Rr-Olefin complexes

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. 

Facile reaction of a carbon nucleophile with an olefinic bond of COD is the first example of carbon-carbon bond formation by means of Pd. COD forms a stable complex with PdCl2. When this complex 192 is treated with malonate or acetoacetate in ether under heterogeneous conditions at room temperature in the presence of Na2C03, a facile carbopalladation takes place to give the new complex 193, formed by the introduction of malonate to COD. The complex has TT-olefin and cr-Pd bonds. By the treatment of the new complex 193 with a base, the malonate carbanion attacks the cr-Pd—C bond, affording the bicy-clo[6.1,0]-nonane 194. The complex also reacts with another molecule of malonate which attacks the rr-olefin bond to give the bicyclo[3.3.0]octane 195 by a transannulation reaction[l2.191]. The formation of 194 involves the novel cyclopropanation reaction of alkenes by nucleophilic attack of two carbanions. 

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. 

From i CO) spectra of [M(CO)5 E=C(Aryl)H ] (M = Cr, W E = S, Se) it follows that the heteroaldehyde ligand acts essentially as a coordination mode. The acceptor properties and thus v(CO) spectra are similar to those of [M(CO)s(thioether)] and [M(CO)s (phosphine)] complexes. In the 7/2-bonding mode the heteroaldehyde ligands are strong rr acceptors and their IR spectra resemble those of, e.g., alkyne and olefin complexes. Because [W(CO)5 Se = C(Aryl)H ] complexes are present in solution as rapidly interconverting mixtures of the 771 isomers and the if isomer their IR spectra are composed of both types of i CO) spectra. The intensity ratio of the v(CO) absorptions depends on the temperature due to the temperature dependence of the 17V172 equilibrium. 

The only way of using exchange with deuterium to solve Problem B is to study compounds that have eclipsed vicinal pairs of hydrogens but with only the remotest chance of forming rr-bonded olefinic complexes. Caged compounds are necessary and a very suitable choice is the heptacyclotetradecane shown in Fig. 9... 

In 1947 Walsh (68) proposed that because the ionization potential of the rr electrons in ethylene and of the lone pair in ammonia are both around 10.5 eV, the it electrons in the olefins should be equally capable of donation to acceptor centers. This implied that olefin complexes should be much more widespread than they were. 

Metal 77-cyclopentadienyls somewhat resemble the rr-allyl complexes. Initially, when the nature of the metal-allyl bond was not sufficiently clear, the similarity was emphasized many times [see the review by E. O. Fischer (425)]. The similarity shows itself, for example, in the equal antisymmetric C—C stretching frequencies (1640 cm ), which indicate that the force constants, hence the bond orders, are close. The central rr-allyl proton absorbs in the same NMR region as do the protons of coordinated cyclopentadienyl. Both ligands display the symmetrical sandwichlike bond with their metals. Today, however, it is clear that the complexes differ significantly in type, the difference being associated first of all with the fact that TT-allyl complexes are much more efficient than 77-cyclopentadienyls at transforming to o-allyl or 77-olefin compounds. This may be due to the difference between the delocalization energies, 2.472 and 0.828 eV for cyclopentadienyl and allyl anions, respectively (426). 

Isomerization of olefins by palladium complexes involves the intermediacy of Tj -allylpalladium hydride complexes. These arise by abstraction of an allylic hydrogen by Pd from the rr-complexed olefin. Collapse of the allylic complex to a new ry -olefin complex results in isomerization ... 

As Scheme 5 illustrates, when a 1-alkene reacts with hydridorhodium complex with a chiral diphosphine ligand 18 two diastereomeric rr-olefin-Rh complexes,... 

There appear to be no known structures of 7>(P)(H)(alkyl) complexes. Table 1 tabulates rr(P)(H)(olefin) complexes. Even for these the tabulation is very small and the results are unsatisfactory. There are only two complexes involving a mono-olefin, 1 and 2. In neither of these structures was the position of the hydride ligand located. But on the basis of the stereochemistry it is clear that H is cis to olefin in 2. In fact, compound 2 represents the best (and only) model for the generally proposed intermediate. That it contains an activated olefin, tetracyanoethylene, and Ir rather than Rh may be the reason for its stability. 

The retention of the (Z)-geometry of the starting olefin in this reaction is rationalized by formation of the conformationally stable rr-allylpaUadium complex with coordination of the sulfinyl sulfur atom to palladium (Scheme 4). 

In the case of (/ s)-(Z)-9, the direct participation of the chiral sulfinyl group to the palladium should be crucial the initially formed rr-allylpaUadium complex (i s)-(Z)-13 is stabilized by the coordination of the sulfinyl sulfur atom to the palladium, forming 14 with the retained (Z)-configuration of the olefin. The subsequent intramolecular nucleophilic substitution from the opposite side of the palladium provides (5,i s)-(Z)-ll in a highly stereoselective fashion (Scheme 5). 

Pd(II) compounds coordinate to alkenes to form rr-complexes. Roughly, a decrease in the electron density of alkenes by coordination to electrophilic Pd(II) permits attack by various nucleophiles on the coordinated alkenes. In contrast, electrophilic attack is commonly observed with uncomplexed alkenes. The attack of nucleophiles with concomitant formation of a carbon-palladium r-bond 1 is called the palladation of alkenes. This reaction is similar to the mercuration reaction. However, unlike the mercuration products, which are stable and isolable, the product 1 of the palladation is usually unstable and undergoes rapid decomposition. The palladation reaction is followed by two reactions. The elimination of H—Pd—Cl from 1 to form vinyl compounds 2 is one reaction path, resulting in nucleophilic substitution of the olefinic proton. When the displacement of the Pd in 1 with another nucleophile takes place, the nucleophilic addition of alkenes occurs to give 3. Depending on the reactants and conditions, either nucleophilic substitution of alkenes or nucleophilic addition to alkenes takes place. 

Schrock RR (1998) Olefin Metathesis hy Well-Defined Complexes of Molybdenum and Tungsten. 1 1-36... 

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