Substituent effect methyl substituted olefin

The survey ends with three examples where the information from one or both effects solved the problem. First, Raman spectra are shown to be helpful in identifying the presence of carbocyclic ring systems in terpene samples (XI, 1). Second, the application of both infrared and Raman spectra of a series of dimer reaction products unambiguously identified the stereochemistry of the product (XI, 2). Third, an examination of the infrared and Raman spectra of methyl substituted olefins can identify the presence of methyl substituents directly substituted on the double bond carbon atoms (XI, 3). 

Previous work has shown that the electronic characteristics of the benzene substituent in triarylphosphine chlororhodium complexes have a marked influence on the rate of olefin hydrogenation catalyzed by these compounds. Thus, in the hydrogenation of cyclohexene using L3RhCl the rate decreased as L = tri-p-methoxyphenylphosphine > triphenylphosphine > tri-p-fluorophenylphosphine (14). In the hydrogenation of 1-hexene with catalysts prepared by treating dicyclooctene rhodium chloride with 2.2-2.5 equivalents of substituted triarylphosphines, the substituent effect on the rate was p-methoxy > p-methyl >> p-chloro (15). No mention could be found of any product stereochemistry studies using this type of catalyst. 

The concerted nature of the reaction is indicated by the fact that a Markovnikoff-type directing effect is essentially absent. Secondary and tertiary products are formed in nearly equal amounts from unsymmetri-cally substituted olefins (9, 13, 14, 15, 17, 27, 34). In addition, substituents on the phenyl group of trimethylstyrene exert no effect on the product distributions (12). Furthermore, values for 2-methyl-2-pentene show only a very small solvent effect (12). All these facts suggest that little polarity is developed at the transition state and are consistent with a concerted reaction. 

Maleic anhydride and its methyl derivatives can dimerise to cyclobutane rings under direct irradiation or in the presence of benzophenone as sensitiser, in benzene ordioxane as solvents. The rate of dimer production is little affected by methyl substitution (a slight decrease is reported) in the sensitised reaction, but more strongly in the unsensitised process In analogous sensitised 1,2-cycloadditions of dimethylmaleic anhydride to several olefins, effects of substituents are also limited, as appears from the few examples below ". 

Huisgen has also studied the effects of substitution in the keten in the reactions of a series of alkylphenyl ketens to ethyl cis- and trans-propenyl ethers. With the cis-enol ethers the thermodynamically less stable cyclobutanone is always produced. This is the same result as that found in the addition of ketens to cyclopentadiene and other cis-olefins, and the mechanistic implications are the same. With the trans-enol ether, the thermodynamically more stable product is formed, and this observation can be rationalized in terms of a [tc2 + k2 J cycloaddition if the preferred orientation complex has the substituent on the keten between the alkoxy-group and a hydrogen rather than between a methyl group and a hydrogen on the enol ether. In all the cases studied, the cis-enol ether reacted more rapidly than its trans-isomer. This cis trans reactivity ratio is not found in [2 + 2] additions proceeding via zwitterionic intermediates. For example, the rate ratio for the reaction of TCNE with cis- and trans-1-alkenyl ethers is very close to unity. 

If steric hindrance is important we should see evidence for this in the effect substitutents in the olefin have on the orientation ratio (see Table 14). The first feature to notice about Table 14 is that the methyl and trifluoromethyl substituent groups have similar directive effects towards the three electrophilic radicals. This is in sharp contrast to electrophilic ionic addition where, for... 

Nitroolefins combine with aminopyridines in the presence of a tetrabu-tylammonium iodide catalyst to form imidazo[l, 2-a]pyridines (Scheme 40). Steric hindrance in the form of a methyl group at C-6 on the aminopyridine decreased the reaction yield. The electron-withdrawing effects of chloride on the aminopyridine also slowed the reaction down. Ort/io-substituents on the aryl group of the nitro olefin also significandy decreased the reaction yield. The use of a heterocyclic substituted nitro olefin worked well in this reaction (14SL718). 

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