Metal olefin complexation, stereoselection

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. 

Inhibition of diazoester decomposition by a large excess of olefin speaks in favor of intermediarily liberated W(CO)5 as direct metal precursor of425. Stereoselectivities in the cyclopropanation reaction are very similar to those observed in the Rh2(OAc)4 catalyzed version, which underlines once more the close relationship of tungsten and rhodium carbene complexes. 

The most famous mechanism, namely Cossets mechanism, in which the alkene inserts itself directly into the metal-carbon bond (Eq. 5), has been proposed, based on the kinetic study [134-136], This mechanism involves the intermediacy of ethylene coordinated to a metal-alkyl center and the following insertion of ethylene into the metal-carbon bond via a four-centered transition state. The olefin coordination to such a catalytically active metal center in this intermediate must be weak so that the olefin can readily insert itself into the M-C bond without forming any meta-stable intermediate. Similar alkyl-olefin complexes such as Cp2NbR( /2-ethylene) have been easily isolated and found not to be the active catalyst precursor of polymerization [31-33, 137]. In support of this, theoretical calculations recently showed the presence of a weakly ethylene-coordinated intermediate (vide infra) [12,13]. The stereochemistry of ethylene insertion was definitely shown to be cis by the evidence that the polymerization of cis- and trans-dideutero-ethylene afforded stereoselectively deuterated polyethylenes [138]. 

In the process of olefin insertion, also known as carbometalation, the 1,2 migratory insertion of the coordinated carbon-carbon multiple bond into the metal-carbon bond results in the formation of a metal-alkyl or metal-alkenyl complex. The reaction, in which the bond order of the inserted C-C bond is decreased by one unit, proceeds stereoselectively ( -addition) and usually also regioselectively (the more bulky metal is preferentially attached to the less substituted carbon atom. The willingness of alkenes and alkynes to undergo carbometalation is usually in correlation with the ease of their coordination to the metal centre. In the process of insertion a vacant coordination site is also produced on the metal, where further reagents might be attached. Of the metals covered in this book palladium is by far the most frequently utilized in such transformations. 

The ease with which the geometry of the metal carbene complexes can adjust to accommodate the incoming olefin may be an important factor in determining the rate and stereoselectivity in a given metathesis reaction124. 

New evidence as to the nature of the intermediates in catalytic diazoalkane decomposition comes from a comparison of olefin cyclopropanation with the electrophilic metal carbene complex (CO)jW—CHPh on one hand and Rh COAc) / NjCHCOOEt or Rh2(OAc)4 /NjCHPh on the other . For the same set of monosubstituted alkenes, a linear log-log relationship between the relative reactivities for the stoichiometric reaction with (CO)5W=CHPh and the catalytic reaction with RhjfOAc) was found (reactivity difference of 2.2 10 in the former case and 14 in the latter). No such correlation holds for di- and trisubstituted olefins, which has been attributed to steric and/or electronic differences in olefin interaction with the reactive electrophile . A linear relationship was also found between the relative reactivities of (CO)jW=CHPh and Rh2(OAc) NjCHPh. These results lead to the conclusion that the intermediates in the Rh(II)-catalyzed reaction are very similar to stable electrophilic carbenes in terms of electron demand. As far as cisjtrans stereoselectivity of cyclopropanation is concerned, no obvious relationship between Rh2(OAc) /N2CHCOOEt and Rh2(OAc),/N2CHPh was found, but the log-log plot displays an excellent linear relationship between (CO)jW=CHPh and Rh2(OAc) / N2CHPh, including mono-, 1,1-di-, 1,2-di- and trisubstituted alkenes In the phenyl-carbene transfer reactions, cis- syn-) cyclopropanes are formed preferentially, whereas trans- anti-) cyclopropanes dominate when the diazoester is involved. 

Cyclopropanation reactions employing an a-diazo carbonyl compound as precursor are almost exclusively performed in the presence of a metal catalyst, The intermediacy of highly reactive metal-carbene complexes is generally accepted, but the details of the carbene transfer are still under discussion7,s. Many examples prove that the configuration of the olefin is maintained during these cyclopropanations (see Vol. E19b, p 1099). This mechanism-bascd stereoselectivity is therefore not further discussed in this section. 

The ease with which the geometry of the metal-carbene complexes can adjust to accommodate the incoming olefin may be an important factor in determining the rate and stereoselectivity in a given metathesis reaction (Lee, J.B. 1981). Extended Hiickel MO calculations on Ti(=CH2)L2, where L = H, Cl, Cp, have shown that the completely planar molecule is easily distorted into a flattish pyramid with Ti at the apex, ready to receive the incoming donor olefin (Gregory 1985). Similarly, calculations employing the self-consistent-field-Xa-scattered-wave method on Mo(=CH2)(=NH)(OMe)2 show that the reaction with ethene at the COO faee to... 

A similar type of chiral rhodium porphyrin was found to be effective for the carbene-insertion reaction to olefins, where formation of the carbene complex takes place. Chiral rhodium complexes for catalytic stereoselective-carbene addition to olefins were prepared by condensation of a chiral aldehyde and pyrrole. Formation of the metal-carbene complex and substrate access to the catalytic center are crucial to the production of optically active cyclopropane derivatives. Optically active a-methoxy-a-(trifluoro-methyOphenylacetyl groups are linked witfi the amino groups of a,p,0L,p isomers of tetrakis-(2-aminophenyI)por-phyrin through amide bonds. Oxidation reactions of the... 

Information about the stability and stereoselectivity of coordinately unsaturated metal carbene complexes is important for understanding a number of reactions, including olefin metathesis and cyclopropanation. A study of CO exchange in [(CO)5MC(OMe)(Ph)] (M = Cr, Mo, or W) shows that the carbene ligand is cis labilizing compared to CO. Ratedetermining CO loss is inferred from the similar rates of "CO exchange and cis-CO substitution by phosphines. Measurements of the ratio cis- CO/frans- CO as a function of percent CO incorporation imply that... 

Once again the main emphasis of this Report is on the formation of carbon-carbon bonds. Of note is the increasing use of metal alkenyl complexes for the synthesis of olefins and dienes with high stereoselectivity. Transition-metal-catalysed asymmetric synthesis continues to develop, and optical yields of 90% are frequently reported, particularly in hydrogenation reactions. Although this is an expanding area of organic synthesis most of the reactions reported this year are not new, and hence only a few references to asymmetric catalysis are discussed. 

Perhaps the most important chemical property of these complexes is their potential as catalysts, particularly of the early transition metal isoleptic compounds for a-olefin polymerization. This arises because unlike the methyls, they are sufficiently stable to be used at temperatures where polymerization rates are adequate. Some data are summarized in Table VIII ( 9) TT-acceptor ligands are clearly disadvantageous. It will be seen that some of the systems are more active than Ziegler types, although stereoselectivity is poorer. 

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-... 

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