Free-radical addition

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In

free radicals
. Radical additions are known for a variety of unsaturated substrates, both olefinic or aromatic and with or without heteroatoms.

Free-radical reactions depend on one or more relatively weak bonds in a reagent. Under reaction conditions (typically heat or light), some weak bonds homolyse into radicals, which then induce further decomposition in their compatriots before recombination. Different mechanisms typically apply to reagents without such a weak bond.

Mechanism and regiochemistry

Radical hydrobromination of an alkene

The basic steps in any free-radical process (the radical chain mechanism) divide into:[1]

In a free-radical addition, there are two chain propagation steps. In one, the adding radical attaches to a

multiply-bonded precursor to give a radical with lesser bond order. In the other, the newly-formed radical product abstracts another substituent from the adding reagent to regenerate the adding radical.[2]
: 743–744 

In general, the adding radical attacks the alkene at the

Morris Kharasch called the "peroxide effect".[3] Reaction is slower with alkynes than alkenes.[2]
: 750 

In the paradigmatic example, hydrogen bromide radicalyzes to monatomic bromine. These bromine atoms add to an alkene at the most accessible site, to give a bromoalkyl radical, with the radical on the more substituted carbon. That radical then abstracts a hydrogen atom from another HBr molecule to regenerate the monatomic bromine and continue the reaction.[2]: 758 

Compounds that add radically

See also: Hydrohalogenation

Radical addition of

endothermic and chemically disfavored.[2]: 692–694  Hydrogen bromide is incredibly selective as a reagent,[2]: 687–688  and does not produce detectable quantities of polymeric byproducts.[4]
: 156–157 

The behavior of hydrogen bromide generalizes in two separate directions. Halogenated compounds with a relatively stable radical can dissociate from the halogen. Thus, for example,

sulfenyl, and other sulfur halides can add radically to give respectively β‑halo sulfones, sulfoxides, or sulfides.[4]
: 200, 204, 206 

Separately, unsubsituted compounds with a relative stable radical can dissociate from hydrogen. In general, these reactions risk polymerized byproducts (see § Side reactions). For example, in the thiol-ene reaction, thiols,[4]: 165–166  disulfides,[4]: 207  and hydrogen sulfide[4]: 191  add across a double bond. But if the unsaturated substrate polymerizes easily, they catalyze polymerization instead.[4]: 171–172  In thermal silane additions, telomerization usually proceeds to about 6 units.[4]: 211 

In the case of silicon, germanium, or phosphorus, the energetics are unfavorable unless the heavy atom bears a pendant hydrogen.[4]: 209, 217–219  Other electronegative substituents on silicon appear to reduce the barrier.[4]: 213, 217–224 

Although nitrogen oxides naturally radicalize, careful control of the radical species is difficult.

nitrite ester.[4]
: 225 

To aryl radicals

See also: Radical substitution

Although aromatic resonance stabilizes aryl radicals, bonds between arenes and their substituents are (in)famously strong. Radical reactions with arenes typically present retrosynthetically as instances of nucleophilic aromatic substitution,[citation needed] because generating the aryl radical requires a strong (radical) leaving group.[2]: 686–687  One example is the Meerwein arylation.

Side reactions

A radical addition which leaves an unsaturated product can undergo radical cyclization between the two propagation steps.[2]: 744  In general, radical additions can also start radical polymerization processes.[4]: 171–172 

With stable inorganic radicals

A self-terminating radical cyclization reaction scheme
Self-terminating oxidative radical cyclization

In self-terminating oxidative radical cyclization,

monatomic oxygen.[5]

In

fragment to a ketone and the stable radical nitrogen dioxide.[5]

hydroxyl radicals show similar reactivity.[5]

See also

The other radical reactions:

References

  1. ^ L.G. Wade's Organic Chemistry 5th Ed. (p 319) – Mechanism supplements original.
  2. ^
    ISBN 0-471-60180-2
  3. doi:10.1021/ja01333a041
    .
  4. ^
    ISBN 978-0-471-26418-7
    .
  5. ^ a b c Dreessen, Tim; Jargstorff, Christian; Lietzau, Lars; Plath, Christian; Stademann, Arne; and Wille, Uta (2004). "Self-Terminating, Oxidative Radical Cyclizations". Molecules, issue 9, pp. 480–497.
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