Nitrostyrenes 2-bromo-substituted

Bromo-j3-nitrostyrene and triphenylphosphine in dry benzene gave the phosphonium bromide (47). Using methanol as the solvent, the rearranged product (48) was formed, possibly via an azirine intermediate. Substituted -bromo-/3-nitrostyrenes yield the phosphoranes (49) and a phosphonium salt. When the aryl group is electron-donating, the reaction follows a different course to form the styrene (50) by initial attack of the phosphine on halogen. 

To reduce to the active formula, see the zinc reduction as given in the reduction section, CPB, 16, 217 (1968). This reduction is specifically matched to 3-bromo-4,5-methylenedioxy-B-nitrostyrene and other highly substituted ring type styrenes and propenes. Zinc reductions carried out properly are very gentle and do not destroy delicate ring substituents, while some reductions do. Zinc reductions can reduce any nitrostyrene or propenes, but some of these compounds must use the zinc reduction. Which compounds Compounds with lots of ring substituents, like 2,5-dimethoxymethylenedioxy, 3-methoxy-4-0-carbethoxy, etc. This is not to say that some of the other reductions are not capable of gentle reductions. 

The reactivity of the halide ions could not be evaluated directly since they have not been studied with the same substrate. However, -toluene-thiolate ion is nine orders of magnitude more reactive than chloride ion towards 2-chloro-l,l-diarylethylenes in dimethylformamide. Although comparison may not be justified (see below), a similar reactivity ratio exists for the reactions of j8-bromo-j -nitrostyrene with iodide ion in butyl cellosolve and thiophenoxide ion in methanol. Bromide ion is 0-6 times as reactive as chloride ion towards l-anisyl-l-phenyl-2-chloroethylene. These relative reactivities of the halide ions should be regarded only as rough estimates. Their very low reactivity is also shown by the chloride exchange in ethyl /3-chlorocrotonate, which is at least 106 times slower than the substitution by thioethoxide ion (Jones et al., 1960) while trichloroethylene does not exchange at all even at 245° (Bantysh et al., 1962). 

Table IV compares the reactivity ratios of a soft (PhS-) to a hard (MeO-) nucleophile in vinylic substitution. PhS is always more reactive, and ratios lower than unity, as for 4, X = Br (4), are certainly due to elimination-addition with MeO . The ratios change by >2000-fold and are sensitive to the geometry of the substrate. An important feature is that for (3-halo-p-nitrostyrenes the ratio decreases strongly with the increased hardness of the (3-halogen (38). The lowest ratios are for the (3-fluoro derivative, whereas the differences between the chloro and bromo compounds are not so large. This behavior is similar to that in SNAr reactions. This behavior can be rationalized by symbiotic effects, which favor the soft-soft PhS--Br interaction and the hard-hard MeO-F interaction. A reactivity-selectivity relationship for vinyl bromides of different electrophilicities does not exist.

The crystal and molecular structure of 2-(a-p-bromophenyl-P-nitro)-ethyl-5-methylcyclohexanone (6), a product from reaction of l-morpholino-3-methylcyclo-hexene with p-bromo-P-nitrostyrene, has been determined. Although a cis-configu-ration was anticipated on a mechanistic basis, the n.m.r. spectral analysis did not provide an unequivocal answer. X-Ray analysis indicated a chair cyclohexanone ring with the methyl group axial and the substituted ethyl group equatorial. 

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