Olefin structures mechanisms

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... 

Allylic hydroperoxides are primary products in the autoxidation of - olefins, and lack of definite information on their reactivity and chemical behavior has hampered efforts to understand olefin oxidation mechanisms (2). This deficiency is most strongly felt in determining the relative rates of addition and abstraction mechanisms for acyclic olefins since assignment of secondary reaction products to the correct primary source is required. Whereas generalizations about the effect of structure on the course of hydroperoxide decompositions are helpful, most questions can be answered better by directly isolating the hydroperoxides involved and observing the products formed by decomposition of the pure compounds. 

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. 

The di-f-butylhydroxylamine and di-/-butylhydroxylamine ethers probably result from DTBN scavenging of radicals produced by hydrogen atom abstraction from the olefins by excited 3-ethoxyisoindolenone (50). The observed destruction of DTBN as a function of olefin structure is consistent with this mechanism. Based upon allyl radical stability and the statistical factor, excited 3-ethoxyisoindolenone should abstract hydrogen atoms more rapidly from tetraroethylethylene than from ds-2-butene. 

The observations (29) on the homogeneous reduction of coordinated olefins confirms the possibility of mechanism (1). It seems unlikely that TT-allyl-adsorbed olefin [Structure (C)] will pass directly into an alkyl radical without either (A) or (B) intervening as in mechanism (1) or (2), although the process... 

In a wide variety of cases, application of this model allows the desired cross product to be generated in high yield. In order to generate the product as a single stereoisomer, an understanding of the relationship between catalyst structure in Ru-based catalysts and cis-trans selectivity is necessary. This requires a closer examination of the olefin metathesis mechanism. 

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

Bowmer and O Donnell studied the degradation of a variety of poly (olefin sulfone) s with different olefin structures (2,3,9-11). The predominant process was C-S bond scission producing free radical and cationic fragments followed by depropagation. A general mechanism was presented which involved (1) depropagation via both radical and cationic species, (2) oligomerization of free... 

This mechanism involves cis insertion of the olefin into the Pd—OH bond. Since the reaction rate has only a slight dependence on the olefin structure, it has been suggested that the transition state has little carbonium ion character, and the tt-ct rearrangement may proceed via a concerted, nonpolar, four-center transition state (32) (13). 

Olefins are more reactive with oxygen than are paraffins, and therefore olefins can be converted over milder catalysts and with much better selectivity. The main points to be considered here are the influence of olefin structure on the reactions, the influence of catalyst composition, and evidence regarding mechanisms. Since the catalysts are of paramount importance in obtaining selective oxidation reactions, we have subdivided the material according to catalyst types. The types are defined by the nature of the products obtained. Under each catalyst type the influence of olefin structure will be discussed to some extent. Therefore some preliminary remarks about the reactivity of olefins are in order. 

As mentioned above, hydroformylation reactions occur under atmospheric pressure at normal temperature with stoichiometric amounts of cobalt carbonyls. However, with catalytic amounts of cobalt catalysts a minimum CO partial pressure is necessary for reformation and stability of Co2(CO)8, or HCo(CO)4, as the case may be (see page 15). A small increase of the CO partial pressure above this value first results in an increase of the reaction velocity until a maximum is reached depending on temperature and olefin structure. However, further increase of the CO-partial pressure causes a decrease in the reaction velocity [38, 40, 120], (see also section on reaction mechanism). 

In the course of the polymerization, the monomer insertion may lec a either to a or to an n -allyl oonplex. In the first case, several mononner insertions may take place before going back to the stable dormant syn n -allyl isomer of the growing chain-end (17). In the latter case, only the formation of a very reactive anti n -adLlyl structure will ensure the formation of the cis-1,4-polybutadiene often observed es jerimentally such a stereoselectivity would inply the absence of any rapid anti-syn isomerization, in accordance with our WR studies on ANiUA (see above), and with the SurCollette diene-olefin dimerization mechanism (29). It is difficult to decide v ch one of these two possible pathways is actually followed, but v tever it can be, the genercLL schonne will not be changed. 

