Triafulvenes synthesis

Alkylation of cyclopropenones — effected by means of trialkyloxonium tetra-fluoroborates42,119) — leads to the easily hydrolyzable O-alkyl cyclopropenium cations 295, which are potential sources for triafulvene 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. 

The potassium triafulvene thiolate K[C3(p-MeSC6H4) C(CN)2 S] is prepared similarly (equation 369)432. The synthesis of the lithium salt of an analogous cyclic hydrazidocalicene has also been reported (equation 370)433. 

The third method for alkylidenecycloproparene synthesis provides triafulvene derivatives by way of cycloproparenylidene dimerization166. The reaction is limited in scope by the availability of appropriate carbene (carbenoid) precursors and thus far has provided 98a,b only (equation 22). Attempts to extend the method into the naphthalene series167 by using the known89 naphthalene analogue of 97a provides a very sensitive (air/light) deep red material that is fluorescent and, while uncharacterized, is likely the analogue of 98a. 

After several unprofitable attempts to produce triafulvene (1) since 1972, the first successful synthesis was achieved via dehydrohalogenation of l-halo-2-methylenecyclo-propanes 5, by passage through a column packed with potassium tert-butoxide on Chromosorb W at room temperature under reduced pressure, and trapping the product below — 78... 

Base-induced dehydrobromination was also used in the synthesis of 1 -bromo-2-/e/ t-butyl-3-(2,2-dimethylpropylidene)cyclopropene (9). This triafulvene was stable enough to be detected in solution by NMR and IR spectroscopy at ambient temperature, but was extremely reactive towards water. 

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

Eicher, T., and Stapperfenne, U., Reaction of triafulvenes with isonitriles. A simple synthesis of diphenyl-substimted functionalized cyclobutcne derivatives and related products. Synthesis, 619, 1987. 

Triafulvene (methylenecyclopropene) 2, the smallest member of the fulvenes, is of particular interest not only for its aromatic character but also for its highly strained nature. Since the first synthesis of derivatives of 2 about 50 years ago [16],... 

The reactions of triafulvene and its derivatives have scarcely been studied, and most of the reports were published before 1980. A review on the synthesis and the reactive behavior of triafulvene was reported by Eicher and coworkers [78]. Most of the triafulvenes undergo [2+2]-, [4+2]-, and [3+2]cycloaddition and subsequently suffer fragmentation processes to give the corresponding cycloadducts. The cycloaddition reactions of triafulvenes are summarized in this section. 

In addition, the same group reported on a synthesis of heptafulvenes and related seven-membered ring systems (e.g., 363) from triafulvene 357 (Scheme 7.80) [91]. A series of donor-substituted 1,3-dienes, when reacted with tria-fulvenes by a Diels-Alder addition, yielded bicyclic methylene cyclopropanes, which were subsequently transformed to 1,6-diphenyl-substituted heptafulvenes or seven-membered ring derivatives. 

Finally, Gomaa and Dopp reported on a synthesis of [2-arylamino-4(l//)-pyridinylidenejpropanedinitrile 378 by reaction of Ar,AT-diarylamidines 377 and triafulvene 357 (Scheme 7.84) [95]. The mechanism of the reaction, proposed by authors, involves a formal [3-1-3] cycloaddition reaction of ylidene 379, arising from ring-opening of triafulvene 357, and the ketene aminal 380 (tautomer of amidine 377), followed by a sequence of thermal dehydrogenation steps. 

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