Synthesis and Reactions of Dibenzo[a,e]pentalenes

Pentalene has recently received a considerable amount of attention as a ligand in sandwich-type transition metal complexes. In contrast, dibenzo[a,e]pentalene (hereafter denoted as dibenzopentalene), which is more -extended than pentalene, has received less attention, despite its potential usefulness as a building block of ladder-type -conjugated molecules, which have recently received growing interest. However, very recently, several novel efficient methods for the synthesis of dibenzopentalenes have been reported. This review surveys recent advances in the synthesis and reactions of dibenzopentalenes and describes the aromaticity of their ionic species.


Introduction
The parent pentalene has 8 electrons, which indicates its antiaromatic nature, and is very highly reactive, having only been observed using a matrix isolation technique at 196 °C (Chart 1) [1,2].In contrast, its dianion has 10 electrons and is calculated to have considerable aromaticity [3].The stable pentalene dianion was first synthesized in 1962 [4,5] and the X-ray crystal structure of its dilithium salt was reported in 1991 [6].On the other hand, no reports on the generation of a pentalene dication have appeared to date, and its aromaticity is still controversial [3,7].The pentalene dianions are now widely used as ligands of sandwich-type transition metal complexes [8][9][10][11].However, the dibenzo[a,e]pentalenes (hereafter denoted as dibenzopentalenes), which are more -extended than the pentalenes have received less attention despite their potential usefulness as building blocks of laddertype -conjugated molecules, which are of growing recent interest (Figure 1).The main reason for this OPEN ACCESS is a limited number of methods for the synthesis of dibenzopentalene frameworks, even though synthesis of a dibenzopentalene was first reported in 1912 [12].However, very recently, efficient and unique methods for the synthesis of dibenzopentalenes together with reports of their reactivity have successively appeared.It is therefore time to briefly survey the recent advances in the synthesis and reactions of dibenzopentalenes and to discuss the aromaticity of their derivatives.

Preparation of Dibenzopentalenes by Thermolysis
Thermolysis, such as the flash vacuum pyrolysis (FVP) technique, is sometimes a very powerful tool for the creation of rigid -frameworks through somewhat unexpected and complex reaction pathways.
Thermolysis is indeed efficient for the preparation of dibenzopentalenes.However, it needs a special apparatus: hence, it is rather difficult to carry out.Moreover, it is difficult to prepare functionalized dibenzopentalenes using thermolysis.

Preparation of Dibenzopentalenes from Dibenzocyclooctene Derivatives through Skeletal Rearrangement
Anionic dibenzocyclooctene derivatives are known to undergo skeletal rearrangements to afford the corresponding dibenzopentalenes.

