Fulvenes reactions with carbonyls

Condensation via the Methyiene Group. Because of the resonance stabilization of the 71 electron system, the cyclopentadienyl anion forms readily from CPD after the dissociation of the acidic methylene proton. Under alkaline conditions, CPD undergoes condensation reaction with carbonyl compounds, such as ketones and aldehydes, yielding a family of highly colored fulvene derivatives (26). 

Table 8.17 shows the scope of the reaction of acetylcobalt tetracarbonyl with polyenes. The reactions are regiospecific with the acetyl group adding to the terminal unsaturated carbon atom of the ir-electron system to produce the E-a,p-unsaturated ketones [9]. In the reaction with fulvenes [10], only the 1-acetyl and 1,4-diacetyl derivatives are formed, with no evidence of the 2-isomer. This is an indication of the relative stabilities of the cyclic it-allyl complexes, compared with the exocyclic complex. It has been postulated that, in the reactions of conjugated systems, the initial o-allyl adduct proceeds to the products via the it-allyl complex (cf Scheme 8.1), whereas in the case of unconjugated tt-systems, the initial o-adduct is more stable and tends to undergo a further carbonylation reaction. 

When a 2-thiomethyl-l,3-selenothiolium iodide such as (84) reacts with an aryl ketone (85) in pyridine-acetic acid, fulvenes such as (86) are obtained reaction with a Wittig-Homer reagent takes place at both carbonyl groups (Equation (8)) <90PAC473,91JCS(Pl)157>. 

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]. 

Muthusamy and co-workers have demonstrated [82] the reactions of the bicyclic ylide 57, generated from the diazocarbonyl compoimd 56, with symmetrical and unsymmetrical dipolarophiles. Thus, exposure of the cyclohexanone-substituted a-diazocarbonyl compound 56 to DMAD in the presence of Rh2(OAc)4 as the catalyst has furnished the cycloadduct 58 (Scheme 16). This cycloaddition was diastereoselective and, in the case of unsymmetrical dipolarophiles such as methyl methacrylate and propargyl bromide, they were regioselective and afforded oxygen heterocycles 59 and 60, respectively. The same research group has reported the 1,3-dipolar cycloaddition of the bicyclic carbonyl yUde 57 with other dipolarophiles, namely fulvenes [83]. In these tandem cycUzation-cycloaddition reactions involving fulvenes, four stereocenters and two new C-C bonds are formed in a single step. Symmetrical dipolarophiles such as macrocycHc olefins were also used for diastereoselective 1,3-dipolar cycloaddition reaction with 56 [84]. 

Methylcyclopentadiene readily undergoes reaction with metal carbonyls 47) and even 1,3-diphenylcyclopentadiene reacts with iron carbonyl 4S). A novel method for obtaining a ferrocene with one substituted ring, is the reaction of 1,3-diphenylcyclopentadiene with the dimer of cyclopentadienyl-iron dicarbonyl at 170° C to produce 1,3-diphenylferrocene in 15% yield. Other substituted cyclopentadienes which have been used are tetraphenyl-cyclopentadienone in the preparation of tetraphenyl(hydroxy)cyclopenta-dienylmanganese tricarbonyl 49) and indene in the preparation of the dimer of indenylmolybdenum tricarbonyl (50). A variety of substituted fulvenes have been used in the preparation of substituted cyclopentadienyl metal tricarbonyl compounds of Cr, Mo, and W (57, 52). This latter reaction proceeds best in the presence of a solvent, such as 1,2-dimethoxyethane, to permit easy abstraction of hydrogen. 

Substituted fulvenes react with iron carbonyls in an abnormal maimer these reactions are discussed in Section IV. 

The reactions of fulvenes with iron carbonyls have been studied by Weiss and Hiibel 113,114). Upon reaction with Fe(CO)s, cyclopentadienyl iron compounds of type (XLIV) are obtained, but under the mild conditions possible with the use of Fe2(CO)9 a variety of products, some of unknown structure, are obtained. The co,a>-diaryl-substituted fulvenes may be caused to yield complexes of type (XCV) or (XCVI) depending on the duration of the reaction. In contrast, alkyl-substituted fulvenes give products for which the general structure (XCVII) has been proposed the parent complex is obtained from reaction of acetylene with iron carbonyls (75, 115). 

Reaction between fulvenes and metal carbonyls. Fulvenes may react with metal carbonyls giving fulvene metal carbonyl complexes (see Chap-... 

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). 

The PTC reaction of fulvenes with CO/Mel gives monoacylated derivatives as the major products [155]. Metal-carbonyl catalyzed reactions of allenes have also been affected under PTC conditions (eq. (14)). Nickel cyanide catalyzes the re-giospecific carbonylation of allenes to y9,y-unsaturated acids [ 156] while hydroxy-ketones are usually the main products when Co2(CO)g and Mel are used [157]. The PTC reaction of allenes with CO in the presence of Mn2(CO)io and Mel afford (Z)-a,y5-unsaturated ketones [158]. 

Novel photochemical (and thermal) reactions of macrocyclic oxa-sila-acetylenic ring systems (expected to show unusual optical properties because of electronic effects arising from orbital overlap of the acetylenic n system with the silicon a bonds and the oxygen lone-pair electrons) were described. While thermolysis in the presence of a transition metal carbonyl compound gave cyclization to both benzenoid and fulvene species, photolysis in the presence of the transition metal carbonyl compound (which catalyzes 1,2-silyl shifts across a carbon-carbon triple bond) gave fulvene and vinylidene products, the latter being readily photolyzed to the fulvene 159 (equation 101). 

Stable at room temperature for a short while. Detailed NMR experiments have demonstrated the non-aromatic property of this brr-electron system [8]. In contrast to the isomeric benzene, fulvene 1 has localized double bonds [9]. Therefore, it behaves like a diene or like a dienophile, and it readily oligomerizes at temperatures above 0°C [7]. In contrast, terminally disubstituted fulvene derivatives such as dimethylfulvene 11 which had been obtained as early as 1906 by Thiele et al. from cyclopentadiene and acetone in methanol in the presence of potassium hydroxide [10], are perfectly stable compounds. A rather efficient new synthesis of 11 and the monosubstituted fulvene derivative 12 by the reaction of cyclopentadiene with unsaturated carbonyl compounds in the presence of pyrrolidine (Scheme 2), has been described by Griesbeck [11]. Dimethylfulvene 11 in this transformation arises via Michael addition of the cyclopenta-dienide ion to the a,j9-unsaturated ketone mesityloxide and a subsequent retro-... 

Some transition metal carbonyls may also form cyclopentadienyl derivatives with fulvenes. Most commonly, the solvent is the source of hydrogen. In the absence of the solvent, disproportionation reactions occur [equations (9.42) and (9.43)]. 

---