Fulvenes triafulvenes

Ketene acetals show a pattern of product formation very similiar to enamines79 Diphenyl-4,4-diacetyl triafulvene is converted to diacetylmethyl cyclopentadiene 529 by S,N-acetals, whilst diphenyl-4,4-dicyano triafulvene undergoes C—C-inser-tion to S,N- and N,N-acetals, e.g. 530/531, resulting in cross-conjugated systems 533/534 by analogy with enamines. Cyclic S,N-acetals 532, however, yield exclusively the bicyclic fulvenes 535 due to additional loss of methyl mercaptan. 

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) ... 

When 4,4-diacyl triafulvenes are reacted, in addition to the deeply colored fulvenes, colorless isomers were also obtained to which structure 540 was assigned. 

The versatility of this triafulvene reaction type is demonstrated by the interaction of ally pyridinium betaines 441 and l,2-diphenyl-4,4-diacetyl triafulvene272, which gives rise to fulvenes 565, benzene derivatives 566, or acyclic systems 567 these products are likely to result from an allenic precursor 563 and its isomer 564 originating from a 1,5-H-shift. 

Methylenecyclopropene (triafulvene) is the simplest member of the cross-conjugated, non-alternant, hydrocarbon family known as the fulvenes. Strong polarization of the tc-system might be expected for such a structure and extensive efforts have been made... 

Many phenoxy-Substituted fulvenes, e.g. 368, are oxidized to the corresponding highly coloured [3]radialenes. Reduction of the latter species can be effected with hydroquinone (equation 107). Oxidative cleavage of some triafulvenes to allenes is... 

The substrates of the present investigation are derivatives of triafulvene ([3] fulvene, alias methylenecyclopropene) (IV), the simplest nontrivial [n] fulvene [8-12]. Interest in these microcyclic three-membered ring systems stems from a possible contribution of the aromatic , dipolar structure (IVa) in the ground-state as well as in the excited state. Aromatic" - in inverted commas - in the sense that the three-membered ring acquires a non-negligible cyclopropenium ion character. It is emphasized from the outset that this attribute should be treated as a matter of gradation [13]. The question to be asked is whether a certain deviation from a complete bond-alternation picture towards aromaticity is indicated. 

This conclusion is supported also by LCAO-HMO calculations. It can be shown that the molecular orbitals of [ ] fulvenes and [n, w] fulvalenes belong either to the A2 representation of the point group C21, or to its representation. In the molecular orbitals of [/i] fulvenes which belong to A2, all the electron density is concentrated in the ring. The A -> transition in triafulvene is from a Bi molecular orbital to an A2 molecular orbital. Thus, in the lowest unoccupied molecular orbital, all the electron density is concentrated in the three-membered ring. Likewise, in 1,2-diphenyltriafulvene, upon A - Fi transition, the calculated dipole moment diminishes. 

Figure 6.1 Pentafulvene (fulvene) 1, triafulvene 2, heptafulvene 3 and their polar structures.

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],... 

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. 

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... 

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