Importance of substituent intramolecular charge-transfer effect on the molecular conformation of diphenyl ethers

The conformational control of diphenyl ethers by the electronic effect of s ubstituents has been demonstrated by spectral measurements and their analys es based on CNDO/S-Cl calculations and by minimum-energy optimization using the method of ab initio MO calculations with 6-31G(d) basis sets. Introduc tion of an electron-accepting substituent at the par a position of either r ing of diphenyl ether fixes the oxygen-bridged phenyl rings in the stable s kew form, whereas unsubstituted diphenyl ether adopts the nonrigid form. Th e conformational stabilization is brought about by an intramolecular charge -transfer interaction of the electron-accepting substituent with a lone pai r pi orbital of the bridging oxygen atom through a pi-electronic system of the benzene ring. On the other hand, the strong electron-donating substitue nt attached at the para position allows the diaryl ether moiety to be rigid in the preferable skew or twist conformation. This behavior is understood in terms of a competition between the electron-donating substituent and the ether oxygen for electron donation to the pi-electronic system rather than a synergistic charge-transfer interaction between them. The electron-donat ing ability of the alkyl groups is insufficient to fix the nonrigid conform ation of diphenyl ether. Thus, it has been demonstrated that the intramolec ular charge-transfer interaction caused by strongly interacting substituent s is important in governing the conformation of diphenyl ethers, and the su bstituent attached at the para position of either ring of diphenyl ether ca n control the conformation of whole molecule, depending on the electron don ating/accepting character of the substituent.