A-Methyl stilbene

TrisubsHtuted alkenes. Negishi et al have extended the reaction of alkynes with trimethylalane and zirconocene dichloride (this volume) to a corresponding addition reaction with trimethylalane-titanocene dichloride. This latter reagent is somewhat more reactive. It reacts with diphenylacetylene to form (Z)-a-methyl-stilbene (2) in high yield. The yield of 2 is <30% when 1 is replaced by the corresponding zirconocene reagent. 

Figure 11.5 Complex dynamic viscosity as a function of temperature for a main-chain polyether consisting of a methyl stilbene mesogen and a mixture of seven-and nine-carbon aliphatic spacers. The polymer has a molecular weight of 36,000. The diamonds and squares are for temperature ramp rates of 0.1 °C and 2.0°C/min, respectively the open and closed symbols are for heating and cooling, respectively. The dashed line marks the isotropic-nematic transition. (From Gillmor et al. 1994, with permission from the Journal of Rheology.)...

With the catalysts in hand, we were able to test their effectiveness in several epoxidation reactions. Initially, we screened the catalysts with our usual test substrates, 1-phenylcyclohexene, a-methylstilbene and triphenylethylene (Table 5.5). Catalyst 33a showed the best reaction profile, being by far the most reactive. For example, in the presence of 33a, 1-phenylcyclohexene oxide was produced in 69% yield with 91% ee in imder 20 min, while the other catalysts (apart from ent-33a) were less selective, and the reactions were slower. The isopinocamphenyl moiety offers little enantiocontrol, leading to epoxides with only moderate ees. The poor reactivity of catalysts 33b-d is highlighted by the attempted epoxidations of a-methyl stilbene and triphenyl ethylene, where no epoxides were formed after 4 h. 

Dehydration of either erythro- or f reo-l,2-diphenyl-l-propanol, (1) and (2), with triphenylphosphite methiodide in HMPT gives a mixture of a-methyl-stilbenes in which the less stable Z-olefin predominates. Dehydration of (1) and... 

Clear evidence that the geometry of the transition state is not constant has been provided by studies of eclipsing effects . Assuming anti stereospecificity, the threo and erythro isomers of the 1,2-diphenyl-l-propyl-X systems undergo bimolecular elimination to give the trans- and cw-a-methyl stilbenes respectively (75). 

Nelson and co-workers [4] tabulated DETA Tg values of semi-fluorinated epoxy thermoplastics containing the a-methyl stilbene linkage. 

Nucleophilic attack by a carbanion on a rhodium-acetylene complex has been postulated in the reaction of tolane with MeMgBr in the presence of (Ph3P)3 RhBr (Michman and Balog, 1971). This interpretation is supported by the known formation of rhodium-alkyne complexes under these conditions (Muller and Segnitz, 1973) and by the predominant formation of trans-a-methyl-stilbene. Additional study of the scope of this reaction would seem worthwhile since recycling of the rhodium-containing by-product (Ph3P)3 RhOH, should be possible. 

Fig. 12 Selectivity-reactivity relationships in arylolefin bromination. Line A correlates the data for styrenes, stilbenes and their a-methyl derivatives, and line B those of 1,1-diarylethylenes and a-methoxystyrenes (Tables 15 and 17).

An informative set of calculations was carried out by Brandt et al, coupled to experimental studies that demonstrated first-order dependence of the turnover rate on both catalyst and H2, and zero-order dependence on alkene (a-methyl-(E)-stilbene) concentration [71]. The incentive for this investigation was the absence of any characterized advanced intermediates on the catalytic pathway. As a result of the computation, a catalytic cycle (for ethene) was proposed in which H2 addition to iridium was followed by alkene coordination and migratory insertion. The critical difference in this study was the proposal that a second molecule of H2 is involved that facilitates formation of the Ir alkylhydride intermediate. In addition, the reductive elimination of R-H and re-addition of H2 are concerted. This postulate was subsequently challenged. For hydrogenation of styrene by the standard Pfaltz catalyst, ES-MS analysis of the intermediates formed at different stages in the catalytic cycle revealed only Ir(I) and Ir(III) species, supporting a cycle (at least under low-pressure conditions in the gas... 

Compounds containing n systems have an affinity to the metallic surface and are arranged on the surface in a parallel manner. The ethylene bond of a-methyl-p-isopropyl stilbene is responsible for adsorption, and the substituents deviate from the metal surface. It is well known that alkali... 

