Thermal Rearrangement of Allyl Silylmethyl Ethers

The working hypothesis that incorporation of the ir-bond of allyl groups in the rearrangement process leads to a symmetry-allowed [o-g -I-o a + reaction was delineated in Section I. Allyl ethers of the type 28 were chosen, in the expectation that the exchange of allyl and silyl should afford the thermodynamically more stable silyl ethers 29 (31, 32). 

As before, R = aryl, providing activation and preventing side reactions such as retro-ene and silanol eliminations. 

Compounds 28a-c were found to rearrange smoothly at 150°-190° to 29a-c, respectively. The reactions are extremely clean, the yields being greater than 96% in all cases (31, 32). Kinetic data for the rearrangements are summarized in Table V. In striking contrast to the silyl-silyl 

Utilizing appropriate compounds in the fluorenyl series it was shown that the silyl groups migrate 100% intramolecularly (ii). On the other hand, allyl migration was not found to be strictly intramolecular, mtermolecularity amounting to about 28%. This means that at least the latter portion does not adhere to a strict dyotropic mechanism. 

In order to test whether the allyl groups migrate with allyl inversion, the a,a-dideuterio compounds 30a-c were synthesized (SS). Thermolysis of 30a proceeds with a high degree of stereospecificity, only 20% of the wrong isomer (32a) being formed. In contrast, 30b and 30c afford statistical amounts of a,a and y,y products. 

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The thermal rearrangement of allyl(silylmethyl) ethers, an example of a dyotropic reaction, in which the silyl and allyl groups exchange their positions, exhibits only a very small solvent dependence this is as expected for a concerted reaction according to Eq. (5-55) [158b] ... 

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