Calicenes synthesis

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. 

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. 

A. Triafulvenes, Calicenes and Quinocyclopropenes Synthesis Houben-Weyl... 

The condensation reactions of cyclopropenones provide a simple and effective method for the synthesis of stable calicenes. Thus, 2,3-diphenyl- and 2,3-dialkylcyclopropenones 8 slowly react with 1,2,3,4-tetrahalocyclopenta-l,3-diene in methanol at around room temperature to precipitate yellow crystals of l,2,3,4-tetrachloro-5,6-diphenylcalicene (9, R = Ph) and colorless crystals (e.g., when R = Pr) of 5,6-dialkyl-l,2,3,4-tetrahalocalicenes 9in moderate yields. "... 

Cyciopropenylium ions with a leaving group provide a straightforward synthesis of calicenes thus, the reaction of tetraphenylcyclopentadienide 17 or dicyanocyclopentadienides 19 and 3,3-dichloro-l,2-diphenylcyclopropene or l-ethoxy-2,3-diphenylcyclopropenylium ion directly yielded calicene derivatives. 

This novel calicene chemistry has been further developed into the synthesis of novel heterocycles (e.g., 31) and cyclic compounds built up of only calicene moieties, i.e. cyclic bicalicene (pen-tacyclo[11.3.0.0. 0- .0 ]hexadeca-l,3,5,7,9,ll,13,15-octaene, 5), tercalicene (6), and quatercalicene (7) as stable compounds. ... 

Polycalicenes are characterized by significant limit hyperpolarizabilities that surpass the hyperpolarizabilities of tran -polyenes, but do not exceed those of some members of the polycyclic family. Some works [75-77] are devoted to the synthesis of polymeric calicene, which offer some potential for use as materials in optical devices. 

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