Imine-Based Catechols and o-Benzoquinones: Synthesis, Structure, and Features of Redox Behavior.

Novel sterically hindered catechols of the type 3-(RN=CH)-4,6-DBCatH2 with iminoalkyl or iminoaryl groups in the third position of the aromatic ring have been synthesized and characterized in detail. The o-benzoquinones 3-(RN=CH)-4,6-DBBQ have been synthesized by the oxidation of the corresponding catechols. The oxidation of methylimino-substituted catechol with K3[Fe(CN)6] in alkaline medium leads to the formation of two products: o-quinone and diene-dione, the product of the water addition to the corresponding o-quinone. Some o-benzoquinones react with water or methanol to yield products of water or methanol addition. A prototropic tautomerism is characteristic of catecholaldimines: a quinomethide form is observed in the case of aliphatic amine derivatives, while aryl-substituted catecholaldimines can exist both in the catechol and quinomethide forms in the crystalline state. The formation of dimeric structures motifs is observed in crystals. The electrochemical oxidation of imino-based catechols proceeds via two one-electron processes; the second wave is quasi-reversible, which is unusual for catechols.

Introduction

Redox-active organic compounds containing several stable redox forms are on class="Chemical">f particular interest to researchers in obtaining complexes of transition and nontransition metals. They are characterized by redox isomerism,[1,2] the ability to reversibly attach small molecules,[3−6] and their activity in redox catalysts.[7,8] These compounds form an important class and may exist in various oxidation states in complexes, such as catecholate , o-semiquinone, and o-quinone forms. Sterically hindered catechols occupy a very important place in the area of redox-active ligands: the presence of sterical hindrances (two tertiary carbons at the aromatic ring of catechol) increases significantly the kinetic stability of the oxidized forms; as a result, 3,5-di-tert-butylcatechols and the corresponding o-benzoquinones are the main objects of research in this field. Catechol derivatives without tert-butyl groups, especially catecholamides and catecholaldimines, are well known as ligands for supramolecules (helicate structures and molecular tetrahedrons)[9−16] and bimetallic catalysts for the Mannich and Henry reactions;[17−22] however, these ligands do not have stable oxidized forms.[23] Salicylaldehyde and its derivatives attract attention due to the accumulated data on their anti-infection properties.[24−26] The structures of hydroxylated salicylaldehydes contain pharmacophores of the natural compound gossypol,[27−30] which is known for its high antiviral activity. Catechol 1 (4,6-di-tert-butyl-1,2-dihydroxybenzaldehyde) was synthesized from 3,5-di-tert-butylcatechol.[31] Variation of the substituents in the sixth position of the spatially shielded catechol/o-quinone allows us to expand the redox properties of these compounds.[32] Compound 1 is a convenient structural block for the synthesis of new sterically hindered catecholaldimines.[33−35]o-Hydroxy Schiff bases tautomerize via an intramolecular proton transfer and they have potential applications in higher energy radiation detectors and memory storage devices.[36]o-Hydroxy Schiff base compounds may exist in two tautomeric forms, enol and ketone, both in solution and in the solid state. Keto/enol tautomerism affects the photophysical and photochemical properties of these compounds.[37] An early imine derivative has already been characterized.[38] We have also synthesized and characterized some derivatives of 3,5-di-tert-butylcatechol, o-quinones, and their antimony(V) catecholate complexes.[39−41] In the present article, we report the structure and electrochemical properties of a number of new alkyl- and aryl-functionalized catecholaldimines and the corresponding 3-iminomethyl-o-benzoquinones.

Results and Discussion

Synthesis and Characterization

The reaction of n class="Chemical">4,6-di-tert-butyl-1,2-dihydroxybenzaldehyde with different primary amines in methanol leads to the formation of catechols 2–11 (Scheme ) with high yields. Reactions of catecholaldehyde 1 with aliphatic amines proceeded more readily and produced higher yields when compared to aromatic amines (Scheme ). Heating these reactions is only necessary to accelerate the process and has little effect on the yield. The reaction proceeds without the use of external acid catalysts. Presumably, catechol 1 and the resulting 3-iminomethylcatechols 2–11 can act as acid catalysts.

Scheme 1

Synthesis of Catecholaldimines 2–11

Catechols 2–11 can be crystallized n class="Chemical">from a methanol solution. Compounds 2–11 are characterized by prototropic tautomerism. In the case of the aliphatic amine derivatives, a quinometide form B is observed, while catecholaldimines based on aromatic amines can be observed in the crystal state both in the catechol form A and quinomethide form B (Scheme ).

Scheme 2

Keto/Enol Tautomerism

The 1H n class="Chemical">NMR spectra of catechols 2–11 have sets of similar signals from the methylene–catechol fragment: two singlets from protons of tert-butyl groups, singlets caused by the proton of the catechol aromatic ring (6.5–6.6 ppm for aliphatic derivatives 2,3 and 6.8–6.9 ppm for aromatic derivatives 4–11), and protons of the CH=N group (8.8–8.9 ppm for 2,3 and 9.3–9.4 ppm for 4–11).

Proton NMR spectra on class="Chemical">f some catechols 2–11 display hydroxyl proton signals downfield between 13.0 and 16.2 ppm, which indicates a strong intramolecular hydrogen bond (O–H···N). For other compounds, this signal is not observed since it is highly broadened. The signals from a second hydroxyl group proton appear as broad singlets in the range of 6.9–7.5 ppm for catechols 2,3 with aliphatic groups and 6.2–6.6 ppm for catechols 4–11 with aromatic groups.

In the 13C{n class="Chemical">1H} NMR spectrum, N–Cam signals are observed at 40.8 and 55.0 ppm for 2 and 3, and 153.3–158.7 ppm for compounds 4–11. The signal of Car–C=N is shifted to a high-field region as compared to the signal of Car–CH=O in the initial catecholaldehyde: 159.6–166.1 ppm for 2–11 vs 195.6 ppm for 1.

Catechol 2 has an intense yellow color in solid, which indicates the presence on class="Chemical">f absorption bands in the visible region. The ultraviolet–visible (UV–vis) spectra are solvent dependent (Figure ). Mainly one intense absorption band is observed in n-hexane (with maximum at λ1 = 281 nm), and two absorption bands are observed in such polar solvents as CH2Cl2, CHCl3, MeOH, and DMSO (with maxima at λ1 = 320–330 nm and λ2 = 430–440 nm). These bands correspond to ππ* and nπ*–electron transitions. The UV–vis spectrum of 2 in CCl4 is a superposition of the spectra in hexane and CH2Cl2: in CCl4, peaks characteristic of both the catecholic form (λ1 = 281 nm) and quinomethide (λ1 = 320–330 nm and λ2 = 430–440 nm) are observed.

Figure 1

UV–vis spectra of 2 in CCl4, CH2Cl2, MeOH, n-hexane, DMSO, and CHCl3, c =1 × 10–4 mol/L.

UV–vis spectra of 2 in n class="Chemical">CCl4, CH2Cl2, MeOH, n-hexane, DMSO, and CHCl3, c =1 × 10–4 mol/L.

In UV–vis spectra of 2 in a mixture on class="Chemical">f “n-hexane–CH2Cl2”, a decrease in the intensity of the absorption band at 281 nm and an increase of absorptions at 321 and 435 nm were observed with the increase of the percentage of CH2Cl2 (Figure ). Three isobestic points are observed at 300, 338, and 375 nm. These data indicate the existence of an equilibrium between the catechol form of compound 2A and quinomethide 2B (Scheme ). Thus, the equilibrium and spectral properties of compound 2 are significantly affected by the nature of the solvent. The change in spectral properties follows relative solvent polarity, in agreement with Meyer’s rule: an increased solvent polarity favors the keto form B.

