Synthesis of POSS-Functionalized Imidazole

The imidazole ring represent an important class of heterocycle frequently found in many natural product.¹ In addition, imidazole derivative have been reported to possess pharmacological properties² such as antiplasmodium,³ antitumor,⁴ and antifungal,⁵ and acting as glucagon receptors,⁶ therapeutic agents,⁷ antibacterial agents,⁸ and inhibitors of p38 MAP kinase⁹ and B-Raf kinase,¹⁰ plant growth regulators¹¹ and fungicides.¹² On the other hand, highly substituted imidazole derivative possess a good photophysical property, which result in their potential in material chemistry application such as organic electroluminescent device (OLED).¹³ The use of the 1H-phenanthro[9,10-d]-imidazol-2-yl group because of its supramolecular property has been reported in a molecular tweezer system for explosives¹⁴ and as a fluorophore in a superradiant laser dye¹⁵ as a result of its photophysical property. Triarylamine-containing imidazole¹⁶ or fused aromatic imidazole such as phenanthroimidazole¹⁷ and pyrenoimidazole¹⁸ material in a solar cells have also been found to serve as promising donors when connected with oligothiophene unit. Consequently, many effort in the past decade have been focused on the preparation of those privileged scaffold. For example, the synthesis of 1,2,4,5-tetraubstuted imidazole have been developed by using various catalytic systems including I₂,^19a BF₃−SiO₂,^19b SBPPSA,^19c alumina,^19d FeCl₃·6H₂O,^19e Cu(OAc)₂,^19f and DABCO.^19g Transition-metal-catalyzed direct C−H or N−H functionalization provides a powerful tool for the formation of multisubstituted imidazole.²⁰

Polyhedral oligomeric silsesquioxane (POSS) is a unique modifier to high-performance materials due to its unique three-dimensional structure.²¹⁻²² In POSS molecule, the rigid inorganic core provide high stiffness, thermo- and photo-stability, the organic corner group offer excellent solubility, processability and compatibility with other materials. Many POSS-based functional material, such as liquid crystal (LC) material,²³⁻²⁶ light-emitting material,²⁷⁻²⁹ dental restorative material³⁰ and ionic liquid³¹ have been prepared. And they are usually based on mono- or octa-substituted POSSs. The incorporation of POSS moieties into organic molecules or polymers may have dramatic effect on their properties, these hybrid materials display superior property to the organic material alone. Especially, aminopropylisobutyl-POSS prepared by employing a facile, corner-capping methodology is one of the most interesting functional POSS because it can be used as the precursor for many functional POSS derivative,³² and it also can be used as organic light-emitting material,³³ fluorescent nano-sensors.³⁴

As part of our further investigations of imidazole derivative and aminopropylisobutyl-POSS, we herein describe the synthesis of N-substituted POSS-imidazole in one pot typically prepared using the Debus−Radziszewski method from diketone such as benzil, phenanthrene-9,10-dione, pyrene-4,5-dione³⁵ and aminopropylisobutyl-POSS (Scheme 1). To the best of our knowledge, POSS-containing molecular hybrid material incorporated to nitrogen of imidazole as organic component remain unknown. Due to the excellent electric properties of these naturally occurring imidazole derivatives, incorporation of imidazole into molecular hybrids of POSS is expected to yield corresponding material with good properties, functionality, and application.

Scheme1.

N-Substituted POSS-imidazole prepared with diketone and aminopropylisobutyl-POSS.

General chemicals were purchased from commercial vendors and used as such without any purification. Reaction progress was monitored using thin layer chromatographic technique (TLC) on pre-coated aluminum sheets silica gel-60/UV254 with I₂ and UV light as detecting agents. NMR analysis was done with Bruker Avance-400 spectrometer using TMS as an internal standard.

With the previously conditions, we have studied the scope and limitations with various aldehyde and diketone. In Table 1, the one-pot approach to synthesize a wide range of imidazole derivatives with various substituents on the aromatic moieties using benzil as diketone was performed.

As shown in Table 1, the first application of the above approach to the preparation of 2,4,5-triphenyl-imidazole-isobutyl-POSS was successfully synthesized 4a-4k in 9-25% yield. Compared with the yield of general organic synthesis, slightly lower yields, but in first synthesis of N-substituted POSS-imidazole in one-pot synthesis, it is the moderate yield. Treatment of benzaldehyde 1a and amino propylisobutyl-POSS 3, benzil 2 with ammonium acetate at reflux for 12 hrs in the presence of acetic acid by one-pot method resulted in conversion to POSS-Imidazole 4a in 11% yield. We next investigated the scope of this three-component reaction of aldehydes bearing both electron-rich and electron-deficient aryl substituents under the same condition. In the reaction proceeded with the aldehyde bearing methyl group as electron donating group at the p-position of phenyl, the yield could be decreased as low as 8%, 4b. Influence of electron-drawing halogen substituents on para-positon of the phenyl ring were also investigated in the reaction. The more electro-deficient fluorobenzal-dehyde proceeded smoothly to give desired product 4c in the moderate 25% yield. Chloro and bromobenzaldehyde afforded also desired product leading to the relatively low isolated yield of 4d-4e. It was found that CF₃-substituted benzaldehyde can also yield the target product 4f. A benzaldehyde 1g-1h with an electron donating group at the paraposition of the aryl ring was furnished the desired product 4g-4h in low yields. The product substituted as 4-formyl-phenylboronic acid ester was isolated as 4i in 16% yield. Furthermore, triphenylaminebenzaldehyde 1j has also been utilized for this reaction. We found that desired product 4j was formed in 15% yield under the same conditions. Dithiophenealdehyde 1k bearing sulfur atom was also resulted in a low isolated 9% yield to give 4k.

Table1.

Substrate scope of one-pot reaction of various benzaldehydes with benzil using aminopropylisobutyl-POSS

Reaction conditions: aldehydes (1a-11a, 1.0 eq.), benzil (2, 1.1 eq.), POSS (3, 2.0 eq.).

^aIsolated yields

The 1H-phenanthro[9,10-d]imidazole unit shows excellent electron injection and transport properties and good thermal stability, making it a suitable component for organic semiconductors used in organic lightemitting diodes (OLEDs)¹ and solar cells.² Herein, in order to further study the reactivity of other diketones, we performed a multicomponent reaction under the same condition by replacing 1H-phenanthro[9,10-d]imidazole 5 instead of benzyl 2. Phenanthrene-9,10-dione proceeded to give the desired product 6, affording in a very low isolated yield (<5%), but it is a very important starting point in the developing of advanced materials (Scheme 2).

Scheme2.

Synthesis of 2-(4-bromophenyl)-1H-phenan thro[9,10-d]imidazole isobutyl-POSS (6).

In conclusion, we developed one-pot method for the synthesis of POSS-imidazoles from aldehydes and aminopropyl- isobutyl-POSS as the corrected compound in the moderate yield (9-25%). The condensation reaction were conducted from aldehydes and amine derivative via the in situ formation of imine intermediate using NH₄OAc. The product could be applied to the synthesis of OLED, a DSSC senstizier.