6- fulvene, reaction

Simple fulvenes can be smoothly hydrogenated over suitable catalysts, all the double bonds being reduced [198], Lithium aluminium hydride specifically reduces the exocyclic bond of fulvenes, reaction proceeding by formation of a lithium cyclopentadienide [238] ... 

The efficient generation of molecular complexity and diversity by cycloaddition reactions of fulvenes, summarized in this review, are noteworthy. Nevertheless, many etforts still need to be explored to develop more sophisticated cycloaddition reactions with elaborate multibond formation in a cascade fashion with high enan-tioselectivity. With the thriving advances in the past decades, it is safe to assume that the subject of fulvene reactions will continue to evolve for years to come. 

Kerber et al. succeeded in the synthesis of the first iron complexes of isobenzo-fulvene. Reaction of the dibromide 89 with Collman s reagent (disodiumtetracar-bonylferrate) afforded isobenzofulvene complexes 90 and 91 in 4 and 2% yield, respectively. 90 is a trimethylenemethane system anellated to an intact benzene ring (Scheme 10.31) [83]. 

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]. 

Oxidation of thiophene with peracid under carefully controlled conditions gives a mixture of thiophene sulfoxide and 2-hydroxythiophene sulfoxide. These compounds are trapped by addition to benzoquinone to give ultimately naphthoquinone (225) and its 5-hydroxy derivative (226) (76ACS(B)353). The further oxidation of the sulfoxide yields the sulfone, which may function as a diene or dienophile in the Diels-Alder reaction (Scheme 88). An azulene synthesis involves the addition of 6-(A,A-dimethylamino)fulvene (227) to a thiophene sulfone (77TL639, 77JA4199). 

Alkylthio groups are replaced in nucleophilic substitutions. Such reactions are easy in cationic derivatives for example, in the 1,2-dithiolylium series (539), substituted cydopen-tadienyl ion gives fulvene derivatives (540) (66AHC(7)39). 2-Methylthio groups in... 

Cycloaddition of 4-acylsubstituted fulvene 425 with the imine 427 in boiling toluene gave the tricyclic pyrroline 428. Treatment of 426 with 427 and subsequent reaction of the product 429 with malononitrile gave 428 (86S908) (Scheme 72). 

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. 

Dimethylamine, reaction with phosphorus trichloride, 46, 42 f-Dimethylami nobenzaldehyde, conversion to 0,/9-dichloro- -dimethyl-aminostyrene, 46, 34 tn-Dimethylaminobenzoic acid, 47, 71 /5-Dimethylaminobenzoic acid, 47, 71 6.(Dimethylamino)fulvene, 47, 52 -Dimethylaminophenylacetic acid, 47, 71... 

Phenylcyclopent[c]azcpine (33a) and 6,7-fused cyclopentazepines 33b-d are formed in moderate yields in a one-pot, two-stage process involving initial condensation of triphenyl-[(l-phenylvinyl)imino]phosphoranes 32 with 6-(dimethylamino)fulvene-2-carbaldehyde (30), followed by an intramolecular aza-Wittig reaction of the iminophosphorane with the pendant aldehyde function.5 The method fails with the unsubstituted vinylphosphorane 32 (R1 = R2 = H). 

Mono- and l,3-di-ter/-butylcyclopentadienyllithium Li(Bu Cp) 4 (n = 1) and 5 (,n = 2) can be obtained directly by addition of methyllithium to 6,6-dimethyl fulvene and 2-/ert-butyl-6,6-dimethyl fulvene, respectively [27a]. Since the fulvenes themselves are easily accessible by base-induced condesation reactions [27b], this method offers a viable alternative to the usual metalation of the substituted cyclopentadienes 1 and 2 (Eq. 2). 

The formation of the tricarbonylchromium-complexed fulvene 81 from the 3-dimethylamino-3-(2 -trimethylsilyloxy-2 -propyl)propenylidene complex 80 and 1-pentyne also constitutes a formal [3+2] cycloaddition, although the mechanism is still obscure (Scheme 17) [76]. The rf-complex 81 must arise after an initial alkyne insertion, followed by cyclization, 1,2-shift of the dimethylamino group, and subsequent elimination of the trimethylsilyloxy moiety. Particularly conspicuous here are the alkyne insertion with opposite regioselectivity as compared to that in the Dotz reaction, and the migration of the dimethylamino functionality, which must occur by an intra- or intermo-lecular process. The mode of formation of the cyclopenta[Z ]pyran by-product 82 will be discussed in the next section. 

