Olefin -Complexes

The first olefin complex and a modern chiral diene for asymmetric catalysis. 

Orbital symmetries and the valence bond description of transition metal olefin complexes. 

Olefins form a great variety of complexes with transition metals. In all these complexes the olefin is bonded to the metal atom by both r-donor and ir-acceptor bonds and this is favored by a low valence state of the metal atom. 

There is often a striking similarity between olefins and carbon monoxide as ligands, and one of the most common ways of preparing olefin complexes is by replacement of one or more carbon monoxide ligands in a metal carbonyl by olefins. Both olefin and carbonyl complexes frequently obey the simple E.A.N. rule, each CO ligand and C C bond contributing two ir-electrons to the metal atom, to enable it to attain the electronic configuration of the next inert gas in the Periodic Table. 

Modern physical methods, e.g., X-ray, infrared, ultraviolet and Raman spectra, dipole moment and magnetic susceptibility measurements, and, more recently, nmr spectra have played a very important part in elucidating the structure and the bonding in these complexes. 

Ultraviolet irradiation of [VC6H6(CO)4] in the presence of certain diolefins gives red complexes of the composition [V(C6H5)(CO)2(diolefin)]. The 

Chelating polyolefins displace carbon monoxide from chromium hexa-carhonyl to form stable olefin complexes. Thus cyclo-octa-1,5-diene gives the yellow complex [Cr(CO)4(C8H12)] for which the cfs-structure (VI M = Cr) is proposed (79). 

It has been established by x-ray analysis that the metal-olefin axis is perpendicular to the plane of the ethylene molecule. The dipole developed from this metal-olefin bond would point along the same direction as in the ethylene molecule. The magnitude of the shift depends in part upon the extent to which the electrical center of the bond is displaced toward the metal atom. The shift is not exceptionally large—only 1.3 t. The presence of the chlorine may partially cancel the shift to higher field. For example, 

There are 8 ethylene hydrogens the reason for the splitting is not clearly understood but probably is the result of Rh-H spin-spin coupling. 

Solutions of cationic olefin complexes of silver exhibit NMR spectra that support the conclusion of IR data that the Ag -olefin interaction is weak. The olefinic protons here are observed at t 3.9, about 0.7 ppm lower than the region observed for resonances of the corresponding free olefins. In addition, no spin-spin coupling between Ag and H was present. 

Norbornadiene [4-7] has been shown by NMR to function both as a mono-olefin ligand and a chelating group. In both the free ligand and its metal complexes in which it coordinates as a bidentate ligand there are three different sets of equivalent protons. In complexes in which only one of the double bonds is attached to a metal, five NMR signals are expected [4-7.5]. 

In Table 4-17 some NMR data of typical norbornadiene complexes are listed. On the basis of these data C7H8Mn(CO)2(7i-C5H5) is formulated as having a norbornadiene bonding mode. The iron derivative displays an accidental equivalence of two types of protons. This also has been observed in other complex organometallic systems and is supported by the relative intensity values. 

Although three proton environments would be anticipated for (CgHg)Fe2(CO)5 if the molecular structure in the crystal were maintained in solution (60) (Fig. 14), only one sharp proton NMR line is observed down to 193 K (the lowest temperature studied). Evidently this complex is also fluxional, and, indeed, ring rotation is rapid even in the solid state at 77 K (19). 

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Eig. 6. Elow diagram for the Shell Chemical alcohol-olefin complex, Geismar, Louisiana, and Stanlow, United Kingdom. 

In most of the rest of the world the olefins industry was originally based on naphtha feedstocks. Naphtha is the dominant olefins feedstock in Europe and Asia. In the middle 1980s several large olefins complexes were budt outside of the United States based on gas Hquids feedstocks, most notable in western Canada, Saudi Arabia, and Scotiand. In each case the driving force was the production of natural gas, perhaps associated with cmde oil production, which was in excess of energy demands. 

The acetaldehyde-forming step (eq. 7) involves nucleophihc attack by hydroxide or water on a coordinated Pd olefin complex followed by P-hydride elimination. 

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). 

Superffex C t lytic Crocking. A new process called Superflex is being commercialized to produce predorninantiy propylene and butylenes from low valued hydrocarbon streams from an olefins complex (74). In this process, raffinates (from the aromatics recovery unit and the B—B stream after the recovery of isobutylene) and pyrolysis gasoline (after the removal of the C —Cg aromatics fraction) are catalyticaHy cracked to produce propylene, isobutylene, and a cmde C —Cg aromatics fraction. AH other by-products are recycled to extinction. 