Figure 2

UV–vis spectra of 2 in the mixture of “n-hexane–CH2Cl2”.

UV–vis spectra of 2 in the mixture on class="Chemical">f “n-hexane–CH2Cl2”.

The UV–vis spectra of n class="Chemical">catechol 4 are weakly dependent on the solvent; absorption bands are observed in the range 327–330 nm (Figure ). Compound 4 is also present in two tautomeric forms, but the small changes in the absorption bands for the two forms do not allow us to clearly visualize the differences as for compound 2 in Figure .

Figure 3

UV–vis spectra of 4 in CCl4, CH2Cl2, n-hexane, and MeOH, c = 2 × 10–5 mol/L.

UV–vis spectra of 4 in n class="Chemical">CCl4, CH2Cl2, n-hexane, and MeOH, c = 2 × 10–5 mol/L.

Single crystals of n class="Chemical">catecholaldimines 2–11 were grown from methanol solutions. The molecular structures of these compounds in the crystal state have been established using the single-crystal X-ray crystallography (see Crystal Structures). The selected bond lengths for all catecholaldimines are listed in Table and the molecular structures are shown in Figure .

Table 1

Selected Bond Lengths (Å) of 2–11

bond	2	3	4	5	6
C(1)–O(1)	1.2977(15)	1.3000(11)	1.3423(14)	1.3388(12)	1.3257(13)
C(2)–O(2)	1.3688(15)	1.3658(11)	1.3747(14)	1.3639(12)	1.3629(12)
C(1)–C(2)	1.4195(18)	1.4286(13)	1.4000(17)	1.4012(14)	1.4019(14)
O(1)–H(1)	1.62(2)	1.682(16)	1.019(16)	1.024(15)	1.25(3)
N(1)–H(1)	1.01(2)	0.939(16)	1.493(16)	1.479(15)	1.29(3)
O(2)–H(2)	0.87(2)	0.884(16)	0.868(17)	0.853(19)	0.881(18)
H(2)–O(1)	1.95(2)	2.116(15)	2.040(16)	2.121(18)	2.165(17)
H(2)–O(1A)	-	2.044(16)	2.316(17)	2.039(19)	1.963(19)

Figure 4

Molecular structures of catechols 2–11 (a–j) from the single-crystal X-ray diffraction analysis. Probability ellipsoids for non-hydrogen atoms are drawn at the 30% level. The hydrogen atoms are omitted for clarity (except for H atoms in the imino group and O(H)···O(H)···N-fragments).

Molecular structures of n class="Chemical">catechols 2–11 (a–j) from the single-crystal X-ray diffraction analysis. Probability ellipsoids for non-hydrogen atoms are drawn at the 30% level. The hydrogen atoms are omitted for clarity (except for H atoms in the imino group and O(H)···O(H)···N-fragments).

The oxidation of n class="Chemical">catechols 4, 5, 8–11 with K3[Fe(CN)6] in alkaline medium leads to the formation of the corresponding o-benzoquinones 12–17 of the type 3-(RN=CH)-4,6-DBBQ (X = Ph, p-Tol, p-F-C6H4, p-Cl-C6H4, p-Br-C6H4, p-I-C6H4) (Scheme ). Compounds 12–17 were isolated from diethyl ether as amorphous powders.

Scheme 3

Synthesis of o-Benzoquinones 12–17

Under oxidation of 4, 5, 8–11 with the subsequent n class="Chemical">formation of the corresponding o-benzoquinones 12–17, the singlets of the tert-butyl groups in 1H NMR spectra shift to an upfield region of 1.27–1.28 and 1.33–1.34 ppm in comparison with those for the initial catechols (1.44–1.49 and 1.50–1.54 ppm). The proton signals from the C6H1 moiety in o-quinones are shifted to a downfield region: they are observed in the range of 7.10–7.15 ppm against 6.8–6.9 ppm in the corresponding catechols. This indicates a change in conjugation of the ring system. The signals from hydroxyl group protons are absent.

The 13C signals n class="Chemical">for the two carbonyl carbons of o-benzoquinones 12–17 appear at 178.4–178.6 and 181.2–181.5 ppm, which is consistent with the presence of an o-quinone structure. In the IR spectra of o-quinones, the stretching vibrations of carbonyl groups are observed at 1620–1690 cm–1.

It should be noted that the oxidation of n class="Chemical">catechol 2 with K3[Fe(CN)6] in alkaline medium leads to the formation of two products—o-quinone 18 and diene–dione 19—the product of water addition to o-benzoquinone 18 (Scheme ).

Scheme 4

Nucleophilic Addition of Water and Methanol to o-Benzoquinones

The accumulation of product 19 occurs under the reaction conditions (in the n class="Chemical">Et2O–H2O/KOH system), and in a solution of CDCl3 with traces of water. After 24 h, no NMR signals corresponding to 18 were observed.

Compound 18 cannot be isolated, while aromatic amine-based o-benzoquinones 12–17 were successn class="Chemical">fully isolated. It was also found that in a solution of CDCl3, compound 12 is able to add water at room temperature to form 20; however, this reaction proceeds by only 11% in 24 h. Upon recrystallization of compounds 12 and 15 from methanol, compounds 21 and 22 were isolated (Scheme ). The aldimine group is a key element in this reaction. An increase in the basicity of the nitrogen atom accelerates the interaction of nucleophiles with o-quinones.

It was previously shown that deprotonation of n class="Chemical">catecholaldimines[42,43] to o-quinones[38] causes a change in the position of the aldimine fragment relative to the catechol/quinone ring. In catechols 2–11, the aldimine fragment is located practically in the plane of the catechol aromatic ring and directed toward oxygen atoms as a result of the stabilization due to the formation of an intramolecular hydrogen bond. In o-benzoquinones, the oxidized and deprotonated forms of catecholaldimines, such an intramolecular hydrogen bond is absent. As a result, the repulsion between the electron pairs of the O and N atoms stabilizes the conformation with the aldimine fragment turned away from the oxygen atoms toward tert-butyl groups. A nucleophilic attack is facilitated in such a conformation. We suppose that the factor enabling the attack of the nucleophile is the formation of a hydrogen bond between the nitrogen atom of the aldimine fragment and the nucleophile (Scheme S1). The stronger this hydrogen bond, the easier the nucleophilic addition to these derivatives should be. The strength of the hydrogen bond correlates with the electron-donor properties of the substituent at the nitrogen atom and should be greater when the substituent at the nitrogen atom has a stronger donating ability.

Thus, it was not possible to obtain o-benzoquinones n class="Chemical">from catechols 2 and 3 based on the aliphatic amine. The use of electron-donor substituents (alkyl) reduces the stability of the corresponding o-quinone form in the reaction with nucleophiles since the basicity of nitrogen increases: thus, the corresponding o-quinones are less stable and react more easily with nucleophilic agents. The fewer donor substituents (aromatic substituents) lead to the stabilization of the catechol form. Due to stabilization of the catechol form, the nucleophilic attack proceeds more slowly and allows the isolation of stable o-benzoquinones.

