Radicals, acyl

The general chemistry of acyl radicals has been recently reviewed/88 Acyl radicals have nucleophilic character. Absolute rate constants for substituted phenacyl radical addition to BA have been reported to be in the range 1.3-5.5xl05 

Acyl radicals undergo dccarbonylation. For aliphatic acyl radicals the rate constant for decarbonylation appears to be correlated with the stability of the alkyl radical formed. Values of the dccarbonylation rate constant range from 4 s for CH3C 0) to 1.5x10s s l [lor (CH.,)2C(Ph)C( )0] at 298 °C.3M The loss of carbon monoxide from phenacyl radicals is endothermic and the rate constant is extremely low (ca 10 8 s 1 at 298 nC).388 Consequently, the reaction is not observed during polymerization experiments. 

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Acyl Halides. Acyl halides, in which the hydroxyl portion of a carboxyl group is replaced by a halogen, are named by placing the name of the corresponding halide after that of the acyl radical. When another group is present that has priority for citation as principal group or when the acyl halide is attached to a side chain, the prefix haloformyl- is used as, for example, in fiuoro-formyl-. 

When another group having higher priority for citation as principal group is also present, the ketonic oxygen may be expressed by the prefix 0x0-, or one can use the name of the carbonyl-containing radical, as, for example, acyl radicals and oxo-substituted radicals. Examples are... 

Reaction 21 is the decarbonylation of the intermediate acyl radical and is especially important at higher temperatures it is the source of much of the carbon monoxide produced in hydrocarbon oxidations. Reaction 22 is a bimolecular radical reaction analogous to reaction 13. In this case, acyloxy radicals are generated they are unstable and decarboxylate readily, providing much of the carbon dioxide produced in hydrocarbon oxidations. An in-depth article on aldehyde oxidation has been pubHshed (43). 

Protonated pyridazine is attacked by nucleophilic acyl radicals at positions 4 and 5 to give 4,5-diacylpyridazines. When acyl radicals with a hydrogen atom at the a-position to the carbonyl group are used, the diacylpyridazines are mainly converted into cyclo-penta[ f]pyridazines by intramolecular aldol reactions (Scheme 43). 

Similar reactions occur with acyl radicals, for example with the CONH2 radical from formamide (74AHC(16)123). 

The same is true for decarbonylation of acyl radicals. The rates of decarbonylation have been measured over a very wide range of structural types. There is a very strong dependence of the rate on the stability of the radical that results from decarbonylation. For example, rates for decarbonylations giving tertiary benzylic radicals are on the order of 10 s whereas the benzoyl radical decarbonylates to phenyl radical with a rate on the order of 1 s . ... 

Other functional groups provide sufficient stabilization of radicals to permit successful chain additions to alkenes. Acyl radicals are formed by abstraction of the formyl hydrogen from aldehydes. As indicated in Table 12.7, the resulting acyl radicals are... 

Acyl radicals can fragment with toss of carbon monoxide. Decarbonylation is slower than decarboxylation, but the rate also depends on the stability of the radical that is formed. For example, when reaction of isobutyraldehyde with carbon tetrachloride is initiated by t-butyl peroxide, both isopropyl chloride and isobutyroyl chloride are formed. Decarbonylation is competitive with the chlorine-atom abstraction. 

Cushny has compared the action of d- and Z-hyoscyamines with that of atropine, and of d-homatropine with that of dZ-homatropine in antagonising the action of pilocarpine, and finds that the order of activity of the first three is in the ratio 1 40 20, and of the second two in the ratio 4 2-5. He drew attention also to the important influence of the acyl radical in the tropeines, which exercises the maximum effect when it is a hydroxyalkyl aromatic residue and is laevorotatory and in illustration of this point gives the following table of relative activities on the basis of capacity to antagnonise pilocarpine in the salivary fistula dog —... 

A possible mechanism for the formation of the furanones 6 and 7 is illustrated in Scheme 2. The initial alkoxy radical generated from the alcohol 5 and lead tetraacetate (LTA) undergoes /3-scission to produce the acyl radical intermediate 9. Subsequent cyclization to 10 proceeds through attack of the radical at the carbonyl oxygen. The resulting Pb(IV) intermediate 11 finally collapses via the reductive... 

The acyl radical 2 can abstract a /3-hydrogen from the radical 3, to give an aldehyde 10 and an alkene 11 ... 

Radicals with adjacent Jt-bonds [e.g. allyl radicals (7), cyclohexadienyl radicals (8), acyl radicals (9) and cyanoalkyl radicals (10)] have a delocalized structure. They may be depicted as a hybrid of several resonance forms. In a chemical reaction they may, in principle, react through any of the sites on which the spin can be located. The preferred site of reaction is dictated by spin density, steric, polar and perhaps other factors. Maximum orbital overlap requires that the atoms contained in the delocalized system are coplanar. 

In this section wc consider the properties and reactions of three classes of carbon-centered radicals alkyl radicals (3.4. l.l), aryl radicals (3.4.1,2) and acyl radicals (3.4.1.3). 

Phenacyl radicals are produced by photodecomposition of initiators containing the phenone moiety (Scheme 3.74). These initiators include benzoin derivatives and acylphosphine oxides (see 3.3.4.1.1). Acyl radicals can be formed by... 

A1BN, iw azobisisnbutyronilrile aldehydes, acyl radicals from 1 18 alkanelhiyl radicals Irom ally] sulfides 300 from disunities 291-2 from thiols 290, 291 polarity 290... 

Radical-based carbonylation procedures can be advantageously mediated by (TMSlsSiH. Examples of three-component coupling reactions are given in Reactions (74) and (75). The cascade proceeds by the addition of an alkyl or vinyl radical onto carbon monoxide with formation of an acyl radical intermediate, which can further react with electron-deficient olefins to lead to the polyfunctionalized compounds. ... 

Radical cascades that feature a 7-exo acyl radical cyclization followed by a 6-exo or 5-exo alkyl radical cyclization proceed with very good yields and diastereoselectivities. Two examples are shown in Reaction (80), where treatment of 100 with E3B, air, and (TMS)3SiH provided the tricycle 101 in excellent yields as a single diastereomer. Interestingly, the bulky silyl ether moiety is not required to achieve stereoselectivity in this process. 

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