Aryl-Substituted Olefins

The compound gives a powerful hyacinth aroma for floral fragrances. Hydrogenation provides dihydrocinnamyl alcohol, which has a sweet and floral odor. Up to now, the usual method of preparation of the latter was by hydrogenation of cin-namaldehyde [157]. 

R = H Aldehydic, green, grassy, ozonic, slightly metallic, cuminic, very powerful, aggressive. 

R = Me Aldehydic, floral-muguet, soapy, fatty, slightly lemon. 

R = Me2CH Aldehydic, green, citrus, ozone, cyclamen aldehyde, metallic, Bourgeonal but weaker. 

The monohydroformylation could also be selectively conducted at 35 atm syngas pressure and 80 °C with a heterogenized Si02-tethered Rh(acac)(CO)2 catalyst, provided the progress of the hydroformylation was carefully controlled [113]. When the reaction was stopped after about 50% conversion, the amount of formed dialdehyde could be minimized to less than 8%. Alternatively, the reaction has been carried out with chiral phosphines as ancillary ligands, but only low enantiomeric excess values (2-5%) were noted [161]. 

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Asymmetric epoxidation of olefins with ruthenium catalysts based either on chiral porphyrins or on pyridine-2,6-bisoxazoline (pybox) ligands has been reported (Scheme 6.21). Berkessel et al. reported that catalysts 27 and 28 were efficient catalysts for the enantioselective epoxidation of aryl-substituted olefins (Table 6.10) [139]. Enantioselectivities of up to 83% were obtained in the epoxidation of 1,2-dihydronaphthalene with catalyst 28 and 2,6-DCPNO. Simple olefins such as oct-l-ene reacted poorly and gave epoxides with low enantioselectivity. The use of pybox ligands in ruthenium-catalyzed asymmetric epoxidations was first reported by Nishiyama et al., who used catalyst 30 in combination with iodosyl benzene, bisacetoxyiodo benzene [PhI(OAc)2], or TBHP for the oxidation of trons-stilbene [140], In their best result, with PhI(OAc)2 as oxidant, they obtained trons-stilbene oxide in 80% yield and with 63% ee. More recently, Beller and coworkers have reexamined this catalytic system, finding that asymmetric epoxidations could be perfonned with ruthenium catalysts 29 and 30 and 30% aqueous hydrogen peroxide (Table 6.11) [141]. Development of the pybox ligand provided ruthenium complex 31, which turned out to be the most efficient catalyst for asymmetric... 

In contrast to the simple olefins, aryl-substituted olefins dissolve in sulfuric acid to give comparatively stable carbonium ions, as is shown by the -factors, the spectra, and the recovery of the olefin on dilution.262 In some cases it is neccessary to extrapolate the freezing point depression to zero time owing to a slow sulfonation. Because of the similarity in the spectra it is believed that these carbonium ions have the classical structures shown below.263... 

Bis(oxazoline)-copper complexes 158 have been used by Evans group as chiral catalysts for the enantioselective aziridination of olefins.116 Aryl-substituted olefins have been found to be particularly suitable substrates, which can be efficiently converted to A-tosylaziridines with ee of up to 97% (R = Ph... 

Arylidene-,6-ionones, triplet oxygen cycloaddition, 199, 201 Aryl phosphites, ozone adducts, 732 Aryl-substituted olefins, selenide-catalyzed epoxidation, 384-5 Ascaridole... 

All types of olefins can serve as substrates. Suitable acyclic olefins include ethylene, terminal and internal monoenes up to and including tetrasubstituted-double bonds, and aryl-substituted olefins. With dienes (and polyenes) an additional, intramolecular reaction pathway becomes available which leads to cyclic olefins (Reaction 2). 

As shown in Scheme 1.30, the chiral titanocene catalyst 34 hydrogenates unfunctionalized, disubstituted styrenes under 136 atm of hydrogen at 65°C to give the saturation products with 83 to >99% ee [156]. A high enantioselectivity is now realized only with aryl-substituted olefins. The enantioselectivity of 41% ee attained 2-ethyl-1-hexene and 34 as catalyst is the highest for hydrogenation of non-aromatic olefins. 

Diaryl ditellurium compounds catalyzed the oxidation of aryl-substituted olefins by /-butyl hydroperoxide, hydrogen peroxide, 3-chloroperbenzoic acid, pure oxygen, or air in the presence of sulfuric acid. When the reactions were carried out in refluxing methanol, the vic-dimethoxy compounds were obtained. With acetic acid as the reaction medium, the vic-diacetoxy derivatives were isolated, text.-Butyl hydroperoxide was the best oxidizing agent with yields approaching 100%. ... 

