Fragmentation-recombination

M. Odelius, M. Kadi, J. Davidsson, and A. N. Tarnovsky, Photodissociation of diiodomethane in acetonitrile solution and fragment recombination into iso-diiodomethane studied with ab initio molecular dynamics simulations. J. Chem. Phys. 121(5), 2208-2214 (2004). 

Carbonaceous solids appear as a result of retrogressive reactions, in which organic thermal fragments recombine to produce insoluble semi-cokes (59,65). Coke formation is observed during liquefaction of all coals and its extent can vary widely according to the coal, the reaction solvent, and reaction conditions. The predominant inorganic species produced during the process of coal... 

Scheme 5.22. Domino Wolff/Cope rearrangement/Norrish type I fragmentation/recombination process.

A few selected types of such relationships between different structures will be presented in this paragraph and in the following ones ( 3.9 and 3.10). Examples will be considered of possible (virtual and/or real) transformation mechanisms from one structure type to another (degenerate, derivative structures) examples will also be discussed of groups of structures interrelated because based on the assembly of similar blocks and fragments (recombination structures). 

Furthermore, the oxy-Cope rearrangement of allenic cycloheptane alcohol 47 (NaOEt, THF, 20 °C, 12 h, 80%) gave rise only to ring-enlarged product 48 without transannular cyclization (equation 15)29. The above transformations can be rationalized either by the fragmentation-recombination mechanism or by a concerted oxy-Cope mechanism29. 

Because a benzilic acid rearrangement is the last step in the formation of saccharinic acids the difference between these results could be due to complexation of hydroxyl groups and alkoxide anions by the divalent calcium ions. Such a complexation apparently promotes isomerization and, hence, a fragmentation-recombination to the xylosaccharinic acid. This route has been questioned, and yet it provides a rational explanation of the results. 

Isomerization of the molozonide appears to occur by a fragmentation-recombination reaction, as shown in Equations 11-7 and 11-8 ... 

As we shall see, the components of the chains are such that the strands can be held together efficiently by hydrogen bonds. In agreement with this structure, it has been found that, when DNA is heated to about 80° under proper conditions, the strands of the helix unwind and dissociate into two randomly coiled fragments. Furthermore, when the dissociated material is allowed to cool slowly under the proper conditions, the fragments recombine and regenerate the helical structure. 

The Skraup-Doebner-Von Miller synthesis of quinolines - involving condensation of an aniline with an ,/i-unsalurated ketone - has been investigated using 13C-labelled ketones in cross-over experiments a complex fragmentation-recombination mechanism involving imine intermediates is indicated.17... 

Thermolysis of 2-acetoxy-2-methoxy-5,5-dimethyl-A3-l,3,4-oxadiazoline affords acetoxy(methoxy) carbene.60 The thermal rearrangement of acetoxy(methoxy) car-bene to methyl pyruvate was studied by DFT at the B3PW91/6-31G(d,p) level. The conformation of the carbene was considered, as were competing fragmentations to radical pairs. The authors concluded that the reaction is a concerted 1,2-migration rather than a fragmentation-recombination process. 

A maltol-ammonia browning reaction produced thirteen pyrazines, two pyrroles, two oxazoles, and one pyridine (12). The major products of this system were 2-ethyl-3-hydroxy-6-methylpyridine and 2-ethyl-3,6-dimethylpyrazine. It is difficult to construct possible formation mechanisms for these compounds from maltol and ammonia. All the carbon atoms must come from maltol. It is possible, then, that maltol degrades into smaller carbon units and that these fragments recombine to form larger carbon units, producing these compounds. Recently, the formation of thiophenones and thiophenes from the reaction of 2,5-dimethyl-4-hydroxy-3(2H)-furanone and cysteine or cystine was reported (13. 14). All these reaction mixtures were reported to possess a cooked meat-like flavor. 

The reactions that are observed are unimolecular fragmentations. Recombinations are impossible since the collisions, under the usual conditions, are non-existent. The rearrangements and the opening of rings are detectable only if they are followed by a fragmentation. These reactions thus depend only on the structure and on the energy contained within the molecule. 

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