Olefins chirally modified

Asymmetric versions of the cyclopropanation reaction of electron-deficient olefins using chirally modified Fischer carbene complexes, prepared by exchange of CO ligands with chiral bisphosphites [21a] or phosphines [21b], have been tested. However, the asymmetric inductions are rather modest [21a] or not quantified (only the observation that the cyclopropane is optically active is reported) [21b]. Much better facial selectivities are reached in the cyclopropanation of enantiopure alkenyl oxazolines with aryl- or alkyl-substituted alkoxy-carbene complexes of chromium [22] (Scheme 5). 

Platinum(II) and ruthenium(II) complexes with chiral modified diphosphines like 47 or tetradentate P2N2 ligands like 48 have been used for the asymmetric epoxidation of olefins with hydrogen peroxide with ee values of 18-23%, which increased up to 41% when cationic solvato derivatives such as P2Pt(CF3)(CH2Cl2)(BF4) are used . Similar chiral inductions were reported for Ru derivatives, although the nature of the active intermediate was still in question. ... 

Model of a chirally modified dinuclear osmium complex reacting with olefins at 0+. 

In the pioneering studies of Homer et al. [57] and Knowles and Sabacky [58], chirally modified Wilkinson catalysts were introduced in the homogeneous enantioselective hydrogenation of prochiral olefins. To this end, in Wilkinson-type catalysts the triphenylphosphine ligand was replaced by the optically active phosphine ligands (-i-)-PMePr"Ph and H-PMePfPh, chiral at the phosphoms atom. 

The Michael-type addition reaction of a carbonucleophile with an activated olefin constitutes one of the most versatile methodologies for carbon-carbon bond formation [1]. Because of the usefulness of the reaction as well as the product, many approaches to the asymmetric Michael-type addition reactions have been reported, especially using chirally modified olefins [2-8]. However, the approach directed towards the enantioselective Michael-type addition reaction is a developing area. In this Chapter, the recent progress of the enantioselective Michael-type addition reaction of active methylene compounds and also organometallic reagents with achiral activated olefins under the control of an external chiral ligand or chiral catalysts will be summarized [9]. 

Chirally modified ruthenium clusters have also been used for a kinetic resolution of enantiomers in the catalytic hydrogenation of non-functiona-lized terpene olefins (Scheme 5) The hydrogenation of a-pinene (IR,5R 27 S,5S 28) leads in principle to the cis- and /rans-pinanes (11 ,25,5/ 29 15,21 , 55 30 IR,2R,5R 31 IS",25,55 32). When a racemic mixture of both a-pinene enantiomers (27 and 28) is hydrogenated in the presence of the (15,25,35,5.R)-isopinocampheyl cluster HRu3(CO)9[//3, //2-NEt-... 

Scheme 5. Enantiomer discrimination in the hydrogenation of olefins by chirally modified cluster catalysts.

This method can also be applied to complexes of conjugated trienes. The uncomplexed double bond may be constructed within the complex via Wittig olefination. Cyclopropanation of the uncomplexed double bond of the resulting tricarbonyliron-triene complexes and oxidative decomplexation leads to dienylcyclopropane products. In this manner chiral dienyl-cyclopropanes 4 and 5 were prepared in high enantiomeric excess (> 90% ee) starting from optically active tricarbonyliron-hexatriene complexes 3 obtained from chirally modified sorbic aldehyde complexes. 

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.). 

Hydroformylation of various heterofunctionalized olefins can be carried out with a number of chirally modified catalysts. Asymmetric induction is usually higher than with unfunctionalized hydrocarbons, presumably, due to additional binding of the substrate to the catalysts65,75. Thus, this method is applicable to the synthesis of hydroxy and amino carboxylic acids and other conversion products of primary functionalized aldehydes. These results are compiled in Table 7. 

Pd modified by cinchona, vinca, or ephedra alkaloids is a moderately efficient catalyst but Pd is still the catalyst of choice for the enantioselective hydrogenation of olefins with a functional group in the a position [8,20]. Modification of Pd with cinchonidine is as simple as for Pt, but Pd requires a considerably lower substrate/ modifier ratio than Pt, probably because of weaker adsorption and/or partial degradation (hydrogenation) of the modifier during reaction. Another drawback is that the reactions are not accelerated but decelerated by the chiral modifier (by a factor of up to 140 [21]). This phenomenon can rationalize the moderate performance of chirally modified Pd. 

