Monoolefin ligands

M. Herberhold, Metal-Ji-Complexes, Vol. II, Complexes with Monoolefinic Ligands, Part I. Elsevier, Amsterdam, London, New York, 1972. 

Herberhold, M., Metal-7r-Complexes, Yol. II Complexes with Monoolefinic Ligands. Elsevier, Amsterdam, 1972. 

The selectivity for hydrogenation of dienes in the presence of monoolefins arises from the exceptional stability of jr-ally 1 complexes. In the case of Pt catalysts the reactions shown can compete with one another (equation 6)14. The second pathway is favored, especially when the olefin or diene must compete with excess ligands (phosphine, CO, SnCp ) for a coordination site. This is why the diene is almost completely hydrogenated before the concentration of olefin increases to the point that the olefin gains access to the catalyst. A similar phenomenon can be responsible for selectivity in hydrogenation of dienes with heterogeneous catalysts. 

Palladium (II)-Nucleophile Addition across Olefins. Adding palladium complexes to olefins, either in the presence of an external nucleophile or a ligand which is attached to palladium, produces a palladium-carbon sigma-bonded complex which is not usually isolated in the case of monoolefins. Instead it decomposes and in doing so oxidizes the olefin to an organic carbonyl compound or a vinyl compound, exchanges a substituent group on the olefin, isomerizes the double bond, arylates (alkylates) the olefin, or carboxylates the olefin (2, 3). 

In the case of certain diolefins, the palladium-carbon sigma-bonded complexes can be isolated and the stereochemistry of the addition with a variety of nucleophiles is trans (4, 5, 6). The stereochemistry of the addition-elimination reactions in the case of the monoolefins, because of the instability of the intermediate sigma-bonded complex, is not clear. It has been argued (7, 8, 9) that the chelating diolefins are atypical, and the stereochemical results cannot be extended to monoolefins since approach of an external nucleophile from the cis side presents steric problems. The trans stereochemistry has also been attributed either to the inability of the chelating diolefins to rotate 90° from the position perpendicular to the square plane of the metal complex to a position which would favor cis addition by metal and a ligand attached to it (10), or to the fact that methanol (nucleophile) does not coordinate to the metal prior to addition (11). In the Wacker Process, the kinetics of oxidation of olefins suggest, but do not require, the cis hydroxypalladation of olefins (12,13,14). The acetoxypalladation of a simple monoolefin, cyclohexene, proceeds by trans addition (15, 16). 

The unpromoted hydrocyanations of monoolefins discussed so far generally involved only a few catalytic cycles on nickel. The development of a practical commercial process depended on getting many cycles. Certain Lewis acids are quite remarkable in increasing (1) catalyst cycles, (2) the linearity of products obtained, and (3) the rates of reaction. The effects depend on the Lewis acid, the phosphorus ligand used, and the olefin substrate (72). 

Another route to Pd(0) complexes reported recently is by reaction of [Pd(CNPh)2] with activated monoolefins 18, 220). With fumaronitrile maleic anhydride and p-quinones, 1 1 adducts are formed, whereas, with tetrachloro-p-benzoquinone, an adduct is formed in which the Pd-to-ligand ratio is 2 1. Another new procedure is the use of bis-(dibenzlideneacetone) palladium(O) which reacts directly with olefins 145). 

For an unsymmetrically substituted monoolefin dual modes of insertion into the M—H bond and an M—C bond are possible (Scheme 2). This can lead to various linear or branched dimers and oligomers. Regioselectivity in these reactions depends in a complicated manner on the metal, the ligands applied, and the reaction conditions. Furthermore, most of the transition metal systems also catalyze isomerization of olefins and therefore the primarily formed products may be converted to other isomers. 

We can visualise the capability of suitable lanthanide (Ln) compounds (J, 6), e.g. as homogeneous catalysts with respect to olefins, by invoking similar intermediates. Although the series of reportedly catalytically active Ln-complexes spans from the pure trihalide via tris(B-diketonato)complexes to the organo-metallic tris(cyclopentadienyl) and tetra(allyl)complexes (8), respectively, no really optimal combination of ligands on a Ln-element has been found so far. Promising aspects are, however, based on some evidence for "reaction steering" in that either cis- or trans-polybutadienes can be obtained from 1,3-dienes, and either polymers or metathesis products from monoolefins, respectively (Table I). 

In 5.8.2.3.1 it is pointed out that olefins are relatively weak ligands for transition metals. Diolefins that can act as bidentate ligands form stronger complexes than monoolefins owing to the chelate effect. As a consequence most of the methods described in 5.8.2.3.1 have been used to prepare diolefin complexes. Thus, for example, diolefins displace anionic ligands from Pt(II) , Pd(II) , Rh(I) and Ir(I) ... 

An exception to the generalization that diolefins rarely displace monoolefins to form chelate complexes is where an anionic chloride ligand is lost as hydrogen chloride from Pt(II) together with ethylene when H[PtCl3(G2H4)] is treated with a diolefin such as norbomadiene ... 

Several transition-metal phosphine complexes, including RuCl2(PPh3)3, are effective for the selective reduction of dienes to monoolefins.It is generally believed that this selectivity arises from the exceptional stability of an intermediate 7r-allyl complex. The key intermediate in this process may be a 7r-allyl hydride complex. Complex 6, as we indicated in the Introduction, is the only known example of such a complex. The hydride ligand is cis to the allyl group. 

Oxygen nucleophiles, such as water, alcohols, and carboxylic acids, also attack coordinated olefins in simple (monoolefin)palladium(II) complexes. These nucleophiles do not displace the olefin from the metal because they are poor ligands for the soft palladium(II). 

Hydrocarbons. A variety of unsaturated hydrocarbons such as monoolefins, linear and cyclic dienes, cyclic tetraenes etc. have been found and structurally characterized as ligands bonded to metal clusters. 

The selective hydrogenation of non-conjugated dienes to monoolefins Is catalysed by Ru2(cod)2(02CCF3)2( t-02CCF3)2(p-0H). 1,5-cyclooctadlene Is reduced to cyclooctene Isomericatlon does not take place, and further reduction only occurs once all the diene has been consumed. By contrast, palladium complexes In the presence of the ferrocenyl ligands (1) reduces conjugated dienes to monoolefins, e. 1,3-cod to cyclooctene.21... 

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