Olefin structures reaction mechanisms

Although the actual reaction mechanism of hydrosilation is not very clear, it is very well established that the important variables include the catalyst type and concentration, structure of the olefinic compound, reaction temperature and the solvent. used 1,4, J). Chloroplatinic acid (H2PtCl6 6 H20) is the most frequently used catalyst, usually in the form of a solution in isopropyl alcohol mixed with a polar solvent, such as diglyme or tetrahydrofuran S2). Other catalysts include rhodium, palladium, ruthenium, nickel and cobalt complexes as well as various organic peroxides, UV and y radiation. The efficiency of the catalyst used usually depends on many factors, including ligands on the platinum, the type and nature of the silane (or siloxane) and the olefinic compound used. For example in the chloroplatinic acid catalyzed hydrosilation of olefinic compounds, the reactivity is often observed to be proportional to the electron density on the alkene. Steric hindrance usually decreases the rate of... 

Computation allows one to circumvent nature s reluctance to offer the dihydride to direct detection. The first papers using molecular mechanics to study asymmetric hydrogenation appeared in the late 80 s [53-55], However, molecular mechanics is not the ideal technique for any reaction that involves bond-breaking or bond-forming, such as all catalytic reactions, and only a limited amount of reliable information was obtained from these early studies. An MP2/QC/5IXT) study of (PH3)2Rh(olefin) structures was published in... 

The examples of ex situ steady-state X-ray photodiffraction utihzed to follow the photodimerizations of olefin bonds in a single-crystal-to-single-crystal (or nearly so) manner are ubiquitous in the chemical literature. The interest of sohd-state chemists in this reaction dates back to the work of Cohen and Schmidt [30, 31], and it has become much of a guinea pig in organic solid-state photochemistry. In 1993, Enkelmann and collaborators published two seminal papers in the Journal of the American Chemical Society [32] and in Angewandte Chemie [33], where they presented a series of structures of a-tra s-cinnamic acid crystals reacted to various extents. These reports laid the way for a plethora of later studies on the olefin photodimerization reaction. The convenience of the high conversion and the simple mechanism, combined with the relatively small structural perturbation that it requires, has turned this reaction into a very useful tool to probe intermolecular... 

The conversion of methanol to hydrocarbons (MTHC) on acidic zeolites is of industrial interest for the production of gasoline or light olefins (see also Section X). Upon adsorption and conversion of methanol on calcined zeolites in the H-form, various adsorbate complexes are formed on the catalyst surface. Identification of these surface complexes significantly improves the understanding of the reaction mechanism. As demonstrated in Table 3, methanol, dimethyl ether (DME), and methoxy groups influence in a characteristic manner the quadrupole parameters of the framework Al atoms in the local structure of bridging OH groups. NMR spectroscopy of these framework atoms under reaction conditions, therefore, helps to identify the nature of surface complexes formed. 

This mechanism is similar to the olefin metathesis reaction. When the molecule structure permits formation of 1,3-diadsorbed species, this reaction can occur via Ti-allyI adsorbed complexes.271... 

Investigations into the scheelite-type catalyst gave much valuable information on the reaction mechanisms of the allylic oxidations of olefin and catalyst design. However, in spite of their high specific activity and selectivity, catalyst systems with scheelite structure have disappeared from the commercial plants for the oxidation and ammoxidation of propylene. This may be attributable to their moderate catalytic activity owing to lower specific surface area compared to the multicomponent bismuth molybdate catalyst having multiphase structure. 

T he epoxidation of olefins using organic hydroperoxides has been studied in detail in this laboratory for a number of years. This general reaction has also recently been reported by other workers (6,7). We now report on the effects of five reaction variables and propose a mechanism for this reaction. The variables are catalyst, solvent, temperature, olefin structure, and hydroperoxide structure. Besides these variables, the effect of oxygen and carbon monoxide, the stereochemistry, and the kinetics were studied. This work allows us to postulate a possible mechanism for the reaction. 

