Sumanenes

Keywords Buckybowl Carbon nanotube Corannulene Sumanene... 

The chemistry of corannulene-based [4—12] and sumanene-based buckybowls [4, 12-15] has been described in several review articles. In 2006 we published a review article focusing on the solution-phase synthesis of buckybowls, and the structures and physical properties of compounds thus prepared [7]. Since then, this research field has expanded dramatically and many important results have appeared we update the contents herein, and introduce some important compotmds, which are not famished by solution-phase synthesis. Due to page limitations, some... 

Sumanene (tricyclopenta[ /e/jl/,/t(7r]triphenylene, 2) is a C21-hydrocarbon containing three periphery methylene bridges, and it is also a basic bowl-shaped fragment of buckminsterfullerene. Sumanene (2) is named after suman , the Hindi and Sanskrit word for a type of flower [133], In contrast to corannulene, the chemistry of sumanene is relatively new because this compound was first successfully synthesized in the laboratory in 2003 [134]. 

In 1993, an attempt was made to synthesize sumanene (2) from tris(bromomethyl)-triphenylene under FVP conditions [133, 135]. Unfortunately, this protocol closed up to two five-membered rings. Ten years later, Sakurai et al. provided a new synthetic approach to prepare 2 in solution phase under mild conditions [134], Similar to Lawton s synthetic strategy for corannulene, they first constmcted the three-dimensional fi amework using tetrahedral sp carbons and later aromatized the... 

Like fluorene, the three methylene moieties in sumanene can undergo alkylation or aldol condensation (Scheme 38). In the presence of a mixture of aqueous NaOH and Bu4NBr, the reaction of 2 with allyl bromide or 4-methoxybenzyl chloride gave the corresponding hexa-alkylated sumanenes 118 [137]. Similarly, aldehydes yielded 119 in two regioisomeric forms [138]. 

In the presence of RUCI3 and oxidant t-BuOOH, 2 was converted to triketone 120 in a good yield (Scheme 39) [139]. Consistent with the preferred exo attack at trigonal carbons of sumanene, reaction of 120 with methylmagnesium bromide selectively gave the product 121 as a single isomer, where all three methyl groups are at the exo positions. 

Hexafluorosumanene 123 was prepared from sumanenetrione 120 via cyclic dithiane 122 (Scheme 40) [140]. Reaction of 122 with pyridine hydrofluoride (Olah s reagent) in the presence of iV-iodosuccinimide gave 123 in 24% yield. The strong electron-withdrawing properties of fluorine substiments cause the first reduction potential of 123 to be shifted to —1.79 V from —3.21 V (vs ferronce) of the parent sumanenes (2). 

Recently, a new synthetic protocol for synthesis of 9-arylfluorenes or 9,9-diarylfluorenes was reported wherein these products were furnished in good to excellent yields by Pd-catalyzed cross-coupling reactions of fluorene with haloarenes [141]. This synthetic method was also applied in the preparation of aryl-substituted sumanenes. Palladium-catalyzed arylation of sumanene (2) was conducted with l-bromo-2,6-dimethylbenzene, and the desired xylylsumanene 124-xyl was... 

A recent study indicated that the stereoelectronic effect of a curved aromatic stmcture dominates the stability of endo/exo-l24-R (Scheme 41) [142]. Steric repulsion is the main factor to facilitate the conformer cxo-124-SiMe3, whereas some mono-substituted sumanenes, such as methylsumanene 124-Me, hydroxy-sumanene 124-OH, and 2,6-xylylsumane 124-xyl, favor formation of the stable enrio-conformation. In methylsumanene and hydroxysumanene, the hyperconjugation between the benzylic proton and the sumanenyl convex face causes the endo conformer to be more stable than the exo conformer. In 124-xyl, the CH-jt interaction, which is caused by the methyl group in xylyl substituent and the concave face of sumanene, favors the cnrto-conformer. 

