Fulvene generation

More recently, a very efficient asymmetric carbolithiation of N,N-dimethyl-aminofulvene 30, leading to a chiral cyclopentadienide anion, was reported by Hayashi et al. [6] for the synthesis of chiral metallocenes (Scheme 6). By adding an aryl lithium such as 31 complexed with a chiral ligand on fulvene 30, a cyclopentadienide ion 32 bearing a stereogenic center at the a position was generated. This anion was reacted with [RhCl(nbd)]2 to yield... 

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]). 

Novel double-deckers illustrated by 30 and 31 have been generated from 1,4-diborapentafulvene ligands.33 In 31 a second metal center is coordinated to the B=C bond of the fulvene moiety. 

When the cyclic acetylene 260 is generated from a suitable precursor, it undergoes an isomerization reaction spontaneously generating naphthalene (263) and benzo-fulvene (264) as the finally isolable products. Very likely the process begins with a retro-Diels-Alder reaction to the [3]cumulene 261, which in a cascade reaction via the semicydic allene 262 rearranges to 263 and 264 [111]. 

Kato et al. (151,152) explored the chemistry of 2-ferf-butylMvenes with isomiinchnones, as well as with several other mesoionic compounds, in a novel approach to pseudo-hetero-azulenes. Thus, isomtinchnone 51a, generated as before in situ from A-benzoylphenylglyoxyanilide 253 with triethylphosphite, reacts with 2-ferf-butyl-6-(dimethylamino)fulvene to give the [47i+6ti] adduct diphenylcyclo-penta[c]pyran in low yield. Likewise, reaction of 51a with dimethylfulvene gave a mixture of two adducts, one of which arises from a [47i+2ti] cycloaddition. 

A less usual method for the generation of starting dienones is thermal rearrangement of fulvene-l,2-dioxetanes 139a,b, leading to 2//-pyrans (140a,b).182... 

Sequential double Michael addition of cyclic dienolates, generated from (107) or (108), to fulvene (109) has been developed as a method for rapid access to the tricyclo[5.3.0. 2,5]alkane system (Scheme 24).90... 

Various cross-conjugated enediynes undergo Bergman-type cycloaromatizations upon reduction with potassium metal, generating anions of fulvenes and fulvalene derivatives (Scheme l).6 Not all cross-conjugated enediynes yield cyclized dianions upon reduction some give uncyclized, Y-shaped, cross-conjugated dianions, whereas others apparently yield radical anions that either dimerize or persist as monomers. 

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... 

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 formation of 133 was rationalized by a stepwise mechanism. The initial addition of the fulvene to the activated 2H-azirine generates the zwitterionic intermediate 135 which then cyclizes to [2]p3U indine 133 via intermediate 136 (Scheme 36). 

Isolobal replacement of the exocyclic methylene group of fulvene with a cobalt fragment generates cobaltafiilvenes ... 

Another reaction can be used to generate a two-carbon bridge between the Cp hgands. In tetrahydrofuran (THF), the reaction of ZrCLi with two equivalents of n-BuLi, begun at —78 °C and completed at 25 °C, results in a black suspension of ZrCl2 and LiCl, which reacts with fulvenes to provide a high yield of ansa zirconocene dichloride (equation 4). 

Diazenes derived from cycloadduct of fulvenes have been used for the generation of diyls and their further cycloadditions (cf. Section D.1.6.1.2.7.). 

A non-Kekule molecule, a /n-xylylene (88), has been generated by the Bamford-Stevens reaction of a fulvene-ketene adduct (85), via the homofulvene (87) from the intermediate carbene (86). This unstable intermediate readily dimerizes to octamethyl[2.2]metacyclophane (89 Scheme 9). ... 

Disubstituted fulvenes are a common source for alkyl-substituted Cp ligand. The reaction of the in-situ generated Bu ZrCV species with 2equiv. of substituted fulvenes in hydrocarbons produces alkyl-substituted zirconocene dichlorides 575 in high yields (Scheme 130).403 6,6-Disubstituted fulvenes were also used to synthesize sterically hindered metallocene dichlorides 576 and 577 via the salt metathesis route.404... 

Addition of dibromomethyllithium (generated from dibromomethane and lithium diiso-propylamide) to fulvenes, and cyclization of the dibromolithium intermediates 2 is an efficient route for the preparation of spiro[2.4]heptadienes 3. ... 

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