Olefins bonding model

The crystal structures of Zeise s salt and of two analogous palladium-olefin complexes were published,shortly after the publication of the Chatt-Duncanson paper. They confirmed the structural proposals made by Chatt, but none of these papers cites Dewar,though metal-olefin bonding models were not discussed. Two short reviews on the history of Zeise s salt, (one part of a more general discussion of the history of organometallic chemistry) also only refer to Chatt s contribution, though neither specifically address questions of bonding. 

In order to rationalize such characteristic kinetic behaviour of the topochemical photoreaction, a reaction model has been proposed for constant photoirradiation conditions (Hasegawa and Shiba, 1982). In such conditions the reaction rate is assumed to be dependent solely on the thermal motion of the molecules and to be determined by the potential deviation of two olefin bonds from the optimal positions for the reaction. The distribution of the potential deviation of two olefin bonds from the most stable positions in the crystal at OK is assumed to follow a normal distribution. The reaction probability, which is assumed to be proportional to the rate constant, of a unidimensional model is illustrated as the area under the curve for temperature Tj between 8 and S -I- W in Fig. 7. 

The coordination of styrene is expected to be strongly influenced by substituents that are neglected in the minimal QM model A. Thus, for sake of clarity, we do not present the styrene coordination results using model A. Depicted in Figure 8 are the three most stable styrene coordinated isomers, 8a-c. The coordination energies, which are also shown in Figure 8 in kcal/mol, reveal that the initial formation of the tt-complex is slow and reversible. In fact, only for isomer 8a is the styrene coordination exothermic and here it is only exothermic by 0.5 kcal/mol. Isomers 8a-c all have the olefinic bond of the styrene lying parallel to the plane defined by the P-Pd-Si atoms. No other sterically accessible isomers could be located where this bond lies parallel to this plane. Due to steric reasons, complexes with the olefinic bond perpendicular to this plane were found to be at least 8 kcal/mol less stable. 

The dynamic behavior of the model intermediate rhodium-phosphine 99, for the asymmetric hydrogenation of dimethyl itaconate by cationic rhodium complexes, has been studied by variable temperature NMR LSA [167]. The line shape analysis provides rates of exchange and activation parameters in favor of an intermo-lecular process, in agreement with the mechanism already described for bis(pho-sphinite) chelates by Brown and coworkers [168], These authors describe a dynamic behavior where two diastereoisomeric enamide complexes exchange via olefin dissociation, subsequent rotation about the N-C(olefinic) bond and recoordination. These studies provide insight into the electronic and steric factors that affect the activity and stereoselectivity for the asymmetric hydrogenation of amino acid precursors. 

The course of modern organometallic chemistry has been greatly influenced by three simple generalizations the Dewar-Chatt-Duncanson synergic bonding model for metal-olefin complexes (40, 72) Pauling s electroneutrality principle (174), and the 18-electron or inert gas rule (202). In this section the impact of recent theoretical calculations on these important generalizations will be evaluated. 

The SCF-A a-SW calculation also confirms the importance of the back donation component of this model, and a contour plot of the relevant molecular orbital is shown in Fig. 3. Johnson and his co-workers (193) have estimated that this component may contribute up to 25% of the total metal-olefin bond energy. These calculations have given a more satisfactory account of the electronic absorption characteristics of this... 

Fig. 16. Observed and calculated carbon-carbon bond lengths for tetrahaptometal-olefin complexes (156), based on a topological Hiickel ir-bonding model.

The situation is different with monophosphines as far as the prevailing isomer is concerned. In fact it appears (see Table III) that the branched isomer prevails over the linear one (17), contrary to the predictions of the model formulated for the palladium-(-)DIOP catalytic system. The available experimental data are not sufficient to allow us to formulate a model for the transition state in the case of palladium monophosphine-catalytic systems. To attempt a preliminary explanation of the isomeric composition found, we suggest a transition state having a geometry approximating a square pyramid in which the olefinic bond of the substrate interacts with three and not with four substituents (see Figures 2a and 2a ). 

