Structurally degenerate Cope rearrangements

If the vinyl groups of c/. s-l,2-divinylcyclopropane are joined by a methylene bridge. Cope rearrangement of the resulting homotropylidene yields a product identical with the starting material. 

This reaction is said to be a structurally degenerate Cope rearrangement.  

Non-reactions of the sort exemplified by the valence tautomerism of 3,4-homotropylidene obviously cannot be studied by conventional methods which involve measuring the rate of disappearance of a starting material or rate of formation of a product. However, many of these degenerate rearrangements occur so rapidly that they cause averaging of some or all of the nmr frequencies of protons in the substrate at temperatures which are experimentally accessible. The rates of these processes can be estimated by use of temperature-dependent line broadening or spin-echo nmr techniques. 

The Cope rearrangement of 3,4-homotropylidene is facilitated by both pre-orientation and steric strain effects, and should occur quite rapidly. In fact, NMR spectroscopy shows that a molecule of 3,4-homotropylidene rearranges about a thousand times per second at 180°C, and about once per second at — 50°C  

8-Bridged derivatives of homotropylidene also undergo extremely facile degenerate Cope rearrangements. Perhaps the most intriguing of these reactions involves the substance tricyclo[3.3.2.0 ]deca-2,7,9-triene, or bull-valene (XIII). Bullvalene can have its ten carbon atoms arranged in more than 

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Doering and Roth, at the time of their first investigations of 86 in 1963, proposed that the structure 90, which they named bullvalene, should undergo degenerate Cope rearrangements that would make each of the ten CH groups equivalent.179 Equation 12.109 illustrates just a few of these transformations.180... 

Figure 4. Calculated transition state structures for degenerate Cope rearrangements of hydrocarbon 41 via transition state 43 and 2,3-epoxyoxepin (18) via transition state 46 (see ref. 34).

The structural peculiarity of trilithium heptaphosphide is its fluctuating anion.On account of the structural similarity to the hydrocarbon bullvalene, the P7 ion is capable of a degenerate Cope rearrangement and at room temperature has to be described by 1680 valence-tautomeric forms of equal structure. 

Figure 1.6 Part of the extended reaction network connecting multiple isomers of bullvalene via degenerate Cope rearrangements. Aithough the structure seems to remain unchanged, note that the blue carbon atom moves away from the black atom.

Doering revealed that degenerate Cope-rearrangement (isomerization of identical structures) occurs in homotropilidjene ... 

The way in which all 10 hydrogens (or carbons) in bullva-lene become equivalent involves a series of degenerate (i.e., leading to an equivalent structure) isomerizations known as Cope rearrangements (or [3.3] sigmatropic shifts). One such rearrangement is shown below (the dot is there to keep track of Cd) ... 

A search for possible degenerate behavior in bicyclo[3.3.0]octa-2,6-dienes164 has not produced clear results due to structural misassignments.126 The normal Cope rearrangement of 112 can be redirected through coordination to rhodium (I) to... 

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