77Se NMR chemical shifts of 9-(arylselanyl)triprycenes: New standard for planar structures of ArSeR and applications to determine the structures in solutions

Article abstract of DOI:10.1021/jo801786j

(Chemical Equation Presented) A set of new δ(Se) parameters is proposed as a standard for the planar (pl) orientational effect of p-YC ₆H₄ (Ar) in ArSeR, employing 9-(arylselanyl)triptycenes (1: p-YC₆H₄SeTpc). The Se-C_R bond in ArSeR is placed on the Ar plane in pl and it is perpendicular to the plane in pd. Large upfield shifts are observed for Y = NMe₂, OMe, and Me (-22 to -6 ppm) and large downfield shifts for Y = COOEt, CN, and NO₂ (19-37 ppm), relative to Y = H, with small upfield and moderate downfield shifts by Y of halogens (-1 ppm for Y = F and 4 ppm for Y = Cl and Br). This must be the result of the p(Se)-π(C₆H₄)-p(Y) conjugation in 1 (pl). While the character of δ(Se) in 1 (pl) is very similar to that in 9-(arylselanyl)anthracenes (2 (pl)), it is very different from that of 1-(arylselanyl)anthraquinones (3 (pd)). Sets of δ(Se) of 1 and 2 must serve as the standard for pl and that of 3 does for pd in solutions. Structures of various ArSeR in solutions are determined from the viewpoint of the orientational effect based on the standard δ(Se) of 1-3. While the structure of 2-methyl-1-(arylselanyl)naphthalenes is concluded to be all pl in solutions, those of 8-chloro- and 8-bromo-1-(arylselanyl)naphthalenes are all pd, except for Y = COOEt, CN, and NO₂: The equilibrium between pd and pl contributes to those with Y = COOEt, CN, and NO₂. The structure of 1-(arylselanyl)naphthalenes changes depending on Y. The structures of ArSeMe and ArSeCOPh are shown to be pl and pd, respectively, in solutions. Those of ArSePh and ArSeAr seem to change depending on Y. δ(Se) of 1-3 are demonstrated to serve as the standard to determine the structures in solutions. The rules of thumb derived from the characters in δ(Se) for 1-3 are very useful to determine the structures of ArSeR in solutions, in addition to the analysis based on the plots.