Alkenes radical formation from

More relevant to our consideration now is the radical addition of hydrogen bromide to an alkene. Radical formation is initiated usually by homolysis of a peroxide, and the resultant alkoxyl radical may then abstract a hydrogen atom from HBr. 

Stereoselective preparation of ( )-allyl alcohols via radical elimination from anti-j-phenylthio-P-nitro alcohols has been reported.154 The requisite anti-P-nitro sulfides are prepared by protonation of nitronates at low temperature (see Chapter 4), and subsequent treatment with Bu3SnH induces anti elimination to give (E)-alkenes selectively (see Eq. 7.112). Unfortunately, it is difficult to get the pure yyw-P-nitro sulfides. Treatment of a mixture of syn- and anti-P-nitrosulfides with Bu3SnH results in formation of a mixture of (E)- and (Z)-alkenes. 

Further evidence for the formation of alkene radical cations derives from the work of Giese, Rist, and coworkers who observed a chemically induced dynamic nuclear polarization (CIDNP) effect on the dihydrofuran 6 arising from fragmentation of radical 5 and electron transfer from the benzoyl radical within the solvent cage (Scheme 6) [67]. 

A concise collection of some modem methods of radical formation via rupture of C-E, C-G and C-A bonds, as well as from alkenes and cyclopropanes, by metals, organometallic hydrides, and photochemical and electrochemical means, are given in the last Chapter of Giese s book [28]. For some examples of radical generation and reactions leading to cyclic and polycyclic compounds see next Chapter (Heading 6.1.3). 

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... 

Much evidence has been accumulated that the ozone-olefin reaction has a predominant role in aerosol formation from alkenes, cyclic olefins, diolefins, and other unsaturated compounds. Free radicals are formed in the reaction and can react further, along with nitric oxide and nitrogen dioxide, either with the various intermediates or with the olefin itself (see the recent review by Pitts and Finlayson ). 

An extremely interesting feature of these mechanisms is the fact that superoxide and the alkene radical cation are both formed in the reduction (Fig. 20) and also in the Frei oxidation (Fig. 19). In the Frei photo-oxidation, however, they are formed concurrently in a tight ion pair and collapse to product more rapidly than their diffusive separation. In the reduction (Fig. 20), the formation of the radical cation and superoxide occur in independent spatially separated events allowing the unimpeded diffusion of superoxide which precludes back-electron transfer (BET) and formation of oxidized products. The nongeminate formation of these two reactive species provides the time necessary for the radical cation to abstract a hydrogen atom from the solvent on its way to the reduced product. 

The addition to alkenes of radicals derived from an a-nitroketone is also catalysed by manganese(III) [30]. During the reaction between a-nitroacetophenone 8 and m-but-2-ene, the stereochemical relationship between the methyl substituents is not preserved. The process terminates with the formation of a nitrone. A related process will generate nitromethyl radicals from nitromethane and these add to benzene to give phenyinitromethane [31],... 

There continues to be an increasing level of activity centered about the use of porphyrin catalysts for the epoxidation of alkenes of various configurations. For example, the sterically encumbered fra/w-dioxoruthenium(VI) porphyrin (26) was found to catalyze the epoxidation of a variety of alkenes in yields from fair to excellent e.g., 27 -> 28). Kinetic studies on a series of para-substituted styrenes point to a mechanism which proceeds via a rate-limiting benzylic radical formation. The high degree of stereoretention in cir-alkenes was attributed to steric crowding which prevents C-C bond rotation of the intermediate radical. This same steric bulk prevents the familiar side-on approach of the alkene substrate, so that a head-on approach is postulated <99JOC7365>. 

For the /3-hydroxyalkoxy radicals formed from alkenes C5 and larger, there is experimental evidence that isomerization starts to dominate (e.g., see Atkinson et al., 1995d Kwok et al., 1996b). Thus, isomerization followed by reaction with 02, NO, etc., ultimately leads to the formation of dihydroxycarbonyl compounds. For the reaction of OH with 1-butene, for example, isomerization of one of the alkoxy radicals ultimately leads to 3,4-dihydroxybutanal in competition with its decomposition and reaction with 02 ... 

Chew, A. A., and R. Atkinson, OH Radical Formation Yields from the Gas-Phase Reactions of 03 with Alkenes and Monoterpenes, J. Geophys. Res., 101, 28649-28653 (1996). 

The photoinduced electron transfer (PET) initialed cyclodimerization was first studied with 9-vinylcarbazole as substrate1 and characterized mechanistically as a cation radical chain reaction.2 The overall reaction sequence3-4 consists of a) excitation of an electron acceptor (A), b) electron transfer from the alkene to the excited acceptor (A ) with formation of a radical ion pair, c) addition of the alkene radical cation to a second alkene molecule with formation of a (dimeric) cation radical, and d) reduction of this dimeric cation radical by a third alkene molecule with formation of the cyclobutanc and a new alkene cation radical. Steps c) and d) of the sequence are the chain propagation steps. The reaction sequence is shown below. 

Another mode for catalyzed cycloaddition involves the generation of radical cations from electron-rich alkenes with single-electron oxidants such as tris(4-bromophenyl)amminium hexachloroantimonate (TBAH). An equivalent reaction involves the photosensitized electron transfer (PET) process (see Section 1.3.2.3.). These processes have been recently reviewed,9 and are limited to electron-rich alkenes capable of producing radical cations. Furthermore, some of the cyclobutanes themselves undergo secondary isomerization under the oxidative conditions, e.g. formation of 31-35.10-12... 

For the HO-substituted alkoxy radicals formed from the higher alkenes, Atkinson et al. [112], Akimoto et al. [111], and this group [109] have shown that unimoiecular dissociation, analogous to reaction (34), is dominant over reaction with 02, leading to the formation of aldehydes and HOO. For instance, the reaction sequence for the HO-initiated oxidation of 2-butene in the presence of NO is shown below. 

Crich D, Huang W (2001) Dynamics of alkene radical cations/phosphate anion pair formation from nucleotide C4 radicals. The DNA/RNA paradox revisited. J Am Chem Soc 123 9239-9245 Das S, Deeble DJ, Schuchmann MN, von Sonntag C (1984) Pulse radiolytic studies on uracil and uracil derivatives. Protonation of their electron adducts at oxygen and carbon. Int J Radiat Biol 46 7-9... 

Neutral aminyl radicals generated from tin hydride-mediated reactions of sulfenamides (Section II,F) have been shown to undergo cyclizations when energetically favored by addition to a strained alkene or by formation of a stabilized intermediate benzylic radical. In both cases, the reverse reaction, cleavage of the /3-amino radical, apparently did not occur (92TL4993). 

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