Revisit of the leaving group (LG)/halogen exchange in 1,2‐dimesityl‐2‐phenylvinyl–LG systems: a caveat about ‘mistaken identity’ by X‐ray diffraction

The unusual report that Z ‐1,2‐dimesityl‐2‐phenylethenol ( Z ‐4‐OH ) affords with POBr 3 the crystallographically identified E ‐1,2‐dimesityl‐2‐phenylvinyl bromide ( E ‐4‐Br ) was reinvestigated together with related reactions. Reaction of Z ‐4‐OH with tosyl chloride gave Z ‐4‐OT s, Z ‐4‐Cl with SOCl 2 and Z ‐4‐Br with Me 3 SiCl/LiBr, PBr 3 or PPh 3 /Br 2 . Reaction of Z ‐4‐OT s with Br − or I − in MeCN gave Z ‐4‐Br or Z ‐4‐I respectively. The reaction of Z ‐4‐OH with POBr 3 was repeated under several conditions and gave Z ‐4‐Br , which was identified by X‐ray diffraction. The discrepancy with the previous experiment was ascribed to the formation of a minute percentage of E ‐4‐Br within the bulk of Z ‐4‐Br and to analysis of a crystal of E ‐4‐Br by X‐ray diffraction. The generalization that MesC(Ph) = C*‐Mes (* = · or +) species capture reagents (e.g. Br − , I − for 2 when * = +) from the face carrying the β ‐mesityl ring is corroborated and extended. It is suggested that the reactions of Z ‐4‐OH with SOCl 2 , PBr 3 or POBr 3 proceed via initial formation of chlorosulfite or phosphorus‐containing ester leaving groups that solvolyze to 2 or its ion pair, which react with halide ions formed from the anionic leaving group to give Z ‐4‐X (X = Cl, Br). From the X‐ray data of MesC(Ph) = C(X)Mes, (X = Br, OAc, E and Z isomers; X = OTs, Cl, I, Z isomer; X = H, E isomer) the effect of X on bond lengths, angles and dihedral angles, as well as on the difference between the E/Z isomers (X = OAc, Br), is determined and discussed. Although steric crowding is important, its effect is significant mainly on the ArCC dihedral angles. 1 H δ values for the nine systems show free rotation of all rings around the ArCC bonds on the NMR time scale. Higher field o ‐Mes‐Me, m ‐Mes‐H signals and lower field Ph‐H signals for the Z ‐ than for the E ‐isomers for X = OAc, Br were observed. Copyright © 2001 John Wiley & Sons, Ltd.