Asymmetrical alkene

The complex [Fe(D4-TmAP)Cl] with Halterman s porphyrin ligand can effect asymmetric alkene cyclopropanation with diazoacetate in high product yield and high stereoselectivity [57]. The reaction occurs smoothly at room temperature without the need for addition of CoCp2, affording the cyclopropyl esters... 

For asymmetric alkenes such as 90, the diastereoselectivity of the initial mercuration directs the stereochemistry in the final product. The presence of a coordinating ether group in 90 increases the selectivity as illustrated in Equation (35).114 The coordination of the mercury by the oxygen of the OTBDPS functionality tends to favor the ry -diastereomer 91 over the A-diastereomer 92.115... 

Chiral C2-symmetric ansa-metallocenes, also referred to as bridged metallocenes, find extensive use as catalysts that effect asymmetric C—C bond-forming transformations [4]. In general, bridged ethylene(bis(tetrahydroindenyl))zirconocene dichloride ((ebthi)ZrCl2) 1 or its derived binaphtholate ((ebthi)Zrbinol) 2 [5] and related derivatives thereof have been extensively utilized in the development of a variety of catalytic asymmetric alkene alkylations. 

The reaction between an asymmetrical alkene and an aromatic ketone gives two different orientations of cycloaddition through two different 1,4-biradicals. The route through the more stable biradical produces the major product ... 

The product of an addition reaction depends on the symmetry of the reactants. A symmetrical alkene has identical groups on either side of the double bond. Ethene, CH2 = CH2, is an example of a symmetrical alkene. An alkene that has different groups on either side of the double bond is called an asymmetrical alkene. Propene, CH3CH=CH2, is an example of an asymmetrical alkene. 

Arrhenius theory (1887) the theory stating that acids and bases are defined in terms of their structure and the ions produced when they dissolve in water defines an acid as a substance that produces hydrogen ions in water and a base as a substance that produces hydroxide ions in water (8.1) asymmetrical alkene an alkene that has different groups on either side of the double bond (2.2) atom the basic unit of an element, which still retains the element s properties (Review)... 

SCHEME 7. Sugar-derived ketones as catalysts for asymmetric alkene epoxidation... 

Ruthenium complexes mediate the hydroamination of ethylene with pyridine.589 The reaction, however, is not catalytic, because of strong complexation of the amine to metal sites. Iridium complexes with chiral diphosphine ligands and a small amount of fluoride cocatalyst are effective in inducing asymmetric alkene hydroamination reaction of norbomene with aniline [the best enantiomeric excess (ee) values exceed 90%].590 Strained methylenecyclopropanes react with ring opening to yield isomeric allylic enamines 591... 

Scheme 10.1 Comparison of routes to epoxides from carbonyl compounds. Top Asymmetric carbonyl epoxidation. Bottom Wittig olefination followed by asymmetric alkene epoxidation.

Scheme 12.10 Ammonium-catalyzed asymmetric alkene epoxidation.

Fig. 3.50. Regioselective methanol addition to an asymmetric alkene via solvomercuration/reduction.

Muller, A., Hauer, B., and Rosche, B. 2007b. Asymmetric alkene reduction by yeast old yellow enzymes and by a novel Zymomonas mobilis reductase. Biotechnol. Bioeng., 98, 22-29. 

Figure 9.10 Catalytic cycle for Os04-catalyzed asymmetric alkene dihydroxylation. The dashed line represents the phase boundary between the organic and the aqueous phase. L is the chiral ligand, e.g., 9.44.

A number of metals catalyze the hydroformylation reaction, of which rhodium is by far the most active, Rh >> Co > Ir, Ru > Os > Pt. Platinum and ruthenium are mainly of academic interest, although L2PtCl(SnCl3) complexes with chiral ligands find use in asymmetric alkene hydroformylations.59 In most cases, and certainly in industrial processes, cobalt has now been replaced by rhodium. 

Hoveyda and co-workers have developed chiral catalysts for asymmetric alkene metathesis. They have demonstrated that with their chiral molybdenum catalyst asymmetric syntheses of dihydrofurans through catalytic kinetic resolution by RCM and enantioselective desymmetrization by RCM are feasible processes (Scheme 40) <1998JA9720>. The use of Schrock s molybdenum catalysts for asymmetric alkene metathesis has been reviewed <2001CEJ945>. 

Studies undertaken in connection with the amine-catalyzed asymmetric alkene osmylation have recently clarified the peculiar mechanism of the hexacyanoferrate/CO2 mediated osmium re-oxidation in biphasic conditions36. Reversal of the osmate oxidation/hydrolysis sequence with respect to the previously described R3NO-mediated conditions was noted with this system. Thus, the monodiolate(amine) osmium(VI) ester 9 appears to be first hydrolyzed, releasing the diol and the amine ligand to the organic phase, and the resulting [0s02(0H)4]2 into the aqueous phase. 

Asymmetric Alkene Isomerization. The chiral titanocene reagent (1) serves as precatalyst for the isomerization of alkene (4) (eq 3). Active isomerization catalyst is obtained by in situ reduction of (1) with Lithium Aluminum Hydride (164 °C, 30 min). Treatment of the achiral substrate (4) with 2 mol % catalyst produced axially dissymmetric product (5)-(5) in 44-76% ee (100% yield). The reaction is slow at room temperature (120 h required for complete reaction) faster rates are obtained at higher temperatures, but at the expense of lower product enantiomeric purity. 

Kinetic Resolution. When the reacting partners allow for a high degree of stereodifferentiation in the transition state, it is possible to achieve asymmetric alkenation by kinetic resolution (eq 2). This is best done with a-alkyl substituted cyclohexanones and bulky anions such as that derived from (4). In a typical procedure, ( )-2-methylcyclohexanone (1 mmol) is treated with the anion of (3) (0.5 mmol, -78 °C, THF, 1 h then AcOH, 25 °C, and workup), to give (E,25 )-(2-methylcyclohexylidene)benzene [a]p —86.4° (c 1, CHCI3), and the (Z)-isomer (>98 2 by capillary GC 63% based on the reagent). 

For asymmetric alkenes, two adducts are formed for each radical X undergoing addition. As at least 4 species of radical, H, OH, HO2 and RO2 will be mechanistically important, a minimum of 8 different adducts are formed. At higher temperatures (ca. 1000 K), the addition of O atoms must also be considered. When coupled with abstraction, then the number of possible radicals formed from the alkene is very large. Even for propene, three C3H5 radicals may be formed, completing a total of 11 species if only 4 radicals are involved in addition. 

Dalton, C. T., Ryan, K. M., Wall, V. M., Bousquet, C., Gilheany, D. G. Recent progress towards the understanding of metal-salen catalyzed asymmetric alkene epoxidation. Top. in Cat. 1998, 5, 75-91. 

S. Bhor, M. K. Tse, M. Klawonn, C. Dbbler, W. Magerlein, M. Beller, Ruthenium-catalyzed asymmetric alkene epoxidation with tert-butyl hydroperoxide as oxidant, Adv. Synth. Catal. 346 (2004) 263. 

Application to the synthesis of medicinal compounds Asymmetric Alkene Metathesis Asymmetric Pericyclic Additions to Carbonyl Groups... 

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