Application of Electron-Withdrawing Coordinatively Unsaturated η6-Arene β-Diketiminato-Ruthenium Complexes in Lewis Acid Catalyzed Diels-Alder Reactions

Utilizing the aza-Wittig reaction involving the ylid 3,5-(CF3)(2)C6H3NPPh3 and 1,1,1,5,5,5-hexafluoro-2,4-pentanedione, a highly fluorinated and electron-withdrawing beta-diketiminate was obtained. Using strong bases, nBuLi, Ag2O, or TlOEt, the corresponding beta-diketiminato-Li, -Ag, or -Tl chelated complexes were prepared. Subsequent in situ transmetalation with (Ru(eta(6)-C6H6)Cl-2)(2) or (Ru(eta(6)-p-cymene)Cl-2)(2) afforded the half-sandwich chloro-substituted Ru(II) beta-diketimino complexes in high yield. The synthesis of the Lewis acidic catalysts featuring a vacant coordination site at the metal center was accomplished using [Na]BArF (BAIT = tetrakis[3,5-bis(trifluoromethyl)phenyl]boron). These complexes are active for the Lewis acid catalyzed Diels-Alder reaction between alpha,beta-unsaturated aldehydes, that is, methacrolein, acrolein, and dienes, that is, cyclopentadiene and 2,3-dimethyl-1,3-butadiene, with conversions in the range of 66-98% under mild conditions. Whereas the herein described catalysts generally promote exo selectivity of the [4 + 2] cycloaddition between methacrolein and cyclopentadiene, the reaction involving acrolein shows predominantly the formation of the endo adduct, similar to that observed for the noncatalyzed reaction. Importantly, the coordinatively unsaturated complexes demonstrate moderate Lewis acidity, which allows for the controlled reaction between methacrolein and 2,3-dimethyl-1,3-butadiene to 1,3,4-trimethyl-3-cyclohexene-1-carboxaldehyde without further isomerization to the bicyclic ketone, which is in contrast to strong Lewis acidic catalysts based on transition metals or main-group elements reported in the literature.