Caged radicals

The results were interpreted on the basis of a mechanism that starts with the photolytic formation of a radical cage consisting of an aryldiazenyl and and arylthiyl (Ar - S ) radical, followed by diffusion of both radicals out of the cage. Three reactions of the aryldiazenyl radical are assumed to occur bimolecular formation of the azoarene and N2, or of biphenyl and N2 (Scheme 8-37), the monomolecular dediazoniation (Scheme 8-38), and recombination with the thiyl radical accompanied by dediazoniation (Scheme 8-39). In addition, two radicals can react to form a di-phenyldisulfide (Scheme 8-40). 

Table III and the plot of Figure 3 compare values of kd (obs.) and kd (calc.) in several solvents. The values of kd (obs.) were taken from the data of Szwarc (7) and Eirich (2) as well as from our laboratory and involve an extrapolation from 60° to 80°C. for the data of Eirich. This was done by making the crude approximation that AG varies with solvent in the same way at the two temperatures. A close parallel exists between the observed and calculated rates. This suggests that the assumption equating ki and kg is not bad and provides a firmly established case in which the dependence of the rate of decomposition of a radical initiator on solvent viscosity in a homologous series of solvents results from the varying importance of cage recombination in the various solvents. Pryor and Smith (19) using data from p-nitrophenylazotriphenylmethane decompositions have suggested that a general criterion for radical cage...

Fig. 5 Rabinowitch and Wood apparatus for demonstrating the radical cage effect (reproduced from reference 189 with the permission of the Royal Society of Chemistry).

To explain the high chemical yield in the transformation A - D, coupled with the fact that the quantum yield for the photodecomposition of nitrites is less than unity, it was suggested that the Barton reaction might take place through a radical "cage mechanism.2 However, recent studies at the Institute on the mechanism of nitrite photolysis have shown this not to be so. Photolysis of an equimolecular mixture of 3/3-acetoxy-androstan-6/8-yl nitrite (102) and 3/8-acetoxy-cholestan-6/8-yf nitrite containing 98% of nitrogen as N16 (103) in iso-octane or toluene... 

Braden DA, Parrack EE, Tyler DR. Solvent cage effects. I. Effect of radical mass and size on radical cage pair recombination efficiency. II. Is geminate recombination of polar radicals sensitive to solvent polarity Coord Chem Rev 2001 211 279-94. 

Jarett JT, Drennan CL, Amaratunga M, Scholten JD, Ludwig ML, Matthews RG. A protein radical cage slows photolysis of methylcobalamin in methionine synthase from Escherichia coli. Bioorg Med Chem 1996 4 1237-46. 

Radical cage effect and coupling (recombination) Radical coupling reactions do not dominate free radical chemistry as most radicals have very short lifetimes and are present in very low concentrations. Consequently, if short-lived radicals are to contribute to useful synthetic procedures by way of a radical coupling, all the events leading up to the coupling must take place in a solvent cage. 

Efficient preparative sequences involving radical decarboxylation followed by carbon-nitrogen bond formation are rare. Acyl nitrates decompose at elevated temperatures to give nitroalkanes (equation 46), but are unfortunately explosive and have to be prepared in situ and stored in solution. A noteworthy exception is found in the thermal or photochemical decarboxylation of tetrahydro-l,2-oxazine-3,6-diones leading to -lactams (equation 47). Doubtless a key factor in this reaction, considered to proceed via a radical cage mechanism, is the intramolecular nature of the carbon-nitrogen bond formation. 

Alkylcobalts form in stoichiometric reactions of pentacyanocobalt(III) hydride and alkenes. This reaction occurs both for halogenated alkenes such as tetrafluoroethylene and for alkenes that contain other electron-withdrawing groups such as carbonyls, nitriles and arenes as substituents (see Table 6) . The addition is regiospedfic, forming the more substituted alkylcobalt. Prior coordination of alkene to cobalt to form an alkene(hydrido)cobalt complex, an intermediate in hydrometalation reactions, is not important. This reaction is a radical process however, by NMR, additions of [HCo(CN)5 ] " to diastereomeric alkenes such as fumaric and maleic add salts lead to a cr-alkylcobalt by stereospecific cis addition of Co and H to the double bond . The overall reduction is not stereospecific. (r-Alkylcobalt bond formation proceeds by either a concerted addition or a rapid collapse of a radical cage. 

The chemiluminescent autoxidation of tetrakis(dimethylamino)-ethylene (35) is spectacular.62 Ground-state oxygen is sufficiently reactive to produce the dioxetane laq, presumably via electron transfer, radical cage coupling, and cyclization [Eq. (22)]. Besides tetramethyl-... 

N-[(/-Butylsulphinyl)oxy] sulphonamides (59) rearrange with N—O cleavage via sulphonamidyl radicals 60 to give 61 in ca 20% yield in a radical cage recombination process, along with products formed after escape from the cage (equation 36)162. 

Photolysis at 254 nm of sodium N-phenylsulphamate (39) gives three isomeric anilinesulphonic acids, viz. orthanilic, methanilic and sulphanilic acids and aniline65. The involvement of an intramolecular radical cage mechanism is supported by the absence of a substrate concentration effect and a considerable lowering of sulphamic acid yields in the presence of a radical scavenger. Stern-Volmer plots have provided evidence for involvement of two triplets in the reaction. 

---