Aryl-Substituted Ethylenes

6 CONJUGATED DOUBLE BONDS 3.6.1 Aryl-Substituted Ethylenes 

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Several aryl-substituted ethylene oxides may be cited m connexion with epoxide hydration. For instance, a-methylstyreoe oxide Fives j. 2-dihydroxy-2-phenyl propano on careful treatment with cold dilute hydrochloric acid.4 9 The isomeric substance -methylstyrene oxide, which can exist in two geometric modifications, is reported to... 

A number of aryl-substituted ethylene oxides have been exposed to tiie action of carboxylic adds during the peroxy arid oxidation of olebns. Among these (Eq. 763) may be cited / -metby)styrene, ... 

Similar decompositions have been observed with 3-acylthio-propylene sulfides, l-octene sulfide, and a number of aryl-substituted ethylene sulfides .iU... 

Polar solvents also may affect the geometrical isomerization of ethylenes by affecting the efficiency of intersystem crossing [96,97]. Thus, even in the absence of polar substituents, solvent polarity can be of importance in the photochemical isomerization of aryl-substituted ethylenes. For example, cis-l-(9-anthryl)-2-(l-naphthyl)ethylene 74 in cyclohexane isomerizes to the trans-isomer 75 with a quantum yield of 0.05, while the quantum yield in acetonitrile solution is 0.31. Any trans->cis isomerization 75—74 in cyclohexane has not been observed, but the reaction does proceed with a preparatively useful quantum yield of 0.05 in acetonitrile [98],... 

Tn previous work it has been shown that a competition exists during - ozonation of olefins between ozonolysis and epoxide formation (I). As steric hindrance increases around the double bond, the yield of epoxide or subsequent rearrangement products increases. This is illustrated with both old (1) and new examples in Table I for purely aliphatic olefins and in Table II for aryl substituted ethylenes. It was suggested that the initial attack of ozone on an olefinic double bond involves w (pi) complex formation for which there were two fates (a) entrance into 1,3-dipolar cycloaddition (to a 1,2,3-trioxolane adduct), resulting in ozonolysis products (b) conversion to a o- (sigma) complex followed by loss of molecular oxygen and epoxide formation (Scheme 1). As the bulk... 

Table II. Percentage Epoxide from Ozonation of Aryl Substituted Ethylenes...

VIII. PHOTOISOMERIZATION IN ARYL-SUBSTITUTED ETHYLENE DERIVATIVES... 

Novel one-way cis —> trans isomerization is discussed by Arai and Tokumaru [94] in anthryl and other aryl-substituted ethylenes and will not be discussed here. The trans —> cis isomerization generally does not occur from the triplet excited state [94] in anthryl ethylene derivatives and it may be possible from the singlet excited state. A variety of anthryl-substituted ethylene derivatives have been prepared (Exhibits 6 and 7) to study the photoisomerization process. 4-Substituted styrylanthracene (Exhibit 6) was found to undergo trans-to-cis and cis-to-trans photoisomerization [95], whereas the parent styrylanthracene... 

Formation of ketones by bromination (chlorination)-rearrangement of aryl substituted ethylenes (Kakis) (see 1st edition). Conversion of 1-arylalkenes to 2-arylaldehydes with l2 and Ag20 at room temperature, via aryl migration (Kikuchi). 

Apart from exceptional cases,30 isolated double bonds and monocyclic aromatic systems are not attacked. Conjugated double bonds,4,31 however, aryl-substituted ethylenes,32 and polycyclic aromatic compounds4,33 can be partially hydrogenated with considerable success. Dimerization or further chain-formation of the starting material often occurs as a side reaction. 

Ishikawa and co-workers also reported a class of structurally modified guanidines for promotion of the asymmetric Michael reaction of ierf-butyl-diphenylimino-acetate to ethyl acrylate [124,125]. In addition to a polymer support design (Scheme 69), an optical resolution was developed to achieve chiral 1,2-substituted ethylene-l,2-di-amines, a new chiral framework for guanidine catalysis. The authors discovered that incorporating steric bulk and aryl substituents in the catalyst did improve stereoselec-tivitity, although the reactivity did suffer (Scheme 70, Table 4). 

When an alkyl or aryl ketone, or an aryl aldehyde, reacts with an alkyl-substituted ethylene, or with an electron-rich alkene such as a vinyl ether, the mechanism involves attack by the (n,n triplet state of the ketone on ground-state alkene to generate a 1,4-biradical that subsequently cyclizes. The orientation of addition is in keeping with this proposal, since the major product is formed by way of the more stable of the possible biradicals, as seen for benzophenone and 2-melhylpropene (4.64). As would be expected for a triplet-state reaction, the stereoselectivity is low, and benzophenone gives the same mixture of stereoisomers when it reacts with either trans or... 

Cyano-substituted ethylenes react in a different way with aliphatic ketones. The orientation of photochemical cycloaddition (4.661 is the opposite of that found for electron-rich alkenes, and the reaction is highly stereoselective (4.69) in the early stages. These processes involve the formation and subsequent decay of an excited complex (exciplex) from the (n,n ) singlet state of the ketone and the alkene. Aryl ketones undergo intersystem crossing so efficiently that such a singlet-state reaction is rarely observed, but the reaction of a benzoate ester with an electron-rich alkene 14.70 rnay well be of this type, with the ester acting as electron-acceptor rather than electron-donor. 

