ECLIPSED ISOPROPYL CONFORMATIONS OF 2 DIMERIC HYDRAZINES

The X-ray structures of 4,10-di-tert-butyl-5,9-diisopropyl-4,5,9,10- t etraazatetracyclo[6.2.2.2(3,6)]-tetradecane (s4iPr) and its 4,9-di-ter t-butyl-5,10-diisopropyl isomer (a4iPr) are reported. Both compounds a re in conformations having their in-N-alkyl groups (directed toward,th e central CH-CH bond of the molecule) anti to each other, as expected from previous work. The principal feature of interest is that one in-i sopropyl group in each compound is in an eclipsed conformation, NN,C(a lpha)Me twist angle -0.5(5)degrees for s4iPr and -6.4(4)degrees for a4 iPr. Low energy (somewhat less) eclipsed in-isopropyl conformations ar e predicted by both molecular mechanics (MM2) and semiempirical quantu m mechanical (AM1) calculations. The asymmetry of the potentially C-2 symmetric a4iPr because the two in-isopropyl groups are in different r otamers is apparently not a result of crystal packing forces, because a conformation with different isopropyl rotamers is the more stable on e by at least 1.0 kcal/mol in solution, determined by C-13-NMR spectro scopy. This result is not predicted by either calculation method. The ''monomer'', rt-butyl-3-isopropyl-2,3-diazabicyclo[2.2.2]octane (3). p roves to be a poor model for the conformations of 4iPr.