Prochiral olefins stereoselective olefin

The origin of the remarkable stereoselectivities displayed by chiral homogeneous catalysts has occasioned much interest and speculation. It has been generally assumed, using a lock-and-key concept, that the major product enantiomer arose from a rigid preferred initial binding of the prochiral olefin with the chiral catalyst. Halpren 48) on the basis of considerable evidence, reached the opposite conclusion the predominant product enantiomer arises from the minor, less stable diastereomer of the olefin-catalyst adduct, which frequently does not accumulate in sufficient concentration to be detected. The predominant adduct is in essence a dead-end complex for it hydrogenates at a much slower rate than does the minor adduct. 

When an appropriate chiral phosphine ligand and proper reaction conditions are chosen, high enantioselectivity is achievable. If a diphosphine ligand with C2 symmetry is used, two diastereomers for the enamide-coordinated complex can be formed because the olefin can interact with the metal from either the Re- or Sf-face. Therefore, enantioselectivity is determined by the relative concentrations and reactivities of the diastereomeric substrate-Rh complexes. It should be mentioned that in most cases it is not the preferred mode of initial binding of the prochiral olefinic substrate to the catalyst that dictates the final stereoselectivity of these catalyst systems. The determining factor is the differ-... 

P is an optically active tertiary phosphine, likely will resemble the RhCl(PPh3)3 system (23). However, even in this exhaustively studied system, both hydride and/or unsaturate routes are feasible (23, 24) by varying conditions, the choice of route could affect stereoselectivity. Most asymmetric hydrogenations have used prochiral olefinic acid substrates, and these systems have not been thoroughly studied even with nonchiral catalysts. 

The Jacobsen-Katsuki-catalysts (Fig. 13) have recently received much attention as the most widely used alkene epoxidation catalysts. An example of Jacobsen s manganese-salen catalyst is shown in Fig. 13. They promote the stereoselective conversion of prochiral olefins to chiral epoxides with enantiomeric excesses regularly better than 90% and sometimes exceeding 98%.82,89,92,93,128 The oxidation state of the metal changes during the catalytic cycle as shown in Scheme 8. 

The use of soluble rhodium catalysts containing chiral ligands to obtain high stereoselectivity in the asymmetric hydrogenation of prochiral olefins represents one of the most important achievements in catalytic selectivity, rivaling the stereoselectivity of enzyme catalysts [J. Halpem, Science, 217 (1982) 401]. Many chiral ligands... 

Applying these methods for the epoxidation of prochiral olefins without additional measures racemic epoxides are obtained. In most cases, the idea is to make the method stereoselective and thus obtain pure or enriched enantiomers of epoxides by using chiral reagents or by addition of optically active auxiliaries. Some of the results obtained by various groups will be discussed. 

Circular Dichroism Spectra of Square Planar Complexes Containing Prochiral Olefins and Their Stereoselective Olefin Exchange... 

A prerequisite for effective asymmetric hydrogenation is that the prochiral olefin is bound stereoselectively to metal at the rate-determining transition-state (Scheme 1). It is therefore of interest to consider stable metal-olefin complexes which may exist as diastereomers by virtue of alternative modes of prochiral olefin complexation. Most work has been done with comparatively simple asymmetric sulfur or nitrogen ligands, and selectivity is usually low. With simple olefins this is not surprising, since discrimination depends on rather small differences in steric bulk in the absence of polar interactions. 

The choice of chelating diphosphines or diphosphites gives additional variations to the coordination chemistry of the catalyst complexes. Beyond their particular coordinating ability and steric demand, these ligands are characterized by their natural bite angle (see the section Theoretical Calculations). By using chiral modificators, the hydroformylation of prochiral olefins can become stereoselective (3-6). Beside the addition of modifiers, the variation of the application phase has been the subject of intense research. For reviews of this topic, see Refs. (242-244). 

Currently, a small number of chiral ligands that meet the requirements of highly efficient and stereoselective hydroformylation are available. Noteworthy, only a few produce satisfactory results for several substrates most of them are limited to a very small range of prochiral olefins. With respect to the huge number of phosphorus ligands that have been synthesized and screened in the last three... 