The higher activity of the catalyst [(mall)Ni(dppmo)][SbFg] in [BMIM][PFg] (TOF = 25,425 h ) relative to the reaction under identical conditions in CFF2C12 (TOF = 7591 h ) can be explained by the fast extraction of products and side products out of the catalyst layer and into the organic phase. A high concentration of internal olefins (from oligomerization and consecutive isomerization) at the catalyst is known to reduce catalytic activity, due to the formation of fairly stable Ni-olefin complexes. 

The addition of carhon monoxide to isobutylene under high pressures and in the presence of an acid produces a carhon monoxide-olefin complex, an acyl carhocation. Hydrolysis of the complex at lower pressures yields neopentanoic acid ... 

As was the case for the Ni (123) and Pd/C2H4 (140) systems, each of the binary olefin complexes isolated has associated with it a moderately intense, UV band, the bands for Pd complexes lying at higher energy than those of the nickel complexes in addition, for each olefin sys-... 

The Chemistry of Complexes Containing 2.2 -Bipyridy 1, 1,10-Phenanthroline, or 2.2. 6. 2"-Terpyridyl as Ligands W. R. McWhinnie and J. D. Miller Olefin Complexes of the Transition Metals... 

By analogy with hydroformylation, dicobalt octacarbonyl has been examined as a hydrosilylation catalyst. Various silanes and a-olefins react, often exothermically. Thermal deactivation occurs above 60° C hence, large exotherms and high temperatures must be avoided (56, 57,130). Isomerization is more pronounced than for the bridged olefin complexes of Pt(II) and Rh(I) (see below) it even occurs with trialkoxysilanes (57). Though isomerization is faster than hydrosilylation, little variation in the relative rates of these two processes with the nature of the silane is observed this is in marked contrast to the bridged systems (55). 

For catalysis by Pt(II) and Rh(I) w-olefin complexes (those containing chelating diolefin ligands were less effective), three types of reaction have been observed depending on the nature of the silane (55). 

But it should be emphasized that there is no proof that the rr-olefin complex is an essential intermediate in any or all of these reactions. 

Synthesis, structure and reactions of chelate metal-olefin complexes... 

Yasuda S, Yorimitsu H, Oshima K (2008) Synthesis of aryliron complexes by palladium-catalyzed transmetalation between [CpFe(CO)2l] and aryl Grignard reagents and their chemistry directed toward organic synthesis. Organometallics 27 4025 027 Jonas K, Schieferstein L (1979) Simple route to Li- or Zn-metalated r -cyclopentadien-yliron-olefin complexes. Angew Chem Int Ed Engl 18 549-550... 

A family of cyclopentadiene(Cp)-containing iron-olefin complexes has been pioneered by Jonas [13-15]. The complexes 38-40 (Scheme 7) can be obtained in a large scale from ferrocene 37 under reducing conditions in the presence of suitable coordinating olefins. Complex 38 is a highly air-sensitive, crystalline material, whereas complexes 39 and 40 are more robust due to their cyclooctadiene (cod)... 

A key question remains how is the olefin formed in the overall process Molecular tantalum complexes are known to undergo facile a- and transfer processes, leading to tantalumalkylidene and tantalum tt-olefin complexes, respectively (mechanism 9, Scheme 29) [98]. Moreover, olefin polymerization with tantalum complexes belongs to the rare case in which the Green-Rooney mechanism seems to operate (Eq. 10, Scheme 29) [102]. Finally, intramolecular H-transfer between perhydrocarbyl ligands has been exemplified (Eq. 11, Scheme 29) [103,104]. 

Nickel(O) reacts with the olefin to form a nickel(0)-olefin complex, which can also coordinate the alkyl aluminum compound via a multicenter bond between the nickel, the aluminum and the a carbon atom of the trialkylaluminum. In a concerted reaction the aluminum and the hydride are transferred to the olefin. In this mechanistic hypothesis the nickel thus mostly serves as a template to bring the olefin and the aluminum compound into close proximity. No free Al-H or Ni-H species is ever formed in the course of the reaction. The adduct of an amine-stabihzed dimethylaluminum hydride and (cyclododecatriene)nickel, whose structure was determined by X-ray crystallography, was considered to serve as a model for this type of mechanism since it shows the hydride bridging the aluminum and alkene-coordinated nickel center [31]. 

In the case of r)2-coordination of the exocyclic C=C bond, it becomes substantially elongated compared with the double bond of free alkenes, as a result of back donation from the metal to the 7t orbitals of the double bond. For instance, in complex 17b the coordinated bond length is 1.437 A (see Fig. 3.2).18 This is also reflected in the loss of planarity around the quaternary exocyclic carbon, the methylenic carbon being bent out of the ring plane by 10.78°.18 Similar structural features were also observed with other P2Pd conjugated olefin complexes.39... 

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