Nonchain Free-Radical Reactions

Radical initiators are molecules bearing one or several weak bonds with a BDE of about 100-200 kJ mol-1. When the temperature of the reaction is sufficiently high, the initiator decomposes with homolysis of the weakest bond and produces free radicals, which initiate a chain or nonchain free radical reaction. 

The major classes of nonchain free-radical reactions are photochemical reactions, reductions and oxidations with metals, and cycloaromatizations. 

Rhodium(I) or polymer supported rhodium(I) compounds catalyzed the formation ofFefCO - CNR L (x = 1 - 3 R = Bu, xylyl L = MA, citra-conic anhydride, acrylamide) (29, 30), and the dimer [CpFe(CO)2]2 catalyzed the stepwise substitution of carbonyl groups in CpFeI(CO)2 to give CpFeKCO - CNR) (x = 1,2 R = Bu, xylyl) in 60-80% yields. A nonchain free-radical mechanism was proposed for the latter reaction (28). The compounds CpFeX(CO)2 x(CNR)x (x = 1,2 X = halide, SiMe3) are known for a range of alkyl and aryl isocyanides (169-171). 

A free-radical reaction may proceed by a chain or a nonchain mechanism. There are many experimental methods for determining whether a chain or a nonchain mechanism is operative in a reaction, but these methods don t help much in pen-cil-and-paper problems such as the ones in this book. Luckily, the reagents or reaction conditions will usually indicate which type of mechanism is operative. 

The present second edition of this book corrects two major errors (the mechanisms of substitution of arenediazonium ions and why Wittig reactions proceed) and some minor ones in the first edition. Free-radical reactions in Chapter 5 are reorganized into chain and nonchain processes. The separate treatment of transition-metal-mediated and -catalyzed reactions in Chapter 6 is eliminated, and more in-text problems are added. Some material has been added to various chapters. Finally, the use of italics, especially in Common Error Alerts, has been curtailed. 

Not all free-radical mechanisms are chain reactions, and those that are not do not require initiators. Reductions with metals such as Li, Na, or Sml2 (often in liquid NH3) and light-promoted rearrangements of carbonyl compounds proceed by nonchain free-radical mechanisms. Compounds containing weak cr bonds (typically either heteroatom-heteroatom bonds or very strained bonds) can undergo intramolecular rearrangements by nonchain free-radical mechanisms upon heating. 

Many hydroperoxides have been prepared by autoxidation of suitable substrates with molecular oxygen (45,52,55). These reactions can be free-radical chain or nonchain processes, depending on whether triplet or singlet oxygen is involved. The free-radical process consists of three stages ... 

Some polymer deterioration reactions occur without loss in molecular weight. These include a wide variety of reactions where free radicals (most typical) or ions are formed and cross-linking or other nonchain session reaction occurs. Cross-linking discourages chain and segmental chain movement. At times this cross-link is desired such as in permanent press fabric and in elastomeric materials. Often the cross-links bring about an increased brittleness beyond that desired. 

In both light and high energy irradiation induced reactions, phenyl-thiol, 2-mercaptomesitylene, and their sulfides inhibit nonchain processes,201202 e.g., the light-induced conversion of benzophenone in 2-propanol to benzopinacol and acetone.99 217 The reaction converts compounds into radicals by removal or addition of hydrogen atoms. The sulfur compounds in rapid hydrogen transfer processes convert the free radicals to stable molecules and may do so repeatedly.46 46... 

Nonchain mechanisms, though, often involve radical-radical combinations and disproportionations. Nitroxyls can be used to trap alkyl radicals that may be present in a reaction medium by a radical-radical combination reaction, giving an electron-sufficient species that is more easily studied than the free radical. The photochemical reactions of carbonyl compounds often involve radical-radical combination and disproportionation steps. 

One could write a perfectly reasonable chain mechanism for this reaction, but a nonchain mechanism is appropriate because NO is a stable free radical, and it hangs around until it is able to combine with the alkyl radical to form a strong C-N bond. 

In the Barton reaction, an alkyl nitrite is converted into an alcohol-oxime. The nitrite is photoexcited to a 1,2-diradical. It fiagments to NO and an alkoxy radical (RO-), and the latter abstracts H- from the nearest C—bond. The resulting alkyl radical then combines with NO to give a nitroso compound, which then tautomerizes to the oxime, probably by a polar stepwise mechanism. One could write a perfectly reasonable chain mechanism for this reaction, but a nonchain mechanism is appropriate because NO is a stable free radical, and it hangs around until it is able to combine with the alkyl radical to form a strong C—N bond. The Barton reaction has been used for the remote functionalization of hydrocarbons, especially steroids. 

Oxidative addition is often quoted as proceeding by four recognized mechanisms concerted, Sf 2, radical chain, and radical nonchain. In principle reductive elimination can go by any of the mechanisms, but there are reactions for which designation is more complicated. For example, the change of a metal dialkyl to a metallacycloalkane and free alkane can involve an individual step in which reductive elimination occurs, see for example equation (75) in the thermal decomposition of various platinum compounds, as discussed in Volume 2, on p. 287 [equations (73)-(76)]. In (75) the hydrogen elimination can be /3, y, or even 5. However,... 

---