pi-facial selectivity in Diels-Alder reactions of cyclopentadienes having pi-systems at 5-positions and the solvent effect

The orbital mixing rule is successfully applied to the prediction of the pi -facial selectivity in the Diels-Alder reactions of cyclopentadienes having pi-systems at 5-positions, by taking into consideration the relative energ ies of the pi-HOMO of the diene and the pi and pi* orbitals on the substitu ent. When the pi orbital lies high enough to interact with the pi-HOMO, the participation of the pi* orbital in the mixing is negligible. The rule giv es a prediction of syn pi-facial selectivity. The FMO extends and distorts inwardly to favor the reaction at the syn side of the substituent (Case A: Psi(FMO)=pi-HOMO-pi+sigma). On the other hand, when the pi* orbital lies lo w enough to interact with the pi-HOMO, the pi* orbital plays a predominant role. The rule gives a prediction of anti pi-facial selectivity (Case B: Ps i(FMO)=pi-HOMO+pi*-sigma). The prediction was examined by theoretical calcu lation of the FMO's of the model dienes 1d, e, and f (Cp-X: X=CHO, CH=NOH, and CH=CH2), and substantiated by the reactions of the corresponding pentam ethylcyclopentadienes 2d, e, and f (Cp*-X: X=CHO, CH=NOH, and CH=CH2) with N-phenylmaleimide at 25 degrees C in CCl4 to give products with syn:anti ra tios of 0:100, 50:50 and 34:66, respectively. The new modeling of the solve nt effects on the pi-facial selectivity in the reactions of the amphoteric diene 2e was proposed as an application of the orbital mixing rule. Enhance ment of syn pi-facial selectivity is expected in a solvent which is a Lewis base. Under the conditions, the orbital mixing of Case A would be enhanced , since the hydrogen-bond formation between the solvent and the hydroxy hyd rogen of 2e should raise the pi and pi* orbitals. The prediction was substa ntiated by the observation of considerable enhancement of syn pi-facial sel ectivity in solvents such as CF3CH2OH, pyridine, THF, MeOH, EtOH, Et3N, TME DA and HMPA relative to CCl4, toluene and AcOH.