Olefin unfunctionalized

In 1990, Jacobsen and subsequently Katsuki independently communicated that chiral Mn(III)salen complexes are effective catalysts for the enantioselective epoxidation of unfunctionalized olefins. For the first time, high enantioselectivities were attainable for the epoxidation of unfunctionalized olefins using a readily available and inexpensive chiral catalyst. In addition, the reaction was one of the first transition metal-catalyzed... 

Ten years after Sharpless s discovery of the asymmetric epoxidation of allylic alcohols, Jacobsen and Katsuki independently reported asymmetric epoxidations of unfunctionalized olefins by use of chiral Mn-salen catalysts such as 9 (Scheme 9.3) [14, 15]. The reaction works best on (Z)-disubstituted alkenes, although several tri-and tetrasubstituted olefins have been successfully epoxidized [16]. The reaction often requires ligand optimization for each substrate for high enantioselectivity to be achieved. 

The past thirty years have witnessed great advances in the selective synthesis of epoxides, and numerous regio-, chemo-, enantio-, and diastereoselective methods have been developed. Discovered in 1980, the Katsuki-Sharpless catalytic asymmetric epoxidation of allylic alcohols, in which a catalyst for the first time demonstrated both high selectivity and substrate promiscuity, was the first practical entry into the world of chiral 2,3-epoxy alcohols [10, 11]. Asymmetric catalysis of the epoxidation of unfunctionalized olefins through the use of Jacobsen s chiral [(sale-i i) Mi iln] [12] or Shi s chiral ketones [13] as oxidants is also well established. Catalytic asymmetric epoxidations have been comprehensively reviewed [14, 15]. 

In the area of the asymmetric hydrogenation of C=C double bonds, the reduction of unfunctionalized olefins has been scarcely explored, in comparison with... 

The insoluble polymer-supported Rh complexes were the first immobilized chiral catalysts.174,175 In most cases, however, the immobilization of chiral complexes caused severe reduction of the catalytic activity. Only a few investigations of possible causes have been made. The pore size of the insoluble support and the solvent may play important roles. Polymer-bound chiral Mn(III)Salen complexes were also used for asymmetric epoxidation of unfunctionalized olefins.176,177... 

The 1-t-butylphospholane sulfide intermediate to TangPhos was also used to prepare the P,N ligands 48 by reacting the lithium complex with C02 and then oxazoline formation with a range of chiral amino alcohols [69b, 74]. The Ir complexes of these ligands have been successfully used in the reduction of / -methylcinnamic esters (80-99% ee) and methylstilbene derivatives (75-95% ee), a particularly challenging class of unfunctionalized olefins [4 c]. 

Substrate 2 has also been used as a test substrate HPLC separation methods exist for 2, while ee-value determination of 1 is more difficult [6, 17]. Reflecting the general recent interest in the hydrogenation of unfunctionalized olefins, the past few years have seen the publication of a number of results for this substrate [15, 18-26]. The highest enantioselectivities were achieved using catalysts 12b [22] and 14a [26],... 

Recently, a breakthrough in the hydrogenation of unfunctionalized olefins was made [51]. For the first time, high enantioselectivities with purely alkyl-substituted alkenes such as 72-74 could be achieved using pyridine-phosphinite catalysts 75 and 76. 

During recent years, substantial progress has been made in the hydrogenation of unfunctionalized alkenes. With iridium complexes derived from chiral phos-phino-oxazolines and related ligands, excellent enantioselectivities and high TON/TOF values can now be obtained for a wide range of unfunctionalized olefins. Most substrates studied to date have at least one aryl substituent at the... 

Scheme 36.4 Lib raries of P,N- and C,N-ligands in the Ir-catalyzed enantioselective hydrogenation of unfunctionalized olefins.

Following the success with the titanium-mediated asymmetric epoxidation reactions of allylic alcohols, work was intensified to seek a similar general method that does not rely on allylic alcohols for substrate recognition. A particularly interesting challenge was the development of catalysts for enantioselective oxidation of unfunctionalized olefins. These alkenes cannot form conformationally restricted chelate complexes, and consequently the differentiation of the enan-tiotropic sides of the substrate is considerably more difficult. 

TABLE 4-18. Epoxidation of Unfunctionalized Olefins Catalyzed by 119 substrate catalyst iodosylbenzene = 1 0.025 1... 

Many efforts have been made to develop salen catalysts for the epoxidation of unfunctionalized olefins, and such work has been well documented.93 Very recently, Ito and Katsuki94 proposed that the ligand of the oxo salen species is not planar, but folded as shown in Figure 4-7 (R/ / H, R2 = H, L = achiral axial ligand). This folded chiral structure amplifies asymmetric induction by the Mn-salen complex. This transition state proposed by Ito and Katsuki is not compatible with the proposal by Palucki et al.95 that the salen ligands of oxo species are planar. 

Chiral Ketone-Catalyzed Asymmetric Oxidation of Unfunctionalized Olefins... 

S.3.2 A C2 Symmetric Chiral Ketone for Catalytic Asymmetric Epox-idation of Unfunctionalized Olefins. Yang et al.105 reported the use of... 

Following their success with chiral ketone-mediated asymmetric epoxidation of unfunctionalized olefins, Zhu et al.113 further extended this chemistry to prochiral enol silyl ethers or prochiral enol esters. As the resultant compounds can easily be converted to the corresponding a-hydroxyl ketones, this method may also be regarded as a kind of a-hydroxylation method for carbonyl substrates. Thus, as shown in Scheme 4-58, the asymmetric epoxidation of enol silyl... 

The reaction was carried out by addition of 1.95 equivalents of -BuLi to a THF solution of 22 at 0°C to generate the active catalyst, which was then combined with substrate (S/C about 20 1) under an inert atmosphere using phenylsilane as the stabilizing agent. Trisubstituted unfunctionalized olefins can be hydrogenated in good yield with high ee. Representative results are listed in Table 6-3. 

The requirement for the presence of an adjacent alcohol group can be regarded as quite a severe limitation to the substrate range undergoing asymmetric epoxidation using the Katsuki-Sharpless method. To overcome this limitation new chiral metal complexes have been discovered which catalyse the epoxidation of nonfunctionalized alkenes. The work of Katsuki and Jacobsen in this area has been extremely important. Their development of chiral manganese (Ill)-salen complexes for asymmetric epoxidation of unfunctionalized olefins has been reviewed1881. 

Figure 10. Buchwald s catalyst system for the enantio-selective hydrogenation of unfunctionalized olefins such as 52 and 53.

Scheme 15. Ir-Catalysed hydrogenation of unfunctionalized olefins and imines using PHOX-ligands.

Recently, Buchwald has developed an analogous, cationic zirconium system which is, to date, the most effective catalyst for the hydrogenation of tetrasubsti-tuted, unfunctionalized olefins.1461... 

The product of this preparation is the most enantioselective catalyst developed to date for asymmetric epoxidation of a broad range of unfunctionalized olefins.6 The procedure includes a highly efficient resolution of trans-1,2-diaminocyclohexane as well as a convenient analytical method for the determination of its enantiomeric purity. This method is general for the analysis of chiral 1,2-diamines. The Duff formylation described in Step B is a highly effective method for the preparation of 3,5-di-tert-... 

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