Radical cage mechanism

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... 

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. 

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. 

The peresters (758 X = H, Ph, or Me) showed first-order kinetics on thermal decomposition in cyclohexene. Rate and activation data were indicative of a radical mechanism. Similarly, in the thermolysis of the peroxide (759) in carbon tetrachloride, the scrambling of oxygen in the products when 0-labelled peroxide was used indicated a radical cage mechanism. The products obtained on irradiation of the azaspriranes (760 n = 1 or 2) may be rationalized by homoallyl rearrangements of the intermediate spiro-radicals. ... 

The Use of Primary Kinetic Isotope Effects to Probe the Mechanism of Aliphatic Hydroxylation by Iron(III) Porphyrins One reaction that is uncommon in organic chemistry but is common in biological systems in the hydroxylation of alkanes to form alcohols. Cytochrome P-450, a heme containing enzyme, catalyzes this reaction. Large primary isotope effects are found, as well as an absence of carbocation-like skeletal rearrangements and loss of stereochemistry. These observations led researchers to conclude that a radical cage mechanism is operative. 

A radical cage mechanism is one in which radicals are created and react together before diffusing apart. 

In certain cases, e.g. with Z = tert-butyl, the experimental findings may better be rationalized by an ion-pair mechanism rather than a radical-pair mechanism. A heterolytic cleavage of the N-R bond will lead to the ion-pair 4b, held together in a solvent cage ... 

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). 

The other mechanism which has been advocated58 is that known as the radical-pair mechanism , in which two cation radicals are thought of as intermediates held in a solvent cage so preserving the intramolecularity of the reaction, viz. 

Further evidence consistent with the polar radical pair mechanism was provided by a crossover experiment (Scheme 6.26). A 1 1 mixture of labeled 8Z /8 and unlabeled 8Z/8E was heated in xylene at 125 °C for 2h and at 135 °C for 4h to afford hydroxypyrimidinones 3 and 3. Analysis of the products by high resolution mass spectrometry showed no crossover between the labeled and unlabeled fragments. This result reinforces the computational results discussed previously wherein PRP recombines to give product within the solvent cage (Scheme 6.24). 

The rigidity of the hexacyclic cage structure of koumine (18) renders some of its chemical behavior quite unusual, for instance, the resistance to Hofmann degradation shown by /Va-acetyldihydrokoumine methyl hydroxide (27). However, owing to the presence of a /J-aromatic imino system in 18, reductive cleavage by sodium-alcohol to yield dihydrokouminol (39) proceeds smoothly. This reaction has been considered to occur through a radical-anion mechanism as indicated in Scheme 12 (27). 

If the photo-Fries reaction would occur via a concerted mechanism, the absence of solvent should be of minor importance for the formation of rearranged products. However, conclusive evidence supporting the radical pair mechanism arises from the experiments carried out with phenyl acetate (10) in the vapor phase. The major product in the irradiations of 10 is phenol (13), which accounts for 65% of the photoproducts. Under these conditions, less than 1% of ortho -hydroxyace-tophenone (11) appears to be formed [19,20]. Conversely, when a high cage effect is expected, as in rigid matrixes (i.e., polyethylene), the result is completely different, and phenol is practically absent from the reaction mixtures [29]. In the intermediate situation (liquid solution), both rearranged products and phenol are formed in variable amounts depending on solvent properties. These observations... 

Which, then, are the reactions of a pair of radicals remaining in the solvent cage The almost independent quantum yield of the formation of 56 from the solvent viscosity favors the possibility that this or/Ao-rearrangement product is formed mostly by other than a radical recombination mechanism, i.e., probably via the concerted Path B. However, one might assume that in the difference to the diffusion from the solvent cage (eventually in the difference to the para-product formation, i.e., 67 + 68), the increasing viscosity is... 

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