Olefin arylation

This modification of the Prins2 reaction has been applied to other olefins.3 The aryl olefins give the best yields see the... 

Table 2 A tandem hydrosilylation cross-coupling approach to aryl olefins...

Photochemical addition of ammonia and primary amines to aryl olefins (equation 42) can be effected by irradiation in the presence of an electron acceptor such as dicyanoben-zene (DCNB)103-106. The proposed mechanism for the sensitised addition to the stilbene system is shown in Scheme 7. Electron transfer quenching of DCNB by t-S (or vice versa) yields the t-S cation radical (t-S)+ Nucleophilic addition of ammonia or the primary amine to (t-S)+ followed by proton and electron transfer steps yields the adduct and regenerates the electron transfer sensitizer. The reaction is a variation of the electron-transfer sensitized addition of nucleophiles to terminal arylolefins107,108. 

The direct carboxylation of unsaturated compounds has already been reported [107,108]. The process is however limited to reducible alkenes like aryl olefins or activated olefins. On the other hand, only a few examples of direct electrocarboxylation of alkynes have been reported [109],... 

In contrast to normal trisubstituted aryl olefins, a strong pressure effect was observed in this case. Interestingly, for substrate 44a catalysts hydrogenation with Ir(13d) demonstrated opposite pressure-dependant enantioselectivity of substrate 45a with catalyst Ir(46). Poor enantioselectivity was obtained for substrates that did not contain an aromatic ring adjacent to the olefin. 

The chemical entrainment method was used by Ono et al. (1979) to eliminate the nitro group in nitroalkene derivatives. On simple mixing with thiophenol and sodium sulfide in DMF, nitro aryl olefins substitute hydrogen for the nitro group (Scheme 5.9). 

Table 2. Synthesis of Benzo- and Double Helicenes from Aryl Olefinic Precursors...

The [Con(bipy)2 ]2+ species has also been reported to activate hydrogen peroxide and ter -butyl hydroperoxide for the selective ketonization of methylenic carbons, the oxidation of alcohols and aldehydes, and the dioxygenation of aryl olefins and acetylenes (36). Later reports (37), however, while confirming that the cobalt complexes did indeed cata-... 

Arylation, olefins, 187, 190 Arylketimines, iridium hydrogenation, 83 Arylpropanoic acid, Grignard coupling, 190 Aspartame, 8, 27 Asymmetric catalysis characteristics, 11 chiral metal complexes, 122 covalently bound intermediates, 323 electrochemistry, 342 hydrogen-bonded associates, 328 industrial applications, 8, 357 optically active compounds, 2 phase-transfer reactions, 333 photochemistry, 341 polymerization, 174, 332 purely organic compounds, 323 see also specific complexes Asymmetric induction, 71, 155 Attractive interaction, 196, 216 Autoinduction, 330 Axial chirality, 18 Aza-Diels-Alder reaction, 220 Azetidinone, 44, 80 Aziridination, olefins, 207... 

A typical example for type 1 is the dimerization of electron-rich olefins like aryl olefins, enol ethers, enol acetates, conjugated dienes, or enamino esters [39] ... 

Rate constants for quenching of 1-7 by methanol and acetic acid in hexane solution from fluorescence quenching and quantum yield data are 10 M l-s-l-. Limiting quantum yields for adduct formation are 0.1. The observation of reactions of protic solvent with 1-7 but not 1-t may reflect the longer lifetime and/or enhanced reactivity of the cyclic molecule. While photo-induced nucleophilic addition is a common reaction of aryl olefins, it is normally observed to occur only under conditions of electron-transfer sensitization (139). Under these conditions, it is the aryl olefin cation radical which undergoes nucleophilic attack. The reaction of 1-7 with protic solvents appears to be the only reported example of nucleophilic trapping of an aryl olefin it,it singlet state (140). 

Acyloxylation of aryl olefins probably involves radical cations as intermediates. Acetoxylation of frans-stilbene in anhydrous acetic acid/sodium acetate yields mainly meso-diacetate, while in moist acetic acid mainly threo-2-acetoxy-l,2-di-phenylethanol is formed 100 Anodic oxidation of trans- and ds-stilbene in ace-tonitrile/benzoic acid produces with both olefins the same mixtures of meso-hydrobenzoin diacetate (62) and f/ireo-2-benzoyloxy-l,2-dip]ienylethanol (63) l01 Product formation is best rationalized by a ECiqE-sequence leading to theienerge-tically most favorable acyloxonium ion (64) (Eq. (125) ) ... 

The dimerization of butadiene, aryl olefins and ethyl vinyl ether is best rationalized by postulating a radical cation 89 (Eq. (174) ) as first intermediate. As the fi -carbon of 89 has the highest free valence, the highest positive charge density and the lowest atom localization energy radical or electrophilic reactions of 89... 