Crystal Structures

The crystal structures of n class="Chemical">catechols 2–11 were determined by single-crystal X-ray diffraction. The crystals suitable for X-ray diffraction analysis were grown from methanol solutions. The molecular structures of catechols 2–11 are shown in Figure . The selected bond lengths and angles are listed in Tables and 2, respectively.

Table 2

Selected Angles (deg) of Catechols 2–11

angle	2	3	4	5	6
O(2)–H(2)···O(1)	126.4(17)	116.4(12)	122.4(14)	117.2(15)	112.1(14)
N(1)···H(1)···O(1)	142.8(17)	143.6(14)	156.5(14)	154.5(13)	153(2)
O(2)–H(2)···O(1A)	-	144.0(13)	137.7(14)	143.8(17)	144.6(16)

According to X-ray data, the bond lengths in the OCCO-fragments in 2–11 (C–O 1.2977(15)–1.3747(14) Å; C–C 1.394(2)–1.4286(13) Å) correspond to the n class="Chemical">catecholate form.[38−41,44]

It should be noted that the H(2) n class="Chemical">hydrogen atom is always located at the O(2) atom (O(2)–H(2) 0.73(3)–0.884(16) Å; H(2)···O(1) 1.95(2)–2.20(3) Å), while the H(1) atom can migrate to the nitrogen atom. The close distances O(1)···N(1) are in the range of 2.4435(12)–2.5125(12) Å for all catecholaldimines due to the strong intramolecular hydrogen bond O(1)···H(1)···N(1).[45]

Compounds 2, 3, and 11 have been found in the n class="Chemical">quinomethide form. The distances N(1)–H(1) are in the range of 0.86(2)–1.01(2) Å and the distances O(1)···H(1) are in the range of 1.62(2)–1.682(16) Å for 2, 3, and 11. At the same time, compounds 4, 5, and 7–10 have been found in the crystal state in a catechol form. In these compounds, the O(1)···H(1) distances range from 0.93(3) to 1.024(15) Å. The distances N(1)–H(1) are in the range 1.479(15)–1.64(2) Å. The most controversial situation is observed in 6: according to the X-ray analysis, atom H(1) forms a distorted six-membered cycle CCCOHN with distances O–H and N–H of 1.25(3) and 1.29(3) Å, respectively.

In the transition from the n class="Chemical">quinomethide to the catechol form, the most significant change is the change of the bond length in the salicylaldimino fragment: the C(1)–O(1) bonds are extended from 1.3188(17) to 1.3497(12) Å with simultaneous shortening of the C(1)–C(2) bonds from 1.407(2) to 1.3961(14) Å and C(1)–C(6) bonds from 1.4249(19) to 1.4111(14) Å. The structural changes reflect the migration of a proton from a nitrogen atom to an oxygen atom with the preservation of molecule electroneutrality.

The distance Ph–X (X = F (8), Cl (9), Br (10), I (11)) increases systematically (Table ) in accordance with an increase on class="Chemical">f the halogen radius (F < Cl < Br < I).[46]

In crystals, molecules of 3–6 and 8–11 n class="Chemical">form dimeric pairs through the formation of two intermolecular O···H hydrogen bonds (Figure ). The distances O(2)–H(2)···O(1A) in 3–6, 8–11 range from 1.963(19) to 2.316(17) Å. These values are comparable with the intramolecular O(2)–H(2)···O(1) interactions in these compounds (2.039(19)–2.20(3) Å).

Figure 5

Dimeric pairs of catechols 3–6 (a–d) and 8–11 (e–h).

Dimeric pairs of n class="Chemical">catechols 3–6 (a–d) and 8–11 (e–h).

In the crystal of 7, the intermolecular O···H n class="Chemical">hydrogen bonds form infinite chains (Figure ); the O(2)–H(2)···O(4A) distance and the O(3A)···H(18B)–C(18A) distance are equal to 2.189(19) and 2.715(16) Å, respectively.

Figure 6

Fragment of crystal packing of 7.

It is interesting to note that the only compound in which no intermolecular shortened O···H distances were found is n class="Chemical">catechol 2 with a methyl substituent at the nitrogen atom. However, the shortest intramolecular O(2)–H(2)···O(1) contact (1.95(2) Å) is also observed in this compound.

Thus, the X-ray difn class="Chemical">fraction data confirm the presence of a strong hydrogen bond N(1)···H(1)···O(1). The existence of different structural forms, even for the same class of compounds, indicates an easy migration of a H(1) atom from an oxygen atom to a nitrogen atom. The quinomethide form is characteristic of an aliphatic amine, while the aromatic amine derivatives can exist both in the catechol and quinomethide forms in crystals.

Cyclic Voltammetry

The electrochemical properties of compounds 2–5, 8–17 have been investigated by cyclic voltammetry in n class="Chemical">dichloromethane solutions containing 0.1 M NBu4ClO4 (TBAP) as the supporting electrolyte at a glassy carbon working electrode (Tables and 4 and Figures and 8). The redox reactions for the “catechol–o-quinone” system can be easily represented using Scheme .[47]

Table 3

Values of the Redox Potentials of Catechols According to CV Dataa

compound	Epox1 (V)	E1/2ox2 (V)	Ic/Ia
2	0.73	1.40	0.93
3	0.77	1.50	0.93
4	0.91	1.42	0.48
5	0.84	1.45	0.85
8	0.93	1.49	0.43
9	0.92	1.47	0.71
10	0.92	1.48	0.43
11	0.90	1.45	0.68

c = 5 × 10–3 M, argon, CH2Cl2, 0.1 M NBu4ClO4, vs Ag/AgCl/KCl(sat.).

Table 4

Values of the Redox Potentials of Quinones According to CV Dataa

compound	E1/2red1 (V)	Ipa/Ipc	Epred2 (V)
12	–0.38	0.79	–0.97
13	–0.38	0.83	–1.04
14	–0.34	0.77	–1.00
15	–0.35	0.83	–0.98
16	–0.36	0.71	–0.96
17	–0.37	0.80	–0.95

c = 5 × 10–3 M, argon, CH2Cl2, 0.1 M NBu4ClO4, vs Ag/AgCl/KCl(sat.).

Figure 7

CV curve of 2 in CH2Cl2 in the range from 0.0 to 2.0 V (c = 5 × 10–3 M, argon, 0.1 M NBu4ClO4, 100 mV/s, vs Ag/AgCl/KCl(sat.)).

Figure 8

CV curves of 13 in CH2Cl2 in the range from −1.5 to 0.0 V (1) and in the range from −0.8 to 0.0 V (2) (c = 5 × 10–3 M, argon, 0.1 M NBu4ClO4, 100 mV/s, vs Ag/AgCl/KCl(sat.)).

Scheme 5

Redox Reactions for the System “Catecholaldimine–Imine-Substituted o-Benzoquinone”

CV curve of 2 in n class="Chemical">CH2Cl2 in the range from 0.0 to 2.0 V (c = 5 × 10–3 M, argon, 0.1 M NBu4ClO4, 100 mV/s, vs Ag/AgCl/KCl(sat.)).

CV curves of 13 in n class="Chemical">CH2Cl2 in the range from −1.5 to 0.0 V (1) and in the range from −0.8 to 0.0 V (2) (c = 5 × 10–3 M, argon, 0.1 M NBu4ClO4, 100 mV/s, vs Ag/AgCl/KCl(sat.)).

c = 5 × 10–3 M, argon, n class="Chemical">CH2Cl2, 0.1 M NBu4ClO4, vs Ag/AgCl/KCl(sat.).

c = 5 × 10–3 M, argon, n class="Chemical">CH2Cl2, 0.1 M NBu4ClO4, vs Ag/AgCl/KCl(sat.).

