Substitution aryl-substituted olefins

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... 

The highest enantioselectivity in the dialkyl-substituted olefines has been obtained with the aryl ethers of DHQD 94a and DHQ 94b. With potassium ferri-cyanide as secondary oxidant, it is possible to carry out the reaction at room temperature, and slow addition of the olefins is not required. Under these conditions, the diols can be obtained in 85-90% yield and excellent enantioselectivity. 

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... 

Knoke and de Meijere [60] recently developed a highly flexible domino Heck-Diels-Alder reaction of a symmetrically substituted cumulene 125, which also involves cross-couplings of an allene at the central position. Both aryl and hetaryl halides react efficiently with l,3-dicyclopropyl-l,2-propadiene (125) and furnish 1,3,5-hexatriene derivatives 126 as intermediates, which are usually trapped by acceptor-substituted olefins in a subsequent cycloaddition, providing adducts 127a/b in moderate to good overall yields (Scheme 14.30). 

Nickel(O) triphenylphosphine can be used to couple aryl halides and alkenes to synthesize substituted olefins [149], 1,2-bis[(di-2-propylphosphino)benzene]nick-el(0) can be used to couple aryl halides [150], and l,2-bis[(diphenylphos-phino)ethane]nickel(0) can be used to prepare benzoic acid from bromobenzene in the presence of carbon dioxide [151]. 

Moreover, looking for more effective ligands, Sharpless and his group prepared and tested a number of cinchona alkaloid derivatives, first in the stoichiometric ADH process [33] and then in the catalytic process. They found that aryl ethers of dihydroquinidine, as 4a and 4b, are excellent ligands for ADH of dialkyl substituted olefins (Table 10.3). 

Sharpless concludes that ligand 4c is preferable for the ADH of arylsubstituted olefins, whereas the aryl ethers 4a or 4b are better ligands for the reaction of dialkyl or alkylcarboalkoxy substituted olefins. 

As mentioned in Sections 3.1.6 and 4.1.3, cyclopropenes can also be suitable starting materials for the generation of carbene complexes. Cyclopropenone di-methylacetal [678] and 3-alkyl- or 3-aryl-disubstituted cyclopropenes [679] have been shown to react, upon catalysis by Ni(COD)2, with acceptor-substituted olefins to yield the products of formal, non-concerted vinylcarbene [2-1-1] cycloaddition (Table 3.6). It has been proposed that nucleophilic nickel carbene complexes are formed as intermediates. Similarly, bicyclo[1.1.0]butane also reacts with Ni(COD)2 to yield a nucleophilic homoallylcarbene nickel complex [680]. This intermediate is capable of cyclopropanating electron-poor alkenes (Table 3.6). 

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. 

As a result, cycloaddition between excited aryl groups and simple alkyl-substituted olefins generally leads to the formation of meta cycloadducts, whereas ortho cycloaddition predominates from the singlet state when excited charge transfer is energetically favorable. In fact, direct irradiation of donor-acceptor bichromophores 206 and 207 affords no meta cycloadduct [261],... 

Ni(0) obtained from electrochemical reduction of NiBr2 in THF/HMPA acts as an efficient catalyst for the electroreductive coupling of ethylene with aryl halides to give 1,1-diarylethanes 217 (equation 109). By proper control of reaction conditions, such as reduction potential, solvent and supporting electrolyte, it can be shown that substituted olefins can be prepared from aromatic halides and alkenes164. 

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. 

The imexpectedly smooth formation of the unequivocally characterized disiletane 5 is presumably attributable to the imsymmetrical substitution at the C=C double bond in combination with the aryl substituent. In support of this hypothesis, reactions of 1 with the symmetrically substituted olefins cyclopentene and cyclohexene give rise only to the siliranes 6 and 7 [10, 11], which were in part characterized previously, together with the cyclotetrasilane 8. 

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