Rearrangements degenerate

The first observation of the proton transfer in pyrazoles in the solid state was made for the intermolecular tautomerism in 3,5-dimethylpyrazole 10b (85JA5290). The degenerate rearrangement was recorded using the... 

Degenerate rearrangements of the l-phenylmercury-3,5-dimethylpyr-azoles 31 are apparently the first example of metallotropy in the series of azoles (71DOK110) (Scheme 15). 

Derivatives of imidazole 41 also exhibit fast degenerate rearrangement, whieh, unlike those of pyrazole derivatives, are exelusively intermoleeular and also very sensitive to the influenee of traees of water (Seheme 23) [74JOM(70)347 76IC3054 77JOM(132)69]. 

Numerous data on kinetics of annular tautomerism have been obtaifled for the degenerate rearrangements of pyrazole derivatives. Since all these rearrangements were found to be intramolecular, we can compare kinetic measurements carried out in different media. The following order of increase in migration ability of various groups is established ... 

Parallel and reversible reactions. The isomerization of allyl phenyl sulfide is a degenerate rearrangement made detectable by isotopic labeling of one end of the allyl group, permitting kinetic monitoring by NMR techniques.12... 

Carbocations that rearrange to give products of identical structure (e.g., 5 5, 6 6 ) are called degenerate carbocations and such rearrangements are degenerate rearrangements. Many examples are known. ... 

Scheme 27 Degenerate rearrangement of the three-membered ring...

Bulo et al. <2004AG732> have studied the degenerate rearrangements that occur among bicyclic hydrocarbons with three-membered rings. Extending the work to include phosphorus resulted in the formation of compound 197. Compound 198 was heated to 50 °C and the Woodward-Hoffmann-allowed product for a [ l,5]-shift (compound 199) was produced (Scheme 17). 

The cyclopentyl cation (39) undergoes a rapid degenerate rearrangement which can be frozen out at cryogenic temperatures as shown by solid state CPMAS 13C NMR spectra.57 MP2/6-31G(d,p) calculations show that cyclopentyl cation has a twisted conformation 4058 in which the axial hydrogens are bend toward the carbocation center. This is due to the pronounced geometrical distortion caused by the hyper-conjugative interaction of the /i-cr-C-H-bond with the formally vacant 2pz-orbital at the C+ carbon of this secondary carbocation. 

It has been found by NMR that carbocations la-d and 3 are prone to undergo degenerate rearrangements [automerization reactions (26)] via 1,2-shifts of the vinyl and the methyl-substituted vinyl groups (Schemes 5, 6), with... 

This conclusion is in accord with the results of MINDO/3 quantum-chemical calculations. Cation 7 is also prone to undergo a rather slow degenerate rearrangement by 1,2-bridge shift (AG is about 60 kJ/mol) (Scheme 8). This rearrangement was probed via 2H NMR spectroscopy, when the 9-CD3-analog of carbinol 5 was used as precursor for carbocation generation. At -50 °C cations 6-8 transform firstly into l,l,2,3,3a,4,8b-heptamethyl-l,3a,8b-trihydrobenzopentalenium ion (9) and then (partially) into 1,1,2,3,4,4,8b-heptamethyl-l,4,8b-trihydrobenzopentalenium ion (10) (Scheme 9). 

NMR spectra show that complexes 52a,b undergo fast degenerate rearrangement proceeding by intra- and/or intermolecular transfer of NO group (Scheme 21). 

Table 4. Calculated (MINDO/2) and Observed Activation Energies for Degenerate Rearrangements in the Bullvalene Series...

The preparation of conjugated diene complexes will be presented by groups. In addition, isomerization reactions, or degenerate rearrangements with activation energies >25 kcalmol-1, will be considered in this section. 

A few examples, illustrating the principle of this type of degenerate rearrangement, are the thermo-induced equilibrium shift of 3-benzoylamino-5-methyl-l,2,4-oxadiazole into 3-acetylamino-5-phenyl-l,2,4-oxadiazole and the isomerization of 5-benzoyl-methylfuroxan oxime into 4-[a-nitroethyl]-3-phenylfurazan (82G181) (Scheme 1.8). They are extensively discussed in Chapter IV. 

Degenerate rearrangements have also been found during hydrazinolysis of 2-thioalkyl-3-phenylquinazolin-4(3//)-one (42). The product obtained is... 

Spectroscopic and kinetic studies were undertaken to get a better understanding of the mechanism of this degenerate rearrangement. From studies... 

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