Abstract
The halides, the methyl, the methoxy and the trimethylsilyloxy cyclododecanes are prepared and their conformations analyzed by low temperature 13CMR spectroscopy. In all cases two conformers are observed and characterized by ground state energy differences (ΔG 0 = 200–400 cal/mole) which are assigned to D 4 conformations bearing substituents either in corner(isoclinal) or noncorner(exo) positions. It is shown how the stereospecific substituent increments observed with cyclohexanes can be used to analyze quantitatively the experimental cyclododecane shifts (standard deviation 0.5 ppm) and to identify the conformations. Line shape analysis with the chloro compound furnishes a conformational barrier of 8.5±0.5 kcal/mole. With conformationally inhomogeneous compounds (measurements at room temperature) the α shifts of all derivatives (14 compounds, amongst others tert. butyl cyclododecane) can be correlated linearly to cyclohexane shifts. This is not so for β- and γ-shifts, which are determined by very stereoselective mechanisms.
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