Prochiral alkenes/olefins

The use of prochiral alkenes such as propene, 2-butene, tran -dideutereoethylene, dialkyl fumarates, and trani-dimethoxyethylenes, have allowed detailed structural and mechanistic studies of alkene complexes. The diastereomers produced upon binding of prochiral alkenes to CpM(CO)X centers provided the key complexes to prove that interconversions occurred by rotations about the metal-(C=C) axis. Thus observations that neither the chirality at the metal nor at the alkene is changed in the rearrangement of a trans-substituted alkene provided proof that the nature of the dynamic process was a propeller rotation. Note that in equation (9) the equilibrium between (20) and (21) averages enviromnents b and d separately from a and c. A key feature is that olefin rotation does not alter the chirahty at the olefin because of the olefin-metal bond. 

NMR studies have shown the reactions of the chiral hydrido clusters [(At3-RC)RuCoM(Cp)(CO)8H] (R = Me, Ph M = Mo, W) with the prochiral alkene H2C=C(C02Me)NHC0Me to be diastereoselective. This formal insertion process produces only one enantiomeric pair of the possible four diastereomeric products, in which the terminal CH2 of the intermediate alkene complex becomes a CH3 group via Markownikow-type addition of the coordinated hydride. This preferred product stereochemistry may arise from the mutual approach during olefin coordination of the two most polar units, the MoCp and C02Me groups, on the same side of the molecule. 

From all of them, highly enantioseleetive eyelopropanes are formed by asymmetric catalytic insertion of carbene to prochiral olefins. This method, first employed by Noyori in 1966 [72] consists in the metal-catalyzed decomposition of substituted diazo compounds in the presence of various prochiral alkenes. 

A variation within the osmium-catalysed asymmetric dihydroxylation (AD) of alkenes has been described that yields cyclic boronic esters from alkenes in a straightforward manner. A protocol based on the Sharpless AD conditions (for enantiose-lective oxidation of prochiral olefins) has been developed that gives cyclic boronic esters, rather than free diols, with excellent enantiomeric excesses. Some of the... 

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. 

Because most olefins are prochiral starting materials, the dihydroxylation reaction creates one or two new stereogenic centers in the products. Since the discovery of the first stoichiometric asymmetric dihydroxylations [7], catalytic versions with considerable improvements in both scope and enantioselectivity have been developed [8]. From the standpoint of general applicability, scope, and limitations, the osmium-catalyzed asymmetric dihydroxylation (AD) of alkenes has reached a level of effectiveness which is unique among asymmetric catalytic methods. As there are recent reviews in this field [9], this section is primarily oriented toward a summary of aspects of fundamental understanding and interesting practical application of catalytic dihydroxylations. 

An enantioselective hydroalumination of a prochiral olefin was first reported by GiacomeUi and coworkers. The authors surveyed a number of chiral amines in the nickel catalyzed hydroalumination of 1,1-disubstituted alkenes. Of the amines examined, N,N-dimethylmenthylamine (DMMA) in combination with triisobutylalane and catalytic amounts of Ni(mesal)2 (mesal = methylsaHcyHde-neimine) (5) effected the hydroalumination of 2,3,3-trimethylbutene which, fol-... 

Hanessian and Beaudoin [310] have performed the diastereoselective condensation of lithiated phosphonamides 1.74 with prochiral or chiral cyclohexanones. Each enantiomer of the nonracemic alkene product is prepared from the appropriate reagent (Figure 6.34). Denmark and Chen studied similar reactions of oxazaphosphorinanes 1.96 (R = Ph, SPh) [356], These reactions are performed in a two-step fashion, and olefin formation requires activation with PhjCOTf in the presence of 2,6-hrtidine. 

When an appropriate chiral ligand is introduced to a catalyst, the differentiation of two enantiofaces of a prochiral olefin is conceptually possible in the hydroformylation reaction. There are three classes of alkenes from which enan-tiomerically enriched aldehydes can be obtained (Eqs. 4-6). The asymmetric hy-... 

The investigated supported complexes 22 and 23, outlined in Fig. 8, were used for hydrogenations of alkenes, nitriles and a,jS-unsaturated ketones. Furthermore, 23 was used in the reduction of different heterocycles like benzoth-iophene, quinoline, indole, dibenzothiophene and acridine. The supported chiral Rh complexes, depicted in Fig. 9, were used for hydrogenation reactions with prochiral olefins. 

Asymmetric Hydrogenation. One of the most important developments of homogeneous catalysis is the synthesis of optically active compounds by asymmetric hydrogenation of prochiral substrates (3,111-119). Thus, any alkene having two different substituents (Ri, R2) on one of the olefinic carbons is prochiral and can give two enantiomers on hydrogenation as shown in equation 26. 

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 )... 

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