1.2- Divinylcyclobutane Cope rearrangement

The Cope rearrangement is of great importance as a synthetic method e.g. for the construction of seven- and eight-membered carbocycles from 1,2-divinylcyclopropanes and 1,2-divinylcyclobutanes respectively (e.g. 11 12),... 

Not all Cope rearrangements proceed by the cyclic six-centered mechanism. Thus c/5-1,2-divinylcyclobutane (p. 1445) rearranges smoothly to 1,5-cycloocta-... 

The Cope and oxy-Cope rearrangement are very useful in organic synthesis, particularly when the 1,5-diene system is incorporated in a ring, then intringuing cyclic compounds may result. Also, the Cope and oxy-Cope rearrangements are greatly facilitated for a cw-1,2-divinylcyclobutane (Eq. 15)), resulting in the formation... 

The dicyclopenta[a,characteristic element of some terpenoids was obtained in 100% yield by a very facile Cope rearrangement of the highly functionalized divinylcyclobutane derivative 533 on heating in benzene at 55 °C for 4 h. The mild conditions can be due to participation of the lone pair of the sulfur atom or to the strain energy of the divinylcyclobutane fragment (equation 206)259. 

Not all Cope rearrangements proceed by the cyclic six-centered mechanism. Thus cis-1,2-divinylcyclobutane (p. 1131) rearranges smoothly to 1,5-cyclooctadiene, since the geometry is favorable. The trans isomer also gives this product, but the main product is 4-vinylcyclohexene (resulting from 8-33). This reaction can be rationalized as proceeding by... 

Cyclization of butadiene catalysed by Ni(0) catalysts proceeds via 7r-allylnickel complexes. At first, the metallacyclic bis-7i-allylnickel complex 6, in which Ni is bivalent, is formed by oxidative cyclization. The bis-7r-allyl complex 6 may also be represented by cr-allyl structures 7, 8 and 9. Reductive elimination of 7, 8 and 9 produces the cyclic dimers 1, 2 and 3 by [2+2], [2+4] and [4+4] cycloadditions. Selectivity for 1, 2 and 3 is controlled by phosphine ligands. The catalyst made of a 1 1 ratio of Ni and a phosphine ligand affords the cyclic dimers 1, 2 and 3. In particular, 1 and 3 are obtained selectively by using the bulky phosphite 11. 1,2-Divinylcyclobutane (1) can be isolated only at a low temperature, because it undergoes facile Cope rearrangement to form 1,5-COD on warming. Use of tricyclohexylpho-sphine produces 4-vinylcyclohexene (2) with high selectivity. 

Oxy-Cope rearrangements are popular in the divinylcyclobutane series, because several good methods of preparation of 2-vinylcyclobutanones are available. A number of these ketones, such as (105), can be... 

Ozkan, I., Zora, M. Transition Structures, Energetics, and Secondary Kinetic Isotope Effects for Cope Rearrangements of cis-1,2-Divinylcyclobutane and cis-1,2-Divinylcyclopropane A DFT Study. J. Org. Chem. 2003, 68, 9635-9642. 

For a stereoconvergent approach to dicyclopenta[tf,d]cyclooctane terpenoids via a Cope rearrangement of a highly functionalized divinylcyclobutane derivative see ref 1013. 

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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