Organometallic compounds asymmetric catalysis, 11, 255 chiral auxiliaries, 266 enantioselectivity, 255 see also specific compounds Organozinc chemistry, 260 amino alcohols, 261, 355 chirality amplification, 273 efficiency origins, 273 ligand acceleration, 260 molecular structures, 276 reaction mechanism, 269 transition state models, 264 turnover-limiting step, 271 Orthohydroxylation, naphthol, 230 Osmium, olefin dihydroxylation, 150 Oxametallacycle intermediates, 150, 152 Oxazaborolidines, 134 Oxazoline, 356 Oxidation amines, 155 olefins, 137, 150 reduction, 5 sulfides, 155 Oxidative addition, 5 amine isomerization, 111 hydrogen molecule, 16 Oxidative dimerization, chiral phenols, 287 Oximes, borane reduction, 135 Oxindole alkylation, 338 Oxiranes, enantioselective synthesis, 137, 289, 326, 333, 349, 361 Oxonium polymerization, 332 Oxo process, 162 Oxovanadium complexes, 220 Oxygenation, C—H bonds, 149... 

The aziridination works for both aromatic and aliphatic olefins, including less active linear terminal olefins. Most reactions proceed in good yield at room temperature. The use of ci.v-stilbene at 0°C gives predominately cis aziridine product in about 90 10 cis trails ratio (Table 6.1). The conservation of cis structure suggests that a discrete silver nitrene intermediate is involved in the reaction path. Because of the unique disilver structure and unlikely formation of a silver(III) species, the authors suspect that a bridged nitrene intermediate between the two silver atoms may be responsible for this transformation in which each silver atom donates one electron to the nitrenoid. However, further research is necessary to prove this hypothesis and a fast radical reaction mechanism cannot be eliminated on the basis of current evidence. 

The skeletal isomerization of C4 and C5 n-olefins is an acid-catalyzed reaction requiring relatively strong acid sites that proceeds via carbenium ion intermediates formed upon protonation of the double bond (17). Double bond cis-trans isomerization usually occurs on the acid sites before skeletal isomerization. The general reaction mechanism for branching isomerization is depicted in Fig. 2 2. Protonation of the double bond leads to a secondary carbenium ion, which then rearranges into a protonated cyclopropane (PCP) structure. In the case of n-butenes,... 

Now, the question arises—which is the correct structure, II or III A final choice with 100% certainty between II and III cannot be made on the basis of a reaction mechanism alone, because both pathways go through rough terrain. However, the lack of precedent for step A of Scheme 55.3, the actual existence of bridgehead olefins, and a great deal of prejudice on our part, caused by our knowing beforehand that III is the correct structure, make us favor Scheme 55.4 as the more reasonable mechanistic interpretation. 

Structures I and II are reported as the main contributing forms. Meinwald (5) used these to explain the addition of ozone to certain olefins. A similar mechanism can be postulated to explain the reaction of ozone on tertiary amines ... 

Product formation was elucidated by closer examination of the reaction mechanism. The reason for the unavailability, for decades, of more pertinent data was the instability of the reactive intermediates and the lack of suitable precursors for isolable intermediate catalyst species. Mechanistic considerations had to explain the question of the stereoselectivity and to give a valid concept for the dependence between catalyst-olefin intermediate structures and product formation. 

On the basis of the structures of the three isolated important intermediates 119-121, we proposed a reaction mechanism for the production of lactone 114 (Scheme 41). In this mechanism, catalytic amounts of water or TfOH would play an important role. Oxonium 120 may be obtained by an intramolecular aldol reaction of the intermediate 122. Compound 122 may be derived from olefin 119 by acidic hydration, subsequent fragmentation, and isomerization. Oxonium 120 may undergo intramolecular cyclization to give acetal 123 then fragmentation... 

FIGURE 11.21 Reaction mechanism for oxidation of an olefin structure with chlorine dioxide. Lignin end-groups,... 

Two reaction mechanisms have been proposed for these dihydroxylations (pathway a or b, Figure 7.23), either a concerted [3+2] cycloaddition of the olefins on osmium-diamine complex 7.33 or a stepwise reversible [2+2] cycloaddition followed by a rearrangement [559,1350], An X-ray crystal structure of the resulting osmic ester 2.89A shows its symmetrical structure. Houk s calculations [1351] are in favor of a concerted reaction, and his transition state model is reactant-like, with steric interactions dictating the face selectivity of osmylation. 

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