Stilbenes hydration

As discussed in Section 10.3, the system consisting of a diazonium ion and cuprous ions can be used for hydroxy-de-diazoniation at room temperature in the presence of large concentrations of hydrated cupric ions (Cohen et al., 1977 see Schemes 10-7 to 10-9). With (Z)-stilbene-2-diazonium tetrafluoroborate under these conditions, however, the major product of ring closure of the initially formed radical was phenanthrene (64%). When the cupric nitrate was supplemented by silver nitrate the yield increased to 86% phenanthrene. Apparently, the radical undergoes such rapid ring closure that no electron transfer to the cupric ion takes place. 

Stilbenes that are used as fluorescent whitening agents are photolytically degraded by reactions involving cis-trans isomerization followed by hydration of the double bond, or oxidative fission of the double bond to yield aldehydes (Kramer et al. 1996). 

Thus for these reactions m is necessarily less than unity, a result that has now been widely observed in practice,117,118,120,161,180,181 and thus the m1 value offers a clear distinction between the A1 and A-SE2 mechanisms, which is not the case with the H0 correlations discussed above. A number of different excess acidity plots according to equation (56), covering a wide reactivity range, are shown in Fig. 9. These are for the hydration of oc-methylstyrene,120 equation (58), and the mechanistically similar hydration of phenylacetylene 118 for the isomerization of m-stilbene 120 and for the detritiation of tritiated benzene, equation (28) above."7 As can be seen, all four plots are good straight lines the references cited may be consulted for the details. The slopes look steep, but m values for carbon protonation approximate 1.8,36 and the nfi values are all calculated to be... 

Fig. 9 Excess acidity plot against X for some A-SE2 reactions at 25°C a-methylstyrene hydration (open circles) 120,182 phenylacetylene hydration (open and closed squares) 118,160,183,184 cw-stilbene isomerization (closed circles) 120,185 and tritiated benzene T-exchange (open and closed triangles).117,147,186,187...

The hydration of stilbene oxide also demonstrates unusual stereochemical characteristics [175][182][183]. Indeed, stilbene oxide exists as two /ranx-stilbene oxide enantiomers ((R,R) and (S,S)-10.121) and as the meso,... 

Fig. 10.28. The stereochemistry of enzymatic hydrolysis of stilbene oxide. The achiral (meso) r/.v-sli Ibene oxide (10.7) yields only one of the two enantiomers of IhreoA,2-diphcnylethane-1,2-diol (10.122). In contrast, the chiral trans-stilbene oxide (10.121) is hydrated exclusively to meso-l -diphenylethane-l -diol [182][183]. 

Resistance to hydration was elucidated with tricyclic model compounds that lack the side chain and, hence, pharmacological activity. In this context, a useful comparison has been made between two meso compounds, namely 5W-dibenz.oja, dIcycloheplene 10,11-oxide (10.130, X = CH2) and d.v-slilbcnc oxide (10.7) [195]. The former compound proved to be a very poor substrate for rabbit liver microsomal EH, with a Km value comparable to that of cis-stilbene oxide, but Emax ca. 100-fold lower. This indicates that the two compounds have a comparable affinity for the enzyme, but that nucleophilic attack in the catalytic step is much less efficient for dibcnzo[ // cycloheplcnc 10,11-oxide than for d.v-slilbcnc oxide. This implies that the former compound acts better as an inhibitor than as a substrate of microsomal EH. Furthermore, there was also a fundamental steric difference in the reaction course of the two substrates, since the predominant stereoisomer formed from dibenzo //]cyclohep(ene 10,11-oxide had the (I OS, 11. -configuration,... 

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 ... 

Styrene and 1-hexene have been selectively hydrogenated as well as substituted acetylenes, alkyne diols, stilbene and other unsaturated hydrocarbons with these palladium montmorillonites. A size selectivity was invoked to explain the enhanced hydrogenation activity of certain clay catalysts presumably due to the differences in interlamellar spacings of the clay which will depend on degree of hydration, concentration of Pd(II) complex, dielectric constant of the solvent used to disperse the reactants and other factors. 

In addition to the complexes of platinum(II), a number of olefin complexes of platinum(O) have been prepared. Chatt, Shaw, and Williams (103) have described the complex (olefin)bis(triphenylphosphine)-platinum(O) prepared by reaction of the olefin at 60°C with cis-dichloro-bis(triphenylphosphine)platinum(Il) in ethyl alcohol containing hydrazine hydrate. The complex was formed only with ethylene complex (C2H4)Pt[P(CgH5)3]2 has been prepared (121) by reduction of the corresponding oxygen complex with sodium borohydride in the presence of the olefin. 

A, while the latter consist of octahedral molecules, (d). The compounds studied were the 3,5-dimethylpyridine compound, (c), and the 3,4-dimethyl-pyridine compound, (d). The compounds containing the bidentate ligand meso-stilbene diamine, CgHs. CH(NH2)—CH(NH2). C Hs, are still more interesting. The blue and yellow forms are interconvertible in solution, and while the hydrated blue form (/n = 3-16 BM) of the dichloroacetate contains octahedral ions, (e), the yellow form (ju = 2-58 BM) contains planar ions of type (f) and octahedral... 

A final comment is merited on the influence of substituents on the correlation of the rate coefficient with Hammett acidity functions . For the series of substituted m-stilbenes the response of rate coefficients of hydration to // varied markedly and slopes (log/c against —Ho) ranging from 0.88 to 1.27 were observed. The change in electron density, in going from reactant to transition state, at the heteroatom centres of the substituents obviously gives rise to further possible variations in activity coefficient behaviour. 

The metabolism of stilbenes has been studied in the rat and in vitro with rat liver microsomes. Aromatic ring hydroxylation and reduction of the double bond are known to occur (35, 36). The following sequence has been established (37) for the vitro metabolism of trans-stilbene epoxidation of the internal double bond, hydration of the epoxide to the diol which can be oxidized to the benzoin and this in turn can be oxidatively cleaved to give the benzoic acid. 

Pyridones carrying an N-alkenyl substituent such as 94 photodimerize (Figure 103.1) and photoi-somerize (Figure 103.5), but in water the alkene undergoes hydration to give 149. Performance of this reaction in the presence of perchloric acid leads to an isolable intermediate 148. When the alkene is a styrene, (E),(Z)-isomerization and stilbene-hke cyclization reactions are also observed. ... 

---