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An unprecedented copper(i)-catalyzed asymmetric inverse electron demand Diels–Alder reaction of azoalkenes with fulvenes is reported. This methodology offers a directed entry to synthesize bicyclic tetrapyridazine derivatives in good yield with exclusive regioselectivity and excellent stereoselectivity.

Acetylene gas has been applied as a feedstock under transition-metal catalysis and photo-redox conditions to produce important chemicals including terminal alkynes, fulvenes, and fluorinated styrene compounds. The reaction discovery process was accelerated through the use of “stop-flow” micro-tubing reactors. This reactor prototype was developed by joining elements from both continuous micro-flow and conventional batch reactors, which was convenient and effective for gas/liquid reaction screening. Notably, the developed transformations were either inefficient or unsuccessful in conventional batch reactors. Its success relies on the unique advantages provided by this “stop-flow” micro-tubing reactor system.

An unprecedented Ag(i)-catalyzed tandem [6+3] cycloaddition/isomerization of isocyanoacetates with fulvenes has been developed, affording the fused dihydropyridine derivatives in good yields with exclusive regioselectivities.

A detailed investigation on the reaction of pentafulvenes with N-substituted-1,2,4-triazoline-3,5-diones leading to the formation of five- and seven-membered azapolycycles is described. The observed reactivity is explained based on the electronic and frontier molecular orbital features of pentafulvene and triazoline dione, which suggested that the latter molecule can add to the former one via a [4 + 2] cycloaddition or it can undergo a nucleophilic reaction at the exo carbon atom of pentafulvene. These reactions would ultimately give the same product and the energetics of them suggested that both mechanisms are operative in the reaction conditions.

6-Substituted fulvenes are interesting and easily accessible starting materials for the synthesis of novel substituted titanocenes via reductive dimerisation, carbolithiation or hydridolithiation reactions, which are followed by a transmetallation reaction with titanium tetrachloride in the latter two cases. Depending on the substitution pattern, these titanocenes prove to be bioorganometallic anti-cancer drugs, which have significant potential against advanced or metastatic renal-cell cancer. Patients bearing these stages of kidney cancer have a poor prognosis so far and therefore real progress in the area of metal-based anti-cancer drugs may come from this simple and effective synthetic approach. This tutorial review provides an insight into the synthesis of fulvene-derived titanocenes and their activity in preclinical experiments.

7-Oxabicyclo[2.2.1]heptadiene derivatives can be converted to phenols, fulvenes and/or the products from a retro-Diels–Alder-like reaction by treatment with Bronsted acids; the outcome of the reaction depends on the experimental conditions and the nature of the Bronsted acid used.

In contrast to the Diels–Alder reaction of fulvenes and benzoquinone, 6-dimethylaminofulvene reacts with benzoquinones to give the hetero [6+3] cycloaddition adduct, constituting an efficient and novel route to cyclopenta[c]chromenes including 11-oxasteroids.

In contrast to the Diels–Alder reaction of fulvenes and pyrones, fulveneketene acetal reacts with α-pyrone to give the [6 + 4] cycloaddition adduct, an efficient and novel route to the azulenols.

Muon avoided level crossing spectroscopy (ALC) shows that in muonium atom addition to fulvenes, radicals are formed exclusively by addition to the 1 and 2 positions.

Pentacarbonyliron and fulvenes, C5H4CR1R2(R1,R2= Me, Et, or Ph) react in diglyme under reflux to give [{Fe(π-R1R2CH·C5H4)(CO)2}2] these compounds exist as their trans-bridged isomers in the solid state, but primarily as a mixture of these with their cis-counterparts in solution. The relative proportions of these species depends upon the solvent, and upon R1 and R2. The complex obtained from 6-dimethylaminofulvene is a ‘linked’ dimer, [Fe2(π-C5H4·CHNMe2·CHNMe2·C5H4)(CO)4] with a cis-conformation. Its i.r. spectrum is independent of solvent and shows that although [{Fe(π-C5H5)(CO)2}2] does not exist solely as its cis-isomer in, for example, heptane solution, this is the principal tautomer present in acetonitrile and related solvents.

Condensation of pyromellitic anhydride with phenylacetic acid (2 mol. equiv.) gave the two isomeric di-γ-lactones of 4,6-bis-(α-hydroxystyryl)isophthalic acid (IX) and 2,5-bis-(α-hydroxystyryl)terephthalic acid (X) in the ratio of 5:3. 4,6-Bis-(2-phenylethyl)isophthalic acid (XI) and 2,5-bis-(2-phenylethyl)terephthalic acid (XIII), formed by reduction of (IX) and (X), respectively, have been cyclized and dehydrogenated to bisbenzo[4,5]-cyclohepta[1,2-a:2′,1′-d]benzene-5,7-dione (III) and bisbenzo[4,5]cyclohepta[1,2-a:1′,2′-d]benzene-5,13-dione (IV).

The arene rings of W(η-C6H5Me)2 and Mo(η-C6H6)2 are readily displaced by the fulvenes C5H4CR2(R = Me or Ph) giving M(η5:η1-C5H4-µ-CR2)(η-arene), M = Mo or W; when M = Mo and R = Me then treatment with HCl gives the binuclear [Mo(η-PriC5H4)(µ-Cl2)]2 whose crystal structure has been determined and which is a convenient precursor to mono-cyclopentadienylmolybdenum derivatives.

Cocondensation of 6,6-dimethylfulvene and iron vapour yields 1,1′-tetramethyleneferrocene and 1,1′-di-isopropylferrocene.

