Alkenes with radicals

Syntheses of alkenes with three or four bulky substituents cannot be achieved with an ylide or by a direct coupling reaction. Sterical hindrance of substituents presumably does not allow the direct contact of polar or radical carbon synthons in the transition state. A generally applicable principle formulated by A. Eschenmoser indicates a possible solution to this problem //an intermolecular reaction is complex or slow, it is advisable to change the educt in such a way. that the critical bond formation can occur intramolecularly (A. Eschenmoser, 1970). 

A chlorohydrin has been defined (1) as a compound containing both chloio and hydroxyl radicals, and chlorohydrins have been described as compounds having the chloro and the hydroxyl groups on adjacent carbon atoms (2). Common usage of the term appHes to aUphatic compounds and does not include aromatic compounds. Chlorohydrins are most easily prepared by the reaction of an alkene with chlorine and water, though other methods of preparation ate possible. The principal use of chlorohydrins has been as intermediates in the production of various oxitane compounds through dehydrochlorination. 

Alkene polymers—large molecules resulting from repetitive bonding together of many hundreds or thousands of small monomer units—are formed by reaction of simple alkenes with a radical initiator at high temperature and... 

Various ab initio and scmi-cmpirical molecular orbital calculations have been carried out on the reaction of radicals with simple alkenes with the aim of defining the nature of the transition state (Section 1.2.7).2I>,j , 6 These calculations all predict an unsymmetrical transition state for radical addition (i.e. Figure 1.1) though they differ in other aspects. Most calculations also indicate a degree of charge development in the transition state. 

Chidsey and coworkers made pioneering works in preparing covalently bonded monolayer films on silicon surfaces by the radical-initiated reaction of 1-alkenes with the H-Si(lll) surfaces. Reactions were carried out in neat deoxygenated alkenes using thermal decomposition of diacyl peroxides as the... 

Reactions of alkenes with H-Si(l 0 0)-2 x 1 surfaces have been shown to yield films with one-dimensional (ID) molecular lines through Si-C linkages, contrary to formation of the islands observed on H-Si(l 11). The reaction can be initiated from isolated surface silyl radicals created using the tip of the STM. The STM images showed molecular lines running along and across the dimer rows depending on the chemical constituent of R in the CH2 = CH-R molecules. 

Thus the observed orientation in both kinds of HBr addition (Markovnikov electrophilic and anti-Markovnikov free radical) is caused by formation of the secondary intermediate. In the electrophilic case, it forms because it is more stable than the primary in the free-radical case because it is sterically preferred. The stability order of the free-radical intermediates is also usually in the same direction 3°>2°>1° (p. 241), but this factor is apparently less important than the steric factor. Internal alkenes with no groups present to stabilize the radical usually give an approximately 1 1 mixture. 

RSO2, and so on. Addition of Ph3SiSH to terminal alkenes under radical conditions also leads to the primary thiol.With alkynes it is possible to add 1 or 2 mols of RSH. 

Free-radical addition of an aryl group and a hydrogen has been achieved by treatment of activated alkenes with a diazonium salt and TiCls. ... 

The compound ArX can be added across double bonds, in a free-radical process, by treatment of alkenes with diazonium salts, although Meerwein arylation (substi-... 

Free radical cyclization of alkenes with tin or mercury halides... 

Addition of phosphonyl radicals onto alkenes or alkynes has been known since the sixties [14]. Nevertheless, because of the interest in organic synthesis and in the initiation of free radical polymerizations [15], the modes of generation of phosphonyl radicals [16] and their addition rate constants onto alkenes [9,12,17] has continued to be intensively studied over the last decade. Narasaka et al. [18] and Romakhin et al. [19] showed that phosphonyl radicals, generated either in the presence of manganese salts or anodically, add to alkenes with good yields. 

At low temperature (375 and 400 °C), the product distribution obtained with the catalysts is very different from the one obtained under thermal cracking. With the catalytic cracking (ZSM-5), the obtained products are mainly n-alkanes, isomerised alkanes and alkenes with a carbon number between 1 to 6 whereas with the thermal cracking the whole range of n-alkanes with 1 to 9 carbon atoms and the 1 -alkenes with 2 to 10 carbon atoms are observed. This difference of product distribution can easily be explained by the cracking mechanisms. In one hand, the active intermediate is a carbocation and in the other hand it is a radical. 

