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Sterically Congested Molecules, 11. The 2,6‐Diisopropylphenyl Substituent at CC and CN Groups
Author(s) -
Knorr Rudolf,
Ruhdorfer Jakob,
Böhrer Petra,
Bronberger Hildegard,
Räpple Edith
Publication year - 1994
Publication title -
liebigs annalen der chemie
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 0170-2041
DOI - 10.1002/jlac.199419940417
Subject(s) - chemistry , substituent , steric effects , aryl , imine , bromine , molecule , double bond , reagent , medicinal chemistry , benzophenone , bond length , stereochemistry , crystallography , alkyl , photochemistry , organic chemistry , catalysis
Two series of new compounds with sterically shielded, sp 2 ‐hybridized C‐α atoms are prepared from bromo(chloro)‐2,6‐diisopropylbenzene. The α‐(2,6‐diisopropylphenyl) substituent common to all of them is used as an NMR‐spectroscopic indicator for the ease of aryl‐α single‐bond rotation. The corresponding ΔG ≠ values are >23.8 kcal/mol with methyl or bromine as additional α substituents at a CC bond, but <8.5 kcal/mol with α‐H. The analogous 2,6‐diisopropylacetophenone imine has ΔG * = 18.0 kcal/mol while the N ‐substituted imines prefer the (Z) conformations with ΔG ≠ > 23.3 kcal/mol, and ΔG ≠ ≈ 31.5 kcal/mol for the (Z)‐hydrazone. Comparisons of these data with reaction conditions show that chemical transformations are not seriously impeded unless they depend on reagent attack at a π face of the C‐α atom in a CC or CN group which rotates slowly about the bond to 2,6‐diisopropylphenyl.