The cycloprop anation of fulvenes has been effected with alkynylcarbene complexes (see Sect. 2.1.1). However, this reaction is inhibited in the presence of CO and under these conditions a formal [4S+3C] cycloadduct is formed [15a]... 

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. 

Cyclopentadienyl radical can combine with methyl radicals to form fulvene, along with a subsequent loss of H atom. Fulvene may also be formed by reactions of the iso isomer of C4H5 with aeetylene. Once fulvene adds an H atom, subsequent rearrangement to benzene has been shown to be thermodynamically favorable using quantum chemieal methods.Cyelopentadienyl radicals may also combine with eaeh other and then rearrange to form naphthalene. 

The ferrocenyldiphynylpropargyl cation, 77, has an intrinsic delocalization nature exhibiting a valence tautomerization band at 856 nm, and its nucleophilic trapping reactions give rise to the formation of ferrocenyldiphyenylallenes (173). The bis(acetylide) mixed-valence complexes of ferrocene and the Ru complex moiety, 78, also behave as a fulvene-cumulene structure, 79, showing a u(M=C = C—C) band at 1985 cm-1 (174). Related alleylidene and cumulenylidene complexes of transition metals have been reviewed by Bruce (175). 

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

The study of the cycloaddition behavior of l,l-dichloro-2-neopentylsilene, C Si =CHCH2Bu (2) [3], reveals the high polarity of the Si=C bond and a strong electrophilicity. The [4+2] cycloaddition reactions with anthracene (3), cyclopentadiene (4) and fulvenes (5) proceed as expected surprising, however, the Diels-Alder reactions with dienes are of lower activity, like naphthalene (6) and furans (7). 

Two equiv. of 6,6-di(cyclopropyl)fulvene react at 60 °C over a period of a week with Ca[N(SiMe3)2]2-(THF)2 bis in THF to yield the metallocene 170. The heteroleptic amido complex 171 is detected as an intermediate with 111 and 13C 1H NMR spectroscopy. A 1 1 reaction of the calcium amide and 170 also produces 171 in solution, an equilibrium involving these three derivatives exists (Equation (30)). The calcocene 170 crystallizes at — 20 °C from THF as colorless cuboids. The metal center is surrounded by the four ligands in a distorted tetrahedral manner, and the cyclopentadienyl group and the propylidene fragment are coplanar with each other.393... 

The various transitions of triafulvenes to pentafulvenes achieved by addition of electron-rich double bonds is complemented by the reaction of triafulvenes with ynamines and yndiamines299, which gives rise to 3-amino fulvenes 539. This penta-fulvene type deserves some interest for its merocyanine-like inverse polarization of the fulvene system and its formation is reasonably rationalized by (2 + 2) cycloaddition of the electron-rich triple bond to the triafulvene C /C2 bond (probably via the dipolar intermediate 538) ... 

The versatility of this triafulvene reaction type is demonstrated by the interaction of ally pyridinium betaines 441 and l,2-diphenyl-4,4-diacetyl triafulvene272, which gives rise to fulvenes 565, benzene derivatives 566, or acyclic systems 567 these products are likely to result from an allenic precursor 563 and its isomer 564 originating from a 1,5-H-shift. 

Hong and coworkers have investigated the cycloaddition chemistry of fulvenes with a wide variety of alkenes and alkynes in great detail [191]. As one example, the reaction of 6,6-dimethylfulvene with benzoquinone is shown in Scheme 6.92. Under microwave conditions in dimethyl sulfoxide (DMSO) at 120 °C, an unusual hetero-[2+3] adduct was formed in 60% yield, the structure of which was determined by X-ray crystallography. The adduct is a structural analogue of the natural products aplysin and pannellin and differs completely from the reported thermal (benzene, 80 °C) Diels-Alder cycloaddition product of the fulvene and benzoquinone (Scheme 6.92) [191]. 

Scheme 6.92 Thermal versus microwave-assisted Diels-Alder cycloaddition reactions of fulvenes with benzoquinones.

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