Enantioselective Isomerization of Olefins

Enantioselective isomerization of olefins for preparation of optically active olefins has great synthetic potential with a long tradition [1] and the non-stereoselective version is probably the most intensively investigated reaction in transition metal catalysis [2]. In particular, stereoselective hydrogen migration in a-functionalized olefins, for example allyl alcohols and allylamines [3], affording optically active aldehydes, ketones and amines, is part of the standard repertoire of enantioselec-... 

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 1. Enantioselective isomerization of non-functiona-lized olefins (a) catalyst generation 2%Ti-precatalyst, 8% LiAIH, mesitylene, 164°C reaction performed at 23 °C, 67 h, sealed tube.

So far, only one example of enantioselective desymmetrization of olefins with (q -arene)Ru(II) half-sandwich complex 78 has been reported [43]. It was found, that isomerization of 2-n-butyl-4,7-dihydro-l,3-dioxepin 79 with a plane of symmetry, leads to the (-1-)-enantiomer of 2-n-butyl-4,5-dihydro-l,3-dioxepin... 

Mikami and Hatano70 demonstrated the efficiency of the dicationic [Pd(MeCN)4](BF4)2/BINAP catalyst system in DMSO with the highly enantioselective synthesis of a variety of quinoline derivatives, including spiro-compound 99 (Equation (63)), resulting from olefin isomerization of the Alder-ene product. 

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). 

Isomerization is a frequent side-reaction of catalytic transformations of olefins, however, it can be a very useful synthetic method, as well. One of the best-known examples is the enantioselective allylamine enamine isomerization catalyzed by [Rh (jR)-or(S)-BINAP (COD)] which is the crucial step in the industrial synthesis of L-menthol by Takasago [42]... 

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,... 

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). 

Although various transition-metal complexes have reportedly been active catalysts for the migration of inner double bonds to terminal ones in functionalized allylic systems (Eq. 3.2) [5], prochiral allylic compounds with a multisubstituted olefin (Rl, R2 H in eq 2) are not always susceptible to catalysis or they show only a low reactivity [Id]. Choosing allylamines 1 and 2 as the substrates for enantioselective isomerization has its merits (1) optically pure citronellal, which is an important starting material for optically active terpenoids such as (-)-menthol, cannot be obtained directly from natural sources [6], and (2) both ( )-allylamine 1 and (Z)-allylamine 2 can be prepared in reasonable yields from myrcene or isoprene, respectively, The ( )-allylamine 1 is obtained from the reaction of myrcene and diethylamine in the presence of lithium diethylamide under Ar in an almost quantitative yield (Eq. 3.3) [7], The (Z)-allylamine 2 can also be prepared with high selectivity (-90%) by Li-catalyzed telomerization of isoprene using diethylamine as a telomer (Eq. 3.4) [8], Thus, natural or petroleum resources can be selected. 

This isomerization is enantioselective when optically active BINAP is used and provides practical access to optically active aldehydes and alcohols such as L-menthol, which is a key fragrance chemical [297—299], The proposed mechanism involves amine, iminium, and enamine as complex intermediates [300], Extension of this olefin isomerization is realized in the isomerization of an alkyne to a conjugated diene (Scheme 1-47) [301], High chemoselectivity is achieved when Pd(OAc)2 or [Pd2(dba)3]/HOAc, in the presence of phosphine, is used as catalyst (Table 1-14). The phosphine of choice is dppb although dppf could give a similar yield. 

---