Vacuum distillation of the benzyl derivative 77 gave a mixture of equal amounts of four products (Scheme 19) stilbene was also formed, again suggesting a radical mechanism.7 A dilute solution in boiling tetralin, however, gave only 76 (60%) and 78 (18%).13 In the case of 59 both a methyl and a phenyl group were observed to migrate.13... 

Acid-catalyzed hydration of a-methyl-lruTw-stilbene oxide W been reported by Tiffeneau and Levy,1 and more recently by Brewster, 49 to give a 1,2-diol whose structure likewise suggest ... 

Adsorption forces cause a-methyl-(3-isopropyl stilbene to be arranged in such a way that the substituents at the ethylenic bond deviate from the metal surface. The reaction under consideration consists of the two consecutive one-electron steps resulting in the formation of the dianion. The dianion accepts two protons, from the metal side. Therefore, regardless of the configuration of the initial stilbene, the reduction proceeds with a predominant formation of the erythro product (Scheme 6-1). 

Athene —r alcohol The reagent prepared from SnCla (1 equiv.) and NaBH (4 equiv.) in THF reacts with olefins to afford anti-Markownikoff alcohols after aqueous work-up. Examples include 2-phenylpropane-l-ol (58%) from a-methyl-styrene, isopinocampheol (40%) from a-pinene, cw-myrtanol (43%) from / -pinene, 1,2-diphenylethanol (53%) from frans-stilbene, and 1-hydroxy-acenaphthene (60%, together with acenapthene, 15%) from acenaphthylene. The reaction with dienes such as limonene and 1,5-cycIooctadiene gave complicated results. The mechanism of this hydroxylation is uncertain. 

Two examples of 1,4-addition to the naphthalene ring have been reported. In one. the substituent effects upon the reaction between the triplet excited state of 1-acetyinaphthalene and variously substituted alkenes has been examined. while in the other, the 1.4-addition of a-methyl styrene and of 1,1-dlphenyl ethylene to the phthallmide (125) is shown to proceed by electron transfer from the alkene to the excited state of the phthallmide. The azaindone (126), which is Isoelectronic with naphthalene, also forms addition products when Irradiated with stilbene. The addition products are assigned the structures... 

While all attempts to avoid formation of the hydride cluster or to convert it back to a catalytically active species failed, the solution to the deactivation problem turned out to be as simple as surprising. Exchange of the counterion from PF to tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BAtp) strongly increased conversion (Scheme 5). Using catalyst 9 with BArp as the counterion, several thousand turnovers could be obtained in the hydrogenation of frani -methyl stilbene 10 under optimized conditions. 

Exciton-coupled circular dichroism (ECCD) spectroscopy was performed on the Aib-Stb-D peptide to fiirther substantiate the interaction between the methyl stilbene side-chains, which was observed via PL experiments. The interaction between the excited state of chromophores in a chiral environment causes split Cotton effects upon absorption of circularly polarized light by the chromophores (29, 30). It can be observed from Figure 7 that Aib-Stb-D exhibits a split CD Cotton effect presumably because of the chiral presentation of the methyl stilbene molecules on the same side of the a-helical peptide the asymmetric nature of the split observed may be due to some other electronic transitions or of additional background ellipticity, as has been observed in other systems (29, 31). The ECCD results confirm the close proximity of the methyl stilbene side chains mediated by the peptide backbone, which permits interaction between the side-chains in the excited state. 

The photoisomerization of the push-pull azobenzene derivative disperse red 1 (DRl) has also been shown recently to occur efficiently in poly-(methyl metacrylate) (PMMA) thin films at room temperature [23,24]. DRl is a pseudo-stilbene type azobenzene molecule, as classified by Rau. This means that the high-energy n-n transition is overlapping the low energy n-n transition, which leads to a large structureless band in the trans isomer and... 

Benzylamino-2-(trimethylsilylmethyl)-but-l-ene cyclizes in the presence of acid to afford the azepine (138) <86TL5067>, and 7V-benzyl-6-(phenylsulfinyl)-hexanamide reacts with O-methyl-O-t-butyldimethylsilyl ketene acetal to afford the lactam (139) <90CPB1473>. Direct irradiation of ortho-(A -methylaminoethyl)stilbene in acetonitrile affords Af-methyl-2-phenyl-2,3,4,5-tetrahydro-benz[primary product but which undergoes demethylation upon further irradiation <92TL4249>. The demethylated product may also be obtained by irradiation of o-(3-amino-n-propyl)stilbene in the presence of w-dicyanobenzene as electron transfer agent. 

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