Fragmentation-recombination process

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

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. 

Complications first arise when one notices the separation of nitrile and ester functions by a three-carbon chain in the product while there is only a methylene in this space in starting material II. This can only be understood in terms of a fragmentation-recombination process. That is, either the C-CN or C-COOMe fragment must maintain its integrity through the reaction, but not both. 

In the synthesis depicted in Fig. 10, White et al (52) converted the protected methoxymethyl (MOM) ether (30) derived from (29) into the aldehyde (31), followed by a Wittig reaction to obtain the E a,P-unsaturated ester (32). Subsequent reaction of the bis-MOM derivative (33) with lithium diisopropylamide furnished, via a fragmentation-recombination process, two diastereoisomers (34) and (35). After chromatographic separation of (34), its bis-ether derivative (36) was converted into... 

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... 

On the other hand, a double asymmetric version where both radical fragments are chiral has been investigated (equation 12) . The stereochemistry of the rearrangement products 20a and 20b is not the result of chelation control, but is determined by the stereochemistry of the hthium-bearing terminus. The significant difference in the stereospecificity of 19a and 19b reveals that a substantial level of mutal recognition of the radical enantiomers takes place during the recombination process. 

Ca and 03 are not bound directly to each other, so the (-resonance integral during the recombination process. It is zero when the fragments are separated and 0.8P when bound (see the parameter Table 2.2). Thus ... 

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