Fulvenes pentafulvenes

Amongthe various types of fulvenes, pentafulvenes stand out as the most widely studied. They serve as 2it-, 4jt-, 6k-, or even three-carbon components in annula-tion processes, depending on the reaction partners. The periselectivity of these reactions depends upon both reaction partners. For decades, pentafulvenes have... 

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) ... 

Photoreactions of [Cr(CO)3( /6-C7H8)] (41) with 6-mono- and 6,6-disub-stituted pentafulvenes (59a-59f) preferentially yield dicarbonyl complexes with substituted tj3 5-2-cyloheptadienylene-2-cyclopentadienylidenemethane chelate ligands (82,83). In the course of the reaction, C-6 of the fulvene forms a C—C bond to C-l of the 1,3,5-cycloheptatriene ligand, and one CO ligand is displaced. This reaction is of the same type as the formation of the f/3 5-[ 1 -(3-butene-1,2-diyl)-7-isopropylidenecycloheptadienyl] complexes 47c, 47e and 47t. The fulvene unit is transformed into a monosubstituted cyclo-pentadienyl entity, / -coordinated to the chromium, with the 1,3,5-cyclo-... 

Consider the case of triapentafulvalene (XIV) (calicene), a combination of tria-fulvene and pentafulvene. Hexaphenyltriapentafulvalene (XV) [37] was taken as a representative. This compound has the highest experimental dipole moment in the ground state ever recorded in a hydrocarbon - 6.3D (benzene, 30°C) [37]. A similar value was obtained in the case of the l,2-dimenthyl-3,4,5,6-tetraphenyl derivative [38]. 

Fulvene Complexes.—Several ( -fulvene)Cr(CO)3 complexes have been obtained by photochemical ligand-exchange reactions of benchrotrenes with 6,6-disubsti-tuted pentafulvenes. The crystal structure and C n.m.r. spectrum of the 6-(dimethylamino) complex have shown that it is best formulated as a... 

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

This section focuses on new developments in the area of pentafulvene synthesis. The traditional methods (condensation between cyclopentadienyl (CP) anion and ketones, and nucleophilic substitution reaction of six-substituted fulvenes) are still often used for the synthesis of pentafulvene derivatives (Scheme 6.24) [1], and some improved procedures for these transformations will be described... 

Another versatile synthetic method for pentafulvenes was employed by a formal nucleophilic substitution reaction (addition and subsequent elimination reactions) of fulvenes carrying leaving groups at the 6-position [1]. While... 

Fulvenes, cyclic molecules with odd number of carbon atoms in the ring, represent one of the fundamental categories of cross-conjugated systems (see Chapters 1 and 6). According to the ring size of these molecules, the compounds are classified astriafulvenes 1, pentafulvenes 2, heptafulvenes 3, and so on (Scheme 7.1). Owing to their dipole moments, fulvenes possess unique reactivity, different from that of open-chain polyolefins and aromatic compounds. In 1989, Neuenschwander... 

Hong and coworkers [18] also reported an azadiene Diels-Alder cycloaddition involving fulvenes (Scheme 7.17). Pentafulvenes 67 was reacted with A/ -sulfonyl-l-aza-l,3-butadiene 68 to give the tetrahydro-[l]pyridine system 69 efficiently. The reactions provided 69 in moderate yields at room temperature the yields could be improved by high-pressure reactions and microwave conditions. Formally, this reaction involves a regio- and diastereoselective inverse electron-demand Diels-Alder reaction. 

Lehn and coworkers reported a series of reactive diene and pentafulvenes involving reversible Diels-Alder reactions at ambient temperature. The results provide a foundation for the implementation of the reversible Diels-Alder reaction in constitutional dynamic chemistry (Scheme 7.26) [27]. With variation of the functionalized substituted fulvenes 108 and cyanoolefin esters 109, the methodology may be applied for the preparation of dynamic combinatorial libraries and provides an entry to reversible dynamic polymers and the discovery of biologically active substances. 

The first [6+3] cycloaddition of pentafulvene was reported by Kanematsu et al. [36] by the reaction of 2-oxyallyl action and 6-(dimethylamino)fulvene 155,... 

Liu and coworkers [77] studied intramolecular [8+6]- and [4+2]cycloadditions of electron-deficient 8-cyanoheptafulvenes with pentafulvenes at various temperatures (Scheme 7.67). The stereoselectivity and periselectivity of these intramolecular cycloaddition depend on the side chain and the substituents at the fulvene systems. 

Although pentafulvenes are the by far most common fulvenes, there are some publications describing the synthesis of heptafulvene complexes. The reaction of (cycloheptatrienyl-7-methyl) phenylsulfonate (92) with sodium [cyclopentadi-enyldicarbonylferrate(II)] afforded complex 93 in 45% yield, from which hydride was abstracted quantitatively by treatment with triphenylcarbenium ions. The heptafulvene complex formed was described by resonance formulas 94 and 95, the latter with an intact aromatic cycloheptatrienylium substructure (Scheme 10.32) [84, 85]. 

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