Fulvenes 6- - from

SCHEME 2.19 Generation of fulvene from the decarbonylation of o-QM, via two different pathways. Enthalpies of formation for the intermediates and activation enthalpies (data above the arrows) are reported in kcal/mol (data have been taken from Ref. [23]). 

The photochemical formation of fulvene from benzene in the region 2400-2700 A has been reported previously, particularly in the liquid phase.51... 

In several synthetic studies, cyclopropane derivatives were used as synthones or building elements for ring enlargement steps, e.g. reaction of enamines with cyclopropenone [65], synthesis of 2,3-dihydro-l,4-diazepine by thermal isomerization of 1,2-diamino-cyclopropanes [32] [66], and preparation of 3-amino-fulvenes from methylencyclopropenes with alkynamines [67]. 

Dreyer, J., Klessinger, M., The Photochemical Formation of Fulvene from Benzene via Prefulvene A Theoretical Study, Chem. Eur. J. 1996, 2, 335 341. 

Diselanium Fulvenes from Phenylacetylene with Selenium and Base under PTC-Ultrasound Conditions, Synth. Commun., 26, 1617 19%). 

Scheme 29. Fulvenes from Schmittel cyclization of enyne-allenes...

Phase-transfer catalysis has also been found to be superior to NaK or Na/Hg for generation of the cyclopentadienyliron dicarbonyl anion from bis(dicar-bonylcyclopentadienyliron), [C5H5Fe(CO)a]a.i Cetyltrimethylammonium bromide, Ci6H33N(CH3)a Br , and 18-crown-6 are about equally satisfactory as catalysts. The anion was used in a synthesis of fulvenes from thiobenzophenones (equation II). 

Triethylamine (s. a. under ZnCl2) Fulvenes from 1,1-acoxyhalides s. 27, 834... 

Schreiner,P.R., Prall,M. and Lutz, V. (2003) Fulvenes from enediynes regioselective electrophilic domino and tandem cyclizations of enynes and oligoynes. Angewandte Chemie International Edition, 42(46), 5757-5760. 

In a photochemical experiment, irradiation of benzene leads to Sj, which connects to the ground-state surface via the conical intersection shown. Benzene, the much more stable species, is expected to be recovered preferentially, but the prebenzvalene structure which hansfomis to benzvalene is also fomied. Another possible route from the prebenzvalene, along a different coordinate, will lead to fulvene [90, p.357] after a hydrogen-atom transfer from... 

The crude product is crystallized after treatment with activated carbon from ca. 1.5 1. of petroleum ether (b.p. 60-80°) or 800 ml. of cyclohexane. From the orange-yellow solution 84 g. (69%) of 6-(dimethylamino)fulvene separates in yellow leaflets, m.p. 67-68° (Note 4). Concentration of the filtrate and further recrystallization of the residue from petroleum ether or cyclohexane gives an additional 8 g. of product. The combined yield is 92 g. (76%). 

The formation of the tricarbonylchromium-complexed fulvene 81 from the 3-dimethylamino-3-(2 -trimethylsilyloxy-2 -propyl)propenylidene complex 80 and 1-pentyne also constitutes a formal [3+2] cycloaddition, although the mechanism is still obscure (Scheme 17) [76]. The rf-complex 81 must arise after an initial alkyne insertion, followed by cyclization, 1,2-shift of the dimethylamino group, and subsequent elimination of the trimethylsilyloxy moiety. Particularly conspicuous here are the alkyne insertion with opposite regioselectivity as compared to that in the Dotz reaction, and the migration of the dimethylamino functionality, which must occur by an intra- or intermo-lecular process. The mode of formation of the cyclopenta[Z ]pyran by-product 82 will be discussed in the next section. 

Scheme 17 Formation of the (tricarbonylchromium)-complexed fulvene 81 and the cyclo-penta[b]pyran 82 from the 3-dimethylamino-3-(2 -trimethylsilyloxy-2 -propyl)propenylidene complex 80 and 1-pentyne [76]...

The unconventional structure of fulvenes with a unique C=C bond conjugation leads to unusual cycloaddition reactions with other unsaturated systems. For example, alkenylcarbene complexes react with fulvenes leading to indanone or indene derivatives which can be considered as derived from a [6S+3C] cycloaddition process [118] (Scheme 72). The reaction pathway is well explained by an initial 1,2-addition of the fulvene to the carbene carbon followed by [1,2]-Cr(CO)5-promoted cyclisation. 

Figure 9.12. Potential energy profile along (adapted from reference 10) near the fulvene conical intersection. The branching space consists of stretching and skeletal deformation of the five-membered ring.

F ure 9.13. Potential energy profile (adapted from reference 10) for fulvene in the space spanned by Xj and the coordinate (torsion). 

Two equiv. of 6,6-di(cyclopropyl)fulvene react at 60 °C over a period of a week with Ca[N(SiMe3)2]2-(THF)2 bis in THF to yield the metallocene 170. The heteroleptic amido complex 171 is detected as an intermediate with 111 and 13C 1H NMR spectroscopy. A 1 1 reaction of the calcium amide and 170 also produces 171 in solution, an equilibrium involving these three derivatives exists (Equation (30)). The calcocene 170 crystallizes at — 20 °C from THF as colorless cuboids. The metal center is surrounded by the four ligands in a distorted tetrahedral manner, and the cyclopentadienyl group and the propylidene fragment are coplanar with each other.393... 

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. 

Hong and coworkers have investigated the cycloaddition chemistry of fulvenes with a wide variety of alkenes and alkynes in great detail [191]. As one example, the reaction of 6,6-dimethylfulvene with benzoquinone is shown in Scheme 6.92. Under microwave conditions in dimethyl sulfoxide (DMSO) at 120 °C, an unusual hetero-[2+3] adduct was formed in 60% yield, the structure of which was determined by X-ray crystallography. The adduct is a structural analogue of the natural products aplysin and pannellin and differs completely from the reported thermal (benzene, 80 °C) Diels-Alder cycloaddition product of the fulvene and benzoquinone (Scheme 6.92) [191]. 

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