高等有机化学_2011[精选].ppt

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高等有机化学_2011[精选]

Migratory Aptitude of pinacol rearangement Longer Nucleophilic Rearangements The question as to whether a group can migrate with its electron pair from A to C in W-A-B-C or over longer distance has been much debated. Although claim has been made that alkyl groups can migrate in this way, the evidence is that such migration is extremely rare, if it occurs at all. One experiment that demonstrated this was the generation of 3,3-dimethyl-1-butyl cation: No product arising from 2-methyl-2-pentyl cation were found. However, the following reaction do take place: The following reaction did took place. Free-radical Rearrangements Free-radical rearrangements are much less common than the nucleophilic type previously discussed, for the reason mentioned before. Where they do occur, the general pattern is similar. There must first be generation of a free radical, and then the actual migration in which the migrating group moves with one eletron: Finally, the new free radical must stabilize itself by a further reaction. Migratory Aptitude of free-radical rearangements The order of radical stability leads us to predict that here too, as with carbocation rearrangements, any migrations should be in the order primary secondary tertiary, and that the logical place to look for them should be in neopentyl and neophyl system . Many other case of free-radical migration of aryl groups have been found. It is noteworthy that the extent of migration is much less than the corresponding carbocations: thus in the example given, there only about 50% migration, whereas the carbocation would have given much more. Also noteworthy is that there was no migration of the methyl group. In general, it may be said that free-radical migration of alkyl group does not occur at ordinary temperatures. Another type of migration that is very common for corresponding carboncations, but which is not observed for free radical, is 1,2 migration of H. Some Examples of That Free-radical Migration of Alkyl G

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