Aryl tellurium trichlorides arylated styrene and other olefins in palladium-catalyzed reactions. The arylated olefins were obtained in low yields. Elemental tellurium precipitated during the reactions7. 

In addition, diaryl tellurium dichlorides were used to arylate olefins. 

Diaryl tellurium dichlorides react with olefins in the presence of palladium(II) chloride to give arylated olefins in moderate yields. The reactions are catalytic with respect to palladium chloride when copper halides are added as oxidants. An aryl palladium compound was postulated as the intermediate5 ... 

Table Vic. Ketonization of Methylenic Carbons and Dioxygenation of Aryl Olefins, Acetylenes, and Catechols via the Fe2+(DPAH)2, 16, Induced Activation of Dioxygen in 1.8 1 py/HOAc...

More interestingly, no reaction products are formed in an oxygen-saturated acetonitrile solution of 8 containing triplet excitable sensitizers, such as rose bengal (RB) or methylene blue (MB+), irradiated at X > 500 nm. It therefore seems that, in the DCA-sensitized photooxygenation of 8 and similar aryl-olefins, singlet oxygen is not involved at least as reactive intermediate vs the specific substrates [84,95],... 

In the effort to find confirmation on Foote s original mechanistic proposal [84] and discriminate among these two different pathways, a great deal of experimental proofs were achieved. First of all, the DCA and/or 9-cyanoanthracene (CNA)-sensitized reactions on aryl-olefins were studied under inert atmosphere by flash spectroscopic techniques obtaining clear evident for the formation of both olefin radical cations and cyanoaromatic radical anions [95]. In the presence of oxygen, the cyanoaromatic radical anions were rapidly removed, supporting the very rapid formation of superoxide ion and so its direct involvement in these photoinduced oxygenations. 

However, as we will see later on, other modes of evolution of the primary intermediate radical ions can be suggested to explain some oxidation reactions mediated by electron-transfer processes. In fact, several exceptions to the Foote s BQ-controlled photooxygenation procedure have been reported during the last years on several electron-rich substrates. Thus, the involvement of superoxide ion, as an oxygen active species, in all of the DCA-sensitized photooxygenations, remains questionable [96,105,112,115,128]. Schaap and co-workers [98] recorded under inert atmosphere the characteristic ESR spectrum of the (DCA ) radical anion. On the other hand, the involvement of aryl-olefin radical cations and their reactions with superoxide ion was easily observed by quenching experiments with compounds exhibiting lower oxidation potentials than those of aryl-olefins [85, 95, 98],... 

In fact, the addition of 1,4-dimethoxybenzene (DMB) and/or several similar compounds, at concentrations as low as 10 4 M, to a mixture of aryl-olefins and DCA almost completely inhibits the reactions. Concentration dependence and flash photolysis studies confirm that the primary electron-transfer process occurs between the singlet excited sensitizer and DMB (E01 = 1.34 V vs SCE) with the generation of the corresponding radical ion pair. As a consequence, quantum yields lower than 1, even at infinite substrate concentration, are measured [95]. In this regard, valuable confirmations came from the cyanoaromatic photoinduced electron-transfer oxygenation of alkynes [99], Farid and Mattes reported that the photooxygenation of diphenylacetylene DPA (E° = 1.85 V vs SCE) 25, leading to a mixture of benzil 26 and benzoic acid 27, was efficiently sensitized by DCA ( = 0.15), but poorly by TCA ( > < 0.001) [Eq. (12)] [99]. 

QCjimJtPFe] CjimlfSbFs] 0s04 NMO Enantioselective dihydroxylation of aryl olefins with acetone-H20 as co-solvent 2.5-5 equivalents of chiral ligand added products extracted with Et20 catalyst reused four times, marked decrease in activity after the third run. [67]... 

A commercial synthesis (Albemarle) of naproxen (a 2-aryl propionic acid anti-inflammatory related to ibuprofen) involves palladium catalyzed hydroxycarbonylation of an aryl olefin which is itself made in a palladium catalyzed Heck coupling reaction (Figure 6b). Resolution is needed to obtain the (5)-enantiomer of naproxen since its optical isomer is a liver toxin. 

Reactions with stable radical species, e.g. oxygen and NO2, occur readily, although oxygenation rates of aryl olefin radical cations (ca 10 s [310]) are two or... 

The synthetic and mechanistic aspects of photoaddition reactions of amines with excited states of arenes and aryl olefins have been extensively investigated [10-12, 277],... 

A new cyclising reagent is proposed for the synthesis of 5-unsubstituted 1,3,4-thiadiazoles (133). The latter are formed in good yield by the reaction of thiohydrazides (134) with diethyl chlorophosphate (Scheme 39). A useful, one-pot protocol has been developed for the conversion of enolizable ketones (135) to alkylated or arylated olefins (136) by Pd-catalysed cross coupling of in-situ generated enol phosphates (137) with Grignard reagents (Scheme 40). ... 

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