STERICALLY CONGESTED MOLECULES .11. THE 2,6-DIISOPROPYLPHENYL SUBSTITUENT AT C=C AND C=N GROUPS

Two series of new compounds with sterically shielded, sp hybridized C- alpha atoms are prepared from bromo(chloro)-2,6-diisopropylbenzene. Th e alpha-(2,6-diisopropylphenyl) substituent common to all of them is u sed as an NMR-spectroscopic indicator for the ease of aryl-alpha singl e-bond rotation. The corresponding DELTAG(not-equal) values are >23.8 kcal/mol with methyl or bromine as additional alpha substituents at a C=C bond, but <8.5 kcal/mol with alpha-H. The analogous 2,6-diisopropy lacetophenone imine has DELTAG = 18.0 kcal/mol while the N-substitute d imines prefer the (Z) conformations with DELTAG(not-equal) > 23.3 kc al/mol, and DELTAG(not-equal) almost-equal-to 31.5 kcal/mol for the (Z )-hydrazone. Comparisons of these data with reaction conditions show t hat chemical transformations are not seriously impeded unless they dep end on reagent attack at a pi face of the C-alpha atom in a C=C or C=N group which rotates slowly about the bond to 2,6-diisopropylphenyl.