The substituted catechols can be oxidized in a one-stage two-electron process. With increasing pH, the oxidation peak becomes irreversible[47] and an unstable n class="Chemical">QH22+ form is immediately deprotonated twice. However, compounds 2–5 and 8–11 are oxidized in two stages. This difference is due to the introduction of the aldimine fragment in the third position with strong intramolecular hydrogen bonds.

The first oxidation wave n class="Chemical">for all of the considered catecholaldimines is electrochemically irreversible. This behavior is typical for the oxidation of spatially shielded phenols without strong intramolecular hydrogen bonds. The cause of irreversibility is the fast deprotonation stage of the cation radical after electron transfer. The second stage is quasi-reversible. For aliphatic amines, the degree of reversibility (Ic/Ia = 0.93) is greater than that for aromatic derivatives (Ic/Ia = 0.43–0.85). A possible cause is the electrochemical processes associated with the electrooxidation of anilines.[48] This behavior was previously observed for phenolic derivatives[49] containing additional intramolecular hydrogen bonds. Thus, the oxidation process of catecholaldimines can be described by the following sequence of reactions: at the first stage, one-electron oxidation of catechols to the form QH2.+(Scheme ) occurs, followed by fast-stage deprotonation of the oxidized phenolic O(2)H(2) fragment with the formation a phenoxyl radical[50] containing a salicylaldimine fragment (form QH., Scheme ). At the second stage, this phenoxyl radical is oxidized to produce a protonated form of o-quinone QH+ (Scheme ). The deprotonation in this case is not observed due to the presence of a strong intramolecular hydrogen bond O(1)···H(1)···N(1) in the salicylaldimine fragment, which makes it possible to observe a reversible oxidation process.

Compound 8 has the weakest oxidizing ability in the catechol line under investigation. This n class="Chemical">fact can be explained by the presence of fluorine in the N-substituent and the contraction of the electron density on it.

Compound 2 has the greatest oxidizing ability. Compounds 2 (Figure ) and 3 have reversible second oxidation peaks. This is rationalized by the presence in the substituent on class="Chemical">f a donor methyl (compound 2) and tert-butyl (compound 3) groups.

The electrochemical properties of n class="Chemical">o-quinones 12–17 were also investigated by means of CV. The CV of 13 is shown in Figure . In the case of these o-benzoquinones, quite a normal electrochemical behavior is observed. The first stage at E1/2red1 = −0.38 to −0.32 V (Table ) is reversible and leads to the formation of a relatively stable o-benzosemiquinone anion. The current ratio (Ia/Ic) falls in the range of 0.71–0.83 (Table ).

The second redox process (Ered2) is irreversible and corresponds to the further reduction on class="Chemical">f the radical anion SQ•– to dianion Cat2– (Scheme ). The irreversibility of this process indicates the presence of a chemical stage immediately after electron transfer. The first half-wave potential (E1/2) is weakly dependent on the nature of the aldimine substituent in the X–N=CH group.

Scheme 6

Redox Processes of Imine-Substituted o-Benzoquinones

The tendency to shift to the cathode region is observed in the series: −n class="Chemical">C6H4–F > −C6H4–Cl > −C6H4–Br > −C6H4–I > Ph > −C6H4–Me (Table ) and agrees with the change of the inductive effect of substitutes in aniline fragments.

Electron Paramagnetic Resonance (EPR)

The ability of new n class="Chemical">o-benzoquinones to be chemically reduced was investigated by means of X-band EPR spectroscopy on the examples of o-quinones 12–14. The o-quinones 12–14 were stirred with potassium in a THF solution, and the X-band EPR spectra of the resulting monoanions were recorded (Figure S64). The EPR spectral parameters are typical for potassium o-semiquinonato derivatives, which is consistent with the electrochemical results. The EPR spectra are doublets (1:1) due to hyperfine splitting (HFS) of the signal from an unpaired electron on the proton in the fifth position of the aromatic ring of the o-benzosemiquinone ligand 4,6-DBSQ. The values of the g-factor (g = 2.0046 for all three compounds 12–14) and the HFC constants (a(H) = 2.72, 2.64, and 2.65 G for o-semiquinone derivatives of 12, 13, and 14, respectively) in the EPR spectra indicate the localization of the unpaired electron in the aromatic cycle of a redox-active o-benzosemiquinone ligand.

Conclusions

In the present article, new derivatives of n class="Chemical">3,5-di-tert-butylcatechol containing additional functional acceptor R–N=CH-groups at the third position of the aromatic ring have been synthesized (catecholaldimines 2–11). Like 3,5-di-tert-butylcatechol and its oxidized o-benzoquinone form, these compounds are prospective redox-active ligands in coordination chemistry; however, the presence of the aldimine fragment in these compounds changes qualitatively and quantitatively the properties of the redox pair “catechol/o-quinone,” which should also be observed in complex compounds. In these catecholaldimines, a strong intramolecular hydrogen bond N(1)···H(1)···O(1) was found. An easy hydrogen atom transfer from the oxygen to the nitrogen atoms indicates the presence of different structural forms (catechol and quinomethide) in this class of compounds. The quinomethide form is characteristic of the aliphatic amine derivatives, while the aromatic amine derivatives exist both in the catechol and in the quinomethide form in crystals.

Under the oxidation of n class="Chemical">catecholaldimines, the formation of corresponding o-quinones takes place. These o-quinones are more active in reactions with nucleophiles (water, methanol) as compared with the parent 3,5-di-tert-butyl-o-quinone; moreover, an increase in the basicity of the nitrogen atom accelerates the interaction of these o-quinones with nucleophiles.

The electrochemical transformations on class="Chemical">f compounds 2–5 and 8–17 were studied in detail by means of CV. The first reduction potentials E1/2 for o-quinones are shifted to a more positive region as compared to 3,5-di-tert-butyl-o-benzoquinone due to the acceptor nature of the imine substituents. In the case of catecholaldimines, two separate electrochemical stages are observed during the oxidation. For quinones, the presence of an aldimine fragment does not significantly affect direct oxidation. However, for catechols, the aldimine fragment plays an essential role. The two-stage one-electron oxidation of catecholaldimines is not typical for classical catechol compounds. All of these changes are related to the nature of the substituents: the presence of a strong intramolecular hydrogen bond O(1)···H(1)···N(1) in the salicylaldimine fragment, that prevents the deprotonation on the second stage, making it quasi-reversible.

The structural features on class="Chemical">f these compounds and the possibility of stabilizing their various forms (catecholic, quinomethide) due to intramolecular hydrogen interactions make these compounds prospective objects for the design of chelate complexes with customizable properties. The chemical (incl. redox) properties of this series of imino-substituted catechols/ quinomethides/o-benzoquinones may be tuned to some extent, and this possibility can be exploited for the design of catalysts, etc.

Experimental Section

General Considerations

Solvents were purified n class="Chemical">following standard methods.[50] 3,5-Di-tert-butyl-catechol was obtained in the laboratory of chemistry of organoelemental compounds of IOMC RAS by the reduction of 3,5-di-tert-butyl-o-benzoquinone. The 1H and 13C{1H} NMR spectra of 2–19 were registered using a Bruker AVANCE DPX-200 spectrometer, and the 1H and 13C{1H} NMR spectra of 20 were registered using a Bruker ARX 400 instrument with tetramethylsilane (TMS) as the internal reference. IR spectra were monitored in the 400–4000 cm–1 range by an FSM 1201 Fourier-IR spectrometer in Nujol mulls and reported in cm–1. The C, H, and N elemental analysis was performed on an Elemental Analyzer Euro EA 3000 instrument. The X-band EPR spectra were obtained using a Bruker EMX spectrometer (∼9.75 GHz).