The reactivities of the substrate and the nucleophilic reagent change vyhen fluorine atoms are introduced into their structures This perturbation becomes more impor tant when the number of atoms of this element increases A striking example is the reactivity of alkyl halides S l and mechanisms operate when few fluorine atoms are incorporated in the aliphatic chain, but perfluoroalkyl halides are usually resistant to these classical processes However, formal substitution at carbon can arise from other mecharasms For example nucleophilic attack at chlorine, bromine, or iodine (halogenophilic reaction, occurring either by a direct electron-pair transfer or by two successive one-electron transfers) gives carbanions These intermediates can then decompose to carbenes or olefins, which react further (see equations 15 and 47) Single-electron transfer (SET) from the nucleophile to the halide can produce intermediate radicals that react by an SrnI process (see equation 57) When these chain mechanisms can occur, they allow reactions that were previously unknown Perfluoroalkylation, which used to be very rare, can now be accomplished by new methods (see for example equations 48-56, 65-70, 79, 107-108, 110, 113-135, 138-141, and 145-146)... 

The mechanism for such a process was explained in terms of a structure as depicted in Figure 6.5. The allylic alcohol and the alkyl hydroperoxide are incorporated into the vanadium coordination sphere and the oxygen transfer from the peroxide to the olefin takes place in an intramolecular fashion (as described above for titanium tartrate catalyst) [30, 32]. 

Like the examples above, dihydroxyacetanilide epoxidase (DHAE) uses an olefin as the substrate for epoxidation. Its mechanism, however, is fundamentally different from those of cytochrome P450 or flavin-dependent enzymes. Dihydroxyacetanilide is an intermediate in the biosynthesis of the epoxyquinones LL-C10037a, an antitumor agent produced by the actinomycete Streptomyces LL-C10037 [75, 76], and MM14201, an antibiotic produced by Streptomyces MPP 3051 (Scheme 10.20) [77]. The main structural difference between the two antibiotics lies in the opposite stereochemistry of the oxirane ring. 

The reaction of methyl acrylate and acrylonitrile with pentacarbonyl[(iV,iV -di-methylamino)methylene] chromium generates trisubstituted cyclopentanes through a formal [2S+2S+1C] cycloaddition reaction, where two molecules of the olefin and one molecule of the carbene complex have been incorporated into the structure of the cyclopentane [17b] (Scheme 73). The mechanism of this reaction implies a double insertion of two molecules of the olefin into the carbene complex followed by a reductive elimination. 

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... 

What concerns us here are three topics addressing the fates of bromonium ions in solution and details of the mechanism for the addition reaction. In what follows, we will discuss the x-ray structure of the world s only known stable bromonium ion, that of adamantylideneadamantane, (Ad-Ad, 1) and show that it is capable of an extremely rapid degenerate transfer of Br+ in solution to an acceptor olefin. Second, we will discuss the use of secondary a-deuterium kinetic isotope effects (DKie) in mechanistic studies of the addition of Br2 to various deuterated cyclohexenes 2,2. Finally, we will explore the possibility of whether a bromonium ion, generated in solution from the solvolysis of traAU -2-bromo-l-[(trifluoromethanesulfonyl)oxy]cyclohexane 4, can be captured by Br on the Br+ of the bromonium ion, thereby generating olefin and Br2. This process would be... 

The present results show that the first step of the interaction between olefins and Bt2 is the formation of CTCs, whose Kf are highly sensitive to structural effects. Both Kf ratios and reactivity ratios of olefins are scarcely affected by the solvent. An increase by two in number of alkyl substituents on the double bond increases both Kf and kobsd roughly by a factor of 103. Therefore, at variance with the expectation for an AdgCl mechanism, substituent effects are not much more influential on k tsd than on Kf. This suggests that the rates of CTC ionization be actually reduced by reversal. 

Selective oxidation and ammoxldatlon of propylene over bismuth molybdate catalysts occur by a redox mechanism whereby lattice oxygen (or Isoelectronlc NH) Is Inserted Into an allyllc Intermediate, formed via or-H abstraction from the olefin. The resulting anion vacancies are eventually filled by lattice oxygen which originates from gaseous oxygen dlssoclatlvely chemisorbed at surface sites which are spatially and structurally distinct from the sites of olefin oxidation. Mechanistic details about the... 

Nickel(O) reacts with the olefin to form a nickel(0)-olefin complex, which can also coordinate the alkyl aluminum compound via a multicenter bond between the nickel, the aluminum and the a carbon atom of the trialkylaluminum. In a concerted reaction the aluminum and the hydride are transferred to the olefin. In this mechanistic hypothesis the nickel thus mostly serves as a template to bring the olefin and the aluminum compound into close proximity. No free Al-H or Ni-H species is ever formed in the course of the reaction. The adduct of an amine-stabihzed dimethylaluminum hydride and (cyclododecatriene)nickel, whose structure was determined by X-ray crystallography, was considered to serve as a model for this type of mechanism since it shows the hydride bridging the aluminum and alkene-coordinated nickel center [31]. 

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