Asymmetric isomerization of olefins

Rhodium (l)-Cata yzed Asymmetric Isomerization of Olefins 83 Tab. 4.1 Rh(l)/PF-P(o-Tol)2-catalyzed isomerization of E-allylic alcohols. 

S. Akutagawa, Asymmetric Isomerization of Olefins in E. N. Jacobsen, A. Pfaltz, H. Yamamoto, Comprehensive Asymmetric Catalysis, Vol III, Springer-Verlag, Berlin 1999, p. 1461-1477. 

In 2000, we demonstrated that the planar-chiral phosphaferrocene PF-PPhj is a useful ligand for rhodium-catalyzed asymmetric isomerizations of several allylic alcohols, providing the first catalyst system that furnishes the target aldehyde in >60% ee (Eq. 6) [7]. It appears that, in order to obtain high enantiomeric excess (>0% ee), the olefin should bear a relatively bulky substituent (for example, Pr Eq. 6). 

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. 

Olefinic double-bond isomerization is probably one of the most commonly observed and well-studied reactions that uses transition metals as catalysts [1]. However, prior to our first achievement of asymmetric isomerization of allylamine by optically active Co(I) complex catalysts [2], there were only a few examples of catalytic asymmetric isomerization, and these were characterized by very low asymmetric induction (<4% ee) [3], In 1978 we reported that an enantioselective hydrogen migration of a prochiral allylamine such as AVV-diethylgerany-lamine, (1) or N V-diethylnerylamine (2) gave optically active citronellal ( )-enamine 3 with about 32% ee utilizing Co(I)-DIOP [DIOP = 2,3-0-isopropylidene-2,3-dihydroxy-l,4-bis(diphenylphosphino)butane] complexes as the catalyst (eq 3.1). 

Optically active aldehydes can be obtained by asymmetric hydroformylation of olefinic substrates when at least one asymmetric carbon atom is formed either by addition of a formyl group or of a hydrogen atom to an unsaturated carbon atom (Scheme 1, reactions (1) and (2)). In the case of trisubstituted olefins, two new asymmetric carbon atoms can form due to the cis stereochemistry of the reaction10), in the absence of isomerization, the formation of only one epimer is expected. 

In 1976 the first example of the asymmetric isomerization of prochiral allyl alcohols to aldehydes was reported [26]. The isomerization proceeds by migration of the olefinic double bond of allyl alcohol 12 from the 2,3 position to the 1,2 position to give enol 13, which transforms rapidly to aldehyde 14 (Scheme 2). It was claimed that DIOP (15)-modified rhodium catalysts (Fig. 1) exemplified the enantiorecognition in 2 to 4% ee. After their successful use for allylamines, BINAP (16)-coordinated rhodium catalysts (Fig. 1) were applied for the isomer-... 

Isomerization via homogeneous catalysts occurs, for instance, as 2ui intermediate step in catalytic processes. Thus in Shell s hydroformylation route, which converts internal olefins to primary alcohols, isomerization takes place prior to CO-insertion. Homogeneous isomerization of 2-me-thyl-3-butenenitrile to the linear nitrile is an essential step in du Font s hydrocyanation. Noteworthy is the recent commercial asymmetric isomerization of neryl and geranyl amines [3]. 

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. 

Mazet et al. have reported an efficient asymmetric isomerization reaction of allylic alcohols [60, 61]. In a preliminary report they utilized the BArp analog of Crabtree s complex to efficiently catalyze a hydride transfer from the a position of the allylic alcohol to the p position of the olefin with a concomitant formation of a formyl group. A subsequent report detailed a remarkable enantioselective variant of this process catalyzed with Ir(12g) and (12h) (Scheme 12). 

The orientation on addition of asymmetric electron-rich olefins has also been studied by Padwa et al. (83JOC2330). Only one regioisomer could be isolated from such reactions. These reactions were assumed to be products resulting from the attachment of the olefinic electron-rich moiety to the exocyclic nitrogen. This assignment was based on H-NMR, which revealed the triazine H-4 in 96 at 88.5. This, however, does not rule out isomeric 97, as Ege and Gilbert (81JHC675) reported a shift of 8.7 for H-3 in a system believed to be a 3-unsubstituted-4-arylpyrazolo[5,l-c]-l,2,4-triazine (Scheme 13). 

The asymmetric synthesis of (+)-Codeine 432 devised by White and colleagues included a Beckmann rearrangement to introduce the nitrogen atom in the carbocyclic structure (equation 182). Even though two isomeric lactams 430 and 431 were obtained as a result of the rearrangement, the preferential migration of the bridgehead carbon atom produced 430 as the predominant isomer. The synthesis of the non-natural enantiomer of Codeine was completed after oxidation, olefin formation and reduction. 

Stability, activity and chemo- and enantioselectivity increased with increasing steric demand of the ortho substituent R. Introduction of the trimethylsilyl group at this position (ligand 38) therefore resulted in an excellent enantioselective system which belongs among the best Pd catalysts described so far for asymmetric hydrovinylation. Almost 70% conversion was observed within 15 min. The product was obtained in 78.5% ee and only a small amount of the isomerization products was detected in the reaction mixture. However, at higher conversions, isomerization of the product to the internal achiral olefin took place. Therefore,... 

Asymmetric epoxidation of prochiral olefins is a powerful strategy for the synthesis of enantiomericaUy enriched epoxidesJ Previously, we reported a fructose-derived catalyst (1) that gives high ee for a wide variety of trans- and trisubstituted olefins (Figure 6.5). " Recently, we discovered a new catalyst (2) derived from D-glucose that can epoxidize many c -olefins with high enantioselectivity and no c w/rran -isomerization. ... 

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. 

II(S)) and/or to a different reaction rate of the two diastereomeric 7r-olefin complexes to the corresponding diastereomeric alkyl-rhodium complexes (VI(s) and VI(R)). For diastereomeric cis- or trans-[a-methylbenzyl]-[vinyl olefin] -dichloroplatinum( II) complexes, the diastereomeric equilibrium is very rapidly achieved in the presence of an excess of olefin even at room temperature (40). Therefore, it seems probable that asymmetric induction in 7r-olefin complexes formation (I — II) cannot play a relevant role in determining the optical purity of the reaction products. On the other hand, both the free energy difference between the two 7r-olefin complexes (AG°II(S) — AG°n(R) = AG°) and the difference between the two free energies of activation for the isomerization of 7r-com-plexes II(S) and II(R) to the corresponding alkyl-rhodium complexes VI(s) and VI(R) (AG II(R) — AG n(S) = AAG ) can control the overall difference in activation energy for the formation of the diastereomeric rhodium-alkyl complexes and hence the sign and extent of asymmetric induction. 

---