Radical-chain addition to alkenes and

On descending groups IVB, VB, and VIB, the increasing weakness of the M—H bond favors formation of the M radical and therefore facilitates radical chain additions to multiple bonds. However, the increasing rate of homocoupling of M- radicals relative to their rate of attack on alkenes leads to increasing loss of reagent via ... 

Reduction of the halides with a metal hydride such as lithium aluminium hydride, sodium borohydride, or poly(methylhydrosiloxane) gives the corresponding organotin hydrides These have an important place in organic synthesis for the reduction of halides to hydrides (hydrostannolysis) and the addition to alkenes and alkynes (hydrostannation), by radical chain reactions. Further reactions may intervene between the pairs of reactions shown in Equations (1.1.3) and (1.1.4), and (1.1.4) and (1.1.5), and these reactions are particularly useful for inducing ring-closure reactions. 

Aryl selenides have also proven to be excellent reagents in group transfer reactions. Photolysis of selenides in an inert solvent such as benzene can initiate chain reactions. Various substituted radicals can be generated in this manner by using a-selenenyl derivatives of esters, nitriles, malonates, P-ketoesters, a-methoxyesters, and phosphonates. The resulting radicals undergo addition to alkenes to generate y-seleno derivatives. 

In contrast with the great wealth of information on both cationic and free-radical additions to alkenes and alkynes, not too much is known about analogous cation radical reactions. The most commonly known among these are additions of aminium radicals. The overall reaction is that of, say, an N-chloramine or N-nitros-amine in acid solution, brought about photochemically or by reaction with ferrous ion (eq. 23) ( ). A chain reaction occurs in... 

In radical chain adflitions to alkenes, the chain carrier adds to the rr bond to create the more highly substituted radical. This method allows for the anti-Markovnikov hydrobromination of alkenes, as well as the addition of thiols and some halomethanes. 

The regioselectivity of addition of Itydrogen bromide to alkenes can be complicated if a free-radical chain addition occurs in competition with the ionic addition. The free-radical reaction is readily initiated by peroxidic impurities or by light and leads to the anti-Markownikoff addition product. The mechanism of this reaction will be considered more fully in Chapter 12. Conditions that minimize the competing radical addition include use of high-purity alkene and solvent, exclusion of light, and addition of free-radical inhibitors. ... 

Michael Faraday reported in 1821 that chlorine addition to alkenes is Stimulated by sunlightand today this is taken to indicate the involvement of a free radical process (equation 26). Free radical chain mechanisms were proposed in 1927 by Berthoud and Beraneck for the isomerization of stilbene catalyzed by Br2 (equation 27), and by Wachholtz for bromine addition to ethyl maleate (equation 28).Later studies showed inhibition of halogen addition by reaction of the intermediate radicals with oxygen, and a free radical chain mechanism for solution and gas phase halogenations as in equation (26) was shown (equation 29). Kinetic and mechanistic... 

Here Q denotes an alkyl radical with two unpaired electrons (in QOOH and QO) which may rearrange to form a stable alkene. The compound QO is a cyclic ether3 (which may break down to form an aldehyde4 and a smaller alkene). The sequence of reactions (R64) to (R67) is chain propagating, in that the initial alkyl radical has produced one HO2 or OH radical in addition to one or more stable components. However, it is also possible that a second oxygen molecule may add to QOOH to form a peroxy alkyl hydroperoxy radical,... 

Clearly, the formation of ROH and a bromine atom is energetically more favorable. The overall process of decomposition of peroxide and attack on hydrogen bromide, which results in the formation of a bromine atom, can initiate a radical-chain addition of hydrogen bromide to an alkene. 

There are many reagents that add to alkenes only by radical-chain mechanisms. A number of these are listed in Table 10-3. They have in common a relatively weak bond, X—Y, that can be cleaved homolytically either by light or by chemical initiators such as peroxides. In the propagation steps, the radical that attacks the double bond does so to produce the more stable carbon radical. For addition to simple alkenes and alkynes, the more stable carbon radical is the one with the fewest hydrogens or the most alkyl groups at the radical center. 

The best procedure to get the desired product is to generate the 1-alkene from the borane with 1-decene (Section 11-6C) and then add hydrogen bromide by a polar mechanism (Section 10-4). Incursion of radical-chain addition must... 

It was reported by Rozhkov and Chaplina130 that under mild conditions perfluorinated r-alkyl bromides (r-RfBr) in nonpolar solvents can be added across the n bond of terminal alkenes, alkynes and butadiene. Slow addition to alkenes at 20 °C is accelerated in proton-donating solvents and is catalyzed by readily oxidizable nucleophiles. Bromination of the it bond and formation of reduction products of t-RfBr, according to Rozhkov and Chaplina, suggest a radical-chain mechanism initiated by electron transfer to the t-RfBr molecule. Based on their results they proposed a scheme invoking nucleophilic catalysis for the addition of r-RfBr across the n bond. The first step of the reaction consists of electron transfer from the nucleophilic anion of the catalyst (Bu4N+Br , Na+N02, K+SCN , Na+N3 ) to r-RfBr with formation of an anion-radical (f-RfBr) Dissociation of this anion radical produces a perfluorocarbanion and Br, and the latter adds to the n bond thereby initiating a radical-chain process (equation 91). 

Organotin hydrides add to alkenes and alkynes (equations 22 and 23). Unless strongly electron-withdrawing groups are present in the unsaturated species, the additions have radical-chain mechanisms (Scheme 2). For l-aUcenes, tin usually adds predominantly to the terminal carbon (equations 24 and 25). 

These results clearly indicate that in the tellurium tetrachloride addition to alkenes the main reaction pathway does not occur via a telluronium ion intermediate a radical addition pathway could explain the poor selectivity observed in many of these additions (see Tables 15 and 16). The strong effect of 4-benzoquinone on the syn/anti ratio supports the involvement of a radical pathway in chloro-telluration with tellurium tetrachloride. A possible radical chain reaction is shown87. 

Both intermolecular and intramolecular additions of carbon radicals to alkenes and alkynes continue to be a widely investigated method for carbon-carbon bond formation and has been the subject of a number of review articles. In particular, the inter- and intra-molecular additions of vinyl, heteroatomic and metal-centred radicals to alkynes have been reported and also the factors which influence the addition reactions of carbon radicals to unsaturated carbon-carbon bonds. The stereochemical outcome of such additions continues to attract interest. The generation and use of alkoxy radicals in both asymmetric cyclizations and skeletal rearrangements has been reviewed and the use of fi ee radical reactions in the stereoselective synthesis of a-amino acid derivatives has appeared in two reports." The stereochemical features and synthetic potential of the [1,2]-Wittig rearrangement has also been reviewed. In addition, a review of some recent applications of free radical chain reactions in organic and polymer synthesis has appeared. The effect of solvent upon the reactions of neutral fi ee radicals has also recently been reviewed. ... 

The radical chain addition of RSH to an alkene is an energetically reasonable process because there are no steps more than 20 kcal/mol (84 kJ/mol) endothermic, and the entire process is exothermic. 

---