Three kinds of mesoionic analogues of heptafulvene have been synthesized by the base-induced reaction of active methylene compounds with the dicationic ether salts derived from mesoionic olates and trifluoromethanesulphonic anhydride. Based on their spectroscopic properties, it is shown that dipolar structures make an important contribution to the ground state of the synthesized fulvenes.

Frontier M.O. theory rationalises the effects of aza-substitution on the dimerisation tendency in 3-oxidopyridiniums, the 1-(2-pyrimidinyl) betaine displaying the predicted high reactivity; fulvenes add as 6 π-electron systems across the 2,6-positions of 3-oxido-pyridiniums.

The reactions of fulvenes with several mesoionic compounds have been investigated. The mesoionic dithiolones (7) gave regio- and stereo-selective [4π+ 2π] cycloadducts (11)–(13) across the endocyclic double bonds of fulvenes, but no periselectivity was observed with unsymmetrical fulvenes. Several other mesoionic ring systems failed to react or gave complex reaction products.

Syntheses of 4,9-dimethyl-13-diphenylmethylene-5,6,7,8-tetradehydrocyclotridecene (7), 4,9-dimethyl-15-diphenylmethylene-5,6,7,8-tetradehydrocyclopentadecene (8), 6,11-dimethyl-17-diphenylmethylene-7,8,9,10-tetradehydrocycloheptadecene (9), and 6,11-dimethyl-19-diphenylmethylene-7,8,9,10-tetradehydrocyclononadecene (10) are described. Although examination of the 1H n.m.r. spectra suggested that all of the fulvenes (7)—(10) are atropic, the benzannelated derivatives of pentadecafulvene (8), i.e. 13-methyl-7-diphenylmethylene-14,15,16,17-tetradehydrobenzocyclopentadecene (25), 13-methyl-9-diphenylmethylene-14,15,16,17-tetradehydrobenzocyclopentadecene (26) and 7-diphenylmethylene-16,17,18,19-tetradehydrodibenzo[a,g]cyclopentadecene (27) were prepared in order to examine the tropicity of (8) more closely. Comparison of the 1H n.m.r. spectra of these benzannelated fulvenes (25)—(27) with that of (8) reveals that the nonbenzannelated fulvene (8) is atropic. The influence of benzannelation upon the structure of the molecular skeleton of the tetradehydropentadecafulvene system is also discussed.

Thiobenzophenones react with the dicarbonylcyclopentadienyliron anion with coupling to afford fulvenes, via a novel desuphurization and metal–ring cleavage reaction.

The condensation of diphenylcyclopropenone with cyanoacetic acid leads unexpectedly to 4-cyano-4-cyanoacetyl-1,2-diphenyltriafulvene (IV). A possible mechanism for this condensation is suggested.

A series of fulvenes derived from isodicyclopentadiene has been prepared and their n.m.r. spectra studied. The simple low-field spectrum enabled immediate assignment of Hα and both Hγ-cis. and Hγ-trans. The shifts are correlated with π-electron densities calculated for a model compound and the transmission of charge from the α-carbon to the five-membered ring was found to be between that for styrenes and trans-cinnamic acids.

The reaction of ferrocenylcarbonium ions with cyclopentadiene proceeds stereospecifically to give 1 : 1 cycloaddition products.

The mass spectra of a number of fulvene derivatives are reported. The fragmentation shows the stability of the fulvene compared with the heptafulvene skeleton. The principal fragmentation pathways are discussed.

Attempts to obtain 1,2-diphenylcyclopentadiene led to its Diels–Alder dimer. Evidence for dissociation of this dimer has only been obtained in its reaction with molybdenum carbonyl. By contrast, 1,4-dimethyl-2,3-diphenylcyclopentadiene and 1,2,3-trimethyl-4,5-diphenylcyclopentadiene exist as stable monomers, like all other di-and polyarylcyclopentadienes. Exact structures have been assigned to these and other arylcyclopentadienes as well as several related cyclopentene derivatives on the basis of their n.m.r. spectra.

The energy barriers restricting rotation about the C(6)–N bond and the C(1)–C(6) bond of a number of 6-dialkylaminofulvenes have been examined using nuclear magnetic resonance spectroscopy. The steric and electronic effects of substituents upon the rotational barriers are discussed, and it is found that solvent effects are consistent with a dipolar transition state for rotation about the C(1)–C(6) bond. From the magnitude of the C(6)–N rotational barrier in 6-dimethylaminofulvene and previous estimates of the delocalisation energy of simple fulvenes, it is concluded that 6-dimethylaminofulvenes possess a substantial delocalisation energy of the order of 25–32 kcal. mole–1.

The reaction between diphenylcyclopropenone and (2-naphthylmethylene)triphenylphosphorane does not give the expected triafulvene, but 2-(2-naphthylmethyl)-3,4-diphenylphenanthrene.

Thermal rearrangements of 6,7,8,9-tetrahydro-2,4-dimethyl-3H-3-benzazepine-1,5-dicarbonitrile (XVI) yields 4,5,6,7-tetrahydro-1-(1-methyliminoethyl)-2-methylindene-1,3-dicarbonitrile (XVIII; R = :NMe). Compounds of the latter type are considered to be intermediates in the thermal rearrangement of 1-substituted 1H-azepines to fulvenes, where a subsequent [1,5] sigmatropic rearrangement of the imine group is also involved.

The electronic structure of the 2-benzylideneindane-1,3-dione system has been studied by a combination of the ωβ and SCF π-electronic methods. Results pertaining to the geometry, ground-state charge distribution and electronic transitions are discussed.

The monochloro- and dichloro-derivatives of 5H-dibenzo[a,d]cyclohepten-5-one and 10,11-dihydro-5H-dibenzo-[a,d]cyclohepten-5-one have been synthesised. Dipole moments of the compounds show that their molecules cannot be planar.