Cyclopentadiene metal carbonyl derivatives

The preparation of 5-ACETYL-l,2,3,4,5-PENTAMETHYLCYCLO-PENTADIENE is of value in the synthesis of pentamethyleyclo-pentadiene and many pentamethylcyclopentadienyl metal carbonyl derivatives that are more soluble in organic solvents than those derived from cyclopentadiene. Simple preparations of 5,6-DIHYDRO-2-PYRAN-2-0NE and 2-//-PYRAN-2-ONE make these hitherto rather inaccessible intermediates available for cycloaddition and other reactions. The already broad scope of the Michael reaction has been widened further by including an efficient preparation of ETHYL (E)-3-NITROACRYLATE. Workers in the field of heterocyclic chemistry will find a simplified method for the preparation of 2,3,4,5-TETRA-HYDROPYRIDINE of help. 

This review deals with metal-hydrocarbon complexes under the following headings (1) the nature of the metal-olefin and -acetylene bond (2) olefin complexes (3) acetylene complexes (4) rr-allylic complexes and (5) complexes in which the ligand is not the original olefin or acetylene, but a molecule produced from it during complex formation. ir-Cyclopentadienyl complexes, formed by reaction of cyclopentadiene or its derivatives with metal salts or carbonyls (78, 217), are not discussed in this review, neither are complexes derived from aromatic systems, e.g., benzene, the cyclo-pentadienyl anion, and the cycloheptatrienyl cation (74, 78, 217), and from acetylides (169, 170), which have been reviewed elsewhere. 

Some heterocyclic cyclopentadiene derivatives form n complexes with metal carbonyls (47, 55, 373, 384) ... 

Cyclopentadiene, CsHg, is a colourless liquid which readily forms a monosodium derivative. This in turn reacts with anhydrous transition-metal halides to form derivatives M(CsHs) , some of which may be made direct from the hydrocarbon and the metal carbonyls at about 300°C. Some of these compounds, including Fe(CsHj)2 ( ferrocene ) can be oxidized to cations. The following are examples of cyclopentadienyl compounds ... 

Organometallic Compounds The most widely used compounds [Is,2m] are the titanocene derivatives (10.36), which exhibit an excellent absorption around 500 nm and very efficiently eliminate an aryl and a cyclopentadienyl ligand yielding an unsaturated titanium species and an arylated cyclopentadiene. The Ti structure adds to the CO group of one acrylate unit (may be two) and generates the initiating radicals. Other systems include metal carbonyl (Mn, Fe, Mo, Cr, Os, Re, Ru, etc. [75,76]). 

Of particular interest is the oxidation of the complexes Fea CjoH R Og (XXXI R = R = CH3 or H) with aqueous ferric chloride to give a product of composition (RC2R)Fe(CO)6 [190). Oxidation of the dimethyl derivative with cold concentrated nitric acid gives dimethylmaleic anhydride (XXXII). The derivative (HC2H)Fe(CO)6 forms a Diels-Alder adduct (XXXIII) with cyclopentadiene the adduct may be oxidized to succinic acid. All of these data indicate that the derivatives (RC2R)Fe(CO)6 are cyclic derived from maleic acid derivatives. Four of the six carbonyl groups are metal... 

H4Ru4(CO)i2 yields (136) and (137) as two previously uncharacterized organo-metallic products, and both may also be prepared by reaction of Ru3(CO)ia with cot (137) is fluxional, and may be carbonylated to yield an Ru2(CO) -(C sHie) derivative. Reaction of (cot)Fe(CO)3 with cot yields (138). H4RU4-(CO)x2 or Ru3(CO)i2 reacts with a cot-cyclopentadiene mixture to yield the fluxional complex (139), and structurally related derivatives may be obtained using chpt-cyclopentadiene and cot-chpt mixtures. Oxidation of (cot)Fe(CX))8-L [L=CO, PPhg, P(OPh)3] with AgPF yields the dimeric cation (140), characterized crystallographically, where L=P(OPh)3. ... 

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