Unsaturated substituent groups

Hydrocarbon chains with free valences at each terminal C atom are frequently still called trimethylene-, tetramethylene-, etc. instead of alkane-l-n>-diyl groups, the systematically correct designation. Unsaturated substituent groups of this kind are named by replacing the terminal syllable. .. ene with. .. enylene. The trivial terms ethylene for -CH2-CH2-, propylene for H3C-CH-CH2-, and vinylene for -CH=CH- are still frequently... 

Unsaturated substituents of dioxolanes 36-38 and dioxanes 39-41 are prone to prototropic isomerization under the reaction conditions. According to IR spectroscopy, the isomer ratio in the reaction mixture depends on the temperature and duration of the experiment. However, in all cases, isomers with terminal acetylenic (36, 39) or allenic (37, 40) groups prevail. An attempt to displace the equilibrium toward the formation of disubstituted acetylene 41 by carrying out the reaction at a higher temperature (140°C) was unsuccessful From the reaction mixture, the diacetal of acetoacetaldehyde 42, formed via addition of propane-1,3-diol to unsaturated substituents of 1,3-dioxanes 39-41, was isolated (74ZOR953). 

The unsaturated substituent in the carbene complex 1 often is aromatic or heteroaromatic, but can also be olefinic. The reaction conditions of the Dotz procedure are mild various functional groups are tolerated. Yields are often high. The use of chromium hexacarbonyl is disadvantageous, since this compound is considered to be carcinogenic however to date it cannot be replaced by a less toxic compound. Of particular interest is the benzo-anellation procedure for the synthesis of anthra-cyclinones, which are potentially cytostatic agents. ... 

A ketone vvith a substituent group in its /3 position might be prepared by a conjugate addition of that group to an a,/3-unsaturated ketone. In the present instance, the target molecule has a propyl substituent on the /3 carbon and might therefore be prepared from 2-methyl-2-cyclopentenoneby reaction with lithium dipropylcopper. 

Maleimides Alkyl and aryl maleimides in small concentrations, e.g., 5-10 wt% significantly enhance yield of cross-link for y-irradiated (in vacuo) NR, cw-l,4-polyisoprene, poly(styrene-co-butadiene) rubber, and polychloroprene rubber. A-phenyhnaleimide and m-phenylene dimaleimide have been found to be most effective. The solubihty of the maleimides in the polymer matrix, reactivity of the double bond and the influence of substituent groups also affect the cross-fink promoting ability of these promoters [82]. The mechanism for the cross-link promotion of maleimides is considered to be the copolymerization of the rubber via its unsaturations with the maleimide molecules initiated by radicals and, in particular, by allyfic radicals produced during the radiolysis of the elastomer. Maleimides have also been found to increase the rate of cross-linking in saturated polymers like PE and poly vinylacetate [33]. 

Stereoisomerism Substituents Parent Unsaturation Functional group... 

The enantioselective hydrogenation of a,p-unsaturated acids or esters, using 5wt% Pt/Al203 or Pd/Al203 commercial catalysts doped with cinchonidine (CD), was deeply investigated to evidence the specific activity of Pd or Pt and the role of the reaction parameters and solvent polarity. Finally, the steric and electronic effects of different substituent groups were also studied. 

A direct connection between the allene group and another unsaturated functional group is no prerequisite for interesting chemical behavior, as illustrated for hydrocarbons 28-31 in Scheme 5.3, in which the allene unit and the above four substituents are connected either by a methylene group or by an ethano bridge, as for allenes 32-35. 

However, dipole stabilization is less significant as an aid to carbanion formation with unsaturated azaheterocycles, since the nitrogen lone-pair electrons are normally incorporated into the 77-electron system of the heterocycle, and are therefore less readily available for donation to the substituent group. In fact, dipole-stabilization can even be a hindrance in some systems where exocyclic ip -carbanion formation is competitive with internal ip -carbanion formation. In these cases it is the heterocycle itself that is responsible for the dipole-stabilization of the external carbanion. 

Typical examples of n >n UV spectra are illustrated in Tables XIII-XVI. The use of absorption spectroscopy for diagnostic purposes is limited. Thus simple 2//-pyrans, as a rule, show more complex and red-shifted absorption curves in comparison to those of similar 4//-pyrans (Tables XIII and XIV). Analogous spectral behavior for simple 2H- and 4H-thiopyrans is perceptible from Tables XV and XVI. The introduction of conjugating aryl groups and other unsaturated substituents causes a significant overlap of original absorption bands with those of the aromatic systems as well as a bathochromic shift of the longest wave absorption maxima. 

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