X-ray Diffraction Studies

Intensity data were collected on an Oxford Xcalibur E (n class="Chemical">for 3) and a Bruker D8 Quest (2, 4–11) diffractometer (graphite-monochromator, Mo Kα-radiation, ω-scan technique, λ = 0.71073 Å). The intensity data were integrated using the CrysAlisPro[51] (3) and SAINT[52] (2, 4–11) programs. All structures were solved using a dual-space algorithm[53] and were refined on F2 using all reflections with the SHELXTL package.[54] All non-hydrogen atoms were refined anisotropically. All hydrogen atoms, except H(1) and H(2), were placed in calculated positions and were refined in the riding model (Uiso(H) = 1.5Ueq(C) in CH3 groups and Uiso(H) = 1.2Ueq(C) in other groups). SCALE3 ABSPACK[51] (3) and SADABS[55] (2, 4–11) were used to perform absorption corrections.

Electrochemistry

The voltammetric measurements were recorded using an Elins P-45X potentiostat with a standard three-electrode configuration. The glassy n class="Chemical">carbon (d 1.6 mm) was used as a working electrode. A platinum wire and a 3.5 M Ag/AgCl/KCl(sat.) were used as the counter and reference electrodes, respectively. All measurements were carried out under argon. The rate scan was 100 mV/s. n-Bu4NClO4 (0.1 M) was used as an electrolyte. The concentration of the compounds was 5 mM.

Syntheses

General Synthetic Procedure to Obtain Catechols

The mixture of 1 (0.02 mol) and corresponding substituted n class="Chemical">amine (0.02 mol) in methanol (20 mL) was stirred at 60 °C for 3 h (24 h for 7). The reaction mixture was cooled to room temperature, and the precipitate formed was filtrated. This crude product was recrystallized from methanol solution after slow evaporation, and isolated as yellow-orange to cherry-red crystalline powders.

4,6-Di-tert-butyl-3-(methyliminomethyl)catechol (2)

Yellow powder. The yield is 96%. mp 145–146 °C. 1H n class="Chemical">NMR (200 MHz, CDCl3): δ = 1.40 (s, 9H, tBu), 1.42 (s, 9H, tBu), 3.44 (s, 3H, CH3), 6.57 (s, 1H, arom. C6H1), 6.96 (br.s, 1H, OH), 8.85 (s, 1H, CH=N), 15.66 ppm (br.s, 1H, OH). 1H NMR (200 MHz, DMSO-d6): δ = 1.31 (s, 9H, tBu), 1.36 (s, 9H, tBu), 3.44 (s, 3H, CH3), 6.42 (s, 1H, CH), 7.95 (br.s, 1H, OH), 8.95 (s, 1H, CH=N), 15.42 ppm (br.s, 1H, OH). 13C{1H} NMR (50 MHz, DMSO-d6): δ = 29.56, 33.26, 34.96, 35.49, 40.83, 110.25, 110.69, 133.99, 139.18, 143.93, 162.33, 165.79 ppm. IR (nujol): v = 465 (w), 542 (w), 648 (w), 676 (w), 771 (m), 815(w), 854 (m), 924 (w), 981 (m), 1026 (w), 1074 (w), 1136 (m), 1165 (m), 1229 (s), 1305 (m), 1358 (s), 1367 (m), 1385 (s), 1408 (s), 1634 (m), 3260 (m) cm–1. Elemental analysis calcd (%) for C16H25NO2: C, 73.00; H, 9.57; N, 5.32; found (%): C, 73.05; H, 9.51; N, 5.33.

4,6-Di-tert-butyl-3-(tert-butyliminomethyl)catechol (3)

Yellow powder. The yield is 86%. mp 152–154 °C. 1H n class="Chemical">NMR (200 MHz, CDCl3): δ = 1.41 (s, 9H, N-tBu), 1.43 (s, 9H, tBu), 1.47 (s, 9H, tBu), 6.50 (s, 1H, arom. C6H1), 7.0–7.5 (br.s, 1H, OH), 8.87 (s, 1H, CH=N), 15.92 ppm (br.s, 1H, OH). 13C{1H} NMR (50 MHz, CDCl3): δ = 29.23, 29.76, 33.22, 34.94, 35.33, 55.13, 108.62, 110.87, 134.41, 138.96, 144.43, 158.74, 166.15 ppm. IR (nujol): v = 478 (w), 579 (w), 676 (w), 785 (w), 853 (w), 924 (w), 990 (w), 1030 (w), 1040 (w), 1077 (w), 1166 (m), 1188 (m), 1223 (m), 1242 (m), 1301 (w), 1357 (m), 1398 (m), 1524 (w), 1620 (m), 3292 (m) cm–1. Elemental analysis calcd (%) for C19H31NO2: C, 74.71; H, 10.23; N, 4.59; found (%): C, 74.65; H, 10.32; N, 4.38.

4,6-Di-tert-butyl-3-(phenyliminomethyl)catechol (4)

Orange powder. The yield is 93%. mp 184–185 °C. 1H n class="Chemical">NMR (200 MHz, CDCl3): δ = 1.45 (s, 9H, tBu), 1.51 (s, 9H, tBu), 6.44 (br.s, 1H, OH), 6.81 (s, 1H, arom. C6H1), 7.22–7.35 (m, 3H, Ph), 7.40–7.52 (m, 2H, Ph), 9.38 (s, 1H, CH=N), 15.86 ppm (br.s, 1H, OH). 13C{1H} NMR (50 MHz, CDCl3): δ = 29.24, 33.36, 35.31, 35.66, 112.76, 113.90, 120.83, 126.78, 129.76, 137.21, 140.58, 142.51, 146.85, 155.09, 161.38 ppm. Elemental analysis calcd (%) for C21H27NO2: C, 77.50; H, 8.36; N, 4.30; found (%): C, 77.54; H, 8.40; N, 4.25.

4,6-Di-tert-butyl-3-(p-tolyliminomethyl)catechol (5)

Orange powder. The yield is 89%. mp 184–185 °C. 1H n class="Chemical">NMR (200 MHz, CDCl3): δ = 1.49 (s, 9H, tBu), 1.54 (s, 9H, tBu), 2.42 (s, 3H, CH3), 6.37 (br.s, 1H, OH), 6.83 (s, 1H, arom. C6H1), 7.17–7.37 (m, 4H, arom. C6H4), 9.40 (s, 1H, CH=N), 16.15 ppm (br.s, 1H, O···H···N). 13C{1H} NMR (50 MHz, CDCl3): δ = 20.99, 29.19, 33.30, 35.23, 35.60, 112.57, 113.63, 120.55, 130.23, 136.79, 140.26, 142.56, 143.91, 155.52, 160.38 ppm. IR (nujol): v = 495 (w), 531 (w), 612 (w), 637 (w), 671 (w), 709 (w), 746 (w), 786 (w), 817 (m), 825 (w), 859 (w), 901 (w), 959 (w), 982 (m), 1015 (w), 1024 (w), 1037 (w), 1076 (w), 1165 (m), 1190 (w), 1220 (m), 1259 (m), 1294 (w), 1309 (w), 1363 (s), 1396 (m), 1510 (m), 1547 (w), 1593 (m), 1613 (w), 3370 (s) cm–1. Elemental analysis calcd (%) for C22H29NO2: C, 77.84; H, 8.61; N, 4.13; found (%): C, 77.76; H, 8.70.

