Synthesis of a C₇₀ Fragment Buckybowl C₂₈H₁₄ from a C₆₀ Fragment Sumanene

Abstract

A C 70 fragment (1) (C 28 H 14 ) was synthesized by conversion of a C 60 fragment sumanene, via ring expansion and annulation in 3 steps, the key step being the WagnerMeerwein rearrangement of a five-membered ring to a six-membered one. Both theoretical and experimental investigations exhibited the shallower bowl structure of 1, resulting in the lower bowl inversion barrier than sumanene. It was also revealed that the indenopyrene skeleton largely contributed to the physical properties of 1. Bowl-shaped π-conjugated aromatic hydrocarbons, corresponding to the fragments of fullerenes (buckybowls), have been attracting widespread interest owing to their unique chemical and physical properties.1 Buckybowls, which possess the fragment skeleton of C 60 , such as corannulene and sumanene, have been extensively studied and various unique properties have been reported. 2,3 In contrast, buckybowls bearing C 70 fragment have not drawn much attention and only few examples have been reported to date. 4 The feature of C 70 is that the presence of additional ten carbons, which connect the two C 60 -like hemispheres and provide the shallower curved structure partially than in C 60 skeleton. A simple and effective method to achieve the formation of the fragment skeleton of C 70 is the conversion of the fused five-membered ring in the fragment structure of C 60 to the fused six-membered ring, which realizes the bowl-depth change from deeper to shallower.Here we show the synthesis of a new buckybowl C 28 H 14 (1) (Figure 1), corresponding to a 40% fragment of C 70 from the C 60 fragment sumanene (2) via ring expansion and annulation, in which WagnerMeerwein rearrangement of a five-membered ring to a six-membered one was the key step.The synthetic plan from sumanene 2 to 1 is shown in Scheme 1 according to Bavin's report on the transformation from the fluorene skeleton to the phenanthrene one via Wagner Meerwein rearrangement (Scheme 1).5 Generation of benzylic carbanion by the addition of n-BuLi (150 mol %) to 2 in THF at ¹80°C, followed by the addition of aryl or heteroaryl aldehydes (150 mol %) quantitatively afforded the precursors 3a3c for the rearrangement reaction (Table 1). The WagnerMeerwein rearrangement was carried out by the reaction of 3a3c with stoichiometric p-TsOH in toluene under reflux condition, giving the benzopyrene derivative 4a4c in good yields. X-ray singlecrystal analysis of 4b confirmed its shallower curved structure

Synthesis of C70-fragment buckybowls bearing alkoxy substituents

“…showing two strong bands observed at around 280-300 nm and 330-350 nm, and a broad one at around 350-480 nm, which was attributable to the indenopyrene moiety of 1 (Figure 6a) [18]. Meanwhile, 5a and 5b showed emission bands at 564 nm and 566 nm, respectively, which were red shifted around 50 nm from that of 1, clearly indicating the effect of the introduction of dialkoxides (Figure 6b).…”

“…Dialkoxides 5a-c were prepared according to the previous report on the synthesis of 1 (Scheme 1) [18,20]. The benzylic carbanion generated by the addition of 130 mol % n-BuLi to 2 in THF at −78 °C was treated with 150 mol % of the corresponding arylaldehydes to afford 3a and 3b quantitatively.…”

“…Although the relatively low diffraction data quality prohibited the detailed discussion about the interaction distances, both π-π (C9 5b also gave the mixture of the two enantiomers, however, they were disordered with 50% site occupancy (Figure 2a). The POAV (π-orbital axis vector) pyramidalization angle φ [27], which is often used for quantifying the curvature of curved π-conjugated materials ( Figure 3a) showed 6.5° at C1 as the minimum value, and 7.6° at C3, which is surrounded by two hexagonal rings and one pentagonal ring, as the maximum value, while the none-substituted 1 shows 6.2° and 7.6°, respectively (Table 2) [18]. Bowl depths, defined by the length of the perpendicular lines (Figure 3b, double-headed arrow) from its peripheral carbons to the bottom hexagonal ring's plane (Figure 3c, red coloured part) in 5b were 0.80-0.84 Å from the peripheral benzylic carbons and 0.80-0.89 Å from the peripheral aromatic carbons, respectively, while 0.74-0.79 Å and 0.79-0.99 Å in 1, respectively ( Table 2) [18].…”

“…The POAV (π-orbital axis vector) pyramidalization angle φ [27], which is often used for quantifying the curvature of curved π-conjugated materials ( Figure 3a) showed 6.5° at C1 as the minimum value, and 7.6° at C3, which is surrounded by two hexagonal rings and one pentagonal ring, as the maximum value, while the none-substituted 1 shows 6.2° and 7.6°, respectively (Table 2) [18]. Bowl depths, defined by the length of the perpendicular lines (Figure 3b, double-headed arrow) from its peripheral carbons to the bottom hexagonal ring's plane (Figure 3c, red coloured part) in 5b were 0.80-0.84 Å from the peripheral benzylic carbons and 0.80-0.89 Å from the peripheral aromatic carbons, respectively, while 0.74-0.79 Å and 0.79-0.99 Å in 1, respectively ( Table 2) [18]. As observed in the crystal of 5a, 5b formed convex-to-concave type stacking columns along the c axis while the stacking mode was eclipsed manner, in which molecular skeletons were completely overlapped (Figure 4b Figure 4c).…”

“…Among them, buckybowls having a C 70 fragment are expected to exhibit different properties from that with C 60 fragment because most of them consists of acene and/or pyrene units, which might give unique photochemical and electrochemical properties. Recently, we synthesized a buckybowl C 28 H 14 1, which is corresponding to a 40% fragment of C 70 , from C 60 -fragment sumanene (2) in three steps via ring expansion by Wagner-Meerwein rearrangement, followed by Pd-catalyzed annulation (Figure 1) [18]. An UV-vis spectroscopy study revealed that the electronic character of 1 rather resembled that of an indenopyrene moiety than that of benzopyrene.…”