Fulvenes with carbenes

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. 

Kinetic analysis shows that the formation of tropone through a hydroxyphenyl-carbene intermediate (which exhibits the lowest activation energy 69.3 kcal/mol) dominates o-QM decomposition process up to 1500 K, with fulvene + CO formation becoming competitive at higher temperatures. In fact, the latter decomposition mode although disfavored by its higher activation enthalpy (75.4 versus 69.3 kcal/mol) becomes competitive due to its more positive activation entropy. 

Alkenyl Fischer carbene complexes can serve as three-carbon components in the [6 + 3]-reactions of vinylchro-mium carbenes and fulvenes (Equations (23)—(25)), providing rapid access to indanone and indene structures.132 This reaction tolerates substitution of the fulvene, but the carbene complex requires extended conjugation to a carbonyl or aromatic ring. This reaction is proposed to be initiated by 1,2-addition of the electron-rich fulvene to the chromium carbene followed by a 1,2-shift of the chromium with simultaneous ring closure. Reductive elimination of the chromium metal and elimination/isomerization gives the products (Scheme 41). 

A carbene intermediate (28) was proposed in the photo-decomposition of the antibiotic agent lomefloxacin.39 Treatment of isoxazole-containing tetrachlorocyclo-propanes such as (29) with MeLi gave rise to 1-aza fulvenes via postulated vinylcarbene intermediates (30).40... 

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 formation of benzvalene is formally an x[2 + 2] cyclo-addition. The S, (Bju) reaction path from benzene toward prefulvene starts at an excited-state minimum with symmetry and proceeds over a transition state to the geometry of prefulvene, where it enters a funnel in S, due to an S,-So conical intersection and continues on the Sg surface, mostly back to benzene, but in part on to benzvalene (Palmer et al., 1993 Sobolewski et al., 1993). At prefulvene geometries, Sg has a flat biradicaloid region of high energy with very shallow minima whose exact location depends on calcula-tional details (Kato, 1988 Palmer, et al., 1993, Sobolewski et al., 1993). Fulvene has been proposed to be formed directly from prefulvene or via secondary isomerization of benzvalene (Bryce-Smith and Gilbert, 1976). Calculations support the former pathway with a carbene intermediate (Dreyerand Klessinger, 1995). 

At still higher temperatures, 3,4-dimethylenecyclobutene gave both fulvene and benzene in a 2 1 ratio, respectively, in a flow system. Further, when the 1,2-dideuterio material was pyrolyzed, the fulvene appears to have deuterium on the 2 and 3 positions while the benzene is 93% ortho labeled with only 5% meta labeled and 2% para labeled. The proposed mechanism for these conversions invoked reversion to the bisallene follwed by recyclization in two different modes to produce biradicals whose resonance structures are carbenes which undergo insertion(s) into adjacent CH bonds (Scheme 7.17). 

Treatment of the anion [ Mo (CNMe ) (CO) 2Cp ] with 1(0112)31 leads tc cis-MoI(CO)2[=C(CHq)3NMe]Cp rather than the anticipated trans species.This compound and other related carbene complexes undergo reduction with NaC2 QHg resulting in the carbene anions [M(C0)2(carbene)Cp] .The fulvene dianion [(C QHg) Mo(CO)3 2l reacts with I[CH2]3l to form the fluxional carbene (43) and with Mel to form the bis-methyl (Cj gHg ) [MoMe (CO) 3 ] 2 Thermolyis or photolysis results in evolution of propene. ... 

Barluenga and coworkers [32] reported on [2+1]- and [4+3]cyclization reactions of alkyl-, aryl-, and acetoxyfulvenes with Fischer carbene complexes (Scheme 7.31). Unlike the previous cyclopropanation reactions of dienes with Fischer carbene complexes, the cyclopropanation reactions of fulvenes therein... 

---