Addition reactions, to alkenes

The regioselectivity in radical addition reactions to alkenes in general has successfully been interpreted by a combination of steric and electronic effects1815,47. In the absence of steric effects, regiochemical preferences can readily be explained with FMO theory. The most relevant polyene orbital for the addition of nucleophilic radicals to polyenes will be the LUMO for the addition of electrophilic orbitals it will be the HOMO. Table 10 lists the HOMO and LUMO coefficients (without the phase sign) for the first three members of the polyene family together with those for ethylene as calculated from Hiickel theory and with the AMI semiempirical method48. 

Therefore summarizing, the problem of addition reactions to alkenes is not so simple and is more complicated than what it looks. It depends on so many factors, e.g., the nature of alkene, the addendum and on the reaction conditions. If the addition proceeds through the formation of bridged ions, then trans addition is the rule. But if it involves a classical carbocation, then cis addition is the mechanism. 

We have here a mixture of electrophilic and radical addition reactions to alkenes. Remember the guidelines that radical reactions are characterized by the inclusion of radical initiators, such as light or peroxides. In the absence of such initiators, consider only the alternative electrophilic mechanisms. 

The selective oxidation of C—H bonds in alkanes under mild conditions continues to attract interest from researchers. A new procedure based upon mild generation of perfluoroalkyl radicals from their corresponding anhydrides with either H2O2, m-CPBA, AIBN, or PbEt4 has been described. Oxidation of ethane under the reported conditions furnishes propionic acid and other fluorinated products.79 While some previously reported methods have involved metal-mediated functionalization of alkanes using trifluoroacetic acid/anhydride as solvent, these latter results indicate that the solvent itself without metal catalysis can react as an oxidant. As a consequence, results of these metal-mediated reactions should be treated with caution. The absolute rate constants for H-abstraction from BU3 SnH by perfluorinated w-alkyl radicals have been measured and the trends were found to be qualitatively similar to that of their addition reactions to alkenes.80 a,a-Difluorinated radicals were found to have enhanced reactivities and this was explained as being due to their pyramidal nature while multifluorinated radicals were more reactive still, owing to their electrophilic nature.80... 

Biological organic chemistry takes place in the aqueous medium inside cells rather than in organic solvents, and it involves complex catalysts called enzymes. Nevertheless, the kinds of biological roactions that occur are remarkably nimiUr to laboratory reactions. Thus, there are many cases of biological addition reactions to alkenes. For example, Che enzyme fumarase... 

Another family of addition reactions to alkenes involves a cyclic intermediate or a cyclic transition state. These commonly lead to cis... 

In the remainder of Chapter 12, the oxidation of alkenes, alkynes, and alcohols— three functional groups already introduced in this text—is presented (Figure 12.8). Addition reactions to alkenes and alkynes that increase the number of C—O bonds are described in Sections 12.8-12.11. Oxidation of alcohols to carbonyl compounds appears in Sections 12.12-12.14. 

In many synthetically useful radical chain reactions, hydrogen donors are used to trap adduct radicals. Absolute rate constants for the reaction of the resulting hydrogen donor radicals with alkenes have been measured by laser flash photolysis techniques and time-resolved optical absorption spectroscopy for detection of reactant and adduct radicals Addition rates to acrylonitrile and 1,3-pentadienes differ by no more than one order of magnitude, the difference being most sizable for the most nucleophilic radical (Table 8). The reaction is much slower, however, if substituents are present at the terminal diene carbon atoms. This is a general phenomenon known from addition reactions to alkenes, with rate reductions of ca lOO observed at ambient temperature for the introduction of methyl groups at the attacked alkene carbon atom . This steric retardation of the addition process either completely inhibits the chain reaction or leads to the formation of rmwanted products. 

Aspects of stereochemistry are explored through consideration of addition reactions to alkenes and carbonyl groups, nucleophilic substitution, and reactions (and interactions) involved in the resolution of racemic mixtures. 

Metal-catalyzed living radical polymerization can be traced back to metal-catalyzed radical addition reactions to alkenes, sometimes collectively called Kharasch or atom-transfer radical addition (ATRA) reactions in organic chemistry (Scheme 2).33 Thus, a... 

In a large number of carbene and carbenoid addition reactions to alkenes the thermodynamically less favored syn-isomers are formed 63). The finding that in the above cyclopropanation reaction the anti-isomer is the only product strongly indicates that the intermediates are organonickel species rather than carbenes or carbenoids. Introduction of alkyl groups in the 3-position of the electron-deficient alkene hampers the codimerization and favors isomerization and/or cyclodimerization of the cyclopropenes. Thus, with methyl crotylate and 3,3-diphenylcyclopropene only 16% of the corresponding vinylcyclopropane derivative has been obtained. 2,2-Dimethyl acrylate does not react at all with 3,3-dimethylcyclopropene to afford frons-chrysanthemic add methyl ester. This is in accordance with chemical expectations 69) since in most cases the tendency of alkenes to coordinate to Ni(0) decreases in the order un-, mono-< di- < tri- < tetrasubstituted olefines. 

Vladimir W. Markovnikov (1838-1904). Russian chemist. Markovnikov s observations of the addition reactions to alkenes were published a year after his death. 

Radicals of the norbornyl (bicyclo[2.2. l]hcptyl) type also show high facial selectivity in addition reactions to alkenes 34,54. In solvomercuration/rcductivc alkylation reactions of norbornene34 or 2-norbornanone hydrazones54 it is found that 2-norbornyl radicals substituted at the radical center- 4 or in the / -position34 yield only e.vo-addition products. The addition of alkenes, therefore, proceeds more selectively than hydrogen, or other atom abstraction reactions, in which small amounts of endo attack are also found (see Section D.2.2.). 

Two equatorial /i-substituents are present in cyclic carbohydrate radicals, in which the pyranose ring forms a heterocyclic cyclohexyl radical37. The glucosyl radicals A, B, and C below all show preferred equatorial attack in addition reactions to alkenes the actual selectivity depends on the radical and alkene substituents. In reactions of cyclohexyl radicals with only one equatorial /1-substituent, addition of 1,2-disubstituted alkenes yields only the equatorial addition product. 

A summary of the stereochemistry of the products obtained from addition reactions to alkenes is given in Table 5.2. 

Although some l,2,4-triazoline-3,5-diones can be isolated, these reagents are generally formed in situ as mentioned earlier. TADs undergo several types of addition reactions to alkenes and/or dienes, for example, [2-1-4] Diels-Alder additions, homo Diels-Alder additions, ene reactions, and [2-1-2] additions. The adducts, in many cases, can be further used for a wide variety of chemical transformations. TADs also add to a wide range of organic compounds to give polycyclic urazoles. Very often several possible reactions compete, and under different conditions the composition of such reaction mixtures often differs. It is necessary to say that even the presence of a reactive diene system does not entirely eliminate the possibility of [2-1-2] addition and/or ene reaction as documented in the appropriate sections. 

Thermodynamic Feasibility of Addition Reactions Carbon-carbon ir bonds are weaker than c bonds. Addition reactions to alkenes are generally exothermic, because one of the bonds broken is weak (the ir bond), but both new bonds formed are strong. 

FluorofunctionaUzation of alkenes. leOH), an addition reaction to alkenes occi... 

In this work, the MP2 and B3LYP methods are used to study reaction (2) for the xylenes, including addition at the Ci carbon atom. The study of reaction (2) is the natural extension of our work on OH-addition reactions to alkenes [18] and to toluene... 

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