J8 -Methylstyrene

The results of the olefin oxidation catalyzed by 19, 57, and 59-62 are summarized in Tables VI-VIII. Table VI shows that linear terminal olefins are selectively oxidized to 2-ketones, whereas cyclic olefins (cyclohexene and norbomene) are selectively oxidized to epoxides. Cyclopentene shows exceptional behavior, it is oxidized exclusively to cyclopentanone without any production of epoxypentane. This exception would be brought about by the more restrained and planar pen-tene ring, compared with other larger cyclic nonplanar olefins in Table VI, but the exact reason is not yet known. Linear inner olefin, 2-octene, is oxidized to both 2- and 3-octanones. 2-Methyl-2-butene is oxidized to 3-methyl-2-butanone, while ethyl vinyl ether is oxidized to acetaldehyde and ethyl alcohol. These products were identified by NMR, but could not be quantitatively determined because of the existence of overlapping small peaks in the GC chart. The last reaction corresponds to oxidative hydrolysis of ethyl vinyl ether. Those olefins having bulky (a-methylstyrene, j8-methylstyrene, and allylbenzene) or electon-withdrawing substituents (1-bromo-l-propene, 1-chloro-l-pro-pene, fumalonitrile, acrylonitrile, and methylacrylate) are not oxidized. 

The separated diastereoisomers 4a and 4b, and also the optically active derivatives 5, with Br instead of Cl, have been used as methylene-transfer reagents for the preparation of cyclopropanes in the reaction with trans-j8-methylstyrene (30, 31). The optical yields obtained (9-38%) are much... 

For styrene [32] bromination, (23) can be applied more appropriately the curvature of the plot (Fig. 19) is not very significant when compared with that in stilbene bromination. Styrene bromination proceeds predominantly via a carbocation intermediate and is accompanied by a bromonium ion pathway which becomes increasingly competitive as X becomes more EW the second term in (23) should reflect the bromonium ion pathway, whereas the Cp carbocation is too unstable to compete with the formation of the carbocation. Exactly the same correlation is obtained for j8-methylstyrene, Ar-CH=CH(CH3). 

In nonprotic solvents, alkenes are stoichiometrically oxidized by V -peroxo complexes to epoxides and consecutive oxidative cleavage products in a nonstereoselective fashion. For example, cis-2-butene gave an approximately 2 1 mixture of cis- and trans-epoxides (equation 37). The reactivity of alkenes increases with their nucleophilic nature. Alkenes containing phenyl substituents such as styrene, a- and j8-methylstyrene are also very reactive and mainly give oxidative cleavage products. 

High yields are obtained with terminal alkenes (styrene and a-methylstyrene) and with the strained acenaphthylene, whereas stilbene and j8-methylstyrene are barely reduced. 

Reaction of allylbenzene or trans-j8-methylstyrene with IrCl(N2)(PPh3)2 produces the 7t-allyl complex XX in 71-78% yield" ... 

A similar reaction affords a simple route to cyclopent[b]indoles. Indole-3-carbinol (68) reacts with j8-methylstyrene in the presence of titanium tetrachloride to give compound (70) <93TL8527>. The presumed four-membered spiroindolenine (69) in this sequence receives support from the observation that the conversion of the D-tryptophan derived compound (71) to its mesylate leads to the formation of racemic cyclopent[b]indole (73) via the achiral intermediate (72) (Scheme 17) <93TL439>. 

The discussion to this point has focused on the addition of bromine to double bonds bearing alkyl substituents. There are significant differences in the addition of bromine to alkenes with aryl substituents. For example, the addition of bromine to aryl-substituted alkenes is not stereospecific. Reaction of ds-j8-methylstyrene (13) with bromine in CCI4 led to the formation of 17% of erythro- (14) and 83% of f/ireo-(l,2-dibromopropyl)benzene (15, equation 9.12). Reaction of frans-jS-methylstyrene under the same conditions yielded 88% of the erythro and 12% of the threo product, When the benzene ring in the trans reactant was substituted with a 4-methoxy group (frans-anethole), the reaction became even less stereoselective, giving 63% erythro and 37% threo product. ... 

