ABSTRACT

Alkyl xanthines underwent selective homolytic aromatic substitution at C-8 position with alkyl groups of pentaamminecobalt(III) complex. In this process of synthesis, we used monoalkyl hydrazines as the radical source in aqueous ammonia solution. Evidence supporting coordination of the alkyl hydrazine to pentaamminecobalt(III) complex by radical trapping was in good agreement with literature. The products were characterized using GC-MS and ¹H, ¹⁴N and ⁵⁹Co NMR spectroscopy.

1. INTRODUCTION

Alkyl xanthines belong to purine group of molecules and are of interest due to their therapeutic value. A number of xanthines are used as adenosine receptor antagonists to treat neurodegenerative diseases in humans [1]. Several xanthines are known to inhibit cells at the G2 checkpoint in the cell cycle, thereby making cells more sensitive to DNA damage [2]. Xanthines, such as caffeine, theophylline, theobromine (see Figure 1) and its derivatives have been used for antihyperuraemic therapy, inhibition of monoamine oxidase B [3], besides serving as anticancer agents. Xanthines are known to enhance affinity for certain receptors selectively as well. This has led to an interest in synthesizing substituted alkyl xanthines.

Aqueous organometallic chemistry and its catalysis have attracted much interest, partly due to the reduced requirement for organic solvents [4]. Many organocobalt(III) complexes are sensitive to oxygen or moisture. Hydrolysis of a cobalt(III)-carbon bond is dependent on the nature of the ligand and requires mild conditions. Alkylcobalt(III) complex acts as a potential radical source, e.g. in organic synthesis [5]  such as oxidation, reduction, thermolysis, photolysis and sonolysis. Homolytic aromatic substitution is a well-known method for the preparation of 8-substituted xanthines. Previously, 8-methylcaffeine was known to be prepared by irradiation of a mixture of caffeine and tert-butyl peracetate with ultraviolet light [6]. Similarly, 8-(1-adamantyl) caffeine and 8-cyclohexyl caffeine were obtained by employing photochemically prepared radicals [7] while other 8-alkyl xanthines were known to be synthesized by reaction with solvent-derived alkyl radicals using benzoyl peroxide as a radical initiator [8].

Till date, no studies have been focused on Cobalt catalysed synthesis of C-8 alkyl xanthines. In this study, an attempt was made to synthesize and purify C-8 substituted alkyl xanthines.

2. MATERIAL AND METHODS

Concentrated aqueous NH₃ (5 mL) was added to a solution of Co (NO₃)₂·6H₂O (30 mg, 0.1 mmol), the monoalkyl hydrazine (2.0 mmol), and methyl xanthine (1.0 mmol) in H₂O (10 mL). The mixture was stirred for 8 - 10 h at room temperature in presence of atmospheric dioxygen. The reaction was monitored by GC-MS after extraction into CH₂Cl₂. Following completion, the product was extracted into CH₂Cl₂ (25 mL) and the solvent was removed by rotary evaporation. The resulting solid was dissolved in a 2:3 mixture of ethoxyacetate, n-hexane (5 mL) and purified by column chromatography using silica gel (mesh, 11, 3.5 cm) and a 2:3 mixture of ethoxyacetate and n-hexane as eluent. The pure compounds were isolated and further characterized by GCMS, ¹H NMR and elemental analysis (See Figure 2).

3. RESULTS AND DISCUSSION

The 8-alkyl xanthines have been synthesized and characterized using GC-MS and ¹H NMR. Further elemental analysis of the isolated compounds was done which were

in agreement with respect to molecular structures. While the following seven compounds synthesized, viz. 8-tertbutyl caffeine, 8-tertbutyl theophylline, 8-tertbutyl-3- isobutyl-1-methyl-theophylline,8-isopropyl -3- isobutyl- 1-methyl theophylline, 8-isopropyl caffeine,8-isopropyl theophylline and 8-isopropyl theobromine; had a conversion of 60% - 90% (See Table 1). These were further used to study their effect in cancer cell lines. However, two compounds, viz. 8-ethyl caffeine and 8-ethy tertbutyl theobromine were not considered due to their insolubility in aqeous solution. Although we tried to synthesize many compounds, we wereable to get good yields for secondary and tertiary alkly hydrazines which have predominantly yielded 8-substituted alkly xanthines The reaction of monoalkyl hydrazines with cobalt (III) in aqueous ammonia in the presence of atmospheric oxygen was analysed by ¹⁴N and ⁵⁹Co NMR. A solution of 0.1 M Co(NO₃)₂ in 4 M NH₃ provided a ⁵⁹Co NMR signal at 8759 ppm (line width at half height, Δυ½ = 11.2 kHz), which was assigned to the diamagnetic [(NH₃)₅CoOOCo(NH₃)₅]⁴⁺ and is in agreement with the literature [11]. Addition of a stoichiometric amount of a methyl hydrazine (or another alkyl hydrazine) to the solution resulted in immediate disappearance of the ⁵⁹Co NMR signal. Rapid gas evolution (presumably O₂) together with the disappearance of [(NH₃)₅CoOOCo(NH₃)₅]⁴⁺ was consistent with methyl hydrazine displacing the coordinated dioxygen to give a cobalt(III) compound. This was oxidized slowly to the [Co(NH₃)₅ (CH₃)]²⁺ cation which was evident by the appearance of a ⁵⁹Co signal at 7370 ppm (Δυ½ = 13.2 kHz) (Figure 3) [9].

