Olefin metathesis stereospecificity

Stereospecificity as applied to olefin metathesis may be considered in two ways (a) How does the cis/trans isomer ratio of a product olefin compare with its equilibrium ratio, or (b) how does this cis/trans value differ from 1.0, which is the statistically expected value in terms of probabilities. In the present discussion, the latter definition applies. 

Several reactions in organometaUic chemistry also appear to contravene the rule, but which can be explained in a somewhat similar way. Hydrometallation [5.45, see (Section 5.1.3.4) page 162], carbometallation, metallo-metallation, and olefin metathesis reactions are all stereospecifically suprafacial [2 + 2] additions to an alkene or alkyne, for which the all-suprafacial pathway is forbidden. Hydroboration, for example, begins with electrophilic attack by the boron atom, but it is not fully stepwise, because electron-donating substituents on the alkene do not speed up the reaction as much as they do when alkenes are attacked by electrophiles. Nevertheless, the reaction is stereospecifically syn—there must be some hydride delivery more or less concerted with the electrophilic attack. The empty p orbital on the boron is the electrophilic site and the s orbital of the hydrogen atom is the nucleophilic site. These orbitals are orthogonal, and so the addition 6.126 is not pericyclic. 

Historically one of the first asymmetric methods to be explored, cyclopropanation came of age32 with box and salen ligands on Cu(I). Diazo compounds, particularly diazoesters 138, react with Cu(I) to give carbene complexes 140 that add to alkenes, particularly electron-rich alkenes to give cyclopropanes 141. The reaction is stereospecific with respect to the alkene -1runs alkenes giving trans cyclopropanes - and reasonably stereoselective as far as the third centre is concerned. Any enantioselectivity comes from the chiral ligand L. You have already seen the Ru carbene complexes are intermediates in olefin metathesis (chapter 15). 

When metathesis is effected with tra i-2-pentene, rather than cis-, and (diphenylcarbene)pentacarbonyltungsten is the initiator, the 2-butene and 2-hexene products are largely trans. The stereospecificity (73-83% trans) is not as great as for cw-olefin metathesis, but it is appreciable (63). The ratios of the stereoisomers in the products are close to the equilibrium ratios, but they probably are not determined by the products equilibrating, for in the short time the metathesis was run to determine the stereochemistry of the initial product, the precursor, tranj-2-pentene, underwent only negligible isomerization. The stereochemistries therefore are determined by the kinetics, which in turn should be affected by conformational factors similar to those in Scheme... 

With but few notable exceptions (75-77), an inherent characteristic of the metathesis of acyclic olefins with both homogeneous and heterogeneous catalysts is the tendency for attainment of thermodynamic equilibrium in the composition of cis and trans isomers in reactions carried to high conversion. Therefore, any inherent stereospecificity can only be evaluated by extrapolating compositional data to zero percent reaction. 

Soluble metathesis catalysts yield trans products in reactions with // / v-2-pentene, but generally are not very stereospecific with c/.v-2-pen-tene. In the latter case, the initially formed butenes and hexenes are typically about 60 and 50% cis, respectively. Basset noted (19) that widely diverse catalyst systems behaved similarily, and so it was suggested that the ligand composition about the transition metal was not a significant factor in the steric course of these reactions. Subsequently, various schemes to portray the stereochemistry have been proposed which deal only with interactions involving alkyl substituents on the reacting olefin or on the presumed metallocyclobutane intermediate. 

An alternative approach has been formulated by Katz and co-workers (28). They consider the latent cis specificity in reactions of c/ s-olefins with metathesis catalysts to be quite high, based on their results with a catalyst having very low Lewis acidity. They further propose that the frequently observed lack of stereospecificity is mainly the consequence of cationic processes inherent in the metathesis step which mask stereospecificity. 

In this scheme, a highly puckered metallocycle was envisioned, possessing pseudoaxial and equatorial substituents, and reaction pathways were said to be favored which minimized the following effects (a) 1,3-diaxial interactions of substituents on the two a-carbons (b) axial substituent interactions with juxtaposed ring carbons and (c) 1,2-diequatorial interactions. This scheme predicts relatively nonstereospecific metathesis of rra/j.v-olefins but highly stereospecific metathesis of ra-olefins. For example, the following pathways for reactions of m-olefins were proposed ... 

Recently special interest in the Schrock carbenes, especially those of Ta, Mo, W, Re, and Ru, have been focused on the stereospecific polymerization of the cyclic olefins or dienes which is called a ring opening metathesis polymerization (ROMP), A typical example of ROMP is shown in the following equation [18]. 

Carbene Mechanism.—Green and co-workers have suggested a novel mechanism for the stereospecific polymerization of olefins by Ziegler-Natta catalysts which differs significantly from the more accepted mechanisms in that it is proposed that a 1,2-hydrogen shift from the a-carbon of the polymer chain occurs as in metathesis reactions. 

Finally, actinide complexes such as [MCp 2R2] (M = Th or U, R = alkyl or H) are very active catalysts for the hydrogenation and polymerization of olefins. The complexes [U(allyl)3X] (X = Cl, Br, I) are excellent initiators for the stereospecific polymerization of butadiene, which produces rubbers that have remarkable mechanical properties. Some other complexes are active for the heterogeneous CO reduction and alkene metathesis. The field of catalysis using organoactinide complexes should considerably expand in the near future. 

Stereospecific ring-opening polymerization of cyclic olefins by metathesis catalysts... 

Incoronata Tritto studied stereospecific olefin polymerization in the group of Prof. Adolfo Zambelli at the Institute for Macromolecular Chemistry of the CNR and received her degree in polymer science at the Specialization School Giulio Natta at Politecnico di Milan (Italy) in 1981. In 1982, she joined as permanent researcher the Institute for Macromolecular Chemistry of the CNR In 1988, she spent 1 year in the group of Prof. Robert H. Grubbs at Caltech (USA), where she studied the relationship between metathesis and addition olefin polymerization. She is currently a senior research chemist at ISMAC-CNR. Her research interests focus on (1) synthesis and microstmctural characterization of stereospecific olefin and cyclic olefin homo- and copolymers by transition metal catalysts (2) activation and deactivation reactions ofthe homogeneous catalytic systems by in situ multinuclearNMR analysis and (3) synthesis of block copolymers and nanostructured hybrid polymers, in situ polymerization on clay and carbon nanotubes. 

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