Isomerization degenerate

The planar civ-butadiene is not a minimum but rather represents the transition structure for the degenerate isomerization of the two identical gauche isomers. 

Early kinetic studies on the structural isomerization of cyclopropane to propene provided estimates of activation parameters73 75 and prompted speculation that the reaction might well involve a trimethylene diradical intermediate. This possibility seemed reinforced when the thermal interconversion of the els and trans isomers of l,2-d2-cyclo-propane at 414 to 474 °C (equation 1) was reported in 195876. This structurally degenerate isomerization was found to be substantially faster than conversion to deuterium-labeled propenes—about 24 times faster at the high pressure limit76 77. 

Vigorous conditions have been found to cause similar skeletal rearrangements in adamantane itself. Thus, treatment of adamantane-2-14 C48) with aluminum bromide in carbon disulfide at 110° C for 8 hours gives rise to nearly 80 % net scrambling of the tertiary and secondary carbons 49 The mechanism for this degenerate isomerization possibly is similar to that depicted in Scheme 5. 

Low-temperature NMR spectrum of triaryl-A3-iodane 73, prepared from iodo-A3-iodane by the ligand exchange with aryllithium, shows two methyl singlets at 6 2.24 and 2.41 [112]. Simultaneous broadening of the methyl and biphenyl aromatic resonances at higher temperatures demonstrates that the temperature dependence is caused by the degenerate isomerization of p-tri-fluoromethylphenyl substituent between two equivalent sites (73 73 ). The... 

IR spectra of dialkynyl-A3-iodane 75 differentiate the two alkynyl groups, occupying apical (2187 cm-1) and equatorial (2152 cm-1) sites, while the and 13C NMR spectra display only single resonances for each of the different protons and carbons, probably due to degenerate isomerization by rapid pseudorotation on the iodine [114]. 

Consider a polyhedral isomerization sequence P, rp2 S3 in which Pi and are combinatorially equivalent. Such a polyhedral isomerization sequence may be called a degenerate polyhedral isomerization with Pj as the intermediate polyhedron. Structures undergoing such degenerate isomerization processes are often called fluxional. A degenerate polyhedral isomerization with a planar intermediate polyhedron (actually a polygon) may be called a planar polyhedral isomerization. The simplest... 

Scheme 28 Transition structures (112a-d) corresponding to the degenerate isomerization of imines (108a-d)...

Several important spectroscopic structural studies of polyvalent iodine compounds in solution have been published [108-112, 189]. Reich and Cooperman reported low-temperature NMR study of triaryl-X -iodanes 27 (Scheme 1.1), which demonstrated that these compounds have a nonsymmetrical planar orientation of iodine-carbon bonds and that the barrier to unimolecular degenerate isomerization between 27 and 27 is greater than 15 kcal mol The exact mechanism of this degenerate isomerization is unknown both pseudorotation on iodine(III) and intermolecular ligand exchange may account for the isomerization of these compounds [189]. 

Scheme 1.1 Degenerate isomerization of triaryl-X -iodanes 27 in solution.

Scheme 1.2 Degenerate isomerization of (diacetoxyiodo)binaphthyl 28 due to rapid pseudorotation on iodine.

Finally, a small amount of degenerate isomerization of 1-isopropyl-3,3-dimethylcyclopropene was observed, which could only reasonably arise via the vinylidene derived from cleavage of the less substituted bond (Scheme 4.5). ... 

The reaction of aluminium trichloride and 2,8-decadiyne gave the complex (312). At room temperature the H n.m.r. spectrum of (312) showed signals at 8 2.13 and 8 2.21 due to non-equivalent methyl groups. The signals coalesced at 92 °C. This behaviour has been ascribed to the degenerate isomerization of (312) via transi-... 

Figure 1.8 (a) A forbidden single dsd degenerate isomerization of a tricapped trigonal prism, (b) An allowed double dsd degenerate isomerization of a tricapped trigonal prism. 

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