Magnetic susceptibility measurements in solution following Evans method [10] using tert-butanol with ¹H NMR detection showed that a solution of 0.1 M Co(NO₃)₂ in 4 M NH₃ was essentially diamagnetic and consistent with formation of the [(NH₃)₅CoOOCo(NH₃)₅]⁴⁺ cation. Addition of a stoichiometric amount of methyl hydrazine resulted in a paramagnetic species. This was consistent with the disappearance of the ⁵⁹Co NMR signal. Coordination of dioxygen to yield diamagnetic cobalt(III) complexes, as indicated by the ⁵⁹Co NMR signals, was conceivable indicating that dioxygen oxidizes the alkyl hydrazine via simultaneous coordination to the cobalt ion. Although many cobalt-dioxygen complexes are known to form in aqueous solution [11], to our knowledge, there are no reports on cobalt coordination compounds with both alkyl hydrazines and dioxygen ligands. Nevertheless, in view of many studies on cobalt-dioxygen complex formation with nitrogen donor ligands [12], it appears plausible that such species may form as intermediates. The ¹H NMR spectra of the reaction mixtures showed that oxidation, e.g. of ethyl hydrazine gives a [Co(NH₃)₅(CH₂CH₃)]²⁺ cation prior to xanthine alkylation. The ethyl ¹H NMR resonance signals of the ethyl hydrazine gradually decreased and instead, two new resonances at 3.90 and 3.97 ppm appeared. These were assigned to the [Co(NH₃)₅(CH₂CH₃)] ²⁺ cation by comparison with data for the isolated coordination compound [12].

The latter compound disappeared slowly and the formation of 8-ethylcaffeine was observed by the presence of appropriate ¹³C and ¹H resonance signals along with ¹⁴N NMR studies. A solution of methyl hydrazine in 6 M NH₃ yielded a broad ¹⁴N signal (−299 ppm); addition of a 25 M solution of Co(NO₃)₂ yielded signals correlating with -NH-NH₂ (−262 ppm, −331 ppm). Thus we observe that the oxidation of hydrazine with cobalt(III) takes place prior to alkylation of the alkylated xanthine.

This interpretation implies coordination of the alkyl hydrazine to cobalt(III) and is supported by the fact that methyl and ethyl hydrazine have been demonstrated to act as unidentate or bidentate bridging ligands towards cobalt(III) [13,14]. The rapid exchange reactions studied in alkylcobalt(III) complexes allow detection of alkyl radical released during the decomposition in aqueous solution. For example, in the case of pentaammine methylcobalt(III) complex [Co(NH₃)₂ (CH₃)(NO₃)₂] the methyl radical can be trapped by α-phenyl-N-tert-butylnitrone (PBN) with the appearance of a (¹⁴N): 16.89 G signal, and furthermore, on addition of caffeine, there were no signals observed due to a methyl adduct of PBN.

4. CONCLUSION

We report that cobalt(III) in aqueous ammonia solution serves as a catalyst for obtaining new carbon-carbon bonds by homolytic aromatic substitution. The amminecobalt(III)-promoted aerial oxidation of alkyl hydrazines afforded alkyl radicals, and some primary alkyl radicals were trapped by pentaamminecobalt(III) to form alkyl cobalt(III) cations [15]. However, these compounds are labile and decomposed to return alkyl radicals. It has been previously shown that the [Co(NH₃)₅(CH₃)]²⁺ cation acts as a methylating agent toward the C-8 atom of purine nucleotides [16-18]. We have applied a number of alkyl radicals for the preparation of C-8 substituted alkyl xanthines. We were unable to obtain evidence of a cobalt(III) species with both an alkyl hydrazine ligand and a peroxo ligand. However, this does not exclude the possibility of such a species existing as a reactive intermediate. It may be speculated that a cobalt(III) species with both an alkyl hydrazine ligand and a peroxo ligand is very shortlived due to rapid oxidation of alkylhydrazine.

5. ACKNOWLEDGEMENTS

Grateful appreciations towards financial support for the work carried out were rendered to the Danish Natural Science Research Council. The author thanks Drs. Pauli Kofod and AS Kumbhar for reviewing the manuscript.

REFERENCES