In both cases, therefore, the bromoacetoxy products were those expected from anti addition, while formation of the dibromo products was much less stereoselective. Furthermore, the fraction of dibromide formed by apparent syn addition to cis-j8-methylstyrene varied with solvent polarity, from 27% in acetic acid to 55% in nitrobenzene. The results were discussed in terms of initial formation of an intimate bromo-carbocation-bromide ion pair that can undergo attachment of acetic add from the face of the ion away from the bromide ion, thus leading to anti formation of bromoacetoxy compounds. Reorientation of the bromide ion within the solvent shell for backside attachment to the bromonium ion provides time for rotation about the carbon-carbon single bond of the bromocarbocation, so the dibromo products are formed with less stereoselectivity. 

Alkenes, such as (E)- and (Z)-j8-methylstyrene, are converted stereo-specifically by chromyl nitrate to the corresponding epoxides with high selectivites in aprotic media/ Cosolvents (DMF, acetone, pyridine) are required for effective epoxidation. Products derived from a Cr(IV) intermediate are absent suggesting that the active species is oxochromium(V), formed in situ by the prior one-electron oxidation of solvent. 

Despite the difficulties arising from competing pathways, the substituent effects on the pathway can be estimated quite successfully and the correlations obtained (Table 11) are reasonably interpreted in comparison with those for the uncomplicated systems. Simple styrenes [32] where a-R = H, irrespective of the j8-substituents R, give a constant p value of -4.45 and constant r of 0.93. All tertiary a-methylstyrene [33] series, irrespective of j8-substituents, give a similar p value (-4.4 0.2) but a slightly lower r value of 0.74-0.82. For the trans-stilbeiiQ series [37], Ruasse assumed that the balance between polar and resonance effects does not differ from that in the reaction defining a, i.e. that there is no need to use a Y-T equation. 

Thermolysis of terminal vinyl azides has generally led to the isolation of products other than azirines. A number of j8-azido-styrene derivatives have, on pyrolysis, yielded indoles. Thus /S-azido-a-methylstyrene, (60) when boiled in mesitylene, produced an 80% yield of 3-methylindole (61) and 9% of a-phenylpropionitrile (62) Decomposition of the same compound in ethanol, however, gave only a trace of the indole the major product, isolated in about... 

In the presence of bromide, the oxidation of alkenes by Mn(OAc)3 in acetic acid readily occurs It 70-80 °C and produces allylic acetates in good yields. Thus cyclohexene is oxidized to cyclo-lexenyl acetate in 83% yield," and a-methylstyrene to j8-phenylallyl acetate in 70% yield," vith a mechanism involving allylic hydrogen abstraction by bromine atoms coming from the ixidation of bromide by Mn (equation 206). 

Similarly, in spite of extensive experimental study of the complexes of styrene and methylstyrene with j8-CyD, the claim by Cao and coworkers [70] on the basis of PM3 calculations that only van der Waals attractions and dipole-dipole interactions influence the complexes geometry seems ill-founded. 

Synthesis of j8,y-Unsaturated Amines. 1,1-Disubstituted alkenes can be converted into the corresponding 8, /-unsaturated amines, valuable synthons in medicinal chemistry, and versatile synthetic intermediates, via a three-step protocol with moderate to good yields. The initial formation of a, 8-unsaturated diphenyl-sulfonium triflates from alkenes, such as a-methylstyrene, and the combination of diphenyl sulfoxide and TfzO, is followed by double bond migration to form /3,y-unsaturated sulfonium triflates. Subsequent nucleophilic substitution with primary or secondary amines such as fert-butylamine leads to the corresponding p,y-unsaturated amines (eq 10). ... 

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