Effect of olefin substituents

Some unconjugated two X 2-electron olefins isomerize when they react with a metal complex forming complexes containing 4-electron, conjugated olefin ligands. Presumably in these cases the conjugated olefin forms the 

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A high level of enantioselectivity in the rearrangement of an acyclic system has been reported with tricarbonylchromium(O) complexes of allyl benzyl ethers, using chiral lithium amide base 21 [13] [Eq. (8)]. Upon treatment with 1.1 equiv. of lithium amide 21 and 1 equiv. of LiCl at -78 to -50°C, ether 22 afforded the rearrangement product R)-23 in 80% yield with 96% ee. The effect of olefinic substituents on the chemical yields and enantioselectivity of this [2,3]-Wittig rearrangement was also studied [see Table 3, Sect. 1.3.3]. 

The direct reaction is first-order in the concentrations of the metal complex and the olefin and the second-order rate constant depends on the nature of olefin, as indicated by some of the data in Table S.9. Strain in the olefin appears to increase its reactivity, as shown for norbomene and cyclopentene. There may be some steric effects of olefin substituents, but these effects may be attenuated by better electron donation from the substituents. It was found also that the reaction rate with 2,3-dimethyl-2-butene was insensitive to the polarity of the solvent, with relative values of 1 0.6 0.8 in cyclohexane. Tiff and methanol, respectively. This seems to rule out an ionic or polar transition state or intermediate, and the authors favor a concerted cycloaddition mechanism. 

Styrenes may act as 2n and 4n components of the Diels-Alder reaction depending on the substitution site and the electronic effects of the substituent. Electron-donating groups at the a-carbon of the olefinic double bond enhance the dienic reactivity of styrenes [30]. 

The very small p- and m-values observed for the fast bromination of a-methoxystyrenes deserve comment since they are the smallest found for this electrophilic addition. The rates, almost but not quite diffusion-controlled, are amongst the highest. The sensitivity to polar effects of ring substituents is very attenuated but still significant that to resonance is nil. These unusually low p-values for a reaction leading to a benzylic carbocation are accompanied by a very small sensitivity to the solvent. All these data support a very early transition state for this olefin series. Accordingly, for the still more reactive acetophenone enols, the bromination of which is diffusion-controlled, the usual sensitivity to substituents is annulled. 

Normally, additions depicted by model C lead to the highest asymmetric induction. The antiperiplanar effect of OR substituents can be very efficient in the Houk model B ( , , , , ) however it plays no role in model C. Furthermore, the Houk model B must be considered in all cycloaddition-like reactions. The Felkin-Anh model A is operative for nucleophilic additions other than cuprate additions ( ). The epoxidation reactions are unique as they demonstrate the activation of one diastereoface by a hydroxy group which forms a hydrogen bridge to the reagent ( Henbest phenomenon ). The stereochemical outcome may thus be interpreted in terms of the reactive conformations 1 and 2 where the hydroxy function is perpendicular to the olefinic plane and has an optimal activating effect. 

Effect of Olefin Structure. The reaction rate of the epoxidation depends on olefin structure. In general, the more alkyl substituents bonded to the carbon atoms of the double bond, the faster the reaction rate. This was shown by a reaction of 2-methyl-2-pentene, cyclohexene, and 2-octene with cumene hydroperoxide under the same conditions (Table V). The yield of epoxide was quantitative. The results indicate that 2-methyl-2-pentene reacts faster than cyclohexene and 2-octene. 

According to the rule formulated in [15], the combined a- and /1-effects (of fluorine substituents) imply that fluoroolefins will react with electrophiles so as to minimize the number of fluorines f to electron-deficient carbon in the transition state. In accordance with this rule, reaction of CH2=CF2 with HF starts as an attack of electrophile (H+) on the CH2 group of ethylene (Eq. 32, pathway A), since this process leads to carbocation 12 stabilized by two a-fluorines in contrast to the much less stable intermediate 13 containing two /1-fluorines and derived from the initial attack of H+ on the CF2 group of the olefin ... 

Although proton chemical shifts are influenced significantly by factors other than electron density, they also reflect the polarization of the enamine framework and the degree of n,n interaction. Thus, the chemical shifts of the vinylic protons are modulated by the same factors discussed for the chemical shifts of the corresponding olefinic carbons, such as amine component, steric and electronic effects of the substituents and ring size effects. In particular, the chemical shift of the proton(s) at C(2) is lowered by increasing njt interaction, in parallel with what has been observed for < C(2). No general correlation exists between the chemical shifts of both nuclei probably as a consequence of their different sensitivity to steric, electronic and, particularly, anisotropic effects of the substituents. Nevertheless, for sets of structurally related compounds, reasonable linear correlations can be found between <5C(2) and <5H(2) (see below). Since the XH-NMR data available for enamines are more abundant than those for 13C and 15N, more complete structural information can be obtained for wider sets of compounds. 

Thus, recent experimental data and calculationsshow the important role of the end of the growing polymer chain on the stereospecificity of the olefin addition. This role is due to the influence of the carbon atom (Cp, C ) of the main chain on the isospecificity of the olefin addition and not to the effect of the substituent of the last monomeric unit as in the case of the syndiospecific addition. The growing polymer chain is one of,the ligand which, together with other ligands of titanium ions, determines the chirality of the active center. 

Several oxovanadium(IV) complexes (141) with ON-donor ligands were prepared from [YOCl2(thf)2] (thf = tetrahydrofuran) and /// /////-substituted phenols to study the electronic effects of para substituents in order to develop better [VCl2(OR)2]-type olefin polymerization catalysts.676 The hyperfine coupling constants, the HOMO-LUMO transitions, and the oxidation potentials were all found to be linearly related to the Hammett a constant of the substituent on the monoanionic aryloxy ring.676... 

Sulfonyl halides, particularly arenesulfonyl halides, can afford radical species much faster than carbon halides by the assistance of a metal complex and efficiently add to olefins with little dimerization of sulfonyl radicals in comparison to carbon-centered radicals. Another feature of the compounds is that there is little effect of the substituents on the rate of addition to an olefin. These properties make sulfonyl halides an efficient and universal series of initiators for the metal-catalyzed living radical polymerizations of various monomers including methacrylates, acrylates, and styrenes (Figure g). 52.175-177... 

None reported (Zr treated as a pseudoatom) Investigate the effects of alkyl substituents on the Cp rings and the olefin substrate 55 ... 

The isomerization mechanism can be estimated from the structure of the compounds on the basis of the above-mentioned effect of the substituent on the mode of isomerization. Furthermore, the triplet lifetime and the efficiency of isomerization could be estimated for some olefinic compounds. 

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