A Fragmentation reactions

Figure 7.44 Thermal bond rupture (a) fragmentation reaction, (b) oxidation degradation.

A fragmentation reaction occurs if one of the oxime substituents can give rise to a relatively stable carbocation. Fragmentation is very likely to occur if a nitrogen, oxygen, or sulfur atom is present a to the oximino group. 

The last few years have seen numerous applications of spin trapping to biological systems, and in these the trapping of hydroxyl radicals has assumed some importance. This work has been confined almost exclusively to nitrone scavengers 4 the fact that the hydroxyl adduct [6] of DMPO is much more persistent than that [7] of the commonly used nitrone, benzylidene-t-butylamine-N-oxide ( phenyl t-butyl nitrone ,3 or PBN) [3], may be due to a fragmentation reaction, with subsequent oxidation of the cr-hydroxybenzyl radical, as shown. 

A plausible reaction mechanism is that oxidative cleavage of tosylhydrazones is initiated by oxidation of the imino double bond of the tosylhydrazones with 51 to an oxaziri-dine derivative, which may be immediately converted to the carbonyl compound by a fragmentation reaction induced by 02 (equation 81). 

A fragmentation reaction which appears to proceed via the generation of 1,4-diradicals is the decomposition of 1,1-tetramethylenediazenes. Unlike the more stable 1,2-diazenes (tetrahy-dropyridazines, see Section 4.2.1.), the 1,1-isomers are not usually isolated or characterized by physical methods but are proposed as intermediates in the thermal decomposition of iV-phenyl-sulfonamidopyrrolidines 1, giving 1,4-diradicals which recombine to yield cyclobutanes 3 and 4. 39 These intermediates are also formed in the photochemical decomposition at low temperature of 1,1-tetramethylenediazenes, prepared in situ from 1-aminopyrrolidines and /er/-butyl hypochlorite.141... 

Fragmentation is another possible unimolecular reaction. A fragmentation reaction may be expressed as... 

Use of the appropriately substituted glycosyl chloride 22 in a fragmentation reaction... 

A fragmentation reaction on the oxygen atom of the peroxyacid group ... 

B in Figure 14.19). The cleavage products are formed by a fragmentation reaction in which four electron pairs are shifted simultaneously. 

As an alternative to the lead tetraacetate oxidation, (diacetoxyiodo)benzene can be used to initiate a fragmentation reaction which leads to unsaturated medium-sized lactones [110]. The structures of the starting materials are similar to those of compounds VII/157, VII/160, and VII/163. The same stereochemical consequences are observed as mentioned above. 

The decomposition of VIII/50 of known configuration, prepared from VIII/49, gives to the ten-membered VIII/51 by a fragmentation reaction [29] [30],... 

An excellent example comes from the intramolecular Diels-Alder reactions explored by Dreiding in 1983. One particular substituted cyclopentadiene was made by a fragmentation reaction (see Chapter 38). It might have been expected to give a simple Diels-Alder adduct. 

We should perhaps remind you here of the reversibility of the aldol reaction (Chapter 27) a retro-aldof is a fragmentation reaction with a carbonyl group as electron sink and OH as electron source. The aldof reaction usually goes in the other direction of course, but where steric or ring-strain factors are involved, this may not be the case. 

Now, the cis isomer has clearly undergone a fragmentation reaction and, as usual, numbering the atoms can help to identify the bond that breaks. The nitrogen lone pair pushes, the departing tosylate pulls, and the resulting iminium ion hydrolyses to the product aldehyde. 

Most syntheses make the side-chain alkene by an elimination reaction so the first disconnection is an FGI adding HX back into the alkene. The last C-C bond-forming operation in most syntheses is an intramolecular aldol reaction to make the enone so that can be disconnected next. It is the starting material for the aldol, a simple monocyclic diketone, which is usually made by a fragmentation reaction because this is a good way to set up the stereochemistry. 

Why is this important Well, if you cast your mind back to Chapter 38, you will remember that tlie overlap of parallel orbitals was very important in fragmentation reactions. Here, for example, is a fragmentation reaction that goes very well, but that can take place only if the nitrogen s lone pair is equatorial, because only an equatorial (black) lone pair can overlap with the antibonding orbital of the C-C bond that breaks. The chloride leaving group must be equatorial as well. 

STIRS Prediction of Substructures Not Present. CONGEN can utilize a BADLIST of features known not to be present in the molecule. Negative information is more difficult to determine from an unknown mass spectrum because other parts of the molecule may suppress a fragmentation reaction characteristic of a specific function. However, this is less true of substructures such as amino which strongly influence ion decomposition pathways. Thus it might be useful to carry out an evaluation of the ability of STIRS to identify the absence of specific substructures in the same way used to find the 589 substructures whose presence is best found by STIRS (8). 

---