Phenylpropenes

The oxidative coupling of alkenes which have two substituents at the 2 posi-tion, such as isobutylene, styrene, 2-phenylpropene, 1,1-diphenylethylene, and methyl methacrylate, takes place to give the 1,1,4.4-tetrasubstituted butadienes 285 by the action of Pd(OAc)2 or PdCF in the presence of sodium acetate[255-257]. Oxidation of styrene with Pd(OAc)2 produces 1.4-diphenylbutadiene (285, R = H) as a main product and a- and /3-acetoxystyrenes as minor pro-ducts[258]. Prolonged oxidation of the primary coupling product 285 (R = Me) of 2-phenylpropene with an excess of Pd(OAc)2 leads slowly to p-... 

A significant modification in the stereochemistry is observed when the double bond is conjugated with a group that can stabilize a carbocation intermediate. Most of the specific cases involve an aryl substituent. Examples of alkenes that give primarily syn addition are Z- and -l-phenylpropene, Z- and - -<-butylstyrene, l-phenyl-4-/-butylcyclohex-ene, and indene. The mechanism proposed for these additions features an ion pair as the key intermediate. Because of the greater stability of the carbocations in these molecules, concerted attack by halide ion is not required for complete carbon-hydrogen bond formation. If the ion pair formed by alkene protonation collapses to product faster than reorientation takes place, the result will be syn addition, since the proton and halide ion are initially on the same side of the molecule. 

The stereochemistry of chlorination can be explained in similar terms. Chlorine would be expected to be a somewhat poorer bridging group than bromine because it is less polarizable and more resistant to becoming positively charged. Comparison of the data for bromination and chlorination of E- and Z-l-phenylpropene confirms this trend (see Table 6.2). Although anti addition is dominant in bromination, syn addition is slightly preferred... 

Addition of HBr to 1-phenylpropene yields only (l-bromopropyl)benzene. Propose a mechanism for the reaction, and explain why none of the other regioisoiner is produced. 

However, a number of examples have been found where addition of bromine is not stereospecifically anti. For example, the addition of Bf2 to cis- and trans-l-phenylpropenes in CCI4 was nonstereospecific." Furthermore, the stereospecificity of bromine addition to stilbene depends on the dielectric constant of the solvent. In solvents of low dielectric constant, the addition was 90-100% anti, but with an increase in dielectric constant, the reaction became less stereospecific, until, at a dielectric constant of 35, the addition was completely nonstereospecific.Likewise in the case of triple bonds, stereoselective anti addition was found in bromination of 3-hexyne, but both cis and trans products were obtained in bromination of phenylacetylene. These results indicate that a bromonium ion is not formed where the open cation can be stabilized in other ways (e.g., addition of Br+ to 1 -phenylpropene gives the ion PhC HCHBrCH3, which is a relatively stable benzylic cation) and that there is probably a spectrum of mechanisms between complete bromonium ion (2, no rotation) formation and completely open-cation (1, free rotation) formation, with partially bridged bromonium ions (3, restricted rotation) in between. We have previously seen cases (e.g., p. 415) where cations require more stabilization from outside sources as they become intrinsically less stable themselves. Further evidence for the open cation mechanism where aryl stabilization is present was reported in an isotope effect study of addition of Br2 to ArCH=CHCHAr (Ar = p-nitrophenyl, Ar = p-tolyl). The C isotope effect for one of the double bond carbons (the one closer to the NO2 group) was considerably larger than for the other one. ... 

On the other hand, addition of DBr to acenaphthylene (5) and to indene and 1-phenylpropene gave predominant syn addition. ... 

Vinyl cations also have been invoked as intermediates in the electrophilic addition of HCl to 1-phenylpropyne in acetic acid. Fahey and Lee (30,31) have shown that the reaction is first order in 1 -phenylpropyne and between first and second order in HCl. The observed products were ds-( 75%) and frans-( 10%) 1-chloro-l-phenylpropene, along with about 15% propiophenone. Control experiments demonstrated that the propiophenone arose from cis- and trans-1-acetoxy-l-phenylpropene. The results were best explained by rate-determining... 

N. Hayashi et al. (1984) described a study of phenylpropene derivatives in 44 species of Heterotropa Morren Dene, growing in Japan. According to Mabberley (1997, p. 339), Heterotropa is a synonym of Asarum L., which consists of some 70 North Temperate species, 30 of which occur in Japan. Regardless of the generic... 

A substantial amount of syn addition is observed for Z-l-phenylpropene (27-80% syn addition), E-1 -phenylpropcnc (17-29% syn addition), and ra-stilbene (up to 90% syn addition in polar solvents). 

The acetoxy group is introduced exclusively at the benzylic carbon. This is in accord with the intermediate being a weakly bridged species or a benzylic cation. The addition of bromide salts to the reaction mixture diminishes the amount of acetoxy compound formed by shifting the competition for the electrophile in favor of the bromide ion. Chlorination in nucleophilic solvents can also lead to solvent incorporation, as, for example, in the chlorination of 1-phenylpropene in methanol.37... 

Subtle ligand changes have pronounced effects in hydroboration of 1,1-disubstituted alkenes. Addition of HBcat across the C=C bond in 2,3,3-trimethyl-l-butene is catalyzed by [Rh(COD)Cl]2/DIOP at —5°C affording 2-t-butyl-l-propanol in 69% ee upon oxidation. In the case of 2-phenylpropene, [Rh(C2H4)2Cl]2 combines with either DIOP or BINAP to provide higher enantiomeric excesses than with [Rh(COD)Cl]2.46... 

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