Reactions of Ruthenium Benzylidene Complexes with Cyclic and Acyclic Imines: Oligomerization of 1-Pyrroline and Metathesis via Tautomerism

Abstract

At room temperature, NMR spectroscopy indicates that the ruthenium benzylidene complex (Cl)2(PCy3)2RuCHPh reacts with 1-pyrroline to yield (Cl)2(PCy3)(1-pyrroline)RuCHPh. Heating a solution of (Cl)2(PCy3)2RuCHPh with excess 1-pyrroline to 90 °C results in ring-opening oligomerization of the cyclic imine. The combination of ruthenium carbene complexes (Cl)2(PCy3)2RuCHPh and (Cl)2(PCy3)(H2IMes)RuCHPh (H2IMes = 1,3-dimesityl-4,5-dihydroimidazolylidene) with acyclic imines of the type (R)NCH(R‘) results in metathesis reactions when the imine possesses a C−H bond α to the imine carbon. Imines that lack C−H bonds α to the imine carbon do not react with (Cl)2(PCy3)2RuCHPh. The primary products from the reactions of (Cl)2(PCy3)2RuCHPh and (Cl)2(PCy3)(H2IMes)RuCHPh with acyclic imines are olefins and new Fischer carbene complexes of the type (Cl)2(L)(L‘)RuCH{N(H)R} (L = L‘ = PCy3; L = PCy3, L‘ = H2IMes). The ruthenium complex (Cl)2(PCy3)2RuCH{N(H)Pr} has been isolated from the reaction of (Cl)2(PCy3)2RuCHPh with (Pr)NCH(i-Pr) and has been fully characterized. A possible pathway for the reactions of (Cl)2(PCy3)2RuCHPh and (Cl)2(PCy3)(H2IMes)RuCHPh with acyclic imines that involves imine to enamine tautomerism followed by CC bond metathesis reactions is discussed. The failure of the ruthenium benzylidene complexes (Cl)2(PCy3)2RuCHPh and (Cl)2(PCy3)(H2IMes)RuCHPh to react with the CN bonds of acyclic imines in combination with observed reactivity between Ru(Cl)2(PPh3)3 and 1-pyrroline indicates that the oligomerization of 1-pyrroline with (Cl)2(PCy3)2RuCHPh likely proceeds via a Lewis acid catalyzed mechanism.