Synthesis, structure and aromaticity of carborane-fused carbo- and heterocycles

The results of molecular structures, NMR data, and NICS (nucleus-independent chemical shift) and ISE (isomerization stabilization energy) values as well as molecular orbital analyses clearly suggest the presence of considerable aromatic character in the exo five-membered ring of carborane-fused carbo- and heterocycles and considerable conjugation between a 3-D carborane and a fused 2-D π-ring system.


Introduction
Icosahedral carboranes are carbon-boron molecular clusters, featuring a fully delocalized system of 26 skeletal electrons via 3c-2e bonding between tangential p orbitals and radial sp orbitals. 1 Such clusters are aromatic molecules having 3-D aromaticity (s-aromaticity), 2 which are different from classical 2-D aromatic molecules such as benzenes (p-aromaticity). 3 On the other hand, both classes of molecules share some common features such as thermal stability and ability to undergo electrophilic substitution reactions. 4 Conjugation between a 3-D carborane and an exo-X atom via cage CX vertex (X ¼ atom with p-donor ability) has been evidenced by experimental and theoretical results, and such a unique s-p conjugation is enhanced as the p-donor ability of X atom increases. 5 However, conjugation between a 3-D carborane and a fused 2-D p-ring system is ambiguous. No s-p conjugation is observed between the C 2 B 10 cage and the diene moiety in benzocarborane (A in Fig. 1), in which the C-C double bonds are localized with the NICS(0) (NICS: nucleusindependent chemical shi) values of À0.7 to À3.4 ppm for exo-C 6 H 4 six-membered ring in benzocarborane derivatives. 6 In contrast, a considerable s-p conjugation is reported for CB 11 cage-fused heterocyclic anion [1,2-N 3 R-1-CB 11 Cl 10 ] À (B in Fig. 1), leading to an aromatic exo-CBN 3 ve-membered ring with a NICS(0) value of À7. 8 ppm. 7 In view of wide applications of carboranes as a unique electronic sink and transmitter in optoelectronic materials, 8 we initiated a research program to study the s-p conjugation in o-carborane-fused ve-membered carbo-and heterocycles, as well as the aromaticity of fused exo ve-membered rings (C in Fig. 1). The results of this work would shed some light on the design of new carborane-fused p conjugated materials.

