Calicene

FIGURE 11 17 Elec trostatic potential map of calicene (Problem 11 47)... 

Tlie name fulvaleiie was first mentioned by R. Brown (49TFS296), who expanded the class of fulvenes for those compounds containing two cyclic poly-enic systems with a central double bond. Tlius, depending on the ring size, cy-clopropylidenecyclopropene 1, fulvaleiie 2, heptafulvalene 3 and the unsymmetrical hybrid molecules triapentafulvalene (calicene) 4, triaheptaful-valene 5, and pentaheptafulvalene (sesquifulvalene) 6 are members of this class of cyclic cross-conjugated systems (Scheme 1). 

Calicene. like azulene (Problem 15.17), has an unusually large dipole moment for a hydrocarbon. Explain, using resonance structures. 

Triafulvene 69 was synthesized by an interesting route starting with diphenyl cyclopropenium cation and lithio ethyl acetate72. Although widely used in calicene chemistry (see later) this reaction principle — like the Wittig reaction — did not find general application in methylene cyclopropene synthesis. 

Since the chemistry of calicenes has been the subject of former reviews5,8 only the principal features for synthesis of calicenes and their mono- and dibenzo derivatives need to be discussed. In general, methods from (a) are used with appropriate variations. 

Li, Na, or Grignard compounds of cyclopentadiene, indene, or fluorene add to di-substituted cyclopropenium cations forming cyclopentadienyl cyclopropenes, which can be transformed to calicenes by subsequent hydride abstraction and deprotonation, as shown by the following examples 9S91 and 9992 (cf. p. 13) ... 

Analogously the ethoxy cation 75 was found to be valuable for the synthesis of calicenes when combined with cyclopentadienyl anions93 Hexaphenylcalicene (100) was prepared by this route from tetraphenylcyclopentadienyl-Li94,76) and the highly polar dicyanocalicene 10195 from the tetramethylammonium salt of dicyano-cyclopentadiene. 

As was recently found by Murata96, Na-tetrachlorocyclopentadienide yields the calicene 103 only as a minor product when reacted with ethoxy cation 102 the main reaction consists of an unexpected ring expansion giving rise to dihydropenta-lenone 104 ... 

Derivatives of calicene that are unsubstituted at the five-membered ring are unknown with the exception of the cationic complex 105 prepared from ferrocene and 3,3-dichloro-1,2-diphenyl-cyclopropene97 ... 

Some aspects of the UV spectroscopy of calicenes and quinocyclopropenes deserve attention. Thus, the UV data of the tetrachloro compound 184 and the thioketal 196 suggest93 that the absorption maximum of the parent pentatriafulvene (197) should Ue close to 300 nm ... 

The di-tert. butyl substituted calicene 230 was calculated to possess considerable non-bonding (steric) interactions in the planar geometry184. The relief of strain when going to a perpendicular transition state is reflected by the low coalescence temperature of tert. butyl signals found on temperature-dependent H-NMR spectroscopy187. ... 

Tetrachloro-5,6-diphenyl- and -5,6-di-n-propyl calicene were found to yield intensely colored radical anions when interacted with alkali metals (Na, K)291. As ESR-spectroscopic investigation showed an unpaired electron spin is located on three-ring carbons Cs/C6 and their substituents. 

In contrast, with the calicene 230 TCNE is attached to five- and three-membered rings in a more complicated cycloaddition mode giving rise to 496s With a series of other calicenes no cycloaddition, but formation of stable charge-transfer complexes was observed2 93 ... 

Acetylene dicarboxylate and maleic anhydride failed to react with simple methylene cyclopropenes, but reacted readily with calicene derivatives, as shown by Prinz-bach293. Thus ADD combined with benzocalicene 497 to give the dimethyl tri-phenylene dicarboxylate 499, whose formation can be rationalized via (2 + 2) cycloaddition across the semicyclic double bond as well as (4 + 2) cycloaddition involving the three-membered ring (498/501). The asymmetric substitution of 499 excludes cycloaddition of ADD to the C /C2 triafulvene bond (500), which would demand a symmetrical substituent distribution in the final triphenylene derivative. 

The addition of maleic anhydride to dibenzo calicene 49 7293 proceeds according to the same (2 + 2) or (4 + 2) mode discussed for the addition of ADD, giving rise to the dihydro triphenylene dicarboxylic anhydride 505, which is capable of addition of a second molecule of the dienophile 506). 

In some cases the C /C2 double bond in methylene cyclopropenes and calicenes was found to show dienophilic functionality towards diene components. Thus, di-ethylamino butadiene combines with 497 to give the Diels-Alder adduct 507, whose proton-catalyzed elimination of amine interestingly did not lead to the dibenzo heptafulvalene 508, but to the methylene norcaradiene derivative 509293 ... 

The calicene derivative 185 shows ambiguous behavior toward ynamines. Whilst reacting with yndiamine 542 according to the above (2 + 2) mode to give the fulvalene 545300), with ynamine 544 a (4 + 2) cycloaddition mode appears to operate which leads to the naphthalene derivative 545301. This is in accordance with the reactivity of other calicenes toward ADD shown earlier (p. 93). 

With calicenes, diazoalkanes were found293 to react in a different manner from other triafulvenes. Thus, dibenzocalicene 497 together with diazomethane gives the product of addition of two moles of the diazo compound 558, which is likely to arise from primary attack of the 1,3-dipole via (3 + 2) cycloaddition to the triafulvene C3/C4 bond (557). 

Application of the original sequence to cyclic ketones, such as fluorenone, benzotropone, and dibenzotropone afforded calicene 242 and the triaheptafiilva-lenes243,244. ... 

Similariy, a series of the related diaminocalicene carbonyl complexes (triapentaful-valenes) of group 6 was reported to be formed upon reaction of the parent bis(dialky-lamino)calicene with M(CO)6 (M = Cr, Mo, W). Likewise, a ferrocene analogue incorporating two diaminocyclopropenylium rings was synthesized by reaction of two equivalents of calicene with FeCl2 (equation 366). ... 

---