4,6-Di-tert-butyl-3-(p-methoxyphenyliminomethyl)catechol (6)

Orange powder. The yield is 88%. mp 152–153 °C. 1H n class="Chemical">NMR (200 MHz, CDCl3): δ = 1.51 (s, 9H, tBu), 1.56 (s, 9H, tBu), 3.86 (s, 3H, OMe), 6.58 (br.s, 1H, OH), 6.86 (s, 1H, arom. C6H1), 7.00 (d, 2H, 3J(H,H) = 8.8 Hz, arom. C6H4), 7.30 (d, 2H, 3J(H,H) = 8.8 Hz, arom. C6H4), 9.40 (s, 1H, CH=N), 16.19 ppm (br.s, 1H, OH). 13C{1H} NMR (50 MHz, CDCl3): δ = 29.19, 33.27, 35.20, 35.56, 55.44, 112.74, 113.62, 114.83, 121.82, 136.66, 139.67, 140.13, 142.44, 154.65, 158.68, 159.56 ppm. IR (nujol): v = 463 (w), 503 (w), 537 (w), 571 (w), 588 (w), 639 (w), 675 (w), 716 (w), 746 (w), 787 (w), 832 (m), 864 (w), 902 (w), 984 (m), 1032 (m), 1076 (w), 1111 (w), 1168 (m), 1181 (m), 1223 (m), 1255 (s), 1300 (w), 1356 (m), 1367 (m), 1397 (s), 1440 (m), 1456 (m), 1510 (m), 1557 (m), 1614 (s), 3352 (m) cm–1. Elemental analysis calcd (%) for C22H29NO3: C, 74.33; H, 8.22; N, 3.94; found (%): C, 74.24; H, 8.01; N, 4.13.

4,6-Di-tert-butyl-3-(p-nitrophenyliminomethyl)catechol (7)

Red powder. The yield is 87%. mp 224–225 °C. 1H n class="Chemical">NMR (200 MHz, CDCl3): δ = 1.44 (s, 9H, tBu), 1.51 (s, 9H, tBu), 6.25 (br.s, 1H, OH), 6.89 (s, 1H, arom. C6H1), 7.36 (d, 2H, 3J(H,H) = 8.9 Hz, arom. C6H4), 8.33 (d, 2H, 3J(H,H) = 8.9 Hz, arom. C6H4), 9.42 (s, 1H, CH=N), 13.0 ppm (br.s, 1H, OH). 13C{1H} NMR (50 MHz, CDCl3): δ = 29.11, 33.48, 35.50, 35.73, 113.16, 114.94, 121.67, 125.44, 139.10, 141.48, 142.16, 145.99, 152.93, 153.54, 164.35 ppm. IR (nujol): v = 528 (w), 555 (w), 572 (w), 632 (w), 663 (w), 696 (w), 755 (w), 769 (w), 859 (m), 868 (m), 906 (w), 978 (m), 994 (w), 1071 (m), 1109 (m), 1167 (m), 1207 (m), 1219 (m), 1276 (m), 1343 (s), 1424 (m), 1511 (s), 1558 (m), 1585 (m), 1601 (m), 3492 (m) cm–1. Elemental analysis calcd (%) for C21H26N2O4: C, 68.09; H, 7.07; N, 7.56; found (%): C, 67.91; H, 7.57; N, 7.67.

4,6-Di-tert-butyl-3-(4-fluorphenyliminomethyl)catechol (8)

Red powder. The yield is 94%. mp 180–182 °C. 1H n class="Chemical">NMR (200 MHz, CDCl3): δ = 1.44 (s, 9H, tBu), 1.51 (s, 9H, tBu), 6.36 (br.s, 1H, OH), 6.85 (s, 1H, arom. C6H1), 7.07–7.31 (m, 4H, arom. C6H4), 9.36 ppm (s, 1H, CH=N). 13C{1H} NMR (50 MHz, CDCl3): δ = 29.22, 33.40, 35.37, 35.68, 113.03, 114.23, 116.26, 116.71, 122.31, 122.48, 137.65, 140.72, 142.28, 153.36, 161.85 ppm. IR (nujol): v = 528 (w), 636 (w), 767 (m), 827 (m), 853 (w), 866 (w), 902 (m), 942 (m), 982 (m), 1003 (m), 1026 (w), 1058 (m), 1078 (w), 1167 (m), 1192 (m), 1222 (m), 1260 (m), 1296 (m), 1366 (m), 1395 (m), 1555 (m), 1558 (m), 1604 (m), 3401 (m) cm–1. Elemental analysis calcd (%) for C21H26FNO2: C, 73.44; H, 7.63; N, 4.08; found (%): C, 73.46; H, 7.62; N, 4.13.

4,6-Di-tert-butyl-3-(4-chlorophenyliminomethyl)catechol (9)

Red powder. The yield is 93%. mp 179–181 °C. 1H n class="Chemical">NMR (200 MHz, CDCl3): δ = 1.44 (s, 9H, tBu), 1.50 (s, 9H, tBu), 6.35 (br.s, 1H, OH), 6.85 (s, 1H, arom. C6H1), 7.21 (d, 2H, 3J(H,H) = 8.7 Hz, arom. C6H4), 7.42 (d, 2H, 3J(H,H) = 8.7 Hz, arom. C6H4), 9.36 ppm (s, 1H, CH=N). 13C{1H} NMR (50 MHz, CDCl3): δ = 29.20, 33.42, 35.41, 35.70, 113.02, 114.38, 122.22, 129.80, 132.46, 138.00, 140.88, 142.29, 145.97, 153.42, 162.19 ppm. IR (nujol): v = 529 (w), 636 (w), 771 (w), 831 (m), 903 (w), 979 (m), 1011 (m), 1090 (m), 1168 (m), 1192 (m), 1199 (m), 1224 (m), 1263 (m), 1293 (m), 1299 (m), 1366 (m), 1397 (m), 1418 (m), 1479 (m), 1557 (m), 1590 (m), 1605 (m), 3430 (m) cm–1. Elemental analysis calcd (%) for C21H26ClNO2: C, 70.08; H, 7.28; N, 3.89; found (%): C, 70.07; H, 7.23; N, 3.73.

4,6-Di-tert-butyl-3-(4-bromophenyliminomethyl)catechol (10)

Cherry-red powder. The yield is 82%. mp 194–196 °C. 1H n class="Chemical">NMR (200 MHz, CDCl3): δ = 1.46 (s, 9H, tBu), 1.51 (s, 9H, tBu), 6.34 (s, 1H, OH), 6.86 (s, 1H, arom. C6H1), 7.15 (d, 2H, 3J(H,H) = 8.4 Hz, arom. C6H4), 7.56 (d, 2H, 3J(H,H) = 8.2 Hz, arom. C6H4), 9.38 (s, 1H, CH=N), 15.48 ppm (br.s, 1H, O···H···N). 13C{1H} NMR (50 MHz, CDCl3): δ = 29.23, 33.43, 35.41, 35.71, 113.07, 114.34, 120.24, 122.62, 132.75, 137.83, 140.81, 142.29, 146.61, 153.47, 162.24 ppm. IR (nujol): v = 527 (w), 574 (w), 602 (w), 636 (w), 669 (w), 706 (w), 768 (w), 827 (m), 853 (w), 866 (w), 902 (w), 941 (w), 982 (m), 1006 (w), 1025 (w), 1071 (m), 1166 (m), 1191 (m), 1221 (s), 1261 (m), 1297 (m), 1395 (s), 1555 (w), 1559 (w), 1571 (w), 1604 (m), 3387 (m), 3392 (m) cm–1. Elemental analysis calcd (%) for C21H26BrNO2: C, 62.38; H, 6.48; N, 3.46; found (%): C, 62.33; H, 6.49; N, 3.34.