Chirally modified Pd is the best catalyst for the enantioselective hydrogenation of olefins, ee > 50 % has been achieved in two types of reaction in both the reactant has an interacting functional group in the a-position. 

Asymmetric hydroformylation of prochiral olefins has been investigated both for the elucidation of reaction mechanism and for development of a potentially useful method for asymmetric organic synthesis. Rhodium and platinum complexes have been extensively studied, and cobalt complexes to a lesser extent. A variety of enantiopure or enantiomerically enriched phosphines, diphosphines, phosphites, diphosphites, phosphine-phosphites, thiols, dithiols, P,A-ligands, and P,5-ligands have been developed as chiral modifiers of rhodium and platinum catalysts. - " ... 

Asymmetric induction can be also accomplished through the use of a chirally modified nitro olefin. Sugar-based nitroalkenes participate in thermal [4 + 2] cycloaddition to form enantiomerically pure nitronates [55,97]. Alternatively, diastereoselective cycloadditions are possible with chiral nitroalkenes as illustrated on Scheme 16.15 [47]. The tandem double intramolecular cycloaddition of enantiopure nitro-alkene 62 containing a single stereogenic center provides nitroso acetal 63 with high diastereoselectivity (relative to the existing center) in moderate yield. The product is isolated as a mixture of isomers that is formed due to epimerization of the intermediate nitronate (not shown) and used toward total synthesis of daphnilactone B. 

High enantioselectivities and regioselectivities have been obtained using both mono- and 1,2-disubstituted prochinal olefins employing chiral phosphine phosphite (33,34) modified rhodium catalysts. For example, i7j -2-butene ia the presence of rhodium and (12) (33) gave (3)-2-meth5ibutanal ia an optical yield of 82% at a turnover number of 9.84. ... 

In some reactions intramolecular chalcogen nitrogen interactions may lead to stereochemical control. For example, selenenyl bromides react with C=C double bonds, providing a convenient method of introducing various functional groups. The reaction proceeds readily, but affords a racemic mixture. The modified reagent 15.22 contains a chiral amine in close interaction with the selenium atom. It reacts with olefins affording up to 97% ee of isomer A (Scheme 15.2). ... 

If the chiral auxiliary in Eq. 4.96 is modified by changing MeO into more bulky groups such as trityl (Tr) or t-butyldimethylsilyl (TBS) group, an improved asymmetric nitro-olefination of a-alkyl-y- and 8-lactones is possible (Eq. 4.97).120... 

This chapter describes atropisomeric biaryl bisphosphine ligands modified DIOP-type ligands P-chiral bisphosphane ligands other bisphosphane ligands and their applications in the enantioselective hydrogenation of olefins. 

An excellent review of the problems of the enantioselective heterocatalytic hydrogenation of prochiral double bonds, covering the literature up to 1970, has been compiled by Izumi57). Raney nickel catalysts modified with chiral amino acids or dipeptides gave only very moderate enantiomeric excesses of between 0 and 10% in the hydrogenation of olefins, carbonyl compounds or oximes 57). Only Raney nickel modified with (S)-tyrosine furnished a higher enantiomeric excess in the products58). 

Hanson et a/.149 hydrogenated the prochiral olefin methyl a-acetamidocinna-mate using rhodium catalysts modified with the tenside chiral sulfonated diphosphine 34 (Table 2) in an ethylacetate/H20 micellar system at 25° C and 1 bar H2. The yield (100%) and enantiomeric excess (69%) were considerably higher than with the tetrasulfonated diphosphine 31 (Table 2 m=0, n=0) which gave 32% yield and 20% e.e. and the reaction time was shorter (1.5 versus 20h). Rh/34 and Rh/31 (m=0, n=0) gave nearly the same results (100% yield and 72-75% e.e. within < lh) in homogeneous methanol solutions.149... 

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