Asymmetric olefin isomerization

There is one strong hydrogen bond in each of the transition states TS1-TS4. However, a detailed analysis showed that the preference for the formation of S-product rather originates from the number and strength of the weak noncovalent interactions that stabilize a transition state and help to keep the reagent properly oriented. Thus, in the most stable among the four TSl, five noncovalent C—H... O interactions can be found, whereas in any of TS2-TS4 only four such interactions are present. 

It is concluded that the selectivity of the cinchona alkaloids catalyzed asymmetric olefin isomerization of p,y- to a,p-unsaturated butenolides arises mainly from the multiple nonconventional C—H...O hydrogenbonding interactions. 

2 Friedel-Crafts alkylation of indoles with a, -unsaturated ketones 

six pathways were analyzed in detail involving activation of the nitrostyrene either by single phenoxy hydroxyl or by both hydroxyl 

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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. 

Scheme 40.30 Asymmetric olefin isomerization of butenolides via proton-transfer organocatalysis.

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. 

S. Otsuka and K. Tani, Asymmetric Catalytic Isomerization of Functionalized Olefins, in J. D. Morrison, ed., Asymmetric Synthesis, Vol. 5, Chap. 6, Academic Press, New York, 1985. 

BINAP complexes have been used extensively in asymmetric synthesis, for example in hydrogenations,389,390 olefin isomerizations,390 arylation of olefins,391 and enantioselective allylation of aldehydes.392 Palladium or platinum complexes of (165) find important applications in enantioselective C—C bond formation,393-396 whilst iridium complexes are catalysts for the hydrogenation of nonfunctionalized tri- and tetrasubstituted olefins. 97... 

The first successful achievements using asymmetric homogeneous transition metal catalysis were obtained in the asymmetric hydrogenation of alkenes24 25, This method has been successfully used in many synthetic applications (Section D.2.5.1.)26-29. In addition, chirally modified versions of the transition metal catalyzed hydrosilylation of olefins and carbonyl compounds (Sections D.2.3.1. and 2.5.1.) and olefin isomerization (Section D.2.6.2.) have been developed. Transition metal catalyzed asymmetric epoxidation constitutes one of the most powerful examples of this type (Section D.4.5.2.). 

In addition to the configurational isomerism encountered in polymers derived from asymmetric olefins, geometric isomerism is obtained when conjugated dienes are polymerized, e.g., (CH2=CX—CH=CH2). Chain growth from monomers of this type can proceed in a number of ways, illustrated conveniently by 2-methyl-1,3-butadiene (isoprene). Addition can take place either through a 1,2-mechanism or a 3,4-mech-anism, both of which could lead to isotactic, syndiotactic, or atactic structures, or by a 1,4-mode leaving the site of unsaturation in the chain. 

Scheme 6.23 olefin isomerization/asymmetric Pictet-Spengler cascade reported by You. 

Very recently, the enantioselective total synthesis of hexahydropyrrolo[2,3-h]-indole alkaloids, (-)-pseudophynaminol, through tandem olefination, isomerization and asymmetric Claisen rearrangement was reported [81]. Using a 3-ketodi-hydroindole derivative 107, the reaction smoothly proceeded under extremely... 

There are few examples of asymmetric catalytic processes which are of practical use. A new study on the catalytic olefin isomerization of, A-diethylneryl-amine (32) or A,JV-diethylgeranylamine (33) has now shown that optically active iV,A-diethylcitronellal-(E)-enamine (34) can be obtained with excellent enantioselectivity (> ca. 95 % e.e.) and chemoselectivity (> ca. 98 %) (Scheme 51). 

Allyl p-tolyl sulphoxide 535 reacts with sodium methoxide in methanol by initial prototropic isomerization and subsequent addition of methanol to give 536 (equation 333). Protic solvents are photochemically incorporated by the open chain olefinic bond of trans methyl )S-styryl sulphoxide 537 in a Markovnikov regiospecificity (equation 334). Mercaptanes and thiophenols add to vinyl sulphoxides in a similar manner (compare also Reference 604 and Section IV.B.3) to give fi-alkylthio(arylthio)ethyl sulphoxides 538 (equation 335). Addition of deuteriated thio-phenol (PhSD) to optically active p-tolyl vinyl sulphoxide is accompanied by a low asymmetric a-induction not exceeding 10% (equation 336) . Addition of amines to vinyl sulphoxides proceeds in the same way giving )S-aminoethyl sulphoxides in good to quantitative yields depending on the substituents at the vinyl moiety When optically active p-tolyl vinyl sulphoxides are used in this reaction, diastereoisomeric mixtures are always formed and asymmetric induction at the p- and a-carbon atoms is 80 20 (R = H, R = Me) and 1.8 1 (R = Me, R = H), respectively (equation 337) ... 

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