4,6-Di-tert-butyl-3-(4-iodophenyliminomethyl)catechol (11)

Cherry-red powder. The yield is 91%. mp 202–204 °C. 1H n class="Chemical">NMR (200 MHz, CDCl3): δ = 1.44 (s, 9H, tBu), 1.50 (s, 9H, tBu), 6.34 (br.s, 1H, OH), 6.85 (s, 1H, arom. C6H1), 7.02 (d, 2H, 3J(H,H) = 8.6 Hz, arom. C6H4), 7.76 (d, 2H, 3J(H,H) = 8.5 Hz, arom. C6H4), 9.36 ppm (s, 1H, CH=N). 13C{1H} NMR (50 MHz, CDCl3): δ = 29.22, 33.42, 35.39, 35.69, 91.19, 113.05, 114.33, 122.91, 137.88, 138.73, 140.85, 142.29, 147.27, 153.58, 162.21 ppm. IR (nujol): v = 528 (w), 636 (w), 767 (m), 827 (m), 853 (w), 866 (w), 902 (m), 942 (m), 982 (m), 1003 (m), 1026 (w), 1058 (m), 1078 (w), 1167 (m), 1192 (m), 1222 (m), 1260 (m), 1296 (m), 1366 (m), 1395 (m), 1555 (m), 1558 (m), 1604 (m), 3401 (m) cm–1. Elemental analysis calcd (%) for C21H26NO2I: C, 61.02; H, 6.34; N, 3.39; found (%): C, 61.01; H, 6.41; N, 3.46.

General Synthetic Procedure to Obtain o-Quinones

The solutions of n class="Chemical">catechol (0.003 mol) in Et2O and K3Fe(CN)6 (10 eq) with KOH (2.2 eq) in water (60 mL) were mixed and vigorously stirred for 30 min. The mixture was then washed with water (3 × 50 mL) and the extract was dried with Na2SO4. The solvent was evaporated. The residual product was crystallized from hexane solution (50 mL). The powder was filtered, washed with cold hexane, and dried under vacuum. A light brown powder was obtained.

4,6-Di-tert-butyl-3-(phenyliminomethyl)-o-benzoquinone (12)

Prepared from 4. The yield is 40%. n class="Chemical">1H NMR (200 MHz, CDCl3): δ = 1.28 (s, 9H, tBu), 1.35 (s, 9H, tBu), 7.12 (s, 1H, C6H1), 7.15–7.22 (m, 2H, Ph), 7.24–7.31 (m, 1H, Ph), 7.35–7.45 (m, 2H, Ph), 8.43 ppm (s, 1H, CH=N). 13C{1H} NMR (50 MHz, CDCl3): δ = 29.13, 30.49, 35.58, 38.45.120.73, 126.65, 129.24, 132.97, 136.21, 149.34, 151.21, 158.33, 159.08, 178.60, 181.40 ppm. Elemental analysis calcd (%) for C21H25NO2: C, 77.98; H, 7.79; N, 4.33; found (%): C, 77.96; H, 7.87; N, 4.28.

4,6-Di-tert-butyl-3-(p-tolyliminomethyl)-o-benzoquinone (13)

Prepared from 5. The yield is 44%. n class="Chemical">1H NMR (200 MHz, CDCl3): δ = 1.28 (s, 9H, tBu), 1.34 (s, 9H, tBu), 2.37 (s, 3H, Me), 7.15 (m, 5H, arom. C6H4 and C6H1), 8.43 ppm (s, 1H, CH=N). 13C{1H} NMR (50 MHz, CDCl3): δ = 20.97, 29.10, 30.46, 35.54, 38.41, 120.71, 125.69, 129.42, 130.42, 133.11, 136.27, 136.64, 148.56, 149.17, 157.34, 158.98, 178.61, 181.46 ppm. IR (nujol): v = 512 (w), 530 (w), 569 (m), 603 (m), 812 (m), 826 (m), 837 (m), 862 (m), 883 (w), 905 (w), 923 (w), 951 (w), 996 (m), 1021 (w), 1041 (w), 1075 (w), 1110 (w), 1126 (w), 1173 (w), 1214 (m), 1242 (m), 1279 (s), 1359 (s), 1365 (s), 1506 (s), 1559 (s), 1622 (s), 1633 (s), 1663 (s), 1683 (s), 3357 (m) cm–1. Elemental analysis calcd (%) for C22H27NO2: C, 78.30; H, 8.06; N, 4.15; found (%): C, 77.96; H, 8.24; N, 4.21.

4,6-Di-tert-butyl-3-(p-flourophenyliminomethyl)-o-benzoquinone (14)

Prepared from 8. The yield is 46%. n class="Chemical">1H NMR (200 MHz, CDCl3): δ = 1.27 (s, 9H, tBu), 1.34 (s, 9H, tBu), 7.12 (m, 5H, arom. C6H4 and C6H1), 8.40 ppm (s, 1H, CH=N). 13C{1H} NMR (50 MHz, CDCl3): δ = 29.07, 30.40, 35.55, 38.44, 115.76, 116.21, 122.19, 122.36, 132.63, 136.18, 147.09, 149.34, 158.09, 159.28, 164.12, 178.47, 181.28 ppm. IR (nujol): v = 466 (w), 531 (m), 566 (m), 602 (m), 667 (w), 790 (s), 809 (w), 838 (s), 842 (s), 864 (w), 883 (w), 906 (w), 919 (w), 838 (w), 996 (m), 1022 (w), 1077 (w), 1094 (w), 1125 (w), 1150 (m), 1158 (w), 1200 (s), 1230 (s), 1276 (s), 1297 (s), 1367 (s), 1414 (w), 1502 (s), 1572 (m), 1594 (m), 1620 (m), 1638 (m), 1657 (s), 1682 (s) cm–1. Elemental analysis calcd (%) for C21H24FNO2: C, 73.88; H, 7.09; F, 5.56; N, 4.10; found (%): C, 74.15; H, 6.86; F, 5.64; N, 4.15.

4,6-Di-tert-butyl-3-(p-chlorophenylliminomethyl)-o-benzoquinone (15)

Prepared from 9. The yield is 42%. mp 137–139 °C. n class="Chemical">1H NMR (200 MHz, CDCl3): δ = 1.28 (s, 9H, tBu), 1.34 (s, 9H, tBu), 7.12 (s, 1H, C6H1), 7.11 (d, 2H, 3J(H,H) = 8.5 Hz, arom. C6H4), 7.36 (d, 2H, 3J(H,H) = 8.5 Hz, arom. C6H4), 8.40 ppm (s, 1H, CH=N). 13C{1H} NMR (50 MHz, CDCl3): δ = 29.12, 30.46, 35.61, 38.48, 122.06, 129.38, 132.39, 132.60, 136.12, 149.50, 149.62, 158.85, 159.37, 178.49, 181.28 ppm. IR (nujol): v = 595 (w), 835 (w), 860 (w), 996 (w), 1092 (w), 1168 (w), 1203 (w), 1241 (w), 1275 (w), 1298 (w), 1359 (w), 1367 (w), 1483 (m), 1619 (w), 1638 (w), 1662 (m), 1682 (m) cm–1. Elemental analysis calcd (%) for C21H24ClNO2: C, 70.48; H, 6.76; N, 3.91; found (%): C, 70.43; H, 6.71; N, 3.83.

