TpRu complexes as recoverable Lewis acid catalysts for enantioselective solvent-free cycloaddition reactions (Tp = hydrotris(pyrazolyl)borate)

The enantiomerically and diastereomerically pure dinitrogen-bridged complexes [{TpRu(L)}2(μ-N2)][PF6]2 (L = R,R- or S,S-1,2-bis(diphenylphosphinoamino)cyclohexane (R,R- or S,S-dppach)) were prepared by reaction of the corresponding chloro-complexes [TpRuCl(L)] with NaPF6 in dichloromethane under dinitrogen. The dinitrogen adducts react with neat methacrolein furnishing the labile complexes [TpRu(methacrolein)(L)][PF6] (L = R,R- or S,S-dppach). Both the dinitrogen and methacrolein derivatives are catalysts for the solvent-free regio- and enantioselective Diels–Alder reactions between methacrolein and cyclopentadiene or pentamethylcyclopentadiene, with moderate enantiomeric excesses ranging from 36 to ca. 70%. The metal complex can be easily recovered and re-utilised for further reactions. The dinitrogen complexes also catalyse the 1,3-dipolar cycloaddition reaction between methacrolein and benzylidenephenylamine N-oxide to yield 5-methyl-2-N-3-diphenyl-isoxazolidine-5-carbaldehyde with very high regioselectivity and 32% enantiomeric excess.

Graphical abstractThe bridging dinitrogen complexes [{TpRu(L)}2(μ-N2)][PF6]2 (L = R,R- or S,S-dppach) are catalyst precursors for the solvent-free enantioselective Diels–Alder reaction between methacrolein and cyclopentadiene or pentamethylcyclopentadiene; they have shown to be also active for the enantioselective 1,3-dipolar cycloaddition reaction between methacrolein and benzylidenephenylamine N-oxide. Figure optionsDownload full-size imageDownload as PowerPoint slide