Iron complexes, with butadiene

A carbon-iron bond is also formed by the reaction of the cyclopropenium salt 185 with dicarbonyl(i/5-cyclopentadienyl)(trimethylsilyl)iron [92], (Scheme 69) In the reaction with benzocyclobutenylidene- 5-cyclopentadienyliron(II) hexafluorophosphate 186, CpFe(CO)2R (R=cyelo-C3H5, CH2-cyclo-C3H5) is converted to the allene and butadiene complexes, 187 and 188, respectively [93]. (Scheme 70)... 

Cycloaddition of 2-alkoxy-l,3-butadienes, H2C=C(OAlk)CH=CH2, and nitrile oxides to give isoxazolines 51 proceeds with the participation of only one of the conjugated C=C bonds. With benzonitrile oxide, only the vinyl group in alkoxydienes participates in cycloaddition reactions while in the case of phenyl-glyoxylonitrile oxide both double bonds react (222). Nitrile oxides RC=NO react with iron complexed trienes 52. The reaction proceeds with good yield and diastereoselectivity ( 90/10) to give isoxazolines 53 (223). 

The use of palladium(II) sulfoxide complexes as catalyst precursors for polymerization has met with mixed results thus a report of a palla-dium(II) chloride-dimethyl sulfoxide system as a catalyst precursor for phenylacetylene polymerization suggests similar results to those obtained using tin chloride as catalyst precursor (421). However, addition of dimethyl sulfoxide to solutions of [NH fPdCh] decreases the activity as a catalyst precursor for the polymerization of butadiene (100). Dimethyl sulfoxide complexes of iron have also been mentioned as catalyst precursors for styrene polymerization (141). 

The straight-chain 1- and 2-butenes can be converted into more butadiene when they are preheated in a furnace, mixed with steam as a diluent to minimize carbon formation, and passed through a reactor with a bed of iron oxide pellets. The material is cooled and purified by fractional distillation or extraction with solvents such as furfural, acetonitrile, dimethylformamide (DMF), and N-methylpyrrolidone (NMP). The conjugated n system of butadiene is attracted to these polar solvents more than the other C4 compounds. Extractive distillation is used, where the C4 compounds other than butadiene are distilled while the butadiene is complexed with the solvent. The solvent and butadiene pass from the bottom of the column and are then separated by distillation. 

The X-ray crystal structure of the 1 -tosyl-1H-1,2-diazepine (10) shows a boat conformation with N(l) at the prow (Figure 1) (72T581). Double bonds are clearly localized at N(2)—C(3) 1.255 A, C(4)—C(5) 1.326 A, and C(6)—C(7) 1.333 A, and the imine bond is isolated, whereas bond delocalization is present in the butadiene-like part C(4)—C(5)—C(6)—C(7). In contrast, when the diazepine is complexed with Fe(CO)3, for example (11), the iron atom is bound to the butadiene moiety C(4) to C(7), as also occurs in the lf/-azepine and l-methoxycarbonyl-l//-azepine complexes the N(l) atom adopts a planar sp2 configuration and the seven-membered ring consists of the two planar parts bent along the C(4)... C(7) line by 140° (70AG(E)958). 

Dehydration of (2-hydroxymethyl-1,3-butadiene)iron complexes or (hydroxymethyltrimethylenemethane)iron complexes with fluorosulfonic acid/liquid sulfur dioxide generates the corresponding (cross-conjugated dienyljiron cations (194) (equation 72). The H and NMR spectral data for these cations favor an " -TMM-methyl cationic structure (282) over an ) " -isoprenyl cationic stmcture (283). These cations react with water or alcohols to afford butadiene products via nucleophilic attack at C-5. As indicated earlier (Section 6.1.1), the cross-conjugated dienyl cations are believed to be intermediates in the substitution of (193) with weak carbon nucleophiles (Scheme 53). In these cases, nucleophihc attack occurs on C-4 to give predominantly TMM products. ... 

Although few examples of acylations of 1,3-butadienes have been described, Friedel-Crafts acylations of diene complexes, in particular iron tricarbonyl derivatives, can give synthetically useful yields. In acylations of iron tricarbonyl complexes with the Perrier reagent from acetyl chloride and aluminum chloride, acylation occurs only at unsubstituted terminal carbons (Scheme 18). ° The primary product is... 