4,6-Di-tert-butyl-3-(p-bromphenylliminomethyl)-o-benzoquinone (16)

Prepared from 10. The yield is 44%. n class="Chemical">1H NMR (200 MHz, CDCl3): δ = 1.28 (s, 9H, tBu), 1.34 (s, 9H, tBu), 7.05 (d, 2H, 3J(H,H) = 8.5 Hz, arom. C6H4), 7.11 (s, 1H, C6H1), 7.51 (d, 2H, 3J(H,H) = 8.6 Hz, arom. C6H4), 8.40 ppm (s, 1H, CH=N). 13C{1H} NMR (50 MHz, CDCl3): δ = 29.07, 30.41, 35.58, 38.46, 120.23, 122.40, 132.31, 132.49, 136.08, 149.42, 150.00, 158.94, 159.36, 178.36, 181.15 ppm. IR (nujol): v = 485 (w), 534 (s), 591 (s), 644 (s), 670 (w), 700 (w), 745 (w), 758 (w), 814 (m), 834 (s), 860 (m), 882 (w), 906 (w), 922 (w), 939 (w), 997 (m), 1009 (s), 1022 (w), 1072 (s), 1100 (w), 1125 (w), 1169 (m), 1204 (m), 1241 (m), 1275 (m), 1298 (w), 1367 (s), 1466 (s), 1482 (s), 1577 (w), 1619 (m), 1638 (m), 1661 (s), 1683 (s) cm–1. Elemental analysis calcd (%) for C21H24BrNO2: C, 62.69; H, 6.01; Br, 19.86; N, 3.48; found (%): C, 62.73; H, 6.14; Br, 19.81; N, 3.54.

4,6-Di-tert-butyl-3-(p-iodophenyliminomethyl)-o-benzoquinone (17)

Prepared from 11. The yield is 48%. mp 139–141 °C. n class="Chemical">1H NMR (200 MHz, CDCl3): δ = 1.27 (s, 9H, tBu), 1.33 (s, 9H, tBu), 6.92 (d, 2H, 3J(H,H) = 8.4 Hz, arom. C6H4), 7.10 (s, 1H, C6H1), 7.70 (d, 2H, 3J(H,H) = 8.4 Hz, arom. C6H4), 8.39 ppm (s, 1H, CH=N). 13C{1H} NMR (50 MHz, CDCl3): δ = 29.12, 30.45, 35.62, 38.49, 91.28, 122.74, 132.56, 136.11, 138.35, 149.52, 150.80, 159.00, 159.38, 178.46, 181.23 ppm. IR (nujol): v = 533 (w), 591 (w), 636 (w), 815 (w), 839 (w), 858 (w), 922 (w), 939 (w), 997 (w), 1005 (w), 1055 (w), 1171 (w), 1203 (w), 1213 (w), 1242 (w), 1275 (w), 1298 (w), 1367 (m), 1479 (m), 1579 (w), 1619 (w), 1638 (w), 1660 (m), 1682 (m) cm–1. Elemental analysis calcd (%) for C21H24NO2I: C, 56.13; H, 5.38; N, 3.12; found (%): C, 56.08; H, 5.41; N, 3.16.

3,5-Di-tert-butyl-5-hydroxy-6-methylaminomethylene-cyclohex-3-ene-1,2-dione (19)

Prepared from 2. The yield is 21%. mp 169–170 °C. n class="Chemical">1H NMR (200 MHz, CDCl3): δ = 0.97 (s, 9H, tBu), 1.33 (s, 9H, tBu), 3.18 (d, 3H, 3J(H,H) = 4.8 Hz, CH3), 3.82 (s, 1H, OH), 5.82 (s, 1H, C6H1), 7.66 (d, 1H, 3J(H,H) = 13.0 Hz, =CH–N), 10.70 ppm (s, 1H, NH). 13C{1H} NMR (50 MHz, CDCl3): δ = 25.43, 30.92, 36.04, 36.59. 40.78, 87.42, 103.17, 114.87, 155.57, 166.08, 199.91, 200.30 ppm. UV/vis (MeOH) λmax (ε) = 331 (22100), 392 nm (4700 mol–1 dm3 cm–1). Elemental analysis calcd (%) for C16H254NO3: C, 68.79; H, 9.02; N, 5.01; found (%): C, 69.01; H, 8.95; N, 4.86.

3,5-Di-tert-butyl-5-methoxy-6-(phenylamino)methylene-cyclohex-3-ene-1,2-dione (21)

The compound was isolated after the recrystallization on class="Chemical">f 12 in methanol. The yield is 76%. mp 161–162 °C. 1H NMR (400 MHz, CDCl3): δ = 0.94 (s, 9H, tBu), 1.28 (s, 9H, tBu), 3.22 (s, 3H, OMe), 6.87 (s, 1H, C6H1), 7.17 (m, 3H, 3J(H,H) = 7.9 Hz, arom. C6H5), 7.39 (m, 2H, 3J(H,H) = 7.9 Hz, arom. C6H5), 7.79 (d, 1H, 3J(H,H) = 12.5 Hz, =CH–N), 13.10 ppm (d, 1H, 3J(H,H) = 12.5 Hz, NH). 13C{1H} NMR (100 MHz, CDCl3): δ = 25.92, 29.12, 35.27, 41.84, 52.04, 81.87, 105.66, 117.62, 125.34, 129.98, 139.47, 148.29, 148.43, 149.80, 180.24, 183.53 ppm. Elemental analysis calcd (%) for C22H29NO3: C, 74.33; H, 8.22; N, 3.94; found (%): C, 74.40; H, 8.20; N, 3.90.

3,5-Di-tert-butyl-5-methoxy-6-(p-chlorophenylamino)methylene-cyclohex-3-ene-1,2-dione (22)

The compound was isolated after the recrystallization on class="Chemical">f 15 in methanol. The yield is 93%. mp 155–157 °C. 1H NMR (400 MHz, CDCl3): δ = 0.93 (s, 9H, tBu), 1.28 (s, 9H, tBu), 3.22 (s, 3H, OMe), 6.87 (s, 1H, C6H1), 7.08 (d, 2H, 3J(H,H) = 8.4 Hz, arom. C6H4), 7.36 (d, 2H, 3J(H,H) = 8.4 Hz, arom. C6H4), 7.69 (d, 1H, 3J(H,H) = 12.5 Hz, =CH–N), 13.05 ppm (d, 1H, 3J(H,H) = 12.2 Hz, NH). 13C{1H} NMR (100 MHz, CDCl3): δ = 25.85, 29.06, 35.28, 41.78, 52.09, 81.75, 105.87, 118.69, 130.05, 130.58, 138.02, 147.95, 148.40, 149.78, 180.46, 183.27 ppm. Elemental analysis calcd (%) for C22H28NO3Cl: C, 67.77; H, 7.24; N, 3.59; found (%): C, 67.36; H, 7.72; N, 3.48.