Circula:r Dichroism of Optically Active 1,2-Disubstituted 1,2-Diphenyl Ethanes Part II: Compounds without COOR-group at Benzylic CY

(R,R)-Hydrobenzoin and homochirally analogous thTeo-compounds of formula Ph-CHX-CHY-Ph (X = Y; X rO Y; Y = H) show anegative O-O CD-line below 37200 crn? and/er a positive one just above it, which can be explained by conformational equilibria around the Ph-C bonds. The Cotton effect at 220-210 nm is negative for this same configuration, if X rO Y (also Y = H), in these other cases there no such simple correlation. Cotton effects within the Elgor the E2g-transitions might be the reason for this, although otherwise no indication of them could be found. The same rules hold if a ring is closed between X and Y, the Cotton effect at 220-210 nm has thus the same sign as the torsional angle (Ph-)C-C(-Ph), and the same is also true of the next Cotton effect at 198-191 nm. The temperature dependence of the CD can be explained only by assuming conformational as well as solvational equilibria. Introduction of asingle p-chloro substituent shifts the borderline between the two O-O CD-lines of opposite signs to 36200 cm-lo It causes a sign inversion of the CD within the «-band of a »half'-molecule« of type CI-C6H4-C(H, Bn)-OZ, not however in the case of CI-C6H4(H, Bn)-CH20Z. Such p-chloro substitution does not influence the Cotton effects below 220 nm. p,p'-Disubstitution by the CH3CO-group shifts the p-band strongly batbochromically, the corresponding intense CD-couplet prooves the preponderance of that conformation deduced already from NMR-spectra, and prooves as well the absolute configuration.