Despite its complicated nature, the Scholl reaction has been extensively used in organic synthesis [1-11]. In Scholl oxidation, a new C-C bond between two aryl moieties is generated under the influence of Friedel-Crafts catalyst. A Lewis acid and an oxidant is required to accomplish this reaction. Transition metal halides, such as, FeCl₃, MoCl₅, etc., which act both as Lewis acid and oxidant are often applied to achieve this condensation. Though this reaction was discovered more than a century ago [12], its mechanism is still under debate. Two possible reaction mechanisms, arenium cation mechanism [9] and radical cation mechanism [4], have been discussed in literature. Recently, the Scholl coupling has been well exploited in the synthesis of discotic liquid crystals (DLCs). DLCs are formed due to self-organization of disc-like molecules [13]. Depending upon the strength of molecular interactions, disc-like molecules can form nematic or columnar phases. The columnar phases formed by these molecules have been extensively studied for one-dimensional conducting properties and their applications in devices like, phovoltaic solar cells, light emitting diodes, sensors, thin film transistors, etc., have been sought. The chemistry and physics of DLCs have recently been reviewed in several research articles [14-31].

Since the discovery of DLCs, triphenylene-based DLCs remained the focal point of research in this field [32-37]. This is mainly because these materials are chemically and thermally stable, their chemistry is relatively easy and they show many different mesophases. Moreover, their one-dimensional conducting properties offer many potential applications. Accordingly, a large number of triphenylene (TP) discotics have so far been prepared to investigate their mesomorphic properties [15]. Hexaalkoxy-TPs are the most widely synthesised and studied discotic mesogens and a number of methods have been developed for their synthesis [34]. The synthesis of hexaalkoxy-TPs involves oxidative trimarization of 1,2-dialkoxybenzene which is one of the unusual cases of Scholl reaction where more than one aryl-aryl bonds are formed. 1,2-dimethoxybenzene (veratrole) was oxidatively trimerized using chloranil or FeCl₃ in concentrated H₂SO₄ at room temperature to obtain hexamethoxy-TP [38]. Initially TP discotics were prepared using this sluggish low yielding methodology but later the Leeds group developed an efficient methodology to prepare triphenylene hexaethers via oxidative trimerization of 1,2-dialkoxybenzene using FeCl₃ using only a catalytic amount of H₂SO₄ (0.3%) in dichloromethane followed by a reductive work-up using methanol [39]. Previously we have reported two other reagents, molybdenum pentachloride [40] and vanadium oxytrichloride [41], to be highly efficient for the preparation of triphenylene hexaethers. Here we discovered yet another reagent, antimony pentachloride (SbCl₅), which can be efficiently used to prepare hexaalkoxy-TP in good yield. Antimony pentachloride is a strong Lewis acid and its application in Friedel-Crafts reaction is well documented [42]. However, its application as Scholl oxidant for the synthesis of triphenylene discotics has so far not been reported. Here we report the synthesis of symmetrical and unsymmetrical triphenylene discotics using this new reagent.

In a typical reaction (Scheme 1), SbCl₅ (4.48 g, 0.015 mol) was added to a solution of 1,2-dibutoxybenzene (1.5 g, 0.0067 mol) in 15 ml of dry CH₂Cl₂. The reaction mixture was stirred at room temperature for 30 min under anhydrous conditions. It was then poured over cold MeOH (50 ml), diluted with water (50 ml) and extracted with hexane (4×30 ml). The combined extracts were washed with water and brine, dried over anhydrous sodium sulphate, the solvent was removed under vacuum and the crude product was purified by column chromatography over silica gel, yielding 1.15 g (78%) of hexabutoxytriphenylene. All the symmetrical hexaalkoxy-triphenylenes were prepared in the same manner and their yield is reported in Table 1. They were fully characterized from their spectral and elemental analysis. All the hexaalkoxy-TP exhibit similar ¹H NMR differing only in the aliphatic protons.

Scheme 1: Synthesis of symmetrical hexaalkoxy-TPs via oxidative trimerization of 1,2-dialkoxybenzene using antimony pentachloride.

Table 1: Preparation of hexaalkoxy-TPs using SbCl₅ in CH₂Cl₂ solution.

