Fulvenes preparation

Interesting formation of the fulvene 422 takes place by the reaction of the alkenyl bromide 421 with a disubstituted alkyne[288]. The indenone 425 is prepared by the reaction of o-iodobenzaldehyde (423) with internal alkyne. The intermediate 424 is formed by oxidative addition of the C—H bond of the aldehyde and its reductive elimination affords the enone 425(289,290]. 

Of the fundamental nonalternant hydrocarbons, only two prototypes were known about fifteen years ago azulene (XI, Fig. 5), the molecular structure of which was determined by Pfau and Plattner and fulvene (XIX) synthesized by Thiec and Wiemann. Early in the 1960 s many other interesting prototypes have come to be synthesized. Doering succeeded in synthesizing heptafulvene (XX) fulvalene (XXI) and heptafulvalene (XXIII). Prinzbach and Rosswog reported the synthesis of sesquifulvalene (XXII). Preparation of a condensed bicyclic nonalternant hydrocarbon, heptalene (VII), was reported by Dauben and Bertelli . On the other hand, its 5-membered analogue, pentalene (I), has remained, up to the present, unvanquished to many attempts made by synthetic chemists. Very recently, de Mayo and his associates have succeeded in synthesizing its closest derivative, 1-methylpentalene. It is added in this connection that dimethyl derivatives of condensed tricyclic nonaltemant hydrocarbons composed of 5- and 7-membered rings (XIV and XV), known as Hafner s hydrocarbons, were synthesized by Hafner and Schneider already in 1958. 

Y.W. Park, Fulvene, metallocene catalysts and preparation method thereof, and preparation of polyolefines copoljmer using the same, US Patent No. 20050004385 A1 (2005). 

Attack on Unsaturated Carbon. The annual addition of phosphites to every variety of activated double bond continues. These include nitro-alkenes,9 a/S-unsaturated carboxylic acid derivatives,10 maleimides,11 fulvenes,12 and pyridinium salts.13 The reaction of diethyl phosphite with keten 0,N-, S,N, and Al,AT-acetals has been used to prepare the enamine phosphonates (19).14... 

The fulvene route was also successfully employed in the preparation of a compound, which can be regarded as one of the most advanced molecular models for a catalytically active titanium center on a silica surface. When Cp Ti(C5Me4CH2) was reacted with the monosilylated silsesquioxane precursor 12 in refluxing toluene a color change from deep purple to amber was observed. Crystallization afforded a bright-yellow material, which was subsequently shown to be the novel mo o(pentamethyleyclopentadienyl) titanium(IV) silsesquioxane complex 126 (69% yield). Its formation is illustrated schematically in Scheme 42. 

Cyclopentyl isoxazolidine cycloadduct 324 was prepared by intramolecular nitrone cycloaddition by Baldwin et al. (280,281,352,353) as part of studies toward a total synthesis of pretazettine (Scheme 1.69). Related adducts have been prepared elsewhere (354—356) including fluorine-substituted carbocycles (357) and the adducts prepared by lOAC by Shipman and co-workers (333,334) who demonstrated their potential as a route to aminocyclopentitols (Scheme 1.66, Section 1.11.2). Such bicyclic structures have been prepared in rather unique intermolecular fashion by Chandrasekhar and co-workers (357a) from the cycloaddition of C,N-diphenyl nitrone to fulvene (325). 

The substituted cyclopentadienylmagnesium compounds 62a and 62b have been prepared from the corresponding fulvenes (equation 8) and were structurahy characterized in the solid state by X-ray crystallography . The structures are, as expected, (tj -bonded cyclopentadienyl groups) for bis(cyclopentadienyl)magnesium compounds. 

The preparation of fulvene enamines by the reaction of sodium cyclopent-adienide with dimethyl sulfate complexes of amides or lactams has recently been reported by several investigators [77, 78] (Eq. 22). 

The preparation of system 57 from cyclopentadienyl anion and s-tetrazines151154 and the synthesis of the same system from fulvenes and s-tetrazines143 are based on a similar principle. By its nature, the preparation of 4//-cyclopenta[c]cinnolines (68) from nickelocene and o-dihalo-benzenes163 is analogous. 

The catalyst Col2(dppe)-Zn/ZnI2 catalyses the 6 + 2-cycloaddition of cyclohepta-triene with terminal alkynes to afford 7-alkylbicyclo[4.2.1]nona-2,4,7-trienes in good yields 239 The intermolecular 6 + 3-cycloaddition of fulvenes (213) with 3-oxidopy-rylium betaines (212) yields (214), which, after a 1,5-hydrogen shift, yields the 5,8-fused oxabridged cyclooctanoids (215). This methodology can be used for the preparation of fused cyclooctanoid natural products such as dactylol and precapnella-diene (Scheme 60).240... 

