Rearrangements small rings

A related reaction is the oxa-di-n-methane rearrangement, where one of the C=C double bonds is replaced by a C=0 double bond. The substrates are thus /3,y-unsaturated ketones. The rearrangement proceeds from the triplet state. This oxa-variant gives access to highly strained molecules containing small rings, as has been demonstrated by irradiation of norborn-5-ene-2-one 10 ... 

The retro-ene reaction also is of synthetic importance. While the application of high pressure facilitates the ene reaction, the retro-ene reaction is favored at higher temperatures. Furthermore small-ring strain can shift the equilibrium towards the side of the dienes. The vinylcyclopropane 11 rearranges by a synchronous process to the open-chain diene 12. Formally this process is the reverse of an intramolecular ene reaction ... 

Two of the three general types of secondary reactions resulting from photochemical a-cleavage of carbonyls, namely molecular rearrangement and hydrogen transfer to yield aldehydes or ketenes, have been discussed. The third type of reaction observed, decarbonylation, will be discussed in this section. The discussion will begin with the decarbonylation of small ring carbonyls. By way of example of this type of reaction, diphenylcyclopropenone decarbonylates upon photolysis to yield diphenylacetylene(57) ... 

Once we leave the realm of small-ring propellanes5 and post-1975 reports41,42 , it appears that it should be possible to convert suitably disposed sets of dispiran moieties into their respective propellane counterparts 39,40) because of the relative stabilities of the two families. By the same token, there are plenty of examples of acid-catalyzed rearrangement of propellanes, through which systems of yet greater stability may be obtained 5 41,42). 

Bond cleavage in ion radicals often involves the expulsion of an ionic moiety from the parent molecules. Rearrangements of ion radicals involving an intramolecular bond cleavage, as for example the opening of a small ring,78-81 belong to the same class of reactions. Their dynamics should therefore lend themselves to the same type of analysis. 

Table II also lists several isomerizations and skeletal rearrangements (examples 4-7) which are related to butadiene-ethylene dimerization. Protonation of phosphorus-containing nickel(O) complexes is sufficient to achieve skeletal rearrangement of 1,4-dienes in a few seconds at room temperature, probably via cyclopropane intermediates (example 6, Table II). For small ring rearrangements see Bishop (69).

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