No abstract

The Cotton effects of A-salicylidene derivatives of chiral a-and /3-arylalkylamines are interpreted in terms of a coupled oscillator mechanism. The exciton splittings in the circular dichroism (CD) spectra of the Ar,Ar'-di(5-bromosalicylidene) derivative of (R)-(-)-imns-l ,2-cyclohexanediamine and the N,N'-di(5-bromosalicylidene) and A^A'-disalicylidene derivatives of (R)-(-)-l ,2-propanediamine are utilized to determine the transition moment directions for the salicylidenimino chromophore. A simple chirality rule is then devised for A-salicylidene-/3-arylalkylamines, stating that the sign of the Cotton effects near 255 and 315 nm (or the long wavelength band in the case of exciton splitting) is the same as the sign of the chirality (right-handed screw for positive chirality) of the attachment bonds to the aryl and salicylidenimino groups. Agreement is shown with the CD data for ßarylalkylamine derivatives for which the aryl substituent is a phenyl, p-nitrophenyl, p-hydroxyphenyl, or p-chlorophenyl group. For the A-salicylidene derivatives of a-arylalkylamines, the sign of chirality is determined by placing the transition moments at the center of the aryl group and of the phenyl ring of the salicylidenimino chromophore. Again positive chirality gives positive Cotton effects near 255 and 315 nm if the interaction energy is positive. Agreement is shown with the CD data for chiral -arylalkylamine derivatives for which the aryl substituent is a 1-naphthyl, phenyl, 2-pyridyl, or 4-pyridyl group. For the A-salicylidene derivatives of chiral alkylamines, the one-electron mechanism accounts for the observed Cotton effects. Consequently, the C, symmetry of the salicylidenimino chromophore suggests the application of a planar sector rule.

The sign of the 1 L b Cotton effects (CEs) from about 240 to 270 nm in the circular dichroism (CD) of enantiomers of chiral benzylcarbinamines and benzylcarbinols is correlated with their absolute configurations using the benzene sector rule and a consideration of the equilibrium between their two conformers of lowest energy and of oppositely signed rotatory powers. For chiral benzylalkylcarbinamines, carbinamine salts, and carbinols, which show a single sign for their 1 L b CEs, a shift in the conformational equilibrium can explain a sign reversal of the CEs with a change in solvent, (R)-2-amino-1-phenylpropane showing negative 1 L b CEs in methanol but positive ones in cyclohexane. L-Phenylalanine in water shows positive 1 L b CEs, but in methanol it shows both negative and positive CD maxima. In methanol, the two longest wavelength maxima comprise a double CE associated with the band origin absorption maximum at 267 nm, the conformational equilibrium shifted from the positive conformer of L-phenylalanine to its negative one and the greater amount of the latter is now detected at slightly longer wavelength in the CD spectrum.

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Codes: MCG and MCP losv/l y alamos scientific laboratory of the University of California LOS ALAMOS, NEW MEXICO 87544 UNITED STATES ATOMIC ENERGY COMMISSION CONTRACT W-7403-ENG. 36 &r THIS oocu&EJjTis DMiiHnra This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Atomic Energy Commission, nor any of their employees, nor any of thair contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights. In the interest of prompt distribution, this LAMS report was not edited by the Technical Information staff.