The anomeric effect in a series of 2-(4-substituted-phenylseleno)-1,3-dithianes (substituents, R = N02, CF3, H, OMe, NMe2) decreases in the sequence N02>CF3>H>OMe>NMe2, pro- viding systematic experimental evidence for the role of stabilizing orbital interactions.
The anomeric effect' has been the subject of much investigation in recent years. Although several explanations have been advanced to account for this conformational effect, that formulated within the framework of perturbational molecular orbital theory has been widely accepted3. According to this proposal, favoured conformations of X-C-Y fragments are those which permit the expression of stabilizing orbital interactions between the p-type orbitals on X and Y, nX and qy, with the vacant o&Y and o&X orbitals, respectively. Such hyperconjugative interactions have been substantiated by theoretical investiga-tions3 s4. However, apart from crystallographic evidence5, there has been little6 experimental support for the role of orbital interactions in dictating the conformational preferences in X-C-Y fragments. As part of a program designed to investigate the nature and origin of conformational effects, we have recently reported7 the axial conformational preference of the arylseleno moiety in two 2-arylseleno-1,3-dithianes, both in solution and in the solid state. In order to assess the relative importance of orbital-interaction effects in S-C-Se fragments, we now report the quantitation of the anomeric effect in these and related compounds (1: -R = NMe2, OMe, H, CF3, N02). Careful analysis of the 'H nmr spectra (400 MHz) of the conformationally-averaged systems, measured in CDC13 at ambient temperature, revealed 1) significant chemical-shift differences for the H-4,6 axial and equatorial proton signals, A6a,/eq, (avg. 0.55 ppm), 2) the appearance of the H-2 signals as broad singlets (avg. WI/~ = 2.7 Hz) and 3) the presence of significant long-range 45 2,~~ and 5J 2,~~ couplings (avg. 1 Hz). These data are consistent only with the preponderance of a conformation in which the arylseleno moiety adopts an axial orientatiod-'.