Mono-substituted sumanenes 125 can easily be obtained by electrophilic aromatic substitution reaction [143]. However, this protocol is unsuitable for preparation of di- and trisubstituted sumanenes due to the low regioselectivity. Trisubstituted sumanenes, such as C3 symmetric triformylsumanene and its derivatives, were synthesized regioselectively by using suitable reaction intermediates syn-tri(norbomeno)benzenes (cf. Scheme 36) [144]. 

The enantioselective synthesis of a chiral buckybowl was reported by Sakurai and Higashibayashi in 2008 [145]. As in the synthesis of sumanene, C3 symmetric iyn-tris(norbomeno)-benzene 126 is the key intermediate in this synthetic approach (Scheme 42). The synthesis started with enantiopure iodonorbomanone. The... 

The sumanene-type heterobuckybowls such as trithiasumanene 132 and trisUasumanene 133 have also been prepared. The former was synthesized from a regioisomeric mixture of tris(chloroethenyl)benzotrithiophenes under FVP conditions at 1,0(X) °C [150]. In contrast, trisUasumanene 133 was prepared by threefold sila-Friedel-Crafts reactions [151]. The two heterobuckybowls provide a suitable model for systematically smdying the influences of the ring size on the structure (Table 7). 

Table 7 POAV pyramidalization angles and bowl depths of sumanene and its derivatives...

POAV angles at the hub six carbons of the sumanenes, and the values obtained by averaging the... 

POAV angles of symmetry-related carbon atoms of sumanenes... 

Based on the crystallographic analysis, sumanene (2) and trimethylsumanene 129 have similar bowl depths (1.11 A) and comparable POAV pyramidalization angles at the hub six carbons with a value around 8.8° [152]. Hexafluorosumanene 123 (bowl depth 1.24 A, POAV angle 9.6°) is more curved than 2 and 129. The... 

Sumanene (2) and sumanenetrione (120) have similar bowl structures, but different electronic properties [155]. Their nanocrystals can form core-shell assemblies, and aggregates of 120 are wrapped by nanocrystals of 2. The assemblies showed that amplified photoluminescence of sumanenetrione nanocrystals was caused by energy transfer from sumanene nanocrystals through the nanocrystaUine interface. 

The methylene bridge of sumanene can be used for C-C bond formation for expanding the backbone of the n -bowl. The synthesis started with bromosumanene 125, which was generated by selective monobromination of 2 with HBr3 pyridine complex (Scheme 44) [156]. The Suzuki coupling of 125 with 2-formylphenylboronic... 

In 2011, Wu [43] applied intramolecular C-H arylation to the synthesis of highly curved buckybowls, which contain corannulene and sumanene fragments. Several polycyclic aromatic hydrocarbons (PAHs) to prepare less strained bowls have aheady been synthesized using palladium-catalyzed intramolecular arylation reactions [45]. 

SCHEME 17.14 Synthesis of sumanene. (Adapted with permission from Ref. [10]. Copyright (2009) American Chemical Society). 

Stepwise Anion Generation at Benzylic Positions of Sumanene... 

Bowl-to-bowl Inversion of Sumanene s Benzylic Anion Species... 

Bowl-to-bowl inversion is one of the characteristic behaviors for some flexible tt bowls such as corannulene (2) and sumanene (1) (Fig. 35.5). The bowl-to-bowl inversion of the corannulene derivative 9 (Fig. 35.6) occurs rapidly with an activation barrier of 10-11 kcal/mol [31]. The close relationship between the inversion energy barrier and the bowl depth was investigated using corannulene derivatives [32]. An equation between the bowl depth x and the energy barrier (E = Cx ) has been suggested [32]. A relatively deep bowl sumanene, as compared to 2, exhibits much slower inversion. The inversion barrier of 1 was estimated by a variable-temperature NMR study [20]. More detailed investigation was carried out by 2D... 

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