Olefins structure-reactivity relationship

Ni catalysts for olefin polymerization incorporating a-iminocarboxamide ligands are activated by the formation of borane-carbonyl adducts (153).542 Structure/reactivity relationships are similar to Brookhart s dimine catalysts. 

Hickinbottom and co-workers have reported several instance in which olefins were converted into epoxides with chromic oxide in ocetir anhydride. 76 42 776-718 Rarely were the desired epoxides the only products formed, however. Two or more carbonyl compounds were usually produced as well. A few illustrations will suffice to demonstrate the subtleties of the structure-reactivity relationship for this reaction. 

T. Inui, "Structure-Reactivity Relationship in Methanol to Olefin Conversion on Various Microporous Crystalline Catalysts," paper presented at... 

Structure-Reactivity Relationship of Olefins. The relative reactivity of a series of olefins toward the potent oxidizing species, X, formed by the interaction of TPP Mn(II) with 02, was investigated by means of a competitive reaction technique. As shown in Table VII, the relative reactivity of an olefin, as followed by gas-liquid chromatographic determination, increases on introduction of an alkyl substituent onto the olefinic carbon atom other than the reacting carbon atom. However, the introduction of an alkyl substituent onto the reacting carbon atom reduces (or compensates) the accelerative electronic effect, as seen in the comparison between cyclohexene and n-hexene. This situation becomes clearer if one compares the two dialkyl ethylenes, cyclohexene and methylenecyclohexane, where the former has a single substituent on the reacting carbon and the other has none the observed relative reactivity is 1 27.2. 

Phosphorus-based olefinations continue to be the most widely used methods for the synthesis of alkenes. An understanding of most facets of the mechanism of the Wittig reaction seems to have been achieved and this has been summarised in a substantive review. Some of the principles established in these mechanistic studies can be applied to phosphorus-based olefinations other than the Wittig reaction. However, substantive mechanistic studies of phosphine oxide-based and phosphonate-based oiefinations are urgently required. A combination of the variety of phosphorus-based methods and the improved understanding of their mechanisms now aliows a substantiai degree of controi of both reactivity and stereochemistry in olefin synthesis. However, studies are required of the applications of established structure-reactivity relationships in ylides and of the various carbon and nitrogen ylide-anions recently reported. 

So far we have proposed a refined catalytic reaction cycle for the co-oUgomer-ization process and have furthermore analyzed the catalytic structure-reactivity relationships for the regulation of the Cm-olefin product selectivity. This, together with our recent theoretical-mechanistic exploration of the [Ni ]-catalyzed 1,3-butadiene cyclooUgomerization affording Cij-cycloolefins [4c,d], enables us now to undertake a comparison of crucial mechanistic aspects of the two reactions. Furthermore, the interplay of the two alternative reaction channels for co-oligomerization and cyclooligomerization wiU be analyzed (Scheme 4). 

This leads us to propose a theoretically verified, refined catalytic cycle for production of linear and cycHc CiQ-olefin products (cf. Scheme 3). Furthermore, a detailed comparison of crucial mechanistic aspects of the catalytic reaction course for co-oligomerization of butadiene and ethylene and for cyclooligomerization of butadiene promoted by zerovalent bare nickel complexes was undertaken. These contribute (first) to a more detailed understan(fing of mechanistic aspects of the [Ni"]-mediated co-oHgomerization of 1,3-dienes and olefins and (second) to a deeper insight into the catalytic structure reactivity relationships in the transition-metal-assisted co-oHgomerization and oligomerization reactions of 1,3-dienes. 

A new structure-activity relationship, Xjj =yS+l, where y is a negative constant, S is the total steric effect, and 4 is the total inductive effect, correlated strongly with available measurements of ozonolysis. New rate coefficients were measured for ozonolysis of a number of unsaturated heteroatomic compounds and it has been emphasized that the inductive effect rather than the steric effect is important in predicting their reactivity %, the inductive effect index, was compared with the Taft a constant and rates of reaction of hydroxyl radical with a given species it correlated strongly in both cases (which should be unaffected by steric factors) suggesting a universal response by olefinic species towards electrophilic addition. ... 

