Cope systems

Earlier dynamic NMR studies had already demonstrated that the substituted semibullvalenes [87], [88] and [89] were indeed rapidly equilibrating Cope systems and not ground-state homoaromatics (Anet and Schenck, 1970 Moriarty et al., 1972 Russell et al., 1973). The bis(trifluoromethyl)semi-bullvalenes [90] and [91] had also been formulated as non-equilibrating Cope systems (Liu, 1968 Liu and Krespan, 1969). 

A selective and brief survey of some of the more recent studies follows. Askani et al. (1984), using isotope perturbation methods (Siehl, 1987), showed that both the dimethyl [92] and the tetracarbomethoxy [93] semibullvalenes are Cope systems. In a related study, mild perturbation of the symmetrical semibullvalenes [94] and [95] is achieved by replacing one of the methyl groups with an ethyl group [94a] and [95a]. Again these systems were shown to be rapidly equilibrating tautomers and not neutral homoaromatics (Gompper et al., 1985). 

Solid-state UC CP-MAS NMR spectroscopy was used to study the dinitriles [97] and [98] both were shown to be rapidly equilibrating Cope systems (Jackman et al., 1989). Of particular interest is the fact that dinitrile... 

Barbaralane 548 (tricyclo[3.3.1.04,6]nona-2,7-diene) was first described in 1967 (see Reference 80 and literature cited therein). Barbaralane and its derivatives functionalized in the C(9) position (550a,b) are degenerate Cope systems whose equilibrium can be frozen... 

Temperature-dependent 13C NMR has been used to investigate Cope systems such as homotropilidene [243], semibullvalene [244], and bullvalene [245] (Table 4.12). Activation parameters have been determined. 

The oxyanion of the enolate (or a silyloxy group) appears to speed up this reaction relative to an unadorned Claisen system, but an even more dramatic acceleration is seen when Cope systems have an oxyanion substituent on the tetrahedral carbon, as in the reaction 5.59 —> 5.61, which takes place at a... 

Colwall and O Bara s weep, 303,359 COPE system. 170 Cox chart, 8, 9... 

One of the double bonds in the Cope system can be replaced by a triple bond, Scheme V/4. The five-membered V/24, heated to 170° for 20 min only, gave the nine-membered V/26, containing a (Z)-double bond and an a,(3- unsaturated carbonyl group in a yield of 62 % [17]. This reaction step was part of a synthesis of phoracantholide I (compare Chapter VII.3). 

Attempts have been made to find reaction sequences which allow the introduction of more than four atoms into a ring by a Cope rearrangement. Two of these methods should be mentioned, both quite different. The first method uses an enlarged Cope system , which forms bigger rings than the normal Cope system. In the second method the product of one Cope rearrangement can be easily transformed into the starting material for a second Cope shift sequence. 

Hctcro-Cope systems bearing a nitrogen in the 3-position lead to N-substituted allylic amines by forming a new C - N bond, new double bonds and, in most cases, anew stereogenic center10. 

Swaminathan, S. Base catalyzed rearrangements of oxy-Cope systems. J. Indian Chem. Soc. 1984, 61, 99-107. 

There are other types of proton shift tautomers, such as phenol/dienone, nitroso/oxime and imine/enamine, but these are less often encountered. There are also valence tautomers that exhibit fluxional structures, which undergo rapid sigmatropic rearrangements. Molecules that exhibit this type of isomerism are very interesting, but are not often encountered in undergraduate courses. An example of a valence tautomer is illustrated by the Cope system. 

Sigmatropic shifts. The oxy-Cope system (1) undergoes a [3,3] sigmatropic shift within minutes at 65" to give (2) when treated with potassium hydride in HMPT or THF. The reaction of the sodium salt of (1) is markedly slower the lithium alkoxide shows no evidence of rearrangement after 24 hours. ... 

Double bonds being part of hetero-Cope systems can be generated by Michael addition of nucleophiles to electron-poor carbon-carbon triple bonds. 

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