Conjugated aromatic alkenes

Table 9.2 Selective reduction of conjugated aromatic alkenes catalyzed by Pd-PMHS nanocomposites. (Reprinted with the permission of the American Chemical Society [52])...

Thallium(III) acetate reacts with alkenes to give 1,2-diol derivatives (see p. 128) while thallium(III) nitrate leads mostly to rearranged carbonyl compounds via organothallium compounds (E.C. Taylor, 1970, 1976 R.J. Ouelette, 1973 W. Rotermund, 1975 R. Criegee, 1979). Very useful reactions in complex syntheses have been those with olefins and ketones (see p. 136) containing conjugated aromatic substituents, e.g. porphyrins (G. W. Kenner, 1973 K.M. Smith, 1975). 

This synthesis method can be utilised by any alkene or alkyne, but steric hindrance on internal double bonds can cause these reactions to be quite slow. Conjugated dienes and aromatic alkenes are not suited for the ultraviolet light-initiated process. The use of other free-radical initiators is required in free-radical-initiated reactions involving these species. 

Nevertheless, in those cases in which the proportion of hydrate in equilibrium with the aldehyde is low, it is possible to obtain a useful yield of aldehyde.60,61 Electron donating groups,68,69 conjugation with alkenes and aromatic rings5 and steric hindrance69 decrease the proportion of hydrates in equilibrium with aldehydes. This explains the fact that alcohols successfully transformed into aldehydes by Jones oxidation, normally belong to the allyl,70 benzyl71 or neopentyl kind.72... 

This hetero-Pauson-Khand reaction is a complete diastereoselective synthesis of y-butyrolactones from the condensation of an alkene, a carbonyl moiety and CO. In a single process, two carbon-carbon bonds and two rings are constructed. The catalytic variant of this procedure has been also established on conjugated aromatic ketones (Scheme 12.44) [64]. 

C = 0 stretching frequencies are typically lowered by about 20-30 cm 1 from the values given when the carbonyl group is conjugated with an aromatic ring or an alkene group. 

Complexes in Which the Acceptor Is a Metal Ion and the Donor is an Alkene or an Aromatic Ring. Note that n donors do not give EDA complexes with metal ions but form covalent bonds instead. Many metal ions form complexes, which are often stable solids, with alkenes, dienes (usually conjugated, but not always), alkynes, and aromatic rings. The generally accepted picture... 

Saturated hydrocarbons are stable. Only cycloalkanes with a tight ring are unstable. Alkenes and alkynes have a strong endothermic character, especially the first homologues and polyunsaturated conjugated hydrocarbons. This is also true for aromatic compounds, but this thermodynamic approach does not show up their real stability very well. Apart from a few special cases, the decomposition of unsaturated hydrocarbons requires extreme conditions, which are only encountered in the chemical industry. 

Formation of the least-hindered alkylzirconium complex during the course of the hydrozirconation-isomerization process of alkenes can be altered by the presence of aromatic rings, additional conjugative double bonds, chelating heteroatoms [81, 94] or groups that can be lost by elimination [95, 96]. 

Nenajdenko et al. described the first example of addition of a 1,2-dication to C-C mutiple bonds. The only S-S dication found to participate in this reaction was the highly strained dication 115 derived from 1,4-dithiane. The reaction with alkenes 119 proceeded under mild conditions and led to derivatives of dithioniabicyclo[2.2.2]octane 120 as shown in Equation (33) and Table 21 <1998JOC2168>. This reaction was sensitive to steric factors and proceeded only with mono and 1,2-disubstituted ethylenes. Only alkenes conjugated with aromatic or cyclopropane moieties underwent this reaction. For the 1,2-disubstituted alkenes used in this study, the relative configuration of substitutents at the double bond was preserved and only one diastereomer was formed (see entries 2 and 3). 

---