Fig. 2. The Dewar-Chatt-Duncanson model for olefin bonding showing the

In these cases a modification of the rearrangement, the so-called oxy-Cope rearrangement, is preferred. Thermolysis of a 3-hydroxy-l,5-diene results in the expected 1,5-diene system, but one of the olefinic bonds formed in this process is an enol, which can tautomerize to the corresponding ketone. Thus, a reverse Cope rearrangement cannot take place. Examples are shown in Scheme V/2 [13]. This reaction sequence has been investigated using the thermal behavior of two 1,2-divinylcyclohexanols as model compounds. The trans-iso-... 

Fig. 7.7 Bonding model proposed by Dewar for olefin silver(I) compounds, showing the orbitals used in the combination of the olefin with the silver(I) cation...

Sensitized photo-oxygenation of 3-morpholino-5a-cholest-2-ene (320) led to a mixture of the 2,3-dione (322) and the 3-morpholino-3-en-2-one (323). Model experiments at low temperature confirmed the formation of an unstable dioxetan (321) as the key intermediate, formed by addition of singlet oxygen on to the olefinic bond of the enamine.253... 

Ley reported that selenium promoted carbocyclization reactions can also be effected by the enolic olefinic bonds of -dicarbonyl compounds [111]. These reactions occur with N-PSP in the presence of zinc iodide, tin tetrachloride or aluminium trichloride. An example is reported in Scheme 33. In the intermediate 219, derived from the -ketoester 218, cyclization through the oxygen atom to afford 220 is kinetically favoured. This reaction, however, is reversible, and upon prolonged reaction times and in the presence of strong acids the carbocyclization product 221 is formed. This procedure has been recently employed by Ley to effect the conversion of the alkenyl p-keto lactone 222 into the tricyclic selenide 223 (Scheme 33) which is a key intermediate in the preparation of model compounds with antifeedant activity [112]. 

Diffiisional restrictions increase the effectiveness of olefin interception sites placed within catalyst pellets. Very high olefin hydrogenation turnover rates or site densities within pellets prevent olefin readsorption and lead to Flory distributions of lighter and more paraffinic hydrocarbons. Identical results can be obtained by introducing a double-bond isomerization function into FT catalyst pellets because internal olefins, like paraffins, are much less reactive than a-olefins in chain initiation reactions. However, light paraffins and internal olefins are not particularly useful end-products in many applications of FT synthesis. Yet, similar concepts can be used to intercept reactive olefins and convert them into more useful products (e.g., alcohols) and to shift the carbon number distribution into a more useful range. In the next section, olefin readsorption model simulations are used to explore these options in the control of FT synthesis selectivity. 

The reactions of polyhydric alcohols with the hydroxyl radical in aqueous solution have been extensively studied (e.g. in radiolytic and biomimetic systems), mainly because of their suitability as models for more complicated carbohydrate substrates [55] or enzymatic systems involving glycol-type radicals [56, 57]. Because there are no double bonds to which OH could add, only H-abstraction reactions are possible. Because the C-H bond energy is significantly lower than the 0-H bond energy, it is the carbons from which H are abstracted and not the alcohol function. In this type of reaction, a,yS-dihydroxyalkyl radicals are formed. The same radicals could, in principle, be produced by addition of "OH to enols, see Scheme 2, lower part. This shows the complementarity of H-abstraction and OH-addition and thereby the relevance of the former to one-electron oxidation of olefinic bonds (Scheme 2). 

The metal-dihydrogen bonding is similar to the Dewar-Chatt-Duncanson model for metal-olefin bonding [29-31]. There is a donor bond from the electron pair in the bonding orbital of the H2 ligand into an empty metal orbital (Figure 1, left-hand side). 

Figure 5 Bonding models for an Ti -olefin interaction, (a) Shows the actual bonding in the complex, (b) a molecular mechanics model of the metallo-cycle, (c) shows how the two halves of the olefin can rotate relative to each other if a pseudoatom, D, interrupts the bonding, and (d) shows a molecular mechanics model that is used in the literature. (From Ref. 23.)...

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