A continuing interest in this area is to obtain more insight into the factors that govern the stereoselectivity of olefin coordination. NMR studies, including NOE and low-temperature experiments, concerning the effect of the olefin structure on the stability and stereoselectivities of a large number of prochiral olefins (GH2=GHR) in the alkene-amino acid chiral complex cis- (N,olefin)-(i ,Y)[PtX(77 -2-mb)(sarcosinato)] (2-mb = 2-methyl-3-buten-2-ol X = G1 )... 

When a chiral ansa-type zirconocene/MAO system was used as the catalyst precursor for polymerization of 1,5-hexadiene, an main-chain optically active polymer (68% trans rings) was obtained84-86. The enantioselectivity for this cyclopolymerization can be explained by the fact that the same prochiral face of the olefins was selected by the chiral zirconium center (Eq. 12) [209-211]. Asymmetric hydrogenation, as well as C-C bond formation catalyzed by chiral ansa-metallocene 144, has recently been developed to achieve high enantioselectivity88-90. This parallels to the high stereoselectivity in the polymerization. 

An isotactic stereospecific polymerization arises essentially from the favored complexation of one prochiral face of the a-olefin, followed by a stereospecific process. The stereospecific insertion process and the stereospecific polymerization of racemic a-olefins giving isotactic polymers may be expected to be stereoselective whenever the asymmetric carbon atom is in an a- or /3-position relative to the double bond, and when the interaction between the chirality center of the olefin and the chiral catalytic site is negligible. 

Thus, for both chiral and prochiral a-olefins, the isotactic sequence of the stereogenic tertiary carbon atom of the backbone is due to the enantioselectivity of the chiral active sites to the prochiral carbon atom of the monomer. The stereoselectivity (namely the selection, among the enantiomers, of a racemic... 

It is well accepted that two mechanisms of stereocontrol (the chiral induction responsible for selecting the monomer enantioface) are operative in stereoselective a-olefm polymerizations. In the simpler cases, the discrimination between the two faces of the prochiral monomer may be dictated either by the configuration of the asymmetric tertiary C atom of the last inserted monomer unit or by the chirality of the catalytic site. These two different mechanisms of stereocontrol are named chain-end stereocontrol and enantiomorphic-site or site stereocontrol. In the case of chain-end stereocontrol, the selection between the two enantiofaces of the incoming monomer is operated by the chiral environment provided by the last inserted tertiary C atom of the growing chain, whereas in the case of site stereocontrol this selection is operated by the chirality of the catalytic site. The origin of stereocontrol in olefin polymerization has been reviewed extensively.162,172-178... 

Several workers have studied platinum(II) complexes of olefins containing chiral amine or amino-acid ligands. Panunzi(9) observed stereoselectivity in the reaction between cis-(S - a-methylbenzylamine)dichloroplatinum (II) and trans-2-butene with the major diastereomer formed to the extent of 70% of total complex. More recently (10), it has been shown that the replacement of coordinated trans-2-butene by free olefin in (S-prolinato) dichloroplatinum (II) complexes takes place more easily with retention than with inversion. Addition of a large excess of trans-2-butene to solutions of the corresponding ethylene complexes produced first an increase and then a gradual decrease in their circular dichroism. The kinetic stereoselectivity in this reaction (that is, the differing reaction rates of the two prochiral faces of trans-but-2-ene) was 3 1, but at equilibrium the ratio of major and minor diastereomers was 64 36 in the cis-isomer and 59 41 in the trans-isomer. 

Stereoselectivity describes the preferential formation of one stereoisomer when two or more stereoisomers are possible. In the case of polydienes, in addition to the iso-syndio isomerism possible due to the presence of a prochiral carbon atom (as observed for a-olefin monomers), an additional source of stereoisomerism is present. This is the presence of the double bond in the polymer backbone, which can assume two possible configurations (cis-trans). 

---