All reactions of benzotriazole derivatives of the type Bt-CR RbS discussed above are based on electrophilic or nucleophilic substitutions at the ot-carbon, but radical reactions are also possible. Thus, the first report on unsubstituted carbon-centered (benzotriazol-l-yl)methyl radical 841 involves derivatives of (benzotriazol-l-yl)methyl mercaptan. 3 -(Benzotriazol-l-yl)methyl-0-ethyl xanthate 840 is readily prepared in a reaction of l-(chloromethyl)-benzotriazole with commercially available potassium 0-ethyl xanthate. Upon treatment with radical initiators (lauroyl peroxide), the C-S bond is cleaved to generate radical 841 that can be trapped by alkenes to generate new radicals 842. By taking the xanthate moiety from the starting material, radicals 842 are converted to final products 843 with regeneration of radicals 841 allowing repetition of the process (Scheme 134). Maleinimides are also satisfactorily used as radical traps in these reactions <2001H(54)301>. 

The synthesis and properties of heat-resistant polyazomethines containing 2,5-disubstituted oxadiazole fragments, being insulators convertible into semiconductors by doping with iodine, have been described. The radical copolymerization of alkenes with the fluorescent co-monomer 2-/-butyl-5-(4 -vinyl-4-biphenylyl)-l,3,4-oxadiazole has resulted in useful macromolecular scintillators. Anionic polymerization of 2-phenyl-l,3,4-oxadiazolin-5-one has produced a nylon-type product <1996CHEC-II(4)268>. 

The dehalogenation of organic halides by organotin hydrides takes place in most cases with a free-radical mechanism [1, 84, 85], The stereospecific reduction of 1,1-dibromo-l-alkenes with Bu3SnH discovered by Uenishi and coworkers [86-89], however, did not occur in the absence of palladium complexes and did not involve radicals. For the synthesis of (Z)-l-bromo-l-alkenes, [(PPh3)4Pd] proved to be the most effective catalyst which could also be generated in situ. The reaction in Eq. (7) proceeded at room temperature and a wide range of solvents could be used. 

A modified version of the Brown-Negishi reaction using B-alkylcatechol-boranes was reported (Scheme 32). This novel method is based on a simple one-pot procedure involving the hydroboration of various substituted alkenes with catecholborane, followed by treatment with catalytic amount of oxygen/DMPU/water and a radical trap. Efficient radical additions to a,ft-unsaturated ketones and aldehydes have been reported. Primary alkyl radicals are efficiently generated by this procedure and the reaction has been applied to a 300 mmol scale synthesis of the y-side chain of (-)-perturasinic... 

In a preliminary study, in situ generated B-alkylcatecholboranes were allowed to react with PTOC-OMe under irradiation with a standard 150 W lamp. The S-pyridyl products coming from primary, secondary and tertiary alkyl radicals were isolated in moderate to good yields [88]. Based on these initial results, a procedure for conjugate addition to various activated alkenes was developed. A one-pot procedure involving hydroboration of an alkene with catecholborane followed by irradiation in the presence of five equivalents of an activated alkene and three equivalents of the chain transfer reagent PTOC-OMe was developed (Scheme 36) [88]. 

Heteroatom-substituted carbene complexes are less electrophilic than the corresponding methylene, dialkylcarbene, or diarylcarbene complexes. For this reason cyclopropanation of electron-rich alkenes with the former does not proceed as readily as with the latter. Usually high reaction temperatures are necessary, with radical scavengers being used to supress side-reactions (Table 2.16). Also acceptor-substituted alkenes can be cyclopropanated by Fischer-type carbene complexes, but with this type of substrate also heating is generally required. 

Similar to the intramolecular addition of neutral carbon-centered radicals to alkenes, the formation of radical cations starting from alkenes with subsequent cyclization offers a convenient method for constructing carbocyclic ring systems. In contrast to the regioselective 1,5-ring closure (5-cxo-trig cyclization) of the... 

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