Competency traps

In this section indirect methods will be explored. One was given in Section 3.6. There, data were presented for two members of a large family of reactions of the phenyl radical with organic halides. Pairs of halides were used, and the ratio of two competing products was determined. Expanding the general scheme of Eq. (5-1) further, to use a pair of competing traps, we have... 

PTOC (pyridine thiocarbonyl) esters as the radical precursors and competing trapping agents (Scheme 15.4S) [122c]. 

Acyl azides can undergo photolytic cleavage and rearrangement upon irradiation at room temperature or below. In that case acyl nitrenes 8 have been identified by trapping reactions and might be reactive intermediates in the photo Curtius rearrangement. However there is also evidence that the formation of isocyanates upon irradiation proceeds by a concerted reaction as in the case of the thermal procedure, and that the acyl nitrenes are formed by an alternative and competing pathway " ... 

A ring isomerization giving the by-product 158a trapped as its 5,5-dioxide 158b was observed to compete with processes 506 - 152c(Ar = Ph, 87KGS614). 

Among the evidence for this mechanism are the facts that other products are obtained when the reaction is run in the presence of competing nucleophiles, for example, p-ethoxyaniline when ethanol is present, and that when the para position is blocked, compounds similar to 23 are isolated. In the case of 2,6-dimethylphe-nylhydroxylamine, the intermediate nitrenium ion 22 was trapped, and its lifetime in solution was measured. The reaction of 22 with water was found to be diffusion controlled. ... 

Aziridinocyclopropanes 163 derived from 2-phenylsulfonyl-l,3-dienes undergo BF3-induced rearrangement to bicyclic amines 165, which feature the skeleton of the tropane alkaloids. The reaction proceeds via cyclopropyl carbinyl cation 164, an intermediate also invoked in the analogous epoxide rearrangements. Trapping by fluoride ion is a competing pathway <96TL3371>. 

First, the order of ky values in different solvents is quite reasonably interpreted as a nucleophilicity order - e.g. ( 113)280 > HjO CH3NO2 > C2H5OH) and second, the k rate is (as is the k2) greatly reduced by steric crowding . Of course, a nucleophilic attack by solvent is a very likely process, a priori. In any solvent, the solvent itself will be the poorest nucleophile that can be studied since poorer ones will not effectively compete. Thus the k term of equation (21) corresponds to the ki value. The aquo intermediate of scheme (22) has been trapped by using reactions in the presence of OH , a poor nucleophile but good base . ... 

Homoenolate Protonation The p-protonation of homoenolates has been observed by Scheidt and co-workers, resulting in a redox transformation of enals to afford saturated esters 48. This process is catalysed by the NHC derived from imidazolium salt 46 and utilises phenol as a proton source [14]. A range of primary and secondary alcohols, and phenol itself, are competent nucleophiles with which to trap the acylazolium intermediate 47 generated by protonation (Scheme 12.8). 

Small amounts of cyclized products are obtained after the preparation of Grignard reagents from 5-hexenyl bromide.9 This indicates that cyclization of the intermediate radical competes to a small extent with combination of the radical with the metal. Quantitative kinetic models that compare competing processes are consistent with diffusion of the radicals from the surface.10 Alkyl radicals can be trapped with high efficiency by the nitroxide radical TMPO.11 Nevertheless, there remains disagreement about the extent to which the radicals diffuse away from the metal surface.12... 

One of the most promising processes is the active DeNO based on NO -trap materials. It has been developed for lean-burn gasoline engines. Cerium compounds are thought to intervene in different steps of the whole process (1) NO oxidation, (2) NO storage, (3) Nitrate desorption and NO reduction. Most probably, the main role of OSC materials is to accelerate HC partial oxidation during rich-spikes (giving CO and H2 as NO reducers). However, this beneficial effect of OSC compounds competes with a detrimental reaction,... 

---