The pyridine n-oxide group. A potent radical stabilizing function
✍ Scribed by Xavier Creary; M.E. Mehrsheikh-Mohammadi
- Publisher
- Elsevier Science
- Year
- 1988
- Tongue
- French
- Weight
- 228 KB
- Volume
- 29
- Category
- Article
- ISSN
- 0040-4039
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✦ Synopsis
The rate of the methylenecyclopropane rearrangement is greatly enhanced by the 4-pyridyi N-oxide group due to spin delocalization in the transition state which impatfs nitroxide radical character to the biradical intermediate. For a number of years wet and other&a have been interested in the relative ability of various groups to stabilize free radicals. Towards this end, we have developed the methylenecyclopropane rearrangement probe based on the thermal rearrangement of 1 to 3. Substituents on an aromatic ring influence rearrangement rates and CY values, which are a measure of stabilizing or destabilizing effects in the intermediate biradical 2, can be derived from rearrangement rates of 1. The advantages and disadvantages of this probe have been discussed.lolc
It is known that pyridyl and the related N-oxide systems are cation destabilizing relative to the phenyl analog.9 The y+ value'0 of the Cpyridyl group is 1 .16 and that of the 4-pyridyl N-oxide group is 0.45. We therefore wanted to evaluate the effect of these heterocyclic systems on stabilities of free radicals. We have now turned our thermal rearrangement probe to an examination of the heterocyclic pyridine analogs of 1.
The requisite systems were prepared by addition of the carbene derived from 4_pyridyldiazomethane, 4,ll to l,l-dimethylallene, which led to the methylenecyclopropane 5. Oxidation with m-chloroperbenzoic acid gave the corresponding N-oxide 6. The 3-pytidyl systems 7 and the N-oxide 6 were prepared in an analogous fashion starting with 3-pyridyldiazomethane. 11 These systems rearranged thermally in clean first order processes to give the isopropylidenecyclopropanes 3 in high yield.
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