomethylenecyclopropene and 2.2 g. of tetraethylammonium perchlorate in 100 ml. of acetonitrile using a stirred mercury pool cathode at a potential of about —1.70 55 volts (S.C.E.) To effect an electrochemical oxidation the potential then is increased to about +0.3 volt and current passage is continued until another faraday per mole of starting material is consumed, after which the potential is maintained at about —0.1 volt until the current drops to 1% of its initial value at —0.1 volt. The resultant solution which contains the tetraethylammonium salt of the anion radical in a solvated stabilized form exhibits a strong 9line electron spin resonance spectrum indicative of the tetraphenyltetracyanoquinodimethan anion radical: 4 equi65 valent nitrogens (A_N=1.08±0.01); g=2.00265(±0.003);

A_c13 splitting is observed and attributed to four C atoms, three of which are equivalent (A_c13=9.50e). The ultraviolet spectrum of the acetonitrile solution exhibits a X_maxat 890μ, indicative of the anion radical.

The tetraphenyl and tetra (substituted phenyl) tetracyanoquinodimethans of this invention are unique in that their aromatic moieties are susceptible to electrophilic substitution reactions. Hence, although the prior art tetracyanoquinodimethans, when chlorinated in the presence of 75 iron, yield 1,6-addition products, namely, l,4-bis(chloro3,408,367 dicyanomethyl)benzenes, phenyl ring chlorination occurs with the compounds of the instant invention when chlorination is carried out under similar conditions.

As demonstrated by Example III, the tetra substituted tetracyanoquinodimethans of this invention are useful as ultraviolet absorbers. It has been noted that said compounds likewise exhibit utility as polymerization inhibitors. Thus, when a sample of purified acrylonitrile containing a trace quantity of azobisdiisobutyronitirle is refluxed for 5-15 minutes, rapid polymerization occurs; whereas, when purified acrylonitrile containing not only a trace quantity of the azobisdiisobutyronitrile but a trace quantity of e.g., tetraphenyltetracyanoquinodimethan is refluxed for 5—15 minutes, no polymerization is evident. Finally, the tetraphenyl and tetra (substituted phenyl) tetracyanoquinodimethans of this invention are useful in thermographic copying papers. The thermographic process for copying of printed or written records uses a copy sheet bearing a substantially colorless coating which darkens on heating. When such a sheet is placed in contact with a text-bearing master and the assembly is exposed to a source of radiant heat, the portions of the copy sheet in register with text areas, particularly where they contain carbon, are heated much more rapidly than portions in register with blank areas of the master. The copy sheet is darkened only in those areas in register with text areas of the master and a copy is obtained. The color change in the copy sheet must take place at a temperature which otherwise causes no change in the copy sheet or master. The tetracyanoquinodimethans of this invention are useful as an active ingredient of the cpating applied to the thermographic copy sheet.

The foregoing detailed description has been given for clearness of understanding only and no unnecessary limitations are to be understood therefrom. The invention is not limited to the exact details shown and described, for obvious modifications will occur to those skilled in the art.

The embodiments of the invention in which an exclusive