Olefin hydrogenation complexes

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

Olefin Complexes. Silver ion forms complexes with olefins and many aromatic compounds. As a general rule, the stabihty of olefin complexes decreases as alkyl groups are substituted for the hydrogen bonded to the ethylene carbon atoms (19). 

The observation that addition of imidazoles and carboxylic acids significantly improved the epoxidation reaction resulted in the development of Mn-porphyrin complexes containing these groups covalently linked to the porphyrin platform as attached pendant arms (11) [63]. When these catalysts were employed in the epoxidation of simple olefins with hydrogen peroxide, enhanced oxidation rates were obtained in combination with perfect product selectivity (Table 6.6, Entry 3). In contrast with epoxidations catalyzed by other metals, the Mn-porphyrin system yields products with scrambled stereochemistry the epoxidation of cis-stilbene with Mn(TPP)Cl (TPP = tetraphenylporphyrin) and iodosylbenzene, for example, generated cis- and trans-stilbene oxide in a ratio of 35 65. The low stereospecificity was improved by use of heterocyclic additives such as pyridines or imidazoles. The epoxidation system, with hydrogen peroxide as terminal oxidant, was reported to be stereospecific for ris-olefins, whereas trans-olefins are poor substrates with these catalysts. 

The cyclooctene dimer [IrCl(C8H]4)2] can selectively hydrogenate cy-clooctene in mixtures with hex-l-ene, and an unsaturate route [Eq. 1(b)] via a monomeric olefin complex was demonstrated (181). The pentamethylcyclopentadienyl dimer was mentioned at the end of Section II, B, 2. 

This observation may well explain the considerable difference between metal-olefin and metal-acetylene chemistry observed for the trinuclear metal carbonyl compounds of this group. As with iron, ruthenium and osmium have an extensive and rich chemistry, with acetylenic complexes involving in many instances polymerization reactions, and, as noted above for both ruthenium and osmium trinuclear carbonyl derivatives, olefin addition normally occurs with interaction at one olefin center. The main metal-ligand framework is often the same for both acetylene and olefin adducts, and differs in that, for the olefin complexes, two metal-hydrogen bonds are formed by transfer of hydrogen from the olefin. The steric requirements of these two edgebridging hydrogen atoms appear to be considerable and may reduce the tendency for the addition of the second olefin molecule to the metal cluster unit and hence restrict the equivalent chemistry to that observed for the acetylene derivatives. 

The formation of acetaldehyde from the w-olefin complex was shown to involve intramolecular migration of a hydrogen atom from one carbon of the ethylene to the other, rather than 0H attack on a vinyl group generated by hydride abstraction with Pd (CH2=CH+ + OH -> CH2=CH0H) followed by rearrangement of the vinyl alcohol to acetaldehyde, since hydrolysis in DgO yielded acetaldehyde free of deuterium (59). 

The proton-olefin complex is probably responsible for the unusually high cisjtrans ratio 47, 92). These intermediates have to be considered as hydrogen bond-like structures and evidence has been presented for an extremely high mobility of the proton in these structures 98, 99). 

The reaction of an olefin with hydrogen may be analogous to the related homogeneous reaction (44, 46). The olefin may form a n complex with a single center and combine with hydrogen in the sequence shown in Fig. 22A. Besides the reverse of the preceding sequence, an alternative... 

In aqueous hydrochloric acid solutions, mthenium(II) chloride catalyzed the hydrogenation of water-soluble olefins such as maleic and fumaric acids [6]. After learning so much of so many catalytic hydrogenation reactions, the kinetics of these simple Ru(II)-catalyzed systems still seem quite fascinating since they display many features which later became established as standard steps in the mechanisms of hydrogenation. The catalyst itself does not react with hydrogen, however, the mthenium(II)-olefin complex... 

The metal halide thus functions in similar manner to the proton and may be considered to be an acidic catalyst (cf. Luder and Zuffanti, 19). The catalyst-olefin complex differs in one significant respect from the product formed by the addition of the proton (or the corresponding acid) to the olefin the halide catalyst is a neutral but electronically deficient molecule and combines with the pi electrons of the double bond to form a coordinate bond between the carbon atom and the aluminum or boron. On the other hand, the addition of the positive proton to the double bond results in the formation of a true (covalent) link between carbon and hydrogen. In other words, the complex, while it contains an electron-deficient (hence, positive) carbon atom, is in itself electronically neutral the product of the addition of a proton to the alkene contains a similar carbon atom but is itself electrically positive. It has been suggested (Whitmore and Meunier, 20) that this difference is related to the fact that metal halide catalysts tend to yield much higher polymers than do the acid (proton) catalysts. 

Platinum(II) and ruthenium(II) complexes with chiral modified diphosphines like 47 or tetradentate P2N2 ligands like 48 have been used for the asymmetric epoxidation of olefins with hydrogen peroxide with ee values of 18-23%, which increased up to 41% when cationic solvato derivatives such as P2Pt(CF3)(CH2Cl2)(BF4) are used . Similar chiral inductions were reported for Ru derivatives, although the nature of the active intermediate was still in question. ... 

Zirconium(IV) complexes hydrogen peroxide determination, 635-6 olefin epoxidation, 417, 419, 422 Zwitterions... 

The calculated transition state for -hydrogen transfer (Fig. 3) has a non-planar 6-membered ring structure. There is no direct interaction between the aluminium atom and the hydrogen being transferred. Thus, the situation differs sharply from that for transition metal polymerization, where the transition state has some resemblance to a hydride-bis(olefin) complex, as illustrated for a typical metallocene case in Fig. 3. 

Jack Halpern Perhaps I have more reason than anyone else to be disposed to the view that 7r-complexing is an important step of the insertion reaction, because I think that possibly we have the only reasonably clear cut case of an olefin insertion reaction where a complex is clearly implicated. This is the ruthenium chloride-catalyzed hydrogenation of certain olefins, which almost certainly involves the insertion of the olefin into a ruthenium hydrogen bond and where certainly a ruthenium olefin complex is involved as an observable reactant. Nevertheless, I am not at all sure to what extent this is a general or necessary feature of such insertion reactions. The important question is whether one or two coordination positions on the metal ion are involved in the transition state of the insertion reaction. For example, if one considers the insertion of an olefin, say into an M—X bond, then the transition state may look something like ... 

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