Methyl 4-[2-(ethylthiocarbonyl)ethenyl]cinnamate (3 SMe) crystallizes into a typical a-translation-type packing structure in which the distances between the ethylenic double bonds are 3.988 A and 4.067 A, respectively. However, the 3 SMe crystal is entirely photostable even though it should be photoreactive based on the topochemical rule (Sukegawa, 1991). Several examples of exceptionally photostable diolefin crystals have been found in compounds having a thioester moiety. Such anomalous behaviour of crystals such as 2 OMe and 3 SMe cannot be explained simply in terms of the topochemical rule since this rule involves only the positional relationship between the reactive olefin pair. 

The investigation of the mechanism of olefin oxidation over oxide catalysts has paralleled catalyst development work, but with somewhat less success. Despite extensive efforts in this area which have been recently reviewed by several authors (9-13), there continues to be a good deal of uncertainty concerning the structure of the reactive intermediates, the nature of the active sites, and the relationship of catalyst structure with catalytic activity and selectivity. Some of this uncertainty is due to the fact that comparisons between various studies are frequently difficult to make because of the use of ill-defined catalysts or different catalytic systems, different reaction conditions, or different reactor designs. Thus, rather than reviewing the broader area of selective oxidation of hydrocarbons, this review will attempt to focus on a single aspect of selective hydrocarbon oxidation, the selective oxidation of propylene to acrolein, with the following questions in mind ... 

The relationship between the structure of olefins and their reactivities in hydrogenation as described above is complicated by the double-bond migration and the cis-trans isomerization that may accompany the hydrogenation. 

Since the rate of enolisation generally determines the rate of bromination of a ketone [i2g], it should be possible to correlate bromination rates for steroid ketones with their rates of enolisation. These in turn should be related to the thermodynamic stabilities of the respective olefinic bonds, insofar as transition states for enolisation under acidic conditions resemble the structures of the enols (p. 154). The very limited kinetic data available [133,134] confirm this relationship for three 3a-cholestanones whose reactivity falls in the order C(3) > C(e> > C(7). This would reasonably be predicted from consideration of strain associated with the A -, A -, and A -double bonds, coupled with steric hindrance to the transition... 

The relationship between the structure and the reactivity of the ketones has been studied [4cj. When 3-acetylthiophene was used in the coupling reaction, the alkylation took place only at 2-position (Eq. 9.2). In the cases of reactions of the ketones, shown in Scheme 9.1, no coupling product was obtained. Based on these results, Murai proposed that the a,/3-conjugate enone framework is important in the C-H/ olefin coupling reaction. 

This study is a comprehensive review of data reported on the effect of the composition of the reaction mixture on the hydrogenation of olefinic reactants in the liquid phase. It is mainly based on papers published by the authors, which deal with the effect of the structure of the reacting compounds on their reactivity and adsorptivity on hydrogenation catalysts, and with the effect of solvents on hydrogenation in the liquid phase. The majority of these studies were carried out with a view to quantify the particular effects, with the utilization of the LFER (linear free energy relationship) method. On the one hand, new possibilities for the application of these relationships appeared, but on the other, a number of limiting factors were found, connected predominantly with the considerably complex character of the systems involved in catalytic hydrogenation in the liquid phase. 

It was thus early recognized that structural deformations of double bonds are related to instability and enhanced reactivity. It is this relationship that induced extensive studies of strained carbon compounds and distorted olefins. Moreover, it is evident that even today, the inherent chemical information potential of strained olefins has only been partially explored (4-6). 

In 1972, Mock considered double-bond reactivity and its relationship to torsional strain, by which he understood the strain imposed on a double bond in medium-ring fra 5-cycloalkenes or by steric compression of large cis substituents [28]. He argued that the loss of 7t overlap due to a torsion about the double bond can be partially compensated by rehybridization in these two situations, leading, respectively, to syn and anti pyramidalization of the double bond consequently, such bonds will favor different modes of addition (cis and trans). The proposition was supported by examples of X-ray structures of strained olefins, STO-3G energy calculations for the twisted and pyramidalized ethylene geometries, and by analysis of the out-of-plane vibrational frequencies of ethylene. Mock concluded that small ground-state distortions may produce sizable effects in the transition states. 

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