Cage mechanism

Our results on Ar I2 [57] are particularly noteworthy because of the extensive previous work aimed at characterizing the continuum fluorescence signals that were found to extend above the 12(B) dissociation limit. It was proposed that these signals resulted from a one-atom caging mechanism, in... 

I wrant to fill in some of the discussions we had at UCL on what I was calling the cage mechanism, to add still another to the list of names. I think it is better than the SAD mechanism. But the point that I think is essential to the general idea is that which comes out of the following set of numbers. If one considers a bimolecular gas phase reaction, and let s say 0.0121/ reagents, one can expect a molecule to experience something like 109 collisions per second. 

The approximate picture provided by the cage mechanism is then one of a preassembly of the reactants by diffusional encounters (whose duration may be lengthened if attractive forces or chemical bonding ability is present) with subsequent arrival of the energy of activation. 

One important implication of the cage mechanism is that those reactions should be favored for which there is a high probability for the reactants to be in each other s vicinity or cage. Hence, the prevalence of aquation reactions and the importance of ion pairs as intermediates in a nation reactions. I think it is very interesting that John Bailar s examples of stereospecificitv involve rather drastic changes in the immediate environment around the complex since concentrated systems were used in the one set of cases, and dilute ones, in the other. 

Arthur Adamson There has been some discussion and comment on what I called a cage mechanism, and I want to say in a little more organized way what I think is involved. 

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. 

Major polymer applications automotive industry (radiator end tanks, inlet manifolds, rocker covers), electrical components (connectors, switches, motor frames), bearing cages, mechanical handling components, fibers, carpets, tire reinforcement, many other applications... 

The possibility of a cage-mechanism involving a biradical intermediate (102) was excluded on the basis of the reaction of pentaphenylbismuth with a phenol bearing an internal radical trap, which did not afford the expected product from an internal trapping of free radicals. (Scheme 6.20)... 

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. 

A detailed study of the photochemical reaction of naphthaquinone with aldehydes has shown that the reaction occurs via an in cage mechanism at low temperatures but at ambient temperatures a small part of the reaction arises from an out-of-cage process. ... 

Cadmium, tris[4,4,4-trinuoro-l-(2-furyl)-l,3-butanedione]-structure, 65 Cadmium complexes equilibrium constant solvent effect, 516 Cage compounds in gravimetry, 525 Cage mechanism photochemistry, 393 Cages, 135 formation... 

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. ... 

Radical-cations may also be involved in the formation of phenyl esters from aromatic carboxylic acids and benzene in the presence of xenon difluoride, and can apparently be detected by e.s.r. spectroscopy during the reaction of iViV-dimethylaniline with perfluoropiperidine. The last reagent gives a low yield of o- and p-fluorophenol in the ratio 3.5 1 in its reaction with sodium phenoxide, and the predominance of ortho substitution is ascribed to a cage mechanism of the type shown in (3). However, towards 2,4,6-tri-t-butylphenoxyl radicals the piperidine acts as a source of fluorine atoms to give the dienone (4). ... 

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. 

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