A Michael-type addition reaction of substituted ethylenes (39) with 2-phenylindolizines yields 3-substituted indolizines (40) <66AC(R)752). Arylation occurs on treatment of an indolizine with the salt (41) (67TL4321) or with diphenylcyclopropenone (68TL5537), yielding the tetrafluoroborate (42). 

Styrene oxide (Eq. 654) illustrates tbe reaction of the aryl substituted class of ethylene oxides -with albylmercaptans a In this ease ring opening allegedly occurs only by attack of tbe nucleophile on the terminal carbon atom, in contrast with the direction of fission obtained with amines (see section IV.4..B.). 

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). 

Table 2. Generation of aryl-substituted 1,2-ethylene dications.

This type of receptor is represented by compounds 16a,b bearing ruthenium ) bipyridine moieties. Both calixarenes [18] exhibit 1 1 binding of chloride and bromide anions (DMSO-d6), and they are especially suitable for the complexation of H2POj (X16a=2.8-104 M-1 K16b=5.2 103 M"1). On the other hand, if we compare these results with those for similar non-calixarene receptors, where the bipyridine unit is substituted by alkyl, aryl or ethylene glycol substituents, the introduction of calixarene does not bring any substantially new features into the complexation abilities of these derivatives. As shown by X-ray analysis, the anion is encapsulated within the cavity formed by amidic functions with the contributions of CH...anion interactions from the bipyridine unit. 

Lipshutz and colleagues presented recently palladium-catalyzed direct coupling reactions of alkyl iodides and vinyl bromides or iodides catalyzed by 1 mol% Pd(amphos)Cl2 in the presence of zinc and TMEDA in a biphasic aqueous/poly-(ethylene glycol tocopheryl sebacate) reaction medium [198], Internal olefins were obtained in 51-95% yield. For aryl-substituted (Aj-vinyl bromides, retention of double bond geometry was observed, while different degrees of isomerization occurred for (Z)-isomers, which may indicate the intervention of a radical addition process in the course of the coupling process. Alkyl-substituted (Z)-vinyl halides were transformed in contrast with retention of alkene geometry. Aryl halides also reacted [199],... 

Alkenes without allylic H atoms—such as ethylene, acceptor-substituted ethylenes, and styrene—can be alkenylated and arylated by Heck reactions in a clearly predictable fashion. These alkenes can be alkenylated to provide 1,3-dienes, a,/3,y,S-unsaturated carbonyl compounds (Figures 13.27 and 13.28), a,j8,y,8-unsaturated carboxyl compounds (Figures 13.27 and 13.28), as well as aryl-substituted 1,3-dienes. Moreover, the same alkenes can be arylated to give styrenes, a./l-unsaturated /3-arylated carbonyl compounds, or a./l-unsaturated /3-arylated carboxyl compounds (Figure 13.26) and stilbenes (Figure 13.29). 

The transformation of tetrasubstituted ethylenes into 1,2,4-trioxolanes may also be achieved if the ozonolysis is carried out in the presence of a foreign carbonyl compound as described in Section 4.33.3.4. With formaldehyde as added carbonyl compound, 3,3-disubstituted derivatives are obtained, whereas in the presence of excess ketone (e.g. by using the latter as solvent), the ozonolysis gives rise to tetrasubstituted 1,2,4-trioxolanes which are difficult to prepare by other methods. Reactions (163) -> (164) and (165) -> (166) provide two examples of this versatile 1,2,4-trioxolane synthesis. Unlike the parent system (2), alkyl- and/or aryl-substituted 1,2,4-trioxolanes generally are stable, non-explosive compounds. Mixtures of crossed ozonides (cf. Section 4.33.3.1.1) or of cis and trans isomers can be separated by thin layer, column or gas chromatography. The cis isomers of symmetrical 3,5-disubstituted 1,2,4-trioxolanes are meso forms, whereas the corresponding trans isomers represent racemates which in some cases have been resolved into their optical... 

Before discussing structural effects on barrier heights, it is necessary to distinguish between the two processes planar nitrogen inversion and rotation about the C=N bond, which may both lead to interconversion of the isomers. Rotation about a non-activated C=N double bond is expected to be hindered by an energy barrier similar to the ethylene barrier, i.e. of the order of 50—60 kcal/mole 121.132) Thus, alkyl- and presumably also aryl-substituted imines, which show interconversion barriers below 30 kcal/mole (Table 6), undergo nitrogen inversion ). 

Many alkyl- or aryl-substituted pyridazines (14) have been prepared by a direct one-step cyclization from an unsaturated 1,4-diketone (diacylethylene) and hydrazine. In the main symmetrically substituted diketones were employed, carrying aliphatic " or aromatic groups. Ethylene-substituted unsaturated 1,4-diketones afford tri- or tetrasubstituted pyridazines (14, R = =Rg = Rg = alkyl or aryl).2 -23 ... 

Aiyl-substituted ethylene oxides have also been prepared. The simplest of these is styrene oxide, which is prepared by the alkali treatment of the iodohydrin made by the action of iodine, water, and mercuric oxide on styrene (51%). Aryl chlorohydrins resulting from the action of chloro-acetone and aromatic Grignard reagents, C,Hj (CHj)nMgX, have been converted by alkali or sodium ethoxide to oxides of the type... 

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