Reactions of Ruthenium Complex with Propargylic Imine Containing a Phenol Substituent

Compound 2-((prop-2-ynylimino)methyl)phenol (1a), a propargyl aldimine, is prepared from the condensation of 2-hydroxy benzaldehyde and propargyl amine. Three analogous compounds 1b, 1c and 1d are prepared similarly, using different aldehydes as starting materials. The reactions of compounds 1a – 1c with [Ru]-Cl ([Ru] = RuCp(PPh3)2) in the presence of NH4PF6 in CH2Cl2 at room temperature for one day afford the corresponding carbene complexes 3a – 3c as the major products and the vinylidene complexes 2a - 2c as the minor products, respectively. For a longer reaction time, the same reaction afforded, in high yield, the carbene complexes 3a – 3c without 2a – 2c. Interestingly, the overnight reaction of compound 1d under the same reaction condition affords complex 2d as the only product. Lengthening the time for 7 days results in similar carbene complex 3d with some by-products. Structures of complexes 2a – 2d and 3a – 3d are proposed on the basis of spectroscopic data. The vinylidene ligand of complex 2 is from complete conversion of 1. The ligand in complex 3 resulted from addition of a o-cresol moiety to C alpha and C beta of the vinylidene ligand of 2 to yield a substituted 2H-chromene unit, with various substituents on the 3,6-positions. In the mass spectrum of 3b, the parent peak at m/z 1014.24 confirms the proposed structure.
Through our study, the added o-cresol portion comes from the organic imine compounds 1, instead of aldehyde. When two imines with different substituents were used in the reaction, new cross carbene complex formed from coupling of two imine molecules as evidenced by the mass spectrum. Therefore, the imine group plays a decisive role when the addition of an o-cresol moiety to the vinylidene ligand occurs.
Furthermore, the amine analogue 5b, which retains the propargyl group and phenol moiety, is treated with [Ru]-Cl under the same reaction condition. Completely different reactivities are observed when the imine functionality of the Schiff base used in our study is transformed to the corresponding amine group.