The Third Row Anomeric Effect. Conformational Analysis of 2-Phenylthio- and 2-Phenylseleno-1,3-diselenanes

Abstract

The conformational behavior of cisand trans-5-methyl-2phenylthio-and -2-phenylseleno-l,3-diselenanes has been examined by means of nmr and " ~e nmr spectroscopy.Whereas the cis isomers exhibit highly biased equilibria, the trans isomers display readily measurable equilibria at the slow-exchange limit.Direct integration of the individual , conformer resonances in the low temperature spectra of the latter isomers yields A G ' , , , , values of -0.33 f 0.01 (SPhl and -0.08 f 0.01 (SePhl kcal mol-I in favor of the diaxial confomr.The conformational free energy of the methyl group in 5methyl-1.3-diselenane (AG'la,x = '0.87f 0.03 kcal mol-'1 is then used to derive AG'l,,K values of -1.20 f 0.04 and -0.96 f 0.04 kcal mol-' for the equilibrium in 2-phenylthio-and 2-phenylseleno-1,3diselenane, respectively.Since the conformational free energy, AG'147X, of the 2-methyl group in 2-methyl-1.3-diselenane is t1.04 f 0.01 kcal mol-l, it is argued that there exists a significant Se-C-S and Se-C-Se anomeric effect.INTRODOCTION The anomeric effect1 refers to the torsional preferences about the C-X and C-Y bonds in RXCHZYRr m~lecules.~The effect has been rationalized both qualitati~el~~'~ and quantitatively' in tern of stabilizing n,+~*~-, and n , -. , * ., orbital interactions.The replacement of X andlor Y with the cognate atom X' andlor Y' can modify the magnitude of the interactions either as a result of differences in orbital energies (n.pn,.; nyfny,) OX as a result of differences in overlap between the interacting orbitals.There is extensive literature on such studies in which X(X'1 and Y(Y'1 are first-row atom and the extension to 4 .Thls paper is dedicated, with respect, to Professor Sir D.H.R. Barton on the occasion of his 70th birthday.