CONFORMATIONS OF 1,2-DISUBSTITUTED INDANS . ELECTRONEGATIVITY CORRECTIONS TO NUCLEAR MAGNETIC RESONANCE COUPLING CONSTANTS

Nmr spectra of hydrogens on the cyclopentene rings of 23 1,2-disubstituted indans of known stereochemistry have been analyzed completely. Constants in the Karplus equation that relates vicinal coupling constants to substituent electronegativity have been evaluated using data from the bicyclo[2.2.1]heptene system, and the resulting equations were used to correct the indan vicinal coupling constants for electronegativity effects. The corrected coupling constants (Ju values), which should be a function of dihedral angle only, show that 1,2-disubstituted indans exist as a mixture of two puckered conformations. For both cis and trans compounds the degree of pucker is controlled by substituent size and not by dipole-dipole forces. In the trans compounds the population of the diaxial conformation increases relative to that of the diequatorial conformation as the size of the substituents increases, trans compounds bearing a hydroxy group at C1 or C2 shown an unusually strong preference for the diequatorial conformation. Analysis of data in the literature shows that 1-haloindans prefer the conformation in which the halogen atom is axial. © 1969, American Chemical Society. All rights reserved.