Fulvenes, reduction

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]. 

Of the substrates that have worked well, let us first illustrate the 7-alkylidene-2,3-dioxabicyclo[2.2.1]heptane system 10. It was known that fulvenes react with singlet oxygen at low temperatures to afford the corresponding endoperoxides however, attempts to isolate these labile compounds led to decomposition, although NMR identification was possible at —70 °C 19>. When reduction of the singlet oxygenates with diimide was performed at —50 °C, the bicyclic peroxides 10 were obtained in high yield (Eq. 7) 20). 

In certain cases a reductive elimination is prevented due to the unfavorable formation of the antiaromatic benzocyclobutadiene (Equation (155)). Annulated fulvenes and pentalenes are accessible by this methodology, in excellent yields (Equation (156)).130 130a-130d... 

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). 

Whitlock et al.14 discovered a reductive cyclization of enediynes promoted by lithium naphthalenide that provides substituted fulvenes and suggested a dianionic mechanism (Scheme 6). However, even now it is still unclear whether the enediyne dianion is indeed the cyclizing species or whether the initially formed acyclic radical-anion cyclizes first to give a fulvene radical-anion which is further reduced by lithium to give the cyclic dianion. 

The formation of fulvene (450) by sodium borohydride reduction of 2-formyl-4//-pyran (371) in water347 may be explained by the mechanism shown in Scheme 21. 

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. 

A new synthesis of diazenes (azoalkanes) has been developed using 4-(S,S-di-methylsulfoximino)-l,2,4-triazoline-3,5-dione 89.M Treatment of fulvenes 88 with 89 followed by diimide reduction of the carbon-carbon double bond of the resulting cycloadduct and then mild base treatment gave the diazenes 90 in good overall yields. 

Many phenoxy-Substituted fulvenes, e.g. 368, are oxidized to the corresponding highly coloured [3]radialenes. Reduction of the latter species can be effected with hydroquinone (equation 107). Oxidative cleavage of some triafulvenes to allenes is... 

The reductive coupling of fulvenes is a convenient method and a novel and versatile route for the preparation of ansa-bis-Cp titanium complexes with ethylene bridges. The ansa-bis-Cp complexes shown in Scheme 640 have been prepared by reduction of the appropriate fulvene reagent with TiCl2. The complexes in combination with MAO are highly active in ethylene polymerization and give linear, high density polymers.1654... 

Reductive coupling of fulvenes with TiCl2 affords the N,N-dimethylaminophenyl and pentamethylphenyl substituted complexes shown in Scheme 641. The determined cis trans ratios at the bridge were 60 40 and 93 7, respectively. The corresponding dithiocyanato complexes are synthesized by the reaction with KNGS in acetone under reflux. The in vitro cytotoxicity of these complexes has been determined.1655... 

Diazene 19 was synthesized in the manner portrayed below. Thus, treatment of anhydride 20 with sodium borohydride selectively reduces one carbonyl to a methylene unit. Reduction of the resulting lactone with DIBAL followed by a Wittig reaction and oxidation with PCC afforded aldehyde 22. When treated with cyclopentadiene in the presence of diethylamine in methanol, 22 undergoes a smooth and efficient conversion to fulvene 23. Diels-Alder cycloaddition to the azodicarboxylate 24 proceeded rapidly, a characteristic of reactions with this electron deficient chlorinated dienophile [8]. Selective reduction of the endocyclic n bond using diimide generated in situ, followed by the electrochemical reductive cleavage of the biscarbamate led to diazene 19 [6]. 

Treatment of 97 with cyclopentadiene and diethylamine in methanol afforded fulvene 100 in a 90% yield. A Diels-Alder reaction with diethyl azodicarboxylate, followed by reduction of the A-5,6 k bond of the adduct using diimide, led efficiently (>95%) to the biscarbamate 101. The diazene linkage was unveiled in a customary fashion, to provide 20 grams of diazene 96 in an overall yield of 35% from THF. 

A selective reduction that can be readily carried out is involved in the preparation of isopropylcyclopentadiene 94 Dimethylfulvene (53 g) in anhydrous ether (200 ml) is dropped slowly into a solution of LiAlH4 (18 g) in ether (200 ml). The mixture becomes warm, the yellow color of the fulvene disappears, and a colorless precipitate is formed. At the end of the reaction (about 2 h) the addition compound is decomposed by methanol (100 ml) with cooling, and sufficient 2n hydrochloric acid is added to give a clear solution. The ethereal layer is separated, washed with dilute sodium carbonate solution, and dried over sodium sulfate. Then the ether is removed and the residue is fractionated in a vacuum. Isopropylcyclopentadiene (38.4 g, 71 %) is obtained as a colorless liquid of b.p. 32-34°/23 mm. 

Treatment of 6-dimethylamino-6-(2-thienyl)fulvene (157) with lithium naphthalenide (2.2 equiv.) in THF at — 78°C, followed by quenching with water gives (158) in 15-20% yield (Equation (14)) <91TL3499>. This novel conversion involves a reductive ring opening of the dianion to an allene and recyclization with elimination of dimethylamine. 

Reduction of Enamines. The reduction of enamines with NaBHsCN under acidic conditions proceeds via an iminium ion, similar to that seen in the reduction of imines. Examples, such as the reduction of an enamine-fulvene to the corresponding amine target (eq 45) and the reduction of the enamino-butenolide to the expected lactone (eq 46), are abundant in the current literature. 

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