Fulvene rings

Tlie name fulvaleiie was first mentioned by R. Brown (49TFS296), who expanded the class of fulvenes for those compounds containing two cyclic poly-enic systems with a central double bond. Tlius, depending on the ring size, cy-clopropylidenecyclopropene 1, fulvaleiie 2, heptafulvalene 3 and the unsymmetrical hybrid molecules triapentafulvalene (calicene) 4, triaheptaful-valene 5, and pentaheptafulvalene (sesquifulvalene) 6 are members of this class of cyclic cross-conjugated systems (Scheme 1). 

This procedure illustrates formylation by N,N-dimethylamino-methoxymethylium methyl sulfate, a compound which can be produced readily by reaction of easily available materials. 6-(Dimethylamino)fulvene is a useful intermediate for the synthesis of various f used-ring nonbenzenoid aromatic compounds. 

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. 

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.

The authors also considered two additional reaction pathways for the o-QM decarbonylation to fulvene, both via ring opening (Scheme 2.19).23... 

In contrast to the usual (4 + 2) diene reactivity of fulvenes 3-amino fulvenes 539 in some cases are capable of expanding the five-membered ring to heptafulvenes 541 by addition of acetylene dicarboxylate in a (2 + 2) fashion299. ... 

The r2 for this line is 0.9995 with a standard deviation of ca 0.9 kJmol 1 (Figure 1). The deviation for R1 = R2 = Ph (129) is some 43 k,I mol 1 below the line. If the exo-methylene/ring bond is quite polar and resonance structures 128 are significant (130 is easily ignorable), then AHf (127, R1, R2) and A//f(0=CR1R2) would be more likely to be linearly related. Another nearly perfect line, that of carbonyls vs fulvenes, can be drawn through the (H,H), (H,Me) and (Me,Me) points (equation 36). 

Consider now the one ring-substituted fulvene for which we have a measured enthalpy of formation, namely the ring tetramethylated derivative 131 with its value of 83 kJ mol-1. If benzene is the appropriate paradigm for fulvene, then reaction 37 is expected to be... 

From the enthalpies of formation from Roth for the fulvenes and from Pedley for the other hydrocarbons in equations 37 and 38, we find the former reaction is exothermic by 12 kJmol-1 while the latter is endothermic by 12 kJmol-1. Ionic resonance structures analogous to 128 are expected to be of less importance for the ring alkylated species than for the parent species 103 negatively charged carbon is destabilized by adjacent... 

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). 

Other structural motifs for designing molecular glasses have been exploited (Fig. 3.16). Some of them are based on five-membered rings as the central part of the molecule, like 1,2,4-triazoles (48) [94] or the electron-transporting material NAPOXA (49) [95]. Pentaphenylcyclopentane (50) exhibits a Tg of 57°C [27]. Braun et al. [96] synthesized some fulvenes with Tg ranging from 74 to 120°C (51). 

Hence, the anion-radical of diphenyl fulvene acquires a spin-charge distribution dictated by steric shielding at Ph2C node and six-n-electron delocalization in the C5H5 ring. Anion-radicals of sterically congested stilbenes represent examples that are quite different, but at the same time are similar in principle. Let us compare the E structures of stilbene and its congested derivative, namely, a,p-di(tert-butyl)stilbene in their neutral and anion-radical forms (Scheme 3.18). 

Dilithium compound 210, obtained from the lithiation of hexasilylfulvene 209 in THF at room temperature (Scheme 72), represents the first X-ray structural analysis of a dUithi-ated fulvene derivative. In the solid state, one lithium centre is capping the central five-membered ring, while the second lithium centre is located at the exocyclic carbon atom of the fulvene unit (Figure 27) . 

An inter-intra-intramolecular carbopalladation cascade, initiated by the alkenylpalladium bromide intermediate from /3-bromostyrene 74 inserting into one of the triple bonds of the diyne 73, yields the ring-annelated fulvene derivative 75 (Scheme 22). ... 

Ring contractions of yet a different kind are observed with H- azepines of type (54), which on heating yield 6-substituted fulvenes as illustrated in Scheme 3 for the dicyano-1//-azepine (7lJCS(C)l237). Analogous reactions occur with 3//-3-benzazepines. 

---