Caldwell (4) prepared aryl substituted olefins, (IV), which were effective in activating nicotinic cholinergic receptors and used in the treatment of central nervous system disorders including Alzheimer s disease. 

There are some examples known for the cycloaddition of azomethine ylides with nonactivated olefins such as aryl-substituted olefins, strained olefins, acyclic or cyclic olefins, and electron-rich olefins. Stabilized ylide 79 (R = H, R = Et, R = Me), bearing an ester moiety as the only C substituent, can be successfully trapped with styrene when generated by the deprotonation route (Section II,D) from ethyl sarcocinate and paraformaldehyde under reflux in toluene, to give 194 as a mixture of two regioisomers (86CL973). 

Olefins with Heteroatoms and Aryl-Substituted Olefins... 

Jacobsen-Katsuki epoxidation Enantioselective epoxidation of unfunctionalized alkyl-and aryl-substituted olefins. 222... 

Jacobsen reported in 1990 that Mnm complexes of chiral salen ligands (41) were the most efficient catalysts available for the enantioselective epoxidation of alkyl- and aryl-substituted olefins.118 This stimulated a rapid development in the chemistry and applications of chiral SB complexes, which offer promising catalytic applications to several organic reactions, such as enantioselective cyclopropanation of styrenes, asymmetric aziridination of olefins, asymmetric Diels-Alder cycloaddition, and enantioselective ring opening of epoxides.4,119... 

Reissig and coworkers have devised an indirect method of enantioselective alkylation of ketones via cyclopropanation of silyl enol ethers in the presence of the chiral copper catalyst 16, followed by ring opening to provide the substituted ketones. Overall, the transformation corresponds to alkylation of ketones using methyl diazoacetate as the electrophile. Enantioselectivities up to 88% were realized in the cyclopropanation of aryl substituted olefins, Eq. (20) [63,64]. 

Manganese and iron porphyrins substituted by chiral atropisomeric groups are models for cytochrome P-450 and several of such catalysts have been recommended by Groves, Kodadek, Mansuy and their coworkers in asymmetric epoxi-dations of olefins with hypochlorites or ArlO [957, 970], Recently, asymmetric epoxidation of 2-mtrostyrene has been accomplished with 89% enantiomeric excess, but this result lacks generality. A preliminary report on the use of threitol-strapped manganese poiphyrins in enantioselective epoxidations of unfunctionalized aryl substituted olefins appeared in 1993 [971] enantiomeric excesses in the range of 80% were observed. 

Planar, conjugated molecules fluoresce, whereas saturated molecules and those with only one double bond do not usually a molecule must possess at least one aromatic ring if one is to observe fluorescence or phosphorescence. Even conjugated olefins such as 1,3,5-hexatriene do not fluoresce however, aryl-substituted olefins do. For example, rrun -stilbene, which is planar, fluoresces c -stilbene does not, presumably because it is not planar. 

The main drawback in Sharpless epoxidation is that the substrate must bear a functional group to achieve the precoordination required for high enantioselec-tivity (as in the case of allyl alcohol). This restriction is not applicable to the epoxidation of alkyl- and aryl-substituted olefins with manganese complexes of chiral Schiffs bases as catalysts. Very high enantioselectivities can be obtained in these reactions (Jacobsen, 1993). The most widely used catalysts that give high enantioselectivity are those derived from the Schiff bases of chiral diamines such as [SiS] and [RR] 1,2-diphenylethylenediamine and [SS] and [RR] cyclohexyl-1,2-diamine. An example is the synthesis of cromakalim. 

Thus, the differences in activities of protonic acids are due to the quality of the corresponding anion or to its tendency to form chemical bonds with the carbon cation. If the anion is unable to form such bonds without extensive regrouping or decomposition, the addition of the proton is followed by polymerization. Should the reactivity of the anion be suppressed by solvation, the tendency to polymerize is enhanced. Consequently, the efficiencies of protonic acids depend very much upon the polarities of the media and upon the reaction conditions [12, 13]. Also, the stronger the protonic acid the higher the reaction rate and the resultant degree of polymerization [14]. Generally, hydrogen halide acids do not initiate polymerizations of alkyl-substituted olefins. They may, however, initiate polymerizations of aryl-substituted olefins and vinyl ethers in polar solvents. The same is true of sulfuric acid [15]. 

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