Divinylcyclobutanes, rearrangement

A similar acceleration has most recently been observed in the rearrangement of vinylcyclopropanes of type (39 Scheme 8). This fluoride-mediated vinylcyclopropane-cyclopentene isomerization proceeds at -78 C to give (40) in 85% yield this is to date the mildest condition available. Two possible intermediates, the enolate anion (39a) or the diradical anion (39b), may be responsible for such acceleration in analogy to the enolate anion accelerated divinylcyclobutane rearrangement recently reported." The mechanism of this transformation is unclear but may involve anion acceleration similar to that observed in the rearrangement of sulfonyl anions derived from (42 Scheme 8). By comparison the thermolysis of (39) produced exclusively the endo isomer of (41) at 580... 

Finally, a recent study by Majetich and Hull has demonstrated the feasibility of the carbanion-accel-erated divinylcyclobutane rearrangement. Whereas the thermal rearrangement of the divinylcyclobu-tane (273 Scheme 38) required extended heating at 180 C and proceeded in modest yield, the corresponding enolate was shown to rearrange smoothly at -35 C in 90% yield. As illustrated in Scheme 39, the enolate-accelerated DVCB rearrangement can be employed in tandem with the intramolecular fluoride-promoted Michael addition of allylsilanes to provide an attractive route to a variety of fused bi-cyclic cyclooctane derivatives. 

Anion-accelerated divinylcyclobutane rearrangements 8333 Cyclobutene Rearrangements... 

The Cope rearrangement, like the Claisen rearrangement, is a no mechanism reaction and thus does not involve ionic or radical intermediates. For practical purposes the result is that Cope rearrangements are independent of catalysts and of the nature of the solvent, and that substituent effects are slight. Steric influences, however, are considerable cw-1,2-divinylcyclobutane rearranges to 1,5-cyclooctadiene within a few minutes at 120° ... 

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