Importance of the role of secondary orbital interactions in the Diels–Alder reaction. Regioselectivity in the catalyzed and uncatalyzed reactions of juglone and aliphatic dienes

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Abstract
The regioselectivities in the uncatalyzed and catalyzed Diels–Alder (DA) reactions of juglone with piperylene (penta-1,3-diene) or 2,3-dimethylpenta-1,3-diene have been investigated, employing Lewis acids such as boron trifluoride (BF 3 ) and sterically hindered aluminium catalysts in order to verify the role of the secondary orbital interactions (SOI). While the results of the uncatalyzed and catalyzed reactions using BF 3 or aluminium trimethoxide agree with the prediction made by the frontier molecular orbital (FMO) theory considering SOI, steric repulsion of the aluminium catalysts causes orderly changes to the ratios of the product regioisomers, which could be interpreted by diminution of SOI. The transition states are located by AM1 calculations and their energies are estimated by CDNO/2-CI and ab initio 6-31G* calculations. The regioselectivities observed in the experiments are explained by comparison of their energies. A zwitterionic mechanism in the catalyzed reaction was strongly supported and the molecular orbital feature suggests the contribution of SOI which would stabilize the transition state to control the reaction pathway.

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