The best known example is the cyclization of butadiene and acetylene 121 14°). Butadiene forms cyclooctadiene and cyclododecatriene by the catalytic action of nickel, iron, and other metal complexes. By an experiment using an iron complex with deuterated butadiene, it was proved that no hydrogen shift takes place in the cyclization reaction 70>. 

Similar to the iron chemistry (compare Chapter 2.3), also nickel complexes allow the reaction of one molecule of butadiene with two molecules of CO2 yielding a,u-dicarboxylic acids [48]. In the reaction of butadiene and CO2 in the presence of nickelbis(cyclooctadiene) and tetramethylethylenediamine first a nickelamonocarboxylate is formed (Figure 19). By further treatment with carbon dioxide and by addition of pyridine a nickeladicarboxylate complex is obtained in yields up to 72 %. Decomposition of the complex with methanol/hydrochloric acid gives cis-dimethyl-3-hexenedioate. 

Iron and ruthenium also form compounds of the type [M(allyl)cp(CO)] and [Fe(allyl) cp(PPh3)]. Ruthenium furnishes the typical derivative [Ru(allyl)2(COD)], [Ru(allyl)2(PR3)2, and [Ru2Cl2(allyl)2(COD)2]. The reaction of ruthenium trichloride with butadiene gives [RuCl2(Ci2Hi8)], while the reaction of RuCla with isoprene affords the dinuclear complex with the structure (7.70). 

Pentacarbonyliron, Pe(CO)5, is able to form stable jt-complexes with hydrocarbons. Rheilen et al. (1930) reported the first example of such a complex, tricarbonyl 1,3-butadiene] iron. 

The largest group of diene complexes which have been investigated are iron carbonyl compounds. When 1,3-butadiene reacts with Fe lCO), the initial product is a labile f/ -complex, which readily loses carbon monoxide to yield (f/ -C4Hg)Fe(CO)3. Butadiene(tricarbonyl)iron is a yellow, essentially air stable complex, m.p. 19 "C. It was first prepared by Reihlen in 1930 by heating FeiCO), with butadiene in a tube under pressure, but this was an isolated discovery which did not arouse much interest at the time. On account of the ease of preparation and handling of these diene complexes, however, an enormous amount of work has been done in this area since the late 1950s. Only modest precautions are required to protect solutions from oxidation and some operations can be carried out even in air. 

PFj exhibits preference for an apical site in complexes [Fe(CO)(3-a )(PF3)a -(butadiene)] rather than either of the two basal sites and in as3mimetric methyl-substituted butadiene derivatives with x=2 a secondary preference for the basal position irons to the methyl group is found. Intramolecular exchange of PFs groups occurs in compounds with x=2 or 3 and leads to equivalence of the PF3 F n.m.r. resonances, possibly via Berry pseudorotation. [Fe(CO)(dpe)( j-C4H6)] exists in two isomeric forms assigned structures (29) [basal-basal (bb) phosphine] and (30) [apical-basal (ab) phosphine] on the basis of their n.m.r. spectra. Variable-temperature P n.m.r. spectra provide evidence that interconversion of (29) and (30)... 

Ironcarbonyl compounds are liable to form 7r-complexes with unsaturated compounds such as monoenes, butadiene, cyclobutadienes, and cyclopentadiene. In the case of diene, the two double bonds of 1,5-cyclooctadiene are able to coordinate to the metal, without a sterical strain as shown in Table 15.2. The two double bonds of 1,4-cyclohexadiene are difficult to bond to iron without strain, so 1,4-cyclohex-adiene isomerized to 1,3-cyclohexadiene. The iron atoms have such a high reactivity with diene compounds that the isomerization of the diene occurs [13,19,20,25]. 

Anionic (77 -allyl)iron tricarbonyl complexes are easily prepared by the hydride reduction of (butadiene)iron tricarbonyl or (l-phenylbutadiene)iron tricarbonyl complexes with Li[BHEt3] in THF. Tricarbonyl(r7" -l,3-diene)-iron(O) complexes undergo addition reactions with reactive carbanions, such as LiCHPh2, to form anionic tricarbonyl(77 7 -but-3-en-l-yl)iron(0) complexes. 

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