Sigmatropic Rearrangements of Organoboranes

a prominent feature of such allylic boranes is that reaction is accompanied by an allylic skin rearrangement (78) [Eq. (9)]. Such a reaction may be favored over the direct insertion of the alkyne into the 2-butenyl-boron bond because of the possible pericyclic character of the rearrangement. If no intermediate is involved, the reaction in Eq. (9) can be viewed as an allowed ene-reaction (55), a [,2s -l-, 2s -t- process if an intermediate collapse is involved, its collapse to the product can be viewed as a converted process, analogous to that shown in Eq. (8). 

The isomerizing rearrangements of such allylic boranes vary markedly 

Recent research on the chemistry of the boracyclopentadiene or borole system has brought to light further manifestations of such allylic shifts and some novel consequences. Since previous reports on the properties of boroles have proved to be erroneous (4), some comments on their successful synthesis and unusual properties are in order (36). Treatment of 1,4-dilithiotetraphenylbutadiene with phenylboron dihalide in ethyl ether solution will lead to pentaphenylborole as a pale yellow ethyl etherate (S) [Eq. (10)] the ether can be removed under reduced pressure to yield the deep blue-green uncomplexed borole [7 in Eq. 

Treatment of these boroles with alkynes leads to a prompt discharge of the color, even with the ordinarily less reactive diphenylacetylene. 

Long-Wavelength Maxima in the Visible Spectra of Substituted Boroles  

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