Preparation of Dibenzopentalenes in the Presence of Catalysts
Catalytic reactions for the preparation of dibenzopentalenes have also been reported.In early studies, the key starting compound was 1,2-bis(phenylethynyl)benzene (29a), treatment of which with PtCl 4 in benzene afforded a Pt-complex of dibenzopentalene 1bb, an unstable compound that converted to 1bb during purification (Scheme 15) [36,37].The yield of 1bb was 85%.This method was later applied for the preparation of dibenzopentalenes 1cc-1ff (Scheme 15) [38].When PdCl 2 was used as a catalyst, a Pd-complex of 1bb was isolated and treatment of the resulting complex with triphenylphosphine provided 1bb, even though the yield was very low.A mixture of H 2 PtCl 6 ·6H 2 O and Aliquat 336 ® (Aliquat = methyltrioctylammonium chloride) was also used as a catalyst for the preparation of 1bb with 80% yield [39].When the reactions were carried out in the presence of dialkyl acetylenedicarboxylates, the corresponding adducts 1gg-1ii were obtained (2536% yields) together with 1bb (Scheme 16) [40].The cyclization reactions of 1,2-bis(phenylethynyl)benzenes to afford dibenzopentalenes were surprisingly catalyzed by tellurium (Scheme 17) [38].Heating of 29a with a catalytic amount of tellurium in pentachloroethane (PCE) under reflux provided dibenzopentalene 1jj in 61% yield through a halogen transfer reaction (Scheme 17).When 29a was heated with a catalytic amount of tellurium in tetrabromoethane at 170 °C, the corresponding bromodibenzopentalene 1kk was obtained in 6% yield (Scheme 18) [38].Treatment of 29b with a catalytic amount of tellurium in PCE under reflux produced a 1:1 mixture of 1ll and 1mm in 79% yield.Likewise, 29c reacted in PCD in the presence of a catalytic amount of tellurium to afford an 1:1 mixture of 1nn and 1oo in 83% yield.
1cc: 1dd: 1ee: 1ff: Two phenylacetylenes, one of which has a halogen atom and the other of which has a Bu 3 Sn group were coupled in the presence of CsF, t Bu 3 P and Pd 2 (dba) 3 to give dibenzopentalenes (Scheme 19) [41].The coupling reaction of 30a and 31a under the reaction conditions shown in Scheme 19 provided dibenzopentalene 1a in 61% yield.A crossover coupling reaction of 30b and 31a under the same conditions afforded homocoupled products 1a (20% yield) and 1qq (trace), as well as the crossovercoupled product 1pp (40% yield), suggesting that the homocoupling reaction of 31a would proceed.
Heating of 31a and 31b afforded 1a and 1qq, respectively in very high yields with 2 equivalents of hydroquinone, Cs 2 CO 3 and CsF in the presence of catalytic amounts of t Bu 3 P and Pd 2 (dba) 3 in 1,4dioxane at 135 ºC (Scheme 20) [41].This method could be applied for the preparation of a variety of dibenzopentalenes (5572% yields) (Scheme 21) [41].The most striking feature of this catalytic system is that a heteroaromatic analog 34 of a dibenzopentalene was able to be synthesized.1nn: 1oo: Being inspired by this catalytic system, an extremely simple synthesis of dibenzopentalenes from 2bromoethynylbenzenes was reported.Treatment of 2-bromoethynylbenzene 25d with an equivalent of Ni(PPh 3 ) 2 Cl 2 and 1.5 equivalents of Zn at 80 ºC produced dibenzopentalene 1uu in 41% yield (Scheme 23) [43].Reaction of 25d with Ni(cod) 2 and PPh 3 at 50 °C afforded Ni complex 36, which was transformed into 1uu at 80 ºC.In the initial step, a generated intermediate Ni(0) complex Ni(PPh 3 ) 2 therefore underwent oxidative addition to 25d to give 36.The yield of 1uu was improved to 46% in the presence of Ni(cod) 2 , PPh 3 and Zn at 110 ºC.This method enabled the synthesis of a variety of dibenzopentalenes (1324% yields) (Scheme 24 and 25) [43].It is noted that the electronic nature of substituents on the aromatic rings did not affect the formation of dibenzopentalenes.A unique two-step synthesis of a dibenzopentalene was also reported recently.The two starting compounds 37 [44] and 38 were prepared from commercially available 3,5-dimethoxybenzaldehyde in

Preparation of Dibenzopentalene Polymer by Electrochemical Polymerization
Starting from diketone 2, thienyl-substituted dibenzopentalenes were synthesized by the classical method (Scheme 27) [15].Diketone 2 reacted with thienyllithium or thienylmagnesium bromide to afford the corresponding adducts, which were treated with acid to provide thienyl-substituted dibenzopentalenes 1ac-1ae.When a cyclic voltammetric analysis of 1ac was carried out, the growth of a new redox system was observed upon scanning between 1.0 and 1.2 V and the electrode was modified, suggesting polymerization of 1ac on the electrode.The anodic study of the modified electrode between 1.0 and 1.5 V revealed that the original species on the modified electrode was further modified, perhaps due to intramolecular cyclization or polymerization through the benzene moiety (Scheme 28) [15].The band gap of the resulting polymer was estimated to be about 2.2 V.

Aromaticity of Dibenzopentalenes
To evaluate aromaticity of dibenzopentalene, current densities in -systems induced by external magnetic fields were calculated [46].The current pattern of dibenzopentalene has three distinct regions, and the central paratropic ring current in the two five-membered rings is bordered by diatropic ring currents in the two benzene rings.The paratropic ring current in the central five-membered rings is derived from two -electrons in the HOMO, whereas the diatropic benzene ring currents arise from several orbitals that lie just below the HOMO.This regional character of the current patterns is consistent with NICS [47] values for individual rings: the NICS values of the five-and six-membered rings are 7.4 and 9.8 ppm, respectively.1ac: 1ad: 1ae: Redox behavior of dibenzopentalenes is of considerable interest because two-electron oxidation and reduction of dibenzopentalenes afford the corresponding 14 dications and 18 dianions, respectively, which are both expected to be aromatic.
Treatment of dibenzopentalenes 1e and 1g with excess SbF 5 in SO 2 ClF resulted in dark green and violet-purple solutions, respectively, attributed to the formation of dications 40 (Scheme 29) [48,49].Dianion 26b of the parent dibenzopentalene 1g was synthesized by the reaction of dihydrodibenzopentalene 4a with butyllithium (Scheme 30) [48,49].Although the H1 and H9 resonated upfield, compared with those of 1g, the other protons on the benzene rings were deshielded, suggesting diatropic ring current over the perimeter of the framework.It is therefore concluded that 26b should be regarded as a peripheral aromatic dianion.Dianion 26c of dibenzopentalene 1e was synthesized by the reaction of 1e with lithium, and a considerably aromatic character was observed (Scheme 30) [48,49].

