Triafulvenes cycloaddition

Formation of the same dimer from irradiation of benzofulvene 462 suggests that benzofulvene 462 or a photoproduct of it is generated first by photolysis of triafulvene 180 via 461 and then undergoes orbital-symmetry allowed cycloaddition of type 463 to a second benzofulvene molecule. 

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 heptafulvenes 512/513 were believed to originate via dipolar (2 + 2) cycloaddition of a cyclobutadiene to triafulvene CVC2 bond according to the following scheme ... 

The rather complex reactivity exhibited by cyclopropenones on interaction with enamines (see p. 74) is not re-found in the reactions of triafulvenes with enamines and ketene acetals. Instead of a (3 + 3) cycloaddition of enamine C=C—N sequence to the CI(2)/C3 bond of triafulvene (as represented by ylide 51 J) the addition of the enamine double bond to triafulvene C /C2 bond (operating with cyclopropenones only as a minor side-reaction) predominates in all reactions hitherto investigated. 

The various transitions of triafulvenes to pentafulvenes achieved by addition of electron-rich double bonds is complemented by the reaction of triafulvenes with ynamines and yndiamines299, which gives rise to 3-amino fulvenes 539. This penta-fulvene type deserves some interest for its merocyanine-like inverse polarization of the fulvene system and its formation is reasonably rationalized by (2 + 2) cycloaddition of the electron-rich triple bond to the triafulvene C /C2 bond (probably via the dipolar intermediate 538) ... 

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). 

Azomethine ylides such as 412 react with triafulvenes again by analogy with cyclopropenones. (3 + 2) Cycloaddition of the 1,3-dipole to the CVC2 bond and subsequent loss of C02 produces l,4-dihydro-4-methylene-N-alkyl pyridines 559, which as merocyanines show marked solvatochromic and thermochromic effects260. ... 

The reactions of 2,3-diarylcyclopropenones with dicyanoketene or bis(trifluoromethyl)-ketene -" furnished triafulvenes through [2-f 2] cycloaddition followed by elimination. It is notable that 2,3-diphenylcyclopropenone (24) and 2,3-bis(4-methylphenyl)cyclopropcnone (27) followed different routes with bis(trifluoromethyl)ketene. 3-[Bis(trifluoromethyl)methylene]-1,2-bis(4-tolyl)cyclopropene (28) thus obtained had a large dipole amoment (ji = 1.42 D) comparable to that of 4 (/i = 7.9 D). 

On the other hand, 2,3-diphenyl-4,4-dicyanotriafulvene (1) is stable and inert to strong acids such as hydrobromic acid and tetrafluoroboric acid. Chlorine, however, adds to this triafulvene at the exocyclic C — C double bond to give a dichloride which regenerates the triafulvene upon zinc reduction. Although the dicyanotriafulvene 1 is also inert to most dienes, it undergoes [2 + 2] cycloadditions with nucleophilic enamines followed by ring opening. 

Alkylidenecycloproparenes (benzotriafulvenes) are more strained than triafulvenes and readily undergo ring opening or rearrangement upon reaction with electrophiles and nucleophiles. For cycloadditions, different reactivity has been observed between diarylmethylenecyclopropa-benzenes 12 and -naphthalenes 13. While the former compounds give Diels-Aldcr adducts at the endocyclic double bond with diphenylisobenzofuran, the latter compounds produce rearrangement products via Diels-Alder adducts from reaction at the exocyclic C —C double bond. A [2-1-2] cycloaddition has also been observed for a naphtbo compound. 

Simple Diels-Alder Additions.—The [4 + 2] and reverse-[4 + 2] cycloaddition reactions of species containing C=X bonds (X = P, As, Sb, Bi, Si, Ge, etc.) have been discussed and the evidence reviewed.The general chemical reactivities of cyclo-propenones and triafulvenes, including their behaviour in Diels-Alder reactions, have been broadly summarized. Kagi and Johnson have reported extensive studies on the Diels-Alder reactions between cyclopropene and various chloro-cyclopentadienes in which endo-adducts are obtained, selective dechlorination, hydrolysis and other reactions of these adducts, and H n.m.r. spectral correlations. 

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. 

Tsuge and coworkers reported on a [3-1-2]cycloaddition of benzothiazolium N-phenacylide 316 and triafulvene 315 to give the corresponding [3-l-2]adduct 317 (Scheme 7.70) [80]. 

Furthermore, Tsuge and coworkers reportedthe intermolecular 1,3-dipolar and IMDA cycloaddition reactions of imidazolium 334 or 337 and triafulvene 333 or 315 to give the cage compound 336 or 340, respectively (Scheme 7.74) [85]. 

In 1967, Ciabattoni and Nathan [86] described a cycloaddition of triafulvene 315 with enamines (e.g., 341 and 344), followed by ring opening to yield the den-dralene products 343 and 346 (Scheme 7.75) (see Chapter 1). 

Interestingly, the triafulvene exhibits ambiguous behavior toward ynamines. Reactions of triafulvene derivative 357 with ynamine 367 by [2-E2] cycloaddition and subsequent fragmentation gave the fulvene derivative 370. In contrast, reaction of calicene 371 with ynamine 367 yielded the naphthalene derivative 374 by... 

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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