Cycloaddition reactions component analysis

Cycloaddition reactions, 162-165, 197-198 component analysis, 168 Diels-Alder, 162, 198 ethylene + ethylene, 198 orbital correlation diagram, 198 stereochemistry, 162-163 Cycloalkanols, synthesis, 277 Cyclobutadiene barrier, 91 ground state, 91 point group of, 5 self-reactivity, 97 SHMO, 151 structure, 309-310 Cyclobutane... 

The most important competing process to the bond-formation is the complete electron transfer to form ion-radicals, which occurs where no bond formation is possible, for example, for aromatic donor-acceptor pairs. For vinyl copolymerizable pairs, the bond will form between the components to give a diradical tetramethylene. For the ionic homopolymerization system, on the other hand, it is difficult to distinguish the ion-radicals from zwitterionic tetramethylenes by the kinetic analysis. In this case, the accompanying cycloaddition reaction offers powerful evidence for the zwitterion formation, i.e., the bond-formation. 

In cycloaddition reactions, frontier orbital analysis considers the interaction of the HOMO of one component and the LUMO of the other. 

From a more detailed analysis of MO and state correlation diagrams. Woodward and Hoffmann presented a set of selection rules for cycloaddition reactions, which are summarized in Table 11.1. ° Here p and q are the number of electrons in the two n systems imdergoing the cycloaddition reaction. When the sum of p and g is a member of the 4n series, then the reaction is thermally allowed to be suprafacial with respect to one of the n components and antarafacial with respect to the other one. When the sum of p and qisa member of the 4n -h 2 series, then Ihe reaction is thermally allowed when it is either suprafacial with respect to both components or antarafacial with respect to both. As usual, the selection rules are reversed for photochemical reactions. 

The kinetically driven copper(I)-catalyzed cycloaddition of azides and alkynes requires hours of reaction time to obtain quantitative yields [63]. However, in the case of no-carrier added radiochemical synthesis the ratio of reactants and catalysts differs considerably from that in traditional chemistry. In particular, the azide component and catalyst are in huge excess compared with the [18F]fluoroalkyne. The quantitative incorporation of [18F]fluoroalkyne could be achieved in 10 minutes when an optimized catalytic system was used [81]. Re versed-phase HPLC analysis of all 18F-labeled peptides showed only a single product, indicating that the reaction proceeded regioselectively to yield 1,4-disubstituted 1,2,3-triazoles as previously reported [64]. 

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