Scheme 30. Formation of Dianions of Dibenzopentalenes.
Very recently, the first report on the molecular structure of a dianion of a dibenzopentalene has appeared (Scheme 14) [33].The X-ray crystallographic analysis of 26a revealed that the fivemembered rings contain nearly equalized CC bond lengths, whereas a slight bond alternation in those of the six-membered ring is found, suggesting preferable aromatic delocalization in the five-membered ring over benzenoid delocalization.Similar trends were also found in some benzannulated anions [50][51][52][53][54][55][56].Dianion 26a was also synthesized by the reaction of the corresponding dibenzopentalene 1aa with lithium (Scheme 31) [57].Oxidation of 26a occurred by treatment with iodine to provide 1aa.Dications and dianions are formed through the cation radical and the anion radical, respectively.The synthesis of cation radicals 41a and 41b of dibenzopentalenes were accomplished by the reactions of dibenzopentalenes 1e and 1g with aluminum trichloride, the formation of which was evidenced by ESR spectroscopy (Scheme 32) [58].Respective anion radicals 42a and 42b of 1e and 1g were synthesized by the reactions of 1e and 1g with potassium (Scheme 33) [58].Subsequent challenging tasks are to characterize molecular structures of cation radicals, anion radicals and dications of dibenzopentalenes.

Reactions of Dibenzopentalenes
Although a variety of dibenzopentalenes can now be synthesized, their reactivity has been explored very little.Very recently, unique reactions of a dibenzopentalene were reported.Dibenzopentalene 1aa reacted with methyllithium to quantitatively produce lithium 5-methyldibenzopentalenide 42, the structure of which was established by X-ray crystallographic analysis (Scheme 34) [57].Since 6,6-dimethylfulvene derivative reacted with methyllithium to give the corresponding lithium t-butylcyclopentadienide [59], 1aa reacted with methyllithium as a fulvene to give 43.The CC bond lengths of the cyclopentadienyl anion moiety differ slightly, in contrast to dilithium dibenzopentalenide 26a [33], which displays no alternation of the CC bonds in the five-membered ring.The six-membered ring adjoining the anionic five-membered ring also has different CC bond lengths in the anion.On the other hand, remarkable alternation of the CC bonds is found in the cyclopentadiene ring of 43.Nucleus-Independent-Chemical-Shifts (NICS) values calculated at 1.0 Å above (12.5 ppm) and below (12.9 ppm) the cyclopentadienide ring [47,60] of the model compound of 43 are negative, suggesting aromatic character of the cyclopentadienide ring.

Summary and Outlook
Dibenzopentalenes have long been known, and their unique redox behavior leading to 14 and 18 aromatic species is well established.However, their chemistry is still limited because of the lack of versatile synthetic methods.Very recently, there have been reported several efficient methods for the synthesis of a wide variety of dibenzopentalenes, together with their unique reactions.It is therefore appropriate to cultivate a new chemistry of dibenzopentalenes, which would be applied as building blocks of new -extended sandwich complexes and redox-active materials.This paper is dedicated to Professor Robert West on the occasion of his nomination as an honorary member from the Chemical Society of Japan.This work was partially supported by a Grant-in-Aid for Young Scientists (B), No. 17750032 (M.S.) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.M. Saito acknowledges a research grant from the Sumitomo Foundation.

Scheme 19 .
Scheme 19.Coupling of Two Acetylenes in the Presence of CsF, t Bu 3 P and Pd 2 (dba) 3 .
Scheme 14. Formation of Dibenzopentalene by Reduction of Phenylsilylacetylene.
2 =Cl [42]coupling of Acetylenes with hydroquinone, Cs 2 CO 3 and CsF in the Presence of t Bu 3 P and Pd 2 (dba) 3 .Preparation of a Variety of Functionalized Dibenzopentalenes.Another catalytic synthesis of a dibenzopentalene from 2-halophenylacetylene was carried out using Pd(PPh 3 ) 2 Cl 2 and CuI (Scheme 22)[42].Heating of a mixture of 2-iodophenylacetylene 35 with Pd(PhCN)Cl 2 , PPh 3 and CuI in toluene and diisopropylamine at 90 °C provided dibenzopentalene 1tt in 67% yield (Scheme 22).Although this system is simpler than that in Scheme 21, no further applications for the synthesis of dibenzopentalenes have been reported.