The use of copper acetylacetonate induced C-13 nmr line broadening in the determination of the stereochemistry of rigid cyclohexylamines

Two problems were encountered. First, the linewidths [at a given Cu(acac)2 concentration] tended to increase with time. Apparently Cu(acac)2 is an effective catalyst for the reaction of amines with the halogenated solvents (CHCl3, CH2Cl2) used to dissolve the Cu(acac)2. The reasons for this linewidth increase were not completely determined. In the case of cyclohexylamine, the addition of a small amount of cyclohexylisocyanide to a Cu(acac)2-cyclohexylamine solution resulted in an immediate increase in the nmr linewidth. The second difficulty was that the plots of linewidth vs. mole ratio of Cu(acac)2 were curved, the slope becoming less at higher concentrations. This complicated the assignment of values to the amount of line broadening. Various explanations were advanced but none were completely satisfactory. Some other copper B-diketonates were used and they produced plots with both more and less curvature. Similar results were found for the small paramagnetic shifts.

iv In spite of these difficulties, the data were applied to the original stereochemical problem. As expected, the γ carbons were found to broaden considerably more for equatorial cyclohexylamines than for axial cyclohexylamines. . The requirements for maximum line broadening appear to be similar to those for long range nmr coupling. Thus, the equatorial amines have the best "w" configuration between the metal and the γ carbon. While the actual values varied considerably, particularly between amines on secondary and tertiary centers, a good correlation was obtained by expressing the amount of γ carbon line broadening in terms of the β carbon line broadening. When this was done, the amines studied fell into two groups, axial and equatorial, with the amount of γ line broadening differing by about a factor of 10.