Result and discussion
Synthesis Scheme 1 outlines the synthetic route to carborane-fused cyclopentadienyl anions. Compound 1 was synthesized according to the method given in the literature. 9 Solvent-free dehydration of 1 in the presence of 1 equiv. of p-TsOH at 180 C gave 2 as colorless crystals in 66% yield. Treatment of 2 with 1 equiv. of n-BuLi in THF afforded the corresponding lithium salt 3 as a yellow powder in quantitative yield. Recrystallization of 3 from a THF solution containing 12-crown-4 ether produced 4 as yellow crystals in quantitative yield.
Treatment of 1-thio-o-carborane 10 (5) with an excess amount of NaH followed by reaction with a-bromoacetone gave 6 as a white solid in 68% yield. Cyclization of 6 in the presence of tetrabutylammonium uoride (TBAF) generated compound 7 as a white solid in 25% yield. The carborane-fused thiophene (8) was synthesized as colorless crystals in 80% yield via solventfree dehydration in the presence of 1 equiv. of p-TsOH at 180 C for 4 hours (Scheme 2).
In a similar manner, carborane-fused furan (12) was prepared as a colorless liquid via concentrated H 2 SO 4 mediated dehydration of 11 in 50% yield (Scheme 3).
The reaction of o-carboranyl triate (13) with lithium Nallylmethylamide afforded the corresponding amine 14 as a colorless liquid in 53% yield. 11 Ruthenium-catalyzed olen isomerization 12 and cage carbon iodination gave compound 16 as a light yellow solid in 50% yield. UV irradiation of 16 produced carborane-fused pyrrole (17) as colorless crystals in 30% yield (Scheme 4). 13 Similarly, iodination of 18, 11 followed by UV irradiation of 19 generated a carborane-fused indole (20) as pale yellow crystals in 50% yield (Scheme 5). 13 Structural characterization NMR spectroscopy serves as a useful tool to determine whether a compound has aromatic character. 14 The fused vemembered ring proton was observed at 6.53 ppm for 3 and 6.10 ppm for 4, which was shied downeld compared to that of 5.75 ppm in 2, suggesting that the fused ve-membered ring in 3 and 4 has some aromatic character. On the other hand, the CH proton in carborane-fused heterocycles was observed at 6.27 ppm in 8, 6.58 ppm in 12 and 6.05 ppm in 17, which was compared to that of 6.87 ppm in 3-methylthiophene, 15 7.16 ppm in 3-methylfuran 15 and 6.35 ppm in 3-methyl-N-methylpyrrole, 16 respectively. These measured proton chemical shis were considerably downeld shied in comparison to those observed in the corresponding dihydrothiophene, 17 dihydrofuran, 18 and dihydropyrrole, 19 indicating that the fused ve-membered rings in 8, 12 and 17 have some aromatic character.
Single-crystal X-ray analyses conrmed the molecular structures of 2, 4, 8, 17 and 20. Their representative structures are shown in Fig. 2. The results clearly indicate that the fused vemembered rings in 4, 8, 17 and 20 are co-planar with the sum of the internal pentagonal angles being 540 .
As shown in Fig. 3, the bond distances of the fused vemembered ring in 4 are averaged in comparison with those observed in 2, suggesting the presence of some degree of delocalization within the ve-membered ring. Except for the cage C(1)-C(2) distance, the measured distances and angles of the fused ve-membered heterocycles in 8, 17 and 20 are comparable to those observed in thiophene, 20 pyrrole 21a and indole. 21b The UV-Vis spectra of 2, 4, 8, 12, 17 and 20 in THF were obtained, and they are shown in Fig. S7 and S8 in the ESI. † The UV-Vis spectrum of 4 (see Fig. S7 in the ESI †) displayed a new absorption band centered at 423 nm, which was assigned as p / p* absorption of the delocalized system of the exo vemembered ring.
The UV-Vis spectra of carborane-fused heterocycles (see Fig. S8 in the ESI †) showed absorptions between 287 and 290 nm attributable to n / s* transition and between 306 to 330 nm corresponding to the p / p* transition of the delocalized exo ve-membered ring system, which were red-shied compared to those observed in heteroarenes (l max ¼ 205-218 nm) 22 and benzene-fused heterocycles (l max ¼ 282-298 nm). 23 These assignments were supported by time-dependent DFT (TD-DFT) calculations (see ESI †).
The cyclic voltammograms of 8, 17, 20, indole and benzothiophene obtained from solutions in THF are shown in Fig. S10 (ESI †). The absorption edge and reduction potentials of the aforementioned compounds are summarized in Table S1 in the ESI. † Based on these data, the energy levels of LUMO (the Lowest Unoccupied Molecular Orbital) and HOMO (the Highest Occupied Molecular Orbital) of these compounds were estimated and are shown in Fig. 4 (also see Table S1 in the ESI †). These results indicate that the electron-withdrawing nature of carboranyl moiety decreases the LUMO energy levels of 8, 17 and 20 compared to those of benzo-fused heterocycles. The DFT-calculated (at the B3LYP/6-311++G(d,p) level of theory) HOMO energy levels are in general agreement with the corresponding experimental values, whereas the DFT-calculated LUMO energy levels are higher than the corresponding    Computational studies NICS values have been used extensively for the identication of aromatic properties of molecules. 25 In this regard, NICS values of carborane-fused ve-membered carbo-and heterocycles were calculated at the B3LYP/6-311++G(d,p) level of theory. For comparison, NICS values of cyclopentadiene, cyclopentadienide, thiophene, furan, pyrrole and their benzo derivatives were also calculated at the same level of theory. The results are compiled in Table 1. The data (À9.0 to À9.9 ppm) suggest that carborane-fused ve-membered carbo-and heterocycles have considerable aromatic character. These results are consistent with those obtained from the aforementioned NMR data and structural parameters. Another parameter used to estimate aromatic stabilization energy is the isomerization stabilization energy (ISE). 26 The calculated ISE data for carborane-fused heterocycles and the related systems are summarized in Table 2. The results clearly show that the ISE values of carborane-fused thiophene, furan and pyrrole are about half (À6.4 to À8.8 kcal mol À1 ) of those calculated for benzo-thiophene, -furan and -pyrrole that are typical aromatic molecules.
To understand the electronic structures of carborane-fused heterocycles, DFT calculations at the B3LYP/6-311++G(d,p) level of theory were performed for both 8 and benzothiophene. The optimized bond distances and angles are in very good agreement with the experimental values of 8 (see Table S5 in the ESI †). Their selected molecular orbitals (MOs) are shown in Fig. 5, revealing the signicant mixing between the carborane cage and the fused exo ring. It is noteworthy that the ve MOs (HOMOÀ1, HOMOÀ6, HOMOÀ10, LUMO+4 and LUMO) in 8 resemble those found in benzothiophene, corresponding to p and p* MOs. These results again suggest that the carborane moiety can utilize its cage carbon p orbitals to participate in the exo p bonding interactions, resulting in considerable conjugation between a 3-D cage molecule and a 2-D p system.

Conclusions
Several new carborane-fused cyclopentadienyl anion, furan, thiophene, pyrrole and indole have been prepared. NMR data, X-ray structural parameters, NICS and ISE values as well as molecular orbital analyses prove that the fused ve-membered rings in the aforementioned carborane-fused cyclics have considerable aromatic character, and the conjugation between a 3-D cluster and a fused 2-D p system has been realized by the p donation of the exo heteroatom/carbanion. These results will shed some light on the design of new carborane-based p molecules for material applications.

Conflicts of interest
There are no conicts to declare.