Compounds of special interest whose preparation is described include 1,2,3-benzothiadiazole 1,1-dioxide (a benzyne precursor under exceptionally mild conditions), bis(l,3-diphenylimida-zolidinylidene-2) (whose chemistry is quite remarkable), 6-(di-methylamino)fulvene (a useful intermediate for fused-ring non-benzenoid aromatic compounds), diphenylcyclopropenone (the synthesis of which is a milestone in theoretical organic chemistry), ketene di(2-melhoxyethyl) acetal (the easiest ketene acetal to prepare), 2-methylcyclopentane-l,3-dione (a useful intermediate in steroid synthesis), and 2-phenyl-S-oxazolone (an important intermediate in amino acid chemistry). 

The final option available to a u-alkylpalladium intermediate from Heck alkylation occurs if another alkene or alkyne function is situated properly to participate in a further Heck-type carbopalladation (equation 161)318,319. In properly constructed systems, more than one further carbopalladation is feasible, and many examples of these cascade car-bopalladations have been reported. Several have been quite spectacular (equation 162)320. Fused, spirocyclic and bridged bicyclic ring systems have been prepared in this manner. The process may also create as many as five rings in one step, with five-,six- and three-membered rings321 being the most suitable for preparation (equation 163). Alternatively, the proper orientation of double and triple bonds allows cyclotrimerization to highly functionalized arenes or fulvenes (equation 164)322,279. 

Common to these molecules with their cyclopentadiene moieties is the so-called fulvene subunit 27. The first fulvenes, 6,6-dialkylfulvenes, were prepared as early as 1906 by Thiele et al. from sodium cydopentadienide and ketones [16]. The parent hydrocarbon 27 and many other derivatives have been thoroughly studied since the 1960s [17-19]. Diazocyclo-pentadiene (28), which is also easily prepared from cydopentadienide, is a heteroanalogue of fulvene. It has frequently been used as a precursor to other theoretically interesting molecules containing annelated cydopentadiene moieties, because its irradiation readily generates the cyclopentadienylidene 29. This carbene has, for example, been trapped with alkynes to form spiro-annelated cydopentadiene derivatives 30 (Scheme 5) [20]. It has been proved by UV spectroscopy [21] and supported by calculations [22] that these spiro[2.4]heptatrienes (so-called [1.2]spirenes) 30 experience a spedal kind of electronic... 

The question of aromaticity versus antiaromaticity and delocalized versus localized double bonds in pentalene (2) dates back to 1922, when Armit and Robinson compared it with naphthalene and postulated that the former might be similarly aromatic [32, 33]. While the first synthesis of a non-fused hexaphenylpentalene (38) [30] provided only some clues as to the non-aromatic reactivity of the pentalene skeleton, the tri-tert-butyl derivative 39, prepared and studied by Hafner et al. in great detail [31], gave a better insight. The ring-proton signals of this alkyl-substituted pentalene 39 are shifted upfield compared to those of fulvene (27) and other cyclic polyenes. This observation led to the conclusion that the pentalene derivative 39 should be an antiaromatic species. However, the results did not permit a distinction... 

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]. 

The diazo-route uses a classical entry via the fulvene 17 into the diazoalkane series 14. Identical conversions via 10 or 11 (azinc) lead to the DHI 7 in 30-50% yields (Scheme 6). Recently, new compounds of type 7 belonging to molecules with supramolecular properties have been prepared.25... 

Stoichiometric lithium naphthalenide has been used for the lithiation of 6,6-bis(dimethylamino)fulvene 687, generating at —78 °C the lithioenamine 688, which has been used for the preparation of 6-aminofulvenes in moderate yields993 (Scheme 178). 

The dienylic cations 44 with cyclopropyl and phenyl groups were also prepared and characterized by the protonation of respective fulvenes. Other cyclopropyl substituted allyl cations include acyclic 1,3- and 1,4-disubstituted allyl cations 45 and 46 The charge in these cations is localized mainly on the carbon adjacent to the cyclopropyl group. The rearrangement of these cations at higher temperatures was also studied. ... 

Methyl groups in the cobaltocenium ion are sufficiently acidic to be deprotonated by strong bases. The ensuing fulvene complexes are not isolated, but can be alkylated in situ with aUcyl iodide. Starting from pentamethylcobaltocenium, derivatives which bear five or 10 alkyl, benzyl, or other functionalized organic groups have been prepared (Scheme 29). ... 

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