L-Phenylpropen

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... 

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. ... 

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... 

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). 

Enantioselective osmylotion of alkenes. Osmium tetroxide forms a 1 1 wine-red complex with the chiral diamine l2 that effects efficient enantioselective dihy-droxylation of monosubstituted, oww-disubstituted, and trisubstituted alkenes (83-99% ee) at -110° in THF. The enantioface differentiation in all cases corresponds to that observed with t/mr-3-hexene and the complex with (-)-l. Essentially complete asymmetric induction is observed with frans-l-phenylpropene (99% ee). 

Scheme 6.37 One-pot reactions, starting from 1,3-cyclopentadiene, to prepare the products of the interception of 1,2,4-cyclohexatriene (162) by ( )-l-phenylpropene, furan and 1,3-cyclopentadiene.

Scheme 6.76 Generation of l-oxa-2,3-cyclohexadiene (351) from 5-bromo-3,4-dihydro-2H-pyran (376) and trapping products of351 obtained from furan, 2,3-dimethyl-1,3-butadiene, 1,1-diphenyl-ethylene, ( )-l-phenylpropene, ( )-2-butene, (Z)-2-butene and tert-butyl alcohol , according to Schlosserand co-workers.

Scheme 6.77 Proposal for the mechanisms of the addition of l-oxa-2,3-cyclohexadiene (351) to (Z)-l-phenylpropene and of the thermolysis of the product 378c.

For (E)-l-phenylpropene the e.e. was 48%, but for the (Z) isomer the e.e. was 26% only. This remained a general feature of the reaction cis- alkenes give moderate results (note that (Z)-stilbene would give a non-chiral meso-product). 

SCHEME 90. Epoxidation of Iratts-l-phenylpropene using chiral catalyst 173a... 

Other free carbenes that have been generated in this way and their reactions compared with those from other precursors include bicyclo[2.2.2]octylidene, cyclopropylmethylcarbene, and 1-phenylpropylidine (104). " In the case of 104, the same ratio of (E) to (Z)-l-phenylpropene was produced when the carbene was generated by deoxygenation of the corresponding ketone and by the phenylcar-bene rearrangement (Eq. 60). " Thus, the method represents a viable alternative to... 

The treatment of tran.v-l-phenylpropene and /rans-stilbene with xenon difluoride in dichloromethane catalyzed by trifluoroacetic acid leads to an approximately equimolar mixture of fluorination and fluorotrifluoroacetoxylation products, the latter process proceeds regio-spccifically.34... 

Reaction of AT-fluorobis(trifluoromethylsulfonyl)amine (Id) with alkenes gives various products, depending on the reaction conditions and the structure of the substrate. In solvents of higher nucleophilicity such as water, acetic acid, aqueous hydrochloric acid, and 70 % hydrogen fluoride/pyridine, a-fluorohydrins or their acetates, a-chloro-fl-fluoroalkanes or a,/ -difluoroal-kanes, e.g. 14. are obtained.146 Reaction of styrene and ( >l-phenylpropene with Id in dich-loromethane/acetic acid gives l-acetoxy-2-fluoro-l-phenylethane and -propane, in 92 and 99 % yield, respectively, the latter product in a ratio (erythrojthreo) 1 l.146... 

The use of solvent has also proved to be necessary in the fluorination of 3,3,3-trichloro-l-phenylpropene18 and l-(4-bromophenyl)-3,3,3-trichloropropene19 with antimony(III) fluoride. 3,3,3-Trifluoro-l-phenylpropene and l-(4-bromophenyl)-3,3,3-trifluoropropene are obtained in 80 and 65% yield, respectively, by refluxing the reactants in dioxane for 7 hours. Tarring occurs in the absence of solvent.15... 

Triphenylvinylphosphonium salts are usually used as dipolarophiles against azides and diazoalkanes30. 2,3-Diphenyl-2//-azirine and 1-azido-l-phenylpropene as precursors of 2-methyl-3-phenyl-2H-azirine produce, after irradiation, 2,5-diphenylpyrrole and 2-methyl-5-phenylpyrrole, respectively29. 

The anti-Markownlkov orientation of addition in the presence of electron-acceptor sensitizers applies also to intramolecular reaction, and 5,5-dipheny pent-4-en-1-ol gives a tetrahydrofuran (2.SI) when irradiated in solution with 9,10-dicyanoanthracene, whereas its thermal reaction under proton-acid catalysis leads to 2,2-diphenyltetrahydropyran by Markownikov addition. Sometimes an added sensitizer is not required, if the alkene itself can act as a good electron-donor or electron-acceptor, and this is likely to be the reason why 1-lo-methoxyphenyl)propene adds photochemically to acetic acid (2.52), whereas l-phenylpropene does not. 

The condensation (already mentioned in Section IV,B,2,a of 2-nitro-l-phenylpropene with dimedone leads to 6,7-dihydro-3-phenyl-2,6,6-trimethyl-4(5/f)-benzofuranone (321).705 c. Ichikawa Reaction.731,732 The oxymercurial derivative of propylene... 

Methoxy-l-phenylpropene, AR85 (3 Methoxypheny )propenoic acid,... 

This is not the precise value for the hydrogenation enthalpy as reported by Abboud and coworkers94, 105.9 1.2 kJmol-1, but, as indicated above in the text, a composite value was used to generate the enthalpy of formation of the unsaturated hydrocarbon. As such, the hydrogenation enthalpy employed here was the difference of the suggested enthalpy of formation of ( )-l-phenylpropene from this source and the archival value for propylbenzene. 

This procedure is similar to the previous one, except that the cyclization happens on the carbon at the /3-position of the nitrogen atom. The synthesis of oxazepine 145 started with the reaction of 4,4 -dichloro-2-methyl-3-(2//)-pyridazinone 142 with 3-benzylamino-l-phenylpropen-l-ol 143. The obtained product 144 was cyclized in basic conditions to the desired 1,4-oxazepine 145 (Scheme 20) <1996JHC583>. 

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