Equilibria in Ring Compounds

Conformational analysis of 6-membered rings dominates this field. Evidence that as-l,4-dihalogenocyclohexanes (Br2 Clj Br, Cl) exist in chair conformations has been found.Variable-temperature studies of the spectra of cis, anti, cis- and cis, syn, cis- 1,2,4,5-tetra-acetoxy[3,3,6,6- H4,]cyclohexanes (48 and 49, R = OAc) can yield estimates of 1,3-diaxial interactions.  

4-tetramethylcyclohexane (50) the rate of ring inversion has been studied by line shape analysis of the methyl resonance at various temperatures. The results are compared with those obtained for the structurally similar acetone peroxide (51). In these two systems (50) and (51), the equilibria are between two identical chair conformers and single coalescence temperatures are observed ( —61°C and — 66°C at 60 Mc./sec., respectively). In duplodithioacetone (52) the more stable twist-boat conformer enters into the equilibrium, and 

Spectra of cis-, 2-bismethoxycarbonyl-[3,3,4,5,6,6- H6]cyclohex-ane (53, R = C02Me), have been studied at various temperatures, two coalescence temperatures being observed for the single inversion process. The authors conclude that the isomerism goes via a half-chair form. Conformational studies on 2-chlorocyclohexanols and on mono-, di- and trimethyl-jS-cyanoethylcyclohexanones have been reported. 

The ring inversion of 9,10-disubstituted as-decalins (54, R = R = CO2CH3 R = R = Br R = Br, R = CN) has been studied. At low temperatures, the CH2 protons of the 9,10 substituents are nonequivalent and give rise to AB spectra. As the temperature is raised the quartets collapse to broadened singlets showing typical AB exchange patterns. 

Spectra of the furan analogue of metacyclophane (55) and the corresponding pyridine analogue (56) show variation with temperature, the AjB, patterns at low temperature collapsing to single line at 63° and 13-5°C, respectively. 

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