Olefins asymmetrical

Asymmetric olefination using optically active P,0-heterocycles as reagents 98YGK521. 

The orbital mixing theory was developed by Inagaki and Fukui [1] to predict the direction of nonequivalent orbital extension of plane-asymmetric olefins and to understand the n facial selectivity. The orbital mixing rules were successfully apphed to understand diverse chemical phenomena [2] and to design n facial selective Diels-Alder reactions [28-34], The applications to the n facial selectivities of Diels-Alder reactions are reviewed by Ishida and Inagaki elesewhere in this volume. Ohwada [26, 27, 35, 36] proposed that the orbital phase relation between the reaction sites and the groups in their environment could control the n facial selectivities and review the orbital phase environments and the selectivities elsewhere in this volume. Here, we review applications of the orbital mixing rules to the n facial selectivities of reactions other than the Diels-Alder reactions. 

A second observation was the fact that isomerization of the starting asymmetric olefin was much faster than the formation of new symmetric olefins. In fact, 40% of the initial cis olefin (Fig. 1) had isomerized to trans after only 4% conversion to new olefins. This result formally parallels the highly selective regenerative metathesis of a-olefins (60, 61), except that steric factors now prevail, because electronic effects should be minimal. Finally, the composition of the initially formed butene from r/j-4-methyl-2-pentene was essentially identical to that obtained when cA-2-pentene was used (18). When tra .v-4-methyl-2-pentene was metath-esized (Fig. 2), the composition of the initially formed butenes indicated a rather high trans specificity. 

Their interactions with the more hindered side of an asymmetrical olefin determine the orientation of the substrate during its approach to the metal-oxo bond and the subsequent enantiofacial selective oxygen transfer. 

In 1959, Kharasch et al.43 reported an allylic oxyacylation of olefins. In the presence of f-butyl perbenzoate and a catalytic amount of copper salt in refluxing benzene, olefin was oxidized to allyl benzoate, which could then be converted to an allyl alcohol upon hydrolysis. It is desirable to introduce asymmetric induction into this allylic oxyacylation because allylic oxyacylation holds great potential for nonracemic allyl alcohol synthesis. Furthermore, this reaction can be regarded as a good supplement to other asymmetric olefinic reactions such as epoxidation and dihydroxylation. 

Scheme 21. Zr-catalyzed asymmetric olefin alkylation is used in conjunction with Pd-catalyzed addition of an arylox-ide to an epoxide and Mo-catalyzed olefin metatheses in Hoveyda s total synthesis of nebivolol (1998).

Virtually every iridium catalyst of the formula [Ir(L )(COD)] [X] for asymmetric olefin hydrogenation that has appeared after the initial counterion effect studies was based on BArp as the preferred anion [14]. The anion effect is broadly applicable in iridium-catalyzed reductions as experiments with a direct analog of the Crabtree catalyst of the formula [Ir(pyridine)(Cy3P)(COD)]BArp indicates (Fig. 2). 

Tab. 11.13 Catalytic asymmetric olefin metathesis reactions promoted by supported chiral catalyst (81). >...

Fig. 11.2 Difference in the appearance of solutions of asymmetric olefin metathesis product (97) obtained from reaction of (78a) (left) and supported catalyst (81) (right).

For a review of asymmetric Mo-catalyzed metathesis, see Catalytic Asymmetric Olefin Metathesis, A. H. Hoveyda, R. R. ScHROCK, Chem. Eur. J. 2001, 7, 945-950 for reports on chiral Ru-based complexes, see (b) Enantioselective Ruthenium-Catalyzed Ring-Qosing Metathesis, T.J. Sei-DERS, D.W. Ward, R.H. Grubbs, Org. Lett. 2001, 3, 3225-3228 (c) A Recyclable Chiral Ru Catalyst for Enantioselective Olefin Metathesis. Efficient Catalytic Asymmetric Ring-Opening/Cross Metathesis In Air, J. J. Van Veldhuizen, S. B. 

Asymmetric Olefin Metathesis Using Ruthenium Catalysts Ruthenium catalysts for asymmetric synthesis have also been developed (Figure 6.3). 

Recent Advances in Rhodium(l)-Catalyzed Asymmetric Olefin Isomerization and Hydroacylation Reactions... 

One of the landmark achievements in the area of enantioselective catalysis has been the development of a large-scale commercial application of the Rh(I)/BINAP-catalyzed asymmetric isomerization of allylic amines to enamines. Unfortunately, methods for the isomerization of other families of olefins have not yet reached a comparable level of sophistication. However, since the early 1990s promising catalyst systems have been described for enantioselective isomerizations of allylic alcohols and aUylic ethers. In view of the utility of catalytic asymmetric olefin isomerization reactions, I have no doubt that the coming years will witness additional exciting progress in the development of highly effective catalysts for these and related substrates. 

Certain tertiary amines such as pyridine or a-quinuclidine accelerate the stoichiometric reaction between osmium tetroxide and olefins (86). An asymmetric olefin osmylation using stoichiometric amounts of cinchona alkaloids as the chiral ligands was described in 1980 (87a). Optical yields of up to 90% were attained with frans-stilbene as substrate. 

The arena in which catalytic asymmetric olefin metathesis can have the largest impact on organic synthesis is the desymmetrization of readily accessible achiral molecules. Two examples are illustrated in Scheme 4. Treatment of achiral triene 18 with 2 mol % 4a leads to the formation of (R)-19 in 99% ee and 93% yield [ 12]. The reaction is complete within 5 min at 22 °C and, importantly, does not require a solvent. Another example is illustrated in Scheme 4 as well here, BINOL complex 11a is used to promote the formation of optically pure (R)-21 from siloxy triene 20 in nearly quantitative yield. Once again, solvent is not needed [15]. Readily accessible substrates are rapidly transformed to non-ra-cemic optically enriched molecules that are otherwise significantly more difficult to access without generating solvent waste. 

Scheme 15. Enantioselective synthesis of carbocyclic tertiary ethers and spirocycles through Mo-catalyzed asymmetric olefin metathesis...

It is not only that catalyst 82 is more easily prepared than 4 more importantly, as illustrated in Scheme 19, a solution of 82, obtained by the reaction of commercially available reagents bis(potassium salt) 83 and Mo triflate 84 (Strem), can be directly used to promote enantioselective metathesis. Similar levels of reactivity and selectivity are obtained with in situ 82 as with isolated and purified 4a or 82 (cf. Scheme 19). Moreover, asymmetric olefin metatheses proceed with equal efficiency and selectivity with the same stock solutions of (R)-83 and 84 after two weeks. The use of a glovebox, Schlenck equipment, or vacuum lines is not necessary (even with the two-week old solutions). 

For a previous brief overview of this Mo-catalyzed asymmetric olefin metathesis, see Hoveyda, AH, Schrock RR (2001) Chem Eur J 7 945... 

Shibahara F, Nozaki K, Hiyama T (2003) Solvent-free asymmetric olefin hydroformylation catalyzed by highly cross-linked polystyrene-supported (R, S)-BfNAPHOS-Rh(I) complex. J Am Chem Soc 125(28) 8555-8560... 

Catalytic Asymmetric Olefin Metathesis Chiral Biphen-Mo Catalysts... 

The arena in which catalytic asymmetric olefin metathesis can have the largest impact on organic synthesis is the desymmetrization of readily accessible achiral molecules. Two examples are illustrated in Scheme 4. Treatment of achiral triene 18 with 5 mol% 4a leads to... 

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