PREPARATION AND ISOMERIZATION OF 1-PHENYLSELENO 1,3-DIENES

The preparation of a series of 1-phenylseleno 1,3-dienes is described starting from enyne derivatives via a hydrozirconation/transfer sequence. Hydrozirconation of a series of conjugated enynes having the general formula HC = CCR1 = CR2R3 (3a: R1 = R2 = R3 = H; 3b: R1 = R3 = H, R2 = OMe; 3c: R1 = R2 = H, R3 = OMe; 3d: R1 = R2 = -(CH2)4-, R3 = H) leads to the formation of zirconium dienyls of the formula (eta-5-C5H5)2Zr(Cl)-CH = CHCR1 = CR2R3, 4a-d, respectively; this reaction is both completely stereoselective and chemoselective. Use of the deuterium-substituted reagent (eta-5-C5H5)2Zr(D)Cl generates the corresponding isotopomers (eta-5-C5H5)2Zr(Cl)-CH = CDCR1 = CR2R3, 4a-d-d1. Addition of PhSeX (X = SePh, Cl, or N-phthalimido) to the zirconium dienyl derivatives 4a-d at low temperature (-20-degrees-C) and in the dark results in the formation of the 1-phenylseleno 1,3-dienes (PhSeCH = CHCR1 = CR2R3 (5a: R1 = R2 = R3 = H; 5b: R1 = R3 = H, R2 = OMe; 5c: R1 = R2 = H, R3 = OMe; 5d: R1 = R2 = -(CH2)4-, R3 = H). This transfer of the dienyl unit from zirconium to selenium proceeds with complete stereoselectivity and with retention of configuration at the 1-position as long as light is excluded and the reaction is carried out at low temperatures. In the presence of room light (fluorescent), mixtures of stereoisomers are obtained for the seleno dienes 5a-c; no apparent isomerization is observed for 5d; similar results are obtained upon thermolysis (80-degrees-C for 24-48 h). The mechanism of this isomerization process was determined to be intermolecular on the basis of crossover experiments; in addition, the use of radical traps established that the process was a radical chain mechanism, probably via addition of PhSe. to the seleno diene followed by single-bond rotation.