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

At room temperature, NMR spectroscopy indicates that the ruthenium benzylidene complex
(Cl)2(PCy3)2RuCHPh reacts with 1-pyrroline to yield (Cl)2(PCy3)(1-pyrroline)RuCHPh.
Heating a solution of (Cl)2(PCy3)2RuCHPh 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)2RuCHPh and (Cl)2(PCy3)(H2IMes)RuCHPh (H2IMes = 1,3-dimesityl-4,5-dihydroimidazolylidene) with acyclic imines of the type (R)NCH(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)2RuCHPh. The primary
products from the reactions of (Cl)2(PCy3)2RuCHPh and (Cl)2(PCy3)(H2IMes)RuCHPh with
acyclic imines are olefins and new Fischer carbene complexes of the type (Cl)2(L)(L‘)RuCH{N(H)R} (L = L‘ = PCy3; L = PCy3, L‘ = H2IMes). The ruthenium complex (Cl)2(PCy3)2RuCH{N(H)Pr} has been isolated from the reaction of (Cl)2(PCy3)2RuCHPh with (Pr)NCH(i-Pr) and has been fully characterized. A possible pathway for the reactions of
(Cl)2(PCy3)2RuCHPh and (Cl)2(PCy3)(H2IMes)RuCHPh with acyclic imines that involves
imine to enamine tautomerism followed by CC bond metathesis reactions is discussed.
The failure of the ruthenium benzylidene complexes (Cl)2(PCy3)2RuCHPh and (Cl)2(PCy3)(H2IMes)RuCHPh to react with the CN 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)2RuCHPh likely proceeds via a Lewis acid
catalyzed mechanism.