Typical ¹H NMR data for the compound 2a: δ_H(CDCl₃) 7.83 (s, 6H), 4.23 (m, 12H), 1.94 (m, 12H), 1.6-1.3 (m, 12H), 0.9 (t, 18H). Elemental anal.: 2a: calculated for C₄₂H₆₀O₆, C 76.33, H 9.15; found, C 76.45, H 8.94%; 2b: calculated for C₄₈H₇₂O₆, C 77.38, H 9.74; found, C 77.62, H 9.44%; 2c: calculated for C₅₄H₈₄O₆, C 78.21, H 10.21; found, C 78.4, H 10.44%. Thermal behaviour: 2a: Cr 89.9 Col_p 145.6 I; 2b: Cr 71.4 Col_h 121.5 I; 2c: Cr 67.4 Col_h 100.0 I.

To prepare an unsymmetrical heptaalkoxy-TP derivative 5 (Scheme 2), SbCl₅ was added to a solution of 3,3’,4,4’-tetrabutyloxybiphenyl 3, and 1,2,3-tributyloxybenzene 4 in dichloromethane. Typical work-up give 1,2,3,6,7,10,11-heptabutyloxy-TP 5 in about 25% yield (Table 1, entry 7). ¹H NMR: δ_H(CDCl₃) 9.2 (s, 1 H), 7.8 (s, 2 H), 7.8 (s, 1 H), 7.7 (s, 1 H), 4.2 (m, 12 H), 4.0 (t, J 7.1, 2 H), 1.9 (m, 14 H), 1.6 (m, 14 H) and 1.0 (m, 21 H). Elemental anal.: 5: calculated for C₄₆H₆₈O₇, C 75.37, H 9.35; found, C 75.0, H 9.44%. Thermal behaviour: Cr 65.5 Col 70.1 I.

Scheme 2: Synthesis of unsymmetrical heptaalkoxy-TPs via oxidative coupling of a 1,2,3-trialkoxybenzene with a tetraalkoxybiphenyl.

The trimerization of 1,2-dialkoxybenzene to hexaalkoxy-TP is presented in Scheme 1. Addition of antimony pentachloride (2.5 equivalent) to a solution of 1,2-dialkoxybenzene in dichloromethane yields 2,3,6,7,10,11-hexaalkoxy-TPs in 30 min (Table 1). It has been reported that the addition of a catalytic amount of mineral acid improves yields in some cases [15,16]. However it is also known that Lewis acids can complex with mineral acids [4], reducing the effective amount of the oxidant and thus yield of the final product. Therefore, reactions were carried out under both conditions and results are collected in Table 1. As can be seen from Table 1, entry 2, addition of acid decreases the yield, indicating complex formation between the Lewis acid and mineral acid. Further, increasing the amount of SbCl₅ from 2.5 eq to 3.2 eq (Table 1, entry 4) does not increase the yield of final pure product probably due to the formation of other side products at the expense of hexaalkoxy-TP. All the products were characterized from their spectral data, phase behaviour and a direct comparison with an authentic sample and found to be in full agreement with literature data [15].

To compare the potential of SbCl₅ with respect to other three Lewis acids viz, FeCl₃, MoCl₅ and VOCl₃, known for the synthesis of hexaalkoxy-TPs, reactions were carried out under identical conditions using 1,2-dibutyloxybenzene as substrate (Table 1, entries 8-10). It may be noted that the optimised reaction conditions for different reagents could be different. Therefore, the best yield for various reagents could be different under optimised conditions. From the data, it is clear that under the one set of identical reaction conditions, SbCl₅ is slightly better than FeCl₃ but not the best reagent known in literature for this trimerization.

As reported for other reagents viz, FeCl₃, MoCl₅ and VOCl₃, this reagent can also be used for the synthesis of unsymmetrical TPs, albeit in poor yield, under similar reaction conditions (Scheme 2). Thus, when 3,3’,4,4’-tetrapentyloxybiphenyl 3, was coupled with a 1,2,3-trialkoxybenzene 4, 1,2,3,6,7,10,11-heptabutyloxy-TP 5 is formed in about 25% yield (Table 1, entry 7).

In conclusion, Antimony(V) chloride, a new reagent, was found to be useful in the synthesis of symmetrical and unsymmetrical alkoxytriphenylene DLCs. The reagent may be explored for the synthesis of various other discotic liquid crystals involving iner- or intramolecular Scholl oxidation.