Radicals, anti-Markovnikov benzylic

A stereo specific addition of BufOI to /1-methylstyrene was observed in the presence of BF3, yielding Markovnikov products. This result contrasts with the non-stereospecific addition of Bu OCl and Bu OBr. It has been suggested that the bridging in the intermediate chloronium and bromonium ion derived from PhCH=CHMe is not as symmetrical as in the iodonium ion. Consequently, charge develops on the benzylic carbon in the first two cases, and rotation occurs about the C—C bond190. By contrast, a radical mechanism is assumed in the absence of BF3 as anti-Markovnikov products are formed (both in the dark and upon UV irradiation)190. 

The functionalisation of these residual pendant vinyl group by its extremity would be a convenient way to incorporate a dimethylene spacer for more stable attachments of the grafted species through a non-benzylic position. Thiols adding to olefins under an anti-Markovnikov way by free radical mechanism8, this approach was used to diversely functionalise our polyHIPEs (scheme 2). 

Cation radicals of arylcyclopropanes, produced by photoinduced one-electron transfer to an electron acceptor, undergo ring-opening reactions accompanied by regioselective polar additions. 1,4-Dicyanobenzene (5) sensitized photolysis of l,l-bis(4-tolyl)cyclopropane (4) in methanol/acetonitrile resulted exclusively in anti-Markovnikov addition of methanol to the cyclopropane. It is assumed that excitation of 5 followed by electron transfer from 4 gives a dicyanobenzene radical anion 5(- ) - cyclopropane radical cation 4(4-) pair. Attack by methanol on the radical cation 4(4-) followed by proton loss affords benzylic radical 7 that is reduced by 5( —) to give benzylic anion 8 and quenched by methanol to afford product... 

As noted above the anti-Markovnikov addition of alcohols to alkenes has attracted considerable mechanistic and synthetic interest. Absolute rate constants for the reaction of alcohols with styrene radical cations have been measured and representative data are summarized in Tables 3 and 5. In this case the high oxidation potentials of simple alcohols effectively rule out the possibility of electron transfer as a competing reaction. Furthermore, transients assigned to the benzyl radical produced by... 

The authors proposed a mechanism based on a cage-mediated guest-to-host electron transfer (Fig. 9.30) in which the cage acted as a photosensitizing molecular flask. Excitement of the coordination cage, followed by electron transfer from alkyne to an electron-deficient cage and the reaction of a molecule of water (solvent) with the obtained phenyl alkyne radical cation, results in benzylic radicals and subsequently the anti-Markovnikov product. 

Another three components synthesis is involved in the extensively investigated NOCAS process (nucleophile olefin combination aromatic substitution, path h) [89,90], In this case, a nucleophile adds to an alkene radical cation and again the interaction between the resulting radical and the radical anion of the aromatic nitrile may follow two paths. The first is electron transfer, which results in sensitized anti-Markovnikov addition onto the alkene, and is favored with stabilized, reducible radicals such as the benzyl radicals obtained from aryl olefins. The latter one is... 

The mechanism for the addition of hydrogen bromide to 1-phenylpropene in the presence of peroxides is a chain mechanism analogous to the one we discussed when we described anti-Markovnikov addition in Section 10.9. The step that determines the orientation of the reaction is the first chain-propagating step. Bromine attacks the second carbon atom of the chain because by doing so the reaction produces a more stable benzylic radical. Had the bromine atom attacked the double bond in the opposite way, a less stable secondary radical would have been formed. 

---

See also in sourсe #XX -- [ , , ]

---